Sung-Won Youn

Size Dependence of Quick Cavity Filling Behavior in Ultraviolet Nanoimprint Lithography Using Pentafluoropropane Gas

Rapid bubble elimination using pentafuoropropane (CFH2CH2CF3, HFC-245fa, CAS No. 460-3-1) condensing gas is one of the most promising methods to realize ultrahigh-speed ultraviolet nanoimprint lithography (UV-NIL). In this study, we investigated the shrinkage behaviors and elimination time of bubbles for different cavity sizes and resist thicknesses by employing a UV-NIL stepper with a real-time monitoring system. As predicted, a smaller cavity size resulted in a faster filling. Unlike the prediction from an analysis model based on Stefan’s equation, a nonlinear decrease in bubble elimination time was observed in accordance with the decrease in cavity size (area). Instead, a linear relationship between bubble elimination time and cavity width was found for a certain range of cavity widths (W ). In the cavity width, range from 25 to 340 mm, bubble elimination time was almost proportional to cavity width and could be defined as 0:00145 � W (s). When the cavity width was 25 mm, the complete filling time was less than 0.033 s, indicating the potential to realize a ultrahigh-throughput nanopatterning process. Regarding the effect of initial resist thickness on a bubble shrinkage behavior, bubble elimination time tended to increase with the decrease in resist thickness. # 2010 The Japan Society of Applied Physics

Flexible Polyimide Micropump Fabricated Using Hot Embossing

Micropumps are important components of advanced microfluidic systems. Here, polyimide (PI) as an advantageous structural material for flexible micropumps was focused on. This is because PI has many advantageous properties such as high thermal stability and superior mechanical strength. However, the difficulty in realizing an all-PI micropump lies in fabricating microstructures on PI film surfaces. In this paper, we present a novel all-PI micropump fabricated using hot embossing. The micropump had diffuser/nozzle valves and functioned by vibrating a 2-µm-thick PI diaphragm with alternating air pressures between 0 and 10 kPa at a frequency of 3 Hz. The height and diameter of the PI micropump chamber were 200 µm and 5 mm, respectively. The flow rate of water in the micropump was 34 µl/min. This micropump is suitable for flexible microfluidic systems.

45 nm hp line/space patterning into a thin spin coat film by UV nanoimprint based on condensation

Applicability of UV nanoimprint based on gas condensation to 45 nm line and space (L/S) patterning using a spin-coat UV curable resin was evaluated. Although high capillary pressure is generated in such fine L/S patterns when UV curable resin liquid comes in contact with the groove surface, the trapped air in those grooves is not completely compressed and results in the formation of bubble defects. Bubbles much larger than the pattern size were generated at the middle of the L/S patterns for a 108 nm thick, UV curable resin film, and incomplete filling may occur on a large area of the L/S patterns for a 19 nm thick film. Since nearly bubble-free L/S patterns have been fabricated for 48 nm thick, UV curable resin film, it might be possible to make bubble-free UV nanoimprint by prolonging the imprinting time. On the contrary, when UV nanoimprint was carried out in pentafluoropropane (PFP), no bubbles were created for all thicknesses of the UV curable resin films. Uniformity of residual layer looked quite good judging from the color uniformity of the area around the L/S patterns. It was found, from the residual layer measurement by a reflective thickness monitor, that bubble-free UV nanoimprint was carried out with a residual layer thickness of 15.2 nm. UV nanoimprinted samples were inspected with a scanning electron microscope and it was verified that UV nanoimprint using PFP is viable for the fabrication 45 nm L/S patterns.Applicability of UV nanoimprint based on gas condensation to 45 nm line and space (L/S) patterning using a spin-coat UV curable resin was evaluated. Although high capillary pressure is generated in such fine L/S patterns when UV curable resin liquid comes in contact with the groove surface, the trapped air in those grooves is not completely compressed and results in the formation of bubble defects. Bubbles much larger than the pattern size were generated at the middle of the L/S patterns for a 108 nm thick, UV curable resin film, and incomplete filling may occur on a large area of the L/S patterns for a 19 nm thick film. Since nearly bubble-free L/S patterns have been fabricated for 48 nm thick, UV curable resin film, it might be possible to make bubble-free UV nanoimprint by prolonging the imprinting time. On the contrary, when UV nanoimprint was carried out in pentafluoropropane (PFP), no bubbles were created for all thicknesses of the UV curable resin films. Uniformity of residual layer looked quite go...

