Boisen, Anja

Davis, Zachary James

We have fabricated Al nanocantilevers using a simple, one mask contact UV lithography technique with lateral and vertical dimensions under 500 and 100 nm, respectively. These devices are demonstrated as highly sensitive mass sensors by measuring their dynamic properties. Furthermore, it is shown that Al has a potential higher sensitivity than Si based dynamic sensors. Initial testing of these devices has been conducted using a scanning electron microscope setup were the devices were tested under high vacuum conditions. The Q factor was measured to be approximately 200 and the mass sensitivity was measured to 2 ag/Hz by depositing electron-beam-induced carbon at the end of the nanocantilever. (c) 2005 American Institute of Physics

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Aluminum nanocantilevers for high sensitivity mass sensorsDavis, Zachary James; Boisen, AnjaPublished in:Applied Physics LettersLink to article, DOI:10.1063/1.1984092Publication date:2005Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Davis, Z. J., & Boisen, A. (2005). Aluminum nanocantilevers for high sensitivity mass sensors. Applied PhysicsLetters, 87(1), 013102. DOI: 10.1063/1.1984092Aluminum nanocantilevers for high sensitivity mass sensorsZachary J. Davisa and Anja BoisenDepartment of Micro and Nanotechnology (MIC), Technical University of Denmark (DTU),Building 345 East, 2800 Kgs. Lyngby, DenmarkReceived 8 February 2005; accepted 24 May 2005; published online 27 June 2005We have fabricated Al nanocantilevers using a simple, one mask contact UV lithography techniquewith lateral and vertical dimensions under 500 and 100 nm, respectively. These devices aredemonstrated as highly sensitive mass sensors by measuring their dynamic properties. Furthermore,it is shown that Al has a potential higher sensitivity than Si based dynamic sensors. Initial testing ofthese devices has been conducted using a scanning electron microscope setup were the devices weretested under high vacuum conditions. The Q factor was measured to be approximately 200 and themass sensitivity was measured to 2 ag/Hz by depositing electron-beam-induced carbon at the endof the nanocantilever. © 2005 American Institute of Physics. DOI: 10.1063/1.1984092Cantilever based dynamic sensors have in recent yearsbeen a platform for highly sensitive mass sensors1–3 and havedemonstrated higher sensitivities than commercial quartz mi-crobalance technologies.4 A cantilever based mass sensorworks by measuring changes in its resonant frequency due tomass changes. Intuitively, by decreasing the size of the can-tilever it is possible to measure smaller mass changes. Themass sensitivity can be derived using the equation for theresonant frequency and is given by:1–3,5  fm = f2m⇔ mm  = 2ff , 1where f is the resonant frequency and m is the mass of theresonator. This equation confirms that by decreasing themass and increasing the frequency of the mechanical resona-tor the mass sensitivity is increased.Current published works have reported ag/Hz masssensitivity.1,3 This has been possible by miniaturizing thecantilever by means of using the emerging nanolithographytechniques such as electron beam lithography on single crys-tal Si substrates.6,7 In this work we report on Al nanocanti-levers as a suitable replacement for Si based mechanicalresonators. It will be shown that higher mass sensitivities canbe achieved using Al and a very simple process flow is pre-sented, which uses standard contact UV lithography toachieve sub 500 nm wide, 100 nm thick cantilevers, whichcan fabricate thousands of nanocantilevers on a wafer scale.Finally, initial testing of the Al nanocantilevers using a scan-ning electron microscope setup will be presented, which hasbeen used to measure Q factor and mass sensitivity.By looking at the device as a whole, the cantilever mustbe excited into resonance in order to detect the change inresonant frequency. Making the cantilever very short wouldincrease the mass resolution of a cantilever based sensor;however, this is not a viable design. By decreasing the lengthof the cantilever the stiffness is increased and thus largerforces are needed for dynamic actuation. However, by re-writing the equation for the first mode resonant frequency ofa rectangular cantilever f =0.162E / · t / l2,7 it can be shownthat the resonant frequency can be increased by decreasingboth the Young’s modulus E and the density  of thecantilever material, if the thickness t, width w, and springconstant k of the cantilever are unchangedf = 0.408 k2/3t w2/3·11/2E1/6. 