CHAMPAIGN, Ill. -- University of Illinois researchers have developed a way to make metal and polymer films stick together better. The films are used in making integrated circuits.
"The mechanical integrity of metal-polymer, multi-layered structures is a major reliability concern in the microelectronics industry," said Marlon Menezes, a U. of I. graduate student who developed the embedded-cluster adhesion technique as a part of his doctoral thesis. "Failure of the packaged circuit often results from poor adhesion between the copper and polymer films."
The new technique utilizes partially embedded copper clusters that act as "nano-nails," anchoring a continuous metal coating to a polymer substrate. Copper is replacing aluminum as the metal of choice for making connections between components in large, multi-chip modules, Menezes said.
Not only does copper conduct better than aluminum, it also offers superior resistance to the effects of electromigration - thus permitting a greater level of miniaturization in the circuitry. Copper has at least one serious drawback, however.
"Because copper is a non-reactive metal, it does not chemically bond to the polymer films currently used to separate the wires between the chips," Menezes said. "Thermal stresses created during fabrication and operation can cause the films to delaminate, ruining the device."
To increase the adhesion at the film interface, Menezes and advisers Howard Birnbaum, professor of physical metallurgy and director of the Materials Research Laboratory on campus, and Ian Robertson, professor of materials science and engineering, developed a novel method for physically interlocking the two surfaces. The technique involves embedding copper clusters in the polymer and then coating the clusters and substrate with a continuous layer of metal.
"First, we form isolated nano-sized copper clusters on the polymer surface using carefully controlled deposition rates and temperatures," Menezes said. "Then, by controlling the time and temperature of the annealing process, we partially embed the clusters into the polymer before depositing the metal layer. The clusters act as nano-nails, anchoring the metal layer above in much the same way that anchor bolts secure a wall to a foundation."
Bonds formed by the embedded-cluster adhesion technique are stronger than those formed by other methods, such as surface roughening, which increase the available surface area for bonding but provide no mechanical interlocks.
"Our technique can work with a number of metal-deposition processes, including e-beam evaporation, sputter deposition and chemical vapor deposition," Menezes said. "And, in addition to copper, it can also bond other non-reactive metals, like gold and silver, to a variety of polymer films."
The researchers have applied for a patent.