JP3539867B2 - Scanning probe microscope - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention changes the amplitude of the cantilever by vibrating the cantilever having a small probe at the tip and changing the amplitude of the cantilever by the physical characteristics acting between the probe and the sample surface when approaching the sample when approaching the sample. The present invention relates to a scanning probe microscope for measuring the surface properties of a sample from the amount of change of the sample.
[0002]
[Prior art]
A conventional scanning probe microscope is configured by a mechanism described below. A cantilever having a small probe at the tip is fixed to the cantilever holder, and the vibration is applied at a frequency near the resonance frequency of the cantilever by a vibration device such as a piezoelectric element, and the amplitude at that time is detected by a displacement detection device such as an optical lever method. measure. The sample is placed on a three-axis fine movement mechanism composed of a piezoelectric element or the like.
[0003]
The sample is brought close to the probe by the coarse movement mechanism, and when the probe and the sample are sufficiently close to each other, a physical force such as an atomic force acts between the sample and the probe. This physical force changes the cantilever amplitude.
Since the force acting at this time depends on the distance between the probe and the sample, the probe and the sample are brought close to the region where the atomic force acts, and while scanning in a two-dimensional plane by the fine movement mechanism, By controlling the distance between the sample and the probe so that the amplitude of the cantilever is always constant, an uneven image of the sample surface is imaged.
[0004]
In addition, a scanning near-field microscope, a type of scanning probe microscope, uses a probe made of an optical fiber with a sharpened tip and a fine aperture to simultaneously measure the unevenness image of a sample and optical characteristics. Is performed.
In a scanning near-field microscope, light is incident on the probe, an evanescent field is formed near the aperture, the probe tip is brought close to the sample at a wavelength or less, and the evanescent light is scattered on the surface of the sample, Convert to propagating light.
[0005]
At this time, a technique of a scanning probe microscope is used for a technique of bringing the probe and the sample close to each other.
That is, when the probe is brought close to the sample while being excited near the resonance point, the amplitude changes due to a force such as an atomic force acting on the probe tip and the sample surface. If control is performed so as to keep this amplitude constant, the probe tip can be brought closer to a distance shorter than the wavelength.
[0006]
In this case as well, the sample is placed on the three-axis fine movement mechanism, scanning is performed in a two-dimensional plane while keeping the distance between the sample and the probe constant, and the control amount of the fine movement mechanism in the height direction is monitored. As a result, an uneven image of the sample is obtained.
Further, by collecting light signals scattered on the sample surface and measuring the light intensity with a photomultiplier or the like, local optical characteristics of the sample surface are imaged.
[0007]
When the measurement is performed with a scanning probe microscope or a scanning near-field microscope, frequency characteristics near the resonance point of the cantilever or probe as shown in FIG. 4 are obtained prior to the measurement. From the waveform of the frequency characteristic, the resonance frequency ω ₀ and Q value of the cantilever or the probe are obtained. Usually, the Q value is obtained from the following formula.
[0008]
Q = ω ₀ / (ω ₂ −ω ₁ )
Here, ω ₀ is the resonance frequency of the cantilever, and ω ₁ and ω ₂ are A / √2 (√2 represents the square root of 2) when the amplitude at the resonance frequency is A, and the frequency characteristics. Is the frequency at the intersection of the curves.
From this equation, it can be seen that the Q value is determined by the width of the resonance point peak, and the value increases as the peak becomes steeper.
[0009]
When the cantilever or the probe is brought close to the sample while being vibrated at a frequency near the resonance point, the resonance frequency shifts as shown in FIG. Changes. The change in amplitude at this time is detected by displacement detection means such as an optical lever method, and the detected amount is processed by a control system such as a PI control system, so that the distance between the probe or probe and the sample is constant. A feedback voltage is applied to the fine movement mechanism.
[0010]
[Problems to be solved by the invention]
As described above, when the measurement is performed by the scanning probe microscope, the distance between the sample and the probe or the probe is controlled using the frequency characteristics near the resonance point of the cantilever or the probe.
The larger the Q value of the cantilever or probe, the steeper the slope of the waveform, and the higher the sensitivity.
