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Application of wide angle beam spectroscopic ellipsometry for quality control in solar cell production
Abstract
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.
... 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 [1][2][3][4]. 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 [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. ...
... In situ measurements on both roll-to-roll polymer and rigid glass will be possible once the instrument is installed on the cluster tool chamber at PVIC. A relatively simple optical model was used to analyze the mapping measurements: opaque silver as a substrate, a ZnO:Al layer with a Cauchy dispersion model [4], and a-Si:H with a Tauc-Lorentz oscillator dispersion model. The parameters of the Cauchy dispersion and the Tauc-Lorentz oscillator were taken from separate SE measurements. ...
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 [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.
... 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. ...
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.
... For the elimination of the problem, we developed a modified version of expanded beam spectroscopic ellipsometry that uses whole spectra of a large number of sample points (along a line) in the near UV-VIS range (presently 350-630 nm) acquired in one measuring cycle (minimum two rotation periods of polarizer or analyzer). The spectral technique can speed up the mapping of physical properties (such as thickness, transparency, conductance) of transparent conductive oxides such as Al-doped ZnO layers [6]. ...
... As reported in refs. [6,13,14], the amplitude (A k ) and the exponent (B k ) of the imaginary part of the complex refractive index function depend on the transparency and are well correlated with the specific resistance of the layers, so expanded beam spectral ellipsometry is a good method to check the homogeneity of such layers. ...
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.
... 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. ...
... 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. ...
... Ellipsometric imaging has been realized in many configurations including microscopic imaging combined with spectroscopic ellipsometry for e.g. biosensors or graphene mapping [6][7][8][9], using translation stages or moving the complete ellipsometry heads over the sample to map it spot-by-spot [10,11], or imaging large areas using extended illumination, as demonstrated in the case of divergent light source ellipsometry [12][13][14][15][16][17][18] in the next chapter. ...
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.
... Taking advantage of its sensitivity, speed and non-destructive manner, ellipsometry is an attractive tool for in line monitoring of thin film properties. The research group of M. Fried at the MFA developed a patented mapping ellipsometry concept for large area thin film characterizations [2][3][4][5][6]. The basic idea is the use of a divergent light source. ...
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.
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.
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 paper reports on InAs/InGaAs quantum dot solar cells (QDSCs) deposited by molecular beam epitaxy (MBE) on (001) n-type silicon (n-Si) substrates. In-situ RHEED measurements show that InAs/InGaAs QDs SC has a high crystalline structure. The dislocation density in the active layer of the InAs/InGaAs QDSC and the lattice mismatch in the GaAs layer can be reduced by using an Si rough surface buffer layer (RSi). To show the effect of the QD layers, a reference SC with the same p-i-n structure as the InAs/InGaAs QDSC, but without InAs QDs, is also grown. The two SCs were studied by sepectroscopic ellipsometry (SE), in the 1–6 eV photon energy range, photoluminescence and photocurrent measurements. The optical constants of the two devices are determined in the photon energy range 1–6 eV from the SE data. The dominant features in the dielectric function spectra at ~ 3 and ~ 4.5 eV are attributed, respectively, to the E1 and E2 critical point structures of GaAs and InAs. The low-temperature photoluminescence spectrum of the InAs/InGaAs QDSC shows ground-state emissions, respectively, from the relatively small QDs near 1081 nm and from the large QDs near 1126 nm. Photocurrent measurements confirm the improved absorption performance (up to 1200 nm) of the InAs QDs SC which is ascribed to the optical absorption from the InAs/InGaAs QDs and the Si substrate as demonstrated by SE and photoluminescence measurements.
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.
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.
An expanded-beam spectroscopic ellipsometer has been developed and applied toward in situ high-speed imaging/mapping analysis of large area spatial uniformity for multilayer coated substrates in roll-to-roll thin-film photovoltaics (PV). Slower speed instrumentation available in such analyses applies a 1-D detector array for spectroscopic mapping and involves width-wise translation of the ellipsometer optics over the moving coated substrate surface, measuring point-by-point in a time-consuming process. The expanded-beam instrument employs instead a 2-D detector array with no moving optics, exploiting one array index for spectroscopy and the second array index for line imaging across the width of a large area sample. Thus, the instrument enables imaging width-wise and mapping length-wise for uniformity evaluation at the high linear substrate speeds required for real-time, in situ, and online analysis in roll-to-roll thin-film PV. In this investigation, we employ the expanded beam technique to characterize the uniformity of the Ag, ZnO, and n-type hydrogenated amorphous silicon (a-Si:H) layers of an a-Si:H n-i-p structure deposited on a flexible polyimide substrate in the roll-to-roll configuration. Spectroscopic ellipsometry data across a line image were collected as the substrate was translated by a roll-to-roll mechanism. Coated areas as large as 12 cm × 45 cm were analyzed in this study for layer thickness and optical properties by applying the appropriate analytical models for the complex dielectric functions of the Ag, ZnO, and n-type a-Si:H layers.
This study demonstrates the feasibility of improving the performance of a quantum dot (QD) intermediate band solar cell (SC) by capping an InGaAs layer on the InAs QDs and inserting GaAs spacer layers. For comparison, a GaAs reference SC of the same p-i-n structure but without InAs QDs is grown. The two devices were grown by solid source molecular beam epitaxy (SS-MBE) on epiready (001) n+-GaAs substrates and the InAs QDs structure is highly stacked and well-aligned. The two SCs were investigated by sepectroscopic ellipsometry (SE), in the photon energy range of 1–6 eV, photoluminescence (PL) and photo-absorption measurements. The two major spectral features observed in the dielectric function spectra of the InAs QDs SC at ∼3 eV and ∼4.5 eV are attributed to the E1 and E2 critical point structures of GaAs and InAs, respectively. The PL spectrum of the InAs QDs in the GaAs matrix is higher and presents an asymmetric shape, which indicated the growth of a high-quality multistacked InAs QDs structure and the contribution of larger and relatively smaller QDs to PL spectrum. The electrical conversion at the infrared range of λ=850–1300 nm for the InAs QDs SC is demonstrated by photocurrent spectra. The enhanced absorption performance (up to 1280 nm) of the QDs SC was attributed to the optical absorption from InAs QDs, wetting layer and InGaAs layer of the InAs/InGaAs/GaAs QD heterostructure.
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.
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)
Divergent illumination optical testing device
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