Page 45 - A Practical Guide from Design Planning to Manufacturing

The Evolution of the Microprocessor 21

at the end of processing is the actual gate length (L GATE ). Also, the source
and drain regions within the silicon typically reach some distance under-
neath the gate. This makes the effective separation between source and
drain in the silicon less than the final gate length. This distance is called
the effective channel length (L EFF ). It is this effective distance that is the
most important to transistor performance, but because it is under the
gate and inside the silicon, it can not be measured directly. L EFF is only
estimated by electrical measurements. Therefore, L GATE is the value most
commonly used to compare difference processes.
Gate oxide thickness is also measured in more than one way as shown
in Fig. 1-10. The actual distance from the bottom of the gate to the top of
the silicon is the physical gate oxide thickness (T OX-P ). For older processes
this was the only relevant measurement, but as the oxide thickness has
been reduced, the thickness of the layer of charge on both sides of the oxide
has become significant. The electrical oxide thickness (T OX-E ) includes the
distance to the center of the sheets of charge above and below the gate oxide.
It is this thickness that determines how much current a transistor will pro-
duce and hence its performance. One of the limits to future scaling is that
increasingly large reductions in the physical oxide thickness are required
to get the same effective reduction in the electrical oxide thickness.
While scaling channel length alone is the most effective way to reduce
delays, the increase in leakage current prevents it from being practical.
As the source and drain become physically closer together, they become
more difficult to electrically isolate from one another. In deep submicron
MOSFETs there may be significant current flow from the drain to the
source even when the gate voltage is below the threshold voltage. This
is called subthreshold leakage. It means that even transistors that
should be off still conduct a small amount of current like a leaky faucet.
This current may be hundreds or thousands of times smaller than the
current when the transistor is on, but for a die with millions of tran-
sistors this leakage current can rapidly become a problem. The most
common solution for this is reducing the oxide thickness.
Moving the gate terminal physically closer to the channel gives the
gate more control and limits subthreshold leakage. However, this

+ + + + + + +
+ + + + + + + + Poly

T OX-P T OX-E SiO 2
− − − − − − − − − − −
− − − − Si Figure 1-10 Gate oxide thickness.
− −