Optical Monitoring for Plasma-Etching Depth Process

In this letter, a novel technique for monitoring the opaque thin-film etching depth during the plasma-etching process is presented. The feature of this technique is that a small Laminar grating is overlaid on the empty region of the wafer along with the etching pattern. By coherent illumination, a reflected intensity distribution from the Laminar grating for different orders of diffraction can be obtained. Since the etching depth and the diffraction distribution are related, by analyzing the intensity profiles due to Laminar depth grating, the etching depth of the specimen can be measured. We have shown that the accuracy of this measurement can be as high as 100 Å.