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Notes Atomic Force Microscopy-Based Nanolithography on Silicon Using Colloidal Au Nanoparticles As a Nanooxidation Mask Jiwen Zheng, Zhucheng Chen, and Zhongfan Liu* Center for Nanoscale Science & Technology (CNST), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Received May 22, 2000. In Final Form: September 14, 2000
Introduction Atomic force microscopy (AFM) lithography has attracted much attention for its nanometer-scale surface structuring capability. The basic strategy of AFM lithography is to utilize the spatially localized force or electric field generated by the AFM tip to induce a physical or chemical change on surfaces. Mamin et al.,1 Bouchiat and Esteve,2 and others have demonstrated the formation of nanopits and grooves on soft polymer surfaces by AFMbased mechanical indentation or scratching. Most of the AFM lithography is focused on the direct electrical modification of semiconductor and metal surfaces for the purpose of nanodevice fabrication. Typical work along this line has been performed on silicon,3-7 titanium,8 gallium arsenide,9 and chromium10 by Snow and Campell,3 Avouris et al.,4 Minne et al.,5 Stievenard and Legrand,6 Garcia et al.,7 Matsumoto et al.,8 Okada et al.,9 Wang et al.,10 etc. Instead of using the unmodified semiconductors and metals, Sugimura and Nakagiri11 and Kim et al.12 have performed AFM nanooxidation of silicon surfaces via organic self-assembled monolayers (SAMs) of organosilanes in ambient air. These investigators found that the monolayer at the tip-scanned area had been degraded as a result of AFM oxidation and that the patterned monolayer can be employed as a self-developing positive resist for the following chemical etching. On the other hand, colloidal nanoparticles have received renewed interest recently from the viewpoint of nanoscale science and technology. Using chemical assembling techniques, one can fabricate well-organized nanoparticle * To whom correspondence should be addressed. Tel and fax, (+86) 10-6275-7157; e-mail,
[email protected]. (1) Mamin, H. J.; Ried, R. P.; Terris, B. D.; Rugar, D. Proc.sIEEE 1999, 87, 1014. (2) Bouchiat, V.; Esteve, D. Appl. Phys. Lett. 1996, 69, 3098. (3) Snow, E. S.; Campell, P. M. Appl. Phys. Lett. 1994, 64, 1932. (4) Avouris, Ph.; Martel, R.; Sandstrom, R. Appl. Phys. A 1998, 66, S659. (5) Minne, S. C.; Flueckiger, Ph.; Soh, H. T.; Quate, C. F. J. Vac. Sci. Technol., B 1995, 13, 1380. (6) Legrand, B.; Stievenard, D. Appl. Phys. Lett. 1999, 74, 4049. (7) Garcia, R.; Calleja, M.; Murano, F. P. Appl. Phys. Lett. 1998, 72, 2295. (8) Matsumoto, K.; Takahashi, S.; Ishii, M.; Hoshi, M.; Kurokawa, A.; Ando, A. Jpn. J. Appl. Phys., Part 1 1995, 34, 1387. (9) Okada, Y.; Amano, S.; Kawabe, M.; Shimbo, B. N.; Harris, J. S. Jpn. J. Appl. Phys. 1998, 83, 1844. (10) Wang, D.; Tsau, L.; Wang, K. L.; Chow, P. Appl. Phys. Lett. 1995, 67, 1295. (11) Sugimura, H.; Nakagiri, N. Nanotechnology 1997, 8, A15. (12) Kim, J. C.; Oh, Y.; Lee, H. W.; Shin, Y. W.; Park, S. W. Jpn. J. Appl. Phys. Part 1 1998, 37, 7148.
