Radar and sonar applications are signal-processing intensive and heavily rely on the efficient implementation of such digital signal-processing (DSP) algorithms as filtering, transforms and modulation. In past systems, conventional digital signal processors were used to perform many of these algorithms. However, field-programmable gate arrays (FPGAs) deliver an order of magnitude higher performance than traditional DSPs. A key reason is that an FPGA can side step the classic Von Neumann architecture's instruction — fetch, load/store bottleneck — found in most DSPs. Another reason is the FPGA's lower power consumption.
When approaching the problem of implementing signal-processing functions within an FPGA, designers have developed the mindset that these functions must be optimally coded from the ground up for their application or significantly modified. However, silicon-optimized, high-precision math functions are being developed for specific applications as part of the programmable logic product offering of many vendors, including Altera, making complex systems easier to manage and lower risk.
Changing requirements in radar applications
Modern military radar systems have evolving requirements, both in how the systems are designed and how the end user uses the data. This results in some of the same design changes in electronic systems affecting both the military and commercial design communities. That is, the need for smaller, energy-efficient systems with high processing-power requirements. This makes low power consumption a key driver in most designs.
With warfare having become more urban, ground clutter and background noise take on additional significance for the radar operator, thus demanding more processing power and better algorithms. Overlaying data from multiple sensors and known terrain features is one approach to increasing resolution, but this too has impacts on system-processing requirements and user-decision models.
High-speed digital systems make new digital beam-forming technologies possible, increasing the number of beams and nulls available for warfighting and surveillance missions. More digital logic also allows designers to make early decisions on actionable intelligence and to meta-tag sensor data earlier for more efficient analysis. These and other emerging techniques will allow for the creation of better radar or sonar systems, but each requires additional signal-processing resources.
One of these resources is the emerging class of high-performance FPGAs. One of the primary differences in the past between FPGAs and application-specific integrated circuits (ASICs) has been greater complexity in the latter class of devices. However, with the 65 nm generation of FPGAs and 45 nm devices on the horizon, FPGAs in sensor systems have become nearly as complex as ASICs. This complexity comes from rapidly increasing logic density, as well as from the integration of the many different processing functions now integrated into one device.
Modern approaches to radar DSP
As FPGAs increase in density and performance capability, more signal-processing functions can be incorporated and migrated to the front end containing the exciter/receiver of the radar (or sonar) system. This may include waveform generation, filtering, matrix-inverse operations, and signal correlation.
A representative multi-element radar element is shown in Figure 1, with multiple signal-processing and beam-forming elements represented in a single logic element. The design of this single FPGA quickly becomes complex, particularly if the beam-forming algorithms allow for multiple beams and nulls in the active array.
An all-FPGA design for signal processing
Fitting multiple DSP functions into a single FPGA has many integration challenges, but also offers significant advantages to the designer in performance and flexibility.
The primary reasons for integrating DSP functions into a single FPGA are system-level reductions in size, weight and power. For example, eliminating the transfer pathways between separate FPGAs and DSPs significantly reduces power consumption and, therefore, heat. This, in turn, reduces the system-cooling burden of the design. Recent releases of design and place-and-route software, such as Altera's Quartus II design suite, have advanced power-awareness features that significantly reduce dynamic power use of the FPGA. These options can be important to the designer; the benchmark of device logic density among competitive FPGA providers is beginning to give way to functionality-per-watt metrics, due to the sensitivity of power and cooling requirements in emerging systems.
Performance is also a key driver as FPGA-pipelined signal processing has become more reliable and faster than traditional processing technologies. In applications where performance is the driving parameter, efficiency can be sacrificed for application speed, where a memory-intensive, massively parallel floating-point math operation is desired. Alternatively, highly iterative DSP calculations can be implemented for applications where moderate performance is allowable, but where logic-element usage is limited.
This leads to the advantage of flexibility. The designer has the flexibility to decide between high-speed performance and the number of logic elements in every DSP operation, whereas calculation bandwidths and iterations would be more difficult and costly to modify in a dedicated DSP device. In addition, consolidating DSP functions within an FPGA allows for post-design system changes in the signal-processing architecture, whereas using separate DSPs locks the designer into a fixed set of chip interfaces once the board is designed. FPGA designers can alternately switch between 9-bit, 18-bit or 36-bit or 18-bit complex math functions without changing the system hardware.
Additional flexibility can be designed into the system when the designer uses fast-embedded processors for the execution or routing of complex floating-point operations. These functions are useful for radar applications.
FPGA DSP functions in radar/sonar applications
Several DSP functions are needed for radar or sonar processing near the receiver element. Each function should be closely examined to determine whether the application will show substantial speed and performance improvements through implementation in an FPGA. In some cases, these operations can be efficiently implemented using an FPGA embedded processor, even for highly complex and adaptive operations.
When a radar or sonar application calls for these operations to be performed with floating-point arithmetic, FPGAs have significant flexibility advantages if the design team takes advantage of a strong architecture-based design approach. Large floating-point math operations can be performed in standard logic cells (the least efficient option), in dedicated reduced-instruction-set-computer (RISC) embedded processors (the most flexible option), or in dedicated floating-point multiplier logic (the most efficient option).
FPGA providers and third-party developers offer efficient and accurate floating-point operators, Fourier-transform tools and filter compilers to FPGA designers as intellectual property (IP). Engineers should conduct their own research on the current availability of advanced DSP functions, but a great deal of preliminary information can be obtained through the technical representatives of programmable logic device (PLD) vendors.
Digital up/digital down signal conversion
The upconversion and downconversion of high-frequency signals are experiencing a dual migration, into the digital domain, and into the same monolithic device (either the ASIC or FPGA) that performs the baseband processing. This push toward more digital, software-radio-style signal-processing techniques provides significant advantages to the system in signal accuracy and speed.
The closer to the RF front end (or the acoustic transceiver front end in sonar systems) that signals can be digitized, the fewer the analog-signal vulnerabilities that are introduced to the system. This includes high-order mixing products, error-vector-magnitude (EVM) impairments due to phase/magnitude imbalance, carrier feed-through, harmonics, and sideband noise.
More important than signal integrity, however, is the design flexibility that the digital domain allows the radar-system designer. Dynamic filtering and conditional signal-processing algorithms significantly improve performance, as well as reduce implementation losses and the time required for the design cycle. While these advantages involve trade offs between power consumption and digital bandwidth, modern FPGAs provide designers much greater flexibility in mitigating power consumption, including the support of selectable core voltages, or critical-path power analyses.
The greater the numbers of on-chip resources available in FPGAs, the more designers are enabled to incorporate polyphase filtering and downconversion in the digital domain, as reflected in Figure 3. Multiple onboard or external numerically controlled oscillators (NCOs) can allow very high phase discrimination with high-capacity FPGA devices. This application is useful for prototyping, research and development, where designers can incorporate and test multiple-phase resolutions without significant hardware investments by using hardware-in-the-loop test methodologies.
Algorithmic functions
Examples of algorithmic math functions in radar systems include recursive least-square and square-root operations. Many designers have implemented these functions in C-based processors (in fixed-decimal and floating-point operations), or with proprietary FPGA VHDL operations. The current generation of FPGA devices include embedded processor and logic-cell resources to efficiently implement these processes; future generations will also have these capabilities. Additionally, IP cores and reference designs are becoming available to transition anywhere from dozens to hundreds of these operations into a single FPGA.
Tools are available to translate processor-based algorithms from C code to hardware languages, such as very high-level descriptive language (VHDL). These tools can be used to optimize certain logic functions from a standard main processor into an FPGA co-processor operating in parallel with the main processor, or to move entire operations from the main processor to the FPGA hardware. This provides an additional dimension of flexibility to the radar- or sonar-architecture designer's toolkit.
Complex matrix inversion
Matrix inversion is an important element of adaptive-array designs and standard spatial-transceiver-array processing (STAP). These operations are commonly performed in fixed hardware elements, though efficiently implemented embedded processing has been demonstrated in some radar/sonar development programs.
The logic-element size and potential parallelism of a matrix inversion engine depends on the size of the array used in the radar system. As the size of the array is increased, so does the number of floating-point multiplications required by the system. Therefore, in larger arrays, there are more trade-off options between the speed of the system and the number of logic elements required by the system (both of which increase as the parallelization of the architecture increases).
