WO2020232491A1 - Cantilever for an atomic force microscope - Google Patents

Abstract

The invention relates to a cantilever (1) for an atomic force microscope, having a base body (2), which is excitable by an external periodic force at at least one frequency, wherein, in the vibrating state, the base body (2) vibrates in a resonance frequency of a predefined, two-dimensional vibration mode, and the vibrating base body has a first dynamic spring constant, and a beam element (3), which is attached to the base body (2) at an attachment point of the base body (2). A first portion (3a) of the beam element (3) protrudes over an edge of the base body (2). The beam element (3) has a measurement tip (8) protruding from the beam element (3) and arranged on the first portion (3a). The beam element (3) is set into motion by the vibration of the base body (2), wherein the moved beam element has a second dynamic spring constant, wherein the base body vibrating at a resonance frequency and the moved beam element form a coupled, vibrating system.

Description

CANTILEVER FOR A LASTING FORCE MICROSCOPE

The invention relates to a cantilever for an atomic force microscope, the cantilever having a base body which can be excited in at least one frequency by an external periodic force provided by an excitation device, the base body in the vibrating state in a resonance frequency of a predetermined two-dimensional vibration mode oscillates so that the oscillating base body has a first dynamic spring constant.

The invention also relates to an atomic force microscope and a use of one

Cantilever and an atomic force microscope.

Atomic Force Microscopy (AFM) is a widespread technology for the topographical investigation and material characterization of

Surfaces in the (sub) nanometer range. When measuring a sample material with an atomic force microscope, so-called cantilevers are used, which in the prior art are designed as microscopic bars clamped on one side, which vibrate vertically to a sample during the measurement. The beam is thus a mechanical resonator which, through a tip attached to the beam, with the

examining sample surface interacts.

The aim of today's developments is to increase the scanning speed in order to achieve the

To shorten measurement times, whereby dynamic processes can be measured almost in real time with the atomic force microscope. High scan speeds require one

Excitation of vibrations in the cantilever at a high resonance frequency. The

Resonance frequencies T of the beam result from Euler and Bernoulli

Wobei a, die Eigenwerte des Differentialoperators der Euler-Bernoulli Gleichung darstellt. Je kürzer die Länge L des Balkens, desto höher liegen demnach seine Resonanzfrequenzen E.

Where a, represents the eigenvalues of the differential operator of the Euler-Bernoulli equation. The shorter the length L of the beam, the higher its resonance frequencies E.

A resulting disadvantage is, however, that a short beam length L results in a high spring constant ki of the beam. This results from the context

w width

However, shortening the length of the bar limits its maximum achievable deflection, which is necessary for examining surfaces of a certain roughness, and also reduces the sensor measurement signal in the case of a measurement based on amplitude modulation. Furthermore, sensitive surfaces such as those found in biological samples can be influenced by excessive spring constants in such a way that the measurement results are falsified. Furthermore, samples, in particular biological or organic samples, are often examined in liquids, whereby the cantilevers known in the prior art are strongly attenuated by using the standard out-of-plane mode and their Q-factor, the resonance frequencies and the signal to-noise ratio can be reduced significantly.

The object of the present invention is to create a cantilever which eliminates the disadvantages of the prior art.

This object is achieved by a cantilever with the features of claim 1 and an atomic force microscope with the features of claim 11. Preferred

Implementations are given in the dependent claims.

According to the invention, the cantilever has a beam element which is attached to a

Attachment point of the base body is attached to the base body, wherein a first portion of the bar element protrudes over an edge of the base body, wherein the bar element has a measuring tip, which is preferably orthogonal to the

Bar element protrudes and is arranged on the first section of the bar element, which protrudes over the edge of the base body, wherein in the oscillating state of the Base body, the beam element is set in motion by the vibration of the base body, the moving beam element having a second dynamic spring constant, the base body oscillating at a resonance frequency and the moving beam element forming a coupled oscillating system.

