Study on Field Evaluation of the 8" ILD CMP

Abstract

PETEOS and HDPCVD oxide films are widely used for interlayer deposition materials between interconnect lines and gap-filling materials in STI process. Particularly since these films have the capability of filling sub-half-micron for high-aspect-ratio device and circuit structures, they are thought to be the most promising gap-filling materials in the current STI process. In this paper, the effect of film type on CMP removal rate and non-uniformity was investigated. Mean thickness of as-deposited films is about 7000A for PETEOS film and about 6000A for HDPCVD film. Polishing was done on a Strasbaugh 6EC machine. An IC1000/Suba IV K-Grooved stacked pad from Rodel Co. and SS25 slurry from Cabot Co. were used in this study. Pre-polishing and polished wafers were measured by an Opti-ProbeTM 3260UV the wafer at 121 points distributed radially across the wafer. Figure 1 shows the removal rates of PETEOS and HDPCVD films for the different spindle speeds. At the spindle speed of 50 rpm, average removal rate for PETEOS films (1916A) is a little higher than that of HDPCVD films (1715A). This would be partly because film density of HDPCVD film is higher than that of PETEOS film, which can be confirmed with refractive index (RI) values measured by Opti-ProbeTM 3260UV. Average RI is 1.459 for PETEOS and 1.462 for HDPCVD. Furthermore, by the detailed analysis of as-deposition profiles, we have confirmed that center-thin profiles of HDPCVD makes removal rate lower due to its relatively smaller effective contact area between pad and wafer. As shown in Fig. 1, removal rate decreases up to the spindle speed of 50 rpm and then slightly increases at 60 rpm. This trend of polishing can be explained by the complex effects of the relative velocity and slurry transport action to the edge of wafer on the local removal rate depending on spindle speed. Figure 2 exhibits the calculated relative velocities across the wafer as a function of spindle speed at a fixed table speed of 30 rpm. This shows that center-to-edge variation of relative velocity increases exponentially with spindle speed. For high relative velocity, centrifugal force on the slurry would result in large slurry loss by rejecting it out of wafer/pad interface near the wafer edge. However, above a critical spindle speed, relative velocities on the most part of the wafer become higher drastically so that mechanical polishing action by the increase of relative velocity becomes more dominant than the slurry loss effect. Figure 3 shows that within wafer non-uniformity (WIWNU) increases with removal rate. But uniformity of PETEOS (about 6%) is better than that of HDPCVD (about 8%). The reason why the uniformity of PETEOS is better than HDPCVD, even though the removal rate of HDPCVD is lower than PETEOS, can be explained by the difference in as-deposition film profiles as shown in Fig.4. It is thought that center-thin profile of HDPCVD makes effective wafer-pad contact area smaller so that makes WIWNU aggravate and removal rate even smaller. In this study, we investigated the CMP characteristics for PETEOS and HDPCVD oxide films from the point of view of removal rate and WIWNU. The experimental results clearly show that CMP characteristics is a function of process conditions, film characteristics, and as-deposition profiles.