AFM Exclusive

17 Mar 2021

Making the connection. Atomic force microscopy correlates Graphene’s functional properties on the nanoscale

Ilka M. Hermes,1 Simonas Krotkus,2 Ben Conran,3 Clifford McAleese,3 Xiaochen Wang,3 Oliver Whear,3 Michael Heuken2 1 Park Systems Europe GmbH, Mannheim, Germany 2 AIXTRON SE, Herzogenrath, Germany 3 AIXTRON Ltd, Cambridge, United Kingdom Introduction The implementation of graphene in devices for nanoelectronics or energy conversion often requires material modifications e.g., via covalent binding or adsorption. However, the local surface inhomogeneities of pristine graphene such as wrinkles can affect the uniformity of these modifications. Therefore, accurate nanoscale characterization of the graphene topography in combination with the material’s functional properties is essential. Atomic force microscopy (AFM) unites real-space topography imaging with the detection of functional surface properties, including the electronic potential, adhesion, and modulus, and thus offers a holistic approach to the nanoscale characterization of graphene and other 2D materials. When materials are reduced to two dimensions, unique properties emerge. As such, graphene displays a variety of exceptional physical characteristics, including astonishing charge carrier dynamics, high thermal conductivity, and mechanical strength. Future applications of 2D materials and in particular graphene range from flexible electronics to optoelectronics and electrochemical energy storage. Moreover, the low dimensionality and light weight of 2D materials have attracted the attention of nanoelectronics researchers due to the continuous downscaling of electronic devices.1 Since an industrial application of graphene in electronic devices requires large wafer-scale graphene films, researchers have focused their efforts on improving monolayer growth procedures, with chemical vapor deposition (CVD) on catalytic copper (Cu) as the most explored route. However, graphene growth on Cu substrates entails a subsequent transfer onto an insulating substrate, which can damage the monolayer and introduce contaminations. Therefore, a process for direct growth on insulating substrates is an important step towards future graphene applications.2 In this study, the wafer-scale graphene was grown on LED grade c-plane sapphire in an AIXTRON CCS R&D reactor. To investigate the morphology and functional properties of CVDgrown graphene on insulating sapphire, AFM with its unique versatility is ideally suitable. Therefore, we have characterized graphene on sapphire with sideband Kelvin Probe Force Microscopy (KPFM) on a Park Systems NX20 AFM. We were able to observe a distinct contrast in the surface potential between graphene and underlying sapphire surface as well as observing variations of surface potential over graphene wrinkles and step edges. The correlation of topographic features and the KPFM surface potential becomes apparent in Figure 1 – a 3D overlay of topography and surface potential. This overlay shows a considerably lower surface potential on graphene wrinkles and around sapphire steps than on the sapphire terraces. Furthermore, the surface potential distribution corresponds to mechanical features, resolved via Park’s PinPoint nanomechanical mode. The correlation of the KPFM and the nanomechanical signals indicates a possible connection between the graphene’s mechanical and electronic properties, thereby demonstrating the potential of AFM as a holistic characterization technique for 2D materials. Figure 1. 3D overlay of the graphene topography, which displayed wrinkles and underlying sapphire steps as indicated, with the surface potential imaged via sideband KPFM. Surface potential imaging via sideband KPFM KPFM, an AFM technique, captures the surface topography simultaneously to the surface potential. As a conductive cantilever and conductive or semiconductive sample are electrically connected, their Fermi levels align and a contact potential difference (CPD) arises corresponding to the materials’ work functions. This CPD between tip and sample introduces an additional electrostatic force. In KPFM, the conductive cantilever scans the surface, while applying an AC voltage to detect local changes in this electrostatic force caused by local variations of the CPD according to the surface potential distribution.3 To measure the surface potential locally, a DC bias counteracts the CPD at each point of the scan. Based on the applied DC bias, the surface potential distribution of the sample is reconstructed in the KPFM signal. For the resolution and accuracy of the surface potential in KPFM, the detection method of the electrostatic force is decisive. In the widely applied off-resonance KPFM, the AC voltage modulates the electrostatic force at a frequency far from the resonance of the cantilever, used for topography imaging. The schematic frequency spectrum of cantilever oscillations in Figure 2a visualizes the separation of the electrical and mechanical excitation at frequencies fAC and f0, respectively. The electrostatic force is detected via the oscillation amplitude at the AC frequency. By applying a DC bias that matches the potential difference between tip and sample the amplitude at the AC frequency and therefore the electrostatic force is nullified. However, the dependence of the KPFM signal on the long-ranged force lowers the sensitivity of the measurement: non-local interactions between sample and cantilever superimpose on the local signal, which in turn impairs the spatial resolution and accuracy of the surface potential measured via off-resonance KPFM.3 To improve the resolution and accuracy of the KPFM signal, Park Systems has implemented an easy-to-use sideband KPFM mode in their NX research AFMs. In sideband KPFM, a low-frequency AC voltage (2-5 kHz) is applied to the tip to modulate the electrostatic force gradient. The modulated force gradient introduces frequency sidebands on the left and right of the cantilever resonance as shown in the schematic cantilever frequency spectrum in Figure 2b. Analogous to off-resonance KPFM, the feedback of sideband KPFM nullifies the amplitude of these sidebands by applying a DC bias matching the potential difference at each measurement position. By detecting the short-range force gradient instead of the force, long-range crosstalk decreases and the lateral resolution and local potential sensitivity improve significantly.3 By measuring sideband KPFM on the CVD-grown graphene film on a sapphire substrate, we resolved a distinct potential contrast that directly correlates with the topography of the sample (Figure 3). The topography signal showed underlying sapphire terraces and varying step heights of up to 3 nm as well as graphene wrinkles with heights ranging between 0.5 and 2 nm. Interestingly, each sapphire terrace featured finer substructures with elevations of 0.6 nm in proximity to the step edges, visible in the enlarged image detail in Figure 3. The simultaneously recorded surface potential displayed a clear contrast between two discrete states: we observed a low potential state at the position of the elevations close to the sapphire steps as well as on the graphene wrinkles, and a high potential state on the bulk film on underlying sapphire terraces. For the wrinkle captured in the line profile of Figure 3, we detected a potential contrast of around 0.5 V with respect to the bulk film on sapphire terraces.4 The potential on the elevations close to the sapphire steps, on the other hand, differed by around 0.7 V from the potential of the bulk film on the terraces. Figure 2. Schematic frequency spectra of the cantilever oscillation for off-resonance KPFM in a) and sideband KPFM in b).   Figure 3. Topography and surface potential captured via sideband KPFM on CVD-grown graphene on a sapphire substrate. The line profiles of the topography in green and the surface potential in red show a correlation of the two signals with a distinct potential contrast between underlying sapphire steps and terraces as well as graphene wrinkles. Correlation of surface potential and nanomechanics To further investigate the graphene wrinkles and the elevations around the sapphire steps as well as their surface potential contrast, we imaged the sample’s adhesion force and modulus in the same position using Park System’s PinPoint nanomechanical mode. PinPoint provides accurate and quantitative nanomechanical images via fast force spectroscopy mapping as shown in Figure 4: the cantilever approaches and retracts at each pixel of the entire scan area to simultaneously acquire 3D topography and nanomechanical information of the sample surface. At each pixel, the XY scanner stops and high-speed force-distance curves are acquired with well-defined control of contact force and contact time between the tip and the sample. The automated analysis of each force curve allows for realtime visualization of sample deformation, modulus, and adhesion force simultaneously to the topography imaging. Figure 4. Schematic diagram of Park Systems’ PinPoint nanomechanical mode. In this force spectroscopy technique, the tip approaches the sample and retracts at each pixel before moving to the next pixel, as indicated by positions 1, 2, 3, and 4. The resulting force curves and their automated analysis allow real-time visualization of nanomechanical properties including adhesion force, modulus, stiffness, and deformation.   Figure 5. Adhesion force and modulus acquired on graphene on a sapphire substrate via Park Systems’ PinPoint nanomechanical mode and the corresponding surface potential imaged via sideband KPFM at the same measurement area. The white box highlights the same sapphire terrace featuring a higher adhesion force, deformation, and surface potential compared to surrounding sapphire steps. The adhesion force and modulus acquired via PinPoint nanomechanical mode show a contrast between the underlying sapphire terraces and the steps and graphene wrinkles (Figure 5). As such, the elevations in proximity to the sapphire steps, which also featured a decreased surface potential, exhibit a lower adhesion force and a higher modulus. The lower adhesion and higher modulus suggest that the graphene film becomes harder at these positions. On the sapphire terraces, on the other hand, the adhesion force increases and the modulus decreases. The correlation of the nanomechanical properties with the surface potential and the sample topography indicates a possible connection of the material’s mechanical and electronic properties. Summary The combination of different AFM techniques including sideband KPFM and PinPoint nanomechanical mode available on Park Systems’ research AFMs allows a holistic and in-depth characterization of 2D materials as demonstrated on a wafer-scale CVD-grown graphene on sapphire produced in an AIXTRON CCS R&D reactor. The measurements showed a distinct correlation of the sample’s surface potential with the adhesion force and modulus indicating a connection of graphene’s electronic and mechanical properties. Characterizing the highly localized mechanical and electronic properties of 2D materials enables a tailored development for future nanoelectronics applications. References [1] S. J. Kim, K. Choi, B. Lee, Y. Kim and B. H. Hong, Annu. Rev. Mater. Res., 2015, 45, 63–84. [2] N. Mishra, S. Forti, F. Fabbri, L. Martini, C. McAleese, B. R. Conran, P. R. Whelan, A. Shivayogimath, B. S. Jessen and L. Buß, Small, 2019, 15, 1904906. [3] U. Zerweck, C. Loppacher, T. Otto, S. Grafström and L. M. Eng, Phys. Rev. B, 2005, 71, 125424. [4] S. Samaddar, J. Coraux, S. C. Martin, B. Grévin, H. Courtois and C. B. Winkelmann, Nanoscale, 2016, 8, 15162–15166.

