10ma publicacion. nanyoly mendez. CAF. 1er parcial

ALD & MLD for MEMS/NEMS Devices

The ability of ALD to grow conformal films on high aspect structures is extremely valuable for microelectromechanical systems (MEMS) applications. ALD coatings can enhance MEMS reliability by providing protecting coatings that minimize mechanical wear . ALD coatings can also be used to insulate, facilitate charge dissipation and functionalize the surface of MEMS devices . Much of our research on ALD for MEMS and nanoelectromechanical systems (NEMS) is conducted in collaboration with Prof. Victor Bright's group in the Dept. of Mechnical Engineering at the University of Colorado.

ALD can also be used as a fabrication tool to create novel structures. New directions include NEMS nanofabrication using ALD and MLD. MLD can deposit precise sacrificial layers that can be removed after ALD and MLD processing . The sacrificial layers become air gaps that are useful for the fabrication of mechanical switches and bridges . The accompanying figure shows a schematic representation and the SEM image of a W ALD bridge defined by a ABC aluconeairgap. ALD-coated carbon nanotubes or nanowires may also be useful for sensors for MEMS/NEMS devices.

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9na publicacion. nanyoly mendez. CAF. 1er parcial

MEMS Technology At The Heart Of Omron Flow Sensors

A major advantage of MEMS Flow Sensors is their ability to measure flow speed from 1 mm/s to 40 m/s. To put this into perspective, this covers a range from the fluttering of a butterfly's wings to the roar of a typhoon. At the heart of the MEMS Flow Sensor, there is a tiny sensor element; the Omron MEMS Flow Sensor chip which is only 1.5 mm square by 0.4 mm thick.

Conventional flow sensors use a resistance measurement method based on a natural characteristic that causes the electrical resistance of a material to change due to changes in temperature. This method has a number of disadvantages, though, such as the high cost required for the extremely time-consuming adjustment of the resistance balance.

In contrast the Omron MEMS Flow Sensor, which by the way was the industry's first to apply this technology, utilizes an element called a thermopile that converts thermal energy into electrical energy. This revolutionary method delivered a variety of previously nonexistent advantages, including low-cost operation because there are few adjustments required, low power consumption, and high sensitivity.

The chip's two sets of thermopiles, located on either side of a tiny heater element, are used to measure the deviations in heat symmetry caused by gas flowing in either direction. A thin layer of insulating film protects the sensor chip from exposure to the gas.

When there is no flow present, temperature distribution concentrated around the heater is uniform and the differential voltage over the two thermopiles is 0V (Diagram1). When even the smallest flow is present, temperature on the side of the heater facing the flow cools, and warms up on the other side of the heater - heat symmetry collapses. The difference of temperature appears as a differential voltage between the two thermopiles, proportional to the mass flow rate.

Diagram 1Omron was the first manufacturer to utilize thermopile technology to measure flow rate.

Omron's unique etching technologies (Diagram 2) were used to create a unique shape that gives their flow sensing chip it's superb characteristics by providing a larger sensing area compared to Conventional Silicon Etching in the same volume. This cavity design enables efficient heating with low power consumption.

MEMS CircuitTo keep the heater temperature above that of the gas being measured, temperature compensating circuitry, which could be described as an "expanded bridge circuit" is incorporated in all Omron flow sensors (except the very economical D6F-V clogged filter/air velocity sensor). This expanded circuit arrangement provides improved temperature characteristics over an ordinary bridge circuit (Diagram 3).

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RF MEMS: a brief history and future trends

One of the most exciting and rapidly growing set of applications in our industry are those involving RF MEMS. We recently spoke with Dr. Dan Hyman, the founder and CEO of XCOM Wireless, about RF MEMS history, current challenges and future trends.

In this thoroughly comprehensive interview, Dr. Hyman shares his expertise and insight, and offers intriguing predictions about the future potential of RF MEMS devices.

MEMS Investor Journal: Why is there a need for RF MEMS? Why are MEMS devices for RF applications better than existing non-MEMS devices?

