There are numerous potential uses for a microwave absorbent material whose absorption can be modified by an applied stimulus. This paper presents work to exploit the dependence of the microwave properties of ferrites on applied magnetic fields, and to develop such an absorbent material. Ferromagnetic resonance (FMR) is known to result in absorption of incident microwave energy at a characteristic frequency for any given ferrite. The FMR frequency is dependent on the anisotropy field H_an, which in turn can be modified by chemical doping. Therefore, ferrite RAMs can be produced to operate at required frequencies through judicious choice of the ferrite. Furthermore, if H_an is supplemented by an applied magnetic field H_app, the FMR frequency will be increased by an amount dependent on the strength of H_app and on the sense of anisotropy. The microwave permeability and permittivity spectra of a range of ferrites were measured as functions of H_app, using a vector network analyzer. These were used as input to a computer model for calculating absorber performance, and the effects of bias fields on a variety of absorber designs were predicted. A novel system was developed to apply large fields in test panels, for validating performance predictions.

In this paper, the feasibility of using electronically steerable antennas for health monitoring of civil structures and early warning of collapsing bridges to the approaching vehicles is presented. These antennas can also be used in automobile collision warning systems. These antennas are lightweight, low volume, low profile and conformal. They have low fabrication costs and are easily mass produced. They are thin and do not perturb the aerodynamics of a host automobile. Linear, circular, and dual polarization are achieved with simple changes in feed position. Beam steering is accomplished by varying the relative phase between radiating elements. In planar array, both horizontal and vertical beam can be combined to provide full scanning capabilities. Tunable ceramic phase shifters are used in these antennas. The dielectric properties of the ferroelectric material are changed by a bias voltage. In the case of health monitoring of civil structures, these antennas are used in conjunction with ferroelectric sensors. The sensors are fabricated with interdigital transducers printed on a piezoelectric polymer or ceramic type film. They are in turn mounted onto an ultra thin Penn State's novel RF antenna. The wave form measurements may be monitored at a remote location via the antenna in the sensors and the electronically steerable antenna outlined above.

Recent developments in smart skins technology at Northrop Grumman have paved the way toward incorporating avionics communication functions, previously provided by blade antennas, into the vertical tail of a military aircraft. Radio frequency communication link ranges can be significantly improved by structurally integrating the antenna radiating element into the tail region. Excitation of the large vertical tail surface improves radiation efficiency in the VHF-FM, VHF-AM, and UHF frequency bands. Analysis shows use of the whole tail region as an antenna would provide the best gain and coverage, however, confining the antenna design to the end cap region alone, also significantly enhances performance compared to blade installations. Near term technology applications to retrofit aircraft with minimum perturbations to the existing tail design are therefore possible. Multidisciplinary aspects of the application approach are discussed under the subheadings (1) antenna design, (2) structures and materials, (3) manufacturing, and (4) weight assessment along with the resolution of key technical road blocks. Finally, recommendations for further work necessary to transition the application to a production aircraft are discussed.

A multifunction structurally embedded antenna panel installation concept is presented which offers a near term solution to the increasing antenna proliferation problem of high technology aircraft. The concept has evolved from the Air Force sponsored program 'Smart Skin Structures Demonstration Program', recently awarded to Northrop Grumman, and is consistent with the Forecast II smart skin initiatives of the early 1990s. Key results from the concept evaluation phase of the program are described in terms of avionics integration and requirements, structural design considerations, and preliminary payoff assessment.

The tunable patch antenna configurations are becoming popular and attractive in many aspects. This was mainly due to the advent of ferrite thin film technology and tunable substrate materials. The integration of monolithic microwave circuits and antennas are becoming easy today. In the development of magnetic tuning of microstrip patch on ferrite substrate is presented by Rainville and Harackewiez. Radiation characteristics of such antennas are presented by Pozer. Band width and radiation characteristics of such tunable antennas are measured and compared. Usually the substrate losses are considered in the analysis and metallization losses are assumed to be ideal. The analysis of magnetic tunable radiator including metallization and ferrite substrate losses are presented. However, all such tuning and integration of circuits and antennas are mainly on ferrite substrate due to magnetic tuning. Recently, Varadan et al. established that the Ba_xSr_1-xTiO₃ series ferroelectric materials such as Barium Strontium Titanate (BST) are well suited for microwave phase shifter applications. It could be possible to change the dielectric constant of these materials more than 50% depending on the BST composition, by changing the applied bias voltage. Also, the porosity of BST can be controlled during processing to produce dielectric constants in the range of 15 to 1500, with some trade off in tunability. In this paper, we are presenting the possibility of designing a microstrip patch antenna on such tunable substrate. Such antennas are having the major advantage of electronic tunability and compact size.

