Test Structures for End-Point Visualization of All-Plasma Dry Release of Deep-RIE MEMS Devices and Application to Release Process Modal Analysis

Test structures were developed to visualize the progress of the dry release of an MEMS device fabricated by deep reactive ion etching (DRIE). Because of the high aspect ratio of DRIE MEMS devices, the undercut progress cannot be observed from the surface. Therefore, a destructive test, i.e., a cross-sectional-view observation of a cleaved sample, has mostly been employed. Using the proposed test structure, the progress of the silicon undercutting can be evaluated in a nondestructive manner, using only an optical microscope. The test structure was not only found to be useful in optimizing the release etching procedure, but also allows us to discover a particular DRIE mode that is useful for dry release. Indeed, most DRIE process engineers have considered and mainly tuned the etching phase; using the test structures, we found that the deposition phase of DRIE can be strategically used to further increase the undercutting speed. The new mode, referred to as the “undercut directional etching mode,” intentionally leaves a fluorocarbon layer for protection of the side-wall, at the bottom center of the trench openings. Because of the deposition layer remaining at the bottom center, the introduced plasma species are concentrated towards the edge of the trench, i.e., at the foot of silicon structure to be released, thereby enhancing the undercut etching. A second test structure identified the appearance of the new mode and visualized the dependence of the etching behavior on the trench opening width. The optimized etching conditions determined using the test structures led to the successful release of a standard in-plane MEMS device.