2In Table I the mass sensitivity m /f is calculated for can-tilevers of different materials. The spring constant of eachcantilever is kept constant by adjusting only the length of thecantilever and maintaining a width of 1 m and a thicknessof 100 nm. By looking at Table I it is seen that Al has thebest mass sensitivity followed by poly-Si and then singlecrystal Si. This is because Al has both a low density and lowYoung’s modulus compared to the other cantilever materials.Among these materials, which are readily available for mi-croelectromechanical systems MEMS fabrication, Al is thebest choice.In Fig. 1 the fabrication of the Al nanocantilevers isshown, which is based on a lift-off technique. The devicesare realized on 4 in. Si wafers, Fig. 1a. First, a resist moldis formed on the Si substrate using UV lithography, Fig. 1b.Then, Al is deposited using electron e-beam evaporation,Fig. 1c. Next, the Al deposited on the resist is removed bylift-off, Fig. 1d. Finally, the metal structure is released bydry etching the underlying Si using an isotropic SF6 baseddry etch, Fig. 1e.The most critical step in the fabrication is the definitionof the resist mold. In order to achieve sub 500 nm cantileverwidths a reverse lithographic process with AZ5214 photore-sist was used.8 Another important aspect of this process isaElectronic mail: zd@mic.dtu.dkTABLE I. The calculated length, resonant frequency, and mass sensitivityfor different cantilever materials. The cantilevers have a spring constant of0.1 N/m, a thickness of 100 nm, and a width of 1 m.Material E GPa  g/cm3 l m f MHz m /f ag/HzAl 74 2.70 5.70 2.61 1.18Poly-Si 160 2.33 7.37 2.49 1.45Si100 180 2.33 7.66 2.44 1.43Ti 110 4.51 6.50 1.89 3.07Cr 140 7.19 7.04 1.44 6.98Ni 200 8.90 7.94 1.22 11.48Au 80 19.30 5.85 0.96 23.19APPLIED PHYSICS LETTERS 87, 013102 20050003-6951/2005/871/013102/3/$22.50 © 2005 American Institute of Physics87, 013102-1Downloaded 07 Jun 2010 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspthat the release step is performed by dry etching, which al-leviates stiction problems that are often seen using wet re-lease techniques.In Fig. 2 scanning electron images SEM are shown ofthe Al cantilever devices before and after dry underetching.From the images it is seen that the width of the nanocantile-ver is below 500 nm and the thickness is approximately 100nm. The length of the realized nanocantilevers range from 1to 10 m.In order to characterize the Al nanocantilevers in avacuum environment the devices where wire bonded to aPCB substrate and placed inside a SEM. Electricalfeedthroughs in the SEM facilitate electrical connection tothe Al nanocantilever chips. On the chip an ac and dc voltageis applied between the driver electrode and the Al nanocan-tilever Fig. 2b, which will actuate vertical motion. Verti-cal actuation is achieved because the electric field is nonuni-form above and below the cantilever due to the Si substrateMoreover, the released nanocantilever bends slightly up-wards due to internal stress, which also creates a nonuniformelectric field. Two SEM based methods of measuring thefrequency response of the nanocantilever were used: 1Sweeping the actuation voltage frequency and visually ob-serving the vibrational amplitude of the nanocantilever and2 focusing the e beam on the cantilever and monitoring thesecondary electron detector signal as a function of the actua-tion voltage frequency.In Fig. 3 two SEM images of an Al nanocantilever areseen at two different actuation frequencies. In this experi-ment an ac actuation voltage of 9 V pp was used. In Fig. 3athe nanocantilever is not in resonance and the image of thecantilever is sharp, whereas in fig. 3b, at an actuation fre-quency of 1.43 MHz, the end of the cantilever is blurred dueto the vibration. In Fig. 3c the frequency response of thisdevice is seen by measuring the amplitude directly from theimages taken at different actuation frequencies. From theSEM images the vibrational amplitude can be measured withan accuracy of approximately 30 nm. In order to measure theQ factor a Lorentzian fit was made and the Q factor wasextracted. In this case the Q factor was measured to be ap-proximately 200. Considering the high vacuum conditionsthis Q factor is very low. However, several factors can con-tribute to this, such as poor anchoring conditions due to un-deretching of the support see Fig. 2b and surface lossesFIG. 