How does increasing the gates oxide thickness in a cpu improve reliability?

[Adul]

While reading this write up on the power 970 from IBM there was mention of this.

Apparently, in order to increase the reliability of the Power4 for the high-end server market, IBM used much thicker gate oxides on the chip's transistors. The trade-off for this decreased failure rate and improved reliability was that the Power4's transistors have slower switching speeds, so even with process shrinks it's harder to push the design to higher clock speeds.

So how does it help improve reliabilty?

 

[theCheetah]

In short:

When you decrease the gate oxide thickness, transistor Vt(think of it as the ease by which the transistor will turn on) decreases hence the transistors turn on faster, conducts more current and hence gives better performance.

The flip side - thinner gates are prone to premature gate oxide breakdown, some of the electrons may get embedded in the oxide causing permanent Vt decrease and hence a leaky transistor.

Hence a process should be tuned somewhere between this two extremes.