Recoil Implantation of 18O from SiO2 by Heavy Projectiles
The recoil implanation yield of oxygen atoms recoiled from thin, 18O enriched SiO2 layers into silicon substrates has been studied using the 180(p,α)15N nuclear reaction. For 24 keV Kr projectiles the 180 yield peaked at a thickness (~150 Å) which approached the expected range of the Kr in SiO2, and ~2.5 oxygen atoms recoiled into the Si for each projectile. The cross section for recoiling into the Si was ~5 × 10-17 cm2/oxygen atom in the near-linear region stated above. For a fixed SiO2 thickness, the yield increased slightly with decreasing projectile energy until the projectile range was no longer greater than the oxide thickness.
The amount of 18O which was backsputtered from the SiO2 layers during the bombardment by Kr was determined. For a fixed Kr energy and a thin SiO2 layer, this amount increased linearly with increasing SiO2 thickness and was nearly 10 times the recoil implantation yield.
A simple cascade model does not describe the Kr data, but the primary knock-on yield from a 1/r2 potential, which is a reasonable approximation to the Thomas-Fermi potential for impact parameters of interest here, predicts the observed yield to better than a factor of two. Making a first order correction for recoils caused by the primary knockons, the calculations give excellent agreement to the yield vs. oxide thickness data.
When using photoresist as a masking material for heavy projectiles, maximum energy carbon recoils have a mean range much larger than the projectile (a factor of ~1.8 for As). Thus, for high dose implantations, where a significant number of carbon atoms are recoiled, the desired photoresist thickness is best dictated by the range and range straggle of the recoils, not the projectiles.
KeywordsOxide Thickness Si02 Layer Projected Range Recoil Atom High Dose Implantation
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