Modeling of rapid thermal diffusion of phosphorusinto silicon and its application to VLSI fabrication

Abstract

The slips on a silicon wafer edge which normally occur during rapid thermal process have been eliminated by forming an oxide ring on the wafer-back surface, thereby reducing the light reflectance from the silcon wafer edge. A simple model which gives the optimum ring oxide thickness and width has been proposed. From the model and experimental results, it has been found that about 3000$\mbox{\AA}$ thcikness of ring oxide gives the best thermal compensation effect. The effect of nonuniform irradiation on the wafer has been incroporated in the model by introducing light power uniformity(LU) factor. In the case of 3300$\mbox{\AA}$-thick, 7mm-wide ring oxide and 0.86$\sim$0.88 of LU, the temperature variation on a 3inch wafer is within$\pm$4$^\circ$C at 1137$^\circ$C, and the slip is completely eliminated. Two-step rapid thermal diffusion (RTD) of phosphorus using a solid diffusion source has been described. Phosphorus profiles in silcon, measured by SIMS, show two distinct regions which are i) constant concentration region near the surface where the phosphorus concentration exceeds the solid solubility and ii) exponentally decaying region forming the diffusion tail. For the quantative analysis of the RTD process, two correction terms for the diffusion time have been introduced. The first correction term incorporates the temperature transient cycle, and the second term is due to the point defect life time during the cooling. From the Boltzmann-Matano alalysis, we have found that the correction term due to the supersaturated point defects during the cooling ($t_{def}$) is about 3 sec. A simple mathmatical modeling shows that one can regard tdef as the life time of point defects. In the case of the boron diffusion, the correction term in the effective diffusion time has a strong dependence on temperature. And the maximum value of $t_{def}$ for boron diffusion has been found to be less than phosphorus diffusion case. The introduction of the additional correction terms ...