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Expanded beam (macro-imaging) ellipsometry
Abstract
Our aim was to make possible to use spectroscopic ellipsometry for mapping purposes during one measuring cycle (minimum one rotation period of polarizer or analyzer) on many sample points. Our new technique uses non-collimated (non-parallel, mostly diffuse) illumination with an angle of incidence sensitive pinhole camera detector system and it works as an unusual kind of imaging ellipsometry. Adding multicolour supplemets, it provides spectral (a few wavelengths on a 2D image or a full spectrum along a line) information from rapid measurements of many points on a large (several dm2) area. This technique can be expanded by upscaling the geometry (upscaling the dimensions of the instrument, and characteristic imaging parameters such as focal lengths, distances, etc.). The lateral resolution is limited by the minimum resolved-angle determined by the detector system, mainly by the diameter of the pinhole. (The diameter of the pinhole is a compromise between the light intensity and the lateral resolution.) Small-aperture (25 mm diameter) polarizers are incorporated into both the polarization state generator (PSG) and polarization state detection (PSD) components of the instrument.
... www.advancedsciencenews.com www.pss-a.com scan of a line on the surface, [16,17] whereas in the usual working modes, HSIM includes both line scanning and area scanning options. ...
... This instrument produces spatial information in 1D of the CCD array simultaneously with spectroscopic information in the perpendicular direction of the array ( Figure 4A,B). Both approaches (multi-angle/multi-wavelength and continuous spectroscopic) are useful, for example, in the analysis of a product moving along a coating line [16,17,[44][45][46] such as in case of an RtR configuration. [47] ...
... The size of the illuminated area of the expanded-beam ellipsometer [17] can be adjusted for the task. Figure 10 shows an extreme case in which the area of mapping was 1000 mm by 500 mm on a glass panel used for solar applications. ...
Thin films covering large surfaces are used in a very wide range of applications from displays through corrosion resistance, decoration, water proofing, smart windows, adhesion performance to solar panels and many more. Scaling up existing thin film measurement techniques requires a high speed and the redesign of the configurations. The aim of this review is to give an overview of recent and past activities in the area, as well as an outlook of future opportunities. This article is protected by copyright. All rights reserved.
... Thus, collimated light beams are conventionally used with a well defined angle of incidence at the reflecting surface. Here we present an ellipsometric method fundamentally different from the conventional techniques [38][39][40][41][42]. In our instrument, the sample is illuminated by an almost diffuse, "divergent beam" of light, providing a collection of rays with diverse angles of incidence at every point of the sample. ...
... A near-ultraviolet-to-visible (nuv-vis) range (350 -630 nm) of the first generation divergent beam instrument [41,42] were built in MFA, Budapest, but this prototype limits potential photovoltaics applications; as a result, an extension into the near-infrared (NIR) region were performed to probe below the band gap of absorber layers in order to measure their thicknesses [42]. Thus, with a broadened spectral range, it became possible to characterize a wider variety of layers and structures. ...
... A near-ultraviolet-to-visible (nuv-vis) range (350 -630 nm) of the first generation divergent beam instrument [41,42] were built in MFA, Budapest, but this prototype limits potential photovoltaics applications; as a result, an extension into the near-infrared (NIR) region were performed to probe below the band gap of absorber layers in order to measure their thicknesses [42]. Thus, with a broadened spectral range, it became possible to characterize a wider variety of layers and structures. ...
Non-destructive analysing tools are needed at all stages of thin film process-development, especially photovoltaic (PV) development, and on production lines. In the case of thin films, layer thicknesses, micro-structure, composition, layer optical properties, and their uniformity are important parameters. An important focus is to express the dielectric functions of each component material in terms of a handful of wavelength independent parameters whose variation can cover all process variants of that material. With the resulting database, spectroscopic ellipsometry coupled with multilayer analysis can be developed for on-line point-by-point mapping and on-line line-by-line imaging. Off-line point-by-point mapping can be effective for characterization of non-uniformities in full scale PV panels or big area (even 450 mm diameter) Si-wafers in developing labs but it is slow in the on-line mode when only 15 points can be obtained (within 1 min) as a 120 cm long panel moves by the mapping station. Last years [M. Fried et al, Thin Solid Films 519, 2730 (2011)], a new instrumentation was developed that provides a line image of spectroscopic ellipsometry (wl=350- 1000 nm) data. Earlier a single 30 point line image could be collected in 10 s over a 15 cm width of PV material. Recent years we have built a 30, a 45 and a 60 cm width expanded beam ellipsometer which speed is increased by 10x. Now, 1800 points can be mapped in a 1 min traverse of a 60*120 cm PV panel or flexible roll-to-roll substrate.
... A near-ultraviolet-to-visible (nuv-vis) range (350 -630 nm) of the first generation divergent beam instrument [4,5] limits potential photovoltaics applications; as a result, an extension into the near-infrared (nir) region is desired to probe below the band gap of absorber layers in order to measure their thicknesses. Thus, with a broadened spectral range, it becomes possible to characterize a wider variety of layers and structures. ...
... Thus, it is impossible to operate the ellipsometer over the full nuv-nir range using one polarizer-analyzer pair. A dual spectral range capability is a convenient solution whereby the optical elements (source, polarizer-analyzer pairs, and optical grating) are automatically interchangeable, and the entire nuv-nir spectra for a line image is detectable in two measurement cycles with one CCD camera [5]. ...
... A smooth variation is expected for the angle of incidence and for the {Re(ρ mirror ), Im(ρ mirror )} values. As a result, smoothing of the dependence of angle of incidence on position as well as the dependence of ρ mirror on position and wavelength reduces errors in the calibration measurement [5]. ...
