Module 13: Soft Lithography. Lecture 19: Soft Lithography 2

Transcription

1 Module 13: Soft Lithography Lecture 19: Soft Lithography 2 1

2 In the previous lecture we have introduced the concept of Soft Lithography and discussed three of the methods, which are Replica Molding, Micro Contact Printing (μcp) and Molding in Capillaries (MIMIC). We have seen some of the major advantages of each of the method. For example, REM is widely used for the fabrication of the cross linked PDMS stamp, which are widely used in almost all other Soft Lithography methods. Similarly, μcp is a unique method that allows the fabrication of chemically patterned substrates. MIMIC is a method primarily aimed at fabricating topographically patterned features with polymers, but can also be used for creating structures with colloids. In this lecture, we will discuss about few more SL techniques, mainly Capillary Force Lithography (CFL), Solvent Assisted Micro Molding (SAMIM), Micro Transfer Molding (μtm) and a few newer developments in this field. Figure 19.1: Schematic showing the key steps of Capillary Force Lithography (CFL) Capillary Force Lithography (CFL) While discussing MIMIC in section 18.3 in the previous lecture, we noted that no external pressure was required for pattern replication. This is indeed a major advantage of MIMIC, over NIL, as it eliminates the hardware arrangements for applying pressure. Further, pattern replication by capillary driven flow also significantly reduces the extent of stresses accumulated with in the replicated structures, enhancing their long term durability. However, the major hindrance of implementing MIMIC is the progressive reduction of the capillary filling rate in long channels. Capillary force lithography essentially combines the advantages of NIL And 2

3 MIMIC and has evolved into a very simple and convenient SL technique for creating 2-D topographic patterns. This method was developed by Hong Lee and colleagues in Korea in early 2000, and thus the only major Soft Lithography technique that was not developed by Whiteside s group. However, experts regard the technique to be a well suited member of the SL family. Similar to MIMIC, CFL also utilizes capillary driven flow for pattern replication. The experimental configuration (shown in figure 19.1) allows overcoming some of the limitations associated with MIMIC. As can be seen in figure 19.1, a flexible crosslinked PDMS stamp (fabricated by REM) is first brought in conformal contact with a thin polymer film. This is in clear contrast with MIMIC, where the stamp is contacted with a bare substrate. Once the contact between the film and the stamp is established, the assembly is heated beyond the glass transition temperature of the polymer. Some groups avoid thermal annealing and instead expose the assembly to the vapor of a solvent of the film material. This way the solvent molecules penetrate into the polymer matrix and position themselves between the polymer molecules which lead to swelling of the film and reduce cohesive force between the polymer molecules. This in turn makes the film liquid like, as the T G of the swollen polymer film becomes lower that the room temperature. As the polymer viscosity drops significantly either due to heating beyond T G, or reduction of T G itself, the liquid polymer starts climbing along the walls of the stamp due to capillary driven flow. This action eventually fills up mold, as shown in inset of C in figure In this technique, pattern replication takes place due to capillary driven vertical rise of the polymer meniscus along the confining stamp feature walls. This is in clear contrast to MIMIC, where capillarity driven flow takes place along the direction of the channel, and therefore the flow path to be covered for complete pattern replication is much larger (~ cm) in comparison to CFL, where the meniscus just have to travel few hundreds of nm for achieving complete mold filling. Thus, CFL eliminates one of the major problems associated with MIMIC, which is drop in capillary flow velocity in long channels. Having said that, one still has to understand that in MIMC, typically a dilute solution or a liquid pre-polymer with significantly lower viscosity 3

4 travels along the channels. In contrast, in CFL, a softened polymer with much higher viscosity undergoes the capillary dynamics. Consequently, the time required for complete pattern replication might still be significantly high. It should be noted that the time requirement of mold filling depends on the viscosity of the polymer melt as well as the operating temperature. The extent of mold filling also depends on the initial film thickness. Still, excellent capillarity driven mold filling in CFL can be achieved even when the groove width is very small (< 50 nm). Thus, replicating finer features becomes possible in CFL, which still remains a matter of major concern in case of MIMIC. Various complex patterns can be engineered by CFL simply by varying the processing conditions and the initial film thickness Once the mold filling is accomplished, the assembly is cooled down to room temperature or removed from the solvent vapor atmosphere. Finally, the flexible stamp is peeled off to reveal the imprinted patterns. There are certain critical aspects associated with CFL which needs to be considered during implementing the process. For example, a sudden increase in the temperature during annealing might dislodge the stamp from the film surface due to mismatch in the rates of thermal expansion of the two and therefore the thermal annealing is performed with a gradual, slow rise of temperature. Use of a softer PDMS stamp (lower cross linker concentration) might help significantly in maintaining a conformal contact at elevated temperatures. While CFL stamps or molds are re-usable, the repeated thermal cycle may have an adverse effect on them and can damage of the mold by additional cross linking with time. Thus, the reusability of a CFL mold is far less as compared to other soft lithography techniques like μcp. Pattern formation by CFL is associated with minimization of the total free energy of the system due to capillary rise of the polymer along the stamp walls. Additional limitations in CFL arise out of the deformability of the PDMS stamp. The problem becomes particularly pronounced when high aspect ratio patterns are made by CFL. The PDMS stamp tends to sag between two stamp protrusions. In order to overcome this limitation, a modified version of CFL with a fluoro-polymer stamp has been developed. However, a fluoro-polymer stamp has a tensile modulus ~ 1 2 GPa and is thus not 4

