Microcontact printing (or μCP) is a form of soft lithography that uses the relief patterns on a master polydimethylsiloxane (PDMS) stamp or Urethane rubber micro stamp to form patterns of self-assembled monolayers (SAMs) of ink on the surface of a substrate through conformal contact as in the case of nanotransfer printing (nTP). Its applications are wide-ranging including microelectronics, surface chemistry and cell biology.
History Both lithography and stamp printing have been around for centuries. However, the combination of the two gave rise to the method of microcontact printing. The method was first introduced by George M. Whitesides and Amit Kumar at Harvard University. Since its inception many methods of soft lithography have been explored.
Procedure
Preparing the master Creation of the master, or template, is done using traditional photolithography techniques. The master is typically created on silicon, but can be done on any solid patterned surface. Photoresist is applied to the surface and patterned by a photomask and UV light. The master is then baked, developed and cleaned before use. In typical processes the photoresist is usually kept on the wafer to be used as a topographic template for the stamp. However, the unprotected silicon regions can be etched, and the photoresist stripped, which would leave behind a patterned wafer for creating the stamp. This method is more complex but creates a more stable template.
Creating the PDMS stamp After fabrication the master is placed in a walled container, typically a petri dish, and the stamp is poured over the master. The PDMS stamp, in most applications, is a 10:1 ratio of silicone elastomer and a silicone elastomer curing agent. This mixture consists of a short hydrosilane crosslinker that contains a catalyst made from a platinum complex. After pouring, the PDMS is cured at elevated temperatures to create a solid polymer with elastomeric properties. The stamp is then peeled off and cut to the proper size. The stamp replicates the opposite of the master. Elevated regions of the stamp correspond to indented regions of the master. Some commercial services for procuring PDMS stamps and micropatterned samples exist such as Research Micro Stamps.
Inking the stamp Inking of the stamp occurs through the application of a thiol solution either by immersion or coating the stamp with a Q-tip. The highly hydrophobic PDMS material allows the ink to be diffused into the bulk of the stamp, which means the thiols reside not only on the surface, but also in the bulk of the stamp material. This diffusion into the bulk creates an ink reservoir for multiple prints. The stamp is let dry until no liquid is visible and an ink reservoir is created.
Applying the stamp to the substrate
Direct contact Applying the stamp to the substrate is easy and straightforward which is one of the main advantages of this process. The stamp is brought into physical contact with the substrate and the thiol solution is transferred to the substrate. The thiol is area-selectively transferred to the surface based on the features of the stamp. During the transfer the carbon chains of the thiol align with each other to create a hydrophobic self-assembling monolayer (SAM).
Other application techniques Printing of the stamp onto the substrate, although not used as often, can also take place with a rolling stamp onto a planar substrate or a curved substrate with a planar stamp.
Advantages Microcontact Printing has several advantages including:
The simplicity and ease of creating patterns with micro-scale features Can be done in a traditional laboratory without the constant use of a cleanroom (cleanroom is needed only to create the master). Multiple stamps can be created from a single master Individual stamps can be used several times with minimal degradation of performance A cheaper technique for fabrication that uses less energy than conventional techniques Some materials have no other micro patterning method available
Disadvantages After this technique became popular various limitations and problems arose, all of which affected patterning and reproducibility.
Stamp Deformation
During direct contact one must be careful because the stamp can easily be physically deformed causing printed features that are different from the original stamp features. Horizontally stretching or compressing the stamp will cause deformations in the raised and recessed features. Also, applying too much vertical pressure on the stamp during printing can cause the raised relief features to flatten against the substrate. These deformations can yield submicron features even though the original stamp has a lower resolution. Deformation of the stamp can occur during removal from the master and during the substrate contacting process. When the aspect ratio of the stamp is high buckling of the stamp can occur. When the aspect ratio is low roof collapse can occur.
Substrate contamination During the curing process some fragments can potentially be left uncured and contaminate the process. When this occurs the quality of the printed SAM is decreased. When the ink molecules contain certain polar groups the transfer of these impurities is increased.
Shrinking/swelling of the stamp During the curing process the stamp can potentially shrink in size leaving a difference in desired dimensions of the substrate patterning. Swelling of the stamp may also occur. Most organic solvents induce swelling of the PDMS stamp. Ethanol in particular has a very small swelling effect, but many other solvents cannot be used for wet inking because of high swelling. Because of this the process is limited to apolar inks that are soluble in ethanol.
Ink mobility Ink diffusion from the PDMS bulk to the surface occurs during the formation of the patterned SAM on the substrate. This mobility of the ink can cause lateral spreading to unwanted regions. Upon the transfer this spreading can influence the desired pattern.
Applications Depending on the type of ink used and the subsequent substrate the microcontact printing technique has many different applications
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