Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 42

etch processes used. Other benefits of surface micromachining are that a large
variety of structural, sacrificial, and etchant combinations can be used, and
some are compatible with microelectronics devices to enable integrated MEMS
devices. Surface micromachining frequently exploits the deposition character-
istics of thin films such as conformal coverage using low-pressure chemical
vapor deposition (LPCVD). Lastly, surface micromachining uses single-sided
wafer processing and is relatively simple. This allows higher integration den-
sity and lower resultant per die cost compared to bulk micromachining.
One of the disadvantages of surface micromachining is that the mechani-
cal properties of LPCVD structural thin-films are usually unknown and
must be measured. Also it is common for these types of films to have a high
state of residual stress, frequently necessitating a high-temperature anneal to
reduce residual stress in the structural layer. Also, the reproducibility of the
mechanical properties in these films can be difficult to achieve. Additionally,
the release of the structural layer can be difficult owing to a stiction effect
whereby the structural layer is pulled down and stuck to the underlying
substrate by capillary forces during release. Stiction can also occur in use,
and an anti-stiction coating material may be needed.
The most commonly used surface micromachining process and material
combination is a phosphosilicate glass (PSG) sacrificial layer, a doped poly-
silicon structural layer, and the use of hydrofluoric acid as the etchant to
FIGuRE 3.7
SEM of a polysilicon micromotor fabricated using a surface micromachining process.