A Systematic Study of Transistor Design Traae-offs
The ability to manufacture circuits at, or near, fundamental density and speed limits is affected by the sensitivity of circuit parameters to variations in the manufacturing process, and by the ability to achieve tight control of the manufacturing process. Modifications of device or circuit design can alter the various dependencies, so that parts of the process that are intrinsically more controllable determine critical circuit properties. Critical dimensions determined by a series of steps (mask-making, exposure, develop, etch) are typically held to + /- 20%. Various ion implant doses and depths can be controlled to + /- 2% to 5%. For example, if there is an absolute minimum for channel length, then the circuit design must be set at L min +ΔL, where the yield of devices with L >L min is satisfactory and ΔL is a measure of the process variation with gate line width. L min must also allow for some overlap of the gate to source and drain. If the minimum channel length is set by a reliability factor, then the design target is determined by allowable early field failures rather than the allowable yield. This illustrates the difficulty in understanding the problems associated with scaling critical dimensions to less than 0.5 μm.
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