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Silicon surface passivation with atomic layer deposited (ALD) thin films has gained more and more interest in the PV community in recent years. With ALD good film quality, accurate thickness control and conformity are reached. Furthermore, ALD is capable of coating difficult substrates such as nanostructured surfaces with the same accuracy as flat surfaces. A variety of materials such as Al2O3, TiO2 and HfO2 demonstrate good surface passivation quality both for front and rear surface of silicon solar cells. In addition of providing good surface passivation, thin films with high refractive index e.g. HfO2, TiO2 and AlN can act simultaneously as antireflection coatings when applied on the front surface of the device. Hence, ALD thin films can reduce both electrical and reflective losses in solar cells.
Thorough investigation of the optical properties of these layers is crucial for several reasons related to their production and use. Optical measurements provide a fast, easy, non-destructive, and in situ capable approach to quality assurance for photovoltaic devices. As the function of the final device is optical, optimisation of the device performance relies strongly on the knowledge of the wide-range dielectric function of the thin layers. In this contribution, we determined the optical constants of ALD generated layers of AlN, Al2O3, TiO2, and HfO2 in a wide spectral range covering the near ultraviolet and the mid-infrared regions by means of spectroscopic ellipsometry. By combining data from a UV-Vis-NIR ellipsometer (Woollam M2000DI) and an FTIR ellipsometer (Sentech Sendira), we can determine the optical constants alongside with the layer thicknesses from one large set of spectroscopic measurements. We consider this a contribution to the metrological treatment of stratified and structured thin films in the optical range by polarisation-sensitive measurement methods.
Industrial quality control (QC) nowadays requires the visualization of surface modifications from the macro-scopic to the microscopic or even nanoscopic scale. This is a prerequisite to the evaluation of functionality and reliability, the detection of defects and their separation of artefacts. The diversity of applications ranges from low-E glazings and solar panels, micro- and optoelectronics, micro- and smart devices to sensor-on-chip and lab-on-chip systems [1]. Optical microcopy (light, confocal laser scanning, white light interference) as established QC-tool is operated at normal incidence, i.e. p- and s-polarization are undistinguishable. Either light-intensity in terms of grey scale and colour or intensity-correlated effects of phase shifts are used. In case of ellipsometry, operated at oblique incidence, p- and s-polarization matter, and amplitude ratios and phase shifts upon reflection are measured. Hence, information content must be much higher.
The visualization of surface modifications may be very challenging for coating/substrate systems of either al-most identical optical constants, e.g. transparent films on substrates of the same material, or minor film thick-ness, substance quantity and affected area, e.g. ultra-thin or island films. Ellipsometry gives access to the con-trast of intensity (I), amplitude ratio (Ψ), and phase shift (Δ) with nanometer-scaled vertical and micrometer-scaled lateral sensitivity, one is able to identify tiny changes within an unmodified surface. As both mapping ellipsometry (ME) and imaging ellipsometry (IE) are operated in the optical far-field, surface inspection is also possible on the macroscopic scale. Near the Brewster-angle of the bare, undamaged, clean, and fresh substrate, the contrast to add-on and sub-off features is superior.
Fig. 1 shows three examples of ellipsometric imaging, i.e. a thin SnO:Ni film on SiO2/Si (Fig. 1a), a dried stain of an anti-body solution on cyclo-olefin-polymer (COP) shown in Fig. 1b, and a polyimide film residue on SiO2/Si (Fig. 1c). For all of these examples, ellipsometry provides much better contrast between substrate and surface modification than optical microscopy, sometimes primarily caused by the oblique incidence (Figs. 1a and 1c), in other cases related to the phase sensitivity of ellipsometry (Fig. 1b). Other examples are laser surface modifications and the corrosion of glass. In these cases, optical microscopy and IE yield to similar results, how-ever only ellipsometry gives access to modelling.
Further investigated coating/substrate systems are 100Cr6 steel, native oxide on silicon, borosilicate glass, and the polymer polycarbonate with deposited films of graphene and ta-C:H, printed and dried pattern of liquids such as water, cleaning agents, and dissolved silicone. Besides imaging ellipsometry, referenced spectral ellipsometry (RSE) has been applied, combining the advantages of both optical microscopy (fast measurement) and ellipsometry (high sensitivity to tiny modifications).
Optical constants of harmful and highly energetic liquids for application to THz screening systems
(2016)
The far-infrared (IR) optical constants of a set of hazardous and flammable liquids have been obtained by means of spectroscopic ellipsometry in attenuated total reflection configuration over a broad spectral range. Such liquids recently became of considerable concern for transportation security measures worldwide. Their optical identification at check-in gates can only become possible if the characteristic spectra are already known. The refractive indices and the extinction coefficients reported here contribute to a spectroscopic data base in the far-IR and terahertz (THz) spectral regions and may support modeling the performance of THz screening systems on liquids for airports and other security sensitive areas. Examples of several container material/liquid systems are discussed. From the measured optical constants typical THz waveforms are calculated and discussed.
