As circuit complexity and density increase, the depth of source drain regions becomes shallower. Further, contact openings formed in dielectric layers in high density circuits typically have high aspect ratios. This results in poor step coverage of the metal layer for formation of the silicide and the overlying common barrier metal layer. These factors can combine to give rise to very high contact resistance.
Depletion of the surface concentration of the conductivity enhancing impurity in the active area has been identified as one possible reason for the adverse high contact resistance. Such is believed to occur at the formation of the contact silicide. Silicides are typically formed between the active area and overlying metal runner to reduce contact resistance. Such are formed by application of a thin metal layer over the wafer which intimately contacts the upper surface of the active area within the contact opening. An elevated temperature anneal reacts the metal with the silicon of the active area (substrate) forming a silicide in the process. A common metal for formation of a silicide is titanium, which forms titanium silicide (TiS.sub.2). However during the silicide formation, especially with shallow active areas, an adverse phenomenon can result which actually increases resistance from the contact to the active area. The problem is described with reference to FIGS. 1, 1A, 1B, 2, and 3.
FIG. 1 illustrates a portion of a semiconductor wafer fragment 10 comprised of a silicon substrate 12 with a source/drain active area 14 provided therein. A polysilicon gate 16 overlies a gate oxide layer 18 adjacent active area region 14. A layer of insulating oxide 20 is provided, with a contact opening 22 having been etched therein to contact active source/drain area 14. A metal layer 24, such as titanium, is applied atop etched oxide layer 20 and contacts source/drain active area 14 for formation of the metal silicide.
FIG. 1A is an enlargement of the FIG. 1 wafer as represented by the dashed circle 1A in FIG. 1. The "dots" within active area 14 in FIG. 1A represents the conductivity enhancing impurity doped within silicon substrate 12 which generally defines a rough boundary for active area 14 after the activation anneal for the active area. The dotted line 26 extending from the base of contact opening 22 into active area 14 roughly represents that area of the silicon from active area 14 that will react with the titanium of layer 24 for formation of TiSi.sub.2. Arrows are shown in FIG. 1A which extend from the conductivity enhancing impurity dots which are in closest proximity to the region defined by dotted line 26. During the elevated temperature anneal to form the silicide, the conductivity enhancing impurity tends to diffuse towards and into the silicide region defined by boundary 26, as indicated by the arrows.
FIG. 1B illustrates the effect at the conclusion of the silicidation anneal. A silicide region 28 is formed which has attracted adjacent conductivity enhancing impurity from within active region 14. This creates a slight void between the outline 26 of silicide region 28 and conductivity enhancing impurity within active area 14, thus significantly increasing resistance of contact through opening 22 to active area 14.
The problem is represented graphically in FIGS. 2 and 3. Each includes a plot of concentration versus depth (elevation) into the substrate from the original upper silicon surface. FIG. 2 illustrates the concentration versus depth profile of a conductivity enhancing impurity, such as boron, after implantation and post implant anneal. FIG. 3 illustrates the curved profile after silicidation. As is apparent, the concentration of boron at the TiSi.sub.2 interface (the location where the boron curve meets the TiSi.sub.2 boundary) is less than what was depicted by the original boron concentration curve. In other words, the boron concentration as a function of depth has been changed by the silicidation which withdraws boron into the silicide region. Such can result in significant resistance and a failure to make electrical contact.
It would be desirable to overcome these and other drawbacks of the prior art.