EP2029998A1

EP2029998A1 - Controlled atomic force microscope

Info

Publication number: EP2029998A1
Application number: EP07766092A
Authority: EP
Inventors: Michal Hrouzek; Alina Anca Voda; Joël CHEVRIER; Gildas Besancon; Fabio Comin
Original assignee: Centre National de la Recherche Scientifique CNRS; Institut Polytechnique de Grenoble; Universite Joseph Fourier Grenoble 1; European Synchrotron Radiation Facility
Current assignee: Centre National de la Recherche Scientifique CNRS; Institut Polytechnique de Grenoble; Universite Joseph Fourier Grenoble 1; European Synchrotron Radiation Facility
Priority date: 2006-05-24
Filing date: 2007-05-23
Publication date: 2009-03-04
Also published as: FR2901601B1; AU2007253164A1; WO2007135345A1; FR2901601A1; JP2009537840A; CA2653116A1; US20100064397A1

Abstract

The invention relates to an atomic force microscope comprising a microtip (1) placed on a flexible support connected to a microscope head (11) facing a surface (5) to be studied, which includes means (31, 32) for controlling the distance between said head and said surface for a given value and means (31, 35) for inhibiting vibration of the microtip.

Description

ASSISTED ATOMIC STRENGTH MICROSCOPE

Field of the invention

The present invention relates to measuring the relief of a surface using an atomic force microscope. Discussion of the Related Art Figure 1 schematically shows the Extremists ^¬ detection moth-eaten an atomic force microscope. This detection end ^¬ consists of a tip 1 disposed at one end of a beam 2 whose other end is embedded at a support 3. The beam has for example a length of 50 to 500 microns, a width of 20 to 60 microns and a thickness of 1 to 5 microns. When the tip is disposed close enough to a surface of a sample to be studied, there appears an atomic interaction force between the tip 1 tip and the sample 5 surface. Also, when the tip is moved transla- tion compared to the sample 5 in the direction of the axis x of Figure 1, or vice versa, the beam is the subject of MOVE ^¬ cements in the direction of the z-axis which are the irregu ^¬ larities surface of the sample 5. in order to measure the position of the beam, various means have been proposed. The most common is an optical detector of a beam reflecting on the beam. The detector optionally comprises interferometric means. Such microscopes, known for twenties, are used for example for measuring surface irregularities having dimensions of the order of a nanometer, that is to say that one reaches observe Mole ^¬ cules or atoms. Two main ways of using an atomic force microscope have been proposed.

In the first case, an extremely flexible beam (of very low stiffness) is used. The tip is in permanent contact with the measured surface and the deflection of the beam is recorded. In this case, there is a strong repulsive interaction with the surface to be measured and this results in the risk of degradation of the tip, and / or the measured surface.

In a second case, the beam is excited in vibra ^¬ near its resonant frequency. Near the swept surface, the attractive and repulsive interaction forces modulate this vibration in phase and / or in frequency. The analysis of the modulation of the vibration of the beam makes it possible to determine said interaction. In this case, the sensitivity of the measurement is fundamentally limited by the thermal noise of the beam. There are various variants depending on whether or not the tip is allowed to strike the studied surface for short periods of time or depending on the control mode obtained: regulated vibration amplitude and constant excitation frequency or permanent search for the resonant frequency taking into account the frequency shift induced by the interaction. Whatever the implementation detail, this permanent vibrating mode of the beam presents problems, inherent in its principle, when it is desired to measure distances and interaction forces in a liquid medium, for example a biological medium. Indeed, this technique is based on the forced vibration of the beam and fundamental problems arise to use such an atomic microscope in liquid medium: how to combine vibration setting and liquid medium, how to reconcile marked resonance necessary for good resolution and damping due to the fluid. Summary of the invention

Thus, an object of the present invention is to provide an atomic microscope structure adapted to a new mode of operation which overcomes at least some of the disadvantages of the previously exposed modes of use and which is furthermore particularly suitable for use in a liquid medium. .

To achieve all or part of these objects, the pre ^¬ feel invention provides an atomic force microscope comprising a microtip provided on a support flexible bound to a microscope head opposite a surface to be examined, comprising means for controlling at a given value the distance between said head and said surface, this distance being measured in line with the tip, and means controlled to inhibit the vibration of the microtip.

According to one embodiment of the present invention, the microtip is disposed at the end of a recessed beam.

According to one embodiment of the present invention, the means for inhibiting vibration of the microtip include an ^¬ NEET means integral conductors of the microscope head, capacitively coupled to the beam and receiving, without high frequency filtering the signal servo used to stabi ^¬ Liser the distance between the microscope head and the surface to be studied.

