Nanofabrication module integrated with optical aligner Colin Stuart,a兲 Qianfei Xu, Ricky J. Tseng, Yang Yang, H. Thomas Hahn, and Yong Chen Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, 420 Westwood Plaza, Los Angeles, California 90095
Wei Wu and R. Stanley Williams Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto, California 94304
共Received 1 September 2005; accepted 19 December 2005; published 8 February 2006兲 In this article, we describe a simple module that can be integrated with a commercial optical aligner for nanoimprint lithography or optical lithography. The module provides a convenient low-cost technique to transform an optical aligner for microfabrication into a nanofabrication machine. This combination enables the creation of nanoscale features and alignment of multiple-layer lithographic patterns with submicron accuracy within one instrument. Imprinting of 30 nm half-pitch lines has been demonstrated by the module, as well as submicron alignment. The module has also been used to fabricate micro- and nanoscale patterns simultaneously by the combination of optical and imprint lithography. © 2006 American Vacuum Society. 关DOI: 10.1116/1.2166861兴
I. INTRODUCTION The last decade has seen a dramatic increase in the demand for tools that can generate structures at the nanometer scale but are also economically viable. Currently, the most ubiquitous lithography method is optical lithography, which is employed extensively by the integrated circuit 共IC兲 industry. Its resolution is ultimately limited by the wavelength of the light used for exposure, which can be overcome by utilizing shorter wavelengths in the extreme ultraviolet and x-ray regions, but with the penalty of a significantly higher cost. Electron-beam lithography and scanning probe lithography, while having excellent resolution, have insufficient throughput for commercial applications due to scanning processes. A promising low-cost, high-throughput alternative that has emerged is nanoimprint lithography 共NIL兲. This technique has been used for pattern transfer of features smaller than 10 nm 共Refs. 1–3兲 and fabrication of numerous devices for various electronic, biological, and optical applications.4–8 NIL essentially involves physical deformation of a resist layer by a mold with topographical features, followed by curing of the resist through thermal or optical methods 共Fig. 1兲. A residual layer remains in the resist layer after demolding, which must be removed by anisotropic etching before the layer is used for further pattern transfer. To achieve the best results a thin and uniform residual layer is desirable, requiring a way to apply an equal force across the surfaces of the substrate and mold. Of the two curing methods, photocurable NIL is preferable due to lower imprinting pressure, less curing time for the resist, and no need for thermal control.9 In order to use the UV light to cure the resist, the mold holder should be transparent to UV light in the area above the mold. Another critical feature for a lithography a兲
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machine is the alignment capability, as the alignment of multiple layers is often required to fabricate nanodevices with complex functions. II. MODULE DESCRIPTION We have developed a nanofabrication module for photocurable NIL that can be integrated with a SUSS MicroTec MA-6 optical aligner. It consists of a mold holder, a substrate holder, and a system of valves and gas lines to regulate gas pressure and vacuum 共Figs. 2 and 3兲. Both holders are made of 6061-T6 aluminum, with dimensions carefully specified to enable interaction with the aligner. Aluminum was chosen for its light weight and ease of machining. The module employs gas pressure on the backside of both the substrate and the mold to achieve homogenous pressure and gap width. When the substrate and mold are brought into contact, an o-ring forms an outer seal so that the air can be evacuated from the space inside the seal. This is necessary to ensure that air bubbles do not destroy the fidelity of the patterns transferred to the resist, and also because oxygen scavenges the free radicals needed for polymerization and slows the curing reaction. The central vacuum area is much larger than the size of the mold and substrate; therefore, it can hold the substrate and mold holders together while pressure is applied to the backside of the substrate and mold. Under normal operating conditions the aligner does not need to supply any additional vertical force to the module. The silicone rubber outer seal is designed with an outward-curving shape so that it can be compressed enough to bring substrate and mold into contact, while still forming a vacuum seal. The system also contains silicone rubber seals for the mold and substrate to prevent nitrogen gas from escaping into the central chamber during the imprinting process. The mold and substrate holders can be conveniently loaded into and detached from the optical aligner like a conventional mask and wafer without changing anything in the optical aligner. The mold holder contains a 25.4⫻ 25.4⫻ 3.18 mm3 borosilicate glass window to enable visual alignment of the sub-
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FIG. 1. Process flow for photocurable NIL. 共a兲 Glass mold positioned over substrate with transfer layer and NIL resist layer, 共b兲 pressing of mold into NIL resist and UV curing, 共c兲 demolding, and 共d兲 etching of residual layer followed by etching of transfer layer.
strate and mold as well as UV exposure. The glass thickness was chosen to provide sufficient safety against fracture by the nitrogen pressure, while remaining within the dimensional constraints of the aligner. The MA-6 provides the function of making the mold and substrate parallel, as well as the function of lateral alignment. The alignment procedure is exactly the same as the normal procedure to align the patterns on an optical mask and a substrate. Once the alignment has been performed, the substrate and mold can be brought into contact without any significant lateral shift. The MA-6 also provides the UV light that flashes through the transparent window above the glass mold to cure the resist. Thus the entire imprinting process can be performed in situ within the optical aligner. Because the nanoimprint module takes advantage of the capabilities of an already existing aligner, it is simple in design and thus very low cost.
