Integrated RF-MEMS Technology for Reconfigurable RF Transceivers

Integrated RF-MEMS Technology for Reconfigurable RF Transceivers

1. Introduction
Various types of wireless communication services are provided around the world through mobile phone networks, wireless local area networks, and RFID (radio frequency identification) systems. With the globalization and diversification of wireless communications, many wireless standards have been established over a wide range of frequencies to provide such services. To make various services available everywhere, we need small low-power RF transceivers capable of multiband operation.
As shown in Fig. 1, currently available RF transceivers consist of several LSI chips and numerous off-chip passive components. Increasing the number of available communication bands will increase the number of components and make RF transceivers larger. It will also increase power consumption and shorten battery life. Reconfigurable RF circuits, which can change circuit configurations according to the wireless standards to be used, would enable multiband operation without increasing the size and power consumption. However, the performance of conventional switches and tunable passive devices needed for such reconfiguration is insufficient. RF-MEMS (radio frequency microelectromechanical systems) technology is attracting interest for the fabrication of high-performance RF devices for reconfigurable RF circuits [1], and several types of RF-MEMS devices, such as switches, variable capacitors (varactors), inductors and filters, have been developed. One problem with conventional RF MEMS devices is that they have been developed as discrete components, and using many RF-MEMS devices increases the size of RF transceivers. Another problem is related to packaging. RF-MEMS devices have movable parts that can be easily damaged or destroyed during packaging processes, such as wafer dicing.

To solve these problems, we have developed integrated RF-MEMS technology for the integration and protection of RF-MEMS devices. For integration, an adaptable multilayer structure and its fabrication process enable fabrication of various types of RF-MEMS devices on the same substrate. For protection, a wafer-level encapsulation process forms small thin capsules to protect RF-MEMS devices.

This paper first explains the concept of our technology. Next, it describes the structure and fabrication process for the devices. Finally, it presents experimental results.

2. Integrated RF MEMS technology
The aim of integrated RF-MEMS technology is to integrate various types of RF-MEMS devices and RF circuits to develop single-chip reconfigurable RF transceivers [2]. The circuit diagram of an RF receiver using RF-MEMS devices is shown in Fig. 2(a). The RF receiver is composed of antennas, RF front-end circuits, an analog-to-digital converter, a baseband processor, and a control circuit. RF-MEMS devices, such as switches, filters, varactors, and inductors, are integrated with the RF front-end circuits. These devices provide reconfigurability and enable multiband operation. For example, switches having low insertion loss and high isolation can reconfigure a circuit according to the communication band, and varactors with wide-range tunability provide wideband oscillators. The integration of RF-MEMS devices and a control circuit also enables on-chip testing and compensation of the devices. This reduces the variation in device characteristics and improves productivity while eliminating the need for extra LSI (large-scale integration) chips.

The configuration for integrated RF MEMS technology is conceptually illustrated in Fig. 2(b).

Several types of RF-MEMS devices are simultaneously formed on top of an LSI and connected to the circuits. This configuration reduces the volume consumed by discrete packaging of the devices and allows the use of a lot of devices without increasing the number of off-chip components. RF-MEMS devices with movable parts, such as switches, are protected by a device-scale capsule to prevent their destruction during packaging. The protective capsule allows the use of conventional LSI packaging technology.
This technology will lead to the fusion of RF-MEMS and circuit technologies and to the development of single-chip reconfigurable RF transceivers.

3. Device structure

The device structure for integrating different types of RF-MEMS devices is illustrated in Fig. 3. The devices are formed in multiple layers and protected by a capsule. The stacking of multiple layers forms various thicknesses and gaps, providing adaptability for integration. Because each layer is fabricated by the same process at the same time, several types of devices can be formed simultaneously.

Fig. 3. Cross-sectional schematic of the integrated RF-MEMS devices.

Each capsule is composed of walls, a roof, and a sealing film. The walls and roof are formed at the same time as the devices, and the etching holes in the roof are sealed with thin film by using a selective sealing technique (see section 4). For compatibility with LSI fabrication, the devices use electrostatic actuation. Applying a voltage between electrodes displaces the movable electrode so that it moves towards the fixed electrode. This enables various electromechanical functions, such as signal switching, capacitance tuning, and frequency filtering. The electrostatic actuation can be implemented without using special materials, such as piezoelectric material, which would make it difficult for the process to be compatible with LSI fabrication.
The device structure enables the fabrication of integrated, protected RF-MEMS devices on an LSI.

4. Fabrication process

The fabrication process for the device structure is based on seamless integration technology [3]. The process flow for a switch in a capsule is shown in Fig. 4. First, the lower structures of the switch, such as lower interconnections, are formed on the substrate by Au electroplating (Fig. 4(a)). Next, the lower structures are planarized with photosensitive polyimide (Fig. 4(b)). The polyimide is a sacrificial layer and is removed later. Then, other parts of the switch and the capsule are formed by repeating the electroplating and planarization (Figs. 4(c) and (d)). This stacking process simplifies the fabrication of the multilayer structure. At this point, the sacrificial layers are removed by dry ashing through etching holes in the roof (Fig. 4(e)). Finally, the etchings holes are sealed with photosensitive polyimide by transferring the sealing film onto the roof. This film transfer is accomplished by using spin-coating film transfer and hot pressing (STP), a technology that we developed ourselves (Figs. 4(f) and (g)) [4]-[6]. The encapsulation protects the integrated RF-MEMS devices without damaging them.

Fig. 4. Fabrication process for integrated RF-MEMS devices.

5. Experimental results
We fabricated the integrated RF-MEMS devices shown in Fig. 5. The images were taken before encapsulation to show the devices. Different kinds of RF-MEMS devices, such as switches, varactors, inductors, and filters, were fabricated on the same substrate by stacking multiple layers. The devices are about 200 µm × 200 µm in size, and the thicknesses and gaps range from less than 0.5 µm to over 10 µm. The multilayer structure enables this wide range of thicknesses and gaps.

Fig. 5. SEM photographs of the fabricated RF-MEMS devices.

The integrated RF-MEMS devices were encapsulated using STP technology. Figure 6 shows SEM photographs of (a) a capsule covering a switch and (b) the inside of the capsule, where a switch was properly encapsulated. Figure 6(c) shows a magnified view of the beam of the switch and the roof of the capsule. It is clear that the etching holes in the roof were sealed with the film. These images show that the encapsulation of RF-MEMS devices was successful.

Fig. 6. SEM photographs of an RF-MEMS switch in a capsule.

The electrical characteristics of integrated RF-MEMS devices were measured. DC-measurement results for a switch are shown in Fig. 7. A voltage of more than 16 V applied between the top and bottom electrodes activated the switch, and low resistance of 1.5 Ω was obtained. The switch was turned on and off and it operated repeatedly.

