Hardness, Abrasive Particle

In CMP, a pad that is made of one or more of polymer materials (sometimes with an embedded inorganic material like glass bead or an abrasive), a substrates with/without a stack of film and the final film to be polished on top, hard abrasive particles, and chemicals in the liquid slurry form the materials group that directly interact, during the process, with each other and with the materials being polished. The materials in the backing film (that supports the substrates in the... 

Zinc coatings on steel (galvanised) are attacked in the same way as iron, but usually more slowly. Very alkaline waters are usually aggressive to zinc and will often remove galvanised coatings the corrosion products consist of basic zinc carbonate or other basic compounds and may take the form of a thick creamy deposit or hard abrasive particles. 

Selection of the most suitable machine for a given requirement is an extremely complex process. Added to variations in the properties of the different materials, many of the machines involved have been specifically developed or adapted to perform only particular tasks. The principal factors which must be addressed are toughness/britdeness, hardness, abrasiveness, feed size, cohesity, particle shape and stmcture, heat sensitivity, toxicity, explodability, and specific surface. 

These statements are only valid as long as there are no abrasive particles in the fluid. If abrasive particles are jammed into the sealing area, the only predominant facts are the difference of hardness between the sealing area and the particles. The best economical solution therefore, in this case, is to use expensive hard and most rigid materials. Experience with silicates and Ranney-Nickel etc. confirm this. Finally by installing valve parts of Silicon-Nitrite or Tungsten Carbide it was possible to achieve quite acceptable life cycle costs. 

We have also conducted adhesion measurements between real CMP abrasive particles (not model particles) and various surfaces as well as particle hardness and elastic modulus measurements, using colloidal AFM and nanoindentation AFM, respectively, in an attempt to correlate CMP defectivity with mechanical and adhesion properties of CMP abrasives [77,78]. For example, in a carefully designed experiment, we have been able to demonstrate that softer particles, indeed, result in fewer scratches [78]. 

Qualitatively, since the contact area increases linearly with applied pressure, the effective pressure is constant for a given pad. Soft, compliant pads have a larger contact area and lower effective pressure whereas hard, stiff pads have a smaller contact area and higher effective pressure. Thus soft pads push abrasive particles against the wafer over a larger area but with less force than hard pads do. 

A wide variety of materials have been implemented as abrasive particles in CMP processes. They include alumina, silica, ceria, zirconia, titania, and diamond. The effectiveness and suitability of these particles in CMP with particular applications are greatly influenced by their bulk properties (density, hardness, particle size, crystallinity etc.) and the surface properties (surface area, isoelectric electric point (lEP), OH content, etc.). This section will focus on the evaluation of alumina, silica, diamond, and ceria as the major abrasives used for the CMP of metals. 

Hardness is one of the most influential bulk properties in metal film CMP. Hardness is defined as the resistance of the material to a localized plastic deformation in the form of a small scratch or indentation. The difference in hardness values between the abrasives and the modified substrate film may determine the removal rate during a CMP process. It is important to emphasize here that, for the substrate film, what matters is the hardness of the modified layer that is in direct contact with the abrasive particles. The hardness of such a modified layer may be significantly different from its bulk film hardness. The... 

One of the most popular techniques used for determining the hardness of a material is the Mohs scale that consists of a qualitative but an arbitrary hardness index scheme ranging from extremely soft materials (value of 1 Moh) to very hard materials such as diamond (10 Moh). Other techniques that are often employed for measuring hardness of substances are developed by Rockwell [72], Brinell [72], Knoop, and Vickers [73]. Over the years, more quantitative methods such as nanoindentation [74] have been developed. This technique applies a small and a controllable load on to the substrate with a probe. The depth of penetration along with a known geometry of the probe provides an indirect way to measure the area of contact at full penetration, which is then used to determine the hardness. The hardness is determined by the ratio of the total force to the contact area. Table 7.2 lists the bulk hardness of different materials, metal films, and abrasive particles, in both Moh and microhardness scales [75]. 

It is important to note that the hardness of the modified substrate may not always be less than the native metal surface. One such example is copper oxide, which has a higher hardness value in comparison to native copper. The resultant oxide might lead to unwanted surface defects such as scratch and redeposition. One way to prevent such defects is to direct these polishing debris to the abrasive particles. The function of abrasive particles in this regard, sometimes, is more important than their duty as a force of abrasion. 

Density of a substance may be defined as the weight of a substance per unit volume. In principle, the bulk density of agglomerated particles in the slurry can offer an indirect measurement of the abrasive particle hardness. The bulk density of the particle can be calculated by using Equation 7.16, excluding the open pores and voids from the volume calculation, where p stands for the specific gravity of the slurry measured using a pycnometer [77]. 

Babu and coworkers [78] established that the abrasive particle density indeed offered a means for characterizing the hardness of submicron abrasive particles based on the material removal rates. The polishing rates of both Cu and Ta were measured for slurries of submicron-sized alumina particles with varying bulk densities ranging from 3.2 to 3.8 g/cm, dispersed in DI water. It was found that the polishing rate increased significantly when the dry powder bulk density exceeded a threshold value. 

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