s6_Three-Dimensional Nanostructures by Reversal UV Imprint

Three-Dimensional Nanostructures by Reversal UV Imprint

S. W. Pang, W. Hu, B. Yang, and C. Peng

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

Three-dimensional (3D) SU-8 micro- and nanostructures were fabricated using a reversal UV imprint process at low temperature and low pressure. The SU-8 polymer was coated on a patterned glass mold and then transferred onto various substrates by reversal UV imprint at a typical temperature of 50 °C, pressure of 1 MPa, and UV exposure of 1 s. The lower temperature and pressure used compared to conventional thermal imprint shorten the imprint time and alleviate pattern distortion. A combination of silanes was used to generate a medium surface energy on the imprint molds to enable polymer spin coating and mold release after imprint. In addition, an O₂ plasma was used for glass mold treatment to improve uniformity of silane coating and to increase substrate surface energy for better polymer adhesion. Using this technology, 100 nm–1 µm wide SU-8 gratings were fabricated on flat or patterned substrates with good fidelity. By repeating this process, multiple-level nanochannels, cavities, or air-bridging polymer structures with 400 nm–10 µm widths have been demonstrated. The surface energy of SU-8 was modified using an O₂ plasma to promote layer adhesion for 3D stacking. This reversal UV imprint technology offers versatility and flexibility to stack polymer layers and multiple-level sealed fluidic channels.

There is a wide range of applications for 3D nanochannels in biomedical systems and fluidic control. A simple and versatile technique to create 3D nanochannels with width from 200 nm to 2 µm is demonstrated using sequentially stacked reversal UV nanoimprint of SU-8. Its advantages include controllable channel profile, low pressure and temperature for imprints, and flexibility in designing 3D channels by stacking. In a typical reversal UV imprint, SU-8 is spin coated on a glass mold and then transferred onto Si substrates by an UV imprint process at low temperature of 55 °C, low pressure of 2 MPa, and UV exposure of 1–4 s. While reversal UV imprinting top SU-8 layer onto bottom SU-8 layer, the UV exposure and imprint sequence of the top SU-8 layer and its effect on channel profile control are investigated. It has been found that initially UV-cured top SU-8 layer is preferred for good channel profile control because UV-cured SU-8 layer is prevented from flowing down into bottom SU-8 layer.

A novel technology combining UV lithography, hybrid mold, and reversal imprint for fabricating 3D structures is developed. A hybrid mold made from quartz is used. The mold has structures patterned by lithography and dry etching. The quartz mold also has selected patterns formed by 50 nm thick Cr. A layer of UV definable SU-8 polymer is spin coated onto the hybrid mold and patterned by optical lithography. The mold with the patterned SU-8 layer and no residue is then transferred to a substrate with topography by reversal imprint with temperatures as low as 50 °C and pressures of nominally 2 MPa. Depending on the dimensions of patterns on the mold compared to the ones on the substrate and the imprint pressure, patterns can be selectively transferred to substrates through reversal imprint. This technology greatly simplifies the fabrication process and provides more flexibility in building complex 3D structures.

Polymer film patterns were formed on substrates with microstructures using an elastomeric polydimethylsiloxane (PDMS) pad, wherein either a continuous or a patterned film on flat PDMS was used for pattern creation. During patterning of the continuous film, a polymer film is first spin coated onto the elastomer pad, and the pad is then brought into contact with the patterned substrate of interest. Since the PDMS can deform elastically around features on the substrate and its surface has low interfacial energy, films on PDMS can be successfully transferred onto the substrate. The resulting profile of the transferred film would depend on the pattern dimensions of the substrate, polymer properties, and conditions for imprinting. Three distinctive film patterns can be achieved, which we designate as continuous film transfer over microstructures, film transfer on both trenches and protrusions, and film transfer on protrusions. Interestingly, a negative replica of patterns on substrates can be simultaneously created on the PDMS pad in the last patterning process and the pattern can be further utilized in subsequent patterning steps. This affords the capability of transferring a patterned film from the PDMS to the sidewalls of the topographic features. Because of the great versatility of this patterning technique, it can be used to rapidly form channels, conformal coating on a patterned substrate, and micro- or nanometer sized patterns inside trenches of a patterned substrate.

