Prebake

The thermal decomposition of the resist photoactive compound or radiation-sensitive components during prebake is modeled using first-order kinetics this often results in a change in the resist s optical properties, mostly the Dill A and B parameters.  

First-order kinetics is used to model the exposure chemistry resulting from the previously calculated light distributions within the resist film. The standard Dill A, B, and C parameters are used.  

The effects of thermally driven diffusion of active chemical species within the resist during the postexposure bake are modeled with appropriate diffusion 

Hopkins, Wave Theory of Aberrations, Clarendon Press (1950). 

A model that relates resist dissolution to the chemical composition of the film is often used in conjunction with an etching algorithm to determine the final resist profile. Effects such as surface inhibition or enhancement are also incorporated into the model.  

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Electrodes. Because of the numerous different processes, there are many different types of electrodes in use (9), eg, prefabricated graphite, prefabricated carbon, self-baking, and composite electrodes (see Carbon). Graphite electrodes are used primarily in smaller furnaces or in sealed furnaces. Prebaked carbon electrodes, made in diameters of <152 cm or 76 by 61 cm rectangular, are used primarily in smelting furnaces where the process requkes them. However, self-baking electrodes are preferred because of thek lower cost. 

A typical large three-phase ferroalloy furnace using prebaked carbon electrodes is shown in Eigure 4. The hearth and lower walls where molten materials come in contact with refractories are usually composed of carbon blocks backed by safety courses of brick. In the upper section, where the refractories are not exposed to the higher temperatures, superduty or regular firebrick may be used. The walls of the shell also may be water-cooled for extended life. Usually, the furnace shell is elevated and supported on beams or on concrete piers to allow ventilation of the bottom. When normal ventilation is insufficient, blowers are added to remove the heat more rapidly. The shell also may rest on a turntable so that it can be oscillated slightly more than 120° at a speed equivalent to 0.25—1 revolution per day in order to equalize refractory erosion or bottom buildup. 

Piebaked anodes aie produced by molding petroleum coke and coal tar pitch binder into blocks typically 70 cm x 125 cm x 50 cm, and baking to 1000—1200°C. Petroleum coke is used because of its low impurity (ash) content. The more noble impurities, such as iron and siUcon, deposit in the aluminum whereas less noble ones such as calcium and magnesium, accumulate as fluorides in the bath. Coal-based coke could be used, but extensive and expensive prepurification would be required. Steel stubs seated in the anode using cast iron support the anodes (via anode rods) in the electrolyte and conduct electric current into the anodes (Fig. 3). Electrical resistivity of prebaked anodes ranges from 5-6 Hm anode current density ranges from 0.65 to 1.3 A/crn. ... 

Furnaces used to bake anodes for prebake cells use the cooling anodes to preheat combustion air. Hot combustion gases from the baking 2one are used to preheat incoming anodes. Using these techniques, about 4.2 MJ/kg (1004 kcal/kg) of anode carbon or only 2520 MJ/1 (6.03 x 10 kcal/t) of aluminum is required to produce anodes. 

Typical raw material mix to produce one metric ton of silicon consists of 2500—3000 kg quartz, 1200—1400 kg of low ashcoal and/or charcoal, and 1500—3000 kg wood chips. From 11 to 14 MWh of electrical power, and prebaked carbon electrodes of 90—140 kg, are consumed. 

A typical 20-MW, a-c furnace is fitted with three 45-in. (114.3-cm) prebaked amorphous carbon electrodes equdateraHy spaced, operating on a three-phase delta connection. The spacing of the electrodes is designed to provide a single reaction zone between the three electrodes. The furnace is rotated to give one revolution in two to four days or it may be oscillated only. Rotation of the furnace relative to the electrodes minimizes silicon carbide buildup in the furnace. 

Pitch. The principal outlet for coal-tar pitch is as the binder for the electrodes used in aluminum smelting. These are of two types. Older plants employ Sn derberg furnaces, which incorporate paste electrodes consisting of a mixture of about 70% graded petroleum coke or pitch coke and 30% of a medium-hard coke-oven pitch. This paste is added periodically to the top of the monolithic electrode as it is consumed. The more modem smelters employ prebaked electrodes requiring less binder, about 18%. 

Furnace Design. Modem carbide furnaces have capacities ranging from 45,000 t/yr (20 MW) to 180,000 t/yr (70 MW). A cross-section of a 40 MW furnace, constmcted in 1981, having a 300 t/d capacity is shown in Figure 2. The shell consists of reinforced steel side walls and bottom. Shell diameter is about 9 m and the height to diameter ratio is shallow at 0.25 1.0. The walls have a refractory lining of 0.7 m and the bottom has a 1-m layer of brick topped by a 1.5-m layer of prebaked carbon blocks. The steel shell is supported on concrete piers and cooling air is blown across the shell bottom. A taphole to withdraw the Hquid carbide is located at the top of the carbon blocks. 

