Al precipitate evolution in epitaxial silicon layers induced by thermal oxidation

Al, the fastest p-type diffuser in silicon, interacts strongly with oxygen, defects, and precipitates, with a detrimental effect on the electrical activity. Although, substrates with low oxygen content, such as epitaxial layers on FZ grown crystals, can be used, it is practically impossible to avoid oxidation steps and then the amount of oxygen that is introduced into the wafer depends on the oxidation temperature. The present work deals with the behaviour of Al implanted at high energies either in bare epitaxial Si or in pre-oxidized epitaxial Si with a 300 nm thick thermal oxide layer. The subsequent annealings were performed under oxygen or nitrogen atmosphere. The SIMS and the spreading resistance analyses indicate that even the oxygen present in the silicon substrates after the thermal oxidation at temperatures above 1000°C causes the precipitation of Al at the depth of the damage peak. The effect has been quantitatively analysed by comparison with thermal processes in nitrogen atmosphere. For the Al implant through the oxide layer, the growth and dissolution of Al-O precipitates has been studied