According to an embodiment of the present invention, said conducting means receive frequencies going beyond the frequency of the third mode of resonance of the beam. According to one embodiment of the present invention, the transverse scanning speed between the microscope head and the surface to be studied is chosen so that the measurement of the variation of relief only has frequency components at frequencies lower than the natural frequency. vibration of the beam. Brief description of the drawings

These and other objects, features, and advantages of the present invention will be set forth in detail in the following description of particular embodiments in a non-limitative manner with reference to the accompanying figures in which: FIG. very schematic the active part of an atomic microscope; FIG. 2 very schematically represents a first embodiment of an atomic microscope according to the present invention; Fig. 3 is a block diagram representation of the present invention; FIGS. 4A to 4D are curves illustrating a first example of use of an atomic microscope according to the present invention; and Figs. 5A to 5D are curves illustrating a second example of use of an atomic microscope according to the present invention. detailed description

FIG. 2 illustrates an exemplary embodiment of an atomic microscope according to the present invention. The tip 1 is disposed at the end of a beam of a conductive material 2, for example highly doped silicon, etched from a silicon support 3. The support is integral with a directional atomic microscope head and adjustable in position 11. In the figure, there is shown an intermediate piece 12 of a conductive material, one end 13 is capacitively coupled with the free end of the beam 2. The intermediate piece 12 is electrically insulated from the support 3 and Preferably also the head 11. The support and the head are for example both grounded. The sample to be measured 5 is placed via a piezoelectric structure 17 on an XY table 19 allowing for example to ensure the displacement in the x direction mentioned in relation to FIG. intermediate piece 12 has an opening allowing the beam 2 to be illuminated by a laser 21 whose reflected beam is detected by a photodetector 22 arranged in a known manner to provide a signal corresponding to the position, z, of the beam.

The present invention provides to maintain constant the distance zd between the beam support (the assembly consisting of the support 3, the intermediate part 12 and the microscope head 11) and the sample 5. The present invention further provides stabilize the beam, that is to say to avoid its vibrations, so that the distance zt between the measuring tip and the surface of the sample 5 is actually constant (and the distance zd is a distance taken to the right from the tip). Indeed, as found by the inventors, normally, in the absence of any action on the beam, it tends to vibrate under the effect of thermal noise at frequencies close to its natural frequency and its harmonics. For a silicon beam having a length L of 50 to 500 μm, a width of 10 to 60 μm and a thickness e of 1 to 5 μm, the natural frequency of the beam will be between 10 and 500 kHz. For example, for a beam having a length L of 125 microns, a thickness e of 4 microns and a stiffness of 40 N / m, the natural frequency will be 300 kHz. According to one embodiment of the invention, the position signal of the beam, Sz, supplied by the measuring device 22 is compared to a desired value SzO, preferably 0, in a stabilization controller 31. The output signal of the controller is provided to a piezoelectric structure control point controller 32 carrying the sample 5. The signal of the controller 32 is amplified by an amplifier 33. This control signal comprises frequency components ranging substantially from continuous to a frequency which depends on the scanning speed of the sample under the microscope and which, as will be seen below, can be of the same order of magnitude as the vibration frequency of the beam but is preferably significantly lower.

The output signal of the stabilization controller 31 is also provided to an amplifier 35 supplying a voltage to the intermediate piece 12 or at least at its end 13 which acts by capacitive effect on the beam 2. The amplifier 35 amplifies the frequencies ranging from a value lower than that of the fundamental resonant frequency of the beam to values as high as possible for correcting higher order resonant frequencies. Preferably, a range of frequencies will be chosen to compensate the vibrations of the beam to high frequencies, typically at least up to the frequency of the third resonance mode of the beam. This channel servo is shown in block diagram form in Figure 3. It includes the photodétec ^¬ tor 22 provides a signal Sz output which is compared with a SZO desired position signal in a comparator 41 followed by a controller stabilization 42, the set of elements 41 and 42 corresponding to the controller 31 of FIG. 2. The output servo signal Sf of this controller is supplied on the one hand to a second comparator 43 followed by a controller 44, the comparator 43 and the controller 44 corresponding to the controller 32 of FIG. 2. The comparator 43 compares the servo signal Sf with a desired signal SO. The controller

44 provides a positioning voltage which is sent via an amplifier 33 to all piézoélec ^¬ stick 17 which provides a signal corresponding to the sample position. Similarly, the signal Sf is supplied to an amplifier 35 and to a capacitive actuator 36 corresponding to the coupling between the intermediate piece 12 and the beam 2. At each instant, the integral of the servo signal Sf constitutes the signal of interaction measurement according to the invention. Figures 4A to 4C show the signal Sz (ω) as it would be in various hypotheses. Figure 4D shows the corresponding signal Sf (ω).

In Figure 4A there is shown what would be the signal Sz (ω) to the input of controller 31 in the absence of any asser ^¬ vissement. This signal would comprise three components 61, 62 and 63. The signal 61 is related to the thermal noise of the system and comprises peaks at the resonant frequency (λg of the beam and at higher resonance modes, (%, (»2 The low frequency signal 62 is related to the electrical and mechanical noise of the system The signal due to the surface interaction between the tip and the sample moving in front of it is contained in the spectral band This surface interaction signal may include frequencies up to a GO ₃ value related to the sample scan rate.

FIG 4B shows the resultant of the three compo ^¬ cient of Figure 4A.

