These optical gauges are essential in the effort to improve the fit-up of automotive body panels to a gap tolerance within 2 mm see Figure 5. An OCMM can measure the dimensions of each automotive body produced. It can also rapidly measure as many as points on the body, providing high-dimensional multivariate data. By investigating the relationship among the various measurement points obtained by the OCMM, the sources of dimensional variation can be readily identified, and appropriate corrective actions can be taken.
The basic principle is cross-correlation among measurements. An OCMM measures critical points on the subassembly or component employing several sensors. During assembly, these points move together in some pattern. Analysis of the degree of correlation and relative variation in the pattern of these points e. The reasons for the dimensional variation can then be addressed to improve the assembly tolerance. This methodology has been successfully applied in automobile assembly plants to reduce variation and shorten the launch times for new body assemblies.
Some, such as laser ultrasound for nondestructive testing of solid materials, offer great potential, but their success will be determined only in the relatively distant future. Thermal imaging systems offer unique capabilities in the area of predictive and preventive maintenance, where hidden problems in machinery or defects in products can often be located and detected prior to failure due to their inducing some form of localized heating.
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Adapting semiconductor processing techniques for scribing, labeling, and trimming to the automobile industry will focus particularly on cost-effectiveness. Other technologies are likely to become important in the nearer term, such as those illustrated by the following three examples:. Rapid prototyping to reduce the time from design to production, now typically on the order of 4 years. The committee believes that the use of modern optics in automobile manufacturing will continue to increase but that the pace of introduction of these methods in the United States is slow in comparison with many foreign competitors.
Improved training in modern optical manufacturing methods is important for efficient insertion and technical development of these new applications. This training should involve all levels of the industry, from the assembler to the plant engineer to plant management to the user and field installation or repair sectors. The immense semiconductor integrated circuit manufacturing industry is powered by optics, optical systems, and optical materials. A modern electronic integrated circuit is a complex three-dimensional structure of alternating patterned layers of conductors, dielectrics, and semiconductor films.
This structure is fabricated on an ultrahigh-purity wafer substrate of a semiconducting material such as silicon or gallium arsenide. The speed and density of the devices is, to a large degree, governed by the size of the individual circuit elements. As a general rule, the smaller the elements are, the faster is the device and the more functions or operations it can perform per chip.
The device structure is produced by a series of steps used to precisely pattern each layer. The patterns are formed by the photolithographic processes discussed earlier in this chapter. For each processing step performed, there are a variety of measurement and inspection techniques many of them optical to ensure that the step was performed correctly. To meet the demand, the capacity for semiconductor chip manufacture will continue to increase. Photolithography is a key enabling technology for the industry. For example, a megabit chip requires about 21 photolithographic steps. In addition to photolithography, there are numerous applications of optics in inspection and process control.
The very first step in IC manufacturing, an initial inspection of the silicon wafers as they are removed from their packages, is performed by shining light at an oblique angle onto the wafer while viewing it from above. Surface debris or flaws will reflect light into the viewing lens and will appear in contrast as light or dark spots against the gray background of the clean silicon wafer.
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Previously, this inspection was performed by a human operator, who determined if the number of defects on the wafer was fewer than an acceptable number. This image analysis is now performed by an automated system, which will produce a defect map of each wafer inspected and will sort wafers without operator intervention into "acceptable" and "unacceptable" bins according to the user's criteria. This process is a very simple application of machine vision to integrated circuit manufacture. Similar inspections are made after almost all of the processing steps, from oxide growth to hin film deposition to etching.
Some of these inspections are automated, but many still require the image analysis powers of the human operator to accomplish because of the increasing complexity of the images as the wafers proceed through the processing line.
Another area in which optics and optical systems contribute greatly to manufacturing process control is in noncontact film characterization—for example, thickness measurements. Integrated circuit manufacturing. Controlling the thickness of many of these films involves empirically characterizing the deposition system by growing a set of films under different processing conditions e. For transparent films, thickness and other characteristics are routinely measured using ellipsometry.
For reflective films, a mechanical step-measurement system is more common. With each wafer lot processed, the film thickness is measured and processing conditions are adjusted controlled to maintain the correct film thickness for the subsequent lot. Diagnostic uses for optical sensors in semiconductor manufacturing can be thought of as addressing three areas of manufacturing: contamination-free manufacturing, adaptive process control, or environmental safety and health. Contamination-free manufacturing includes the detection and analysis of trace impurities, particles, and submonolayer surface contamination.
Adaptive process control includes the use of sensors and feedback systems for photolithography, dielectric film deposition, etching, and metallization processes.