Fig. 7. Measured DC characteristics of a fabricated RF-MEMS switch.

These results confirm that different types of RF-MEMS devices with encapsulation can be fabricated on the same substrate.

6. Conclusion

We have developed elemental techniques for integrated RF-MEMS technology. The multilayer structure and its fabrication process enabled us to fabricate different types of RF-MEMS devices on the same plane. An STP-based encapsulation technique has also been developed to protect the devices. Several types of protected RF-MEMS devices were fabricated, and the operation of a switch was demonstrated. These techniques will pave the way for the development of reconfigurable RF transceivers with integrated RF-MEMS devices.


Ever Teneppe
17767425
Material: EES
Fuente:https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr200710sp4.html

INTEGRACIÓN Viruta-en-MEMS heterogéneo de MEMS y circuitos

INTEGRACIÓN Viruta-en-MEMS heterogéneo de MEMS y circuitos

New verified integration concept Nueva integración verificado concepto
VTI has verified a new heterogeneous integration concept for combining MEMS devices and integrated circuits: chip-on-MEMS or CoM. VTI ha verificado un nuevo concepto de integración heterogénea de la combinación de dispositivos de MEMS y circuitos integrados: Chip-on-MEMS o COM. The concept is based on a combination of VTI's wafer level encapsulated 3D MEMS, wafer level packaging (WLP) technology and chip-on wafer technology. El concepto se basa en una combinación de nivel de oblea de VTI encapsulado 3D MEMS, encapsulado nivel (WLP) y la tecnología de chip de tecnología de las placas. All these elements of CoM have existed for a few years. Todos estos elementos del CdM existen desde hace unos años. Combining them in an innovative way solves the tough packaging problem: how to combine cost efficiently MEMS with circuits. La combinación de ellos en una forma innovadora de resolver el problema de embalaje difícil: cómo combinar de manera eficiente a los circuitos de MEMS.

The technology consists of steps of applying a redistribution and isolation layers on the MEMS wafer, dropping 300 micron solder balls, flip-chipping thinned ASICs and finally passivating the gap between the ASIC and MEMS by underfilling. La tecnología consiste en los pasos de la aplicación de una redistribución y las capas de aislamiento de la oblea de MEMS, dejando caer bolas de soldadura 300 micrones, flip-saltar adelgazado ASICs y finalmente pasivante la brecha entre los ASIC y MEMS por falte. The MEMS-wafer was probed so that only known good sites will be populated. El MEMS-oblea fue probado de manera que sólo conoce los sitios buenos serán pobladas. After completion of the process the wafer will be diced and the final test performed when the dies are still on the dicing tape. Una vez terminado el proceso de la oblea será cortado en cubitos y la prueba final se realiza cuando los dados se siguen en la cinta de corte. Sensors will be also calibrated while still on the tape.. Los sensores se calibra también al mismo tiempo en la cinta ..

The first fully functional MEMS device based on CoM has a foot print of less than 4 mm2 and height 1 mm. El primer dispositivo MEMS completamente funcional basado en COM tiene una huella de menos de 4 mm2 y la altura de 1 mm. The technology is now ready for product design and industrialization. La tecnología ya está lista para el diseño del producto y la industrialización.

A new direction for system integration Una nueva dirección para la integración de sistemas

The flip-chipped CoM is the first step on VTI's heterogeneous integration roadmap. El flip-astillas com es el primer paso en la integración heterogénea hoja de ruta de VTI. It is a radical step away from the conventional packaging, which relies on integration on a carrier, either a pre-molded housing, a lead frame or a substrate. Es un paso radical, pasando de los envases convencionales, que se basa en la integración en un soporte, ya sea una vivienda pre-moldeado, un marco de conexión o sustrato uno. Eventually CoM will result in smaller size and lower cost than any carrier based packaging. Finalmente CdM resultará en un tamaño más pequeño y menor costo que cualquier compañía de embalaje basado. All packaging will be just an extension of the processes of a wafer-fab. Todos los envases serán sólo una extensión de los procesos de un wafer fab.

CoM is not the first ever demonstration of wafer level combination of MEMS and circuits. COM no es la primera demostración alguna vez de la combinación de obleas nivel de MEMS y circuitos. But it solves many issues that are present with the earlier approaches. Pero los que resuelve muchos problemas que se presentan con los planteamientos anteriores. In CoM the MEMS-device and the ASIC are fully isolated in manufacturing: both can be 100% tested prior to combining. En el CdM dispositivo MEMS y ASIC son completamente aislado en la industria manufacturera: ambos pueden ser probados al 100% antes de combinar. No area is wasted due to size mismatch. Ninguna zona se desperdicia debido a la desigualdad de tamaño. No area is wasted for the sealing between MEMS and the circuit. Ninguna zona se pierde para el sellado entre MEMS y el circuito.

The first implementation of CoM requires that the MEMS die is somewhat larger than the circuit and the I/O-count will be limited. La primera implementación del COM requiere que el MEMS morir es algo mayor que el circuito y el I / O-conteo será limitado. After the flip-chipped CoM VTI will implement embedded CoM. Después de que el flip-astillas CdM VTI implementará empotrados Com. Very thin dies will be embedded in polymer layers on the MEMS wafer. Muy delgada muerte, será integrado en las capas de polímero en la oblea de MEMS. Interconnections between layers will be made by deposited metal films. Las interconexiones entre las capas se efectuará por las películas depositadas metal. Several circuits can be stacked. Varios circuitos se pueden apilar. A real microsystem with MEMS and several circuits is possible. Un microsistema real con varios circuitos de MEMS y es posible. This is the technology for smart MEMS. Esta es la tecnología de MEMS inteligente.

Ever Teneppe
17767425
Material: EES
Fuente:http://translate.google.co.ve/translate?hl=es&sl=en&u=http://www.vti.fi/en/products/technology/com/&ei=NKkjTMmBO8K78gaqr4W7BQ&sa=X&oi=translate&ct=result&resnum=9&ved=0CE0Q7gEwCDjIAQ&prev=/search%3Fq%3DMEMS%2BCircuits%26start%3D200%26hl%3Des%26sa%3DN

MEMS-based inertial sensor is not your grandfather's gyroscope

MEMS-based inertial sensor is not your grandfather's gyroscope
The IC Insider looks at a MEMS-based gyroscope in microscopic detail, and finds that the ingenuity in this sophisticated sensor goes far beyond the process technology used to sculpt its mechanical features.

