Langmuir 2005, 21, 6127-6130
6127
Improved Pattern Transfer in Nanoimprint Lithography at 30 nm Half-Pitch by Substrate-Surface Functionalization Gun-Young Jung,† Zhiyong Li,† Wei Wu,† S. Ganapathiappan,† Xuema Li,† Deirdre L. Olynick,‡ S. Y. Wang,† William M. Tong,†,§ and R. Stanley Williams*,† Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto, California 94304, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 02-400, Berkeley, California 94720, and Technology Development Operations, Inkjet Technology Platform, Hewlett-Packard Company, 1000 Circle Boulevard, Corvallis, Oregon 97330 Received January 4, 2005. In Final Form: May 6, 2005 Resist detachment from the substrate during mold-substrate separation is one of the key challenges for nanoimprint lithography as the pitch of features decreases. We analyzed the problem by considering the surface and interfacial free energies of the initial state and the possible final states of the moldpolymer-substrate system and designed the chemistry of the system to provide the desired final state. We dramatically improved the resist adhesion to the substrate by assembling a monolayer of surface linker molecules on the substrate surface. A 37 nanowire pattern at 30 nm half-pitch was imprinted onto the surface-modified substrate.
Nanoimprint lithography (NIL)1,2 has been used to fabricate structures such as a single electron memory device,3 fluidic channels,4 waveguides,5 and protein patterns.6 Recently it has achieved resolution that surpassed that for even the most advanced photolithography. Thus, in 2003 NIL was placed on the International Technology Roadmap for Semiconductors (ITRS) as a candidate for next-generation lithography for the 32 nm node.7 We have reported using NIL to fabricate a nonvolatile crossbar memory device at a 65 nm half-pitch with a thermally cured resist process8 and a 34 × 34 crossbar structure at 50 nm half-pitch with a UV-curable resist.9 In contrast to other approaches that employ bilayer resist processes,2,10 to date our nanoscale circuits were successfully patterned with a single-layer resist. While the bilayer processes have certain advantages, such as undercut resist profiles that are defined by reactive ion etching, single layer processes have far fewer process steps and have the advantage of being more cost-effective to implement for large-volume manufacturing. As the pitch size in our test circuits was reduced, the polymer was more likely to adhere to the nanoscale gap * Corresponding author. E-mail:
[email protected]. Phone: (+1)650-857-6586. Fax: (+1)650-236-9885. † Hewlett-Packard Laboratories. ‡ Lawrence Berkeley National Laboratory. § Hewlett-Packard Company. (1) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272, 85. (2) Colburn, M.; Johnson, S.; Stewart, M.; Damle, S.; Bailey, T.; Choi, B.; Wedlake, M.; Michaelson, T.; Sreenivasan, S. V.; Ekerdt, J.; Willson, C. G. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3676, 379. (3) Wu, W.; Gu, J.; Ge, H.; Keimel, C.; Chou, S. Y. Appl. Phys. Lett. 2003, 83, 2268. (4) Guo, L. J.; Cheng, X.; Chou, C. F. Nano Lett. 2004, 4, 69. (5) Wang, J.; Schablitsky, S.; Yu, Z. N.; Wu, W.; Chou, S. Y. J. Vac. Sci. Technol. B 1999, 17, 2957. (6) Hoff, J. D.; Cheng, L. J.; Meyhofer, E.; Guo, L. J.; Hunt, A. J. Nano Lett. 2004, 4, 853. (7) http://public.itrs.net/Files/2003ITRS/Home2003.html. (8) Chen, Y.; Jung, G. Y.; Ohlberg, D. A. A.; Li, X.; Stewart, D. R.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart, J. F.; Williams, R. S. Nanotechnology 2003, 14, 426. (9) Jung, G. Y.; Ganapathiappan, S.; Ohlberg, D. A. A.; Olynick, D. L.; Chen, Y.; Tong, W. M.; Williams, R. S. Nano Lett. 2004, 4, 1225. (10) Sun, X. Y.; Zhuang, L.; Zhang, W.; Chou, S. Y. J. Vac. Sci. Technol. B 1998, 16, 3922.
