JP5579516B2 - Scanning probe microscope - Google Patents

Description

  The present invention relates to a scanning probe microscope for measuring a magnetic force distribution on a sample surface.

  Conventional surface observation methods for magnetic materials include the Bitter method, Kerr microscope, spin-polarized scanning electron microscope (Spin-SEM), Lorentz electron microscope, magnetic force microscope (MFM). ) Etc. The bitter method is a technique that has been used for a long time, and is a technique in which a colloidal solution of magnetic fine particles is dropped on the surface of a magnetic sample and the pattern of fine particles attracted near the domain wall of the sample is observed with an optical microscope. The Kerr microscope is a magneto-optical technique for observing magnetic domains with a polarizing microscope using the magnetic Kerr effect in which reflected light becomes elliptically polarized light when linearly polarized light is incident on a magnetic sample. This is also known for a long time. . Spin-SEM is a technique for detecting the spins of secondary electrons emitted when a magnetic material is irradiated with an electron beam by decomposing the spins into three-dimensional vector components (for example, Non-Patent Document 1). Since the secondary electrons emitted from the magnetic material have information on the spin magnetic moment in the sample, the magnitude and direction of the magnetization of the sample surface can be obtained as a two-dimensional image by detecting and mapping this information. The Lorentz electron microscope visualizes the domain wall or magnetic domain by utilizing the bending of the electron beam due to the Lorentz force received from the magnetization inside the sample when the electron beam passes through the magnetic sample.

  On the other hand, MFM is based on a technique called an atomic force microscope (AFM) without using light or an electron beam. In this method, magnetic force distribution is detected by scanning the probe surface over the sample surface while detecting the magnetic force acting between the cantilever with the probe coated with the magnetic material and the magnetic sample as the deflection of the cantilever. This is a mapping method (for example, Non-Patent Document 2). In addition to the magnetic force, atomic force is also applied to the probe, and it is necessary to separate the magnetic force from other interactions. To that end, first, the cantilever is vibrated, and the distance between the probe and the sample is adjusted so as to keep constant the vibration amplitude that decreases due to the atomic force acting when the probe and the sample are in contact with each other. As a result, the position of the sample surface in the height direction is determined, and a magnetic force, which is a long-distance force, is detected from the phase change of the cantilever vibration while the probe is separated from the sample surface by a certain distance therefrom.

(K. Koike et al., Jpn. J. Appl. Phys., 24, L542 (1985)), Kazuyuki Koike et al., Japanese Journal of Applied Physics, Volume 24, L542. (Martin et al., Applied Physics Letter 50, 1455, 1987 (Y. Martin et al., Appl. Phys. Lett. 50, 1455 (1987)))

  The bitter method and the Kerr microscope are relatively simple methods, but both use an optical microscope, so the resolution is about 1 μm, and a fine magnetic domain structure cannot be observed. The resolution of Spin-SEM is as high as several nanometers, but since the surface sensitivity is high, it is necessary to expose the observation target by the sample surface treatment, and further, the operation in an ultrahigh vacuum is required. Further, in the Lorentz microscope, since it is necessary to process the sample into a thin film in order to transmit electrons, there is a concern that the sample may be altered by the processing.

  On the other hand, the MFM can be operated in the atmosphere, and a high resolution of about 10 nm can be obtained by a relatively simple operation. However, in order to detect the position of the sample surface, the probe must be brought into contact with the sample surface. If the tip of the probe is broken or a magnetic substance on the sample surface is attached during measurement, it is detected. There has been a problem that the magnetic force signal becomes unstable. In AFM, there is a non-contact method that uses the van der Waals force between the probe and the sample to detect the position of the sample surface without bringing the probe into contact with the sample, but the van der Waals force and the magnetic force are detected. Since it is difficult to separate the forces, a non-contact method cannot be used in MFM.

  An object of the present invention is to provide a non-contact scanning probe microscope capable of measuring a magnetic force distribution with high resolution without processing a sample and without deteriorating a probe tip.