Recently there has been considerable interest toward designing 'smart skins' for aircraft. The smart skin is a composite layer which may contain conformal radars, conformal microstrip antennas or spiral antennas for electromagnetic applications. These embedded antennas will give rise to very low radar cross section (RCS) or can be completely 'hidden' to tracking radar. In addition, they can be used to detect, monitor or even jam other unwanted electromagnetic field signatures. This paper is designed to address some technical advances made to reduce the size of spiral antennas using tunable dielectric materials and chiral absorbers. The purpose is to design, develop and fabricate a thin, wideband, conformal spiral antenna architecture that is structurally integrable and which uses advanced Penn State dielectric and absorber materials to achieve wideband ground planes, and together with low RCS. Traditional practice has been to design radome and antenna as separate entities and then resolve any interface problems during an integration phase. A structurally integrable conformal antenna, however, demands that the functional components be highly integrated both conceptually and in practice. Our concept is to use the lower skin of the radome as a substrate on which the radiator can be made using standard photolithography, thick film or LTCC techniques.

This paper will describe a new approach to the integration of electronics with MEMS, or Smart MEMS. Flip chip solder bumping of integrated circuits is routinely used for packaging purposes and has now been extended to the placement of electronics in close proximity to MEMS devices. The flip chip approach separates the fabrication of the MEMS and electronic devices, allowing both the IC's and MEMS to be fabricated of many different substrate materials, not just single crystal silicon. The close proximity of the electronics to the MEMS devices is very desirable to improve signal to noise performance, and provide higher levels of systems integration. This new approach provides batch fabrication capability as opposed to the serial hybrid approach, without having to fabricate the electronics and MEMS on the same chip. Results on the attachment of surface micromachined structures to glass and silicon substrates will be reported.

There has been an increasing need for the design and fabrication of large (few millimeters) MEMS made of micron-size features for a variety of applications. Mechanical design of large MEMS structures needs careful consideration of the forces developed during and after processing to ensure their survivability and reliable performance. forces generated to actuate a device, e.g., capacitive force, are well characterized. However, the forces developed due to conditions of processing, such as fluid forces, have not yet been fully characterized. In this paper, we propose simple models to estimate thermal and intrinsic stresses and the corresponding deformations of large MEMS, as well as fluid forces during wet processing. We also discuss the issue of stability of large MEMS during actuation.

This paper describes the use of MEMS in smart structures. A smart skin concept is described in which an array of sensors and actuators may be incorporated onto the surface of a structure. A hear flux sensor array concept is described, and shear stress sensors are presented as examples of MEMS devices for structure surfaces. MEMS devices may also be embedded into a structure. For example, pressure and temperature memory sensors are presented for a smart-tire application, in which the MEMS device may be embedded into the tire to provide tire-monitoring capabilities.

Future advanced fixed and rotary-wing aircraft, launch vehicles, and spacecraft will incorporate smart MEMS devices to monitor structural integrity and manage overall structural health. These smart structures will be capable of assessing vehicle structural damage in real time and ultimately reconfiguring the vehicle flight control to prevent catastrophic failures. This paper describes an overview of Honeywell's MEM technology for new and aging aircraft applications to assess preflight readiness, in- flight structural integrity, and postflight time-based maintenance. Honeywell's MEMS approach combines silicon micromachining, free-space optical waveguides, high-speed optical interconnects, and supervisory sensor management to monitor structural integrity and health. A unique second-generation polysilicon resonance microbeam sensor is described. It incorporates a micron-level vacuum-encapsulated microbeam to optically sense structural-integrity parameters such as acoustic- mission, strain, and to optically power the sensor pickoff. Its principle of operation and significant payoffs and benefits are summarized.

