Soft pads

Urethane gels and ultrasoft elastomers are a more recent development.18 They are made primarily by reacting high-molecular-weight polyether polyols with a stoichiometric deficiency of isocyanate. The low NCO-to-OH ratio allows for a wide latitude in hardness adjustment. These low-hardness elastomers are used for seating applications (such as gel bicycle seats), shoe inserts, and soft padding for orthopedic devices. 

The reduction of blood loss during or after surgical procedures where suturing or hgature is either impractical or impossible can often be accomphshed by the use of sterile, absorbable haemostats. These consist of a soft pad of sohd material packed around and over the wound which can be left in situ, being absorbed by body tissues over a period of time, usually up to 6 weeks. The principal mechanism of action of these is the ability to encourage platelet fiacture because of their fibrous or rough surfaces, and to act as a... 

F. G. Shi, B. Zhao, S.-Q. Wang, A New Theory for CMP with Soft Pads, Proceedings of International Interconnect Technology Conference, San Francisco, CA, pp. 73-75, June 1998. 

FIGURE 2.17 Schematic diagram of filling cavities in porous soft pads with water. 

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. 

Since the abrasive-wafer surface contact area increases with the force applied, harder pads will remove more material per abrasive than soft pads, but soft pads—with larger contact areas—can remove more material than hard pads at low pressures. 

The limiting behavior, for large Pv, isR = a. When bE Pv, which is the case for hard pads, Equation 5.3 becomes R = (ajbE) Pv, which identifies the Preston constant as fcpreston = ajbE). Equation 5.3 thus predicts that hard pads do obey the Preston equation, whereas soft pads do not, except at very low pressures. This is illustrated by Fig. 5.14 for tungsten CMP. 

The qualitative explanation of Fig. 5.14 is that as pressure increases, the contact area will be much larger for soft pads than for hard pads, so the removal rate of soft pads will increase faster than the removal rate of hard pads. At low pressures, the mechanical removal step dominates the chemical oxidation step because only a very small fraction of the surface is unoxidized... 

Zhang F, Busnaina A, Ahmadi G. Particle adhesion and removal in chemical mechanical polishing and post-CMP cleaning. J Electrochem Soc 1999 146 2665-2669. Shi FG, Zhao B. Modeling of chemical-mechanical polishing with soft pads. App Phys A 1998 67 249-252. 

Shi F, Zhao B. Modeling of chemical mechanical polishing with soft pads. Appl Phys A Mater Sci Process 1998 67 249-252. 

In general use the word "burnish" means to produce a shiny or glossy surface on a material by rubbing. This may be achieved by a variety of techniques from the use of a soft pad to the use of a hard burnishing tool, depending on the nature and material of the surface. 

The CMP process is composed of a chemical effect from nanosize ceramic particles and a physical effect from the pressed pad. Pads and slnrries are the consumables of a CMP process. The polishing pads consist of polynrethane. Generally two types of pads (hard and soft types) are simultaneously used in the CMP process. A hard pad gives better local (within die) planarity, bnt a soft pad gives better uniformity of material removal across the entire wafer. A hard pad is mounted onto a softer pad to form a stacked pad. Figure 15.1 shows a stacked CMP pad the hard pad is the Rodel 1C-1000 and the soft pad is the Rodel Snba-I V. ... 

The control of dynamic effects at impact rates up to 1 m/s (in some instances somewhat higher) frequently makes use of mechanical damping in the load transmission by placing a soft pad (elastomer or grease) between the striker tup and the specimen [3,5], Above about 1 m/s inertia effects overshadow the true mechanical response of the specimen. Due to such dynamic effects, the applicability of FBA is limited to loading rates up to about 1 to 2 m/s for bending type fracture specimens. 

Suppose a thin hard pad is on top of a thick soft pad. In this case it is concluded from figure 1 the top pad bends easily. As a consequence the bottom pad will be deformed non uniformly. Typical values for the bottom pad are a Young modulus (E) of 20 MPa and a thickness (1) of 1 mm. The maximal change in thickness for the bottom pad can be deduced from the results for the within die non-uniformity. A typical value is 300 nm. In the perfect pad bending assumption this level difference is entirely translated into thickness variations of the bottom pad (Al). Hooke s law allows to calculate the additional pressure resulting from these deformations, i.e. 

FIGURE 2.10 Correlation between standard deviations of nanotopography and film thickness variation for soft pad test. 

FIGURE 2.12 Power spectral densities for (a) soft pad test and (b) hard pad test. 

FIGURE 18.24 Illustration of the polishing process using abrasive grains held in a soft pad. This method is the one most often used in university metallography laboratories. 

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