Pad Types

Figure 5-41. Self-equalizing tilting pad type thrust bearing. (Courtesy of Elliott Company ...

F(ir axial compressors, the journal bearings are of the plain sleeve type for the larger, slower speed compressors. They are of the tilting pad type for the smaller, higher speed machines. The sleeve bearing is normally housed io a spherically seated carrier. The bearings require pressure lubrication as do most of the other compressors. 

Effective inlet air filtration is required to ensure satisfactory operation of the engine. The location of the unit determines the most appropriate filter system to use. Desert environments where a large amount of sand particles could be expected in the ambient air may use an automatic roll type of filter that allows new filter material to be rolled in front of the inlet without frequent shut-downs to change filters. Arctic or extremely cold locations may use pad type filters, snow hoods to prevent blockage, and exhaust recirculation to prevent icing. Filter assemblies for offshore marine environments may include weather louvers, demister pads, and barrier elements for salt and dirt removal. Screens may be u.sed for insect removal prior to filtration in areas with bug problems. 

Bearings shall be split for ease of assembly, precision bored, and of the sleeve or pad type, with steel-baeked, babbitted replaceable liners, pads, or shells. The bearings shall be equipped with antirotation pins and shall be positively secured in the axial direction. 

Figure 17 shows the effective density using the elliptic filter with a characteristic length of 2.9 mm. The optimal length must be determined for each consumable set and process conditions since the planarization length is dependent not only on the polish pad type but also on the polish process conditions, notably the down force. 

A tabular summary of these pad types, with subcategories, common trade name varieties, applications, and property ranges is given in Table I. 

All commercially available polishing pads are relatively complex composite materials, as evidenced in photomicrographic cross sections of the major pad types illustrated in Figs. 5-7. The signature structural characteristics of each class of pads (Table I) are readily apparent. The impact of manufacturing process on microstructure is sufficiently strong that the manufacturing process used to produce an unknown pad sample can be readily determined from microscopic examination. 

Despite its importance as a process step, there have been relatively few publie reviews of pad effects in metal CMP. The pronounced effect of the choice of pad type on metal CMP proeess yields has undoubtedly inhibited... 

Pad type dewatering is similar to pit dewatering. The major difference is that the coke and water drop onto a grade level pad. Traditionally, water drains through coke packed ports in the pad wall and is then clarified of remaining fines in a settling maze. 

Pad Porosity/Density Type I, II, and III pads all have pores although their microstructures are different. Type IV pads (noncell, nonporous, solid) do not have native porous microstructure except those generated by the conditioning process. Typical physical properties of ICIOOO pad (type III, porous) and IC2000 pad (type IV, noncell, nonporous, solid) are listed in Table 5.2. 

According to the Preston equation (Equation (4.1)), the polish rate varies linearly with pressure and velocity. In general, the Preston equation describes the pressure and velocity dependence of oxide CMP rate well, as shown in Figure 5.14. However, the theoretical value of the Preston coefficient, = 1/2E, does not explain the polish rate variation with other important process variables such as pad type, pad condition, slurry abrasive, and slurry chemicals. 

A planarization monitor has been applied to the copper system to investigate pattern dependencies during copper overburden planarization. Conventional profilometry and a noncontact, acousto-optic measurement tool, the Insite 300, are utilized to quantify the planarization performance in terms of the defined step-height-reduction-ratio (SHRR). Illustrative results as a function of slurry, pad type and process conditions are presented. For a typical stiff-pad copper CMP process, we determined the planarization distance to be approximately 2mm. 

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