Page 135 - An Introduction to Microelectromechanical Systems Engineering

114 MEM Structures and Systems in Industrial and Automotive Applications

Aluminum Polysilicon

Silicon
dioxide
Silicon substrate
2. Anisotropic etch from backside;
1. Deposit oxide and polysilicon; pattern and etch polysilicon
deposit and pattern aluminum
Bondpad Cap

Glass
3. Sacrificial etch of oxide; 4. Bond cap wafer;
DRIE of silicon anodic bond glass
Figure 4.28 Illustration of the fabrication process for the yaw-rate sensor from Robert Bosch
GmbH. (After: [32].)

protective silicon cap wafer that contains a recess cavity is bonded on the front side
using a low temperature seal glass process. A glass wafer anodically bonded to the
back side seals the device. The final assembly brings together the silicon sensor and
the electronic circuits inside a metal can whose cover holds a permanent magnet.
The sensitivity of the device is 18 mV/(º/s) in the range of ±100 º/s over –40° to
+85ºC. The temperature dependence of the uncompensated sensor causes an offset
amplitude of 0.5 º/s over the specified temperature range, but signal conditioning
circuits reduce this dependence by implementing appropriate electronic temperature
compensation schemes.

Carbon Monoxide Gas Sensor
Many gas sensors operate on the principle of modulating the resistance of a metal-
oxide element by adsorption of gas molecules to its surface. The adsorbed gas mole-
cules interact with the surface of such a wide-bandgap semiconductor to trap one or
more conduction electrons, effectively reducing the surface conductivity. The resis-
tance is inversely proportional to a fractional power of the gas concentration. The
class of sensor materials include the oxides of tin (SnO ), titanium (TiO ), indium
2 2
(In O ), zinc (ZnO), tungsten (WO ), and iron (Fe O ). Each metal oxide is sensitive
2 3 3 2 3
to different gases. For example, tin oxide is effective at detecting alcohol, hydrogen,
oxygen, hydrogen sulfide, and carbon monoxide. Indium oxide, by contrast, is sensi-
tive to ozone (O ); zinc oxide is useful for detecting halogenated hydrocarbons.
3
Unfortunately, most are adversely affected by humidity, which must be controlled at
all times. In addition, variations in material properties require that each sensor is
individually calibrated.
The MiCS series of carbon monoxide sensors from MicroChemical Systems SA
of Switzerland [33] is based on an earlier implementation by Motorola that incorpo-
rated a tin-oxide, thin-film sense resistor over a polysilicon resistive heater [34]. The
role of the heater is to maintain the sensor at an operating temperature between 100°