Dr. Dan Hyman: Developing RF circuits and subsystems requires a series of engineering trade-offs that are limited by the technology you are using. This is true at the device, component, and circuit level, and this is a part of RF engineering life that has been true for many decades. This is "easy" to deal with for single-mode systems like an old-fashioned cell phone, or modern Bluetooth circuit, but this gets harder and harder to do as frequencies get higher, data bandwidth gets larger, and, most of all, when multiple broadband signals have be handled in the same device. This is a defining trend in the wireless industry, and one that is taxing the limits of conventional technologies and "old-school" radio architectures. Wireless engineers inthe broadband, multi-standard world need a better way of doing things, and better technologies that can easily handle these widely varying signals. Enter RF MEMS.

Essentially, RF MEMS devices offer "best of breed" in a host of performance and usage parameters out of all possible technologies you would reasonably consider to be your alternatives in a very wide variety of RF applications. The most widely recognized advantages are low loss, high isolation, near-perfect linearity, and unbelievably large instantaneous bandwidth that conventional mechanical and semiconductor technologies simply can't even compete with. On top of this are a host of usage parameters like cost, size, speed, ruggedness, reliability, repeatability, and lifetime, that each range from fantastic to poor depending specifically on what you are comparing them to and depending on whether or not a customer values that particular specification. A good RF MEMS Ohmic relay, for example, has fantastic repeatability and ruggedness compared to any semiconductor technology, and has a good (or great) price, size, speed, reliability, and lifetime compared to any conventional mechanical technology.

There are a staggering number of RF applications where RF MEMS are superior in every measurable technical way to incumbent technologies, and it is this variety that allows for a substantial amount of non-competitiveness in the marketplace. Business concerns of supplier networks, fulfillment, qualification & supplier chain, etc. are still real challenges for many applications and customers, however, so niche applications are still the best bet for near-term product releases. Also note that there are many applications where RF MEMS devices are not a good choice. The ubiquitous and over-lauded "T/R switch in a GSM handset," for example, was never a good idea to consider for RF MEMS, and probably never will be.

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MEMS and technology gap

NTT Microsystem Integration Laboratories has the mission of creating key hardware-oriented technologies to achieve a network environment that can provide safe, secure, and enjoyable broadband and ubiquitous services, as promoted by the NTT Group. At these laboratories, MEMS is positioned as a platform technology for creating ubiquitous services.

The graph in Fig. 1 shows device design rule on the horizontal axis and film thickness on the vertical axis. RF-MEMS and optical-MEMS devices lie in the intermediate region between miniature devices like LSIs (large-scale integrated circuits) and Jisso technologies*1 like multichip modules and printed wired boards. Jisso technologies have an important role in enabling MEMS devices with microscopic movable mechanical elements to interface with the outside world. Existing technologies used for Jisso, however, target elements with sizes of several micrometers to several millimeters or more, and they cannot by themselves be used to fabricate tiny MEMS devices.

Fig. 1.Device technology and dimensions.

If, however, submicrometer LSIs and MEMS devices using LSI fabrication technology can be fabricated and integrated on a silicon substrate, we can expect even greater downsizing and greater functionality including enhanced intelligence in MEMS devices. Yet, technology for fabricating LSIs is specialized for ultramicrofabrication, and it too is not necessarily applicable to the fabrication of MEMS devices. This leaves MEMS devices lying in a technology gap between LSIs and Jisso technologies. Consequently, despite the great expectations that MEMS has been generating, only a few MEMS devices have been successful in business, such as inkjet printer heads.

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6ta publicacion. nanyoly mendez. CAF. 1er parcial

Seamless integration technology

To fill in this technology gap, NTT Microsystem Integration Laboratories is developing technology that uses the silicon technology cultivated with LSIs as a core technology for fabricating and converging ultrafine LSIs and MEMS devices on a silicon wafer while providing a seamless bridge to Jisso technologies. This seamless integration technology (SeaiT), as it is known, aims to achieve smooth convergence between the heterogeneous functions performed by microelectronics and micromachines [2], [3].

As shown in Fig. seamless integration technology for converging LSI and MEMS is intended to fabricate microscopic MEMS devices and wiring with a size of 10 µm to 1 mm made of metal or silicon in a layered manner. This calls for fabrication technology that will produce little damage to the LSI. Specifically, it must be a low-temperature process and must not use dry etching, which could damage LSIs, for example. Accordingly, to form MEMS structures without incurring such damage, we are developing low-temperature plating techniques as well as 10-µm-level thick-film multilevel interconnection technology using photosensitive organic resins and other advanced materials.

