3.6.1 Basic n-well CMOS process

In a standard n-well process, one of the first things made is the n-well in a p type substrate. Once the n-well is created, the active areas can be defined. The MOSFET is build within this active area. A very thin layer of silicon dioxide is grown on the surface. This will be used to insulate the gate from the surface. The thin layer of SiO₂ is grown and covered with Si₃N₄. This will act as a mask during the subsequent channel stop implant and field oxide growth. The channel stop implant is to prevent conduction between unrelated transistor source/drains. A thick additional layer oxide grows in both directions vertically where Si₃N₄ is absent. Layer of silicon dioxide under the polysilicon gate (which will be created later) is known as gate oxide and that is not directly under the gate of a transistor is known as field oxide. The field oxide provides isolation between transistors. A threshold adjustment implant would be the next process step. This is carried out to balance off the threshold voltage differences. The P-MOS results in a higher threshold voltage level than nMOS with normal doping concentrations. With additional negative charges buried inside the channel, V_T for pMOS could be controlled.