Langmuir 2004, 20, 2445-2448
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Sub-100 nm Patterning with an Amorphous Fluoropolymer Mold Dahl-Young Khang and Hong H. Lee* School of Chemical Engineering, Seoul National University, Seoul, 151-742, Korea Received October 7, 2003. In Final Form: December 1, 2003 A fluoropolymer mold is introduced and used to pattern sub-100 nm features with the characteristics that cause problems in patterning with a mold. The low surface energy and inertness, stiffness, and permeable nature of the mold material make it possible to pattern without surface treatment densely populated very fine features, mixed patterns of small and large features, and features with a high aspect ratio, when the mold is used with a polymer solution for the patterning. The ultraviolet transparency of the mold material also allows for patterning with photocurable pre-polymers.
Introduction Lithography, or patterning, is one of the key steps in manufacturing many modern products such as electronic, optical, micro-electromechanical system (MEMS), and biological devices. With ever-increasing demand for smaller devices and more economic processes, there has been a need for new technologies for patterning very small features (30 s.). After the molding, the mold was detached manually. No adhered polymer was found on the mold surface as observed by optical microscopy. The same mold was used many times, only with nitrogen blowing to eliminate particle contamination from the laboratory ambient. Shown in Figure 2 are the representative atomic force microscopy (AFM) images of a sample patterned with an amorphous Teflon mold. Densely packed 1 µm × 1 µm square features were well patterned. A drop of PMMA/ TCE polymer solution was dispensed onto a cleaned Si substrate and then the mold was placed upon it, with its patterned face down and slight pressing, for ∼10 min (Route II). The solvent in the solution was absorbed into the permeable amorphous Teflon mold, leaving behind dried and patterned polymer layer that is the negative replica of the mold, as in soft-molding.8 Figure 3 shows the scanning electron microscopy (SEM) images of patterned small (∼80-nm line width, dense lines) and large (∼80 µm × 80 µm isolated rectangle) features on the same sample surface. This result demonstrates that dense 80-nm lines can be patterned with the amorphous Teflon mold, which is not possible with any lithographic technique based on an elastomeric mold because of mold deformation problems. The success lies in the stiffness/hardness of the mold that is coupled with its high permeability. It also shows that a mixed pattern of small and large features that are different in size by
3 orders of magnitude can be molded without any material transport problems that are typically encountered in imprint lithography for such a mixed pattern.15 The permeable nature of the mold material that allows the patterning in a solution state removes the material transport limitation. The patterning result that was obtained according to Route I is shown in Figure 4. Here again, mixed patterns of small and large features on scales of 80 nm (Figure 4a) and 50 µm (Figure 4b) have been molded without material transport problems. For the patterning, a PMMA/TCE 10 wt % solution was spin-coated onto a Si substrate at 500 rpm for 3 s, and then the coated polymer layer was immediately molded with a slight pressing for conformal contact. It is well known that the pattern depth of sub-100 nm features is shallow particularly with the lithographies based on a mold, with the exception of the isolated, largespaced simple pattern features. Even with photolithography, the aspect ratio, which is the ratio of the depth to the feature width, rarely exceeds 3. Shown in Figure 5 are the SEM images of a sample patterned according to Route III in Figure 1. For the patterning, a drop of photocurable pre-polymer was dispensed on a cleaned Si substrate. An amorphous Teflon mold was then placed on it, followed by UV illumination through the mold for >30 s. As can be seen from the magnified version (Figure 5b), the molded pattern height or depth is about 500 nm for the dense lines with about 80-nm width, yielding an aspect ratio larger than 6. It is remarkable that such a high aspect ratio can be achieved for 80-nm lines. Conclusions An amorphous fluoropolymer as a mold material for patterning has been presented. The unique properties of
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Langmuir, Vol. 20, No. 6, 2004
Khang and Lee
Figure 4. SEM images of molded patterns using amorphous Teflon mold (Route I). PMMA/TCE 10 wt % solution was spin-coated at 500 rpm for 3 s onto Si and then molded. (a) ∼80-nm line width, dense lines, and its large-area view (inset), (b) large area (∼50 µm) interdigital patterns and its low magnification image (inset).
Figure 5. Perspective SEM images of molded patterns using amorphous Teflon mold (Route III).18 Photocurable pre-polymer solution was dropped onto Si and then UV illuminated through the mold for 30 s. (a) ∼80-nm line width, dense lines, and (b) magnified view of a. The pattern height is ∼500 nm, which leads to an aspect ratio of ∼6:1.
the mold material allow one to pattern various cases of sub-100 nm features. These include the following: (1) Patterning of high-density small features down to 80 nm without mold deformation problem because of its high stiffness. In comparison, our experimental results showed that such high-density features could not be patterned with PDMS molds including the composite mold. (2) Clean mold release or de-molding without any surface treatment because of its low surface energy and inertness. (3) Patterning in liquid state by absorbing the solvent as in soft molding because of highly permeable structure, thus eliminating material transport problems that are typically encountered in imprint lithography in patterning combinations of small and large features or density variations on the same mold. In addition, any organic solvents, except the fluorinated ones, can be used in patterning without the mold swelling that is problematic in patterning with a PDMS mold. (4) Patterning of photocurable pre-polymer
by UV illumination because of its transparency to UV light. It is quite remarkable that the mold would allow one to achieve an aspect ratio larger than 6 for sub-100 nm features which, for example, may be particularly useful in the manufacturing of ultra-hydrophobic surfaces.19 These results indicate that the novel mold could open the path to patterning any type of sub-100 nm features with some characteristics that are not conducive to generalpurpose patterning. Acknowledgment. This work was supported in part by the Brain Korea 21 project. LA0358668 (18) The slight variation of pattern thickness in the figure was originated from the SiO2/Si master mold that was used for the replication of amorphous Teflon mold, as confirmed from separate SEM and AFM measurements (not shown here). (19) Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hasimoto, K. Langmuir 2002, 18, 5818.