19 Feb 2021

Surface Potential Imaging via Sideband Kelvin Probe Force Microscopy

  Armando Melgarejo, Ben Schoenek, and Byong Kim Technical Services, Park Systems, Inc., USA Introduction From materials science to biological research, scientists have adopted Kelvin Probe Force Microscopy (KPFM) to measure surface potential and work functions. KPFM follows the Electrostatic Force Microscopy (EFM) operation principle. It measures a contact potential difference (CPD) and determines the sample's work function using DC bias feedback. However, conventional Off-resonance KPFM produces low spatial resolution and low signal-noise ratios due to long-range crosstalk caused by the interaction between the sample and the total body of the cantilever. Sideband KPFM has an unparalleled spatial resolution and centers on the local interaction between the tip apex and sample, making it useful for analyzing grain boundaries, semiconductor junctions, photovoltaics materials, and even molecular structures [1-3]. KPFM operates by measuring the CPD. CPD is an electrostatic potential that exists between samples of two dissimilar, electrically connected materials. Considering the atomic force microscopy (AFM), the two different materials are the AFM tip and the sample. Each one of the materials has a specific conduction band and a separate work function. The work function of a material is the amount of energy required to remove an electron to infinity from the surface of a given solid [2]. If the tip and the sample are electrically connected, the electrical connection will induce a natural flow of electrons and will create a potential difference between the two materials (Figure 1). This potential difference is measurable by applying the theoretical formula of contact potential difference: where ϕtip & ϕsample are the work function of the sample and tip, respectively, and “e” corresponds to the electronic charge. Obtaining the work function of the material is possible by multiplying the surface potential of the materials by a single electron charge. Equation (1) can be explained using the Kelvin Probe method, which relies on detecting the electric field of the materials composing the AFM tip and sample. By modifying the voltage CPD, the electric field varies. Thereby, when applying an external bias (VDC) of the same magnitude to the AFM tip but opposite direction, the surface charge of the contact area nullifies. Once there are no electrostatic forces, the work function difference has the same value as the applied voltage CPD; consequently, the work function of the sample can be calculated if the work function of the tip is known (Equation (2)) [3]. Figure 1. Contact Potential Difference between an AFM tip and sample. Off-resonance KPFM When considering the AFM tip and the sample as a capacitor, analyzing the electrostatic forces between them is straightforward, as the amount of energy can be calculated with the difference in voltage and the capacitance. KPFM uses two frequencies to acquire topography and surface potential simultaneously. The standard procedure for KPFM consists of using two lock-in amplifiers (Figure 2A). The first lock-in amplifier modulates the frequency used to oscillate the cantilever at its mechanical resonant frequency, using a piezoelectric material to obtain a topography image. The second lock-in amplifier regulates the frequency, generally at 17 kHz, to measure the surface potential. This technique is denoted “Off-resonance KPFM,” as we can theoretically choose any resonance frequency as long as it does not match the cantilever's mechanical resonance [2]. Additionally, Off-resonance KPFM gets the surface potential information by measuring the total interaction of the complete cantilever with the sample, limiting its spatial resolution. Figure 2. Sideband KPFM feedback loop. a) Off-resonance KPFM Loop. b) Sideband KPFM Loop. Sideband KPFM Sideband KPFM is a technique that exploits the intermodulation of an electrostatic drive force and a mechanical drive force to upconvert the electrostatic frequency to the first flexural resonance, where the high Q “quality factor” yields a more sensitive measurement [3]. Additionally, Sideband KPFM signal is calculated by using a local interaction between the tip apex and the sample rather than a total interaction between the cantilever and the sample, improving the spatial resolution over Off-resonance KPFM. Local interaction allows the system to measure the surface potential of localized features with high resolution, due to the reduced interaction with other parts of the cantilever. The reason for the substantial reduction of the interaction, induced by the cantilever and the tip, is the downward distance dependence of the lever and cone forces on the experimentally important range [1]. The electrical driving frequencies of Sideband KPFM appear at the sidebands of the mechanical oscillation of the cantilever (Figure 3), reducing the long-range crosstalk [3]. The sideband frequencies of the mechanical oscillation and the local interaction of the tip apex allow Sideband KPFM to accurately measure the surface potential with an increased resolution compared to Off-resonance KPFM. Figure 3. Fourier Transform of the vertical deflection comparing Off-resonance KFPFM vs Sideband KPFM. When performing Sideband KPFM, instead of using a frequency of 17 kHz, the KPFM mode works with the sideband’s frequencies, usually 1-5 kHz Off from the cantilever’s mechanical resonance. This method uses two lock-in amplifiers (Figure 2B) to lock on the sideband frequencies. Figure 2B shows that the second lock-in amplifier modulates the left sideband, while the third modulates the right sideband. The surface potential is obtained by averaging the DC voltage feedback from both sideband frequencies [1]. EFM Amplitude Sensitivity The KPFM signal-to-noise ratio reflects the magnitude of the electrostatic interaction, or Electrostatic Force Microscopy (EFM) amplitude sensitivity. The higher the EFM amplitude sensitivity, the higher the KPFM signal-to-noise ratio response can be. The EFM amplitude sensitivity can be defined as ΔEFM amplitude / Δ tip bias. When the cantilever’s resonance frequency equals the electrical driving frequency, the EFM amplitude can be defined as: where X is the amplitude of the cantilever displacement, Q is the cantilever’s Q factor, and “k” is the spring constant of the cantilever. Thereby, the EFM amplitude is directly proportional to the VAC and inversely proportional to the cantilever’s spring constant. Another factor that affects the EFM amplitude sensitivity is the laser displacement on the photo detector (Δa) [4]. It is defined as: where Lx is the distance between the tip and the cantilever, Lc the cantilever’s length, and ΔZ the displacement of the cantilever oscillation. Therefore, the longer the cantilever, the smaller the displacement. The EFM amplitude sensitivity, thus the KPFM signal sensitivity, increases when the cantilever has a short length and/or a small spring constant [4]. F14H20 KPFM not only measures the CPD on metals or semiconductors, but it can also measure the surface potential of organic or soft samples like the self-assembled molecules of some alkanes. Eicosylperfluorotetradecane, F(CF2)14(CH2)20H, or its abbreviated form F14H20, is a semifluorinated alkane with a perfluorinated chain segment. It consists of two incompatible subunits that segregate into distinct domains. The mismatch in the cross-section between the perfluoro and the perhydro alkyl tail result in distinctive superstructure formations (Figure 4). The mechanism on how these compounds order in specific polymorphism is yet not fully understood. The investigation of semifluorinated alkanes, such as F14H20, contributes to self-assembly knowledge [5]. Figure 4. Schematic pictures of the ribbon and the spiral morphology for F14H20 Experimental In this study, a Park NX10 AFM system, equipped with SmartScanTM software, measures the local contact potential difference and topography of F14H20 molecular structures. The study shows a comparison between Sideband KPFM and conventional Off-resonance KPFM image resolution. For the Sideband KPFM experiments, lock-in amplifiers send a 3 kHz AC signal to the cantilever, while the standard technique uses a 17 kHz AC signal. SmartScan automatically selects the phase and amplitude of the resonances. For these experiments, the average signal of both sidebands is recorded. Two different types of conductive cantilevers, NSC14/Cr-Au and NSC36-C/Cr-Au, are used to analyze the correlation between EFM amplitude sensitivity, the cantilever’s length, and the spring constant. NSC14/Cr-Au and NSC36-C/Cr-Au are 125 and 130 μm long and have a spring constant of 5 and 0.6 N/m, respectively. Results Figure 5. Sideband and Off-resonance comparison. (a) Sideband topography image. (b) Sideband surface potential image. (c) Off-resonance topography image. (d) Off-resonance surface potential image. (e) Line profiles taken along red line in (b) and along green line in (d). Figure 5 shows a surface potential comparison between Sideband and conventional Off-KPFM images obtained with a NSC36-C/Cr-Au cantilever. The Sideband KPFM measurement of the F14H20 aggregates shows a surface potential contrast of 600-650 mV between the fluorinated parts of the molecules and the substrate, along with a clear lateral resolution where even the in-between space of the aggregates is distinguished. Looking at the conventional Off-resonance KPFM measurements, the image shows that the technique successfully measures the potential difference of the aggregates and the substrate; however, the potential difference gives a value of 300 mV. The spatial resolution is not as well defined. The improvement of resolution when using the Sideband mode relies on the local interaction between the tip apex and sample. The topography images indicate that the F14H20 aggregates of this sample form an incomplete spiral superstructure (Figure 5). These incomplete spirals tend to form more compacted superstructures, reducing the in-between spaces of the aggregates [5]. Figure 6. Surface Potential Difference of F14H20 via Sideband KPFM. (a) Surface Potential imaged with probeNSC14/Cr-Au. (b) Surface Potential imaged with probe NSC36C/Cr-Au. (c) Line profiles taken along redline in (a) and green line in (b). Figure 6 shows the results of the correlation between the spring constant of the cantilever and KPFM signal-to-noise ratio sensitivity. Cantilever NSC36-C/Cr-Au (Figure 6b) shows a higher sensitivity for the surface potential changes than the cantilever NSC14/Cr-Au (Figure 6a). Line profiles taken from both images, shown in Figures 6a and 6b, demonstrate that using either type of cantilever tip allows Sideband KPFM to detect small changes on the surface potential, even in-between the F14H20 superstructures. The fact that NSC36-C/Cr-Au shows a higher KPFM signal sensitivity and has a spring constant that is almost ten times smaller than NSC14/Cr-Au indicates a correlation between the spring constant of the probe and the sensitivity. Equation 4 establishes that the length of the cantilever is also crucial for EFM amplitude sensitivity; both cantilevers have almost the same length, so the correlation between cantilever length and EFM sensitivity is not analyzed. Further experiments are needed to investigate the correlation between the cantilever's length, the EFM amplitude, and the KPFM signal sensitivity. Summary In this study, Sideband Kelvin Probe Force Microscopy on a Park NX10 atomic force microscope is used to image F14H20 molecular structures. This study validates that Sideband KPFM is a technique that measures the surface potential with higher sensitivity and spatial resolution compared to the conventional Off-resonance KPFM techniques. The results suggest a correlation between KPFM signal sensitivity and the cantilever spring constant, and that a lower spring constant of the cantilever yields higher sensitivity than stiffer cantilevers. Further study is necessary to understand the correlation between KPFM signal sensitivity and cantilever length. References 1. Park Systems Application Technology Center. A Comparative Study of Atomic Force Microscopy between AM-KPFM and Sideband KPFM, Principles and Applications, https://parksystems.com/images/media/appnote/AppNote-61_sideband-KPFM_A-comparative-study.pdf 2. H. Seo, D. Guo, and G. Jung, How to obtain sample potential data for KPFM measurement. Park Systems Corporation, https://parksystems.com/images/media/appnote/How-to-obtain-sample-potential-data-for-KPFM-measurement-18.pdf 3. A. Cerreta, et al. Exploring the Potential of the Sideband KPFM Mode. Park Systems Webinar, https://parksystems.com/medias/nano-academy/webinars/115-webinars/1850-exploring-the-potential-of-the-sideband-kpfm-mode-april-9-2020 4. T. Fukuma, et al. Development of low noise cantilever deflection sensor for multi environment frequency modulation atomic force microscopy. Review of Scientific Instruments, Vol 76, (2005): 1-8. 5. A. Mourran, et al. Self-Assembly of the Perfluoroalkyl-Alkane F14H20 in Ultrathin Films. Langmuir, Vol 21, (2005): 2308-2316.

