PH. D. THESIS APPLICATION OF CMOS-MEMS INTEGRATED RESONATORS TO RF COMMUNICATION SYSTEMS. Joan Lluís López Méndez PDF

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Universitat Autònoma de Barcelona PH. D. THESIS APPLICATION OF CMOS-MEMS INTEGRATED RESONATORS TO RF COMMUNICATION SYSTEMS Joan Lluís López Méndez 2009 Memòria presentada per optar al Grau de Doctor en

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Universitat Autònoma de Barcelona PH. D. THESIS APPLICATION OF CMOS-MEMS INTEGRATED RESONATORS TO RF COMMUNICATION SYSTEMS Joan Lluís López Méndez 2009 Memòria presentada per optar al Grau de Doctor en Enginyeria Electrònica per la Universitat Autònoma de Barcelona. Directora: Núria Barniol Beumala La Dra. Núria Barniol Beumala, Catedràtica d Electrònica del Departament d Enginyeria Electrònica de la Universitat Autònoma de Barcelona, CERTIFICA que la memòria Application of CMOS-MEMS integrated resonators to RF communication systems que presenta Joan Lluís López Méndez per optar al grau de Doctor en Enginyeria Electrònica, ha estat realitzada sota la seva direcció. Bellaterra, juliol de 2009 Dra. Núria Barniol Beumala A la llum que guia les meves passes: Gisela We shall not cease from exploration And the end of all our exploring Will be to arrive where we started And know the place for the first time. T.S. Eliot AGRAÏMENTS It was the best of times, it was the worst of times, it was the age of wisdom, it wasthe age of foolishness. Amb aquesta frase Dickens començava 'Història de dos ciutats' aquesta mateixa frase va servir a T. Newman i R.F.W. Pease per superar el repte del Dr Feynman: escriure l equivalent de la enciclopèdia britànica en el cap d una agulla. La tesi que tens a les mans és la meva petita (i humil) contribució al fantàstic món dels MEMS. Una tesi no és únicament feina de l'autor: es sustenta en la feina del conjunt de la comunitat científica i en els treballs previs d'aquells que van ser pioners. Tanmateix també representa un sacrifici important per tots aquells que envolten a l'investigador. Per tots ells espero que hagi valgut la pena i espero ser capaç de recompensar-los degudament. Em considero afortunat d'haver començar la meva vida professional al departament d'enginyeria electrònica de la UAB en el que entre tots em veu fer sentir com a casa per tots aquells moments (i anys) gràcies de tot cor. També tinc un agraïment especial als membres del grup ECAS: a en Gabriel, a l'arantxa, a en Joan i en Gonzalo per totes les converses, professionals o no. Especialment a en Francesc per les tasques administratives i els moments que vam compartir a Tres Cantos. També a l'eloi per haver fet part de les mesures de la tesi. A en Jaume i en Jordi per marcar el camí a seguir i especialment a la Núria per la seva paciència i comprensió. Agraeixo també la feina feta per la Marta Duch i Marta Gerbolés del IMB-CNM, en el postprocessat dels xips. Finalment agrair la paciència (infinita) de la Gisela, així com el suport de les nostres famílies (pares, mares, avis,...), durant el llarg procés d escriptura de la tesi. Joan Lluís López Guadalajara, 2009 SUMMARY MEMS devices demonstrated a wide range of sensing and actuation applications. These mechanical elements present nowadays extension to the RF world as key elements for highly reconfigurable systems, frequency references and signal processors. This thesis focuses on some of the applications of MEMS devices in the RF domain: frequency references for oscillators, filters and mixers. The resonators presented in this thesis are completely fabricated in commercial CMOS technologies to take profit of monolithic MEMS and complementary circuitry integration and low cost fabrication inherent of these technologies. Several kinds of MEMS resonators (clamped-clamped beams, free-free beams and double ended tuning forks) were designed and fabricated to evaluate their performance according to different properties. Two different CMOS technologies, from two different foundries and also different technological node (0.35um and 0.18um) were successfully