Fabrication of subwavelength-size aperture for a near ... - IEEE Xplore

Fabrication of subwavelength-size aperture for a near-field optical probeusing various microfabrication proceduresS. S. Choi, a) M. Y. Jung, and D. W. KimDepartment of Physics, College of Natural Science, Sun Moon University, Ahsan, ChungNam336-708, KoreaJ. W. Kim and J. H. BooDepartment of Nanoscience and Technology, Sungkyunkwan University, Suwon, Kyung Gi-Do, KoreaReceived 18 July 2002; accepted 4 November 2002; published 23 December 2002We successfully fabricated subwavelength-size silicon oxide apertures on a cantilever array as anear-field optical probe. Various semiconductor processes were utilized for subwavelength-sizeaperture fabrication. The anisotropic etching of the Si substrate by alkaline solutions followed byanisotropic crystal orientation dependent oxidation, anisotropic plasma etching, and isotropic oxideetching was carried out. 2, 3, and 4 m size dot arrays were initially photolithographically patternedonaSi100 wafer. After fabrication of a V-groove shape by anisotropic etching, oxide growth at1000 °C was performed to have an oxide etch mask. The oxide layer on the Si111 plane has beenutilized as an etch mask for plasma dry etching and water-diluted HF wet etching forsubwavelength-size aperture fabrication. A Au thin layer was deposited on the fabricated oxideaperture on the cantilever array. After this procedure, the initial opening, 300 nm of the oxideaperture was reduced down to 95 nm. © 2003 American Vacuum Society.DOI: 10.1116/1.1534574I. INTRODUCTIONThere has been considerable interest in fabricating a subwavelengthaperture for a near-field optical sensor due to itspotential application for promising near-field optical recordingand other biological applications. 1–3 Recently, there hasbeen a dramatic increase in storage density as high as 100Gbyte/in. 2 in near-field optical recording because it can circumventthe diffraction limit. 4–6 However, the low opticalpower generated through the tip of the fiber probe limits thescanning speed and hinders the development of the practicaloptical storage device. Recently, in order to improve the datarate, a parallel processing technique using a cantilever arrayseems to be the key solution for a faster data rate. 7–9 In thisarticle, nanofabrication techniques of a subwavelength apertureon the cantilever array using directional plasma etchingand isotropic buffered hydrofluoric acid BHF etching arepresented.Fabrication techniques for the subwavelength-size aperturearray using the photoresist removal technique and sputteretching in addition to the focused ion beam milling havebeen reported. 1,3,4,10 The etching of Si using alkaline solutionssuch as KOH, ethylene-diamine/pyrocatechol EDP,and tetramethyl ammonium hydroxide TMAH, will be anisotropicdue to the different atomic densities of the Si crystalsurface. As the etch ratio of the Si100 surface to theSi111 surface is on the order of a few hundred, the intersectionof the Si111 surfaces eventually will form a V-typegroove or a pyramidal shape at the bottom Si100surface. 11–13 The oxidation rate is dependent upon both thecrystal plane and the angle of the plane intersection. Thea Author to whom correspondence should be addressed; electronic mail:sscphy@empal.comhigher packed Si111 surface will have a higher oxidationrate than the Si100 surface, especially for the surfacereactioncontrolled region. Retarded oxide growth from aconcave surface was also reported. The stress-induced retardationof oxidation can result from oxidation at temperaturelower than 950 °C or volume expansion of oxide at a concavesurface during thermal oxidation even at 1000 °C. 13–16The resultant thicker oxide on the Si111 surface than theSi100 bottom surface can be utilized as an etch mask duringeither the dry or wet etching procedure forsubwavelength-size aperture fabrication. Anisotropic dryetching was employed to provide a subwavelength-size apertureon the bottom of the V groove. Chlorine gas alone, orin a mixture with Ar gas is also known to be very effective inproviding an anisotropic directional process. 