12 Jan 2021

A Comparative Study of Atomic Force Microscopy between AM KPFM and Sideband KPFM, Principles and Applications

  Research Application Technology Center, Park Systems Corporation Introduction Since the development of Atomic Force Microscopy (AFM) [1], several measurement modes have been developed to characterize electrical, mechanical, magnetic, and thermal properties. Among them, Kelvin Probe Force Microscopy (KPFM) [2] is a well-known mode that enables the monitoring of both surface morphology and surface potential distribution properties on a nanometer scale. KPFM has been utilized extensively to investigate the localized charge distributions on a surface layer [3], surface potential distributions [4] variations in surface work functions [5] and ferroelectric domains [6], in a variety of research fields. KPFM can simultaneously deliver both surface topographical information and surface potential/work functions of the sample by applying AC and DC voltage. Using the same configuration as Electrostatic Force Microscopy (EFM), KPFM monitors the surface potential and work function of samples by applying a DC bias (VDC) to nullify tip-sample potential difference. The VDC, controlled by the KPFM feedback, equals the surface potential (VS) and thereby generates the surface potential map. In the conventional method, amplitude modulation (AM) KPFM, the measurement signal directly relates to the electrostatic force between sample surface and tip via the amplitude of the AC frequency signal (Figure 1a). This implies that AM KPFM is strongly affected by the whole AFM cantilever, which lowers the resolution and sensitivity due to an averaging effect [7] [8]. Sideband KPFM has an advantage in terms of the spatial resolution and sensitivity of the surface potential measurement compared to AM-KPFM [9]. The AC voltage (VAC) applied between AFM tip and sample, generates and oscillation in the electrostatic force and its gradient. This oscillation leads to a modulation of the resonance frequency of the cantilever (f0). In the frequency spectrum of the cantilever deflection, sidebands appear at f0 + fAC and f0 – fAC caused by the oscillating force gradient (Figure 1b). The application of a DC voltage, which matches the potential difference between the AFM tip and the sample, compensates for the electrostatic force and the sideband disappears. By measuring the DC voltage required to compensate for the electrostatic force, the surface potential distribution of the sample can be imaged, as well as the work function distribution, if the work function of the tip is known. Unlike AM-KPFM, the Sideband KPFM signal depends on the electrostatic force gradient and is therefore mainly affected by the AFM tip apex. The dependence on the force gradient leads to an improved spatial resolution and potential sensitivity. In this study, we present results obtained with AM-KPFM and Sideband KPFM on well-defined samples with extended areas of different surface potentials. From these results, we directly compare the spatial resolution of AM-KPFM and Sideband KPFM under identical conditions. Figure 1. Frequency spectra for AM-KPFM (a) and Sideband KPFM (b). Materials and Methods Sideband KPFM Sideband KPFM is an optional AFM mode used to measure electrical properties of a sample surface. Figure 2 shows the connection diagram of Sideband KPFM, which uses two lock-in amplifiers to measure the amplitude and phase of each sideband. On lock-in amplifiers 2 and 3, a signal with a frequency of f0 ± fAC is used as the reference signal (generated by lock-in amplifier 2) to decouple the sideband signals with a frequency of f0 ± fAC in the deflection signal (detected by lock-in amplifiers 2 and 3). The decoupled signals from the two lock-in amplifiers are averaged and used for the feedback to adjust the DC voltage. The AFM controller applies a corresponding VDC so the averaged sideband peak size becomes zero (as tip bias servo in Park SmartScan). Figure 2. Connection diagram of Sideband KPFM.. Test samples and AFM cantilever ZYH grade highly-ordered pyrolytic graphite (HOPG, SPI Supplies) was utilized for showing layer by layer surface potential differences. A silicone substrate with low conductivity polymer dots was used to display the surface potential contrast of two different materials. An IFX dopant calibration sample (NP20, Infineon) was chosen to monitor the stepwise electrical signal. This sample consisted of 5 different n-type implant areas in order to compare the electrical signals. All imaging was performed using an Au coated NSC36-C (resonance frequency, 65 kHz; spring constant, 0.6 N/m; length, 130 μm) with the same AFM parameters. Results and discussion This study analyzes three different samples to compare the electrical resolution performances of AM-KPFM and Sideband KPFM. Figure 3 shows surface potential images of HOPG by AM-KPFM and Sideband KPFM. Sideband KPFM confirms the sharp contrast due to the HOPG layer, whereas AM-KPFM shows blurry edges. As shown in the line profile analysis, Sideband KPFM has an approximately two-fold higher surface potential difference on layers (~70 mV) than AM-KPFM (~35 mV, red arrow). Also, Sideband KPFM displays small HOPG fragments very clearly, while AM-KPFM shows a blurry image. Figure 3. The comparison of surface potential between AM-KPFM (a) and Sideband KPFM (b) on a HOPG sample with a line profile analysis (c). Next, the analysis of polymer dots on a silicone substrate was performed by AM-KPFM and Sideband KPFM. Additionally, Sideband KPFM both in and out of lift mode were compared to investigate the non-contact feedback performance for Sideband KPFM. In Figure 4, all images show contrasts between the polymer dots and the silicone substrate. However, AM-KPFM indicates a lower surface potential contrast than Sideband KPFM including lift mode. For Sideband KPFM, there is no significant difference between the no-lift mode and a 5 nm lift mode, which the line profile analysis confirms. Surface potential differences between the polymer dots and the silicone substrate are approximately 180 mV in AM-KPFM, ~300 mV for the no-lift Sideband KPFM, and ~330 mV for the 5 nm lift Sideband KPFM. Figure 4. Comparison of surface potential between AM-KPFM (a), Sideband KPFM with no lift (b) and Sideband KPFM with 5 nm lift (c) on polymer dots sample with line profile analysis (d). Lastly, AM-KPFM and Sideband KPFM were used on a IFX dopant calibration sample to compare stepwise electrical resolution. In AM-KPFM, different doping level of n-type implant are recognized; however, measurement of the steps in the electrical signal was difficult. In contrast, the stepwise electrical signal is displayed clearly in both no-lift and 5 nm lift Sideband KPFM. Based on several repetitive tests, all Sideband KPFM analyses show improved spatial resolution and potential sensitivity than those of AM-KPFM. As the line profile suggests, Sideband KPFM can monitor step-by-step surface potential differences, while AM-KPFM only detects a gradual signal decrease. Figure 5. Multiple measurements on AM-KPFM (a), Sideband KPFM with no lift (b) and Sideband KPFM with 5 nm lift (c) on an IFX dopant calibration sample with line profile analysis (d). Conclusions In this study, we compare AM-KPFM and Sideband KPFM using a variety of samples. The results demonstrate that Sideband KPFM has superior spatial resolution and potential sensitivity compared to AM-KPFM. In surface potential images, Sideband KPFM shows a clear electrical contrast and detects relatively small potential changes. From line profile analysis, we find that the surface potential differences measured by Sideband KPFM are higher than that of AM-KPFM, underlining the superior performance of Sideband KPFM. Furthermore, the comparison between no lift and 5 nm lift in Sideband KPFM shows no difference for all tested samples. Due to the accuracy of Park Systems’s AFM feedback system, topography mapping is performed in True Non-Contact Mode without tapping between AFM tip and sample surface. Thus, it is possible to obtain the true surface morphology and a clear KPFM signal without crosstalk. Since the development of KPFM, it has become one of the more useful AFM options utilized by surface material science and in semiconductor engineering. It is a unique technique for surface potential or work function mapping on the nanoscale, and the Sideband KPFM option offers superior spatial resolution and improved electrical sensitivity for material characterizations. References 1. Quate, C. F., Gerber, C., & Binnig, C. (1986). Atomic force microscope. Phys. Rev. Lett, 56(9), 930-933. 2. Nonnenmacher, M., o’Boyle, M. P., & Wickramasinghe, H. K. (1991). Kelvin probe force microscopy. Applied physics letters, 58(25), 2921-2923. 3. Lü, J., Delamarche, E., Eng, L., Bennewitz, R., Meyer, E., & Güntherodt, H. J. (1999). Kelvin probe force microscopy on surfaces: investigation of the surface potential of self-assembled monolayers on gold. Langmuir, 15(23), 8184-8188. 4. Liu, L., & Li, G. (2010). Electrical characterization of single-walled carbon nanotubes in organic solar cells by Kelvin probe force microscopy. Applied Physics Letters, 96(8), 33. 5. Bieletzki, M., Hynninen, T., Soini, T. M., Pivetta, M., Henry, C. R., Foster, A. S., ... & Heiz, U. (2010). Topography and work function measurements of thin MgO (001) films on Ag (001) by nc-AFM and KPFM. Physical Chemistry Chemical Physics, 12(13), 3203-3209. 6. Wu, W., Horibe, Y., Lee, N., Cheong, S. W., & Guest, J. R. (2012). Conduction of topologically protected charged ferroelectric domain walls. Physical review letters, 108(7), 077203. 7. Melitz, W., Shen, J., Kummel, A. C., & Lee, S. (2011). Kelvin probe force microscopy and its application. Surface science reports, 66(1), 1-27. 8. Garrett, J. L., & Munday, J. N. (2016). Fast, high-resolution surface potential measurements in air with heterodyne Kelvin probe force microscopy. Nanotechnology, 27(24), 245705. 9. Stan, G. (2020). High-speed digitization of the amplitude and frequency in open-loop sideband frequency-modulation Kelvin probe force microscopy. Nanotechnology.