Implementing this function using a combination of a DSP and a group of internal memory blocks is the most likely design path for radar-system designers. As these operations are often tailored to the adaptive-array algorithms of the radar system, they are likely to be custom designed in VHDL. However, reference designs that are optimized for the place-and-route capabilities of an FPGA device can be offered or designed to order from the FPGA manufacturer, if required for the radar or sonar system.
Fast-Fourier transforms
The bandwidths of many systems, including radar/sonar and test/measurement systems, are beginning to exceed the capabilities of dedicated DSPs. Implementing fast-Fourier transforms (FFTs) and their inverses in FPGA logic has advantages in prototyping and scalability, and offers design flexibility between a system's speed and the number of required logic elements. For example, massively parallel implementations can be designed and distributed among the logic elements of a single or multiple FPGAs. However, while these implementations can significantly reduce latency, they impose the penalty of a greater number of logic elements.
In fact, the primary flexibility advantage of an FPGA for FFTs is the ability to select the optimal balance between these two parameters in the initial design. This is fortunate, because the implementation of large or complex FFTs should be the primary factor in any design, and the advantages of an FFT implementation in an FPGA (Figure 4) are apparent. However, creating code or modifying existing code from previous designs can be cumbersome when testing and verifying code units. Therefore, what is needed is a comprehensive suite of FFT design tools that allows a nearly infinitely scalable FFT design. These tools should allow scripted logic distribution among multiple FPGAs where necessary. They should also be able to automatically generate numerical coefficients having floating-point accuracy. Customer inputs are being taken now for such tools.
Because radar, sonar and digital-communication system designers must focus on the complications of multi-element beam-forming and waveform generation — not FFT design — programmable logic vendors such as Altera have internal tools and generators for conducting large, difficult element transformations. This includes reference designs and core IP wizards for standard and non-standard designs, as well as FFT co-processors, which are important design aids in the programmable logic offering.
Design flow
DSP logic designs are commonly executed from an initial model in simulation languages, such as Matlab or Simulink. These models are the most common, but not the only sources for designers to access optimized DSP IP offered through FPGA providers. The linkage between modeling and hardware implementation is important, not only for design simplicity, but for simulation and verification against the model.
As the design density for FPGA-based sensor systems increase, full system modeling and simulation will become more time consuming. Compile, simulation, and place- and-route times will increasingly become discriminators when selecting FPGA and design-software vendors. Furthermore, multiprocessor and distributed processing options for design software will be necessary to keep up with design complexity.
To cope with these trends, and to achieve the greatest signal-processing performance in their sonar or radar systems, designers are encouraged to consider options beyond their own VHDL modules or other internally developed IP. Specifically, they should consider working with programmable logic manufacturers to develop tailored DSP cores, or find ways to improve and optimize their designs through advanced place-and-route methods available for FPGA design tools. This is because the advanced capabilities of integrated circuits enabled by increasingly sophisticated fabrication technologies cannot be fully harnessed without flexible and effective design techniques.
Jorge L Polentino U
19769972
CRF
http://mobiledevdesign.com/hardware_news/fpga-signal-processing-sonar-dsp-1207/
domingo, 30 de mayo de 2010
MEMS-Switched Reconfigurable Antennas
The integration and use of RF MEMS switches in microstrip patch antennas and feed structures were investigated for developing reconfigurable multi-band antennas. The current application focuses on the development of a dual L/X-band antenna that would support several satellite or UAV-based communications and radar applications such as SAR, terrain mapping, GMTI, AMTI, etc. A reconfigurable patch module (RPM) was designed and fabricated consisting of a 3x3 array of patches connected together using MEMS switches (we simulated MEMS switches with ideal OPEN/CLOSED switches). This RPM could be used as a basic element in a tile/conformal architecture. Stripline power dividers and blind via transitions were developed to demonstrate feed structures that could be located below the radiating aperture.
Reconfigurable multi-band phased-array antennas are receiving a lot of attention lately due to the emergence of RF MEMS (micro-electro-mechanical systems) switches [1-6]. A MEMS-switched reconfigurable multi-band antenna is one that can be dynamically reconfigured within a few microseconds to serve different applications at drastically different frequency bands, such as communications at L-band (1-2 GHz) and synthetic aperture radar (SAR) at X-band (8-12.5 GHz).
Reconfigurable multi-band phased-array antennas are receiving a lot of attention lately due to the emergence of RF MEMS (micro-electro-mechanical systems) switches [1-6]. A MEMS-switched reconfigurable multi-band antenna is one that can be dynamically reconfigured within a few microseconds to serve different applications at drastically different frequency bands, such as communications at L-band (1-2 GHz) and synthetic aperture radar (SAR) at X-band (8-12.5 GHz).
The Air Force also uses both ground- and airborne- moving target indication (GMTI/AMTI) at these frequencies in order to detect moving targets such as vehicles on the ground and low observables in the air. The RF MEMS switch is attractive because it shows of achieving excellent switching characteristics [2] over an extremely wide band (DC-40 GHz and upwards). These switches can also be used to develop wideband phase shifters [3]. Although there is currently a tremendous amount of research in RF MEMS devices, reliability and packaging of the switches continue to be problematic. The switches are also limited in their power handling capability.In this work, we do not focus on the development of the MEMS switches themselves. Rather, we want to use them as control elements in a reconfigurable antenna. Since the actual MEMS switches were not available to us at the time of this work, we simulated the MEMS switches using ideal OPEN and CLOSED circuits. There is actually a great deal of work to develop optimal radiating elements and feed structures in order to achieve the desired multi-band performance.
Reconfigurable Patch Module (RPM): We investigated the design and fabrication of a dual L/X-band reconfigurable antenna. Microstrip antenna elements were chosen due to their inherent lowprofile, which is suitable for satellite and UAV applications. We used RT/duroid 5880 material with a dielectric constant of 2.2 and a loss tangent of 0.0009 at 10 GHz. Two material thicknesses were investigated: 0.062" and 0.125". The material thickness must be chosen carefully, since it controls both the bandwidth and array scanning performance. The thicker the material, the more bandwidth, particularly at the low frequency end. However, if the substrate becomes too thick, surface waves are generated and array scanning performance and efficiency is lost. Figure 1 shows a picture of the 3x3 RPM fabricated on 0.125" duroid. The patches are 0.370" square and separated by 0.590" on center. Interconnecting tabs are 0.050" wide and 0.085" long. The "reconfigurable" antenna was actually fabricated as two separated prototypes (OPEN and CLOSED configurations) for testing in the laboratory.
shows the measured return loss for the 3x3 RPM in Figure 1 for both the CLOSED (L-band) and OPEN (S-band) configurations. We were able to achieve 1.2% impedance bandwidth for the L band configuration and greater than 7% bandwidth at X-band. This bandwidth was limited rimarily by the substrate thickness. Figures 3 and 4 show the measured radiation patterns at both L-band and X-band. Computer simulation results for both the return loss and radiation patterns agree well with the measurements, and will be shown in the talk.
Jorge L Polentino U
19769972
CRF
http://www.appliedradar.com/Papers/aps01_mems.pdf
MEMS: Diseño de un microrelay realizado con tecnología SOI
El desarrollo de dispositivos MEMS (Micro Electro-Mechanical Systems) ha experimentado un continuo crecimiento con nuevas áreas de aplicación. Paralelamente, la tecnología SOI (Silicon On Insulator) a demostrado ser una interesante opción para ser utilizada en la fabricación de microsensores y MEMS surgiendo en el mercado más opciones de servicios de fabricación en este tipo de tecnología. En este trabajo se presenta el diseño de un Demostrador con tecnología MEMS sobre obleas de tipo SOI para ser fabricado por la empresa Tronic's a través del sistema Europractice.
La industria de dispositivos MEMS sigue experimentando un gran crecimiento y ampliando cada
vez más sus campos de aplicación. Así mismo, lasexigencias del mercado hacen que crezca la demanda de dispositivos de mayor rendimiento y confiabilidad. La utilización de SOI como substrato para la fabricación de dispositivos MEMS a demostrado ser extremadamente versátil [1]. Este material tiene grandes ventajas respecto del silicio policristalino [2]. Sus principales características son el reducido "stress" residual y la posibilidad de combinar circuitos eléctricos con elementos micromecánicos. También se caracteriza por su gran resistencia en ambientes extremos y corrosivos, soportando altas temperaturas y niveles de radiación. Desde el punto de vista del diseño y la producción de MEMS, tiene además la ventaja de requerir un menor número de máscaras para el proceso de fabricación. Como consecuencia de lo expresado surgen en el mercado más alternativas para la fabricación de MEMS utilizando obleas de tipo SOI. Si a eso se le suma la posibilidad de obtener prototipos a bajo costo mediante el sistema de servicios MPW (Multi Project Wafer) la utilización de SOI para el desarrollo de MEMS se convierte en una opción viable e interesante.