By Randy Torrance, Chipworks -- EDN, December 1, 2008

Gyroscopes have been around for 190 years, ever since Johann Bohnenberger built the first one in 1817. For the first 180 years, the basics of the simple and effective design remained much the same. A gyroscope uses a spinning disk whose axis is mounted such that it can take any orientation. Because the spinning disk has a high angular momentum, it is quite resistant to external torque. By mounting the device in gimbals, a gyroscope can maintain an almost fixed orientation in spite of applied torque. By measuring the angle between the spin axis and the fixed gyroscope frame, one can determine the external torque or rotation that has been applied to the frame (see Figure 1).

In recent years people have realized that a gyroscope would be a great addition to many devices that can benefit from information about their orientation. For instance, I'd like one in my coffee cup so that when I'm walking around the office, it can notify me when I tilt it too far. OK, that may be a bit of a stretch, but gyroscopes are now showing up in many applications. How does your car's ESC (electronic stability control) system know that your car is spinning out of control—and how does it figure out how to adjust the brakes and power distribution to correct the situation? A gyroscope. How do some of today's more advanced digital cameras adjust for a user's shaky hand to improve photo quality? A gyroscope.

But how do they get that spinning disk in there? The answer, of course, is that gyroscopes have moved on from that original design and now come in silicon form. To be exact, MEMS (microelectromechanical systems) are now being used to implement gyroscopes in silicon, and they now come integrated on the same ICs as the standard logic that controls them.

Coriolis effect

MEMS gyroscopes use a vibrating structure rather than the traditional rotating disk to determine orientation. These gyroscopes measure angular rate by means of the Coriolis acceleration, which can be explained as follows. Think of a rotating disk, such as a 33-1/3-rpm record (younger readers can try to imagine a DVD spinning inside its player). An object near the center of the disk is moving quite slowly, whereas an object near the outer edge of the disk is moving much more quickly. Both objects are also subject to tangential acceleration due to the rotation. This acceleration, which is proportional to the object's distance from the center of the disk, is known as the Coriolis acceleration. Hence the object near the outside experiences a greater Coriolis acceleration than the object near the center. You can measure this acceleration and use it to calculate the externally applied torque.

Chipworks has analyzed the InvenSenseIDG-300 two-axis gyroscope—procured by removing it from a camera—which operates according to this principle. The InvenSense MEMS gyroscope uses two resonating proof masses that are linked. The proof-mass system can be thought of as sitting on our rotating disk. As it resonates outward it experiences more Coriolis acceleration, and as it moves inward it experiences less. The mass is micromachined into the silicon and allowed to resonate in only one direction (Figure 2).

Interlocking fingers are micromachined in between and connected to the proof-mass system and the support silicon, such that they can measure capacitance changes (Figure 3).

The Coriolis acceleration will move the proof-mass system proportionally to the value of that acceleration, and the capacitance will in turn change and give a measure of the Coriolis force. InvenSense has integrated two of these MEMS structures orthogonally on the die to allow for two-axis detection.

The design uses a bulk micromachining process, vertically integrated electronics, and wafer-scale packaging. The gyro design includes an integrated dual-mass electrostatically driven actuating mechanism with capacitance sensing. The two linked proof masses are driven into an anti-phase oscillation by electrostatic actuators placed beneath them. This out-of-plane resonating proof-mass system senses the rate of rotation in either the X or Y axis.

The product measures angular velocity with a full-scale range of ± 500º/sec and sensitivity of 2 mV/º/sec. The oscillation circuit precisely controls the amplitude to maintain constant sensitivity over the temperature range of 0 to 70ºC. Two externally connected compensation capacitors are used for the amplitude control loops. An external low-pass filter can attenuate high-frequency noise generated by the vibrating proof masses. The sensor bandwidth is limited to 140 Hz by an internal low-pass filter, and the user can achieve lower bandwidth by choosing an appropriate external filter with a cutoff frequency less than 140 Hz. The MEMS device has an onboard EEPROM for factory calibration of the sensor. Factory trimmed scale factors eliminate the need for external active components and end-user calibration.

Figure 4 shows an annotated die photo of the IDG-300.

The two MEMS structures including the vibrating proof masses can be seen in the center of the die. Each MEMS device has its own dedicated drive circuitry (located below the MEMS sensors), and signal-detection and measurement circuitry (above the MEMS sensors). At the top of the die is a large amount of trimming and calibration circuitry, which includes nonvolatile memory arrays. The bottom of this die photo is mainly occupied by voltage- and bias-generation circuitry.

InvenSense has designed a lot of interesting and novel circuits onto this chip. The Coriolis sense circuitry needs to be able to detect very small capacitance changes with high accuracy. High-voltage circuits are required both to drive the MEMS proof masses and for the nonvolatile memory. The proof mass drive circuits and oscillator must be both accurate and powerful. Finally, the voltage- and bias-generation circuitry must be very accurate, and must account for variations in process, voltage, and temperature. Let's consider this circuitry, and how it can maintain accurate voltages, currents, and oscillation frequency to the MEMS devices.

Voltage reference

One of the key components in a MEMS gyroscope is the circuitry used to drive the proof mass into resonance. The proof mass will only vibrate effectively at its resonant frequency. The oscillator subsystem must therefore drive the proof mass at a precise oscillation frequency to maintain the system in resonance. This driver system must include methods to compensate for process, voltage, and temperature variations. The IDG-300 does this using multiple methods. A large percentage of the die is dedicated to on-chip trim and calibration circuits that can be set at the factory to compensate for process variations. An interesting CMOS-compatible OTP (one-time programmable) memory is used to store these factory settings. Figure 5 shows a row of four of these cells.

However this still leaves voltage and temperature variations to handle. InvenSense starts with a bandgap reference to generate a supply- and temperature-independent voltage reference to the oscillator. However, the resonant frequency of the proof mass is a function of temperature.

Hence the oscillator actually needs a temperature-dependent reference in order to maintain the proof-mass resonance. We believe InvenSense has modified the bandgap reference circuit to add in this characteristic.



Referring to Figure 6, we see the heart of the bandgap reference

pNP transistors X1619 and X1167 are used in conjunction with the resistors and op-amp to compensate for temperature variations. It is interesting to note the lengths to which InvenSense went to match these bipolar devices. Referring to the image of the polysilicon layer of the chip shown in Figure 7, we can see these two PNP transistors highlighted with the white rectangles.

X1167 can be clearly seen to be eight unity PNPs, compared with the two of X1619. What is of interest is just how much larger these devices are than a standard MOS device in this process, one of which is highlighted by the small white rectangle in the upper left of the bipolar devices. This is likely done for matching reasons, to ensure the best temperature response possible. It is also interesting to note that the feedback path to the op-amp actually travels through two more PNPs that are also scaled to allow for a large Vbe difference.

Another interesting feature of this circuit is the method by which it creates the bias currents through these two feedback PNP transistors of the bandgap core. The circuit shown in Figure 8 is used to create these currents.