Figure 4C shows the movement of the beam resulting from damping according to the present invention. It has been assumed that this movement is not completely damped and a still relatively large displacement has been shown to better understand the invention. Note, however, that in practice, it will impose a movement attenuation by a factor of about 100 compared to what would be this unamortized movement as shown in Figure 4B.

FIG. 4D shows the signal Sf (ω) measured at the output of the controller 42 of FIG. 3, which corresponds to the enslaving force supplied. Of course, the value of this signal and the efficiency of the damping will depend on the cutoff frequencies chosen and the amplification rates of the various amplifiers.

It will be noted that the evolution of the servo force required for the damping of the beam as a function of the frequency depends on the shape of the response function of the beam. With equal movement amplitude, a much greater force It is necessary to damp a displacement outside a resonance range only in a resonant frequency range (this explains the hollow in the servo force for a constant displacement in the vicinity of the resonance). In other words, the displacement induced by a given amplitude signal at a frequency outside of a resonance range will be practically indistinguishable from the displacement induced by this same signal at a frequency located in a resonance range. On the other hand the forces necessary for the cancellation of displacements will be approximately equal. Thus, the influence of a uniform thermal noise, which prevails at the resonance frequencies in the representation of the displacement of Figure 4C, fades these frequency reso nance ^¬ on the damping force curve of Figure 4D. The integral of the damping energy curve of FIG. 4D will therefore represent the influence of an interaction outside resonance frequency ranges much better than would be the integral of the displacement curve of FIG. 4B in which the influence of the noise component at the resonance frequencies would be far from negligible.

If one wants to further improve the results of the present invention, there can be placed under the conditions illus ^¬ Trees in Figures 5A to 5D which correspond respectively to Figures 4A-4D. The difference between these figures results from the choice of the relative scanning speed between the microtip and the sample from which it results that the interaction signal is not likely to contain components at the resonance frequencies of the beam.

As illustrated in FIG. 5A, the scanning speed between the microtip and the sample is chosen so that the highest frequency component that can result from the surface interaction is less than the natural frequency of the beam. It should be noted that the depreciation effort shown in Figure 5D essentially includes a component related to the surface interaction. We will have a more precise measurement of the interaction.

Depending on the case, it will be possible to choose a fast scan as illustrated in relation to FIGS. 4A to 4D, which nevertheless provides a good measure of the relief of the sample, or a slower scan as illustrated in relation to the figures

5A to 5D if one wants to obtain a homogeneous treatment of all the frequency components of the signal. For example, if one wants to observe surfaces of living matter, in displacement, one will choose a relatively fast scan, corresponding to the conditions of figure 4.

According to a first advantage of the present invention, the absence of vibration of the beam results in the measurement of the interaction force being carried out for a precise distance and not for an average of distances, as in the case where the beam is permanently excited in vibration. This improves intrinsè ^¬ cally the measurement accuracy.

According to a second advantage of the present invention, the absence of vibration of the beam results in the invention being well suited to measurement in a liquid medium. Indeed in such a medium, the vibrations would be disturbed by the environment and the creation of vibrations in the medium can cause various disadvantages.

According to a third advantage of the present invention, the cancellation by the vibration control loop of the beam causes a reduction in thermal noise and therefore a large increase in measurement accuracy. Indeed, in a conventional system, the thermal noise essentially results in an excitation of the beam that starts to resonate. Thus, the vibration damping is equivalent to a cooling of the entire system, which would be impossible in liquid medium.

According to a third advantage of the present invention, it makes it possible to perform sweeps faster than the previous devices. Of course, the present invention is capable of many variants that will occur to those skilled in the art, in particular with regard to the realization of the various electrical and electronic circuits described. Furthermore, the present invention applies to various types of microscopes ato strength ^¬ nomic, e.g. microscopes wherein the microtip, instead of being carried by a beam is carried by another flexible structure, for example a membrane.

Claims

1. Atomic force microscope comprising a micro ^¬ tip disposed on a support flexible bonded to a head microphones ^¬ cope (11) opposite a surface to be investigated (5), comprising: means (31, 32) for slaving at a given value the distance between said head and said surface, this distance being measured at the point of the tip, and means (31, 35) controlled to inhibit the vibration of the microtip.

2. Atomic microscope according to claim 1, in which, at any moment, the signal for measuring the interaction with the surface to be studied consists of the integral of the servo signal (Sf (OD)).

3. Atomic microscope according to claim 1, wherein the microtip (1) is disposed at the end of a recessed beam (2).

4. Atomic microscope according to claim 3, wherein the means for inhibiting the vibration of the microtip comprise conductive means (12) integral with the microscope head (11), in capacitive coupling with the beam (2) and receiving, without high-frequency filtering, the signal enslave ^¬ used to stabilize the distance between the microscope head and the surface to be studied.

The microscope of claim 4, wherein said conductive means receives frequencies beyond the frequency of the third resonance mode of the beam.

6. Microscope according to claim 2, in which the transverse scanning speed between the microscope head and the surface to be studied is chosen so that the measurement of the variation of relief only has frequency components at frequencies lower than the natural frequency. vibration of the beam.