Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 143

structure. A very high aspect ratio leads to the pairing effect, in which two parallel structures attach to
each other, while a very low aspect ratio leads to sagging of noncontact regions, which makes further
steps of soft lithography impossible. The recommended aspect ratios for PDMS structures are between
0.2 and 2 [86] .
There are different techniques to transfer the pattern on this elastomeric stamp: microcontact
printing and replica micromolding [86] . In many applications, the elastomeric PDMS part can be used
directly as a microfluidic device with microchannels on it. Soft lithography is based on an elastomeric
stamp with patterned relief structures on its surface. There are two basic techniques for transferring the
micropatterns: microcontact printing and replica molding. In many polymeric devices, the elastomeric
part can be used directly as the functional material. The fabrication process starts with the fabrication
of a master using established technologies such as DRIE in silicon or photolithography of a thick-resist
film such as SU-8. Polydimethylsiloxane (PDMS) is mixed from two prepolymers. The weight ratio of
the base and the curing agent could be 10:1 or 5:1. Next, the PDMS mixture is poured into the master.
PDMS is then cured at relatively low temperature from 60 Cto80 C for several hours. After peeling
off and having surface treatment with low-temperature oxygen plasma, the structured PDMS
membrane can be brought into contact with clean glass, silica, or another piece of surface-activated
PDMS. The bond can withstand pressures up to five bars. Three-dimensional structures can be formed
by lamination of many PDMS sheets. In this case, methanol helps to promote both bonding and self-
alignment. The surface tension at superimposed holes in the PDMS sheets self-aligns them. Methanol
prevents instant bonding between two PDMS sheets after plasma treatment. After evaporating
methanol on a hot plate, the laminated stack is bonded.
The master for a PDMS device can be fabricated with conventional silicon-based micromachining
technologies. Figure 4.14 depicts the basic steps of the fabrication of a PDMS part. The silicon master
is silanized by exposure to the vapor of CF 3 (CF 2 ) 6 (CH 2 ) 2 SiCl 3 for about 30 minutes [85] . The pre-
polymer is coated on the silicon or glass master. After curing in an elevated temperature, the cured
PDMS layer can be peeled off and is ready for the use in the subsequent steps.
The master can also be fabricated with SU-8 [87] . The PDMS part is used directly as structural
material. The PDMS device is bonded to a glass plate after oxidizing their surfaces with oxygen
plasma. In a similar approach, three-dimensional structures are fabricated by the lamination of
different structured PDMS layers. Fluidic interconnects are embedded directly in the PDMS
device.
FIGURE 4.14
Fabrication of PDMS stamps: (a) DRIE of silicon master; (b) coating; and (c) release.