Bulk micromachining (BMM) describes the fabrication process of a device taking advantage of all three space dimensions. In most applications single crystalline silicon is used as bulk material. The excellent electrical and mechanical properties make it a desirable material in MEMS and have led to a focus on the development of different machining technologies.
In order to structure silicon, two basic methods are used: wet chemical etching and dry etching. The wet chemical etching is performed by anisotropic etchants, such as KOH and TMAH. Both chemicals show different etch rates depending on the crystal structure, whereas the etch rate in <100> direction is much higher than in <111> direction. Geometric limitation of etch holes is possible by this property. The depth of an etch hole in the <100> direction is usually controlled by time or by a functional layer respectively a pn-junction. TMAH etchants are alkaline ion free and are, therefore, highly compatible with CMOS-processes. Thus, they can be integrated in an additive process to an IC fabrication. A limiting factor in the design flexibility of wet chemical etching is the fixed angle of the silicon sidewalls <111>.
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 plasma etch processing (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.