Thin Film

23 Nov 2023

Thickness Maps of Thin Films on the Water Surface by Imaging Ellipsometry

SAMPLE AND SETUP: The water surface is of elementary interest as well in biophysics as in industrial applications. Brewster Angle microscopy is state of the are in label free imaging of the water surface but enable only an estimation of layer thicknesses.   To measure the thickness of ultra thin films, imaging ellipsometry is the method of choice and the nanofilm_EP4 can also be operated as a Brewster angle microscope. To Avoid the disturbents by vibration, an sufficticated anti vibration concept based on active vibrations isolation is requiered.     MEASUREMENT: Droplets of a Ethyl-stearate (1 mg/ml in Hexane) were spread with a microliter syringe at a cleaned water surface. Microscopic Delta and Psi maps were recorded with an single wave imaging ellipsometer equipped with a red laser (IEEP4L658nm) and a 5x objective. The instrument was mounted on an active vibration isolation system (Halcyonics_vario40). A round Teflon trough equipped with a wedge shaped black glass plate was set on top of a table top active vibration isolation system (halcyonics _nano20).     RESULTS: microscopic Δ, Ψ maps Thickness maps on water  

15 Oct 2023

Principles and Application of Brewster Angle Microscopy

INTRODUCTION Reflection of light from surfaces is part of our daily experience. However, an amazing property of optics is the possibility of having zero reflection. In more detail, no reflection occurs from a clean and perfect interface illuminated under a unique angle of incidence with p-polarized light. This well-understood phenomenon is described by Brewster´s law providing the so called Brewster’s angle for the involved optical media. Any subsequent changes of the optical properties at the interface will lead to reflection. This fact is the basis of Brewster angle microscopy, a recent technique for the study of nanofilms at the water/air interface or at the surface of other transparent nonadsorbing dielectric substrates. Even just with a monolayer on an interface Brewster’s law is not fulfilled and the covered part of the surface reflects light, although with very low intensity. By increasing the intensity of the illumination the contrast between the covered and the clean part of the surface can be enhanced. By using a microscope optic and a CCD-detector high contrast images of the lateral morphology of the layer can be taken. Typical fields of application are life sciences, colloid and surface chemistry, and materials research.   BREWSTER’S LAW When a light beam passes the boundary between two media of differing refractive index, generally some of it is reflected. The Brewster's angle (θB) is a particular angle of incidence where light with one particular polarization state cannot be reflected. This state is parallel to the the plane of incidence. Light with this polarization is said to be p-polarized. For an air-water interface (n2 = 1.33) in an ambient of air (n1 = 1), the Brewster's angle for visible light reaches approximately 53° to the normal. Since the refractive index for a given medium is a function of the wavelength of light, Brewster's angle will also vary in wavelength.     BREWSTER ANGLE MICROSCOPY Brewster angle microscopy was introduced in 1991. No light is reflected from the air-water interface under Brewster’s angle of incidence, if p-polarized light is used. With constant angle of incidence, the formation of a monolayer on the water surface modifies the Brewster’s angle condition, and light reflection is observed. Figure 1 illustrates the principle of obtaining the contrast for a surface covered with a nanofilm by Brewster angle microscopy. Figure 1: Principle of Brewster angle microscopy: Making use of Brewster`s Law for imaging ultra-thin films with high contrast. Brewster angle microscopy is an effective visualisation method for substructures with long range orientational order. To obtain a contrast between subdomains of different molecule orientations an analyzer is positioned in the reflected beam path.     INSTRUMENTATION The origin of commercial Brewster angle microscopes was the instrument developed by Dirk Hönig in his diploma-thesis (figure 3). Since the first presentation at the LB6 in Paris Brewster angle microscopy has been established as a worldwide standard technique for the investigation of ultra-thin films. Currently we offer two different Brewster angle microscopes. The nanofilm_ep4bam (figure 2) is based on the ellipsometric platform nanofilm_ep3 and can be upgraded to an imaging ellipsometer. Figure 2: nanofilm_ep4bam with ultraobjective The new nanofilm_ultrabam (figure 3) is based on a complete new optical pathway of light. The unique imaging optic provides fully focused images at max. 35 frames per second. Thus, for the first time in a commercial instrument high resolution AND overall focused real time imaging of monolayers becomes possible. It enables the visualization of Langmuir monolayers or adsorbed films in real time. Figure 3: nanofilm_ultrabam   APPLICATIONS     Morphological features of monolayers during compression/decompression The morphological study of monolayers has led to a more detailed understanding of the two-dimensional condensed phase structure of monolayers. Brewster angle microscopy can be used for direct observations during compression of monolayers in a Langmuir trough. The advantage against epi fluoresence and AFM is that no markers are required and that the film does not need to be transferred to a solid substrate. Figure 4: Ethyl stearate monolayer at ƍ<1mN/m, Field of view ca. 600μm (nanofilm_ultrabam)   Inner structure of 2D condensed phase domains As mentioned, Brewster angle microscopy is an effective method to vizualize the substructure with longrange orientational order by using an analyzer. Typical examples are methyl esters of fatty acids. Figure 5: Monolayer of DMPE during first-order phase transition, contrast in domains caused by long range orientation order (nanofilm_ultrabam).   Formation dynamics, non equilibrium structures The formation of the condensed phase in a fluid monolayer can occur far from equilibrium, and growth kinetics can be so fast that the stable phase does not have time to reach its lowest energy state on the microscopic level. Metastable microstructures are formed under non-equilibrium conditions and the growth patterns of these structures are mainly affected by the complex interplay of microscopic interfacial dynamics and external driving forces. Monolayers & chirality Monolayers of enantiomers of amphiphilic molecules can differ. The interaction of enantiomers and diastereomer pairs can be investigated for specific monolayer states. Figure 6 shows domains of D-Dipalmitoylphosphatidylcholin (D-DPPC), L-DPPC and the racemate. Figure 6: Brewster angle micrograph of of D-Dipalmi-toylphosphatidylcholin and the racemate     INTEGRATION OF OTHER TECHNIQUES The information obtained with the Brewster angle microscopy can be complemented by other optical techniques like imaging ellipsometry or UV/VIS reflection spectroscopy. With imaging ellipsometry, it could be possible to e.g. determine the optical parameters of two phases of a monolayer observed by BAM, like domains and surrounding areas. With UV/VIS reflection spectroscopy it is possible to e.g. observe absorption bands in the monolayers and their growth or possible shift, which can provide valuable information about the molecular interactions happening in the monolayer, like the formation of domains or multilayers.

