Before starting this lab please read the Diffusion and Photolithography sections.
In this laboratory session, we will perform standard photolithographic techniques for patterning a diffusion window through the oxide layer that you grew in Lab 1. Then we will begin the diffusion process. Figure 1 shows a schematic of this sequence.
The photoresist is an organic material that can be selectively applied to a wafer through a sequence of bakes, exposures (with or without light-blocking masks), and develops. Once patterned, the photoresist shields the underlying areas from acid etchants, so that selective etching can occur. We will pattern the oxide for use as a barrier to diffusion, i.e. the etched regions will be openings for the phosphorus diffusion needed to create the n-type diode wells and resistor channels (The photolithography procedure used in this lab and in Lab 4 will create a reverse image of the mask, areas that are black on the mask will be developed allowing the BOE to etch the oxide in this area.)
Diffusion is performed with solid source dopant wafers. The dopant wafers face the silicon wafers in the diffusion boat, and the dopants evolve phosphorus pentoxide, P2O5, when heated to the diffusion temperature. This gaseous oxide reacts at the silicon wafer surface to produce diffusing phosphorus and a glassy residue:
The diffusion process will actually be performed in two steps. First, a "pre-deposition" will put a high dopant concentration into a very thin layer of the silicon wafer. Then, the solid source is removed and the "drive-in" is performed. The drive-in is run at a higher temperature (and longer time) than the pre-deposition, producing a deeper, more evenly spread diffusion profile.
Full cleanroom protocol is required for this lab experiment. Full gowning is required. This is for your safety, and for the cleanliness of the wafers.
Read the section on Safety to understand the hazards involved in this lab.