Test patterns in the form of diffraction gratings are used for testing and monitoring linewidths on integrated circuit structures. The first and second diffraction orders produed by a laser beam are evaluated to give the width of the grating lines. Measurements on chrome masks show that this technique is accurate to 5% down to linewidths of 0.5 microm. The design of a test set for factory type mask testing is presented. Also, experiments are reported on the testing of patterns on Si wafers directly after. photoresist development and after various etching steps, and an-automatic setup for rapid testing of wafers is described.
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 aefine 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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