After analysis, one can see that the current generated by this circuit is proportional to absolute temperature (PTAT) voltage divided by R1257. Assuming that the temperature dependence of the polysilicon resistor is much smaller than the bandgap voltage temperature dependence, the output current is also PTAT. This may be used to compensate for the temperature dependence of the feedback PNPs in the bandgap core of Figure 6. A more likely scenario, however, is that this PTAT current, along with appropriate selection of component values in the bandgap core, creates a temperature-dependent characteristic that will compensate for the temperature dependence of the proof mass resonant frequency. Taken all together, this is one of the most complex bandgap circuits we have ever seen.

The overall MEMS market is a bright spot in the semiconductor industry, and inertial sensors represent one of the fastest-growing subsegments. Driven by accelerometer applications like the Apple iPhone and the Nintendo Wii, and by the coming legislation requiring stability-control systems in all vehicles, these devices have moved out of industrial segments and into consumer ubiquity. While most designers might think that the innovation in the MEMS industry lies in the hands of the technology teams, this device shows that these ICs deliver sophisticated designs of their own. And as the competition heats up, we can only expect that it will get even better.

Ever Teneppe
17767425
Material: EES
Fuente:http://www.edn.com/article/469827-MEMS_based_inertial_sensor_is_not_your_grandfather_s_gyroscope.php

RF MEMS: a brief history and future trends

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 in the 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.

MEMS Investor Journal: When were RF MEMS first introduced and what were the original applications?
Dr. Dan Hyman: RF MEMS came into their own as a nascent industry in the early 90's, which is when most of the classic devices were invented by researchers (myself included) working for major Defense-oriented companies. These device-level development efforts were largely funded by DARPA and the Air Force, with a system-level view of how these nifty gizmos could actually improve radar and communications systems. Soon thereafter, other Federal services and agencies recognized their own needs for improved RF systems, and followed suit with programs directed towards broadband, highly multi-band, and/or millimeter-wave applications such as tactical radio programs (JTRS, WIN-T, etc.), satellite communications, and terrain-penetrating imaging radar (FOPEN, URPEN, etc.). RF MEMS enable switching and tuning of front-end circuits for applications of mode switching, antenna tuning, and antenna beam steering with phase shifters.

The IP space started getting crowded in the late 90's as universities and research organizations began to get involved. The "small company" interest is a fairly new phenomenon, really only starting in 2000 and focusing on a variety of niche applications in Defense, test equipment, and consumer electronics. Critical service first-responder radio interoperability, homeland security communications and radar became another rapidly growing niche since 9/11. Fortunately, so many lessons were already learned about MEMS devices and processes that XCOM and other venture and government-backed small businesses were able to hit the ground running with technology and process licensing, comparatively fast development times (3-4 years), and product releases in these niche applications.

MEMS Investor Journal: How have these original applications been accepted by the market? Which applications have survived and are on the market today?
Dr. Dan Hyman: The original Defense sponsors have so far been very patient and respectful of RF MEMS advances and product offerings over the years. However, they are still skeptical of adopting RF MEMS products for their specific applications because they have heard every promise in the book from researchers, trade rags, analysts, or companies out in the field (sometimes even their own). There are still significant obstacles for RF MEMS to enter conservative application areas with long histories of well-established technologies and very strict screening requirements. RF MEMS received substantial federal money in part because it truly is "DARPA hard" to get these things into production with the ruggedness, reliability, lifetime, repeatability, and performance demanded by the systems in which they would be used. I would even dare say that "ability to deliver" is still a significant concern for potential customers that require hundreds of thousands or millions of components, as there have been a number of aborted product releases in the industry's short history.

Defense product goals are dedicated to product testing and also improvement to overcome failures observed in the course of that testing. This is a natural part of product release, iteration, re-release, and ultimate customer qualification that should come as no surprise to any business. The delay to product adoption is normal, and just happens to be fairly long for Defense applications. Fortunately, so much time and energy has been spent characterizing and experimenting with RF MEMS, which Defense customers are highly educated about "what RF MEMS can do for you". They are a generous and patient, though exacting, group of customers, and they are often willing to help subsidize the cost of development, iteration, iteration, testing, qualification, and manufacturing ramp if they need your component in their system badly enough.

I would assert that the vast majority of the original applications (though not the original programs or envisioned insertion points) for RF MEMS are still valid and interested in the technologies today after 15 years. The subsystem and system engineers are still skeptical with each new company or product release, but they are still eager and willing to try them out, and willing to comment on what they do or don't like with each new RF MEMS component they get their hands on.

MEMS Investor Journal: How would you categorize the main kinds of RF MEMS devices today?
Dr. Dan Hyman: This is an important question for RF MEMS developers and users, because differences in component architectures have huge implications on how they can be used. Each kind of RF MEMS device is different than another, and even devices within the same category can vary wildly in terms of capabilities, performance, cost, size, etc. The best way I like to discuss them is in terms of their circuit configuration, because that is how the customer will be looking at the part relative to semiconductor options they may already be using (or considering). The circuit configurations I like to think about are as follows:

• Ohmic switch – The simplest three-terminal device architecture, directly comparable to a transistor switch and usable in the same way. It is possible to use switches in more complex circuits, but you have to pay careful attention to biasing, especially in very wide-band applications. The device is called "Ohmic" because it makes a genuine Ohmic metal-to-metal electrical and physical contact when closed.

• Ohmic relay – A four-terminal device that separates the control signal from the load signal within the body of the component package or even at the individual MEMS device itself. Many customers prefer relays to switches because of the ease of control design. Ohmic is as defined above.

• Capacitive switch – A three-terminal device architecture, with control problems as with Ohmic switches. These devices form a capacitor with a gap that can be varied with applied voltage. Changing the gap between the electrodes in a well-defined manner allows for a change in capacitance. This is not useful for direct current applications such as telephone line switching, but it is extremely useful in switching or tuning RF circuits.

• Capacitive relay – A four-terminal device that separates the control signal from the load signal, and changes capacitance as with the less capable switch version above. This is a configuration with a simple control, and is attractive for complex RF integrated circuits such as matching networks and phase shifters.

• Mechanical resonator – A micromachined three-terminal device that has a mechanical resonance in the RF regime. The resonance is useful as either a very exact time reference (better than quartz, and suitable for arraying or integration) or as a very sharp filter (that is very difficult to consistently implement).

• Bulk acoustical resonator – A micromachined two-terminal device that is better defined as a micro-system technology (MST) rather than a MEMS device. The best example is the FBAR, already found in many millions of handsets. Think of them as competitors to SAW filters, and you're most of the way there.

MEMS Investor Journal: Which of these are already commercialized? Which are likely to be commercialized over the next 1-2 years? Which are going to be commercialized in the longer term?

