NL2009014C2 - Method for determining a spring constant of a cantilever of an atomic force microscope or scanning probe microscope, and a calibration device for a cantilever. - Google Patents
Method for determining a spring constant of a cantilever of an atomic force microscope or scanning probe microscope, and a calibration device for a cantilever. Download PDFInfo
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- NL2009014C2 NL2009014C2 NL2009014A NL2009014A NL2009014C2 NL 2009014 C2 NL2009014 C2 NL 2009014C2 NL 2009014 A NL2009014 A NL 2009014A NL 2009014 A NL2009014 A NL 2009014A NL 2009014 C2 NL2009014 C2 NL 2009014C2
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- cantilever
- voltage
- electrode
- pull
- spring constant
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
- G01Q40/00—Calibration, e.g. of probes
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- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
- Radiology & Medical Imaging (AREA)
- Measurement Of Length, Angles, Or The Like Using Electric Or Magnetic Means (AREA)
Description
Method for determining a spring constant of a cantilever of an atomic force microscope or scanning probe microscope, and a calibration device for a cantilever 5 The invention relates to a method for determining a spring constant of a cantilever of an atomic force microscope or scanning probe microscope, which cantilever is provided with a scanning probe tip and wherein the cantilever is provided adjacent to an electrode, wherein the method comprises 10 the step of applying a voltage between the electrode and the cantilever until a predetermined condition is met.
Such a method is known from the article "Characterizing size dependent effective elastic modulus of silicon nanocantilevers using electrostatic pull in instability" by H.
15 Sadeghian et al, Applied Physics Letters, AIP, American Institute of Physics, Melville, New York, volume 94, nr 22, 2 June 2009, pages 221903-1 - 221903-3. In this known method the predetermined condition is an electrostatic pull in condition of the cantilever with reference to the electrode, and it is 20 taught to derive Young's modulus of the cantilever, which is a measure for the cantilever's spring constant, from at least the pull in voltage that is required for meeting said electrostatic pull in condition.
The beauty of the approach based on the electrostatic 25 pull in condition is that one can measure the spring constant and avoid the pitfalls of other solutions to measure the cantilever's spring constant or the corresponding Young's modulus. The pull in behavior of the cantilever depends on the interaction of the electrostatic load which is generated by 30 the applied voltage, the cantilever's stiffness and its geometry. It is however independent of the cantilever's mass. The measurement based on the electrostatic pull in condition thus avoids the sensitivity of measuring Young's modulus or the spring constant of the cantilever by the also known method, 35 which is discussed in the above mentioned article, to determine the stiffness by measuring the cantilever's resonance frequency, which measurement is corrupted because of inaccuracies due to mass changes caused by surface contamination, native oxide and other adsorbed layers on the cantilever.
2
The problem with measuring Young's modulus or the spring constant of the cantilever based on the electrostatic pull in condition is the possible occurrence of stiction and the need to apply high voltages when the cantilever is a rela-5 tively stiff. It may also be undesirable as such to bring the cantilever in contact with the electrode.
It is therefore an object of the invention to provide a simple, easy, fast, low-cost and accurate method to determine and calibrate the spring constant of the cantilever of an 10 atomic force microscope or scanning probe microscope which forms a viable alternative for the known method to base the measurement of said spring constant on the electrostatic pull in condition of the cantilever with reference to the electrode .
15 According to the invention the method of measuring the cantilever's spring constant is therefore characterized in that a series of different voltages are applied between the electrode and the cantilever, wherein each voltage is selected to remain below the pull in voltage, and that with each such 20 voltage a corresponding resonance frequency of the cantilever is measured, which series of frequency measurements is used to estimate the pull in voltage at which electrostatic pull in of the cantilever with respect to the electrode will occur, and that the spring constant is thereafter derived from at least 25 this estimated pull in voltage. The manner in which the spring constant according to the invention is determined therefore largely corresponds to the known method to base the measurement of the conditions prevailing when the cantilever is in the pull in position, with the essential difference that the 30 pull in conditions are not actually achieved but estimated from a series of measurements that avoid the pull in position of the cantilever.
The method of the invention is very suited to measure the spring constant of a series of cantilevers in a batch pro-35 cess.
Suitably the pull in voltage is estimated by employing a known relation between the cantilever's Eigen frequency and the cantilever's displacement that is determined by a differential equation describing the relation between the canti 3 lever's position and the voltage applied between the cantilever and the electrode. This differential equation is provided on page 221903 -2, left-hand column of the above mentioned article, which is deemed inserted and fully incorporated herein.
