Oxide polish

For opaque materials, the reflectance p is the complement of the absorptance. The directional distribution of the reflected radiation depends on the material, its degree of roughness or grain size, and, if a metal, its state of oxidation. Polished surfaces of homogeneous materials reflect speciilarly. In contrast, the intensity of the radiation reflected from a perfectly diffuse, or Lambert, surface is independent of direction. The directional distribution of reflectance of many oxidized metals, refractoiy materials, and natural products approximates that of a perfectly diffuse reflector. A better model, adequate for many calculational purposes, is achieved by assuming that the total reflectance p is the sum of diffuse and specular components p i and p. ... 

Check bore and face-end plates for nicked edges, deep scratches, or scoring. Stone or scrape if necessary, and polish with very fine aluminum oxide polishing paper. 

N2. Sulfur containing odorants (mercaptans, disulfides, or commercial odorants) are added for leak detection. Because neither fuel cells nor commercial reformer catalysts are sulfur tolerant, the sulfur must be removed. This is usually accomplished with a zinc oxide sulfur polisher and the possible use of a hydrodesulfurizer, if required. The zinc oxide polisher is able to remove the mercaptans and disulfides. However, some commercial odorants, such as Pennwalf s Pennodorant 1013 or 1063, contain THT (tetrahydrothiophene), more commonly known as thiophane, and require the addition of a hydrodesulfurizer before the zinc oxide catalyst bed. 

The hydrodesulfurizer will, in the presence of hydrogen, convert the thiophane into H2S that is easily removed by the zinc oxide polisher. The required hydrogen is supplied by recycling a small amount of the reformed natural gas product. Although a zinc oxide reactor can operate over a wide range of temperatures, a minimum bed volume is achieved at temperatures of 350 to 400°C (660 to 750°F). 

For example, a COS hydrolysis reactor needs to operate at about 180°C (350°F), the ammonia and acid scrubbers need to be in the vicinity of 40°C (100°F), while the zinc oxide polishers need to be about 370°C (700°F). Thus, gasification systems with cold gas cleanup often become a maze of heat exchange and cleanup systems. 

In general, the rule of thumb is to add the diluting component to the abrasive component. There are, however, exceptions to this rule of thumb, so experience and the recommendations of the slurry manufacturer should be considered. In the case of silica-abrasive oxide-polishing slurries, this means slurry first, water second. Even when practiced in this manner, slurry diluted at the user site will typically have more agglomerates than slurry diluted to use-concentration by the slurry manufacturer. The primary reason... 

Oxide-polishing slurry Potassium hydroxide 1-3 (% by weight)... 

Further work by Cornish and Watt ( ) and Silvernail and Goet-zinger ( ) established the active role played by the presence of water, and these authors concluded that a chemical-mechanical hypothesis would fit the observed data. In the case of ceric oxide polishing of glass, Cornish and Watt suggest the formation of a "CeO-Si" activated complex which permits the rupture of the O-Si-0 bonds by hydrolysis. The complex "CeO-Si" then breaks apart, the hydrated silica is swept away along with alkalis released from the glass surface, and the process repeats. 

Polishing rates with standard processing parameters are in the 100-200 nm/min region for oxide. Polishing times are therefore normally in the order of several minutes. Oxide-to-nitride selectivity depends significantly on the composition of the slurry, the deposition technique, and the thermal treatment of oxide and nitride and polishing conditions. Minimum and maximum values for commercially available slurries are several to one and several hundred to one, respectively. 

Additive, wt% Nitride Polish Rate, nm/min Oxide Polish Rate, nm/min Selectivity (oxide nitride)... 

Abrasive Type Silica (SiOj) is most often used for oxide polishing while alumina (AljOj) and silica are used for metal... 

OXIDE CMP PROCESSES — MECHANISMS AND MODELS 5.1 THE ROLE OF CHEMISTRY IN OXIDE POLISHING... 

Izumitani demonstrated that water is important to the polish mechanism because it provides the chemical component of the polish. During polishing, water entry into the oxide surface has the effect of softening the glass surface (Figure 5.4). The importance of water to glass and oxide polishing has been demonstrated by... 

Because of the accelerated polish rates and increased smoothness observed with ceria, ceria is often used as a polishing compound in glass polishing. However, because oxide CMP evolved from CMP of silicon for the preparation of wafers, the silica abrasive used for silicon CMP has been utilized almost exclusively for oxide CMP. One study by Jairath et al., however, examines the use of ceria in oxide CMP. Figure 5.12 compares the oxide polish rate vs. normalized polishing stress (a function of velocity and pressure) between a slurry of fumed silica abrasive and a slurry of ceria plus silica abrasives. As expected, the polish rate with the ceria slurry is greater (approximately 3 times) than the polish rate with the silica slurry. 

Oxide polish rate vs. normalized polishing stress (a function of velocity and pressure) for slurries of fumed silica abrasive and ceria plus silica abrasives. (From Ref. (11).)... 

Figure 5.14 Oxide polish rate vs. pressuiexvelocity. The polish rate is linear with the pressuiexvelocity product as predicted by Preston. (From Ref. (12).)...

One of Ae biggest concerns with oxide polish is the uniformity of the polish rate across the wafer. The across wafer nonuniformity of the polish rate can be considerable leading to a variation in insulator thickness from die-to-die across the wafer. Consider, when removing 1.5 pm of oxide, that a 3-a nonuniform-... 

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