Electrografting Proposed for Cu Seed Layers

Pete Singer, Editor-in-Chief -- Semiconductor International, 11/1/2007 7:59:00 AM

A new technique called electrografting has been developed by a small French company called Alchimer (Massy, France) that could provide a cost-effective solution to replace standard physical vapor deposition (PVD) for copper seed layers. The company says it ensures extendibility of copper metallization to aspect ratios >10 :1 by enabling electrochemical deposition on virtually any diffusion barrier material (tantalum, TaN, titanium, TiN, tungsten, ruthenium layers deposited by PVD, MOCVD or atomic layer deposition {ALD]). The company is targeting high-aspect-ratio through-silicon vias (TSVs) needed for 3-D integration, as well as conventional dual-damascene interconnects.

Paolo Mangiagalli, business operations manager at Alchimer, said that existing copper PVD techniques could run into limitations with aspect ratios in the 5:1 to 8:1 range, in terms of their ability to provide continuous seed layers with good step coverage and acceptable cost-of-ownership. He said PVD also had the same problem for high-aspect-ratio tantalum- and titanium-based diffusion barrier deposition, but the higher surface mobility of tantalum and titanium atoms should allow better step coverage. “Will they be viable economically? This is another question,” Mangiagalli said.

"Electrografting" is a wet coating process invented in the ‘90s by Christophe Bureau while working at CEA (Saclay, France) to make strongly adherent vinylic polymer layers as anti-corrosion coatings for nuclear applications.

It is a two-step process, where a small current (in the range of a few µA-cm²) intiates a transient chemical bond formed at the surface of the substrate. The amount of current determines the grafting density. “Everything is controlled by the current, but this current is very low,” Mangiagalli noted.

The second step is a purely chemical propagation/polymerization step, the kinetics of which is driven by the composition of the bath — it does not require any current. “This transient chemisorbed radical deactivates or transfers its charge to another monomer that is in the solution, and this propagation of polymerization goes on and on and on. The thicknesses of the layers we put down is fixed by the bath composition. How many propagators we put in the bath vs. how many killers we also put into the path to stop the propogation," Mangiagalli said.

The propagation is known as electrografting because, at that time, it was the first-known reaction electro-initiated that was creating a metal-carbon bond, grafting precursors on the surface.

Mangiagalli said in addition to vinylic polymer, the company has found several other classes of polymers that work well for first electro-initiated step, including diazonium. “We also discovered different classes of consecutive reactions other than polymer propagation. For example, disproportionation can be used for copper layers, and protonation can be used for laying down polymers on metals. Tautomerization and organometallic complex cleavage are other examples." 

The main advantage of the technique is conformal step coverage. The feasibility of the process was demonstrated with a 5-µm-wide, 25-µm-deep via, with a TiN barrier layer deposited in collaboration with IMEC (Leuven, Belgium). As shown in the photo, the conformal copper layer at the bottom of the via is ~50 nm thick. Process time was 5 min.

The feasibility of the process was demonstrated with a 5-µm-wide, 25-µm-deep via, with a TiN barrier layer deposited in collaboration with IMEC. As shown in the photo, the conformal copper layer at the bottom of the via is ~50 nm thick. Process time was 5 min.

The company also demonstrated good results on vias with even higher aspect ratios. Copper was deposited conformally on a 10:1 aspect ratio, 5-µm-wide, 50-µm-deep via, where the TiN barrier was deposited by TiN.

Mangiagalli said Alchimer has also developed a completely electroless grafting method that it calls chemical grafting. “If you cannot change the surface potential by applying a voltage, and you have the ability to modify or create the right reactive species in the bath, then they will have the right potential in order to give or take electrons from the surface. Basically chemical grafting is the same as electrografting, but with the first electro-initiated step, the electron doesn’t come from the surface, but does come from the fact that we put in the bath enough very active reactive species that are willing to take or give an electron from the surface.”