EP2409165A1

EP2409165A1 - Scanning force microscope

Info

Publication number: EP2409165A1
Application number: EP09775848A
Authority: EP
Inventors: Henning Heuer; André STRIEGLER; Jörg Opitz; Malgorzata KOPYCINSKA-MÜLLER; Sascha Naumann
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV; Technische Universitaet Dresden
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV; Technische Universitaet Dresden
Priority date: 2009-03-16
Filing date: 2009-03-16
Publication date: 2012-01-25
Also published as: WO2010105584A1

Abstract

The invention relates to a scanning force microscope, which can be used for different examinations of the surfaces of samples. The aim of the invention is to provide a scanning force microscope, by which surface regions of samples can be detected, which are oriented at a steeply inclined angle with respect to the central longitudinal axis of a cantilever of the scanning force microscope. In the scanning force microscope according to the invention, at least one sensor tip is disposed on the cantilever, the sensitive region of which is disposed at a distance from the cantilever. To this end, the cantilever which is held on a face and can be prompted to oscillate using at least one actuator can be moved with the sensor tip and sample relative to each other along at least one axis. The sensitive region of a sensor tip and/or the base of a sensor tip are disposed on the cantilever with a distance and next to the center longitudinal axis of the cantilever. Either alone or additionally, the sensor tip can be angled or curved at an angle of < 90º with respect to the center longitudinal axis of the cantilever.

Description

Atomic Force Microscope

The invention relates to an atomic force microscope according to the preamble of claim 1, which can be used for investigations on surfaces of samples.

With Atomic Force Microscopes (AFM), which are used in the scanning probe microscopy, various investigations can be performed on samples. For example, in detections made by Scanning Probe Microscopy (SPM), quantitative topographies of sample surfaces with a lateral resolution at ambient pressure conditions in the range of a few nanometers and in a vertical aligned axis with an accuracy in the range of 0.1 nm be determined. In this case, a relative movement of a sensor tip (Tip), which is also referred to collectively as a "sample" on a cantilever (cantilever), and a sample (sample) in at least one axial direction take place. and / or y-direction. The vertically aligned axis is then the z-axis.

However, if depressions or elevations are present on a sample surface which are delimited by strongly inclined surface areas, these can not be detected, or at least with insufficient accuracy. This is the case, in particular, if the flank angle of such inclined surface areas is greater than the cone angle of a sensor tip of a conventional atomic force microscope, which is usually approximately 77 °.

This is also not possible when using a scanning tunneling microscope (STM).

However, such highly inclined surface areas are present in nano- and microelectronic structures (for example, in the manufacture of silicon-integrated circuits) on samples to be detected. You will u.a. in the commonly used etching methods, e.g. in anisotropic etching in which surfaces are undercut, formed. For example, the topography and roughness of nearly vertically aligned surfaces or edges of a locally etched photoresist layer on a photomask have a considerable influence on subsequent processes or process steps, since the topography and roughness of an etching mask on the target structure to be formed on a substrate again place.

In conventional atomic force microscopes, a sensor tip is aligned with a cantilever so that its tip is at an angle of approximately 90 ° with respect to the central longitudinal axis in the direction (i.d.R.

Axis) on the surface of a sample to be detected. be aligned and in the form of a pyramid, which tapers conically in the direction of the surface to be detected, formed and arranged in the plane of the central longitudinal axis of the cantilever. Frequently, sensor tips made of the same material from which the spring bar is formed, such as monocrystalline silicon, are used.

Sensor bar cantilevers may additionally be provided with suitable actuators, e.g. Piezoactuators in

Oscillation are added, this should preferably be done at a resonant frequency.

Investigations on sample surfaces are carried out in different forms, in principle, in three different modes, the contact, the non-contact and the intermittent mode.

The object of the invention is to provide an atomic force microscope with which surface regions of samples can be detected which are oriented at a steeply inclined angle with respect to the central longitudinal axis of a cantilever of the atomic force microscope.

