PORTLAND, Ore. An organic low-k dielectric material called organosilane is capable of forming bonds among copper interconnects that are five times stronger than current tantalum compounds, according to a material scientist at Rensselaer Polytechnic Institute.
"This material is already commercially available, but we discovered quite by accident that it can withstand the high heats used to manufacture semiconductors," said Ganapathiraman Ramanath, who worked with researchers at IBM's T.J. Watson Research Center (Yorktown Heights, N.Y.) and the Technion-Israel Institute of Technology (Haifa, Israel).
"When used as a nanoglue between copper and silicon dioxide, it actually gets stronger above 400 degrees Celsius, which was quite a surprise to us," Ramanath added.
The nanoglue material has a very low dielectric constant (k=2.5) compared to even the most advanced carbon-doped silicon oxides (k=3) used by semiconductor makers. It can also be laid down in atomically-thin monolayers in contrast to the tantalum used today, which is usually deposited in layers thicker than 10 angstroms.
|Nanoglue could replace current IC interconnect techniques|
"Organosilane self-organizes into a monolayer in such a manner that one end adheres to the copper and the other end adheres to the silicon dioxide," said Ramanath. "We believe the material will be an important adhesive in a wide assortment of future nanoelectronic devices where thicker adhesive layers will not fit."
Copper interconnection layers in silicon chips use tantalum because the copper lines are deposited using damascene metallization, an additive process that uses electrodeposition to add the copper to 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. But electrodeposition cannot be used atop organosilane either because it too is a low-k dielectric. In order to use organosilane for copper interconnects, chip makers would have to switch back to a subtractive process for patterning metallic interconnection layers, as was done 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 utilize the high adhesion and low-k dielectric properties of organosilane monolayers. One reason is their extreme thinness compared to other materials like tantalum.
Meanwhile, Ramanath and colleagues are experimenting with using organosilane monolayers as adhesives, lubricants and protective surface coatings for other applications such as jet engines or power turbines which must withstand temperatures as high as 700 degrees C.
At 35 cents a gram, organosilane is relatively inexpensive, but the process described by Ganapathiraman would have to be licensed from Rensselaer Polytechnic Institute, which has filed a disclosure on its findings (usually a preliminary step towards a patent).
Funding for the nanoglue research was provided by the National Science Foundation, the U.S.-Israel Binational Science Foundation, the Alexander von Humboldt Foundation and New York state through its Interconnect Focus Center.