Page 90 - An Introduction to Microelectromechanical Systems Engineering

Combining the Tools—Examples of Commercial Processes 69

All of these processes are compatible with CMOS fabrication and hence allow
the integration of electronic circuits alongside microelectromechanical devices.
Successful integration requires that circuit and structural processing steps do not
adversely affect each other; for example, once aluminum is on the wafer in contact
with silicon, it cannot be heated above 400–450°C. As will be observed, a key dis-
tinguishing feature among the processes is the release step that frees the microstruc-
tures in selected locations from the underlying substrate.

Polysilicon Surface Micromachining
In surface micromachining, thin layers of a material—most commonly polysili-
con—form the structural elements. Originating at the University of California at
Berkeley, polysilicon surface micromachining is an established manufacturing
process at Analog Devices, Inc., of Norwood, Massachusetts, MEMSCAP (formerly
Cronos Integrated Microsystems, Inc.) of Research Triangle Park, North Carolina,
and Robert Bosch GmbH of Stuttgart, Germany. Bustillo et al. present a compre-
hensive review of surface micromachining in a special issue of the Proceedings of the
IEEE on MEMS [35].
Polysilicon surface micromachining combines a stack of patterned polysilicon
thin films with alternating patterned layers of sacrificial silicon dioxide. A single
layer of structural polysilicon is sufficient to make many useful devices, and up to
five polysilicon and five oxide layers are a standard process at Sandia National
Laboratories of Albuquerque, New Mexico. The polysilicon is deposited using
LPCVD, followed by a high-temperature anneal (>900ºC) to relieve mechanical
stress. The silicon dioxide is deposited using LPCVD or PECVD and is often doped
with phosphorus [phosphosilicate glass (PSG)] to increase the etch rate in hydro-
fluoric acid. In the Sandia process, the polysilicon and silicon dioxide layers are each
2 µm thick. By contrast, Robert Bosch uses a process with 10-µm-thick polysilicon
grown by epitaxy over silicon dioxide.
Each of the layers in the stack is lithographically patterned and etched before
the next layer is deposited in order to form the appropriate shapes and to make pro-
visions for anchor points to the substrate (see Figure 3.29). The final release step
consists of etching the silicon dioxide (hence the sacrificial term) in a hydrofluoric
acid solution to free the polysilicon plates and beams, thus allowing motion in the
plane of and perpendicular to the substrate. Small holes are usually added to large
plates to allow the sacrificial etchant access for faster release. To avoid sticking of
compliant structures when drying the wafer, supercritical drying or a self-assembled
monolayer is often used.
Gears, micromotors, beams, simple as well as hinged plates, and a number of
other structures have been demonstrated, though primarily accelerometers and
yaw-rate sensors are currently in high-volume production. Surface micromachining
offers significant flexibility to fabricate planar structures one layer at a time, but
their thinness limits the applications to those benefiting from essentially two-
dimensional forms.
Polysilicon is a useful structural material because integrated circuit processes
already exist for depositing and etching it and because its thermal coefficient of
expansion is well matched to that of the silicon substrate. However, surface
micromachining is not limited to the materials just described. Many systems of