According to the invention this object is achieved with a scanning force microscope having the features of claim 1. Advantageous embodiments and further developments of the invention can be realized with features designated in subordinate claims.

Essential elements known from the prior art may be present on an atomic force microscope according to the invention. These are, for example, the control and evaluation electronics, elements for the execution of movements and the measuring technique. Preferably, an optical measurement of movements of the cantilever, in which a reflected from the cantilever laser beam is directed to an optical detector. Preference is given to spatially resolved detecting optical detectors, such as a four-quadrant photodiode.

The cantilever may be vibrated with at least one piezo actuator when detection is to be performed in a dynamic mode.

According to the invention, a modified arrangement and / or design of at least one sensor tip present on a cantilever is realized with an atomic force microscope in order to be able to detect the steeply inclined surface areas of samples. These surface areas may be oriented at an angle of 90 ° ± 20 ° with respect to a generally horizontally oriented plane (xy plane). For this purpose, the sensitive region of a sensor tip is arranged at a distance to and in addition to the central longitudinal axis of the cantilever. This can also be achieved with sensor tips whose base point is arranged at a distance next to the central longitudinal axis of the cantilever. A combination of sensitive area and foot point arranged next to the central longitudinal axis can also be realized. Alone or in addition to this, a sensor tip can be angled or curved at an angle <90 ° with respect to the central longitudinal axis of the cantilever. The sensitive area of a sensor tip is that part of the sensor tip which points in the direction of the surface to be detected. As with known sensor tips it has the smallest cross-section, since here too a conical taper of the sensor tip is realized in the direction of the surface to be detected.

In an atomic force microscope according to the invention, a plurality of sensor tips can also be present on a spring bar. In this case, two sensor tips may be present on two opposite sides of the cantilever with respect to its central longitudinal axis. Two or three sensor tips can be aligned at angular intervals of 90 ° to each other.

It may be a sensor tip arranged parallel to the central longitudinal axis of the cantilever, the sensitive area but be aligned at an angle less than 90 ° with respect to the longitudinal axis of the cantilever and the unfixed end face of the cantilever. Such a sensor tip is then aligned in the direction of the unfixed end face of the cantilever and can also tower over this with the sensitive area.

In addition, at least one second sensor tip can be present on the spring bar, the sensitive area of which is arranged at a distance and next to the central longitudinal axis of the cantilever. The sensitive area points in a lateral direction next to the spring bar.

Sensor tips can also be dimensioned and designed so that the sensitive area projects beyond an outer edge of the cantilever in the unfixed area of the cantilever. This should be the case if an obliquely inclined surface area is to be detected whose length in this axial direction is greater than the distance between the sensitive area Area of the sensor tip and the spring bar is.

The at least one sensor tip may be aligned or angled with respect to the central longitudinal axis of the cantilever at an obliquely inclined angle.

An orientation of the sensitive area of a sensor tip is preferably at an angle of almost 90 ° with respect to the orientation of the surface area of the respective sample to be detected.

The sensor tip (s) should be able to approach a surface to be detected. This can be achieved by a vertical deflection of the cantilever. With the detection of the mechanical restoring forces of the cantilever, a detection of the topography of the surface area of the sample can be achieved. The sensor tip should therefore be able to absorb forces acting in the vertical direction without being damaged. It should have sufficient rigidity so that it deforms only negligibly during detection itself.

The frequency with which the spring bar with sensor tip (s) are vibrated, can lead to a deformation of the cantilever, which can bend or twist. This should be done in a targeted manner in order to take into account the deformation of the cantilever during the evaluation. However, the sensor tip should not deform or twist.

Sensor tips which can be used in the invention can be angled as an obliquely inclined or tilted pyramid, the tip of which forms the sensitive area, as a one-sidedly bent cone Cone or as a fibrous structure which is bent or inclined in one direction, be formed.

The production can be carried out in such a way that they are produced analogously to conventional standard sensor tips in silicon technology, taking into account the new orientation of the sensitive region.