La industria de dispositivos MEMS sigue experimentando un gran crecimiento y ampliando cada
vez más sus campos de aplicación. Así mismo, lasexigencias del mercado hacen que crezca la demanda de dispositivos de mayor rendimiento y confiabilidad. La utilización de SOI como substrato para la fabricación de dispositivos MEMS a demostrado ser extremadamente versátil [1]. Este material tiene grandes ventajas respecto del silicio policristalino [2]. Sus principales características son el reducido "stress" residual y la posibilidad de combinar circuitos eléctricos con elementos micromecánicos. También se caracteriza por su gran resistencia en ambientes extremos y corrosivos, soportando altas temperaturas y niveles de radiación. Desde el punto de vista del diseño y la producción de MEMS, tiene además la ventaja de requerir un menor número de máscaras para el proceso de fabricación. Como consecuencia de lo expresado surgen en el mercado más alternativas para la fabricación de MEMS utilizando obleas de tipo SOI. Si a eso se le suma la posibilidad de obtener prototipos a bajo costo mediante el sistema de servicios MPW (Multi Project Wafer) la utilización de SOI para el desarrollo de MEMS se convierte en una opción viable e interesante.
En este trabajo se presenta el diseño de un Demostrador con tecnología MEMS sobre obleas de tipo SOI para ser fabricado mediante el uso del servicio MPW [3], para la fabricación de prototipos, que la empresa Tronic's ofrece a través del sistema Europractice.
DISEÑO DEL DEMOSTRADOR
Se diseñó un Demostrador de MEMS conteniendo veintidós variantes de una unidad microrelay tomada como referencia. A partir de la estructura base del microrelay se diseñaron varios dispositivos con diferentes dimensiones y/o estructuras con el objeto de analizar su comportamiento mecánico y eléctrico. El dispositivo de referencia es un microrelay de contactos laterales accionado mediante actuadores electrostáticos de accionamiento lateral. Los actuadores están formados por dos peines, uno fijo y el otro móvil sujeto a un anclaje por medio de un resorte. El accionamiento electrostático presenta ciertas ventajas respecto a otros métodos alternativos. Por su parte la actuación lateral resuelve algunos de los inconvenientes que presenta el accionamiento vertical, como por ejemplo la alinealidad de la fuerza de actuación respecto al desplazamiento [4] [5]. El actuador de tipo peine provee una fuerza electrostática lineal [6] [7], producida por la aplicación de una diferencia de potencial, que depen de en granmedida de las dimensiones del actuador [5]. Un parámetro importante es la tensión que se debe aplicar para producir el cierre de los contactos del microrelay. Se determinó que el aumento en el ancho de los dedos del peine, si bien logra una disminución de dicha tensión, su influencia no es muy significativa comparada con la de otros parámetros, por lo cual se mantuvo constante salvo en uno de los dispositivos solo a los fines de verificar los cálculos realizados. Los parámetros que se tomaron en consideración para el diseño de los diferentes dispositivos del Demostrador son la separación entre dedos del peine, la separación en el extremo de los dedos del peine, la cantidad de dedos y la longitud y espesor del elemento elástico. También se utilizó una estructura alternativa para éste último para evaluar su comportamiento. El tamaño del Demostrador es de 3400x3100 μm y los microrelay dentro de dicha área van desde 500x400 μm a 900x400 μm.
DISEÑO DEL DEMOSTRADOR
Se diseñó un Demostrador de MEMS conteniendo veintidós variantes de una unidad microrelay tomada como referencia. A partir de la estructura base del microrelay se diseñaron varios dispositivos con diferentes dimensiones y/o estructuras con el objeto de analizar su comportamiento mecánico y eléctrico. El dispositivo de referencia es un microrelay de contactos laterales accionado mediante actuadores electrostáticos de accionamiento lateral. Los actuadores están formados por dos peines, uno fijo y el otro móvil sujeto a un anclaje por medio de un resorte. El accionamiento electrostático presenta ciertas ventajas respecto a otros métodos alternativos. Por su parte la actuación lateral resuelve algunos de los inconvenientes que presenta el accionamiento vertical, como por ejemplo la alinealidad de la fuerza de actuación respecto al desplazamiento [4] [5]. El actuador de tipo peine provee una fuerza electrostática lineal [6] [7], producida por la aplicación de una diferencia de potencial, que depen de en granmedida de las dimensiones del actuador [5]. Un parámetro importante es la tensión que se debe aplicar para producir el cierre de los contactos del microrelay. Se determinó que el aumento en el ancho de los dedos del peine, si bien logra una disminución de dicha tensión, su influencia no es muy significativa comparada con la de otros parámetros, por lo cual se mantuvo constante salvo en uno de los dispositivos solo a los fines de verificar los cálculos realizados. Los parámetros que se tomaron en consideración para el diseño de los diferentes dispositivos del Demostrador son la separación entre dedos del peine, la separación en el extremo de los dedos del peine, la cantidad de dedos y la longitud y espesor del elemento elástico. También se utilizó una estructura alternativa para éste último para evaluar su comportamiento. El tamaño del Demostrador es de 3400x3100 μm y los microrelay dentro de dicha área van desde 500x400 μm a 900x400 μm.
PROCESO SOI
El proceso de fabricación SOI utilizado [3], si bien por un lado presentó limitaciones en cuanto a la flexibilidad en el diseño, por otro lado simplificó las tareas, ya que solo fue necesario el diseño de una máscara, correspondiente a la estructura de silicio del conjunto. Las restricciones impuestas por el proceso SOI impidieron obtener la metalización de los contactos laterales del microrelay, por lo que se realizará posteriormente mediante la técnica de evaporación. Para ello se removerá el encapsulado del Demostrador colocado en la etapa final del proceso de fabricación. Se dispusieron, alrededor del área del Demostrador, una serie de pads a los cuales se conectan seis microrelay con el objetivo de ensayar algún tipo de encapsulado posterior. El tamaño de los pads es de 200x200 μm en uno de los laterales y 200x320 μm en el otro con un pitch de 240 μm en ambos casos. La capa estructural de silicio, que forma la estructura móvil de los dispositivos, es de 20 μm de espesor y la capa de óxido de la oblea SOI de 0.4 μm.
DISEÑO DEL LAYOUT
El diseño del Demostrador fue realizado en el IMEC de Leuven-Bélgica ajustándose a las reglas del proceso de fabricación. Para el diseño de la máscara se utilizó el editor de layout de Cadence, el cual está estructurado en forma jerárquica en varios niveles. El nivel más bajo lo constituyen una serie de celdas con estructuras básicas definidas en forma paramétrica para facilitar la construcción de los diferentes dispositivos del Demostrador que difieren básicamente en sus dimensiones. Las celdas definidas corresponden a las estructuras de los peines de los actuadores, a los contactos del microrelay, a los pads de conexionado, a los anclajes de la estructura y a las vigas del resorte. La posibilidad del editor de trabajar con este tipo de celdas facilitó mucho el diseño del layout. El Demostrador diseñado se envió para su fabricación y se prevé realizar en IMEC la caracterización de los prototipos para evaluar sus resultados. Se diseñó un Demostrador de dispositivos MEMS con tecnología SOI utilizando uno de los procesos de fabricación disponibles en el mercado. Se incluyeron en el Demostrador varios microrelay con diferentes estructuras y dimensiones para ensayar sus características eléctricas y mecánicas. Se diseñó una biblioteca de celdas paramétricas para el diseño del layout. El diseño completo del layout requirió solamente una máscara para su implementación.
