The concentration profiles of the recoil implanted oxygen in Si after ion implantations into SiO2-Si substrates

Abstract

The annealing of bare thermal oxide on silicon at 400–500°C in a hydrogen bearing gas results in a reduced density of states N_ss at the substrate silicon/oxide interface. Treatments of this type have played a role in MOS processing schedules for several years. However, a similar approach applied to large areas (cm²) of poly-silicon coated oxide appears to be less effective in reducing N_ss. This may be due to the polysilicon acting as a partially impermeable barrier which tends to starve the substrate/oxide interface of hydrogen.

In the present work hydrogenation of 2-inch diameter, polysilicon coated wafers has been accomplished by hydrogen ion implantation. H2⁺ ions of 135 kV energy were implanted (to a dose of 10¹⁵ cm^?2) through a 7000 Å polysilicon coating into an underlying 1400 Å SiO₂ layer. The polysilicon was removed after 30-min anneals carried out in pure N₂ at 300, 400 or 500°C. Aluminium dots, 1 mm in diameter were then deposited on to the oxide and high frequency (1 MHz) and quasistatic C-V curves recorded for determinations of Nss. Control anneals on unimplanted material were carried out in pure N₂ and N₂-H₂ ambients. Control samples annealed in pure N₂ with their polysilicon coating intact had mid-gap N_ss values of not less than 4 × 10¹⁰ cm^?2 eV^?1. The corresponding value after N₂-H₂ anneals on polysilicon-free wafers was 3 × 10¹⁰. H₂ ⁺ implanted samples annealed in pure N₂ with their polysilicon intact had mid-gap N_ss values of 1 × 10¹⁰ cm^?2 eV^?1.

The effectiveness of ion beam hydrogenation may depend upon confinement of the associated displacement damage to the polysilicon. This allows the implanted hydrogen to be activated within the SiO₂ at temperatures similar to those employed for normal hydrogeneous gas annealing of the substrate silicon/oxide interface.