To investigate broadband operation, we tuned the signal from�s � 1549 nm to 1568 nm and measured the Q-factor and on–offgain in this range. Figure 4(a) shows the results. Q-Factors largerthan 6 (BER � 10�9) were obtained for �s between 1550.5 and1567 nm. Note that the Q-factor is almost constant for signalwavelengths close to �p and decreases rapidly for �s at the outerslopes of the gain spectrum. This strong deterioration of theQ-factor can be originated by two parameters of the pump thatchanges in the time and produces gain fluctuations: (i) the powerPp and (ii) the instantaneous frequency �p. The pump powervaries with time, due to the fact that it beats with ASE noise fromEDFA2, thus resulting in a relative-intensity noise (RIN), whilethe pump’s instantaneous frequency changes due to phase modu-lation. Both factors will produce signal-gain variations that shouldbe stronger when the signal wavelength is tuned at the outer slopesof the gain spectrum, where small variations of power or frequencywill be efficiently transferred as signal-gain variations [8]. Figure4(b) show BER versus received power, measured at the preampli-fier input for �s � 1563.2 nm. The receiver-power penaltyrelative to the back-to-back measurement at a BER � 10�9 isnearly 1.5 dB and is most likely due to the pump RIN and timejitter. By properly filtering the ASE noise from EDFA2, the BERperformance of our system can be improved.

4. CONCLUSION

We have demonstrated the transmission of PRBS 231 � 1 NRZdata streams modulated at 10 Gb/s over 90-km of standard fiberusing mid-span phase conjugation, which provides a parametricnet gain of 9.4 � 1.4 dB over a bandwidth of 17 nm and a powerpenalty �1.5 dB at BER � 10�9. To the best of our knowledge,this is the first demonstration of a fiber-optic parametric device forbroadband simultaneous compensation for fiber loss and chromaticdispersion. Nevertheless, transmission performance still requiresfurther improvements by reducing the induced time jitter and gainvariations due to the phase modulation and pump RIN.

ACKNOWLEDGMENTS

This work was supported by FAPESP, CAPES, CNPq, PRONEXand Fundacao CPqD. The authors would like to thank A. A.Juriollo, F. Simoes, J. B. Rosolem, J. C. Said, and M. R. Horiuchifrom Fundacao CPqD for their help during the experiments.

References

1. S. Watanabe, T. Naito, and T. Chikama, Compensation of chromaticdispersion in a single-mode fiber by optical phase conjugation, IEEEPhoton Technol Lett 5 (1993), 92–95.

2. S.Y. Set, R. Girardi, E. Riccardi, B.E. Olsson, M. Puleo, M. Ibsen, R.I.Laming, P.A. Andrekson, F. Cisternino, and H. Geiger, 40 Gbit/s fieldtransmission over standard fiber using midspan spectral inversion fordispersion compensation, IEE Electron Lett 35 (1999), 581–582.

3. S. Watanabe, S. Takeda, and T. Chikama, Interband wavelength con-version of 320 Gb/s (32 � 10 Gb/s) WDM signal using a polarization-insensitive fiber four-wave mixer, ECOC’98, Madrid, 1998, pp. 83–88.

4. M. Tani and S. Yamashita, Dispersion compensation with SBS-sup-pressed fiber phase conjugator using synchronized phase modulation,IEE Electron Lett 39 (2003), 1375–1377.

5. M. Tani and S. Yamashita, Optical fiber four-wave mixing using syn-chronous modulation with a single phase modulator, in Optical Ampli-fiers and their Applications, Ohtaru, 2003, paper WD3.

6. S. Radic, R.M. Jopson, C.J. McKinstrie, A.H. Gnauck, S. Chan-drasekhar, and J.C. Centanni, Wavelength division multiplexed trans-mission over standard single mode fiber using polarization insensitivesignal conjugation in highly nonlinear optical fiber, Optical Fiber Com-mun Conf, Anaheim, CA, 2003, PD Paper 12.