used to validate the monolithic fabrication approach on future CMOS technologies. The resonance frequencies of these resonators are located on the HF and VHF range. All these devices, based on flexural beams, show superior Q than integrated LC tanks and are also tunable. Moreover, their size is significatively lower than the one of the aforementioned LC tanks. The CMOS-MEMS resonators reported in this thesis show a Qxf value in the range between 1GHz and 10GHz in air and these values are further improved in vacuum up to 100GHz, higher than any other reported resonator based on CMOS technology. Filtering and mixing applications were also studied. The goal in these applications was to define a flat band-pass combining different resonators. A prototype of parallel filter was measured using two CC-beams and a monolithic CMOS differential amplifier. The filter shows a flat bandpass up to 200kHz in air at a center frequency of 21.66MHz. Filtering with a single resonator was also demostrated with a DETF. A mixer based on a 22MHz CC-beam resonator was able to up and downconvert a signal from/to 1GHz. Monolithic oscillators with MEMS elements as frequency references have shown oscillation with a reduced applied DC voltage ( 5V) thanks to the reduction of the gap. The DETF based oscillator shows good phase noise performance of and better than previously reported monolithic oscillators whereas operating at a lower DC voltage. RESUM EN CATALÀ Els dispositius MEMS han demostrat la seva utilitat en un gran ventall d aplicacions de sensat i actuació. L extensió al domini de RF d aquests elements mecànics són ara una de les peces clau per sistemes altament reconfigurables referències de freqüència i processadors de senyals. Aquesta tesi es centra en algunes de les aplicacions dels dispositius MEMS en el domini de RF: referències de freqüència per oscil ladors, filtres i mescladors. Els resonadors que es presenten en aquesta tesi s han fabricat completament en tecnologies CMOS comercials per aprofitar la integració de MEMS i circuiteria complementària i el baix cost de fabricació d aquestes tecnologies. Diferents tipus de ressonadors MEMS s han dissenyat i fabricat a fi d avaluar les seves prestacions en diferents propietats. La validesa de la tècnica emprada per fabricar els MEMS en tecnologies CMOS futures s'ha demostrat fabricant i testant amb èxit resonadors MEMS en dos tecnologies diferents: de diferents fàbriques i nodes tecnològics (0.35um i 0.18um). La freqüència de ressonància d aquests dispositius mecànics es troben a les bandes de HF i VHF. Tots aquests dispositius basats en bigues flexurals, presenten un major factor de qualitat Q que els tancs LC integrats i són a més a més sintonizables en freqüència, amb una mida inferior a la dels citats tancs LC. Els ressonadors MEMS-CMOS descrita a la tesi presenten un valor de Qxf en el rang entre 1GHz i els 10GHz mesurats a l'aire. Aquests valors es milloren mesurant al buit arribant als 100GHz, majors a qualsevol altre ressonador basat en tecnologia CMOS. Les aplicacions de mesclat i filtrat de senyals també s estudien. Dins d aquestes aplicacions, la meta és definir una banda passant plana combinant diferents ressonadors. El prototipus d un filtre paral lel basat en ponts i un amplificador diferencial CMOS monolític presenta una banda passant plana de 200kHz a una freqüència central de 21.66MHz quan es mesura a l aire. També es demostra el filtrat emprant un únic ressonador del tipus tuning fork. Com a mesclador, és destacable la possibilitat de convertir a alta i baixa senyals de 1GHz amb un ressonador de 22MHz Com a oscil ladors monolítics, es mostra un oscil lador operatiu per tensions DC baixes ( 5V), gràcies a la reducció del gap del ressonador. L oscil lador basat en un tuning fork aconsegueix valors de soroll de fase de i millor que altres oscil ladors CMOS monolític reportats. 