arrays on solid surfaces. These arrays can be used as the coulomb islands in the study of room-temperature singleelectron tunneling,13,14 as nanoelectrodes for electrochemical studies,15 as active substrates in surface-enhanced Raman scattering (SERS),16 etc. Recently, Ahmed et al. reported that the gold colloidal nanoparticles could be utilized as reactive ion etching (RIE) masks for the fabrication of silicon nanopillars.17 The diameter of a pillar is directly determined by the nanoparticle size. These investigators have succeeded in making nanopillars as small as 5 nm in diameter. In this letter, we present a novel use of colloidal gold nanoparticles in AFM nanolithography. The nanoparticles are immobilized on a mercaptopropyltriethoxysilane (MPTS)-modified silicon surface via Au-S chemical bonding and are exploited as the lithographic mask to prevent the covered area from AFM tip-induced nanooxidation of MPTS and silicon. By combining this approach with the wet chemical etching, we demonstrate the feasibility of fabricating large-area silicon nanocolumns with a simple one-raster scanning of an AFM tip. Experimental Section Mercaptopropyltriethoxysilane and HAuCl4‚3H2O were purchased from Aldrich and used as received. An n-type silicon wafer with a resistivity of 0.025Ω‚cm was used as the substrate. Ultrapure water (>17MΩ‚cm) was used throughout the experiments. Other reagents used were of analytical grade. The silicon wafer was ultrasonicated successively in acetone, ethanol, and water, and then cleaned in the bath of HCl (36%)/ H2O2 (30%)/H2O (1:1:5, in volume) at 80 °C for 10 min.18 The wafer was then rinsed with water and blown dry with highpurity nitrogen. This treatment made the silicon surface fully hydrophilic because of the growth of a thin oxide layer. MPTS derivatization of the Si surface was performed with the use of a chemical vapor deposition process.19 The Si wafer was suspended 5 cm above a refluxing 10% MPTS/ xylene solution under atmosphere of N2 for 4 h. The vaporized MPTS molecules were reacted with the surface -OH groups forming a monolayer on Si. After being rinsed with ethanol and then blown dry in a stream of high-purity nitrogen, the sample was baked at 120 °C for 30 min to complete the Si-O bond formation.20 The aqueous gold colloids with an average size of 18 ( 2 nm (confirmed by transmission electron micrography (TEM)) were synthesized by Frens’ method.21 The gold nanoparticle mask was prepared by immersion of the MPTS-derived silicon substrate in the colloidal gold suspension for 6 h; the substrate was then rinsed with ultrapure water and blown dry with high-purity nitrogen. (13) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (14) Jiang, P.; Liu, Z. F. Appl. Phys. Lett. 1999, 75, 3023. (15) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313. (16) Zhu, T.; Yu, H. Z.; Wang, J.; Wang, Y. Q.; Cai, S. M.; Liu, Z. F. Chem. Phys. Lett. 1997, 265, 334. (17) Lewis, P. A.; Ahmed, H.; Sato, T. J. Vac. Sci. Technol., B 1998, 16, 2938. (18) Wang, J.; Zhu, T.; Song, J. Q.; Liu, Z. F. Thin Solid Films 1998, 591, 327-329. (19) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (20) Sato, T.; Brown, D.; Johnson, B. F. G. Chem. Commun. 1997, 82, 697. (21) Frens, G. Nat. Phys. Sci. 1973, 241, 20.
10.1021/la000705e CCC: $19.00 © 2000 American Chemical Society Published on Web 11/04/2000
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Figure 2. Transmission electron microscope image of 18-nm colloidal Au nanoparticles synthesized by Frens’ method.
Figure 1. 5 µm × 5 µm contact-mode AFM image of silicon surface covered by 18-nm colloidal gold nanoparticles. The height of the gold nanoparticles is about 18 ( 2 nm. Localized oxidation of nanoparticle-covered silicon was conducted with Nanoscope III AFM (Digital Instruments, Santa Barbara, CA). A constant bias voltage was applied between the AFM tip and the silicon surface (tip negative) during continuous tip scanning in a contact mode. The probe used in the experiments was a gold-coated standard Si3N4 pyramidal tip having a force constant of 0.12 N/m. Repulsive force of 0.5-2 nN was used during lithographing and imaging. All the experiments were performed under ambient conditions, and the relative humidity around the AFM equipment was controlled at 42%. The patterned sample was finally subjected to chemical etching by immersion in NH4F/ H2O2 (30%)/H2O (10:3:100, in weight) for 5 min at room temperature.