Jorge L Polentino U
19769972
CRF
http://www.iberchip.org/VIII/docs/posters/p27.pdf
http://www.iberchip.org/VIII/docs/posters/p27.pdf
SISTEMAS MICRO-ELECTRO-MECÁNICOS PARA DETECCIÓN DE GASES. Sensores avanzados para múltiples aplicaciones
Los Sistemas Micro-Electro-Mecánicos (Micro-electro-mechanical systems (MEMS) )y de tecnologías de película delgada permiten la integración de circuitos electrónicos y matrices de sensores multifuncionales fabricados en sustrato de silicio como detectores químicos, mecánicos y de parámetros físicos. También la integración ofrece la posibilidad de acortar el camino entre los sensores y las técnicas de reconocimiento. Es importante fijarse que los sensores biológicos tienen igualmente la facultad de funcionar como sensores múltiples por ejemplo el sentido del tacto agrupa sensaciones de temperatura, presión, viscosidad de líquidos, además de disponer de un mecanismo de retroalimentación y de memoria.
Los MEMS son sensores avanzados para detectar simultáneamente varios parámetros, temperatura, presión, radiación, gas y concentración de vapor, olor, aceleración, inercia, campos eléctricos y magnéticos y muchos más, proporcionan no solamente alta relación señal ruido en un gran rango dinámico sino que también presentan buena sensibilidad. En general, los sensores consisten en dos elementos: un detector y una plataforma que comunica con el detector a través de un interface activo con variables eléctricas, mecánicas, ópticas o impedancia química. La plataforma debe permitir generar la salida de señales eléctricas que transportan la información proporcionada por el detector.
El principal elemento utilizado en los MEMS es el silicio. La tecnología usada en la fabricación electrónica de las micromáquinas es cualquiera de las utilizadas en la fabricación de circuitos integrados, la fotoeléctrica, difusión ,oxidación , etc. La técnica como silicon-to-silicon y silicon-to-glass es la que se suele emplear permitiendo la realización de sensores de olor.
Sensor de superficie acustica SAW
Los MEMS son sensores avanzados para detectar simultáneamente varios parámetros, temperatura, presión, radiación, gas y concentración de vapor, olor, aceleración, inercia, campos eléctricos y magnéticos y muchos más, proporcionan no solamente alta relación señal ruido en un gran rango dinámico sino que también presentan buena sensibilidad. En general, los sensores consisten en dos elementos: un detector y una plataforma que comunica con el detector a través de un interface activo con variables eléctricas, mecánicas, ópticas o impedancia química. La plataforma debe permitir generar la salida de señales eléctricas que transportan la información proporcionada por el detector.
El principal elemento utilizado en los MEMS es el silicio. La tecnología usada en la fabricación electrónica de las micromáquinas es cualquiera de las utilizadas en la fabricación de circuitos integrados, la fotoeléctrica, difusión ,oxidación , etc. La técnica como silicon-to-silicon y silicon-to-glass es la que se suele emplear permitiendo la realización de sensores de olor.
Sensor de superficie acustica SAW
los sensores mas comunes estan formados por dispositivos piezoelectronicos. la figura muestra la estructura basica de un sensor acustico electronico de gas. este tipo de sensores se utiliza para la realizacion de sensores multifuncionales fisico y quimicos; esta estructura se utiliza para sensores microscopicos de viscosidad, humedad, detectores de humo, sensores de gas, y sensores de campo magnetico-electrico normalmente el deposito de sustancias en la pelicula delgada se utilizan parala medida de estos parametros al producir cambios fisicos y quimicos que hacen variar la frecuencia de resonancia de la pelicula. los cambios en los sensores basados en el desplazamiento de frecuencia de resonancia, son producidos por causas mecanicas, quimicas u otras perturbaciones
Si el resonador se pone junto con una película delgada, las condiciones de funcionamiento varían. Una película dieléctrica modifica su funcionamiento bajo condiciones mecánicas, mientras una película conductora modifica ambos su funcionamiento eléctrico y mecánico. Las perturbaciones mecánicas y eléctricas causan desplazamientos de la frecuencia de resonancia. Si asumimos que la cantidad de corriente interna es aproximadamente y , donde F es el potencial eléctrico, y ho es la densidad de carga eléctrica, los cambios en el funcionamiento y desplazamiento de frecuencia de resonancia vienen dados por:
donde U es la energía acústica cargada en modo resonador, T es la tensión de esfuerzo, y * indica
un conjugado complejo. Las condiciones de la interface son:
Donde H es el campo magnético relativo a al campo eléctrico a través de la ecuación de Maxwell. El régimen de la oscilación del resonador piezoeléctrico puede ser modificado mecánica o eléctricamente. Las perturbaciones eléctricas pueden ocurrir en la película metálica con diferentes valores de conductividad en el resonador o si el resonador se introduce en un electrolito de conducción iónica. La influencia mecánica, química y eléctrica en sólidos y fluidos en la superficie del sensor depende de la interface entre el resonador de cuarzo y la resonancia. Algunos efectos en líquidos y sólidos hacen oscilar el resonador y modificar la resolución del sensor. La resolución del sensor se determina por la respuesta en el desplazamiento de la frecuencia de resonancia, a perturbaciones y la capacidad de monitorizar los cambios en desplazamiento de frecuencia. Cuando un resonador de cuarzo libre se pone en contacto con un sólido o fluido, parte de la energía acústica se trasmite fuera del resonador.
El acoplamiento acústico define el desplazamiento de la frecuencia de resonancia, y modifica el factor Q de calidad.
La siguiente figura muestra la superficie de una plataforma de onda acústica usando varios sensores de película delgada depositados en un resonador piezoeléctrico. En una mezcla de gases, cada película detecta un componente determinado.
La figura muestra otra superficie de onda acústica (SAW) configurada con silicio, en un sustrato no piezoeléctrico. El traductor interdigital esta construido por ZnO, un material piezoeléctrico. La línea SAW es parte de un circuito oscilador. Cuando la sensibilidad cambia los parámetros mecánicos también lo que se produce por la presencia de un gas cuya presión desplaza la frecuencia de resonancia y cambia la velocidad de propagación en el SAW. La línea de referencia del SAW, construida con película de cristal pasivo se usa para calibraciones.
Sensores ElectroquímicosOtra familia de sensores multifunción son los sensores de gas electroquímicos ver Figura , que se usan con celdas galvánicas en estado sólido para media de presiones parciales de gases como CO2, NOx, SOx, y gases de hidrocarburos. El sensor opera como una batería: la fuerza electromotriz cambia la función química modificando el cátodo en presencia de gas de acuerdo a la siguiente reacción:
Pt electrode, gas gas sensitive film ion conductor reference electrode Pt electrode
Por ejemplo, para medir la f.e.m. y la presión parcial del NO2 una celda galvánica, produce:
La reacción en la celda es: 
La relación ente la f.e.m. y la presión parcial de NO2 viene dada por:
Donde G es la energía de Gibbs, P es la presión parcial, k es la constante de Boltzmann, y T la temperatura absoluta.
Las siguiente figura muestra un sensor de gas para medida parcial de presencia de CO2 fabricado usando tecnología de película delgada. El sensor opera a 350°C y toda su estructura se construye encima de una oblea de silicio con platina de película delgada.
utilizando varios sensores sobre un mismo sustrato de silicio con tecnología MEMS, sólo se necesita un electrodo diferente para cada una de las salidas del sensor.
Jorge L Polentino U
19769972
CRF
MEMS, las nanomáquinas que cambiarán al mundo
La electrónica de consumo ha llegado al estado en que se encuentra hoy gracias a la miniaturización. Sin ella, sería imposible crear circuitos integrados con millones de transistores y un tamaño de solo una fracción de centímetro cuadrado. Sin la microelectrónica, el equivalente de un microprocesador como el que tiene tu ordenador ocuparía el volumen de un edificio de 12 o 14 pisos. No habría iPods ni teléfonos móviles.
Sin embargo, y a pesar de los logros obtenidos en la reducción de tamaño de los componentes electrónicos, los sistemas mecánicos aun requieren de piezas cuyo tamaño es varios órdenes de magnitud más grandes que sus contrapartes electrónicas. Cualquier pieza de un reloj mecánico, por ejemplo, es millones de veces más grande que uno de los transistores integrados en un microprocesador. Pero esta situación está cambiando.