15 Oct 2023

Current Applications of Brewster Angle Microscopy

Brewster angle microscopy  Reflection of light from surfaces is part of our daily experience. However, an amazing property of optics is the possibility of having zero reflection. In more detail no reflection occurs from a clean and perfect interface illuminated under a unique angle of incidence with p-polarized light. This well-understood phenomenon is described by Brewster ´ s law providing the so called Brewster’s angle for the involved optical media. Any subsequent changes of the optical properties at the interface will lead to reflection. This fact is the basis of Brewster angle microscopy, a recent technique for the study of nanofilms at the air/water interface or at the surface of other transparent nonadsorbing dielectric substrates (fig. 1). Figure 1: Brewster angle microscopy: Principals (A) and setup - nanofilm_ultrabam, Accurion GmbH, Göttingen (B). Even just with a monolayer on an interface Brewster’s law is not fulfil led and the covered part of the surface reflects light although with very low in tensity. By increasing the intensity of the illumina tion the con trast between the covered and the clean part of the surface can be enhanced. By using a microscope op tic and a CCD-detector high contrast images of the lateral morpho logy of the layer can be taken. The depth-of-focus problem  In Brewster angle microscopy the depth of focus problem is caused by the oblique imaging angle. With conventional optics, focus is achieved only in a narrow stripe. It is disturbing, especially at higher magnifications. A possible solution is a mechanical focus scanning in combination with image processing. Figure 2: Depth of focus. In the nanofilm_ultrabam a new optical approach is implemented. It allows high resolution and overall focused real-time Brewster angle microscopy. 1,2-Dimyristoyl-sn-glycero-3- phosphoethanolamin  In one of the papers introducing Brewster angle microscopy 1,2- Dimyristoyl-sn-glycero-3-phosphoethanolamin (DMPE) was used to illustrate the capability of the technique. To get an impression of the improvement to current ap plications, the same system is used. Over all focused images were recorded (fig. 3) and the growth of a single Domain was followed (fig. 4A and 4B). Figure 3: Monolayer of DMPE during firstorder phase transition, contrast in domains caused by long range orientation. Figure 4: Growth of a single Domain: Trace (A), area/frame-diagram (B). Ethyl-stearate  The condensed phase of ethyl stearate monolayers exhibits long range orientational order. The aliphatic chains are only weakly tilted in these monolayers. This has an effect on the textures of the monolayers. The nanofilm_ultrabam is equipped with a motorized analyzer to visualize optical anisotropy caused by long range molecular orientation order. To quantify the effect, the gray level distribution is displayed for a series of frames (fig. 5). Figure 5: Ethyl-stearate Visualization of long range molecular orientation order: Brewster angle micrographs, gray level distribution 4-(Cyano-4[dodecylsulanylthiocarbonyl)sulfanyl] pentanoic acid Characteriasation of a Raft reagent at the air water interface. Characteriasation of a Raft reagent at the air water interface. Figure 6: A-Isotherm (A), Brewster angle micrographs, gray leveland thickness distribution of the at 0.06 mN/m (B) 0.12 mN/m (expansion) (c) 3.4 mN/m (D)

15 Oct 2023

Characterization of Thin Films on Transparent Substrates with Imaging Ellipsometry