But it can also be applied a fibrous structure. This can be at least one carbon nanotube. One or more of such tubes can be formed by a suitable method, which can be done by a pre-formed in the spring bar opening. An aperture (for example bore) can be formed in a correspondingly inclined angle by the spring bar, with which the orientation of such a sensor tip can be specified.

Extra manufactured sensor tips made of a wide variety of materials, including metals, can also be attached to a spring bar. This can preferably be done cohesively.

The preparation can also be done by silicon etching with subsequent material removal. Thus, a spring bar made of silicon can be etched with an approach for a sensor tip so that a connected to its smallest cross-section with the spring bar truncated pyramid has been formed on the spring bar on which subsequently a targeted shaping material removal takes place. The material removal can be carried out, for example, with an ion beam, which is preferably focused. The spring bar can assume a cross-sectional shape rotated through 180 °. men.

Sensor tips can also be arranged on a spring bar asymmetrically to its central longitudinal axis. In this case, the base point of the sensor tip, are connected to the sensor tip and cantilever with each other or merge into each other, be arranged on one side next to the central longitudinal axis. In such an embodiment, it may be advantageous, in particular to provide a corresponding balancing mass on the spring bar, with a mass balance on the opposite side, seen from the central longitudinal axis of the cantilever, can be reached. For this purpose, however, a second oppositely inclined sensor tip may also be present on a spring bar.

However, the use of a balancing mass can also be favorable in the case of an asymmetrical mass distribution in relation to the central longitudinal axis of the cantilever.

With such an embodiment, two lateral opposite wall regions of a depression can be detected by means of a spring bar and the two sensor tips, without requiring replacement or re-clamping of the sample or the cantilever.

In addition to the determination of surface topographies, further physical properties of samples can be determined. Thus, elastic properties, hardness, electrical and magnetic properties can be determined. Elastic properties can be determined by suitable excitation of contact resonance frequencies (bending, torsional vibrations). It is also possible to apply the known principles of Atomic Force Acoustic Microscopy (AFAM), Ultrasonic Force Microscopy (UFM) or Ultrasonic Atomic Force Microscopy (UAFM).

The invention will be explained in more detail by way of example in the following.

Showing:

Figure I A - D four examples of usable on a cantilever sensor tips in a sectional view;

FIG. 2 three further examples of sensor tips in a sectional view;

FIG. 3 in stages, the procedure for approaching and detecting;

FIG. 4 shows a detection in contact mode and

FIG. 5 shows a detection in an intermittent mode.

In Figure 1, four examples of a cantilever sensor tip unit are shown in sectional views. It is clear from the cross-sectional representations that the sensitive regions 2 'of the sensor tips 2 are arranged offset to one side with respect to the central longitudinal axis of the cantilever 1. The central longitudinal axis of the spring bar 1 is illustrated with the vertically aligned line, in the illustrated cross section of the cantilever 1. Example A shows a pyramid tilted diagonally to the side. In examples A, C and D, the sensor pointed angled and curved in Example B.

Example B may be formed with a fibrous structure which, as explained in the general part of the description, may be formed with at least one carbon nanotube.

A starting from the prior art modified sensor tip 2 is shown with Example C. Here, the sensor tip 2 made of silicon, as well as the cantilever 1 is formed. Only the part of the sensor tip 2 facing the sensitive surface 2 'in the direction of the detecting surface is angled accordingly.

The sensor tip 2 of Example D was brought into the form shown by material removal with a focused ion beam (FIB technology). In this case, prior to processing with the ion beam by an etching process from a semi-finished silicon product of the spring bar 1 was produced with a truncated cone formed thereon, which is indicated by the dashed line. The finished contour of the sensor tip 2 is illustrated by a solid line after the material removal.

In an unillustrated form but other orientations of the sensor tips 2 on the cantilever 1 are possible. Thus, sensor tips 2 can also be aligned at an angle of 90 ° on the spring bar 1, so that the sensitive area 2 'should then be pointed out of the plane of the drawing.

