Page 292 - The Mechatronics Handbook

that the spherical-conical micromotors develop higher electromagnetic torque compared with the con-
ventional design. In addition, improved cooling, reduced undesirable torques components, as well as
increased ruggedness and robustness contribute to the viability of the proposed solution. Thus, using
the classifier paradigm, novel microdevices with superior performance can be devised.

14.3 MEMS Fabrication

Microelectromechanics, which integrates micromechanics and microelectronics, requires affordable, low-
cost, high-yield fabrication technologies which allow one to fabricate 3-D microscale structures and
devices. Micromachining is a key fabrication technology for microscale structures, devices, and MEMS.
Microelectromechanical systems fabrication technologies fall into three broad categories: bulk machining,
surface machining, and LIGA (LIGA-like) techniques [1–3].

Bulk Micromachining

Bulk and surface micromachining are based on the modified CMOS and specifically designed microma-
chining processes. Bulk micromachining of silicon uses wet and dry etching techniques in conjunction
with etch masks and etch-stop-layers to develop microstructures from the silicon substrate. Microstruc-
tures are fabricated by etching areas of the silicon substrate to release the desired 3-D microstructures.
The anisotropic and isotropic wet etching processes, as well as concentration dependent etching techniques,
are widely used in bulk micromachining. The microstructures are formed by etching away the bulk of
the silicon wafer to fabricate the desired 3-D structures. Bulk machining with its crystallographic and
dopant-dependent etch processes, when combined with wafer-to-wafer bonding, produces complex 3-D
microstructures with the desired geometry. Through bulk micromachining, one fabricates microstruc-
tures by etching deeply into the silicon wafer. There are several ways to etch the silicon wafer. The
anisotropic etching uses etchants that etch different crystallographic directions at different rates. Through
anisotropic etching, 3-D structures (cons, pyramids, cubes, and channels into the surface of the silicon
wafer) are fabricated. In contrast, the isotropic etching etches all directions in the silicon wafer at same
(or close) rate, and, therefore, hemisphere and cylinder structures can be made. Deep reactive ion etching
uses plasma to etch straight walled structures (cubes, rectangular, triangular, etc.).

Surface Micromachining
Surface micromachining has become the major fabrication technology in recent years because complex
3-D microscale structures and devices can be fabricated. Surface micromachining with single-crystal
silicon, polysilicon, silicon nitride, silicon oxide, and silicon dioxide (as structural and sacrificial materials
which deposited and etched) is widely used to fabricate microscale structures and devices on the surface
of a silicon wafer. This affordable low-cost high-yield technology is integrated with IC fabrication
processes guaranteeing the needed microstructures-IC fabrication compatibility. The techniques for
depositing and patterning thin films are used to produce complex microstructures and microdevices on
the surface of silicon wafers (surface silicon micromachining) or on the surface of other substrates. Surface
micromachining technology allows one to fabricate the structure as layers of thin films. This technology
guarantees the fabrication of 3-D microdevices with high accuracy, and the surface micromachining can
be called a thin film process. Each thin film is usually limited to thickness up to 5 µm, which leads to
fabrication of high-performance planar-type microscale structures and devices. The advantage of surface
micromachining is the use of standard CMOS fabrication processes and facilities, as well as compliance
with ICs. Therefore, this technology is widely used to manufacture microscale actuators and sensors
(microdevices).
Surface micromachining is based on the application of sacrificial (temporary) layers that are used to
maintain subsequent layers and are removed to reveal (release) fabricated (released or suspended) micro-
structures. This technology was first demonstrated for ICs and applied to fabricate microstructures in
the 80s. On the surface of a silicon wafer, thin layers of structural and sacrificial materials are deposited

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