[0011]
However, when considering the entire system including the control system, if the Q value is too high, the system cannot follow and causes oscillation. Therefore, in order to achieve both responsiveness and sensitivity in a scanning probe microscope system, it is necessary to optimize the Q value.
The Q value is determined by the amount of attenuation of the vibration amplitude of the probe, and the following three factors are the main causes of attenuation.
[0012]
(1) Damping due to surrounding fluid (air or liquid)
(2) Damping due to internal friction of members
(3) Damping from the support of the member.
Of these, the amount of attenuation due to the fluid around (1) is determined by the surrounding environment and the shape of the cantilever or probe, and it is difficult to control unless the shape is changed.
[0013]
Also, the amount of attenuation due to the internal friction of the member (2) is determined by the material of the cantilever or the probe and the manufacturing conditions, and it is difficult to control the Q value after the manufacturing.
The amount of attenuation from the support in (3) is determined by the holding state and the holding force of the cantilever or the probe, but the current device holds the cantilever or the probe with a predetermined force.
[0014]
Therefore, with the current scanning probe microscope, it is impossible to control the Q value after the cantilever or probe is attached, and the measurement environment, material characteristics, shape, and retention of each cantilever or probe are not possible. Variations in Q value occur due to differences in methods, and in many cases, an appropriate Q value cannot always be obtained.
[0015]
[Means for Solving the Problems]
In order to solve the above problems, in the present invention, the holding force of the cantilever or the probe is controlled so that the attenuation of the cantilever or the probe, that is, the Q value can be controlled.
That is, the structure of the cantilever holder 1 of the scanning probe microscope is changed to a substrate 2, a cantilever 5 having a fine probe 6 at the tip, a first actuator 3 for exciting the cantilever, and a cantilever. And a second actuator 4 that changes the holding force of the cantilever, and the first actuator 3 is fixed to the substrate. The cantilever is placed so that the exciting force of the first actuator 3 is transmitted to the cantilever, the flat portion 5b of the cantilever is brought into contact with the elastic member, and the second actuator 4 is interposed on the other flat portion 5a. Then, the cantilever is pressed against the elastic member 7 by the second actuator, and the force for pressing the elastic member by the second actuator is changed. Varying the amount of energy to be attenuated from the support portion of Nchireba, has a structure as attained the optimization of the Q value.
[0016]
In another method, the cantilever holder 1 has a structure in which the first actuator 16 has two functions of vibrating the cantilever 5 and pressing against the elastic member 7.
Further, in a scanning near-field microscope which is a kind of a scanning probe microscope, a probe 20 made of an optical fiber having a sharpened tip is used instead of a cantilever.
[0017]
With the above structure, while changing the pressing force of the cantilever or the probe pressing actuator, the probe is vibrated, the amplitude of the probe is detected, the frequency characteristics are obtained, the Q value is optimized, and scanning is performed. The system was designed to measure with a scanning probe microscope.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, as shown in FIG. 1, a first actuator 3 for vibration is mounted on a substrate 2, a second actuator 4 for pressing a cantilever is mounted thereon, and a cantilever 5 is mounted on a second actuator 4. The cantilever is placed on the flat plate portion 5a in contact with the flat plate portion 5a. Further, the elastic member 7 is arranged so as to be in contact with the other flat plate portion 5b of the cantilever. These members are held on the substrate by the holding plate 8.
[0019]
When measuring with a scanning probe microscope, first, the cantilever 5 is vibrated by using the vibrating actuator 3, and the amplitude of the cantilever 5 is measured by a displacement detecting means such as an optical lever method, and the resonance characteristic is measured. Ask for.
Next, while the cantilever is vibrated at a frequency near the resonance frequency, the pressing force of the cantilever 5 against the elastic member 7 is changed by the second actuator 4. At this time, the energy of the vibration is propagated to the support member side by the elastic force and the moment acting on the support portion, and the vibration amplitude is attenuated. Since the amount of attenuation depends on the pressing force, changing the pressing force changes the amount of attenuation of the cantilever, that is, the Q value. With this method, the Q value is optimized to a value set in advance.