  One embodiment for achieving the above object includes a cantilever, a probe provided at the tip of the cantilever, a force detector that detects displacement of the cantilever as a force, a sample stage on which a sample is placed, A scanning probe microscope for measuring the height distribution and magnetic force distribution of the surface of the sample placed on the sample table in a non-contact manner, and applying an AC voltage to the probe A scanning probe microscope characterized by further comprising an AC voltage source to be applied and a DC voltage source to apply a DC voltage, wherein the height distribution of the sample surface is measured using electrostatic force. .

  Further, in a scanning probe microscope that measures the magnetic force distribution on the surface of the sample using a ferromagnetic probe, the probe is moved relative to the sample surface in an in-plane direction and in a vertical direction. An exciter that vibrates the probe, a force detector that detects a force applied to the probe, an AC voltage source that applies an AC voltage to the probe, and a DC voltage source that applies a DC voltage to the probe A first lock-in amplifier that detects the frequency component of the AC voltage source from the output signal of the force detector, and controls the output of the DC voltage source so that the output from the first lock-in amplifier becomes zero The first control system, the second lock-in amplifier for detecting the double frequency component of the AC voltage source from the output signal of the force detector, and the output from the second lock-in amplifier to be a constant value The moving means in the vertical direction A second control system for controlling, a phase comparator for comparing the phase of the output signal of the force detector and the excitation signal of the exciter, and the second lock-in amplifier while moving the moving means in the in-plane direction. A control device that acquires the output and the output of the phase comparator, and the output of the second lock-in amplifier and the output of the phase comparator acquired by the control device correspond to the position in the in-plane direction of the moving means The scanning probe microscope is characterized by comprising a display device for display.

  According to the present invention, by detecting the position of the sample surface using electrostatic force, the magnetic force of the magnetic material can be measured with high stability and high resolution without bringing the probe into contact with the sample surface.

It is a schematic block diagram of the scanning probe microscope which concerns on embodiment of this invention. 6 is a plot showing the probe-sample distance dependence of the voltage twice differential signal of electrostatic force measured by the scanning probe microscope according to the first example. (A) a two-dimensional image showing the shape of the magnet surface measured by the scanning probe microscope according to the first embodiment, (b) a two-dimensional image showing the magnetic force distribution, (c) a straight line AA in FIG. FIG. 4 is a cross-sectional view taken along the line BB, and FIG. It is the flowchart which showed the measurement procedure of the scanning probe microscope which concerns on embodiment of this invention.

  In the present invention, the electrostatic force acting between the probe and the sample can be separated from other interactions including magnetic force by performing lock-in detection using an alternating voltage, and the electrostatic force is atomic. Since it is a long-distance force compared to an interatomic force, it is based on new knowledge such as the ability to detect the position of the sample surface with the probe away.

Embodiments of the invention relating to a scanning probe microscope will be described below.
FIG. 1 is a schematic configuration diagram of a scanning probe microscope according to an embodiment of the present invention. Sample 1 is a sample made of a magnetic material, and is grounded to the earth potential. A cantilever 6 is disposed facing the surface of the sample 1, and a probe 5 is provided at the tip thereof. The probe 5 itself is made of a ferromagnetic material or coated with a ferromagnetic material and magnetized in a certain direction. The cantilever 6 and the probe 5 are caused to vibrate in the direction perpendicular to the surface of the measurement sample 1 by an oscillating portion (exciter) 7 at or near the natural frequency (within about ± 1% of the natural frequency). It is done. The cantilever 6 is displaced by the interaction with the sample 1 in addition to the forced vibration by the oscillating unit (exciter) 7, and the displacement is detected by using a displacement detecting unit (force detector) 8. The sample 1 is fixed on the XYZ scanning mechanism 3 and the coarse movement mechanism 4 via the sample stage 2 and can be moved in the three-dimensional azimuth direction with respect to the probe 5 by the XYZ scanning mechanism 3. The distance between the sample 1 and the probe 5 can be greatly changed by the moving mechanism 4.

