Slurry Turnover

Regardless of the needs dictated by these factors, the piping system itself will fall into a few general variations as shown in Fig. 4. Dead-head style piping systems are unusual and can only be used with a very well-suspended slurry. This type of system can minimize the loss of volatile components, such as ammonia from a slurry like Rodel Klebosol, since there is no circulation through a daytank or any other head space that would permit ammonia evaporation. It can also be attractive to minimize slurry turnovers to a shear- or gelling-sensitive slurry. Cabot SS-25 is well-suspended, but if... 

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. 

The size of the daytank will affect the life of the slurry, measured in days, but not in turnovers. Since some slurry health measures are a function of time and others a function of turnovers, one must decide which is more important. 

Results of comparable turnovers shown in Figs. 18.23a and 5.3b show a very similar and small growth in LPC for the first 140 turnovers of the slurry, demonstrating that the shearing effect on the slurry pumped by a MLC pump is a function of the number of turnovers rather than the recirculation time. 

FIGURE 18.25 (a) LPC data for silica-M slurry recirculation in AOD pump at 28 psi backpressure, 63.4turnovers/h (Test 4) (b) LPC data for silica-M slurry recirculation in AOD pump at 28 psi backpressure, 31.7 turnovers/h (Test 5). 

Test 8 Silica-H was recirculated in an AOD pump at 28 psi back pressure, 63.4turnovers/h, 81pm, for 6h (380 turnovers). Figure 18.28a shows the LPC distribution, which is significantly different than that of MLC pump Test 7. These LPC results are very similar to those results seen for AOD pump silica-M slurry test earlier (Test 4). There is a larger increase in LPC throughout from 0.56 to 10 pm, but a smaller increase in LPC between 0.56 and 0.60 pm as... 

These results of LPC are supported by mean PSD measurements. PSD data for the AOD pump test samples presented in Fig. 18.28b and c demonstrate relatively smaller changes in PSD during this 380 turnover run. A very small growth of particles in the larger size tail region can be observed, but the mean PSD peak value remains nearly the same (Fig. 18.28b and c). In this test, the slurry viscosity and pump discharge flow rate remained constant within the measurement uncertainties. 

This study shows the significant benefits of an MLP pump in handling shear-sensitive CMP slurries in single-pass applications and under normal turnovers ( 100) expected in a typical fab operation. Since, the MLC pump generated far fewer >lpm particles in shear-sensitive slurries, the filter lifetime for MLC pump-based slurry delivery systems should be longer than the other AOD or bellows pump-based systems, especially when relatively open (> 1 pm nominal rating) filters are used in the global loop and point-of-use (POU) locations. 

Size reduction that is achieved in a wet mill will depend in part on the residence time of the batch in the mill. Residence time can be controlled by operating in single-pass mode, where the batch is pumped through the milling device from one vessel to another, or in recycle mode. For both, it is important to measure slurry flow rate to confirm that residence times are maintained as a process is scaled up. When the batch is recirculated, one way to quantify this is to convert elapsed time to number of batch turnovers (batch turnovers = elapsed time X flow rate through the mill/total batch volume). It can take several passes or batch turnovers to achieve a steady-state particle size distribution, depending on the mill. 

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