US20110302676A1

US20110302676A1 - Method and Device for Examining a Sample with a Probe Microscope

Info

Publication number: US20110302676A1
Application number: US12/602,150
Authority: US
Inventors: Jacob Kerssemakers; Detlef Knebel
Original assignee: JPK Instruments AG
Current assignee: JPK Instruments AG
Priority date: 2007-05-31
Filing date: 2008-05-30
Publication date: 2011-12-08
Also published as: WO2008145109A1; WO2008145110A1; DE102007063066A1; WO2008145108A1; US8415613B2; DE102007063065A1; US20100251437A1

Abstract

The invention relates to a method for examining a sample by using probe microscopy, in particular scanning probe microscopy in which a sample is examined by way of a probe microscope with a multi-part measuring probe comprising a probe element and a guide clement guiding the probe element during the probe microscopy examination with the method furthermore comprising the following steps: capturing of noise measuring signals for the measuring probe in a non measuring configuration in which the probe clement is arranged separately from the guide element, capturing of measuring signals for the measuring probe in a measuring configuration in which the probe element is guided by the guide element, and analysing the measuring signals by at least partially assigning the measuring signals to the noise measuring signals. Further, the invention relates to a device for examining a sample with a probe microscope.

Description

The invention relates to a method and a device for examining a sample with a probe microscope, in particular a scanning probe microscope.

