Deposition and electrical properties of in situ phosphorus-doped silicon films formed by low-pressure chemical vapor deposition

Abstract

The low-pressure chemical vapor deposition of phosphorus-doped silicon films on oxidized silicon wafers was investigated as a function of phosphine/silane mole ratio, silane partial pressure, temperature, and wafer spacing. The deposition rate decreases, concomitant with increased phosphorus incorporation, as the mole ratio increases. The deposition rate tends to saturate or vary linearly with silane partial pressure for undoped and heavily doped films, respectively. This, together with differing deposition-rate activation energies of 1.5 and 2.0 eV for undoped and doped films, respectively, is indicative of different reaction mechanisms in the two regimes. As the mole ratio increases, the deposition rate becomes increasingly dependent on the wafer spacing and the radial position on a wafer. Because phosphorus incorporation varies inversely with deposition rate, it develops similar dependencies on wafer spacing and radial position. The majority of these observations are interpretable in terms of a model that has been defined for the growth of oxide from the silane-oxygen reaction where the phosphorus and oxygen play analogous roles. The resistivity of annealed films decreases with increased phosphorus incorporation (mole ratio) and for 0.5-μm-thick films reaches a minimum value of approximately 440 μΩ cm at about 1021 phosphorus atoms cm-3. The resistivity decreases with increasing deposition temperature which may be attributable to one or a combination of the decreased phosphorus incorporation at higher temperature or deposition rate, or decreased grain size at higher temperature. The decrease in resistivity with increased thickness is attributed to increased grain size with increasing thickness. At least for thicknesses less than 0.5 μm, lower resistivity is achieved by in situ doping than by doping of films subsequent to deposition.