10 October 2011

 

Op-tection offers PL-based wafer inspection platform for efficiency forecast on as-cut wafers

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The PL inspection platform was developed in cooperation with Schott Solar Wafer, which bundles solid metrology experience in one hand with in depth process and material knowledge in the other hand. The result of this effort is the capability to quantify the achievable efficiency of solar wafers before they enter into the cell process line.

Problem

Solar cell makers sell their solar cells based on efficiency. Higher cell efficiency in general equals more money earned for the cell batch at hand. Wafer makers are often qualified and judged by cell makers based on the performance their wafers can deliver. Key point in both cases is that the wafer itself is often the limiting factor for achievable final solar cell efficiency. This applies in particular to polycrystalline as well as the new quasi-monocrystalline wafer type. A reliable forecast of the achievable efficiency that a wafer can achieve is important for both wafer and cell makers, as it will help wafer makers prove the value of their product to cell makers. Vice versa the cell maker is required to continuously increase process and production yield and required good wafer to be successful doing so.

Solution

To enable wafer and cell makers to achieve this goal, Op-Tection offers PL-based sorting platform that automatically inspects and segregates wafers. The wafer under test is illuminated with strong laser light source to excite the luminescence response of the silicon. A sensitive and high-resolution camera combined with adapted filters records the PL image. This 2D image is a measure for the spatially resolved carrier lifetime of the silicon. Subsequent image processing and analysis of the various defects and structures, such as dislocations and material contaminations, is performed with proprietary algorithms that calculate metrics such as Edge contamination fractions, Dislocation clusters, PL intensity & distribution. Based on the calculated and individually marked wafer metrics, an efficiency forecast is made. Op -Tection integrates this capability in sorters that allow segregation of wafers in multiple classes based on forecasted achievable efficiency. Sorters can be configured to handle various throughputs ranging from few hundred wafers per hour up to 1s capture time to meet current line cycle times. The wafer handling can be from stack-box to stack-box or from cassette to cassette or a combination of both to ensure a smooth handover to the next process step.

Applications

Op-Tection`s PL technology can be used for all types of solar wafers. The strongest benefit of this capability however applies to: Poly-Wafers: dislocation clusters, edge contamination fractions and corner brick metrics are weighted and used for a precise forecast.

Pseudo-Mono-Wafers: this new generation of wafer material has some distinct characteristics that require detailed investigation and efficiency forecasting. The very small amount of grain boundaries make this type of wafer look nearly like a single crystal from the outside. However, dislocations are present in this material, and unlike regular poly-wafers, there are just few grain boundaries. This can result in over-proportional growth/presence of dislocations in pseudo-mono material, which strongly impacts the achievable cell efficiency.