MANUFACTURINGMETROLOGY
In addition to electrical qualities, optical and mechanical aspects are also important. If the shape or thickness varies, or the wafers break, this jeopardizes process and damages yield, too. There are ways of reducing the probability of these, of course with the deployment of proper inspection tools. Advanced vision systems are available to perform so-called 2D and 3D examinations, i.e. to check for irregularities in shape, deviations from the expected square or pseudo-square specifications, and bending, bow and warp. Similar optical methods can also be applied to find saw damage which may result from erroneous wafering. Thickness uniformity is typically monitored by capacitive distance gauging.
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The possibility of the wafer breakage can also be reduced. In many cases the cause of this is the occurrence of micro-cracks in the incoming material. Micro-cracks are usually formed at grain boundaries in polysilicon material generally used for photovoltaic purposes, and despite their small size they can severely affect the mechanical integrity of a wafer. Unfortunately since they are not visible to the naked eye or an ordinary vision system, special tools are required to catch them. One of the widely used and most effective methods is to have intensive illumination on one surface and cameras on the other. From the transmission image micro-cracks can be identified by advanced image processing algorithms. A leading provider of such tools was the German- based company Basler AG, a developer of advanced camera systems. Recently their Solar Wafer Inspection Business was acquired by Semilab, already a leading provider in front-end electrical characterization equipment and also
process monitoring tools. From Q3 2010, Semilab will develop and sell such optical tools including the Microcrack Inspection Equipment.
Other imaging techniques may also be used such as photoluminescence imaging, but current tools are not capable of working in-line. The benefit of this approach is the extremely rapid acquisition time for a high resolution wafer map, but it should be taken into account that an image by itself only provides relative information about material quality distribution within each wafer. To compare wafers, and thus provide a feasible production control solution, these images need to be calibrated to an absolute, unambiguous physically measurable quantity like carrier lifetime.
Once starting material has been characterized and enters into the manufacturing process, all steps including emitter diffusion, application of the anti- reflective coating and metallization should be precisely monitored to check the performance of the different production tools with suitable techniques for each.
Diffusion is conventionally controlled based on sheet resistance value of the diffused layer, which may be measured by four point probe. However to do this the sample has to be removed and measured offline, due to the time needs of the metrology tool. This can be avoided if non-contact sheet resistance measurement is used, which is fast enough to be measured in-line. Additionally on-the-fly metrology allows several linescans of data to be collected in-line, which can also identify issues with edge isolation.
Anti-reflective coatings and dielectric layers are usually monitored using optical methods. The simplest way is to look at the colour of the wafer after the nitridation process, however this can only reveal major processing errors. Most cell producers require finer control such as reflectometry or ellipsometry. The latter involves shining polarized light on the sample and detecting changes in intensity and polarization degree after reflection. This technique offers many options regarding wavelength range used and data evaluation. Most common is to use spectral measurements in the visible (possibly extended to the near infra-red) range, but tools are available from the vacuum-UV to the deep infra red (FTIR) range. Of course such complexity adds cost, which might be affordable and necessary for R&D and process development, but may be not economical
www.solar-pv-management.com Issue VI 2010
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