Introduction Thin films on transparent substrates are of increasing interest in advanced technologies like flexible displays, plastic solar cells, functional windows or new concepts in illumination. State of the art in thin film metrology is spectroscopic ellipsometry, but transparent substrates are still an issue because of the backside reflected light that after a long pathway through a transparent substrate gets incoherent or partly coherent with the consequence that the phase information gets completely or partly lost and the determination of metrological parameters of the layer of interest is difficult or impossible. A number of workarounds are established to get rid of backside reflection, like mechanical grinder or a number of different treats of the backside with different types of tape, with glues, with toothpaste etc. A number of these approaches are working well but in most cases, the sample cannot be used for further investigations. The main question is, how to suppress backside reflection in a nondestructive manner to optimize ellipsometric measurements on transparent substrates   Knife edge illumination A very convenient and effective way to suppress backside reflected light is knife edge illumination to get a defined shadow behind the beam cutter By moving the knife edge of the beam cutter into the beam of an imaging ellipsometer, the sample is illuminated with a well-defined edge. Also the backside reflected light still has a well defined edge and in relation to the internal angle of incidence it hits the surface in a distance to the knife edge. The shadow inbetween is free of contributions of backside reflected light. For illustration purposes a backside roughened glass plate was used. The illuminated part of the backside as well as the shadow can be seen clearly. The same principle works also for much thinner substrates where the shadow is in the microscopic range. With an imaging ellipsometer, the measurements can also be performed in the microscopic shadow. Ellipsometric enhanced contrast micrographs In the ellipsometric enhanced contrast micrographs you can see clearly that the image quality in the shadow is much better than in the area with contributions of backside reflected light. In figure c, you see also disturbance from the backside like brighter strips and shadows of surface contaminations. All these effects disturb the measurement. It is obvious that the shadowed area is free of this disturbance.   a) Ellipsometric enhanced contrast micrographs of Photoactive layer on PEDOT:PSS | ITO | PET (a) patterned coating on glass (b) and SiO2 (Nanoparticles) | glass (c)   Regions of Interest (ROI) The detector of an imaging ellipsometer is a 2D CCD. Pixels, that display the region of interest on the sample can be selected to detect the ellipsometric angles only for this region of interest. In case of transparent substrates, the technique enable the measurement of delta and psi directly in the “shadow” of the beam cutter. The results like wavelength spectra of Delta and Psi can directly be used for the optical modeling.   Example: The image shows data of PEDOT:PSS on an ITO-PET substrate. Image scan (a) - 10x Objective - Lambda = 550 nm - AOI = 55° - FOV = 540 x 400 nm Two layers model, with PET as substrate, ITO layer, PEDOT:PSS layer on top and air as the ambient. The PEDOT:PSS dispersion is modeled by a Tauc-Lorentz and a Drude term. The PET layer and the ITO layer were characterized in independent measurements.   Micro maps of Delta and Psi A unique option of imaging ellipsometry is to use stacks of images at different orientations of the optical components to detect the delta and psi in pixel resolution. Depending on the objective the lateral resolution can be down to 1 nm. Example: MoS2-2D crystallites on a Sapphir substrate. - The Delta and Psi maps:  10x Objective  Lambda = 400 nm  AOI = 50°  FOV = 510 x 435 nm In the shadowed area, the values have a low noise level and the step between substrate and monolayerMoS2 monolayer is significant.   The new beam cutter for the nanofilm_ep4   The open design of the nanofilm_ep4 enabled a new, very convenient setup: After an optimization it can be moved up and down easily. Changes in AOI and y,x-position can be done independently from the beam cutter.  

15 Oct 2023

Analysis of Thin-Film Coatings on Curved Surfaces by Spectroscopic Imaging Ellipsometry

