Cooperative effects between arsenic and boron in silicon during simultaneous diffusions from ion implanted and chemical source predepositions

Abstract

Simultaneous diffusions of As and B from predeposited layers (chemical source or ion implantation) have been used in order to fabricate the emitter and base regions, respectively, of microwave transistors. Mathematical simulations of the doping profiles in these transistors have shown that the cooperative diffusion effects that occur in sequentially diffused As-B structures (chemical sources) are noticeably absent in the predeposited-diffused structures. The result is that transistors that are fabricated via this technique show no base retardation, and the active base doping concentrations are higher than predicted by previously established diffusion equations. In order to determine the extent to which cooperative effects are important, transistor doping profiles were measured and compared with calculated profiles. By including the electric field interaction, the vacancy undersaturation condition due to [ V SiAs 2] complex formation, and ion pairing, it was possible to estimate the significance of each effect upon the B diffusion. It is shown that the electric field interaction is two- to three-times smaller in a predeposited-diffused structure than in a constant surface concentration diffusion (non-depleting source). More importantly, a negligible undersaturation of vacancies occurs during simultaneous As-B diffusions from predeposited layers. In the case of a chemical source As predeposition, this is due to the fact that a quasi-equilibrium concentration of [ V SiAs 2] complexes is achieved during the predeposition (before the simultaneous diffusion with B). In the case of an As implantation predeposition, complexes appear to be formed either during implantation or very rapidly during annealing for doses ≲ 3 × 10 15 cm −2. Data is presented which suggests that no inactive As complexes are formed in As implanted-annealed structures in which the maximum solubility of As + ions is approached (3·8 × 10 20 atoms/cm 3 at 1000°C, or a dose of 5–8 × 10 15 cm −2). This result pertains to the As dose used in this study. Since this result has not been observed in As-doped layers that were diffused from chemical sources, further work is needed in order to explain this anomaly.