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Harnessing Light: Optical Science and Engineering for the 21st Century 5 Optics in Industrial Manufacturing Modern manufacturing is being revolutionized by the use of optics, which can both improve current manufacturing capabilities and enable new ones. Light can be used to process or probe materials remotely, even through windows isolating harsh or vacuum environments. With no surface contact, there is no contamination of the process by the probe beam and no wear of tool edges. Scanning provides action over large areas. Light can be used to induce photochemistry, for example, in photolithography to produce submicron features in thin films of photoresist or in rapid prototyping where liquid polymers are solidified by lasers to form a three-dimensional piece from a computer-aided design database. Light can cast images, making it possible to inspect a part or use the image to guide the working tool to the correct area of the work-piece. Images of the surface topology can be compared to the topology of the ''perfect" image captured in a database or the topology of an identical piece to ensure consistent component fabrication. For these many reasons, optics has reached into every aspect of manufacturing and promises to increase in use with improvements in speed, control, precision, and accuracy. Numerous optical techniques are used throughout industry and are critical to the manufacture of such diverse and basic products as semiconductor chips, roads and tunnels, and chemicals. Optical techniques, grouped by function, fall into two broad classes: Performing manufacturing: Light interacts directly with the finished or intermediate product to change its physical properties, as in the case of photolithography or materials processing. Controlling manufacturing: Optics is used to provide information about a manufacturing process, as in the chemical industry's use of optical sensors for in-line process control, or to inspect a manufactured
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Harnessing Light: Optical Science and Engineering for the 21st Century product, as in the semiconductor industry's use of optical inspection tools to characterize particulate contamination. Some applications may be relatively familiar, such as the use of high-power lasers for cutting, drilling, or welding steel. Others are less familiar, such as the use of optical sensors to monitor chemical processes in real-time or the use of lasers for alignment and control in the construction industry. Some of the challenges that these applications face are unique to a particular industry, but others, such as the need for trained optics technicians or the importance of making equipment robust and reliable, are universal. Table 5.1 shows the most important uses of these optical manufacturing techniques for five major U.S. industries—automotive, semiconductor, chemical, aerospace, and construction. These industries in aggregate account for approximately $1 trillion, or 17% of the 1992 U.S. gross domestic product (GDP). Each has a critical dependence on one or more optical manufacturing techniques. Because of the diversity of U.S. industry, this chapter cannot address the use of optics in every single branch of manufacturing. It endeavors instead to cover a representative sample, including those applications TABLE 5.1 Major Uses of Optics in Industrial Manufacturing Automobiles Semi-conductors Chemicals Aircraft and Aerospace Construction Value of shipments (billions of 1992 dollars)a 152.9 32.2 305.4 131.9 391.2 Photolithography — Critical — — — Laser materials processing Critical Major — Major Significant Rapid three-dimensional prototyping Emerging — — Emerging — Metrology (location, position, dimension, and alignment) Major Critical — Critical Critical Machine vision (features, orientation, and defects) Emerging Significant — — — Optical sensors (composition, temperature, pH, etc.) — Significant Critical — Major NOTE: Critical means that a technique is used pervasively and cannot be replaced by alternative nonoptical techniques without major negative economic impact to the entire industry. Major means that a technique is used pervasively and adds significant economic value to the entire industry. Significant means that a technique is used for specialized niche applications within an industry and adds significant economic value to those niche sectors. Emerging means that a technique is being put to increasing use in an industry and has the potential to be of at least significant importance. a All shipment values are from the 116th Edition of the Statistical Abstract of the United States, 1996.
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Harnessing Light: Optical Science and Engineering for the 21st Century that represent large markets for optical systems and devices. An illustrative selection of other applications with significant potential for growth is also given. This chapter is organized in five sections. Two explore the use of light to perform manufacturing and the use of optics to control manufacturing, respectively. Industry-by-industry examples follow to highlight the interplay between the various applications of optics to perform manufacturing in each industry. Prospects for increasing the use of optics in manufacturing are discussed in the next section. Findings, conclusions, and recommendations are gathered in the last section. Use of Light to Perform Manufacturing Because of the many unique properties of light and the manner in which light interacts with matter, optics offers a rich variety of application options for manufacturing processes. The imaging properties of light and its ability to induce photochemical reactions allow highly complex mask patterns to be transferred to photoresist in the optical lithography process. Tightly focused laser beams can deliver thermal energy to the workpiece for cutting, welding, or drilling with a precision and accuracy unmatched by any other technique; they can also induce localized photochemical reactions to generate solid three-dimensional prototype parts. Additional advantages are the ability to deliver this energy at a distance in a noncontact manner through windows and in various atmospheres. Some of light's diverse range of utility is illustrated in the following applications. Photolithography Photolithography plays an essential enabling role in integrated circuit processing. Photolithography requires both an optical system—the step-and-repeat camera (stepper) that is the workhorse of the integrated circuit (IC) industry—and an optical material—the light-sensitive photoresist used to transfer the desired pattern to the silicon substrate or thin film of interest (Figure 5.1). As the demand for faster processing speeds continues, increasing pressure will be put on photolithographic processes to produce smaller feature dimensions, requiring new photolithographic tools, new materials, shorter wavelength light sources, and other more advanced optical system designs. At present, photolithography requires the use of three elements: The mask, which defines which areas of the film to be patterned will be exposed to light; The exposure tool, which images the pattern from a mask onto the substrate; and