2. SEM images of Al nanocantilever before a and after b dryunderetching.FIG. 1. Process flow of Al nanocantilevers.FIG. 3. SEM images of a nanocantilever out of resonance a and in resonance b. c The measured frequency response and a Lotentzian fit. The cantileveris vibrating vertically.013102-2 Z. J. Davis and A. Boisen Appl. Phys. Lett. 87, 013102 2005Downloaded 07 Jun 2010 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspdue to the large surface to volume ratio of thesenanocantilevers.9,10By using the second measurement method the frequencyresponse was measured using the exact same device as theprevious experiment. In Fig. 4c the detector signal is seento shift abruptly at an actuation frequency of 2.76 MHz. Thereason for this abrupt shift is not fully understood, however,it could be due to a nonlinear behavior that is seen for largevibrational amplitudes.11 Another aspect of this measurementis that the resonant frequency is measured here to be approxi-mately double the first experiment. The reason is because inthe first experiment the actuation of the cantilever was donewith a pure ac signal between the driver electrode and can-tilever, thus the actuation happened at half the mechanicalfrequency since the forces are proportional to the voltagesquared. In the second experiment a dc voltage of 10 V andan ac voltage of 1 V were applied, thus the electrical andmechanical frequencies are the same. The theoretical ex-pected resonant frequency neglecting spring softening ef-fects of this Al cantilever is approximately 3.0 MHz, whichcorresponds well to the experimental values. The slight dif-ference in the measured and theoretical values can be attrib-uted to electrostatic spring softening and added mass duringSEM imaging.Finally, a mass measurement has been performed depos-iting carbon by electron-beam-induced deposition EBID.12In Fig. 4 images of the Al nanocantilever are seen both be-fore a and after b the EBID. The frequency responses ofthe nanocantilever before and after are shown in Fig. 4c.By measuring the frequency shift 6.5 kHz and using arough estimate of the mass of the carbon deposit 14 fg themass sensitivity of the device is calculated to approximately2 ag/Hz. The expected theoretical value, using Eq. 1, isapproximately 1 ag/Hz, which is in good agreement with theexperiment. In this case the minimal frequency shift df thatcan be measured is approximately 500 Hz, which leads to anabsolute minimal detectable mass of approximately 1 fg inthese experiments. However, this number can be improvedby improving the Q factor and optimizing the electron detec-tor signal with respect to noise.It has been demonstrated that using a simple batch pro-cess, cantilevers with sub 500 nm widths can be fabricatedon a wafer scale. Furthermore, Al nanocantilevers can bevertically excited into resonance with a lateral electrode.Through a simple characterization setup, involving a SEM,the Q factor was measured to approximately 200, which islower than expected. However, more work needs to be doneto investigate why, by improving current anchoring condi-tions, etc. Finally, measurements of EBID carbon have beendone and a mass sensitivity of 2 ag/Hz has been measured.The SEM characterization technique has shown to be a veryversatile and fast tool for measuring nanocantilevers invacuum condition. Furthermore, this work demonstrates thata very simple device can be used for high sensitivity massmeasurements.The authors would like to acknowledge both NANO-MASS II EU, IST-2001-33068 and Forskningsrådet forTeknologi og Produktion FTP-26-04-0058 for financialsupport of this work.1K. L. Ekinci, X. M. H. Huang, and M. L. Roukes, Appl. Phys. Lett. 84,4469 2004.2H. P. Lang, R. Berger, J. K. Gimzewski, P. Fornaro, and E. Meyer, Anal.Chim. Acta 393, 59 1999.3J. Yang, T. Ono, and M. Esashi, Sens. Actuators, A 82, 102 2000.4M. Campione, M. Cartotti, E. Pinotti, A. Sassella, and A. Borghesi, J. Vac.Sci. Technol. A 22, 482 2004.5G. Abadal, Z. J. Davis, B. Helbo, X. Borris, R. Ruiz, A. Boisen, F. Cam-pabadal, J. Esteve, E. Figueras, F. Prez-Murano, and N. Barniol,Nanotechnology 12, 100 2001.6Y. T. Yang, K. L Ekinci, X. M. H. Huang, L. M. Schiavone, and M. L.Roukes, Appl. Phys. Lett. 78, 162 2001.7A. N. Cleland and M. L. Roukes, Appl. Phys. Lett. 69, 2653 1996.8E. W. Balch, S. E. Weaver, and R. J. Saia, Proc. SPIE 922, 387 1988.9C. T.