Nanomechanical resonators have been monolithically integrated on preprocessed complementary metal-oxide-semiconductor(CMOS) chips. Fabricatedresonatorsystems have been designed to have resonance frequencies up to 1.5 MHz. The systems have been characterized in ambient air and vacuum conditions and display ultrasensitive mass detection in air. A mass sensitivity of 4ag/Hz has been determined in air by placing a single glycerine drop, having a measured weight of 57 fg, at the apex of a cantilever and subsequently measuring a frequency shift of 14.8 kHz. CMOS integration enables electrostatic excitation, capacitive detection, and amplification of the resonance signal directly on the chip

Abadal Berini, Gabriel

American Physical Society

Barniol i Beumala, Núria

Boisen, A.

Campabadal Segura, Francesca

Esteve, Jaume

Figueras Costa, Eduardo

Forsen, E.

Ghatnekar-Nilsson, S.

Montelius, L.

Pérez Murano, Francesc

Sandberg, R.

Svendsen, W.

Teva Meroño, Jordi

Verd Martorell, Jaume

E. Forsen

G. Abadal

S. Ghatnekar-Nilsson

J. Teva

J. Verd

R. Sandberg

W. Svendsen

F. Perez-Murano

J. Esteve

E. Figueras

F. Campabadal

L. Montelius

N. Barniol

A. Boisen

Crossref

Ultrasensitive mass sensor fully integrated with complementary metal-oxide-semiconductor circuitry

