New Product: BT Imaging detects wafer electrical material defects

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Product Briefing Outline: BT Imaging Pty launched what it claims is breakthrough product for qualifying the electrical quality of as-cut wafers in production. The QS-W1, is said to be the industry’s first automated electrical wafer quality sorter, enabling the sort and rejection of as-cut wafers based on photovoltaic performance, which maximizes solar cell yield and efficiency. 

Problem: Photovoltaic solar cells are sold based on dollars per watt. Ideally, all solar cells would have maximum efficiency and revenue would be predictable. Today, however, there is a spread in processed cell efficiencies. Wafers are the largest cost element in solar cell manufacturing and can reduce absolute efficiency of a processed solar cell by as much as one percent. However, wafer electrical quality is not monitored today, and wafers are purchased at the same price based on physical specifications, regardless of final cell efficiency. Currently, wafers are checked at the end of the wafer manufacturing line for physical attributes like size, thickness, thickness variation, cracks, and roughness. Electrical defects such as dislocations and impurities—which significantly impact cell electrical performance—are not seen by current inspection techniques. These electrical defects remain in the cell and reduce the cell’s performance.  Cell electrical performance is determined only at the end of the cell process, in the IV tester; this final sorting is mandatory due to the inherent spread of wafer quality. At this stage, all wafers—even the bad ones—have been processed, so cell manufacturers have no means to further optimize yield and profits.

Solution: The QS-W1 can take high-resolution (1 MP) PL images of as-cut wafers in one second. Automated algorithms classify and report the different electrical defect types seen in as-cut silicon wafers. Defects that reduce efficiency and yield—such as dislocations, impurities, and cracks—are automatically separated and reported. User controlled binning is then used to grade the wafers appropriately based on defect density. The QS-W1 includes a crack and chip inspector, and an integrated thickness and resistivity measurement unit, which is used to normalize PL data to ensure accurate grading and sorting. It is possible to inspect all wafers incoming to the cell line and sort based on electrical quality, or reject wafers below a user defined electrical quality specification. In this way, wafers can be rejected before the cell processing cost is spent. This sorting of wafers enables cell makers to increase the cell conversion efficiencies and electrical yield of the line. The best wafers can be processed into high efficiency cells, poor quality wafers are rejected to wafer suppliers, and wafers with certain types of defects can go through extra processing to improve quality.

Applications: Initially targeting as-cut multi-crystalline silicon photovoltaic wafers, BTi has a roadmap in place to address the needs of mono-crystalline wafers and all photovoltaic cells at various stages of production. The QS-W1 is currently in extensive testing with leading photovoltaic wafer and cell makers.

Platform: Utilizing BTi’s proprietary second-generation patented photoluminescence (PL) imaging technology, the QS-W1 is ten times faster than previous generation systems, enabling inspection at production speeds for the first time. Fully automated, the base-model QS-W1 includes a coin stack loader that accommodates up to two load stations, fully integrated conveyor belt, input and output reject stations, and a coin stack sorter with six stations. Various handling options are available to meet specific customer needs.

Availability: The QS-W1 will start shipping to beta customers in the third quarter of 2010.

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