A process of glassy carbon etching without the micro masking effect for the fabrication of a mold with a high-quality surface

The surface quality of a mold is one of the major factors in imprint lithography as it determines the final surface quality and the minimum size of the replicated patterns. This paper describes a process for O2-based reactive ion etching (RIE) of glassy carbon (GC) without encountering any micro masking effect. Glassy carbon, because of its attractive properties such as its surface inertness, thermal stability and extraordinary hardness, has drawn much interest as a mold material for high-temperature imprinting on glasses and metals. Etch profiles with highly smooth surfaces free of micro masking effects were achieved by adding SF6 to the etching gas. The fraction of SF6 in the gas mixture (ranging from 0.1 to 0.6) showed little change in the quality of the etched surface, but it did lead to a proportional decrease in the etch rate of GC. A reasonable GC etch rate (115?120 nm min?1) and a smooth etch surface were obtained using SF6 at a fraction 0.2 or below. Using electron beam lithography (EBL), and processing under the established SF6/O2 RIE conditions, GC molds were fabricated and successfully applied to thermal imprinting onto glass and metals.

Fabrication of a micro patterned parylene-C master by hot-embossing and its application to metallic mold replication

This study demonstrates a replication process for metallic micro mold that combines poly-chloro-p-xylylene (parylene-C) hot-embossing and electroplating techniques. First, the replication characteristics of parylene-C were investigated and compared with those of poly-methyl-methacrylate (PMMA) in terms of the dimensional accuracy of replication. For a given mold (aspect ratio 1:1), the complete filling conditions were 150 °C–50 kgf–200 s for PMMA and 260 °C–200 kgf–900 s for parylene-C, respectively, implying the significantly better durability and heat resistance of parylene-C. The results of surface topological analysis revealed that the dimensional deviation between the mold and the resulting embossed profiles was <2.94% for PMMA and <2.32% for parylene-C, indicating that a hot-embossed parylene-C could be an alternative master for electroplating. Then, a micro-patterned parylene-C master was prepared by hot-embossing with a mold (aspect ratio 1:2.5) and was applied to metallic mold replication. The patterned fields composed of lines of structures in the Ni mold with 25 µm height and 10 µm width/spacing were successfully replicated on parylene-C with a dimensional deviation of about 1.4%. The electroplated Cu successfully filled parylene-C replica master patterns with an aspect ratio of 2.5 without void formation by both adding organic addictives and controlling the seed layer thickness. After electroplating, the Cu micro mold could be successfully separated from the parylene-C replica master.

Dynamic mechanical thermal analysis, forming and mold fabrication studies for hot-embossing of a polyimide microfluidic platform

In this study, we have explored the hot-embossing of polyimide films through dynamic mechanical thermal analysis (DMTA), forming and mold fabrication studies. First, the relationship between the formability and the temperature for the polyimide film was investigated by DMTA and hot-embossing tests. The DMA results represented that, as a polyimide goes through its glass transition, it exhibited dramatic decreases in storage elastic modulus, storage shear modulus and viscosity as well as the peak of tangent delta, and continued to show strong dependence on frequency and temperature. The filling characteristics of the polyimide film investigated by hot-embossing tests showed a sharp increase in the replicated depth near the glass transition temperature. Second, silicon microfluidic platforms and molds were prepared by electron beam lithography (EBL) combined with inductively coupled plasma (ICP). An ICP etching condition to prevent reverse taper of a microstructure was investigated, and Si microfluidic channels and high aspect-ratio microstructures with nearly vertical sidewall were structured. Finally, the channel structures could be successfully replicated on the polyimide surface by hot-embossing with the Si mold prepared by using EBL combined with ICP plasma etching.

