JP2008122091A - Method for evaluating cantilever resonance characteristics - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for readily acquiring the resonance characteristics of a cantilever of an atomic force microscope. <P>SOLUTION: When observing a sample 2 in a dynamic mode of the atomic force microscope, (1) an exciting element is driven by a combined electrical signal acquired, by adding an additional electrical signal having a plurality of frequency components near the frequency component of the main component of an electrical signal to a position control electrical signal having a single frequency component as the main component, for controlling a cantilever 5 position; vibration of the cantilever 5 is excited; and (2) vibration of the cantilever 5 is detected by a vibration detecting element and converted into a detection signal, to thereby acquire a frequency distribution of the detection signal; and (3) the resonance characteristic of the cantilever 5 is derived from the acquired frequency distribution. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention belongs to a scanning probe microscope (SPM), and relates to an atomic force microscope (AFM), and more particularly, to a method for evaluating a cantilever resonance characteristic of an AFM used in a dynamic mode. .

  The scanning probe microscope is classified into a scanning tunneling microscope (STM) and an atomic force microscope. Atomic force microscopes are widely used because they can be observed at the atomic size by stroking the surface of an insulator sample or the surface of a sample in a liquid using a probe. The observation probe is fixed to the tip of the cantilever, and the movement of the probe is detected from the movement of the cantilever. The form of observation is classified into a static mode and a dynamic mode, or an intermittent contact mode, depending on the operation of the cantilever. Moreover, it is classified into a contact (contact) mode in which the sample and the probe are in contact and a non-contact (non-contact) mode in which the sample and the probe are not in contact.

  In addition, as a special method of use, there is an example in which local elasticity is detected using an ultrasonic microscope by using an AFM probe used in a dynamic mode in contact with a sample. The present invention can also be used for such applications.

  For example, in the static mode, no special vibration is given to the cantilever. The distance between the sample and the probe is controlled using a force measured from the deflection of the cantilever. The relationship between the force and the distance between the probe and the sample can be measured by obtaining a graph (force curve) of the relationship between the distance between the probe and the sample and the force.

  Imaging in AFM, when used in the static mode, detects and maps cantilever deflection caused by the atomic or intermolecular force that the probe receives from the surface of the sample. Usually, the distance between the probe and the sample is controlled so that the deflection of the cantilever is constant, and the control signal is mapped and imaged.

  On the other hand, in the dynamic mode, a minute vibration is given to the cantilever. This vibration is used to control the distance between the sample and the probe to be constant while the probe is brought close to the sample or in intermittent contact. More specifically, the vibration of the probe is detected and used as a feedback signal, thereby controlling the vibration amplitude and resonance frequency of the cantilever to be constant. For example, in the dynamic mode in which the control of the cantilever amplitude is constant, the distance between the probe and the sample and the interaction (resonance frequency and resonance constant) are obtained by obtaining the resonance characteristics in a state where the probe and the sample interact. It becomes possible to obtain the relationship. By mapping these results, the surface state can be imaged.

More specifically, it is as follows. In other words, when used in the dynamic mode, the cantilever is vibrated finely and the vibration amplitude is observed, but at that time, the cantilever is excited near its natural vibration frequency to maintain stable vibration near the natural vibration. Can be made. Generally, it is known that the natural frequency of the cantilever shifts to the low frequency side or the high frequency side when the probe receives an external force such as attractive force or repulsive force. A change in amplitude that occurs because the difference between the excitation frequency and the natural vibration frequency changes due to the deviation of the natural vibration is detected and used. In other words, the feedback circuit is used to control the distance between the probe and the sample so that the deviation is offset, and the control signal or the cantilever position information (up and down on the Z axis) is imaged. For the feedback signal of this feedback circuit, the movement of the resonance center point, the change in the vibration amplitude of the cantilever, or the phase value near the resonance center point is used.

