NANO LETTERS
Nanomechanical Oscillators Fabricated Using Polymeric Nanofiber Templates
2004 Vol. 4, No. 3 437-439
David A. Czaplewski, Scott S. Verbridge,* Jun Kameoka, and H. G. Craighead School of Applied Physics and the Nanobiotechnology Center, Cornell UniVersity, Ithaca, New York 14853 Received December 10, 2003; Revised Manuscript Received February 2, 2004
ABSTRACT We have used deposited polymer nanofibers as templates for the formation of functional nanomechanical devices, interfaced to lithographically defined surface microstructures. The deposition of oriented poly(methyl methacrylate) nanofibers, combined with contact photolithography, created silicon nitride nanomechanical oscillators with dimensions on the order of 100 nm. We used contact photolithography to define the support structures for the oscillators. The moving beams were defined in the device layer by using the deposited fiber as a mask for reactive ion etching using a CF4 plasma chemistry, followed by removal of a sacrificial underlying layer. After releasing the devices, the frequencies of the modes of oscillation of the beams were determined by laser interference techniques. The devices would self-oscillate due to the interferometric effects of the continuous wave (CW) detection laser. The oriented polymeric nanofiber deposition method, used in this experiment, offers an approach for the rapid formation of arrays of nanomechanical devices, connected to micromechanical structures, that would be difficult to form using a completely self-assembled or completely lithographic approach. This direction may provide a useful method for realizing nanoscale device architectures in a variety of active materials.
Recently, there has been great interest in nanomechanical devices, for a range of applications and studies. Carbon nanotubes have been investigated for their mechanical properties and as possible nanomechanical device components.1 Mechanical oscillators have been used as highly sensitive mass detectors of individual cells2 and adsorbed chemicals.3 Nanooscillators have also been used to study mechanical loss mechanisms4,5 with dimensions such that surface-to-volume ratios become important. Additionally, the damping effects of air on nanomechanical oscillators have been investigated where the amplitude of motion of the oscillator is comparable to the mean free path of the gas molecules.6 The oscillators used in these studies were made from single crystalline silicon,7 polycrystalline silicon,2 silicon nitride, and nanocrystalline diamond films.8 They were fabricated using either electron beam lithography or high-resolution projection lithography.7 While these methods have afforded the investigators the freedom to customize the dimensions of the oscillators, the fabrication of the devices can be lengthy and involved. We have created nanomechanical oscillators by incorporating poly(methyl methacrylate) (PMMA) nanofibers, deposited by a scanned electrospinning source, with lithographically defined support structures. Without the need for electron beam or high-resolution projection lithography, we have fabricated doubly clamped beams made from silicon nitride with widths down to 100 nm (Figure 3c), and lengths up to 30 µm. We have measured the resonant frequency of 10.1021/nl035149y CCC: $27.50 Published on Web 02/25/2004
© 2004 American Chemical Society
Figure 1. Schematic detailing the electrospinning process used. (a) Simple schematic of electrospinning setup. (b) Scanning electron micrograph showing the source with the channels created by the overhang.
devices with widths as small as 165 nm using laser interference techniques. The nanofibers, used to define the nanomechanical oscillators, were deposited on the target substrate by electrospinning a polymer solution from a microfabricated source (Figure 1a). The electrospinning process is similar to the processes described in previous work;9-11 however, the microfabricated electrospinning source has been modified to include channels on the sides of the source. These channels help transport the polymer solution to its apex during electrospinning, which has increased the number of nanofi-
Figure 2. Schematic diagram showing the process steps used to fabricate the nanomechanical oscillators. (a) Target substrate, consisting of a silicon nitride device layer and a sacrificial silicon dioxide layer deposited on a silicon substrate. (b) Polymer nanofibers were deposited on the target substrate and baked (support structures, not visible in cross-section schematic, defined and patterned with contact lithography). (c) The device layer was etched to define both the oscillator and the supports. (d) The remaining nanofibers and photoresist were removed in an oxygen plasma. (e) The devices were released in a hydrofluoric acid etch, rinsed in DI water and dried with nitrogen. (f) Angled schematic of an oscillator showing the device elevated above the surface of the silicon substrate by the height of the sacrificial silicon dioxide layer.
