Page 51 - A Practical Guide from Design Planning to Manufacturing

The Evolution of the Microprocessor 27

used, but this has a K value of 4. New materials are being tried to
reduce K to 3 or even 2, but these materials tend to be very soft and
porous. When heated by high electrical currents the metal wires tend
to flex and stretch and soft dielectrics do little to prevent this. Future
interlevel dielectrics must provide reduced capacitance without sacri-
ficing reliability.
One of the common sources of interconnect failures is called electro-
migration. In wires with very high current densities, atoms tend to be
pushed along the length of the wire in the direction of the flow of elec-
trons, like rocks being pushed along a fast moving stream. This phe-
nomenon happens more quickly at narrow spots in the wire where the
current density is highest. This leads these spots to become more and
more narrow, accelerating the process. Eventually a break in the wire is
created. Rigid interlevel dielectrics slow this process by preventing the
wires from growing in size elsewhere, but the circuit design must make
sure not to exceed the current carrying capacity of any one wire.
Despite using new conductor materials and new insulator materials,
improvements in the delay of interconnects have continued to trail
behind improvements in transistor delay. One of the ways in which
microprocessors designs try to compensate for this is by adding more
wiring layers. The lowest levels are produced with the smallest dimen-
sions. This allows for a very large number of interconnections. The high-
est levels are produced with large widths, spaces, and thickness. This
allows them to have much less delay at the cost of allowing fewer wires
in the same area.
The different wiring layers connect transistors on a chip the way
roads connect houses in a city. The only interconnect layer that actually
connects to a transistor is the first layer deposited, usually called the
metal 1 or M1 layer. These are the suburban streets of a city. Because
they are narrow, traveling on them is slow, but typically they are very
short. To travel longer distances, wider high speed levels must be used.
The top layer wires would be the freeways of the chip. They are used to
travel long distances quickly, but they must connect through all the
lower slower levels before reaching a specific destination.
There is no real limit to the number of wiring levels that can be added,
but each level adds to the cost of processing the wafer. In the end the
design of the microprocessor itself will have to continue to evolve to
allow for the greater importance of interconnect delays.

Microprocessor scaling
Because of the importance of process scaling to processor design, all
microprocessor designs can be broken down into two basic categories:
lead designs and compactions. Lead designs are fundamentally new