Daytanks

Commercial suppliers of bulk slurry delivery equipment have taken several different approaches to measuring slurry constituents. In addition, there are different approaches to replenishing a daytank as its slurry is consumed by the polishing process. To further complicate things, measure-... 

Periodic addition of large (relative to the daytank size) batches... 

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... 

Dispense unit and daytank above polisher floor... 

Dual daytanks. Dual daytanks are especially useful for concerns about slurry life or assay. One daytank is consumed to completion and then the tank may be cleaned before a fresh batch is mixed. The second daytank can be blended, checked for assay, and sit ready to use after the first tank has been consumed. 

Redundant piping system. Redundant piping systems can be installed, especially in conjunction with dual daytanks, to enable a complete cleanout of the delivery system between slurry batches. When this degree of redundancy is chosen, a totally separate slurry dispense machine is included to permit a backup for all major components. 

Storage tanks, or daytanks, are used to hold a supply of slurry for circulation to a fab via a global delivery loop. Since slurry must be kept moving, rather than delivered in a deadhead delivery fashion, a slurry dispense engine will typically draw from a storage tank and circulate the slurry back to that storage tank, as depicted in Fig. 6. 

Fig. 6. Slurry storage tank or daytank (Courtesy of BOC Edwards).

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. 

Supply drum filtration before transfer to daytank or blending... 

Filtration of diluted or blended slurry before transfer to daytank... 

Filtration of diluted or blended slurry before transfer to daytank avoids some of the pitfalls just described, but still suffers from intermittent flow... 

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