Campabadal, Francesca

Diposit Digital de Documents de la UAB

Ultrasensitive mass sensor fully integrated with complementary metal-oxide-semiconductor circuitryE. Forsen, G. Abadal, S. Ghatnekar-Nilsson, J. Teva, J. Verd, R. Sandberg, W. Svendsen, F. Perez-Murano, J.Esteve, E. Figueras, F. Campabadal, L. Montelius, N. Barniol, and A. Boisen  Citation: Applied Physics Letters 87, 043507 (2005); doi: 10.1063/1.1999838 View online: http://dx.doi.org/10.1063/1.1999838 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/87/4?ver=pdfcov Published by the AIP Publishing                      This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:158.109.223.71 On: Fri, 21 Feb 2014 14:51:02Ultrasensitive mass sensor fully integrated with complementarymetal-oxide-semiconductor circuitryE. ForsenDepartment of Micro and Nanotechnology, Technical University of Denmark, Kgs. Lyngby,DK-2800, DenmarkG. AbadalDepartment of Electronic Engineering, Universitat Autonoma de Barcelona, 08193 Bellaterra, SpainS. Ghatnekar-NilssonSolid State Physics and The Nanometer Consortium, University of Lund, 22362 Lund, SwedenJ. Teva and J. VerdDepartment of Electronic Engineering, Universitat Autonoma de Barcelona, 08193 Bellaterra, SpainR. Sandberg and W. SvendsenDepartment of Micro and Nanotechnology, Technical University of Denmark, Kgs. Lyngby,DK-2800, DenmarkF. Perez-Murano, J. Esteve, E. Figueras, and F. CampabadalNational Microelectronics Centre, Universitat Autonoma de Barcelona, 08193 Bellaterra, SpainL. MonteliusSolid State Physics and The Nanometer Consortium, University of Lund, 22362 Lund, SwedenN. BarniolDepartment of Electronic Engineering Universitat Autonoma de Barcelona, 08193 Bellaterra, SpainA. BoisenDepartment of Micro and Nanotechnology, Technical University of Denmark, Kgs. Lyngby,Dk-2800, DenmarkReceived 17 March 2005; accepted 7 June 2005; published online 21 July 2005Nanomechanical resonators have been monolithically integrated on preprocessed complementarymetal-oxide-semiconductor CMOS chips. Fabricated resonator systems have been designed tohave resonance frequencies up to 1.5 MHz. The systems have been characterized in ambient air andvacuum conditions and display ultrasensitive mass detection in air. A mass sensitivity of 4 ag/Hzhas been determined in air by placing a single glycerine drop, having a measured weight of 57 fg,at the apex of a cantilever and subsequently measuring a frequency shift of 14.8 kHz. CMOSintegration enables electrostatic excitation, capacitive detection, and amplification of the resonancesignal directly on the chip. © 2005 American Institute of Physics. DOI: 10.1063/1.1999838The advances of nanotechnology can be utilized in de-veloping portable sensor systems for applications in biologi-cal, physical, or chemical sensing, achieving ultrasensitivedetection with low analyte consumption. One approach indeveloping such a system is to make a nanoresonator devicewhere a change in the mass of the resonator is detected as achange in the resonance frequency of the resonator. In orderto achieve the highest possible mass sensitivity, research onfabrication, integration, and development of nanoelectrome-chanical resonator systems is pursued.1–3 The most fre-quently used techniques for measuring the resonance fre-quency of a cantilever are based on optical detection.4 Theadvantage is its inherent simplicity and high sensitivity.Some disadvantages are problems with alignment capability,miniaturization and portability. Resonator systems canbe actuated by piezoelectric, magnetic, or thermalactuation.5–7 Another option is to use electrostatic actuationand capacitive readout. This is achieved by connectingnano/microstructures with standard microelectronics.Previously,8–11 we have reported on the principle of designand fabrication of cantilever resonators integrated with stan-dard complementary metal-oxide-semiconductor CMOScircuitry. CMOS integration enables simple electrostatic ac-tuation, capacitive read-out, and signal amplification. In thisletter, we present the functional evaluation of CMOS inte-grated cantilever structures achieving attogram/Hz massresolution in air.The sensing principle is based on monitoring the reso-nance frequency change of a cantilever as a function of massadsorption, e.g., due to the adsorption of molecules. Thenanoresonator structures are excited into lateral vibration byapplying an ac and dc voltage between the suspended canti-lever and a fixed parallel electrode. The frequency shift uponadded mass is measured on-chip by capacitive resonance fre-quency detection. CMOS integration reduces the parasiticcapacitance contribution and is hence crucial for our choiceof cantilever readout. Furthermore, CMOS integration allowsfor increased functionality in terms of frequency trackingand Q-factor enhancement,12,13 and can be used as a compo-nent in a portable device.The nanoresonators are defined by combined electron-beam lithography and direct write laser lithography on a pre-processed CMOS chip.11 Examples of fabricated 425 nmwide, 600 nm thick, and 20 m long polycrystalline siliconAPPLIED PHYSICS LETTERS 87, 043507 20050003-6951/2005/874/043507/3/$22.50 © 2005 American Institute of Physics87, 043507-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:158.109.223.71 On: Fri, 21 Feb 2014 14:51:02poly-Si cantilevers, integrated on a CMOS chip, aredemonstrated in Fig. 1. We have used