Slurry characteristics

FIGURE 4.8 Stribeck curves for slurries with 13-nm abrasives (left) and 35-nm abrasives (right) (from Ref. 17). 

CMP correlated more with the variation in frictional forces (stick-slip) rather than the COF value itself. 

Investigations in the past also emphasized the effect of various additives of the slurry on the tribology during CMP [18,19]. It was observed that the presence of a flocculent reduces the surface frictional force (see Fig. 4.9a). Also, the ionic strength of the slurry has a significant impact on the frictional force (shown in Fig. 4.9b). It can be noted from these data that the friction at the surface depends significantly on the chemical interaction at the interface, besides the mechanical aspects. 

FIGURE 4.9 (a) Friction force dependence on flocculant (from Ref 18). (b) Friction 

The wafer contour determines the area of contact between the wafer and the pad along with the abrasives. Thus, the amount of surface asperity interaction and the particle-wafer interaction also depends on the wafer contour. The fluid film that is in contact with the wafer surface is also dependent on the wafer contour. Thus, the pressure experienced by the wafer at different applied pressures and velocities changes with the shape of the surface. Scarfo et al. [20] conducted polishing tests on wafer samples with concave, convex, and intermediate surface contours and noted that the shape of the wafer affects the coefficient of friction. 

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Slurry characteristics determine whether a clarifying or a cake filter is appropriate and if the latter, they determine the rate of formation anci nature of the cake. They affect the choice of driving force and cycle as well as specific design of machine. 

Slurry characteristics settling rates, filterability, drying rates and final moisture eontents (Chapter 4). 

The overriding factor will be the filtration characteristics of the slurry whether it is fast filtering (low specific cake resistance) or slow filtering (high specific cake resistance). The filtration characteristics can be determined by laboratory or pilot plant tests. A guide to filter selection by the slurry characteristics is given in Table 10.3 which is based on a similar selection chart given by Porter et al. (1971). 

Slurry characteristics Fast filtering Medium filtering Slow filtering Dilute Very dilute... 

A compressibility-permeability test cell has been developed by Ruth(7) and Grace(8) for testing the behaviour of slurries under various conditions of filtration. A useful guide to the selection of a filter type based on slurry characteristics is given in Volume 6. 

The optimum design parameters for aerobic treatment must be those which select for the most desirable mixture of microbes and metabolic activity that brings about a degree of degradation so that the treated slurry characteristics meet the treatment objectives. 

In Section V-D it was shown that Slurry sensitivity and performance depend on Slurry characteristics such as d, particle size of solids and compn, as well as external factors such as pressure and temp. To a considerable degree the effects of internal and external factors can be optimized by control of the rheology and d of Slurries. We shall now examine the rationale behind these control methods... 

Kim NH, Seo YJ, Lee WS. Temperature effects of pad conditioning process on oxide cmp polishing pad, slurry characteristics, and surface reactions. Microelectron Eng 2006 83 362-370. 

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. 

Slurry characteristics filtering filtering filtering Dilute dilute... 

Please complete flow diagram below.) Feed slurry characteristics... 

Slurry Characteristics Fast Filtering Medium Filtering Slow Filtering Dilute Very Dilute... 

To keep the particles in suspension, the flow should be at least 0.15m/sec faster than either 1) the critical deposition velocity of the coarsest particles, or 2) the laminar/turbulent flow transition velocity. The flow rate should also be kept below approximately 3 m/sec to minimize pipe wear. The critical deposition velocity is the fluid flow rate that will just keep the coarsest particles suspended, and is dependent on the particle diameter, the effective slurry density, and the slurry viscosity. It is best determined experimentally by slurry loop testing, and for typical slurries it will lie in the range from 1 m/s to 4.5 m/sec. Many empirical models exist for estimating the value of the deposition velocity, such as the following relations, which are valid over the ranges of slurry characteristics typical for coal slurries ... 

Various small scale test units and procedures are available to determine slurry characteristics and suitability for a particular application. Buchner funnel, and vacuum leaf test units can be purchased or rented fi om vendors to perform in-house tests, or one can have tests conducted at the vendor s facility. Pilot testing on the actual equipment would be the optimum with a rental unit in the plant. In either case, slurry integrity must be maintained to ensure accurate filtration data. 

Slurry characteristics Concentration, pH, conductivity, zeta potential, particle settling velocity, rheology characteristics, etc. 

To correct na unfavorable situation often requires an adjustment to the slurry characteristics rather then modification of the separation device. Increasing solids particle drainage characteristics via size enlargement techniques, conoenrmiing the sluny, heating the slurry, nadfor re arriving "fines" is the usual approach. 

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