UV Nanoimprint in Pentafluoropropane at a Minimal Imprint Pressure

UV nanoimprint in air and in pentafluoropropane (PFP) was carried out at imprint pressures ranging from 200 down to 10 kPa using a mold with fractal structure recesses with nominal sizes of 1.3, 3.9, 11.7, 35, 105, and 316 µm. While UV nanoimprint in air suffered severe bubble defect problems, UV nanoimprint in PFP was free from the bubble defect problem even at 10 kPa. From real-time monitoring of recesses being filled with resin for UV nanoimprint in PFP, it was found that the recesses are filled in the ascending order of sizes and filling is completed in 2–4 s. No imprint pressure by the imprint apparatus is needed for UV nanoimprint in PFP, and a full wetting can be spontaneously realized even at a concave area where a large amount of gas is liable to be trapped. This suggests that a capillary force must play an important role of filling at the initial stage of contact and filling between unparallel surfaces of the mold and wafer.

Control of Parameters Influencing the Thermal Imprint of Parylene/Silicon

The study aims to investigate the possible defects that may occur during imprinting of poly(chloro- p-xylylene) (parylene-C) film (thermal oxidation, delamination, thermal cracking and insufficient filling at the periphery) and to overcome them by modifying the process conditions and mold design. X-ray diffraction (XRD) analyses results for the parylene-C films indicated that higher deposition pressure leads to a lower crystallinity of parylene-C film. By tuning the process conditions and mold design, patterned fields (composed of arrays of 25-µm-high, 10-µm-wide and 1-mm-long lines with 10 µm spacing) in 0.4-mm-thick and 20-mm-sized nickel molds could be successfully replicated on 60-µm-thick parylene-C films deposited at both 25 and 45 mTorr. Complete filling over the whole imprint area could be achieved at 175 °C, followed by an oxygen plasma etching.

Residual layer uniformity using complementary patterns to compensate for pattern density variation in UV nanoimprint lithography

How to form a thin and uniform residual layer, which is difficult to be created for a feature with nonuniform pattern densities, is of critical importance in nanoimprint lithography since residual layer removal by a reactive ion etching process will result in poor pattern transfer fidelity for nonuniform residual layers. A capacity-equalized mold using complementary cavity patterns to balance the imprinted volume of the feature with pattern density variation was verified to be a good method against pattern density variation for producing a uniform residual layer. To obtain an insight into the validity of the capacity-equalized mold, the effects of the introduced complementary patterns for capacity equalization on the residual layer, at locations that were not only at a distance away from the introduced complementary patterns but also the ones that were very close to the introduced complementary patterns, were carefully studied. It was found that a capacity-equalized mold with even very coarse complementar...

Microstructuring of 45-µm-Deep Dual Damascene Openings in SU-8/Si by UV-Assisted Thermal Imprinting with Opaque Mold

In this study, we demonstrated a process for fabricating a dual damascene opening in a chemically amplified negative UV photoresist, such as SU-8. The process is based on a UV-assisted thermal-imprinting process, which involved UV pretreatment and thermal imprinting using a Ni mold. After establishing process parameters (e.g., adhesion enhancing, UV pre-treating, thermal imprinting, and etch back conditions) for the given geometry and dimensions of the mold pattern, we could successfully form 25-µm-deep trenches with a 10 µm line-and-space pattern and 45 µm-deep via holes in SU-8/Si samples by UV-assisted thermal imprinting, followed by O2/CHF3 reactive ion etching (RIE). Delamination defects could be avoided with the help of adhesion enhancement treatment. The process studied here showed great potential in reducing time and cost compared with the standard photolithographic dual damascene process.