How Do We Make Small Things? - Introduction to BioMEMS - page 20

George Whitesides is also a scientist with a deep appreciation for imagery, photogra-
phy, and writing (as testiied by the overall style and clarity of his papers—Whitesides' stu-
dents have shared online his cleverly simple and very helpful “manual of style”). Together
with his long-time collaborator and pioneer scientiic photographer, Felice Frankel, he
has published two beautiful photography topics on his experiments, On the Surface of
hings (2008) and No Small Matter (2009). he irst is arguably the most delightful topic
on microfabrication ever published.
[ Excerpt adapted from Wikipedia. ]

Sot lithography difers fundamentally from photolithography in the way surfaces are pat-
terned, as illustrated in Figure 1.18 . Two classes of sot lithographic patterning strategies can be
distinguished depending on where the material is selectively deposited: in the areas where the
stamp contacts the surface (“ microstamping ” or “ microcontact printing ”) or where it does not
contact the surface (“ microluidic patterning ”). hese techniques will be reviewed in detail later.
Sot lithography has gained widespread popularity among MEMS researchers because the
methods are inexpensive and straightforward once the mold is fabricated: the replication pro-
cedure requires neither expertise nor equipment—unlike other micromolding methods—and
PDMS is commercially available by the gallon at low cost. In addition, PDMS has particu-
larly interesting optical, surface, and mechanical properties that make it an ideal material for
many applications in biomedicine for which traditional MEMS materials (silicon and glass) are
unsuited. his means that the devices can be disseminated among a wider community than was
possible before. hus, sot lithography represents an enabling technology.

1.6.1 Basics of Soft Lithography
he starting procedure in sot lithography is the microfabrication of a master mold. he next
step, which is the molding of PDMS, is deceptively simple, as shown in Figure 1.19 . A PDMS
prepolymer is prepared as a ~10:1 ratio by weight mixture of two components, the monomer
solution and the cross-linker, and is poured over the master. Because the monomer solution
is viscous, both mixing of the two components as well as the pouring can cause air bubbles to
be trapped at the corners of small features, so it is standard practice to place the prepolymer-
covered mold in a house vacuum until the bubbles rise to the surface of the liquid and pop. he
cross-linking reaction—which is slow at room temperature but can be sped up by heating (e.g.,
~4 hours at 65°C)—causes the PDMS to solidify to a material that feels and looks very much like
a transparent rubber. Ater curing, the molded block is manually peeled of.
As opposed to other micromolding methods, the fact that the replication starts from a room
temperature luid that has time to conform to the smallest features ensures that the replication is

Photolithography

Microstamping Microfluidic patterning

Mask

I. Expose I. Ink I. Fill

Microchannels

PDMS stamp

Photoresist

2. Develop

2. Transfer 2. Remove microchannels

FIGURE 1.18 Photolithographic.versus.soft.lithographic.patterning.methods.