We have developed a prototype spectroscopic ellipsometer for imaging/mapping purposes requiring only one measurement cycle (one rotation period of a polarizer or analyzer) for the acquisition of a two-dimensional array of data points. Our new measurement technique serves as a novel form of imaging ellipsometry, using a divergent (uncollimated, diffuse) source system and a detection system consisting of an angle-of-incidence-sensitive pinhole camera. By incorporating broad-band sources and wavelength dispersion optics, the instrument provides continuous high-resolution spectra along a line image of the sample surface. As a result, information on multilayer photovoltaics stacks can be obtained over large areas (several dm 2) at high speed. The technique can be expanded to even larger areas by scaling-up the optical geometry. The spatial resolution of the line image is limited by the minimum resolved-angle as determined by the detection system. Small-aperture polarizers (25 mm diameter) are incorporated into the instrument, which reduces its cost. Demonstration mapping measurements have been performed ex situ on a multilayer sample deposited on a polymer substrate, including an intentionally graded 80-350 nm thick hydrogenated amorphous silicon (a-Si:H) layer and an intended uniform 400-500 nm thick transparent conducting ZnO:Al layer, both on opaque silver. Alternative commercial instruments for ex situ SE mapping must translate the sample in two dimensions. Even a 15 x 15 cm 2 sample requires > 200 measurements with cm-resolution and at least 15 min. By collecting ex situ data in parallel along one dimension through imaging, the divergent-beam system can measure with similar spatial resolution in < 2 min. In situ measurements on both roll-to-roll polymer and rigid glass will be possible in the future.
... Thus, collimated light beams are conventionally used with a well defined angle of incidence at the reflecting surface. Here we present an ellipsometric method fundamentally different from the conventional techniques [38][39][40][41][42]. In our instrument, the sample is illuminated by an almost diffuse, "divergent beam" of light, providing a collection of rays with diverse angles of incidence at every point of the sample. ...
... A near-ultraviolet-to-visible (nuv-vis) range (350 -630 nm) of the first generation divergent beam instrument [41,42] were built in MFA, Budapest, but this prototype limits potential photovoltaics applications; as a result, an extension into the near-infrared (NIR) region were performed to probe below the band gap of absorber layers in order to measure their thicknesses (Fig. 2). Thus, with a broadened spectral range, it became possible to characterize a wider variety of layers and structures. ...
... Divergent beam ellipsometers (using uncollimated beam) employ film polarizers, which exhibit a wide semi-field angle but limited spectral range. In earlier state, dual spectral range capability was a convenient solution whereby the optical elements (source, polarizer-analyzer pairs, and optical grating) were automatically interchangeable, and the entire nuv-nir (350-1000 nm) spectra for a line image was detectable in two measurement cycles with one CCD camera [42], see Fig. 2. ...
Non-destructive analyzing tools are needed at all stages of thin film photovoltaic (PV) development, and on production lines. In thin film PV, layer thicknesses, micro-structure, composition, layer optical properties, and their uniformity (because each elementary cell is connected electrically in series within a big panel) serve as an important starting point in the evaluation of the performance of the cell or module. An important focus is to express the dielectric functions of each component material in terms of a handful of wavelength independent parameters whose variation can cover all process variants of that material. With the resulting database, spectroscopic ellipsometry coupled with multilayer analysis can be developed for on-line point-by-point mapping and on-line line-by-line imaging. This work tries to review the investigations of different types of PV-layers (anti-reflective coating, transparent-conductive oxide (TCO), multi-diode-structure, absorber and window layers) showing the existing dielectric function databases for the thin film components of CdTe, CuInGaSe2, thin Si, and TCO layers. Off-line point-by-point mapping can be effective for characterization of non-uniformities in full scale PV panels in developing labs but it is slow in the on-line mode when only 15 points can be obtained (within 1 min) as a 120 cm long panel moves by the mapping station. In the last years [M. Fried et al., Thin Solid Films 519, 2730 (2011)], instrumentation was developed that provides a line image of spectroscopic ellipsometry (wl = 350-1000 nm) data. Up to now a single 30 point line image can be collected in 10 s over a 15 cm width of PV material. This year we are building a 30 and a 60 cm width expanded beam ellipsometer the speed of which will be increased by 10x. Then 1800 points can be mapped in a 1 min traverse of a 60 * 120 cm PV panel or flexible roll-to-roll substrate.
... Generally, the ellipsometry technique uses collimated light beams with a well-defined angle of incidence to illuminate the sample. Nevertheless, a nonparallel expanded light beam with precision suitable for real-time mapping, production lines, and quality control can also be used [1]. ...
... In the case of light reflection, the measured complex reflectance ratio ρ is equal to ≡ tan exp ∆ ≡ (1) where tan ψ is the amplitude ratio, Δ is the phase difference, rp = reflection coefficient for p-polarized light, and rs = reflection coefficient for s-polarized light. During data analysis, information about the sample under study is obtained by fitting measured ellipsometry spectra to optical and structural models, as ellipsometry does not give direct information about the sample in consideration. ...
We developed and applied a new calibration method to make more accurate measurements with our multi-color ellipsometric mapping tool made from cheap parts. Ellipsometry is an optical technique that measures the relative change in the polarization state of the measurement beam induced by reflection from or transmission through a sample. During conventional ellipsometric measurement, the data collection is relatively slow and measures one spot at a time, so mapping needs a long time compared with our new optical mapping equipment made by an ordinary color LED monitor and a polarization-sensitive camera. The angle of incidence and the incident polarization state is varied point by point, so a special optical calibration method is needed. Three SiO2 samples with different thicknesses were used for the point-by-point determination of the angle of incidence and rho (ρ) corrections. After the calibration, another SiO2 sample was measured and analyzed using the calibrated corrections; further, this sample was independently measured using a conventional spectroscopic ellipsometer. The difference between the two measured thickness maps is less than 1 nm. Our optical mapping tool made from cheap parts is faster and covers wider area samples relative to conventional ellipsometers, and these correction enhancements further demonstrate its performance.
