Microelectromechanical accelerometer for automotive applications

DETAILED DESCRIPTION Process For Making a Microelectromechanical Accelerometer In one embodiment of the invention, capacitance-based accelerometers are fabricated utilizing a modified version of the process known as Single Crystal Reactive Etching And Metalization (SCREAM-I), which is a low temperature, single-mask micromachining process which permits construction of high aspect ratio single crystal silicon beams and other structures in or from a single wafer.
The typical maximum temperature at which the SCREAM-I process operates is 300.
degree.
C.
Although it is possible to use multiple masking steps to make an accelerometer of this invention, a single mask that establishes the overall structure of the accelerometer is preferred in order to reduce the number of processing steps and to provide accurately-formed accelerometers from one batch to the next.
The process is one that utilizes process steps typically encountered in semiconductor fabrication, allowing single or multiple accelerometers to be fabricated simultaneously in a single wafer, and enabling fabrication of an accelerometer in a wafer that is to have other electronic components fabricated in or on it, or in a wafer that has electronic components in or on it that are partially or fully fabricated.
A basic, single-mask reactive-ion etching process of this invention is illustrated in FIG.
4 and may be summarized as follows.
Photoresist is deposited and patterned on top of a layer of silicon dioxide (a).
After the pattern is transferred using reactive ion etching, the silicon dioxide serves as a hard mask during a deep silicon trench etch (b).
A layer of silicon dioxide is conformably deposited over the entire structure, and unwanted oxide on the floor of the trenches is removed using another reactive ion etch step (c).
The silicon mesa, to become a released cantilevered beam, is undercut with a nearly isotropic reactive ion etch.
The etch undercuts those areas not protected by silicon dioxide, leaving the beam in the center free to move laterally (d)