Deep alumninum via process

04/01/1997

Deep aluminum via process

Researchers at Fujitsu have developed a novel approach to aluminum plug fill that relies on the same driving force that causes aluminum junction spiking. Called polysilicon-aluminum substitution (PAS), the process creates single-crystal aluminum in vias with aspect ratios up to 10:1. Other aluminum via-fill processes, including CVD, high-temperature reflow, and high-pressure aluminum extrusion, have reported aspect ratio limitations of less than 7:1. Hiroshi Horie, et al., from Fujitsu Labs first presented the process at the 1996 IEEE International Electron Devices Meeting in San Francisco.

After vias are etched in oxide, a 600?C LPCVD process fills the vias with polysilicon. The poly-Si on the surface is removed with CMP, and then blanket PVD aluminum coats the tops of the vias (see figure`s panel a). Leaving the vias at 500?C for 180 minutes in ambient nitrogen allows the poly-Si to diffuse up into the aluminum bulk, and aluminum substitutes into the vias. The poly-Si agglomerates at the top of the vias in rough clumps (panel b).

Cross-sectional transmission electron microscopy (TEM) shows that the aluminum plugs are single-crystal. The atomic composition of the plugs, as measured by energy dispersive x-ray spectroscopy, is pure aluminum with approximately 0.4% residual silicon. Even if the initial poly-Si plug has a pre-substitution void, the final Al is void free.

An improvement to the standard process is to deposit a thin coating of titanium above the aluminum before the substitution step (panel c). Then, during the substitution step, instead of merely agglomerating within the aluminum, it is more energetically favorable for the poly-Si to react with the Ti at the Al-Ti boundary. A rough titanium silicide layer then grows out of the bottom of the Ti layer (panel d).

The Ti layer modification makes process integration easier as the silicide can be more easily removed by CMP, compared to the substituted poly-Si agglomerates in residual Al. Also, since the silicide formation is more energetically favorable, the via substitution requires less driving force and the process can occur at lower temperatures. The aggressive 10:1 structure shown in panel d was completely filled using a 450?C, 60 minutes in ambient nitrogen PAS process. The process has filled less than 0.1-micron diameter holes with single-crystal aluminum. Also, there is a limit to the solubility of polysilicon in the top aluminum, so thicker top aluminum is needed when the total volume inside the vias in an actual device increases. This implies that there could be micro-loading effects, and experiments are also underway to determine the extent of any such limitations to incorporation into actual devices. E.K.

Polysilicon-aluminum substitution (PAS) vias a) before annealing of Al and b) after annealing of Al. c) A thin coat of Ti is deposited above the Al before annealing. d) After annealing, a roughTiSix layer grows out of the bottom of the Ti layer.Some questions remain for complete integration of the PAS process. For metal-one contact, a barrier metal will be needed between silicon and the polysilicon to prevent the aluminum from diffusing into the silicon. Fujitsu Labs is evaluating different barrier metal systems.