s2_Reversal Nanoimprint over Topography to Form Multilayer Three-Dimensional Nanostructures

Reversal Nanoimprint over Topography to Form Multilayer Three-Dimensional Nanostructures

S. W. Pang, X. D. Huang, L.-R. Bao, X. Cheng, L. J. Guo, and A. F. Yee

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

In reversal imprinting technique, a polymer layer was first spin coated on a patterned hard mold, and then transferred to a substrate under an elevated temperature and pressure. The reversal imprinting method offers an advantage over conventional nanoimprinting by allowing imprinting onto substrates that cannot be easily spin coated, such as flexible polymer substrates. Another unique feature of reversal imprinting is that three different pattern-transfer modes can be achieved by controlling the degree of surface planarization of the mold after spin coating the polymer resist as well as the imprinting temperature. ‘‘Embossing’’ occurs at temperatures well above the glass transition temperature (T_g) of a polymer; ‘‘inking’’ occurs at temperatures around T_g with nonplanarized polymer coating surface on the mold; and ‘‘whole-layer transfer’’ occurs at temperatures around T_g but with a somewhat planarized surface. These three imprinting modes have been quantitatively correlated with the surface planarization of the mold after polymer coating and the imprinting temperature.

Reversal imprinting technique also allows patterning over a nonflat substrate without the need for planarization. In this process, a polymer film is spin coated onto the mold and then transferred to a patterned substrate by imprinting. By selecting polymers with different mechanical properties, either suspended structures over wide gaps or supported patterns on raised features of the substrate can be obtained with high uniformity. It is found that imprinting at a temperature well above the T_g of the polymer causes the thin residue film between features to dewet from the mold, which can greatly simplify the subsequent pattern transfer process. Multilayer three-dimensional polymer structures have also been successfully fabricated using this new imprinting method. The yield and dimensional stability in the multilayer structure can both be improved when polymers with progressively lower T_g are used for different layers. Compared to existing techniques for patterning on nonflat substrates, the current method has a number of advantages, including simplicity, versatility, high resolution, and low pattern distortion.

An example of reversal imprinted patterns is shown in Figure 1(a). An important feature of the whole-layer transfer mode is the low residue thickness. While the whole-layer transfer mode requires adequate surface planarization of the coated mold, larger step height after coating is advantageous to the inking mode. Figure 1(b) shows the inking result at 105 °C with a step height of 305 nm.

Figure 1. Examples of polymethylmethacrylate (PMMA) imprints in different modes. (a) Imprint in the whole-layer transfer mode at 105 °C with a 350 nm deep grating mold, 7% coating, and R_max=75 nm. (b) Inking at 105 °C with a 650 nm deep grating mold, 6% coating, and R_max=305 nm.

In the reversal imprinting process, since there is no need to spin coat a polymer layer onto the substrate, imprinting on a flexible substrate is quite straightforward. Figure 2 shows PMMA patterns created by reversal imprinting at 175 °C after spin coating the mold with micrometer-sized features and a 350 nm deep grating mold with a 7% solution.

Figure 2. Patterns in PMMA created by reversal imprinting at 175 °C on a 50 µm thick Kapton film: (a) 190 nm deep micrometer-sized mold coated with a 7% solution and (b) 350 nm deep grating mold coated with a 7% solution.

Figures 3(a) and 3(b) demonstrate the imprinted pattern in PMMA (M_w=15 000, T_g =105 °C), which is a relatively brittle polymer (tensile elongation at break=2%), over SiO₂ lines with different widths and spacings. The imprinting was performed at 145 °C with 5 MPa pressure. Figure 3(c) demonstrates the effect of polymer selection on the imprint result. In Figure 3(c), polycarbonate (PC) (M_w=18 200 and T_g=150 °C), an extremely tough polymer with relatively high ductility (tensile elongation at break>110%), was coated on the mold and imprinted at 160 °C under 5 MPa pressure.

Figure 3. Imprinting of 700 nm period grating on 2-µm-deep SiO₂ line patterns with various linewidths and spacings. (a) PMMA pattern imprinted across lines with spacings of less than 2 µm. (b) PMMA pattern imprinted on the protruded surfaces for line spacing of greater than 3 µm. (c) PC pattern imprinted across 5 µm gaps, and 10 µm gaps as shown in the inset.

Another interesting behavior observed in this new imprinting method is the effect of temperature. Figure 4 shows the imprinted PMMA grating pattern on a SiO₂ substrate with 350 nm line spacing at two different imprinting temperatures.

Figure 4. Imprint of 700 nm period grating in PMMA on another 700 nm period grating in SiO₂. (a) Imprinting at 90 °C, continuous residue is formed between lines; (b) imprinting at 175 °C, dewetting on the mold removes the residue during imprinting.

This new imprinting method can also be used to fabricate multilayer, three-dimensional (3D) structures by imprinting over existing polymer patterns. Figure 5 shows a three-layer structure fabricated using three polymers with progressively lower T_g. The residue layer in each layer has been removed by O₂ reactive ion etching (RIE).

Figure 5. Three-layer polymer structure fabricated using the process of building up 3D structures by imprinting over existing polymer patterns. From the top to the bottom, the polymers used are Poly(t-butyl acrylate)(PBA), PMMA, and PC. After imprinting of each layer, the residue film was removed by O₂ RIE.

References

Last Updated: November 19, 2007

E-Mail: pang@umich.edu

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