Dr. Dan Hyman: Well, the two resonator types are already commercialized and shipping in high volumes at this moment. The mechanical resonators have not yet ramped up to the level of "Dick Tracy watch" market, but they are quite aggressively penetrating the reference oscillator market, replacing quartz crystal technologies at a truly impressive rate. The FBARs, of course, are also shipping high volumes and still growing, so the question of "can a MEMS gizmo make it?" has a clear answer of "yes."

For more traditional RF MEMS switches and relays, the definition of the word "commercialized" is a topic of discussion. A typical definition would mean that a product has been created and released, to one or more customers, or to the open market. The part would be manufactured in a process that has been "qualified" with a proven reproducibility in manufacturing and performance, with that qualification being performed by the company itself (least desirable), by one of its customers (better), or by a third-party incumbent in the industry (best yet). With this reasonable definition, it can be said that four products have been commercialized in the test, instrumentation, and Defense communities: XCOM and Panasonic/Matsushita have commercial Ohmic relay products, and Radant MEMS and TeraVicta have Ohmic switch products.
In the next 1-2 years, other Ohmic switches and relays will be commercialized, as well as other niche variants of existing products re-focused for a particular application area or operational requirement. You will probably also see the first few capacitive switches or relays released inside larger circuit products such as tunable matching networks and phase shifters. In the longer term, you will see variants of these devices, and the actual manufacturing ramp of these devices. The analysts are a bit aggressive with predictions of adoption of RF MEMS by the marketplace, but I do not think they will be far off.

MEMS Investor Journal: In terms of taking RF MEMS devices from lab prototypes to mass production, what do you see as the main challenges?

Dr. Dan Hyman: There are a number of challenges that face anyone who wishes to take an RF MEMS device or circuit prototype into component production. The most significant challenges are those associated with packaging. MEMS actuators are intrinsically challenging devices to package, because you have to have a tiny, fragile device actually affect the environment or signal in some way. The purpose of the package is to interfere with this task in a minimal way, and protect the device from all other aspects of the environment. For RF MEMS devices, this means the package needs to be invisible to RF in the desired signal path (low insertion loss), be opaque to RF in every other possible signal path (good isolation, low cross-talk), defend the device from internal and external heat problems (including solderability, temperature cycling, power handling, etc.), defend it from environmental problems (fatigue, shock, vibration resilience), and defend the device from internal and external contamination (hermeticity, gettering, etc.). The package needs to do these things to an acceptable level, and also cost as little as possible.

These packaging aspects are designed to enable the high RF performance of the MEMS devices inside to shine, yet still provide customer-demanded levels of four critical operational characteristics: lifetime, repeatability, reliability, and ruggedness. These four separate characteristics have widely varying requirements (many orders of magnitude different) for different applications and customers. Lifetime is the number of times you can carry operational loads in service before a particular likelihood of a defined failure event. Repeatability is how similarly each part operates each time it operates, and sometimes how each part operates similarly to other parts throughout their lifetime. Reliability is how likely the part is going to operate properly throughout its lifetime given particular environments or usage. Ruggedness is how well the part survives harsh environments and conditions of storage or operation. All of these things are important at different levels to different customers, and the fact of the matter is that RF MEMS developers tend to focus on the most challenging specifications posed to them by their highest paying customers!

MEMS Investor Journal: What are the key challenges with marketing and selling RF MEMS devices?

Dr. Dan Hyman: Fortunately, the early marketing and sales problems of "customer outreach" and "user education" are not really the significant impediments they used to be. RF MEMS has, fortunately, passed through and survived the "nascent hype" phase of the industry growth cycle. There are no longer announcements of "wiggling beams" and vaporware products "sampling soon." As for education, there are not very many RF MEMS suppliers in the industry compared to the large number of customers who want them, and it is interesting to continuously learn that every single customer is intimately familiar with every product.

Customer adoption for any of the "high-end" application areas still has taken a long time, and will continue to take a long time. The present marketing and sales efforts in our industry is simply about products or test results that address specific customer concerns and impediments to adoption. All of the developers in the field, whether small business or large, are facing the same questions; we need to address them to continue to make traction, and get our sales victories as we go.

There is, however, one downside to all of the progress RF MEMS developers make, which is that as an industry we continue to sell next years' parts before we sell this years' parts. Customers are learning that next year's model really will handle more power, switch faster, last longer, and cost half as much! While personally satisfying to receive this vote of confidence, I'd rather have a second big order right now. . .

MEMS Investor Journal: In general, could you comment on the lessons learned and future trends for RF MEMS?
Dr. Dan Hyman: Lessons learned:

• Hermeticity – You can't get too clean, too hermetic, too "well-sealed". A perfect seal with a well-controlled environment inside is needed for contacting MEMS devices. Know your seal and perfect it, or use a highly trusted service provider to do it for you, because it can very easily make or break your product.

• Don't over-design – Everything is an engineering tradeoff, so improving something that isn't asked for by a customer is probably sacrificing something that they care about. Focus on product adoption.

• Don't over-test – There are millions of possible tests and test conditions you could put a part through, but your customer probably only cares about ten. Perform them wisely, and don't agonize over failure to meet an esoteric salt spray test, or an irrelevant burn-in test designed for semiconductor devices, or any other test that isn't specifically asked for by a customer. Invest time and money carefully.

Future trends:
• Shipping quantities - More parts are going to be shipped; that's a trend that we have seen in 2006 that we are certain is going to continue. It only takes two flagship customers to make a product line successful.

• Invisible usage – More devices are being developed out of existing processes, and I expect RF MEMS will become a "secret sauce" for circuit and subsystem products that will simply not advertise the technology. I will consider this to be an important milestone for the RF MEMS industry. . . "Oh, this thing here? It just works better, it doesn't matter what's inside!"

Dr. Hyman is an internationally recognized expert in RF MEMS packaging and contact reliability, with numerous papers, invited lectures, and patents in RF MEMS devices and applications. He holds a M.S. and Ph.D. in Electrical Engineering and Applied Physics from Case Western Reserve University, and a B.S. in Engineering from Harvey Mudd College. He is an active member of the IEEE MTT-S Subcommittee for RF MEMS since his appointment in 2001, and has served as a reliability and commercialization advisor to the NSF and Department of Defense. Dr. Hyman entered the RF MEMS industry in 1995 as a Hughes Fellow for HRL Laboratories, where he designed RF MEMS for U.S. military communications programs. He left HRL to co-found XCOM Wireless and address broader RF MEMS commercialization opportunities.