12 Jan 2021

Contact AFM Nanolithography Based on Anodic Oxidation

  Armando Melgarejo, Ben Schoenek, Jiali Zhang, and Byong Kim Park Systems, Inc., Santa Clara, CA, USA Introduction The field of nanotechnology has diversified into different areas of research, from materials science to biotechnology. Many of these applications are based on the capability to fabricate or manipulate nanostructured materials [1]. One convenient technique for structuring, manipulation, and fabrication at the nanometer scale is atomic force microscopy (AFM) nanolithography. ll AFM nanolithography techniques can be classified into two general groups in terms of their operational principles: force-assisted and bias-assisted nanolithography [2]. The force-assisted method consists of applying a large force to a sharp tip to mechanically modify the surface atoms or molecules of the sample and produce trenches on the surface. In this case, the interaction between the tip and the sample is purely mechanical [2]. On the contrary, the bias-assisted method entails applying a voltage between the AFM tip and the substrate in contact with the sample. The tip-sample voltage induces an electrochemical reaction that produces oxide on the surface of the substrate [1]. This application note demonstrates an electrochemical process called anodic oxidation using the bias-assisted lithography method to create oxide patterns on the surface of a silicon substrate. The success of this technique relies on using the AFM tip as a biased cathode to the sample surface. Also, the water meniscus around the sample acts as an electrolyte for the chemical reaction. The environment humidity directly influences the size of the meniscus [1]. Therefore, this application shows nanoscale oxide line formation using AFM anodized lithography on a Park NX10 AFM system using Park SmartLithoTM, the new nanolithography software developed by Park Systems. Park SmartLitho [3] can also be used for nanomanipulation, scratching techniques, and high voltage lithography. Experimental For this experiment, a Park NX10 AFM was used to perform bias-assisted nanolithography to draw nanopatterns onto a bare silicon substrate. The oxide patterns were formed using Park SmartLithoTM. Non-contact mode imaging was conducted after the lithography process to confirm the successful fabrication of the oxide patterns on the surface as well as to distinguish the surface difference between the oxide layers and silicon substrate. A conductive AFM cantilever probe (Multi 75G) with a nominal spring constant (k = 3 N/m) and resonance frequency (f = 75 kHz) was utilized in these experiments. The Park SmartLitho consists mainly of five windows and panels that allow the operator to control the complete lithography process (Figure 1). In the first step, the appropriate alignment of the super-luminescent diode (SLD) and position-sensitive photodetector (PSPD) is verified (Figure 1A). After confirming the correct alignment, the operator selects the control mode; in this case, the setpoint mode was selected. For the anodic oxidation, a bias has to be applied; in this case, the tip was chosen as the desired channel (Figure 1B). In the design area (Figure 1C), the operator can insert the desired figures and shapes using a previous AFM image as a baseline. All embedded figures will appear in the object list (in this case, just one line) (Figure 1D). The operator can change the formation order, and the estimated lithography time for each figure is provided. Finally, for any desired feature, parameters can be changed in the object edit panel (Figure 1E). Parameters such as voltage, load force, stroke speed, extend speed, or lift speed can be modified to obtain different features within the same designed figures. Figure 1. Park SmartLithoTM software. A) Vision & Monitoring view. B) Nanolithography mode panel. C) Lithography design area. D) Object list. E) Objects edit panel. To start a lithography or manipulation process, a baseline image is needed. In this case, the baseline image already contains one previously-drawn line to compare with the line about to be drawn (Figure 1C). The image is 1.5 µm by 1.5 µm. The sample features small nanoparticles on its surface according to the image taken using non-contact AFM mode. The lithography process is planned by drawing a shape on the surface utilizing the software. The AFM operating software, Park SmartScanTM, provides the operator with offset and easy positioning features. The DC bias lithography process is done in contact mode, and in this case, a bias of negative 5.0 Volts is applied between the tip and sample. For this experiment, a line was drawn at 0.2 µm/s with a 200 nN load force. The drawn line is approximately 1.8 microns in length (Figure 2, Top Left). The process takes a couple of seconds to complete. As the cantilever is compressed to the loading force of 200 nN, the PSPD signal moves up, which visually verifies the applied pressure. Results and Discussion Following the lithography process and using Park SmartScan, the system is switched back to non-contact mode. An AFM cantilever probe is mounted on a piezoelectric bimorph shaker, attached to the AFM head Z piezoelectric scanner. The shaker vibrates the AFM cantilever probe at its resonant frequency, which is automatically selected along with the vibration amplitude. As the probe nears the sample surface, the vibration amplitude decreases. Likewise, as the probe moves away from the surface, the amplitude increases. The Z piezo scanner moves up and down and adjusts its height to maintain a constant vibration amplitude. The height adjustment of the scanner thus reflects the height change on the sample surface. The AFM raster scans and detects the height change point-by-point in the XY-direction to map out the three-dimensional surface topography. The tip approaches the surface, and the image is retaken promptly with the same parameters as the baseline AFM image. The newly drawn line is clearly visible and parallel to the previously drawn line (Figure 2, Top Left). Notice the image position remains the same, showing the control of the XY-scanner, separate from the Z-scanner, even at smaller scan sizes. In this example, the lithography process is done in contact mode, and therefore, imaging in non-contact mode is an option. However, after the manipulation, it is important to take the image using non-contact mode to avoid disturbing the particles present on the sample. After verifying the success of lithography process, the final image is taken and exported to the analysis software called Park XEI™ [4]. In this case, we can see that the previously-drawn line is around 35 nm in width, and the newly-drawn one is 47 nm (Figure 2, Bottom). Both lines are drawn using the same lithography parameters. Nevertheless, the line on the right side, drawn first, is observed to be narrower than the oxide line on the left. This might be due to performing the lithography in contact mode. In contact mode, the tip is rubbed against the sample surface as the lines are drawn. The rubbing can cause the end of the tip to wear out and blunt. The blunted tip can fabricate wider oxide lines than the ones drawn with a sharper tip [1,5]. We can see that the resulting line-height is approximately 1.4 nm for both lines when adding another cursor. Park XEI software is capable of showing a three-dimensional rendering of the image taken (Figure 2, Top Right). With the 3D representation, we notice that the lines drawn during this oxide growth process have a lower height than the nanoparticles on the surface. Figure 2. Nanolithography process. (Top Left) Post-scan image. Top Right) 3D View. (Bottom) Line profile plotted of green line seen in Top Left. Conclusion This application note discusses how to use Park SmarthLithoTM software to easily design and customize nanoscale oxide patterns using Park NX10 AFM. With only a few seconds of runtime, this simple test case demonstrates a bias-assisted nanolithography process, which successfully generates oxide patterns as narrow as 35 nm in width. Although the process itself employs contact mode, true non-contact mode before and after the operation validates that the new oxide line is indeed parallel to the line in the baseline image. This test case also demonstrates the scanning and imaging precision of Park’s XY-scanner. Overall, this study shows that bias mode in Park SmartLitho software is excellent tool for generating well-defined nanoscale patterns and features.   References 1. E. Pinilla-Cienfuegos, et al. Local Oxidation Nanolithography on Metallic Transition Metal Dichalcogenides Surfaces. Applied Science, (2016): 250. 2. JP. Pineda, et al., Nanoscale material patterning using atomic force microscopy nanolithography. NanoScientific, Vol 18, (2020): 21-23. 3. Park Systems. (2020). Park SmartScanTM. California, USA. https://parksystems.com/products/operating-software/park-smartscan 4. Park Systems. (2020). XEI Data Processing and Analysis. California, USA. https://parksystems.com/manuals-software 5. J. Voves. Nanoelectronics and nanolithography. Nanocon 20, (2009): 22-30.

5 Jul 2020

Carrier Profiling in High Vacuum Using Scanning Spreading Resistance Microscopy and Scanning Capacitance Microscopy

Lennaert Wouters1, Albert Minj1,2, Umberto Celano1, Thomas Hantschel1, Wilfried Vandervorst1,2, Kristof Paredis1 1IMEC, Leuven, Belgium 2Department of Physics and Astronomy, University of Leuven, Leuven, Belgium Introduction Scanning Spreading Resistance Microscopy (SSRM) and Scanning Capacitance Microscopy (SCM) are both established scanning probe-based methods for two-dimensional carrier profiling. Their initial development was mainly pushed by the microelectronics industry looking for 2D alternatives for their 1D carrier/dopant profiling techniques, such as Capacitance-Voltage (C-V) measurements, Secondary Ion Mass Spectrometry (SIMS), and Scanning Resistance Profiling (SRP). In SSRM, the resistance of the current spreading through the nanoscale tip-sample contact is measured while scanning the probe. The key to its large sensitivity and spatial resolution lies in the presence of a pressure induced metallic pocket below the tip apex resulting in a (nearly) ohmic tip sample contact. As a result, the measured resistance is dominated by the spreading resistance and hence dependent on the local sample resistivity ρ: Rspreading = ρ/4a, Where a represents the contact size. The main advantages of SSRM include a very high spatial resolution (~ 1 nm) and a large sensitivity and dynamic range (1014-1020 cm-3). In SCM, small capacitance variations (~10-21 F) between the tip and the sample are measured using a high frequency capacitance sensor while scanning the probe. Highly doped regions show a low differential capacitance dC/dV, while lowly doped areas exhibit a relatively larger capacitance change. The main advantages are the large dynamic range and the carrier type sensitivity as n- and p-type show opposite phases in their dC/dV signals. In both methods, it is not straightforward to convert the measured values directly into quantitative values for carriers because detailed information on the probe and sample surface are missing. Hence, a calibration against a known standard is generally the most straightforward way for converting resistance/differential capacitance to resistivity/carrier concentrations. In fact, both techniques exhibit pros and cons, but are quite complementary; SSRM provides the highest spatial resolution while SCM provides carrier type sensitivity. As current state-of-the-art devices are entering the sub 10 nm nodes, the requirements for materials and device characterization are being pushed to their limits, requiring every aspect of the measurement to be optimized for successful measurements, for instance the probe, the sample preparation, and the measurement environment. In this application note, we review the performance of the Park NX-Hivac atomic force microscope (AFM) from Park Systems [1] for SSRM and SCM applications, based on the following three samples: 1. A p-type doped silicon calibration sample (imec CS08-SiB): a sample consisting of various Boron doped epitaxial silicon layers (~600 nm) with known doping concentration (Figure 1). 2. An n-type doped silicon calibration sample (imec CS01-SiAs): a sample consisting of various arsenic doped epitaxial silicon layers with known doping concentration (Figure 5). This sample additionally has a p-type doped layer embedded in the stack. 3. Buried oxide sample: a 0.5 nm oxide layer sandwiched in between a highly doped Si and a poly Si layer (Figure 2). These samples are measured on the Park NX-Hivac AFM system, operating in vacuum conditions (~5e-5 mbar) using imec Full Diamond probes [2]. SSRM – Basic Performance Firstly, the p-type calibration sample is measured to investigate the repeatability of SSRM measurements. Figure 1 shows that excellent repeatability is achieved in SSRM measurements on silicon. Note that the repeatability is better in the highly doped layers. As a reference, for identical scanning parameters, typically a repeatability of 15-20 % is achieved in air highlighting the added value of high vacuum [3]. Figure 1: SSRM measurement on a p-type doped silicon calibration sample performed in vacuum. The coloured resistance cross-sections are taken from single line scans and the black line is the average over those 5 scan lines. The repeatability is plotted in purple (right Y-axis). Secondly, to assess the resolution of SSRM the buried oxide sample is measured. Figure 2 demonstrates that the 0.5 nm oxide layer embedded in silicon can easily be observed in SSRM measurements as an increase in the resistance. The doubling of the resistance on the thin oxide indicates that the electrical contact radius is similar as the oxide thickness, highlighting a sub-nm resolution [2]. Figure 2: High resolution SSRM measurement on a 0.5 nm oxide layer sandwiched in between highly doped Si and poly Si layers. The resistance cross-section is taken from a single scan line. Finally, the p-type calibration sample is measured in air and in vacuum with the same probe to verify the advantages of performing SSRM measurements in a vacuum environment. The results are summarized in Figure3. As expected, in order to obtain an image of similar quality, the applied force to the tip required for a good electrical tip-sample contact had to be increased by almost 50 % when moving from vacuum to ambient environment. This observation suggests that the metallic pocket below the tip apex is induced at a lower pressure in vacuum as compared to in air. Therefore, measurements in vacuum environment can be performed with a lower force applied to the tip which will result in less tip wear and thus higher resolution [2]. Figure 3: SSRM measurements on a p-type doped silicon calibration sample performed in vacuum and in air with the same probe. The resistance cross-sections are the average over 50 scan lines. From the results shown above it can be concluded that the repeatability and resolution of SSRM measurements performed on the Park NX-Hivac AFM are state of the art, and that the vacuum conditions in the chamber are beneficial for conducting repeatable SSRM experiments. SSRM – Device measurements & calibration In general, the calibration samples allow to convert the resistance map of an unknown sample to a carrier concentration map. To do so, the calibration sample is measured with the same probe and scan parameters as the measurement on the sample of interest. As an example, in Figure 4, the resistance and carrier concentration maps of an n-type doped silicon layer in a solar cell are shown. Figure 4: SSRM resistance and carrier concentration maps of an n-type doped silicon layer in a solar cell sample. The carrier concentration map is calculated from the resistance map by using the calibration curve that is shown in Figure 5. The resistance data points are taken from the average resistance values measured over the 5 layers with known doping levels. The calibration curve is obtained by interpolation of these data points. Figure 5: SSRM measurement on an n-type doped silicon calibration sample. The resistance cross-section is taken from the average of 50 scan lines. The resistance versus dopants data points are taken from the average resistance values measured over the 5 layers with known doping levels in the calibration sample. It can be concluded that the good measurement repeatability on both the sample and the standard allows for an accurate determination of the active carrier concentration in a sample with unknown carrier levels. SCM The n-type doped silicon calibration sample is used to evaluate the performance of SCM on the Park NX-Hivac AFM. This sample has, next to the n-type epitaxial layers, also a p-type doped layer in between the substrate and the n-layers. Figure 6 shows respectively the dC/dV amplitude and the dC/dV phase map obtained from an SCM measurement on the n-type calibration sample. From the phase map, one can clearly distinguish the n and p-layers. The various n-type layers with different doping concentrations show also a clear contrast in the dC/dV amplitude map. These results demonstrate the carrier concentration sensitivity and the carrier type sensing capability of SCM, even though the spatial resolution is not as good as in SSRM. Figure 6: SCM dC/dV amplitude and phase measurements on ann-type doped silicon calibration sample. The dC/dVamplitude cross-sectionis taken from the average of 50 scan lines. Besides Si, another interesting application of SCM is on III-V material stacks. In Figure 7 the results of an SCM measurement on a doped InGaAs stack are shown and they allow to extract information about the carrier concentration and the carrier type. Figure 7: SCM measurement on a doped InGaAs stack. The dC/dV amplitude and phase cross-sections are taken from the average of 100 scan lines. Conclusions This study shows that scanning spreading resistance microscopy (SSRM) and the scanning capacitance microscopy (SCM) can meet the challenges of carrier profiling the state-of-the-art devices entering the sub 10 nm nodes given the right microscopy tool. This investigation found that Park NX-Hivac is the powerful tool that meets the challenges of ever shrinking devices with its implementation of electrical SPM modes, including SSRM and SCM, in a high vacuum environment. Furthermore, we conclude that the use of Park NX-Hivac vacuum aids significantly to reduce the noise, resulting in an average repeatability of 6.1 % for SSRM measurements. References [1] Park NX-Hivac, Retrieved from https://parksystems.com/products/small-sample-afm/park-nx-hivac?highlight=WyJueC1oaXZhYyIsIm54LWhpdmFjJ3MiXQ== [2] Hantschel, T., et al. Diamond scanning probes with sub-nanometer resolution for advanced nanoelectronics device characterization, Microelectronic Engineering 159, 46-50 (2016). [3] Eyben, P. et al. Fundamentals of Picoscience, Subnanometer Characterization of Nanoelectronic devices. Edited by Sattler, K.D., CRC Press (2014)