BACKGROUND OF THE INVENTION

Scanning Probe Microscopy (SPM) is a measuring and analysing technique by which a measuring probe is used to scan a sample of a medium to be examined by means of interaction between the measuring probe and sample which depends on the distance between the two, resulting in the production of a topographical representation of the sample. Additional measured data that can be obtained by this technique are the measurements of material constants or other sample parameters or characteristics. The most prominent devices using this technique are the Atomic Force Microscope (AFM) and the Scanning Tunneling Microscope (STM). Other such devices arc the Scanning Near Field Microscope (SNOM) and the Scanning Photon Force Microscope (SPhM) or Photon Force Microscope (PFM). One method of examination assigned to scanning probe microscopy is the measuring of force-distance curves, by which the measuring probe is essentially moved only in a vertical direction relative to the sample examined.
A further method used in probe microscopy, scanning probe microscopy being part of, involves examining the influence of an environment on a probe not being actively moved. In this case usually no scanning movement between sample and measuring probe takes place.
In scanning photon force microscopy, the measuring probe used for scanning the sample is designed as being a multi-part measuring probe. It comprises a particle element in the form of a small particle of a material the refractive index of which differs advantageously from the refractive index of the medium surrounding the particle. For examinations with a probe microscope, this multi-part measuring probe is formed by guiding the particle within an optical trap which is formed by focussing laser beams. The size of the particle element is often in the size of the wavelength of the laser light used for the optical trap which provides the field of force which guides the particle clement. However, it can be noticeably smaller or greater. Thus it is possible to move entire cells with the aid of such an optical trap which then form the particle element. It is also possible to capture a large number of particles by means of the field of force created by the optical trap. With the assistance of the optical trap which forms a guide clement of the measuring probe which guides the particle, the particle can be held in the vicinity of a focus due to the light gradients formed by the optical trap. Thus, the particle element can be guided to the location where the examination will take place. In scanning photon force microscopy the position of the measuring probe is determined by the position of the optical trap and the position of the measuring probe in the optical trap. To determine the position of the measuring probe in the optical trap, the influence of the particle on the optical rays of the optical trap is measured and analysed. Further information on the technology of scanning photon force microscopy can, for example, be found in DE 199 39 574 and U.S. Pat. No. 6,833,923.
A substantial disadvantage of known methods in the various probe microscopy technologies is that when capturing measuring or detection signals the measuring signals are often ambiguous. Quite frequently, there are two possible positions for each measuring signal. In a symmetrical field distribution, the signal from both positions can produce the same value. This causes problems particularly if on receiving a certain measuring signal one cannot be certain whether the measuring probe element is located in the desired measuring position or not. According to the present state of the art such verification is usually carried out by using a light-optical microscope.
SUMMARY OF THE INVENTION
The object of this invention is to present a method and a device for examining a sample with a probe microscope and in particularly with a scanning probe microscope where the captured measuring signals can be analysed with improved reliability.
According to the invention this objective is achieved by a method for examining a sample with a probe microscope, particularly a scanning probe microscope, in accordance with claim 1 and by a device for examining a sample with a probe microscope, in particular a scanning probe microscope according to the independent claim 10. Advantageous embodiments of the invention are subject of dependent sub-claims.
The invention comprises the idea of a method for the examining a sample by means of probe microscopy with a multi-part measuring probe formed by a probe element and a guide element guiding the probe element during the probe microscopy examination and the method additionally comprising the following steps: capture of noise measuring signals for the measuring probe in a non-measuring configuration, the probe element being separated from the guide element; capture of measuring signals for the measuring probe in a measuring configuration, the probe element being guided by the guide clement, and analysis of the measuring signals by at least partially assigning the measuring signals to the noise measuring signals.
Furthermore, the invention comprises the idea of a device for the examination of a sample with a probe microscope, in particular by a probe microscope with a multi-part measuring probe, said device comprising a probe element and a guide element guiding the probe element during probe microscopy examinations; said device being configured to examine a sample with a probe microscope; as well as a control unit coupling to the multi-part measuring probe and being configured to capture noise measuring signals for the measuring probe in an non-measuring configuration, the measuring probe element being separated from the guide element and also configured to capture measuring signals for the measuring probe in a measuring configuration wherein the probe element is guided by the guide element with the said control unit having also the task of analysing the signals by at least partially assigning the measuring signals to the noise measuring signals. In a preferred embodiment the probe microscope can be of the scanning probe microscope design.
For examining samples with a probe microscope, in particular when using scanning probe microscopes, it is suggested that in addition to measuring signals captured with the aid of a multi-part measuring probe in a measuring configuration, noise measuring signals are detected during the capture of which the probe element is kept separate from the guide element of the measuring probe. i.e. where the measuring probe element has been detached from the measuring probe. In the case of scanning photon microscopy this means for example that the particle or particles are not located in the optical trap and therefore do not interact with it. With the assistance of the noise measuring signals, additional measuring data assigned to the measuring signals are made available, which on their part can indicate different measuring parameters such that the measuring signals based on this additional information can be analysed with greater reliability. Prior art usually treats noise only as interference which must be avoided or at least suppressed as much as possible or eliminated from the measuring signals. In a number of measuring situations and phases the captured signals can he analysed as suggested here by taking into consideration the noise measuring signals. Part of such analysing procedures are not only the probe microscopy examination as such but also the downstream or upstream process phases such as approaching the measuring probe to the sample, the positioning of the measuring probe relative to the sample or the removal of the measuring probe from the sample. An embodiment can provide for storing the noise signals in electronic form in order to make them available for later measuring procedures. Generally speaking, the quality of the measuring method and optionally the analysing options are improved with the aid of the captured noise measuring signals.