standard CMOStechnology14 which leads to cantilever design constraints,limiting the highest detectable resonance frequency to ap-proximately 2 MHz. The on-chip frequency response of sucha cantilever, as a function of applied dc voltage Vdc, ismeasured using a gain-phase analyzer, see Fig. 2a. Theresonance frequency is reduced when the Vdc is increased,due to electrostatic spring softening.15 The unperturbed reso-nance frequency of the system is determined to be f0=1.487 MHz from the intersection of the linear fit with they axis at zero-applied voltage, Fig. 2b. Electrical measure-ments performed on equivalent cantilevers without an inte-grated CMOS circuit did not show any appreciable reso-nance frequency signal, due to the screening effect of theparasitic capacitance introduced by the pads and externalwires. The Young’s modulus of the poly-Si cantilever is cal-culated using Eq. 1 to have a value of E=175±18 GPaRef. 16E =1624n1f02w2, 1in which =20 m is the length and w=425 nm is the widthof the cantilever, =2.33106 g/m3 is the mass density ofthe poly-Si, n10.2427 is a geometrical form factor for thefundamental vibration mode originating from the Euler–Bernoulli beam equation.17 The calculated Young’s modulusis consistent with measurements on similar poly-Si thin filmswith E=158±7 GPa.18Resonator structures have been characterized in a cham-ber in which the pressure has been controlled in the range of1013 mbar–0.1 mbar. The quality factor as a function of thepressure displays a log-log dependence, as shown in Fig.2c. Given a dc+ac actuation voltage of 1 Vdc and 0.9 Vpeak-peak VPP, a quality factor of approximately 5000 isdetermined at a pressure of 0.4 mbar. The quality factor isreduced to 30 at ambient air conditions. This is comparableto previous visual characterization of similar sized equivalentresonators that have not been CMOS integrated. Here, aquality factor of 70 was found for air operation14 and thevalue increased to 28 000 for operation at a pressure of2 bar.8In order to characterize the mass sensitivity of the reso-nator device in air, punctual masses are placed on the canti-lever. Single glycerine drops are placed selectively on theapex of the cantilever using a glycerine coated scanning tun-neling microscope STM tip. The deposited glycerine drophas a diameter of approximately 500 nm, estimated by com-paring the drop size with the known width of the cantilever.Assuming a hemispherical volume and =1.26106 g/m3,the mass of the glycerine drop is estimated to be 41 fg.Figure 3 shows the on-chip readout before and after the con-trolled positioning of a single glycerine drop at the apex ofthe resonator. An ac voltage of 6 Vpp and a dc voltage of14 V is applied to a 20 m long, 425 nm wide, and 600 nmthick nanoresonator, similar to the resonator shown in Fig.1c. A resonant frequency of 1.453 MHz is measured beforethe deposition of the glycerine drop. Directly after the depo-sition of the glycerine, a frequency shift in air of f=14.8 kHz is determined from the shift in phase. From thefrequency shift, an added mass of 57 fg is calculated usingEq. 219m =2meffff0, 2where meff=n1wt is the effective mass of the cantilever forthe fundamental vibration mode, calculated using a densityof poly-Si =2.33106 g/m3, and a resonance frequencyFIG. 1. SEM images of fabricated poly-Si cantilever structures on a CMOSchip. a Top view image of a defined cantilever structure. The cantilever isexcited into lateral resonance by applying an ac and dc voltage between thedriver electrode and the cantilever. The cantilever is connected to a combcapacitor in order to polarize the CMOS circuitry. b Tilted view image ofa 20 m long, 425 nm wide, and 600 nm thick cantilever.FIG. 2. a On-chip readout using a poly-Si cantilever with a width of 425nm, a thickness of 600 nm and a length of 20 m, at a pressure of 0.4 mbar.The frequency signal from the CMOS circuitry was analyzed using a gain-phase analyzer. The inset shows the fluctuation of the phase signal 0.5° . b The effect of electrostatic spring softening is shown. The un-perturbed resonance frequency of the system is determined from the inter-section of the linear fit at zero-applied dc voltage. The fundamental reso-nance frequency is 1.487 MHz. c The quality factor dependence on thepressure displays a log-log relationship. A quality factor of approximately5000 is determined at 0.4 mbar and 1 Vdc and 0.9 VPP.043507-2 Forsen et al. Appl. Phys. Lett. 87, 043507 2005 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:158.109.223.71 On: Fri, 21 Feb 2014 14:51:02f0=1.487 MHz. Hence, the calculated mass sensitivity of thesystem is 4±1 ag/Hz.Inevitably, the ultimate mass resolution is limited by thenoise of the system. In order to get an estimate of the mini-mal detectable mass, M, which is the mass change thatresults in a frequency shift that overcomes the noise floor, aninvestigation of the phase noise of the system needs to beconducted. From the inset in Fig. 2a, the magnitude of thephase noise—caused by both intrinsic and extrinsic noisesources from the transducer and the readout circuitry—canbe determined to be of the order of 0.5° at a pressure of0.4 mbar. Since the slope of the phase signal at the frequencyof the resonance peak was  /f =−0.108, the correspondingminimum frequency shift is f4.6 Hz. Hence, the minimaldetectable mass is calculated as Mnoise=2mefff / f018 ag.The ultimate resolution