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Harnessing Light: Optical Science and Engineering for the 21st Century FIGURE 5.1 The photolithography process: (a) coat, (b) expose, (c) develop. Advances in the resolution and depth of focus of photolithography systems drive increases in the speed and performance of computers and computer-controlled systems. The photoresist, which changes solubility when exposed to light and transfers the pattern on the mask to the film or layer below the photoresist. Effective combination of these three elements, with appropriate integrated circuit design, has resulted in tremendous decreases in the minimum size of features and increases in the number of elements on a chip, allowing for increased speed and number of computational operations. In the early 1980s, state-of-the-art IC devices contained as many as 8,000 transistor elements and had minimum feature dimensions of 5-6 µm. Today, devices with several million transistor cells are commercially available and are fabricated with minimum features of 0.5 μm or smaller. Indeed, the decrease follows an almost perfectly exponential trend known as Moore's law. The steady decrease in integrated circuit linewidths or feature size has largely been fueled by improvements in the resolution of optical lithography. This improved resolution, in turn, has been enabled by the use of shorter and shorter wavelengths for the exposure tools. Deep ultraviolet (UV) lithography using 248-nm wavelength light is just coming into production use for chips with minimum dimensions as small as 0.25 µm. A lithography roadmap prepared by SEMATECH (1997) projects the minimum feature sizes desired in the future and the technologies that must be developed to achieve them. Exposure Tools The workhorse of photolithography is the step-and-repeat camera. The optical imaging system of this device is the most demanding application of commercial lens design and fabrication today and can cost in excess of $1 million. Four competing demands on lens performance are (1) increasing resolution, (2) increasing depth of focus, (3) increasing field size, and (4) decreasing aberrations. Maximum resolution and depth of field are determined primarily by the wavelength of the imaging light and the numerical aperture of the projection lens, with changes that increase resolution and result in decreasing depth of field. The trade-off of resolution with depth of field
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Harnessing Light: Optical Science and Engineering for the 21st Century has driven many process changes, for example, the development of planar metallization. Industry anticipates a transition to an exposure wavelength of 193 nm by a change in the excimer laser light source from KrF to ArF. Generations of exposure tools have relied on high-quality fused-silica refractive lenses. Due to issues of compaction and color center formation with fused silica, which are not adequately understood, the 193 nm exposure tools will likely use, for the first time, some reflective elements as well as CaF refractive elements. The lack of materials that are adequately transparent at 157 nm or 126 nm is a barrier to further reduction in wavelength, necessitating all-reflective exposure tools for use at these wavelengths. All reflective optical systems with high numerical aperture (0.6) are prohibitively difficult to fabricate because of the large number of aspherical reflectors required and the stringent specifications for these reflectors. This situation speaks to the issue of the manufacture of optics covered in Chapter 6. Step-and-scan systems offer another alternative to the step-and-repeat equipment common today. Because of the difficulty of making bigger lenses, an alternate approach is to combine modest-sized lenses with scanning systems to increase the field size. By synchronously scanning the mask and the wafer through an illuminated area corresponding to the corrected field of the lens, it is possible to achieve patterning over large areas. The synchronization between the mask and the wafer stages must be kept well under 100 nm, which is not easy. However, for 256-megabit DRAM chips and beyond, step-and-scan technology will likely prove more cost-effective than step-and-repeat because of the smaller optical system employed. Photo Masks In the past 10 years the transition from 1x to 4x and 5x optical systems has provided a technology respite to the mask-making industry, but the recent emphasis on optical proximity correction combined with the relentless trend toward smaller geometries and more complex structures has accelerated mask making requirements. The mask-making industry generates insufficient revenue to cover the cost of developing new generations of mask-making tools. Given the current direction, mask making will almost certainly be a major impediment in only a few years, although there are some initiatives under way aimed at alleviating this. Mask alignment is also a critical issue. Subsequent masks must be precisely aligned with patterns on the silicon wafer with a precision far beyond that of the minimum feature dimensions. New metrology will be required for next-generation systems. One interesting possibility is the conversion to maskless systems that have a large micromirror array or similar device in the lens focal
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Harnessing Light: Optical Science and Engineering for the 21st Century plane. In this case the mask pattern is simply a data file stored on an array of hard disks or other high-speed data storage device, which feeds pattern data to the mirror array. The flexibility of an electronic mask would be unprecedented and could correct for small imperfections in the imaging system. Photoresist The pattern on the photomask is transferred to the silicon wafer by means of a light-sensitive polymer that is spun uniformly onto the wafer surface. Exposure to UV light changes the solubility of the polymer such that the exposed (positive photoresist) or unexposed (negative photoresist) regions can be removed in a solvent after exposure. Optimum materials exhibit high photosensitivity and uniform absorption of the UV light for uniform solubility and contrast. The key to developing an effective photoresist is to develop a material with excellent etching resistance combined with good imaging characteristics. This combination presents a significant challenge and is the focus of several research efforts today. Present conventional photoresists are not appropriate for use with the nonconventional lithographic technologies that will be necessary for sub-0.5 μm lithography. The most notable deficiencies of the conventional novalac-quinonediazide resist are the exposure sensitivity and absorption