1 INTRODUCTION MEMS IN RF OVERVIEW AND RESEARCH MOTIVATION MARKET PERSPECTIVE FOR RF MEMS RF MEMS APPLICATIONS ON A RF FRONT END FABRICATION TECHNOLOGIES SYSTEM IN PACKAGE SYSTEM ON CHIP PACKAGING RESEARCH FRAMEWORK OF THE THESIS NANOSYS (TIC C03 02) MEMSPORT (TEC /MIC) THESIS: OBJECTIVES AND OUTLINE 37 2 MEMS RESONATORS CHARACTERISTICS AND APPLICATION IN RF SYSTEMS WORKING PRINCIPLE ELECTROSTATIC EXCITATION EQUATION OF RESONATOR MOVEMENT SPRING SOFTENING PULL IN AND COLLAPSING VOLTAGE CAPACITIVE READOUT RLC ELECTRICAL EQUIVALENT MODEL KEY PARAMETERS OF MEMS FOR RF APPLICATIONS RESONANCE FREQUENCY QUALITY FACTOR FREQUENCY TUNING MOTIONAL RESISTANCE NON LINEAR RESONANT BEHAVIOR MEMS RESONATORS APPLICATIONS IN A RF FRONT END FREQUENCY REFERENCES FILTERING FILTERING MIXING ( MIXLING ) STATE OF THE ART CONCLUSIONS 73 3 CMOS MEMS FABRICATION CMOS MEMS FABRICATION PROCESSES: 79 3.2 CMOS TECHNOLOGIES OVERVIEW SPACER TECHNIQUE TECHNOLOGY COMPARISON FURTHER CONSIDERATIONS DOWNSCALING PROOF OF CONCEPT: UMC 0.18UM CC BEAMS RESONATORS HF RESONATOR VHF RESONATOR CONCLUSIONS 99 4 MEMS RESONATORS CLAMPED CLAMPED BEAMS LINEARITY CONSIDERATIONS FOR CLAMPED CLAMPED BEAMS CLAMPED CLAMPED BEAMS DIMENSIONING MHZ CLAMPED CLAMPED BEAMS MHZ CLAMPED CLAMPED BEAM FREE FREE BEAMS MHZ FREE FREE BEAM THIRD MODE 48MHZ FREE FREE BEAM RESONATOR DOUBLE ENDED TUNING FORKS DETF D1U COUPLED MHZ DETF CONCLUSIONS FILTERING, MIXING AND OSCILLATOR APPLICATIONS PARALLEL FILTER SINGLE RESONATORS MEASUREMENT COMBINED RESONATOR RESPONSE: FILTERING SINGLE RESONATOR FILTER SINGLE RESONATOR MIXING HF CC BEAM RESONATOR VHF DOUBLE ENDED TUNING FORK FILTER MIXER VHF CC BEAM RESONATOR PARALLEL MIXING OSCILLATOR: S=100NM CLAMPED CLAMPED BEAM RESONATOR OSCILLATOR DETF OSCILLATOR CONCLUSIONS FILTERS 168 5.6.2 MIXERS OSCILLATORS CONCLUSIONS 173 MEMS RESONATORS 173 MEMS RF SIGNAL PROCESSORS 174 CMOS MEMS OSCILLATORS 175 FINAL CONCLUSIONS 175 REFERENCES 175 PUBLICATION LIST 176 JOURNALS 176 CONFERENCES 176 ANNEX 1: MEMS DESIGN EQUATIONS 179 FLEXURAL MODES IN BEAMS 179 CLAMPED CLAMPED BEAMS 180 FREE FREE BEAMS 183 DOUBLE ENDED TUNING FORKS (DETF) 186 ANNEX2: MEMS CHARACTERIZATION EQUIPMENT AND TECHNIQUES 191 MEASUREMENT TEST SETUP 191 MEASUREMENT TECHNIQUES 192 DIRECT S21 MEASUREMENTS 192 MIXING MEASUREMENTS 193 Q MEASUREMENT TECHNIQUES 194 MOTIONAL RESISTANCE EXTRACTION 194 ADVANCED MEASUREMENT TECHNIQUES 195 CALIBRATION 195 ADS FITTING 195 TERMINATION 198 ANNEX 3: RUN DESCRIPTION 201 AMS RUN UMC RUN 202 AMS RUN 2 203 1 Introduction MEMS is an acronym of MicroElectroMechanical Systems, however there are more denominations for the same kind of devices, as Micromachines (in Japan) or the European MST (Micro System Technology or even Microsystems). As the proper name indicates, MEMS are simply electrically actuated or sensed mechanical devices with microscale dimensions. The earliest reference of the power of miniaturization, which is also considered as one of the most enlightening visions on MEMS topic, was provided by Dr. Feynmann in a conference given in the annual meeting of the American Physics Society in 1959 ( There s plenty of room at the bottom ), which was later reprinted in the Journal of MEMS in [1]. Only 6 years later (in 1965), the very first MEMS work was published: the resonant gate transistor [2]. Unfortunately, due to technical problems, and lack of the appropriate techniques for MEMS fabrication, MEMS technology did not take off in that moment. It was in the early 1990, that several commercial MEMS products became available in mass market applications: ink-jet printers heads [3] and airbag accelerometers [4, 5]. Nowadays applications of MEMS are widespread and include sensing: pressure [6] and gyroscopes [5]; digital light processors (DLP) [7]; memories [8]; chemical and biological applications [9, 10] as well as actuators as microphones [11]. The great advantage of miniaturization provided by MEMS is driving the research and application of these devices in RF applications. 