A remote local and global sensing and control of aerospace structures using advanced polymeric smart materials, MEMS, and built-in antennas is presented. The sensors are fabricated with interdigital transducers printed on a piezoelectric polymer. They in turn are mounted onto an ultrathin Penn State novel RF antenna (patent filed). The sensors are designed to measure both pressure and shear of the fluid flow on aerospace structures. The wave form measurements may be monitored at a remote location either at the cockpit or elsewhere via the antennas in the sensors and an outside antenna. The integrated MEMS actuators, which are comprised of cantilever-, diaphragm-, and microbridge-based MEMS with suitable smart electronics etched onto the structure, are controlled by the built-in antennas through feedback and feedforward control architecture. The integration of such materials and smart electronics into the skin of airfoil is ideal for sensing and controlling drag. The basic idea of this concept involves detection of the point of transition from laminar to turbulent flow and transmitting acoustical energy into the boundary layer so that the low-energy fluid particles accelerate in the transverse direction and mix with the high energy flow outside of the boundary layer. The use of the present smart materials and electronics for active noise control and EMI suppression in aircraft and helicopters is also outlines.

Silicon micromachining is, at the present time, the tool of choice for the fabrication of microelectromechanical systems (MEMS) and, in general, miniature devices. Silicon micromachining techniques include some of the basic processing steps of IC technology especially plasma etching for the fabrication of submicron structures. However, inherent to plasma etching are damage effects to Si that can negatively influence the MEMS device performance. In this study we have explored the effects of conventional reactive ion etching (RIE) and magnetically enhanced RIE (MERIE) on the properties of Si exposed to fluorocarbon based oxide etch chemistries. By using combinations of spectroscopic ellipsometry (SE), secondary ion mass spectrometry (SIMS), and Schottky diode current-voltage-temperature (IVT) measurements, we show that damage in the form of thin layers of residual polymer and heavily damaged silicon can be induced by plasma exposure on the Si surface. SE and IVT measurements have shown that the thickness of the heavy damage layer decreases with magnetic field. This layer is proposed to comprise amorphous Si and voids and the density of the latter increases with the magnetic field. The polymer residue layer, detected by both SE and SIMS on the plasma-exposed Si surface, is observed to be thinner the higher the magnetic field.

Recent development of silicon micromachining technology has made possible the fabrication of many micromechanical devices. Applications of these micromechanical devices are many, but their use for smart structures and materials has just begun. Here, an updated report on the development of a drag-reducing smart skin is given. In order to facilitate the fabrication of the smart skin, we have first developed a new sacrificial-layer etching model for etching phosphosilicate-glass using hydrofluoric acid. This model then leads to the development of two key devices for the skin, including a shear-stress sensor and a magnetic microactuator.

An overview of the major sensor and actuator projects using the micromachining capabilities of the Microelectronics Development Laboratory at Sandia National Laboratories will be presented. Development efforts are under way for a variety of micromechanical devices and control electronics for those devices. Surface micromachining is the predominant technology under development. Pressure sensors based on silicon nitride diaphragms have been developed. Hot polysilicon filaments for calorimetric gas sensing have been developed. Accelerometers based upon high-aspect ratio surface micromachining are under development. Actuation mechanisms employing either electrostatic or steam power are being combined with a three-level active (plus an additional passive level) polysilicon surface micromachining process to couple these actuators to external devices. The results of efforts toward integration of micromechanics with the driving electronics for actuators or the amplification/signal processing electronics for sensors is also described. This effort includes a tungsten metallization process to allow the CMOS electronics to withstand high-temperature micromechanical processing.

Integrated Force Arrays (IFAs) are thin, flexible, metallized membranes which may be configured as actuators or sensors. The current prototype structures are approximately 1 cm long by 1 mm wide and designed for deformations of 2 mm. In this paper we will discuss how the devices may be scaled-up for extended range and force.