properties of the materials. New photolithographic tools in general have low-brightness sources, and high-sensitivity resists are highly desirable. Additionally, the absorption of conventional photoresists is too high to allow uniform imaging through practical resist film thicknesses, usually on the order of 1 µm. For 248-nm lithography, these challenges were accommodated by application of chemically amplified resist technology, which greatly enhances photosensitivity. However, hydroxystyrene polymers, which form the basis for this technology, are effectively opaque at 193 nm. Thus, new polymer materials are required for 193-nm single-layer resists that possess high optical transparency at the exposure wavelength, combined with good etching resistance and functionality that will effect a change in solubility of the exposed regions. No matter what technology becomes dominant when today's photolithography capabilities have reached their limits, new optical materials and processes will be required, necessitating enormous investments in research and process development. The introduction of new resist materials and processes will also require a considerable lead time to bring them to the performance level currently realized by conventional materials, as has been the case with new photolithography techniques. For example, the printing of 0.5-µm features, common in manufacturing since 1993, was the result of more than 12 years of development of
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Harnessing Light: Optical Science and Engineering for the 21st Century new photoresists, designed to respond to 248-nm light, and new step-and-repeat cameras producing that illumination. The next big decrease in resolution, which is expected to be in production in 2001, is the production of 0.18-µm features; this technology will use photoresists that have been under development since 1992, as well as new steppers operating at 193 nm. Future of Photolithography What are the alternatives for future advances in photolithography? There are currently several possibilities: Wavefront Engineering. Because integrated circuit design uses a limited set of objects with limited dimensions, the limitations of classical imaging can be overcome by appropriate design of a mask feature, use of phase-shift masks, or modifying the illumination to change the amplitude and phase of the optical wavefront. Extreme Ultraviolet (EUV). At wavelengths as short as 14 nm, small numerical aperture reflective systems can provide high-resolution and depth of focus. Hurdles to overcome include EUV-robust and reliable x-ray sources, defect-free EUV masks, aspheric reflective optics, and surface imaging photoresists. Electron Beam. Electron beam projection lithography offers promise for resolution as fine as 30 nm with a depth of focus as high as 75 μm for 0.25-µm features. This approach would, however, require a significant departure from current industry processing; for example, electron beam lithography requires processing under vacuum. High cost and low throughput continue to limit the use of this technology. At this time, considerable progress can still be made with optical lithography—previous predictions of its demise have proven completely wrong. It is important, however, to recognize that the risk is too great for a new technology to be introduced in a single generation of devices. Whatever the technology of choice, it must be developed and put in limited production with operational experience well before full implementation. Appropriate metrology tools for process control and evaluation must be developed in parallel with improved lithography equipment. Laser Materials Processing Laser materials processing offers many powerful advantages for manufacturing applications. Unlike competitor technologies such as resistance welding, plasma arc cutting, and flame hardening, lasers deliver energy to the workpiece without physical contact, provide high localized energy densities, and are remarkably versatile in their energy delivery. Although capital equipment acquisition costs can be high, once installed the ease of application, high-speed processing, reproducibility,
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Harnessing Light: Optical Science and Engineering for the 21st Century reliability, greatly reduced distortion especially in thin sheets, ability to interact with complicated shapes (joints in restricted areas can be welded provided a line of sight to the weld is available), and environmental advantages (especially compared with chemical processing) make laser-materials processing increasingly attractive for commercial applications. Such applications range from macroscopic processing (e.g., metal welding, cutting, drilling, slitting), where the thickness of material processed can be several millimeters, to new advances in micromachining where dimensions range from 1 mm down to 1 μm, and finally to the submicron processing of semiconductors. Although laser types abound, there are two that dominate the laser materials processing field (Bell and Croxford, 1995): CO2 and Nd:YAG. At present only these two types provide sufficient power and a usable beam as a package that can be integrated economically into a production line. Of the two, CO2 lasers have tended to dominate the higher-power market, whereas Nd:YAG is favored for high-precision, low-heat-input applications. Another type of laser, the excimer laser, is beginning to make an impact in industrial processing (Weiss, 1995). These lasers operate in the ultraviolet part of the spectrum (as contrasted with the infrared), and favored types include KrF (krypton fluoride) and XeCI (xenon chloride). Besides the potential for ultrasmall feature sizes due to the short wavelengths, an advantage of the excimer is the way it interacts with materials. Materials processing using an infrared laser is a thermal process, whereas the laser-materials interaction with high-power pulses of UV radiation is a "cold process" that uses energy to break chemical bonds rather than heat the material. Thus, excimer lasers are particularly useful for processing polymer-based materials and ceramics to avoid problems of ablation, charring, or gasification that often accompany the heating of these materials to high temperatures. Box 5.1 notes the use of excimer lasers to clean ancient metal art objects. There are two areas in which the general field of laser materials processing could benefit from advances in optical technology. The first is BOX 5.1 LASERS FOR ART RESTORATION Cleaning ancient and antique art objects can be a tricky task. Chemical cleaning can harm surfaces and be hazardous to the environment. In one alternative method, an excimer laser is used to clean items such as ancient Roman coins by exciting particles in microcontaminants and breaking their bonds with the surface. A flowing inert gas then blows away the contaminants, leaving the surface undisturbed. This process is expected to reveal details by removing oxidation products.
