Dr. Ken Gilleo “The Coming of Copper UBM,” excepted from Dr. Ken
Gilleo’s book on flip chip and related technologies, summarizes
the choices and tradeoffs in flip chip substrate materials
Several developments in the past few years have solved the
numerous problems associated with using copper metal in place of
aluminum as the IC interconnect metal. Copper is about three
times more conductive than aluminum, and allows higher
frequencies to be used with smaller line widths. Many
fabricators are converting to copper not just for speed, but
also for cost reduction. Thinner conductors allow closer spacing
and smaller chips. The switch to copper allows many times more
dies per wafer, and this is where the savings come from. Since
copper is much more compatible with bump metals than aluminum,
the transition to copper is expected to boost flip chip
technology, possibly by eliminating the UBM step. It remains to
be seen what the final finish will be for copper ICs. The
industry will continue to use wire bonding as the