for a nanoresonator, equivalent withthe one presented in this letter, only taking intrinsic phasenoise into account and excluding extrinsic noise limitationscan be approximated using Eq. 3:19Mintrinsic = 2meffET/Ecf/Q2f0  1 ag, 3where ET=kBT is the thermal energy at room temperature,Ec=meff42f02xc2	 is the driving energy, xc	100 nm is theapproximate root-mean-square drive amplitude in the direc-tion of vibration still consistent with producing a linear re-sponse, and f =1 kHz is the measurement bandwidth. Thisis consistent with the theoretical/experimental work byEkinci et al.4,19 regarding noise limitations for nanoresonatormass sensor systems.The phase noise is increased as one increases the pres-sure, mainly due to the reduced quality factor but alsoadsorption/desorption processes on the cantilever and addi-tional drift of the circuit are limiting factors. At ambientconditions, the minimum frequency shift that can be moni-tored is of the order of f1 kHz, which corresponds to anultimate mass resolution in air of the order of a few femto-grams.In conclusion, the characterization of a fully integratedresonator mass sensor system in air and at low pressure con-ditions has been discussed. CMOS integrated cantilevershave been electrostatically excited into mechanical resonanceand the resonance frequency has been detected on-chip bycapacitive readout. The mass sensitivity of the system hasbeen determined by controlled positioning of glycerine dropson a cantilever, whereby a mass sensitivity of the order of4 ag/Hz is measured for a resonator system with a funda-mental resonance frequency of 1.487 MHz. At 0.4 mbar, thequality factor is determined to be approximately 5000. Thesensitivity of the system is comparable to recent resultsachieved with resonator structures based on external actua-tion and readout.4,20 Nanoresonator devices should be func-tionalized in order to enable selective mass sensing. The ul-timate goal is to develop nanoresonator devices as integralparts of a portable sensor system.The authors would like to acknowledge funding grantedby the European Union IST project NANOMASS II IST-2001-33068, NANOSES MEYT-MC2003-07237, andNANOSYS TIC2003-07231N. V. Lavrik and P. G. Datskos, Appl. Phys. Lett. 82, 2697 2003.2J. Yang, T. Ono, and M. Esashi, Sens. Actuators, A 82, 102 2000.3K. L Ekinci, X. M. H. Huang, and M. L. Roukes, Appl. Phys. Lett. 84,4469 2004.4R. Wiesendanger. Scanning Probe Microscopy and Spectroscopy Cam-bridge University Press, Cambridge, U. K., 1994.5A. N. Cleland, M. Pophristic, and I. Fergurson, Appl. Phys. Lett. 79, 20702001.6H. J. Cho and C. H. Ahn, Proceedings of the International ConferenceMEMS 2000 2000, p. 686.7C. Hagleitner, A. Hierlemann, D. Lange, A. Kummer, N. Kerness, O.Brand, and H. Baltes, Nature London 414, 293 2001.8Z. J Davis, G. Abadal, B. Helbo, O. Hansen, F. Campabadal, F. Perez-Murano, J. Esteve, E. Figueras, J. Verd, N. Barniol, and A. Boisen, Sens.Actuators, A 105, 311 2003.9E. Forsén, S. G. Nilsson, P. Carlberg, G. Abadal, F. Pérez-Murano, J.Esteve, J. Montserrat, E. Figueras, F. Campabadal, J. Verd, L. Montelius,N. Barniol, and A. Boisen, Nanotechnology 15, 628 2004.10S. Ghatnekar-Nilsson, E. Forsén, G. Abadal, J. Verd, F. Campabadal, F.Pérez-Murano, J. Esteve, N. Barniol, A. Boisen, and L. Montelius,Nanotechnology 16, 98 2005.11J. Verd, G. Abadal, J. Teva, M. Villaroya Gaudó, A. Uranga, X. Borrisé, F.Campabadal, J. Esteve, E. Figueras Costa, F. Pérez-Murano, Z. J. Davis,E. Forsén, A. Boisen, and N. Barniol, J. Microelectromech. Syst. 14, 5082005.12G. Abadal, Z. J. Davis, B. Helbo, X. Borrisé, R. Ruiz, A. Boisen, F.Campabadal, J. Esteve, E. Figueras, F. Pérez-Murano, and N. Barniol,Nanotechnology 12, 100 2001.13D. Lange, C. Hagleitner, C. Herzog, O. Brand, and H. Baltes, Sens. Ac-tuators, A 103, 150 2003.14CNM-CMOS twin-well 2-poly 2-metal 2.5 micron technology.15Z. J. Davis, G. Abadal, O. Kuhn, O. Hansen, F. Grey, and A. Boisen, J.Vac. Sci. Technol. B 18, 612 2000.16The error was determined using the following measurement uncertainties:= ±200 nm, = ±1105 g/m3, f = ±5 kHz, and w= ±40 nm.17W. T. Thomsson, Theory of Vibration with Applications, 4th Ed. StanleyThornes Ltd., 1993.18D. Maier-Schneider, A Köprülülü, B. S. Holm, and E. Obermeier, J. Mi-cromech. Microeng. 6, 436 1996.19K. L. Ekinci, Y. T. Yang, and M. L. Roukes, J. Appl. Phys. 95, 26822004.20B. Ilic, H. G. Craighead, S. Krylov, W. Senaratne, C. Ober, and P. Neuzil,J. Appl. Phys. 95, 3694 2004.FIG. 3. The diagram shows the electrical resonance signal for a poly-Siresonator structure as measured through the CMOS. At ambient conditions,the resonance frequency was 1.453 MHz. The cantilever is 20 m long, 425nm wide, and 600 nm thick. The cantilever is excited into resonance byapplying 14 Vdc and 6 VPP ac. The inset shows two glycerine droplets, one atthe apex of the cantilever and the other is on the parallel actuation electrode.This is due to the fact that the glycerine-coated STM-tip touched the elec-trode on the first approach, resulting in a glycerine drop on the electrode.After deposition of a single glycerine drop onto the cantilever, the resonancefrequency was reduced by 14.8 kHz, which yields a mass sensitivity on theorder of 4 ag/Hz at ambient conditions.043507-3 Forsen et al. Appl. Phys. Lett. 87, 043507 2005 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:158.109.223.71 On: Fri, 21 Feb 2014 14:51:02