5 Ultimately this means that deriving the spring constant is based on said differential equation describing the relation between the cantilever's position and the voltage applied between the cantilever and the electrode. Albeit that this equation may be used for the determination of the spring constant, 10 this does not mean that it is required to measure the cantilever's deflection; it suffices to measure the cantilever's resonance frequency with different voltages applied between the cantilever and the electrode.
It may be advantageous that an initial distance be-15 tween the cantilever and the electrode prior to applying the series of voltages is varied, and that different series of measurements are made starting each time with a different initial distance between the cantilever and the electrode. This may be used to improve the reliability of the measurement and 20 rule out or suppress the influence of measurement noise.
The invention is also embodied in a microscope, such as an atomic force microscope or scanning probe microscope, having a cantilever that is provided with a scanning probe tip, and is also embodied in a calibration device for such a 25 cantilever.
The microscope and the calibration device of the invention are embodied with means for determining a spring constant of the cantilever comprising means to apply a voltage between the electrode and the cantilever, and calculating 30 means to determine the spring constant from at least a pull in voltage that is required for meeting an electrostatic pull in condition, wherein the means to apply a voltage are embodied to apply a series of different voltages between the electrode and the cantilever, wherein each voltage is selected to remain 35 below the pull in voltage, and that measurement means are provided to measure with each such voltage a corresponding resonance frequency of the cantilever, and that estimator means are provided to estimate the pull in voltage at which electrostatic pull in of the cantilever with respect to the electrode 4 will occur based on the measurement means providing said series of frequency measurements, and that the calculating means determine the spring constant from at least this estimated pull in voltage.
5 The invention will hereinafter be further elucidated with reference to the drawing.
In the drawing: -figure 1 schematically shows apparatus features that may be embodied in a microscope or calibration device for a 10 cantilever according to the invention; and -figure 2 represents a graph showing the relation between the square of the cantilever's Eigen frequency and its tip displacement.
In the drawing of figure 1 reference 1 denotes a can-15 tilever and reference 2 denotes the electrode adjacent to which the cantilever 1 is provided. The cantilever 1 is provided with a scanning probe tip 3.
The cantilever 1 and the electrode 2 are connected to supply means 4 for applying a voltage between the cantilever 1 20 and the electrode 2, and these supply means 4 preferably are controllable so that different voltages can be applied to the cantilever 1 and electrode 2.
Figure 1 further shows that measurement means 5 are provided for measuring the resonance frequency of the cantile-25 ver 1. Both the measured resonance frequency recorded by the measurement means 5 and the corresponding voltages that are applied to the cantilever 1 and electrode 2 are for instance stored in memory 7 of a computer 6. The computer 6 is also provided with estimator means 8 which uses the measured reso-30 nance frequencies and the corresponding voltages that collectively are stored in the memory 7 to estimate the pull in voltage that is required to ensure electrostatic pull in of the cantilever 1 with reference to the electrode 2. In the invention pull in of the cantilever 1 with reference to the 35 electrode 2 is however never achieved, since the applied voltages are always kept at a level below the voltage that is required for meeting the electrostatic pull in condition.
The estimated pull in voltage of the cantilever 1 is subsequently provided by the estimator means 8 to calculating 5 means 9 forming part of the computer 6, and based thereon the calculating means 9 provide at output 10 a calculated spring constant of the cantilever 1 employing the method known from the prior art for calculating the Young's modulus of the can-5 tilever. In this connection it is further remarked that it is also possible to vary the initial distance between the cantilever and the electrode prior to applying the series of voltages and to make different series of measurements starting each time with a different initial distance between the canti-10 lever and the electrode. In this manner the reliability of the measurement is increased and the influence of measurement noise can be suppressed.
Determining the spring constant according to the method known for determining Young's modulus of the cantilever 15 is theoretically based on a differential equation describing the relation between the cantilever's position and the voltage applied between the cantilever and the electrode. More in particular the pull in voltage is estimated by employing a relation as shown in figure 2 between the (sguare of the) cantile-20 ver's Eigen frequency and the cantilever's displacement that is determined -as is known by the person skilled in the art-by said differential equation describing the relation between the cantilever's position and the voltage applied between the cantilever and the electrode. Concerning the differential 25 equation reference is made to the article mentioned in the introduction "Characterizing size dependent effective elastic modulus of silicon nanocantilevers using electrostatic pull in instability" by H. Sadeghian et al, Applied Physics Letters, AIP, American Institute of Physics, Melville, New York, volume 30 94, nr 22, 2 June 2009, pages 221903-1 - 221903-3.