The coupled oscillating system, which is formed from the base body and the beam element attached to it, can have a dynamic spring constant which is in particular smaller than the first and the second dynamic spring constant. The spring constant that an oscillating system has is called the dynamic spring constant. The base body and the beam element preferably differ both in their dimensions and in their material parameters. The base body is preferably plate-shaped or a plate element which can have a rectangular, square or trapezoidal shape, the plate element having a greater length, width and thickness compared to the beam element. The beam element is in particular shorter, narrower and thinner than the base body. In other words, the base body is preferably larger in its dimensions and more rigid in its elastic properties than the beam element. Conversely, the beam element is preferably smaller in its dimensions and softer in its elastic properties than the base body. The base body can be excited in two-dimensional oscillation modes, which, compared to a simple one

Bending bars, high-frequency eigenmodes with high quality factors Q, can be excited. The two-dimensional oscillation modes can be calculated with the Kirchhoff-Love plate theory. The larger one compared to the beam element

For certain two-dimensional oscillation modes, the base body can contain a great deal of elastic energy (in relation to Euler-Bernoulli modes with a comparable maximum frequency of the spatial mode modulation), but this also increases the dynamic spring constant of the base body. If the base body vibrates at a resonance frequency, the beam element coupled to the base body has a smaller dynamic spring constant compared to the base body. By coupling the base body with the beam element, which is much smaller relative to the base body, the beam element can be excited or moved at high frequencies, on the one hand, and, on the other hand, have a lower dynamic spring constant compared to the base body. The cantilever according to the invention can thus be used for high scanning or

Measurement speeds required high resonance frequencies are excited, with at the same time the dynamic spring constant of the coupled oscillating system is kept low. The dynamic spring constant of a system represents the

Proportionality factor between quadratic deflection and elastic energy, so that the energy of the corresponding point-mass system and the continuum mode are the same. The base body preferably oscillates at a high rate during a measurement

Resonance frequency, so that the small bending beam is not brought into resonance and only follows the movement of the base body quasi-statically. The term “quasi-static” is to be understood in such a way that the bar element preferably does not vibrate in resonance, whereby its dynamic spring constant is correspondingly low. The bar element can, however, also vibrate in resonance. The bar element is preferably attached to the base body in such a way that the base body and In other words, a first section of the bar element protrudes over an edge or an edge region of the base body and a second section of the bar element is attached to a surface region of the base body

The edge area of the base body to which the second section of the beam element is attached is referred to in this context as the overlap area. In this case, the bar element and the base body each lie in one plane, the two planes being shifted parallel to one another. The bar can essentially be used as a

Extension arm of the base body are viewed. The beam element also has a measuring tip which can interact with a sample surface during a measurement. The measuring tip is preferably on a surface of the first section of the

Fastened bar element, which faces away from the base body. The measuring tip can, for example, comprise diamond and be glued to the beam element, or it can be attached to the beam element, for example, by means of focused electron beam induced deposition. As an excitation device, for example, one of the

Basic body spatially separated, external piezo actuator, a photothermal one

Activation device, or a piezoelectric layer, which is applied in an integrated manner to the base body, can be provided. At this point it should be mentioned that one

A person skilled in the art of atomic force microscopy knows how a measuring tip can be attached to a cantilever and the person skilled in the art also knows the possibilities of vibrating a cantilever in an atomic force microscope and their technical implementation, which is why this disclosure does not explain this in detail

is received. The main body can comprise a first material and the beam element can comprise a second material, wherein the first and the second material can be different. This has the advantage that the first and the second material can be selected according to (for an AFM measurement) optimal mechanical properties. Alternatively, the two materials can also be of the same type. The base body can in particular consist of a material that is more rigid than the beam element. The base body can for example comprise silicon or silicon carbide and have a modulus of elasticity between 150 GPa and 600 GPa, preferably between 130 GPa and 170 GPa, particularly preferably 160 GPa. The beam element can in particular comprise gold, silver, aluminum, silicon, polymers or titanium and have a modulus of elasticity between 2 GPa and 170 GPa, preferably 70 GPa. The ratio of

The modulus of elasticity of the base body and the beam element can be between 0.75 and 300.

