Material removal mechanism dissolution

The material removal mechanism of the CMP process was relatively well explained by the previous scientists. The material removal mechanism of dielectric CMP is further well explained by Cook in his paper pubhshed in 1990 [7]. It was explained that the rate of mass transportation during glass pohshing is determined by five factors the rate of water diffusion into the glass surface, the dissolution of the glass under the applied load, the adsorption rate of the dissolved material onto the abrasive surface, the redeposition of the dissolved material onto the surface of the work piece, and the aqueous corrosion between particle impacts. Water diffuses into sUoxane bonding (Si—O—Si) and the diffusion rate is controlled by multiple process conditions such as pressure or temperature. This hydrated oxide surface is removed by an abrasion process. The indentation process by each abrasive was modeled by Hertzian contact and their contact stress was calculated from the theory of elasticity. 

CMP processes for oxide planarization (ILD and STI) rely on slurry chemistry to hydrolyze and soften the Si02 surface. Mechanical abrasion then controls the actual material removal. Thus, the key process output control variables (i.e., removal rate and nonuniformity) are strong functions of the mechanical properties of the system, namely, the down force and the relative velocity between the pad and the wafer. Metal CMP processes such as copper CMP rely more on chemical oxidation and dissolution of the metal than mechanical abrasion to remove the metal overburden. Consequently, careful control of the chemistry of the CMP process is more important for these CMP processes than it is for oxide CMP. Thus, CMP tools and processes optimized for ILD may not be optimal for metal CMP and vice versa. 

We utilize the Kaufman model as a basis for understanding the removal mechanism in the CMP of copper. Specifically, we hypothesis that a CU2O film forms on the copper and that just as with tungsten, the surface film prevents the removal of the low-lying copper. However, we believe that the dominant mechanism for removal in the case of copper CMP is abrasion of material from the surface by the mechanical action, rather than direct dissolution of material at the metal surface. 

In the copper slurry formulations described below, we have attempted to maximize the solubility and dissolution rate of the copper in the slurry, and therefore dissolution of the abraded material is expected to be the dominant removal mechanism. However, the removal of the abraded material may occur as a combination of several of the above mechanisms. For example, the abraded material may initially fall into the pad undissolved, where it then dissolves. Such a scenario was described in Section 4.6.2 where it was observed that when the concentration of polish by products in the pad is high, the slurry initially turns black, indicating incomplete dissolution and the formation of copper oxide precipitates. With time, however, the slurry turns blue, indicating dissolution and the formation of the Cu(NH3)2 complex. Thus, the optimum polishing conditions may provide for a combination of removal mechanisms. 

Electrochemical interaction between the oxidizer and the metal is believed to play a key role in material removal in tungsten CMP. In this study, we use X-ray Photoelectron Spectroscopy (XPS) in conjunction with electrochemical measurements in both in-situ polishing conditions as well as in static solutions, to identify the passivation and dissolution modes of tungsten. Dissolution of tungsten oxides was found to be the primary non-mechanical tungsten removal mechanism in CMP. 

In all these derivations, the role of the slurry chemicals during the polish process is not apparent. Even under static conditions, some of the chemicals can dissolve the material as in the case of ferric nitrate and copper or even H202/glycine and copper. This effect can, in principle, be easily included in a model description by adding a nonzero, velocity and pressure independent, intercept to the polish rate expression. In practice, it is more complicated since the relation between this nonzero intercept and static dissolution rates is not simple and is unknown due to, among other things, the effects of the polishing pad. In such cases, the role of a threshold pressure, while perhaps obvious when mechanical abrasion is the only mechanism for material removal, is not evident unless the removal rate can be broken neatly into two independent terms, one for the mechanical abrasion and the second for the chemical removal. Such is the case for the... 

Materials implanted into bone are modified by degradation and the action of different cell types. Degradation occurs by dissolution and the action of osteoclasts. Osteoclasts are very effective in material removal and provide a material resorption mechanism at the surface of the ceramic and particulate removal by phagacytosis (Heymann et al. 2001). Degradation of calcium phosphates as obtained from in vitro studies employing cells or animal studies has been discussed in several reviews (Frayssinet et al. 1993, Le Geros 1993, Heymann et al. 1999). 

The three-dimensional displacements inherent to NIL require resist materials that easily deform under an applied pressure and/or elevated temperature. These resists must have a low viscosity during imprinting, a Young s modulus less than that of the mold, and a low sheer modulus. It should also be mentioned that the resist material should have excellent adhesion to the substrate, provide high-quality, uniform film thickness through deposition via spin-coating, and have sufficient thermal and mechanical properties for subsequent processes. When determining the type of NIL resist to use, one should consider the critical dimensions of the pattern, pattern density, release properties from the mold, required imprint temperature and pressure, etch selectivity for subsequent pattern transfer, and route to eventual removal by dissolution or other processes. 

Leaching is the removal of a soluble fraction, in the form of a solution, from an insoluble, permeable sohd phase with which it is associated. The separation usually involves selective dissolution, with or without diffusion, but in the extreme case of simple washing it consists merely of the displacement (with some mixing) of one interstitial liquid by another with which it is miscible. The soluble constituent may be solid or liquid and it may be incorporated within, chemically combined with, adsorbed upon, or held mechanically in the pore structure of the insoluble material. The insoluble sohd may be massive and porous more often it is particulate, and the particles may be openly porous, cellular with selectively permeable cell walls, or surface-activated. 

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

The most widely used positive resists are those that operate on the basis of a dissolution inhibition mechanism. Such resists are generally two-component materials consisting of an alkali soluble matrix resin that is rendered insoluble in aqueous alkaline solutions through addition of a hydrophobic, radiation-sensitive material. Upon irradiation, the hydrophobic moiety may be either removed or converted to an alkali soluble species, allowing selective removal of the irradiated portions of the resist by an alkaline developer. 

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