Solar cell quality control is done at the end of the production chain, testing each individual cell for efficiency. The optical surface metrology system Leica DCM 3D combines confocal and interferometry technology and offers the possibility to check silicon surface texture, roughness, pyramid statistical characterization and metal contact in a few seconds.
Unlike the time-consuming scanning electron microscope method, the wafer is simply placed under the Leica DCM 3D and a 3D measurement taken in less than 10 seconds. The high local slope of the pyramid faces demands the use of objectives with a high numerical aperture, which are only available in confocal technology.
Figure 4 shows a 3D measurement of a monocrystalline silicon wafer after pyramid etching. For a 3D measurement of this kind, a 150x objective with a numerical aperture of 0.95 was used. As a result, the size of the visual field was reduced to a few tens of microns, which is roughly equivalent to the visual field of an SEM. The surface is scanned a few microns along the focus position of the objective, collecting the confocal images plane by plane. The result is a high-resolution image similar to that generated by an SEM with infinite focus and precise 3D information on the height of the pyramids.