  A voltage signal obtained by adding the AC voltage output from the AC voltage application unit (AC voltage source) 9 and the DC voltage output from the DC voltage application unit (DC voltage source) 12 is applied to the probe 5. At this time, an electrostatic force corresponding to the difference between the voltage signal and the work function of the surface of the sample 1 is applied between the grounded sample 1 and the probe 5, and the cantilever 6 is displaced.

The amplitude of the AC voltage is a preset value, but the value of the DC voltage is determined as follows. The displacement signal of the cantilever 6 from the displacement detector (force detector) 8 is input to the lock-in amplifier A (first lock-in amplifier) 10 and the AC voltage output from the AC voltage application unit (AC voltage source) 9. The intensity of the same frequency component as that of the alternating voltage in the displacement signal is detected using a signal having the same frequency synchronized with the reference signal as a reference signal. When the distance between the probe 5 and the sample 1 is z, the potential difference is V, and the dielectric constant is ε, the electrostatic force F applied to the cantilever 6 is F∝εV ² / z ² . Since the signal output from the lock-in amplifier A (first lock-in amplifier) 10 is differentiated with respect to the voltage V of the electrostatic force, it becomes dF / dV∝εV / z ² , and the distance z and the dielectric constant ε are constant. For example, the value is proportional to the potential difference. Therefore, a DC voltage application unit (DC voltage source) is used by using the feedback control unit A (first control system) 11 so that the output signal from the lock-in amplifier A (first lock-in amplifier) 10 becomes zero. ) By adjusting the DC voltage output from 12, the potential difference between the probe 5 and the sample 1 is always kept at zero. Thereby, the electrostatic force applied to the cantilever 6 can be made zero regardless of the potential of the sample surface. That is, the potential difference between the probe and the sample can be controlled by adjusting the DC voltage of the DC voltage application unit by the potential difference control unit including the lock-in amplifier A and the feedback control unit A.

Further, the displacement signal of the cantilever 6 from the displacement detector (force detector) 8 is input to the lock-in amplifier B (second lock-in amplifier) 13 and output from the AC voltage application unit (AC voltage source) 9. The intensity of the double frequency component of the alternating voltage in the displacement signal is detected using the double frequency signal synchronized with the alternating voltage as a reference signal. Since the signal output from the lock-in amplifier B (second lock-in amplifier) 13 is twice differentiated with respect to the voltage V of the electrostatic force, it is d ² F / dV ² ∝ε / z ² and the dielectric constant ε is constant. Then, the value is inversely proportional to the square of the distance z between the probe 5 and the sample 1. Therefore, the Z drive unit is used by using the feedback control unit B (second control system) 14 so that the output signal from the lock-in amplifier B (second lock-in amplifier) 13 becomes a set constant value. The distance between the probe 5 and the sample 1 is maintained at a constant value by driving the XYZ scanning mechanism 3 in the Z direction via 15 and adjusting the position of the sample 1 in the Z direction with respect to the probe 5. Thereby, the distance between the probe 5 and the sample 1 is always constant regardless of the unevenness of the sample surface. That is, the distance between the probe and the sample surface can be controlled by adjusting the position of the sample in the Z direction with respect to the probe by the distance control unit including the lock-in amplifier B and the feedback control unit B.

  Further, the displacement signal of the cantilever 6 from the displacement detector (force detector) 8 is input to the phase detector (phase comparator) 17, the excitation signal from the oscillator (exciter) 7, and the displacement signal of the cantilever 6. The phase of the signal having the same frequency component as the excitation signal is compared, and a phase signal proportional to the phase difference is output. The phase signal changes in accordance with the force applied to the probe 5, but the potential difference between the probe 5 and the sample 1 is kept at zero by feedback control, so no electrostatic force is included, and the probe 5 and the sample are not included. The probe 5 leaks from the sample 1 because the influence of the mechanical contact force and van der Waals force can be neglected by separating sufficiently enough that 1 does not contact (1 nm or more, practically 5 nm or more). It reflects the magnetic force received from the magnetic field. That is, the magnetic force and other forces can be separated.