Matthias Duwe Table of Contents Introduction Imaging ellipsometry: concepts and setup Imaging ellipsometry: concepts and setup - Layer-thickness of ITO coating - Layer-thickness of SiO2 top-layer antireflection coating Summary Imprint     Introduction This slide-show shall provide a brief introduction for our proof-of-concept studies regarding the application of Spectroscopic Imaging Ellipsometry (SIE) for the analysis of thin-film coatings on curved surfaces. The primary scope of this study is to demonstrate measurements of layer-thickness distributions across microscopic lenses, i.e. the quantitative characterization of the metrology of the respective anti-reflection coatings. As a case study, we will present layer-thickness measurements of one-layer and two-layer coatings on two different microlens arrays.     We chose micro optics for this case study as our company’s flagship product, the imaging ellipsometer ep4, copes with this task without any need for special hardware adaptions. The general concepts developed, however, may also be applied to larger-scale, curved optics or other free-form objects. In those cases, application-specific hardware adaptions (e.g. sample positioning) might be required. Abstract of Results Using Spectroscopic Imaging Ellipsometry (SIE), we managed to measure the thickness distributions of thin-film coatings on two microlens arrays (MLA) with sub-nanometer thickness resolution and x-y-resolution of 4 µm or better   Measured thickness distribution of custom-made Indium-Tin Oxide (ITO) coating on square-grid MLA. / Measured thicknesses of top-layer antireflection coating (SiO2) on commercially available MLA     Imaging ellipsometry: concepts and setup Ellipsometry - Ellipsometry analyzes the change of polarization of light reflected from a sample  - yields information about film properties like thickness, refractive index and absorption - optical components polarizer (P), compensator (C) and analyzer (A) are used   - change on polarization is described by the ellipsometric angles Delta(△) and Psi (ψ) - △ and ψ are measured against wavelength (λ) and/or angle of incidence (AOI) - film properties are obtained by fitting a numerical model to measurement data     Imaging Ellipsometry Ellipsometry + Microscopy  => lateral ellipsometric resolution down to 1 µm - ψ(Psi),  △(Delta) describe the change in the state of polarization induced by investigated sample - ψ and △ are recorded for the each pixel of the field of view - fitting of ψ and △ to a numerical model yields maps of: - thickness (left) - optical-constants (n, k) (right) volume-fraction etc. - volume-fraction etc.     Micromapping of △ and ψ   - △ and ψ maps are recorded for different wavelengths to obtain spectroscopic maps - with the pixelshot tool, any region can be selected after the measurement to extract the corresponding spectra of △ and ψ - these spectra of△ and ψ can be used to fit for layer thickness, optical parameters, etc. - accordingly, spectra extracted on a pixel-by-pixel basis convert into maps of layer thickness, optical properties, etc.     Pixelshot on Spectroscopic Maps   - △ and ψ maps are recorded for different wavelengths to obtain spectroscopic maps - with the pixelshot tool, any region can be selected after the measurement to extract the corresponding spectra of △ and ψ - these spectra of △ and ψ can be used to fit for layer thickness, optical parameters, etc! - accordingly, spectra extracted on a pixel-by-pixel basis convert into maps of layer thickness, optical properties, etc.     EP4 Spectroscopic Imaging Ellipsometer Spectroscopic configuration* • Xenon lamp + monochromator (360-1000 nm) • LED hub (6 wavelengths) Objective lenses* • NA 0.13 • NA 0.21 • NA 0.35   Imaging ellipsometry: concepts and setup Layer-thickness of ITO coating Microlens Array 1: ITO-Coating Analysis Here, we present a study of a custom-made*, single-layer coating of ITO (nominal thickness 150 nm) deposited on commercially available MLA of spherical lenses (radius of curvature 6.5 mm, square grid with 500 µm pitch, without apertures, substrate: fused silica). For thickness analysis, we acquired hyperspectral images of A & 4 within the spectral range of 385 to 940 nm (in steps of 15 m) using a field of view of 1×1 mm² at an x-y-resolution of 4 um.     For data analysis, we first selected the (approximate) lens apex region (see highlighted regions) from the stack of hyperspectral ǻ & Ȍ images, and extracted the corresponding ellipsometric spectra. We used a numerical model of the sample to fit for ITO layer thickness along with the AOI-offset which accounts for unknown misalignment of the evaluated sample spot. Results:     Secondly, we used the established numerical model to perform this fit procedure on a pixel-by-pixel basis Thus, we converted our stack of hyperspectral A & W images into maps of ITO layer thickness (left) and AOl-offset* distribution (right). The obtained layer thickness distribution features a gradient for each microlens that is oriented along the image's horizontal axis (left figure). This gradient is on the order of just 1 nm in total across each lens. This confirms our technique's capability of performing thin-film measurements on a curved surface with sub-nanometer thickness resolution.     Layer-thickness of SiO2 top-layer antireflection coating Microlens Array 2: SiO2-Coating Analysis As a second sample application, we present a study on a two-layer antireflection coating of a commercially available MLA that has a significantly smaller radius of curvature (0.7 mm) than the one presented in the first example. That is why this sample required a MuellerMatrix based measurement combined with a proprietary analysis for curved-surface metrology that was developed by Accurion* The inter-grid spacing between the circular microlenses feature Chromium apertures, and the lenses are made of fused silica. The MLA’s antireflection coating is a two-layer design, with the top layer made of SiO2. The thickness distribution of this top layer is the subject of this case study.     Imaging Mueller-Matrix Ellipsometry (IMME) is an advanced measurement mode that is available as an upgrade package for the imaging ellipsometer ep4. This mode measures 11 images of the smaple's Mueller matrix* whereas the standard mode would only yield the two images of △ & ψ. This additional data provides the essential information which is needed to measure thin-film coatings on *The Mueller matrix is the most general description of the VDPSOHµV interaction with polarized light. coatings on strongly curved surfaces.   IMME images at a single wavelength (selected from stack of hyperspectral IMME images).   For data analysis, we use Accurion's proprietary algorithm to extract isotropic sample parameters* △ & ψ from the measured Mueller matrix (PATENT PENDING). This algorithm works without any prior knowledge of the surface geometry. Its only input is the measured Mueller matrix micrograph.   MM micrograph (left) and ǻ & Ȍ maps extracted thereof (top right and bottom right images, respectively). For the measurement of the SiO2 top-layer thickness, we acquired hyperspectral images of the Mueller matrix within the spectral range of 380 to 1000 nm (in steps of 20 nm) using a field of view of approximately 0.5x0.5 mm2 at an x-y-resolution better than 4 µm. The set of hyperspectral Mueller matrix micrographs then translated into a set of hyperspectral ǻ & Ȍ micrographs. From hereon, the analysis procedure is essentially the same as for the first application: 1. Establishing a model at the lens apex. 2. Calculating SiO2 layer thickness pixel-by-pixel from hyperspectral stack of ǻ & Ȍ micrographs   Similar to the first application example, the pixel-by-pixel analysis yielded images of the SiO₂ top-layer thickness (left) and again a map of the AOI-offset as a side product (right). The analysis reveals a gradient of SiO2 layer thickness across the microlenses. Compared to the first case study, edge effects at the outer borders between the lenses and the surrounding chromium apertures cause the thickness map to appear "less clean." However, the resulting thickness values across the individual lenses are not affected by these edge effects.   Summary   The imaging ellipsometer ep4 measures hyperspectral sets of micrographs of the sample's polarization properties. Computational modelling translates these data into layer-thickness micrographs. This proof-of-concept study demonstrated unique layer-thickness measurements on microlenses with radii of curvature above 1 mm (ITO coating) and below 1 mm (SiO2 coating). Imprint    

