WINNERSOLAR INDUSTRY AWARD 2011
measurement removes operator variability with less training demands
Step and trench measurement Optimising the trench depth of the cells is important to minimise expensive silver trace usage. For controlling the trench dimensions the CCI offers the ability to measure silver trace height and width allowing optimisation of cell efficiency thereby reducing cost. The high measurement speed offers high sample throughput and the automation gives the ability to sample multiple sites and/or stitch together large datasets leading to more representative sampling. Controlling the parameters of etched scribe lines gives improves manufacturing efficiency by identifying line depth errors such as shallow lines which give problems with conductivity and deep lines which lead to electrical shorts.
Roughness measurement
The relationship between the surface roughness and efficiency is complex. Rough surfaces trap more light than smooth surfaces but surfaces that are too rough reduce the efficiency due to scattering. Other interfaces need to be smooth to reduce scattering and absorption. In the past traditional parameters such as Ra has been used but they are inadequate to fully describe the important features of the solar cell surface. Some of the more advanced 3D parameters now appear to offer correlation between the surface roughness and efficiency. For example valleys on the surface help to trap light so parameters such as Ssk can show strong correlation with cell efficiency.
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 different characteristics then any incompatible measurement reports will cause major problems for the optimisation of the surface. The CCI algorithm provides sub-angstrom 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 systems any field of view optics required leads to lower lateral resolution and reduced angle 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 of the surface properties necessary to improve efficiency.
Film Thickness
A recent breakthrough in interferometry is the ability to measure film thicknesses less than 1 micron. Film thickness is an important parameter in all photovoltaic technologies. CCI can be used to measure the thickness of semi-transparent films. 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 1-2µm. Upon measurement of the film, two or more localised fringes appear, each corresponding to a surface interference. CCI provide accurate thick film thickness measurements, which are relatively straightforward by locating the position of the two envelope maxima.
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.
© 2012 Angel Business Communications. Permission required.
Dr. Daniel Mansfield and Mr. Andrew Bankhead of Taylor Hobson accepting their award
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