Asperity structure

Conditioning, an important technology in the CMP process, is to maintain the asperity structures on the pad surface, which force the abrasive particles against the wafer. A... 

For polishing to proceed and create a nearly planar surface, interactions and material removal must occur in or near a plane. For a pad and slurry system as used in CMP, the pad is in contact with abrasive particles which are in contact with the material covering the wafer which is to be removed. This is qualitatively pictured in Figure 1. Here, the particle is idealized to be spherical, which is usually not the case, and the asperity structure of the pad is crudely simplified, except to show that the tops of the asperities are flattened somewhat, which is often observed. 

Interferograms and taper section profiles also can be analyzed for asperity structure. Table 12-5 gives a survey of the characteristics of the asperities in the surface whose interferogram and profile diagrams are shown in Fig. 12-4. Approximately 25-30 asperities were examined in... 

This section provides an overview of CMP models that focus on how the wafer surface material is removed. These include models focused on slurry particle effects, as well as those that incorporate pad asperity structure effects. Our purpose is not to be comprehensive rather, our goal is to focus on selected models that convey physical understanding and represent the state of the art in CMP modeling. 

Once it is recognized that particles adhere to a substrate so strongly that cohesive fracture often results upon application of a detachment force and that the contact region is better describable as an interphase [ 18J rather than a sharp demarcation or interface, the concept of treating a particle as an entity that is totally distinct from the substrate vanishes. Rather, one begins to see the substrate-particle structure somewhat as a composite material. To paraphrase this concept, one could, in many instances, treat surface roughness (a.k.a. asperities) as particles appended to the surface of a substrate. These asperities control the adhesion between two macroscopic bodies. 

Figure 3.12 Illustration of the supposed structure of a typical STM tip at the atomic level showing a number of asperities through which tunnelling might be expected to occur.

It is frequently asserted that two weaknesses of STM are first that all atomic asperities in images need not necessarily correspond to atom surface positions and second that it is inherently difficult to establish the identity of imaged atoms when two or more surface species are involved. The latter need not, however, be a problem. In a study (for example) of the oxidation of ammonia at Cu(110) the oxygen and nitrogen adatoms form separate individual structures which run in the < 100 > and < 110 > directions, respectively, whereas under ammonia-rich conditions only imide species are formed, running in the < 110 > direction, with in situ XPS confirming their presence and the absence of surface oxygen (Chapter 5). 

Binder A material with a high bonding ability can be used as a binder to increase the mechanical strength of the tablet. A binder is usually a ductile material prone to undergo plastic (irreversible) deformation. Typically, binders are polymeric materials, often with disordered solid-state structures. Of special importance is the deformability of the peripheral parts (asperities and protrusions) of the binder particles [5],... 

In the asperity filtering regime, the Greenwood and Williamson theory no longer properly models the local contact pressure since the model contains no notion of asperity width. We describe a simple statistical method for incorporating width effects in two-dimensional polishing. A more sophisticated but more complex approach based on elasticity theory that takes into account asperity shape and the interaction of the asperity with trench structures of similar size can be found in Reference 27. The statistical approach assumes that... 

The particles will undergo rearrangement to form a less porous structure. This will take place at very low forces, the particles sliding past each other. This stage will usually be associated with some fragmentation, as the rough surfaces move relatively to one another and asperities are abraded away. 

Unlike Ni(lll), Ag(lll) surfaces flashed repeatedly (ca. 20 times) to high temperature (1000 K) or aimealed at high temperature developed white (or cloudy) areas as judged by a visual inspection at appropriate angles of incidence. Analysis of such surfaces with optical microscopy and scanning Auger microscopy indicated that this effect was purely structural, involving pits and other asperities with characteristic dimensions of about... 

We need to understand the force-bearing characteristics of the asperity-rich pad surface. The scale of this perspective shrinks another order of magnitude from that used to examine the pad s pore structures. As seen in Figure 6.16, the wafer compression action on the polymer material is likely to show a viscoelastic response (the compression span of the traveling wafer is sufficiently long for the polymer material to respond viscoelastically). The asperities in contact with the wafer surface will then suffer a viscoelastic modification. The nature of this modification will be defined by the forces at work on the asperity profiles. 

If the fluid film separating two metallic bounding surfaces is an electrical insulator, then loss of insulating behavior and appearance of conductance can be interpreted as contact between these surfaces. This is the premise behind the electrical conductance method of detecting lubricant film failure. A practical limit on the applicability of this premise is electrical breakdown of the fluid as thinning of the film concentrates the field intensity. Another complication arises from the fact that for the structured surfaces of every day experience first contact is at the highest asperities rough surfaces may therefore come into initial contact sooner than indicated by the overall trend of the resistance measurements. 

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