27 Apr 2020

Nanoscale Material Patterning using Atomic Force MicroscopyNano-Lithography

John Paul Pineda, Charles Kim, Cathy Lee, Byong Kim, and Keibock Lee Park Systems Inc., Santa Clara, CA USA INTRODUCTION Nanotechnology has received increased attention within the scientific community due to its application in a number of fields ranging from electronics to biomedical technologies [1]. Progress in many of these applications depends mainly on the capability to fabricate nanostructured materials that include polymers and semiconductors, among others [2, 3]. Several methods have been introduced for the fabrication of nanostructures; the more common ones are electron beam lithography and focused ion beam lithography. However, these methods are not straightforward and are expensive to operate. One powerful method designed to overcome these problems is atomic force microscopy (AFM) nanolithography [1-3]. This technique is simple and less expensive [2]. AFM nanolithography is divided into two general groups based on their mechanistic and operational principles, bias-assisted and force-assisted AFM lithography [2]. In bias-assisted method, a bias voltage is applied to the tip to generate an oxide pattern on metallic or semiconductor substrate [4, 5]. Whereas, force-assisted method applies a large force to the tip to produce fine grooves on the surface of polymer like samples by mechanically scratching, pushing or pulling the surface atoms and molecules with a sharp tip; the interaction between the tip and the sample is mainly mechanical [2, 3]. In this technical note, we demonstrate the capability of AFM nanolithography to generate nanopatterns by utilizing Park NX10 Atomic Force Microscope. The tip bias mode is used to create oxide patterns on the silicon substrate. Successful pattern fabrication is confirmed by Lateral Force Microscopy (LFM). The integration of lithography and LFM mode with AFM enables it to fabricate nanopatterns and simultaneously acquire both topography and frictional data. The results obtained in this experiment confirms the fabricated oxide nanopatterns are successfully executed. Experimental A Park NX10 AFM was used for AFM nanolithography to draw nanopatterns onto a silicon substrate. LFM imaging was conducted after the lithography process to confirm the successful fabrication of the oxide patterns on the surface as well as to distinguish the surface difference between the oxide layers and silicon substrate. A PPP-CONTSCPt cantilever with a nominal spring constant (k = 0.2 N/m) and resonance frequency (f = 25 kHz) was utilized in the experiment. Results and Discussion Bias-Assisted AFM Nanolithography Bias-Assisted AFM Nanolithography is a technique described as material modification (oxidation) through the application of voltage bias to the AFM tip. The applied bias on the tip results in the generation of an electric field between the tip and the sample. The field ionizes water molecules formed between the tip and the sample, which leads to local anodic oxidation (LAO) used to fabricate nanoscale oxide features on the surface. In the lithography process, the tip acts a nanoscale electrode for current injection or collection [1, 2]. The amount of oxide layer formed on the surface depends on the magnitude of the applied bias and humidity. Previous study [2] reported that the thickness of a pre-designed nanoscale oxide pattern of device structure increased due to the increment of applied tip voltages. Therefore, it concluded the tip voltage influenced the amount of oxide formed in the surface. Figure 1 shows a schematic illustration of bias-assisted AFM nanolithography. In this study, Park lithography software was used to design and apply nanolithography to the sample surface. A silicon substrate was used as a sample. The application of –10 V resulted in a high electric field around the tip which led to cleavage of water molecules into ions H+, OH- and O- ions [5]. The OH- and O- ions are then pushed away from the biased tip and react with silicon to form SiO2. The nanoscale oxide pattern forms on the surface of the silicon substrate along the scan area of the lithography scan. Figure 1. Schematic illustration of Bias-Assisted AFM Nanolithography [4, 5].   Lateral Force Microscopy In Lateral Force Microscopy (LFM) mode, the vertical deflection and the torsion of the cantilever are measured and utilized to acquire topography and surface frictional characteristics of the sample. The cantilever movement in both the vertical direction and the torsion is tracked via a position sensitive photo detector (PSPD) that consists of four domains (a quad-cell), shown in Figure 2. Surface topographical information and frictional information are obtained using Equation 1 and 2. Equation 1: Topographic information = (A+C) - (B+D) Equation 2: Frictional information = (A+B) - (C+D) Figure 2. Schematic illustration of laser position on PSPD in the operation of AFM (top) and LFM (bottom).   Figure 3 shows the topography, frictional data and equivalent line profile of the images after the lithography process. The images were taken using LFM mode. Since LFM is a contact mode, both topography and frictional data can be acquired simultaneously, allowing for correlation between topographic and frictional properties. The AFM topography image (Figure 3b) reveals that the design is a Christmas ball pattern with diameter of approximately 17 µm and composed of multiple tiny structures with heights ranging from 0.2–1.5 nm. Figure 3e represents the line profiles that were generated using the Park XEI software to provide a more direct signal comparison. By analyzing the line profiles obtained from the LFM forward (green line) and backward (blue line) scans, we gain insight into the frictional characteristics of the sample. In the forward scan, the LFM signal shifted upward, indicating that the movement of the cantilever was hindered due to change in frictional force as the tip bypassed the oxide layer. Conversely, the LFM signal shifted downward during the backward scan, which, again, was a result of the cantilever being dragged by the surface because of the larger frictional interaction between the cantilever and the surface. As a result, we can conclude that the frictional coefficient of the oxide layer is higher than that of the silicon substrate. Conclusion Here we have demonstrated the use of AFM nanolithography to generate nanoscale oxide patterns in a silicon substrate using the Park NX10 system. Voltage bias-mode technique was used in the AFM nanolithography process. The application of –10 V voltage bias on the tip led to successful fabrication of the nanoscale oxide features in the surface. LFM mode was also conducted and the results confirmed the successful fabrication of the oxide patterns. Overall, the AFM nanolithography described in this study is an effective method to fabricate next generation materials and devices with nanoscale features. REFERENCES 1. A. Pimpin, et al., Review on Micro- and Nanolithography Techniques and their Applications.Enginering Journal. 2. S. Hutagalung, et al., Nanoscale Patterning by AFM Lithography and its Application on the Fabrication of Silicon Nanowire Devices. Sains Malaysiana 43(2), (2014): 267–272. 3. R. Garcia, et al., Advanced scanning probe lithography. Nature Nanotechnology 9, 577–587 (2014); doi:10.1038/nnano.2014.157 4. J. Voves, Nanoelectronics and nanolithography. https://www.researchgate.net/publication/228859246_Nanoelectronics_and_nanolithography 5. A. Bernal et al., LOCAL ANODIC OXIDATION ON SILICON (100) SUBSTRATES USING ATOMIC FORCE MICROSCOPY. http://www.scielo.org.co/pdf/dyna/v79n174/v79n174a08.pdf

