Atomic force microscope

Abstract

The present invention relates to an atomic force microscope for evaluating a surface of a sample, comprising a sample holder, having a first zone suitable for receiving the sample mounted in a stationary manner, a probe having a tip able to be positioned facing the surface of the sample, the microscope being configured to allow an adjustment of a position of the tip relative to the surface, and a support, the sample holder having at least one second zone, separate from the first zone and stationary relative to the support, the sample holder being deformable so as to allow a relative movement of the first zone with respect to the second zone, and the microscope comprising a detector able to detect a movement of the first zone relative to the second zone.

Claims

1. An atomic force microscope for evaluating a surface of a sample, comprising: a sample holder comprising a first area configured to receive the sample fixedly mounted with respect to the first area, a probe comprising a tip configured to be positioned facing the surface of the sample, the microscope being configured to allow an adjustment of a position of the tip with respect to the surface, a support, wherein the sample holder has at least a second area distinct from the first area and fixed with respect to the support, the sample holder being deformable so as to authorize a relative displacement of the first area with respect to the second area, the microscope comprises a detector configured to detect a displacement of the first area with respect to the second area, the microscope being configured to evaluate an interaction between the surface and the tip base on a measurement of the displacement of the first area of the sample holder by the detector.

2. The atomic force microscope according to claim 1, wherein the sample holder is a harmonic oscillator.

3. The atomic force microscope according to claim 1, wherein the detector is fixedly mounted on the sample holder.

4. The atomic force microscope according to claim 3, wherein the detector is fixedly mounted on the first area.

5. The atomic force microscope according to claim 1, comprising an actuator configured to vibrate the sample holder at a predetermined frequency.

6. The atomic force microscope according to claim 1, comprising a closed-loop servo-control regulator, the detector being configured to transmit to the regulator a signal representative of a measurement of the displacement of the first area and the regulator being configured to transmit a regulation signal to the actuator.

7. The atomic force microscope according to claim 1, wherein the sample holder has a length greater than 2 mm.

8. The atomic force microscope according to claim 7, wherein the sample holder has a length greater than 1 cm.

9. The atomic force microscope according to claim 8, wherein the sample holder has a length greater than 3 cm.

10. The atomic force microscope according to claim 9 wherein the bending is greater than 105 N.Math.m−1.

11. The atomic force microscope according to claim 1, wherein a bending stiffness of the sample holder between the first area and the second area is greater than 103 N.Math.m−1.

12. The atomic force microscope according to claim 11 wherein the bending is greater than 104 N.Math.m−1.

13. The atomic force microscope according to claim 1, comprising several probes, each probe having a tip, the microscope being configured to allow an independent adjustment of each of the positions of the tips with respect to the surface.

14. The atomic force microscope according to claim 13 wherein at least one of the probes is made of a material different from another probe.

15. The atomic force microscope according to claim 1, comprising a cell configured to contain a liquid medium.

16. The atomic force microscope according to claim 15 wherein the cell is fixedly mounted with respect to the first area and the sample is fixedly mounted on the cell.

17. The atomic force microscope according to claim 1, wherein the second area is one among a plurality of second areas comprised in the sample holder.

18. The atomic force microscope according to claim 17 wherein the first area is arranged between two second areas among the plurality of second areas, the first area being at equal distance from each of the two second areas.

19. A method for evaluating a surface of a sample by an atomic force microscope according to claim 1, comprising the steps of: a) positioning the tip so as to face the surface, b) measuring the displacement of the first area with respect to the second area by the detector so as to evaluate an interaction between the surface and the tip.

20. The method according to claim 19, wherein the repetition of steps a) and b) defines a scanning of the surface by the tip.

21. The method according to claim 19 further comprising a step d) of actuating the sample holder, concomitant with the measurement step b), wherein the actuator is actuated so as to vibrate the first area of the sample holder at a predetermined frequency between 500 Hz and 10 MHz.

22. A method for evaluating a surface of a sample according to claim 21, wherein the actuator is actuated so as to vibrate the first area of the sample holder at several predetermined frequencies.

23. The method according to claim 21, wherein the sample holder has at least one natural resonant frequency fk, so as to vibrate the first area at a frequency of between (fk−0.5.Math.fk) and (fk+0.5.Math.fk).

24. The method according to claim 19, wherein step a) is performed so as to position the tip at a distance of less than 100 nm from the surface.

