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.