Thin film thickness measurements using Scanning White Light Interferometry

Scanning White Light Interferometry is a well-established technique for providing accurate surface roughness measurements and three dimensional topographical images. Here we report on the use of a variant of Scanning White Light Interferometry called coherence correlation interferometry which is now capable of providing accurate thickness measurements from transparent and semi-transparent thin films with thickness below 1 μm. This capability will have many important applications which include measurements on optical coatings, displays, semiconductor devices, transparent conducting oxides and thin film photovoltaics. In this paper we report measurements of thin film thickness made using coherence correlation interferometry on a variety of materials including metal-oxides (Nb2O5 and ZrO2), a metal-nitride (SiNx:H), a carbon-nitride (SiCxNy:H) and indium tin oxide, a transparent conducting oxide. The measurements are compared with those obtained using spectroscopic ellipsometry and in all cases excellent correlation is obtained between the techniques. A key advantage of this capability is the combination of thin film thickness and surface roughness and other three-dimensional metrology measurements from the same sample area

Optical optimization of high resistance transparent layers in thin film cadmium telluride solar cells

Thin film photovoltaic devices are multilayer opto-electrical structures in which light interference occurs. Light reflection at the interfaces and absorption within the window layers reduces transmission and, ultimately, the conversion efficiency of photovoltaic devices. Optical reflection losses can be reduced by adjusting the layer thicknesses to achieve destructive interference within the structure of the cell. The light transmission to the CdTe absorber of a CdS/CdTe cell on a fluorine doped tin oxide transparent conductor has been modeled using the transfer matrix method. The interference effect in the CdS layer and high resistance transparent buffer layers (SnO2 and ZnO) has been investigated. The modeling shows that due to relatively high absorption within the SnO2 layer, there are modest benefits to engineering anti-reflection interference in the stack. However, a ZnO buffer layer has limited absorption and interference can be exploited to provide useful anti-reflection effects. Optical modeling and optimization shows that for a 50 nm CdS layer, a maximum transmission of 78.5% is possible using ZnO as a buffer layer at 58 nm thickness, and 78.0% for a SnO2 buffer layer at a thickness of 48 nm

High temperature stability of broadband Anti-Reflection coatings on soda lime glass for solar modules

Reflections from glass surfaces reduce the efficiency of photovoltaic devices. Reflections can be reduced using a broadband Multi-layer Anti-Reflection (MAR) coating. For thin film CdTe modules, the glass is also the substrate. Manufacturers would prefer to use pre-MAR coated glass, so it is essential to establish if the MAR coating can withstand the module production process conditions. Thin film CdTe module fabrication requires temperatures up to ~500°C. Crazing may occur due to mismatch of the thermal expansion coefficients between the glass and the coating materials. The resilience of MAR coatings on soda lime glass, Eagle 2000™ Glass, and NSG TECTM 7 has been tested by exposure to increasing temperatures up to 800°C to establish the point of failure. SEM imaging and reflection measurements were used to observe the damage caused. Surprisingly, the MAR coating is unaffected up to a temperature of 590°C on soda lime glass substrates and up to 800°C on Eagle Glass. This provides confidence that thin film CdTe module manufacturers can use existing processes with pre-MAR coated glass

Passivation of silicon wafers by Silicon Carbide (SiCx) thin film grown by sputtering

Silicon Carbide films for silicon solar cell application were deposited by means of RF sputtering process. Films were deposited from mixed Silicon – Graphite target onto silicon Cz wafers. Samples were characterized by Photo Conductance Decay (PCD) method to measure the effective lifetime. The thickness and refractive index of the films deposited were measured using a spectroscopic Ellipsometer. X-Ray Diffraction (XRD) was performed to measure the crystallinity of the samples. Results have indicated that the deposited films were mainly amorphous. The crystalline fraction was present in samples with a better passivation level. Results from PCD show that the effective lifetime improved up to 38 μs which corresponds to a Voc=641 mV. Deposition rates up to 30 nm/min were obtained for samples at 0.9 kW bias power

Optical optimization of perovskite solar cell structure for maximum current collection

High conversion efficiency has been recently demonstrated for Perovskite thin film photovoltaic devices. Perovskite thin film solar cells are multilayer opto-electrical structures in which light interference occurs. This phenomenon can be used to maximise the light transmission into the absorber material and increase the device efficiency. Fine tuning of the layer thicknesses within the stack can be used to control interference at the interfaces. Optical reflection losses can be reduced by achieving destructive interference within the structure of the cell. The light transmission to the Perovskite absorber of a thin film solar cell on a fluorine doped tin oxide transparent conductor has been modelled using the transfer matrix method. Alternative transparent conductor materials have been also investigated including AZO and ITO. The modelling showed that replacing FTO with ITO could increase the photocurrent by as much as 4.5%. The gain can be further increased to 6.5% by using AZO as the TCO material. Fine tuning of the TiO2 layer thickness can increase the current density by 0.3%. Furthermore, the current density of a Perovskite solar cell can be increased by application of a multilayer anti-reflective coating by another 3.5%. Optical optimisation of the stack design offers a significant increase in conversion efficiency

High temperature stability of broadband Anti-Reflection coatings on soda lime glass for solar modules

