Page 114 - An Introduction to Microelectromechanical Systems Engineering

Sensors and Analysis Systems 93

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piezoresistors is above 5 × 10 cm , or below 3 × 10 cm . Compensation over
intermediate ranges of dopant concentration requires sophisticated electronic cir-
cuits that continuously monitor the temperature of the Wheatstone bridge.
There has been recently a shift within the industry to provide the calibration
and compensation functions with specially designed application-specific integrated
circuits (ASICs). The active circuits amplify the voltage output of the piezoresistive
bridge to standard CMOS voltage levels (0–5V). They also correct for tempera-
ture errors and nonlinearities. Error coefficients particular to individual sensors
are permanently stored in on-board electrically programmable memory (e.g.,
EEPROM). Most sensor manufacturers have developed their own proprietary
circuit designs, and some have even integrated the circuitry onto the pressure-
sensor chip. A few general-purpose signal conditioning integrated circuits are
commercially available; one example is the MAX1457 from Maxim Integrated
Products of Sunnyvale, California.

High-Temperature Pressure Sensors
The temperature rating of most commercially available silicon micromachined pres-
sure sensors is –40° to +125ºC, covering the automotive and military specifications.
The increased leakage current above 125ºC across the p-n junction between the dif-
fused piezoresistive element and the substrate significantly degrades performance.
Silicon-on-insulator (SOI) technology becomes very useful at elevated temperatures
because the thin silicon sense elements exist over a layer of silicon dioxide, thus
eliminating all p-n diode junctions. Adjacent silicon sense elements are isolated from
each other by shallow moat-like trenches. The dielectric isolation below the sense
elements completely eliminates the leakage current through the substrate, as long as
the applied voltages are below the breakdown voltage of the insulating oxide layer.
A high-temperature pressure sensor from GE NovaSensor utilizes SOI technol-
ogy to form thin p-type crystalline silicon piezoresistors over a thin layer of silicon
dioxide. Gold metallization and bond pads provide electrical contacts to the sense
elements (see Figure 4.11).

Figure 4.11 Photograph of an SOI-based pressure sensor rated for extended temperature
operation up to 300°C. (Courtesy of: GE NovaSensor of Fremont, California.)