Fluid Based Wear

Section 4.4.2 further separates the polishing mode of abrasion into two submodes, that of Hertzian indentation based wear and fluid-based wear. The difference between these two polishing modes is the nature of the slurry fluid layer between the pad and the wafer. This area of CMP is still poorly understood, yet has important implications as to the removal mechanisms of CMP. 

Whether CMP occurs as Hertzian indentation or fluid-based wear is not clear and has been the subject of some debate. The difference between the two wear modes is in the slurry fluid layer between the pad and wafer. As discussed in Section 4.2, if the fluid layer is not continuous, then pad-wafer contact occurs. Note, however, that the pad does not contact the wafer surface directly, but rather the pad presses abrasive particles against the surface. In such instances, the pad will drag the abrasives across the surface, resulting in Hertzian indentation. 

If the fluid layer is continuous, then the pad does not contact the wafer surface, and Hertzian indentation will not occur. Instead, the collisions between abrasive particles and the pad accelerate the abrasive particles. The particles then impinge on the wafer surface, resulting in fluid-based wear. The velocity and angle of approach of the abrasive particles will determine the kinetic... 

There are several models discussed in the literature of CMP planarization of oxides. In this section we shall review two of these models that assume different modes of polishing, i.e., Hertzian indentation, by Wamock and fluid based wear, by Runnels. In addition to predicting planarity, these models gives good insight into the planarization mechanisms for CMP. However, these models are not necessarily the most accurate models. Other models include those by Burke et al., Yu et al, and Renteln and Coniff.< >... 

This model implicitly assumes that, at least to some extent, the pad makes contact with the wafer surface (i.e., the pad directly presses the abrasive against the surface) and exerts pressure directly to the surface. The abrasive then moves across the surface as a Hertzian indenter. As discussed in Chapter 4 however, it is also possible that a continuous fluid layer exists between the wafer and the pad. The pad compresses the fluid layer, which in turn exerts hydrodynamic pressure on the surface. The existence of a hydrodynamic fluid layer is an important distinction because the wear mechanisms are different for fluid-based wear as opposed to Hertzian indenter-based wear (see Chapter 4). 

Runnels assumes the existence of a continuous fluid layer between the pad and the wafer and models planarization using a feature scale fluid-based-wear model. Runnels uses fluid mechanics to model the normal and shear stresses that are developed at the feature scale. Material removal rate is assumed to depend only upon the shear stress, Ot, according to ... 

High Water-Base Fluids. These water-base fluids have very high fire resistance because as Httle as 5% of the fluid is combustible. Water alone, however, lacks several important quaUties as a hydrauHc fluid. The viscosity is so low that it has Httle value as a sealing fluid water has Httle or no abiHty to prevent wear or reduce friction under boundary-lubrication conditions and water cannot prevent mst. These shortcomings can be alleviated in part by use of suitable additives. Several types of high water-based fluids commercially available are soluble oils, ie, od-in-water emulsions microemulsions tme water solutions, called synthetics and thickened microemulsions. These last have viscosity and performance characteristics similar to other types of hydrauHc fluids. 

Bannerjee, J.K., Some Effects of Molybdenum Disulphide (MoSj) - Based Cutting Fluids on the Workpiece Surface Finish in Grinding and Conventional Machining Processes, Proc. Conf. Wear of Materials 1981, New York, (1981), p. 496. 

You should advise the patient that cystitis is quite common among women but it is important to drink plenty of fluids to ensure the bladder is flushed thoroughly. Offer a potassium-based proprietary cystitis treatment such as Cystopurin. Advise the patient about the importance of wiping herself from front to back after visiting the toilet and that wearing loose clothing and cotton underwear is sometimes helpful. 

Wang et al. (2004a) reported that excessive eccentric sucker-rod wear with beam pumps occurred when the produced fluid contained more than 100 mg/L polymer. The service life of rods was approximately only half a year, a 75% reduction compared to the rod life in waterflooding (waterflood producers with beam pumps had a service life of approximately 2 years). Wang et al. (2004c) also reported that based on the statistical data in the year 2000, 880 wells had been eccentrically worn in 2,365 rod-pumped wells, producing fluids containing polymer. Compared to the wells with no polymer before the year 1997, the number of wells with eccentric wear had increased by 840. Thirty-six percent of wells suffered the eccentric wear. The pump service work wells had also increased from 3.2% in 1997 to 27.1% in 2000 because of eccentric wear in rod-pumped wells. The main cause of eccentric wear on sucker rods was the normal stress of polymer solution, as explained next. 

Modern lubricants are formulated from a range of base fluids and chemical additives. The base fluid has several functions but it is primarily the lubricant which provides the fluid layer to separate moving surfaces. It also removes heat and wear particles whilst minimizing friction. Many properties of the lubricant are enhanced or created by the addition of special chemical additives to the base fluid, as described in later chapters. For example, stability to oxidation and degradation in an engine oil is improved by the addition of antioxidants whilst extreme pressure, EP, anti-wear properties needed in gear lubrication are created by the addition of special additives. The base fluid acts as the carrier for these additives and therefore must be able to maintain them in solution under all normal working conditions. 

Lubricity Ester groups are polar and therefore affect the efficiency of anti-wear additives. When a base fluid is used which is too polar, it and not the anti-wear additives will be adsorbed onto, and cover, the metal surfaces, giving higher wear characteristics. Consequently, although esters have superior lubricity properties compared to mineral oil, they are less efficient than anti-wear additives. Esters are classified in terms of polarity or non-polarity by the van der Waal formula [48], Equation (2.1) ... 

Phosphate esters have been produced commercially since the 1920s and now have important applications as plasticisers, lubricant additives and synthetic-based fluids for hydraulic and compressor oils. Their first use in lubrication was as anti-wear additives. Later developments in aircraft hydraulic control systems, particularly during the Second World War, introduced phosphate esters as less flammable hydraulic fluids. As esters of orthophosphoric acid they have the general formula OP(OR)3, where R represents an aryl or an alkyl group or, very often, a mixture of alkyl and/or aryl components. The physical and chemical properties of phosphate esters can be varied considerably depending on the choice of substituents [59, 60], selected to give optimum performance for a given application. Phosphate esters are particularly used in applications that benefit from their excellent fire-resistant properties, but compared to other base fluids they are fairly expensive. 

Van der Waal, G. (1985) The relationship between chemical structure of ester base fluids and their influence on elastomer seals and wear characteristics. J. Syn. hub. 1(4), 281. 

Antioxidant technology To indicate the oxidation and thermal stability of a variety of ZnDTP-containing and ashless anti-wear hydraulic fluids, TOST and Cincinnati Milacron data - both relevant tests for this type of fluid - have been compiled and summarised. Table 4.9, from both published [80, 81] and in-house results. The data show that oxidation resistance and thermal stability of the premium fluids, both ashless and stabilised ZnDTP, are comparable and clearly outperform the normal-grade fluids. The normal ashless grade at 0.4-0.5% additives is superior to the normal ZnDTP-based grade, especially for thermal stability, which may be due to the use of thermally unstable secondary ZnDTPs. 

Although the performance of ZnDTP-based fluids is acceptable, their environmental aspects may bring about a change to ashless technology. In this context, a major European OEM has recently replaced a ZnDTP-based, anti-wear hydraulic fluid, AWHF, in its factories by an ashless speciality fluid meeting, amongst others. 

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