An organic low-k dielectric material has been found capable of forming bonds to copper interconnects that are five times stronger than the tantalum compounds semiconductor makers use today.
Dubbed "nanoglue" by the researchers who demonstrated its potential for semiconductor manufacturing, the material, called organosilane, self-organizes into a very thin monolayer in such a manner that one end adheres to the copper interconnect and the other to the silicon dioxide.
"We believe the material will be an important adhesive in a wide assortment of future nanoelectronic devices, where thicker adhesive layers will not fit," said materials scientist Ganapathiraman Ramanath at the Rensselaer Polytechnic Institute. Ramanath and colleagues at IBM's T.J. Watson Research Center and the Technion-Israel Institute of Technology "discovered quite by accident" that this commercially available material can withstand the high heats used to make semiconductors, said Ramanath.
"In fact, when used as a nanoglue between copper and silicon dioxide, it actually gets stronger above 400°C, which was a surprise to us," he said.
Organosilane has a very low dielectric constant (k=2.5) compared with even the most advanced carbon-doped silicon oxides (k=3) that semiconductor makers use today. In addition, it can be laid down in an atomically thin monolayer, in contrast to the metal tantalum, which is usually deposited in layers thicker than 10 angstroms.
In the fabrication of high-performance silicon chips, the copper lines are deposited by means of the damascene process, an additive technique that uses electrodeposition to add the copper into grooves lithographically patterned into the silicon dioxide dielectric. Electrodeposition cannot be used directly on silicon dioxide, because it does not conduct electricity; consequently, a metallic tantalum layer is put down first.
Unfortunately, electrodeposition cannot be used atop organosilane either. So to use organosilane for copper interconnects, semiconductor makers would have to switch back to a subtractive process for patterning metallic interconnection layers, as was done in the past when aluminum was used for interconnections.
"The biggest obstacle to using organosilane to bond copper interconnects is getting semiconductor makers to change the way they do things now," said Ramanath. Nevertheless, future nanoscale devices could someday use the high-adhesion and low-k-dielectric properties of organosilane monolayers, because of their extreme thinness compared with materials like tantalum.
Organosilane is relatively inexpensive at 35 cents a gram, but the process Ramanath described would have to be licensed from Rensselaer Polytechnic Institute (Troy, N.Y.), which has filed a disclosure on the findings--a preliminary step toward a patent.