Page 293 - The Mechatronics Handbook

Sacrificial Layer Sacrificial Layer 1. Deposition and patterning
of the sacrificial layer
Silicon Substrate

Structural Layer
Sacrificial Layer Sacrificial Layer 2. Deposition and patterning
of the structural layer
Silicon Substrate
Micromachined Structure
Structural Layer
3. Etching of the sacrificial
layer
Silicon Substrate

FIGURE 14.3 Surface micromachining.

Permanent Stator
r Magnet Windings
Bearing Flange
Stator Rotor Bearing Post Rotor Stator
Insulating Insulating
Silicon Substrate
ICs

FIGURE 14.4 Cross-section schematics for slotless permanent-magnet brushless micromotor with ICs.

and patterned. Then, the sacrificial material is removed, and a micromechanical structure or device is
fabricated. Figure 14.3 illustrates a typical process sequence of the surface micromachining fabrication
technology.
Usually, the sacrificial layer is made of silicon dioxide (SiO 2 ), phosphorous-doped silicon dioxide, or
silicon nitride (Si 3 N 4 ). The structural layers are then typically formed with polysilicon, and the sacrificial
layer is removed. In particular, after fabrication of the surface microstructures and microdevices (micro-
machines), the silicon wafer can be wet bulk etched to form cavities below the surface components, which
allows a wider range of desired motion for the device. The wet etching can be done using hydrofluoric
and buffered hydrofluoric acids, potassium hydroxide, ethylene-diamene-pyrocatecol, tetramethylam-
monium hydroxide, or sodium hydroxide. Surface micromachining technology was used to fabricate
rotational micromachines [6]. For example, heavily-phosphorous-doped polysilicon can be used to
fabricate rotors and stators, and silicon nitride can be applied as the structural material to attain electrical
insulation. The cross-section of the slotless micromotor fabricated on the silicon substrate with polysilicon
stator with deposited windings, polysilicon rotor with deposited permanent-magnets, and bearing is
illustrated in Fig. 14.4. The micromotor is controlled by the driving/sensing and controlling/processing
ICs. To fabricate micromotor and ICs on a single- or double-sided chip (which significantly enhances
the performance), similar fabrication technologies and processes are used, and the compatibility issues
are addressed and resolved. The surface micromachining processes were integrated with the CMOS
technology (e.g., similar materials, lithography, etching, and other techniques). To fabricate the integrated
MEMS, post-, mixed-, and pre-CMOS/micromachining techniques can be applied [1–3].

LIGA and LIGA-Like Technologies
There is a critical need to develop the fabrication technologies allowing one to fabricate high-aspect-
ratio microstructures. The LIGA process, which denotes Lithography–Galvanoforming–Molding (in
German words, Lithografie–Galvanik–Abformung), is capable of producing 3-D microstructures of up
to centimeter high with the aspect ratio (depth versus lateral dimension) more than 100 [2,7,8]. The
LIGA technology is based upon X-ray lithography, which guarantees shorter wavelength (in order from

©2002 CRC Press LLC