-C. Nguyen and R. T. Howe, IEEE J. Solid-State Circuits 34, 4401999.10J. Yang, T. Ono, and M. Esasjo, J. Microelectromech. Syst. 11, 7752002.11A. Husain, J. Hone, Henk W. Ch. Postma, X. M. H. Huang, T. Drake, M.Barbic, A. Scherer, and M. L. Roukes, Appl. Phys. Lett. 83, 1240 2003.12K. Ingolf Schiffmann., Nanotechnology 4, 163 1993.FIG. 4. SEM images of a nanocantilever before a and after b carbon deposition and c the measured frequency responses both before dashed and afterfull carbon deposition.013102-3 Z. J. Davis and A. Boisen Appl. Phys. Lett. 87, 013102 2005Downloaded 07 Jun 2010 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Aluminum nanocantilevers for high sensitivity mass sensors

We have fabricated Al nano-cantilevers using a very simple one mask contact UV lithography technique with lateral dimensions under 500 nm and vertical dimensions of approximately 100 nm. These devices are demonstrated as highly sensitive mass sensors by measuring their dynamic properties. Furthermore, it is shown that Al has a potential higher sensitivity than Si based dynamic sensors. Initial testing of these devices has been conducted using a novel scanning electron microscope setup were the devices were tested under high vacuum conditions. The Q-factor was measured to approximately 200 and the mass sensitivity was measured to 2 attograms/Hz by depositing electron beam induced carbon at the end of the nano-cantilever

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 17, 2017Aluminum nano-cantilevers for high sensitivity mass sensorsDavis, Zachary James; Boisen, AnjaPublished in:5th IEEE Conference on Nanotechnology, 2005.Link to article, DOI:10.1109/NANO.2005.1500709Publication date:2005Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Davis, Z. J., & Boisen, A. (2005). Aluminum nano-cantilevers for high sensitivity mass sensors. In 5th IEEEConference on Nanotechnology, 2005. IEEE. DOI: 10.1109/NANO.2005.1500709Aluminum nano-cantilevers for high sensitivity mass sensors Zachary J. Davis and Anja Boisen Department of Micro and Nanotechnology (MIC), Technical University of Denmark (DTU), building 345east, 2800 Kgs. Lyngby, Denmark  Abstract – We have fabricated Al nano-cantilevers using a very simple one mask contact UV lithography technique with lateral dimensions under 500nm and vertical dimensions of approximately 100nm.  These devices are demonstrated as highly sensitive mass sensors by measuring their dynamic properties.  Furthermore, it is shown that Al has a potential higher sensitivity than Si based dynamic sensors.  Initial testing of these devices has been conducted using a novel scanning electron microscope setup were the devices were tested under high vacuum conditions.  The Q-factor was measured to approximately 200 and the mass sensitivity was measured to 2 attograms/Hz by depositing electron beam induced carbon at the end of the nano-cantilever.    Index Terms – Nanoelectromechanical systems (NEMS), mass sensor.  I. INTRODUCTION Cantilever based dynamic sensors have in recent years been a platform for highly sensitive mass sensors [1]-[3] and have demonstrated higher sensitivities than commercial Quartz microbalance technologies [4].  A cantilever based mass sensor works by measuring changes in its resonant frequency due to mass changes.  Intuitively, by decreasing the size of the cantilever it is possible to measure smaller mass changes.  The mass sensitivity can be derived using the equation for the resonant frequency and is given by [1]-[3],[5]: 2m mf f∂ =∂ .   (1) f is the resonant frequency and m is the mass of the resonator.  This equation confirms that by decreasing the mass and increasing the frequency of the mechanical resonator the mass sensitivity is increased.   Current published works have reported attogram/Hz mass sensitivity [1],[3].  This has been possible by miniaturizing the cantilever by means of using the emerging nano-lithography techniques such as electron beam lithography on single crystal Si substrates [6],[7].  In this work we report on Al nano-cantilevers as a suitable replacement for Si based mechanical resonators.  