We present a detailed experimental and theoretical study of the charging of silicon nitride and silicon oxide thin films following focused ion beam irradiation. The samples were irradiated using 30keV Ga+ ions at different ion doses and their consequent work function changes were measured by Kelvin probe force microcopy. The surface potential of both samples increased following the ion irradiation up to a critical ion dose, and then moderately decreased. The dependence of the sample surface potential on the irradiated ion dose is analyzed by taking into account all the main factors affecting charging in dielectric thin films: electron-hole generation by the incident fast ions, secondary ion-electron emission, sputtering of surface atoms, electron-hole recombination, electron recombination with the incident stopped ions, hole leakage current to the Si substrate, and various charge trapping processes. It was found that the much larger surface potential induced in Si3N4 in comparison to SiO2 is associated with the different resistance to the Ga+ ion bombardment. Under equal ion irradiation dose, a larger concentration of shallow traps is created in SiO2 than in Si3N4. This leads to an increased hole capture in shallow traps versus deep traps, and a consequent decrease in the surface potential.
The application of focused ion beams to the repair of defects in x-ray masks is described. An image of the defective region on the mask is obtained using the ion beam in a manner analogous to a scanning electron microscope. Opaque defects are removed by physical sputtering of extra absorber. Clear defects are repaired by ion-beam-induced decomposition of an organometallic compound to form an opaque film on the substrate. Examples illustrating the repair process and demonstrating submicron spatial resolution are presented. The effect of ion channeling on imaging and opaque repair is also described.
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The influence of diffraction on the shape and size of features printed using x-ray proximity printing is reviewed, and the effect of image blurring on these results is described. Diffraction can alter the shape of a printed feature, and the systematic shape changes observed in resist images can be explained using simple scaling based on Fresnel diffraction. In addition, the linewidth change with exposure dose is independent of feature type and size, and depends only on the square root of the mask to wafer gap. The shape of printed features and the linewidth change with dose can be modified by smearing the aerial image at the wafer plane. This can be achieved by adding beam divergence or by varying the angle of incidence of the x-ray beam onto the mask (wobbling). A technique for incorporating wobble into an exposure system is described, and exposures of contact holes, spaces, lines, and line-space arrays using this technique are presented. For example, 0.35 μm square contact holes normally print diamond shaped at a 40 μm gap. However, the same contact holes are round in resist when 4 mrad of wobble is incorporated into the exposure. The linewidth change for a 10% increase in dose is 24 nm at a 40 μm gap with 5 mrad of wobble. This linewidth change with exposure dose is larger than the 20 nm measured for exposures at a 40 μm gap without wobble. Finally, wobbling during exposure can either increase or decrease the absolute linewidth of a feature in resist at a given exposure dose.
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