bers deposited on the target substrate from the same volume of polymer solution. In fabricating the tips used, the back of the wafer was patterned and etched separately from the front to define a slightly larger shape of the tip on the back side, creating the channels along the sides of the tip. The bottom pattern extended 100 µm further out along the length of the tip and at the apex than the top pattern. A tilted scanning electron micrograph of the tip (Figure 1b) shows the shape of the tip and the thin silicon extension at the bottom of the tip protruding from the two sides and the apex. During the electrospinning process, an applied voltage established an electric field between the source and the counter electrode, which formed a Taylor cone at the apex of the source. At a voltage of 4000 V, a liquid jet was extracted from the Taylor cone and traveled to the counter electrode. Often, several Taylor cones formed at different locations on a single tip, leading to the deposition of several parallel fibers at once. Furthermore, a single jet can lead to several parallel fibers deposited on a substrate through a translation of the entire substrate stage perpendicular to its axis of rotation during the spinning process. The fabrication of the nanomechanical oscillators began with the deposition of PMMA nanofibers on the surface of the target substrate (Figure 2b). The target substrate consisted of two films, the device layer on top of a sacrificial oxide layer, deposited on a silicon substrate (Figure 2a). After the fibers were deposited on the target substrate, the substrate was heated to 115 °C to increase the adhesion of the PMMA fibers to the substrate surface. Then photoresist was spun over the fibers and soft-baked at 90 °C, for definition of the oscillator support structures. Conventional photolithography was used to expose an area of the resist where the nanofibers defined the width and orientation of the nanomechanical oscillators and the remaining resist served as the supports for the oscillators. After the photolithography processing, the fibers adhered to the substrate, without moving during subsequent processing. Next, the exposed areas of the device layer were etched in a CF4 plasma (Figure 2c), and then the remaining photoresist and nanofibers were removed in an 438
Figure 3. (a) Scanning electron micrograph of the nanomechanical oscillator. A 10 µm scale bar is shown. The beam length is 30 µm. (b) Close-up view of the oscillator whose frequency data is shown in Figure 4. The width and thickness of the oscillator were 165 and 95 nm, respectively. A 200 nm scale bar is shown. (c) Closeup view of a suspended beam of width just below 100 nm. A 100 nm scale bar is shown.
oxygen plasma (Figure 2d). The devices were released by etching the sacrificial oxide in hydrofluoric acid, rinsing in water, and drying with nitrogen (Figure 2e). A schematic of a beam suspended over the substrate after release can be seen in Figure 2f. Doubly clamped beams as long as 30 µm were fabricated and released by nitrogen drying (critical point drying is not necessary). A scanning electron micrograph of a beam created by this method can be seen in Figure 3a, with a close-up for size determination shown in Figure 3b. The thickness and width of the beam are 95 and 165 nm, respectively. Devices of significantly longer lengths than 30 µm could be fabricated using this method because the diameter of the nanofibers varies by only 2-3% over a 2 cm distance.9 However, supercritical CO2 drying would need to be used in order to ensure release of longer structures. Devices with widths smaller than 100 nm were fabricated using this procedure (Figure 3c); however, we were not able Nano Lett., Vol. 4, No. 3, 2004
interference techniques, with self-oscillations observed. Polymer nanofibers offer an alternative method to electron beam lithography and high-resolution photolithography to realize nanodevice architectures, which can be interfaced with lithographically defined microstructures.
Figure 4. Resonance data for the 165 nm oscillator fundamental mode. The resonant frequency was determined to be 1.28 MHz.
