CMOS Manufacturing

Why poly but not metal as M in MOS transistors?

The primary criterion for the gate material is that it is a good conductor. Highly-doped polysilicon is an acceptable, but certainly not ideal conductor, and it also suffers from some more technical deficiences in its role as the standard gate material. So why use polysilicon instead of a metal like aluminum? The reason is simple: in the MOSFET IC fabrication process, the gate material must be deposited prior to high-temperature steps that would melt metals. To improve the performance of the gate, some manufacturers form a silicide by blending a metal into the polysilicon. Such a silicide has better electrical properties than polysilicon but doesn't melt in subsequent processing.

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What is latch-up?

Reasons:
- Parasitic circuit effect that causes V_DD and GND to short.
- Can result in self-destruction, at best a malfunction requiring a power cycle.
- Parasitic bipolar transistors formed between n and p MOS transistors.

Prevention:
- Latch-up is triggered by supply voltage transients which cause V_DD to overshoot or GND to dip by ~0.7V above or below the supply.
- Not likely to occur in core logic.
- You can prevent this condition by making liberal use of substrate contacts:
- Every well MUST have a substrate contact.
- Every substrate contact should be connected to metal directly to a supply pad.
- Place substrate contacts as close as possible to the source contacts.
- I/O circuitry is more susceptible and must be protected.
- Guard rings are used in the I/O pads to reduce gain of parasitic transistors.
- I/O pad design should be left to experts.

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