Abrasive Concentration

Sundararajan et al. [131] in 1999 calculated the slurry film thickness and hydrodynamic pressure in CMP by solving the Re5molds equation. The abrasive particles undergo rotational and linear motion in the shear flow. This motion of the abrasive particles enhances the dissolution rate of the surface by facilitating the liquid phase convective mass transfer of the dissolved copper species away from the wafer surface. It is proposed that the enhancement in the polish rate is directly proportional to the product of abrasive concentration and the shear stress on the wafer surface. Hence, the ratio of the polish rate with abrasive to the polish rate without abrasive can be written as... 

Philipossian, A. and Olsen, S., Effect of Pad Surface Texture and Slurry Abrasive Concentration on Tribological and Kinetic Attributes of ILD CMP," Materials Research Society Symposium Proceedings, Vol. 161,2003, F2.8. 

The data in Fig. 5.18 can be understood as showing a rapid increase in removal rate with increasing concentration at low abrasive concentrations when a large fraction of the pad surface is available, slowing toward an asymptotic maximum removal rate at higher concentrations when the pad is largely saturated by abrasive particles. 

As the abrasive size increases, the values of ATpad and of fe in Equation 5.5 both decrease. Use this information to sketch R versus %A or R versus [A] curves for the polishing rate of an abrasive larger than the 20 nm as shown in Eig. 5.17. Qualitatively explain the difference in pohshing rate changes as abrasive concentrations increase at both low and high concentrations. 

Philipossian A, Olsen S. Effect of pad surface texture and slurry abrasive concentration on tribological and kinetic attributes of ILD CMP. Mater Res Soc Symp Proc 2003 767 F2.8.1-2.8.7. 

FIGURE 7.20 Removal rate versus downforce on 8" Cu blanket test wafers polished at 75/65 rpm table/carrier speed and 200ml/min slurry flow rate on Strasbaugh u-Hance polisher. Diamond data points indicate the removal rate values with the organic particles, and square data points denote removal rate values for silica particles, both polished under identical formulation and abrasive concentration (from Ref. 110). 

Abrasive Concentration Units of weight percent (wt% in liquid volume). The concentration of abrasive affects the number of cutting tools at the surface. Generally, higher abrasive concentrations leads to higher polish rates. 

Figure 5.17 Polish rate vs. pH and abrasive concentration for (a) 30 nm silica abrasive and (b) 7 nm silica abrasive. (From Ref. (11).)...

Figure 5.20 At low abrasive concentrations, when the particle fill factor is much less than unity, decreasing particle size does not increase the number of particles contacting the surface.

The above results seem to indicate a transition from one removal mechanism to another as particle size is increased. In another paper [8], we have described two mechanisms for polishing, an indentation-based mechanism and a surface area based mechanism. The mathematical expressions for particle size and solids loading dependence have been derived from an expression for penetration depth of an abrasive particle, which was developed by Brown et al. [9]. According to the surface area based model, polishing rate depends on the total contact area between the abrasive particles and the surface being polished. The contact area as a function of particle size and abrasive concentration is given by the following expression ... 

The concentrations as well as the size of the abrasives have an effect on MRR. There are three distinct regions of MRR dependence on abrasive concentration (Lee et al., 2009 Luo and Dornfeld, 2003) (Figure 11.11). First, MRR rapidly increases with an increase in abrasive concentration, which indicates that chemical removal of material is dominant at low concentrations. Second, as mechanical removal becomes dominant, MRR is proportional to the abrasive concentration. And third, the dominant mechanical effect is saturated because the contact area between abrasive and the wafer surface is at a maximum. Although the MRR varied with the abrasive concentration depending on the types of abrasives, films, and additives, their trends are quite similar. In practice, the silica slurries have high solids contents of 5—50 wt% whereas the ceria and alumina slurries have low solid contents ( 5 wt%). 

Figure 12.6 Material removal rate (MRR) as a function of abrasive concentration. Reproduced from Luo Jianfeng, Integrated Modeling of Chemical Mechanical Planarization/ Polishing for Integrated Circuit Fabrication (PhD Dissertation), 2003, Figure 5.11, page 178, with permission from University of California at Berkeiey.

Figure 14.23 ATR-absorbance spectra in lower MIR range of an alkali-silicate sol in dependence of the abrasive concentration.

The removal rate and its profile of the slurry change depending on the abrasive size, the abrasive concentration (wt%), or the material of the abrasive or liquid. Grasp the features of the slurry to improve the CMP removal rate uniformity. For example, changing the method for supplying the slurry may improve the removal rate uniformity. 

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