Electron beam ablation of materials.

Abstract

A low accelerating voltage, high current electron beam accelerator, called the channelspark, has been used for ablation of materials applied to thin film deposition. The channelspark is a pseudospark-like device with the multi-gap arrangement of pseudosparks replaced by a continuous dielectric tube. The channelspark operates at accelerating voltages of 15 to 20 kV with &sim;1500 A beam currents. The electron beam ionizes a low pressure gas fill (10 to 20 mTorr Ar or N₂) to compensate its own space charge, allowing for such large currents. Ablation of Fe, Ti, TiN, Si, and fused silica have been studied through plasma diagnostics including optical emission spectroscopy, dye laser resonance absorption photography (DLRAP), non-resonant interferometry and resonant ultra-violet interferometry (RUVI). In addition, thin films Of SiO₂ have been deposited. The morphology and stoichiometry of these films has been studied via scanning electron microscropy (SEM) and x-ray energy dispersive spectroscopy (XEDS) respectively. Strong optical emission from ionized species, persisting for several microseconds, was observed in the electron beam ablated plumes. Free electron temperatures were inferred from relative emission intensities to be between 1.1 and 1.2 eV. DLRAP showed plume expansion velocities from 0.38 to 1.4 cm/mus for several pressures of Ar or N₂ background gas. A complex, multilobed plume structure was also observed. Nonresonant interferometry yielded electron densities of about 2.5 x 10<super>17</super> cm<super>-3</super> near the target surface. RUVI performed on Si neutral atoms generated in the ablation plumes of fused silica measured line integrated densities of up to 16 x 10₁₅ cm<super>-2</super>, with the total number of ablated silicon neutrals, calculated to be in the range 20 x 10<super> 14</super> to 0.5 x 10<super>14</super>. Electron beam deposited films of fused silica were microscopically rough, with a thickness variability of 7%. The average deposition rate was found to be about 0.66 shot The electron beam deposited fused silica films had accurately maintained stoichiometry. Ablated particulate had an average diameter near 60 nm, with a most probable diameter between 40 and 60 nm. The mass of material ablated in the form of particulate made up only a few percent of the deposited film mass, the remainder being composed of atomized material.