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Microcontact Printing Self-assembled Monolayer Films: Dow Corning-is cast against a master whose surface has been patterned with a complementary relief Microcontact Printing structure using conventional microlithographic tech- niques such as photolithography, micromachining, or , Microcontact printing (PCP), one of the non-photo- high-resolution printing. The stamp may also be cast lithographic techniques that make up "soft lithogra- from preformed microstructures such as optical dif- phy" (Xia and Whitesides 1998), uses an elastomeric fraction gratings, metal grids for transmission electron stamp to generate two-dimensional patterns by print- microscopy (TEM), compact disks (CDs), corner cube * ing the molecules of an "ink" onto the surface of a reflectors, anti-reflection structures, and assembled solid substrate. In its most common configuration, the structures of colloidal particles. The only requirement stamp is fabricated from an elastomeric polymer, seems to be that the surface of a master does not react poly(dimethylsiloxane) (PDMS), the "ink" is an with the PDMS prepolymer to form covalent bonds. alkanethiol, and the substrate on which the printing In most cases, it is necessary to treat the master with occurs is a thin, evaporated film of gold or silver. the vapor of octadecyltrichlorosilane (or a perfluoro- Alkylsiloxanes are also printed on Si/Si02 and glass. silane such as C,FI3C2H,SiCl3) to cap any polar Microcontact printing is an experimentally simple reactive group (e.g., -OH) on the master with a methodology; the elastomeric stamp is easily fabri- chemically inert group such as -CH3 or -CF,. cated by molding a liquid prepolymer against a master Several properties of PDMS are instrumental in the that has the desired pattern in relief on its surface. formation of high-quality patterns during PCP. First, Once the master is available, multiple copies of the the softness of PDMS can be easily tailored by varying pattern can be produced reproducibly, using straight- the amount of curing agent. The stamp can be made forward experimental procedures. The initial products flexible enough to conform to the surface of the of PCP are usually patterned regions of self-assembled substrate over a relatively large area, with little or no monolayers (SAMs). These printed SAMs can subse- applied pressure. This conformal contact can even be quently beused as ultrathin (2-3nm) resists in selective achieved on surfaces that are nonplanar on the scale of wet etching, or as templates to control the wetting, microns. Second, the surface properties of PDMS can dewetting, adsorption, nucleation, growth, and de- be readily modified using a combination of oxygen position of a rich variety of materials. The printing plasma treatment and formation of alkylsiloxane process is inherently parallel-that is, it forms the SAMs. Tailoring the surface of the PDMS is necessary pattern over the entire area of the substrate in contact to accommodate ink molecules and solvents that with the stamp at the same time-and thus allows for themselves have a wide range of interfacial free patterning over a relatively large area in a single energies. Third, PDMS is a durable, chemically inert impression. The minimum size of features that can be elastomer: the same stamp can be used at least 100 directly generated using PCP still has not been com- times in PCP over a period of several months without pletely defined. It is possible to print routinely noticeable degradation in performance. Fourth, - 300nm features of hexadecanethiolate SAMs over PDMS is highly transparent down to - 300nm; this - 50cm2 areas (Xia et al. 1997), and it has also been optical transparency allows one to observe alignment demonstrated that PCP is capable of generating under an optical microscope (in the transmission parallel lines of dodecanethiolate SAMs on gold that mode). This elastomer has also been used to construct are - 35nm in width and separated by - 350nm deformable components for adaptive optics, and (Biebuyck et al. 1997). photomasks for phase-shift or conventional photo- lithography (Xia et al. 1999). Fifth, PDMS is elec- trically insulating; it can be directly employed to fabricate microfluidic devices for use with capillary electrophoresis (CE). I. The Elastomeric Stamp PDMS also has several disadvantages that may Conformal contact between the stamp and the surface limit its performance in certain types of applications. of the substrate is the key to the success of micro- For example, it shrinks by a factor of - 1% upon contact printing. This conformal contact has been thermal curing, and it can be readily swelled by achieved by fabricating the stamp in an elastomeric nonpolar solvents such as toluene and hexane. The polymer. A variety of elastomers-for example, elastomeric character of PDMS also poses technical poly(dimethylsiloxane)s.and polyurethanes, as well as challenges for the use of PCP in printing certain kinds harder polymeric materials such as polyimides and of features. For example, the