Recoil Implantation of ¹⁸O from SiO₂ by Heavy Projectiles

Abstract

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.