Effect of surface treatment during Ge+/B+ two step ion implantation

For ultra-low energy implantation it is known that the wafer surface condition is very important. Thin oxide formation, for example, significantly affects the dose retention and activation of ultra-low energy processes, as the projected range is comparable to the thickness of the oxide film. In the case of 200 eV implants, Rp is less than 2 nm and typically the native oxide thickness is around 1 nm. This means that a large percentage of implanted boron cannot penetrate the native oxide and these boron atoms may become trapped and not activate. Thus the thickness of the surface film does have a significant affect on the effective dose. B⁺/0.2 and 1 keV implants have been studied before and after rapid thermal annealing, and the effect of pre-amorphisation using Ge⁺ investigated. It is found that pre-amorphised wafers have lower activation (higher sheet resistance), especially for very low B⁺ implant energies. Analysis has correlated this shift with an increase in surface oxide thickness, as well as an increase in surface B concentration. Total B dose, measured by SIMS after annealing also decreases. It is therefore concluded that the increased resistance may be attributed to trapping and segregation of dopant into the surface oxide, associated with the increased oxide thickness, and evaporation from the surface. Use of a wet chemical treatment between Ge⁺ and B⁺ implants prevents the activation change, confirming the above mechanism and offering one processing strategy to achieve higher activation efficiency when using pre-amorphisation