FIG. 2 shows three further examples of spring beam sensor tip units B1 to B3. Again, cross-sectional views were chosen. At the Example Bl it can be seen that the sensitive area 2 'of the sensor tip 2, the cantilever 1 protrudes on one side beyond the edge, but is mounted symmetrically with respect to the central longitudinal axis of the cantilever 1 at this. The examples B2 and B3 show correspondingly asymmetrical arrangements of sensor tips 2 on cantilever 1, in which the bases of the sensor tips 2 are arranged offset to the central longitudinal axis of the cantilever 1 at a distance from the longitudinal axis. They differ by the inclination angle and the dimensioning of the sensor tips 2 and also by the orientation of the cantilever 1, in which once the narrower side is arranged vertically below at B2 and once vertically above at B3.

For a detection, it is possible to proceed in such a way that, as usual, first a movement is carried out for approaching a sample 3 of the spring beams 1 with sensor tip 2. The movement takes place in the z-axis direction. The contact of the sensor tip 2 with the sample 3 can be detected by the vertical deflection of the cantilever 1 as a result of mechanical restoring forces in a horizontal region of the sample 3. This is illustrated by the steps 1 and 2 shown in FIG. Subsequently, sensor tip 2 with spring bar 1 is moved horizontally in the x-axis direction to a surface area of sample 3 which is vertically aligned here (see step 3) until the sensitive area 2 'of sensor tip 2 touches this surface area or has been recognized by the atomic force microscope , With simultaneous movement in the z-axis direction - step 4, the determination can be made on the vertically aligned surface area of the sample 3 in modes known in atomic force microscopes. When the contact mode is used, a detectable torsional force acts upon impact of the sensitive area 2 'of the sensor tip 2 on the surface of the surface area to be detected on the spring bar 1.

It is also used in e.g. From DE 696 29 475 T2 known "tapping mode" method in which the spring beam 1 oscillates about its central longitudinal axis, a reduction of the vertical oscillation amplitude can be detected and evaluated.

It is also often useful to carry out the determination on the same surface area several times but parallel to one another in order to avoid errors, e.g. be able to recognize and take into account by adhering to a sample particles.

A region of the surface region of the sample 3 to be detected can thus consist of several detected lines. It is approximately in the y-z plane, which corresponds to a vertical surface / plane. The surface can be scanned in this way.

The spring constant or spring characteristic of the cantilever 1 influences the measuring accuracy and the contact force between the sensor tip 2 and the surface of the sample 3. The stiffness in the vertical bending direction (z-axis) should be higher than that in the lateral direction. A possibly occurring vertical deflection of the cantilever 1 can be detected during the measurement and compensated in a suitable form during the evaluation.

For the measurement of a torsion of the cantilever 1, a spatially resolved measuring optical system should preferably be used. shear detector, such as a four-quadrant photodiode can be used.

The implementation of a modified contact mode is to be illustrated with FIG. During the detection, the sensor tip 2 is kept in permanent contact with the surface to be detected.

In a conventional procedure, a constant contact force between the contacting parts of the sensor tip 2 and the sample 3 is set by a constant held deflection of the cantilever 1. If a change in the height occurs during the movement of the sensor tip 2 over the surface area, a change in the contact force occurs, which can be compensated by a change in the position of the clamping of the cantilever 1. From this height tracking, a quantitative statement can be made regarding the surface topography of a surface to be detected in the case of several measurement runs to be performed.

In a detection, as can be carried out with the invention, but the torsion of the cantilever 1 should be kept constant about its central longitudinal axis. In this case, the contact forces acting between the sensor tip 2 and the surface of the sample 3 can be calculated from the respective torsion angles with the spring stiffness of the cantilever 1 about its central longitudinal axis.

A constant torsion can be observed during detection by changing the position of the clamping of the cantilever 1 in the horizontal x-axis direction, in which it is tracked. The course of the surface contour can thus be detected by determining the change in position of the clamping of the cantilever 1.

The course in a measurement run is shown in FIG. 4 with the dotted line.