  For example, FIG. 6A relates to frequency control for controlling the frequency of the resonance center point. A resonance frequency difference δf is generated, as shown by a resonance characteristic a when the probe is separated from the sample to some extent and a resonance characteristic b when the probe and the sample are further closer. When the distance between the probe and the sample is controlled so that this difference δf does not occur, the control signal becomes a signal related to the unevenness of the sample. In order to keep the resonance frequency constant in this way, (1) the entire image of the resonance characteristics or a part thereof is drawn, and the distance between the probe and the sample is controlled so that the center does not change, The frequency control method is to image the distribution of the control signal.

  FIG. 6B relates to amplitude control. It can be seen that when the resonance characteristics are shifted in the same manner as described above, when the excitation is performed at the frequency f near the center frequency of the resonance characteristics, the vibration amplitude increases when the resonance frequency is closer to the frequency f. The vibration amplitude is detected and used as a feedback signal. Similarly to the above, the distance between the probe and the sample is controlled so as to cancel the difference δf, and the control signal is used so that the vibration amplitude becomes constant. The amplitude control method is used as a signal relating to the unevenness of the above. In this case, the amplitude is controlled including the attenuation of the amplitude of the cantilever due to the interaction of the probe with the sample (Van der Waals force) or contact.

  An example of the AFM in the dynamic mode is shown in FIG. In this configuration, the light reflected by the cantilever is converted into an electric signal by a photoelectric converter having a two-part or four-part structure, and this is used for Z-axis control. The Z-axis control signal and the signal for scanning the XY plane are used to perform an imaging process to display a three-dimensional limit. In this Z-axis control, amplitude control and frequency control as described above are performed.

  In addition, a method using a phase change occurring at the resonance point is also conceivable. Comparing the excitation signal and the detection signal, it is known that the polarity of the phase changes before and after the resonance point, and the distance between the probe and the sample is controlled to cancel this phase shift value. Obviously, it may be imaged.

  Patent Document 1 discloses a sample observation method using an atomic force microscope. In this method, with the probe attached to the cantilever in contact with the sample, the cantilever is excited so that the cantilever is always in the resonance state, the amplitude of the cantilever in the resonance state is detected, and the detected amplitude The Q value of the cantilever is obtained based on the above.

  In Patent Document 2, an amplitude-modulated excitation signal is used, and the excitation signal is applied as an electric field between the sample and the probe or as a drive signal for the magnetic field generating means.

In the present invention, the resonance characteristic is directly measured, and the Q value is not indirectly determined as described above. In the disclosure of Patent Document 1, if the shape of the resonance characteristic is obtained in advance, it is possible to evaluate the Q value almost accurately, but if the fixed shape is not shown, an incorrect Q value is obtained. It will be. Further, in the disclosure of Patent Document 2, a conductive probe or a magnetic probe is used, and the material of the probe is limited.
JP 2002-277378 A JP 2004-294218 A

The resonance characteristics of the AFM cantilever are sharp when separated from the measurement sample, but the resonance characteristics when used as an AFM are known to be gentle depending on the sample. . This is to dissipate vibration energy from the probe to the sample in addition to the characteristics of the cantilever itself. Conversely, by evaluating the resonance characteristics of the cantilever, a distribution map regarding dissipation can be created, and knowledge about the surface structure or internal structure of the sample can be obtained.

  A circuit that excites the cantilever and detects the vibration can be regarded as an electric circuit having resonance characteristics. In general, the resonance characteristics of an electric circuit are measured using a network analyzer. However, in this case, it is problematic to measure the transfer characteristics by changing the frequency. That is, when applying a network analyzer to a cantilever, pay attention to its resonance frequency. For this reason, the transfer characteristic is measured by changing the frequency near the resonance frequency. As a matter of course, when the frequency for exciting the cantilever is changed, the vibration amplitude of the probe changes. For this reason, the distance between the sample and the probe changes, and the interaction changes. In order to measure the resonance characteristics while keeping the vibration amplitude of the probe constant, it is necessary to adjust the intensity of the signal applied in order to excite the vibration of the cantilever without changing the vibration amplitude. However, as the intensity of the excitation signal is increased, higher-order vibration modes are likely to occur, which is an obstacle to obtaining the frequency characteristics. As described above, it is difficult to obtain the resonance characteristics while keeping the contact state with the sample constant.