... The lateral resolution is usually not better than one µm 2 , except for the time-consuming and expensive cross-sectional methods : or special approaches such as the scanning Auger electron spectroscopy. While macro imaging ellipsometry can map surfaces of square meters [18], microscopic imaging ellipsometry [19] works down to the above-mentioned diffraction limit (not much better than one micron). Besides the cross-sectional approach, better lateral resolution can only be achieved using tapping mode methods, such as atomic force microscopy or scanning tunneling microscopy (with special features as nano-lithography [20]) and numerous emerging optical methods including the near-field approach, evanescent-wave enhancement [21], scatterometry [22][23][24] or nanospheres [25,26]. ...
... 190-1700 nm) within just one second, sometimes utilizing both dual light sources and detectors. The high acquisition rate and sensitivity combined with the non-destructive measurement capability can be utilized in many fields from in situ bioellipsometry [28] to large area mapping [18,29], although the increase of lateral resolution remains a challenge. ...
Understanding interface processes has been gaining crucial importance in many applications of biology, chemistry, and physics. The boundaries of those disciplines had been quickly vanishing in the last decade, as metrologies and the knowledge gained based on their use improved and increased rapidly. Optical techniques such as microscopy, waveguide sensing, or ellipsometry are significant and widely used means of studying solid‐liquid interfaces because the applicability of ions, electrons, or X‐ray radiation is strongly limited for this purpose due to the high absorption in aqueous ambient. Light does not only provide access to the interface making the measurement possible, but utilizing the phase information and the large amount of spectroscopic data, the ellipsometric characterization is also highly sensitive and robust. This article focuses on ellipsometry of biomaterials in the visible wavelength range. The authors discuss the main challenges of measuring thickness and optical properties of ultra‐thin films such as biomolecules. The authors give an overview on different kinds of flow cells from conventional through internal reflection to combined methods. They emphasize that surface nanostructures and evaluation strategies are also crucial parts of in situ bioellipsometry and summarize some of the recent trends showing examples mainly from their research.
... Spectroscopic ellipsometry belongs to the group with limited lateral resolution and high sensitivity (for layer thickness, composition and vertical resolution), but its most important feature to our topic is the high speed combined with the non-destructive nature. This allows either the mapping of surfaces (instrumentation for imaging and mapping have long been existing [2,3]) and the real time monitoring of solid state [4] or bio processes [5]. This article provides a short review of both bio and inorganic applications of ellipsometry. ...
... The most perspective capability of the optical techniques like ellipsometry is not the characterization of a completely unknown sample or structure, but the high throughput and high sensitivity testing and monitoring of processes, as in the above examples for bio processes, and also for solid state processes e.g. in photovoltaics. Figure 8 shows a prototype of a divergent light source ellipsometer that has been developed in our group for more than a decade [3]. The latest versions are capable of mapping a 30-by-30 point area within 2 minutes, even on surfaces as large as 90-by-90 cm [41]. ...
Optical methods have been used for the sensitive characterization of surfaces and thin films for more than a century. The first ellipsometric measurement was conducted on metal surfaces by Paul Drude in 1889. The word 'ellipsometer' was first used by Rothen in a study of antigen-antibody interactions on polished metal surfaces in 1945. The 'bible' of ellipsometry has been published in the second half of the '70s. The publications in the topic of ellipsometry started to increase rapidly by the end of the '80s, together with concepts like surface plasmon resonance, later new topics like photonic crystals emerged. These techniques find applications in many fields, including sensorics or photovoltaics. In optical sensorics, the highest sensitivities were achieved by waveguide interferometry and plasmon resonance configurations. The instrumentation of ellipsometry is also being developed intensively towards higher sensitivity and performance by combinations with plasmonics, scatterometry, imaging or waveguide methods, utilizing the high sensitivity, high speed, non-destructive nature and mapping capabilities. Not only the instrumentation but also the methods of evaluation show a significant development, which leads to the characterization of structures with increasing complexity, including photonic, porous or metal surfaces. This article discusses a selection of interesting applications of photonics in the Centre for Energy Research of the Hungarian Academy of Sciences.
... Imaging ellipsometry provides a concept of measuring in different lateral positions simultaneously. Both microscopic [15,137,138] and macroscopic [49,50,67,87,88] concepts have been demonstrated. The size of the measured area and the resolution ranges from several microns to several millimeters. ...
... For divergent light source non-collimated beam macroscopic ellipsometric configurations there is theoretically no limit of the maximum size-the resolution can be defined as the viewing angle of a pixel on the CCD camera. The concept is shown in Fig. 17.11 [49] demonstrating a new design, in which all the polarizing optical components can be small-the only component that scales with the mapping size is the spherical mirror. ...
In this chapter we make an attempt to give a comprehensive overview on the optical modeling of layer structures that accommodate or are entirely composed of semiconductor nanocrystals. This research field is huge both in terms of the theories of effective dielectric functions and applications. The dielectric function of single-crystalline semiconductors can be determined on high quality reference materials. The accuracy of the reference data depends mostly on the numerical or experimental elimination of the surface effects like oxides, nanoroughness, contamination, etc.
... Generally, ellipsometry technique use collimated light beams with a well-defined angle of incidence to illuminate the sample. Nevertheless, a non-parallel expanded light beam with precision suitable for real-time mapping, production lines and quality control can also be used [1]. ...
We developed and applied a new calibration method to make the measurement more accurate with our multi-color ellipsometric mapping tool from cheap parts. Ellipsometry is an optical technique that measures the relative change in polarization state of the measurement beam induced by reflection from or transmission though a sample. During the conventional ellipsometric measurement, the data collection is relatively slower and measures one spot at a time, so mapping needs long time compared with our new optical mapping equipment made by an ordinary color LED monitor and a polarization sensitive camera. The angle-of-incidence and the incident polarization state is varied point-by-point so a special optical calibration method is needed. Three SiO2 samples with different thicknesses were used to determine point-by-point the angle-of-incidence and rho corrections. After the calibration, another SiO2 sample was measured and analyzed using the calibrated corrections and this sample further was independently measured using a conventional spectroscopic ellipsometer. The difference between the two measured thickness maps is less than 1 nm. Our optical mapping tool from cheap parts is faster and covers wider area samples in relative to the conventional ellipsometers and these correction enhancements further signify its performance.
