Page 119 - An Introduction to Microelectromechanical Systems Engineering

98 MEM Structures and Systems in Industrial and Automotive Applications

reach a dynamic peak of 10,000G! In addition to achieving a large impact, the drop
test excites the various modes of resonance that are liable to cause catastrophic
failure.
While many companies offer micromachined acceleration sensor products, a
representative set of only four accelerometers follows next, each unique in its design
and fabrication.

Piezoresistive Bulk Micromachined Accelerometer
Until only recently, piezoresistive-type acceleration sensors were widely available.
Many companies, including GE NovaSensor of Fremont, California, and IC Sensors
of San Jose, California (a division of Measurement Specialties, Inc., of Fairfield,
New Jersey), offered products using anisotropically etched silicon inertial mass and
diffused piezoresistive sense elements. But these products were retired because they
could not meet the aggressive pricing requirements of the automotive industry. The
product introduction in 1996 by Endevco Corp., of San Juan Capistrano, Califor-
nia, indicates that piezoresistive accelerometers remain in this highly competitive
market (see Figure 4.15).
The Endevco sensor consists of three substrates: a lower base; a middle core con-
taining a hinge-like spring, the inertial mass, and the sense elements; and finally a top
protective lid [19]. The inertial mass sits inside a frame suspended by the spring. Two
thin boron-doped piezoresistive elements in a Wheatstone bridge configuration span
the narrow 3.5-µm gap between the outer frame of the middle core and the inertial
mass. The piezoresistors are only 0.6 µm thick and 4.2 µm long and are thus very sen-
sitive to minute displacements of the inertial mass. The output in response to an
acceleration equal to 1G in magnitude is 25 mV for a Wheatstone bridge excitation of
10V. The thick and narrow hinge structure allows displacement within the plane of

Acceleration

Outer frame
Base

Mass

Hinge
Bondpads

{110} plane
Lid
Boron doped
piezoresistor
Figure 4.15 Illustration of a piezoresistive accelerometer from Endevco Corp., fabricated using
anisotropic etching in a {110} wafer. The middle core contains the inertial mass suspended from a
hinge. Two piezoresistive sense elements measure the deflection of the mass. The axis of sensitivity
is in the plane of the middle core. The outer frame acts as a stop mechanism to prevent excessive
accelerations from damaging the part. (After: [19].)