  When AFM is used in the non-contact dynamic mode, in order to keep the distance between the sample and the probe constant, the intensity of the excitation signal is made constant and the vibration amplitude of the probe is made constant. Adjust the distance between the probe and the probe (amplitude control method). Alternatively, the distance between the sample and the probe is adjusted so that the resonance frequency of the cantilever is constant (frequency control method). However, when measuring an inhomogeneous sample, the dissipative characteristics of the vibration energy of the cantilever are also inhomogeneous, so it is impossible to keep the distance between the sample and the probe constant with constant amplitude control. The image data may not correspond to the height change. In the frequency control method, the change in the resonance frequency is a result of the interaction between the probe and the sample. In this case, the distance between the sample and the probe cannot be kept constant. In order to correct this, it is necessary to obtain the relationship among the force, the amount of energy dissipation, and the distance between the sample and the probe. The present invention proposes a method for easily obtaining the resonance characteristics as basic data for such discussion.

  The AFM of the present invention can easily obtain the cantilever resonance characteristics when used in the dynamic mode.

First, the present invention is a cantilever resonance characteristic evaluation method, comprising a cantilever with a probe, an excitation element for exciting the vibration of the cantilever, an electric signal generator for driving the excitation element, and a cantilever An interatomic unit comprising: a vibration detection element that detects vibration; a spectrum analyzer that analyzes a frequency distribution of a signal from the vibration detection element; and an arithmetic unit that derives a value to be measured from the output of the spectrum analyzer. Assume that a sample is observed using a force microscope in dynamic mode. In observation using this atomic force microscope,
(1) An additional electric signal having a plurality of frequency components in the vicinity of the frequency component of the main component of the electric signal is added to the position control electric signal for controlling the position of the cantilever whose main component is a single frequency component. Exciting the vibration of the above cantilever with the excitation element that is driven by the added composite electrical signal,
(2) The vibration of the cantilever is detected by a vibration detecting element and converted into a detection signal,
Obtain the frequency distribution of the above detection signal,
(3) The resonance characteristic of the cantilever is derived from the obtained frequency distribution.

  Further, according to the present invention, the additional electric signal is in a frequency band including a frequency of a main component of the position control electric signal, and the amplitude distribution with respect to the frequency in the amplitude distribution with respect to the frequency of the additional signal has the spectrum described above. By dividing by the strength, the resonance characteristics of the cantilever are obtained.

  Further, the additional signal is a signal whose frequency periodically changes in a predetermined frequency region.

  In particular, the amplitude of the additional electrical signal is smaller than that of the position control electrical signal.

  The synthesized electric signal is obtained by modulating the position control electric signal with a predetermined modulation signal composed of a plurality of frequency components.

  The modulated signal is a noise signal having a predetermined density distribution.

Furthermore, the cantilever resonance characteristic evaluation method of the present invention is:
A cantilever with a probe, an excitation element for exciting the vibration of the cantilever, an electric signal generator for driving the excitation element, a vibration detection element for detecting the vibration of the cantilever, and from the vibration detection element Applicable when observing a sample using an atomic force microscope equipped with a spectrum analyzer that analyzes the frequency distribution of the signal and an arithmetic unit that derives a value to be measured from the output of the spectrum analyzer in a dynamic mode What to do
(1) Exciting the vibration of the cantilever with an excitation element driven by an electrical signal for excitation whose frequency changes in a frequency band of a predetermined width including the resonance frequency of the cantilever,
(2) The vibration of the cantilever is detected by a vibration detecting element and converted into an electric detection signal,
(3) Obtaining the frequency distribution of the above electric detection signal,
(4) From the obtained frequency distribution, find the resonance point of the cantilever,
(5) The distance between the cantilever and the observation sample is controlled using the frequency of the resonance point as feedback information.

  The excitation electrical signal may be any signal having a spectrum showing a plurality of peaks or a noise signal spectrum with a clear frequency distribution in the frequency band of the predetermined width.

  In the present invention, the above-described cantilever resonance characteristic evaluation method is performed for a plurality of predetermined observation points of the observation sample, and further displayed at points corresponding to the observation points.

  Embodiments of the present invention will be described below in detail with reference to the drawings. In the following description, devices having the same function or similar functions are denoted by the same reference numerals unless there is a special reason.

