Optical Monitoring of the Etching of SiO2 and Si3 N 4 on Si by the Use of Grating Test Patterns

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Abstract
An optical technique is described which monitors both the etch depth and the amount of lateral underetching during the etching process. This technique applies to wet chemical etching as well as to plasma etching. For this purpose a test pattern containing one or more diffraction gratings is included on the mask, which is used to define the particular etching step. During the etching process a laser beam is aimed at this test pattern on the Si wafer and the reflected first‐order diffraction intensities are monitored. As the etching progresses and the grating profile deepens, the diffracted intensity goes through oscillations which give an in‐process indication of etch depth and etch rate. In the wet process the complete underetching underneath the photoresist bars of the grating is signaled by a drastic drop in the diffraction intensity. The simultaneous monitoring of a number of gratings with different bar widths permits one to follow the underetching in fine steps.

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