Use of Interferometry for Optimization of PV Cell Performance

by the surface, optimizing the surface texture. (See Box 3.)

Film Thickness
A recent breakthrough in interferometry is the ability to measure film thicknesses less than 1 micron. The technique is suitable for measuring different materials such as a-Si, CdTe, CdS, CIGS, ITO, ZnO2, TiO2 plus many others. There are two different approaches depending on the thickness of the film. Films can be considered as “thick films” for physical thickness values higher than 1-2 µm. Measurement of these thick films produces two or more localized fringes, each corresponding to a surface. Knowledge of the refractive index or the film allows accurate 3D film thickness to be calculated as well as the roughness of the top surface and interface.

For thinner films, it is very difficult to identify the envelope maxima, so a different technique is required. If a priori knowledge of the dispersive film index is known, it is possible to use the patented film thickness software of the CCI to measure thicknesses down to around 50 nm.

Instrumentation
An important consideration when looking at the control of the surface properties is the ability to use the same technique for all the measurements. If multiple inspection routines are required for samples with slightly different characteristics, then any incompatible measurement reports will cause major problems for the optimization of the surface. Modern techniques such as coherence correlation interferometry provides sub-angstrom surface resolution regardless of scanning range, so that all surfaces at any stage in production can be measured on the same instrument using the same measuring technique.

For many older system designs, any field-of-view optics required leads to lower lateral resolution and reduced angular sensitivity. The increase in missing data gives poorer surface understanding and therefore less process control. Using a high-resolution camera, typically 1 million or 4 million pixels combined with a large-area objective is essential for understanding the surface properties necessary to improve efficiency.

Conclusions
Non-contact metrology can play a vital role in the development and production of all designs of photovoltaic devices. Interferometry provides a 3D surface analysis providing a rapid means of characterizing a number of important parameters including surface roughness, step height, trench depth and surface texture. The option to measure film thickness has increased the parameters that can be investigated to optimize solar cell efficiency.

Acknowledgments
Thanks to Yang Yu, Danny Mansfield and many others at Taylor Hobson for their help with measurements and analysis. I would also like to thank Professor Mike Walls and his colleagues at CREST, Longborough University, U.K., for enhancing our knowledge of solar cell technology.

Endnotes
1. Y.N. Denisyuk, “On the reproduction of the optical properties of an object by the wave field of its scattered radiation. Pt. II,” Opt. Spectrosc. (USSR) 18 (1965) 152.
2. B.J. Chang et al. “Space-invariant achromatic grating interferometers: theory (TE),” Appl. Opt. 14 (1975) 1592.
3. E.N. Leith and G.J. Swanson, “Achromatic interferometers for white light optical processing and holography,” Appl. Opt. 19 (1980) 638.
4. Y.-S. Cheng and E.N. Leith, “Successive Fourier transformation with an achromatic interferometer,” Appl. Opt. 23 (1984) 4029.
5. E.N. Leith and R.R. Hershey, “Transfer functions and spatial filtering in grating interferometers,” Appl. Opt. 24 (1985) 237.
6. A. Bankhead et al. “Interferometric Surface Profiling,” GB2390676 (2004).
7. D. Mansfield, “Thin film extraction from scanning white light interferometry,” 229 Proceedings of the 21st Annual ASPE Meeting (October 2006), 230
8. D. Mansfield, “Extraction of film interface surfaces from scanning white light interferometry,” Proc. SPIE 7101, 71010U (2008)
9. M. Conroy, “Advances in thick and thin film analysis using interferometry,” Wear (2008), 79 n

About the Author
Mike Conroy – Following a master’s and doctorate in polymer science, Mike spent 10 years as a researcher at different academic institutions. From 2000 to 2005, he was UK applications manager at Veeco Instruments. Since then, Mike has worked at Taylor Hobson as non-contact applications scientist and for the last four years, as business development manager for non-contact metrology. He is currently based in Shanghai.

FPV7_N_WP_Conroy_MFA.pdf