25. The method according to claim 24, wherein the distance is less than 10 nm from the surface.

Description

DESCRIPTION OF THE FIGURES

(1) Other characteristics, aims and advantages of the invention will emerge from the following description which is purely illustrative and not limiting and which should be read in relation to the appended drawings in which:

(2) FIG. 1 schematically illustrates an atomic force microscope according to one embodiment of the invention,

(3) FIG. 2 is a photograph of an atomic force microscope according to one embodiment of the invention,

(4) FIG. 3 illustrates a method for evaluating a surface of a sample according to one embodiment of the invention,

(5) FIG. 4 schematically illustrates part of a microscope according to one embodiment of the invention suitable for evaluating the surface of a sample in a liquid medium,

(6) FIG. 5 is an image of a surface measured by an atomic force microscope according to one embodiment of the invention.

(7) In all of the figures, similar elements have identical references.

DETAILED DESCRIPTION OF THE INVENTION

(8) General Architecture of the Microscope 1

(9) Referring to FIG. 1 and FIG. 2, the atomic force microscope 1 comprises a sample holder 3. The sample holder 3 supports a sample 2 having a surface 9 to be measured. Particularly, the sample holder 3 comprises at least two distinct areas: a first area 4 and a second area 7. The first area 4 is adapted to receive the sample 2 fixedly mounted with respect to the first area 4. The microscope also comprises a support 6. The support 6 is fixedly mounted on the ground or on the reference frame of the measuring location. The second area 7 is fixedly mounted on the support. The second area 5 can only be made in one piece with the support 6 or welded to the support 6.

(10) The sample holder 3 is deformable so as to authorize a relative displacement of the first area 4 with respect to the second area 7. The part of the sample holder 3 comprised between the first area 4 and the second area 7 can be preferably deformable so that the relative movement of the sample 2 with respect to second area 7 results from a deformation of the whole part of the sample holder 3. Preferably, the bending stiffness of the sample holder 3, and in particular of the part(s) located between the first area 4 and the second area(s) 7, has a bending stiffness greater than 10.sup.3 N.Math.m.sup.−1 in particular greater than 10.sup.4 N.Math.m.sup.−1 and more preferably greater than 10.sup.5 N.Math.m.sup.−1. In addition, the bending stiffness of the sample holder 3 and in particular of the part(s) located between the first area 4 and the second area(s) 7 has a bending stiffness of less than 10.sup.8 N.Math.m.sup.−1 is preferably less than 10.sup.7 N.Math.m.sup.−1. The sample holder 3 can for example be made of aluminum. Thus, even though the sample holder 3 is deformable, it may have a higher rigidity than that of the probes of the art prior while remaining sufficiently deformable to allow an evaluation of the surface.

(11) The sample holder 3 has at least one macroscopic dimension, that is to say greater than 2 mm in particular greater than 1 cm and preferably greater than 3 cm. The sample holder 3 can for example have the shape of a 7 cm long, 12 mm thick and 7 mm wide aluminum cuboid bar. The first area 4 then corresponds to one of the ends of the bar and the second area 7 corresponds to the other end of the bar, fixedly mounted on the support. At a minimum, the dimensions of sample holder 3 must allow the sample holder to support the sample 2.

(12) The sample holder 3 is preferably a harmonic oscillator. The sample holder 3 can have a natural frequency between 500 Hz and 10 MHz preferably between 1 kHz and 1 MHz. Thus, the measurement of the frequency of the sample holder 3 is not disturbed by a surrounding noise, for example caused by an electrical or acoustic noise. The sample holder 3 has for example a quality factor greater than 10 and preferably greater than 100. For example, the sample holder 3 has a natural frequency of 2 kHz and a quality factor of 100.

(13) The sample holder 3 can also be in the form of a macroscopic sized tuning fork preferably of a length greater than 1 cm. The stem of the tuning fork corresponds to the second area 7 and at least one blade of the tuning fork corresponds to the first area 4. Thus, the quality factor of the sample holder 3 can be maximized compared to a beam-shaped sample holder 3 of the same length.

