Within wafer nonuniformity

The year 2000 will mark 15 years since the initial CMP patents were filed by IBM. Opportunities for expanding use of CMP in existing chip technology continue to flourish. In addition, the challenges ahead for CMP technology to keep pace are formidable in this third wave of the evolution of the technology. Increasing concern about improved within-wafer nonuniformity, better planarity (flatter surfaces), and lower defectivity levels are all requirements for advanced, sub-0.25-micron devices. In addition. 

These carriers were generally capable of providing 5-7% nonuniformity at 6- to 7-mm edge exclusion. The nonuniformity is more precisely called the within-wafer nonuniformity (WIWNU) and is defined by the standard deviation of a set of film thickness measurements on a wafer divided by the mean of that set. Smaller numbers denote better process control. Better performance was limited in part by the manner in which the carrier held the wafer during polish. 

The chemical-mechanical polishing or planarization (CMP) process is a complex interplay between the wafer and the consumables involved. The consumables include slurry, pad, conditioner, and so on. During polishing, the pad carries the slurry and delivers it to the wafer surface. It also transmits the normal and shear forces from the polisher to the wafer. Therefore, polishing pad plays a critical role in the CMP process and influences the outcomes such as material removal rate (MRR), within-wafer nonuniformity (WIWNU), wafer-to-wafer nonuniformity (WTWNU), step height reduction efficiency (SHRE), and defect counts. 

Residual tungsten and barrier layer are also viewed as defects (Fig. 9.9). Residual W and barrier layers are usually caused by insufficient polishing that can be attributed to inadequate end-point detection and high within-wafer nonuniformity (WIWNU) due to process drift or poor optimization of polishing parameters. Insuffieient polishing times are usually solved by the implementation of end-point detection. For more detailed analysis of tungsten defects, see Chapter 17. 

As mentioned above, the dispersion stability of the slurry is directly related with CMP performance, removal rate, within-wafer nonuniformity (WIWNU), which is defined as the standard deviation divided by the average of remaining thickness after CMP, microscratching, and the remaining particle on the wafer. To avoid poor CMP performance, the dispersion stability of the slurry must be controlled by preventing silicon ion dissolution. Surface modification of the silica particle was produced by addition of an organic additive. Without surface modification, the amount of silicon dissolution was 1.370 0.002 mol/L, while surfaces modified with poly(vinylpyrrolidone) (PVP) polymer yielded a dissolution of 0.070 0.001 mol/L, almost 20 times less than the unmodified surface. 

Figure 7.14 MRR as a function of conditioner downforce for various conditioners.14 (WIWNU = within wafer nonuniformity).

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