NANO LETTERS
Patterned Transfer of Metallic Thin Film Nanostructures by Water-Soluble Polymer Templates
2003 Vol. 3, No. 9 1305-1309
Charles D. Schaper* Department of Electrical Engineering, Stanford UniVersity, Stanford, California 94305-9510 Received June 17, 2003; Revised Manuscript Received June 23, 2003
ABSTRACT Water-soluble polymer templates are replicated from surface patterns by spin-casting a poly(vinyl alcohol) film-forming solution that undergoes a room-temperature evaporation process to solidify in less than 1 min with lateral resolution of sub-100 nm. The patterned polymer templates are deposited with metal and coupled to substrate surfaces by methods that may include photocurable materials, two-component reaction systems, or direct bonding without an intervening adhesion layer. Water is used to dissolve the flexible polymer templates at the conclusion of the transfer process.
High-resolution printing is an important step in the nanoscale manufacturing of electronics, data storage devices, and displays, in addition to emerging applications in microfluidics, MEMS, optoelectronics, plastic electronics, and biological sensors. Current research focuses on printing processes other than standard photolithography to lower the costs of nanofabrication and to increase the range of patterning applications.1 These methods include soft lithography that relies on a reusable template, generally fabricated of poly(dimethylsiloxane) (PDMS), to transfer patterned materials directly from the template to the substrate.2 Significant work in this area has been done, and challenges remain, on achieving balanced interfacial forces to affect a transfer of materials by adhesion to the substrate with subsequent release from the template.3 The properties and processing requirements for the soft lithography template require a stable “release layer” that may limit such schemes in resolution and in material/substrate combinations that are conveniently transferable.4-7 In this laboratory, a new class of printing processes that use a soluble polymer template for pattern formation has been demonstrated. The template dissolves in water after template/ substrate coupling, thus eliminating the potential complications associated with physical release and template reuse while providing several advantages including resolution, application range, and alignment.8 The class of printing strategies that use these water-soluble templates is collectively referred to as molecular transfer lithography (MxL). The low-cost strategy has been proven to transfer patterns * E-mail:
[email protected]. Tel: 650-723-2873. Fax: 650-7238473. 10.1021/nl034412s CCC: $25.00 Published on Web 07/26/2003
© 2003 American Chemical Society
in organic adhesive and etch-resist layers by bonding the templates to the substrate surface through an intervening adhesive layer of photocurable or reactive two-component systems, followed by dissolution of the template with water. This paper demonstrates for the first time that the MxL strategy of employing patterned water-soluble templates is useful not only for transferring patterns but also for transferring structured materials, in particular, metal thin films. This specific demonstration of a new category of application for MxL, namely, patterned materials transfer, allows a comparison with soft lithography schemes as described in ref 3 that show metal structure transfer from soft PDMS, as well as hard GaAs and Si templates, using a process called nanotransfer printing (nTP). The first phase of the material-transfer process of MxL involves the replication of prefabricated surface patterns as water-soluble templates, as depicted in Scheme 1A. The master surface pattern may be produced using standard lithography techniques and materials such as patterned silicon wafers. The MxL approach begins with a spin-casting of poly(vinyl alcohol) (PVA) film-forming solution onto the surface of the master pattern. The PVA solution undergoes solidification involving evaporation of the casting solvents at room temperature to form a thin film with a thickness and drying period dependent upon the rotation speed, time, and ambient conditions. Resolution is achieved by exploiting the ability of spin-casting to fill the voids and recesses of the master surface pattern. The room-temperature solidification process eliminates the effects of thermal expansion mismatch between the polymer and master pattern substrate. The next step of the manufacturing sequence involves
Scheme 1. Procedure to Replicate Surface Topography in the Form of Metallized Water-Soluble Templatesa
a (A) Procedure begins with spin-casting a poly(vinyl alcohol) (PVA) film-forming solution onto a master pattern, the binding of solid PVA sheet, and the removal from the master pattern to form a water-soluble template, followed by (B) deposition of a metal thin film.