  FIG. 1 is a block diagram of an AFM for applying the present invention. The cantilever 5 having the probe 6 placed on the observation sample 2 reflects the light from the laser light source 3, the reflected light is photoelectrically converted by the quadrant detector 8, and the component extractor 9 The displacement component of the displacement to be extracted is extracted. The extracted displacement component is converted into a target control signal by the Z control signal discriminator 22. The control signal is compared with the target value set by the control target value setting unit 20 and the Z control signal output generation unit 16 to generate an error signal. This error signal is applied to the Z-axis drive unit 18 for the observation table 1 to keep the distance between the observation sample 2 and the tip of the probe 6 constant.

  The vibrator 7 is an excitation element and is a vibrator using an electrostrictive effect or a magnetostrictive effect. The cantilever 5 is excited by the vibration of the vibrator. A signal obtained by amplifying the signal from the Z control signal generator 14 by the signal synthesis / cantilever driving unit 12 is applied to the vibrator 7. The signal synthesis / cantilever drive unit 12 modulates the signal from the Z control signal generator 14 with the signal from the resonance characteristic measurement signal generator 13 or synthesizes them by combining them.

  The reason why the signal from the Z control signal generator 14 is applied to the Z control signal output generator 16 is to regenerate the Z control signal from the signal discriminated by the Z control signal discriminator 22. In other words, the Z control signal output generator 16 performs, for example, synchronous detection with the Z control signal in the amplitude control method. In the frequency control method, the Z control signal discriminator 22 finds the frequency position of the maximum amplitude of the spectrum distribution, compares it with the signal from the Z control signal generator 14, and the Z control signal output generator 16 generates an error. Generate a signal. As the spectrum distribution, an output from the spectrum analyzer 10 may be used.

  The output from the component extractor 9 also analyzes the spectrum in the frequency band in which the cantilever 5 is excited by the spectrum analyzer 10. Since this spectrum intensity depends on the spectrum intensity of the excitation signal included in the resonance characteristic measurement signal generator 13, the spectrum intensity is normalized by the spectrum intensity of the resonance characteristic measurement signal generator 13 and then displayed on the display / transmission unit 23. Or transmit.

  Since the Z-axis drive signal represents the surface unevenness of the observation sample 2, the image display unit 19 maps it on a two-dimensional plane corresponding to the signal from the XY scan unit 18.

  FIG. 2 is a block diagram showing a configuration particularly when amplitude control is performed. The configuration shown in FIG. 1 is specialized in that the signal from the Z control signal discriminator 22 is subjected to RMS-DC conversion to generate an error signal. In this case, the Z control signal discriminator 22 is a bandpass filter in the case of a simple configuration.

  FIG. 3 is a block diagram illustrating an example in which the respective resonators for resonance characteristic measurement and Z control are used. A signal from the resonance characteristic measurement signal generator 13 is amplified by the cantilever driving unit 12a and applied to the vibrator 7a. Similarly, the signal from the Z control signal generator 14 is amplified by the cantilever driving unit 12b and applied to the vibrator 7b. As a result, an electric circuit for multiplexing signals can be omitted, and a cantilever driving unit suitable for each can be used.

  When the sample surface is observed using the AFM, the Z control signal and the resonance characteristic measurement signal can be selected from a plurality of combinations. Examples of this are shown in FIGS. In the figure, the broken line indicates the resonance characteristic of the cantilever, and the triangle indicates the frequency of the Z control signal. A is for the amplitude control method, and B is for the frequency control method.

  In FIG. 4A, a signal from a sweep oscillator that sweeps between frequencies f1 and f2 is used as a resonance characteristic measurement signal, and a signal of frequency fb is used as a Z control signal in the frequency control method. These two signals are combined for the AFM of the configuration of FIGS. 1 and 2, but in the case of the AFM of FIG. 3, there is no need to electrically combine them. fa is a frequency at which the resonance characteristic of the cantilever peaks. This frequency sweep is preferably faster than the spectrum analyzer sweep. In the case of using the amplitude control method, a signal located on the shoulder of the resonance characteristic such as a signal of frequency fa is used instead of fb. This is for facilitating feedback to keep the distance between the probe and the observation sample constant. If nonlinear control such as integral control or differential control is possible, it is obvious that the peak position may be used.