Nanomechanical resonators have been monolithically integrated on preprocessed complementary metal-oxide-semiconductor (CMOS) chips. Fabricated resonator systems have been designed to have resonance frequencies up to 1.5 MHz. The systems have been characterized in ambient air and vacuum conditions and display ultrasensitive mass detection in air. A mass sensitivity of 4 ag/Hz has been determined in air by placing a single glycerine drop, having a measured weight of 57 fg, at the apex of a cantilever and subsequently measuring a frequency shift of 14.8 kHz. CMOS integration enables electrostatic excitation, capacitive detection, and amplification of the resonance signal directly on the chip. (c) 2005 American Institute of Physics

Forsén, Esko Sebastian

Abadal, G.

Teva, J.

Verd, J.

Sandberg, Rasmus Kousholt

Svendsen, Winnie Edith

Perez-Murano, F.

Esteve, J.

Figueras, E.

Campabadal, F.

Barniol, N.

Boisen, Anja

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Ultrasensitive mass sensor fully integrated with complementary metal-oxide-semiconductor circuitryForsén, Esko Sebastian; Abadal, G.; Ghatnekar-Nilsson, S.; Teva, J.; Verd, J.; Sandberg, RasmusKousholt; Svendsen, Winnie Edith; Perez-Murano, F.; Esteve, J.; Figueras, E.; Campabadal, F.;Montelius, L.; Barniol, N.; Boisen, AnjaPublished in:Applied Physics LettersLink to article, DOI:10.1063/1.1999838Publication date:2005Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Forsén, E. S., Abadal, G., Ghatnekar-Nilsson, S., Teva, J., Verd, J., Sandberg, R. K., ... Boisen, A. (2005).Ultrasensitive mass sensor fully integrated with complementary metal-oxide-semiconductor circuitry. AppliedPhysics Letters, 87(4), 043507. DOI: 10.1063/1.1999838Ultrasensitive mass sensor fully integrated with complementarymetal-oxide-semiconductor circuitryE. ForsenDepartment of Micro and Nanotechnology, Technical University of Denmark, Kgs. Lyngby,DK-2800, DenmarkG. AbadalDepartment of Electronic Engineering, Universitat Autonoma de Barcelona, 08193 Bellaterra, SpainS. Ghatnekar-NilssonSolid State Physics and The Nanometer Consortium, University of Lund, 22362 Lund, SwedenJ. Teva and J. VerdDepartment of Electronic Engineering, Universitat Autonoma de Barcelona, 08193 Bellaterra, SpainR. Sandberg and W. SvendsenDepartment of Micro and Nanotechnology, Technical University of Denmark, Kgs. Lyngby,DK-2800, DenmarkF. Perez-Murano, J. Esteve, E. Figueras, and F. CampabadalNational Microelectronics Centre, Universitat Autonoma de Barcelona, 08193 Bellaterra, SpainL. MonteliusSolid State Physics and The Nanometer Consortium, University of Lund, 22362 Lund, SwedenN. BarniolDepartment of Electronic Engineering Universitat Autonoma de Barcelona, 08193 Bellaterra, SpainA. BoisenDepartment of Micro and Nanotechnology, Technical University of Denmark, Kgs. Lyngby,Dk-2800, DenmarkReceived 17 March 2005; accepted 7 June 2005; published online 21 July 2005Nanomechanical resonators have been monolithically integrated on preprocessed complementarymetal-oxide-semiconductor CMOS chips. Fabricated resonator systems have been designed tohave resonance frequencies up to 1.5 MHz. The systems have been characterized in ambient air andvacuum conditions and display ultrasensitive mass detection in air. A mass sensitivity of 4 ag/Hzhas been determined in air by placing a single glycerine drop, having a measured weight of 57 fg,at the apex of a cantilever and subsequently measuring a frequency shift of 14.8 kHz. CMOSintegration enables electrostatic excitation, capacitive detection, and amplification of the resonancesignal directly on the chip. © 2005 American Institute of Physics. DOI: 10.1063/1.1999838The advances of nanotechnology can be utilized in de-veloping portable sensor systems for applications in biologi-cal, physical, or chemical sensing, achieving ultrasensitivedetection with low analyte consumption. One approach indeveloping such a system is to make a nanoresonator devicewhere a change in the mass of the resonator is detected as achange in the resonance frequency of the resonator. In orderto achieve the highest possible mass sensitivity, research onfabrication, integration, and development of nanoelectrome-chanical resonator systems is pursued.1–3 The most fre-quently used techniques for measuring the resonance fre-quency of a cantilever are based on optical detection.4 Theadvantage is its inherent simplicity and high sensitivity.Some disadvantages are problems with alignment capability,miniaturization and