Figure 1. Water-soluble template replicated from a 100-mm silicon master pattern. The template contains multilayer structures and is sputter coated with a 55-nm-thick film of gold. The template includes a plastic sheet backing for improved stability and handling.
binding a preformed solid sheet of PVA to the outer surface of the spin-cast thin film. This step is achieved by rolling the sheet on the surface of the thin film prior to full drying of the applied liquid solution, allowing for sufficient time to adhere the materials together through intermixing of the polymer surfaces. The solid water-soluble template is then detached from the master pattern by peeling. The addition of the solid preformed sheet reduces potential distortion. An alternative method of constructing the water-soluble template involves using the prefabricated soluble sheet as a tape and placing the adhesive side on the dried film after spin-casting. After the fabrication of the water-soluble template, a metallic thin film is deposited on the surface as shown in Scheme 1B. The metal film may be deposited using a sputtering technique. In Figure 1, a 100-mm-diameter water-soluble template is presented with a thin film coating of gold approximately 55 nm thick. The master pattern consisted of a multilevel 100-mm silicon wafer pattern generated from a four-masklevel process. A plastic backing adhered to the water-soluble template was provided commercially with the solid soluble sheet and aided with rigidity and handling. The overall thickness of the template was 100 µm. The replication materials in liquid solution form were obtained from Fiberlay, Inc. as their PVA product in an alcohol blend including ethyl alcohol, acetic acid ethenyl ester polymer with ethonol, and water. PVA in solid sheet form was purchased as large-area 1306
water-soluble tapes on rolls from Shercon, Inc. Spin-casting was performed at roughly 3000 rpm for 12 s. It was noted that faster spin speeds toward 6000 rpm resulted in a dry film at the conclusion of the rotation period. The binding process involved placing a preformed square 150-mm solid sheet of PVA on the liquid surface. The binding period was 4 min in duration for solid-liquid adhesion but took less than 20 s for the dry adhesion procedure. For dissolvable template use to be feasible for high-volume production, the replication procedure must be rapid to achieve high throughput rates by making the master pattern available for the next replica fabrication. Metal thin films were deposited on the template surface using a sputtering tool. The water-soluble polymer templates were secured to a solid support of a silicon wafer using Kapton tape during the deposition procedure. Several metal targets were tested, including copper, chromium, and aluminum; however, gold seemed to produce the best results for all experiments, as discussed below. A wide variety of patterns were replicated on the watersoluble templates and then coated with a thin film metal. To form advanced nanodevices and structures, a range of geometries and dimensions is needed. A representation of patterns replicated on the template surface included multilayer complex structures as depicted in Figure 2A. Highresolution lines measured as 53 nm are depicted in Figure 2B. Posts are shown in Figure 2C. Midrange resolution lines of 135 nm are depicted in Figure 2D. The master surface patterns were obtained from optical lithography (Figure 2A) or from electron-beam lithography in PMMA over silicon. The PMMA photoresist was baked for 6 h prior to electronbeam lithography patterning to remove residual casting solvent. The patterned metal thin films on the water-soluble templates may be transferred to substrates using chemical bonding techniques involving an intervening adhesive layer. In this approach, the patterned metal thin film surface is connected via the liquid adhesive from the template to the substrate. The intervening liquid adhesive material, which may contain additional properties such as plasma etch resistance, should not appreciably dissolve or distort the metal film. The chemical bonding mechanism of the adhesive layer may be photoinitiated or achieved through a reaction mechanism such as hardening over time by interaction with the moisture in the ambient. This transfer approach is depicted in Scheme 2A. After adhesion is achieved, water is added to dissolve the template and leave the thin film bonded to the substrate. Another approach is presented in Scheme 2B where the metal film is bonded directly to the substrate surface. The final result of both approaches is a patterned thin film structure in-phase and in the same orientation as the master pattern used for replication. The processes that may generate defects include the detachment of the template from the master pattern, the contact of the template to the substrate, the bonding procedure, and the water dissolution of the template. With regard to template fabrication, there are at least two issues to consider for a successful replication: (a) aspect ratios greater than about 5 in the master pattern tend to result in a Nano Lett., Vol. 3, No. 9, 2003
Figure 2. Replicated patterns with a sputtered gold film on a water-soluble template demonstrating (A) pattern replication complexity, (B) sub-100-nm lines, (C) an array of posts, and (D) 135-nm lines with measuring markers shown on each line. Scheme 2.
