Invited paper. Issues on nanoimprint lithography with a single-layer resist structure. Applied Physics A Materials Science & Processing

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1 Appl. Phys. A 81, (2005) DOI: /s g.y. jung 1 w. wu 1 s. ganapathiappan 1 d.a.a. ohlberg 1 m. saif islam 2 x. li 1 d.l. olynick 3 h. lee 4 y. chen 5 s.y. wang 1 w.m. tong 1,6 r.s. williams 1, Issues on nanoimprint lithography with a single-layer resist structure Applied Physics A Materials Science & Processing 1 Hewlett Packard Laboratories, 1501 Page Mill Road, Palo Alto, CA 94304, USA 2 Department of Electrical and Computer Engineering, University of California, Davis, CA 95616, USA 3 Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS , Berkeley, CA 94720, USA 4 Division of Materials Science and Engineering, Korea University, 5Ga-1, Anam-Dong, Sungbuk-Gu, Seoul , Korea 5 Department of Mechanical and Aerospace Engineering, UCLA, CA 90024, USA 6 Technology Development Operations, Inkjet Technology Platform, Hewlett Packard Company, 1000 Circle Boulevard, Corvallis, OR 97330, USA Invited paper Received: 30 May 2005/Accepted: 3 June 2005 Published online: 4 August 2005 Springer-Verlag 2005 ABSTRACT We summarize our key developments in nanoimprint lithography (NIL) that employs a single layer resist lift-off process: lowering of the imprint temperature (for thermal imprint) and pressure, achieving uniform resist thickness and low residual resist layer thickness in the trenches, and eliminating metal rabbit ears for the single-layer lift-off. In thermal NIL, our requirements for lower operating temperature and pressure motivated us to develop an alternative resist that is a viscous fluid at room temperature and cures at a lower temperature of 70 C than the operating temperature of the conventional thermal NIL ( 200 C). For UV NIL, we devised a method to dispense the resist onto a hydrophobic mold and use the hydrophilic substrate surface to spread the resist via surface wetting to engineer a continuous and uniform film. We also explored the use of Si(110) substrates as molds to produce features with perfectly vertical side walls, and the use of aqua regia to directly etch away rabbit ears. PACS h; Nd; Rf 1 Introduction Nanoimprint lithography (NIL) [1, 2] has gained acceptance in recent years as a viable low-cost alternative to photolithography for the patterning of nanoscale features. It uses direct contact between the mold (or template) and the thermoplastic or UV-curable resist to imprint the pattern and, unlike photolithography, does not require expensive optics to image sub-wavelength features. At Hewlett Packard Labs, we were motivated to develop NIL because we wanted to fabricate nanoscale devices and circuits with feature sizes beyond those obtainable by commercially available photolithography. We have reported using a single-layer NIL process to fabricate non-volatile cross-bar memory devices at 65 nm half pitch (hp) with a thermally cured resist process [3] and at 50 nm hp with a UV resist process [4]. We have encountered issues that were universal for NIL: lowering of the imprint temperature (for thermal imprint) and Fax: , stan.williams@hp.com pressure, achieving uniform resist thicknessand low residual resist layer thickness in the trenches, and eliminating metal rabbit ears for the single-layer lift-off. This paper summarizes the results of our development. 2 New thermal NIL resist and process to lower imprint temperature and pressure Poly(methyl methacrylate) (PMMA) has been widely used as the thermoplastic polymer in NIL despite its requirements for high operating temperature ( 200 C)and pressure ( 2000 psi). In the PMMA process, a spin-coated film is baked to remove any residual solvent, resulting in a hard polymer film. Such a film is essentially a solid, requiring a high imprint temperature (typically more than 90 C above the glass-transition temperature T g ) to make it soft, and a high imprint pressure to ensure that it flows into the grooves of the mold. High temperatures cause layer-to-layer misalignment and are detrimental to our molecular switching media. High pressures are detrimental to the nanoscale features on the mold. These reasons motivated us to seek a different approach to the thermal nanoimprint process. We employed as our resist poly(benzyl methacrylate) (PBMA) that was dissolved in its own monomer (BMA) rather than in a separate solvent. The PMBA/BMA mixture was formulated to contain mostly the monomer so that, like PMMA, it could be spin coated on the substrate. A key advantage of our process is that the resist film, which contains mostly the monomer BMA, is a viscous fluid at room temperature and can fill the fine openings of the mold easily under lower hydrostatic pressure. The percentage of polymer in the monomer solution is critical. A solution with high polymer content would be too viscous to spread evenly under low pressure, leading to a non-uniform film. However, a solution with a low polymer content would dry too quickly before imprinting. We found from a series of experiments that an 8% solution of polymer in monomer was optimal. An initiator was added to this solution to polymerize the monomer at a lower setting temperature. The procedure in our approach is different from that for the conventional thermal imprint with PMMA: in our method, the resist is a liquid instead of a solid at room temperature, so a lower pressure of 200 psi is applied before instead of

