Page 82 - An Introduction to Microelectromechanical Systems Engineering

Advanced Process Tools 61

(MPa) 10 Heat Supercritical

pressure 8 6 Liquid region
Critical point
Absolute 4 2 Fill Vent (31°C, 7.4 MPa)
Gas
0
0 10 20 30 40 50
Temperature (ºC)
Figure 3.21 The path taken on the carbon dioxide pressure-temperature phase diagram during
supercritical drying.

formation of a meniscus and the corresponding stiction. Finally, the dried wafer is
removed from the chamber.

Self-Assembled Monolayers

The stiction problem during drying that was presented earlier can also be avoided if
a hydrophobic layer is coated onto the structure. One method of doing this is the
application of a self-assembled monolayer (SAM) [25]. The SAM precursors used
for this application are straight-chain hydrocarbons, such as octadecyltrichlo-
rosilane (OTS, CH (CH ) SiCl ), with a chemical group at one end that adheres to
3 2 17 3
silicon, silicon-dioxide, and silicon-nitride surfaces. These head groups naturally
pack tightly onto the surface and crosslink, leaving the tails sticking straight up
away from the surface. The coating self-limits at one molecule of thickness and is
hydrophobic.
In a SAM-coating process, the structures are released and rinsed in water as
usual, then soaked in a solvent miscible with water. The wafer may be moved to an
intermediate solvent compatible with the first solvent and the subsequent SAM sol-
vent. The wafer is then placed in a solution containing the SAM precursor and held
for a few minutes, during which the coating occurs. Finally, it is rinsed and dried,
which may be done on a hot plate or under a heat lamp. Due to the hydrophobicity
of the SAM-coated surface, the contact angle changes, and the water does not pull
compliant structures down to the substrate. An added benefit is that if the structure
ever does touch down during operation, it will not stick, as it might otherwise do
without the coating. SAM coatings have also been studied as a dry lubricant and
found to prolong the life of micromachined parts sliding in contact, eventually
wearing out [25]. SAMs decompose at high temperatures (~350°C).

SU-8 Photosensitive Epoxy
Epoxies have been in use for decades for joining sections of material together and as
a structural component of composites. Some epoxies are formulated to be sensitive
to ultraviolet light, allowing photolithographic patterning. SU-8 is a negative-acting
photosensitive epoxy intended for use in fabricating microstructures. Originally
developed by International Business Machines Corp., it is commercially produced
under license by two companies, MicroChem Corp. of Newton, Massachusetts, and
SOTEC Microsystems of Renens, Switzerland.