The measurement means 5 for measuring the resonance frequencies of the cantilever 1 can be selected from any known means to record these resonance frequencies. Information on this can be derived from the following citations:
Analytical beam model C. A. Clifford, M. P. Seah, "The determination of atomic force microscope cantilever spring constants via dimensional methods 35 6 for nanomechanical analysis", Nanotechnology 16 (9) (2005) 1666.
Cleveland 5 J. P. Cleveland, S. Manne, D. Bocek, P. K. Hansraa, "A nondestructive method for determining the spring constant of cantilevers for scanning force microscopy", Review of Scientific Instruments 64 (2) (1993) 403-405.
10 Sader J. E. Sader, J. W. M. Chon, P. Mulvaney, "Calibration of rectangular atomic force microscope Cantilevers", Review of Scientific Instruments 70 (10) (1999) 3967-3969.
15 Thermal noise J. L. Hutter, J. Bechhoefer, "Calibration of atomic-force microscope tips", Review of Scientific Instruments 64 (7) (1993) 1868-1873.
20 Added mass (zelfde paper als Cleveland) J. P. Cleveland, S. Manne, D. Bocek, P. K. Hansma, "A nondestructive method for determining the spring constant of cantilevers for scanning force microscopy", Review of Scientific Instruments 64 (2) (1993) 403-405.
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Reference cantilever R. S. Gates, M. G. Reitsma, "Precise atomic force microscope cantilever spring constant calibration using a reference cantilever array", Review of Scientific Instruments 78 (8) (2007) 30 086101.
and C. A. Clifford, M. P. Seah, "Improved methods and uncertainty analysis in the calibration of the spring constant of an atomic force microscope cantilever using static experimental 35 methods", Measurement Science and Technology 20 (12) (2009) 125501.
Electroactuation S. Rana, P. M. Ortiz, A. J. Harris, J. S. Burdess, C. J.
7
McNeil, "An electrostatically actuated cantilever device capable of accurately calibrating the cantilever on-chip for afm-like applications", Journal of Micromechanics and Microengineering 19 (4) (2009) 045012.
5 Turning now to Figure 2 a series of measurements 21- 26 are shown reflecting measured resonance frequencies. These frequencies are stored in memory 7 together with the corresponding excitation voltages that are applied between the cantilever 1 and the electrode 2. Using the known relation be-10 tween the displacement w of the cantilever 1 as shown on the X. axis and the square of the resonance frequency shown on the Y. axis, the estimator 8 makes an estimation of the pull in voltage that is required to establish electrostatic pull in of the cantilever 1 to the electrode 2. When pull in occurs by 15 definition the resonance frequency becomes zero due to loss of stability. The pull in situation symbolized by arrow 27 happens when applying the pull in voltage results in a critical deflection of the cantilever 1. The thus estimated pull in voltage is subsequently used in known manner by calculating 20 means 9 for determining the cantilever's spring constant.
Claims (7)
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NL2009014A NL2009014C2 (en) | 2012-06-15 | 2012-06-15 | Method for determining a spring constant of a cantilever of an atomic force microscope or scanning probe microscope, and a calibration device for a cantilever. |
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Citations (1)
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WO2012064193A1 (en) * | 2010-11-12 | 2012-05-18 | Technische Universiteit Delft | Method for determining a spring constant for a deformable scanning probe microscope element, and scanning probe microscope and calibration device arranged for determining a spring constant for a probe element |
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WO2012064193A1 (en) * | 2010-11-12 | 2012-05-18 | Technische Universiteit Delft | Method for determining a spring constant for a deformable scanning probe microscope element, and scanning probe microscope and calibration device arranged for determining a spring constant for a probe element |
Non-Patent Citations (2)
Title |
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J. P. CLEVELAND ET AL: "A nondestructive method for determining the spring constant of cantilevers for scanning force microscopy", REVIEW OF SCIENTIFIC INSTRUMENTS, vol. 64, no. 2, 1 January 1993 (1993-01-01), pages 403 - 405, XP055024743, ISSN: 0034-6748, DOI: 10.1063/1.1144209 * |
SUNIL RANA ET AL: "An electrostatically actuated cantilever device capable of accurately calibrating the cantilever on-chip for AFM-like applications; An electrostatically actuated cantilever", JOURNAL OF MICROMECHANICS & MICROENGINEERING, INSTITUTE OF PHYSICS PUBLISHING, BRISTOL, GB, vol. 19, no. 4, 1 April 2009 (2009-04-01), pages 45012, XP020153379, ISSN: 0960-1317, DOI: 10.1088/0960-1317/19/4/045012 * |
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