s4_Stability of Functional Polymers after Imprinting

Stability of Functional Polymers after Imprinting

S. W. Pang, L. Tan, R. M. Reano, W. Hu, Y. P. Kong, H. Y. Low, and A. F. Yee

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

R. L. Jones, C. L. Soles, E. K. Lin, H. W. Ro, A. Karim, and W. L. Wu

National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA

S. J. Weigand, D. T. Keane, J. P. Quintana, and D. M. Casa

Argonne National Laboratory, Argonne, Illinois 60439, USA

A method to pattern polymeric materials, including nonthermoplastic polymers and biomaterials, at low temperature and low pressure was developed. In this method, plasticizers are added to increase the chain mobility of the polymers, resulting in lower imprinting temperature and/or pressure. Three established imprinting and transfer techniques were chosen to demonstrate this method: conventional nanoimprint lithography (NIL), microcontact printing (µCP), and soft ink-pad (SIP). These three techniques were used to pattern poly(3,4-ethylenedioxythiophene) (PEDOT) and chitosan. PEDOT and chitosan were chosen because both of them are nonthermoplastic polymer and therefore cannot be easily patterned using conventional NIL. Imprinting of PEDOT and chitosan films from the polydimethylsiloxane (PDMS) mold was achieved at a low pressure of 10 kPa and 25 °C by controlled addition of glycerol as a plasticizer using conventional NIL; well-defined arrays of 2 µm wide, 185 nm high PEDOT dots have also been demonstrated by µCP; and residual-layer-free recessed PEDOT circles 2 µm in diameter were formed by SIP. In contrast, patterning of PEDOT film without plasticizer requires higher temperature (80 °C) and pressure (10 MPa), which could cause severe deformation of the transferred patterns. This method of plasticizer-assisted imprint lithography broadens the applicability of NIL to a wide range of polymeric materials.

PEDOT and chitosan structures on Si, patterned by plasticizer-assisted imprint lithography (PAIL), are examined under a variety of imprinting conditions. The stabilities of pattern dimension and chemical functionality of these polymers are studied. Thermal annealing for 5 min at 80 °C is found to be an effective method to stabilize imprinted PEDOT patterns. Biofunctionality in chitosan as a function of imprint temperature and pressure is examined through fluorescence spectroscopy. The accessibility of the amine group of chitosan is observed to decrease for imprint temperatures above 80 °C, whereas the chemical functionality is not affected by pressure up to 1 MPa. Fluorescence spectra of the chitosan are observed to be strong functions of exposure time to O₂ plasma.

The primary measure of process quality in NIL is the fidelity of pattern transfer, comparing the dimensions of the imprinted pattern to those of the mold. Routine production of nanoscale patterns will require new metrologies capable of nondestructive dimensional measurements of both the mold and the pattern with subnanometer precision. A rapid, nondestructive technique termed critical dimension small angle x-ray scattering (CD-SAXS) is used to measure the cross sectional shape of both a pattern master, or mold, and the resulting imprinted films. CD-SAXS data are used to extract periodicity as well as pattern height, width, and sidewall angles. Films of varying materials are molded by thermal embossed NIL at temperatures both near and far from the bulk glass transition (T_G). The polymer systems include a photoresist and two homopolymers. Our results indicate that molding at low temperatures (T-T_G < 40 °C) produces small-aspect-ratio patterns that maintain periodicity to within a single nanometer, but feature large sidewall angles. While the observed pattern height does not reach that of the mold until very large imprinting temperatures (T-T_G ≈ 70 °C), the pattern width of the mold is accurately transferred for T-T_G > 30 °C.

NIL is used to fabricate parallel line and space gratings into polymethylmethacrylate (PMMA). CD-SAXS reveals that the periodicity of the grating is (325 ± 1) nm with a trapezoidal cross section of the lines. The average line width, height and sidewall angle of the trapezoid are (141 ± 1) nm, (210 ± 10) nm, and (5 ± 1) °, respectively. CDSAXS also monitors the real time shape evolution of the nanostructures as they are annealed just above T_G of the PMMA. During this anneal the patterns decay with an exponential time dependence. However, in the early stages of annealing, the patterns reduce their height at a rate that is one to two orders of magnitude faster than the rate at which they broaden in width. Studies on several different PMMA samples with different molecular masses indicate that this fast reduction in height relative to the broadening reflects residual flow stresses in the materials created by the imprinting process.

Figure 1(a) and 1(b) show the atomic force microscope images of the imprinted 1:1 glycerol:PEDOT and 1:1 glycerol:chitosan line patterns using NIL. They were generated using a 6 µm deep Si mold under a pressure of 10 kPa at 25 °C.

Figure 1. Atomic force microscope images of (a) 1:1 glycerol:PEDOT line patterns imprinted using NIL; (b) 1:1 glycerol:chitosan line patterns imprinted using NIL.

Imprinting of PEDOT/glycerol mixture by µPC results in well-defined arrays of 2 µm wide and 185 nm high dots, as shown in Figure 2(a). Figure 2(b) shows the atomic force microscope image of 2 µm recessed circles created by SIP, where the negative replica on the PDMS ink-pad was imprinted onto a Si wafer at 25 °C under a pressure of 1 MPa.

Figure 2. Atomic force microscope images of (a) 3:1 glycerol:PEDOT patterns imprinted using µCP; and (b)1:1 glycerol:PEDOT patterns using SIP.

When the glycerol was heavily loaded (glycerol:PEDOT=3:1) into the PEDOT, significant polymer flow occurred and a smooth and continuous film coating on features with high aspect ratio was observed, as shown in Figure 3(a). In contrast, the transfer of plain PEDOT film without plasticizer requires high pressure and temperature, which causes bridging of the transferred film over gaps between protrusions, as shown in Figure 3(b).

Figure 3. Scanning electron micrographs of PEDOT on Si substrate with 920 nm deep patterns. The film was transferred from a flat PDMS mold. (a) Plasticizer was added into PEDOT (W_Plasticizer:W_PEDOT=3:1) and imprinted with a pressure of 10 kPa at 80 °C for 2 min; and (b) no plasticizer was added and imprinted at 10 MPa at 80 °C for 2 min.

Fluorescence micrographs of chitosan imprints of 20 µm circular cylinders are shown in Figure 4. The imprinting temperature was 80 and 160 °C. The fluorescence micrographs clearly show that the fluorescence intensity is temperature dependent, decreasing as a function of temperature.

Figure 4. Fluorescence micrographs of chitosan imprints. (a) Pillars and recessed cylinders imprinted at 80 °C and (b) 20 µm circular cylinders imprinted at 160 °C.

To achieve highly conformal imprints, large values of T-T_G (≈ 40 °C or larger) are suggested for many commercial NIL imprinters. In Figure 5, data from both polystyrene (PS) and PMMA films imprinted at higher temperature demonstrate significant increases in pattern height and follow the dimensions of the mold very closely.

Figure 5. CD-SAXS data from nanoimprinted PMMA films shown as a function ω and q_x. (ω is sample rotation angle and q_x = 2π/L, where L is the repeat period of the pattern.) Shown are data from a film imprinted at T-T_G= 70 °C (top Left) and T-T_G= 80 °C (bottom left). Data are compared to results of fitting to a trapezoidal model for each film (middle) and cross sectional scanning electron micrographs of a sample prepared under nominally identical conditions (right). The white bar in the images represents a length scale of 100 nm.

References

Last Updated: November 19, 2007

E-Mail: pang@umich.edu

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