XCOM Wireless is a small business that specializes in RF MEMS devices, packaging, and circuitry. Most MEMS devices in the industry are expensive, fragile, environmentally-sensitive, and must be treated with great care in laboratory environments. XCOM RF MEMS circuits, by comparison, can be soldered, baked, frozen, drop-kicked, and shaken, which is an absolute requirement for rugged industrial, military, aerospace, and automotive electronics applications

Ever Teneppe
17767425
Material: EES
Fuente: http://www.memsinvestorjournal.com/2006/10/rf_mems_a_brief.html

MEMS to Replace Quartz Oscillators as Frequency Sources

MEMS to Replace Quartz Oscillators as Frequency Sources

A new encapsulation and packaging technology for MEMS resonators provides a clean, stable environment for reference oscillators.

Research on microelectromechanical system (MEMS) resonators dates back to the mid-1960s, but MEMS timing references are only now being introduced commercially. In this time, two fundamental technical challenges have been overcome: finding a stable and predictable material from which to build the resonators, and developing a sufficiently clean hermetic packaging system.

This research was motivated by a desire to replace quartz crystal resonators. Quartz crystals are manufactured in very high volume; about eight billion will be built this year. While they have many excellent properties, they also have significant limitations. For instance, quartz crystals cannot be integrated into silicon circuits, they are often large, and they have manufacturing yield and quality problems. These limitations are becoming more apparent and constraining in modern products.

The first problem MEMS researchers overcame was finding a material that was sufficiently stable to maintain a precise resonant frequency. In one year of continuous use, a typical resonator must flex a million-billion (1015) times, while only changing a few parts per million in resonant frequency. The strain levels can be large, in many cases approaching the non-linear limits of the material. In addition to quartz, the MEMS community has tested many alternatives, including metals, nitrides, oxides, silicon, polysilicon, and various combinations. Silicon has been found to be an exceptionally good material for resonators

The toughest problem in the intervening 40 years was developing a suitable hermetic packaging technology. MEMS resonators require exceptional cleanliness because the small devices are especially sensitive to surface contamination — a single monolayer adsorbed on or desorbed from the resonator surface can push the resonant frequency out of specification. This packaging also must provide a robust mechanical cover over the MEMS structure, be small and preferably CMOS integrable, and be low cost. The recently developed encapsulation and packaging technology provides all of these and opens the way for commercial devices.

Quartz crystals generally are packaged in metal or ceramic vacuum enclosures, but MEMS resonators packaged this way usually do not show the required stability. Also, using the old packaging techniques would not leverage the MEMS strengths, namely small size, potential CMOS integrability, and low costs derived from integrated circuit manufacturing technologies. Therefore, MEMS researchers have looked beyond metal and ceramic vacuum enclosures.

Potential packaging and sealing technologies include bonded covers attached with anodic, frit glass, solder, or gold compression techniques. While covers attached this way provide mechanical protection, they do not provide clean enough environments for timing references. Bonded covers also double the thickness of the MEMS components, use significant die area for sealing rings and bondpad access, and incur significant system costs, usually greatly exceeding the cost of the resonators. For example, with bonded covers, the seal rings and bondpad routing can consume as much as 80 to 90% of the die area, and can account for over 80 to 90% of the cost of the packaged MEMS resonators.

Thin film encapsulations are an alternative to bonded covers. These usually are based on deposited layers — for example, deposited nitride or polysilicon, or on plated metal. These techniques do not burden the encapsulated part with large seal rings and restrictive bondpad layouts, but they are not intended to withstand the full pressure of plastic molding and are not generally clean enough for frequency references.

Getters have been investigated to scrub encapsulated environments and have yielded high-quality vacuums, but the added cost and complexity is significant, and integration problems exist.

The new encapsulation described here does not require sealing rings or restrictive bondpad routing, and the electrical connections are brought to the surface wherever appropriate, enabling efficient use of die area. The encapsulation is mechanically strong and survives over 100 atmospheres pressure in the plastic injection molding used in chip packaging. The encapsulation also provides a vacuum environment that is exceptionally clean and stable, and is suitable for reference oscillators. Finally, the encapsulation is highly economical.

The production process is based on common CMOS production tools and is performed in common CMOS foundries. Briefly, resonator structures are lithographically patterned and plasma-etched into silicon wafers. The structures are covered with layers of oxide and silicon, then released by removing the surrounding oxide to form freestanding resonators. The freed resonators are annealed at high temperature in an extremely clean reactive gas and sealed under an additional layer of silicon. Contacts are made to the resonators, and interconnect wiring is built with normal integrated circuit processes. The completed MEMS devices, now enclosed in small chip-level vacuum chambers, are diced from their wafers and packaged with drive electronics to form complete oscillators.

The final chip packaging deserves a special note since packaging usually dominates the cost of resonators and oscillators. The resonators with their drive circuits are molded into standard, inexpensive plastic packages developed for integrated circuits. These packages are in high-volume production, and have proven to be very reliable and economical in integrated circuit applications. This is only possible because the MEMS resonators are safely protected under durable silicon covers.

These MEMS oscillators have been tested for frequency accuracy, frequency stability, package hermeticity, temperature cycle durability, accelerated aging, and across various other performance and reliability regimes. In these and other measures, the MEMS resonators and drive circuits perform exceptionally well, meeting or exceeding the requirements for many consumer and industrial applications.

The MEMS oscillators now being sampled are the world's smallest programmable oscillators. They meet or exceed specifications for a wide variety of applications, and they cost less to manufacture than the quartz parts they replace. They are being manufactured with high yield in standard CMOS foundries, and packaged in standard integrated circuit production lines. They will be in volume production in only a few months, and will be built into a wide range of products a few months later.

This article was written by Aaron Partridge, Chief Technical Officer, and John McDonald, Vice President of Sales and Marketing, at SiTime Corporation of Sunnyvale, CA. For more information, contact Courtney Dimpel at 408-328-4427; email cdimpel@sitime.com ; or visit http://info.ims.ca/5659-140.

Ever Teneppe
17767425
Material: EES
Fuente: http://www.techbriefs.com/component/content/108?task=view

A matter of timing: Labs' micromachine prototype performs job of quartz crystals

A matter of timing: Labs' micromachine prototype performs job of quartz crystals

Within the next few years, your watch, television, and computer may all contain microelectromechanical systems (MEMS), micron-size machines being developed at Sandia.
Jim Smith, Manager of Intelligent Micro- machine Dept. 1725, together with colleagues from Sandia and Trey Roessig, Al Pisano, and Roger Howe from the University of California at Berkeley, have built a MEMS prototype that functions as a clock source. The minuscule machines with moving parts the size of a pollen grain perform the same job as quartz crystals, the traditional technology used in timing devices in all digital electronics.

