Abrasive slurry

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. 

Pump Comfortable technology High shear on slurry Abrasive wear of pumps... 

Typical industrial plastic selection criteria have focused on pin-on-disk tests (involving plastic sliding over steel) and sand slurry abrasion tests. The CMP environment, however, is very different from these typical industrial tests. In CMP, the retaining ring plastic is subjected to a plastic-to-plastic adhesive force component involving the polyurethane pad, chemical attack from the chemicals in the slurry, as well as an abrasive component associated with slurry particles. 

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 17.7 Slurry abrasive effect on scratching performance. Ultrahigh-purity (TEOS base) colloidal silica slurry has an excellent oxide scratch behavior. On the contrary, the fumed silica has a tendency to scratch the oxide surface. A step-by-step dilute HF etching reveals smaller and smaller scratches. 

FIGURE 17.8 Slurry abrasive residues. In case the particle adhesion on the oxide wafer is stronger than the cleaning scrubbing force, abrasive particles remain on the wafer surface. 

TABLE 18.2 Required Minimum Flow Velocity (RMFV) of Slurry Blend in the Global Loop, Abrasive Settling Rate (SR), and Stokes Number (St) for Tested CMP Slurries. SR Values Depend on Selected Transmission and Back-Scattering Zones. (A) = Slurry Abrasive Component Only and (B) = Slurry Blend. 

Present results suggest that most of the silica slurry abrasives can be kept in dispersion by maintaining a RMFV value of 0.5ft/s, whereas alumina and ceria slurries require 1.5 to 2.5ft/s, depending on abrasive and chemical composition of the slurry. Next generation slurry delivery systems should be able to measure and control abrasive particle and chemical characteristics (to ensure mix accuracy and control of time sensitive components such as hydrogen peroxide, ammonium hydroxide, etc.) of the slurry on a real-time basis and consistently provide good slurry at the CMP tools. 

CMP slurry delivery system employing filtration for LPC eontrol should consider slurry characteristics including—abrasive type(s) and composition, LPC, PSD, wt% solids, viscosity, chemical composition and the distribution system characteristics—specific pump type and the pumping effects on slurry abrasive, pump size and speed, global distribution loop backpressure, slurry usage and replenishment cycles, slurry turnover rate and typical turnovers before consumption, filter ratings for various locations, allowable pressure drop for filters, and the slurry flow and temperature consistency needs. 

Discuss various filtration approaches used in CMP slurry abrasive particle management. How are the requirements of new slurries filtration likely to change for future low-defectivity CMP processes What considerations should be made to get stable and extended lifetime from CMP slurry filters How may next-generation slurries low abrasive content and/or complex abrasives change filtration optimization ... 

Singh RK. Pumping effects on CMP slurry abrasive particle characteristics. Semiconduct Manuf 2006 7(l) 40-43. 

Singh RK, Hagerman S, Wargo C, Roberts BR. Effective dispersion of CMP slurry abrasive particles. Proceedings of the 23rd International VMIC. 2006. p 143-146. 

Slurry Abrasive The slurry abrasive provides the mechanical action of CMP. Size and concentration slurry have a different effect on mechanical abrasion. However, the abrasive can also have a chemical effect as in the case of glass polishing with ceria abrasive where the ceria forms a chemical bond with the glass surface or in the case of alumina, which seems to create surface defects on SiOj films polished in pH, in the range of 5 to 8. 

According to the Preston equation (Equation (4.1)), the polish rate varies linearly with pressure and velocity. In general, the Preston equation describes the pressure and velocity dependence of oxide CMP rate well, as shown in Figure 5.14. However, the theoretical value of the Preston coefficient, = 1/2E, does not explain the polish rate variation with other important process variables such as pad type, pad condition, slurry abrasive, and slurry chemicals. 

One of the most important results of Roy et al. is the need to keep the wafer surface wet through all clean steps. If the slurry abrasive is allowed to dry on the wafer surface, a chemical bond apparently forms between the particle and the surface. Once this bond forms, removal of the particle from the surface is virtually impossible. Therefore, the wafer should be sprayed with DI water immediately upon removal from the polishing pad and transported quickly between soluions of the clean sequence to ensure that the wafer remains wet. 

The effect of slurry abrasive size on polish rate is not very clear. Different results have been reported for oxide polishing showing contradicting conclusions. Results obtained by Jairath et al. [S] suggested that the oxide polish rate increases with both abrasive particle size and concentration. However, other reports found that glass polish rate is constant with abrasive size [6], [7] or even decreases with abrasive size [8]. 

The selection of slurry abrasives is one of the most important task in CMP process development. It will determine the removal rate and the level of defects such as particles and scratches. In this study various slurry particles and surfaces to be polished were chosen to measure their electrical properties in aqueous solutions. The harder particles, the greater the removal rates. Table 1 shows the hardness of materials of interest to CMP process. Among particles in Table 1, y-alumina, Ce02, Mn02, fumed and colloidal silica particles were used to measure their zeta potentials as a function of solution pH. 

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