between features on the mold and detach from the substrate surface during the mold separation. Surface modification techniques have been used to provide a specific function on various surfaces such as water repellency,11 adhesion improvement,12 corrosion inhibition,13 and biocompatibility.14 In this letter, we report surface modification of the substrate for NIL to enhance the resist adhesion to the substrate, leading to fine pattern transfer in the polymer layer. The silicon mold was patterned by e-beam lithography at Lawrence Berkeley National Laboratory.15 Prior to the first use, it was exposed to the vapor of a releasing agent, CF3(CF2)5(CH2)2SiCl3 (tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane), to form a self-assembled monolayer (SAM) that assists mold-resist detachment.16 The formulation of a UV-curable monomer solution for NIL was optimized through many experimental trials. Its formulation was composed of three ingredients. Irgacure 184 (absorption peak: 280 and 320 nm, Ciba) was used as the UV-sensitive free radical generator and dissolved into benzyl methacrylate monomer solution (Aldrich). In addition, 2-hydroxyethyl methacrylate (Aldrich) was added to lower the surface energy to allow facile resist release from the mold. The composition of the solution was 77% monomer, 20% releasing promoter, and 3% free radical generator by weight. Prior to use, the resist solution was filtered through syringe filters with a 0.2µm pore size to remove residual particles. The borofloat glass substrate was soaked in piranha solution and then treated with water vapor plasma for 10 (11) Mayer, T. M.; de Boer, M. P.; Shinn, N. D.; Clews, P. J. J. Vac. Sci. Technol. B 2000, 18, 2433. (12) Ohashi, K. L.; Yerby, S. A.; Dauskardt, R. H. J. Biomed. Mater. Res. 2001, 54, 419. (13) Ramachandran, S.; Tsai, B. L.; Blanco, M.; Chen, H.; Tang, Y. C.; Goddard, W. A. Langmuir 1996, 12, 6419. (14) Pfohl, T.; Kim, J. H.; Yasa, M.; Miller, H. P.; Wong, G. C. L.; Bringezu, F.; Wen, Z.; Wilson, L.; Kim, M. W.; Li, Y.; Safinya, C. R. Langmuir 2001, 17, 5343. (15) Jung, G. Y.; Ganapathiappan, S.; Li, X.; Ohlberg, D. A. A.; Olynick, D. L.; Chen, Y.; Tong, W. M.; Williams, R. S. Appl. Phys. A 2004, 78, 1169. (16) Jung, G. Y.; Wu, W.; Li, Z.; Chen, Y.; Wang, S. Y.; Tong, W. M.; Williams, R. S. Langmuir 2005, 21, 1158
10.1021/la050021c CCC: $30.25 © 2005 American Chemical Society Published on Web 05/28/2005
6128
Langmuir, Vol. 21, No. 14, 2005
Letters Scheme 1 a
a For a single-layer process, as the feature pitch decreases, the interface area between the resist and the mold Ar-m increases while that between the resist and the substrate Ar-s remains constant. This causes the left-hand term in the free-energy inequality to increase. At a certain pitch, the left-hand term will overtake the right-hand term, resulting in the resist detaching from the substrate and adhering to the mold. The red line denotes the self-assembled releasing layer on the mold surface.
min at a power of 10 W and a pressure of 0.7 Torr to produce a hydroxyl-terminated surface, resulting in a very hydrophilic surface. The drop method was used for the film application as detailed in a previous report.9 A drop of UV-curable monomer solution was placed on the hydrophobic mold surface and stayed as a small droplet because of its high surface tension. The hydrophilic substrate was placed on top of the mold. The monomer solution was spread uniformly across the region between the mold and the substrate through a combination of the weight of the substrate pressing down on the solution and the surface tension causing the solution to wet the substrate. During this process, all the air was expelled from between the mold and the substrate, and the final solution film uniformity was such that no interference fringes were observable over the entire contact area after 30 min.9 Another advantage of the drop method was that the residual layer thickness under the imprinted trenches was negligible after imprinting, as determined by thickness measurements. The gap distance between the substrate and the mold can be controlled by adjusting the viscosity and volume of the monomer solution dispensed on the mold. After the resist had spread uniformly, the sample was loaded into a custom-built nanoimprinter, where the monomer solution was cured by UV irradiation through the glass substrate for 15 min at a hydrostatic pressure of 20 psi. The excess resist solution under the mold features was displaced to the edges of the contact area while applying imprinting pressure. Resist adhesion to the mold during mold-substrate separation is one of the key challenges to nanoimprinting. An important guide for controlling the release is to consider the thermodynamics of the process governed by surface and interfacial free energies. The initial state is the unseparated substrate-resist-mold system; the two extreme final states are resist detachment from the mold and resist adhesion to the mold. The state with the lower ∆G will be preferred. We present a simple model (Scheme 1) to assist in this discussion. The total free energy of the surfaces and interfaces of the initial state before mold-substrate separation can be expressed by