7. J.M. Chavez Boggio, S. Tenenbaum, and H.L. Fragnito, Four-wavemixing induced changes in the noise spectrum in an optical fiber,Optical Fiber Commun Conf, Optical Society of America TechnicalDigest, Washington, DC, 2001, paper WDD24.

8. P. Kylemark, T. Torounidis, P.O. Hedekvist, H. Sunnerud, and P.A.Andrekson, Noise figure characterization of fiber optical parametricamplifier, Euro Conf Opt Commun, Granada, Spain, 2003, PaperWe1.6.4.

© 2004 Wiley Periodicals, Inc.

A FEASIBILITY STUDY OF ZnO-BASEDFBAR DEVICES FOR AN ULTRA-MASS-SENSITIVE SENSOR APPLICATION

Linh Mai, Dong-Hyun Kim, Munhyuk Yim, and Giwan YoonInformation and Communications University (ICU)103-6 Munji-dongYusong-guDaejon 305-714, Korea

Received 1 March 2004

ABSTRACT: We present a feasibility study of ZnO-based FBAR de-vices and their fabrications for the ultra-mass-sensitive sensor applica-tion. In this work, a considerable shift in the resonance frequency isobserved due to the mass-loading effect by a mixture of ink and volatilemethanol. A high sensitivity of 0.05 � 105 Hz � cm2/ng is obtained inthe FBAR-based device, which is �5 orders of magnitude higher thanthat of 0.057 Hz � cm2/ng reported in the conventional 5-MHz quartzcrystal microbalance. This approach seems useful for the ultra-high-mass resolution chemical sensor or biosensor applications. © 2004Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 505–507, 2004;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.20351

Key words: FBAR devices; ZnO; resonance characteristics; Q-factor;return loss

1. INTRODUCTION

Recently, mass-sensitive sensors based on acoustic wave-reso-nance phenomena have been investigated widely due to their highsensitivity for transducers for various chemical and biologicalenvironment monitorings [1]. The representative sensors are sur-face acoustic wave (SAW) sensors and bulk acoustic wave (BAW)sensors. Unfortunately, a further reduction in the SAW devicedimension is limited due to the length of the delay line for theinterdigital transducer (IDT) [1]. Also, the BAW sensors, wellknown as a quartz crystal microbalance (QCM), are shown to havea limitation in achieving an even higher sensitivity due to theirlow-frequency operation of mostly several hundred MHz. On theother hand, the film bulk acoustic resonator (FBAR) has recentlyemerged as an attractive device, mainly because it has a strongpotential of enabling the realization of the microwave monolithicintegrated circuits [2]. Reportedly, a membrane-type FBAR deviceusing micromachining technology has been applied to the chemi-cal vapor and liquid-mass sensing devices [1]. In spite of thiseffort, to the best of our knowledge, no reports have been made onthe feasibility study of the solidly mounted resonator (SMR)-typeFBAR devices for ultra-mass-sensitive sensor (UMS) applications.In this paper, for the first time, FBAR-based UMS devices andtheir feasibility for ultra-mass-sensitive sensor applications arepresented.

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 42, No. 6, September 20 2004 505

2. EXPERIMENT

For this feasibility study, we fabricated two FBAR-based UMSdevices of different structures to investigate their performance orsensitivity. Basically, these FBAR-based UMS devices were fab-ricated based on the SMR-type FBAR technology [2]. A thermalannealing was also performed to improve the as-deposited W/SiO2

multilayer reflector [3]. In this experiment, two different types ofthe bottom electrodes were designed to investigate their resonancecharacteristics in the FBAR-based UMS devices. In the first struc-ture, the nonpatterned (blanket-type) bottom electrode acts as thefloating ground, which has a semi-infinite ground effect. In thesecond structure, the patterned bottom electrode defines the reso-nance area by overlapping with the top electrode on the piezoelec-tric layer. The three-dimensional (3D) schematic diagrams of thefabricated FBAR-based UMS devices are shown in Figure 1. Thecross-sectional view for the single-resonance peak and top-viewimages for the two resonance peaks of the fabricated FBAR-basedUMS device are shown in Figures 2(a) and 2(b), respectively.

