Wafer fabrication is a complicated process involving repeated sequences of circuit placement that is essential for the preparation of circuit boards. The process is also essential in the production of photonic circuits. The process is essential in the production of such systems as light emitting diodes, computer Central Processing Units, all optical components in computers as well as radio frequency amplifiers. The complexity of the wafer is determined by the number of signals that the designer intends it to transmit or the number of circuits that are required in the system. The layers can be made to overlap to accommodate even more circuits in cases where the circuit boards required more complex. The rooms where these circuits are built in layers are called clean rooms.
Silicon is a very important component of the wafers. This is because the silicon is a semiconductor, and it permeates currents to pass through it selectively. The silicon is coated with a base oxide to relieve stress that could damage the silicon. An additional layer of Nitride coats the base oxide layer and protects the silicon from oxidation even at high temperatures. Patterns of photoresist are etched into the nitride to protect the part of the nitride that is important to remain. The other parts of the Nitride are then removed and replaced with phosphorus. An oxide mask (known as N-Tub) is then grown on the phosphors side. The N-Tub protects the phosphorus side from misalignment and oxidation. Boron and BF2 are then used to implant the Nitride side (NMOs side ). A thin Pad Ox layer is used to randomize implantation and in channeling prevention. The wafer is then exposed to a predetermined amount of heat to ensure that the device works correctly. The Phosphorous doped side is called the P-well while the Nitride side is known as the N-well. Following oxidation, the Nitride layer is removed is then removed leaving thin Pad Ox layer, Thick Field Ox and the N-well and P-well.
After the Oxidation step, the Nitride is no longer needed and can be removed. After the Nitride strip, we are left with a thin Pad Ox layer, a thick pattern of Field Ox, a P-Well and an N-Well. The gate is then created through threshold voltage adjustment, an addition of a Gate Oxide layer and application of a doped poly pattern. Implantation of BF2 ions sets the threshold voltage for both transistors. Gate oxidation insulates the space between the poly gate and silicon substrate. Gate oxidation is the most important oxidation process. The insulation prevents electrons from leaving the silicon layer and travelling to the gate. A polysilicon layer is then applied which is a semiconductor and is doped using phosphorous to enable it conducts electricity.
A doped space layer is added on top of the polysilicon layer. The unimportant parts of the spacer oxide layer are removed forming oxide spacers over gates. The polysilicon that will not be part of the gate in a plasma etch. Ash is then used to remove the photoresist. Some of the transistors are doped using Arsenic instead of Phosphorous because arsenic atoms are heavier and, therefore, do not move. Contact oxide layer application then prepares the wafer for application of the metal layer insulating the wafer. Two layers of metal are added on the wafer surface. The wafer is then cleaned to remove any foreign oxides from the wafer.
The steps are repeated hundreds of times to create an overlapping layer of semiconductors circuits. The process of wafer creation is an evolving process, and new methods of accomplishing this process emerge almost every year. These new methods allow circuits to be embedded more densely. More density of packaging allows for smaller chips and circuit boards as well as microprocessors.