(14) The microscope 1 also comprises a detector 8 suitable for detecting a displacement of the first area 4 with respect to the second area 7. The second area 7 being fixed with respect to the ground, it may be sufficient for the detector 8 to detect the absolute movement of the first area 4. The detector 8 can be an accelerometer, for example manufactured in MEMS technology, fixedly mounted with respect to part of the sample holder 3 and preferably with respect to the first area 4 of the sample holder 3. Thus, it is possible to maximize the amplitude of the movement of the measured sample holder 3. Alternatively or in addition, the detector 8 can be an optical interferometer, a capacitive detector, a piezoelectric detector, a laser deflection detector and/or a tunneling detector. With reference to FIG. 2, the detector 8 is for example mounted fixedly facing the sample 2 on the first area 4 of the sample holder 3. The movement frequency range detectable by the detector 8 must comprise the natural frequency of the sample holder 3. The detector 8 can advantageously measure movements corresponding to very low amplitude vibrations of the sample holder 3 preferably of an amplitude of less than 1 nm and in particular of an amplitude of less than 500 pm. Thus, it is possible to use a sample holder 3 having a higher rigidity than the probes of the prior art.

(15) Probe

(16) The microscope 1 comprises at least one probe 5. The probe 5 comprises a tip 13 suitable for being positioned facing the surface 9 of the sample. The microscope 1 comprises means for positioning the tip 13 with respect to the surface 9. The probe 5 can for example comprise an electrochemically etched tungsten tip fixedly mounted on means for positioning the tip 13 with respect to a direction tangential to the surface 9, allowing a control of the position having a sub-micrometric accuracy preferably less than 100 pm. Such means can for example comprise a piezoscanner. Unlike the prior art, the probe may not comprise a sensor and thus be passive. Thus, the microscope 1 can preferably comprise a control unit configured to control a fixed position of the tip 13 and at the same time measure a signal emitted by the detector 8.

(17) The microscope 1 can also comprise a plurality of probes 5. The different probes 5 can be installed so as to be interchangeable. Several probes 5 can also be presented concomitantly facing the surface 9. The microscope can for example comprise a rotating table to which various probes 5 are fixed. By way of example, the tip 13 of a probe 5 can be able to experience a repulsive, chemical, magnetic, thermal, ionic, fluidic type interaction and/or a tunneling current. Preferably, at least one of the probes 5 is made of a material different from another probe 5. Each probe 5 can comprise positioning means independent of each other.

(18) The inventors discovered that the sample holder 13 can be used to detect the interactions between the surface 9 and the tip 13 of the probe 5. Indeed, the tip 13 can be brought closer to the surface 9 at a sufficiently small distance, for example between 10 nm and 1 μm, to increase the interaction between the tip 13 and the surface 9 so that the mechanical properties of the sample holder 3 are modified. Thus, unlike the atomic force microscopes of the prior art, in which the sensor forms part of the probe 5 or is fixed thereto, the interactions between the surface 9 and the tip 13 are detected by the sample holder 13. The sample holder 13 is mechanically decoupled from the probe 5. Thus, it is possible to significantly reduce the cost of a probe 5, because the probe 5 does not necessarily comprise a sensor. In addition, the cost of the microscope 1 as a whole can also be reduced, the sample holder 3 being reused for each measurement. Finally, the evaluation of the surface 9 can be implemented in media other than air in a simplified manner: indeed, the manufacture of the sensor no longer has to take into account the dissipation of the energy transmitted to the medium during the movement of the probe 5 in a medium with properties different from air such as a liquid, because the movement allowing the detection of the interaction between the tip 13 and the surface 9 is carried out by the sample holder 3. In addition, even if the medium in contact with the surface 9 is not such as to cause more frictional forces with the probe 5 than air, as it is the case for a partial vacuum, the integration of a probe 5 without sensor in an enclosure adapted to said medium is simplified.

(19) Referring to FIG. 3, another aspect of the invention is a method for evaluating the surface 9 by the atomic force microscope 1. The method comprises a step 301 of positioning the tip 13 at a point facing the surface 9, preferably at a distance of less than 100 nm from the surface and in particular less than 10 nm from the surface 9. The method comprises a step 302 of measuring the displacement of the first area 4 of the sample holder 3 by the detector 8 so as to evaluate an interaction between the surface 9 and the tip 13. Thus, the microscope 1 is configured to evaluate an interaction between the surface 9 and the tip 13 from the detector 8. The method preferably comprises the repetition 303 of steps 301 and 302, step 301 being carried out at other points facing the surface 9. The repetition 303 can for example be implemented by scanning the surface 9 to be evaluated by displacement of the tip 13. The scanning can be implemented by repeating steps 301 and 302 at successive points separated for example by a sub-nanometric distance, for example of between 100 μm and 1 nm. The control unit can be configured to calculate an interaction between the surface 9 and the tip 13 at least from the output signal of the detector 8. Preferably, the control unit is configured to calculate an interaction between the surface 9 and tip 13 at least from the output signal of the detector 8 and from a signal representative of the position of the tip 13 with respect to the surface 9.