Interlaboratory comparison: optical property classification of carbon-based films by ellipsometry
(2017)
The paper addresses the “INTERLABORATORY COMPARISON: OPTICAL PROPERTY CLASSIFICATION OF CARBON-BASED FILMS BY ELLIPSOMETRY” and the following points are discussed in more detail: Established classification for mechanical properties, Complementary classification for optical properties, Ellipsometry for determination of optical constants n-k plane as material fingerprint, Samples, participants & set-ups (samples: Japan, participants: Japan, Germany, France, set-ups: Japan, Germany, USA) and Results of interlaboratory comparison: thickness, n & k.
Ellipsometry is a useful tool for studying the optical properties of thin films such as photovoltaic devices. We employ Müller matrix ellipsometry to study the thin film photovoltaic material copper indium gallium selenide Cu(In,Ga) Se2 (CIGS), a commercially relevant material with high energy conversion efficiency. Confocal microscopy reveals an rms roughness of 68 nm, which greatly affects the ellipsometry data. Rayleigh-Rice theory is employed to account for the optical properties of the surface roughness in the ellipsometry experiment, and a library search method is used to compare Müller parameters calculated for various CIGS compositions, to the measured data. The Müller parameters calculated with the Rayleigh-Rice model are found to correspond well with the measured data, and a surface roughness of 37nm and a correlation length of 125nm are extracted.
Background: Gas sensors are very important in several fields like gas monitoring, safety and environmental applications. In this approach, a new gas sensing concept is investigated which combines the powerful adsorption probability of metal oxide conductive sensors (MOS) with an optical ellipsometric readout. This concept Shows promising results to solve the problems of cross sensitivity of the MOS concept.
Results: Undoped tin oxide (SnOx) and iron doped tin oxide (Fe:SnOx) thin add-on films were prepared by magnetron sputtering on the top of the actual surface plasmon resonance (SPR) sensing gold layer. The films were tested for their sensitivity to several gas species in the surface plasmon resonance enhanced (SPREE) gas measurement. It was found that the undoped tin oxide (SnOx) shows higher sensitivities to propane (C3H8) then to carbon monoxide (CO). By using Fe:SnOx, this relation is inverted. This behavior was explained by a change of the amount of binding sites for CO in the layer due to this iron doping. For hydrogen (H2) no such relation was found but the sensing ability was identical for both layer materials. This observation was related to a different sensing mechanism for H2 which is driven by the Diffusion into the layer instead of adsorption on the surface.
Conclusion: The gas sensing selectivity can be enhanced by tuning the properties of the thin film overcoating. A relation of the binding sites in the doped and undoped SnOx films and the gas sensing abilities for CO and C3H8 was found. This could open the path for optimized gas sensing devices with different coated SPREE sensors.
The discussions on the nanoconfinement effect on the glass transition and glassy dynamics phenomena have yielded many open questions. Here, the thickness dependence of the thermal glass transition temperature of thin films of a PVME/PS blend is investigated by ellipsometry. Its thickness dependence was compared to that of the dynamic glass transition (measured by specific heat spectroscopy), and the deduced Vogel temperature (T0). While and T0 showed a monotonous increase, with decreasing the film thickness, the dynamic glass transition temperature () measured at a finite frequency showed a non-monotonous dependence that peaks at 30 nm. This was discussed by assuming different cooperativity length scales at these temperatures, which have different sensitivities to composition and thickness. This non-monotonous thickness dependence of disappears for frequencies characteristic for T0. Further analysis of the fragility parameter, showed a change in the glassy dynamics from strong to fragile, with decreasing film thickness.
The talk addresses the STANDARDIZATION OF ELLIPSOMETRY and the following points are discussed in more detail: historical background of ellipsometry, history of International Conferences on Ellipsometry, Workshops Ellipsometry in Germany and Europe, information on German/European Working Group Ellipsometry, technical/industrial importance of ellipsometry, applications on non-ideal material systems and standardization activities on ellipsometry.
Transparent conductive oxide (TCO) films are a vital part of a large part of modern technology. The production of TCO materials has sparked much development in plasma coating technology. Quality control measurements of these layers are therefore important in many fields of optics and electronics such as high efficiency thin film photovoltaics. In this presentation, we report on optical measurements of ZnSnOx layers generated by DC/RF magnetron plasma co-sputtering. By changing the respective power on two different targets, the overall power, the gas composition and post-treatment, the properties of this type of layers can be varied in a number of parameters. The optical, electrical, and chemical properties of TCO layers are the technically most important properties together with the layer thickness. The dielectric function of layers is accessible by means of spectroscopic ellipsometry, which also yields the very important value for the layer thickness at the same time. It would be a significant step forward in quality control to use this non-destructive method also as a fast test for electrical properties. Therefore, we report on the optical properties connected to the production parameters, and also on our preliminary results connecting the optical dielectric function (in the visible and near infrared) to the electrical conductivity of the layers. We discuss the effect of deposition parameters on the optical properties of the layers and present an approach for correlating optical and electrical properties. Further, we discuss the question of accuracy of optical properties gained from model-fit-based optical methods and the use of different parameterised models for the dielectric function to achieve this.