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--> 8 Integrated Processes Integrated processes are those that combine more than one specific unit process into a single piece of equipment or into a group of work stations that are operated under unified control (NRC, 1992). Within the context of the unit process families defined in Chapter 2, integrated processes can combine multiple processes that fall within the same family, such as different material removal processes, or they can combine processes that are in different unit process families, such as a mass-change process and a structure-change process. A number of factors are accelerating the push toward integrated unit processing. These include the need for reduced equipment and process cost, shorter processing times, reduced inspection time, and reduced handling (NRC, 1992). On the other hand, by their very nature, integrated systems require a higher level of synthesis than does a single unit process, such as for in situ process control. Therefore, development of integrated processes will generally be more complex than that of individual unit processes, but it could provide simplified, lower-cost manufacturing. Microelectronics fabrication of integrated circuits employs beam-processing ''cluster tools'' to perform multiple process steps. The initial tools were multichamber etch or deposition systems. The experience of the microelectronics industry is that the development of integrated processing tools has been constrained by many factors, including high development cost, limited range of process expertise at a typical equipment vendor, unknown market requirements, and lack of industry-wide equipment interface standards.1 1 The Modular Equipment Standards Committee of Semiconductor Equipment and Materials International was formed to develop standards for mechanical, utility, software, and control interfaces for future integrated processing systems. These standards will enable circuit manufacturers to choose
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--> Processes based on directed-energy beams lend themselves to integrated processing. The characteristics that favor their use in integrated processing systems include short interaction times with the part due to high, directed-energy densities; a beam that does not require contact with the part; and operational flexibility in the processing environment. Atomic and molecular material beam technologies of interest include physical vapor deposition (direct evaporation processes, direct reactive evaporation processes, direct sputtering, reactive sputtering, ion beam sputtering) molecular-beam epitaxy, chemical vapor deposition, microwave plasmas, ion beams, and directed-energy beams (electron, laser, x-ray, and microwave). Laser beams in particular offer new opportunities for integrated processing. The advantages of lasers include (1) decreasing cost while durability improves some ability to tune the wavelength of a laser so as to maximize the absorption of the energy by the material processing that can be conducted in a variety of atmospheres and (2) processing that can be automated. The amount of energy deposited in a region in an interval of time determines the temperature change and the effect on materials. Thus there are three characteristics of lasers that are very important for applications in an integrated processing system: the spatial and temporal energy intensity distributions and the wavelength. The distribution of intensity in space and time determines the degree of localization of the effect. The spatial distribution of intensity depends on the mode of the laser—it can be highly localized or spread-out (i.e. defocused). The temporal distribution can be a continuous wave or pulsed; a variety of wave shapes and duty cycles are possible. Laser-beam technologies have already found many applications in the traditional manufacturing environment, as is discussed in Chapters 3 (machining and cutting), 5 (heat treating and surface modification), and 7 (welding). As mentioned in Chapter 3, an emerging area of development exploits the flexible nature of laser machine tools; a laser beam can perform a variety of processes on many classes of engineering materials by changing process parameters (e.g., beam diameter, scanning velocity, beam focus, assist gas, etc.) instead of changing machine tools. This is an example of an integrated process. There are several reported examples of integrated processing systems in use. Many employ directed-energy beams, such as lasers. Several examples are discussed below, and a vision of what could be possible in the future is presented. the component systems necessary for their fabrication processes and have assurance that the systems can be integrated into a working whole (Dorsch, 1991).
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--> A fully integrated laser processing system has been developed to manufacture precision gears (Storma and Chaplin, 1987). As indicated in Table 8-1, the operation is simpler than the standard approach, which requires multiple steps accomplished by many different processes; the integrated process is also reportedly less expensive. It is interesting to note the similarity of steps involved in gear making to those involved in integrated-circuit fabrication. In gear making, the use of masks and copper plating corresponds to the use of masks and photoreist in integrated-circuit chip making. The purpose in both cases is to produce localized property changes on the surface. In gear making, the aim is to locally change the surface hardness, thereby increasing the resistance of the gear to fatigue and wear failure. In integrated-circuit chip making, the goal is to produce a localized change in electrical properties. Table 8-1 Comparison of Processes to Produce Precision Gears CONVENTIONAL PROCESS INTEGRATED PROCESS - A low-carbon steel preform is initially rough machined into a gear blank. - The blank is then annealed to remove residual stresses. - The gear teeth are machined by milling, hobbing, or broaching. - The gear is carburized to increase its hardness, strength, wear resistance, and fatigue resistance in the contact areas. - Prior to carburizing, the gear is copper plated in those areas in which the increased carbon content is not wanted. - After the carburization, the gear must be reheated and then slow quenched in oil in order to develop a fully hardened case layer at the surface. - Finally, the gear is ground to its finished shape - The gear blank is made from a high-grade carbon steel, avoiding the need for carburization to achieve required strength and wear resistance. - A laser beam is used to harden the gear at a station in a flexible machining system that is used for shaping the gear.
