Predeposition step diffusion

Figure 1 is a plot of boron concentration versus ps that illustrates Henry s law. When the solid solubility of boron in silicon is reached, Henry s law no longer applies. Thus, most predeposition steps are operated at a sufficiently high partial pressure in the dopant gas phase to achieve solid solubility of the dopant in the silicon. This requirement provides a natural control for reproducible diffusion results. 

Typically, two heat treatments are nsed in this proeess. In the first, or predeposition step, impurity atoms are diffused into the silicon, often from a gas phase, the partial pressure of which is maintained constant. Thus, the surface composition of the impurity... 

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. 

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