Yoshiaki Hirata

A new z-axis capacitive accelerometer with high impact durability

This paper reports a newly developed z-axis capacitive accelerometer with high durability to impacts. Our accelerometer has a unique sensing mechanism by which the displacement of an inertial mass caused by an acceleration in the z-axis direction is converted into the rotational displacement of a pair of detection plates. Our accelerometer has a high degree of design freedom in terms of selecting the resonant frequency and sensitivity. While maintaining a constant sensitivity, the resonance frequency and etching hole within the detection plate were designed so as to have an appropriate damping effect. As a result, it achieved an output saturation level higher than 5000 G under the impact acceleration of 5 kHz.

Effects of metal impuritites on the etch rate selectivity of (110)/(111) in (110) Si anisotropic etching

We have investigated the effects of metal impurities and thermal oxidation process on Si anisotropic KOH etching characteristics for fabricating microsensors and microactuators having precise deep narrow grooves or channels. The effect of metal impurities such as Al, Cr, Fe, Pb and Cu are evaluated using 30 wt percent KOH etchant at 60 degrees C. Al, Cr and Fe have no relation to the etching characteristics, however, Pb and Cu strongly affect the etching characteristics. The etching rate of Si decreases with increasing Pb and Cu concentration. The etch rate selectivity , which is an anisotropy ratio defined by the etch rate ratio of (110) and (111) plane, is not changed by Pb impurity, but extremely decreases with increasing Cu impurity. It dips form 150 to 80 when the Cu concentration is over 50ppb, and saturated over the concentration. With respect to the surface roughness within our evaluated metal impurities, Cu impurity also strongly affects the roughness of the etched surface. In addition to the effect of metal impurities, we have experienced the effect on the etching selectivity by the thermal oxidation process. The selectivity is decreased with repeating thermal oxidation process, and this tendency is more pronounced with higher oxygen concentration of evaluated Si wafers. As a result, controlling Cu impurity concentration and using a low oxygen concentration wafer are crucial considerations when fabricating sensors and actuators with narrow grooves or channels by Si anisotropic KOH etching.

Airbag accelerometer with a simple switched-capacitor readout ASIC

A bulk micromachined capacitive accelerometer for airbag applications based on (110) silicon anisotropic KOH etching is presented. The sensor is a two-chip accelerometer that consists of a glass-silicon-glass stacked sense element and an interface ASIC containing an impedance converter for capacitance detection, an EPROM and DACs for digital trimming, and a self-test feature for diagnosis. A simple switched-capacitor readout circuit with DC offset error cancellation scheme is proposed as the impedance converter. The dependence of narrow gap etching, surface roughness, and uniformity of the groove depth on the KOH concentration are also investigated for the fabrication of the device, and it is shown that the etch rate of the plane intrinsically controls the depth of the narrow gap with a KOH concentration of over 30 wt. percent, and smooth surface and uniformity of groove depth are obtained at 40 wt. percent KOH. The nonlinearity of the output is about 1.5 percent FS. The temperature coefficient of sensitivity and the off-axis sensitivity are 150 ppm/degree C and 2 percent respectively. The dimensions of the sensor are 10.3 X 10.3 X 3 mm.