ABSTRACT
Surface microengineering (Linder et al., 1992) is similar to con-
ventional semiconductor fabrication in that a series of thin films
are deposited, lithographically patterned and selectively etched. In
the case of microelectromechanical systems (MEMS), a typical thin-
film process is to deposit polysilicon layers (1-5 μm) onto a silicon
dioxide sacrificial layer (1-5 μm). The silicon dioxide layer can then
be selectively removed to leave a freestanding structure. Commercial
foundry processes exist with up to five layers of polysilicon
(see Chapter 11). Devices manufactured exclusively with surface
microengineering are often referred to as ‘complementary-metal-
oxide semiconductor (CMOS)-MEMS’. Surface-microengineered de-
vices are limited to near-planar options where the requirement
is to detect extremely low capacitance changes (e.g., 10−1510−18 F) in an MEMS accelerometer or gyro. Compared with a surface-microengineered version, a high-aspect-ratio (HAR) sensor
fabricated with deep microstructures will offer much higher signal
levels (in proportion to their relative depth) and hence will require
less demanding design.
