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 There are many steps involved in building the CMOS device. A layer of high-quality silicon is deposited on a silicon wafer base using a process called epitaxial deposition. To separate each transistor, a series of process steps are completed to form what is known as shallow-trench isolation (STI). A layer of oxide is then deposited. Next, a photoresist layer is added, and the first layer of the transistor pattern is transferred through a mask onto the photoresist. Areas of photoresist not protected by the mask are chemically removed, and the resulting exposed areas of the oxide layer are removed in a process known as etch. To form a PMOS transistor, an n-type well is created by doping, or implanting, small amounts of impurities into the opening to the surface of the wafer. Next, a thin layer of high-quality oxide is grown by exposing the silicon to oxygen at high temperature, and a layer of polysilicon is deposited. Photoresist is added, patterned, and the polysilicon and oxide stack is etched. This forms the gate of the transistor. Then, the source and drain are formed by implanting the surface of the epitaxial layer to either side of the gate. Because the CMOS device has both NMOS and PMOS type transistors, the implantation steps must be repeated to create both types of wells and source/drains. Next, a layer of insulating oxide is deposited, patterned and etched to reveal a path to contact the gate, source and drain. Finally, a metal layer is deposited, and additional processing steps are completed to create the gate and source/drain contacts. The transistor is now complete. |
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