The Semiconductor Research Corporation (SRC), now entering its thirteenth year, is implementing an innovative form of industry—university cooperation in research that is directed to integrated circuits. SRC's mission is to plan, promote, coordinate and sponsor research that will result in: (1) new knowledge of semiconductor materials and phenomenon, and of related scientific and engineering subjects that are required for the useful applications of semiconductor I.C.'s; (2) the development of new and more efficient design and manufacturing technologies for semiconductor devices; (3) timely and efficient transfer of the results of the research to its industrial members; (4) an increase in the number of scientists and engineers who are proficient in all aspects of semiconductor technology, design and manufacture. The era of microelectronics started with the invention of transistors in the 1940's; but the revolution was really ignited with the invention of the integrated circuit in the 1950's. Since that time, the product development has been characterized by explosive world-wide growth. The SRC is a natural derivative of this global growth.

One feature of a smart structure implies that some computational and signal processing capability can be performed at a local level, perhaps integral to the controlled structure. This requires electronics with a minimal mechanical influence regarding structural stiffening, heat dissipation, weight, and electrical interface connectivity. The Advanced Controls Technology Experiment II (ACTEX II) space-flight experiments implemented such a local control electronics scheme by utilizing composite smart members with integral processing electronics. These microelectronics, tested to MIL-STD-883B levels, were fabricated with conventional thick film on ceramic multichip module techniques. Kovar housings and aluminum-kapton multilayer insulation was used to protect against harsh space radiation and thermal environments. Development and acceptance testing showed the electronics design was extremely robust, operating in vacuum and at temperature range with minimal gain variations occurring just above room temperatures. Four electronics modules, used for the flight hardware configuration, were connected by a RS-485 2 Mbit per second serial data bus. The data bus was controlled by Actel field programmable gate arrays arranged in a single master, four slave configuration. An Intel 80C196KD microprocessor was chosen as the digital compensator in each controller. It was used to apply a series of selectable biquad filters, implemented via Delta Transforms. Instability in any compensator was expected to appear as large amplitude oscillations in the deployed structure. Thus, over-vibration detection circuitry with automatic output isolation was incorporated into the design. This was not used however, since during experiment integration and test, intentionally induced compensator instabilities resulted in benign mechanical oscillation symptoms. Not too surprisingly, it was determined that instabilities were most detectable by large temperature increases in the electronics, typically noticeable within minutes of unstable operation.

A demonstration of the capabilities of an active AlGaAs based 1 X 10 photonic coupler is presented along with an Optically Coherent Frequency-Domain Reflectometer (OCFDR) Fabry-Perot demodulation system. The increased capability of the active 1 X 10 photonic coupler allows for the simultaneous multiplexing and demodulating of up to ten parallel Intrinsic Fabry-Perot Interferometric (IFPI) fiber optic sensors. A demonstration of the amplification and switching ability of the photonic device is also presented along with a comparison of the active photonic device with current polarization preserving fiber optic couplers. Future developments and improvements of photonic devices and their effect on future fiber optic sensors systems will also be discussed.

Recent advances in ferroelectric and high temperature superconductive technology have enabled new phase shifters incorporating ferrite, ferroelectric, and high temperature superconductor materials to be developed. Each material offers important and unique properties which may be combined with one of the other materials to obtain useful phase shifters. Ferrite materials with tunable magnetic permeabilities have classically been used for phase shifters. Ferrites offer large phase shifts with small changes in insertion loss. Ferroelectric materials employing variable dielectric constants can also be used as phase shifters. One advantage of ferroelectric phase shifters over ferrite phase shifters, ultra-wideband performance, enables coverage of the entire band from 2 to 18 GHz. Superconductors offer low losses and unique properties including the kinetic inductance effect and SQUIDs. The physics of the three materials will be reviewed briefly, and the difference between ferroelectricity and ferromagnetism will be discussed. Combination of magnetic and superconducting materials is possible, despite the fact that magnetic fields suppress superconductivity. A comparison of variable time delay and phase shift devices will show interesting contrasts between these devices and MMIC components. Superconducting phase shifters using only superconducting properties, and phase shifters combining superconductivity with ferrites or ferroelectrics will be described.