La miniaturización de máquinas electromecánicas ha dado lugar a los MEMS, que silenciosamente han ocupado un lugar en nuestra vida cotidiana. De hecho, el dispositivo capaz de medir la aceleración a la que sometes el mando de tu Wii (un acelerómetro) es un MEMS. Se trata del mismo dispositivo que, instalado en el airbag de un coche determina el momento justo en que se produce un choque y dispara el mecanismo de inflado de las bolsas.
Pero si bien los acelerómetros son quizás los dispositivos basados en MEMS mas difundidos, no son los únicos. Existen sensores de presión, de temperatura y de humedad construidos a partir de piezas que tienen un tamaño similar al de un glóbulo rojo. Forman parte del sistema de control de los más modernos marcapasos, censando la actividad física del paciente para modificar su ritmo cardíaco. También se emplean MEMS en los cabezales de las impresoras de inyección de tinta, como parte del dispositivo que produce la evaporación controlada de la tinta en el momento justo.
Por lo general, estos mecanismos tienen un tamaño mayor al micrómetro (millonésima de metro) y menor al milímetro. Lo que los hace tan particulares es que, a estas escalas, el comportamiento físico que rige a las maquinas convencionales no siempre funciona como la intuición puede indicar. Efectivamente, el incremento en la relación entre la superficie y el volumen de las piezas de un MEMS hace que los efectos electrostáticos y térmicos predominen sobre la inercia o la masa térmica.
Para fabricar las pequeñas piezas que conforman estas maquinas se utiliza una tecnología que, en esencia, es la misma que la empleada para la fabricación de los circuitos integrados. La posibilidad de "integrar" piezas móviles es lo que ha hecho posibles maquinas a escala nanométricas. Existen motores a vapor del tamaño de un grano de polen, engranajes y palancas cuyo tamaño de mide en diámetros atómicos, y hasta pequeños espejos montados sobre soportes móviles, con un tamaño mucho menor al diámetro de un cabello, capaces de enfocar o corregir una imagen.
Los MEMS permiten cada día la creación de dispositivos sorprendentes. Por ejemplo, para evitar la falsificación de una firma, es posible incorporar acelerómetros en una lapicera, para que además de escribir sea capaz de registrar las velocidades y aceleraciones que le imprimió la mano mientras se firmaba. Esto hace prácticamente imposible una falsificación.
Dentro de poco, será factible la fabricación de un dispositivo, que ubicado en el cuerpo de un paciente, analice su sangre y que, en función de los resultados, inyecte los fármacos necesarios en las dosis adecuadas. En caso de ser necesario, hasta podría enviar una señal de alerta para que el paciente fuera atendido de urgencia. Estas máquinas funcionarán como pequeños robots, capaces de realizar tareas que resultan imposibles a una escala mayor.
Se trata de una ciencia que, a pesar de habernos brindado ya una cantidad de soluciones concretas a problemas de ingeniería, recién está naciendo. Pero tiene el potencial de, como decíamos al comienzo, cambiar el mundo.
acelerometro comercial
Jorge L Polentino U.
19769972
CRF
ACTUADORES MEMS EN EL SECTOR AERONÁUTICO
ACTUADORES MEMS EN EL SECTOR AERONÁUTICO
En Octubre de 2006, el CIMTAN realizó un estudio titulado: "Actuadores MEMS en el sector Aeroespacial: Control de flujo". En él se hacía un estudio detallado de los diferentes tipos de actuadores, así como los distintos usos que se le daba a estos sistemas en el campo de la aeronáutica. El estudio se centró en particular en el control de flujo, por ser ésta la temática que más estaba en expansión con estos actuadores MEMS.
La principal conclusión a la que se llegó en el pasado estudio es que la aplicación de MEMS al control de flujo está madura, es decir, es una tecnología ampliamente estudiada porque su reducido tamaño les permite moverse y actuar dentro de la capa límite, a la vez que las frecuencias en las que trabajan son del mismo orden que las de los torbellinos que se crean en esta región. Ya se han llevado a cabo ensayos en túnel de viento y los resultados muestran una reducción de la resistencia y un aumento de la sustentación.
Durante el estudio se identificaron varios tipos de actuadores dependiendo de su interacción con la capa límite: Superficie móvil, inyectores de aire, actuadores térmicos, actuadores de burbuja y de interacción eléctrica y magnética. De ellos, el que más posibilidades de implantación a corto plazo tiene es el denominado "Synthetic jets" enmarcado en los actuadores de inyección de aire. Consiste en pequeñas cavidades con un orificio de salida hacia la capa límite que inyectan aire con la frecuencia resonante de la membrana inferior de la cavidad.
Los actuadores MEMS se enfrentan a dos problemas importantes. Por un lado está el hecho de que para lograr maniobrabilidad se requiere una gran capacidad de computación. Cada actuador MEMS necesita instrucciones individuales en función del flujo que detecta, lo que hace que se necesiten muchos cálculos en paralelo y a gran velocidad. El otro gran problema es su elevado consumo energético. El estudio muestra, que los "Synthetic Jets" son los que menos energía consumen, por lo que siguen siendo los mejores candidatos, pero para su implantación final es necesario aún realizar los test en túneles de viento.
Otras aplicaciones de los actuadores MEMS en el sector que se detectaron a lo largo de la realización del informe fueron: Sistemas de propulsión, sistemas de acoplamiento de satélites, sistemas de control de la orientación, sistemas RF MEMS, actuadores inerciales
Tras el analizar los resultados obtenidos desde Octubre de 2006 hasta Mayo de 2008 (19 meses), se ha obtenido un total de 25 resultados que se han distribuido, según las aplicaciones para las que han sido desarrollados, siguiendo los siguientes resultados :
Control de Flujo47% Control Térmico 20% Control de Estructuras13% Otras aplicaciones20%
Como quiera que el informe previo se dedicara explícitamente a actuadores MEMS para control de flujo, en este se va a seguir la misma línea para analizar las tendencias. Del resto de aplicaciones, como se hizo anteriormente, se hará mención pero no análisis.
Adan F Chaparro Castillo
CI:17501640
EES
Seccion:1
RF MEMS SWITCHES: STATUS OF THE TECHNOLOGY
This paper presents the latest accomplishments in RF MEMS switches, and at the same time, an assessment of their potential applications in defense and commercial systems. It is seen that RF MEMS devices offer spectacular performance at microwave frequencies,but suffer from reliability problems and the potential of relatively high-cost hermetic packaging. Still, this technology offers such tremendous advantages over GaAs and silicon switching devices that, in the author's opinion, it will find many applications in satellite, base-station and defense applications, particularly at high microwave frequencies.
PROS AND CONS OF RF MEMS
SWITCHES
MEMS switches are surface-micromachined devices which use a mechanical movement to achieve a short circuit or an open circuit in the RF transmission-line (Figs. 1-2). RF MEMS switches are the specific micromechanical switches which are designed to operate at RF to mm-wave frequencies (0.1 to 100 GHz). The advantages of MEMS switches over PIN diode or FET switches are [1]: Near-Zero Power Consumption: Electrostatic actuation requires 30-80 V, but does not consume any current, leading to a very low power dissipation (10-100 nJ per switching cycles). On the other hand, thermal magnetic switches consume a lot of current unless they are made to latch in the down-state position once actuated. Very High Isolation: RF MEMS metal-contact switches are fabricated with air gaps, and therefore, have very low off-state capacitances (2-4 fF) resultingin excellent isolation at 0.1-60GHz. Also, capacitive switches with a capacitance ratio of 60-160 provide excellent isolation from 8-100Hz. Very Low Insertion Loss: RF MEMS metal-contact and capacitive switches have an insertion loss of 0.1dB up to 100GHz. Linearity and Intermodulation Products: MEMS switches are extremely linear devices and therefore re-
SWITCHES
MEMS switches are surface-micromachined devices which use a mechanical movement to achieve a short circuit or an open circuit in the RF transmission-line (Figs. 1-2). RF MEMS switches are the specific micromechanical switches which are designed to operate at RF to mm-wave frequencies (0.1 to 100 GHz). The advantages of MEMS switches over PIN diode or FET switches are [1]: Near-Zero Power Consumption: Electrostatic actuation requires 30-80 V, but does not consume any current, leading to a very low power dissipation (10-100 nJ per switching cycles). On the other hand, thermal magnetic switches consume a lot of current unless they are made to latch in the down-state position once actuated. Very High Isolation: RF MEMS metal-contact switches are fabricated with air gaps, and therefore, have very low off-state capacitances (2-4 fF) resultingin excellent isolation at 0.1-60GHz. Also, capacitive switches with a capacitance ratio of 60-160 provide excellent isolation from 8-100Hz. Very Low Insertion Loss: RF MEMS metal-contact and capacitive switches have an insertion loss of 0.1dB up to 100GHz. Linearity and Intermodulation Products: MEMS switches are extremely linear devices and therefore re-
sult in very low intermodulation products in switching and tuning operations. Their performance is 30-50 dB better than PIN or FET switches. Potential for Low Cost: RF MEMS switches are fabricated using surface micromachining techniques and can be built on quartz, Pyrex, LTCC, mechanicalgrade high-resistivity silicon or GaAs substrates. RF MEMS switches also have their share of problems, and these are: Relatively Low Speeds: The switching speed of most electrostatic MEMS switches is 2-40 μs, and
High Voltage or High Current Drive: Electrostatic MEMS switches require 30-80 V for reliable operation, and this requires a voltage up-converter chip when used in portable telecommunication systems. Thermal magnetic switches can be actuated using 2-5 V, but require 10-100 mA of actuation current. Power Handling: Most MEMS switches cannot handle more than 200 mW although some switches have shown up to 500 mW power handling (Terravicta and Raytheon). MEMS switches that handle 1-10 W with high reliability simply do not exist today. Reliability: The reliability of mature MEMS switches is 0.1-40 Billion cycles. However, many systems require switches with 20-200 Billion cycles. Also, the long term reliability (years) has not yet been addressed. It is now well known that the capacitive switches are limited by the dielectric charging which occurs in the actuation electrode, while the metalcontact switches are limited by the interface problems between the contact metals, which could be severe under low contact forces (in electrostatic designs, the contact forces are around 40-100 μN per contact).