  The measurement is performed according to the procedure shown in FIG. First, after setting the cantilever 6 in the cantilever setting step S401, the sample 1 is set on the sample stage 2 in the sample setting step S402. Further, in the sample position adjusting step S403, the position of the sample 1 is adjusted so that the desired position to be measured of the sample 1 is directly below the probe 5. Next, in the probe voltage application step S404, a voltage signal obtained by adding the AC voltage output from the AC voltage application unit (AC voltage source) 9 and the DC voltage output from the DC voltage application unit (DC voltage source) 12 together. Is applied to the probe 5, and the probe-sample potential difference control by the feedback control unit A (first control system) 11 is started in the potential difference control start step S405. In this state, in coarse movement step S406, the control unit 21 performs coarse movement by driving the coarse movement mechanism 4 via the coarse movement unit 20, and brings the probe 5 close to the sample 1 to about several hundred nm. Coarse movement stops when the signal output from the lock-in amplifier B (second lock-in amplifier) 13 exceeds a preset threshold value, and in the next distance control start step S407, the feedback control unit B (first control) The control of the probe-sample distance by the second control system 14 is started. At this time, the distance between the probe 5 and the sample 1 is controlled to be constant, and the potential difference between the probe 5 and the sample 1 is controlled to be zero.

  In scanning and data acquisition step S408, the control unit 21 drives the XYZ scanning mechanism 3 via the scanning unit 19 to scan the sample 1 in the XY directions with respect to the probe 5. Note that the distance between the sample 1 and the probe 5 is kept constant, and unlike the conventional contact-type MFM, it does not require time for the cantilever to move up and down (Z direction), so that the measurement time can be shortened. Further, since the probe does not touch the sample, there is no adhesion of the magnetic substance on the sample surface to the probe, and the magnetic force signal does not become unstable.

  When scanning the sample 1 with respect to the probe 5 in the X and Y directions, the control unit 21 outputs an output signal from the feedback control unit B (second control system) 14 at each measurement point via the surface shape acquisition unit 16. Then, it is captured as surface shape data, and the output signal from the phase detector (phase comparator) 17 is captured as magnetic force data via the magnetic force acquisition unit 18. In surface shape data display step S409, the surface shape data is two-dimensionally mapped using the XY coordinates, and an image of the surface shape is displayed on a display device (not shown). Further, in the magnetic force data display step S410, the magnetic force data is two-dimensionally mapped using the XY coordinates, and an image of the magnetic force distribution is displayed on a display device (not shown).

  The invention is explained in detail using the following examples. Note that matters described in the mode for carrying out the invention and not described in this embodiment can be applied to this embodiment.

  Using the scanning probe microscope according to the present example, the magnetic force distribution on the surface of a neodymium-iron-boron magnet as a magnetic material sample was measured. The configuration of the scanning probe microscope is the same as that shown in FIG. For neodymium-iron-boron magnets, a thin film with a thickness of 200 nm formed on the surface of an amorphous carbon substrate is laminated via a tantalum film with a thickness of 20 nm, and the outermost surface is coated with a tantalum film with a thickness of 40 nm. It was. The sample was demagnetized in the direction perpendicular to the sample surface. A cantilever having a length of 110 μm, a width of 35 μm, and a force constant of 0.95 N / m was used, and the cantilever was vibrated at 105 kHz, which is the natural frequency of the cantilever, by an excitation piezoelectric element provided in the cantilever holder. The vibration amplitude was 5 nm. A tip having a length of 20 μm was provided at the tip of the cantilever, and the surface was coated with 60 nm thick cobalt as a ferromagnetic material, and further coated with 20 nm thick chromium as a protective layer.