1.1 MEMS IN RF OVERVIEW AND RESEARCH MOTIVATION Considering the benefits obtained in better performance or reduction of fabrication costs by the use of MEMS in other areas, it was only a matter of time that manufacturers and researchers tried to apply these devices in the RF world. Moreover, the increasing number of wireless protocols and the search for more compact, less power-hungry and higher performance communication systems become one of the most important driver in consumer electronics research [12]. Figure 1.1 shows a diagram of the wireless services present (or on the road) on cellular handsets. Figure 1.1: Wireless handsets services. Some of the most relevant wireless protocols are shown. 19 Chapter 1. Introduction With this high amount of protocols and services, the challenge is to integrate all of these in a cellular handset maintaining its size and reduced power consumption. The goal is to design a single reconfigurable RF radio capable to deal with all these protocols and services. The current bottleneck in miniaturization of these portable systems is situated in the off-chip passives (in concrete in filters and resonators) rather than in the integrated circuits, as can be observed in Figure 1.2:(a). In this figure, (b) and (c) shows the evolution from 1998 to 2004 of a RF chip. The 1998 chip is a GSM power amplifier (the BGY241 from Philips), whereas the 2004 chip is a quadband transmitter module (BGY504 also from Philips). It can be observed that the second module offers more performance in a reduced place, thanks to the integration of passives in chips (shown with arrows), with the PASSI technology process [13] [14]. This example shows the important trends on RF technology, and is in this miniaturization, and performance improvement, MEMS are expected to provide an added value on both fields. As an example of these achievements we want to mention the case of MEMS-based oscillators, as the ones recently commercialized from Discera [15]. This company, among others mentioned in next sections, has achieved a fully MEMS oscillator which reduces size and power consumption compared with traditional quartz crystal oscillators (see Figure 1.3 in which a comparison between both approaches is shown). (a) (b) (c) Figure 1.2: (a) Photograph of a iphone mobile phone circuitry, RF radio section is highlighted.. (b) Image of a decapsulated GSM commercial power amplifier (BGY241 from Philips), 1998 and (c) Image of an evolved RF transmitter (BGY504) with increased performance and functionality on reduced size, (b) and (c) images are from [16]. 20 ASIC Plastic package MEMS wafer-level Packaged resonator 5 mm ASIC Ceramic substrate Hermetic package Quartz resonator 7 mm Figure 1.3: Comparison between the size of a MEMS-based oscillator and quartz crystal resonator. (from Discera web page) The MEMS devices being used for RF applications are simply called RF-MEMS. Under this label, a wide range of devices for different applications (including basically switches, oscillators, filters and mixers) can be found. These RF mechanical devices can be divided in: RF switches, tunable capacitors, high-q inductors and MEMS resonators (Figure 1.4). Figure 1.4: Several examples of RF MEMS: a) Suspended inductor [17], (b) DC tunable capacitor [18], (c) RF MEMS switch [19] and (d) MEMS resonator [20] 21 Chapter 1. Introduction It is interesting to note, however, that although high-q inductors are considered RF-MEMS, they are not movable mechanical parts (they do not present displacement) but take advantage of microfabrication techniques for the building of the inductors MARKET PERSPECTIVE FOR RF MEMS It is evident that the wireless market has become one of the most important drivers in consumer electronics. Starting from GSM mobile phone to wireless technologies like WI-FI, Bluetooth and RF-ID, wireless became an important pie in which every company wants its part, and so MEMS designers and manufacturers. Moreover, the use of MEMS is claimed to bring enhancements on RF systems and provide new applications nowadays unpractical by using traditional circuit approaches. Among the most successful MEMS devices (and the first ones to be commercialized) are Bulk Acoustic Wave (BAW) filters and duplexers. The BAW filters are the most mature MEMS devices, however, the