  Further, in FIG. 4A, the resonance characteristic measurement signal is written almost symmetrically with respect to fb, similarly to the resonance characteristic of the cantilever. However, these need not be symmetrical, and FIG. As shown in b), it suffices if the signal band for measuring resonance characteristics covers the band of resonance characteristics of the cantilever.

  FIG. 4C shows an example in which the Z control signal is amplitude-modulated with noise of the bandwidth W1. In this case, the signal synthesis and synthesis performed by the cantilever drive unit 12 in FIG. 1 is modulation. It is also clear that modulation may be performed with a non-noise signal having a bandwidth W1.

  FIG. 4D shows an example in which the Z control signal is amplitude-modulated with a signal of the bandwidth W1. Also in this case, the signal synthesis and synthesis performed by the cantilever driving unit 12 in FIG. 1 is modulation. It is also clear that modulation may be performed with a noise signal having a bandwidth W1.

  In FIG. 4E, the Z control signal is pulse-modulated to generate a comb signal, or the comb control signal from the comb signal generator is used to generate the Z control signal and the resonance characteristic measurement signal. An example of simultaneously generating In this case, the signal of the frequency fb that is the strongest component signal located at the peak of the resonance characteristic of the cantilever is used for the frequency control method, and the signal of the frequency fa is used for the amplitude control method. Since the signal intensity for measuring the resonance characteristics is not uniform, the vibration spectrum of the cantilever is normalized with the signal intensity for measuring the resonance characteristics.

  FIG. 4 (f) shows an example in which the signal of the frequency fa, which is the strongest component signal located at the peak of the resonance characteristic of the cantilever, is used for the amplitude control method, and the signal of the frequency fb deviated from it is used for the frequency control method. . In this case, by analyzing and interpolating the spectrum in the frequency band where the cantilever 5 is excited by the spectrum analyzer 10, it is not necessary that fb matches any of the comb signals.

  FIG. 5G shows an example in which only the resonance characteristic measurement signal is applied by analyzing and interpolating the spectrum in the frequency band where the cantilever 5 is excited by the spectrum analyzer 10. By returning the intensity at the position of fa or fb in FIG. 5G, an amplitude control method and a frequency control method can be configured, respectively.

  FIG. 5 (h) shows an example in which one-side band modulation is performed with a noise signal having a bandwidth W1 and combined with a signal from a sweep oscillator that sweeps between frequencies f1 and f2. Although the amplitude control method is shown in the figure, the frequency control method can also be applied.

  It is known that when the Q value of the resonance characteristic is deteriorated, the excitation efficiency is deteriorated substantially in proportion to the Q value. Therefore, it is understood that the distance between the probe and the sample depends on the Q value when the sample whose Q value changes is observed, for example, when amplitude control is performed. From this, it can be seen that it is desirable to reflect information related to the width of the resonance characteristic in the feedback signal. In other words, when the width of the resonance characteristic increases and the Q value decreases, it cannot be easily determined by the detected intensity alone, so the resonance characteristic when there is dissipation, the reference resonance characteristic, and the amplitude are detected. It is desirable to adjust the distance between the probe and the observation sample in consideration of the three frequency positions. However, since this adjustment is generally complicated, when the change in the Q value is large, that is, when the dissipation is very large compared to the reference resonance characteristic, the feedback signal is the center position of the resonance characteristic or the resonance point. It is desirable to use the amount of phase shift from the center point.

