Resistless Patterning of Hydrogenated Amorphous Silicon Films

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scanning probe technique that allows optical excitation of materials at spatial resolutions well below the diffraction limit. An optical fiber is tapered to give an aperture typically 100nm or less in diameter. The fiber tip is scanned across the sample surface while maintaining a fixed tipsample separation using shear force feedback.4 The separation and aperture are much less tham the wavelength of light and, hence, in the near-field with lateral resolution comparable to the aperture diameter. This allows NSOM to be used as a direct write tool for photolithography on the submicron length scale. Amorphous silicon has been shown to be a potentially useful resist for nanoscale lithography based on scanning probe techniques.3' 5' 6' 7 Madsen et al. have shown that NSOM can be used to pattern a-Si:H optically with dimensions less than 100nm.8 They have also observed that simple proximity of the fiber tip to the a-Si:H surface can lead to patterning even in the absence of optical illumination. EXPERIMENT In this work, a-Si:H films were grown on crystalline silicon wafers, Corning 7059 glass, or HgCdTe to a thickness of 100-800 nm. A Materials Research Group multichamber, parallel plate, capacitively coupled plasma enhanced chemical vapor deposition (PECVD) system operated at 13.56 MHz was used for both deposition with pure silane and etching with pure hydrogen. 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. Laser sources used for optical irradiation included a KrF excimer laser as the source of 248 nm light with a nominal pulse length of 20 ns and Ar ion and HeNe lasers for continuous wave (cw) irradiation from 350 to 633nm. A Cobilt mask aligner with peak emission at 360nm has also been used to generate patterns. Optical irradiation occurred with the sample in various atmospheres, including air, 5% 02

in Ar, and nitrous oxide

(N 2 0).

NSOM tips were prepared both by chemical etching and by pulling with a Sutter Instruments P2000 micropipette puller. The sides of the probes were coated with approximately 100nm of aluminum. For proximity effect studies uncoated tips were also used. The fiber optic probe was attached to a piezoelectric tube which was oscillated (dithered) parallel to the sample surface. As the tip approached the sample surface, shear force damping of the amplitude provided the necessary signal for maintaining a constant probe-sample separation of approximately 5-20nm. The dither amplitude was monitored using conventional optical detection of laser light scattered off the tip.4 Since the relationship between the absolute dither amplitude and this signal depends on factors such as the tip taper and the location of the light on the tip, only relative dither amplitudes were determined. For a given tip, however, this method allows relative changes in dither amplitude to be easily determined. RESULTS AND DISC

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