Boron Autodoping during Silane Epitaxy

In order to calculate the redis tr ibut ion of boron in silicon by both diffusion and autodoping dur ing epitaxial growth, certain materials parameters must be known as a function of temperature. The parameters are the diffusion coefficient of boron, the evaporat ion coefficient of boron from silicon, and the silicon evaporation rate in a hydrogen ambient. The value of the diffusion coefficient of boron is already well known while the values for the other two parameters have not been determined over the temperature range usual ly used for epitaxial growth. The evaporation coefficient of boron in silicon in a hydrogen ambient was exper imenta l ly determined to be h = 1.674 • 10 T e -2.4s/kT where h is in microns per minute, the energy is expressed in electron volts, and tempera ture is in degrees Kelvin. The evaporation rate of silicon in a hydrogen ambient was measured as 0.013 __ 0.003 mic rons /min in a hydrogen ambient. These results were obtained in the tempera ture range from 1190 ~ to 1380~ These results allowed the accurate prediction of boron back surface "autodoping" effects dur ing epitaxial growth using a numerical solution to Fick's second law. Radial as well as in -dep th concentrat ion vs. distance plots were exper imenta l ly determined i l lustrat ing boron autodoping for both l ight ly and heavily doped substrates. Reduction of boron autodoping was obtained by the addit ion of HC1 to the ambient dur ing epitaxial growth, permi t t ing the growth of l ightly doped layers on heavily boron-doped substrates. In order to calculate the impur i ty redis t r ibut ion dur ing epitaxial growth or other high temperature semiconductor device processing steps, accurate values of the diffusion coefficient, segregation coefficient, and evaporation coefficient must be known for each of the dopant elements in silicon. Any time a l ightly doped region is exposed to a heavily doped region t ransport of dopant will occur both through the solid and gaseous phases. The transport of dopant from one wafer to another or from one region on a wafer to another region on the same wafer has been known as "autodoping." The presence of excess dopant via autodoping in regions where it is unwanted leads to the inabi l i ty to fabricate certain types of transistors and integrated circuit structures. The epitaxial growth process has long been the principal processing step in which autodoping may occur. Especially in the fabrication of in tegrated circuits, autodoping must be suppressed or at least controlled. The purpose of this paper is to first obtain the physical constants necessary to characterize boron autodoping. With these constants available, predictions of the boron dis tr ibut ion during epitaxial growth are made. These predictions are then compared with experiments as an over-al l test of the mathematical model being employed. Background : Epitaxial Growth The existence of dopant in an epitaxial layer over and above that dopant which was transported by solidstate diffusion from the substrate has been a subject of interest from the earliest work on epitaxial deposition. Thomas et al. (1) and Grossman (2) explain the existence of this extra dopant by a mechanism which involves t ransfer of dopant from the front surface of the wafer, mixing of this dopant with the gas phase, and reincorporat ion of some fraction of the dopant in the gas phase back into the wafer. Thomas worked exclusively with arsenic-doped substrates. Since solidstate diffusion was ignored in this model, application near the subs t ra telayer interface was difficult. An" E l e c t r o c h e m i c a l S o c i e t y A c t i v e Member.