Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 43

FIGuRE 3.8
SEM of a polysilicon resonator structure fabricated using a surface micromachining process.
remove the PSG sacrificial layer and release the device. This type of sur-
face micromachining process is employed in the widely used MEMSCap
MUMPS process technology [22]. Figure 3.7 and Figure 3.8 are SEMs of two
surface micromachined polysilicon MEMS devices.
Another variation of the surface micromachining process is to use a metal
structural layer, a polymer layer as the sacrificial layer, and an O 2 plasma as the
etchant. The advantage of this process is that the temperature of the sacrificial
and the structural layer depositions are sufficiently low so as not to degrade
any microelectronics in the underlying silicon substrate, for processes that inte-
grate MEMS with electronics. Also, since the sacrificial layer is removed with-
out immersion in a liquid, problems associated with stiction during release are
avoided. A process similar to this is used to produce the Texas Instruments
digital light processor (DLP) device used in projection systems [22].
Wafer Bonding
Wafer bonding is a micromachining method that is analogous to welding in
the macroscale world and involves the joining of two (or more) wafers together
to create a multi-wafer stack. There are three basic types of wafer bonding,
including direct or fusion bonding; field-assisted or anodic bonding; and