Biomedical Engineering Reference
In-Depth Information
Microcontact printing utilizes the relief structures on the surface of the PDMS stamp to transfer
a pattern of SAMs to the substrate surface by contact ( Fig. 4.15 (a)). SAMs can be created by
immersion of the substrate in a solution containing a ligand Y(CH 2 ) n X, where X is the head group and
Y is the anchoring group. The head group determines the surface property of the monolayer. The stamp
is wetted with the above solution and pressed on the substrate surface ( Fig. 4.15 (b)). Usually, the SAM
layer is too thin for some applications. Thus, SAM can be used as a mask to transfer the pattern to an
underlying functional layer. Because of its small thickness, SAM can be quickly destroyed by ion
bombardment. Therefore, a SAM mask is not suitable for reactive ion etching but for wet chemical
etching ( Fig. 4.15 (c)). The thicker functional layer, in turn, can be used as a mask for the more
aggressive RIE. The resolution of microcontact printing depends on the properties of the stamp
material, and can reach several tens of nanometers.
Surface properties of patterned SAM can be used as templates for selective deposition of other
materials. For example, a patterned hydrophilic SAM traps liquid prepolymer on its surface. After
curing, a polymer structure is formed on top of the patterned SAM. Furthermore, patterned SAM can
be used for controlled deposition of metals and ceramics by selective CVD.
Micromolding with a PDMS master can be categorized as replica molding, microtransfer molding,
micromolding in capillaries, and solvent-assisted micromolding [86] . Replica molding uses the PDMS
stamp as a replica master for a prepolymer, which is cured by UVexposure or by elevated temperature.
This technique can achieve resolutions of less than 10 nm. In microtransfer molding , liquid prepol-
ymer is applied on the PDMS master. The prepolymer layer is planarized by removing the excess
prepolymer. Only prepolymer trapped between the relief structures remains on the surface of the
PDMS master. The master is then placed on a planar substrate. UV exposure or heating solidifies the
prepolymer. Peeling off the elastic PDMS master results in polymer structures on the substrate surface.
This method does not completely remove the excess prepolymer on top of the PDMS stamp. A thin
polymer layer on the order of 100 nm remains on the substrate surface. If the patterned polymer is to be
used as a mask for subsequent etching, this thin polymer layer should be removed by oxygen plasma
[86] . Micromolding in capillaries uses capillary forces to fill the gaps between the substrate and the
PDMS master. First, the PDMS master is pressed tightly on a planar substrate. Elastic PDMS seals off
walls and creates capillary channels. A drop of liquid prepolymer is placed at the ends of these
FIGURE 4.15
Microcontact printing with PDMS stamps: (a) immersion; (b) stamping; and (c) etching.
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