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Harnessing Light: Optical Science and Engineering for the 21st Century improvements in optomechanics to solve beam delivery problems for applications such as pipe welding. A second area is adaptive optics to make possible laser surface treatments of arbitrarily shaped surfaces through obscuring, turbulent, or aberrating intervening regions between the laser and the workpiece. Welding Applications In the automotive industry, lasers have been used to join stamped steel panels to form underbodies (American Society for Metals, 1983). The process is computerized, and welding is performed at a rate of 1,000 to 1,150 cm/s. Laser welds are continuous, which results in high structural integrity and eliminates the need for a sealing operation. The ability to program a laser welding system has the advantage that underbodies for any car model can be welded by calling up the correct program from the computer memory. A growing application for laser materials processing is the welding of zinc-coated (galvanized) steel for car bodies. The low melting point of zinc (419°C versus 1535°C for steel) greatly changes the characteristics of the process (Bell and Croxford, 1995). If allowed to stay in the molten weld pool, zinc can alloy with steel during the welding process to produce unacceptable welds. The zinc also forms vapor pockets that, when trapped, expel molten material out of the weld, thus resulting in weld porosity. The solution to this problem includes the use of a pulsed laser that rapidly vaporizes the zinc out of the weld zone before joining the steel. In the electronics industry, laser welding is used to seal electronic devices that are either high-value, low-quantity production devices or welds that must meet stringent reliability and other special end use requirements (American Society for Metals, 1983). Examples of the latter type include hermetically sealed devices for commercial and military aircraft applications. These devices must maintain highly reliable operating performance under extremely severe environments. For instance, the laser welding of relay containers processed according to military specifications has proven an effective way of sealing each package. Laser welding has been quite useful in such applications because of its ability to produce welds near heat-sensitive, glass-to-metal seals. The production of heart pacemakers is another application requiring high-quality welded construction (American Society for Metals, 1983). Today, laser welding has become a widely accepted technique for producing hermetic welds in titanium and stainless steel pacemaker cases. A principal power source for pacemakers is the lithium battery, which because of its highly reactive nature must be hermetically sealed. Lithium cells have also entered the consumer and industrial markets
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Harnessing Light: Optical Science and Engineering for the 21st Century as long-lived power sources for such applications as watches, calculators, and backup power for computer memories. The small size, coupled with requirements for a fusion welded seal in close proximity to the reactive contents of the cell, again make the laser ideal for this job. Although most industrial laser applications are autogenous, lasers are increasingly used in the production and refurbishment of components by adding material during processing (Azer, 1995). For example, prealloyed or mechanically mixed powders can be added to a weld pool. By scanning a laser beam across the workpiece, a weldment is created that is metallurgically bonded to the base metal. The automotive, aerospace, oil, and nuclear industries are benefiting from such welding and cladding techniques. Cutting and Drilling Applications In laser beam machining and drilling, material is removed by melting. Such melting does not involve mass material removal since only a very thin layer is actually melted. The technique has the advantage of rapid material removal with an easily controlled, noncontact, nonwearing tool. A major application of lasers is in metal cutting, primarily two-axis profiling of sheet goods that otherwise would be blanked out by punch presses or fabricated by hand after laborious layout of the pattern (American Society for Metals, 1989). Laser cutting and drilling is ideal for batch processes, just-in-time, or low- to medium-volume production. Sheet thicknesses up to 13 mm can be processed. A recent publication describes one of the first uses of an Nd:YAG laser (instead of the CO2 laser) for cutting sheet metal (Industrial Laser Review, 1997). The application is for assembling burner systems and high-pressure cleaning machines. Other applications include the production of continuous-flow oil heaters and exhaust mufflers. Burst disks for hydraulic systems represent another product. If a fault develops, these disks are designed to burst at a given pressure to vent the system. They are made of high-grade steel, 1 mm thick, and are laser-cut at a precise location to break under a specific pressure. Laser beams are used also to drill small-diameter holes in stainless steels. Advantages compared to other techniques include lower aspect ratios, less deformation of hole walls, higher accuracy, less taper, and most important of all, high production rates (one hole per second). Micromachining with excimer lasers is becoming increasingly popular (Weiss, 1995). Applications include using an excimer laser as an alternative to ion milling to pattern thin films onto disk-drive heads. Areas other than semiconductor and electronic applications are growing as well and include flow orifices, such as nozzles for inkjet printers and automobile fuel injection; optical fiber positioning ferrules and waveguides; devices for DNA and other biomedical or biotechnology
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Harnessing Light: Optical Science and Engineering for the 21st Century research; and medical devices. Lasers may be the only technology that can process such materials as chromium- and titanium-based metals for orthopedic implants without unacceptable levels of corrosion. Laser cutting and welding techniques are used for nonmetallic materials as well. They are highly effective in cutting hard workpieces with low electrical conductivity such as cubic boron nitride, an ultrahard tool material. Alumina likewise can be cut or drilled with lasers rather than diamond saw blades and drills that rapidly dull or wear out. Surface Hardening Applications The flexibility of laser delivery systems has made lasers very effective in selective hardening of steel surfaces, especially those subject to wear or fatigue. Although in such applications the heat generated by the laser at the surface is controlled to avoid melting, a steep temperature gradient is set up between the surface and the interior. Selective austenization (change from body-centered iron to face-centered iron) occurs at the surface, which transforms to martensite (a hard form of iron resulting from a diffusionless phase transformation) as a result of rapid quenching (self-quenching) through the conduction of heat into the workpiece. Because the process is all solid-state, no change in chemistry is produced at the surface by this laser transformation hardening. Laser transformation hardening is often used to harden the surfaces of automobile components such as camshafts and crankshafts. High hardness and good wear resistance with less distortion result from the process. Also, the laser