MANUFACTURINGMETROLOGY
Metallization control may also be carried out via optical instruments: high resolution images are taken and processed to find spots where the metal fingers are damaged
in routine process monitoring. For monitoring a well-known material such as silicon nitride, simple single-wavelength ellipsometry may be sufficient for production control. With careful evaluation, taking into account the depolarization effects caused by the textured wafer surface, the two most important parameters can be obtained: thickness and optical index (at 633nm).
In addition to economic considerations, laser ellipsometry has another advantage over spectral ellipsometry: which is speed. According to Semilab’s experience laser ellipsometry is the best option when fast and accurate measurements are required in-line production control.
Metallization control may also be carried out via optical instruments: high resolution images are taken and processed to find spots where the metal fingers are damaged.
End-product classification is also essential and has to be carried out under standard conditions to ensure that efficiency meets the specification. Each cell is put into a test circuit and flashed, and generated current is measured. However, this is a mere go – no go test, not revealing many details. If there is doubt about efficiencies, high-resolution LBIC (laser-beam induced current) mapping has to be performed. In this case, small-spot laser beam is scanned over the sample, and together with generated current, direct and scattered reflection is recorded. From these data, internal quantum efficiency can be extracted (at the given laser wavelength). If repeated with multiple lasers, carrier diffusion length in the final cell can also be obtained. Measurements with 125 µm resolution are possible within several hours, but provide extremely valuable data and images.
For the reasons described above, the development of integrated metrology modules capable of performing such monitoring is a key interest to everyone in the PV value chain. A metrology tool supplier needs to provide equipment which can be tailored to the customers’ specific process requirements in these fields. Moreover, the techniques should be capable of both real time process monitoring and also process development
with little or no changes, to ensure that lab results will be useful directly in the production line. R&D tools will always be slower, require more knowledgeable personnel to operate and analyze data, but in return they offer more flexibility in measurement conditions and provide more valuable, detailed analysis.
Semilab’s goal is to provide unique optical and electrical characterization tools with flexible integration possibilities to the customers. Most of the techniques can be used independently for R&D and engineering purposes, or at critical steps of the production line. They may also be utilized together where multiple parameter monitoring is required at one production stage (for example integrated thickness and resistivity measurement, complemented by optical inspection for incoming wafers).
This is achieved via a modular system design. Key components of the measurement units are designed to be compact in size, such that they can be incorporated into different systems depending on application. This may be a stand alone high resolution mapping type system for R&D, or simple instruments for single-point measurements, or in- line metrology equipment built into the production line, which can work “on-the-fly” without stopping a conveyor.
This way, at each step all necessary electrical and optical data are available in real time, and can be sent to the production control system through a number of state-of-the-art communication interfaces to enhance production, and, in the long run lead to a “sunnier” future.
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www.solar-pv-management.com Issue VI 2010
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