MKS Instruments GLM-2000 measures photoconductance in c-Si wafers

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MKS Instruments has introduced the GLM-2000 PV and Silicon Wafer Monitor for quality control in photovoltaic and crystalline silicon wafer processing for solar applications. The multi-function GLM-2000 uses a unique, high speed, non-contact RF detection technology to measure photoconductance in PV and silicon feedstocks and partially processed cells, either in-process or post-process.


The quality of silicon used for crystal silicon PV cells has a direct influence on the quality, or conversion efficiency, of the final product. The objective is to transfer as much of the incoming energy, or carriers, to the output of the circuit or cell. This is achieved, in part, by minimizing any recombination of the carriers. Carriers recombine when they come in touch with defects or impurities in the silicon. Recombination reduces the potential output current from the cell.


Measuring minority carrier lifetime is a way to quantitatively assess the quality of the base silicon, since lower minority carrier lifetime is an indication of impurities and defects in the silicon. Additionally, measuring sheet resistance and Photoconductance Decay (PCD) give further information on surface recombination effects and the quality of the PV cell manufacturing process from as-cut inspection through ARC deposition.
The GLM-2000 metrology tool employs a novel implementation of the current RFPCD measurement technique to instantaneously measure sheet resistance, minority carrier lifetime (Gtau), photoconductance decay (PCD), and photoconductance rise (PCR), key quality control parameters for PV processing. The GLM-2000 employs a programmable LED array that permits measurements at different light intensities, allowing the user to determine the true steady state minority carrier lifetimes. The use of LEDs also allows flexibility in the choice of light color and this can be useful in distinguishing surface effects from bulk characteristics.


High accuracy monitoring using True Steady State (Gtau) provides detection of changes in lifetime caused by crystal imperfections and impurities and a simple, deterministic value for c-Si or poly-Si quality and for recombination lifetimes. Photoconductance Rise (PCR) and Photoconductance
Decay (PCD) measurements are used to detect standard changes in lifetime of partially fabricated solar cells (pre-metallization). Comparing PCR to PCD signals to distinguish shallow trapping effect that happen with plague standard PCD measurements. Detect changes in lifetimes due to surface conditions or surface preparation steps in standard solar cell manufacturing.


The flexibility of the sensor design permits easy integration into the production line as either a standalone benchtop unit or integrated within OEM or custom system components. The unit is compact, fast, and contactless and uses a robust LED illuminator for measurements near the surface or through the complete bulk of the substrate. Information is displayed via easy-to-use software that works with 32 or 64 bit ‘Windows’ XP and higher operating systems.


June 2011 onwards.

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