For the purpose of accuracy it should be noted that these processing steps are simplified for this paper. In actual application various masks would be implemented to allow for the creation of multiple p-n junctions on the surface of the wafer. The process illustrated below will focus on the thermal diffusion of a liquid p-type dopant into an extrinsic silicon wafer (already n-doped to a certain concentration) to create a p-n junction of certain depth.4 .

             Process Overview.

             To form a p-n junction on the surface of a silicon wafer, a source of dopant is applied to the surface of the wafer. Next, the wafer is loaded into a furnace at a certain temperature and time to deliver the total dose of dopant to the wafer. Subsequent drive in diffuses the dopant further into the wafer to push the p-n junction depth further into the wafer.

             Predeposition.

             The process starts with the application of a liquid dopant spun onto the wafer to distribute it evenly across the wafer surface. Once the dopant has dried, the wafer is loaded into a furnace to diffuse the dopant. The spun on layer acts as an "infinite " source of dopant atoms with respect to the intrinsic wafer (the concentration of the "infinite " source is the solid solubility limit of the dopant). The time and temperature in the furnace determine the total dose (D) of the dopant within the wafer (Equation 1).

             Equation 11.

             Within Equation 1, the D0 refers to the diffusion coefficient, which is based on the comparative lattice differences between the dopant and silicon substrate. Activation energy is represented by Ea and is in electron volts. The Boltzmann constant is expressed as 8.62E-5 eV/K. With the total dose (D) calculated, the concentration of dopant at any depth can be calculated (Equation 2).

             Equation 21.

             Equation 2 represents the concentration at a specified depth and time based on the surface concentration (Csurface) and total dose (D). Different dopants have different solid solubility limits (Table I), which represents the "infinite " source of dopant.