(20) With reference to FIG. 1 and FIG. 2, the microscope 1 preferably comprises an actuator 10 configured to vibrate the sample holder 3 in a controlled manner and at a predetermined frequency. The actuator 10 can for example be a piezoelectric actuator (or dither) able to vibrate the sample holder 3 at its natural frequency. The actuator 10 can be fixedly mounted on the sample holder 3, for example supported by the second part 7 of the sample holder 3. The method according to one aspect of the invention can comprise a step, for example simultaneous with the step of measuring the displacement of the first area 4, in which the actuator 10 is actuated so as to vibrate the first area 4 of the sample holder 3 at a predetermined frequency of between 500 Hz and 10 MHz. For a natural resonant frequency f.sub.0 of the sample holder 3, the actuator 10 is preferably actuated so as to vibrate the first area 4 at a frequency of between f.sub.0 0.5.Math.f.sub.0 and f.sub.0+0.5.Math.f.sub.0, in particular between f.sub.0−0.1.Math.f.sub.0 and f.sub.0+0.1.Math.f.sub.0. Thus, it is possible to measure a variation in the amplitude of the vibrations of the first area 4 or in the frequency of the vibrations of the first area 4 during an interaction between the surface 9 and the tip 13. The actuator 10 allows controlling the deformation of the first area 4 with respect to the second area 7. The control unit can be connected to the actuator 10 and be configured to control the actuator 10 so as to control the deformation of the first area 4 with respect to the second area 7 and at the same time to receive the output signal from the detector 8 so as to calculate an interaction between the surface 9 and the tip 13 of the probe.

(21) The actuation of the first area 4 can also be implemented at several predetermined frequencies. It is thus possible to evaluate the behavior of a sample 2 under a stress at different frequencies or speeds.

(22) The microscope 1 can also comprise a regulator 11 by a closed-loop servo-control. A signal representative of the displacement of the first area 4 can be transmitted by the detector 8 to the regulator 11. The regulator 11 can then transmit a regulation setpoint to the actuator 10 and/or to the means for positioning the tip 13 so to regulate the interactions between the tip 13 and the surface 9.

(23) The microscope 1 preferably comprises a tip positioning actuator for positioning the tip 13 of the probe 5 facing the surface 9. The tip positioning actuator can be a piezomotor. The regulator 11 may be adapted to transmit a regulation signal to the tip positioning actuator so as to maintain the tip 13 at a constant and predetermined distance from the average surface 9 over time.

(24) Configurations of the Sample Holder 3

(25) The quality factor (defined by the ratio between the resonant frequency and the width of the Lorentzian resonance at half height) can be controlled by the shape of the used sample holder 3. Particularly, the sample holder 3 can have the shape of a beam fixedly mounted at its two ends on the support 6 by the second areas 7. The first area 4 is then arranged in the middle of the beam, at an equal distance from each of the second areas 7. Thus, the quality factor of the sample holder 3 can be maximized. The sample holder 3 can also be in the form of a membrane.

(26) In this case, the first area 4 is arranged at the center of the membrane and the second area 7 is arranged at the edge of the membrane.

(27) Measurement in a Liquid Medium

(28) The microscope 1 is particularly advantageous to implement measurements of a surface 9 in a liquid medium. With reference to FIG. 4, the microscope 1 can comprise a cell 12. The cell 12 is adapted to contain a liquid or gelled medium. The cell 12 is fixedly mounted on the first area 4. The sample is fixedly mounted on the cell 12. By integrating the sensor into the sample holder 3, the measurement of a surface 9 in a liquid medium is simplified. Indeed, it is not necessary for the probe 5, comprising the tip 13, to oscillate. Thus, the measurement is not affected by any frictional forces which can be exerted by the liquid medium on the probe 5 during the evaluation of the surface 9, as it is the case in the microscopes of the prior art. This type of configuration is particularly advantageous for the evaluation of biological objects attached to the surface 9. In addition, since the detector 8 is not mounted in a submerged probe 5, it is possible to avoid a drift of the output signal of the detector 8. Indeed, the sample holder 3 and the detector 8 can be kept out of contact with the liquid medium.

(29) Result of a Surface Measurement 9

(30) With reference to FIG. 5, the surface 9 of a silicon oxide calibration network is measured by the microscope 1 according to the embodiment illustrated in FIG. 2. The pitch of the network is equal to 5 μm and the depth of the blind hole is equal to 100 nm (calibration network).