Concept of seamless integration technology.

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MEMS Device Technologies

MEMS stands for microelectromechanical systems. It is an enabling technology that uses semiconductor microfabrication processes to develop integrated devices containing movable mechanical elements and electrical elements from the submicrometer level up to sizes of about one millimeter. MEMS promises to provide key high-performance devices featuring energy-saving characteristics, compact configurations, and high accuracy applicable to a diverse range of fields such as information communications, security, and biotechnology.

No doubt many readers have heard of the 1966 science fiction film "Fantastic Voyage". To save an important person suffering from a blood clot in the brain, a submarine with a crew including doctors and scientists is shrunk to microscopic size and injected into the person's bloodstream in an effort to destroy the clot. At the time of the film's release, this was a completely fantastic idea, as the name of the movie implies. Of course, sending people into a person's body is just a fantasy, but the idea of treating the body from the inside is starting to take shape through devices such as ingestible endoscopy capsules.

The microdevice concept dates back nearly 50 years to a famous lecture entitled "There's Plenty of Room at the Bottom" given by Richard Feynman, who later was a joint recipient of the Nobel prize, at a 1959 meeting of the American Physical Society held at the California Institute of Technology . In that lecture, Feynman mentioned the possibility of micromachines consisting of several thousand atoms. The lecture title means that there are still unexplored regions at the submicroscopic scale that mankind should investigate through science and technology. A chronology of inventions and discoveries that have become a basis for MEMS-related technologies is given in Fig. 1. Also given are the years of Nobel prizes awarded in recognition of these ground-breaking efforts. We can see that a number of great inventions and discoveries in the microworld that have become the basis for information communications and biotechnology of the present were made in the ten-year periods before and after Feynman's lecture. These include the invention of the transistor, proposal of basic principles underlying integrated circuits, discovery of the double-helix structure of DNA, and invention of the laser.

Fig. 1. Major scientific/technological inventions and discoveries related to MEMS.

We can view MEMS as technology for integrating in the microworld these inventions and discoveries of 20th-century science and technology for use in information communications, biotechnology, and other fields, and as a means for fabricating 21st-century devices and systems. At present, MEMS-related development proceeds on a field-by-field basis, such as optical-MEMS for optical communications, RF-MEMS for wireless communications including cell phones (RF: radio frequency), and bio-MEMS for medical care and biotechnology.

MEMS devices in development
Seamless integration technology enables the formation on LSIs with structures having dimensions or thicknesses that are hard to achieve by existing technologies. This should lead to highly functional MEMS chips featuring an array of MEMS devices or MEMS that integrate new functions that could not be achieved with LSIs or existing MEMS devices alone.With the aim of providing safe and secure ubiquitous services, NTT Microsystem Integration Laboratories is developing a MEMS fingerprint authentication sensor [7], a MEMS mirror array as an element of a MEMS optical switch module [8] (a key device in the next-generation network), and an RF-MEMS chip for wireless terminals [9]. The other selected papers in this issue describe seamless integration technology in more detail and introduce these MEMS devices.

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MEMS and Microsystems in Europe

The term MEMS (micro-electro mechanical systems) is heard frequently in Europe, but most of the organizations our group visited prefer "microsystems" (or the acronym MST — microsystems technology) to define the domain of interest. MST has a significantly broader meaning than MEMS. While devices fabricated with IC technology that include moving or moveable parts for actuation or sensing are of course included, so are other categories of very compact device types where shape is critical to functionality, including both passive and active devices. The field of MST also includes work that seeks to incorporate such devices into highly compact systems.1

A useful operative definition of the scope of MST as understood in Europe was developed by a NEXUS task force in 1998 (see below for a brief description of NEXUS):

"Microstructure products have structures in the micron range and have their technical function provided by the shape of the microstructure. Microsystems combine several microcomponents, optimized as an entire system, to provide one or several specific functions, in many cases including microelectronics."

As evidenced by this definition, microsystems in Europe do not necessarily include integrated circuits, nor are they always monolithically integrated. As this report will show, the European technical community envisions numerous types and varieties of devices and applications for future microsystems in Europe, making them a pervasive influence in many product sectors ranging from automotive and domestic electronics to the medical and pharmaceutical sector.MEMS and Microsystems in Europe.