A preferred further development of the invention provides that the the noise measuring signals are captured as rms (root means square) signals.
A convenient embodiment of the invention can provide that analysing the measuring signals contains a step for deriving probe microscopy measuring results by capturing probe microscopy measuring signals during the capture of the measuring signals. The probe microscopy measuring signals supply the measuring information by scanning the sample to he examined. With the assistance of the combined analysing which includes the noise measuring signals, the unambiguity of certain measuring results can, for instance, be improved or established in the first place.
An advantageous embodiment of the invention includes a step for deriving adjustment information For analysing the measuring signals by capturing adjustment measuring signals during the capture of measuring signals. Adjustment measuring signals deal for instance with the process of adjusting certain measuring positions of the measuring probe relative to the sample or other adjustment processes, in particular before and after the scanning of the sample as such with the measuring probe. This includes among other things the approach of the measuring probe to the sample to be examined. With the assistance of the noise measuring signals it is possible to ascertain better in which relative position and at what distance the measuring probe and in particular the measuring probe clement is positioned in relation to the sample.
A preferred development of the invention provides that the analysis of the measuring signals contains a step of deriving measuring probe checking information by capturing measuring probe checking signals during the capture of measuring signals. Measuring probe checking signals characterise one or several parameters which indicate the state of the measuring probe. One such parameter indicates the correct guidance of the measuring probe through the associated displacement facility for instance. Provision can also he made to ascertain whether the measuring probe works properly with the aid of measuring probe checking signals. Thus probe wear can be detected which would necessitate the replacement of the measuring probe or parts of it. In connection with the embodiment of the measuring probe for scanning photon microscopy the provision can be made to ascertain whether one or more particles have been captured in the optical trap on the basis of the measuring probe checking signals. This embodiment can also include a method for finding the associated measuring probe element with the help of the measuring probe checking signals.
A preferred embodiment of the invention can provide for analysing the measuring signals to include a step of deriving positional information by capturing position signals during the capture of the measuring signals. The position measuring signals can contain information as to whether or not the measuring probe element is in contact with the sample to be examined. A combined analysis of the signals including the noise signals facilitates a conclusion to that.
A further development of the invention can make provision for analysing the measuring signals to contain a step for standardising the measuring signals. For the purpose of standardising measuring signals, different relationships between variables can he used for the different embodiments. In one embodiment, noise signals are preferred for this purpose. Thus, for example, the quotient K as stated below can be worked out in the analysing process.
K−(σ(t)−σ₀)/(σ₁−σ₀).
where σ₀represents the noise signals and σ₁the measuring signals which were captured by the measuring configuration for the measuring probe where the measuring probe was positioned at a sufficient distance from the sample to be examined so that no interaction would take place. Finally σ(t) designates measuring signals from probe microscopy examinations of the sample, in particular scanning probe microscopy examinations, The mean of K equals 1 if the measuring probe is far removed from a position where interaction would take place. The mean of K equals 0 after the measuring probe has left a detection volume. By detection volume we mean the volume in which the measuring probe still exerts a measurable influence on the measuring signal so that a relative scattering of the light to be measured can occur for example. Standardisation in this or another manner is meaningful for several reasons. In one respect it enables a conclusion about a useful setpoint for a control system since σ₁as well as σ₀are largely dependent on the measuring probe element used and the environment, for instance, i.e. in the case of scanning photon microscopy therefore on the particle used. Even the inversion of K can be taken into consideration.
The standardised values can also he used for checking purposes, to find out whether the measuring probe, in particular the probe element is located in a detection volume. If K equals 0 and the control system is not able to reach a set point which differs from 0 then the measuring probe is not located in the detection volume.
A preferred further development of the invention provides that the capturing of the measuring signals comprises a step for the capturing of optical force measuring signals by the guide element which constitutes a field of force guiding a particle probe which forms the measuring probe element in the measuring configuration. In the case of the scanning probe microscope, the field of force has the form of an optical trap in which a particle measuring probe is guided with one or several particles.
The measuring information obtained in the previously described various embodiments can also he used in different ways for an automatic control of the displacement or positioning of the measuring probe, in particular relative to the measuring probe. Current measuring signals, whether in their pure or standardised form can be compared with setpoints in order to produce control signals as a function of the comparison which would in particular serve to displace the measuring probe in its complete form or incomplete embodiment. To this end the usual displacement devices are used which as such are known to be used for scanning probe microscopy device of different types. Examples are the so-called piezo elements winch allow very precise positioning of individual elements of the measuring device in relation to each other, particularly in the case of relative displacement between sample and measuring probe.

DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

Below the invention is explained in more detail by means of example embodiments and reference to the figures of a drawing.
FIGS. 1A to 1C show in a schematic illustration a multi-part arrangement of a measuring probe of a scanning photon microscope and a sample to be examined;
FIG. 2 shows a magnified illustration of the multi-part measuring probe as shown in FIG. 1A to 1C, which, in addition, illustrates the shape of a measuring signal and a noise measuring signal, and
FIGS. 3A and 3B show the multi-part measuring probe of FIG. 1A to 1C with and without a probe particle.
FIGS. 1A to 1C show the schematic illustration of an arrangement with a multi-part measuring probe of a scanning photon microscope and a sample 20 to be examined. The illustration shows a laser focus which in this example is shown as Gaussian beam 1 together with a probe particle 10 which is trapped therein. The laser focus is still some distance from the sample 20 to be examined which is transparent to the laser light. The probe particle 10 is located in the mean position in the centre of the laser focus 11 (cf. FIG. 1A).
In FIG. 1B the laser focus has been brought so close to the sample to be examined 20 that the probe particle 10 assumes a centre position 12 which deviates from the centre of the laser focus 11.
In FIG. 1C the influence of the laser focus on the measuring probe particle 10 has finally disappeared since the probe particle 10 cannot penetrate into the sample 20 to be examined. The result is a centre position 13 which deviates considerably from the centre of the laser focus 11 and which can vary constantly in the case of free diffusion of the measuring probe particle 10.
FIG. 1A to 1C show by way of example the measuring probe getting closer with laser focus and probe particle 10 in the lateral direction. In the same way the approach can also be from any other direction in space, in particular in the vertical direction.
FIG. 2 shows an enlarged view of the multi-part measuring probe of FIG. 1A to 1C where in addition the curve of a measuring signal 60 and a combination of the measuring signal and a noise signal 70, for example quotient K, are shown. Below the combination 70 is also being referred to as optimised measuring signal.
Within a radius 30, a force acting on the probe particle 10 is essentially linear. For illustration purposes the lower part of FIG. 2 shows a sectional view along line 40. In a coordinate system 50, 51 curve 60 plots the typical characteristics of the measuring signal as the probe particle 10 passes through the laser focus. The shown characteristics occur, for instance, when the measuring probe particle 10 cannot hollow the centre of the laser focus because of an interaction taking place with sample 20. The measuring signal 60 shown can also be interpreted in terms of force characteristics.
Outside a radius 31 the measuring signal 60 decreases. Dotted lines 41 show that the radius 31 matches the maxima of the measuring signal curve 60. At the point 01 origin in the co-ordinate system 50, 51, which is identical with the centre of the laser focus and which is also indicated by dotted line 42, the value of the measuring signal is just 0 V.
An offset can be superimposed on the measuring signal 60 which, however, does not change the characteristics of the curve. What is problematic now for the interpretation of the measuring signals in particular is that in the case of probe particle 10 being in position 80, the curve of the measuring signal 60 gives value 61 but also value 62 the latter also corresponding to another position 81. It is therefore not possible to infer from the value of the measuring signal 60 the actual position of the probe particle 10 in the laser focus. If, for example the measuring probe approaches sample 20 as shown in FIG. 1, this can cause problems. If a measuring probe positioning control system (not shown) were to assume a wrong position, the corrective action would be carried out in the wrong direction.
FIG. 2 in its lower section furthermore shows the shape of the optimised measuring signal 70 which was obtained for the measuring probe in FIG. 1A to 1C without the probe particle 10 being located in the optical trap. At least in the vicinity of the centre the shape or the optimised measurement signal 70 shows at least an unambiguous location in its positive and negative phase. Therefore for position 80 there is only one value 71 On the positive axis. This information is sufficient if it is clear from the movement or the measuring probe on which side an interference is to be expected. The analysis of the shape of the measuring signal 60 and the shape of the optimised measuring signal 70 gives an unambiguous indication regarding the position of the measuring probe.
FIGS. 3A and 3B show the multi-part measuring probe from FIG. 1A to 1C with and without probe particle.
For standardisation purposes of the measuring signals different relationships between variables can be used for the different embodiments. For one embodiment the optimised measuring signal 70 is used. As an example, the following quotient K can be calculated:
K=(σ(t)−σ₀)/(σ₁−σ₀),
where σ₀represents noise measuring signals and σ₁measuring signals 60 which were captured by the measuring configuration for the measuring probe with the measuring probe being located at a sufficiently large distance from the sample 20 to be examined so that no interaction would take place between the two. Finally σ(t) stands for measuring signals generated during probe microscopy examination of the sample 20, in particular during a scanning probe microscopy examination. The mean of K equals 1 if the measuring probe is located at a considerable distance away from where interaction would take place. The mean of K equals 0 if the measuring probe has left a detection volume. Standardisation in this or another way makes sense for a number of reasons. Because of the unambiguity it makes it possible to derive a set point for a control system, for example for the approach of the measuring probe to the sample. It is also possible to use the inverted value of K.
If a measuring signal is registered and its noise determined, for example as standard deviation, this would then represent σ₁. In FIG. 3B the measuring probe particle 10 is removed from the laser focus and the noise of the measuring signal results in σ₀. Preferably both measurements should he carried out in the same location which should be kept free from interferences as much as possible. Examples of interferences can be particles flying past.
The features of the invention disclosed in the above description, the claims and the drawing can be significant individually as well as in any combination in the realisation of the invention in its various embodiments.