Methods of Metal Thin Film Pattern Transfera
a (A) Liquid adhesive is coated on the surface of the substrate, and then a water-soluble template is placed into liquid, followed by bonding through UV or two-part reactive schemes. Water is then used to dissolve the template, leaving the metal thin film bonded to the substrate surface. (B) Direct bonding of the patterned thin film to the substrate in the absence of an intervening liquid adhesive. After the soluble template has been bonded to the surface, it is dissolved with water, leaving the metal thin film adhered to the substrate.
tearing of the spin-cast film during removal because of lateral shear forces and (b) patterned photoresist surfaces as master patterns need to be free of residual solvents because there may be interaction with the spin-cast PVA, preventing replication. It is interesting that the master surface pattern apparently does not require special release layers because the replication material is nominally recommended as a parting barrier between various resins and mold surfaces, thus acting as a protection material for the expensive master surface pattern. Moreover, the PVA template is sufficiently elastic and can be tailored through proper selection of the water content of the prefabricated sheet or with the addition of plasticizers to permit conformable contact during both the replication and transfer of the pattern and materials. The prefabricated sheets are delivered as a roll and are soft enough to be indented with a pointed object. Conventional contact aligners utilizing vacuum pressing techniques are particularly effective in aiding transfer by removing air. Alternatively, simply rolling the template onto the substrate using a cylindrical rod is effective in achieving reliable Nano Lett., Vol. 3, No. 9, 2003
contact and minimal air bubbles. The presence of micro air bubbles may also be limited because of their inclusion within the liquid adhesive. In Figure 3A, a metal thin film transfer of a blazed diffraction grating of gold is shown bonded via an adhesive layer to a silicon substrate. The intervening adhesive layer (MB300, Master Bond, Inc.) with 2-3 cps viscosity was first spin-coated onto the silicon substrate at 3500 rpm for 12 s. The replicated grating pattern was placed on the surface and allowed to set by waiting for 30 s. The system was placed in a water bath for 10 min, after removing the plastic sheet, to dissolve the template, leaving the thin film of gold bonded to the substrate through the adhesive. This process was repeated using a master pattern consisting of an array of holes etched in silicon dioxide approximately 100 nm thick. The metal thin film replication transferred to a silicon substrate is depicted in Figure 3B. Other adhesive materials tested that yielded similar results included UV-curable epoxy (UV15, Master Bond), a 50:50 mixture of UV15 and the siliconcontaining material Zipcone UA (Gelest), Norland Optical Adhesive 6001 and Summers SK-9 optical cement obtained from Edmund Industrial Optics, Inc. The development of distortions and cracks in the metal thin film may be of concern because of stresses during the phases of deposition, transfer, and bonding. For the applications involving gold, this issue was not seen to be a problem as confirmed by electrical continuity tests. For chromium and aluminum, microscopic examination and electrical resistivity measurements suggested distortions and cracks in the patterned thin films after sputter deposition, with the greatest density of defects apparent on flat topography. However, the films did stay intact during the transfer process, as seen in Figure 3C for an aluminum thin film transfer that used MB300 as the intervening adhesive layer. Copper thin films demonstrated electrical continuity after sputter deposition and transfer. However, copper deposited on a 50 mm × 31 mm patterned polymer template showed distortions and electrical discontinuity after 9 months. This result was 1307
Figure 3. Thin film transfer of (A) a gold pattern of a diffraction grating with an intervening adhesive layer on top of a silicon substrate, (B) a hole pattern in gold of approximately 100-nm depth, and (C) a transfer of a multilevel aluminum thin film where distortions and cracks were noted after sputter deposition; the film was maintained intact during the transfer process.