24 Jun 2019

Advantages of High Vacuum for Electrical Scanning Probe Microscopy

Jonathan Ludwig1,2, Marco Mascaro1,2, Umberto Celano1, Wilfried Vandervorst1,2, Kristof Paredis1 1. IMEC, Leuven, Belgium 2. Department of Physics and Astronomy, University of Leuven, Leuven, Belgium.    Introduction The discovery of graphene as the prototype of a new class of materials in 2004 has led to an enormous scientific interest in two-dimensional (2D) layered materials.1 Since then, a wide variety of 2D materials has been synthesized and explored.2-6Among them, the family of transition metal dichalcogenides (TMDs) has attracted specific interest from the semiconductor industry owing to an inherent band gap, a small dielectric constant, high mobilities, and ultrathinmaterials, which are heralded as promising candidates for scaling logic technology beyond the 5nm node. However, the integration of such materials in a 300mm compatible manufacturing environment still faces many challenges. As the praised properties have mostly been observed very locally, in flakes or single grains, the controlled growth, transfer and processing of high quality TMD layers remain a key hurdle. Scanning probe microscopy, as an inherent high resolution 3D technique, is a powerful enabler for investigating the morphological and electrical properties of TMDs. In this technical note, we will illustrate the advantages of high vacuum for electrical measurements utilizing the capabilities of the Park NX-Hivacatomic force microscopy system (Park Systems) using MoS2 as an example material. MoS2 A series of MoS2 samples with varying layer thickness were grown by metal organic chemical vapor deposition (MO-CVD) on sapphire substrates. All measurements are performed on the as-grown, un-transferred MoS2/sapphire. The room temperature mobility for devices made from the same material are found to be up to μm ~ 30 cm2/Vs, with higher average mobilities for thicker samples.12 Atomic force microscopy (AFM) images of all the samples measured are shown in figure 1. In total, three samples were measured with layer thicknesses of 1-2 layers, 3-4 layers, and one with pyramid structures, referred to here as multi-layered MoS2. The 1-2 layer sample consists of a completely closed monolayer MoS2 film with additional monolayer islands forming on top. These monolayer islands constitute the beginning of the growth of the second layer and can be identified as the light-colored regions in the topography image. Similarly, the 3-4 layer sample consists of a completely closed tri-layer MoS2 film with additional monolayer islands. An example of the sample structure for the 3-4 layer sample is shown in figure 1(d). Here, each green layer represents one layer of MoS2. In addition to MoS2 islands, we also see diagonal lines running across each sample. These are terraces from the sapphire substrate, which can be seen through the 2D film. The sapphire terraces can be unambiguously distinguished from MoS2 layers by the step height, 0.2nm for c-plane sapphires. 0.6nm for a monolayer MoS2 step, as can be seen from the cross section in figure 1 (f).13,14 The multi-layer sample differs from the other two in that the MoS2 surface is characterized by 3D pyramid-like structures. These pyramids sit on a completely closed tri-layer, and their formation is due to a change in the growth mechanism from layer-by-layer to 3D with increasing layer thickness. Details of the growth can be found in ref.12. Fig. 1. (a-c) AFM topography images of the samples studied. (d) Schematic of C-AFM setup used to measure multilayer MoS2 on a sapphire. (e) Cartoon showing how the cantilever twists as it scans over a high friction region. (f) Cross section of topography corresponding to black line in (b) showing 0.6 nm step at the MoS2 island edge and 0.2nm step at the sapphire terrace. All images are plotted in Gwyddion. Scale bar is 500 nm. Conducting Scanning Probe Microscopy Here we employ two conducting scanning probe microscopy (SPM) techniques to characterize the electronic properties of MoS2: Conductive Atomic Force Microscopy (C-AFM) and Scanning Tunneling Microscopy (STM). In C-AFM, the cantilever is in contact with the surface of the material and we simultaneously record topography and current. To measure the electrical current, a bias is applied to the sample chuck and current is measured by an external current amplifier connected to a conducting AFM probe. Electrical contact to the material is made by applying silver paint to the top and sides of the material. We use commercially available Pt-Ir coated probes, such as PPP-CONTSCPt or PPP-NCSTPt, with nominal spring constants in the range of 0.2 to 7 N/m. Since C-AFM is a contact-based AFM technique, it also allows the lateral force to be recorded along with the other C-AFM channels. Lateral force microscopy (LFM) measures the lateral deflection of the laser on the PSD due to torsion, or twisting, of the cantilever as it scans across the surface, as depicted in figure 1(e). The difference between the forward and reverse LFM images is proportional to the friction of the materialist differs from C-AFM in that a conductive wire, cut Pt-Ir in our case, is used to measure the tunneling current between the probe and the sample when the probe is a few angstroms above the surface. STM can be performed by either keeping the height constant and recording the current (referred to as constant height mode) or using the feedback to keep the current level constant and recording the height (constant current mode). In constant current mode, the height image contains both topographic and electronic information. C-AFM in Air vs High Vacuum In order to demonstrate the importance of the water layer on the surface on 2D materials, we performed C-AFM on the same MoS2 sample in air and high vacuum (HV), figure 2 (a-b) and (c-d), respectively. While the topography images for the scan in airand HVare quite similar, the C-AFM images differ considerably. Most notably, the measured current increases by three orders of magnitude in HV. The average current level in air is 1.4 nA at 5 V bias, while in HV it is 1.1 μA. The increase in current level is due to the removal of the thin water layer that is always present on the surface of the sample in air. This water layer is particularly problematic for MoS2 as it p-dopes the material, effectively turning it off electrically. From electrical transport of similar CVD grown MoS2 devices, the on state current is severely degraded and mobility decreases by 40% after exposure to DI water for two hours.15 Fig. 2. C-AFM from the same 3-4 MoS2 sample showing the increased current level and sensitivity under high vacuum. (a) Topography and (b) current images in air at 5 V bias. (c) Topography and (d) current images taken immediately after pumping to high vacuum at 0.5 V bias. The data taken in air and high vacuum was acquired using identical parameters: same probe with spring constant k of 7 N/m, set point of 10 nN, and 1 Hz scan rate. Scale bar is 500 nm. Besides just an increase in current, the C-AFM image in HV shows much more detail. From the image in air, it appears that current is relatively homogeneous. Other than the current level, not much information can be extracted for the C-AFM in air on this sample. From the current map taken in HV, in contrast, we can clearly see the grain boundaries in the MoS2 layers. Despite the fact thatthe C-AFM probe is in direct contact with the material, the low force appliedresults in no removal of MoS2 material over the repeated scans. Figure 3 shows the topography image from the same sample after 5 scans in HV at ~30 nN force with a probe with nominal spring constant at ~7 N/m. Fig. 3. Topography image of a3-4 layer MoS2 (a) initially and (b) after 5 successive scans at 0.1 V set point using a PPP-NCSTPt probe with spring constant of ~7 N/m. Scale bar is 50 nm. C-AFM + LFM for Grain Boundary Analysis When imaging with a low spring constant probe, such as PPP-CONTSCPt with nominal spring constant of 0.2N/m, we can acquire frictional data simultaneously with C-AFM, allowing for correlation between topographic, electrical, and material properties. Figure 3 shows the height, friction, and current images from a 1-2 layer MoS2 sample. The 1st layer and 2nd layer regions are labeled as 1Ly and 2Ly, respectively, in figure 3(a). The friction at grain boundaries is higher than the pristine regions, so that they show up as dark lines in the friction. By comparing the current and friction, we can see that the dark lines in the friction image match the dark lines in the current. However, the current image shows additional features due to the influence of the substrate on the local conductivity of the 2D film. Fig. 4. (a) Topography, (b) Friction, and (c) current acquired simultaneously on an 1-2 layer as-grown MoS2 / sapphire sample. The layer thickness of each region is indicated in (a). Scale bar is 200 nm. Scanning Tunneling Microscopy on MoS2 With the Park NX-Hivac, we were able to also acquire high quality STM images without the need for complicated UHV systems and special sample prep/handling. Figure 4 shows a 500 nm scan of the multi-layer MoS2 sample imaged in constant current mode, with Iset=0.5 nA and Vbias = 1 V. Since STM gives a convolution of topography with electronic structure, we see both islands and grain boundaries in the height image. Fig. 5. STM image of multilayer MoS2 / sapphire.Cut Pt-Ir wire in constant current mode. Iset = 0.5 nA, Vbias = 1 V. Scale bar is 200 nm.   Conclusion In this study, molybdenum disulfide (MoS2), one of the highly interesting 2D materials of transition metal dichalcogenides (TMDs) family, was investigated on morphological and electrical aspects using Park NX-Hivac AFM (Park Systems). The differences between single and multi-layer was observed on AFM topography images. Furthermore, the details of 3D pyramid-like structures due to layer-by-layer growth mechanism was determined at multi-layer image. Using conducting SPM (C-AFM and STM), electrical properties of MoS2 were studied both in ambient and high vacuum (HV) condition. Despitethe presence of oxide layer, clear, homogeneous and higher current signal was measured while in HV condition. Lastly, topographical, electrical, and mechanical information were obtained for grain boundary analysis using combination of C-AFM and LFM. This approach allowed the finding of more specific and detailed structures on grain boundary. 2D layered material is widely used for various research field both industry and academy. The characterization and exploration of electrical and mechanical properties of 2D material are one of the most critical pointsin materials research field. The atomic force microscope, a versatile imaging and measurement tool, allows us to evaluate 2D materialsin multidirectional viewpoints using various imaging modes. This study underlines the improving strategy of material analysis. Moreover, these results emphasize the importance of multidirectional and multichannel analysis on 2D materials, including the transition metal dichalcogenides that are of high interest by the semiconductor industry. References 1. K. S. Novoselov, A. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, &A. A. Firsov. Electric field effect in atomically thin carbon films. Science306, 666–669 (2004). 2. A. K. Geim & I. V. Grigorieva. Van der Waals heterostructures. Nature499, 419–425 (2013). 3. K. F. Mak, C. Lee, J. Hone, J. Shan, & T. F. Heinz. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys Rev Lett105,136805 (2010). 4. H. Liu, A. T. Neal, Z. Zhu, Z. Luo,X. Xu, D. Tománek,&P. D. Ye. Phosphorene: an unexplored 2D semiconductor with a high hole mobility. ACS Nano8, 4033–4041 (2014). 5. J. Zhao, H. Liu, Z. Yu, R. Quhe, S. Zhou, Y. Wang, C. C. Liu, H. Zhong, N. Han, J. Lu, Y. Yao,&K. Wu. Rise of silicene: A competitive 2D material. Prog Mater Sci83, 24–151 (2016). 6. C. R. Dean, A. F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei, K. Watanabe, T. Taniguchi, P. Kim, K. L. Shepard, & J. Hone.Boron nitride substrates for high-quality graphene electronics. Nat Nanotechnol5, 722–726 (2010). 7. X. Xu, W. Yao, D. Xiao, &T. F. Heinz. Spin and pseudospins in layered transition metal dichalcogenides. Nat. Phys.10, 343–350 (2014). 8. G. Fiori, F. Bonaccorso, G. Iannaccone, T. Palacios, D. Neumaier, A. Seabaugh, S. K. Banerjee,& L. Colombo. Electronics based on two-dimensional materials. Nat Nanotechnol9, 768–779 (2014). 9. X. Xi, L. Zhao,Z. Wang, H. Berger, L. Forró, J. Shan,& K. F. Mak. Strongly enhanced charge-density-wave order in monolayer NbSe2. Nat. Nanotechnol.10, 765–769 (2015). 10. S. Manzeli, D. Ovchinnikov, D. Pasquier, O. V. Yazyev, &A. Kis. 2D transition metal dichalcogenides. Nat. Rev. Mater.2, 17033 (2017). 11. W. Choi, N. Choudhary, G. H. Han, J. Park, D. Akinwande,&Y. H. Lee. Recent development of two-dimensional transition metal dichalcogenides and their applications. Mater. Today20, 116–130 (2017). 12. D. Chiappe, J. Ludwig, A. Leonhardt, S. El Kazzi, A. Nalin Mehta, T. Nuytten, U. Celano, S. Sutar, G. Pourtois, M. Caymax, K. Paredis, W. Vandervorst, D. Lin, S. Degendt, K. Barla, C. Huyghebaert, I. Asselberghs, and I. Radu, Layer-controlled epitaxy of 2D semiconductors: bridging nanoscale phenomena to wafer-scale uniformity. Accepted Nanotechnology (2018). 13. E. R. Dobrovinskaya, L. A.Lytvynov,& V. Pishchik. Sapphire: material, manufacturing, applications. Springer Science & Business Media, 2009. 14. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, & A. Kis. Single-layer MoS2 transistors. Nat Nanotechnol6, 147–150 (2011). 15. A. Leonhardt, D. Chiappe, I. Asselberghs, C. Huyghebaert,&I. Radu. Improving MOCVD MoS2 Electrical Performance: Impact of Minimized Water and Air Exposure Conditions. IEEE Electron Device Lett38(11) 1606-1609 (2017).  