portability. Resonator systems canbe actuated by piezoelectric, magnetic, or thermalactuation.5–7 Another option is to use electrostatic actuationand capacitive readout. This is achieved by connectingnano/microstructures with standard microelectronics.Previously,8–11 we have reported on the principle of designand fabrication of cantilever resonators integrated with stan-dard complementary metal-oxide-semiconductor CMOScircuitry. CMOS integration enables simple electrostatic ac-tuation, capacitive read-out, and signal amplification. In thisletter, we present the functional evaluation of CMOS inte-grated cantilever structures achieving attogram/Hz massresolution in air.The sensing principle is based on monitoring the reso-nance frequency change of a cantilever as a function of massadsorption, e.g., due to the adsorption of molecules. Thenanoresonator structures are excited into lateral vibration byapplying an ac and dc voltage between the suspended canti-lever and a fixed parallel electrode. The frequency shift uponadded mass is measured on-chip by capacitive resonance fre-quency detection. CMOS integration reduces the parasiticcapacitance contribution and is hence crucial for our choiceof cantilever readout. Furthermore, CMOS integration allowsfor increased functionality in terms of frequency trackingand Q-factor enhancement,12,13 and can be used as a compo-nent in a portable device.The nanoresonators are defined by combined electron-beam lithography and direct write laser lithography on a pre-processed CMOS chip.11 Examples of fabricated 425 nmwide, 600 nm thick, and 20 m long polycrystalline siliconAPPLIED PHYSICS LETTERS 87, 043507 20050003-6951/2005/874/043507/3/$22.50 © 2005 American Institute of Physics87, 043507-1Downloaded 07 Jun 2010 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsppoly-Si cantilevers, integrated on a CMOS chip, aredemonstrated in Fig. 1. We have used standard CMOStechnology14 which leads to cantilever design constraints,limiting the highest detectable resonance frequency to ap-proximately 2 MHz. The on-chip frequency response of sucha cantilever, as a function of applied dc voltage Vdc, ismeasured using a gain-phase analyzer, see Fig. 2a. Theresonance frequency is reduced when the Vdc is increased,due to electrostatic spring softening.15 The unperturbed reso-nance frequency of the system is determined to be f0=1.487 MHz from the intersection of the linear fit with they axis at zero-applied voltage, Fig. 2b. Electrical measure-ments performed on equivalent cantilevers without an inte-grated CMOS circuit did not show any appreciable reso-nance frequency signal, due to the screening effect of theparasitic capacitance introduced by the pads and externalwires. The Young’s modulus of the poly-Si cantilever is cal-culated using Eq. 1 to have a value of E=175±18 GPaRef. 16E =1624n1f02w2, 1in which =20 m is the length and w=425 nm is the widthof the cantilever, =2.33106 g/m3 is the mass density ofthe poly-Si, n10.2427 is a geometrical form factor for thefundamental vibration mode originating from the Euler–Bernoulli beam equation.17 The calculated Young’s modulusis consistent with measurements on similar poly-Si thin filmswith E=158±7 GPa.18Resonator structures have been characterized in a cham-ber in which the pressure has been controlled in the range of1013 mbar–0.1 mbar. The quality factor as a function of thepressure displays a log-log dependence, as shown in Fig.2c. Given a dc+ac actuation voltage of 1 Vdc and 0.9 Vpeak-peak VPP, a quality factor of approximately 5000 isdetermined at a pressure of 0.4 mbar. The quality factor isreduced to 30 at ambient air conditions. This is comparableto previous visual characterization of similar sized equivalentresonators that have not been CMOS integrated. Here, aquality factor of 70 was found for air operation14 and thevalue increased to 28 000 for operation at a pressure of2 bar.8In order to characterize the mass sensitivity of the reso-nator device in air, punctual masses are placed on the canti-lever. Single glycerine drops are placed selectively on theapex of the cantilever using a glycerine coated scanning tun-neling microscope STM tip. The deposited glycerine drophas a diameter of approximately 500 nm, estimated by com-paring the drop size with the known width of the cantilever.Assuming a hemispherical volume and =1.26106 g/m3,the mass of the glycerine drop is estimated to be 41 fg.Figure 3 shows the on-chip readout before and after the con-trolled positioning of a single glycerine drop at the apex ofthe resonator. An ac voltage of 6 Vpp and a dc voltage of14 V is applied to a 20 m long, 425 nm wide, and 600 nmthick nanoresonator, similar to the resonator shown in Fig.1c. A resonant frequency of 1.453 MHz is measured beforethe deposition of the glycerine drop. Directly after the depo-sition of the glycerine, a frequency shift in air of f=14.8 kHz is determined from the shift in phase. From thefrequency shift, an added mass of 57 fg is calculated usingEq. 