26 Sep 2023

Imaging Ellipsometry: a Tool for Studying Microstructured Surfaces

Christian Röling1,2 11 Institute of Physical Chemistry, Georg-August-University Göttingen, Tammanstraße 6, 37077 Göttingen, Germany; 2Accurion GmbH, Stresemannstraße 30, 37079 Götingen, Germany Imaging Ellipsometry Imaging ellipsometry, combines the power of ellipsometry with optical microscopy. It achieves the highest lateral resolution in the field of ellipsometry and offers a very sensitive imaging technology for thin films. Typical application is the characterization of microstructured thin films. The ep4 is the latest microscopic imaging spectroscopic ellipsometer with unique measurement capabilities. It can provide real-time ellipsometric enhanced contrast images, and maps of the ellipsometric angles Δ and Ψ. These data can be transferred into d and the optical properties by computerized optical modeling.   Setup of an imaging ellipsometer including objective and 2D CCD. Imaging spectroscopic ellipsometer nanofilm_ep4.   Microscopic Mapping Microscopic mapping offers the highest lateral resolution in the field of ellipsometry (down to 1 µm). Each pixel works as a single detector.       Optical Modeling Fitting of optical- and metrological parameters for ultrathin films. Effective medium approach (EMA) also offers information about chemical composition     The Drude equation combines the reflection coefficients of p- (rp) and s-polarized (rs) with the ellipsometric angles Δ and Ψ. Hence rp and rs depend on complex refractive index and thicknesses of the investigated layers conclusions on n and k and thickness can be done. By varying wavelength and angle of incidence more independent values are obtained to improve the results.     Microcontact Printing Pattern of OTS on SiO2 surfaces by using microcontactprinting method on SiO surfaces [1]. Fast recording of Δ-maps (λ = 658 nm, AOI = 50°) using 10x objective on 25 postions of 4 samples. Calculate Δ-maps into thickness-maps by interpolation using dispersion für SiO2 and Si(100) from database.   Left: Cropped Δ-map and line-profile of sample micropatterned with OTS (c = 5 mM) on SiO2/Si(100). Right: Correlating histogram of the Δ-map inclusive Gaussian fit for substrate and patterned SAM layer   3D-plot of thickness-map (c (OTS)=5 mM) on SiO2/Si(100) Quantitative study (n=100) of the pattern formation regarding the ink concentration. Results of quantitative histogram analysis (Gaussian Fit) of 100 thickness-maps varying the ink concentration. Literature: [1] Kumar, Amit, Hans A. Biebuyck, and George M. Whitesides. "Patterning selfassembled monolayers: applications in materials science." Langmuir 10.5 (1994): 1498- 1511.     Laser Ablation Microstructured PMMA layer on SiO2/Si(100) formed by laser ablation (pulsed argon ion laser (λ = 193nm)). Variation in fluence and number of pulse by laser ablation on the sample. Recording Δ-maps (λ = 500 nm, AOI = 50°) using 50x objective on 12 postions of the sample. Calculate Δ-maps into thickness-maps by interpolation using dispersion for SiO2 , Si(100) and PMMA from database.   Left: d-map (3D-Plot) of microstructured thin PMMA layer (d = 21.9 nm) on SiO2/Si(100) Right: Correlating histogram of the thickness-map inclusive depth of ablation for PMMA using Gaussian fit analysis. Determination of the depth of ablation regarding fluence and number of pulse using Gauss Fit Histogram Analysis of thickness-maps. Calculated depth of ablation depending on pulse and energy obtained by histogram analysis of thickness-maps.   Exponential plot of the depth of ablation per pulse depending on the energy.     Holographic Interferometry Formation of photoinduced diffraction grating in amorphous As2S3 thin film on transparent subtrate by holographic interferometry [2] Mapping the photoinduced changes of refractive index (λ= 658 nm, AOI = 50°, 10x) regarding expose time. Δ-map (left) and Ψ-map (right) of microstructured amorphous As2S3 thin film on glass (d = 949.4 ± 0.1 nm) with proper-exposed gratings.   Determine the thickness by Spectrosopic Ellipsometry (380-800 nm) using Tauc-Lorentz-Model. Calculate n+k values for each pixel using point to point method with fixed thickess (constant, measured by AFM).     Obtained n-map (left) and k-map (right) of microstructured amorphous As2S3 thin film on glass (d = 949.4 ± 0.1 nm) with proper-exposed gratings. Gaussian Fit Analysis of n+k-maps regarding photoinduced changes in the optical parameters depending on the exposure time.   Results of quantitative histogram analysis of the n+k-maps with different exposure time. Literature: [2] Röling, C., et al. "Imaging ellipsometry mapping of photo-induced refractive index in As2S3 films." Journal of Non-Crystalline Solids 365 (2013): 93-98.

26 Sep 2023

Current Trends in Microscopic Characterization: Spectroscopic Imaging Ellipsometry (SIE)

The intensity changes are recorded only for the pixels, that represent the Region of Interest (ROI). With this method, a much higher lateral resolution is available and similar areas can be measured in parallel.

26 Sep 2023

Principles of Nulling and Imaging Ellipsometry

Ellipsometry is a very sensitive optical method, which has been used for about a hundred years to derive information about surfaces. It makes use of the fact that the polarization state of light may change when the light beam is reflected from a surface.