MEMS/Microsystems Device and Process Technologies: The State of the Art
On a worldwide basis, MEMS devices or microsystems which have successfully established high-volume commercial markets include accelerometers and pressure sensors for automotive applications, inkjet print heads, and digital micro-mirrors for image projection. The automotive MEMS supplier group has strong European representation by companies such as Bosch, TEMIC, SensoNor, and VTI-Hamlin, all of which are major players in this market. The other two device classes are mostly supplied by U.S. and Japanese companies.

In addition to major industry players, our team visited with research institutes, government laboratories, and universities pursuing new and emerging technologies and applications, in order to find out about the future of MEMS and microsystems in Europe.

* Among the institutions and companies the MCC team visited in central Western Europe (Germany, Switzerland, France, Belgium, and the Netherlands), the device classes being pursued mostly fell into the following categories:

Fluidic MEMS: The majority of sites visited had a significant level of effort in fluidic devices such as pneumatic valves, membrane pumps, chemical reactors, and flow and pressure sensors. The application targets range from medical and biological, to pharmaceutical and chemical. Miniaturization here increases portability, reduces cost, increases accuracy, reduces the amount of chemical or biological sample material required for analysis, and also decreases measurement time.

Mechanical Transducers: With major applications and markets already established in this device class, the work in the research and development laboratories we visited focuses on further integration for cost reduction, including work with side-impact sensors, or specialty application niches with less price pressure, ranging from instrumentation, aeronautics, and down-hole sensing for drilling equipment, to medical patient monitoring. Most of the sites we visited continue to work in this class of devices.

Optical MEMS: Some of the laboratories visited are working on micro-opto-electro-mechanical devices such as micromirrors for scanning or imaging applications, temperature IR sensors, as well as miniaturized devices such as connectors and switches for fiber applications.
Electrical MEMS Switches: Several European groups are competing in this area, with Siemens leading the way towards commercialization.

Others: Many of the laboratories visited presented work on passive miniaturized components without any moving part such as integrated inductors, magnetic devices, trench capacitors, ISFETs, etc.

Device technology areas where the MCC team did not see evidence of significant level of activity are fuel cells, micro-motors, and wireless communication (RF/microwave), although Europe is said to have a significant amount of activity in the RF/microwave area.

* The process and device technologies used in these efforts include all approaches and a variety of materials. The European microsystems researchers are pragmatic in the ways they approach miniaturization; they are not partisan to a particular process or technology but usually have a variety of process capabilities at their disposal, either internally or through collaboration within the European and national programs. They focus on the goals (products), not the means (processes) to achieve them. This having been said, there appears to be much work in progress in Europe on high-aspect-ratio devices through processes such as deep reactive ion etching (DRIE) and LIGA-like processes.

* Silicon micromachining is only one of the tools employed in the quest for miniaturization. Our team heard of work on polymers, glass, even metals. CMOS-based research centers (such as IMEC, the University of Delft, or Siemens/Infineon) are naturally approaching the microsystems field from the point of view of enhancing the integrated circuit capabilities through back-end micromachining and forming an integrated, low-cost device. However, the majority of the institutions visited which do not have the silicon CMOS focus place less emphasis on silicon integration, as they are targeting small- to medium-volume applications, such as medical applications, with less restrictive cost constraints.In summary, the focus in the European microsystems community is on device and system miniaturization using the processes and materials that fulfill particular application requirements. The device domains receiving the greatest attention in Europe appear to be those that are relatively simple, and that have limited requirements for movement. The push is toward simple miniaturized devices which provide incremental performance advantages over their traditional counterparts, not revolutionary new concepts and break-through applications. There is a large amount of work going on in the micro-fluidics area with medical, biological, and chemical applications targeted as the next area where microsystems will move into commercial markets.

 

 

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3ra publicacion. nanyoly mendez. CAF. 1er parcial

MEMS Switches: On or Off?

MEMS switches have led a short, interesting life so far. A lot of hope was heaped on these components, but six years after the first commercial announcements were made in 2003, there are still no significant revenues, and the field has already claimed a number of victims. The most well-known casualty is Magfusion; more recently, Teravicta, Simpler Networks, and Siverta have fallen by the wayside as well. Such fallout does not even take into account the RF MEMS switch programs that have closed down at companies like Infineon, ST, Motorola, Alcatel, and more recently, RFMD.

