SENSOR MATERIALS R&D OPPORTUNITIES
This chapter synopsizes the sensor materials R&D needs and opportunities that were identified in the examples presented in the four chapters of Part II: Chapter 3, "Selected Sensor Applications in Manufacturing"; Chapter 4, "Selected Sensor Applications for Structural Monitoring and Control"; Chapter 5, "LWIR"; and Chapter 6, "Chemical Sensors.''
These opportunities are illustrative of a process of matching technologies to needs to determine application opportunities and of matching needs to technologies to determine gaps in capabilities. They are not prioritized and are only representative of the cases examined by the committee.
SELECTED MANUFACTURING APPLICATIONS
Curing of Thermosetting Resins
Sensors currently used to monitor the curing of thermosetting resins have serious shortcomings. The principal opportunities lie in developing sensor systems that can measure, in real time, several different microstructural parameters of the curing materials without requiring a number of separate sensors. Such multiuse sensors would enable low cost intelligent processing of a range of polymeric composites.
Photon-scattering sensor technology has the potential to satisfy many of these needs cost-effectively. The near-term research opportunity involves extending this technology, using laser-fiber optics, to measure changes in several key parameters during cure such as resin temperature, resin viscosity, degree of cure, and amount of surface stress. The materials challenges result from applying this sensor system in a severe manufacturing environment.
Consolidation of Thermoplastic Resins
Void content sensing is currently the most common measure of laminate quality for thermoplastic structural composite parts. Void content can be measured many different ways, including x-rays, ultrasonic pulses, and local electrical or thermal conductivity gradients. Thermal conductivity, as measured with an infrared scanner, is an attractive option. The presence of voids affects the cooling rate of the newly laid down tape, and thus a thermal image of the part indicates void areas as hot spots. This sensor approach is noncontacting and does not interfere with the process.
A commercially available scanning array thermal imager of sufficient resolution may be suitable. The primary implementation issues arise from considerations of optical access and processing speed. The fiber optics must be able to conduct infrared energy without severe loss. Various geometrical and environmental constraints imposed by the process equipment configuration and operation will be of paramount importance in the sensor design.
To allow control of consolidation processes in real-time, both temperature and pressure must be accurately measured during processing. Temperature
could be measured by the infrared scanner mentioned above. Point measurements of pressure must be made at small intervals. A pressure-conductive matrix pad would be geometrically suitable and possess a fast enough response time; but its operating temperature range would have to be enhanced through use of temperature-resistant materials and/or coatings.
Manufacturing Integrated Circuits
Accurate control of rapid thermal processing and plasma deposition and etching requires advanced sensor materials and approaches. For example, the back surface emission technique for measuring and controlling temperature in today's rapid thermal processing equipment can lead to temperature errors as much as 200 ¹C. LWIR sensors could provide superior temperature measurement capability.
Improved sensors to monitor gas and chemical purity/cleanliness are of major interest. Gas analyzers, mass controller calibrators, chemically selective sensors, and particle detectors are all essential to maintaining the required process cleanliness. Environmentally conscious manufacturing requires recycling and reuse of chemicals, not only for waste minimization but also for cost reduction. Chemical generation and reuse will require sensors that can detect impurities at the part-per-billion level for on-line monitors of chemical purity.
Processing of Artificially Structured Semiconductors
A high-priority requirement is the development of noninvasive sensor technologies to sense and control the thickness (down to one atomic layer) of films, especially "superlattice" structures. The use of optical technologies (such as ellipsometry, laser-induced fluorescence, and fiberoptic probes) as an energy transduction medium is rapidly growing in capability and popularity.
A material application critical to all optical technology is the optoelectronic modulator, which serves as the interface between optical and electronic components. The material currently used in optical modulators is lithium niobate, which is extremely costly. A lower cost replacement represents a significant materials development opportunity.
SELECTED STRUCTURAL MONITORING AND CONTROL APPLICATIONS
Sensors constitute an enabling technology for this application area, although specific requirements are still evolving for many specific applications. For the near term, the critical materials issues are oriented at solving particular implementation problems.
Condition monitoring applications generally enjoy an abundance of available sensing options. The challenge is to assess these options against the requirements, so that the most appropriate selections can be made. Incremental improvements to these sensor technologies can involve materials improvements. An example is video spectroscopy used to examine hard-to-inspect structures for signs of corrosion. Flexible, broad-passband optical fibers would be required to allow all the measurements to be made effectively.
Life-cycle management (LCM) has several challenging sensor needs for in-service monitoring that are affected by sensor materials selection: very long sensor lifetimes with high reliability, and integration of the sensor with the product. LCM will provide a technology pull for incremental materials development efforts that are needed to solve the profusion of problems that arise as sensors are incorporated into structures.
LCM and smart materials have many similar sensor requirements. Fiberoptic sensors, both intrinsic and extrinsic, are very important, and their potential has yet to be fully exploited. Chemical sensors, possibly used in conjunction with fiber optics, have
significant potential. Piezoelectric polymers, ceramics, and composites offer advantage for strain measurements. Smart materials have the added requirement for actuation, and these piezoelectric materials can do "double duty." Active and passive taggants placed in structures offer the promise of low-cost sensing; their potential is only beginning to be explored.
LWIR sensor materials must satisfy a multitude of criteria in addition to their wavelength sensitivity. The performance of a sensor cannot surpass that determined through an understanding of a material's fundamental limitations, which include such factors as carrier interaction, absorption cross-section, and electron-phonon couplings, as well as material stability. In practice, maximum theoretical performance is rarely attained for large LWIR arrays due to the limitations of the materials synthesis and processing operations. For example, detector performance is extremely sensitive to defects and inhomogeneities that are not "fundamental" but which can be ubiquitous unless the materials growth and processing is extremely well controlled. Improvements in LWIR sensor materials thus depend to some extent on the development of LWIR and other sensor technologies that can be used for intelligent process control. This is particularly true for those processes that require atomic scale control during the growth process.