21 Feb 2022

Capturing conductive ferroelectric domain walls

Ilka Hermes and Andrea Cerreta1 1 Park Systems Europe, Germany Introduction: Ferroelectric materials typically feature wide band gaps and are therefore classified as insulators. They carry a spontaneous electrical polarization that exists in two or more orientation states and can be switched by applying an external bias. Areas that feature a uniform polarization orientation are called ferroelectric domains, separated by domain walls. These domain walls can be charged, if the polarization orientation of the two adjacent domains features a normal component with respect to the domain wall. For example, domains with a 45° polarization orientation may display positive 90° head-to-head walls or negative 90° tail-to-tail walls (Figure 1a). To compensate the bound domain wall charge and thereby lower the consequently increased energy, free electronic or ionic charge carriers accumulate at charged domain wall.1 The high density of free electronic charges and/or the ionic doping at domain walls lead to localized two-dimensional conductivities approaching those of metals.2–4 This 2D conductivity sandwiched between insulating domains and the possibility to tailor the domain wall arrangement as required makes conductive domain walls attractive systems for nanoelectronic applications.2 To image ferroelectric domain patterns and the position of domain walls on the nanoscale, piezoresponse force microscopy (PFM), featured in all of Park Systems’ NX and FX research atomic force microscopes (AFM), is ideally suited. In PFM, a conductive cantilever scans the surface in contact mode, while an AC voltage is applied between tip and sample. This AC voltage induces a periodic sample oscillation that leads to a periodic deflection of the cantilever during the scan. This sample oscillation is called piezoresponse, which captures the domain arrangement, polarization orientation, and piezoelectric behavior in PFM phase and amplitude. An AC excitation frequency close to the contact resonance of the cantilever can improve the signal-to-noise ratio of the piezoresponse for the characterization of sensitive materials or thin films.5 By correlating PFM domain imaging with conductive atomic force microscopy (C-AFM), the researcher can directly correlate local 2D conductivity with the position of charged ferroelectric domain walls. Figure 1: a) Example of positive and negative 90° charged domain walls (CDW) in a head-to-head and tail-to-tail arrangements, respectively. b) Schematic of experimental setup for conductive domain wall imaging, in which an AC/DC voltage coupler allows simultaneous or subsequent PFM and C-AFM measurements without changes. In C-AFM, a conductive cantilever scans the surface in contact mode while a DC voltage is applied between tip and sample. The consequent current response of conductive samples areas is detected by a current amplifier during the scan and correlated to the sample topography. In Park Systems AFMs, the current response of samples with low conductivity can be enhanced via an ultra-low noise current amplifier with a gain setting of 1011 V/A that can detect currents down to femtoampères. Typically, PFM and C-AFM measurements require different experimental setups: In C-AFM the cantilever is directly connected to the current amplifier, which prevents the application of an AC voltage as required for PFM. The intermediate change of the setup extends the measurement time. Moreover, subsequent correlation of data necessitates that the same area is imaged after changing from PFM to C-AFM or vice versa. Here, we present how the experimental flexibility of Park Systems’ AFMs allows combining PFM and C-AFM in the same setup for subsequent or even simultaneous PFM and C-AFM measurements without intermediate hardware changes (Figure 1b): The AC/DC voltage coupler allowed applying an AC voltage for PFM as well as a DC voltage for C-AFM to the sample, while the tip was connected to the current amplifier for conductivity mapping. Therefore, we were able to image conductive domain walls in a ferroelectric Bismuth Ferrite (BFO) thin film sample,6 provided Dr. Akash Bhatnagar at Centre for Innovation Competence SiLi-nano, Halle, Germany.   Experimental: To combine PFM and C-AFM measurement in the same experimental setup on a Park Systems NX10 AFM, we used an AC/DC voltage coupler available at Park Systems to apply the PFM AC excitation as well a DC bias for C-AFM imaging to the sample stage. For PFM imaging, we used resonance enhancement by applying an AC voltage with 2 V amplitude at a frequency of 355 kHz, close to the vertical contact resonance of the ElectriMulti75-G cantilever (k=3 N/m, f=75kHz from Budgetsensors). For C-AFM imaging we applied a sample bias of 2.5 V and detected the current with an ultra-low noise current amplifier series LCA by FEMTO Messtechnik GmbH, Berlin, Germany.   Results: Figure 2 summarizes the results of subsequent C-AFM (a-c) and PFM measurements (d-e) on the same scan area of the BFO sample, without intermediated setup changes. The C-AFM data in Figure 2b captured localized conductive areas with current reaching more than 1 pA in an otherwise insulating background. By comparing the current image to the PFM phase in Figure 2e, which was measured immediately afterwards, we find that the positions of the local current maxima coincide with the boundaries of the domain in the image center. Between the domain and the surrounding, the PFM phase displayed a contrast inversion. This contrast inversion is characteristic for a change in the ferroelectric polarization orientation by 180°. The line profile extracted from the current image in Figure 2c features local current maxima of up to 0.5 pA at the position of the ferroelectric domain walls. Hence, this data demonstrates that the flexible experimental setup in Park Systems’ AFMs facilitates a straight-forward combination of C-AFM and PFM without intermediate hardware changes for reliable and fast characterization of conductive ferroelectric domain walls. Figure 2: Results of subsequent C-AFM and PFM measurements on BFO thin film with a Park Systems NX10 AFM. a) Sample topography, b) corresponding current signal of C-AFM measurement at a DC voltage of 2.5 V and c) the line profile extracted along the red line in the current image features current maxima at the position of domain walls of up to 0.5 pA. d) Sample topography and e) corresponding PFM phase of subsequent PFM measurement detected at an AC voltage of 2 V at 355 kHz showing the contrast inversion on the oppositely polarized domain in the center of the image. Figure 3: Results of simultaneous C-AFM and PFM measurement on BFO thin film with a Park Systems NX10 AFM, with the respective sample height in a, current signal in b and PFM phase in c at an AC voltage of 2 V at 355 kHz and a DC voltage of 2.5 V. The correlation of current and PFM phase captures the increased conductivity at the position of ferroelectric domain walls. Following the subsequent measurements, we additionally performed simultaneous C-AFM and PFM measurements on the same domains as shown in Figure 3a-c. We again observe distinct maxima in the current signal in 3b at the position of the domain walls, resolved in the PFM phase in 3c. However, the applied DC sample voltage of 2.5 V for the current detection appeared to introduce a change in the polarization orientation of the center-domain as visible in the more blurred phase contrast in the area highlighted by the red box. Hence, for BFO thin films, which exhibit comparably low coercive fields,7 simultaneous PFM and C-AFM measurements would require even lower DC voltages.   Conclusion: In this application note, we demonstrate that the experimental flexibility available in Park Systems AFMs facilitates a straightforward combination of C-AFM and PFM measurements without intermediate hardware changes. Therefore, Park Systems AFMs allow for a fast, accurate and reliable correlation of current detection and ferroelectric domain imaging as required to characterize conductive ferroelectric domains walls, which are promising systems for next-generation nanoelectronics.   References: 1 A. K. Tagantsev, L. E. Cross and J. Fousek, Domains in ferroic crystals and thin films, Springer, 2010, vol. 13. 2 P. S. Bednyakov, B. I. Sturman, T. Sluka, A. K. Tagantsev and P. V Yudin, npj Comput. Mater., 2018, 4, 1–11. 3 T. Sluka, A. K. Tagantsev, P. Bednyakov and N. Setter, Nat. Commun., 2013, 4, 1808. 4 J. Seidel, L. W. Martin, Q. He, Q. Zhan, Y.-H. Chu, A. Rother, M. E. Hawkridge, P. Maksymovych, P. Yu and M. ea Gajek, Nat. Mater., 2009, 8, 229–234. 5 E. Soergel, J. Phys. D Appl. Phys., 2011, 44, 464003. 6 A. Bhatnagar, A. Roy Chaudhuri, Y. Heon Kim, D. Hesse and M. Alexe, Nat. Commun., 2013, 4, 2835. 7 V. Shelke, D. Mazumdar, G. Srinivasan, A. Kumar, S. Jesse, S. Kalinin, A. Baddorf and A. Gupta, Adv. Mater., 2011, 23, 669–672.