Despite this gloomy picture, MEMS switches continue to fascinate the industry, and like salmon springing upstream against the odds, there are always companies launching new MEMS switch development programs or products. Examples include Omron (see November Market brief), Maxim (December brief), and new start-up companies like MultusMEMS (February brief).

Definition
For the purpose of this article, iSuppli provides an extended definition of RF MEMS. These are components such as:
RF MEMS relays from DC to microwave frequencies up to 100 GHz
RF MEMS switched capacitors, sometimes called varactors
Simple on-off MEMS switches that do not transmit a signal, as a replacement of conventional Reed-, mechanical snap- or even Hall switches

Status of MEMS Switches Today
The current status of the MEMS switch market is summarized in the table. Essentially, RF MEMS switches are produced in appreciable volume by Advantest, mostly for its own Automated Test Equipment (ATE) applications.

Table caption: Overview of activities in RF MEMS and MEMS switches (source, iSuppli) Click image for larger version.

Non-RF applications are essentially limited to medical applications such as smart pills, which can accommodate higher costs. Asulab and MEMSCAP are players here, although other companies like OKI are targeting this space.

Sampling today, in small volumes for evaluation at end customer but not yet implemented in series in real applications, are Panasonic and Omron for ATE and RF test, Radant for test in defense and civilian applications (Radant was unable to export outside the United States in the past due to its military funding and is now free to sell on the free market). Also sampling are WiSpry, with a varactor component for cell-phone applications and serial production expected this year (see this issue, news); and French utility company Schneider Electric. Schneider owns Kavlico (MEMS pressure sensors) and SDA (MEMS gyroscopes), and develops MEMS switches for its own industrial equipment and outside the group.

Development is very active at a number of companies like X-COM for military and instrumentation. Commercial samples should be available this year through its partner Teledyne, in addition to products being available from Baolab, a Spanish start-up looking to service cell-phone applications and planning to sample first devices in 2010. Maxim is aiming at instrumentation (ATE and RF), but there are also development programs at several semiconductor groups, including Fujitsu, Toshiba, Mitsubishi and Freescale, ADI, Alps, OKI, and at defense and aerospace companies like Raytheon, Rockwell, and EADS. The early stage of development of RF MEMS switches and varactors is taking place at Protronor Swedish start-up MultusMEMS.

Market Outlook
The market for MEMS switches remains small (see Figure 1). But though still at the beginning of the curve, revenue dropped in 2008 as Magfusion went off the radar and the ATE equipment market collapsed in the second half of the year.

Market for MEMS Switches, 2006-2013

However, iSuppli expects the market to pick up again this year as WiSpry goes into serial production for cell-phone applications. The market should subsequently accelerate in 2011 and 2012 as additional suppliers like EPCOS, Baolab, and Skyworks break into the cell-phone business and as ATE starts to generate significant revenues followed by RF MEMS switches for phased array antennas and tunable/switchable filters in military systems in 2012 and 2013.
In 2013, iSuppli expects the total market to be in the range of $160 million. The main application fields will be mobile handsets, instrumentation, and defense and telecom infrastructure. What some consider an optimistic forecast is actually very conservative compared to projections from other analysts, who see an explosion of the market from $5 million in 2007 to $700 million in just five years (a CAGR of 168%)!

Cell-phone Applications
The major trends for RF components in mobile handsets are summarized as follows (from iSuppli's wireless team in its recent Topical Report, RF Components: Adapt or Die – Maybe Darwin Was Un to Something):
Tunable matching circuitry is a must for multimode, multiband handsets
New RF technologies for high data rates (HSPA+ and LTE) require higher linearity
Multiband PAs are being introduced to support both polar and linear transmit architectures
Considering its superior performance in linearity, RF MEMS switches and varactors are ideal candidates for impedance matching networks in front of a reconfigurable power amplifier, as demonstrated by RFMD and WiSpry, or for the antenna matching function, which is being pursued by EPCOS.

Caption: Tunable Impedance Matching Device (TIM), Tunable Digital Capacitor (TDC), and Antenna Tuner (TA) on 8" CMOS wafer (Courtesy: WiSpry)

iSuppli does not expect RF MEMS to be adopted by the majority of the market in the next four years for this function The reason is that emerging technologies such as ferroelectric varactors as well as CMOS switches from companies like Peregrine, which not only have made great strides but also have performed as well as MEMS in terms of linearity.