FIG. 2. Photograph of module mold holder. J. Vac. Sci. Technol. B, Vol. 24, No. 2, Mar/Apr 2006
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FIG. 3. Photograph of module substrate holder.
III. EXPERIMENTAL PROCEDURE A typical imprinting run with this module is as follows. We have used bare 共100兲 silicon as the substrate and Borofloat® borosilicate flat glass as the mold material. The substrate is first prepared by spin coating with a layer of ⬃100 nm polymethyl methacrylate 共PMMA兲 having a molecular weight of 495 000 g / mol, which serves a dual function as a transfer layer for post processing and an adhesive layer between the NIL resist and the substrate. The PMMA is baked at ⬃180 ° C for ⬃90 s to drive out solvents, and is followed by spin coating of the UV resist. For our experiments we have used NXR-2010, a resist available for purchase from Nanonex. The mold is treated with a surfactant, tridecafluoro-共1,1,2,2兲-tetrahydrooctyl trichlorosilane 共F13-TCS兲, which reduces its surface energy and makes it easier to detach it from the wafer after imprinting without contamination from the resist. This surfactant layer lasts for many imprint runs before needing retreatment. The substrate and mold are secured on their respective holders, which are then loaded into the MA-6. After the alignment is performed on the aligner, the wafer and mold are brought into rough contact, and the central chamber is evacuated. Application of nitrogen pressure up to 4.5 bars behind both substrate and mold minimizes the gap between them. Schematic cross sections of the module before and after the substrate and mold
FIG. 4. Cross section of module with substrate and mold 共a兲 separated, just after loading and 共b兲 in contact, during imprinting.
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FIG. 5. Atomic force microscopy 共AFM兲 image showing the pattern of parallel nanowires with a 30 nm half-pitch generated on a resist by using NIL.
are brought into contact are shown in Fig. 4. The resist is cross-linked by the UV exposure at an intensity of 17.0 mW/ cm2 for 5 s, and then the substrate and mold are separated and unloaded. At this point the residual layer and the underlying transfer layer can be etched, and further processing to transfer the resist patterns to metal and/or semiconductor patterns is performed. IV. RESULTS AND DISCUSSION Using this module, we have demonstrated a precise transfer of parallel nanowire patterns with 30 nm width and 60 nm pitch from a mold onto a resist on a 1 in.2 silicon wafer by NIL 共Fig. 5兲. Also, through integration with the MA-6 aligner, we have demonstrated an alignment of two layers of patterns generated by NIL with sub-1 m alignment accu-
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FIG. 7. 共a兲 Optical image of a dog-bone pattern produced in NIL resist using a hybrid optical/imprint mold. The large 100⫻ 100 m patterns were produced by photolithography, and the connecting nanowire by photocurable NIL. 共b兲 AFM image of the 100-nm-wide nanowire.
racy 共Fig. 6兲. The module has also been used to simultaneously fabricate patterns by optical lithography and photocurable NIL. It has been observed that molds having large and small features in close proximity can lead to a nonuniform thickness of the imprinted resist layer.10–12 This problem has been previously addressed by employing a technique described as combined nanoimprint and photolithography 共CNP兲.13 This method eliminates the large-scale protrusions from the mold and replaces them with opaque photolithography patterns, since high resolution is not necessary. To test the CNP technique in our module, a mold was fabricated that consists of several “dog-bone” structures with nanoscale protrusions for the wires and large opaque chromium photolithographic pads for the contact pads. After coating the mold with a release layer, it was used to imprint the patterns using the same procedure and resist as described before. The photolithographic patterns were also generated at the same time with the exact same procedure and resist. Following a rinse in isopropanol to remove the uncured resist under the photolithography patterns, the resulting structure is shown in Fig. 7. The 100⫻ 100 m2 contact pads were successfully transferred along with the nanowire having 100 nm width and 80 m length. V. CONCLUSIONS
FIG. 6. Optical image demonstrating the alignment of 共a兲 a layer imprinted in NIL resist 共square patterns兲 to 共b兲 an underlying metal layer 共cross patterns兲. The Vernier marks indicate an alignment error of approximately 0.5 m. JVST B - Microelectronics and Nanometer Structures
In summary, we have designed and constructed a nanofabrication module that can be integrated with a SUSS MA-6 optical aligner. The module provides a convenient and lowcost technique to transform an optical aligner for the fabrication of micron scale patterns to a nanofabrication machine for the fabrication of nanoscale patterns. We have demonstrated the imprinting of 30-nm-wide lines by using this module, as well as submicron alignment overlay. By using a
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mold with both large opaque areas and nanoscale features, we have demonstrated simultaneous optical and imprint lithography using the module. ACKNOWLEDGEMENTS This research was supported in part by DARPA, the California NanoSystems Institute, and the NSF Scaleable and Integrated Nano Manufacturing Center. 1
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