Page 57 - An Introduction to Microelectromechanical Systems Engineering

36 Processes for Micromachining

RF planar sputtering and ion-beam methods work for the deposition of both con-
ducting and insulating materials, such as silicon dioxide. In planar and cylindrical
magnetron sputtering, an externally applied magnetic field increases the ion density
near the target, thus raising the deposition rates. Typical deposition rates are 0.1–0.3
µm/min, and can be as high as 1 µm/min for aluminum in certain sputtering tools.
Nearly any inorganic material can be sputtered. Sputtering is a favored method
in the MEMS community for the deposition at low temperatures (<150ºC) of thin
metal films such as aluminum, titanium, chromium, platinum, palladium, tungsten,
Al/Si and Ti/W alloys, amorphous silicon, insulators including glass, and piezoelec-
tric ceramics (e.g., PZT and ZnO). In a variation known as reactive sputtering, a
reactive gas such as nitrogen or oxygen is added during the sputtering of a metal to
form compounds such as titanium nitride or titanium dioxide.
The directional randomness of the sputtering process, provided that the target
size is larger than the wafer, results in good step coverage—the uniformity of the thin
film over a geometrical step—though some thinning occurs near corners. The depos-
ited film has a very fine granular structure and is usually under stress [2]. The stress
levels vary with the sputter power and chamber pressure during deposition, with
tensile stress occurring at lower power and higher pressure, and compressive stress
occurring at higher power and lower pressure. The transition between the compres-
sive and tensile regimes is often sharp (over a few tenths of a Pa), making the
crossover—an ideal point for zero-stress deposition—difficult to control. Heating the
substrate during deposition is sometimes used to reduce film stress.
Many metals, particularly inert ones such as gold, silver, and platinum, do not
adhere well to silicon, silicon dioxide, or silicon nitride, peeling off immediately
after deposition or during later handling. A thin (5- to 20-nm) adhesion layer, which
bonds to both the underlying material and the metal over it, enables the inert metal
to stick. The most common adhesion layers are Cr, Ti, and Ti/W alloy. The inert
metal must be deposited on the adhesion layer without breaking the vacuum, as oxy-
gen in the air would immediately oxidize the adhesion layer, rendering it useless.

Evaporation
Evaporation involves the heating of a source material to a high temperature, gener-
ating a vapor that condenses on a substrate to form a film. Nearly any element (e.g.,
Al, Si, Ti, Au), including many high-melting-point (refractory) metals and com-
pounds (e.g., Cr, Mo, Ta, Pd, Pt, Ni/Cr, Al O ), can be evaporated. Deposited films
2 3
comprised of more than one element may not have the same composition as the
source material because the evaporation rates may not correspond to the stoichio-
metry of the source.
Evaporation is performed in a vacuum chamber with the background pressure
typically below 10 −4 Pa to avoid contaminating the film. Target heating can be done
resistively by passing an electrical current through a tungsten filament, strip, or boat
holding the desired material. Heating can alternatively be done by scanning a high-
voltage (e.g., 10-kV) electron beam (e-beam) over the source material. In this case,
the carrier is usually made of tungsten, graphite, alumina, or copper (copper is an
excellent thermal conductor, but it can only be used if it is not wetted by the molten
source). Resistive evaporation is simple but can result in spreading impurities or
other contaminants present in the filament. E-beam evaporation, by contrast, can