There is a trend in microsensor and microsystem development to increase the aspect ratio of the device as high as possible. Higher aspect ratios result in better device performance, for example, by increasing device sensitivity, increasing the signal-to-noise ratio, and increasing the micro-actuator force. In wet chemical etching process, the silicon is etched preferentially along a certain silicon crystal plane and very high aspect ratios cannot be achieved. Dry reactive ion etching (DRIE), capable of delivering such high aspect ratios, is required to maintain a high etch rate and vertical profile inside the high aspect ratio structure. If such features are closely packed, the undercut must be controlled tightly to prevent the adjacent features from merging into one another.