Results and Discussion Figure 1 shows a contact-mode AFM image of a silicon surface covered with gold nanoparticles. This image was obtained in ambient air. A clear and stable image could be obtained only when relatively weak loading force, 0.5 ∼ 2 nN, was applied. When the loading force is raised, the image shows an increasing number of horizontal lines (streaks), indicative of the mechanical sweeping-out of Au nanoparticles by the AFM tip. The result shows that a uniform gold nanoparticle submonolayer can be formed on the silicon surface and that the Au-S chemical bonds provide strong enough binding for the nanoparticle to tolerate the contact-mode tip scanning, which is essential for the following nanoparticle-based AFM lithography. The measured width from the AFM topographic cross section is approximately 101 nm, remarkably larger than the value determined from the TEM image as shown in Figure 2 (18 ( 2 nm). This width is attributed to the
Figure 3. 7 µm × 7 µm contact-mode AFM image of particlecovered silicon surface after AFM tip oxidation. The AFM oxidation was performed by applying continuous 9 V sample bias voltage with 512 lines raster-scanned at a scanning rate of 1 µm/s in 42% RH air. The height profile showed that the oxidized region not covered by nanoparticles increases its height by 3.0 nm, and the height of nanoparticles relative to the surrounding oxidized area reduces to 15 nm.
magnification effect of the AFM tip.22 Averaging 25 particles’ height from the AFM image yields a value of 18 ( 2 nm, well consistent with the TEM result. Figure 3 gives a 7 µm × 7 µm AFM image of the nanoparticle-covered silicon surface after oxidation treatment in the 5 µm × 5 µm central area. Treatment was delivered by continuous application of a sample bias voltage of +9 V during tip scanning at a scanning rate of 1 µm/s. The oxidized region clearly exhibits higher (22) Grabar, K. C.; Freeman, R. G.; Hommer, M.; Natan, M. J. J. Anal. Chem. 1995, 67, 735.
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Figure 4. 7 µm × 7 µm tapping-mode AFM image of a sample surface etched in NH4F/H2O2 solution for 5 min. The height of the silicon columns, including Au nanoparticles, is 30 nm.
topography (bright contrast) than the surrounding area. Sugimura and colleagues have studied the AFM-based nanooxidation of hexamethyldisilazane (HMDSN)-modified silicon surfaces and concluded that the HMDSN monolayer exposed to a high-voltage pulse is degraded, and the underlying silicon is also oxidized following an electrochemistry-like mechanism.11,12 We believe that the same mechanism works in the present case at a region not covered by gold nanoparticles. Actually, the region not covered by nanoparticles increases its height by 3.0 ( 0.2 nm after it is exposed to high-voltage bias, suggesting the formation of an oxide layer. On the contrary, the measured height of the gold nanoparticle relative to its surrounding area within the voltage-exposed area decreases to 15 ( 2 nm. This outcome strongly suggests that the underlying organic monolayer and silicon substrate were not oxidized, or, in other words, that the gold nanoparticles function as an antioxidation mask during tip scanning. When the imaging loading force is increased, some nanoparticles could occasionally be swept away, leaving some pits on the surface. The depth of the pits was ca. 3 nm, corresponding to the thickness of the oxide layer, in nice agreement with the above interpretation. To obtain further evidence of the lithographic masking effect of gold nanoparticles, we performed a wet chemical etching experiment. Figure 4 shows the AFM image of a partially oxidized sample after it was dipped in NH4F/ H2O2 solution for 5 min. Obviously, the high-voltageexposed area becomes etched, forming a depressed feature with a depth of 12 ( 0.4 nm. However, the particles inside the etched region showed no change in position or height, suggesting that the nanoparticle-covered area was not etched out by the etchant. This result indicates that only the MPTS layer not coated with nanoparticles has been degraded as a result of AFM nanooxidation and become an etching window, leading to the formation of nanocolumn
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Figure 5. Schematic illustration of AFM-based nanolithography on an MPTS-modified silicon surface using colloidal Au nanoparticles as a mask.
