Slurry distribution

Lime stabilization originated in Texas after World War II, and now it is used throughout the world. Lime is most commonly applied at a 4 wt % application or ca 11 kg/m (20 Ib/yd ) for 15 cm of compacted depth. It can be applied dry as hydrated time or granular quicklime or as a wet slurry. Distribution of the latter form is dusfless. Copious amounts (as much as 5—10%) of water are always needed in excess of the optimum moisture content of the soil. Then, a requisite for success is intimate mixing with a rotary mixer, followed by compaction to a minimum of 95% Proctor density. 

By delivering slurry directly to the pad-wafer interface, process engineers have a great deal of latitude in controlling slurry distribution across the wafer during polish. In other words, they can design processes that do not suffer the limitations of pH or oxidizer concentration gradients across the wafer. Oxide and metal CMP processes are very different, so it is useful not only to be able to inject slurry directly to the wafer surface, but also to control where on the wafer the slurry is delivered. 

The need for a reliable source of slurry during CMP stimulated the growth of a whole industry of bulk chemical distribution (BCD) systems. An extended discussion of BCD systems is beyond the scope of this work, but a few words are in order. Slurry distribution systems vary in sophistication from the simple laboratory system consisting of little more than a barrel of slurry and a pump to sophisticated delivery systems designed to supply slurry to tens of CMP tools in a high-volume production environment [56]. 

A slurry distribution system is a part of the infrastructure of a semiconductor fab, rather than a stand-alone piece of equipment. Even so, it can be thought of as a collection of pieces that comprise the whole. Many of these individual components can be sized for the specific system or left out... 

Support facilities and services associated with the slurry distribution system may include... 

Fig. 1. Generalized CMP slurry distribution system with metrology, assay control, and filtration.

CMP slurry is typically shipped to user sites in 20-liter (5-gal) pails for R D evaluation or 200-liter (55-gal) drums or 1000-liter (265-gal) totes for manufacturing. The difference in handling ease of each container varies due to the size, shape, and weight of the container the number, size, and location of openings in the container and the need to insert stirrers, diptubes, or other devices into the container. The variety and nonstandardization of containers present a challenge for slurry distribution equipment manufacturers and additional cost for users every different container potentially requires a different type of connection to the dispense equipment. 

Slurry distribution loop with dual containment... 

J. P. Bare, B. Johl, and T. A. Lemke, Comparison of Vacuum-Pressure vs. Pump Dispense Engines for CMP Slurry Distribution, Proceedings of SEMlCONtWesl Contamination in Liquid Chemical Distribution Systems Workshop (July, 1998). 

Slurries, distribution of fine particles in a liquid, may be analyzed rather than clear solutions. Graphite furnace atomic absorbance analysis is particularly suited to this method. Slurries have also been introduced into ICP-AES and ICP-MS instruments. There are both advantages and concerns when slurries are used. The preparation is simple, so contamination can be... 

To ensure uniform slurry distribution across the pad surface and delivery of slurry to the wafer center in order to maintain uniform polishing from wafer center to edge. This is especially important for larger size wafer (300 mm) and for polishing reactive metals such as Cu, since chemical reaction of slurry with metal layer is as critical to polishing rate as mechanical action ... 

Similar to oxide CMP, a basic W CMP process uses a polisher equipped with a polishing pad, a cleaning unit, and a slurry distribution system. Owing to the need for an oxidizer, the slurry distribution system also includes a premixing sequence. As the lifetimes of these oxidizers are usually short, the... 

The slurry distribution and mixing system operates in the background of the actual wafer processing. However, a slurry with consistent physical and chemical properties is critical for maintaining a repeatable W CMP process with steady yield. Two of the most important parameters to monitor are etchant (oxidizer) and solid (abrasive particle) concentrations. 

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