Microstructuring of gold on x-ray masks with focused Ga+ ion beams

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Abstract
For repairing opaque defects on x-ray masks by focused gallium ion beams (Ga-FIB), it is important to achieve nearly vertical slopes in the remaining absorber material and to avoid the redeposition of sputtered particles. The sputter yield depends on the angle of incidence. Since this angular dependence influences the slope angle, th sputter yield of a 100 keV focused Ga ion beam on polycrystalline gold, as used on x-ray masks, was measured at five different angles of incidence (8 =0 , 30 , 45 , 60 , 80 ). At normal incidence (9 =0 ) the yield is 32. It increases with increasing angle of incidence and becomes 128 at e = 80 By means of computer simulations it could be shown, that the sputter yield at great angles of incidence plays the predominant role for the evolution of the slope, even if the repair process is carried out with nominally vertical incidence. A high sputter yield at a great angle of incidence leads to a precipitous slope (a =85 ). An even better slope angle was achieved when the beam was scanned in an appropriate manner: If the center of the beam, where the ion current density is maximum (Gaussian beam shape), is directed onto the lower parts of the slope, there, an increased sputter erosion takes place. The reason is not only the increased ion current density, but the higher sputter yield at great angles of incidence as well. Experimentally and in a computer simulation it could be shown that a slope angle of a =88 can be achieved in forward direction (direction of scanning). Since redeposition affects the microstructuring of gold substantially, the redeposition characteristics (spatial distributions of sputtered particles) were measured for two angles of incidence (=0 , 45°). In both cases an area of 500 zm x 10 mm of a polycrystalline gold layer was bombarded with a 100 keV Ga+ focused ion beam. The sputtered particles were collected on semi-cylindrical glass screens. The redeposited gold layers on the inner surface of the glass screens were measurable after 40 hours of sputtering each specimen. The maximum thickness was 1.4 nm for the normal incident beam and 2.2 nm for a beam inclination of 45 . The redeposited layers were evaluated locally resolved by an optical transmission measurement system. These measurements show that the spatial distribution of the sputtered gold particles can be approximated by a cos2-function for both, normal and inclined incidence. Taking this distribution, the simulations were carried out.

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