2 1332 Applied Physics A Materials Science & Processing HP resist PMMA Material Temperature Above decomposition 90 C above T g temperature of initiator (> 180 C) (70 C in our case) Dissolved in Its monomer (BMA) Chlorobenzene Post-application Not required Required bake State at room Viscous liquid Solid temperature Process sequence Pressure Temperature Temperature Pressure psi 70 C 190 C 2000 psi TABLE 1 Comparison between the thermal imprint processes that employ the HP resist and PMMA after the heating step. Then, the temperature is raised to the initiator s decomposition temperature, which is significantly lower than the temperature required for the conventional process ( 200 C). In our case, the patterned resist is heated only to 70 C to trigger the polymerization process and solidify the film. Table 1 compares our process with the conventional one [5]. Figure 1 shows a cross-sectional view of imprinted polymer resist patterned with our method and resist formulation. Although no residual layer was observed at the bottom of the trenches by cross-sectional scanning electron microscopy (SEM), a six-second de-scum step was performed before metal deposition to ensure that the trenches were free of resist. The short de-scum time also ensures that there was a high thickness uniformity of the resist film after imprinting. Figure 2 shows an 8 8 cross-bar memory structure at 65 nm hp with a cell density of 6.4Gbit/cm 2. It was fabricated by FIGURE 1 SEM image of sectional view of the thermally imprinted resist, showing no residual layer at the bottom of the trench FIGURE 2 Eight by eight cross-bar memory circuit. The switching molecules were sandwiched between two sets of eight nanowires fabricated by thermal imprint lithography and lift-off. The cell density is 6 Gbit/cm 2 a sequence of two imprints as described above at 90 to each other. 3 Air-free and uniform resist film by surface engineering UV NIL, also known as step and flash imprint lithography (SFIL), has emerged to generate nanoscale patterns at room temperature and at a low pressure by replacing the thermal resist with a UV-curable resist. Its key advantage over thermal NIL is that it does not require a heating step, which is detrimental to our switching molecules and to maintaining good alignment. However, the requirements for high thickness uniformity and for a thin residual layer with a low imprinting pressure remain. We devised a new method to satisfy these requirements. The mold surface by necessity has been rendered extremely hydrophobic (water contact angle > 110 ) because it has been treated with a mold-releasing agent [6] to facilitate mold substrate detachment after imprint. When the liquid resist is directly applied to the mold, it stays as a drop and does not spread. On the other hand, the substrate surface can be made hydrophilic (i.e. with a high free energy) to facilitate the spreading of the resist, such as treating with a piranha solution and/or water-vapor plasma. However, if applied directly to such a substrate, the resist would spread in an uncontrolled manner. Moreover, when the mold is placed onto an alreadyspread liquid resist film, air is inevitably trapped in the resist pattern, particularly in the nanoscale features. We devised a method that takes advantage of the opposing properties of the two surfaces: on the mold we applied a small drop of the liquid resist, which stayed localized as a small bead due to the low free energy of the mold surface; then we placed the substrate on the bead. The hydrophilic surface of the substrate acted to spread the resist droplet uniformly, pulling the resist across the mold surface by the wetting action of the resist and substrate. Figure 3 shows how the resist spread with time after contact. Trapped air was seen near the

3 JUNG et al. Issues on nanoimprint lithography with a single-layer resist structure 1333 FIGURE 4 SEM image of 34-cross-bar structure with an equivalent cell density of 10 Gbit/cm 2, fabricated with the resist-spread process described in Sect. 3 sidual resist, and the sample was irradiated with UV light to cure the resist. Figure 4 shows the cross-bar structure that was fabricated with the above method. Two sets of 34 parallel nanowires at 50 nm hp were successively imprinted at 90 to each other, forming a cross-bar structure that had an equivalent cell density of 10 Gbit/cm 2. 4 Overcoming the challenge of rabbit ears FIGURE 3 Images showing the spread of the resist by the method describedinsect.3.(a) 5min,(b) 10min,and(c) 30 min after substrate/mold contact edges of the sample initially, but was pushed out as the resist occupied the channels. The interference fringes, which were caused by the thickness difference of liquid resist, disappeared with time, illustrating the flow of the liquid from the spreading drop to empty areas to produce a uniform film. The spread time increases with both the amount and the viscosity of the resist used. That is why we employed a small amount of the liquid (0.2 µl for a 1-in 2 substrate) and formulated it to have a low viscosity. On the sample shown in Fig. 6, the resist spread across the entire sample area after 30 min.however,at this stage the contact between the two surfaces was still not perfectly conformal across the entire substrate because of the inevitable nanoscale difference in flatness between the mold and the substrate. This would have resulted in a significant residual resist layer under the trenches of the resist layer. The mold and the substrate, now bonded tightly together by the liquid resist, were placed in the nanoimprinter. A hydrostatic pressure of about 20 psi was applied to squeeze out the re- In any lift-off process, an undercut or a negativesloped resist profile is generally preferred to ensure no deposition on the side wall of the resist. Otherwise, metal protrusions, which look like rabbit ears, can form after lift-off. However, it is very difficult to achieve such profiles in NIL because it will result in mechanical locking between the mold and the resist patterns. Figure 5a shows a SEM image of a typical mold, which has side walls that were neither smooth nor vertical. The slope would be directly imprinted into the resist, resulting in metal deposition onto the side wall. Figure 5b shows an atomic force microscopy (AFM) topograph of 34 metal wires and their fan-out structure after imprinting with such a mold. The bright spots are rabbit ears that would electrically short the top electrodes, so they needed to be removed before the top electrodes were fabricated on top of them. One common approach is to switch to a bilayer process that uses the NIL-patterned top layer as an etch mask to produce an undercut in the bottom, but such a process requires additional processing steps that make it much more time consuming. Viable alternatives that employ a single-layer resist when scaled up would be more cost effective. We discuss our exploratory work on two such alternatives below. The first alternative is to employ a Si(110) substrate as the mold. The advantage of Si(110) is that it can be etched with KOH to obtain trenches with perfectly vertical side walls formed by the Si(111) planes, because the KOH etch rate for the (110) plane is 100 times faster than that for the (111)