219m =2meffff0, 2where meff=n1wt is the effective mass of the cantilever forthe fundamental vibration mode, calculated using a densityof poly-Si =2.33106 g/m3, and a resonance frequencyFIG. 1. SEM images of fabricated poly-Si cantilever structures on a CMOSchip. a Top view image of a defined cantilever structure. The cantilever isexcited into lateral resonance by applying an ac and dc voltage between thedriver electrode and the cantilever. The cantilever is connected to a combcapacitor in order to polarize the CMOS circuitry. b Tilted view image ofa 20 m long, 425 nm wide, and 600 nm thick cantilever.FIG. 2. a On-chip readout using a poly-Si cantilever with a width of 425nm, a thickness of 600 nm and a length of 20 m, at a pressure of 0.4 mbar.The frequency signal from the CMOS circuitry was analyzed using a gain-phase analyzer. The inset shows the fluctuation of the phase signal 0.5° . b The effect of electrostatic spring softening is shown. The un-perturbed resonance frequency of the system is determined from the inter-section of the linear fit at zero-applied dc voltage. The fundamental reso-nance frequency is 1.487 MHz. c The quality factor dependence on thepressure displays a log-log relationship. A quality factor of approximately5000 is determined at 0.4 mbar and 1 Vdc and 0.9 VPP.043507-2 Forsen et al. Appl. Phys. Lett. 87, 043507 2005Downloaded 07 Jun 2010 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspf0=1.487 MHz. Hence, the calculated mass sensitivity of thesystem is 4±1 ag/Hz.Inevitably, the ultimate mass resolution is limited by thenoise of the system. In order to get an estimate of the mini-mal detectable mass, M, which is the mass change thatresults in a frequency shift that overcomes the noise floor, aninvestigation of the phase noise of the system needs to beconducted. From the inset in Fig. 2a, the magnitude of thephase noise—caused by both intrinsic and extrinsic noisesources from the transducer and the readout circuitry—canbe determined to be of the order of 0.5° at a pressure of0.4 mbar. Since the slope of the phase signal at the frequencyof the resonance peak was  /f =−0.108, the correspondingminimum frequency shift is f4.6 Hz. Hence, the minimaldetectable mass is calculated as Mnoise=2mefff / f018 ag.The ultimate resolution for a nanoresonator, equivalent withthe one presented in this letter, only taking intrinsic phasenoise into account and excluding extrinsic noise limitationscan be approximated using Eq. 3:19Mintrinsic = 2meffET/Ecf/Q2f0  1 ag, 3where ET=kBT is the thermal energy at room temperature,Ec=meff42f02xc2	 is the driving energy, xc	100 nm is theapproximate root-mean-square drive amplitude in the direc-tion of vibration still consistent with producing a linear re-sponse, and f =1 kHz is the measurement bandwidth. Thisis consistent with the theoretical/experimental work byEkinci et al.4,19 regarding noise limitations for nanoresonatormass sensor systems.The phase noise is increased as one increases the pres-sure, mainly due to the reduced quality factor but alsoadsorption/desorption processes on the cantilever and addi-tional drift of the circuit are limiting factors. At ambientconditions, the minimum frequency shift that can be moni-tored is of the order of f1 kHz, which corresponds to anultimate mass resolution in air of the order of a few femto-grams.In conclusion, the characterization of a fully integratedresonator mass sensor system in air and at low pressure con-ditions has been discussed. CMOS integrated cantilevershave been electrostatically excited into mechanical resonanceand the resonance frequency has been detected on-chip bycapacitive readout. The mass sensitivity of the system hasbeen determined by controlled positioning of glycerine dropson a cantilever, whereby a mass sensitivity of the order of4 ag/Hz is measured for a resonator system with a funda-mental resonance frequency of 1.487 MHz. At 0.4 mbar, thequality factor is determined to be approximately 5000. Thesensitivity of the system is