  The displacement of the cantilever was detected by irradiating the surface of the cantilever with laser light from the laser diode, detecting the reflected light at that time with a two-divided photodiode, and taking the difference between the two outputs. A voltage of electrostatic force acting between the probe and the sample is applied to the probe by applying an AC voltage with a frequency of 1000 Hz and an amplitude of 1 V and detecting the same frequency component as the AC voltage from the displacement signal of the cantilever using a lock-in amplifier. A signal corresponding to the differentiation was obtained. By adjusting the value of the direct current component of the probe voltage using a feedback circuit so that this signal becomes zero, the potential difference between the probe and the sample was controlled to be zero. In addition, a signal corresponding to the double voltage differentiation of the electrostatic force acting between the probe and the sample is detected by detecting a double frequency component of the AC voltage, that is, a frequency component of 2000 Hz, from the displacement signal of the cantilever using a lock-in amplifier. Got.

  The sample substrate is fixed on an XYZ scanning stage using a piezo element, the entire stage is moved up and down by a coarse movement mechanism using a stepping motor and a screw, and the sample is placed on a cantilever mounted facing the sample surface. Approached. After approaching until a sufficiently large electrostatic force began to act between the sample and the probe, the stepping motor was stopped.

  FIG. 2 is a plot of the results of measuring the probe-sample distance dependence of the twice differential value of the electrostatic force measured by the scanning probe microscope according to this example. When the sample surface approaches the tip of the probe, the double differential signal of the electrostatic force increases in inverse proportion to the square of the probe-sample distance, so the probe is adjusted by making the double differential signal of the electrostatic force constant. -The distance between samples can be kept constant.

  In this embodiment, the voltage applied to the Z-direction piezo element of the XYZ scanning stage is adjusted using a feedback circuit so that the set value of the double differential signal of electrostatic force is 1 (arbitrary unit), and the probe-sample The inter-distance was maintained at 40 nm. At such a distance, the effects of interatomic forces and van der Waals forces acting on the probe are negligible.

  In this state, FIG. 3 shows an image obtained by raster scanning a 10 μm × 8 μm region of the sample surface using the scanning probe microscope according to the present example. FIG. 3A is a surface irregularity image in which the output value from the feedback circuit of the probe-sample distance is read into the control PC by a D / A converter and displayed in shades with respect to the XY coordinates of the sample surface. FIG. 3B shows the phase difference signal value obtained by comparing the phase of the AC signal applied to the piezo element for cantilever excitation and the displacement signal of the cantilever with a phase comparator. This is an image that is read and displayed in shades with respect to the XY coordinates of the sample surface, and the shades correspond to the magnitude and direction of magnetization in the vertical direction of the sample surface. The bright area is the S pole and the dark area is the N pole.

  3C is a cross-sectional view taken along the line AA in FIG. 3A, and FIG. 3D is a cross-sectional view taken along the line BB in FIG. Both in-plane spatial resolutions were about 50 nm, and it was found that the magnetic force distribution can be measured with high resolution, and the position detection accuracy by electrostatic force can be sufficiently obtained. Further, according to this embodiment, since the probe does not come into contact with the sample surface, a protective layer for protecting the ferromagnetic layer coated on the tip of the probe becomes unnecessary. As a result, the radius of curvature at the tip of the probe can be reduced, and the resolution is improved.

  As described above, according to the present embodiment, by detecting the position of the sample surface using electrostatic force, the magnetic force distribution can be measured with high resolution without processing the sample and without degrading the tip of the probe. The scanning probe microscope can be provided. Further, since the probe does not come into contact with the sample, there is no adhesion of foreign matter, and the magnetic force can be detected with high stability. Further, since it is not necessary to move the cantilever up and down for each measurement point, the measurement time can be shortened. Furthermore, since the probe does not come into contact with the sample surface, a protective layer for protecting the ferromagnetic layer coated on the probe tip is unnecessary, and the radius of curvature of the probe tip can be reduced, thereby improving the resolution. .

  It can be applied to the evaluation of the magnetic domain size and structure of the magnet material and the magnetic field around the hard disk drive head.