It is important to note that the reliability and packaging issues have been the limiting factors to the quick deployment of RF MEMS switches, and they are currently under intense investigations. DARPA has initiated two programs in 2002 and 2003 to address these problems, the RF MEMS Improvement program (Dr. Larry Corey), and the HERMIT program (Dr. Clark Nguyen), and it is expected that some of these problems will be solved in the coming 2-3 years. Packaging: MEMS switches need to be packaged in inert atmospheres (Nitrogen, Argon, etc..) and in very low humidity, resulting in hermetic or nearhermetic seals. Hermetic packaging costs are currently relatively high, and the packaging technique itself may adversely affect the reliability of the MEMS switch. Microassembly (Fig. 3) and Analog Devices have both developed excellent packages for RF MEMS switches. The Microassembly package is based on gold-to-gold thermo-compression at 250◦C while the Analog Devices package is based on glass-to-glass seal at 400−450◦C. Other companies which have packaged switches are Terravicta (ceramic package) and Omron (glass-to-glass). Cost: While MEMS switches have the potential of very low cost manufacturing, one must add the cost of the packaging and the high-voltage drive chip. It is therefore hard to beat a $0.3-0.6 single-pole doublethrow 3 V PIN or FET switch, tested, packaged and delivered. It is for this reason that Prof. Rebeiz believes that RF MEMS switches will be first used in defense and high-value commercial applications and not in cellular phones.
DETAILED DISCUSSION OF MEMS
SWITCHES
SWITCHES
Actuation Mechanisms: The actuation forces required for the mechanical movement can be obtained using electrostatic, magneto-static, piezoelectric or thermal designs. To date, only electrostatic-type switches have been demonstrated at 0.1-100GHz with high reliability at low RF powers for metal contact and medium power levels for capacitive contacts (100 4A1.4 Million to 50 Billion cycles depending on the manufacturer) and wafer-scale manufacturing techniques. Other switches which have demonstrated excellent performance are the Microlab Latching switch (up to 100 Million cycles) using magnetic actuation, and the thermal switches developed independently by Cronos Microsystems and the Univ. of California, Davis. It
is hard to test thermal switches for long cycle times
is hard to test thermal switches for long cycle times
Switching Time: Electrostatic switches can be made small and with a very fast switching time (2-30 μs) while thermal/magnetic actuation requires around 100-2, 000 μs of switching time. An excellent metal-contact switch developed by LETI using thermal actuation but with an electrostatic hold, thereby requiring very little switching energy and virtually zero hold-down power. However, its switching time is still relatively slow (300 μs). The LETI switch has been tested to more than 100 million cycles. Contact Type: There are two different contacts in RF MEMS switches, a capacitive contact and a metalto- metal (or DC) contact. The capacitive contact is characterized by the capacitance ratio between the up-state (open circuit) and down-state (short-circuit) positions, and this is typically 80-160 depending on the design. The down-state capacitance is typically 2-3 pF, and is suitable for 8-100GHz applications. In general, it is hard to obtain a large down-state capacitance using nitride or oxide layers, and this limits the low-frequency operation of the device. On the other hand, DC-contact switches with small up-state capacitances (open circuit) can operate from 0.01 to 40GHz, and in some cases, to 60GHz (for example, the Rockwell Scientific switch has an up-state capacitance of only 1.75 fF and an isolation of 23 dB at 60GHz). In the down-state position (short-circuit), the DC-contact switch becomes a series resistor with a resistance of 0.5-2 Ω, depending on the contact metalused. Circuit and Substrate Configurations: As is the case with all two-terminal devices, the switches can be placed in series or in shunt across a transmission line. Typically, capacitive switches have been used in a shunt configuration, while DC-contact switches are placed in series. The reason is that it is easier to get a good isolation with a limited impedance ratio (such as the capacitive switch) in a shunt-circuit than in a series circuit. Also, MEMS switches are compatible with both microstrip and CPW lines on glass, silicon and GaAs substrates, and have been used in these configurations all the way to 100GHz. For low loss applications at microwave frequencies, it is important to use high-resistivity substrates.
CIRCUITS WITH RF MEMS SWITCHES
The near-ideal electrical response of RF MEMS witches (both metal-contact and capacitive) have allowed many designers to build state-of-the-art switching circuits from 0.1GHz all the way to 120GHz. In the past 4 years, these applications concentrated on the replacement of GaAs phase shifters which are commonly used in phased arrays by the thousands of units. A comparison between 3-bit GaAs phase shifters and MEMS phase shifters is shown in Table I and it is seen that MEMS switches provide an immense performance benefit especially at Ka-Band to W-band applications.
The near-ideal electrical response of RF MEMS witches (both metal-contact and capacitive) have allowed many designers to build state-of-the-art switching circuits from 0.1GHz all the way to 120GHz. In the past 4 years, these applications concentrated on the replacement of GaAs phase shifters which are commonly used in phased arrays by the thousands of units. A comparison between 3-bit GaAs phase shifters and MEMS phase shifters is shown in Table I and it is seen that MEMS switches provide an immense performance benefit especially at Ka-Band to W-band applications.
Fig. 4 presents a 4-bit miniature RF MEMS phase shifter developed jointly by the Univ. of Michigan and Rockwell Scientific. It is based on the Rockwell metalcontact switch and on CLC delay lines for miniaturization. The phase shifter results in an average loss of 1.4dB at 10GHz, a ±3◦ phase error, and is matched to −13 dB at the input and output ports from 6-16GHz. This phase shifter represents the smaller design using RF MEMS to-date, and with excellent response. Fig. 5 presents an 885-986MHz 5-pole tunable
filter using switched MEMS capacitors developed by Raytheon Systems Co. In this case, capacitive switches are used to switch fixed-value metalinsulator- metal capacitors in the transmission line. The filter employs 18 switches and is a very complicated circuit with variable resonators and impedance inverters. Its measured response is nearly ideal, with excellent frequency tuning capabilities, very high linearity (in terms of measured IIP3) and a loss of 5- 6 dB due to the finite Q of the planar inductors used (Q = 30 at 0.9GHz). Fig. 6 presents a W-band 3-bit phase shifter developed at the Univ. of Michigan using MEMS capacitive switches [3]. This is the highest frequency MEMS phase shifter to-date and results in an average loss of 2.7-2.9 dB at 77-94GHz with an associated phase error of ±3◦. The results are about 8 dB better than GaAs designs.