Gi ) Ar-sγr-s + Ar-mγr-m
(1)
After mold-substrate separation, two extreme cases
bracket the possible outcomes. If the resist detached from the mold, the total free energy of the final system would be
Gf ) Ar-sγr-s + Ar-m(γr + γm)
(2a)
If the resist detached from the substrate and adhered to the mold, the total free energy of the final system would be
Gf ) Ar-mγr-m + Ar-s(γr + γs)
(2b)
In the above equations, γr, γs, and γm are the surface free energies per area of the resist, of the substrate and of the mold; γr-m and γr-s are interface free energies per area of the resist-mold and of resist-substrate interfaces; and Ar-s and Ar-m are the interface areas between the resist and the substrate and between the resist and the mold. For the resist to detach from the mold and adhere to the substrate, the following must hold:
∆Gresist detaches from mold < ∆Gresist adheres to mold
(3)
S Ar-m(γr + γm - γr-m) < Ar-s(γr + γs - γr-s) (4) Scheme 1 illustrates the tendency of the resist to detach at decreasing pitch length. In a single-layer process, the height of the mold features is held constant as the lateral feature size is decreased to preserve process latitude in subsequent steps. This causes the contact area between the mold and the resist, Ar-m, to increase. As the pitch size decreases for a particular process chemistry, the lefthand term in eq 4 will at some point overtake the righthand term, causing the resist to adhere to the mold and detach from the substrate. Previous approaches have focused on coating the mold with a releasing layer, in effect lowering the γm term to make ∆Gresist detaches from mold smaller. Using this approach, we have successfully imprinted patterns down to 50 nm half-pitch.9 However, at 30 nm half-pitch, it was no longer sufficient, as illustrated by our imprint results shown in Figure 1. A scanning electron microscopy (SEM) image of the resist after mold detachment, Figure 1b, shows that only the patterns of the sparser 60 nm half-pitch fan-out wires remained on the substrate, whereas the denser patterns at 30 nm half-pitch had completely detached from the substrate and adhered to the mold. A similar effect has
Letters
Langmuir, Vol. 21, No. 14, 2005 6129
Figure 2. Representive FTIR spectra in the region 800-1800 cm-1 of methacryloxypropyltrichlorosilane in the transmission mode through a liquid film (labeled “Neat liquid”) and in the reflection mode from a SAM on a silicon dioxide on silicon surface (labeled “Monolayer”). Figure 1. (a) SEM image of the mold in a region in which two wire pitches coexist. The half-pitches are 60 nm in the fan-out region and 30 nm in the nanowire region. (b) SEM image of the imprinted nanowire pattern in the resist.
been observed by Kawai,17 who reported that the static load required to collapse a pattern was proportional to the substrate-resist contact area. A further means to improve the release from the mold is to increase the adhesion between the resist and the substrate. We made ∆Gresist adheres to mold larger by decreasing the free energy of the resist-substrate interface γr-s using a linker monolayer. A good linker molecule should bond covalently to the substrate as a SAM and bond chemically to the resist polymer during UV-curing. After a series of experiments, we found methacryloxypropyltrichlorosilane to be an effective linker. As illustrated in Scheme 2, the trichlorosilane group can bond to the hydroxyl groups on the glass substrate, forming a monolayer through self-assembly (SAM),18,19 and its methacrylate end can copolymerize with the resist during UV exposure. The SAM film on the substrate was produced by first treating the substrate with piranha solution to produce hydroxyl termination and then soaking inside 0.2 wt % methacryloxypropyltrichlorosilane in toluene for 1 h. After the substrate was removed from the solution, it was cleaned thoroughly with fresh toluene in an ultrasonic bath and dried with nitrogen gas. To verify the presence of a SAM of our linker molecule on a suitably prepared substrate surface, both reflection absorption infrared spectroscopy (RAIR) and ellipsometry were utilized. The former method can confirm the existence of the linker molecule on the substrate, while the latter one can physically provide the effective thickness or density of the molecular layer formed. We used heavily doped silicon substrates (As > 1020 cm-3) instead of glass for these experiments, because silicon has a ∼2 nm native oxide that mimics the glass substrate surface but the free carriers in the Si bulk reflects the incoming electromagnetic field to provide monolayer sensitivity to the chemisorbed molecules. The SAM of methacryloxypropyltrichlorosilane was applied to the oxide-covered silicon substrate in the same manner as that for the glass substrate. The RAIR spectra were collected by a Nexus 870 Fourier transform infrared (FTIR) spectrometer (Thermo-Nicolet, (17) Kawai, A. J. Photopolym. Sci. Technol. 2002, 15, 121. (18) Bunker, B. C.; Carpick, R. W.; Assink, R. A.; Thomas, M. L.; Hankins, M. G.; Voigt, J. A.; Sipola, D.; de Boer, M. P.; Gulley, G. L. Langmuir 2000, 16, 7742. (19) Ulman, A. Chem. Rev. 1996, 96, 1533.