... Generally, the aim of this research is to make a prototype optical mapping tool for materials using only cheap parts such as a tablet, monitor, big screen TV (LCD or LED) and a pinhole camera [1][2][3][4][5][6][7][8][9] with CMOS Sensor with Integrated 4-Directional Wire Grid Polarizer Array (Sony's IMX250MYR CMOS), shown in Fig.2. ...
... Imaging spectroscopic optical techniques, such as spectroscopic ellipsometry, photometry and polarimetry, are excellent for the characterisation of non-uniform, structured and patterned thin films and biological specimens [1][2][3][4][5][6][7]. Although film uniformity is usually one of the goals during the development and tuning of a deposition technology, the deposits need to be characterised already during the development. ...
A least-squares data fitting procedure is developed for the analysis of measurements of thin films non-uniform in thickness by imaging spectroscopic reflectometry. It solves the problem of simultaneous least-squares fitting of film thicknesses in all image pixels together with shared dispersion model parameters. Since the huge number of mutually correlated fitting parameters prevents a straightforward application of the standard Levenberg-Marquardt algorithm, the presented procedure exploits the special structure of the specific least-squares problem. The free parameters are split into thicknesses and dispersion model parameters. Both groups of parameters are fitted alternately, utilising an unmodified Levenberg-Marquardt algorithm, correcting however the thicknesses during the dispersion model fitting step to preserve effective optical thicknesses. The behaviour of the algorithm is studied using experimental data of two highly non-uniform thin films of different materials, SiO x C y H z and CN x :H, and by numerical simulations using artificial data. It is found that the optical thickness correction enables the procedure to converge rapidly, permitting the analysis of large imaging spectroscopic reflectometry data sets with reasonable computational resources.
... Recently, there has been a great demand for innovative SE inspection tools that provide a rapid SE mapping capability with current commercial SE performance, such as high precision and microspot focusing to a few tens of micrometers. Some attempts have been made to achieve high-throughput targets [8,9]. However, previous high-throughput SE imaging schemes have typically employed mechanically driven polarization modulation mechanisms, resulting in limited performance, i.e., poor precision or low spatial resolution. ...
A dynamic spectroscopic imaging ellipsometer (DSIE) employing a monolithic polarizing interferometer is described. The proposed DSIE system can provide spatio-spectral ellipsometric phase map data Δ(λ, x) dynamically at a speed of 30 Hz. We demonstrate the ultrafast mapping capability of the spectroscopic ellipsometer by measuring a patterned 8-inch full wafer with a spatial resolution of less than 50 × 50 µm² in an hour.
... Thus, collimated light beams are conventionally used with a well defined angle of incidence at the reflecting surface. Here we present an ellipsometric method fundamentally different from the conventional techniques [26][27][28][29][30][31][32] In our instrument, the sample is illuminated by an almost diffuse, "divergent beam" of light, providing a collection of rays with diverse angles of incidence at every point of the sample. Precise "angle-selection" is performed on the detector side by a pin-hole camera. ...
... The first proposal of a microscopic imaging ellipsometer with a CCD camera as a detector of the output signal has been published in 1981 22 . In the last decade imaging versions of conventional single spot techniques such as null-ellipsometry 23 , of-null ellipsometry 24 , or expanded-beam (macroimaging) ellipsometry 25 have been developed. Applications of imaging ellipsometry in biology and biomedicine are very promising [26][27][28][29] . ...
It is possible to encounter thin films exhibiting various defects in practice. One of these defects is area non-uniformity in optical parameters (e.g. in thickness). Therefore it is necessary to have methods for an optical characterization of nonuniform thin films. Imaging spectroscopic reflectometry provides methods enabling us to perform an efficient optical characterization of such films. It gives a possibility to determine spectral dependencies of a local reflectance at normal incidence of light belonging to small areas (37 μm × 37 μm in our case) on these non-uniform films. The local reflectance is measured by individual pixels of a CCD camera serving as a detector of an imaging spectroscopic reflectometer. It is mostly possible to express the local reflectance using formulas corresponding to a uniform thin film. It allows a relatively simple treatment of the experimental data obtained by imaging spectroscopic reflectometry. There are three methods for treating these experimental data in the special case of thickness non-uniformity, i.e. in the case of the same optical constants within a certain area of the film - single pixel imaging spectroscopic reflectometry method, combination of single-pixel imaging spectroscopic reflectometry method and conventional methods (conventional single spot spectroscopic ellipsometry and spectrophotometry), and multi-pixel imaging spectroscopic reflectometry method. These methods are discussed and examples of the optical characterization of thin films non-uniform in thickness corresponding to these methods are presented in this contribution.
... Imaging spectroscopic optical techniques, such as spectroscopic ellipsometry, photometry and polarimetry, are excellent for the characterisation of non-uniform, structured and patterned thin films and biological specimens [1][2][3][4][5][6][7]. Although film uniformity is usually one of the goals during the development and tuning of a deposition technology, the deposits need to be characterised already during the development. ...