structures on the silicon surface. The size of these columns is 71 ( 2 nm in diameter (full width at half-maximum (FWHM) measured from the height profile) and ca. 30 nm in height, including the gold nanoparticles left on the top. However, the true lateral dimension of these nanocolumns would be far smaller than 71 nm, because AFM measurement involves a convolution with the tip geometry. We noted that at longer etching times, the MPTS monolayer resist was also attacked by the etchant, as evidenced by the formation of pinholes in the area unexposed to high-voltage bias, indicative of the loosely packed structure of the MPTS monolayer. This situation can be improved by using longer-chain silane molecules, or by using a relatively dilute etching solution. Figure 5 shows the schematic illustration that explains the lithographic masking effect of gold nanoparticles during the AFM oxidation process. According to the electrochemical mechanism proposed by Sugimura and Nakagiri11 and by Kim et al.,12 a water column is created between the AFM tip and the sample surface because of the capillary effect in ambient air, which acts as a minute electrochemical cell (see Figure 5a). When the sample is positively biased, at the region not covered by nanoparticles, an electrochemical degradation/oxidation of the organic monolayer and the underlying silicon occurs at the point beneath the AFM tip, leading to the formation of the silicon oxide layer (see Figure 5b). The condensed water is essential here for AFM nanooxidation. Actually, no obvious topographic change was observed at extremely low humidity in our experiments. When the AFM tip is moved over the gold nanoparticle, the situation changes completely. Because of the physical shielding of the nanoparticle, a water column cannot form between the nanoparticle and the organic monolayer. Therefore, the electrochemical reaction cannot occur at such a location, leading to a site-specific oxidation of the silicon surface
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Figure 6. Hard-ball model for interpreting the effect of nanoparticle size and tip curvature radius on particle masked area.
(see Figure 5b). Combining with chemical etching technique, a patterned nanostructure is formed on the silicon surface as illustrated in Figure 5c. The lateral dimension of the nanostructure fabricated in this manner is determined mainly by the nanoparticle size, tip curvature radius, and relative humidity. A hardball model is presented for interpreting the effect of these parameters, as shown in Figure 6, where r is the radius of nanoparticle and R is the curvature radius of the tip. Assuming that the tip-induced electrochemical oxidation occurs only when the AFM tip directly contacts the monolayer surface, the nanoparticle-masked lateral distance is then equivalent to that between two points of contact, which can be written as d ) 4(R*r)1/2, neglecting a formation of oxide layer thickness (typically a few nanometers). This expression gives the largest estimation of the nanoparticle-masking area, equivalent to the convolution of the AFM tip and the gold nanoparticles. Clearly, reducing the size of either the nanoparticle or (23) Sheiko, S. S.; Moller, M.; Reuvekamp, E. M. C. M.; Zandbergen, H. W. Phys. Rev. B 1993, 48, 5675. (24) Sugimura, H.; Nakagiri, N. J. Vac. Sci. Technol. A 1996, 14, 1223.
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the AFM tip radius will result in the decrease of the masking area, leading to the formation of smaller nanocolumns. Using colloidal chemistry, one can easily prepare varisized gold nanoprticles with sharp size distribution. So we have much freedom to design the column size by regulating the diameter of the gold nanoparticles, which is actually the strong advantage of this lithography technique. Using the known particle size of 18 nm, we estimated the curvature radius of the AFM tip used for nanooxidation,23 which gives a value of 128 nm. This considerably large tip size originates from the metalcoating treatment. A smaller tip radius can be obtained by reducing the coating layer thickness. In fact, the direct tip-sample contact is not a prerequisite for tip-induced oxidation. The earlier work on contact AFM oxidation shows that under ambient conditions, the diameter of the water column formed between the tip and the sample surface may reach several hundreds of nanometers.24 As a consequence, the oxidation induced under the tip is extended over a large area. This results in the effective decrease of the nanoparticle-masking area. In addition, the following chemical etching proceeds isotropically and leads to the formation of small columns. Work along this line is currently under way in our laboratory. In addition to the easy control of nanostructure size that comes from using different-sized nanoparticles, the important advantage of this nanoparticle-masked AFM lithography is its batch like operation, which is suitable for large-area lithography. One can simply raster-scan the biased AFM tip on a large area. This technique is much faster than the conventional point-by-point AFM nanolithography. Acknowledgment. The authors gratefully acknowledge the financial support from the National Science Foundation of China (69890222, 29973001, 599101061982). LA000705E