roadmap of these devices include cost reduction, increase of performance and integration with CMOS IC, the same challenges to be faced by other MEMS devices (as switches and resonators). For further information see the predictions of the International Technology Roadmap of Semiconductors [21]. Figure 1.5 shows a roadmap of the commercialization of RF-MEMS components from 2002 to 2006 on this roadmap several updates must be highlighted. Only a few of these companies have already reached the commercialization release of their products, like the aforementioned tunable capacitor technology form Philips and included in BGY504. especially relevant is the reference oscillator market. MEMS-based reference oscillators are supposed to help reducing the size of overall RF systems, once that quartz reduction reached a kind of saturation [22]. However, even though forecasts prediction set the start of serial production on 2006, by the hand of Discera, this and other companies have recently started to publish preliminary commercial products datasheets. These delays in the commercialization, can allow purely CMOS high performance oscillators [23, 24] to steal part of the market coveted for MEMS devices. Figure 1.5: Roadmap from 2003 to 2008 of RF MEMS market, extracted from [25] 22 Figure 1.6 shows a forecast of the evolution of RF market from 2004 to 2009 divided in different applications. Figure 1.6: Forecast of evolution of RF MEMS market, from [25]. It can be observed in Figure 1.6 that the highest part of the RF-MEMS market is placed on the mobile telephony, although there is a very important increase in consumer electronics expected beyond Another interesting graphic is the one shown in Figure 1.7, which shows the estimated public investment on MEMS research per target application on different geographic areas: Europe, USA and Asia on Figure 1.7: Estimated distribution of the public funding for RF MEMS research for geographic area and market in 2004, adapted from [26]. 23 Chapter 1. Introduction It is significant the big amount of funding in MEMS technologies on USA and Asia, when compared to Europe. Moreover, the target application is quite different in each area: whereas in USA most of the investment was focused on military and space applications, Asia s effort is focused on commercial applications and in Europe it is significant the ratio of the total funding dedicated to space applications RF-MEMS APPLICATIONS ON A RF-FRONT-END With the successful applications of MEMS in other application markets, and with wireless electronics on the bull s eye of MEMS industry, the question is: how MEMS can improve wireless radios? Even though this question was partially answered in terms of reconfigurability of RF frontends using MEMS switches, some of the potential of MEMS resonators has not yet been described. In particular, these resonators show performance exceeding the obtained using integrated passive LC tank-resonators, especially in terms of the quality factor, Q of the inductor, therefore allowing highly frequency selective devices. Considering the classical RF receiver architectures, they can be divided into: (super) hetherodyne, low-if and zero-if [27]. Every RF receiver works in a similar way: the RF signal received in the antenna is filtered to consider only the application interest band, attenuating all other interferers. The filtered signal is then amplified by the low-noise amplifier and then downconverted in frequency (baseband frequency) for signal processing. It is in this down-conversion where the main difference between the aforementioned architectures exists. In zero-if receivers, the radio signal is directly down-converted to the baseband signal (nearly DC), on low-if the down-conversion is located at low frequency. These two architectures have a single downconversion stage, whereas hetherodyne receivers use two (or more) down-conversion stages. One of the most important problems for the RF receivers is the image frequency, because any nondesired s
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