5 - MEMS devices

Capacitive sensor basic concept

Capacitive single lead

The sensor is a capacitive device suitable for barometric applications. It has a wide dynamic range (500-800 torr) as well as very high resolution (25 mtorr) The working temperature range is from -25°C to 85°C, where the effect of trapped gas expansion is eliminated by one vacuum-sealed reference cavity.

Capacitive multilead

The multilead transfer sensor with multiple stacked polysilicon and dielectric layers reduce the effect of parasitic capatiance in paralell with the measuring capacitor.

Piezoresistive

The multilead transfer sensor with multiple stacked polysilicon and dielectric layers reduce the effect of parasitic capatiance in paralell with the measuring capacitor.

Bulk micromachined

In bulk micromachined capacitive acceleration sensors, the cantilever structure etch out from Si is bonded between an upper and lower wafer or metallized glass substrates. An acceleration perpendicular to the wafer surface moves the seismic mass out of its center position. The displacement is detected by changing the top and bottom capacitors between the wafers or by piezoresistive effect.

Surface micromachined

In surface micromachined sensors movable structures are etched out using sacrificial layers. The seismic mass is suspended by springs. The fingers on both sides of the seismic mass, and fingers attached to the wafer surface form a capacitance for signal pickup, which is changing during acceleration. In contrast to bulk micromachining, surface micomachining uses movements parallel to the wafer surface.

Thermodynamic sensor

In thermodynamic Si sensor a heated air bubble moves opposite direction of acceleration. There are heat sensors on both sides of the heater element for signal pickup. Acceleration can be measured as function of heat difference changing during the motion of hot air bubble.

In surface micromachined Si vibration sensors movable structures are etched out using sacrificial layers. The moving part is suspended by a pre-stressed spring, so that the spring constant is tunable and can change device sensitivity. The fingers to both sides as well as fingers attached to the wafer surface form a capacitance for signal pickup. This capacitance is changing during vibration.