In a dynamic measurement, the non-fixed part of the cantilever 1 is excited to torsional vibrations, as shown in Figure 5. The excitation of the torsional vibrations about the central longitudinal axis can be achieved with a pair of piezoactuators, which oscillate in opposite phase and are arranged laterally. In order to achieve high oscillation amplitudes, a constant excitation frequency should be selected which corresponds to an n-th order torsional resonance frequency of the freely oscillating cantilever 1, but is at least close to this frequency. A possibly existing influence of an asymmetrical mass distribution can be compensated for by a balancing mass to be attached correspondingly to the spring bar 1 or to be formed there.

Arrives in the oscillating state of the spring beam in the vicinity of a surface to be detected, in which case the sensitive area 2 'of the sensor tip 2 impinges on the surface at maximum deflection (maximum vibration), the amplitude of the torsional vibrations reduced by an adjustable factor and it is also the Position of the phase of the oscillation shifted. When the sensitive area 2 'of the sensor tip 2 strikes, no frictional forces occur, which is an advantage compared to operation in the contact mode.

A detected during detection reduced amplitude The torsional vibrations can be taken into account by changing the position of the clamping of the cantilever 1 in the x or y axis direction. There is a tracking according to the surface topography in the detected surface area. By evaluating the correspondingly changing positions of the clamping of the cantilever 1, the respective topography curve can be determined. This is shown by the dotted line in FIG.

During detection, several lines are successively traversed in a surface area. The detected surface area likewise lies at least approximately in the y-z plane (y-z plane = vertical area).

At the same time, an error image can be generated via the changing amplitude of the torsional vibrations, and with the detected phase shift, further information about the respective sample can be obtained. Et al elastic properties of the sample can be determined.

In addition to torsional vibrations, a horizontal movement of the sensor tip 2 in the y-axis direction can also be achieved by exciting bending vibrations. This movement allows the detection of steeply inclined surfaces, which are aligned approximately to the longitudinal axis of the cantilever 1, by tracking the oscillation amplitude. The cross-sectional geometry of the cantilever 1 should take into account whether torsional or bending vibrations are used.

When positioning the clamping of the spring beam 1 in space (x, y and z axis direction), probably in contact mode, as well as in dynamic mode piezo actuators are used, which have an effect on the mechanical forces acting between sensor tip 2 and sample 3 (x-axis direction) as well as during a scanning movement (y- and z-axis direction) achieve high accuracy. In general, conventionally trained atomic force microscopes, which are supplemented only with the invention, can meet these requirements. If mobility in a conventional atomic force microscope is not sufficiently great, in particular in the z-axis direction, a sample stage which can be positioned very accurately can be used. A positioning unit can take into account the required travel, the travel speed and the required positioning accuracy.

Claims

claims

1. Rasterkraftitiikroskop for investigations on surfaces of samples, with at least one arranged on a cantilever sensor tip, the sensitive area is arranged at a distance from the spring beam, while the held on a front side and with at least one actuator vibratable spring bar with Sensor tip (s) and sample are movable relative to each other along at least one axis and an optical measuring system is present; characterized in that the sensitive area (2 ') of a sensor tip (2) and / or the base of a sensor tip (2) on the spring bar (1) at a distance and adjacent to the central longitudinal axis of the cantilever (1) is arranged and / or the Sensor tip (s) (2) with respect to the central longitudinal axis of the cantilever (1) at an angle <90 ° angled or curved. 2. An atomic force microscope according to claim 1, characterized in that the sensitive area (2 ') projects beyond an outer edge of the cantilever (1) in the non-fixed area of the cantilever (1).

3. atomic force microscope according to claim 1 or 2, characterized in that the sensor tip (s)

(2) is oriented or angled with respect to the central longitudinal axis of the cantilever (1) at an obliquely inclined angle. At least one suitable for the excitation of vibrations actuator is present

11. Atomic force microscope according to one of the preceding claims, characterized in that the spring bar (2) and / or its clamping at least one for exciting torsional vibrations about the central longitudinal axis of the cantilever (1) suitable actuator is present.

12. Atomic force microscope according to one of the preceding claims, characterized in that there are two piezo actuators operable in antiphase.