Claims

1. A method for examining a sample by using probe microscopy, in particular scanning probe microscopy in which a sample is examined by way of a probe microscope with a multi-part measuring probe comprising a probe element and a guide element guiding the probe element during the probe microscopy examination with the method furthermore comprising the following steps:

capturing of noise measuring signals for the measuring probe in a non measuring configuration in which the probe element is arranged separately from the guide element,

capturing of measuring signals for the measuring probe in a measuring configuration in which the probe element is guided by the guide element, and

analysing the measuring signals by at least partially assigning the measuring signals to the noise measuring signals.

2. The method according to claim 1, wherein the noise measuring signals are captured as rms signals.

3. The method according to claim 1, wherein analysing the measuring signals comprises a step for deriving probe microscopy measuring results by capturing probe microscopy measuring signals during the capture of the measuring signals.

4. The method according to claim 1, wherein the analysing method of the measuring signals comprises a step for deriving adjustment information by capturing adjustment measuring signals during the capture of the measuring signals.

5. The method according to claim 1, wherein analysing the measuring signals comprises a step for deriving probe checking information by capturing probe checking measuring signals during the capture of the measuring signals.

6. The method according to claim 1, wherein analysing the measuring signals comprises a step for deriving position information by capturing position measuring signals during the capture of the measuring signals.

7. The method according to claim 1, wherein analysing the measuring signals comprises a step for the standardisation of he measuring signals.

8. The method according to claim 1, wherein the capturing of the measuring signals comprises a step for the capturing of optical force measuring signals by the guide element which constitutes a field of force guiding a particle measuring probe which forms the probe element in the measuring configuration.

9. The method according to claim 1, wherein the sample is examined by scanning probe microscopy.

10. A device for probe microscopy examination, in particular by a probe microscope, of a sample with a multi-part measuring probe which is formed with a probe element and a guide element providing guidance during probe microscopy examination and configured to examine a sample by way of a probe microscope, and a control unit coupled to the multi-part measuring probe being configured:

to capture noise measuring signals for the measuring probe in a non-measuring configuration wherein the probe element is arranged separately from the guide element,

to capture measuring signals for the measuring probe in a measuring configuration in which the probe element is guided by the guide element, and

to analyse the measuring signals by at least partially assigning the measuring signals to the noise measuring signals.

11. Device according to claim 10, wherein the control unit is further configured to capture the noise measuring signals as rms signals.

12. Device according to claim 10, wherein the control unit is further configured to derive scanning probe microscopy measuring results during analysing the measuring signals by capturing probe microscopy measuring signals during the capture of the measuring signals.

13. Device according to claim 10, wherein the control unit is further configured to derive information for adjustment during analysing the measuring signals by capturing adjustment signals during the capture of the measuring signals.

14. Device according to claim 10, wherein the control unit is further configured to derive probe checking information during analysing the measuring signals by capturing probe checking measuring signals during the capture of the measuring signals.

15. Device according to claim 10, wherein the control unit is further configured to derive position information during analysing the measuring signals by capturing position measuring signals during the capture of the measuring signals.

16. Device according to claim 10, wherein the control unit is further configured to standardise the measuring signals during analysing.

17. Device according claim 10, wherein the multi-part measuring probe is of the optical force microscope measuring probe type where a particle probe which forms the measuring probe element is guided by a field of force which forms the guide element.