Reflections from glass surfaces reduce the efficiency of photovoltaic devices. Reflections can be reduced using a broadband Multi-layer Anti-Reflection (MAR) coating. For thin film CdTe modules, the glass is also the substrate. Manufacturers would prefer to use pre-MAR coated glass, so it is essential to establish if the MAR coating can withstand the module production process conditions. Thin film CdTe module fabrication requires temperatures up to ~500°C. Crazing may occur due to mismatch of the thermal expansion coefficients between the glass and the coating materials. The resilience of MAR coatings on soda lime glass, Eagle 2000™ Glass, and NSG TECTM 7 has been tested by exposure to increasing temperatures up to 800°C to establish the point of failure. SEM imaging and reflection measurements were used to observe the damage caused. Surprisingly, the MAR coating is unaffected up to a temperature of 590°C on soda lime glass substrates and up to 800°C on Eagle Glass. This provides confidence that thin film CdTe module manufacturers can use existing processes with pre-MAR coated glass

Room temperature surface passivation of silicon for screen printed c-Si solar cells by HiTUS reactive sputter deposition

The dielectric coatings used on silicon solar cells serve a dual purpose: a surface passivation layer and as an antireflection coating. Silicon nitride films were deposited by sputtering, using a HiTUS technology, on crystalline silicon wafers. Films were deposited without substrate heating, which simplifies the deposition process, from a polycrystalline silicon target in a mixed ambient of Argon, Nitrogen and Hydrogen gasses. After the deposition, the minority carrier lifetime, refractive index and deposition rate were measured. Photo conductance decay measurements show that the minority carrier lifetime increased up to 26μs on a 40Ω/□ doped 1 Ω-cm p-type Cz-Si pseudo square wafer (compared to 1μs measured for bare wafer) and up to 984μs for a double-side polished 3 Ω-cm Cz-Si wafer (from ~70μs measured for uncoated wafer). Spectroscopic ellipsometry measurements showed that the refractive index of the deposited films was 2.05 at λ=632.8nm; deposition rate was measured at 22.4nm/min. The films were used to prepare screen-printed c-Si solar cells. The resultant cells showed an efficiency of 15.14% with silicon nitride films grown without the use of silane or substrate heating

An infra-red reflecting optical coating for solar cover glass

A major problem with silicon solar cells is that they lose efficiency with increased operating temperature, at a rate of about 0.5% per 1◦C increase. This causes a significant reduction in power output, particularly in hot climates. A solution in the form of an optical coating is presented, which reflects infrared (IR) radiation to limit the module temperature increase. The optical coating is also anti-reflecting (AR) in the visible wavelength range, increasing the amount of light reaching the cell absorber. Modelling results show that the weighted average reflection (WAR) is reduced to 1.22% in the wavelength range associated with the band gap of silicon. The optical coating then reflects up to 70% of the infra-red. Although the model presented is based on silicon, the coating design can be modified to work with other photovoltaic technologies. The coating design uses only 4 layers and can be deposited using conventional high throughput magnetron sputtering systems already familiar to glass manufacturers. Preliminary work on optimising the coating deposition parameters is also presented here alongside modelling results. Deployment of the infra-red reflecting optical coating on solar cover glass represents a potential breakthrough in solar technology and will result in a significant increase in the power output of photovoltaic modules.<br

High rate deposition of thin film CdTe solar cells by pulsed dc magnetron sputtering

A new high rate deposition method has been used to fabricate thin film CdTe photovoltaic devices using pulsed dc magnetron sputtering. The devices have been deposited in superstrate configuration on to a commercial fluorine doped tin oxide transparent conductor on soda lime glass. The cadmium sulphide and cadmium telluride thin films were deposited from compound targets. The magnetrons were mounted vertically around a cylindrical chamber and the substrate carrier rotates so that the layers can be deposited sequentially. The substrates were held at 200ºC during deposition, a process condition previously found to minimize the stress in the coatings. Optimization of the process involved a number of parameters including control of pulse frequency, power and working gas pressure. The devices deposited using the process are exceptionally uniform enabling the CdTe absorber thickness to be reduced to ~1um. The asdeposited material is dense and columnar. The cadmium chloride treatment increases the grain size and removes planar defects. The microstructure of the films before and after activation has been characterized using a number of techniques including transmission electron microscopy, Energy Dispersive mapping and these measurements have been correlated to device performance. The deposition rate is much higher than can be obtained with radio-frequency sputtering and is comparable with methods currently used in thin film CdTe module manufacturing such as Vapour Transport Deposition and Close Space Sublimation

Refractive index determination by coherence scanning interferometry

Coherence scanning interferometry is established as a powerful noncontact, three-dimensional, metrology technique used to determine accurate surface roughness and topography measurements with subnanometer precision. The helical complex field (HCF) function is a topographically defined helix modulated by the electrical field reflectance, originally developed for the measurement of thin films. An approach to extend the capability of the HCF function to determine the spectral refractive index of a substrate or absorbing film has recently been proposed. In this paper, we confirm this new capability, demonstrating it on surfaces of silicon, gold, and a gold/ palladium alloy using silica and zirconia oxide thin films. These refractive index dispersion measurements show good agreement with those obtained by spectroscopic ellipsometr