Two methods for monitoring structural integrity with microwave transmission-line reflectometry are described. The first approach uses a meandering RF transmission line, while the second consists of a bounded dielectric region such as a parallel-plate waveguide. Either of these devices can be embedded in or adhered to a composite or metal panel of the structure. A microwave signal is introduced on the transmission line and produces a reflection at any discontinuity as the wave propagates. This creates a signature reflection wave whose salient features can be stored for future reference. If the substrate subsequently becomes flawed, an extraneous reflection is created in the vicinity of the flaw. By comparing this reflection with the signature, the presence and location of both new and progressive laws can be determined. Experimental verification of the second technique for the detection of flaws is presented.

Phase shifters are performing very important role in the performance of electronically steerable phased arrays. In this paper we have demonstrated the performance of microstrip line and planar phase shifters on (Ba-Sr)TiO₃ (BST) substrate. It requires very low drive power and has low production cost. Because of the large tunability of the dielectric constant of the BST materials with the dc bias voltage, these materials are suitable for the electronically steerable antenna applications. The performance of the filter phase shifter and microstrip line phase shifter are evaluated and it is observed that the filter phase shifter shows better performance compared to the microstrip line phase shifter.

High rate of areal density growth rate enables the reduction in cost per MB and increasing demand for data storage. Micromechanics is soon likely to be needed to accommodate the increased mechanical position precision needed for reading and writing data. Two examples will be described. A microactuator can be used as the fine actuator of a two-stage actuator servo system for very high bandwidth magnetic head slider positioning. This device is batch-fabricated metal structure using high-aspect-ratio lithography and stencil plating. The fabrication process and characteristics will be described. As a second example, micromachined atomic force microscope (AFM) probes are used to generate and detect fine pits on the surface of a polymer disk. An areal density of 25 Gb/in² is achieved with a data reading rate of above 1 Mb/s. A very low mass (0.3 ng) silicon nitride AFM probe has been used in this study, and the fabrication and performance are described.

Micro riblets have been designed and fabricated by surface- micromachining technology with 3-layer polysilicon and 2-layer PSG. The riblets with rib-peak-widths of 3, 4, 6 micrometers and spacings of 100 micrometers and 300 micrometers have been chosen. Several more complicated surface patterns like fish scale replica, have also been designed. On the same chip, the micromachined hot film shear stress sensor with different geometries (6 X 100, 12 X 100, 18 X 100 micrometers ², etc.) are integrated downstream from the micro riblets. The sensors are made of polysilicon and used for the shear stress measurement of the fluid flowing over micro riblets.

The phased product development approach can be applied advantageously to develop and manufacture automotive microsensors. The phased approach involves a multifunctional team from innovation to development to eventual production and maintenance phases. The key advantage of this approach is the shortened development cycle and fast product introduction, while minimizing waste of resources and lowering risk of product failure. When applied to the product cycles of automotive sensors based on micromachining technology, this approach elucidates several critical considerations. In particular, since industrial application of micromachining technology is still at the infant stage, standards and design rules are not firmly established. Therefore, several important activities must be initiated simultaneously from the start of the innovation phase, which proves to be crucial to the prudent decision of technology alternatives and sensor system configuration. The use of a multifunctional team, as mandated in the phased approach, enables coherent development and optimization of the sense element, the fabrication technology, the packaging approach, the interface circuit configuration, and design features that allow efficient test and assembly flow. Also, with intermediate milestones within each phase, risk assessment and necessary midcourse adjustment to technology trade- offs can be both timely and accurate. Accelerometers, one of the most developed micromachined sensors, serve as representative examples that illustrate how the phased approach can benefits the commercialization of the newly established and rapidly expanding field of micromechanics.