Measurement and analysis of trace impurities and "smokestack" monitoring contribute to improved environmental safety. Common optical measurements used in semiconductor manufacturing include scatterometry for measuring critical dimensions, thicknesses, and sidewall profiles, and interferometry for measuring the alignment of upper layers with those below and for measuring temperature in rapid thermal processing applications.
Particles and contamination are commonly measured using infrared spectroscopy. These many requirements for measurement represent a tremendous impetus for the development of in-line process sensors for immediate feedback and process adjustment. To improve process yield and reduce defects requires sensors that are fast enough to measure and analyze pertinent process parameters during processing and that are as reliable as the processing equipment itself.
Along with the development of sensors that can work in the often hostile atmospheres required for IC processing, nonoptical requirements include increased computational power at the individual tools and throughout the manufacturing facility and improved modeling of manufacturing processes to identify the most effective measurements for process control Brueck, Optical systems that can replace the human operator for inspections or can provide in situ film thickness measurements offer great potential for integrated circuit manufacturing improvement.
The first step is to use optical lithography to produce the hin film transistor arrays and color filter arrays. Optical inspection is then used to monitor the quality of bare substrates, patterned substrates, color filter arrays, and the final display product itself. Diagnostic procedures during processing make use of optics for particulate control.
Ultraviolet light is often used to cure the seals of the liquid crystal cell. Finally, lasers are often used to locate and repair manufacturing defects. Because of the extensive use of such techniques, incremental improvements in the tools for optical lithography, optical inspection, and optical diagnostic procedures during processing have considerable leverage in manufacturing large-volume throughput and hence in the cost and performance of devices such as future flat-panel display products.
Optical inspection tools should be more intelligent, easier to use, cheaper, faster, and better able to operate in concert with other tools. Processing diagnostics must be able to count lower levels of particulate contamination and allow optical testing of starting materials both before and during manufacturing. In addition to these incremental changes, there are several ways in which new inventions in optical technology can have a profound effect on the display industry. For example, if optical inspection tools for display components could be made sufficiently intelligent, the need for costly and more error-prone human involvement in the inspection process could be further reduced.
Optical techniques for liquid crystal alignment could be developed that would diminish the yield-limiting rubbing step in the manufacturing process, or new optical components could be developed, such as compensation films or structured back-lights, that could significantly enhance display efficiency.
Optical diagnostic instrumentation, especially instrumentation for absorption spectroscopy and spectrometry, has long been a mainstay of the chemical industry Box 5. Off-line analytical laboratories were the primary users for applications such as quality control and trouble shooting. Today, optical techniques are used to provide information for process control in-line within the process tool and at-line within the processing area , which allows up-to-the-second evaluation of chemical streams during the manufacturing process, in turn allowing real-time process control.
This ability to maintain optimum operating conditions. Optical instrumentation for monitoring and control is critical to efficiency and quality in the chemical industry.
A key goal is the development of inline optical probes that can withstand adverse environments without degradation. The improved speed and sensitivity that have led to this expanded range of applications within the manufacturing line have been made possible principally by the advent of optical fibers, lasers, or other improved light sources, and new optical detectors, along with advances in computer technology.
In the chemical industry, optical techniques are critical parts of a multifaceted approach to chemical process issues to advance quality, quantity, customization, and reductions in waste while enhancing yield. This has resulted in part from excellent collaboration between industry and the academic sector, where optics have been integrated into process developments and design.
A concept known as Plant , being developed at the University of Washington, the University of Tennessee, and other centers, will use model-based control and on-line simulation to establish expert system supervision of the chemical stream. The use of optical probing techniques has expanded to include using laser diodes, optical fibers, improved dispersive elements, and detector arrays. Optical diagnostics now in use include laser Raman spectroscopy, fluorescence spectroscopy, measurement of turbidity and solids distribution by light scattering, critical angle refractive index measurements, and combinations of these various techniques to name just a few.
New areas will include video, microscopy, and others. These systems can be operated either at-line or remotely and are easily transportable. Optical fiber is key in the transport of information from the probe site to the analysis location Figure 5. However, current optical fibers are not robust in many chemical environments.
Therefore, a serious barrier to the expanded use of optics-related process diagnostics is the development of a robust probe capable of operating in the adverse environments of chemical systems. Window performance is also a critical element, because an appropriate window material can allow the use of less robust probes by providing optical access to a chemical process. Windows must frequently operate.
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Courtesy of R. Harner, Dow Chemical. Improvement to present technology will enhance productivity in the chemical industry. No advance in associated optical technology has been more important than the optical fiber, which both transports light energy into the system as a probe and transports it out of the system as a signal.
The implementation of optical sensors has been found in some instances to double the production output of an existing chemical plant. In other instances, chemical processes could not operate at all without real-time optical sensors. The use of such sensors is currently quite limited, however.