3. RESULTS AND DISCUSSION

The resonance characteristics of the two FBAR-based UMS de-vices at air prior to and after mass-loading on each device surfaceare shown in Figure 3, where the reflection coefficient (S11) valuesare plotted as functions of frequency. To investigate the mass-sensitivity characteristics of the FBAR-based UMS devices, weprepared and used a test solution, consisting of a 10:1 mixture ofmethanol and ink. The results show that the resonance frequencycan be changed significantly by dropping a small amount of themixture solution on the surface of the FBAR-based UMS devices.Prior to the mass-loading, the resonance peaks in both FBAR-based UMS devices have appeared near 2 GHz. By dropping asmall amount of the mixture solution on the surface of the devices,the resonance frequency can be changed due to the mass of the inkparticles that remain dried on the surface of the devices, as thesolvent portion will evaporate quickly. The shifts of resonancefrequency due to the change of mass by the dried ink particles areshown in Figure 3. The resonance frequency was shifted from 1.99to 1.96 GHz in the single-peak structure and from 1.99 to 1.97GHz or 1.88 to 1.86 GHz in the dual-frequency peaks. As a result,the mass-loading effects were comfirmed to occur by this signif-icant shift of resonance frequency. Moreover, we also estimatedsome important parameters using the following Sauerbrey’s for-mula [4] for insight regarding the sensitivity due to the mass-loading effect:

�f � �2f 0

2

A�c66�q

�m � �Sf�m, (1)

where f0 is the reference resonance frequency and f0 � �f is theloaded frequency, c66 is a stiffness constant of the piezoelectricmaterial, given by (c11 � c12)/ 2 in the hexagonal structure, and�q is its density. In the case of ZnO, c66 � 4.43 � 1010 N/m2 and�q � 5680 kg/m3 [5]. We obtained the mass sensitivity Sf of0.05 � 105 Hz � cm2/ng by some calculations putting the area A tounity. This gravimetric sensitivity factor is �5 orders of magnitudehigher than the sensitivity in the 5 MHz quartz crystal of 0.057 Hz �cm2/ng. The ink amount sensed by the UMS devices is estimatedto be 6 �g/cm2 in the first device structure and 4 �g/cm2 in thesecond structure because the changes of resonance frequency (�f )are about �0.03 GHz and �0.02 GHz, respectively.

4. CONCLUSION

We have fabricated two mass-sensitive devices based on the SMR-type acoustic resonator and measured the change of the resonancefrequency due to the mass-loading effect using a mixture ofmethanol and ink. As a result, we obtained a very high sensitivity

Figure 1 3D schematic diagrams for the device structures showing (a)single-frequency peak and (b) dual-frequency peaks

Figure 2 FBAR-based ultra-mass-sensitive (UMS) device fabricated forthe dual peaks: (a) cross-sectional view; (b) top view

Figure 3 Return-loss characteristics showing the shift of the resonancefrequency for (a) single-frequency peak and (b) dual-frequency peaks. Theline with solid circles indicates the frequency shift after mass-loading

506 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 42, No. 6, September 20 2004

of 0.05 � 105 Hz � cm2/ng, which is �5 orders of magnitudehigher than the sentitivity (0.057 Hz � cm2/ng) of the conventional5-MHz QCMs. The ultra-mass-sensitive device will be very usefulfor future chemical-sensor or biosensor applications.

ACKNOWLEDGMENT

This work was partially supported by the Radio Education andResearch Center (RERC) of Information and CommunicationsUniversity.

REFERENCES

1. H. Zhang and E.S. Kim, Vapor and liquid mass sensing by microma-chined acoustic resonator, IEEE Micro Electro Mechanical Systems IntConf MEMS’ 03 Kyoto, 2003, pp 470–473.

2. G. Yoon and J. Park, Fabrication of ZnO-based film bulk acousticresonator devices using W/SiO2 multilayer reflector, Electron Lett 36(2000), 1435–1438.

