Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 68

Wheatstone bridge). Freescale has used two types of piezoresistive trans-
ducers, the “X-ducer ™ ” and the “Picture Frame.” The original “X-ducer”
design resembles an “X” located at the edge of the pressure-deflecting mem-
brane (Figure 3.27a). The X-ducer design had the benefit of reducing the off-
set distribution that is an undesirable attribute of some Wheatstone bridges.
The process technology has been improved over the years, tracking recent
developments in micromachining and design, and Freescale now uses what
is called a “picture frame” piezoresistor configuration that allows approx-
imately 40% greater output signal than the X-ducer design (Figure 3.27b).
Other process improvements include using an electrochemical etch stop to
precisely control the pressure-sensing membrane thickness and reduction of
the area consumed by the sensor [53].
The fabrication process sequence begins with a single crystal p-type silicon
substrate (Figure 3.28) [54]. A diffusion to create an n+ region is performed,
and this will be used to form the buried layer for the bipolar transistor
devices. A 15-micron-thick layer of n-type silicon is epitaxially grown on
the surface of the wafer, and this is followed by a deep diffusion to create
p+ regions to electrically isolate the bipolar transistors (Figures 3.28a, b).
(a)
(b)
FIGuRE 3.27 (See color insert.)
(a) Optical photograph of top surface of Freescale Semiconductor pressure sensor that employs
the original “X-ducer.” An “X” has been added to show the transducer layout. (b) The newer
“picture frame” piezoresistor configuration. (Reprinted with permission, copyright Freescale
Semiconductor, Inc.)