Page 124 - An Introduction to Microelectromechanical Systems Engineering

Sensors and Analysis Systems 103

Capacitive Deep-Etched Micromachined Accelerometer

The DRIE accelerometer developed at GE NovaSensor of Fremont, California,
shares its basic comb structure design with the ADXL and Bosch accelerometers. It
consists of a set of fingers attached to a central backbone plate, itself suspended by
two folded springs (see Figure 4.19). Two sets of stationary fingers attached directly
to the substrate complete the capacitive half bridge. The design, however, adds a
few improvements. By taking advantage of the third dimension and using structures
50 to 100 µm deep, the sensor gains a larger inertial mass, up to 100 µg, as well as a
larger capacitance, up to 5 pF. The relatively large mass reduces mechanical
Brownian noise and increases resolution. The high aspect ratio of the spring practi-
cally eliminates the sensitivity to z-axis accelerations (out of the plane of the die).
Fabrication follows the SFB-DRIE process introduced in Chapter 3.
The sensor, described by van Drieënhuizen et al. [23], uses a 60-µm-thick comb
structure for a total capacitance of 3 pF, an inertial mass of 43 µg, a resonant
frequency of 3.1 kHz, and an open-loop mechanical sensitivity of 1.6 fF/G. The
corresponding mechanical noise is about 10 µG Hz, significantly less than for a
surface-micromachined sensor. The read-out circuitry first converts changes in
capacitance into frequency. This is accomplished by inserting the two variable
capacitors into separate oscillating circuits whose output frequencies are directly
proportional to the capacitance. A phase detector compares the two output frequen-
cies and converts the difference into a voltage. The circuit then amplifies the signal
before feeding it back to a set of actuation electrodes for force balancing. These
electrodes may be distinct from the sense electrodes. Filters set the closed-loop
bandwidth to 1 kHz. The overall sensitivity is 700 mV/G for a ±5G device. Early
prototypes had a dynamic range of 44 dB limited by electronic 1/f noise in the
CMOS circuitry. Recent prototypes with newer implementations of the electronic
read-out circuits demonstrated a dynamic range approaching 70 dB over the
1-kHz bandwidth. The SFB-DRIE process is fully compatible with the integration

1mm
Folded
spring

Bondpad

Capacitive Trench
sense plates isolation

Figure 4.19 Scanning-electron micrograph of a DRIE accelerometer using 60-µm-thick comb
structures. (Courtesy of: GE NovaSensor of Fremont, California.)