21 Jan 2019

Enhanced Surface Potential Detection Study using FM-KPFM

John Paul Pineda, Charles Kim,Cathy Lee, Byong Kim, and Keibock Lee Park Systems Inc., Santa Clara, CA USA Introduction Scanning Kelvin probe microscopy (KPFM) is a tool used to measure work function and local electrical potential distribution of various materials with nanoscale features. The most common application of this technique is the study of electronic properties of semiconductor nanostructures and surfaces. Potential profiling of such materials composing of negatively or positively charged substancesis some of the measurements can be done using this technique with nanoscale lateral resolution. Implementation of such techniqueto unveiling thevariations of charge distribution that occurs on the nanoscale can help better understanding and improving the performance of CMOS semiconductor devices.[1-2] Such KPFM has also been instrumental incharacterizing and determining the quantitative physical information such as the total charge and charge position of polymer materials.[3, 4] There are other surface potential or work function measuring tool that were introduced such as electron beam induced current (EBIC), scanning electron microscopy (SEM), and photoelectron spectroscopy (PES). However, some of these techniques are only applicable for inorganic type semiconductor samples, some are destructive and requires high vacuum, and others do not provide high spatial resolution.[5] Compared to other techniques, KPFM is nondestructive and air-ambient compatible technique which unequivocally is one of the most easy to use electrical failure analysis and nanometrology tool available in the market today. Conventional KPFM technique (also known as Amplitude Modulation AM-KPFM) has alreadymade remarkable contribution in maintaining device reliability and analysis of advance materials compared to other measurement techniques, but there is room for improvement in the signal to noisedetection ability of AM-KPFM technique so that even the featureshaving less optimal surface potential strength can be detectedand mapped out with higher lateralresolution and accuracy. Toward this end, recently, Park Systems developed a technique called Frequency Modulation FM-KPFM with Atomic Force Microscope (AFM). This technique is useful for electrical characterization allowing measurements with better sensitivity compared to AM-KPFM.In this study, a polymer material was measured to compare the performance of AM-KPFM to FM-KPFM. The results acquired in the experiment show that FM-KPFM is significantlymore sensitive compared to AM-KPFM in measuring both work function and surface potential distribution of different materials. Experimental A polymer patterned array material deposited on Silicon substrate was analyzed using a Park NX10 AFM. Two separate scans were conducted to acquire AM-KPFM and FM-KPFM measurements. The scan parameters and the tip used in acquiring images were all the same. A conductive Mikromasch NSC36Cr-Au cantilever (nominal spring constant k = 1 N/m with resonant frequency f = 90 kHz) was used in the experiment. In KPFM mode, there are two interaction forces between the AC biased tip and the sample: the electrostatic force and Van der Waals force. The Van der Waals force is harnessed to generate the sample's surface topography while the electrostatic force between the tip and sample generates data for the sample's electrical properties. The obtained cantilever deflection signal contains both sets of information; therefore, a method that can completely separate these signals is the key to successful imaging. In the Park NX10, lock-in amplifiers embedded to its electronics are used to separate the signals. This allows for the acquisition of both topography and EFM data simultaneously. Two amplifiers are used by the system, named lock-in 1 and lock-in 2. Lock-in 1 obtains the topography information by analyzing the tip motion caused by the Van der Waals interaction, while lock-in 2 obtains electrical property information by analyzing the frequency of the applied AC voltage signal to the tip which, in turn, generates an electrostatic force interaction with the sample. The frequency of the applied AC voltage signal is chosen to be smaller (5 - 20 kHz) than the cantilever oscillation frequency (70- 330 kHz), enough so that the two signals do not interfere each other. In FM-KPFM setup [6], the NCM phase signal of Lock-in 1 is transferred to Lock-in 2 to serve as a source for EFM operation by connecting BNC cable between ‘Aux2 out’ and ‘Aux1 in’ shown in Figure 1. Furthermore, a separate DC bias was applied to the cantilever and controlled for to create a feedback loop that would zero out the electrical oscillation between the tip and the sample caused by the application of an AC bias to the cantilever. The value of this offsetting DC bias that zeroes out the AC bias-induced electrical oscillation is considered to be a measure of surface potential.   Fig. 1. Diagram of FM-KPFM   Results and Discussion The topography data obtained in this experiment shows that polymer patterned array with square-like features were successfully deposited on silicon substrate and no significant information related to its surface potential was provided. In contrast, the surface potential data acquired in AM-KPFM and FM-KPFM shows the surface potential structure, but no significant information related to physical structure of the sample. The domain structure shape observed in KPFM data of the sample was similar to the physical structure observed in topography data, which was a patterned array consisting of square dots. The data acquired in this experiment were all analyzed using Park XEI software developed by Park Systems. Figure 2 shows the topography (left), FM-KPFM (center), and AM-KPFM (right) images of the polymer patterned array sample. The topography data can be acquired simultaneously with the KPFM data. In Figure 2, the topography image presented was acquired simultaneously with the AM-KPFM image. The topography shows a clear image of a well-defined lattice structure. The Park XEI software maps the acquired signals into a color table. In the topography image, the square-like features which appeared to have a brighter color represent the areas with higher height while the flat surface with darker color represents the areas with lower height. The measured peak to valley in the topography image was approximately 150 nm. On the other hand, both AM-KPFM and FM-KPFM data show a patterned array with an irregularity on the surface (pointed by red arrows). In the two KPFM images, the square-like features which appear to have a darker color, represent the areas with relatively lower surface potential while the flat surface with brighter color represent the areas with higher surface potential. By comparing the surface potential results acquired from AM-KPFM and FM-KPFM, one can easily determine that FM-KPFM has better sensitivity in detecting surface potential variation compared to AM-KPFM. In this experiment, FM-KPFM technique provided a higher resolution image that shows sharper edges of the square features compared to AM-KPFM. In addition, FM-KPFM was able to detect weak potential in the irregular surface while AM-KPFM was not able to detect any potential variation.   Fig. 2. 10x10 μm image of polymer patterned array. Topography image (left), FM-KPFM image (center), and AM-KPFM image (right).   The line profile that was generated by the XEI software in Figure 3 (bottom) provides the potential information of the polymer patterned array. The line profile for the FM-KPFM is indicated in color red while the line profile for AM-KPFM is indicated in color green. The line profile of the FM-KPFM image clearly shows that there is a slightly decreasein potential (encircled in red) in the irregular surface compared to the areas next to it. On the other, the line profile of AM-KPFM doesn’t show any changes of potential in that surface. By comparing the line profile of the two techniques, it can be concluded that FM-KPFM is at least an order of magnitude more sensitive than conventional AM-KPFM technique.   Fig. 3. FM-KPFM image (top-left), AM-KPFM image (top-right) and corresponding line profile (bottom) acquired from the polymer sample. FM-KPFM line profile (red line, y-axis on left) and AM-KPFM line profile (green, y-axis on right).   Conclusion The polymer patterned array was successfully characterized by the Park NX10 AFM using AM and FM-KPFM imaging. The topography data revealed that the sample surface is composed of patterned array with square-like features. An irregularity in the patterned array was observed in both AM-KPFM and FM-KPFM images. FM-KPFM technique shows that the irregularity in the surface pattern is a surface with weak potential. AM-KPFM technique was not sensitive enough to detect the weak potential in the same region. Given the above results, it can be concluded that FM-KPFM is an order of magnitude more sensitive compared to AM-KPFM and it is more useful in detecting surface potential variations. The increased in sensitivity of this technique will also increase the chance of detecting defects with improved resolution and accuracy in semiconductor devices and other advanced materials. References 1. LanFei (2018) FUNDAMENTALS OF KELVIN PROBE FORCE MICROSCOPY AND ITS APPLICATIONS IN THE CHARACTERIZATION OF SOLAR CELLS. Doctoral Dissertation, University of Pittsburgh. 2. J.Pineda, et al., Electrical Characterization of Semiconductor Device Using SCM and KPFM Imaging.terization. 3. J. Gonzalez, et al., Charge distribution from KPFM images, PCCP, Issue 40, 2017. 4. M. Ortuño, et al., Conducting polymers as electron glasses: surface charge domains and slow relaxation, Scientific Reports volume 6, Article number: 21647 (2016). 5. W. Melitz, et al., Kelvin probe force microscopy and its application, Surface Science Reports 66 (2011) 1–27. 6. Charles Kim, et al., How to Measure FM-KPFM, Park Systems White Paper, To be published (2018)

