Monitoring Solar Cell Film Thickness

Challenge

A photovoltaic solar cell manufacturer needed a device to monitor film thicknesses over the full width of the product emerging from the thin layer depositing equipment.

Solution

Thin layers can be monitored by detecting the interference fringes across the electromagnetic spectrum. The thickness in this case was best detected in the 1000-2500 nm region, thus a SWIR hyperspectral camera system was mounted about 10 feet above the roll-to-roll manufacturing equipment. A line of halogen lights provide the broadband radiation above the 36” wide product line. Small changes in the spacing of the interference fringes detect the apparent thickness of the layers through which the light penetrates. Having a wide wavelength range and a plurality of spectral points allows for the detection of both thickness and uniformity of multiple layers in real-time.

Farther down in the same manufacturing line, a high-resolution VNIR hyperspectral camera was installed to detect a narrow band in the visible spectrum. A very small 1-2 nm shift in the reflectance peak is a sensitive indicator of the quality of the product and was detected at 1600 points across the manufactured 36” wide solar cell web-surface.  This high spatial resolution is able to notice not just the overall quality trend but even the smallest out of specification areas or stripes on the surface of the product.