THE TIMING IS RIGHT - Jim Smith, Manager of Intelligent Micromachine Dept. 1725, looks through a high-power microscope at a MEMS prototype that functions as a clock source. (Photo by Randy Montoya)

Roessig, accompanied by Jim, made the first public announcement of the prototype in June at the Solid State Sensor and Actuator Workshop in Hilton Head, S.C.

"We have taken the same technology that is now being used in such devices as sensors in car airbags and applied it to a timing device," Jim says. "It looks extremely promising."
Micromachines are made from polysilicon, which is the same material used in manufacturing integrated circuits, the building blocks of digital electronics. Because of this, the micromachines and integrated circuits can be constructed on one chip.

Systems on a chip

The micromachined clock source, conventional integrated circuits, and other micromachined elements can be built simultaneously to form a complete "system on a chip," which if mass produced could yield dramatic reductions in price and increases in reliability, Jim says. Hundreds to thousands can be built on a single silicon wafer. In addition, the cost of manufacturing could be significantly reduced because the need for assembly would be eliminated. Under current production methods, quartz crystal timing devices and integrated circuits are manufactured separately and then assembled. Since the two systems would be on one unit, there would be no need to piece them together, saving a significant cost source.
The system-on-a-chip concept, only about three years old, embeds the micromachines in a shallow trench on a silicon wafer. These wafers with the microelectromechanical devices are then used as the starting material for the conventional complementary metal oxide semiconductor (CMOS) manufacturing process of integrated circuits. The integrated circuits are built on the surface of the wafer, while the MEMS are sealed in the trench.

This technology, which in 1996 won Sandia an R&D 100 award, has already been licensed to industry for use in applications such as computer game joy sticks, automotive stability systems, and airbag deployment sensors. With incorporation of a timing device, applications for this technology will continue to grow.

Currently, quartz crystals - precision-cut and polished single-crystal silicon dioxide (a main ingredient of sand and window glass) - serve as the clock source. A piezoelectric material, the crystals expand and change shape when an electric field is applied, storing up electric charge. When no current is administered, the crystals release the charge. Electrical energy sloshes back and forth at a fixed frequency between the crystal and the timing circuit in a feedback loop. This fixed frequency generates timing signals, which allow calculations in digital electronics to occur in synchronized steps. For example, a modern wristwatch contains a quartz oscillator and a circuit that counts the oscillations. Once the correct number of counts is recorded, the display is advanced one second.

MEMS replacement clock source

The MEMS prototype would serve as a replacement clock source. It is different from the quartz crystals because it is excited and sensed electrostatically instead of piezoelectrically. Unlike their quartz counterparts, which expand or change shape, these polysilicon resonators physically move in much the same way as a tuning fork vibrates.
The prototype also acts somewhat differently from other micromachines used in products currently on the market - such as sensors for pressure and acceleration - that are minute moving gears and pins.

Observed through a high-power microscope, the MEMS timing device prototype looks exactly like a tiny double-ended tuning fork. It consists of two very fine strings or tines - 10 would fit on a pinhead - anchored in parallel to actuator frames the size of red blood cells. Voltage, set up in a continuous feedback loop (the oscillating effect), is applied through the actuator frames, causing the strings to move back and forth. Because they are so very small, the MEMS vibrate extremely fast and generate frequencies of about 1 MHz. Although this is a relatively low frequency for a system clock, the prototype oscillator is the first integrated oscillator that operates above the audio range.

The process for building these devices at Sandia appears capable of fabricating integrated oscillators with frequencies above 10 MHz. Despite the high frequencies, these micromachines are producing very low noise - due primarily to the integration of the mechanical structure with electronics and the design of the electronic circuit.
The frequencies provide the constant timing signals necessary for the digital electronics device to operate. Because of the low noise, the signals are constant and not disrupted, resulting in more accuracy.

Micromachines in the shape of a tuning fork serving as oscillators are not new. The uniqueness is putting the MEMS oscillator on the same chip as the integrated circuits.
Collaboration with Berkeley
Jim says his efforts build on work done at the University of California at Berkeley by Howe and Clark Nguyen (now at the University of Michigan).

"They were able to get a few of the tuning fork oscillators to work," he recalls. "But when they saw the dramatic improvement in manufacturability of devices offered by Sandia's integrated MEMS process, we soon were collaborating with them. They have the expertise in design, and we have the expertise in manufacturing. It's a natural match."
He adds that having the clock source on the same chip as other electronic circuitry is one of the building blocks toward developing complete electromechanical systems in a single monolithic piece of silicon.

"Internally, we are working with Thom Fischer and Kurt Wessendorf from 1732 to adapt these devices to defense program applications," he says. "Industry has already shown great interest in Sandia's ability to use this technology to build accelerometers - sensors that measure acceleration used in airbags - and gyroscopes that sense the rotation of a vehicle. The new ability to use micromachines as timing devices will greatly expand the fields of application for these systems on a chip."

Ever Teneppe
17767425
Material: EES
Fuente: http://www.sandia.gov/LabNews/LN07-03-98/mems_story.htm

MEMS resonators vs. crystal oscillators for IC timing circuits

MEMS resonators vs. crystal oscillators for IC timing circuits

 

by Sergis Mushell and Steve Ohr, Gartner

Since the early 1980s, companies have been trying to replace quartz with silicon MEMS-based oscillators as the frequency reference in clock and timing oscillators. Developments in semiconductor process technology, packaging, and the integration of circuitry have enabled some progress for MEMS resonators. The resonators are effectively time-base generators, or references, similar in operating principle to the mechanical tuning fork used to tune musical instruments. A separate electronic oscillator provides impetus that forces the crystal to vibrate at a precise frequency. Those vibrations are captured and output by a gain buffer, and a phase-locked loop (PLL) captures and distributes the reference signal generated by the resonator-oscillator combination.

SiTime's 8003 low-power programmable oscillator.

One of the most highly touted features of MEMS resonators is the integration they promise for integrated circuit (IC) timing circuits. Fabricated in silicon with bulk etching processes, MEMS resonators can in principal be tied to oscillator circuits and PLLs on the same silicon substrate. This would allow the clock and timing generators–together with the resonator–to occupy a single low-profile semiconductor package. This package, moreover, would support high-volume assembly techniques. Even where MEMS resonators and oscillators are separate chips, they could occupy the same package–one that would be smaller and easier to handle than the metal cans that currently house crystal oscillators. Thus, silicon MEMS-oscillator combinations are promoted as replacements for crystal oscillators in computers, communications equipment and digital consumer devices (like set-top boxes).

The lure of integration

Revenue for IC timing devices, which provide clock and trigger signals for all sorts of electronic circuits, is expected to grow as clock frequencies increase and the sequencing of electronic events becomes ever more precise. The stability of these silicon timing circuits–which constitute an estimated $3 billion market–depends on outboard precision time-based generators, which today are largely quartz crystal.