[0020]
The frequency characteristic near the resonance frequency of the cantilever has the shape shown in FIG. In the figure, when the sample and the cantilever are moved close to each other while vibrating near the resonance point, the resonance frequency shifts and the waveform of the resonance peak changes as a broken line or a dashed line, and the amplitude at the operating point is changed. Changes. In a scanning probe microscope, the distance between the sample and the probe is controlled so that the amplitude amount shows a constant value.
[0021]
When the pressing force of the cantilever is changed, the amount of attenuation of vibration energy from the support changes, and the Q value changes. When the pressing force is increased, the amount of attenuation decreases, and as a result, the Q value increases, and the waveform of the solid line resonance peak shown in FIG. On the other hand, when the holding force is reduced, the amount of attenuation increases, and as a result, the Q value decreases, and the waveform of the resonance peak becomes broad as shown by the two-dot broken line in FIG. Therefore, the Q value can be controlled by changing the pressing force.
[0022]
As another method, as shown in FIG. 6, a method is also conceivable in which the same actuator 16 is used to change the holding force while the cantilever 5 is vibrated by the vibration actuator 16. In this method, the actuator 16 is fixed on the substrate 2, the cantilever is placed so that the plate 5a of the cantilever contacts the actuator, and the elastic member 7 is brought into contact with the other plate 5b. These members are held on the substrate by the mounting plate 8. By applying a sine wave for excitation and a DC voltage for pressing as shown in FIG. 7 to the actuator 16, it is possible to have the functions of excitation and pressing.
[0023]
After determining the resonance frequency and Q value by the method described above, measurement is performed using the waveform of the resonance peak.
Fig. 3 shows an example of a measurement system for a scanning probe microscope. The detection signal of the amplitude of the cantilever 5 is sent to the control system. The signal sent to the control system is subjected to processing such as PI control, and a feedback voltage is applied to the fine movement mechanism 14 so that the distance between the cantilever 5 and the probe 6 becomes constant. By monitoring the signal in the height direction applied to the fine movement mechanism 14 while scanning the sample 12 in a two-dimensional plane, a concavo-convex image of the sample surface is imaged.
[0024]
In the case of performing measurement with a scanning near-field microscope, which is a type of scanning probe microscope, a probe 20 made of an optical fiber having a sharpened tip is used instead of the cantilever.
[0025]
【Example】
FIG. 1 is a schematic view of a cantilever holder of a scanning probe microscope. In this cantilever holder 1, a first actuator 3 for vibration made of a piezoelectric element is fixed on a substrate 2, and a second actuator 4 made of another piezoelectric element is mounted on the piezoelectric element. The cantilever 5 is placed on the second actuator 4 by bringing the flat plate portion 5a into contact therewith. An elastic member 7 made of silicon rubber or the like is attached to the other flat plate portion 5b of the cantilever in contact therewith. The cantilever 5 and the elastic member 7 are held on the substrate by the holding plate 8.
[0026]
Here, as the elastic member 7, a member having an elastic property compared with the substrate portion 2, the first actuator 3, the second actuator 4, and the pressing plate 8 of the cantilever holder 1 is used.
The embodiment shown in FIG. 2 is an example in which a scanning probe microscope is configured using the cantilever holder 1 of FIG. The amplitude of the cantilever 5 attached to the cantilever holder in FIG. 1 is detected by the optical head 9. The optical head 9 uses a detecting means based on an optical lever system. Laser light is applied from the semiconductor laser 10 in the optical head to the back surface 5c of the cantilever, and the reflected light is detected by the four-divided photodetector 11. The sample is mounted on a sample stage 13 provided on a cylindrical piezoelectric actuator 14. The cylindrical piezoelectric actuator 14 can perform three-axis fine movement in the XYZ directions by a single actuator. This cylindrical piezoelectric actuator is attached to a coarse movement mechanism 15, and can roughly move the distance between the sample 12 and the probe 6.