10 Jun 2019

Defect Recognition on Coating Layer using PinPoint Nanomechanical Mode, Atomic Force Microscopy

Moses Lee, Jake Kim and Cathy Lee Park Systems Corporation, Suwon, Korea    Introduction Surface treatment is a critical component in a variety of fields and industries such as automobile1, aerospace2, construction materials3, and mobile and precision4. Surface coating treatment techniques normally involve a coating process, which is widely used industrially to prevent surface abrasion and corrosion. More recently, the ability to convey special properties to coating materials such as anti-static5, antibacterial6 and electromagnetic wave shielding7 have been developed. The popularity of digital devices increased significantly during the 1990’s; this popularity led to rapid developments in industrial coating technologies. These technologies advanced device protection in a number of ways, including scratch resistance, anti-fingerprint, chemical resistance and UV protection.8 Atomic Force Microscopy (AFM), largely used in the field of material surface research, is also utilized extensively on coated surfaces9. From analysis of AFM images, the thickness of the coating can be derived from the morphology of the sample surface while surface roughness yields information about the uniformity of a coating once applied. AFM images allow for a review and inspection of quality control for possible defects. AFM measures both surface morphology and mechanical properties. Using force-distance curves on the coated surface, mechanical properties such as stiffness, adhesion and modulus can be measured10. This application note introduces the latest method developed by Park Systems called “PinPointTM nanomechanical mode” that probes the mechanical properties of a surface. Furthermore, this note describes how the method can be used to analyze the stages at which defects are generated during the material surface processing, specifically during the coating process. In general, defects on the sample surface can be observed through AFM images. However, it is difficult to determine when the defect was generated (during production) solely from surface morphology observation. PinPoint nanomechanical mode resolves this issue: it gathers mechanical information about the coated surface and determines whether defects were created before or after the coating during production. Materials and Methods PinPoint Nanomechanical Mode PinPoint nanomechanical mode collects high resolution topographical data while simultaneously obtaining force-distance data for each pixel of the scan area11. This allows measurement of the sample surface morphology while simultaneously obtaining quantitative nanomechanical properties including modulus, adhesion, deformation, stiffness and energy dissipation. Experimental Setup In order to measure mechanical properties of the sample, the PinPoint nanomechanical mode was employed on Park NX10 AFM from Park Systems coupled with an NSC36 cantilever, acquired from MikroMasch. The resonant frequency (f = 130 kHz) and spring constant (k = 2 N/m) of the cantilever allow adequate sample deformation while maintaining sufficient cantilever deflection. The results were analyzed using the Park XEI data processing software, which provides quantitative data from the images, comparisons of mechanical property values, and characterization of the surface material. Testing Samples Samples were prepared in two ways to test the ability of the PinPoint nanomechanical mode in distinguishing defects generated before and after the coating application. The first test involved a scratch produced over the surface of a coated glass substrate. The second test featured a scratched glass substrate whose surface was subsequently re-coated. The difference between the creation times of the defect can be determined from whether the glass material was exposed to the scratch site. If the scratch was created prior to coating, the glass will not be exposed, as the coating is applied over the scratch. If the scratched was created after coating, the coating will peel off and expose the glass material. By imaging the site using PinPoint nanomechanical mode, surface mechanical properties (such as adhesion and modulus) show a clear contrast when the AFM tip probes different materials (glass and coating). This information cannot be determined from the topography alone, and Figure 1 clearly shows this. Fig. 1. Illustration of a defect created before and after coating. The difference between the materials cannot be seen through the depth of the scratch alone but through the mechanical properties of the material.   Fig. 2. PinPoint nanomechanical mode images of a glass substrate when the scratch was created before coating was applied. No contrast was observed between the coated area and the scratched area, in both adhesion energy and modulus images.roperties of the material. Fig. 3. PinPoint nanomechanical mode images of a glass substrate when the scratch was created after a coating was applied. The contrast between the coating area and the scratched area is clearly visible in both adhesion energy and modulus images. Fig. 4. Region selection applied in Park XEI software, to calculate mean adhesion and modulus values of each distinct area. Results Topography, adhesion energy and modulus images from two samples with defects created before and after coating were obtained using PinPoint nanomechanical mode. The topography images in both cases confirmed the scratch behind a trench (Figure 2a and 3a). Images from this site highlight the mechanical property differences between the scratch area and the unaffected area. Examining the adhesion energy and modulus images of the scratch before coating (Figure 2b and 2c), they show no visible difference in the mechanical properties between the areas of the scratch and