SURFACE-TOPOGRAPHY OF PHOSPHORUS-DOPED POLYSILICON

The surface topography of doped polysilicon films was investigated by atomic force microscopy for a wide range of doping and process conditi ons. These low-pressure chemical vapor deposition silicon films were a pproximately 350 nm thick. The amorphous films were in situ phosphorus doped during deposition at 550 degrees C, while the crystalline films were deposited at 625 degrees C and subsequently diffusion doped usin g either PH3 or POCl3 gases. Measured resistivities ranged from 700 to 10 000 mu Omega cm corresponding to secondary ion mass spectrometry p hosphorus concentrations that ranged from 8.45 to 0.95 X 10(20) cm(-3) . In situ doped films exhibited the smoothest surface topography with a peak-to-valley surface roughness of 11 nm. The surface roughness val ues 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 bimo dal grain size distributions, with the secondary grains measuring 500 nm in size and the normal grains averaging 225 nm. These secondary gra ins increased the surface roughness and their occurrence correlates wi th chlorine concentration. The number of secondary grains and their si ze increases with higher phosphorus content. Following the polyoxide g rowth, the surface roughness increased 3X to 5X with POCl3 doping, but the surface topography increased only slightly for PH3 and in situ do ped poly-Si. After removing the polyoxide, the surface roughness decre ased for the diffusion doped films. In situ doped films retained their smooth surface following the oxidation and removal of the oxide. (C) 1995 American Vacuum Society.