Evaluation of the Surface Concentration of Diffused Layers in Silicon

Recently the investigation of the diffusion of electrically active impurities in semiconducting materials has become important because of the use of diffusion technology in the fabrication of semiconductor devices. A diffused layer is characterized by the impurity distribution; its mathematical form is obtained by solution of the diffusion equation with the appropriate boundary conditions. When the distribution is known, two of the following parameters may be chosen to describe the layer completely: diffusion coefficient -- time product, surface concentration, total number of diffused atoms and concentration at a given depth. Of these quantities, only the depth is easily measured. Another readily measured quantity of the layer is its sheet resistivity. But, from the point of view of the device designer, the junction depth and surface concentration are the most important quantities. The present paper shows how the surface concentration of a diffused layer may be obtained from the sheet resistivity and junction depth if the impurity distribution, the majority carrier mobilities, and the resistivity of the silicon are known. The necessary evaluations are given here for the case of diffusion of Group I I I and V elements into uniformly doped silicon of the opposite conductivity type. 699