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Dielectric Spacer Formation to Eliminate Polysilicon Stringers

IP.com Disclosure Number: IPCOM000036134D
Original Publication Date: 1989-Sep-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 4 page(s) / 90K

Publishing Venue

IBM

Related People

Breiten, C: AUTHOR [+5]

Abstract

To improve circuit density and conserve horizontal device area multiple polysilicon wiring levels are employed. A major problem in polysilicon wiring is the formation of polysilicon stringers.

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Dielectric Spacer Formation to Eliminate Polysilicon Stringers

To improve circuit density and conserve horizontal device area multiple polysilicon wiring levels are employed. A major problem in polysilicon wiring is the formation of polysilicon stringers.

Polysilicon stringers are residual polysilicon frequently left after reactive ion etch (RIE) of the second level blanket polysilicon film. This residual polysilicon 10 occurs at the bottom of the first level polysilicon wiring step, as shown in Fig.
1. Stringers form along these vertical steps because the depth of the second polysilicon film 12 which is etched is at its greatest at these locations (see Fig.
2). The formation of stringers is a function of step sidewall angle and step height. As the step sidewall angle approaches 45 degrees, the

(Image Omitted)

thickness of the polysilicon approaches the thickness of polysilicon on a horizontal surface. In comparison, at a vertical step the thickness of the polysilicon is approximately the step height plus the deposited film thickness.

In the prior art, polysilicon stringer formation is reduced by minimizing the first level step height, by sloping the sidewall or by using a significant overetch. Sloping the first level polysilicon sidewall is not permissible in processes that use self-aligned salicides since such slopes lead to silicide bridging. Therefore, these salicide processes rely heavily on over-etching. Over-etching can cause other problems such as undercutting of the first level polysilicon wiring that leads to performance degradation. Over-etching also destroys all exposed silicide. Further, for many geometry combinations, it is nearly impossible to completely eliminate the stringers regardless of how much overetch is used.

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This article describes several methods of spacer formation to eliminate polysilicon stringers. The general idea of all of the methods of spacer formation is to provide a sloped spacer 13 along the sidewall of the first level polysilicon wiring as shown in Fig. 3, so that the second polysilicon layer 12 is of as uniform thickness as possible. This spacer can be composed of silicon dioxide or silicon nitride or a composite of the two materials.

One approach to making silicon dioxide spacers would be...