FIG. 5A shows the semiconductor device 500 after the diffusion barrier structure 132 is formed on the sidewall and bottom of the contact trench 124, e.g., as previously described herein in connection with FIG. 2C. A conformal spacer oxide 502 is also deposited on the capping layer 140 of Si of the diffusion barrier structure 132. If the capping layer 140 is omitted, the conformal spacer oxide 502 is deposited directly on the uppermost one of the alternating layers of Si 134 and oxygen-doped Si 136 of the diffusion barrier structure 132. In either case, any standard conformal spacer oxide may be used such as, e.g., silicon oxide.

FIG. 5B shows the semiconductor device 500 during anisotropic etching of the conformal spacer oxide 502 from the top to expose the diffusion barrier structure 132 at the bottom of the contact trench 124. The anisotropic etching is represented by the downward facing arrows in FIG. 5B. If the capping layer 140 of Si is provided, the capping layer 140 would be exposed at the bottom of the contact trench 124. Otherwise, the uppermost one of the alternating layers of Si 134 and oxygen-doped Si 136 would be exposed. Any standard dielectric anisotropic etching process may be used to remove the conformal spacer oxide 502 from the bottom of the contact trench 124.