cmut fabrication based on a thick buried oxide layer - Khuri-Yakub ...

acoustic response in the electrical impedance [Fig. 4(a)] anddisplacement at the center of a plate of a single cell [Fig. 4(b)].At a dc bias voltage of 115 V (70% of pull-in voltage)and an ac excitation signal of 1 V pp , the maximum centerdisplacement of the plate is 550 nm peak-to-peak [Fig. 4(b)].At this operation point electrical nonlinearities start to becomevisible [14].The typical total capacitances (measured and calculated) forthese devices are 400 pF and 280 pF for devices with 5 µmand 10 µm thick BOX layer, respectively. These numberscorrespond to 48% and and 63% reduction in total capacitancedue to a lower parasitic capacitance. The assumption forthis calculation are devices of equal cell geometry <strong>based</strong> onthe conventional <strong>fabrication</strong> process [2], i.e. using the cellstructure as shown in Fig. 1(b). Several of these deviceswere tested up to 1000 V dc bias voltage, repeatedly overa time period of six months, without a single occurrence ofan electrical breakdown.V. CONCLUSIONA versatile <strong>fabrication</strong> process for wafer-bonded CMUTs ispresented. The process is simple and improves the reliability ofwafer-bonded CMUTs in terms of electrical breakdown. Theproof-of-concept of the main idea was demonstrated in thiswork by fabricating highly reliable single-element transducers,intended for airborne applications, such as ultrasonic gas flowmetering at elevated temperatures. Furthermore, we expect thisprocess to be ideal for fabricating large and reliable 1D and2D arrays. Thus, at the moment we are fabricating large 2Darrays for medical applications, <strong>based</strong> on this thick BOX layerprocess.ACKNOWLEDGMENTThis work was supported by AVL List GmbH, Graz Austria,Corporate R&D Headquarter, Canon Inc., Tokyo, Japan, andNIH grant 5R01CA134720. The authors thank Dr. MichaelCernusca, Dr. Michael Wiesinger, and Dr. Katarzyna Kudlaty,AVL List GmbH, Graz, Austria, for many fruitful discussions.We also thank Dr. Steve Vargo, ST Systems USA Inc., foradvice on all DRIE steps required for this work and we thankKim Vu, GE Sensing, Fremont, CA, USA, for performing theDRIE steps.REFERENCES[1] A. S. Ergun, Y. Huang, X. Zhuang, Ö. Oralkan, G. G. Yaralioglu, andB. T. Khuri-Yakub, “Capacitive micromachined ultrasonic transducer <strong>fabrication</strong>technology,” Ultrasonics, Ferroelectrics and Frequency Control,IEEE Transactions on, vol. 52, no. 12, pp. 2242–2258, 2005.[2] Y. Huang, A. S. Ergun, E. Haeggstrom, M. H. Badi, and B. T. Khuri-Yakub, “Fabricating capacitive micromachined ultrasonic transducers withwafer-bonding technology,” Journal of microelectromechanical Systems,vol. 12, no. 2, pp. 128-137, 2003.[3] X. Zhuang, I. O. Wygant, D. Lin, M. Kupnik, Ö. Oralkan, and B. T. Khuri-Yakub, “Wafer-bonded 2D CMUT arrays incorporating through-wafertrench-isolated interconnects with a supporting frame,” Ultrasonics, Ferroelectricsand Frequency Control, IEEE Transactions on, vol. 56, no. 1,pp. 182–192, 2009.[4] X. C. Jin, C. H. Cheng, Ö. Oralkan, S. Calmes, F .L. Degertekin, and B.T.Khuri-Yakub “Recent progress in capacitive micromachined ultrasonicimmersion transducer arrays,” Proc. of Eighth International Symposiumon Integrated Circuits, Devices, and Systems, pp. 159-162, 1999.Amplitude (Z)Phase (Z)806040200-20-40-60-80250k 275k 300k 325k 350k 375k(a) LensFrequency (Hz)700Displacement at center of one plate (nm)(b)40k35k30k25k20k15k10k5kProbe from4294A95 V95 V75 V55 V35 V250k 275k 300k 325k 350k 375k650600550500450400350300250200150100505 mmLaserCMUTFrequency (Hz)Vdc = 115 V115 VElectrical nonlinearityis visibleVdc = 115 V95 V75 V55 V35 V75 V55 V1 Vpp ACexcitation only35 V0250k 275k 300k 325k 350k 375kFrequency (Hz)Fig. 4. Two preliminary measurement results (electrical impedance of aCMUT (a) and plate displacement of one single cell of the same device (b)as a function of dc bias voltage). The thick BOX layer for this device is 5 µm.[5] M. Kupnik, A. S. Ergun, Y. Huang, and B. T. Khuri-Yakub, “Extendedinsulation layer structure for capacitive micromachined ultrasonic transducers,”in Proc. IEEE Ultrasonics Symposium, pp. 511-514, 2007.[6] M.-C Ho, M. Kupnik, and B. T. Khuri-Yakub, “FEA of CMUTs suitablefor wide gas pressure range applications,” in Proc. IEEE UltrasonicsSymposium, to appear, 2010.[7] A. Nikoozadeh, and B. T. Khuri-Yakub, “CMUT with substrate-embeddedsprings for non-flexural plate movement,” in Proc. IEEE UltrasonicsSymposium, to appear, 2010.[8] M. Kupnik and B. T. Khuri-Yakub, “Direct wafer-bonded 2D CMUTarray,” US patent, US 2009/0122651 A1, 2009.[9] L. A. Donohue, J. Hopkins, R. Barnett, A. Newton, and A. Barker,“Developments in Si and SiO2 etching for MEMS <strong>based</strong> optical applications,”Micromachining Technology for Micro-Optics and Nano-OpticsII, Proceedings of SPIE, vol. 5347, pp. 44-53, 2004.[10] M. Kupnik and B. T. Khuri-Yakub, “Wafer bonded CMUT meetsCMOS,” Slides for CMOS Emerging Technology Workshop, available atwww.cmoset.com/uploads/Mario Kupnik 2010.pdf, 2009.[11] M. Kupnik and B. T. Khuri-Yakub, “Monolithic integrated CMUTsfabricated by low temperature wafer bonding,” filed for US and PCTpatent application, 2009.[12] J. Köhler, C. Strandman, Ö. Vallin, C. Hedlund, and Y. Bäcklund,“Silicon fusion bond interfaces resilient to wet anisotropic etchants,”Journal of Micromechanics and Microengineering, vol. 11, pp. 359-363,2001.[13] S. J. Cunningham and M. Kupnik, “Wafer bonding,” Chapter 11 inMEMS Material and Process Handbook, Springer, to appear, December2010.[14] V. Kaajakari, T. Mattila, A. Oja, and H. Seppä, “Nonlinear Limits forsingle-crystal silicon microresonators,” Journal of microelectromechanicalSystems, vol. 13, no. 5, pp. 715-724, 2004.