Borosilicate glass nanolayer as a spin-on dopant source: FTIR and spectroscopic ellipsometry investigations

Borosilicate glass is a potential dopant source for producing shallow boron junctions by the use of proximity rapid thermal diffusion. Interest in this technique has increased recently due to its application to the manufacture of solar cells. A borosilicate gel is spun onto a silicon wafer and the layer is rapidly thermally annealed to convert it to a borosilicate glass (BSG). Fourier transform infrared (FTIR) spectroscopy, spectroscopic ellipsometry and sheet-resistance measurements have been used to understand and subsequently optimise the conversion of the gel to a BSG nanolayer. Physical properties of the thin, spun-on layer, such as thickness, refractive index and porosity, were monitored. The optimum conversion step involved rapid thermal annealing for 45 s at 900 °C. This avoided any boron loss from the BSG layer during the thermal processing step. The position of the B–O stretching vibration around 1370 cm⁻¹ was found to be sensitive to boron outdiffusion and it is suggested that FTIR spectroscopy provides a simple method for monitoring the outdiffusion of boron from the spin-on dopant nanolayer. Further FTIR studies using p-polarised light at oblique incidence revealed, for the first time, the LO–TO phonon splitting of the B–O stretching vibration band in the glassy layer. Investigation of the stability of BSG layers over long periods showed that unstabilised (or undensified) BSG films demonstrate a dramatic loss of boron over 6 months.