Re: With aluminum, the capacitance of the wires causes a charging effect, which makes it take a longer amount of time before you get a stable logical value on the wire. With copper, this capacitance is reduced, and the wires themselves can switch faster.
You're confusing the move to copper with the move to SOI. SOI can offer dramatically lower capacitance, which, together with the lower leakage losses, and other beneficial effects of SOI like elimination of the "body effect" is why power consumption can be so much lower while performance increases.
The sources of increased SOI performance are elimination of area junction capacitance and elimination of "body effect" in bulk CMOS technology.
chips.ibm.com
Small surface areas have lower capacitance, and copper can allow for small surface areas, but there is no capacitance reduction from substituting copper for aluminum.
For identical line dimensions, copper and aluminum provide the same side-wall capacitance, but copper has lower resistance. Similarly, for copper and aluminum lines with equivalent resistance, copper lines can be thinner, resulting in lower side-wall capacitance.
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