method differs from induction and flame hardening in that the laser can be located some distance from the workpiece (American Society for Metals, 1991). Molian has tabulated the characteristics of 50 applications of laser transformation hardening. The materials hardened include plain carbon steels, alloy steels, tool steels, and cast irons. Because the absorption of laser radiation in cold metals is low, laser surface hardening often requires energy-absorbing coatings on surfaces (Molian, 1986). Industrial Lasers Market Perspective A special class of lasers, known as ''industrial lasers," has evolved to serve the needs of laser materials processing for manufacturing and exists as an industry in its own right. The United States once dominated this industry, but in recent years has dropped to a minority share (Box 5.2). The three main applications of industrial lasers today are (1) sheet metal cutting, (2) automotive welding, and (3) component marking and product coding. By mid-1995 more than 62,000 industrial lasers had been installed worldwide. The annual worldwide market for industrial lasers has grown to more than $400 million per year (Table 5.2), with the worldwide market for systems that use industrial lasers at approximately $1.5 billion, supplied by 500 separate companies that employ
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Harnessing Light: Optical Science and Engineering for the 21st Century FIGURE 5.9 The Boeing Video Measurement System as used during installation of an AWACS strut assembly. (Courtesy of R. Withrington, Hughes Aircraft Company.) four optical light sources and video detection to accurately position (~0.01 inch) the AWACS strut assembly on a 767 aircraft. The second instrument, the Laser Tracker Instrument, is a real-time coordinate measurement system for accurately mapping large structures. Typical operational range is more than 80 feet. The system uses amplitude modulators based on multiple-frequency semiconductor-lasers. Important uses of this instrument include verification of machine tool accuracy and profiling surfaces to archive engineering models. The third instrument, developed at Boeing, is a scanned laser template generator. Optical templates are used for locating and placing the cut edges of plys to produce ply dropoffs during composite component manufacturing. Planes of light, generated by a rotating laser head, are used for airplane body joint and wing alignment and for the generation of airplane interior reference planes during cabin outfitting. Crossed-fan laser beams are used to align a reference mark on a workpiece with a drill during machining.
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Harnessing Light: Optical Science and Engineering for the 21st Century It appears that aircraft manufacturing problems are the impetus for the use of specialized optical instruments specifically designed for an application as opposed to a particular existing optical technology or apparatus being adapted to this purpose. At McDonnell-Douglas and British Aerospace, new electronic theodolites have been very effective in equipment calibration with reported cost savings of more than 80%. Cost savings at Boeing are proprietary, but the installation of multiple units speaks to the success of the introduction of these optical techniques; active research and development on optical hole diameter measurements, surface profiling, wind tunnel instrumentation, and both cutting and welding indicate a considerable cost payoff in aircraft manufacturing. The Construction Industry New construction accounts for about 9% of the nation's gross domestic product each year. The industry includes the construction of residential and commercial buildings, factories, airports, tunnels, dams, landfills, environmental remediation systems, and a wide variety of other products. The annual market for construction is growing in the United States and worldwide, especially in the Pacific Rim and in Europe. Construction projects typically require the acquisition and assessment of a lot of data. Optical techniques can make this process faster and cheaper by reducing the need for expensive labor or making it more efficient. The use of optical methods in the construction industry is widespread but relatively straightforward. The techniques used fall into four categories: (1) optical systems incorporated into the final constructed product, (2) optical tools used in designing and building the product, (3) optical transducers that monitor activity or conditions at the construction site, and (4) optical elements that monitor the condition of finished structures over time. The optical systems now being incorporated into some buildings and other constructed products can be quite sophisticated. Natural and artificial lighting systems are designed not only to illuminate the interior but also to control the structure's heat loss and gain. They incorporate optical coatings on lights and windows, lenses and mirrors, heliostats, light tubes, and other elements. Illuminating systems are discussed in more detail in Chapter 3. Design and construction tools include optical image scanners, laser guidance systems for construction equipment (see Figure 5.10), geodetic measurements including fly-over and satellite-based mapping systems, and laser tools for precision cutting and welding of construction materials or monitoring of shifting structures and stresses. Laser guidance systems can be used to control the line and grade of tunnels, to control the blade elevation of grading equipment for site earthwork,
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Harnessing Light: Optical Science and Engineering for the 21st Century FIGURE 5.10 Laser-guided tunneling equipment. (Courtesy of B. Dorwart, Shannon and Wilson, Inc) and to control equipment for railroad track maintenance and highway repaving. Optical guiding can sometimes boost tunneling speed by as much as a factor of two, not only because fewer stops are needed for equipment realignment, but more importantly because straighter tunnels allow for faster and more efficient removal of waste material from the tunnel. Laser levels and targeting systems act as templates during construction by placing a spot or a line in an area to be excavated, eliminating the possibility that survey markers might be moved during excavation and often allowing replacement of an expensive survey crew by a less skilled person. Laser-guided tunneling has been accomplished from both tunnel ends, further reducing construction time. At the construction site, optical transducers are used for gathering engineering data and for geodesy (surveying), including making distance measurements, measuring positions, and monitoring physical and chemical properties that can be detected from changes in optical properties. For example, in building a highway or a railroad, laser-based equipment may be used to measure displacements, grades, and loads. The alternatives, such as manual, mechanical, or sonic techniques, are often slower, more expensive, and less accurate. In an environmental remediation project, optical monitors may be used for groundwater, air quality, or stack emission measurements. Here the competing technology is usually manual sampling and analysis in a laboratory, which is often expensive and slow, especially for subsurface measurements such as groundwater, tunnels, or utility pipes. Video systems and stereo photogrammetry are becoming increasingly popular for documenting conditions at the construction site and their change over time.
