Crystal Growth and Evaluation of Silicon for VLSI and ULSI

267Impurities in Silicon Wafers

manifested. A simple stepwise shape found for a lower nitrogen content can be accounted

for, assuming that the nitrogen exists mostly in a precipitated form, as numerous small

clusters, with a low solubility C

Be

. Due to out-diffusion of the dissolved species, their con-

centration C

B

becomes smaller than C

Be

in a near-surface region, which results in dissolu-

tion of the clusters. Eventually, the clusters disappear within some depth L that increases

with time by a normal square law t

1/2

. In this denuded zone z < L, the nitrogen concentra-

tion is remarkably low, less than C

Be

. Deeper in the bulk, the total nitrogen ...