Other circuits, which are not shown due to space constraints, are very wideband SP4T switches, highisolation series/shunt switches covering 0.1-50GHz, double-pole double-throw transfer switches, and a whole range of phase shifters from 8GHz to 120GHz. Also, tunable filters covering 200MHz to 23GHz have been developed by various groups. In general, RF MEMS circuits outperform GaAs FET and PIN diode circuits by a large margin at all frequencies of interest
the RF and microwave communities. Most of the
circuits developed in the world can be found in [1].
circuits developed in the world can be found in [1].
THE FUTURE
It is now clear that we understand RF MEMS switches well, both from the mechanical and electrical/ electromagnetic point of view. We can design complicated circuits using MEMS switches or varactors, and we can accurately predict their performance all the way to 120 GHz. They are still not accepted in the commercial and defense arena due to their need of a hermetic package, and their reliability under medium to high-power conditions. There is currently an intense effort to solve these problems, and the author believes that RF MEMS switches and varactors will play an essential role in future high-value commercial and defense systems.
It is now clear that we understand RF MEMS switches well, both from the mechanical and electrical/ electromagnetic point of view. We can design complicated circuits using MEMS switches or varactors, and we can accurately predict their performance all the way to 120 GHz. They are still not accepted in the commercial and defense arena due to their need of a hermetic package, and their reliability under medium to high-power conditions. There is currently an intense effort to solve these problems, and the author believes that RF MEMS switches and varactors will play an essential role in future high-value commercial and defense systems.
Jorge L Polentino U
19769972
CRF
RF MEMS SWITCHES: STATUS OF THE TECHNOLOGY
This paper presents the latest accomplishments in RF MEMS switches, and at the same time, an assessment of their potential applications in defense and commercial systems. It is seen that RF MEMS devices offer spectacular performance at microwave frequencies,but suffer from reliability problems and the potential of relatively high-cost hermetic packaging. Still, this technology offers such tremendous advantages over GaAs and silicon switching devices that, in the author's opinion, it will find many applications in satellite, base-station and defense applications, particularly at high microwave frequencies.
PROS AND CONS OF RF MEMS
SWITCHES
MEMS switches are surface-micromachined devices which use a mechanical movement to achieve a short circuit or an open circuit in the RF transmission-line (Figs. 1-2). RF MEMS switches are the specific micromechanical switches which are designed to operate at RF to mm-wave frequencies (0.1 to 100 GHz). The advantages of MEMS switches over PIN diode or FET switches are [1]: Near-Zero Power Consumption: Electrostatic actuation requires 30-80 V, but does not consume any current, leading to a very low power dissipation (10-100 nJ per switching cycles). On the other hand, thermal magnetic switches consume a lot of current unless they are made to latch in the down-state position once actuated. Very High Isolation: RF MEMS metal-contact switches are fabricated with air gaps, and therefore, have very low off-state capacitances (2-4 fF) resultingin excellent isolation at 0.1-60GHz. Also, capacitive switches with a capacitance ratio of 60-160 provide excellent isolation from 8-100Hz. Very Low Insertion Loss: RF MEMS metal-contact and capacitive switches have an insertion loss of 0.1dB up to 100GHz. Linearity and Intermodulation Products: MEMS switches are extremely linear devices and therefore re-
SWITCHES
MEMS switches are surface-micromachined devices which use a mechanical movement to achieve a short circuit or an open circuit in the RF transmission-line (Figs. 1-2). RF MEMS switches are the specific micromechanical switches which are designed to operate at RF to mm-wave frequencies (0.1 to 100 GHz). The advantages of MEMS switches over PIN diode or FET switches are [1]: Near-Zero Power Consumption: Electrostatic actuation requires 30-80 V, but does not consume any current, leading to a very low power dissipation (10-100 nJ per switching cycles). On the other hand, thermal magnetic switches consume a lot of current unless they are made to latch in the down-state position once actuated. Very High Isolation: RF MEMS metal-contact switches are fabricated with air gaps, and therefore, have very low off-state capacitances (2-4 fF) resultingin excellent isolation at 0.1-60GHz. Also, capacitive switches with a capacitance ratio of 60-160 provide excellent isolation from 8-100Hz. Very Low Insertion Loss: RF MEMS metal-contact and capacitive switches have an insertion loss of 0.1dB up to 100GHz. Linearity and Intermodulation Products: MEMS switches are extremely linear devices and therefore re-
sult in very low intermodulation products in switching and tuning operations. Their performance is 30-50 dB better than PIN or FET switches. Potential for Low Cost: RF MEMS switches are fabricated using surface micromachining techniques and can be built on quartz, Pyrex, LTCC, mechanicalgrade high-resistivity silicon or GaAs substrates. RF MEMS switches also have their share of problems, and these are: Relatively Low Speeds: The switching speed of most electrostatic MEMS switches is 2-40 μs, and
High Voltage or High Current Drive: Electrostatic MEMS switches require 30-80 V for reliable operation, and this requires a voltage up-converter chip when used in portable telecommunication systems. Thermal magnetic switches can be actuated using 2-5 V, but require 10-100 mA of actuation current. Power Handling: Most MEMS switches cannot handle more than 200 mW although some switches have shown up to 500 mW power handling (Terravicta and Raytheon). MEMS switches that handle 1-10 W with high reliability simply do not exist today. Reliability: The reliability of mature MEMS switches is 0.1-40 Billion cycles. However, many systems require switches with 20-200 Billion cycles. Also, the long term reliability (years) has not yet been addressed. It is now well known that the capacitive switches are limited by the dielectric charging which occurs in the actuation electrode, while the metalcontact switches are limited by the interface problems between the contact metals, which could be severe under low contact forces (in electrostatic designs, the contact forces are around 40-100 μN per contact).
It is important to note that the reliability and packaging issues have been the limiting factors to the quick deployment of RF MEMS switches, and they are currently under intense investigations. DARPA has initiated two programs in 2002 and 2003 to address these problems, the RF MEMS Improvement program (Dr. Larry Corey), and the HERMIT program (Dr. Clark Nguyen), and it is expected that some of these problems will be solved in the coming 2-3 years. Packaging: MEMS switches need to be packaged in inert atmospheres (Nitrogen, Argon, etc..) and in very low humidity, resulting in hermetic or nearhermetic seals. Hermetic packaging costs are currently relatively high, and the packaging technique itself may adversely affect the reliability of the MEMS switch. Microassembly (Fig. 3) and Analog Devices have both developed excellent packages for RF MEMS switches. The Microassembly package is based on gold-to-gold thermo-compression at 250◦C while the Analog Devices package is based on glass-to-glass seal at 400−450◦C. Other companies which have packaged switches are Terravicta (ceramic package) and Omron (glass-to-glass). Cost: While MEMS switches have the potential of very low cost manufacturing, one must add the cost of the packaging and the high-voltage drive chip. It is therefore hard to beat a $0.3-0.6 single-pole doublethrow 3 V PIN or FET switch, tested, packaged and delivered. It is for this reason that Prof. Rebeiz believes that RF MEMS switches will be first used in defense and high-value commercial applications and not in cellular phones.
DETAILED DISCUSSION OF MEMS
SWITCHES
Actuation Mechanisms: The actuation forces required for the mechanical movement can be obtained using electrostatic, magneto-static, piezoelectric or thermal designs. To date, only electrostatic-type switches have been demonstrated at 0.1-100GHz with high reliability at low RF powers for metal contact and medium power levels for capacitive contacts (100 4A1.4 Million to 50 Billion cycles depending on the manufacturer) and wafer-scale manufacturing techniques. Other switches which have demonstrated excellent performance are the Microlab Latching switch (up to 100 Million cycles) using magnetic actuation, and the thermal switches developed independently by Cronos Microsystems and the Univ. of California, Davis. It
is hard to test thermal switches for long cycle times
Switching Time: Electrostatic switches can be made small and with a very fast switching time (2-30 μs) while thermal/magnetic actuation requires around 100-2, 000 μs of switching time. An excellent metal-contact switch developed by LETI using thermal actuation but with an electrostatic hold, thereby requiring very little switching energy and virtually zero hold-down power. However, its switching time is still relatively slow (300 μs). The LETI switch has been tested to more than 100 million cycles. Contact Type: There are two different contacts in RF MEMS switches, a capacitive contact and a metalto- metal (or DC) contact. The capacitive contact is characterized by the capacitance ratio between the up-state (open circuit) and down-state (short-circuit) positions, and this is typically 80-160 depending on the design. The down-state capacitance is typically 2-3 pF, and is suitable for 8-100GHz applications. In general, it is hard to obtain a large down-state capacitance using nitride or oxide layers, and this limits the low-frequency operation of the device. On the other hand, DC-contact switches with small up-state capacitances (open circuit) can operate from 0.01 to 40GHz, and in some cases, to 60GHz (for example, the Rockwell Scientific switch has an up-state capacitance of only 1.75 fF and an isolation of 23 dB at 60GHz). In the down-state position (short-circuit), the DC-contact switch becomes a series resistor with a resistance of 0.5-2 Ω, depending on the contact metalused. Circuit and Substrate Configurations: As is the case with all two-terminal devices, the switches can be placed in series or in shunt across a transmission line. Typically, capacitive switches have been used in a shunt configuration, while DC-contact switches are placed in series. The reason is that it is easier to get a good isolation with a limited impedance ratio (such as the capacitive switch) in a shunt-circuit than in a series circuit. Also, MEMS switches are compatible with both microstrip and CPW lines on glass, silicon and GaAs substrates, and have been used in these configurations all the way to 100GHz. For low loss applications at microwave frequencies, it is important to use high-resistivity substrates.