In addition, there are only two credible candidates for supplying RF MEMS switches and varactors in the next three years—WiSpry and EPCOS—although other companies to watch are Skyworks and Baolab.

INSTRUMENTATION MARKETS
While the potential market for MEMS switches in ATE and RF Test well exceeds $100 million, iSuppli expects that MEMS will have grabbed only a portion of it by 2013. MEMS have been used since 2005, but iSuppli does not expect a significant uptake before 2011 or 2012 for the following reasons:
The ATE market is in a slump along with the entire manufacturing and test equipment chain for semiconductors today. This slows the implementation of new technologies.

While test companies love RF MEMS in theory, iSuppli has noticed a growing skepticism/caution regarding its adoption. This follows the bankruptcy of several suppliers with which test companies have cooperated, as well as repeated issues with MEMS reliability.

Finally, experience shows that a number of iterations occur in the design of MEMS switches and relays for test companies, and that it often takes several years to go from commercial sampling to serial implementation.iSuppli also takes note of a growing interest in MEMS switches (and not relays) as simple on-off switches to function as replacement for Reed switches in other industrial applications. Schneider Electric is the company to watch in this space.

 

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Applications

Applications of RF MEMS resonators and switches include oscillators and routing networks. RF MEMS components are also applied in radar sensors (passive electronically scanned (sub)arrays and T/R modules) and software-defined radio (reconfigurable antennas, tunable band-pass filters).

Antennas
Polarization and radiation pattern reconfigurability, and frequency tunability, are usually achieved by incorporation of lumped components based on III-V semiconductor technology, such as single pole single throw (SPST) switches or varactor diodes. However, these components can be readily replaced by RF MEMS switches and varactors in order to take advantage of the low insertion loss and high Q factor offered by RF MEMS technology. In addition, RF MEMS components can be integrated monolithically on low-loss dielectric substrates, such as borosilicate glass, fused silica or LCP, whereas III-V semiconducting substrates are generally lossy and have a high dielectric constant. A low loss tangent and low dielectric constant are of importance for the efficiency and the bandwidth of the antenna.

The prior art includes an RF MEMS frequency tunable fractal antenna for the 0.1–6 GHz frequency range, and the actual integration of RF-MEMS on a self-similar Sierpinski gasket antenna to increase its number of resonant frequencies, extending its range to 8 GHz, 14 GHz and 25 GHz , an RF MEMS radiation pattern reconfigurable spiral antenna for 6 and 10 GHz, an RF MEMS radiation pattern reconfigurable spiral antenna for the 6–7 GHz frequency band based on packaged Radant MEMS SPST-RMSW100 switches, an RF MEMS multiband Sierpinskifractal antenna, again with integrated RF MEMS switches, functioning at different bands from 2.4 to 18 GHz, and a 2-bit Ka-band RF MEMS frequency tunable slot antenna.

Filters
RF bandpass filters are used to increase out-of-band rejection, if the antenna fails to provide sufficient selectivity. Out-of-band rejection eases the dynamic range requirement of low noise amplifier LNA and mixer in the light of interference. Off-chip RF bandpass filters based on lumped bulk acoustic wave (BAW), ceramic, surface acoustic wave (SAW), quartz crystal, and thin film bulk acoustic resonator (FBAR) resonators have superseded distributed RF bandpass filters based on transmission line resonators, printed on substrates with low loss tangent, or based on waveguide cavities. RF MEMS resonators offer the potential of on-chip integration of high-Q resonators and low-loss bandpass filters. The Q factor of RF MEMS resonators is in the order of 1000-1000 .


Tunable RF bandpass filters offer a significant size reduction over switched RF bandpass filter banks. They can be implemented using III-V semiconducting varactors, BST or PZT ferroelectric and RF MEMS switches, switched capacitors and varactors, and yttrium iron garnet (YIG) ferrites. RF MEMS technology offers the tunable filter designer a compelling trade-off between insertion loss, linearity, power consumption, power handling, size, and switching time .

