Researchers at Imec and KU Leuven managed to electrically insulate on-chip structures using so-called metal-organic frameworks (MOFs). Thanks to their highly porous structure, MOFs possess a low dielectric constant k, making them good insulators. But getting them on the chips has proved an obstacle.
The ultimate low-k material would be air, and indeed air gaps are being used in some chips. However, air alone could never be the answer to the increasing crosstalk that arises when IC structures (components, wires and so on) get smaller and smaller because too much air would compromise mechanical stability. Thus, the search for a reasonably sturdy, yet insulating material was on.
MOFs are a natural candidate for the job, but applying them to electronic material presented a challenge. The Imec and KU Leuven researchers found an industrially relevant solution: first growing an oxide film on the substrate, followed by exposure to vapor of an organic compound. This causes the oxide film to expand, forming the porous structures.
“The main advantage of this method is that it’s bottom-up,” says KU Leuven researcher Mikhail Krishtab. “We first deposit an oxide film, which then swells up to a very porous MOF material. You can compare it to a souffle that puffs up in the oven and becomes very light. The MOF material forms a porous structure that fills all the gaps between the conductors. That’s how we know the insulation is complete and homogeneous. With other, top-down methods, there’s always still the risk of small gaps in the insulation.”
The Flemish research groups have received an ERC Proof of Concept grant to further develop the technique. One issue that still needs solving is the irregular MOF surface. It needs to be smoothed before it can be used in commercial applications.