Reversal UV imprint can pattern polymer structures directly onto devices with topography and create cavities underneath. Sealed cavities were formed for both the 1 and 10 µm wide gratings as shown in Figure 1 (a). Figure 1(b) shows air-bridging structures formed by this method. Figure 1(c) shows two levels of SU-8 sealed channels formed by three layers of SU-8 gratings transferred from the same glass mold using the reversal UV imprint. Imprints were carried out at 50 °C and 1 MPa. The top and middle layers were UV exposed for 1 s and the bottom layer was UV exposed for 10 s.

Figure 1. (a) SU-8 structures transferred onto a Si grating substrate with 1 µm depth, SU-8 heated at 40 °C for 3 min; (b) 1 µm wide, 600 nm deep SU-8 gratings bridging on Si grating substrate after SU-8 residue removal; (c) Two layers of SU-8 grating structures forming two levels of sealed SU-8 channels.

An alternative reversal imprint process to obtain well-controlled channel profile while imprinting at low temperature and pressure is to use a thin SU-8 adhesion layer during the second imprint. Channels fabricated using this process are shown in Figure 2.

Figure 2. Channels fabricated using a thin SU-8 adhesion layer. Patterns in SU-8 are formed by imprinting at 55 °C, 2 MPa, and 1 s UV exposure. The patterned SU-8 layer is reversal UV imprinted on a thin SU-8 adhesion layer at 55 °C, 1 MPa, and 2 s UV exposure. (a) One-level 2 µm wide channels, (b) two-level 2 µm wide channels, (c) four-level 400 nm wide channels, and (d) four-level 200 nm wide channels.

Transferred SU-8 patterns on Si and SU-8 are shown in Figure 3. The reversal UV imprint is carried out at 80 °C for Figure 3(a) and 50 °C for Figure 3(b). For both imprints, the pressure is 5 MPa for 1 min, then 1 s UV exposure, followed by another 1 min at the same temperature and pressure.

Figure 3. Micrographs of reversal UV imprint. (a) 3.5 µm thick SU-8 transferred to flat Si substrate and (b) 3.5 µm thick SU-8 layer with 2 µm diameter interconnect openings over 5 µm half-pitch SU-8 gratings.

Figure 4 shows selective patterning of an array of posts over SU-8 gratings. The SU-8 posts of 1.3 µm diameter and 1.7 µm height are selectively transferred to only the top surface of the SU-8 gratings in the substrate.

Figure 4. Selective transfer of 1.3 µm diameter SU-8 posts onto 5 µm wide, 300 nm deep gratings by reversal UV imprint. The imprint conditions are 80 °C and 5 MPa for 1 min, then 1 s UV exposure, followed by another 1 min at the same temperature and pressure.

Under higher pressure and at imprinting temperature slightly above glass transition temperature (T_g) of the polymethylmethacrylate (PMMA) film, pattern of continuous film transfer over microstructures is achieved. Under lower pressure, the pattern of film transfer on protrusions is observed. Figure 5(a) presents a negative replica of the patterned substrate left on the PDMS pad. Under higher pressure and at imprinting temperature below T_g of PMMA, suspended structures along narrower gaps or the continuous film in contact with the bottom of the trenches with wider gaps are observed, as shown in Figure 5(b). The pattern of film transfer on both protrusions and trenches will be achieved for relatively deep trenches. Circular rings left on the PDMS pad are subsequently inked onto a Si wafer as shown in Figure 5(c).

Figure 5. Atomic force microscope and scanning electron microscope images of PMMA films. (a) PMMA patterns with a thickness of 160 nm, the negative image of the patterned substrate, inked from a PDMS pad at 115 °C under 3 MPa pressure to a Si wafer, (b) PMMA film with a thickness of 320 nm transferred onto a 0.9 µm patterned Si substrate to form continuous film transfer over microstructures, and (c) PMMA circular rings with a thickness of 160 nm, the negative image of the micrometer sized dots on a Si substrate, inked from the PDMS pad at 115 °C under 3 MPa pressure to a Si wafer.

The pattern of film transfer on protrusions offers us the capability to pattern the sidewalls or trenches of microstructures. Figures 6(a) and 6(b) show atomic force microscope images of negative replica on the PDMS pad imprinted onto a patterned Si substrate at 105 °C under pressure of 3 MPa.

Figure 6. Imprinted images of PMMA transferred from a PDMS pad with patterns to a Si substrate with 720 nm deep SiO₂ patterns at 115 °C and 3 MPa. The patterns on the PDMS pad was created by contacting the PMMA coated PDMS with a Si substrate with 2.5 µm deep microstructures at 0.1 MPa and 110 °C.

References

Last Updated: November 19, 2007

E-Mail: pang@umich.edu

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