Figure 1: Architecture of a typical clock generator. (VCO = voltage-controlled oscillator). Source: Gartner, Discera

A number of startup companies have proposed replacing outboard crystals with silicon-based devices, microelectromechanical systems (MEMS) resonators, which would enable coupling with IC timing circuits on a complementary metal-oxide semiconductor (CMOS) system-on-chip (SoC).

But despite significant venture funding, resonator manufacturers have not resolved key technical and manufacturing issues. Consequently, they will be slow to displace less-costly crystal-based timing sources in systems.

Barriers to adoption

The goal of single-chip integration (the "holy grail" for the semiconductor timing industry) would be to include the resonator, the oscillator, the PLL and a temperature compensation circuit (TCC) on a single silicon substrate. The current structure of silicon MEMS-based devices utilizes a stacked-die arrangement, housed in a multi-chip package.

Discera QFN package oscillator.

Single-chip integrations–and market acceptance of MEMS resonators–is impeded by four issues:

• Manufacturing cost. The formation of a mechanical armature requires time-consuming bulk etching. In addition, the performance of the MEMS resonator is impacted by the presence of moisture in the MEMS cavity. Thus, specialized packaging and encapsulation are required.
• Temperature stability. Quartz technology has been around for decades, and offers temperature and long-term stability, as well as high-frequency products (which are offered at a premium). First-generation MEMS resonators do not have the same temperature stability as quartz and require TCCs. This is not likely to occur (enough to generate significant revenue) within the next four or five years. So MEMS resonators are currently useful for lower-frequency timing references (kHz rather than MHz).
• Low phase noise. MEMS resonators suffer from phase noise, which limits their utility in digital-communications applications. Higher market penetration could occur if these issues were resolved.
• Power consumption. The high power consumption and initial frequency stability are major challenges for silicon MEMS in handheld products.

• MEMS solutions currently have an initial frequency accuracy that can vary by as much as 100 parts-per-million (ppm) from their intended center frequency. Some manufacturers are claiming accuracy to within 25ppm, though this would require a costly hand testing and sorting process. Unlike quartz crystals, which can be scored with a laser to precisely trim the resonant frequency, MEMS cannot be fine-tuned. It is possible to utilize a "fractional-N" frequency synthesizer in conjunction with PLL to extract more precise timing signals from the MEMS resonator-oscillator combination, but this will raise the cost of the timing solution, as well as promote a very large die size for the integrated solution. MEMS resonators also will require digital temperature compensation circuitry, which contributes to die size. In addition, the compensation circuits make their own contributions to higher-phase noise and jitter; this makes MEMS timing devices less attractive to the communications market.
• Investments in MEMS resonator technology
• Since early 2003 there have been a slew of startups targeting the replacement of quartz as the sole frequency reference source available to the electronics industry. The quartz crystal is one of the last electronic components that have yet to surrender to silicon integration. The resonating quartz crystal provides the pulses for every electronic circuit, and is inside every electronic device and piece of equipment.
• Quartz has been used as a timing source since the early 1940s. No other material or technology has been able to replace the quartz crystal because of its exceptional temperature stability and low phase noise. Though companies such as Maxim and Micro Oscillator have offered "quartz-less" timing sources in the past, these products did not make any major impact on the marketplace, because of system requirements for precision and issues over their stability.

Conventional beveling process (left) vs. Epson Toyocom's QMEMS process (right). Photo: 2.0mm x 1.6mm AT-cut quartz crystal fabricated by QMEMS.

• Among the newer startups, there are four main approaches to replacing quartz crystals. Since there is no potential for integrating quartz with silicon and quartz large packaging, startups believe that silicon resonators can solve these problems. The four main approaches are:
• Silicon MEMS, with devices offered by SiTime, Discera, and Silicon Clocks;
• A hybrid QMEMS approach combining quartz with micro machine technology, offered by Epson;
• A self-perpetuating Mobius Loop offered by Multigig; and
• Silicon RC oscillators offered by Silicon Laboratories and Mobius Microsystems. [Changed to distinguish properly between Multigig, a supplier of clock generators using "Mobius Loop" technology, and Mobius Microsystems, a manufacturer of CMOS oscillators said to compete with crystal based oscillators in size and performance. -- Ed.]

Note that silicon-based MEMS (Si MEMS) are fabricated on silicon, while QMEMS are fabricated on quartz using a photolithography process. Regardless of their manufacturing technology, the value proposition offered by all these vendors is the same: integration with CMOS, ideally lower costs, and standardized IC packaging–which will enable not just the use of low-profile packaging, but also the use of standardized IC pick-and-place assembly equipment. Regardless of the technology approach, the underlying issues addressed are the same.

The three startup companies using Si MEMS have all based their technology on research from universities and research labs. SiTime's initial research and technology came from Bosch labs and Stanford University, and it has secured three rounds of funding ($12 million in December 2004, another $12M in March 2006, and $20M in May 2007). Discera's research came from the University of Michigan; it raised $4M in July 2001, $12M in April 2004, and $18M in April 2007. Silicon Clocks' research came from UC Berkeley; it secured initial $10M funding in 1Q05, and another $8M in 3Q07. Silicon Clocks also merged with SiSync, a clock synthesizer startup.

Other technologies and vendors

While MEMS technology is being targeted at the displacement of quartz, other companies including Silicon Labs, Mobius Microsystems, and Multigig, are proposing alternative timing circuits, such as free-running LC oscillators. Silicon area is a concern, though. We believe these will have limited success, at least in the near term. [Tone softened to reflect improvements vs. crystal-based forerunners, when packaging benefits are taken into consideration. -- Ed.]

As startup MEMS vendors continue to sample parts, there have been few reports of success in any major applications that would drive high-volume production of MEMS resonators. One manufacturer claims to be sampling parts to a cellular handset manufacturer. There have also been signs of particular interest in this area from timing-circuit vendors, such as IDT, which supply the clock generators and clock buffers for PC, server and consumer markets. By incorporating the technology into its silicon timing solutions, IDT could become a large consumer of MEMS resonators; conversely, it could manufacture its own circuits, in competition with the startups–or perhaps make an acquisition. Either scenario could result in more development and deployment of silicon MEMS technology.

Assuming vendors can deliver on their promises, the market for MEMS resonators could grow to $100 million in 2012. Wide adoption could start in late 2009 or early 2010 if certain technical problems are resolved. Improving the accuracy and stability of MEMS resonators would increase the number of applications and markets that could successfully use them.

 

 EVER TENEPPE

17767425

MATERIA: EES

FUENTE: http://www.electroiq.com/index/display/nanotech-article-display/354176/articles/small-times/volume-9/issue-1/features/mems-resonators-vs-crystal-oscillators-for-ic-timing-circuits.html