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Harnessing Light: Optical Science and Engineering for the 21st Century What are some of the industry's other needs that can be met by further developments of optical equipment or employment? Cost and ruggedness are key. Conditions on a construction site are often extreme, and equipment must be able to tolerate heat, cold, dust, humidity, and vibration. Equipment that requires extremely careful handling or a highly trained operator is unlikely to be accepted for construction use. Survey equipment has to be portable, repairable, or replaceable while in the field, and able to operate 12 hours on a battery charge. Monitoring equipment must be accurate, reliable, low power, and remote-sensing. For soil or water characterization, the most useful parameters to measure include pH, total dissolved solids, identity and concentration of hydrocarbons and chlorinated hydrocarbons, and turbidity. Inexpensive and accurate continuous monitoring of water pressure (to within 0.1 pound per square inch) and distance (to 0.01 mm accuracy over distances up to 1 km) would also be helpful. The quality of the laser spot—including size, clarity, steadiness, and roundness—is important for guidance systems, marking of transfer tools, and welding. Many of the barriers to more widespread use of optics in the construction industry are nontechnical. Equipment suppliers have difficulty finding qualified application engineers that understand the problems of the construction industry. Development has to be directed more closely to usable products, and education should be updated for modern equipment. Specific project goals are important drivers for the introduction and acceptance of optical adjuncts; for example, tunneling and surveying technology advanced tremendously as a result of the Superconducting Super Collider project. The Printing Industry The U.S. printing market, including such documents as periodicals, catalogs, newspapers, financial and legal documents, and greeting cards, represented about $7.5 billion in shipments in 1994, with real growth averaging about 4% per year. Compared to the 4% overall annual growth rate of the printing industry, digital production printing has been growing at 16.5% per year (Box 5.6). Manufacturers of commercial printing equipment are predominantly non-U.S. Based. BOX 5.6 GROWTH OF MARKET FOR OPTICS IN PRINTING The printing industry is large and growing steadily. The market for optical techniques in printing is growing at an annual rate four times that of the industry overall.
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Harnessing Light: Optical Science and Engineering for the 21st Century A strong drive toward shorter production times and just-in-time printing closer to the end user is forcing the printing industry to move from technologies such as traditional platemaking and printing to digital techniques, where the information to be printed is provided as digital input directly to the press. Figure 5.11 shows the current state and projected increase in use of digital technologies for a variety of printing applications. Digital platemaking, used for low-volume, high unit price applications, and digital printing, used for high-volume applications, are predominantly optical processes. Digital platemaking employs 25-to 100-W Nd:YAG, argon, or gallium aluminum arsenide lasers to expose traditional silver halide or a variety of photopolymers. Desktop digital publishing will continue to be dominated by inkjet printing and digital production printing by electrophotography. For digital platemaking, photopolymers currently offer resolutions of 2,500 pixels per inch at a processing speed of 8 square inches per second. Requirements for the next 10 years call for an increase in processing speed to 50 square inches per second. Current electroplating techniques offer faster processing speed, about 20 square inches per second, but the resolution is only 1,200 pixels per inch. For digital printing, desktop applications are anticipated to increase from 30 to 100 square inches per second within 10 years, with production applications increasing from 200 to 1,200 square inches per second. Resolution for both techniques is predicted to increase from 600 to 1,200 pixels per inch. From these figures, it can be seen that the primary opportunity to meet the projected requirements is an increase in pixel rates. However, the anticipation that the printing industry will be fully digital within 10 years offers a variety of opportunities for optics to contribute to the growth of digital printing technologies. Opportunities for optics include the development of higher-powered lasers for use with photopolymer plates, with emphasis on high-efficiency blue-green lasers, and imaging arrays to address the pixel limitations of current electronics. FIGURE 5.11 Market segments adopting digital print production technologies. (Courtesy of M. Fleming, Duplex Products, Inc.)
