NMI3
Fabrication of Polymeric Nanostructures: Techniques and Stability Issues Y. P. Kong1, H. G. Peng2, and A. F. Yee1,2 1
Department of Chemical Engineering and Materials Science University of California, Irvine 916 Engineering Tower, Irvine, CA 92697, USA 2 California Institute for Telecommunications and Information Technology Calit2 Building, Irvine, CA 92697, USA Email:
[email protected]
Abstract— Imprint lithography has become a potential next generation lithography technique for the microelectronics industry. We have developed new imprint lithography techniques that fabricate three-dimensional polymeric nanostructures so that imprint lithography can create a larger impact in other areas of science and technology. We have also studied the stability of imprinted polymeric nanostructures and found that the stability is reduced significantly as the feature size decreases. The cause is probably the interplay between residual stress and viscosity of the polymer at the relaxation temperature and our preliminary results clamor for more studies in this area. Keywords- Lithography; embossing; polystyrene; atomic force microscopy.
I.
polymer
nanostructures such as a lattice structure as shown in Figure 1.[4]
Figure 1: 3-D lattice structure made with reversal imprint lithography.
relaxation;
Since the imprint mold is treated with a low surface energy coating for mold release, the key to a successful spin coat is to use a non-polar polymer and solvent. We have used poly(methyl methacrylate) (PMMA) and polystyrene (PS) in toluene very successfully as resist solutions. One major advantage of using reversal imprinting for patterning polymeric nanostructures is the relatively low imprint pressure and temperature used as compared to conventional imprint lithography. Another advantage when using the inking mode is the fabrication of patterns without the residual layer that is commonly formed with conventional imprint lithography. Duo-mold imprint lithography is an extension of reversal imprinting and can be used to create 3-D polymeric nanostructures.[3] As the name suggests, two molds are used to create the 3-D nanostructures, one defining the bottom features and the other defining the top features. As in reversal imprint lithography, the polymer solution is spin coated onto one mold and the second mold is then pressed onto the polymer coated first mold. One mold is then removed and the 3-D structures are then transferred on a substrate. Figure 2 shows a structure fabricated using duo-mold imprinting.
INTRODUCTION
In 2003 the International Technology Roadmap for Semiconductors (ITRS) named imprint lithography as one of the next-generation lithography candidates for the technology nodes that are at or below 45 nm. This announcement meant that conventional imprint lithography[1] and related imprint lithography techniques have emerged from the realm of academic research to become a technology worthy of serious consideration by the microelectronics industry. However, before imprint lithography techniques are adopted by industry, a number of problems associated with the technology must be addressed such as mask defects and overlay accuracy. In the meantime, the applications of imprint lithography have gone beyond microelectronics. Imprint lithography techniques have been developed and applied in photonics, bioscience, light emitting displays, and data storage. We believe that the greatest impact of imprint lithography techniques will be in the nanopatterning of functional polymeric structures. Imprint lithography has also been extended to the fabrication of three-dimensional (3-D) structures. This ability to create 3-D structures simplifies processing and allows novel structures to be formed. Our group has developed several 3-D fabrication techniques. Reversal imprinting[2] and Duo-mold imprinting[3] were developed to create 3-D polymeric nanostructures that are functional. We present these techniques in brief in this report. The stability of these polymeric nanostructures is important since in certain applications, e.g., photonic and storage devices, they need to remain functional over a long period of time. We present preliminary data that show the effect of decreasing feature dimensions on the stability. II.