Figure 4. Patterned structure in gold transferred directly to a silicon/silicon dioxide topography depicting (A) transfer without an intervening adhesive layer over the topography and (B) deep submicrometer pattern replication and transfer capability.
attributed to thermal expansion effects of the free-standing structure because the template was secured only at the perimeter. A control sample of a copper thin film deposited on a 72 mm × 25 mm patterned template and secured to a solid support exhibited an absence of cracks during that same period. Distortion and cracking of the metal thin film due to shrinkage of the adhesive material during curing are important possibilities to consider. In the experiments, such developments were not observed in gold with the photocuring of UV15, although detachment of this epoxy from silicon topography was noted. Direct coupling of a metal thin film of gold to a silicon substrate was demonstrated without the use of an intervening adhesive layer. The water-soluble polymer template was replicated using a direct dry-adhesion method of removing the spin-cast material from a master pattern of PMMA on silicon produced by electron-beam lithography. The 25 mm × 50 mm template was deposited with a thin gold film of roughly 25 nm using a sputter tool. The target substrate was 1308
of silicon with a 100-nm-thick film of silicon dioxide. Topography on the substrate was fabricated using photolithography to obtain roughly 1-µm-wide oxide lines running on the silicon surface in a square field of 16 mm. An adjacent field consisted of an array of 0.5-µm holes etched into the silicon dioxide and grouped into 100-µm-wide subfields at 1-mm pitch across the 16-mm square field. The patterned gold metal film was brought into contact with the surface, and the system was slightly heated to approximately 60 °C for 2 min during the transfer process, with pressure applied by rolling a metal rod on the backside. The heating step was useful in relaxing the template so that adequate contact with the substrate was obtained. Afterward, the template was dissolved in water, leaving the gold patterned film attached to the surface. Electrical continuity of the gold thin film structure was confirmed. A tape adhesion test showed some flaking at the edges of the transferred pattern, indicating a weak interactive force between the film and substrate. In Figure 4A, a patterned structure in gold on silicon dioxide Nano Lett., Vol. 3, No. 9, 2003
topography over silicon is presented. This pattern is replicated from a master surface pattern used to test the fidelity of the electron-beam lithography tool. A closer view of the structure is depicted in Figure 4B to demonstrate deep submicrometer replication and transfer capability. The MxL printing strategies that employ water-soluble templates provides a practical approach to high-resolution nanofabrication. The tooling required is standard, and the chemicals are readily available, low-cost, nontoxic, biocompatible, and simple to apply and store, with processing outside of a cleanroom feasible. The range of geometries and complexities needed for nanofabrication is achievable with MxL. The method is capable of both pattern definition and, as the paper proves, the transfer of materials such as patterned thin film metals. These attributes of range and resolution at low cost suggest that MxL will have significant application in research and commercial development of nanofabricated device technologies at molecular scales.
Nano Lett., Vol. 3, No. 9, 2003
Acknowledgment. DARPA Advanced Lithography and NSF ECS sponsored this research. Prof. R. F. W. Pease and Prof. T. Kailath provided useful discussion. Dr. J. Conway of the Stanford University Nanofabrication Facility performed the electron-beam lithography work. References (1) Pease, R. F. W. Nature 2002, 417, 802. (2) Xia, Y.; Whitesides, G. M. Annu. ReV. Mater. Sci. 1998, 28, 153. (3) Loo, Y.-L; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. J. Am. Chem. Soc. 2002, 124, 7654. (4) Quake, S.; Scherer, A. Science 2000, 290, 1536. (5) Sprenger, M.; Walheim, S.; Schafle, C.; Steiner, U. AdV. Mater. 2003, 15, 703. (6) Chan-Park, M. B.; Yan, Y.; Neo, W. K.; Zhou, W.; Zhang, J.; Yue, C. Y. Langmuir 2003, 19, 4371. (7) Glasmastar, K.; Gold, J.; Andersson, A.; Sutherland, D. S.; Kasemo, B. Langmuir 2003, 19, 5475. (8) Schaper, C. D. Proc. SPIE Emerging Lithographic Technologies 2003, 5037, 538.
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