It is a block diagram of AFM for applying the present invention. It is a block diagram which shows the structure in particular when performing amplitude control. It is a block diagram showing an example using each vibrator for resonance characteristic measurement and Z control. When observing a sample surface using an AFM to which the present invention is applied, a Z control signal and a resonance characteristic measurement signal are a plurality of combinations. When observing a sample surface using an AFM to which the present invention is applied, a Z control signal and a resonance characteristic measurement signal are a plurality of combinations. (A) is a figure which shows the principle of frequency control, (b) is a figure which shows the principle of amplitude control. It is a block diagram which shows the prior art example of AFM of a dynamic mode.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Observation stand 2 Observation sample 3 Laser light source 4 Optical path 5 Cantilever 6 Probe 7, 7a, 7b Vibrator 8 4 division detector 9 Component extractor 10 Spectrum analyzer 11 Standardization part 12 Signal synthesis and cantilever drive part 12a, 12b Cantilever drive unit 13 Resonance characteristic measurement signal generator 14 Z control signal generator 16 Z control signal output generation unit 17 Z axis drive unit 18 XY scan unit 19 Image display unit 20 Control target value setting unit 21 RMS-DC conversion + Error signal generator 22 Z control signal discriminator

Claims (9)

A cantilever with a probe,
An excitation element for exciting the vibration of the cantilever;
An electrical signal generator for driving the excitation element;
A vibration detecting element for detecting the vibration of the cantilever;
A spectrum analyzer for analyzing a frequency distribution of a signal from the vibration detecting element;
In the observation using the dynamic mode of the atomic force microscope provided with an arithmetic unit for deriving a value to be measured from the output of the spectrum analyzer,
(1) An additional electric signal having a plurality of frequency components in the vicinity of the frequency component of the main component of the electric signal is added to the position control electric signal for controlling the position of the cantilever whose main component is a single frequency component. Exciting the vibration of the above cantilever with the excitation element that is driven by the added composite electrical signal,
(2) The vibration of the cantilever is detected by a vibration detecting element and converted into a detection signal,
Obtain the frequency distribution of the above detection signal,
(3) A method for evaluating cantilever resonance characteristics, wherein the resonance characteristics of the cantilever are derived from the obtained frequency distribution. The additional electric signal is in a frequency band including the main component frequency of the position control electric signal,
The cantilever resonance characteristic evaluation method according to claim 1, wherein in the amplitude distribution with respect to the frequency of the additional signal, the resonance distribution of the cantilever is obtained by dividing the intensity distribution with respect to the frequency by the spectrum intensity.   The cantilever resonance characteristic evaluation method according to claim 1, wherein the additional signal is a signal whose frequency periodically changes in a predetermined frequency region.   The cantilever resonance characteristic evaluation method according to claim 1, wherein the amplitude of the additional electric signal is smaller than that of the position control electric signal.   The cantilever according to any one of claims 1 to 3, wherein the synthesized electric signal is obtained by modulating a position control electric signal with a predetermined modulation signal composed of a plurality of frequency components. Resonance characteristic evaluation method.   3. The cantilever resonance characteristic evaluation method according to claim 1, wherein the modulation signal is a noise signal having a predetermined density distribution. A cantilever with a probe,
An excitation element for exciting the vibration of the cantilever;
An electrical signal generator for driving the excitation element;
A vibration detecting element for detecting the vibration of the cantilever;
A spectrum analyzer for analyzing a frequency distribution of a signal from the vibration detecting element;
When observing a sample using an atomic force microscope equipped with an arithmetic device that derives a value to be measured from the output of the spectrum analyzer in a dynamic mode,
(1) Exciting the vibration of the cantilever with an excitation element driven by an electrical signal for excitation whose frequency changes in a frequency band of a predetermined width including the resonance frequency of the cantilever,
(2) The vibration of the cantilever is detected by a vibration detecting element and converted into an electric detection signal,
(3) Obtaining the frequency distribution of the above electric detection signal,
(4) From the obtained frequency distribution, find the resonance point of the cantilever,
(5) A method for evaluating a cantilever resonance characteristic, wherein the distance between the cantilever and the observation sample is controlled using the frequency of the resonance point as feedback information.   8. The cantilever according to claim 7, wherein the excitation electric signal has a spectrum showing a plurality of peaks or a noise signal spectrum having a clear frequency distribution in the frequency band of the predetermined width. Resonance characteristic evaluation method.   9. The cantilever resonance characteristic according to claim 1, wherein the cantilever resonance characteristic evaluation method is performed for a plurality of predetermined observation points of the observation sample, and further, a display corresponding to the observation point is performed. Evaluation method.

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