Page 83 - An Introduction to Microelectromechanical Systems Engineering

62 Processes for Micromachining

SU-8 is spun onto a substrate in the same manner as photoresist. Different vis-
cosities and a range of spin speeds yield thicknesses from 0.5 to over 250 µm with a
single coating [26]. Multiple spins have been used to coat up to 1 mm. The epoxy is
then exposed, typically with a standard contact lithography system in the near UV
(350–400 nm), but x-rays or an electron beam may also be used. At wavelengths
longer than 350 nm, SU-8 has little absorption, allowing exposure through the
thickness of much thicker layers than are typically used for traditional photoresist.
During exposure, a strong acid is generated where exposed. During the post-
exposure bake, the acid initiates thermally driven crosslinking. Immersion in a
developer then removes the SU-8 that is not crosslinked. At this point, the remaining
material is suitable for many applications, but a hard bake may be performed to pro-
mote further crosslinking.
SU-8 structures are the same thickness as the original spin. Aspect ratios (ratio of
epoxy height to width) of 20:1 are regularly produced. The cured material is resis-
tant to most chemicals and is thermally stable. SU-8 has been used to form microflu-
idic channels and optical waveguides. It has also been used as the mask for thick
electroplating, although stripping the SU-8 is much more difficult than stripping
photoresist.

Photosensitive Glass

Positive-acting photosensitive glass wafers are made commercially under the trade
®
name FORTURAN by Schott Glas of Mainz, Germany, and processed by Mikro-
glas Technik AG of Mainz, Germany. FORTURAN is a lithium aluminum silicate
glass with small amounts of cerium and silver ions. The full thickness of the glass is
exposed with ultraviolet light through a mask, causing the silver ions to form atoms.
Annealing causes these atoms to aggregate into microscopic particles, which then
serve as nucleation sites for lithium metasilicate crystals. The crystallized volumes
are etched relatively rapidly in hydrofluoric acid, leaving holes through the wafer.
Up to 14 patterned or unpatterned glass wafers can be thermally bonded together,
creating complex systems of channels suitable for microfluidic applications.
Substrates 150 to 1,500 µm thick can be processed. The smallest hole that can be
formed in a 400-µm wafer is 60 µm, for an aspect ratio of seven, with a 1.5-µm toler-
ance. Sidewalls are within 2º of vertical [27].

EFAB
EFAB TM is the trade name for an electrochemical fabrication surface micro-
machining process by Microfabrica, Inc., of Burbank, California, under license from
the University of Southern California. In the EFAB process, three-dimensional struc-
tures are created by multilayer depositions of patterned metals. Photolithographic
techniques are used to deposit a patterned layer of metal (see Figure 3.22). While the
details of the process are proprietary, one could accomplish such a structure by elec-
troplating through patterned photoresist. Next, a blanket deposition of a second
metal is performed, which fills in the spaces left from the patterned deposition, as
well as coating the first metal. The structure is then planarized, leaving the entire
substrate covered by patterns of the two metals, all the same thickness. These three
steps are then repeated with different masks as many times as necessary to build the
desired structure. The definition of each layer is arbitrary with respect to the