Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 70

a silicon wafer may be bonded to the backside of the sensor wafer to create
sealed pressure reference cavities for each sensor device (Figure 3.28g). This
would be used for implementing an absolute pressure sensor. Wafer bond-
ing is performed using a glass frit layer between the two substrates.
From a fabrication standpoint, the Freescale pressure sensor example has
several very interesting attributes. First, this technology is unusual in that
it integrates a bipolar transistor microelectronics process technology with
a MEMS bulk micromachining process technology. The bipolar transistors
form amplifiers that convert the millivolt-level transducer output into a volt-
level device output. Nearly all other MEMS-integrated process technologies
that have been demonstrated merge CMOS with MEMS devices. Second, con-
trol over the mechanical dimensions of MEMS devices made by bulk micro-
machining are typically not very precise; however, the Freescale technology
employs an electrochemical etch stop that enables precise control of the mem-
brane thickness, which is extremely important for determining the mechani-
cal stiffness of the pressure sensor membrane (i.e., the amount of membrane
deflection that is to result from a certain level of pressure loading). This etch
stop layer is an epitaxially grown n-type layer that is reverse-biased during
the etching; when the etchant solution reaches this layer, the etch terminates.
This same epitaxial layer in the process sequence is also used in the fabrication
of the bipolar transistors. Third, the microelectronics is fabricated first, and
the MEMS are subsequently fabricated. This is possible since all of the MEMS
processing steps subsequent to the microelectronics fabrication are performed
at relatively low processing temperatures. Fourth, the process uses <100> ori-
ented silicon wafers to enable the micromachining steps to be done. Normally,
<111> oriented wafers are used for the fabrication of bipolar transistor devices,
and therefore Freescale needed to develop a special bipolar transistor process
for this substrate orientation. Fifth, the device wafer can be bonded to another
wafer to form a sealed reference cavity for implementing an absolute pressure
transducer, or it can be left “as is” to implement a differential pressure trans-
ducer without any major changes to the process sequence. Last, the chrome
silicide resistors enable the circuits and sensors to be easily trimmed for cali-
bration, thereby allowing any offsets to be inexpensively eliminated.
The major disadvantages of this process are that it employs wet anisotro-
pic bulk micromachining to implement the pressure sensor, and therefore a
large amount of die area is consumed by the sidewalls of the exposed crys-
tallographic planes in the silicon substrate. This is costly compared to the
area that would be used to implement a surface-micromachined sensor hav-
ing the same membrane dimensions. Also, the wet-etch process must expose
only the back of the wafer to etchant; this requires specialized etch fixturing.
Finally, bipolar transistors consume large amounts of power compared to
CMOS electronics, and thus the Freescale integrated MEMS pressure sensor
has higher power consumption levels than some other technologies. The
bipolar circuitry cannot be used to form complex digital circuits, so this fab-
rication process is limited to analog devices.