17 An isotropicwater-diluted HF solution etching technique has been alsoemployed to fabricate the subwavelength-size aperture at theapex of the oxide pyramid on the cantilever array.II. FABRICATION PROCEDURES ANDEXPERIMENTAL RESULTSThe key fabrication procedures in our experiments includei the deposition of oxide and nitride as an etch mask,ii formation of inverted pyramids or V grooves using alkalinesolution, iii thermal oxidation of V-groove patterns,iv definition of the cantilever patterns, v fabrication of theoxide pyramid, vi fabrication of nanosize apertures, andvii the thin metal deposition process. A detailed schematicof the fabrication procedures is given in Fig. 1.A. Fabrication of subwavelength-size valleysAt first, 2, 3, and 4 m dot arrays were patterned onp-type 5–15 cm 4 in. Si wafers using the conventional118 J. Vac. Sci. Technol. B 21„1…, JanÕFeb 2003 1071-1023Õ2003Õ21„1…Õ118Õ5Õ$19.00 ©2003 American Vacuum Society 118

119 Choi et al.: Fabrication of subwavelength-size aperture 119FIG. 2. a SEM micrograph of KOH solution etched Si100 wafer presentsthe V groove of the 22 m 2 pattern. The Si111 surface intersects eachother at the bottom of the V groove. b The SEM micrograph presents theretarded oxidation on the bottom of the V groove white circled area. Thethicknesses of the oxides on the 54.7° angled Si111 surface and the narrowbottom Si100 surface are 330 and 100 nm, correspondingly.FIG. 1. Schematic of the fabrication procedures for the subwavelength-sizeaperture on the cantilever array. a Opening for the etch window using anitride and oxide double layer as an etch mask, b formation of the invertedpyramidal shape, c thermal oxidation, d definition of the cantilever patternand top view in d-1, c formation of the hollow oxide pyramid, ffabrication of nanosize aperture, and g metal thin film deposition.photolithographic technique. A double layer of 200-nmthicksilicon nitride and 250-nm-thick silicon oxide was dryetched for the opening window using reactive ion etchingRIE, as in Fig. 1a. The RIE etching conditions for themask layer were rf power 600 W, an etching gas mixture ofCHF 3 10 sccm, CF 4 10 sccm, and O 2 20 sccm with an operatingpressure of 100 mTorr.40 wt % KOH solution will anisotropically etch the Sisubstrate with 1 m/s etch rate at 80 °C. KOH etching atlower temperature will have a slower etch rate, while it willresult in a nonuniform etch surface at temperature higherthan 80 °C. On the other hand, 20 wt % TMAH solution at80 °C will etch anisotropically with an etch rate of 0.45m/s. The V-groove formation with 2 m square patternswas carried out using KOH solution as in Fig. 1b. Thewidth of the opening (W m ) is related to etch depth t and thewidth of the etched silicon surface at the bottom (W Si )isasbelow:W m W Si 2t,etched Si111 surfaces will intersect each other to form a Vgroove, when the opening is small enough. Even in the caseof the V-groove formation, the narrow bottom area at theSi100 surface would have a nanoscale area, as seen in Fig.2a.As a next step, Fig. 1c, thermal steam oxidation hasbeen performed at 1000 °C for 72 min. The thickness of theoxide grown at the Si111 and Si100 bottom surfaces wasexamined by field emission scanning electron microscopySEM, and found to be 330 and 100 nm, respectively, asin Fig. 2b. The SEM micrograph reveals that the thicknessof the oxide grown on the Si111 surface becomes thinner asit approaches the bottom of the V groove. This phenomenoncan be attributed to two reasons: i the oxidation rate at theSi100 surface is slower than that at the Si111 surface, andii the fact that the diffusion-controlled mechanism dominateson the concave region of the V-groove bottom. Theoxide growth will be severely retarded in a concave structuredue to the compressive stress since it would hinder the oxygendiffusion to the Si/SiO 2 interface. 13–16 Our experimentalresults for other 3-, 4-, 5-, and 10-m-square pattern Vgrooves also exhibited retarded oxidation of more than 50%.In order to fabricate a subwavelength-size aperture, twotechniques have been tried; one is to fabricate it with directionalplasma etching and the other is to utilize HF isotropicetching. Both techniques utilize the thicker oxide grown onthe Si111 plane as an etch mask. Inductively coupledplasma ICP etching was performed to generatesubwavelength-size apertures at the bottom of the oxidepyramid. The operating conditions were 100 W rf power, 9mTorr operating pressure, 40 sccm Cl 2 feed gas, with a durationof 3–5 min. The rf power applied to supply the biasvoltage on the substrate was set to 200 W. The bias voltageswith coil rf power of 100 W were measured to be 500 V. Theetch rate of SiO 2 at the measured bias voltage of 500 V wasJVSTB-MicroelectronicsandNanometer Structures

