Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 134

the most difficult process, because SU-8 film becomes highly crosslinked after exposure and PEB.
Etching with acid solutions, RIE, and laser ablation [59] are some of the methods for removing SU-8.
Because of its simple processes and the relatively good mechanical properties, SU-8 is used as the
structural material for many microfluidic applications. There are many fabrication examples where
SU-8 was used as spacer or directly as channel material.
The simplest technique to form a microchannel with SU-8 is using the crosslinked SU-8 structure
to define the channel's sidewall. While the bottom wall is the glass or silicon substrate, the channel can
be covered and sealed by another silicon and glass plate. Ayliffe et al. reported an LIGA-like [60]
( Fig. 4.9 (a)). Starting with a glass wafer as substrate material, a metal seed layer was deposited
( Fig. 4.9 (a,1)). Subsequently, SU-8 is spin-coated and structured ( Fig. 4.9 (a,2)). This high-aspect-ratio
SU-8 structure is used as a mold for electroplating of gold or other metals ( Fig. 4.9 (a,3)). In the next
step, SU-8 is etched in oxygen plasma using an aluminum mask to form the actual microchannel
( Fig. 4.9 (a,4) and (a,5)). Finally, a glass plate covers the structure using adhesive bonding.
In the above example, microchannels are etched by oxygen plasma. An alternative is
patterning by photolithography and development. Figure 4.9 (b) shows a fabrication process that
uses two SU-8 layers to form a microchannel with a complex cross-section [61] . To start with, the
first SU-8 layer is coated and exposed with the first mask ( Fig. 4.9 (b1)). The next layer is spin-
coated on top of the first layer ( Fig. 4.9 (b2)). Since the second exposure may affect the structure
defined by the first mask, the mask for the second layer should cover completely the unexposed
areas of the first layer to avoid double exposure ( Fig. 4.9 (b3)). After exposure of the second
layer, the two layers are developed together to form the T-shape microchannel. The channel is
then covered by a glass plate, which has a thin unexposed SU-8 layer as the adhesive layer
( Fig. 4.9 (b4)). This thin adhesive layer is crosslinked by a blanket exposure through the glass
plate. To form an optically transparent device, the silicon substrate can be etched away to yield an
optically transparent device.
Three-dimensional structures can be constructed by multilayer exposure and embedded mask, as
shown in Fig. 4.9 (a). As mentioned above, the problem of multilayer exposure is that the mask of the
later layer should cover completely the previous layers to protect their unexposed areas. This means
that direct fabrication of a closed structure, such as a covered channel, is not possible with conven-
tional glass masks. One solution for the double-exposure problem is the use of an embedded mask
[62,63] . The process starts with the exposure of the first SU-8 layer to form the bottom of the channel
( Fig. 4.10 (a,1)).
After the second layer is coated, the embedded mask is deposited and structured. A thin metal layer,
such as gold [62] , can be sputtered on the second SU-8 layer. This metal layer is patterned by common
photolithography and etching. The patterned metal layer is used as an embedded mask for the
subsequent exposure of the second SU-8 layer ( Fig. 4.10 (a,2)). A third SU-8 layer is spin-coated and
exposed to fabricate the top wall of the channel ( Fig. 4.10 (a,3)). In the final step, all three layers are
developed in a single process, resulting in a covered microchannel. The embedded mask is washed
away after the developing process ( Fig. 4.10 (a,4)). Instead of the embedded metal mask, an antire-
flection film, such as CK-6020L resist (FujiFilm Olin Inc., Japan), can be used for making covered SU-
8 microchannel [64] . The use of antireflection coating ensures that this coating and the structural SU-8
can be developed at the same time.
The penetration depth of an energy beam depends on its intensity and determines the thickness of
the crosslinked layer. A covered channel can be fabricated with selective proton writing or proton