gravity, adhesion, and cross-linked novolac resins (phenol formaldehyde capillary forces exerted on the elastomeric features polymers+have been successfully used to fabricate may cause these features to collapse or distort, and to the stamp. The elastomer commonly used in most generate defects in the printed pattern. demonstrations is PDMS. The PDMS stamp is usually As shown in Fig. 1, if the aspect ratios (Ilh) of the fabricated by replica molding (Fig. 1); a liquid pre- relief structures are too small, the raised portions of polymer of PDMS-for example, Sylgard 184 from PDMS may fall under their own weight, or collapse Self-assembled Monolaver Films: Microcontact Printina 2. Microcontact Printing I Fabrication and silanization of master The concept of microcontact printing was first demon- strated using SAMs of hexadecanethiolate on thin films of gold, supported on silicon wafers (Kumar and . Whitesides 1993). The printing step involves a direct - Pouring of PDMS over master contact between the raised portions of the stamp and I the substrate, and can be routinely performed with one of the following three configurations: printing on - PDMS I a planar surface with a planar stamp; printing on a curved surface with a planar stamp; or printing on a planar surface over a large area with a rolling stamp that has been mounted on a cylindrical support. Figure 2 shows the schematic procedure for printing on a Curing, and releasing of PDMS planar surface of gold with a planar stamp: the stamp 1 is wetted with a solution of hexadecanethiol PDMS (CH,(CH?),,SH) in ethanol (- IOmM) and is then brought into contact with the surface of gold +I ++ +dW for 5-10s. The hexadecanethiol transfers from the fb0.9910) a raised regions of the stamp to the gold, forms a species I Deformation of PDMS commonly represented as hexadecanethiolate [CH,(CH,),,S-Au'], and generates patterned micro- features of SAMs on the surface of the gold. For PCP with a solution of hexadecanethiol in ethanol, a contact time of 5-10s has to be used because longer contact times may result in the destruction of the pattern, due to the transport of hexadecanethiol from the stamp to the surface of gold in noncontact regions by diffusion through the vapor phase, or by lateral movement. Figure I The success of PCP rests on two characteristics of Schematic procedure for casting PDMS stamps from a the system: the rapid formation of a highly ordered master. The bottom part illustrates two possible monolayer, and the autophobicity of the monolayer to distortions that are usually associated with an elastomer the ethanol solution of the alkanethiol; this auto- such as PDMS: the pairing of raised structures and the phobicity blocks the spreading of the excess ink liquid sagging of recessed regions during contact printing. across the surface. The adsorption and self- organization of alkanethiols on gold is a relatively fast process: when a gold substrate is immersed in a due to the forces involved in the inking or printing solution of hexadecanethiol in ethanol, an ordered process. It has been shown that the aspect ratio (llh) SAM of hexadecanethiolate can be formed within a must be >, 0.2 in order to obtain printed patterns few minutes. The formation of highly ordered SAMs largely free of defects caused by mechanical distortions of alkanethiolates during PCP may occur in an interval of the stamp (Delamarche et al. 1997). On the other as short as a few seconds. An STM study showed that, hand, when the aspect ratios (d/h) are too large, the for an ink solution of - lOOmM dodecanethiol in relief structures may not be able to withstand the ethanol, a contact time of > 0.3s was enough to compressive forces typical of printing and/or the generate highly ordered SAMs on polycrystalline gold adhesion between the stamp and the substrate. The that were indistinguishable from those formed by result of these interactions is sagging, a process that equilibration in solution (Larsen et a[. 1997). may prevent microcontact printing from generating Since its first demonstration in 1993, PCP has been patterns in which features are widely separated (e.g., successfully extended to pattern a wide variety of d 2 20h) without defects in the pattern between these materials (Xia and Whitesides 1998). For example, features. A solution to the sagging problem that is this technique has been extended to different systems sometimes possible is to introduce nonfunctional of SAMs, including SAMs of alkanethiolates on silver, . "posts" into the design to provide a support for the SAMs of alkanethiolates on copper, SAMs of alkane- noncontact regions, or to fabricate the stamp on a thiolates on palladium, SAMs of alkanethiolates on rigid support. Many of the problems of mechanical gallium arsenide, SAMs of alkylsiloxanes on HO- distortions in the stamp can be solved by using terminated surfaces, SAMs of alkylphosphoric acids . appropriate materials, new designs, and new on aluminum/aluminum oxide, and SAMs of alkyl- procedures optimized for different fabrication tasks.
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