Figure 2: 3-D structure created using duo-mold imprint lithography. Duo-mold imprint lithography requires both molds to be treated with a low surface energy coating for mold release and the key is to selectively treat one surface to have a lower surface energy than the other. This is achieved by either using different low surface energy silanes or a combination of them.[3] The stability of polymeric nanostructures, as mentioned earlier, is important if these structures need to remain functional, dimension wise, for a considerable amount of time. We study the stability of reversal imprinted polymeric
EXPERIMENTAL DETAILS
Reversal imprint lithography is a variation of conventional imprint lithography. Instead of spin coating the polymer solution onto the substrate, the solution is spin coated onto the mold. Careful control of this spin coating process leads to three different imprint modes: reversal embossing, inking and whole-layer transfer. Using the whole-layer transfer mode of reversal imprint lithography, it is possible to build up 3-D 168
PS films of similar thickness and molecular weight, a Tg suppression of more than 70 K is expected.[6,7] In our experiments, we observe that the 40nm lines start to relax at ~7 0 C lower than the 350 nm lines. The relaxation temperature is, however, still much higher than that observed with freestanding films. Such discrepancy in temperature was also observed in hole growth in free-standing PS films.[8] It is perhaps because pattern height slumping, as in hole growth, involves polymer chain center of mass motion across the entire pattern line thickness and not just segmental motion. In ultrathin 40 nm lines, the temperature dependence of the chain and segmental motion might not be the same, as in the case of bulk. The motion of the entire chain could be suppressed at the measured Tg in the structures. In addition to the shift of relaxation to lower temperatures, the 40 nm lines relaxed with a comparable rate (normalized height shrinkage with respect to temperature change) to the 350 nm lines. From the residual stress perspective, one would expect a much faster relaxation rate in the 40 nm lines. However, we note that the 40 nm patterns did relax at a lower temperature. When chain mobility, not Tg, is the main relaxation mechanism, the viscosity of the 40 nm film is very likely higher than the 350 nm lines during relaxation. Even though the 40 nm film inherently has a larger relaxation driving force, the relaxation rate at lower temperatures can still be slowed by the higher viscosity. The absence of the plateau in the 40 nm grating is likely to be due to the initial high internal stress which is sufficient to overcome the viscosity due to molecular entanglements.
nanostructures with the use of an atomic force microscope (AFM) from NT-MDT(Ntegra Therma). This AFM uses an enclosure to heat the sample which eliminates the problem of heat transfer from a heated sample to a relatively colder AFM probe. The latter problem is encountered with the use of AFMs fitted with heated stages and we have data showing that the structural relaxation of nanostructures studied with such setups is very different from that studied with heated enclosures. The polymers used in this study are PMMA and PS of various molecular weights. The molecular weights of PMMA are 15 kg/mol, 120 kg/mol, and 950 kg/mol and are all of broad molecular weight distribution. The PS samples used are of narrow molecular weight distributions. They are 15.5 kg/mol (Mw/Mn ~1.04), 220.9 kg/mol (Mw/Mn ~1.03) and 1571 kg/mol (Mw/Mn ~1.03). The polymer samples are dissolved in toluene in various concentrations and are reversal imprinted onto gratings of 350 nm and 40 nm linewidths. The 350 nm mold is fabricated from Si by interference lithography and subsequent reactive ion etching. The 40 nm mold is fabricated from Si using a Zeiss 1540XB focused ion beam system. Both molds are treated with 1H,1H,2H,2Hperfluorodecyltrichlorosilane in a nitrogen glove box to form a coating for mold release. Due to space limitations, the results of only two films are shown herein. III.
EXPERIMENTAL RESULTS
The normalized heights vs. annealing temperature for the two PS films (1571 kg/mol) of 350 nm and 40 nm linewidths gratings are plotted in Figure 3. The radius of gyration of PS at this molecular weight is about 35 nm calculated from Polymer Handbook, 4th edition. Note that the 350 nm linewidth is only 5 times that of the molecular size. When the molecules flow into the trenches, orientation of the coils is expected, and this induces residual stresses. The 350 nm lines begin slumping at 101.5 0C, slightly higher than the bulk Tg(100 0C). This is not surprising, as the relaxation driving force has to overcome the viscosity of the film to change the shape of the pattern. After the start of relaxation, the driving force due to orientation is reduced, and thus the temperature needs to be increased to lower the viscosity to allow further relaxation. Therefore, the relaxation persists over a relatively large temperature window of 15 0C. The flat region above 120 °C is probably related to the rubbery plateau due to molecular entanglements.
IV.
REFERENCES [1]
[2]
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PS, Mw ~ 1500k Mw/Mn ~ 1.03 350 nm linewidth 40 nm linewidth
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CONCLUSIONS
Imprint lithography is poised to become a major technique for patterning polymeric nanostructures because of its versatility. The myriad variations of this technique, too numerous to present and discuss here, attest to that statement. Extending imprint lithography into the third dimension, as shown here, adds increasingly to the potential of this technology. However, as we gradually become more adept in molding polymers in the nanoscale into more and more complex structures, we have to keep in mind that the stability of these nanoscale structures may be reduced significantly and a better understanding of this phenomenon is required.
140
[6]
o
Temperature C
Figure 3: Structural relaxation of 350 nm and 40 nm linewidth PS of Mw=1571 kg/mol.
[7]
The relaxation behavior of the 40 nm lines is compared with the 350 nm lines in Figure 3. The aspect ratio of the 40 nm lines is about 2:1 (height over width) so each pattern line is very similar to a finite free-standing film anchored to the polystyrene residue layer on one side wall. For free-standing
[8]
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