Processes to achieve vibrating beams for an angular rate measurement sensor

Abstract

Vibrating beam structures may be used as sensing elements in an angular rate measurement sensor. Achieving a square cross-sectional area with smooth vertical side walls and sharp edges is necessary for these beams. In this work, fabrication of beams with reasonably smooth vertical sidewalls and accurate dimensions, using anisotropic etching of silicon in TMAH, has been investigated. Beams with various thicknesses (500 -100 microns) are fabricated. It is shown that by carefully aligningthemaskat45°fromthe<110> wafer flat, andby maintaining the concentration of TMAH at 25%, it is possible to achieve smooth vertical sidewalls with good uniformity along the length of the beam. Adequate control over the beam lateral dimension is achieved by etching in a 2-step procedure with the second step at a lower temperature (lower etch rate). Single-sided and doublesided masking techniques are investigated in order to fabricate the beams. An alternative design is presented to form the beams using a standard CMOS process.

Fabrication of a new highly-symmetrical, in-plane accelerometer structure by anisotropic etching of (100) silicon

“…That is the reason why, in many conventional bulk-micromachined accelerometers, the seismic mass is pyramidal and not symmetrically shaped. It can be imagined, and it has also been demonstrated, that the emerging planes appearing at the intersection of the (100)-(001) planes makes a 45 • angle with the 110 wafer flat [18,19]. Thus, uniform and flat vertical {100} sidewalls in a (100) wafer are achievable if the pattern is aligned with a ±45 • angle from the wafer flat (see figure 3).…”

“…During the subsequent etching process in the KOH solution, the dimensions of the structure are controlled by time and also visually. When the wafer is etched through, a lower temperature of the KOH solution may be used in order to lower the etching rate and also to achieve a higher accuracy [19]. When the desired dimension is achieved the overhanging oxide is removed with a buffered HF solution.…”