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PHOSPHORUS DIFFUSION IN POLYCRYSTALLINE SILICON: MONTE CARLO SIMULATION OF EXPERIMENTAL DIFFUSION PROFILES

J. P. LAVINE, S.-T. LEE, D. L. BLACK, D. L. LOSEE, AND C. M. JARMAN Research Laboratories, Eastman Kodak Company, Rochester, NY 14650 ABSTRACT Phosphorus ions were implanted into silicon layers deposited by low pressure chemical vapor deposition onto thermally oxidized silicon substrates. Thermal anneals diffused the phosphorus and the resulting depth profiles were determined by secondary-ion mass spectrometry (SIMS). Transmission electron microscopy shows that the polysilicon layers have a multi-layer pattern of grains. The phosphorus profiles are fit by a Monte Carlo simulation technique that includes both grain and grain-boundary diffusion. The grain-boundary diffusion coefficient is found to be thermally activated with an activation energy of 3.3 eV. INTRODUCTION Impurity diffusion in polycrystalline silicon involves both grain and grain-boundary diffusion. Since the impurities diffuse during device processing, a knowledge of the diffusion coefficients is useful for the control of the fabrication process and for the achievement of the desired device properties. The impurity distribution in polysilicon influences the oxide thickness grown during an oxidation step [1], the polysilicon profile after etching [2], and the amount of impurity driven from the polysilicon into the silicon substrate [3]. The electrical [4,5] and optical properties [6] of a device that incorporates polysilicon also depend on the impurity profile. The present work shows how ion-implanted phosphorus profiles evolve during thermal anneals in nitrogen. Diffusion coefficients for both grain and grain-boundary diffusion are extracted from the measured profiles. The sample preparation and measurement techniques are described first. The Monte Carlo approach to grain and grain-boundary diffusion is then outlined and the experimental phosphorus profiles are shown with the calculated profiles. The deduced grain-boundary diffusion coefficients are compared to those determined by other experiments. EXPERIMENTAL TECHNIQUES The p-type silicon substrates were thermally oxidized to produce a 0.1 um silicon dioxide layer. Silicon layers -0.4 ym thick were deposited in a low pressure chemical vapor deposition (LPCVD) system at 580 0 C. The as-deposited silicon films are amorphous [7,8] and the deposition rate was about 0.004 Mm/min. Phosphorus was implanted at 50 keY to a dose of 3 x 101 40 /cm 2 , and a cap layer of 0.5 mm of silicon dioxide was deposited at 425 C. The samples were annealed in nitrogen at temperatures of 699 to 900 0 C. The amorphous silicon films quickly convert to polysilicon during the thermal annealing. Transmission electron microscopy (TEN) shows that the polysilicon films have grains that are equiaxed and random in size and orientation. The films are seen to have multiple layers of grains [8]. Silicon films were0 also deposited at 620 0 C by LPCVD and then treated identically to the 580 C films. The films deposited at 620 0 C h