Surface topography of phosphorus doped polysilicon

Abstract

The surface topography of doped polysilicon films was investigated by atomic force microscopy for a wide range of doping and process conditions. These low-pressure chemical vapor deposition silicon films were approximately 350 nm thick. The amorphous films were in situ phosphorus doped during deposition at 550 °C, while the crystalline films were deposited at 625 °C and subsequently diffusion doped using either PH3 or POCl3 gases. Measured resistivities ranged from 700 to 10 000 μΩ cm corresponding to secondary ion mass spectrometry phosphorus concentrations that ranged from 8.45 to 0.95×1020 cm−3. In situ doped films exhibited the smoothest surface topography with a peak-to-valley surface roughness of 11 nm. The surface roughness values were 50 nm for PH3 doped poly films, and as high as 135 nm for the POCl3 doped films. Atomic force microscopy grain size analysis showed uniform distributions for the in situ and PH3 doped films with grain sizes of 130 and 200 nm, respectively. POCl3 doped poly-Si showed bimodal grain size distributions, with the secondary grains measuring 500 nm in size and the normal grains averaging 225 nm. These secondary grains increased the surface roughness and their occurrence correlates with chlorine concentration. The number of secondary grains and their size increases with higher phosphorus content. Following the polyoxide growth, the surface roughness increased 3× to 5× with POCl3 doping, but the surface topography increased only slightly for PH3 and in situ doped poly-Si. After removing the polyoxide, the surface roughness decreased for the diffusion doped films. In situ doped films retained their smooth surface following the oxidation and removal of the oxide.