Predeposition step

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. 

The goal of the predeposition step is to deposit some number of atoms per square centimeter, Q(t), in the silicon substrate. That number is calculated by integrating the total concentration per cubic centimeter from 0 to 00 as shown in equation 8. 

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... 

If fls Am, misfit is present, positive or negative, and growth proceeds by the Stranski-Krastanov mechanism, which is composed of two steps. In the first step a 2D overlayer of M ij on S is formed, and in the second step 3D crystallites grow on top of this predeposited overlayer (Fig. 7.20). 

Once the predeposition is completed with Q atoms per square centimeter, the next step is to redistribute the atoms to give the desired junction depth. 

Applying semi-on-line coupling techniques, particularly spraying, provides reproducible results. The coupling to an electrospray deposition interface is shown in Figure 5.11. The matrix can be added to the eluent stream by an injection pump with flow rates of 5—10 pi/min" or it can be predeposited on the target in a preceding step. 

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. 

In the simplest, one-step approach, electropolymerization is carried out in a solution of the monomer and enzyme, at a pH sufficiently removed from the pi such that the protein is incorporated in the charged polymer matrix via electrostatic interactions [162-167]. By this method, multilayer architectures with distinct electropolymer and/or encapsulated proteins can be fabricated. Alternatively, or in combination with this, biologicals can simply be adsorbed onto or covalently attached to predeposited electropol5nmer surfaces. 

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