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Harnessing Light: Optical Science and Engineering for the 21st Century Increasing Use of Optics in Industrial Manufacturing The incorporation of optics and optical systems into industrial manufacturing can be divided into two broad categories: (1) optical systems in applications that improve on current practice, and (2) optical systems that provide a new capability. For example, laser levels and targeting systems for tunnel construction provided an improvement over mechanical theodolites in the ability to sight and construct tunnels; three-dimensional rapid prototyping enabled an entirely new method of constructing models without requiring the development of tools to build individual components. For each new situation, developers and users balance the rewards offered by the new technique against the risks inherent in inserting new technology. Because of the overriding importance of maintaining a controlled, reproducible process, incorporating new techniques and technologies into a manufacturing process is often avoided until cost or delivery time pressures from competitors compel manufacturers to change or until their current process is no longer capable of delivering the product with the performance needed. In the former case, manufacturers will often incorporate incremental changes into their process, as in the IC manufacturers' introduction of a chemically amplified photoresist in the 1980s to obtain smaller feature sizes with current photolithography equipment. However, the limits of the current generation of photolithography tools are approaching, and manufacturers are now contemplating the introduction of a new generation of equipment. Over the next few years, several key advances are expected in the use of light to perform manufacturing. In the area of photolithography, a new generation of deep-ultraviolet and extreme ultraviolet photolithography equipment and processes will have to be introduced to produce features in tomorrow's 16-gigabit chips. The use of excimer lasers will make it necessary to develop entire new families of polymer materials for use as photoresists at wavelengths of 193 nm and shorter. Many exciting advances are anticipated in the field of laser materials processing, where new laser sources will provide shorter wavelengths, higher beam intensity, and sharper focus. An exciting possibility is the use of adaptive optics to achieve true diffraction-limited resolution on arbitrarily shaped workpieces in environments with poor optical quality. The use of three-dimensional solid modeling for rapid prototyping and manufacturing will continue to expand as new solid-state, high-power, cw ultraviolet laser sources are developed and improved optics for beam delivery makes it possible to achieve submicron root-mean-square accuracy for surface roughness.
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Harnessing Light: Optical Science and Engineering for the 21st Century FIGURE 5.12 Oxygen is introduced into molten steel in a furnace through the four large holes in the tip of this lance (top). Behind the smaller central hole is optical sensor equipment (bottom) for measuring position and temperature. (Courtesy of the American Iron and Steel Institute and B. Fuchs, Sandia National Laboratories.) Major advances in the use of light to control manufacturing are also expected. Optical metrology should benefit from the development of smart sensors that incorporate data processing capability and from improved optical figure measurement techniques with 1-nm accuracy. Improvements in window and optical fiber materials will make it possible to use optical sensors to control manufacturing processes in increasingly hostile environments such as foundries. Figure 5.12 shows the recent successful use of optical sensors to measure temperature by submerging a probe in a bath of molten steel. Machine vision promises to increase its impact on manufacturing provided improved image processing and pattern recognition algorithms can be developed to make generic or plug-and-play solutions feasible. Summary and Recommendations Photolithography is the single most significant application of optics in industrial manufacturing. Submicron resolution, narrow-field-of-view photolithography is essential for the mass production of semiconductor integrated circuits, a major component of the U.S. and world economies. The resolution achievable by photolithography will continue to be improved as far as possible. Dimensions as small as 0.18 µm will be achieved in the near future using 193-nm excimer laser sources. To
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Harnessing Light: Optical Science and Engineering for the 21st Century achieve even higher resolutions, new technologies such as electron beam projection lithography and extreme-UV projection lithography are being developed. Key technical barriers to be overcome for improved photolithography include the development of new families of photoresist materials for extreme-UV and electron beam sources, development of practical high-resolution masks or of maskless photolithography systems, and realization of all-reflective exposure tools for use at wavelengths of less than 193 nm. Lasers perform a variety of materials processing operations as part of the manufacturing processes currently employed in a wide range of industries, including the semiconductor, aircraft, aerospace, construction, and automotive industries. These processes include cutting, welding, drilling, and surface hardening. Compared to conventional techniques, laser materials processing tools operate without physical contact, provide high localized energy density, and are truly versatile in their energy delivery. The United States once dominated the production of specialized industrial lasers to perform materials processing functions but now has only a minority share of the market. To aid in reestablishing U.S. leadership, the Precision Laser Machining Consortium was formed through a partnership of government, industry, and academia. Key technical barriers to be overcome for laser materials processing include improved optomechanics for beam delivery in adverse environments. An exciting possibility is the use of adaptive optics to correct for thermal and other aberrations in the beam path so that true diffraction-limited application and resolution can be achieved in manufacturing environments. An emerging optical technique for performing manufacturing with great potential importance is laser-based rapid prototyping. Solid three-dimensional structures can be created in several different media by using laser irradiation to build a solid design directly from the information stored on a computer-aided design tape, layer by layer. Among the techniques for using optics to control manufacturing, optical metrology is pre-eminent. Major uses of optical metrology systems include defect detection, inspection, measurement of product dimensions, monitoring manufacturing process conditions, providing real-time manufacturing process feedback control, alignment, and multidimensional measurement. Optical sensors play a major role in many diverse industries. In the chemical industry, the implementation of robust, noninvasive optical sensors has been found in some instances to double the productivity of an existing chemical plant. In the semiconductor industry, optical sensors are used for contamination-free manufacturing, adaptive process