It will be shown that higher mass sensitivities can be achieved using Al and a very simple process flow is presented, which uses standard contact UV lithography to achieve sub 500nm wide cantilevers with thickness of approximately 100nm.  Finally, initial testing of the Al nano-cantilevers using a scanning electron microscope setup will be presented, which has been used to measure the devices resonant frequency, Q-factor and mass sensitivity.    II. DESIGN By looking at the device as a whole, the cantilever must be excited into resonance in order to detect the change in resonant frequency. Making the cantilever very short would increase the mass resolution of a cantilever based sensor; however this is not a viable design.  By decreasing the length of the cantilever the stiffness is increased and thus larger forces are needed for dynamic actuation.  However, by rewriting the equation for the resonant frequency of a rectangular cantilever [7], it can be shown that the resonant frequency can be increased by decreasing both the Young’s modulus (E) and the density (ρ) of the cantilever material, if the thickness (t), width (w) and spring constant (k) of the cantilever are unchanged.   232 113 6210.408 kft w Eρ= ⋅   (2)  TABLE I  The calculated length, resonant frequency and mass sensitivity for different cantilever materials.  The cantilevers have a spring constant of 0.1N/m, a thickness of 100nm and a width of 1µm. Material E(GPa) ρ(g/cm3) l(µm) f(kHz) ∂m/∂f (ag/Hz) Al 74 2.70 5.70 2.61 1.18 Poly-Si 160 2.33 7.37 2.49 1.45 Si(100) 180 2.33 7.66 2.44 1.43 Ti 110 4.51 6.50 1.89 3.07 Cr 140 7.19 7.04 1.44 6.98 Ni 200 8.90 7.94 1.22 11.48 Au 80 19.3 5.85 0.96 23.19 In table 1 the mass sensitivity (∂m/∂f) is calculated for cantilevers with different materials.  The spring constant of each cantilever is kept constant at 0.1N/m by adjusting only the length of the cantilever and maintaining a width of 1µm and a thickness of 100nm.  By looking at table 1 it Proceedings of 2005 5th IEEE Conference on Nanotechnology Nagoya, Japan, July 20050-7803-9199-3/05/$20.00 ©2005 IEEEAuthorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on December 2, 2009 at 05:30 from IEEE Xplore.  Restrictions apply. is seen that Al has the best mass sensitivity followed by poly-Si and then single crystal Si. This is because Al has both a low density and low Young’s modulus compared to the other cantilever materials.  Among these materials, which are readily available for MEMS fabrication, Al is the best choice. III. FABRICATION In fig. 1 the fabrication of the Al nano-cantilevers is shown, which is based on a lift-off technique. The devices are realized on 4” Si wafers, fig. 1a.  First, a resist mold is formed on the Si substrate using UV lithography, fig. 1b. Then, Al is deposited using e-beam evaporation, fig. 1c. Next, the Al deposited on the resist is removed by lift-off, fig. 1d. Finally, the metal structure is released by dry etching the underlying Si using an isotropic SF6 based dry etch, fig. 1e. (a)(b)(c)(d)(e) Fig. 1.  Process flow of Al nano-cantilevers. (a) silicon substrate. (b) photolithography is used to make a resist mold. (c) A 100nm thick Al layer is deposited. (d) the resist mold is etched, lifting off the Al on top of it. (e) the Si substrate is etched to release the Al structures.  The most critical step in the fabrication is the definition of the resist mold.  In order to achieve sub 500nm cantilever widths a reverse lithographic process with AZ5214 photoresist was used [8].  Another important aspect of this process is that the release step is performed by dry etching, which alleviates stiction problems that are often seen using wet release techniques.   (a)(b)400nmAlSiDriver electrodeNano-cantilever Fig. 2.  SEM images of Al nano-cantilever before (a) and after (b) dry underetching. In fig. 2 scanning electron images (SEM) are shown of the Al cantilever devices before and after dry underetching.  From the images it is seen that the width of the nano-cantilever is below 500nm and the thickness is approximately 100nm.  The length of the realized nano-cantilevers range from 1-10µm. IV. CHARACTERIZATION In order to characterize the Al nano-cantilevers in a vacuum environment the devices where wire bonded to a PCB substrate and placed inside a SEM.  Electrical feedthroughs in the SEM facilitate electrical connection to the Al nano-cantilever chips.  