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

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