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Harnessing Light: Optical Science and Engineering for the 21st Century control, and environmental safety and health. Optical sensors are employed in the construction industry to measure groundwater, air quality, and stack emissions as well as for geodesy. Because of the overwhelming importance of photolithography to the U.S. economy, government agencies such as DARPA, in concert with commercial alliances such as SEMATECH, have been motivated to play an essential role in providing high-level oversight and coordination in the development of U.S. photolithography technology. There is a need to continue this role to guide future technical progress. A particular requirement is new families of photoresist materials, especially for extreme-UV and other new lithography technologies. Advanced laser materials processing techniques are widely used in automobile assembly and other factories overseas, but less widely used in the United States. A factor that has undoubtedly aided the more rapid acceptance of laser materials processing in the foreign automobile industry is the greater level of emphasis that European and Japanese universities place on training engineers to be familiar with laser manufacturing techniques. The establishment of an application test facility in a service center setting in the United States would be particularly useful. The use of three-dimensional laser-based or other rapid prototyping tools should be investigated for manufacturing limited quantities of actual working replacement parts. Such a capability would have important logistic benefits for military and other mobile or remotely sited applications. Current problems in metrology for industrial manufacturing include the high-accuracy measurement of dimensions and position, the measurement of complex three-dimensional parts and surfaces, and the inspection of nanoparticle contamination on semiconductor wafers. Improved measurement standards and practices are needed for lithography systems, film thickness measurement, high-definition imaging systems, and colorimetry. More flexible applied optical metrology systems are necessary that do not require customization before each implementation. In the manufacturing environment, the frequent goal of machine vision is replace human inspectors and allow automatic adjustment and optimization of the manufacturing process, quality control, and inspection. Two major limiting factors are the poor performance of presently available image processing and pattern recognition algorithms, and the need for custom algorithms and hardware configurations for each specific task. Nevertheless, tens of thousands of machine vision systems have been used. For expanded utility in the chemical industry, optical sensors require improved optical configurations that can withstand extremely harsh environments involving corrosive materials at elevated temperature and
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Harnessing Light: Optical Science and Engineering for the 21st Century pressure. Standardized optical probes are generally not commercially available. The special need of the construction industry is for portable sensors that are rugged enough to survive field use. To preserve and enhance this critical technology base, coordinated government-industry-university activities are recommended in the following areas: A multiagency-supported application and test facility should be established in a service center setting using the DARPA-sponsored Precision Laser Machining Consortium as a model for extension of laser materials processing and other optically assisted manufacturing techniques. The National Institute of Standards and Technology should support development of optical metrology and machine vision systems with improved performance, with the ultimate objective of plug-and-play capability. References American Society for Metals. 1983. Pp. 647-671 in Metals Handbook, 9th ed., Vol. 6: Welding, Brazing and Soldering. Materials Park, Ohio: ASM International. American Society for Metals. 1989. Pp. 572-576 in Metals Handbook, 9th ed., Vol. 16: Machining. Materials Park, Ohio: ASM International. American Society for Metals. 1991. P. 265 in Metals Handbook, Vol. 4: Heat Treating. Materials Park, Ohio: ASM International. Azer, M.A. 1995. Laser powder welding: A key to component production, refurbishment and salvage. Photonics Spectra 29(10):122-127. Belforte, D. 1995. Presentation to the Committee on Optical Science and Engineering, October 12. Belforte, D. 1997. Belforte Associates. Personal communication to the Committee on Optical Science and Engineering, June. Bell, I., and N. Croxford. 1995. Fiber delivery gives YAGs an edge. Photonics Spectra 29(10):117-120. Brueck, S. 1995. Optoelectronic diagnostics for semiconductor manufacturing. Presentation to the Committee on Optical Science and Engineering, October 12. Dinda, S. 1996. Chrysler Corporation. Personal communication to the Committee on Optical Science and Engineering. Hu, S.J., and S.M. Wu. 1992. Identifying root causes of variation in automotive body assembly using principal component analysis. Trans. NAMRI 20:311-316.
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Harnessing Light: Optical Science and Engineering for the 21st Century Industrial Laser Review. 1997. User profile: Successful solid-state sheet-metal cutting, Vol. 12, No. 5, pp. 15-17. Levenson, M.D. 1993. Wavefront engineering for photolithography. Phys. Today 46:28. Mak, C.A. 1996. Trends in optical lithography. Opt. and Photonics News (April):29. Molian, P.A. 1986. Engineering applications and analysis of hardening data for laser heat treated ferrous alloys. Surf. Eng. 2:19-28. Moore, G.E. 1975. Progress in digital integrated electronics. IEDM Technical Digest 11. Photonics Spectra. 1995. Laser and light sources applications, Vol. 29, No. 10, p. 142. SEMATECH. 1997. Critical level exposure technology potential solutions roadmap. Available online at <http//www.sematech.org/public/roadmap/doc/graphics/lithoro04.gif>. July 22. Semiconductor Industry Association (SIA). 1994. National Technology Roadmap for Semiconductors. San Jose, Calif.: SIA. Weiss, Stephanie A. 1995. Think small: Lasers compete in micromachining. Photonics Spectra 29(10):108-114.
Representative terms from entire chapter: