Page 112 - An Introduction to Microelectromechanical Systems Engineering

Sensors and Analysis Systems 91

The fabrication process of a typical pressure sensor relies mostly on steps
standard to the integrated circuit industry, with the exception of the precise forming
of the thin membrane using electrochemical etching (ECE). An n-type epitaxial layer
of silicon is grown on a p-type, {100} wafer. A thin, preferably stress-free, insulating
layer is deposited or grown on the front side of the wafer, and a protective silicon
nitride film is deposited on the back side. The piezoresistive sense elements are
formed by locally doping the silicon p-type using the masked implantation of boron,
followed by a high-temperature diffusion cycle. Etching of the insulator on the front
side provides contact openings to the underlying piezoresistors. A metal layer, typi-
cally aluminum, is then sputter deposited and patterned in the shape of electrical
conductors and bond pads. A square opening is patterned and etched in the silicon
nitride layer on the back side. Double-sided lithography ensures that the backside
square is precisely aligned to the sense elements on the front side. At this point, elec-
trical contacts are made to the p-type substrate and n-type epitaxial layer, and the
silicon is electrochemically etched from the back side in a solution of potassium
hydroxide. Naturally, the front side must be protected during the etch. One practi-
cal protection method includes coating with wax such as paraffin and clamping in a
fixture. The etch stops as soon as the p-type silicon is completely removed, and the
n-type layer is exposed. The process forms a membrane with precise thickness
defined by the epitaxial layer. Anodic bonding in vacuum of a Pyrex glass wafer on
the back side produces an absolute pressure sensor that measures the pressure on the
front side in reference to the cavity pressure (often, vacuum). For differential- or
gauge-type pressure sensors, previously drilled holes in the glass wafer provide vent
ports (see Figure 4.9).
The advent of silicon-fusion bonding in the 1980s proved very useful to the
design of bulk micromachined pressure sensors. The outward sloping of {111}
planes delineating the sensor’s frame results in an unnecessary increase in die size.
Silicon-fusion bonding allows the forming of the membrane after the etching of a
reference cavity with inward sloping {111} walls. Consequently, extremely small
pressure sensors are feasible. For example, GE NovaSensor of Fremont, California,
manufactures a sensor that is 400 µm wide, 800 µm long, and 150 µm thick, and it
fits inside the tip of a catheter (see Figure 4.10).
The fabrication of a silicon-fusion-bonded sensor begins with the etching of a
cavity in a bottom handle wafer. Silicon-fusion bonding of a p-type top wafer with
an n-type epixatial layer encapsulates and seals the cavity. Electrochemical etching
or standard polishing thins down the top bonded wafer to form a membrane of
appropriate thickness. The remaining process steps define the piezoresistive sense
elements, as well as the metal interconnects, and are similar to those used in the fab-
rication of the standard bulk micromachined pressure sensors described earlier.
Calibration and correction of error sources are necessary for the manufacture of
precision pressure sensors. A specification on accuracy of better than 1% over a
temperature range of –40° to +125°C is typical of many automotive, medical, and
industrial applications. First-order errors include zero offset (the output at no
applied pressure), uncalibrated sensitivity or span (conversion factor between input
pressure and output signal), and temperature dependence of the output signal.
Second-order effects include nonlinearities in the output response, as well as tem-
perature coefficients of some first-order error terms. Compensation and correction