Anthracite is calcined at appreciably higher temperatures (1800—2000°C). The higher calcining temperatures for anthracite are necessary to complete most of the shrinkage and to increase the electrical conductivity of the product for use in either Soderberg or prebaked carbon electrodes for aluminum, siHcon, or phosphoms manufacture. 

Two types of carbon electrodes are in widespread use. Prebaked carbon electrodes (Fig. 5) are those made from a mixture of carbonaceous particles and a coal-tar pitch binder. The electrode is formed by extmsion or mol ding from a heated plasticlike mix and subsequently baked. Final bake temperature is sufficient to carbonize the binder, ie, about 850°C. At this temperature the binder is set, all volatiles have left, and a significant portion of the product shrinkage has occurred. 

Fig. 5. Prebaked electrodes. Courtesy of UCAR Carbon Technology Corp.

Prebaked carbon electrodes are manufactured in all diameters up through 1500 mm. Some prebakes are produced as quadriforms to suit specific furnaces. Self-baking electrodes are in service through 2134 mm diameter. Electrode lengths are as needed for particular appHcations. Rounds are available in lengths up to 2794 mm and quadriforms as long as 3556 mm. Self-baked electrodes are continuous. 

Production of carbon electrodes is a capital-intensive business. Two suppHers dominate the prebaked market. Carbon paste producers are more numerous and tend to serve local markets. There is no international standard for the threaded joints on carbon electrodes. Manufacturers of straight pin carbon electrodes have followed the physical specifications adopted for graphite electrodes (37). Unified standards do not exist for pinless joints resulting in limited interchangeability among brands. Electrode diameters are offered in both English and metric sizes with no restrictions on new or unique diameters. 

Although carbon electrode production has been regarded as a mature business, the steady growth in demand and the need for improved electrodes has prompted ongoing development efforts in these areas (/) cost containment through raw material substitutions and process improvements (2) higher purity electrodes for those processes such as siUcon production (J) improvements in thermal shock resistance to enhance electrode performance and (4) better joining systems for prebakes. 

The waste materials produced during the primary production of aluminum are fluoride compounds. Fluoride compounds are principally produced during the reduction process. One reason that prebaked anodes are favored is that the closure of the pots during smelting facilitates the capture of fluoride emissions, although many modern smelters use other methods to capture and recycle fluorides and other emissions. 

The polymers were dissolved in methylisobutylketone (MIBK) and spin-coated on oxjdized silicon wafers (1100 X thick Si02 layers) to form 5000 A thick films. After a prebaking to improve adhesion to the substrate, the resist samples were irradiated 0 through the mask (A) using the Al K 152 emission line at 8.3 A as X-ray source. The electron beam gun was operated at a 300 W power and the source to sample distance was U.9 cm. Taking into account the absorption of the aluminum foil mask,the different X-ray fluxes available on the sample were calculated from the relation given by (9) ... 

Prebaked carbon electrodes, 12 752, 755 Prebaked cathode blocks, 12 765-766 Prebaked electrodes, 12 758 Precautionary Principle (PP), 10 245-246, 24 188... 

Samples for analysis were prepared by spin-coating Si substrates from cyclohexanone solutions of 2,6-dinitrobenzyl tosylate and TBMS or TBS. The samples were prebaked at 90 C for 15 min in a convection oven, and analyzed after exposure and heating at 120 C for 30 min. The films on each substrate were dissolved with 5 ml of THF and a 1 ml aliquot of 0.1 vol % benzene in THF added. Finally, the solutions were diluted to 10 ml with THF, and IOOjj.1 injected onto the GPC for analysis. 

Solubility of polymers in tetramethyl ammonium hydroxide aqueous solution was measured by dipping the wafer on which the polymer solution was spin-coated, for 1 min. at 25°C. The prebake was carried out at 90°C for 5min. Sensitivity of resists was measured after the exposure with CA 800(Cobilt) or KrF excimer laser(0.9mJ/cm2/1 pulse). The polymer structure was determined by iH-NMR, 13C-NMR(FX90Q apparatus,JEOL) and 2 Si-NMR. The molecular weight distribution was determined with a Toyo Soda Model 801 gel permeation chromatograph at 40°C. The four columns were connected in series, each packed with G-2000H8x3 and G-400H8(Toyo Soda polystylene gel), respectively. 

This indicates that the prebaking temperature higher than the melting point of the azide decomposes the azide (50%) and it totally decomposes upto 100 mJ/cm2 irradiation. It is possible that subsequent reactions of the nitrene, generated from the azide thermolysis and photolysis, with the styrene resin could be responsible for solubility modulation of this type resist (16). 

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