CIRCUITS WITH RF MEMS SWITCHES
The near-ideal electrical response of RF MEMS witches (both metal-contact and capacitive) have allowed many designers to build state-of-the-art switching circuits from 0.1GHz all the way to 120GHz. In the past 4 years, these applications concentrated on the replacement of GaAs phase shifters which are commonly used in phased arrays by the thousands of units. A comparison between 3-bit GaAs phase shifters and MEMS phase shifters is shown in Table I and it is seen that MEMS switches provide an immense performance benefit especially at Ka-Band to W-band applications.
The near-ideal electrical response of RF MEMS witches (both metal-contact and capacitive) have allowed many designers to build state-of-the-art switching circuits from 0.1GHz all the way to 120GHz. In the past 4 years, these applications concentrated on the replacement of GaAs phase shifters which are commonly used in phased arrays by the thousands of units. A comparison between 3-bit GaAs phase shifters and MEMS phase shifters is shown in Table I and it is seen that MEMS switches provide an immense performance benefit especially at Ka-Band to W-band applications.
Fig. 4 presents a 4-bit miniature RF MEMS phase shifter developed jointly by the Univ. of Michigan and Rockwell Scientific. It is based on the Rockwell metalcontact switch and on CLC delay lines for miniaturization. The phase shifter results in an average loss of 1.4dB at 10GHz, a ±3◦ phase error, and is matched to −13 dB at the input and output ports from 6-16GHz. This phase shifter represents the smaller design using RF MEMS to-date, and with excellent response. Fig. 5 presents an 885-986MHz 5-pole tunable
filter using switched MEMS capacitors developed by Raytheon Systems Co. In this case, capacitive switches are used to switch fixed-value metalinsulator- metal capacitors in the transmission line. The filter employs 18 switches and is a very complicated circuit with variable resonators and impedance inverters. Its measured response is nearly ideal, with excellent frequency tuning capabilities, very high linearity (in terms of measured IIP3) and a loss of 5- 6 dB due to the finite Q of the planar inductors used (Q = 30 at 0.9GHz). Fig. 6 presents a W-band 3-bit phase shifter developed at the Univ. of Michigan using MEMS capacitive switches [3]. This is the highest frequency MEMS phase shifter to-date and results in an average loss of 2.7-2.9 dB at 77-94GHz with an associated phase error of ±3◦. The results are about 8 dB better than GaAs designs.
Other circuits, which are not shown due to space constraints, are very wideband SP4T switches, highisolation series/shunt switches covering 0.1-50GHz, double-pole double-throw transfer switches, and a whole range of phase shifters from 8GHz to 120GHz. Also, tunable filters covering 200MHz to 23GHz have been developed by various groups. In general, RF MEMS circuits outperform GaAs FET and PIN diode circuits by a large margin at all frequencies of interest
the RF and microwave communities. Most of the
circuits developed in the world can be found in [1].
circuits developed in the world can be found in [1].
THE FUTURE
It is now clear that we understand RF MEMS switches well, both from the mechanical and electrical/ electromagnetic point of view. We can design complicated circuits using MEMS switches or varactors, and we can accurately predict their performance all the way to 120 GHz. They are still not accepted in the commercial and defense arena due to their need of a hermetic package, and their reliability under medium to high-power conditions. There is currently an intense effort to solve these problems, and the author believes that RF MEMS switches and varactors will play an essential role in future high-value commercial and defense systems.
It is now clear that we understand RF MEMS switches well, both from the mechanical and electrical/ electromagnetic point of view. We can design complicated circuits using MEMS switches or varactors, and we can accurately predict their performance all the way to 120 GHz. They are still not accepted in the commercial and defense arena due to their need of a hermetic package, and their reliability under medium to high-power conditions. There is currently an intense effort to solve these problems, and the author believes that RF MEMS switches and varactors will play an essential role in future high-value commercial and defense systems.
Jorge L Polentino U
19769972
CRF
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the True Time Delay Phased Antenna Array
Beam Steering
The fundamental principles underlying the concept of electronic beam steering are derived from electromagnetic radiation theory employing constructive and destructive interference. These principles can be stated as follows: The electromagnetic energy received at a point in space from two or more closely spaced radiating elements is a maximum when the energy from each radiating element arrives at the point in phase. Controlling the phase through the many segments of the antenna system allows the beam to be rapidly directed in different directions. Figure 1 shows a 4-element linear array of antennas with constant phase difference between neighboring phase shifters, where θ0 is the scan angle, w1 to w4 are amplitude weights, and d is the spacing between adjacent antenna elements.
The angle θ0 of the beam with respect to the antenna axis is determined by the operating wavelength of the microwave signal, the spacing between the antenna elements that is usually half of a wavelength, and the phase shift between the signals in the individual elements. It is given by
(1) 
where l is wavelength, d is the inter-element array spacing, nd is the location of the particular radiating element n being investigated, and f is the phase shift located at the nth element. It is seen that changing the frequency results in a change in steering angle q0, if the phase shift f is fixed. Therefore, the beam squinting arises from this distortion.
Instead, the group time delay t only depends on beam position angle and array length, but not frequency, which is shown in Eq. (2).
Beam shaping
Although steering capability is the most common function, a phased antenna array can also provide beam-shaping capability by appropriate arrangement of the feed signals. A radar system with variable beamwidth can produce a wide beam for the acquisition of targets and a narrow beam for subsequent high-precision tracking. Additionally, a broadcast satellite antenna with variable beamwidth can achieve efficient coverage of irregularly shaped geographical service areas based on the environment or traffic conditions, which is an important feature for communication systems.
The major objectives of beam shaping are to minimize pattern ripples, to reduce sidelobe levels, change null positions, or control output power levels, etc. To vary the antenna beamwidth, there are many methods which may be employed. In this research work, phase-only adjustment is used to vary the beamwidth. Adding a quadratic phase error on the array aperture moves the array's phase center and changes its focusing distance (see Fig. 2). However, for a given size of an array aperture, there is a limit to the movement of the phase center. Beyond a certain value of phase taper across the array aperture, beam bifurcation on the plane of the linear array occurs. In addition, the sidelobe level of an array with equal power feeding increases rapidly as the value of phase taper increases. In order to increase the useful range and control the sidelobe level, a Taylor N-bar amplitude taper is introduced across the array aperture in this design.
True Time Delay Technologies
The bandwidth of a phased antenna array is affected by many factors, including change of element input impedance with frequency, change of array spacing in wavelengths that may allow grating lobes, change in element beamwidth, etc. When an array is scanned with fixed values of phase shift, provided by phase shifters, there is also a bandwidth limitation as the position of the main beam will change with frequency, which is called beam squint. In Eq.1, It is obvious that changing the frequency results in a change of the scan angle for fixed element spacing. In contrast, when the array is scanned with true time delay, the beam position is independent of frequency to first order.Three representative true time delay (TTD) technologies are being investigated in our group. They are:
· Piezoelectric-Bender Controlled Delay Line;
· RF-MEMS Extended Tuning Range Varactor Delay Line;
· Liquid Crystal Phase Shifter
Jorge L Polentino U
19769972
CRF
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