(1) Field of the Invention
The invention relates to the manufacture of integrated circuits, and more particularly, to a process of creating a contact or via opening which allows for improved metal step coverage in the manufacture of integrated circuits.
(2) Description of the Prior Art
The problem of poor metal step coverage within a contact or via opening is an important one in the art. If the metal coating within the opening is too thin, it can cause the resistance within a contact opening to be too high for the circuit to work properly. Current practice calls for the formation of contact or via openings with stepped or sloped sides rather than vertical sides to improve the metal step coverage.
U.S. Pat. No. 4,372,034 to Bohr describes a wet anisotropic etch followed by a dry anisotropic etch, resulting in a "teacup" shape which is wide and sloping at the top and narrow at the bottom, as shown in FIG. 1A. This shape solves the step coverage problem at the top of the opening, but the problem still exists at the bottom. Another method in current practice is the resist pull technique in which a first etching creates a step in the oxide layer, then an oxygen plasma is used to pull back the resist exposing an oxide stair, as in FIG. 1B. Finally, an anisotropic etch chops the stair into a sloped pattern, as shown in FIG. 1C. One problem with this approach is fencing on the sidewall. That is, the first etching creates a polymer which remains on the sidewalls of the opening after the resist pull back step. This polymer acts as a resist mask to block the final anisotropic etch resulting in an oxide fence or shell within the opening. Another problem is the restriction of the resist pull back to 0.15 micrometers per side. This occurs since the etch rate for the resist is normally two times faster than the pull back to keep a margin of resist to prevent etching through to the oxide.
U.S. Pat. Nos. 4,814,041 to Auda, 4,824,767, to Chambers et al, 4,832,788 to Nemiroff, 4,908,333 to Shimokawa et al, and 4,980,316 to Huebner all describe various methods of forming contact openings.