[0027]
FIG. 3 is a system diagram showing a system configuration of the scanning probe microscope. The amplitude of the cantilever 5 is detected by an optical lever type optical head, and a detection signal is sent to a control unit in the control unit. In the control section, PI control is performed so that the distance between the sample 12 and the probe 6 becomes constant, and a feedback voltage is applied to the Z electrode 14a of the cylindrical piezoelectric actuator 14. On the other hand, a scanning voltage for scanning the sample 12 in a two-dimensional plane is applied to the XY electrode 14b of the cylindrical piezoelectric actuator 14. By monitoring the voltages applied to these XYZ electrodes, an uneven image of the sample surface is imaged.
[0028]
Before measurement, a frequency characteristic is obtained from the amplitude detection signal by applying a sine wave to the vibration actuator 3 while sweeping, and an operating point is set near the resonance frequency. A DC voltage is applied to the actuator 4, and a voltage value is set such that an optimum Q value is obtained from the frequency characteristics at that time.
FIG. 6 shows an example in which a single actuator 16 is used as both a vibration actuator and a pressing actuator. In this embodiment, an actuator 16 composed of a piezoelectric element or the like is fixed on a substrate 2, a cantilever is placed on the actuator so that the flat plate portion 5a of the cantilever 5 contacts, and a silicon rubber or the like is mounted on the other flat plate portion 5b. The elastic member 7 is brought into contact. These members are held on the substrate 2 by the mounting plate 8.
[0029]
FIG. 7 shows an operation example when the cantilever holder shown in FIG. 6 is used. In this embodiment, by applying a sine wave for excitation and a DC voltage for pressing to the actuator 16, it is possible to have the functions of excitation and pressing. In this case, the frequency of the sine wave is swept, the amplitude amount at that time is detected, the excitation frequency is determined by obtaining the frequency characteristic, and the magnitude of the DC voltage is determined while comparing with the preset Q value. The Q value is optimized by sweeping the value.
[0030]
FIG. 8 shows an embodiment in which the present invention is applied to a probe holder of a scanning near-field microscope which is a kind of scanning probe. In this embodiment, a bent type probe is used in which the tip 20a of the optical fiber is sharpened, a minute opening is provided, and the tip 20b is bent. In the present embodiment, an actuator 19 such as a piezoelectric element is fixed on a substrate 18, a probe 20 is mounted on the actuator 19, an elastic member 21 such as silicon rubber is arranged so as to sandwich the probe, and a mounting plate 22 is used. Hold on substrate. In this case, as in the embodiment of FIG. 6, the actuator 20 has both a vibration function and a pressing function. Also, as in the embodiment of FIG. 1, it is also possible to separate the vibrating and pressing actuators.
[0031]
FIG. 9 is a configuration diagram when a scanning near-field microscope is configured using the probe holder 17 of FIG. In the present embodiment, the probe 20 is vibrated in a direction perpendicular to the sample 12, and the amplitude at that time is detected by the optical head 9. This optical head utilizes a detection means based on an optical lever system. Laser light is applied from the semiconductor laser 10 in the optical head to the back surface 20c of the probe, and the reflected light is detected by the four-divided photodetector 11. The sample 12 is placed on a sample stage 24 provided on a three-axis fine movement mechanism 23 composed of three cylindrical piezoelectric actuators. The three-axis fine movement mechanism 23 is attached to the coarse movement mechanism 15 and can coarsely move the distance between the sample 12 and the probe 20. The end 20d of the probe is coupled with the laser light source 27, and the laser light is incident. In this case, the sample is irradiated with evanescent light from the minute opening at the tip of the probe, and the light scattered on the sample surface is collected by the objective lens 25 arranged in the space at the center of the fine movement mechanism. The collected optical signal is reflected by a mirror 26, guided to a photodetector such as a photomultiplier 28, and the intensity of the optical signal is converted into an electrical signal. At this time, while applying a feedback voltage to the fine movement mechanism 23 so that the distance between the sample surface and the probe is constant, the fine movement mechanism is scanned in the XY plane, so that the uneven image on the sample surface and the optical image are imaged. Be converted to
[0032]
In the scanning near-field microscope, in addition to the illumination transmission mode shown in the present embodiment, the present invention is also applicable to an illumination reflection mode for condensing light reflected from the sample, and a collection mode for picking up light emission on the sample surface with a probe. It is possible to apply
FIG. 10 shows an embodiment in which a straight type probe is used in a scanning near-field microscope. In this embodiment, the tip is sharpened, the probe 32 is vibrated in a direction parallel to the sample surface using a straight probe 32 having a minute opening, and a lateral force acting on the sample surface and the probe tip is used. In this method, the distance between the sample and the probe is controlled by the amount of attenuation of the probe. In the embodiment shown in the figure, an actuator 31 such as a piezoelectric element is fixed on a substrate 29, a probe is mounted on the actuator, and an elastic member 33 such as silicon rubber is arranged so as to sandwich the probe. By holding the actuator on the substrate, the actuator has both a vibration function and a pressing function.
