s3_Polymer Inking to Form Stacked Patterns with Smooth Edges and Vertical Sidewalls

Polymer Inking to Form Stacked Patterns with Smooth Edges and Vertical Sidewalls

S. W. Pang, L.-R. Bao, Y. P. Kong, L. Tan, X. D. Huang, L. J. Guo, and A. F. Yee

University of Michigan, Ann Arbor, Michigan 48109-2122, USA

A polymer inking technique was developed to form micro- and nanopatterns on a substrate. In this process, a polymer thin film is spin coated on a patterned mold. After contacting the substrate at a suitable temperature and pressure, the polymer on the protruded surfaces of the mold is transferred to the substrate and a positive image of the mold is obtained. A selective surface treatment method has been developed to improve the edge smoothness of the inked pattern. During selective surface treatment, the protruded surfaces of the mold are first treated with a flat polydimethylsiloxane (PDMS) stamp impregnated with a silane that has medium surface energy. The mold is then immersed into the solution of another silane with very low surface energy to treat the trenches of the mold. Because the surface energy of the sidewalls is lower than that on the protrusions, polymer dewetting from the sidewalls is promoted, which makes the polymer film discontinuous along the edges of patterns. Therefore, inked polymer patterns from the protrusions of the mold show very smooth edges and smaller dimensions compared to that of the mold. The dimension change of the inked pattern is dependent on the selection of polymer materials. It was found that patterns inked from polycarbonate (PC) showed larger dimension shrinkage (~75%) compared to that from polymethylmethacrylate (PMMA) (~30%). This offers a viable approach to obtain predictable submicrometer features using a mold with much larger feature sizes.

A soft inkpad imprinting technique to produce stacked micrometer and submicrometer polymer patterns on substrates is also presented. A thin soft inkpad is used to coat a polymer film onto the protrusions of a surface treated hard mold. The polymer film on the protrusions of the hard mold is then transferred to a substrate. Simultaneously, a negative pattern of the hard mold is formed on the soft inkpad that may also be transferred to a substrate. Numerical simulations are used to study the mechanisms of pattern transfer by soft inkpad imprinting. With the use of polymer blends, both positive and negative polymeric gratings with 700 nm period were produced. The soft inkpad allows multiple transfers of polymers with similar solubilities to the hard mold since no chemical solution is used for coating. High aspect ratio polymer stacks can be formed without alignment. This capability is an important advantage when forming submicrometer and nanometer multiple-layered polymer structures because current nanoimprint systems have limited overlay accuracy.

Figure 1 demonstrates PMMA dewetting on a patterned mold, in which the mold has been treated with surfactants. In Figure 1(a), the mold is treated with perfluorodecyltrichlorosilane (FDTS) only. In contrast, the mold in Figure 1(b) has been given selective phenethyltrichlorosilane (PETS)/FDTS treatment.

Figure 1. Dewetting behavior of PMMA film after annealing at 130 °C for 5 min on a 700 nm period grating mold with: (a) FDTS solution treatment and (b) PETS/FDTS selective treatment.

The atomic force microscope image in Figure 2 shows the profiles of dewetted PMMA on a mold with protruded circles (with selective methacryloxypropyltrichlorosilane (MOPTS)/FDTS treatment). A raised rim surrounds the PMMA island on top of the protrusions.

Figure 2. Dewetting behavior of PMMA film after annealing at 130 °C for 5 min on a 450 nm deep mold with MOPTS/FDTS selective treatment.

The polymer features in Figure 1(b) can be easily transferred to a Si wafer at 105 °C and 5 MPa. Figure 3(a) is the scanning electron micrograph of the transferred PMMA grating pattern. The micrometer scale PMMA islands in Figure 2 can also be transferred to the substrate at 115 °C under elevated pressure of 5 MPa. Figure 3(b) is an atomic force microscope image of the transferred pattern. Compared to PMMA inking at 105 –110 °C, PC is inked at a temperature of 150 °C and shows easier dewetting during the imprinting process. Figure 3(c) presents such an example.

Figure 3. (a) PMMA grating pattern formed by inking the PMMA film in Figure 1(b) onto a Si wafer at 105 °C, 5 MPa; (b) PMMA pattern formed by inking the PMMA film in Figure 2 onto a Si wafer at 115 °C, 5 MPa; and (c) submicrometer PC pattern formed by inking the PC coated mold with 2 mm protruded circles at 150 °C, 5 MPa. The PC was inked without annealing.

From a 700 nm period grating Si mold, submicrometer polymeric grating patterns have been successfully replicated using the PMMA–polyvinyl acetate (PVAc) blend and the soft inkpad technique as shown in Figure 4. The gratings formed from the negative patterns on the soft inkpad were wider than the positive gratings inked from the hard mold.

Figure 4. Atomic force microscope images of residue free submicrometer PMMA–PVAc patterns on Si. (a) Positive 700 nm period gratings and (b) their corresponding negative 700 nm period gratings. Height of the PMMA–PVAc patterns is 30 nm.

With the soft inkpad technique, repeated pressing on freshly coated soft inkpads with the same hard mold coats multiple layers of polymer only on the mold protrusion. The multiple-layer polymer film on the mold protrusion is then transferred to a blank substrate to form the stacked polymer patterns as shown in Figure 5.

Figure 5. Atomic force microscope image of micrometer PMMA–PVAc stacked polymer patterns on Si formed without alignment. 5 µm wide protruded circles comprised of four polymer layers with a total thickness of 322 nm.

References

Last Updated: November 19, 2007

E-Mail: pang@umich.edu

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