Device & circuit scaling [Fonstad + Spectrum 05/12] Transistors - - PowerPoint PPT Presentation

Device & circuit scaling

[Fonstad + Spectrum 05/12]

• Transistors. In order to increase speed, let us shorten the

gate by a factor s

and in order to have a constant K In order not to spoil the performances of the FET, we also

reduce the vertical dimension by s

So, higher doping is needed and also a thinner gate oxide This, however, increases K These would give higher I and , => possible breakdown So, it’s better, to reduce the voltages. Let’s suppose then

And it is found that the power per transistor decreases with

s2 (supposing f increases by s)

But we have a higher transistor density (by s2) =>

power density remains roughly constant

(Normally, we are not able to scale V with 1/s, so power

density tends to increase)

Metal connections. Lateral dimension will be scaled by

1/s, but as currents scale with 1/s, height must not be scaled in order not to increase current density (otherwise, electromigration occurs)

Capacitive parasitic effects. If we do not scale the

interlevel oxide thickness (as average length is supposed to remain constant) C decreases

• this is good, as f has increased

voltage scaling

* Technology node=Nominal feature size = DRAM (or minimum metal interconnect) half pitch

As transistors shrink, the problem of chip variability grows Chips have improved because their transistors and

connecting wires have kept getting smaller, but now they’re so small that random differences in the placement

• f an atom can have a big impact on electrical properties.

For a long time, most variation was global, caused by

slight alterations in the manufacturing process. Such changes differentiate one chip from another or all the chips on one wafer from those on another

A second source of variation is often called local process

variation or process variability.

One of the most dramatic sources of local process

variation comes from dopants. Transistor channels once contained tens of thousands of dopant atoms. Nowadays chipmakers produce transistors that can accommodate

• nly a few hundred of them