the coating. Figures 3b and c, where the glass substrate is scratched after the coating, show a very clear contrast between the scratch and the adjacent surface. Comparing the mean adhesion energy and modulus values of the surfaces before and after scratching clearly differentiates the two different surfaces. Using Park XEI data processing software, different regions of the same image can be selected i.e. area of the scratch and area of the coating (Figure 4.). For each region, the mean value of each mechanical property was calculated; those values are listed in Table 1 for comparison. Figure 4a shows the selected coating area which produced a Results 2 Park Systems | Enabling Nanoscale Advances Fig. 1. Illustration of a defect created before and after coating. The difference between the materials cannot be seen through the depth of the scratch alone but through the mechanical properties of the material. Fig. 4. Region selection applied in Park XEI software, to calculate mean adhesion and modulus values of each distinct area. Fig. 2. PinPoint nanomechanical mode images of a glass substrate when the scratch was created before coating was applied. No contrast was observed between the coated area and the scratched area, in both adhesion energy and modulus images.roperties of the material. Fig. 3. PinPoint nanomechanical mode images of a glass substrate when the scratch was created after a coating was applied. The contrast between the coating area and the scratched area is clearly visible in both adhesion energy and modulus images. scratch site. If the scratch was created prior to coating, the glass will not be exposed, as the coating is applied over the scratch. If the scratched was created after coating, the coating will peel off and expose the glass material. By imaging the site using PinPoint nanomechanical mode, surface mechanical properties (such as adhesion and modulus) show a clear contrast when the AFM tip probes different materials (glass and coating). This information cannot be determined from the topography alone, and Figure 1 clearly shows this. Scratch before Coating Scratch after Coating Adhesion Energy Modulus Adhesion Energy Scratch Before Coating 4a (Full Area) 1.22 fj 1.15 fj 2.47 fj 4b (Red) 4b (Green) After Coating Before Coating 4c (Full Area) 3.15 GPa 3.08 GPa 1.79 GPa 4d (Red) 4d (Green) After Coating Figure Average Modulus calculated mean value of 1.22fJ. In figure 4b., the area of the coating and exposed material underneath was selected separately. The adhesion energy value of the coating (1.15 fJ) coincided with the previous value while the adhesion energy of underlying glass substrate exhibited a higher value (2.47 fJ). Also, when comparing the modulus values, the coating from figure 4c gave a calculated value of 3.15 GPa, similar to the modulus value of the coating from figure 4d. (3.08 GPa). Conversely to the adhesion energy values, the modulus of the scratched glass substrate underneath showed a lower value of 1.79 GPa. Conclusion This application note highlights the utility of PinPoint nanomechanical mode from Park Systems to study the mechanical properties of defects to determine whether a surface defect was created before or after the coating process was applied. By measuring the adhesion energy and modulus differences between glass and coating materials, the initial time of the defect formation was successfully identified. AFM can be used for diverse range of applications in the coating industry because of its ability to closely examine the surface properties of samples, properties otherwise not observable by studying morphology alone. By using the PinPoint nanomechanical mode, one can investigate mechanical properties at nanoscale, and study the differences between different coating materials and coating methods. Such benefits will elevate the utility of AFM as a valuable instrument in the coating industry.   References 1. 1. Akafuah, N. K., Poozesh, S., Salaimeh, A., Patrick, G., Lawler, K., & Saito, K. (2016). Evolution of the automotive body coating process—A review. Coatings, 6(2), 24. 2. Benavides, S. (Ed.). (2009). Corrosion control in the aerospace industry. Elsevier. 3. Lee, J., Mahendra, S., & Alvarez, P. J. (2010). Nanomaterials in the construction industry: a review of their applications and environmental health and safety considerations. ACS nano, 4(7), 3580-3590. 4. Leopold, J., Neugebauer, R., Löffler, M., Schwenck, M., &Hänle, P. (2006). Influence of coating-substrate systems on chip and burr formation in precision manufacturing. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, 220(4), 499-506. 5. Qiu, C., Wang, J., Mao, S., Guo, W., Cheng, S., & Wang, Y. (2010). Preparation of poly (3, 4‐ethylenedioxythiophene)/poly (styrene sulfonate)(PEDT/PSS) composite and its applications in anti‐static coating. Polymers for Advanced Technologies, 21(9), 651-655. 6. Ciacotich, N., Din, R. U., Sloth, J. J., Møller, P., & Gram, L. (2018). An electroplated copper–silver alloy as antibacterial coating on stainless steel. Surface and Coatings Technology, 345, 96-104. 7. Kim, C. G., Nam, Y. W., & Choi, J. H. (2018). U.S. Patent Application No. 15/809,345.\ 8. Future Markets. “Automotive.” Nanocoatings. Nov. 2013. 9. Carpick, R. W., & Salmeron, M. (1997). Scratching the surface: fundamental investigations of tribology with atomic force microscopy. Chemical reviews, 97(4), 1163-1194. 10. Butt, H. J., Cappella, B., &Kappl, M. (2005). Force measurements with the atomic force microscope: Technique, interpretation and applications. Surface science reports, 59(1-6), 1-152. 11. John Paul Pineda, Gerald Pascual, Byong Kim, and Keibock Lee (2017). Using AFM PinPointTM Nanomechanical Mode for Quantification of Elastic Modulus in Materials Two Orders of Magnitude Faster than Force Volume Spectroscopy, Park Systems Application note #26