A least-squares data fitting procedure is developed for the analysis of measurements of thin films non-uniform in thickness by imaging spectroscopic reflectometry. It solves the problem of simultaneous least-squares fitting of film thicknesses in all image pixels together with shared dispersion model parameters. Since the huge number of mutually correlated fitting parameters prevents a straightforward application of the standard Levenberg-Marquardt algorithm, the presented procedure exploits the special structure of the specific least-squares problem. The free parameters are split into thicknesses and dispersion model parameters. Both groups of parameters are fitted alternately, utilising an unmodified Levenberg-Marquardt algorithm, correcting however the thicknesses during the dispersion model fitting step to preserve effective optical thicknesses. The behaviour of the algorithm is studied using experimental data of two highly non-uniform thin films of different materials, SiOxCyHz and CNx:H, and by numerical simulations using artificial data. It is found that the optical thickness correction enables the procedure to converge rapidly, permitting the analysis of large imaging spectroscopic reflectometry data sets with reasonable computational resources.
... Mapping of properties of thin plasma jet films using imaging spectroscopic reflectometry ellipsometry [6], off-null ellipsometry [7], or expanded-beam (macro-imaging) ellipsometry [8]. In the case of specimens and thin films with thicknesses in the nanometer to micrometer range that change rapidly in time, fast single-wavelength measurement methods are used, for instance, for monitoring in biomedical applications [9,10] or study of behaviour and time evolution of thin lubricant films [11]. ...
The construction of a normal-incidence imaging spectrophotometer for mapping of thin film properties is described. It is based on an on-axis reflective imaging system, utilising a telescope-like arrangement. A charge-coupled device camera is used as the detector, permitting measurements in the spectral range of 275–1100 nm with resolution of 37 µm. The performance of the instrument is demonstrated by optical characterisation of highly non-uniform thin films deposited from hexamethyldisiloxane on silicon substrates by a single capillary plasma jet at atmospheric pressure. The imaging spectrophotometry is used as a self-sufficient technique for the determination of both the film optical constants and maps of local thickness. The thickness maps are compared with the results of conventional thickness profile characterisation methods, profilometry and atomic force microscopy and the differences and errors are discussed.
... Lateral structures can be measured either by scanning a single spot [36][37][38] or using imaging. [39][40][41][42] In standard ellipsometric configurations the diameter of collimated light beams for the measurement is in the range of several millimeters. For these table top ellipsometers, the sample holder can be moved to distances typically up to the size of a 300-mm silicon wafer. ...
Optical techniques have been intensively developed for many decades in terms of both experimental and modeling capabilities. In spectroscopy and scatterometry material structures can be measured and modeled from the atomic (binding configurations, electronic band structure) through nanometer (nanocrystals, long range order) to micron scales (photonic structures, gratings, critical dimension measurements). Using optical techniques, atomic scale structures, morphology, crystallinity, doping and a range of other properties that can be related to the changes of the electronic band structure can most sensitively be measured for materials having interband transition energies in the optical photon energy range. This will be demonstrated by different models for the dielectric function of ZnO, a key material in optoelectronics and in numerous other fields. Using polarimetry such as spectroscopic ellipsometry, sub-nanometer precision has long been revealed for the thickness of optical quality layers. The lateral resolution of spectroscopic ellipsometry is limited (> 50 μm) by the use of incoherent light sources, but using single-wavelength imaging ellipsometry, a sub-micron lateral resolution can be reached. In case of sub-wavelength structures, the morphology (of e.g. porous or nanocrystalline materials) can be characterized using the effective medium theory. For structure sizes comparable to the wavelength, scatterometry is applied in a broad versatility of configurations from specular to angle resolved, from coherent to incoherent, from monochromatic to spectroscopic, from reectometric to polarimetric. In this work, we also present an application of coherent Fourier scatterometry for the characterization of periodic lateral structures.
... This new device requires only one measurement cycle (one rotation period of a polarizer or analyzer) for the acquisition of a two-dimensional array of data points [1]. Our new measurement technique serves as a novel form of imaging spectroscopic ellipsometry, using a divergent (non-collimated, diffuse) source and a detection system consisting of an angle-ofincidence-sensitive pinhole camera [2]. By adding broad-band lamps on the source side and a concave grating on the detector side, the instrument provides continuous high-resolution spectra along a line image. ...
A prototype expanded-beam spectroscopic ellipsometer has been developed that uses uncollimated (non-parallel, diffuse) illumination with a detection system consisting of an angle-of-incidence-sensitive pinhole camera for high-speed, large-area imaging/mapping applications. The performance of this novel instrument is being tested for imaging/mapping of mixed-phase hydrogenated silicon films having graded amorphous (a-Si:H) and nanocrystalline (nc-Si:H) components throughout the film depth. The speed of the measurement system makes the instrument suitable for use on production lines. The precision enables detection of subnanometer thicknesses, and refractive index and extinction coefficient changes of 0.01. Angle-of-incidence and mirror calibrations are made via well-known sample structures. Alternative commercial instrumentation for mapping by spectroscopic ellipsometry must translate the sample or ellipsometer in two dimensions. For this instrumentation, even a 15 × 15 cm2 sample with cm2 resolution requires > 200 measurements and at least 15 min. By imaging along one dimension in parallel, the expanded-beam system can measure with similar resolution in < 2 min. The focus of recent instrumentation efforts is on improving the overall system spectral range and its performance.