A surface micromachined pressure sensor array is under development at the Integrated Micromechanics, Microsensors, and CMOS Technologies organization at Sandia National Laboratories. This array is designed to sense absolute pressures from ambient pressure to 650 psia with frequency responses from DC to 2 MHz. The sensor is based upon a sealed, deformable, circular LPCVD silicon nitride diaphragm. Absolute pressure is determined from diaphragm deflection, which is sensed with low- stress, micromechanical, LPCVD polysilicon piezoresistors. All materials and processes used for sensor fabrication are CMOS compatible, and are part of Sandia's ongoing effort of CMOS integration with MicroElectroMechanical Systems (MEMS). Test results of individual sensors are presented along with process issues involving the release etch and metal step coverage.

The integration of electronic logic and processing circuits of various complexity with sensing and actuating elements is needed for the realization of 'Smart' structures. Direct bonding techniques relies on topotoaxial integration of two dissimilar materials, one in thin film and the other in bulk forms. These techniques can deliver new smart structures by combining sensing and/or actuation components with high complexity logic circuits while preserving their performance. Integration of ferroelectric ceramics, multiple quantum well modulators and lasers with Si and GaAs electronic circuits is already demonstrated. Potential future implementation of the direct bonding technique will include the bonding of various electronic circuits to nonplanar, flexible and deformable substrates and combination with micromechanical structures.

A sensor fabricated from microminiature free-standing structures which is capable of simultaneous measurements of several different inputs in real-time is described. The small thermal mass and good thermal isolation of free-standing structures have been used to advantage for sensing infrared radiation, ambient pressure and gas flow. The sensory element in all of these detectors is a microthermopile. The hot junctions of the device are made of free-standing wires whereas the cold junctions are thermally attached to the substrate. Energy dissipated in the microthermopile causes a rise in the temperature of the hot junctions relative to the cold junctions and thus produces a thermovoltage across the device. Monitoring the thermovoltage caused by the absorption of incident infrared radiation has resulted in a fast and sensitive thermal infrared detector which can be used for noncontact temperature measurements. For small temperature differences from ambient, the rise in the temperature of hot junctions is determined by the magnitudes of conductive and convection heat losses from the free- standing wires and therefore is a function of the ambient pressure, gas composition and gas flow. These dependencies have been used for sensing pressure, flow and gases. The sensors made from free-standing structures can be monolithically integrated into a sensor microsystem because the techniques used in their fabrication are compatible with silicon microfabrication technology. It should therefore be possible to integrate these sensors with active electronic circuits to make a smart microsystem.

Integrated circuit receivers have been built and tested for applications at millimeterwave frequencies. The receivers utilize a coplanar waveguide-fed double folded-slot (DFS) antenna on a high-resistivity silicon substrate lens. The 22 GHz receiver couples the antenna to a 6- stage MESFET followed by a planar Schottky-diode detector. Measurements of the system performance show a video responsivity close to 1 GV/W and a radiometer sensitivity of 3.8 (mu) V/K. The active MIC receiver can be integrated monolithically and is readily scalable to W-band frequencies. The 94 GHz double folded-slot antenna exhibits excellent millimeterwave patterns, demonstrating the suitability of the DFS for use at W-band. The application areas for this type of receiver re in low-cost millimeterwave linear imaging arrays for remote sensing, automobile collision-avoidance and limited-visibility aircraft landing systems.

Ferroelectric materials are nonlinear dielectrics, having a dielectric constant which is a function of the electric field. The nonlinear behavior of these materials makes them candidates for the realization of advanced high frequency devices which can operate up to the millimeter range. Ferroelectrics have been successfully employed in many optical devices, but their application at high frequency has been limited, mostly due to the large bias voltage required to significantly change the electrical properties of the bulk material. However, today there are several new techniques available to produce ferroelectric thin films and thin ceramics which only require low or medium bias voltage. These processes open the way for development of a new family of devices which are fully compatible with conventional analog and digital electronic circuits. Use of tunable dielectric material finds applications in many new devices including tunable phase shifters, tunable filters, and tunable beam scan antennas. High quality thin films and thin ceramics of PbTiO₃ (PTO), BaTiO₃ (BTO), Ba_xSr_1-xTiO₃ (BST), and Pb_xCa_1-xTiO₃ (PCT) have been produced and characterized. A 2.5 GHz tunable phase shifter, realized using BTO in microstrip configuration, is proposed and tested.