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--> Laser-beam processing can be used to change various structure-sensitive properties, such as corrosion resistance. Laser surface alloying combines laser irradiation with surface alloying and provides a way to produce a broad range of surface compositions and microstructures. For example, a relatively inexpensive mild carbon steel could be processed to have the corrosion resistance of a much higher priced stainless steel by effectively forming a thin surface layer of stainless steel. Alloying material can be either predeposited on the surface in a separate step prior to laser treatment or co-deposited by injection into the melt at the time of laser treatment. The advantage of using direct powder injection is that it eliminates the electroplating step. A high-powered laser melts the injected powder and the surface layer in the relatively wide beam-spot area. A uniform level of laser alloying 1 mm deep has been reported using this technique (Riabkina-Fishman and Zahavi, 1990). Further development of beam technologies may lead to the development of innovative approaches. For instance, a very flexible integrated manufacturing system could use gases at the beginning of the process to produce the starting materials. The gases could react either directly at the surface to produce a deposit, or within the vapor phase to produce nanophase particles that are then deposited on the surface. The deposition would be carried out selectively to produce a three-dimensional structural part in accordance with the design resident in a computer database. It could be possible to rapidly produce both parts made from homogeneous and composite materials that have unique and desirable properties. An integral part of the process would be noncontacting measurement and nondestructive evaluation sensors. Not all integrated processes employ beam technologies. For example, powder processing (see Chapter 7) starts with metal, ceramic, or polymer particles that have specific attributes of size, shape, packing, and composition and converts them into a strong, precise, high-performance shape. Key process steps include the shaping or compaction of the particles and thermal bonding of the particles using sintering. These two steps can be integrated into a single operation, as in vacuum hot pressing. A cost-effective integrated process has been demonstrated to produce dispersion-strengthened copper alloys (Lee et al., 1992). It can be applied to a wide variety of dispersion-strengthened alloys and metal-matrix composites. The process involves two or more crucibles of molten metal of desired composition. These liquids are then injected into a mixing chamber such that the jets impinge on one another, causing localized turbulent flow and mixing at a very fine microlevel. Macrolevel mixing (i.e., avoiding compositional inhomogeneity) occurs as a result of the bulk swirling motion of the fluids in the mixing chamber. The reinforcement phases are generated within the mixing chamber by an exothermic chemical reaction between selected chemical elements. The liquid
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--> mixture then directly enters a mold, which can be designed to provide a near-net shape requiring little final finishing. In theory, a large number of phase-change unit processes are available, such as ingot casting, die casting, and centrifugal casting. The size and distribution of these particles can be tailored, along with the microstructure of the matrix material, by adjusting process parameters. For example, copper with 50 nm TiB2 particles has been produced. The selection of the casting process would depend on the material system and the desired microstructure (Lee et al., 1992). This integrated process is representative of combining several unit processes such that a final component can effectively be produced in a single step that starts with the introduction of basic materials. Research Opportunities There are promising opportunities to develop integrated processes that transcend the capabilities of an individual unit process. The results can lead to significant processing breakthroughs for low-cost, high-quality production. Integrated tools for solid modeling, expert system design assistants, and process development. The goal is to make a complex part right the first time, or with one iteration, using one or more unit processes that are themselves integrated so that outside intervention is not required. This would result in dramatic time savings for the design and manufacturing processes, and would represent realization of concurrent engineering. The various elements of such a system exist, but with the exception of some high-volume electronic components (e.g., integrated circuits) additional research is needed to provide a fully integrated design-manufacturing capability. Architecture and analysis of integrated processes. Significant problems of information flow, process step development throughput, cost, and real-time control must be overcome before significant use of integrated processing can occur. Extensive development programs could accelerate progress in this vital area. Efforts directed at incorporating multiple operations within a single piece of equipment, as well as at integrating multiple pieces of equipment, are required. Development of standards for process integration. Standards must be developed that address mechanical, utility, software, and control interfaces between unit processes that are candidates for integration (similar to the effort currently underway by the Modular Equipment Standards Committee of Semiconductor Equipment and Materials International). These standards will allow process designers the option to select the combinations of unit processes for easy integration.
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--> Control architectures. Other future directions for research include the development of new or improved lasers and control systems that provide a high degree of dimensional accuracy and production tolerance.
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--> References Dorsch, J. 1991. Cluster tools edge toward plant: MESC standards help. Electronic News 37(1868): 17-18. Lee, A.K., L.E. Sanchex-Caldera, S.T. Oktay, and N.P. Suh. 1992. Liquid-metal mixing process tailors MMC microstructures. Advanced Materials and Processes 142(2):31-34. NRC (National Research Council). 1992. Beam Technologies for Integrated Processing. National Materials Advisory Board, NRC. Washington, D.C.: National Academy Press. Riabkina-Fishman, M., and J. Zahavi. 1990. Laser surface treatments for promoting corrosion resistance of carbon steel. Special Issue on Laser Processing. Materials and Manufacturing Processes 5(4):641-660. Storma, J.M., and M.R. Chaplin. 1987. Induction gear hardening by dual frequency method. Heat Treatment Magazine 19(6):30-33.
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