An Understanding of in Situ Boron Doped Polysilicon Films by Characterization and Simulation

A new understanding of the deposition of in situ boron doped polysilicon based on a comparison between experimental results and modeling predictions, is proposed. The depletion of both silane and boron trichloride is put in evidence and the necessity of the use of injectors is established. 1 Introduction The in situ doping of polysilicon is of interest /I/, as this technique makes it possible to reduce, both the number of steps involved in the preparation of the doped layer and the amount of equipments required to process the wafers. For boron doped polysilicon, two ways in industry have been considered: (1)from diborane /2/ B2H6, but covered boats are required to obtain a good uniformity across a wafer, (2)from boron trichloride /1/ BCl3, but the use of injectors is required. In this work, studies are devoted to the Si&-BC13 system /l/, where normally the design of injectors is empirical and consequently seldom satisfactory because the use of such a system requires significant maintenance, due to the frlling-in of the holes. In this paper, a studied approach of the in situ boron doped polysilicon is carried out, taking into account, on the one hand, experimental and simulation results of the deposition step and, on the other hand, electronic and physical characterizations. Comparison is made between results obtained with or without the use of injectors, in order to achieve a better understanding of the physical and chemical phenomena underlying the deposition. 2 Experimental results Deposition is made in a horizontal LPCVD reactor in isothermal conditions at 555OC from S a (200 sccm) and BCl3 diluted at 3% in N2 (90 sccrn), injected from the front door, at a pressure of 46.7 Pa, without injectors and compared with results obtained when 40 sccm of the BCl3 + N2 are spread by the injectors, and 50 sccm injected from the front door 111. The load included in both cases 162 wafers of 5" silicon wafers, separated by a 3.8 rnrn interval. The position of each of the wafers in the batch is indicated by a number starting from 1 (first wafer reached by the gases) to 162 (last wafer reached). For every wafer, the thickness of the deposit layer has been measured by an interferometrics method and the silicon growth rate has been deduced from it.The boron concentration has been evaluated by SIMS experiments and conductivity measurements have also been made. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1991209 C2-80 JOURNAL DE PHYSIQUE IV 2.1Results without injectors Figure 1 shows that the deposition rate varies linearly with the position, from 85 to 32 Qmin. SIMS experiments (see Table 1) indicate a boron concentration of 1.3 1021 cm-3 on the wafer n082, and 5 times lower on the wafer 136. Conductivity measurements are given in figure 2, which shows that a maximum value of 510 s.cm-1 is obtained between position 10 and 50, and that a minimum value of 120 s.cm-1 is obtained on the last wafers of the load. The two mass balances on silane and on boron trichloride, taking into account respectively the silicon and the boron atoms deposited on the solid surfaces, put in evidence conversion rates of 30% and 80%. Fig. 1: Experimental polysilicon growth rate along the reactor with and without injectors E 40 s X X 2 30w 20 -