3. D.-H. Kim, M. Yim, D. Chai, and G. Yoon, Improvements of resonancecharacteristics due to thermal annealing of Bragg reflectors in ZnO-based FBAR devices, Electron Lett 39 (2003), 962–964.

4. G. Sauerbrey, Verwendung von Schwingquarzen zur Wagung dunnerSchichten und zur Mikrowagung, Z Phys 155 (1959), 206–222 (inGerman).

5. B.A. Auld, Acoustic fields and waves in solids, Kreiger Publishing,Malabar, FL, 1990, pp 376–379.

© 2004 Wiley Periodicals, Inc.

HIGH-Q LIGA-MEMS VERTICALCANTILEVER VARIABLE CAPACITORSFOR UPPER MICROWAVEFREQUENCIES

Darcy T. Haluzan1,2 and David M. Klymyshyn1,2

1 TRLabs111–116 Research DriveSaskatoon, Saskatchewan, Canada, S7N 3R32 Department of Electrical EngineeringUniversity of Saskatchewan57 Campus DriveSaskatoon, Saskatchewan, Canada, S7N 5A9

Received 20 February 2004

ABSTRACT: Simulation results for four MEMS variable capacitors suit-able for fabrication using the LIGA process are presented. A Q-factor ashigh as 435.6 was obtained using a copper-device layer. The capacitor witha nominal capacitance of 0.24 pF has a self-resonant frequency of 33.7GHz, making it suitable for operation up to frequencies of approximately 18GHz. Using a “pull-away” actuation design, a tuning ratio of 1.43:1 wasobtained with tuning voltages as low as 6.1 V. © 2004 Wiley Periodicals,Inc. Microwave Opt Technol Lett 42: 507–511, 2004; Published online inWiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20352

Key words: LIGA; MEMS; variable capacitor; X-ray lithography; mi-crowave

INTRODUCTION

The unprecedented performance of microwave microelectrome-chanical systems (MEMS) devices compared to the traditionalnonMEMS approach has led to increased attention in recent years.Examples of these devices include switches, variable capacitors,and inductors found in systems such as voltage-controlled oscil-lators, filters, and phase shifters. They have been developed to

replace their on-chip solid-state counterparts, and certain off-chipcomponents. The use of MEMS devices promises high integration.This is attractive because of the potential to reduce cost, size, andpower consumption. In the case of MEMS variable capacitors, oneof the main advantages is the potential for high Q-factors at highfrequencies.

Most existing MEMS capacitor designs feature parallel plates,where the capacitance is varied by changing the gap between theplates [1–3]. Traditionally, these plates are limited to planar ge-ometries and lie parallel to the substrate. These devices are con-structed from layers that are typically thinner than 5 �m. They arenormally actuated electrostatically, or thermally. Most of thesedevices have been fabricated using silicon-based thin-film pro-cesses, such as MUMPs, which are lossy at high frequencies. Analternate configuration is a lateral-comb structure [4, 5]. In thisgeometry, adjusting the overlap of the capacitor fingers changesthe capacitance. The direction of actuation is perpendicular to theactuation in parallel-plate-type capacitors. Device layers as thickas 80 �m have been reported [5]. These layers were constructedusing a highly refined deep silicon etch.

Of the many variations of MEMS variable capacitors in exis-tence, most are designed to operate at the lower end of themicrowave-frequency range. Very few are able to achieve Q-factors greater than 100 at frequencies above 4 GHz. The reportedvariable capacitors which are capable of high-Q operation at highfrequencies make use of a shunt-mounted design [6, 7]. In thisconfiguration, coplanar-waveguide transmission lines are used. Abridge is created over the center conductor and the two air gaps. Achange in air gap between the bridge and the center conductorchanges the capacitance. In [6], a shunt-mounted capacitor withthermal actuation was designed. A minimum Q-factor of 197 for

Figure 1 Top view of the capacitor

Figure 2 Three-dimensional view of the capacitor

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 42, No. 6, September 20 2004 507