Phase shifters
RF MEMS phase shifters have enabled wide-angle passive electronically scanned arrays, such as lenses, reflect arrays, subarrays and switched beamforming networks, with high effective isotropically radiated power (EIRP), also referred to as the power-aperture product, and high Gr/T. EIRP is the product of the transmit gain, Gt, and the transmit power, Pt. Gr/T is the quotient of the receive gain and the antenna noise temperature. A high EIRP and Gr/T are a prerequisite for long-range detection. The EIRP and Gr/T are a function of the number of antenna elements per subarray and of the maximum scanning angle. The number of antenna elements per subarray should be chosen to optimize the EIRP or the EIRP x Gr/T product, as shown in Fig. 3 and Fig. 4.

Fig. 3: Radar sensor sensitivity: EIRP x Gr/T

Fig. 4: EIRP versus number of antenna elements in a passive subarray

Passive subarrays based on RF MEMS phase shifters may be used to lower the amount of T/R modules in an active electronically scanned array. The statement is illustrated with examples in Fig. 3: assume a one-by-eight passive subarray is used for transmit as well as receive, with following characteristics: f = 38 GHz, Gr = Gt = 10 dBi, BW = 2 GHz, Pt = 4 W. The low loss (6.75 ps/dB) and good power handling (500 mW) of the RF MEMS phase shifters allow an EIRP of 40 W and a Gr/T of 0.036 1/K. The number of antenna elements per subarray should be chosen in order to optimize the EIRP or the EIRP x Gr/T product, as shown in Fig. 3 and Fig. 4. The radar range equation can be used to calculate the maximum range for which targets can be detected with 10 dB of SNR at the input of the receiver.

in which kB is the Boltzmann constant, λ is the free-space wavelength, and σ is the RCS of the target. Range values are tabulated in Table 1 for following targets: a sphere with a radius, a, of 10 cm (σ = π a2), a dihedral corner reflector with facet size, a, of 10 cm (σ = 12 a4/λ2), the rear of a car (σ = 20 m2) and for a contemporary non-evasive fighter jet (σ = 400 m2). A Ka-band hybrid ESA capable of detecting a car 100 m in front and engaging a fighter jet at 10 km can be realized using 2.5 and 422 passive subarrays (and T/R modules), respectively.

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RF MEMS

The RF MEMS acronym stands for radio frequency microelectromechanical system, and refers to components of which moving sub-millimeter-sized parts provide RF functionality. RF functionality can be implemented using a variety of RF technologies. Besides RF MEMS technology, ferrite, ferroelectric, GaAs, GaN, InP, RF CMOS, SiC, and SiGe technology are available to the RF designer. Each of the RF technologies offers a distinct trade-off between cost, frequency, gain, large scale integration, lifetime, linearity, noise figure, packaging, power consumption, power handling, reliability, repeatability, ruggedness, size, supply voltage, switching time and weight.

Microfabrication

An RF MEMS fabrication process allows for integration of SiCr or TaN thin film resistors (TFR), metal-air-metal (MAM) capacitors, metal-insulator-metal (MIM) capacitors, and RF MEMS components. An RF MEMS fabrication process can be realized on a variety of wafers: fused silica (quartz), borosilicate glass, LCP, sapphire, and passivated silicon and III-V compound semiconducting wafers. As shown in Fig. 1, RF MEMS components can be fabricated in class 100 clean rooms using 6 to 8 optical lithography steps with a 5 μm contact alignment error, whereas state-of-the-art monolithic microwave integrated circuit (MMIC) and radio frequency integrated circuit (RFIC) fabrication processes require 13 to 25 lithography steps. The essential microfabrication steps are:

Deposition of the bias lines (Fig. 1, step 3)
Deposition of the electrode layer (Fig. 1, step 4)
Deposition of the dielectric layer (Fig.1, step 5)
Deposition of the sacrificial spacer (Fig. 1, step 6)
Deposition of seed layer and subsequent electroplating (Fig. 1, step 7)
Beam definition, release and critical point drying (Fig. 1, step 8)

RF MEMS fabrication processes, unlike barium strontium titanate (BST) or lead zirconatetitanate (PZT) ferroelectric and MMIC fabrication processes, do not require electron beam lithography, molecular beam epitaxy (MBE), or metal organic chemical vapor deposition (MOCVD). With the exception of the removal of the sacrificial spacer, the fabrication steps are compatible with a CMOS fabrication process.

Fig. 1: RF MEMS fabrication process

 

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