Two approaches are reviewed for the application of spectroscopic ellipsometry (SE) to on-line monitoring of thin film photovoltaics (PV) production. In the first approach, through-the-glass SE is applied for serial point-by-point measurements spanning the area of a thin film PV panel 60 cm × 120 cm in size. An ellipsometer detection system is used that incorporates two one-dimensional detector arrays for spectroscopy over a wide photon energy range (0.75–3.5 eV, limited by glass absorption at high energies). The PV panel in this review is fabricated starting from soda-lime glass with four oxide layers deposited on its surface, including the transparent top contact. A CdS/CdTe semiconductor bilayer is deposited subsequently on the top contact, functioning as the PV heterojunction. In the on-line analysis configuration, the coated glass panel moves along a roller conveyer with the film side facing up and passes a station designed for on-line mapping by SE. The polarization generation and detection arms of the ellipsometer located beneath the panel scan from side to side and acquire SE data in a through-the-glass measurement mode. In this approach, a maximum of ~30 locations can be measured in the one minute time period required for the 120 cm long panel to travel by the SE station; the largest fraction of the time is consumed by ellipsometer translation. The effective thickness of CdS (or CdS material volume/area), which includes bulk and interface layer components, is deduced in SE data analysis. This thickness is found to be a robust parameter that can be used in modeling to predict photo-generated charge carrier collection for the CdTe PV modules. The second approach for on-line monitoring reviewed here employs an instrument with an expanded beam for line imaging across a PV substrate/film-stack structure with a maximum image width of 15 cm. In this approach, a detection system is used incorporating a two-dimensional detector array; the two array indices are exploited for spectroscopy (1.3–3.3 eV) and line imaging in parallel. Thus, imaging width-wise and mapping length-wise is performed without ellipsometer translation, enabling high speed multilayer uniformity evaluations in flexible roll-to-roll PV production. The application reviewed here involves film-side analysis of multilayer fabrication on a moving length of 12.7 cm wide flexible polyimide foil substrate mounted within a cassette for roll-to-roll deposition. Maps are acquired in situ after deposition of individual Ag and ZnO layers, functioning together as the back reflector and back contact, as well as after deposition of n-type doped hydrogenated amorphous silicon (a-Si:H n-layer) as a component of a thin film a-Si:H n-i-p solar cell structure. Areas of the flexible coated PV panels up to 12 cm × 45 cm in size were characterized to determine layer thicknesses and optical properties. Parametric expressions incorporating Drude, critical point oscillator, and modified Lorentz oscillator terms were employed to describe the complex dielectric functions of thin film Ag and ZnO, and the a-Si:H n-layer, respectively. Currently, ~30 point line images can be collected every 20 cm of length when using an average 120 cm/min substrate speed. Prospects exist for increasing length-wise resolution significantly to ~0.5 cm, using high speed detection schemes demonstrated previously.
This chapter focuses on optical characterization of thin films by means of non-microscopic imaging spectroscopic reflectometry. This technique is primarily intended for characterization of thin films with an area non-uniformity in their optical properties. An advantage of the technique is the possibility to measure along a relatively large area of the measured films. The motivation for development and exploitation of this technique is also discussed. Essential features and implementation of the technique are given, as well as the basic experimental set-up of imaging spectroscopic reflectometers and the way the experimental data are obtained. The data processing methods are classified based on the purpose of the thin film measurement. Furthermore, this chapter presents examples of results of imaging spectroscopic reflectometry in the field of thin films. At the end of the chapter, potential applications of imaging spectroscopic reflectometry in other tasks are also briefly mentioned.
The measured signals in imaging ellipsometry are the change in polarization as the incident radiation interacts with the material structure of interest at each point on the surface.
Indirect optical methods like ellipsometry or scatterometry require an optical model to calculate the response of the system, and to fit the parameters in order to minimize the difference between the calculated and measured values. The most common problem of optical modeling is that the measured structures and materials turn out to be more complex in reality than the simplified optical models used as first attempts to fit the measurement. The complexity of the optical models can be increased by introducing additional parameters, if they (1) are physically relevant, (2) improve the fit quality, (3) don't correlate with other parameters. The sensitivity of the parameters can be determined by mathematical analysis, but the accuracy has to be validated by reference methods. In this work some modeling and verification aspects of ellipsometry and optical scatterometry will be discussed and shown for a range of materials (semiconductors, dielectrics, composite materials), structures (damage and porosity profiles, gratings and other photonic structures, surface roughness) and cross-checking methods (atomic force microscopy, electron microscopy, x-ray diffraction, ion beam analysis). The high-sensitivity, high-throughput, in situ or in line capabilities of the optical methods will be demonstrated by different applications.
Re
ection of light measured in a polarimetric, scatterometric and spectroscopic
way allows the measurement of structures in a broad size range from large (meter) scales like
photovoltaic panels down to small (nanometer) scales like nanocrystals. Optical metrology
continues to be improved to measure those materials with increasing sensitivity and accuracy,
typically in a form of thin �lms on high quality substrates. This review provides an overview
of some recently developed or improved methods, e.g. divergent light source ellipsometry
for the mapping of large surfaces for photovoltaic applications, Fourier scatterometry for the
measurement of periodic structures with sizes comparable to the wavelength of illumination,
as well as spectroscopy around the band gap photon energies to characterize nanostructures {
without attempting completeness.
In this study we compare the thicknesses and optical properties of atomic layer deposited (ALD) Al2O3 films measured using table top and mapping ellipsometry as well as X-ray and optical reflectometry. The thickness of the films is varied in the range of 1-50 nm. ALD samples are used as references with well-controlled composition and thickness, as well as with a good lateral homogeneity. The homogeneity is checked using mapping ellipsometry. Optical models of increasing complexity were developed to take into account both the top (surface roughness on the nanometer scale) and bottom interfaces (buried silicon oxide and interface roughness). The best ellipsometric model was the one using a single interface roughness layer. Since the techniques applied in this work do not measure in vacuum, organic surface contamination even in the sub-nanometer thickness range may cause an offset in the measured layer thicknesses that result in significant systematic errors. The amount of surface contamination is estimated by in situ reflectometry measurement during removal by UV radiation. Taking into account the surface contamination the total thicknesses determined by the different methods were consistent. The linearity of the total thickness with the number of atomic layer deposition cycles was good, with an offset of 1.5 nm, which is in good agreement with the sum of thicknesses of the interface layer, surface nanoroughness, and contamination layer.