The bar element is preferably at an edge or an edge region of the

Fixed base body, which in the vibrating state of the base body has a maximum deflection relative to a non-vibrating rest state of the base body. As a result, the beam element can be excited to vibrate particularly efficiently. Two or more bar elements can also be attached to the base body, each bar element protruding from an edge of the base body. The two or more bar elements are preferably each fastened to an edge region which has a maximum deflection in the oscillating state of the base body. The two or more beam members are preferably substantially on the

Amplitude maxima of the oscillation of the edge area attached.

The base body can preferably be excited in the resonance frequency of at least one two-dimensional oscillation mode, and the beam element can vibrate quasi-statically with the vibrating base body or be excited by the vibrating base body in a resonance frequency of the beam element. In the vibrating state of the base body, the beam element can be excited in resonance by the vibration of the base body. This is the case when the

The resonance frequency of the base body, in which the base body is excited, and a resonance frequency of the beam element overlap. When the resonance frequency in which the base body is excited is different from a resonance frequency of the beam element is far away in terms of amount, the beam element is not excited resonantly and follows the oscillation of the base body preferably quasi-statically. In the event that that

If the beam element vibrates resonantly, the beam element can vibrate in one-dimensional or in two-dimensional vibration modes. The oscillation mode of the base body can be, for example, a roof-tile-shaped oscillation mode, for example a 12-mode with resonance frequencies between 200 kHz and 2.5 MHz, preferably 700 kHz or 760 kHz or a 14-mode with resonance frequencies between 500 kHz and 5 MHz, preferably 3 MHz or 2.2 MHz. The vibration modes in which the base body is excited preferably have a high Q factor and are therefore only weakly damped.

The main body can have a first dynamic spring constant and the beam element can have a second dynamic spring constant, the first dynamic spring constant

Spring constant is greater than the second dynamic spring constant. As a result, the base body can vibrate at a high resonance frequency, the beam element being able to be excited or moved at high frequencies, and at the same time having a lower dynamic spring constant compared to the base body. This enables AFM measurements with a high sampling rate, in particular of soft samples, even in liquid, without the samples or their surface properties being changed by the measurement.

The length of the base body is preferably 100 μm to 2000 mhi and the length of the bar element 5 μm to 50 μm. The length ratio of the base body and the bar element can in particular be greater than 2, preferably greater than 50, particularly preferably greater than 200, even more preferably greater than 300.

The width of the base body is preferably 100 μm to 2000 μm and the width of the bar element 5 μm to 50 μm. The width ratio of the base body and the bar element can in particular be greater than 2, preferably greater than 50, particularly preferably greater than 200, even more preferably greater than 300.

The thickness of the base body is preferably 5 μm to 20 μm and the thickness of the beam element is 100 nm to 500 nm. The thickness of the beam element can also be up to 5 μm. The thickness ratio of the base body and the beam element can in particular greater than 10, preferably greater than 50, particularly preferably greater than 100, even more preferably greater than 150.

At least one dimension of the base body and a corresponding dimension of the bar element can have a ratio of greater than 2, preferably greater than 50, particularly preferably greater than 100, even more preferably greater than 300, the

Dimension can be a length, width or thickness or height.

The base body is preferably designed in the form of a plate, with nodal lines of the vibration being oriented essentially parallel and / or orthogonally to the length of the base body in the vibrating state of the base body. The two-dimensional oscillation mode in which the base body is excited during a measurement preferably has parallel and / or orthogonal nodal lines, based on the length of the base body.

The amplitude of the oscillation is zero along the nodal lines. The terms parallel and orthogonal relate to the direction in which the spatial extension of the length of the base body the nodal lines lie. The length of the base body is to be understood as the normal distance between a first edge and a second edge opposite the first edge, the beam element preferably being attached to the first edge and the base body, for example, to the base body on the second edge

Excitation device is attached. There are parallel nodal lines when the nodal lines run parallel to the longitudinal extent or length of the base body. Orthogonal nodal lines are present when the nodal lines run orthogonally to the longitudinal extent or length of the base body. The nodal lines can in particular also have a

have a curved or inclined course.