comparable to recent resultsachieved with resonator structures based on external actua-tion and readout.4,20 Nanoresonator devices should be func-tionalized in order to enable selective mass sensing. The ul-timate goal is to develop nanoresonator devices as integralparts of a portable sensor system.The authors would like to acknowledge funding grantedby the European Union IST project NANOMASS II IST-2001-33068, NANOSES MEYT-MC2003-07237, andNANOSYS TIC2003-07231N. V. Lavrik and P. G. Datskos, Appl. Phys. Lett. 82, 2697 2003.2J. Yang, T. Ono, and M. Esashi, Sens. Actuators, A 82, 102 2000.3K. L Ekinci, X. M. H. Huang, and M. L. Roukes, Appl. Phys. Lett. 84,4469 2004.4R. Wiesendanger. Scanning Probe Microscopy and Spectroscopy Cam-bridge University Press, Cambridge, U. K., 1994.5A. N. Cleland, M. Pophristic, and I. Fergurson, Appl. Phys. Lett. 79, 20702001.6H. J. Cho and C. H. Ahn, Proceedings of the International ConferenceMEMS 2000 2000, p. 686.7C. Hagleitner, A. Hierlemann, D. Lange, A. Kummer, N. Kerness, O.Brand, and H. Baltes, Nature London 414, 293 2001.8Z. J Davis, G. Abadal, B. Helbo, O. Hansen, F. Campabadal, F. Perez-Murano, J. Esteve, E. Figueras, J. Verd, N. Barniol, and A. Boisen, Sens.Actuators, A 105, 311 2003.9E. Forsén, S. G. Nilsson, P. Carlberg, G. Abadal, F. Pérez-Murano, J.Esteve, J. Montserrat, E. Figueras, F. Campabadal, J. Verd, L. Montelius,N. Barniol, and A. Boisen, Nanotechnology 15, 628 2004.10S. Ghatnekar-Nilsson, E. Forsén, G. Abadal, J. Verd, F. Campabadal, F.Pérez-Murano, J. Esteve, N. Barniol, A. Boisen, and L. Montelius,Nanotechnology 16, 98 2005.11J. Verd, G. Abadal, J. Teva, M. Villaroya Gaudó, A. Uranga, X. Borrisé, F.Campabadal, J. Esteve, E. Figueras Costa, F. Pérez-Murano, Z. J. Davis,E. Forsén, A. Boisen, and N. Barniol, J. Microelectromech. Syst. 14, 5082005.12G. Abadal, Z. J. Davis, B. Helbo, X. Borrisé, R. Ruiz, A. Boisen, F.Campabadal, J. Esteve, E. Figueras, F. Pérez-Murano, and N. Barniol,Nanotechnology 12, 100 2001.13D. Lange, C. Hagleitner, C. Herzog, O. Brand, and H. Baltes, Sens. Ac-tuators, A 103, 150 2003.14CNM-CMOS twin-well 2-poly 2-metal 2.5 micron technology.15Z. J. Davis, G. Abadal, O. Kuhn, O. Hansen, F. Grey, and A. Boisen, J.Vac. Sci. Technol. B 18, 612 2000.16The error was determined using the following measurement uncertainties:= ±200 nm, = ±1105 g/m3, f = ±5 kHz, and w= ±40 nm.17W. T. Thomsson, Theory of Vibration with Applications, 4th Ed. StanleyThornes Ltd., 1993.18D. Maier-Schneider, A Köprülülü, B. S. Holm, and E. Obermeier, J. Mi-cromech. Microeng. 6, 436 1996.19K. L. Ekinci, Y. T. Yang, and M. L. Roukes, J. Appl. Phys. 95, 26822004.20B. Ilic, H. G. Craighead, S. Krylov, W. Senaratne, C. Ober, and P. Neuzil,J. Appl. Phys. 95, 3694 2004.FIG. 3. The diagram shows the electrical resonance signal for a poly-Siresonator structure as measured through the CMOS. At ambient conditions,the resonance frequency was 1.453 MHz. The cantilever is 20 m long, 425nm wide, and 600 nm thick. The cantilever is excited into resonance byapplying 14 Vdc and 6 VPP ac. The inset shows two glycerine droplets, one atthe apex of the cantilever and the other is on the parallel actuation electrode.This is due to the fact that the glycerine-coated STM-tip touched the elec-trode on the first approach, resulting in a glycerine drop on the electrode.After deposition of a single glycerine drop onto the cantilever, the resonancefrequency was reduced by 14.8 kHz, which yields a mass sensitivity on theorder of 4 ag/Hz at ambient conditions.043507-3 Forsen et al. Appl. Phys. Lett. 87, 043507 2005Downloaded 07 Jun 2010 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Nanomechanical resonators have been monolithically integrated on preprocessed complementary metal-oxide-semiconductor (CMOS) chips. Fabricated resonator systems have been designed to have resonance frequencies up to 1.5 MHz. The systems have been characterized in ambient air and vacuum conditions and display ultrasensitive mass detection in air. A mass sensitivity of 4 ag/Hz has been determined in air by placing a single glycerine drop, having a measured weight of 57 fg, at the apex of a cantilever and subsequently measuring a frequency shift of 14.8 kHz. CMOS integration enables electrostatic excitation, capacitive detection, and amplification of the resonance signal directly on the chip

Ultrasensitive mass sensor fully integrated with complementary metal-oxide-semiconductor circuitry

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