Direct Patterning of Hydrogenated Amorphous Silicon by Near Field Scanning Optical Microscopy
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cantilevers and a-Si:H resists to pattern silicon oxide, silicon nitride, chromium and titanium metal. 9 Studies of Near-Field Scanning Optical Microscopy (NSOM) for nanolithographic applications are fairly recent. Smolyaninov et. al. and Davy et al. used the light from the NSOM 0 tip to pattern conventional optical photoresists.', " Feature sizes of 100nm were produced.3 2 Massanell et al. patterned ferroelectric surfaces.1 Madsen et. al. used NSOM to pattern a-Si:H .1 In this study, a wet chemical etch was used for pattern development. In agreement with our results they observed that proximity effect lines were written by the tip in the absence of optical illumination. They did not, however, explore the dependence of this effect on write speed, dither amplitude, tip sample separation, or the dose dependence of true optical exposure as done here. Minimum feature sizes near 50nm were produced in their work. FILM DEPOSITION In this work, a-Si:H films were grown on crystalline silicon wafers or Coming 7059 glass to a thickness of 25-800nm. A Materials Research Group multichamber, parallel plate, capacitively coupled plasma enhanced chemical vapor deposition (PECVD) system operated at 13.56MHz was used for both deposition with pure silane and etching with pure hydrogen. The substrates were placed in a stainless steel carrier that formed the ground electrode. The same chamber was used for both deposition and etching. Film thickness was monitored in situ by reflecting a HeNe laser off the surface and counting interference fringes. The films were allowed to cool from the deposition temperature to room temperature in vacuum in order to minimize native oxide growth. Unless other wise specified, all processing was performed at a heater temperature of 50'C. Laser sources used for optical irradiation included a KrF excimer laser as the source of 248nm light with a nominal pulse length of 20ns and Ar ion and HeNe lasers for continuous wave (cw) irradiation from 350 to 633nm. Optical irradiation occurred with the sample in various atmospheres, including air, 5% 02 in Ar, nitrous oxide (N 20), and ultra high vacuum. RESULTS a-Si exposure Direct patterning of a-Si:H films requires a means of modifying the surface or bulk of the film to change its resistance to etching. Two methods of modifying the a-Si:H film to generate patterns (laser crystallization and optically enhanced oxidation) have the potential to meet the throughput requirements of modem factories. Laser crystallization modifies the bulk film through localized melting, requiring high power density. Excimer lasers have sufficient power to crystallize centimeter size areas to a depth of more than 100nm, making these sources suitable 4 for projection lithography applications. Our work in this area has been reported previously.1 "15 Other laser sources can be used for direct writing, but feature sizes will be limited by the spot size that can be realized with a scanned beam system. Selective oxidation of the a-Si:H surface provides an alternative patterning approach that a
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