According to the invention, an atomic force microscope is provided, comprising at least one cantilever according to the invention, the atomic force microscope being set up to measure a sample material, which preferably comprises an organic or biological sample material, the cantilever scanned the sample in a raster shape during the measurement, so that during the scanning of the Sample material the measuring tip of the

Beam element interacts with a surface of the sample material, the base body of the cantilever being able to vibrate in at least one during the measurement

Resonance frequency of the base body is excited, which corresponds to a resonance frequency of a predetermined two-dimensional oscillation mode, so that the oscillating Base body has a first dynamic spring constant, and the beam element is set in motion by the vibration of the base body, the moving

Beam member has a second dynamic spring constant, the in one

The body vibrating at the resonance frequency and the moving beam element form a coupled system.

The beam element can have a piezoelectric layer, wherein the

Change in movement of the bar element due to the interaction of the measuring tip with the sample material, the piezoelectric layer can generate a measuring signal. This can be used to analyze the sample surface.

The change in movement of the beam element due to the interaction of the

The measuring tip with the sample material can also be measured optically, preferably with a laser beam focused on the beam element. This enables the movement of the beam element to be recorded and the sample surface to be measured.

In one possible use of the cantilever according to the invention, it is built into an atomic force microscope. When measuring a sample material with the

Cantilever or the atomic force microscope, the cantilever and the sample material can be immersed in a liquid, preferably completely, during the measurement. Due to the two-part construction of the cantilever according to the invention, its oscillation in a liquid is less strongly damped than in a conventional oscillating beam, whereby the measurement of the sample material in a liquid is favored. Applications in other, in particular gaseous or supercritical media, are of course also possible.

In the context of this description, the terms "top", "bottom", "horizontal", "vertical" are to be understood as indications of the orientation, which relate to a positioning as shown in the figures and not as restrictive with respect to an actual one

Orientation of the cantilever or scanning microscope are to be understood. This corresponds to a cantilever in an atomic force microscope, which scans a horizontal surface of a sample material. The invention is explained in more detail below with reference to a preferred exemplary embodiment to which, however, it is not intended to be restricted. In the drawings shows:

1 shows a plan view of a schematic representation of a cantilever according to the invention;

FIG. 2 shows a side view of the cantilever according to FIG. 1;

3 is a perspective view of a schematic representation of the cantilever;

4 shows a schematic representation of a parallel and an orthogonal nodal line on the base body of the cantilever; and the

Fig. 5-8 different two-dimensional oscillation modes of the base body.

For the sake of simplicity, the figures do not show the components of an atomic force microscope, since the basic designs of an atomic force microscope are well known to those skilled in the art and this disclosure relates primarily to a cantilever. Furthermore, for the sake of clarity, the proportions of the dimensions of the individual components do not correspond to the real dimensional proportions. The figures are therefore to be understood as a strong simplification, which adequately represent the basic principle of the invention.

1, 2 and 3 show different views of an embodiment of the

Cantilever 1 for an atomic force microscope, the canula 1 having a base body 2. In the embodiment shown, the base body 2 is plate-shaped and rectangular. The cantilever 1 additionally has a beam element 3 which is fastened to the base body 2 at a fastening point 4 of the base body 2. A first section 3a of the bar element 3 projects beyond an edge 5 of the base body 2 or projects beyond the edge 5 of the base body 2. A second section 3b of the

Bar element 3, that section that does not protrude beyond the edge 5, is used to fasten the bar element 3 to the base body 2. As can be seen in FIG. 2, the top of the second section 3b of the bar element 3 is on the underside of an edge region of the edge 5 of the base body 2 attached. The bar element 3 can also be fastened with one of its narrow side surfaces 6 directly to a narrow side surface 7 of the base body 2, so that there is no overlap area between the base body 2 and the beam element 3. The beam element 3 has a measuring tip 8 which protrudes orthogonally from the beam element 3 and is arranged on an end region 9 of the first section 3 a of the beam element 3. As can be seen in FIG. 2, the measuring seats 8 are on a

Arranged underside of the beam element 3.