4 1334 Applied Physics A Materials Science & Processing FIGURE 5 (a) SEM image of nanowire features on the mold with rough and sloped side walls; (b) AFM image of 34 metal nanowires and their fanout structure imprinted by the mold in (a), showing rabbit ears at the edge of the wires FIGURE 7 Aqua regia solution etching of rabbit ears. AFM images of metal patterns (a) before etching and (b) after a 5-min dip into the aqua regia solution. (c) High-resolution scan and profile of the area circled in (b), showing a smooth metal surface with no rabbit ears technique does introduce new steps to mold making and is not applicable to complex patterns, our preliminary results show its promise. Another alternative is to directly etch the metal rabbit ears chemically. An aqua regia solution of HCl:HNO 3 (1:3 by volume) can etch platinum at room temperature. Because the rabbit ears are nearly vertical and therefore exposed to the etch solution on both sides, their etch rate should be at least twice that for the nanowires, which are only exposed on the top side. Our experimental results are shown in Fig. 7. Figure 7a shows an AFM topograph of the nanowires immediately after lift-off, showing clear rabbit ears in both the nanowires and fan-out regions. After dipping the sample into the aqua regia solution for 5 min, the rabbit ears were greatly diminished, as can be seen in Fig. 7b. Figure 7c contains the detailed AFM scan and profile of the nanowires in the circled area in Fig. 7b, showing a very smooth surface without any rabbit ears. FIGURE 6 (a) SEM image of Si(110) substrate etched by KOH solution at 110 C with silicon oxide layer as an etch mask. The vertical side walls are along the Si(111) plane. (b) AFM topography of 1.5-µm-wide nanowires patterned with a single-layer resist and the mold shown in (a) plane [7]. A mold thus fabricated (Fig. 6a) was used to perform UV imprint lithography. Figure 6b shows the AFM profile of the imprinted titanium wires after metal lift-off. No rabbit ears were present at the edge of the wires. While this 5 Summary We have summarized the key results of our development of an NIL process that employs a single-layer resist. In thermal NIL, our requirements for lower operating temperature and pressure motivated us to develop an alternative resist that is a viscous fluid at room temperature and can be cured at a lower temperature than the operating temperature of the conventional imprint, which depends on the glass-

5 JUNG et al. Issues on nanoimprint lithography with a single-layer resist structure 1335 transition temperature of PMMA. For UV NIL, we devised a novel technique for dispensing resist that takes advantage of the opposing surface energies of the mold and the substrate to produce a uniform and air-free resist film. We also explored the use of Si(110) substrates as molds to produce features with perfectly vertical side walls to prevent the formation of rabbit ears, and experimented with the use of aqua regia to directly etch away rabbit ears once they were formed. Even though one might obtain superior results with a bilayer process, we believe that our results are important because of the simplicity and the lower cost of a single-layer process. by DARPA. ACKNOWLEDGEMENTS This research was supported in part REFERENCES 1 S.Y. Chou, P.R. Krauss, P.J. Renstrom: Science 272, 85 (1996) 2 M. Colburn, S. Johnson, M. Stewart, S. Damle, T. Bailey, B. Choi, M. Wedlake, T. Michaelson, S.V. Sreenivasan, J. Ekerdt, C.G. Willson: Proc. SPIE 3676, 379 (1999) 3 Y. Chen, G.Y. Jung, D.D.A. Ohlberg, X. Li, D.R. Stewart, J.O. Jeppesen, K.A. Nielsen, J.F. Stoddart, R.S. Williams: Nanotechnology 14, 462 (2003) 4 G.Y. Jung, S. Ganapathiappan, D.A.A. Ohlberg, D.L. Olynick, Y. Chen, W.M. Tong, R.S. Williams: Nano Lett. 4, 1225 (2004) 5 S.Y. Chou, R.K. Peter, J.R. Preston: Appl. Phys. Lett. 67, 3114 (1995) 6 G.Y. Jung, Z. Li, W. Wu, Y. Chen, D.L. Olynick, S.Y. Wang, W.M. Tong, R.S. Williams: Langmuir 21, 1158 (2005) 7 M.S. Islam, S. Sharma, T.I. Kamins, R.S. Williams: Nanotechnology 15, L5 (2004)