to measure their modes of vibration by the methods discussed in the following section. The devices were then placed in a vacuum chamber that was pumped down to pressures below 10-5 Torr. Through a microscope objective with 20× magnification, a CW laser was focused on the center of the beam, the location of the largest out of plane deflection of the beam. The return light was directed into a photodiode by a beam splitter. A spectrum analyzer was used to monitor the voltage output from the photodiode. The CW detection laser provided enough power to the beams to cause them to oscillate in resonance without any other external drive. This phenomenon, termed “selfoscillation”, is the result of the device being heated and cooled in phase with its oscillation due to an interference pattern of the CW detection laser light established with the device and substrate. The laser power absorbed by the beam is strongly dependent on the position of the beam within this interference pattern. This dependence of absorption on device position creates appropriate thermal feedback conditions to excite limit cycle, or self-oscillations. This phenomenon, as well as the resonance detection scheme used, is similar to that described in earlier work.12-14 A plot of one of the self-oscillations from the above imaged beam can be seen in Figure 4. The oscillation frequency of the device was 1.28 MHz, with an effective Q of approximately 130. The mechanical properties of this device are consistent with previous reports.15 In conclusion, we have used PMMA nanofibers in conjunction with contact photolithography to create nanomechanical oscillators. We deposited the nanofibers on the target substrate by electrospinning from a microfabricated source. The target substrate consisted of a device layer and a sacrificial layer on a silicon substrate. The beams were then defined in the device layer by reactive ion etching using a CF4 plasma chemistry. After releasing the devices, the modes of oscillation of the beams were determined by laser
Nano Lett., Vol. 4, No. 3, 2004
Acknowledgment. This work was supported by the Nanobiotechnology Center (NBTC), an STC Program of the National Science Foundation under Agreement No.ECS9876771. One of the authors (S.V.) gratefully acknowledges a GAANN fellowship. This work was performed in part at the Cornell Nano-Scale Science & Technology Facility (a member of the National Nanofabrication Users Network). This research made use of the Cornell Center for Materials Research Shared Experimental Facilities supported through the NSF MRSEC program (DMR-0079992). The authors also thank Keith Aubin, Robert Reichenbach, and Maxim Zalalutdinov for assistance with instrumentation, as well as helpful discussions relating to the phenomenon of self-oscillation. References (1) Fennimore, A. M.; Yuzvinsky, T. D.; Han, W.; Fuhrer, M. S.; Cumings, J.; Zetti, A. Nature 2003, 424, 408-410. (2) Ilic, B.; Czaplewski, D. A.; Zalalutdinov, M.; Craighead, H. G.; Neuzil, P.; Campagnolo, C.; Batt, C. J. Vac. Sci. Technol. 2001, B19(6), 2825-2828. (3) Lavrik, N.; Datskos, P. G. Appl. Phys. Lett. 2003, 82(16), 26972699. (4) Evoy, S.; Olkhovets, A.; Sekaric, L.; Parpia, J. M.; Craighead, H. G.; Carr, D. W. Appl. Phys. Lett. 2000, 77(15), 2397-2399. (5) Cleland, N.; Roukes, M. L. J. Appl. Phys. 2002, 92(5), 2758-2769. (6) Sekaric, L.; Zalalutdinov, M.; Bhiladvala, R. B.; Zehnder, A. T.; Parpia, J. M.; Craighead, H. G. Appl. Phys. Lett. 2002, 81(14), 26412643. (7) Carr, D. W.; Craighead, H. G. J. Vac. Sci. Technol. 1997, B15(6), 2760-2763. (8) Sekaric, L.; Parpia, J. M.; Craighead, H. G.; Feygelson, T.; Houston, B. H.; Butler, J. E. Appl. Phys. Lett. 2002, 81(23), 4455-4457. (9) Czaplewski, D. A.; Kameoka, J.; Craighead, H. G. J. Vac. Sci. Technol. 2003, B21(6), 2994-2997. (10) Kameoka, J.; Orth, R.; Yang, Y.; Czaplewski, D. A.; Mathers, R.; Coates, G. W.; Craighead, H. G. Nanotechnol. 2003, 14(10), 11241129. (11) Czaplewski, D. A.; Kameoka, J.; Mathers, R.; Coates, G. W.; Craighead, H. G. Appl. Phys. Lett. 2003, 83(23), 4836-4838. (12) Aubin, K.; Zalalutdinov, M.; Alan, T.; Reichenbach, R.; Rand, R.; Zehnder, A.; Parpia, J.; Craighead, H., submitted to JMEMS. (13) Zalalutdinov, M.; Zehnder, A.; Olkhovets, A.; Turner, S.; Sekaric, L.; Ilic, B.; Czaplewski, D.; Parpia, J. M.; Craighead, H. G. Appl. Phys. Lett. 2001, 79(5), 695-697. (14) Sekaric, L.; Carr, D. W.; Evoy, S.; Parpia, J. M.; Craighead, H. G. Sens. Actuators, A 2002, 101, 215-219. (15) Ilic, B.; Czaplewski, D. A.; Zalalutdinov, M.; Craighead, H. G.; Neuzil, P.; Campagnolo, C.; Batt, C. Appl. Phys. Lett. 2000, 77(3), 450-452.
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