Page 79 - An Introduction to Microelectromechanical Systems Engineering

58 Processes for Micromachining

planarization method yielding a surface roughness less than 1 nm over large dimen-
sions, but it is slow with removal rates less than 100 nm/min compared to 1 µm/min
for standard polishing.

Sol-Gel Deposition Methods
A sol-gel process is a chemical reaction between solid particles in colloidal suspen-
sion within a fluid (a sol) to form a gelatinous network (a gel) that can be
transformed to solid phase upon removal of the solvent. Sol-gel is not a unique
process, but rather represents a broad type of processes capable of forming glasses
and ceramics in a multitude of shapes starting from basic chemical precursors. A
widespread application of sol-gel processing is in the coating of surfaces with optical
absorption or index-graded antireflective materials. It has been used in research
laboratories to deposit thick piezoelectric films on silicon substrates.
A sol-gel process starts by dissolving appropriate chemical precursors in a liquid
to form a sol (see Figure 3.18). After a time the sol goes through its gel point, the
point at which the sol undergoes polymerization, to change it from a viscous liquid
state to a gelatinous network. Both sol formation and gelation are low-temperature
steps. The gel is then formed into a solid shape (e.g., fiber or lens) or applied as a film
coating on a substrate by spinning, dipping, or spraying. For example, TEOS in
water can be converted into a silica gel by hydrolysis and condensation using
hydrochloric acid as a catalyst. Drying and sintering at an elevated temperature
(200°–600°C) results in the transition of the gel to glass and then densification to
silicon dioxide [22]. Silicon nitride, alumina, and piezoelectric PZT can also be
deposited by sol-gel methods.

Electroplating and Molding
Electroplating is a well-established industrial method that has been adapted in
micromachining technology to the patterned deposition of metal films. A variety of
metals including gold, copper, nickel, and nickel-iron (Permalloy™) have been
electroplated on silicon substrates coated with a suitable thin metal plating base.
Table 3.4 lists some plating solutions.

Sol Wet gel
Precursors
Gelation

Drying

Solids

Coatings Dry gel

Forming
and sintering
Figure 3.18 Basic flow of a sol-gel process.