DESCRIPTION OF SYMBOLS 1 ... Sample, 2 ... Sample stand, 3 ... XYZ scanning mechanism, 4 ... Coarse movement mechanism, 5 ... Probe, 6 ... Cantilever, 7 ... Oscillator (exciter), 8 ... Displacement detector (force detector), DESCRIPTION OF SYMBOLS 9 ... AC voltage application part (AC voltage source), 10 ... Lock-in amplifier A (1st lock-in amplifier), 11 ... Feedback control part A (1st control system), 12 ... DC voltage application part (DC voltage) Source), 13 ... lock-in amplifier B (second lock-in amplifier), 14 ... feedback control unit B (second control system), 15 ... Z drive unit, 16 ... surface shape acquisition unit, 17 ... phase detection unit (Phase comparator), 18 ... Magnetic force acquisition unit, 19 ... Scanning unit, 20 ... Coarse movement unit, 21 ... Control unit, S401 ... Cantilever setting step, S402 ... Sample setting step, S403 ... Sample position adjusting step, S404 ... Probe voltage application step, S 05 ... difference control start step, S406 ... coarse step, S407 ... distance control start step, S408 ... scanning and data acquisition step, S409 ... surface shape data display step, S410 ... magnetic force data display step.

Claims (2)

A cantilever, a probe provided at the tip of the cantilever, a force detector for detecting displacement of the cantilever as a force, a sample stage on which a sample is placed, and a moving means for moving the sample stage, In a scanning probe microscope for non-contact measurement of the height distribution and magnetic force distribution of the surface of the sample placed on the sample stage,
An AC voltage source for applying an AC voltage to the probe and a DC voltage source for applying a DC voltage;
For the height distribution of the sample surface, an output signal from the force detector is input, and a distance control unit for controlling the distance between the probe and the sample surface to be constant is used, and electrostatic force is used. Is measured,
The distance controller is
A second lock-in amplifier that detects a double frequency component of the AC voltage source from the output signal of the force detector;
A second control system for controlling the moving means in the vertical direction so that the output from the second lock-in amplifier becomes a constant value and the distance between the sample and the probe is 1 nm or more,
The voltage of the DC voltage source is set to keep the potential difference between the probe and the sample at zero,
The potential difference between the probe and the sample is controlled by a potential difference control unit to which an output signal from the force detector is input,
The potential difference control unit
A first lock-in amplifier that detects a frequency component of the AC voltage source from an output signal of the force detector;
A first control system that controls the output of the DC voltage source so that the output from the first lock-in amplifier becomes zero, and
When the sample is scanned in the horizontal direction with respect to the probe, the output signal from the second control system is captured as surface shape data at each measurement point. Further, the excitation signal to the cantilever and the signal from the cantilever A scanning probe microscope comprising a control unit that takes in an output signal from a phase comparator that compares phases of excitation signals as magnetic force data. In a scanning probe microscope that measures the magnetic force distribution on the surface of a sample using a ferromagnetic probe,
Means for moving the probe in an in-plane direction and in a direction perpendicular to the surface of the sample;
An exciter for vibrating the probe;
A force detector for detecting a force applied to the probe;
An AC voltage source for applying an AC voltage to the probe;
A DC voltage source for applying a DC voltage to the probe;
A first lock-in amplifier that detects a frequency component of the AC voltage source from an output signal of the force detector;
A first control system for controlling the output of the DC voltage source so that the output from the first lock-in amplifier is zero, and maintaining the potential difference between the probe and the sample at zero;
A second lock-in amplifier for detecting a double frequency component of the AC voltage source from an output signal of the force detector;
A second control system for controlling the moving means in a vertical direction so that an output from the second lock-in amplifier becomes a constant value and a distance between the sample and the probe is 1 nm or more;
A phase comparator for comparing the phase of the excitation signal of the exciter and the output signal of the force detector,
A control device for acquiring the output of the second lock-in amplifier as surface shape data and moving the output of the phase comparator as magnetic force data while moving the moving means in an in-plane direction,
And a display device that displays the output of the second lock-in amplifier and the output of the phase comparator acquired by the control device in correspondence with the position in the in-plane direction of the moving means,
A scanning probe microscope comprising:

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