Drive-in diffusion

The dopants introduced by the ion implantation step can be redistributed deeper in the substrate to lower the concentrations by a drive-in step. In the drive-in step, the total amount of dopant atoms, Q, remains fixed. The concentration profile due to the drive-in diffusion is given by... 

Fig. 14.8. Schematic dopant distributions in a conventional well formed by the drive-in diffusion and in a retrograde well formed by high energy implantation...

The depth of the recrystallised layer is very small, of the order of 0.01 -0.1 pm. Thus, a possible way to eliminate its effect on p-n junction properties is to perform a drive-in diffusion of aluminium from the epitaxial layer. Unfortunately, the diffusivity of aluminium from an epitaxial layer is extremely slow in SiC. The diffusivity is 3 - 5 orders of magnitude lower than that observed for diffusing aluminium from the vapour phase [70]. The authors of [69] had to employ very high diffusion temperatures, over 2500 °C. The anneal produced a shift of the p-n junction into the crystal bulk and the electrical properties were substantially improved. However, this could not provide the elimination of the weak points of the junctions. The characteristics of the p-n junctions were worse than those with the recrystallised layer removed by sublimation etching. In addition, the surface evaporation and graphitisation at temperatures above 2500 °C severely reduces the reproducibility of the results. 

The implantation of As into polycrystalline samples, and drive-in diffusion into substrates, was investigated by back-scattering analysis and electrical measurements. It was found that the effective diffusivity of implanted As could be described by ... 

Solid-state diffusion also plays an important role in the manufacture of semiconductors. To produce the junctions needed in these devices, a dopant such as boron is deposited on the surface of the semiconductor crystal, e.g., silicon or germanium, and is subsequently made to diffuse into the interior. This process, termed drive-in diffusion, is again carried out at elevated temperatures. An analysis of it and some relevant calculations appear in Chapter 4, Practice Problem 4.4. The diffusivity required in these calculations is derived in the short illustration given below. 

A jxanction in silicon is made by doping it with boron. This is done by first depositing a layer of boron on tire chip (predeposition), followed by what is termed drive-in diffusion. If the deposition step requires 5 min, at what distance from the surface is the concentration of boron raised by 3 x 10 afoms/cm during this interval The density of pure boron is 5.1 x 10 atoms/cm, ifs diffusivity 5.8 x Ifb pm /h. 

If we assume that the impurity atoms introduced during the predeposition treatment are confined to a very thin layer at the surface of the silicon (which, of course, is only an approximation), then the solution to Fick s second law (Equation 5.4b) for drive-in diffusion takes the form... 

Figure 5.9 Schematic concentration profiles for drive-in diffusion of semiconductors at three different times, b, t2, and fj.

FigMre 5.10 Schematic concentration profiles taken after (1) predeposition and (2) drive-in diffusion treatments for semiconductors. Also shown is the junction depth, Xj. 

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