Pyrometers improve quality and yield in silicon growth

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Courtesy of LumaSense Technologies, Inc.

Temperature control has always been one of the most critical parameters in optimizing many complicated solar photovoltaic manufacturing processes because it plays an important role in producing higher-quality product and increasing overall yield. Unfortunately, it's also been one of the most difficult things to measure in these processes. Certain applications of radiation pyrometers in crystalline silicon solar photovoltaic

Certain environmental conditions are required to grow silicon - ironically, these conditions cause many challenges to creating a high-quality, high-yield product. manufacturing, however, have proven effective at providing precise measurements.

Radiation pyrometers are particularly well-suited to overcome these challenges for several reasons. First, they provide non-contact measurement, which is important because many processes (such as deposition processes) do not allow contact instrumentation, as it can contaminate and heat-sink the product during process. They also eliminate junction deterioration problems typically seen with thermocouples, and they can achieve better accuracy in higher-temperature ranges. Following are a few examples of how the use of pyrometers can allow equipment designers to improve equipment performance and process engineers to improve quality and yield.

Polysilicon growth

The Siemens process is the traditional way of converting purified trichlorosilane to polysilicon.

The cold-wall reactor is usually water-cooled in this process. Seed rods inside the reactor are heated to process temperature, and chemical vapor deposition (CVD) takes place on the surfaces of the rods. The rods grow as the deposition accumulates, and this growth continues throughout the process until the rods reach the desired diameter. The surface temperature of the growing rods is of the most interest in this process as the deposition occurs on the surface and temperature control is important in CVD processes. Figure 1 shows CVD as part of the Siemens Process [1].

Conventionally, two-color (ratio) radiation pyrometers are used for this application. The optical access for the pyrometer to the rods is through a viewport (window). During the process, the window can get contaminated, which could result in errors in temperature measurement. By taking the ratio of the measurement at two wavelengths, the changing transmission of the window can be canceled out. The measurement therefore can be made reasonably immune to window deposition. This is based on the assumption that the spectral transmission of the coated window is identical at the instrumentation wavelengths or at least with a constant ratio between them at all temperatures. In reality, neither can be completely true. In practice, by carefully choosing the pyrometer wavelengths, the assumption can be satisfied to an acceptable degree. With proper emissivity slope setup, the p3rrometer would serve the purpose.

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