Page 122 - An Introduction to Microelectromechanical Systems Engineering

Sensors and Analysis Systems 101

Capacitive Surface Micromachined Accelerometer

Surface micromachining emerged in the late 1980s as a perceived low-cost alterna-
tive for accelerometers aimed primarily at automotive applications. Both Robert
Bosch GmbH of Stuttgart, Germany, and Analog Devices, Inc., of Norwood, Mas-
sachusetts, offer surface micromachined accelerometers, but it is the latter company
that benefited from wide publicity to their ADXL product family [21]. The Bosch
sensor [22] is incorporated in the Mercedes Benz family of luxury automobiles. The
ADXL parts are used on board Ford, General Motors, and other vehicles, as well as
inside joysticks for computer games. The surface micromachining fabrication
sequence, illustrated in Chapter 3, is fundamentally similar to both sensors, though
the Bosch device uses a thicker (10-µm) polysilicon structural element.
Unlike most bulk-micromachined parts, surface-micromachined accelerometers
incorporate a suspended comb-like structure whose primary axis of sensitivity lies
in the plane of the die. This is often referred to as an x-axis (or y-axis) type of device,
as opposed to z-axis sensors where the sense axis is orthogonal to the plane of the
die. However, due to the relative thinness of their structural elements, surface
micromachined accelerometers suffer from sensitivity to accelerations out of the
plane of the die (z-axis). Shocks along this direction can cause catastrophic failures.
The ADXL device [21] consists of three sets of 2-µm-thick polysilicon finger-like
electrodes (see Figure 4.18). Two sets are anchored to the substrate and are
stationary. They form the upper and lower electrode plates of a differential capaci-
tance system, respectively. The third set has the appearance of a two-sided comb
whose fingers are interlaced with the fingers of the first two sets. It is suspended
approximately 1 µm over the surface by means of two long, folded polysilicon beams
acting as suspension springs. It also forms the common middle and displaceable

Stationary polysilicon fingers
Spring

Displacement
Inertial mass

C 1
C 2
Anchor to
substrate

Figure 4.18 Illustration of the basic structure of the ADXL family of surface micromachined accel-
erometers. A comb-like structure suspended from springs forms the inertial mass. Displacements of
the mass are measured capacitively with respect to two sets of stationary finger-like electrodes.
(After: ADXL data sheets and application notes of Analog Devices, Inc., of Norwood,
Massachusetts.)