On the chip an AC and DC voltage is applied between the driver electrode and the Al nano-cantilever (fig. 2b), which will actuate vertical motion.  Vertical actuation is achieved because of two reasons; (1) the electric field is non-uniform above and below the cantilever due to the Si substrate and (2) the released nano-cantilever bends slightly upwards due to Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on December 2, 2009 at 05:30 from IEEE Xplore.  Restrictions apply. internal stress, which also creates a non-uniform electric field.   Two SEM based methods of measuring the frequency response of the nano-cantilever were used; (1) sweeping the actuation voltage frequency and imaging the nano-cantilever at a given actuation frequency and (2) focusing the e-beam on the cantilever and monitoring the secondary electron detector signal as a function of the actuation voltage frequency using a gain-phase analyser. fac=1447kHzfac=1435kHz(a)(b)1,425M 1,430M 1,435M 1,440M 1,445M-2,00,02,04,06,08,010,012,014,016,0Vibrational Amplitude (a.u.)Driving Frequency (Hz)  Lorentzian fit (c)SEM measurements Fig.  3.   SEM images of a nano-cantilever out of resonance (a) and in resonance (b).  (c) the measured frequency response using the visual measurement method and a Lotentzian fit. In fig. 3 two SEM images of an Al nano-cantilever are seen at two different actuation frequencies.  In this experiment an AC actuation voltage of 9Vpp was used.  In fig. 3a the nano-cantilever is not in resonance and the image of the cantilever is sharp, whereas in fig. 3b, at an actuation frequency of 1435kHz, the end of the cantilever is blurred due to the vibration.  In fig. 3c the frequency response of this device is seen by measuring the amplitude directly from the images taken at different actuation frequencies.  From the SEM images the vibrational amplitude can be measured with an accuracy of approximately 30nm.  In order to measure the Q-factor a Lorentzian fit was made and the Q-factor was extracted.  In this case the Q-factor was measured to approximately 200.  Considering the high vacuum conditions this Q-factor is very low.  However, several factors can contribute to this, such as poor anchoring conditions due to underetching of the support (see fig. 2b) and surface losses due to the large surface to volume ratio of these nano-cantilevers [9,10].  By using the 2nd measurement method the frequency response was measured using the same device and is shown in fig. 4.  In this measurement an AC voltage of 1V and a DC voltage of 10V was used.  In fig. 4 it is seen that the detector signal shifts abruptly at the resonant frequency.  The reason for this abrupt shift is not fully understood, however it could be due to a non-linear behavior that is seen for large vibrational amplitudes [11].  Another aspect of this measurement is that the resonant frequency is measured here to approximately double the first experiment.  This is because in the first experiment the actuation of the cantilever was done with a pure AC signal between the driver electrode and cantilever, thus the AC frequency is half the electrostatic force frequency since the forces are proportional to the voltage squared.  In the second experiment the DC voltage was much larger than the AC, thus the AC signal and the electrostatic force have the same frequency.  The theoretical expected resonant frequency (neglecting spring softening effects) of an Al cantilever with these dimensions is approximately 3MHz, which confirms our measurements. 2.65M 2.70M 2.75M 2.80M 2.85M0.0160.0140.0120.0100.0080.0060.004electron detector signal (V)Freqeuncy (Hz) Fig. 4.  Measured frequency response of the same device shown in fig. 3, using the electron detector signal versus the actuation frequency.   Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on December 2, 2009 at 05:30 from IEEE Xplore.  Restrictions apply. 1µm(a)(b)EBID carbon2,65M 2,70M 2,75M 2,80M 2,85M 2,90M6,0m8,0m10,0m12,0m14,0m16,0m18,0m20,0m22,0m24,0m26,0melectron detector signal (V)Frequency (Hz)(c) Before EBIDAfter EBID Fig. 5.  SEM images of a nano-cantilever before (a) and after (b) carbon deposition and (c) the measured frequency responses both before (red) and after (black) carbon deposition. V. MASS MEASUREMENT A mass measurement has been performed by depositing carbon by electron beam induced deposition (EBID) [12].  