Double polysilicon contact structure and process

A process for fabricating a contact structure for a semiconductor device including at least two superimposed polysilicon conductors comprising the steps of:

• A. forming a first insulating layer 2 (FIG. 3) on a semiconductor substrate A to a preselected thickness, said insulating layer 2 being formed of silicon dioxide and having a thickness in the order of 300 to 700 A,
• B. forming a first conductive polysilicon layer 3 (FIG. 3) on the first insulating layer 2, said polysilicon layer 3 having a thickness in the order of 3000 A to 5000 A,
• C. forming a first dielectric layer 4 (FIG. 4) on the first conductive polysilicon layer 3, said first dielectric layer 4 being formed of silicon dioxide and having a thickness in the order of 1000 A to 4000 A,
• D. patterning the first insulating 2 and dielectric 4 and conductive polysilicon layers 3 into a preselected shape (FIG. 5),
• E. patterning a second dielectric layer 5 (FIG. 5) on the first dielectric layer as an etch barrier to selected etchants, said second dielectric layer 5 being selected from the group consisting of silicon nitride and aluminum oxide, said second dielectric layer 5 having a thickness in the order of 200 to 1000 A,
• F. forming a second conductive polysilicon layer 7 (FIG. 7) on the second dielectric layer 5 and superimposed, in part at least, on the first conductive polysilicon layer 3, said second polysilicon layer 7 having a thickness in the order of 3000 to 5000 A,
• G. forming a third dielectric layer 8 (FIG. 7) on the second conductive polysilicon layer 7, said third dielectric layer 8 being formed of silicon dioxide and having a thickness in the order of 1000 to 2000 A,
• H. etching a contact opening 16' (FIG. 9) in the third dielectric 8 to expose the second conductive polysilicon layer 7 overlying the first conductive polysilicon layer 3 whereby the second dielectric layer 5 prevents the etchant from exposing the first conductive polysilicon layer 3 by way of voids existing in the second conductive polysilicon layer 7 due to enlarged polysilicon grains, and
• I. forming a metal contact 12' in the opening 16' (FIG. 9), whereby the second dielectric layer 5 prevents the etchant from attacking the first dielectric layer 4 overlying the first conductive polysilicon layer 3 and exposing the surface of the first polysilicon layer 3 to the metal 12 thereby physically joining together the first and second conductive polysilicon layers 3, 7.