Surface Layer Formation—Planarization

These removal mechanisms are discussed in detail in Section 7.3.1. In our slurry formulations, we attempt to maximize dissolution of the abraded species. 

As discussed in Section 6.2.1, Kaufman has shown that the formation of a WO3 surface layer during tungsten CMP is 

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In this chapter, we shall first propose a model to explain the removal and planarization mechanisms of copper CMP. Next, we discuss surface layer formation during copper CMP, which is important for planarization, followed by copper dissolution during CMP, which is iii5)ortant to maintain high removal rates. Next a comparison of copper CMP to the Preston equation is made, followed by a discussion of the abrasion mode during copper CMP. Lastly, we investigate the dishing and erosion behavior of copper CMP. 

In the years 1910-1917 Gouy2 and Chapman3 went a step further. They took into account a thermal motion of the ions. Thermal fluctuations tend to drive the counterions away form the surface. They lead to the formation of a diffuse layer, which is more extended than a molecular layer. For the simple case of a planar, negatively charged plane this is illustrated in Fig. 4.1. Gouy and Chapman applied their theory on the electric double layer to planar surfaces [54-56], Later, Debye and Hiickel calculated the potential and ion distribution around spherical surfaces [57],... 

Figure 7.36 Mechanisms of porous layer formation, enriched in noble metal, due to selective dissolution in the overcritical region (a) instability of a planar surface under dissolution conditions controlled by volume diffusion (b) atomic rearrangement by surface diffusion and (c) cracking due to internal stress caused by the coalescence of vacancies.

It was noted in Section 1.3.1 that the free energy of an adatom in the supersaturated distribution on the surface is with respect to its state once entrained within the surface layer. In other words, this is the amount of energy reduction per atom associated with the phase change from a two dimensional gas of adatoms on the surface to a completely condensed surface layer. Suppose that a planar cluster of n atoms is formed on the surface. Will it tend to grow into a larger cluster, and eventually into a film, or will the cluster tend to disperse If all n atoms are fully entrained within the cluster then the free energy reduction due to cluster formation would be —nFph- However, those atoms on the periphery of the cluster are not fully incorporated. They possess an excess free energy compared to those that... 

The maintenance of product formation, after loss of direct contact between reactants by the interposition of a layer of product, requires the mobility of at least one component and rates are often controlled by diffusion of one or more reactant across the barrier constituted by the product layer. Reaction rates of such processes are characteristically strongly deceleratory since nucleation is effectively instantaneous and the rate of product formation is determined by bulk diffusion from one interface to another across a product zone of progressively increasing thickness. Rate measurements can be simplified by preparation of the reactant in a controlled geometric shape, such as pressing together flat discs at a common planar surface that then constitutes the initial reaction interface. Control by diffusion in one dimension results in obedience to the... 

The situations would be totally different when the two surfaces are put in electrolyte solutions. This is because of formation of the electrical double layers due to the existence of ions in the gap between solid surfaces. The electrical double layers interact with each other, which gives rise to a repulsive pressure between the two planar surfaces as... 

Since the equations of state of the system are summarized by the curves in Figure 2, all interesting thermodynamic properties of the interface will have a simple representation in such a diagram. We shall consider the free energy of formation of a single charged surface and the interaction free energy due to the overlap of two identical planar double layers. 

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