Cost is not a fundamental physical limit, but it does impose significant practical constraints. A key consideration is the selection of materials systems that are inherently robust and producible. In terms of operating costs, materials that provide sensors with a uniform response are preferable, as are sensors whose performance does not degrade over time.
Examples of specific materials R&D needs for the three LWIR photodetector materials systems (mercury-cadmium-telluride, multiple-quantum-well, and strained-layer-superlattice) are summarized below. In addition, LWIR bolometers offer attractive alternatives to photodetectors for certain applications, and the committee encourages continued development of those systems.
The quality of LWIR (MCT) has improved over the last decade, and continued incremental improvements may eventually yield temporally stable, uniform detector arrays. However, materials instabilities result in major materials and growth challenges. Very sophisticated materials-processing techniques are being employed to produce low-defect LWIR material. These processes can only be effective if there are sensors for in situ process control. Sensors that detect melt temperature and pressure of the constituent elements (especially mercury) during liquid-phase epitaxy processing are under development. Sensors developed for in situ process control can be applied to molecular beam epitaxy organometallic vapor-phase epitaxy and to metal-organic chemical vapor deposition processing.
Growth of MCT is made more difficult by a paucity of suitable lattice-matched substrates. The most commonly used substrate is CdTe, which is expensive and difficult to produce in large sizes needed for arrays. Development of a low-cost, producible alternative material represents a high-risk, high-payoff research opportunity. Another high-payoff, high-risk opportunity is the development of processing techniques for growth on nonlattice-matched substrates.
The background-limited performance of LWIR sensors based on multiple-quantum-well technology is lower than MCT. Arrays of GaAs/AlGaAs heterostructures exceed the performance MCT arrays for selected applications due to their superior response uniformity. Further improvements in detectivity of quantum-well detectors will require higher efficiency in the optical coupling of the incident radiation to the detector material and an increase in the carrier lifetime. Several promising approaches are under way to improve the optical coupling efficiency, but the definitive solution has not yet been found. At this time, it is not clear how to increase carrier lifetime, and thus this is an area requiring additional basic research.
From consideration of the fundamentals, this materials system offers many attractive advantages for LWIR applications. It is the least mature of the three technologies. Thus, substantial improvements in realized performance appear to be possible if adequate effort is devoted to the technology, since there appear to be no fundamental limitations. Major materials and processing issues remain to be resolved for the manufacture of low-cost, high-performance detector arrays in this materials system. For example, the development of surface passivation is extremely important to device performance.
Most chemical sensor applications have been based on a broad base of measurement principles and chemical reactivity developed through research in different branches of chemistry. Chemical sensor research has also adapted many fundamental ideas, devices, and materials from other sciences and technologies, such as: inexpensive optical fibers from the communications industry, lithographic patterning technology widely used in the manufacture of modern microelectronics, advanced piezoelectric materials, and ultrasensitive light detection using charge coupled devices.
Selective Direct-Reading Chemical Sensors
The most important materials-related opportunities to improve direct-reading chemical sensors involve the choice of materials employed to elicit stable selectivity of interaction with the target analyte. Table 6-4 summarizes materials needs for direct-reading chemical sensors.
Chemical Sensors with Sample Separation
Limitations of the existing chemistry or technology can become apparent at any stage during sensor development. Table 6-5 summarizes some key materials challenges for various chemical sensor technologies employing sample separation. The most frequent materials limitation for chemical sensors probably relates to the chemistry required to fashion an adequately selective response to the target analyte. One strategy to address this is the development of miniaturized high-speed separations-based sensors. These have the potential for avoiding difficulties in molecular selectivity but present major challenges in improving detector sensitivity.
Miniaturized, total analytical systems are a relatively new area of research in analytical chemistry, but their development could greatly supplement the capabilities of existing direct reading sensors. The numerous materials issues in designing and fabricating low-cost, miniaturized separations-based analytical systems include:
coatings and films with improved properties for enhanced sensor performance;
materials that enhance detector sensitivity and increase performance range;
fiberoptic materials with improved performance in the near-infrared and infrared spectral regions;
technologies for cost-effective miniaturization of sensor systems;
on-chip formats for practical applications of miniaturized sensor systems; and
chemical sensor systems with increased ruggedness, reliability, and control.
New materials can lead to improvements in the selectivity of direct chemical sensors. The development of fast, miniaturized chromatographic and capillary electrophoresis systems with detectors that are sensitive to chemical structure is important for environmental monitoring. The requirement to monitor a given analyte over a wide range of concentrations and in a variety of environments places particularly stringent requirements on chemical sensor sensitivity and selectivity. The need to meet regulatory requirements for monitoring toxins is an important driver in the development of environmental chemical sensors. Low cost sensor formats are particularly important for occupational environmental monitoring.
Detection of chemical weapons is a specialized type of environmental sensing. The following materials needs are important in developing suitable
candidate sensor technologies for these applications: fiberoptic coatings with improved chemical selectivity; new membranes and electrode coatings to obtain improved chemical selectivity with electrochemical sensors; and chemically selective films that undergo changes in mechanical and electrical properties following analyte sorption.
The possibility exists of leveraging generic miniaturization techniques, including materials and processing technologies developed for mass market applications, in order to further develop compact, lightweight, hand-held sensor systems for environmental and chemical weapons detection. Miniaturization techniques of particular interest include methods relating to supporting electronics and protective packaging.