5 as flexible as a PDMS stamp. Therefore, to ensure conformal contact, a slight external pressure of 2 3 Bar is often applied on the stamp. This method is known as the pressure assisted Capillary Force Lithography (PA CFL). The use of a very low surface energy fluoro-polymer stamp also allows easy and clean detachment after pattern replication, and thus increases reusability of the stamp. One key requirement for the successful implementation of PA CFL is that the externally applied pressure must be uniformly spread over the entire stamp surface. This is generally achieved by using a PDMS block between the press and the stamp, as a buffer layer with eliminates the possibility of localization of pressure with a flat punch. The pattern fidelity in PA CFL is better as compared to CFL due to the application of the external pressure. CFL as a method works better with finer features and becomes difficult to replicate features with lateral resolution larger than 10 μm, as the capillary driving force reduces as P Cap ~ R 1. In spite of these minor limitations, CFL is one of the most easy to execute SL method that hardly requires any major tools and instruments and can be implemented in a non clean room environment. CFL has been extended to various materials other than polymer. For example, sol gel derived thin films can be patterned by CFL for fabrication of optical wave guides. Figure 19.2: Schematic showing the key steps of SAMIM Solvent Assisted Micro Molding in Capillaries (SAMIM) SAMIM is a Soft Lithography technique that combined the essential components of both CFL and MIMIC. It is closer to CFL, as it also requires the existence of a film coated on the substrate as well as the capillarity driven flow is in the vertical direction along the side walls of the stamp patterns. The method primarily aims at eliminating the problem of slow capillary driven flow of 5

6 high viscosity polymer melt. In this technique, the stamp is first pre-soaked in a solvent of the polymer to be patterned. Then the wet stamp is then brought in conformal contact with a thin film of the polymer which is desired to be patterned. As the stamp is wet, the solvent molecules carried along with stamp come in contact with the polymer molecules of the film and locally dissolve them. This results in significant reduction in viscosity of the polymer. Beyond this stage, the polymer solution climbs up along the walls of the stamp features due to capillary action, resulting in mold filling and pattern replication. The schematic of the process is shown in figure SAMIM indeed reduces the pattern replication time as compared to CFL, as the presence of the solvent significantly reduces the viscosity of the polymer, thereby facilitating rapid capillary rise. However, once the mold filling is complete, then the solvent needs to be evaporated completely before the stamp can be peeled off. This step might take significantly long time, as removal of traces of solvent is often difficult as the solvent molecules tend to remain entrapped within the entangled polymer matrix. Solvent evaporation also introduces the possibility of pattern shrinkage. However, the shrinkage is significantly less as compared to MIMIC. Thus, SAMIM is also a reasonably easy to implement SL technique for creating topographic patterns Micro Transfer Molding (μtm) The last topic in this lecture on conventional SL we are going to discuss is Micro Transfer Molding (μtm). This method is unique and rather distinct from all the previously discussed methods, as it becomes possible to fabricate 3 D micro structures by this method. The key steps of the method are shown in figure First, a few drops of a liquid pre-polymer is dispensed on the patterned face of a cross linked PDMS stamp (frame B, figure 19.3). The liquid is allowed to flow over the stamp surface and fill of the stamp grooves. It is very likely that there will be some excess liquid which will form a drop on the stamp surface (frame C, figure 19.3). The excess prepolymer liquid is subsequently removed by scraping off (frame D, figure 19.3). It is desirable 6

7 Figure 19.3: Schematic showing the key steps of Micro transfer Molding (μtm) that the filled mold has a perfectly flat morphology. The pre-polymer within the grooves at this point of time is solidified, by cross linking by any possible mechanism such as thermal annealing, UV exposure etc. The stamp, with the filled grooves is then placed conformally on a flat substrate. After this, the stamp is withdrawn from the substrate, leaving behind the strips of the cross linked polymer (frame F, figure 19.3). This is typically achieved by swelling the cross linked PDMS stamp by exposing it to solvent vapor (chloroform, in most cases). This allows neat detachment of the stamp, leaving behind the structures on the flat substrate surface. After the first layer patterns have formed (frame F, figure 19.3), a second layer of structures can be made by placing a pre-polymer filled stamp on the first layer of structures (frame G, figure 19.3). A typically two layer structure obtained after peeling the second stamp is shown in frame H of figure In this sequential manner, more numbers of layers can be progressively added and true 3-D patterns can be obtained. Micro transfer molding (μtm) can be used for the fabrication of both interconnected and isolated structures, which is a major improvement over MIMIC. Further, as the structures are fabricated on the stamp, away from the substrate, therefore it becomes possible to build three dimensional micro structures layer by layer. This unique ability is attributed to the purely additive nature of μtm, which allows the 2D and 3D printing steps to be performed several times to build up 7

8 multilayer stacks, even on non planar substrates. The method is capable of generating patterns over relatively large areas within a short period of time, as pattern replication in this process is not limited by confined capillary dynamics like MIMIC. Of course, the critical issue here is to achieve neat detachment of the patterns from the stamp groves and their easy transfer to the substrate, without any distortion of alignment. The method has been extensively used to pattern different classes of polymers including complete devices like optical waveguides, couplers, polymers interferometers etc. Several recent methods like Nanoternsfer Printing (ntp) are extensions of the fundamental concept of μtm. (Zaumseil et al. 2003). Reference: 1. R. Mukherjee, Soft Lithography and Beyond: Some Recent Developments in Meso Patterning, Chapter 4, pages , in Microfluidics and Microscale Transport Process, edited by S. Chakraborty, CRC Press, ISBN: E. Kim, Y. Xia, and G. M. Whitesides, Polymer microstructures formed by moulding in capillaries, Nature 1995, 376, E. Kim, Y. Xia, and G. M. Whitesides, Micromolding in Capillaries: Applications in Materials Science, J. Am. Chem. Soc. 1996, 118, Y. Xia and G. M. Whitesides, Soft Lithography, Angew. Chem. Int. Ed. 1998, 37,