4 May 2018

PinPoint Piezoelectric Force Microscopy

Wenqing Shi, Cathy Lee, Gerald Pascual, John Paul Pineda, Byong Kim, Keibock Lee Park Systems Inc., Santa Clara, CA USA ABSTRACT Electromechanical couplingin materials is a key property that provides functionality to a variety of applications,including sensors, actuators, IR detectors, energy harvesting and biology. Most materials exhibit electromechanical coupling in nanometer-sized domains. Therefore, to understand the relationships between structure and function of these materials, characterization at nanoscale is required. This electromechanical coupling property can be directly measured in a non-destructive manner using piezoelectric force microscopy (PFM), a mode that comes standard in all Park atomic force microscopes (AFMs). Here in this application note, we developed a novel technique termed as PinPoint™ PFM and demonstrated the application of PinPoint PFM in the characterization of annealed phenanthrenethin film on top of an ITO surface. The phenanthrenematerial has been a challenging sample to get quality topographical and piezoelectric response data from using conventional SPM methods. The main difficulty is due to the rod-shaped nanostructures on the sample surface being very susceptible to displacement by a scanning probe's tip. The invention of Park's latest PinPoint PFM technique gives researchers both a friction-less imaging technology that overcomes this difficulty and the means to achieve publication-ready image quality in much less time than previously possible with older methods. Here we demonstrated, not only can image well-resolved individual rod-shaped phenanthrene structures, but also differences in electrical polarization expressed as differences in PFM contrast (brighter areas showing a positive polarization and darker areas a negative polarization) without image distortions. Introduction Piezoelectric effect, is an intrinsic material property, in which the application of an electric field leads to thickness changes and/or shearing of the material. This unique electromechanical coupling property has been employed in a wide range of applications ranging from medical imaging and energy harvesting, to actuators and sensors.1 Example of piezoelectric materials include crystals (i.e., quartz), biological materials (i.e., DNA, bones and proteins) and man-made materials such as synthetic ceramics (barium titanate and zinc oxide) and some organic thin films.3 Driven by the developing nanotechnology and the increasing demands for miniaturization of electronic devices, characterization of piezoelectric effect at micro- and nanoscale has attracted significant interest. Piezoelectric force microscopy (PFM), also termed as dynamic-contact electrostatic force microscopy (DC-EFM) by Park, is an atomic force microscopy (AFM) based method that allows for high-resolution imaging, quantification and manipulation of piezoelectric materials at micron- and nanometer-length scale. Conventional PFM is usually performed in contact mode, and concurrent topographic imaging and piezoresponse measurements is obtained. In conventional PFM operation, an electrically-biased conductive tip is brought in contact with the surface of a piezoelectric material. Through application of an AC modulation to the conductive tip, the piezoelectric response of the material can then be measured by tracking the deflection of the cantilever as a result of sample’s local expansion or contraction based on the applied electric field. As these surface displacement are often very small with a low signal-to-noise ratio, and, thus, a lock-in amplifier is used for the detection of the amplitude and phase of the piezoelectric response signal. In terms of the AC bias frequency selection, a frequency that’s much lower compared to the cantilever’s resonance frequency is used. In addition, a DC bias can be applied to the sample to switch the domains of the piezoelectric material. Since the atomic force microscopy’s photodiode is position-sensitive, piezoelectric force microscopy can also identify the direction of electrical polarization in piezoelectric or ferroelectric domains. There are two modes of PFM imaging: vertical PFM (VPFM) and lateral PFM (LPFM), which are sensitive to domains polarized out-of-plane and in-plane, respectively.2 (Figure 1) In vertical piezoelectric force microscopy, in the presence of piezoelectric domains that point out-of-plane or perpendicular to the sample surface (Figure 1a-b), the cantilever will deflect vertically with respect to the sample surface in response to the applied electric field. Therefore, the PFM signal will appear bright for domains that point upward and dark for domains that point downward. In lateral piezoelectric force microscopy, in the presence of in-plane piezoelectric domains that’s parallel to the surface, a displacement shearing on the surface will occur. As a result, a torsional displacement of the cantilever will be induced, which in turn will be captured by the position sensitive photo detector as a lateral deflection. (Figure 1c-d)   Fig. 1.A schematic representation of (a-b) vertical and (c-d) lateral PFM. The AFM laser shows vertical deflections which correspond with (a) downward or (b) upward out-of-plane electrical polarization. In lateral PFM, the cantilever will exhibit torsion in response to (c-d) lateral in-plane polarization directions. Black arrows indicate the direction of polarization vector in each case assuming that the relationship between polarization and crystal orientation is conserved. In this application note, piezoelectric force microscopy is performed utilizing the newly-developed PinPoint™ mode by Park Systems as opposed to the conventional contact mode. Performance comparison of PinPoint™ PFM and conventional PFM was carried out on annealed phenanthrene film, and improved resolution was observed in both topography and piezoelectric response signal with PinPoint™ PFM. In PinPoint™ PFM mode, the AFM probe monitors its feedback signal, approaches towardsthe sample surface until a predefined force threshold point is reached, measures the Z scanner’s height, then the AFM probe is rapidly retracted away from the surface to a user-defined height. The XY scanner stops during the piezoelectric response acquisition, and the probe-substrate contact time is controlled to allowsufficient time for quality data acquisition (Figure 2). PinPoint PFM allows higher spatial resolution with optimized piezoelectric response measurement over different sample surface.PinPoint PFM was designed to replace the conventional contact PFM and is an enhanced design that eliminates the problems of the tip wearing out during contact mode topography and diminished contact time. The PinPoint™ PFM technology has proven to solve all of the shortcomings of conventional PFM including quick tip wear, degradation of resolution, low signal to noise ratio, and poor reproducibility of data. Fig. 2.A schematic representation of PinPoint mode operation. The probe approaches towards the sample surface until a pre-defined force threshold is reached, then the Z scanner height is recorded. The XY scanner stops and the piezoelectric response is measured. Then the probe retracts away from the surface and move to the next pixel. The process repeats to collect the topography map and the piezoelectric response map. Experimental A Park NX10 AFM was used to image the annealed phenanthrene surface, and the topography signal and PFM quad signal were acquired in both conventional PFM imaging and PinPoint™ PFM imaging. In conventional PFM imaging, a NSC36-C (nominal spring constant k = 0.6 N/m and resonance frequency f = 65 kHz) coated with Cr and Au on both the front and back side was used in conventional PFM imaging. The nominal radius of the tip curvature is ~25 nm. Scan size was 20 μm × 20 μm. Scan rate was 0.2 Hz. The AFM tip was biased with AC potential with an amplitude of 4.5 V and a frequency of 17 kHz. No external bias was applied to the sample during imaging. The force set point used in the experiment was 6.84 nN. In PinPoint™ PFM imaging, a conductive NANOSENSORS™ PointProbe® Plus-Electrostatic Force Microscopy (PPP-EFM) cantilever (nominal spring constant k = 2.8 N/m and resonance frequency f = 25 kHz) coated with Ptlr5 on both the front and back sides was used in PinPoint™ PFM imaging. The nominal radius of the tip curvature is ~25 nm. Scan size was 20 μm × 20 μm. Same as the conventional PFM experiment, a AC potential with an amplitude of 4.5 V and a frequency of 17 kHz was applied to the tip, and no DC potential was applied to the sample. The force set point used in the experiment was 196.8 nN. The retract height was 0.3 μm. The retract/approach speed was 20 μm/s. The contact time between the probe and the sample surface was controlled at 1 ms. Results and Discussion In Figure 3, the topography and the piezoelectric response maps of the annealed phenanthrene thin film on ITO surface imaged with both conventional PFM and PinPoint™ PFM are shown. The images obtained via conventional PFM are shown in Figure 3a (topography) and Figure 3b (piezoelectric response), and the images collected by PinPoint PFM are shown in Figure 3c (topography) and Figure 3d (piezoelectric response). From the topography images (Figure 3a and 3c), the annealed phenanthrene polymer was resolved under both imaging modes and observed to be rod-shaped features with pointy ends. The height of polymer rods was measured to be ranging from 50 to 400 nm, while the width of the polymer rods was measured to be between several hundred of nm to a couple of μm. It is noteworthy that the quality of the topography image obtained via PinPoint PFM is significantly improved compared to that taken with conventional PFM mode. In Figure 3c, under PinPoint PFM imaging conditions, the annealed phenanthrene polymer were well-distinguished from the ITO substrate. However, in Figure 3a, under conventional PFM imaging conditions, image artifacts can be seen throughout the entire scan area, indicating that the probe was scratching on the surface repeatedly. Of note, for conventional PFM measurement, our initial attempt was to use PPP-EFM (f = 2.8 N/m) as the probe, which is the same as the one used in the PinPoint PFM measurements. However, the probe was constantly scratching on the surface and the image quality was very poor. A cantilever with smaller force constant, NSC36-C (f = 0.6 N/m) was used instead, in hope to alleviate the scraping on the surface. Albeit the NSC36-C has a smaller force constant compared to that of PPP-EFM, the probe was still dragging on the surface and resulted in imaging artifacts as seen in Figure 3a. In PinPoint PFM, on the other hand, since the probe was retracted away from the surface to a safe height at every pixel prior to approach towards the surface, the scratching between the probe and the underlying substrate was effectively eliminated, leading to significantly improved imaging quality. Figure. 3.a) Topography image taken with conventional PFM mode; b) PFM quad image taken with conventional PFM mode; c) Topography image taken with PinPoint PFM mode; d) PFM quad image taken with PinPoint PFM mode. Scan size: 20 μm × 20 μm. Similar results were observed in the piezoelectric response images (Figure 3b and Figure 3d).By comparing the results of the two techniques, one can easily determine that PinPoint PFM has better performance compared to conventional PFM in the detection of piezoelectric response as the image obtained under PinPoint PFM was much better compared to that taken with conventional PFM mode. Figure. 4. Hysteresis curves for the amplitude (a) and phase (b) signal taken with the phenanthrene polymer film. Figure 4a and 4b are hysteresis curves for the PFM amplitude and PFM phase signals of the phenanthrene film. These hysteresis curves were obtained by measuring the piezoelectric response at a specific location of the sample while applying the sample bias from -1.5 V to +1.5 V. The hysteresis curves provide localized information with respect to the switching properties of piezoelectric material. In Figure 4a, a characteristic “butterfly” shape that is similar to the ideal strain versus bias curve was observed in the amplitude signal. In addition, the coercive voltage, which is a measure of ability to withstand an external electric field without depolarization, is ~0.3 V. In Figure 4b, the phase hysteresis loop is shown, which is the typical response of a ferroelectric material. Conclusions Here in this application note, imaging performance of PinPoint PFM mode and conventional PFM mode was compared on phenanthrene film annealed on ITO surface. The PinPoint PFM mode introduced by Park Systems has proven to offer significantly-improved quality in both topography and piezoelectric response measurement. The advanced PinPoint mechanism eliminates the frictional force between the probe and the substrate, enabling concurrent high-resolution topography and piezoelectric response mapping of the surface. In addition, the response in strain (PFM amplitude) and polarization (phase) as a function of applied voltage was examined to obtain insights regarding material characteristics such as coercive voltage and hysteresis. Taken in total, PinPoint PFM mode is the ideal approach to characterize and quantify localized piezoelectric response at nanoscale, with maximized tip life and sample originality as a result of minimal frictional force between tip and sample. References [1] Ikeda T. Piezoelectricity. Oxford university press; 1990. [2] Soergel E. Piezoresponse force microscopy (PFM). J Phys D Appl Phys. 2011;44(46):464003. [3] Vijaya MS. Piezoelectric Materials and Devices: Applications in Engineering and Medical Sciences. CRC Press; 2012. .

15 Feb 2018

Simultaneous Topographical and Electrochemical Mapping using Scanning Ion Conductance Microscopy – Scanning Electrochemical Microscopy (SICM-SECM)

Conductive AFM (C-AFM) is uniquely suited to perform electrical measurements such as current distribution with nanoscale precision and can be performed in contact mode, tapping mode, or PinPoint™ mode. Here we perform C-AFM on ZnO nanorods, which have been widely studied due to their remarkable performance in electronics, optics, photonics, and photocatalysis.