Structure and composition of phosphosilicate glass systems formed by POCl3 diffusion

The phosphosilicate glass (PSG) layer system grown on the silicon surface during diffusion processes with phosphorus oxychloride (POCl3) is a two-layer stack system consisting of a PSG and a silicon dioxide (SiO2) layer. Understanding the stack layers’ structure and composition is essential for further optimizing POCl3 diffusion processes. For diffusion processes with in-situ oxidation (i.e. a high oxygen gas flow during the drive-in step), we find that the intermediate SiO2 layer thickness increases significantly from 8 nm (no in-situ oxidation) to 43 nm, while the PSG layer thickness remains constant at about 8 nm. This thick SiO2 layer seems to hinder the diffusion of phosphorus atoms from the PSG through the SiO2 layer into the silicon. Implementation of a second deposition step with active N2-POCl3 flow at the end of another diffusion process type increases the thickness of the PSG layer from 14 nm to 25 nm, while at the same time the intermediate SiO2 layer thickness decreases by about 40% to 5 nm. The total phosphorus dose within the PSG/SiO2 stack layer thereby increases by a factor of three and is determined to be about 1.7·1015 cm-2. These findings make the approach of a second deposition step very interesting for, e.g., laser doping applications to form selective emitters. For diffusion processes with different N2-POCl3 to oxygen (N2-POCl3:O2) ratios during deposition, we find only small influences on the PSG/SiO2 stack layers present after drive-in. However, the resulting emitter sheet resistances are strongly impacted by the N2-POCl3:O2 ratio; the measured sheet resistances continuously decrease from 343 Ω/sq for a N2-POCl3:O2 ratio of 0.8 to 72 Ω/sq for a ratio of 4.0