Page 146 - MEMS Mechanical Sensors

6.5 MEMS Technology Pressure Sensors 135

Resistors
Diaphragm

Pressure
Figure 6.17 Fusion bonded piezoresistive pressure sensor.

thickness, or as an electrical insulator, enabling higher temperature operation
[35–37]. Ultimate high-temperature operation of piezoresistive pressure sensors has
been developed using micromachined silicon carbide [38]. The diaphragms are
etched by a photoelectrochemical process in a diluted HF etchant. A prototype
device has been demonstrated operating at 600°C [39] and in a dynamic sensing
application on a gas turbine engine [40]. Finally, silicon nitride diaphragms have
been realized by bulk wet anisotropic etching. The nitride membrane is formed by
wet etching through the silicon entirely from the back of the wafer. The wet etch
stops upon reaching the nitride, and the piezoresistors are protected due to the
high-dose boron implant used to define them [41]. Nitride membranes are stronger
than their silicon counterpart but may suffer from in-built stresses due to the
deposition process.
The need to reduce the size of devices, and therefore the cost of production, has
led to the use of surface micromachining to fabricate the mechanical sensing ele-
ment and resistors [42]. In addition to reduced size, surface micromachining is more
compatible with IC fabrication technology. It is a flexible fabrication approach ena-
bling the diaphragm to be fabricated from a range of deposited materials such as
polysilicon [43] and silicon nitride [44]. In both cases an underlying sacrificial layer
is removed. For the polysilicon diaphragm the sacrificial material is silicon dioxide
and a wet etch is used to remove it. The nitride membrane uses a polysilicon sacrifi-
cial material. In both cases the lateral dimensions of the membrane are defined by
previous patterning of the oxide, or doping of the polysilicon, respectively. Both
devices use polysilicon resistors to sense diaphragm deflections. Both are absolute
pressure sensors since a CVD process is used to deposit nitride to seal sacrificial
etch holes. The vacuum used in the CVD process is therefore trapped in the
sealed volume under the diaphragm. A cross-section of each device is shown in
Figure 6.18. Other examples of surface-micromachined piezoresistive pressure sen-
sors include a cardiovascular pressure sensor for measurement of blood pressure
inside coronary arteries [45]. This is based on a square polysilicon diaphragm with
edge lengths of 103 µm with a vacuum-sealed cavity underneath. One polysilicon
resistor is used to detect the deflection of the diaphragm, and a second dummy resis-
tor is used for temperature compensation.
As discussed in the earlier analysis, the boundary conditions of the diaphragm
will play an important role in the behavior of the diaphragm. With surface