The manufacturing of optoelectronic thin films is of key importance,
because it underpins a significant number of industries. The aim of the
European joint research project for optoelectronic thin film
characterization (IND07) in the European Metrology Research Programme of
EURAMET is to develop optical and X-ray metrologies for the assessment
of quality as well as key parameters of relevant materials and layer
systems. This work is intended to be a step towards the establishment of
validated reference metrologies for the reliable characterization, and
the development of calibrated reference samples with well-defined and
controlled parameters. In a recent comprehensive study (including XPS,
AES, GD-OES, GD-MS, SNMS, SIMS, Raman, SE, RBS, ERDA, GIXRD), Abou-Ras
et al. (Microscopy and Microanalysis 17 [2011] 728) demonstrated that
most characterization techniques have limitations and bottle-necks, and
the agreement of the measurement results in terms of accurate, absolute
values is not as perfect as one would expect. This paper focuses on
optical characterization techniques, laying emphasis on hardware and
model development, which determine the kind and number of parameters
that can be measured, as well as their accuracy. Some examples will be
discussed including optical techniques and materials for photovoltaics,
biosensors and waveguides.
Damage created by ion implantation into single crystalline silicon was characterized with an optical model based on the coupled half-Gaussian model developed by Fried et al [J. Appl. Phys. 71, 2835 (1992)]. In the improved optical model the damage profile was described by sublayers with thicknesses inversely proportional to the slope of the profile. This method allows a better resolution at the quickly changing parts of the profile, and a better approximation of the Gaussian profile with the same number of sublayers. A fitting procedure, which we call “multipoint random search,” was applied to minimize the probability of getting in a local minimum. The capabilities of our method were demonstrated for amorphizing doses using different ions and energies. The improved fit quality and the correlation with results of backscattering spectrometry basically supported the optical model. © 2003 American Institute of Physics.
In the past years spectroscopic ellipsometry (SE) was applied to materials science problems as an optical technique for non destructive depth profiling and characterization of multilayer structures and interfaces with considerable success. The measured optical response of the multicomponent and/or multilayer structure under investigation can only be related to actual material properties by a model calculation. The successful application of ellipsometry is not only determined by the quality of the measurements, but more importantly by the quality of the optical model. Several examples for the different application of SE are reviewed. Two recent examples of multilayer analysis illustrate possibilities: in the first example damage created by ion implantation in single-crystalline silicon and in silicon carbide was characterized using ellipsometry and Rutherford Backscattering Spectrometry (RBS) in combination with channeling. In the second example electrochemically prepared porous silicon layers (PSL) were investigated by SE.
Al doped ZnO (ZAO) thin films were deposited by reactive magnetron sputtering from Al:Zn metallic alloy target for copper–indium–gallium–diselenide (CIGS) solar cell contacts. The effect of variation of technological parameters (power, plasma Ar/O2 ratio and target voltage) was studied.A novel approach for controllable reactive sputter deposition is suggested using a hysteresis loop in the target voltage vs. oxygen gas flow. As a result of this optimization a 1.85×10−4 Ω cm layer with 90% transparency was obtained without plasma emission monitoring and substrate heating.Thickness and specific resistance were measured on the samples prior to spectroscopic ellipsometry (SE) analysis. Evaluation of SE data was done by least square fitting the Cauchy dispersion relation using polynomial functions. Subsequently a two-step “screaning” method, based on the parameters of the complex refractive index, is suggested for evaluation of the process. Furthermore, surface roughness data can also be determined by SE, as verified by comparison with SEM morphologies. Using this method SE offers a non-destructive in-line characterization tool for the deposition process.
Optical, compositional, and structural properties of strontium bismuth tantalate (SBT) films deposited in a high throughput low-pressure chemical vapor deposition reactor using liquid metal-organic precursors were characterized using three non-destructive techniques, spectroscopic ellipsometry (SE), Rutherford backscattering spectrometry (RBS), and X-ray diffraction (XRD). The thicknesses and the refractive indices of the SBT layers were calculated with SE using different parametric dielectric function models. The samples were characterized with RBS using different tilt angles and probe ions to enhance the depth resolution and the mass separation. Comparison with SE measurements supports the results of Bahng et al. revealing an increasing refractive index (n) with increasing Bi/Sr ratio. The decreasing grain size measured by XRD was reflected as a decrease of n in the SE measurement. We show that RBS, XRD, and SE supply a wide range of information about the SBT layers, which can be used for qualification as well as for feedback to layer production. The results suggest that by SE, being used as in situ or in line characterization tool, the control of even complex MOCVD deposition looks feasible.
Al doped ZnO (ZAO) thin films (with Al-doping levels 2 at.%) were deposited at different deposition parameters on silicon substrate by reactive magnetron sputtering for solar cell contacts, and samples were investigated by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS) and spectroscopic ellipsometry (SE). Specific resistances were measured by the well known 4-pin method. Well visible columnar structure and in most cases voided other regions were observed at the grain boundaries by TEM. EELS measurements were carried out to characterize the grain boundaries, and the results show spacing voids between columnar grains at samples with high specific resistance, while no spacing voids were observed at highly conductive samples. SE measurements were evaluated by using the analytical expression suggested by Yoshikawa and Adachi [H. Yoshikawa, S. Adachi, Japanese Journal of Applied Physics 36 (1997) 6237], and the results show correlation between specific resistance and band gap energy and direct exciton strength parameter.