In fig. 5 images of the Al nano-cantilever are seen both before (a) and after (b) the EBID.  Also, the frequency responses of the nano-cantilever before and after are shown in fig. 4c.  By measuring the frequency shift (6.5kHz) and using a rough estimate of the mass of the carbon deposit (14fg) the mass sensitivity of the device is calculated to approximately 2ag/Hz.  The expected theoretical value, using equation 1, is approximately 1ag/Hz, which is in good agreement with the experiment.  In this case the minimal frequency shift (df) that can be measured is approximately 500Hz, which leads to an absolute minimal detectable mass of approximately 1fg in these experiments.  However, this number can be improved by improving the Q-factor and optimizing the electron detector signal with respect to noise.    VI. CONCLUSIONS    It has been demonstrated that using a relatively simple process, cantilevers with sub 500nm widths and 100nm thickness can be fabricated on a wafer scale using UV lithography.  It was found that Al nano-cantilevers can be vertically excited into resonance with a lateral electrode by electrostatic actuation.  Through a novel characterization setup, involving a SEM, the Q-factor was measured to approximately 200, which is lower than expected.  However, more work needs to be done to investigate why, by improving current anchoring conditions.  Finally, measurements of EBID carbon have been done and a mass sensitivity of 2ag/Hz has been measured.  The SEM characterization technique has shown to be a very versatile and fast tool for measuring nano-cantilevers in vacuum condition.  Furthermore, this work demonstrates that a very simple device can be used for high sensitivity mass measurements.        REFERENCES  [1] K.L. Ekinci, X.M.H. Huang and M.L. Roukes, Appl. Phys. Lett. 84(22), 4469 (2004). [2] H.P. Lang, R. Berger, J.K. Gimzewski, P. Fornaro, and E. Meyer, Analytica Chimica Acta, 393(1-3), 59 (1999). [3] J. Yang, T. Ono, and M. Esashi, Sensors and Actuators A 82, 102 (2000). [4] M. Campione, M. Cartotti, E. Pinotti, A. Sassella, and A. Borghesi, J. Vac. Sci. Technol. A 22(3), 482 (2004). [5] G. Abadal, Z.J. Davis, B. Helbo, X. Borris, R. Ruiz, A. Boisen, F. Campabadal, J. Esteve, E. Figueras, F. Prez-Murano and N. Barniol, Nanotechnology 12, 100 (2001). [6] Y.T. Yang, K.L Ekinci, X.M.H. Huang, L.M. Schiavone, and M.L. Roukes, Appl. Phys. Lett. 78(2), 162 (2001). [7] A.N. Cleland and M.L. Roukes, Appl. Phys. Lett. 69(28), 2653 (1996). [8] E.W. Balch, S.E. Weaver, and R.J. Saia, Proceedings of the SPIE - The International Society for Optical Engineering 922, 387 (1988). [9] Clark T.-C. Nguyen and Roger T. Howe. Integrated cmos micromechanical resonator high-q oscillator. IEEE Journal of Solid-State Circuits 34(4), 440 (1999). [10] J. Yang, T. Ono and M. Esasjo, Journal of Microelectromechanical Systems 11, 775 (2002). [11] A. Husain, J. Hone, Henk W. Ch. Postma, X. M. H. Huang, T. Drake, M. Barbic, A. Scherer and M. L. Roukes, Appl. Phys. Lett. 83(6), 1240 (2003). [12] Kirsten Ingolf Schiffmann., Nanotechnology 4(3), 163 (1993).      Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on December 2, 2009 at 05:30 from IEEE Xplore.  Restrictions apply. 

Aluminum nano-cantilevers for high sensitivity mass sensors

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We have fabricated Al nanocantilevers using a simple, one mask contact UV lithography technique with lateral and vertical dimensions under 500 and 100 nm, respectively. These devices are demonstrated as highly sensitive mass sensors by measuring their dynamic properties. Furthermore, it is shown that Al has a potential higher sensitivity than Si based dynamic sensors. Initial testing of these devices has been conducted using a scanning electron microscope setup were the devices were tested under high vacuum conditions. The Q factor was measured to be approximately 200 and the mass sensitivity was measured to 2 ag/Hz by depositing electron-beam-induced carbon at the end of the nanocantilever

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Aluminum nano-cantilevers for high sensitivity mass sensors

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Online Research Database In Technology

Online Research Database In Technology

Crossref

Online Research Database In Technology