122 Choi et al.: Fabrication of subwavelength-size aperture 122electron microscropy and a deposition system with betteruniformity, it is expected that a diameter of the aperture ofeven less than 50 nm can be obtained.ACKNOWLEDGMENTThis work was supported by Grant No. KOSEF 2001-1-11400-008-2 from the Basic Research Program of the KoreaScience and Engineering Foundation.Presented at the 46th International Conference on Electron, Ion, and PhotonBeam Technology and Nanofabrication, Anaheim, CA, 28–31 May 2002.FIG. 7. SEM micrographs show the fabricated cantilever array with theAu-coated 160-nm-diam aperture. The double layer of 350 nm oxide and1 m nitride layer were initially utilized. The remaining oxide layer wouldbe 200 nm after the isotropic etching process. The Au thin film was sputterdeposited on the outside surface area of the oxide pyramidal probe.ICP etching, isotropic HF etching, and metal sputter deposition.Wet chemical etching of alkaline solution using thedouble layer of silicon nitride and SiO 2 as an etch maskprovides an inverted pyramid or a V-groove shape due to theanisotropic crystal orientation dependent etching. After theV-groove formation, stress-dependent oxidation at 1000 °Cwas performed and resulted in a thicker oxide on the inclinedSi111 surface than the bottom Si100 surface, regardlessof the pattern size. This phenomenon can be attributed tocompressive stress-induced retarded oxide growth from theconcave surface.We have employed the isotropic HF etching techniqueusing 10:1 diluted HF to open the pyramidal oxide apertureson the cantilever array generated after removing bulk Si fromthe backside. The isotropic etching technique using waterdilutedHF solution requires careful handling because of thethin edges of the oxide tetrahedron. In order to reduce thefabricated oxide aperture size, in a controllable way, the Authin film was sputter deposited and the size of the openingwas reduced down to 95 nm with Au deposition of 170 nm.With proper instruments such as high resolution transmission1 R. C. Davis, C. C. Williams, and P. Neuzil, Appl. Phys. Lett. 66, 23091995.2 A. G. T. Ruiter, M. H. P. Moers, N. F. van Hulst, and M. de Boer, J. Vac.Sci. Technol. B 14, 597 1996.3 C. Mihalcea, W. Scholz, S. Werner, S. Munster, E. Oesterschulze, and R.Kassing, Appl. Phys. Lett. 68, 3531 1996.4 A. Partovi, D. Peale, M. Wuttig, C. A. Murray, G. Zydzik, L. Hopkins, K.Baldwin, W. S. Hopson, J. Wynn, J. Lopata, L. Dhar, R. Chichester, andJ. H. Yeh, Appl. Phys. Lett. 75, 1515 1999.5 Y. Martin, S. Rishton, and H. K. Wickramasinghe, Appl. Phys. Lett. 71, 11997.6 E. Betzig, J. K. Trautman, R. Wolfe, E. M. Gyory, P. L. Finn, M. H.Kryder, and C. H. Chang, Appl. Phys. Lett. 61, 142 1992.7 H. J. Mamin, B. D. Terris, L. S. Fan, S. Hoen, R. C. Barret, and D. Rugar,IBM J. Res. Dev. 39, 681 1995.8 P. Vettiger, M. Despont, V. Dreschler, U. Duerig, W. Haeberle, M. I.Lutwyche, H. E. Rothuize, R. Stutz, R. Widmer, and G. K. Binnig, IBMJ. Res. Dev. 44, 323 2000.9 Y. Mitsuhashi, Jpn. J. Appl. Phys., Part 1 37, 2079 1998.10 M. Y. Jung, I. W. Lyo, D. W. Kim, and S. S. Choi, J. Vac. Sci. Technol. A18, 1333 2000.11 H. Seidel, L. Csepregi, A. Heuberger, and H. Baumgartel, J. Electrochem.Soc. 137, 3612 1990.12 G. T. A. Kovacs, N. I. Maluf, and K. E. Peterson, Proc. IEEE 86, 15361998.13 M. Y. Jung, J. W. Lee, J. W. Kim, D. W. Kim, and S. S. Choi, Proceedingsof the European Surface Science Meeting, Madrid, Spain, 6–9 September2000.14 D. B. Kao, J. P. McVittie, W. D. Nix, and K. C. Saraswat, IEEE Trans.Electron Devices ED-35, 251988.15 D. B. Kao, J. P. McVitte, W. D. Nix, and K. C. Saraswat, IEEE Trans.Electron Devices ED-34, 1008 1987.16 H. L. Liu, D. K. Biegelson, F. A. Ponse, N. M. Johnson, and R. F. W.Pease, Appl. Phys. Lett. 64, 1383 1994.17 H. F. Winters and J. W. Coburn, J. Vac. Sci. Technol. B 3, 13761985.J. Vac. Sci. Technol. B, Vol. 21, No. 1, JanÕFeb 2003