Scheme 2. Schematic Diagram Illustrating the Mechanism of the Silanation Reaction of the Linker with Hydroxyl Groups on the Glass during Initial SAM Formation and of the Copolymerization between the Methacrylate Groups in the Linker and That in the in the Resist during Subsequent UV Irradiation
Madison, WI) equipped with a liquid nitrogen cooled mercury cadmium telluride detector. The incident radiation was p-polarized, 80° from the surface normal. Typically 1000 acquisitions were used to collect the RAIR spectra for the molecular film. Figure 2 shows a representative RAIR spectrum of the SAM and a transmission spectrum of a neat film of methacryloxypropyltrichlorosilane in the range of 800-1800 cm-1 for comparison.
6130
Langmuir, Vol. 21, No. 14, 2005
The majority of the peaks from the neat film spectrum were also present in the SAM spectrum. In particular, the peak at 1726 cm-1, which is the characteristic vibrational mode of the carbonyl group in the methacryloxy molecules, was observed with a strong intensity in the molecular layer. Also, peaks at 1635 cm-1 (CdC stretch), 1300 cm-1 (C-O-C stretch), and 1173 cm-1 (CsO stretch) were found in the molecular film. The C-H vibrational peaks in the range 2800-3000 cm-1 were poorly resolved in the molecular film (result not shown). The thickness measurements were made using a Gaertner ellipsometer (Chicago, IL), operating at an incident angle of 70° with three different wavelengths (543.5, 632.8, and 832.2 nm). A bilayer model built into the equipment software was used to obtain the thickness of the molecular layer with an approximated refractive index of 1.46. We measured the thicknesses of the surface layer for the bare substrate, which contains the native oxide layer, and for the substrate with the molecular layer. The difference between the two measurements, 0.91 ( 0.15 nm and 2.13 ( 0.19 nm, was ∼1.2 nm, which is consistent with the length of the linker molecule, confirming the formation of a closely packed single molecular layer on the substrate with the molecules approximately normal to the surface. We imprinted a glass substrate that had a SAM of the linker under NIL process conditions that were otherwise identical to the unsuccessful attempt shown in Figure 1b. Figure 3a is the SEM image of a resist layer with 37 distinct nanowire impressions at 30 nm half-pitch and the transition to the fan out on top of a linker-functionalized substrate. The SAM dramatically improved the nanoimprint pattern transfer to the substrate surface. Figure 3b displays a high-resolution image of the nanowire impressions. The thin residual layer under the trenches after imprinting was etched away with oxygen plasma (50 W, 20 mTorr) for 10 s. Then Ti (4 nm) and Pt (6 nm) were deposited by e-beam evaporation at a rate of 0.3 Å/s at a base pressure of 1 × 10-6 Torr. A subsequent lift-off process was performed with acetone in an ultrasonic bath to form the nanoscale metal wire patterns, as shown in Figure 3c, signifying a successful nanoimprint process. We demonstrated a new approach for reducing detachment of nanoimprint resist from the substrate during mold separation by chemically functionalizing the substrate surface. A SAM of a linker molecule (methacryloxypropyltrichlorosilane) was used to lower the interface energy between the substrate and the resist. As a result, a successful imprinting of 30 nm half-pitch nanowire patterns on the polymer resist layer was achieved with improved resist adhesion to the substrate. It is important to analyze and engineer the chemistry of all the surfaces and interfaces involved in a complex NIL process (and not
Letters
Figure 3. (a) SEM image of the imprinted resist layer on the surface treated with the linker molecule, demonstrating good resist adhesion to the substrate and detachment from the mold in both dense and spare regions. Close-up SEM images in the 30 nm half-pitch nanowire region showing high fidelity pattern transfer: (b) the imprinted resist layer and (c) the Pt nanowires after lift-off.
just the polymer, for instance) to construct a system with free energies that yield the desired result. Acknowledgment. This research was supported in part by the Defense Advanced Research Projects Agency (DARPA). LA050021C
