Author: Tom Kerrigan
Date: 15:06:42 12/04/99
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So would a superconductor transmit signals faster? I figure the wires on an IC are like transmission lines, so the speed of propogation c = 1/sqrt(e0eru0ur). This gets close to the speed of light for a lot of metals, if I remember correctly. (?) -Tom On December 04, 1999 at 12:15:31, Tom Likens wrote: >On December 04, 1999 at 09:01:46, Tom Kerrigan wrote: > >>I don't see that superconductors would decrease the amount of metal necessary... >>And even if signal transmission in the wires takes more time than FET switching >>(it does??), the relative mu and epsilon of aluminum is almost ideal, so >>superconductors won't transmit THAT much faster... > >The wire delay is the biggest problem these days in most ASIC processes. >In fact the problem everyone is wrestling with now is the fact that the >wireload models used by the Verilog/VHDL RTL synthesis engines are so >inaccurate that we have to iterate too many times through the synthesis, >placement and routing cycle to meet our timing requirements. > >Copper wasn't used earlier because it is a *MUCH* more difficult metal to >work with then aluminum. Until recently only IBM was having any luck getting >yields up. Many fabs are playing a wait and see game, before making the >tremendously expensive move to copper interconnect. Look for most 0.13 micron >and smaller processes to use copper. > >Superconductors would help with power/heat problems, which is the other >major problem we are all dealing with as the geometries and frequencies >increase. Unfortunately, room temperature superconductors seem to be the >stuff of science fiction currently. Hopefully, one day (soon) that will >change. > >Interestingly, the other major piece of this that no one has mentioned is the >capacitance of the wires. This is just as important as the wire resistance. >Actually, because of the large fringe capacitances for deep-submicron designs >you could argue that it may be more important. Many fabs are experimenting >with low K dielectrics to lower the capacitance of the substrates. > >It's an interesting time in the semiconductor business. > >--Tom > >> >>If higher conductivity is so great for modern-process ICs, then everybody would >>have switched to copper a long, long time ago. >> >>I'm not arguing that superconductors are bad. I'm just saying that as far as I >>know, they wouldn't help modern processes. (They may be great for future >>processes though...) >> >>-Tom >> >>On December 03, 1999 at 00:35:54, David Blackman wrote: >> >>>On December 02, 1999 at 04:54:52, Tom Kerrigan wrote: >>> >>>>I'm not an expert, but I think metals on chips are already almost ideal. The way >>>>to make a chip faster would be to use substrates that allow channels to form >>>>faster. Superconductors wouldn't help with this. >>>> >>>>Notice that IBM is using a copper interconnect process for G4s. Copper is almost >>>>twice as conductive as aluminum, but they're not getting very high clock speeds. >>>> >>>>Better conductors have lower resistance, which does mean less heat... >>>> >>>>-Tom >>> >>>These days on most CPUs the "wires" take up more chip space, more power, produce >>>more wast heat, and cause more of the delay time, than the transistors. >>>Superconductors would be a huge breakthrough if they could figure out how to >>>make them as cheap, reliable, and convenient as the metal layers used now. But >>>as far as i know no-one is close to doing that, and there are probably not even >>>any major development efforts trying any more.
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