Slurry stability

J. P. Bare and T. A. Lemke, Monitoring slurry stability to reduce process variability, MICRO, pp. 53-63 (September 1997). 

The neutralising agent as well as the salt content influences the slurry ageing. A good dispersing agent must maintain constant viscosity over time. We know that a lower monovalent cation content improves slurry stability. 

Although the Ohio line operated successfully, transporting 7 million tons (6.3 million metric tons) of coal, some unexpected operating problems had to be resolved. Much investigation was conducted with variables, such as size consist and slurry concentration and the resultant effect on slurry stability. After 7 years of operation, the line was shut down in 1963, when the unit train concept resulted in much lower freight rates on signilicantly higher amount of coal movement. 

The added surfactant molecules intended for CMP slurry stabilization can adsorb not only onto the abrasive particle but also onto the surface of the wafer to be polished. Depending on the extent of such adsorption, the added surfactant may influence the CMP process in several ways such as change in friction behavior of the slurry, modification of removal rate and selectivity, alteration of defectivity level, and shift in post-CMP profile. In this section the impact of surfactant adsorption on the removal rate, selectivity, and post-CMP cleaning characteristics will be discussed. 

The pH of a slurry has a profound influence on its colloidal stability and CMP performance. Strong correlations have been established between the particle isoelectric point (lEP) and the optimal pH for slurry stability. The general rule is that the slurry is more stable at a pH that is away from the lEP, so the zeta potential of the particles is greater than 20 mV. The focus of this section is on the influence of pH on the slurry performances such as material removal rate and defectivity. In order to examine the impact of slurry pH on these two important performance features, we first take a closer look at the interaction between abrasive particles and the surface to be polished. There is a vast amount of literature on the interaction between abrasive particles and silicon dioxide surface [26]. The discussion below will focus on the interaction between ceria abrasive particles and the silicon dioxide surface to be polished. The basic principles and conclusions can be easily extended to other pairs of abrasive particles and surfaces. 

Slurry formation is strongly affected by diverse physical and chemical variables including particle size, slurry concentration and the faotors governing slurry stability. 

Water-solid Coal slurry stabilization" Coal dewatering"... 

Stability The technique and results of stability experiments were described explained in an earlier paper [I ]. In brief, a series of conunercially available homologous nonionic surfactants of the polyoxyethylene alkyl ether type and two nonionic surfactants were examined for their ability to affect the CMP process with respect to enhancing slurry stability. The salient features of the stability study are incorporated in the following discussion. 

As indicated above, the effect of surfactants on slurry stability is reported elsewhere [1]. Summarized below are the essential features as well as the rationale for the surfactants selected for the further study. 

The main variables influencing this sampling mode are slurry stability and concentration, and particle size. 

Slurry stability Slurries can be maintained stable for insertion into ETAs and ETVs using various approaches, namely ... 

Another aspect of cement slurry stability is the stability under dynamic conditions. Dynamic conditions are usually more severe than static ones because cement slurries are shear-thinning. This is a problem in the laboratory as the solid particles may settle while the fluid is being sheared (thickening time, rheology), and also in the field especially if the well is deviated from vertical. But there is currently no standard test in the industry to evaluate the stability of cement slurries under dynamic conditions. 

A binder developed at DuPont called Ludox SK [8,10] avoids the need for pH adjustments in order to maintain slurry stability. The product is based on aluminate modified colloidal silica. The aluminated product is deionized and because the alumina sites provide sufficient charge density the product is stable. Deionization to remove sodium hydroxide results in a pH which is acidic. As long as the pH remains above about 3, there is little danger of the alumina being leached from the product surface. 

Hazardous Decomp. Prods. Exposed to fire or heat, aluminum trihydroxide dec. forming aluminum oxide and water vapor b inning at 200 C Storage Keep material dry 30 days slurry stability Hydral Coat 5 [Alcoa]... 

Surfactants such as CTAB, ammonium dodecyl sulfate, and SDS were also investigated as corrosion inhibitors for Cu film. However, these surfactants may cause poor slurry stability and insufficient corrosion inhibition. Among various corrosion inhibitors reported in the literature, BTA is still the most efficient corrosion inhibitor. Although an addition of a corrosion inhibitor causes a decrease in MRR by preventing the dissolution of Cu film, it is one of essential components of high planarization and improved surface quality. 

In all the slurry stability tests in the Low Intensity Test Reactor (see Section 4-6) where enriched uranium was present, sufficient catalyst was added to prevent any net radiolytic-gas production. Both PdO and M0O3 were used for this purpose. No radiolytic gas (<25 psi) in excess of steam pressure was observed in these experiments. In the tests where only ThOa was used, no catalyst was necessary. 

FIG. 29 Mechanisms of slurry stabilization with SDS (anionic) and Tween 80 (CisPEO, nonionic) surfactants. (From Ref. 90.)... 

FIG. 30 (a) Material removal rate response of the tungsten CMP slurries stabilized... 

---