DS 7-7R 17-12R Semiconductor Fabrication Facilities ... - FM Global

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7-7R REFERENCE DOCUMENT 17-12R SEMICONDUCTOR FABRICATION FACILITIES Page 18 Fig. 4. Semiconductor fabrication facility systems diagram. In addition to the production of electronic circuits, electro-optical and electro-magnetic devices are also produced. These devices are made on wafers in cleanrooms with similar processes. Photocells for converting light energy to electrical energy and sensors for measuring UV, visible, and IR electromagnetic waves are made using the deposition, photolithography, and etching processes. Electromagnetic devices, such as read-write heads for magnetic disc drives are also made using similar processes in cleanrooms. Crystal production involves the growing of silicon crystals in electrically heated, argon-atmosphere vacuum furnaces operating at a temperature above 1400°F (760°C). As with all crystal growing, a seed crystal is required to set the process in motion. When the growing process is complete, the silicon ingot is brought to room temperature and the seed is removed from the crystal. Years ago the diameter of the ingot was only 1/2 in. (13 mm). Six in. (150 mm) and 8 in. (200 mm) ingots are common today and 12 in. (300 mm) versions are in the development stages. In the cut and grind operation, the ends of the polysilicon crystal are removed and the uneven exterior is ground to achieve uniformity. The silicon ingot is then sliced into wafers. This can be done using either multiwire saws that make numerous cuts at once, or with a diamond edge circular saw. These slice the ingot into 14 to 30 mil (0.36 to 0.76 mm) wafers. (This is about the thickness of a business card.) About 28 wafers are cut from each inch of the ingot. After slicing, the wafers are lapped to remove the saw marks. The wafers are mounted to the equipment which features an abrasive slurry on a revolving disc. An acid etch process performed in plastic wet benches follows to remove the lap marks. Wafers are then polished with a diamond paste to a mirror-like finish. Finally, the wafers are either given a thin surface layer of silicon dioxide in an oxidation furnace (metal oxide semiconductor [MOS] process) or silicon in a epitaxial reactor (bipolar process). At this point, the wafers are ready for building the circuits on ©2003 Factory Mutual Insurance Company. All rights reserved.
REFERENCE DOCUMENT 7-7R SEMICONDUCTOR FABRICATION FACILITIES 17-12R Page 19 the silicon substrate. Most chip manufacturers purchase wafers from an outside supplier, but some facilities make a small portion of wafers needed for processing. Gallium arsenide crystal is more brittle and it is more difficult to grow a single crystal than silicon; so 2 in. (50 mm), 3 in. (75 mm), and 4 in. (100 mm) wafers are typically used. Gallium arsenide circuits are sometimes grown on germanium wafers due to the lower cost of germanium. Mask production involves transferring a large circuit drawing to a glass plate called a mask. The mask contains hundreds of exact reproductions of the original art work and is used later to recreate the pattern on the wafer surface. Each mask contains the pattern for a single layer of the circuit, so many masks are used to fabricate the entire integrated circuit or chip. The mask surface may be an emulsion, chrome, iron oxide or silicon monoxide. Most masks are fabricated from chrome on glass. Two different techniques used to produce masks are known as reticle and electron beam technology. The circuit design process starts with a determination of the functioning of the circuit. A logic diagram of the circuit is developed and then translated to a schematic diagram which shows the location of the various components. The circuit components are then translated to their relative final dimensions, as they will be formed in and on the wafer surface. A sophisticated computer-aided design (CAD) system then draws a composite picture of the circuit surface showing all of the sublayer patterns. The reticle is a miniaturized reproduction of one layer of the circuit. The actual size of the pattern on the reticle is normally ten times the final size (10 X) of the pattern on the wafer. A reticle is an emulsion or chrome photo plate that is selectively exposed to light in a pattern generator. The computer tape from the digitizing operation instructs the shutter system to open and close, exposing the reticle in the exact pattern of the original drawing. The pattern on the reticle is transferred to the mask in the step and a repeat operation. The reticle is positioned over one corner of the photoresist coated mask blank and a light source transfers the pattern on the reticle into the photoresist. After the first pattern is transferred, the machine ‘‘steps’’ the reticle to the next position and repeats the pattern in the next location. This process continues until the entire mask surface is filled with the reticle pattern. Electron beam technology is used to make masks which produce more advanced circuits. An electron beam writer is similar to a scanning electron microscope. The coated mask is placed in a vacuum chamber and an electron beam directed at it. The pattern information stored on the tape at the digitizing operation is used to direct the electron beam to the correct locations to expose the photoresist. The pattern is written onto the mask without a reticle. The fabrication or main part of the process involves repeated steps of photoresist, masking, etching, doping, and deposition. These processes are typically performed in cleanrooms. Photoresist and its developer are the largest volume solvents within the fabrication area. Negative photoresist is a photosensitive polymer suspended in a flammable organic solvent base such as xylene or toluene. It is used to coat the wafer in preparation for transferring the pattern of the circuit from the mask to the wafer. The wafers are coated by dispensing a small quantity of photoresist on the wafer and rapidly rotating on a ‘‘spinner’’ which spreads a thin uniform layer. Photoresist materials are classified as either negative or positive resists, depending on whether the solubility in the developer decreases (negative) or increases (positive) upon exposure to a UV light source. Since photoresist is sensitive to light, it is shipped, stored and dispensed to the areas in brown glass or plastic bottles. Photoresist adjuncts, a variety of chemical liquids and gases, are used to promote the adhesion of the photoresist coating to the wafer. Hexamethyldisilazane (HMDS) is the most widely used chemical for adhesion and is spun onto the wafer surface prior to photoresist application. After the wafers are coated with photoresist, they are ‘‘soft baked’’ to evaporate a portion of the solvents in the photoresist. Methods used to soft bake include hot plates and the following different type ovens: convection, vacuum, moving belt IR, microwave and conduction belt. After the baking has been concluded, the actual photomask process takes place. Photomasking is a process of alignment and exposure. The different types of equipment used for this process can vary in size, overall appearance, method of operation and equipment cost. This equipment includes contact aligners, projection aligners, and wafer steppers. The function is the same in that the wafer is placed onto this machine and a specific patterned ‘‘mask plate’’ is placed over the wafer. The wafer is then aligned ©2003 Factory Mutual Insurance Company. All rights reserved.
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DS 7-7R 17-12R Semiconductor Fabrication Facilities ... - FM Global

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