When measuring a sample material with the cantilever 1, the base body 2 can be excited in a frequency by an external periodic force which is provided by an excitation device (not shown). The swinging one

Base body 2 is excited in such a way that it is in a vibrating state in a

The resonance frequency of a given two-dimensional vibration mode oscillates.

The base body 2 preferably has parallel 10 and orthogonal nodal lines 11 in the oscillating state. The preferably plate-shaped base body 2 has a

Muzzle extension or catches and a transverse extension or width. The parallel nodal lines 10 preferably run along or parallel to the muzzle of the base body 2 and are accordingly oriented parallel to a lateral edge 2a of the base body. The orthogonal nodal lines 11 preferably run along the width of the base body 2 and are accordingly orthogonal to the lateral edge 2a (or parallel to a front edge 2b) of the base body 2. An exemplary arrangement of parallel 10 and orthogonal node lines 11 on the base body 2 are shown in fig. FIG. 4 is greatly simplified and shows, for example, the base body 2 without a bar element 3.

FIGS. 5-8 show examples of two-dimensional oscillation modes in which the base body 2 can be excited during a measurement. The resonance frequencies of the corresponding vibration modes are also shown. The bar element 3 (not shown in FIGS. 5-8) is preferably at an edge region of the edge 5 of the

Base body 2 attached, which in the vibrating state of the base body 2 has a maximum deflection relative to a non-vibrating rest state of the base body 2. It is also possible for two or more beam structures 3 to be attached to the base body 2, with each beam structure 3 preferably each being one

Maximum deflection is attached.

5-8 also show the non-oscillating state of rest of the base body 2 so that the areas of maximum deflection can be seen. The 12 mode at 700 kHz (FIG. 5) has, for example, an area in which the edge 5 of the base body 2 is maximally deflected, this area corresponding to the center of the front edge 2b of the base body 2. If the 12 mode at 700 kHz is used for the excitation of the

Chosen base body 2, the beam element 3 is therefore preferably attached in the middle of the front edge 2b. The 14 mode at 3.0 MHz (Fig. 6) has several

Displacement maxima, for example at one, several or each

Auslenkungsmaxima a beam element 3 can be attached.

In the vibrating state of the base body 2, the beam element 3 is through the

Vibration of the base body 2 set in motion. The beam element 3 can be excited in one of its resonance frequencies by the vibrating base body, or it can follow the vibration of the base body quasi-statically. The resonant vibrating one

Beam element 3 vibrates in particular in a one-dimensional vibration mode, which can be calculated using beam theory.

The base body 2 can be made of silicon or silicon carbide, for example, and have a modulus of elasticity of, for example, 160 GPa. The beam element 3 can be made of gold, silver, aluminum, silicon, polymers or titanium and have a modulus of elasticity of, for example, 70 GPa. The base body 2 can have a first dynamic spring constant and the beam element 3 can have a second dynamic

Have spring constant, wherein the first dynamic spring constant is greater than the second dynamic spring constant. The effeküve dynamic spring constant of the coupled system is in particular smaller than the respective dynamic

Spring constants of the base body 2 and of the beam element 3.

In the exemplary embodiment shown, the length of the base body 2 can be 100 μm to 2000 μm and the length of the bar element 3 can be 5 μm to 50 μm. The width of the

The base body 2 can be 100 μm to 2000 μm and the width of the bar element 3 can be 5 μm to 50 μm. The thickness of the base body 2 can be 5 μm to 20 μm and the thickness of the beam element 3 can be 100 nm to 5 μm. The overlap area can comprise 10% to 50% of the total length of the beam element 3.

