Transistors are a common component formed into integrated circuits. A transistor is typically formed by disposing a conductive material to form a gate over a semiconductor substrate, with an intervening layer of gate oxide. The portions of the semiconductor substrate on opposite sides of the gate are doped to form the source and drain regions of the transistor.
Transistor gates are typically made from a non-metallic conductive material such as polysilicon. This is because the interface between polysilicon and gate oxide can be beneficial. But, the conductivity of the poly-silicon layer is relatively low, which can cause a low charge accumulation. This, in turn, can lead to unwanted delays in circuits. Moreover, use of a polysilicon gate can cause a depletion region on the polysilicon-oxide interface which can have an adverse effect on the channel formation. Consequently, some circuit designers are looking into metal gates.
Metal gates, however, have issues that have to be overcome by designers. For example, the metal material is prone to damage from various lithographic processing steps. Specifically, a thermal annealing process performed on the semiconductor substrate can damage a metal gate. Thus, dummy gates are often formed in place of metal gates. After the procedures which have a potential to damage a metal gate have been performed, the dummy gate can be replaced with a metal gate. It is desirable to be able to remove and replace the dummy gates in a clean, precise, and cost-efficient manner.