Memory Chip Manufacturing Part 2
At the beginning of the production process, the bare silicon wafer is covered with a thin glass layer followed by a nitride layer. The glass layer is formed by exposing the silicon wafer to oxygen at temperatures of 900 degrees C or higher for an hour or more, depending on how thick a layer is required. Glass (silicon dioxide) is formed in the silicon material by exposing it to oxygen. At high temperatures, this chemical reaction (called oxidation) occurs at a much faster rate.
Next, the wafer is uniformly coated with a thick light-sensitive liquid called photoresist. Portions of the wafer are selected for exposure by carefully aligning a mask between an ultraviolet light source and the wafer. In the transparent areas of the mask, light passes through and exposes the photoresist.
Photoresist undergoes a chemical change when exposed to ultraviolet light. This chemical change allows the subsequent developer solution to remove the exposed photoresist while leaving the unexposed photoresist on the wafer. Wafers are exposed to a multiple-step photolithography process that is repeated once for each mask required by the circuit.
The wafer is subjected to an etch process (either wet acid or plasma dry gas etch) to remove that portion of the nitride layer that is not protected by the hardened photoresist. This leaves a nitride pattern on the wafer in the exact design of the mask. Hundreds of memory chips can be etched onto each wafer. The hardened photoresist is then removed (cleaned) with another chemical.
Dopants are frequently introduced as part of the layer formation in high temperature diffusion operations or with ion implanters. These dopants tailor the silicon's conductive characteristics making it either negative (n-type) or positive (p-type). These basic steps are repeated for additional layers of polysilicon, glass, and aluminum.
The finished wafer is an intricate sandwich of n-type and p-type silicon and insulating layers of glass and silicon nitride.