Claims

PATENT CLAIMS 1. Cantilever (1) for a scanning force microscope, the cantilever (1) being a Has base body (2) which can be excited in at least one frequency by an external periodic force which is provided by an excitation device, the base body (2) in the vibrating state in a resonance frequency of a predetermined two-dimensional oscillation mode oscillates, the oscillating base body having a first dynamic spring constant, characterized in that the cantilever (1) has a beam element (3) which is attached to the base body (2) at a fastening point (4) of the base body (2) is, wherein a first section (3a) of the beam element (3) protrudes over an edge (5) of the base body (2), wherein the beam element (3) has a measuring tip (8) which preferably protrudes orthogonally from the beam element (3) and is arranged on the first section (3a) of the bar element (3), the bar element (3) being set in motion by the oscillation of the base body (2) in the oscillating state of the base body (2), the moving bar element (3) has a second dynamic spring constant, the base body (2) oscillating at a resonance frequency and the moving beam element (3) forming a coupled oscillating system. 2. Cantilever (1) according to claim 1, wherein the base body (2) has a first material and the beam element (3) has a second material, the first and the second material being different. 3. Cantilever (1) according to one of the preceding claims, wherein the beam element (3) is attached to an edge (5) of the base body (2), which in the oscillating state of the base body (2) has a maximum deflection relative to a non-oscillating rest state of the base body (2). 4. Cantilever (1) according to one of the preceding claims, wherein the base body (2) can be excited in the resonance frequency of at least one two-dimensional vibration mode and the beam element (3) with the vibrating base body (2) oscillates quasi-statically or through the oscillating base body (2) in one Resonance frequency of the beam element (3) is excited. 5. Cantilever (1) according to one of the preceding claims, wherein the base body (2) has a first dynamic spring constant and the beam element (3) has a second dynamic spring constant, the first dynamic spring constant being greater than the second dynamic spring constant. 6. Cantilever (1) according to one of the preceding claims, wherein the length of the base body (2) is 100 pm to 2000 mhi and the length of the beam element (3) is 5 mhi to 50 pm. 7. Cantilever (1) according to one of the preceding claims, wherein the width of the base body (2) is 100 pm to 2000 pm and the width of the beam element (3) is 5 pm to 50 pm. 8. Cantilever (1) according to one of the preceding claims, wherein the thickness of the base body (2) is 5 pm to 20 pm and the thickness of the beam element (3) is 100 nm to 500 nm. 9. Cantilever (1) according to one of the preceding claims, wherein at least one dimension of the base body (2) and a corresponding dimension of the beam element (3) have a ratio of greater than 5, preferably greater than 50, particularly preferably greater than 100, wherein the dimension can be length, width, or height. 10. Cantilever (1) according to one of the preceding claims, wherein the base body (2) is plate-shaped and in the oscillating state nodal lines (10, 11) are oriented essentially parallel and / or orthogonally to the length of the base body (2). 11. Atomic force microscope, comprising a cantilever (1) according to one of the preceding claims, wherein the atomic force microscope is set up to measure a sample material, which preferably comprises an organic material, the cantilever (1) preferably scanning the sample in a grid shape during the measurement , wherein during the scanning of the sample material, the measuring tip (8) of the bar element (3) with a Surface of the sample material interacts, whereby during the measurement the Base body (2) of the cantilever (1) is excited to vibrate in at least one resonance frequency of the base body, which is a resonance frequency of a predetermined corresponds to two-dimensional oscillation mode, the oscillating base body having a first dynamic spring constant, and the beam element (3) being set in motion by the oscillation of the base body (2), the moving The beam element (3) has a second dynamic spring constant, the base body (2) oscillating at a resonance frequency and the moving beam element (3) forming a coupled oscillating system. 12. Atomic force microscope according to claim 11, wherein the bar element (3) of the canula (1) has a piezoelectric layer, wherein the change in movement of the bar element (3) due to the interaction of the measuring tip (8) with the Sample material the piezoelectric layer can generate a measurement signal. 13. Atomic force microscope according to claim 11, wherein the change in movement of the Bar element (3) due to the interaction of the measuring tip (8) with the Sample material can be measured optically, preferably with a laser beam focused on the beam element (3). 14. Use of a cantilever (1) according to one of claims 1 to 10 for measuring a sample material, the cantilever (1) and the sample material being immersed, preferably completely, in a liquid during the measurement. 15. Use of an atomic force microscope according to one of claims 11 to 13 for measuring a sample material, the sample material being immersed, preferably completely, in a liquid during the measurement.