Our aim was to make possible to use ellipsometry for mapping purposes during one measuring cycle even on large wafers or panels (several dm2 area). The new technique (Patent pending: P0700366, 2007 [1]) (based on our wide-angle beam ellipsometry solution) uses non-collimated illumination with special mirror arrangement giving multiple-angle-of-incidence information. The prototype uses a so called RGB-laser (658, 532, 474 nm) as light source. The detection is almost without background. One rapid measuring cycle is enough to determine the polarization state at all the points inside the illuminated area. The collected data can be processed very fast providing nearly real-time thicknesses and/or refractive index maps over a large (several dm2) area of the sample surface even in the case of multi-layer samples. The method can be used for mapping (quality) control purposes in the case of large area solar cell table production lines even in vacuum chamber with 5-10 mm lateral resolution. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
A new method of polarization reflectometry for mapping purposes is presented. Two different optical arrangements were built to study the specific features of the new technique that uses non-collimated illumination giving multiple-angle-of-incidence information from rapid measurements of the whole area. The prototypes were built in the form of wide-angle 3-wavelength ellipsometers using film polarizers. Using pin-hole-CCD-matrix detector arrangement, the detection is almost background free. It can provide real-time polarization state parameter maps (and thicknesses and/or refractive index maps) over a relatively large area of the surface with 0.5-1 mm lateral resolution. The speed of the measuring system makes it suitable for use even on production lines. The accuracy of the device is not higher than that of standard ellipsometers, but it is enough for determining the thickness of the silicon-dioxide film with subnanometer and the angle-of-incidence with subtenthdegree precision. We used the prototype for mapping purposes in the case of homogeneity check of ion implantation in silicon, thickness and porosity mapping on electrochemically etched porous silicon layers, thickness mapping on a polysilicon/silicondioxide layer structure. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Wide angle beam ellipsometry developed by our group uses non-collimated illumination with a special light source and arrangement giving multiple-angle-of-incidence and multiwavelength information. Our aim was to make our wide angle beam ellipsometer suitable for spectral measurement and to obtain the spectra of many points along a long line (presently 0.2 m but it could be increased up to 1 m if necessary) of an entire sample simultaneously. The prototype uses a xenon lamp as a light source with film polarizers and a concave optical grating to reach the desired 6 nm spectral resolution over the range of 360–630 nm. This new technique mixed with an appropriate ellipsometric model has the capability to make “in situ” control in solar cell fabrication. In order to demonstrate the ability of our instrument, wide angle beam spectroscopic ellipsometry measurements were carried out on Al-doped ZnO samples, which have different physical properties such as specific resistance and transparency.
A several‐parameter fitting of spectroscopic ellipsometry data is developed to characterize near‐surface layers in semiconductors damaged by implantation. The damage depth profiles are described by either rectangular, trapezoid‐type, or coupled half‐Gaussian (realistic) optical models. The rectangular model has three parameters: the average damage level, the effective thickness of the implanted layer, and the thickness of the native oxide. The trapezoid‐type model is enhanced with a fourth parameter, the width of the amorphous/crystalline interface. The realistic optical model consists of a stack of layers with fixed and equal thicknesses. The damage levels are determined by a depth profile function (presently coupled half‐Gaussians). Five parameters are used: the position of the maximum, the height, and two standard deviations of the profile, plus the thickness of the native oxide. The complex refractive index of each layer is calculated from the actual damage level by the Bruggeman effective medium approximation. The optical models were tested on Ge‐implanted silicon samples and cross checked with high‐depth‐resolution Rutherford backscattering spectrometry and channeling.
Imaging ellipsometry is presented as a technique for quantification and visualization of the lateral thickness distribution of thin (0–30 nm) transparent layers on solid substrates. The main advantage of imaging ellipsometry is that every point on a surface is measured at the same time with a high lateral resolution. The method is based on the use of combined null and off‐null ellipsometry at an incident angle close to the pseudo‐Brewster angle of a high index substrate such as silicon. In the present experimental setup, a xenon lamp, a collimator, and a wavelength‐selective filter provide an expanded collimated probe beam with a diameter of 25 mm. Other major components in the system are a polarizer, a compensator, and an analyzer. In this way, a 15×30 mm2 image of a sample surface can be focused onto a charge‐coupled‐device video camera and transferred to a computer for further evaluation by image processing. Thickness measurements are performed for calibration purposes with ordinary null ellipsometry. The imaging ellipsometer has an accuracy of better than 0.5 nm at a lateral resolution of 5 μm in the present configuration, but improvements of at least a factor of 5 can be foreseen. Several aspects of the ellipsometric imaging system are illustrated in selected applications including continuous protein thickness distributions, stepped silicon dioxide thickness distributions, and visualization of protein patterning of surfaces. The latter can be used in a biosensor system as illustrated here by antigen–antibody binding studies. © 1996 American Institute of Physics.
A microscopic imaging ellipsometer has been constructed based on use of a CCD camera and framegrabber board in a PC computer. The performance (sensitivity and speed) are described.
Zn and Pb ions were implanted at 100 keV into silicon, in order to form amorphous layers. Ion beam induced epitaxial crystallization of the amorphous silicon was induced at 400 °C using 3 MeV Si+ ion beam. Rutherford Backscattering Spectrometry and channeling techniques were applied to assess the complete amorphization and to follow the epitaxial crystallization and the redistribution of the Zn and Pb atoms. Spectroscopic ellipsometry measurements were performed to determine the thickness of the surface oxide layer and that of the amorphous silicon layers. The evaluation of the ellipsometry data was done using the Tauc–Lorentz model for description of the complex dielectric function of the amorphous silicon. A dose of 3 MeV Si+ ion beam was sufficient for almost complete recrystallization of the ion implantation amorphized silicon layer.
A new method is described for calculation of the real and imaginary parts of the dielectric function of semiconductors at energies below and above the lowest band gaps, in which the model is based on the Kramers-Kronig transformation and strongly connected with the electronic energy-band structures of the medium. This model reveals distinct structures at energies of the E0, E0+Delta0, E1, E1+Delta1, and E2 critical points. Analyses are presented for GaP, GaAs, GaSb, InP, InAs, and InSb, and results are in satisfactory agreement with the experimental information over the entire range of energies. The model is able to properly give the optical constants, such as the refractive indices and the absorption coefficients, which are important for a variety of optoelectronic device applications.
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The prototype is under work and will be installed on the experimental vacuum chamber of the PhotoVoltaic Innovation and
- Fig
Fig. 8. The prototype is under work and will be installed on the experimental vacuum
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Toledo (Ohio).
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