[0033]
【The invention's effect】
As described above, in the present invention, the cantilever holder of the scanning probe microscope includes the substrate, the cantilever having the fine probe at the tip, the first actuator for exciting the cantilever, and the cantilever. A holding plate, an elastic member having elasticity with respect to the substrate and the holding plate, and a second actuator for changing a holding force of the cantilever; the first actuator is fixed to the substrate; The cantilever is placed so that the exciting force of the actuator is transmitted to the cantilever, the flat portion of the cantilever is brought into contact with the elastic member, and the second actuator is interposed on the other flat portion, and the second actuator is interposed. By pressing the cantilever against the elastic member and changing the force to press against the elastic member, the cantilever is attenuated from the support part That was the amount capable of changes configuration of energy. With this configuration, it is possible to control the amount of attenuation of the cantilever, that is, the Q value, and it is possible to maximize the system capability in response and sensitivity.
[0034]
In addition, in the above-mentioned cantilever holder, the first actuator has two functions, that is, vibration of the cantilever and pressing against the elastic member, thereby enabling control of the Q value and simplification of the mechanism. Rigidity is improved.
Furthermore, by using a probe made of a sharpened optical fiber or the like instead of the cantilever, the Q value of the probe can be optimized even in a scanning near-field microscope.
[Brief description of the drawings]
FIG. 1 is a schematic view of a cantilever holder section of a scanning probe microscope according to one embodiment of the present invention.
FIG. 2 is a configuration diagram when a scanning probe microscope is configured using the cantilever holder of FIG. 1;
FIG. 3 is a system diagram of a scanning probe microscope.
FIG. 4 is an explanatory diagram for explaining the operation principle of the scanning probe microscope.
FIG. 5 is an explanatory diagram showing a difference in frequency characteristics when the pressing force of the cantilever is changed.
FIG. 6 is a schematic view of a cantilever holder of a scanning probe microscope according to another embodiment of the present invention.
FIG. 7 is a schematic diagram for explaining the operation of the cantilever holder of FIG.
FIG. 8 is a schematic view of a probe holder of a scanning near-field microscope according to another embodiment of the present invention.
FIG. 9 is a configuration diagram illustrating a configuration of a scanning near-field microscope using the probe holder of FIG. 8;
FIG. 10 is a schematic view of a probe holder of a scanning near-field microscope according to another embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Cantilever holder 2 Substrate 3 Vibration actuator 4 Pressing actuator 5 Cantilever 6 Probe 7 Elastic member 8 Holding plate 9 Optical head 10 Semiconductor laser 11 Photodiode 12 Sample 13 Sample stage 14 Cylindrical piezoelectric actuator 15 Coarse movement mechanism 16 Actuator 17 Probe holder 18 Substrate 19 Actuator 19 Probe 21 Elastic member 22 Pressing plate 23 3-axis fine movement mechanism 24 Sample stage 25 Objective lens 26 Reflecting mirror 27 Laser light source 28 Photomultiplier 29 Probe holder 29 Substrate 31 Actuator 32 Probe 33 Elastic member 34 Pressing plate

Claims (4)

While vibrating a cantilever with a small probe at the tip, the amplitude of the cantilever is changed by the physical characteristics acting between the probe and the sample surface when the probe is brought close to the sample. In a scanning probe microscope for measuring the surface properties of a sample, a cantilever holder for holding the cantilever includes a substrate, a first actuator for exciting the cantilever, a holding plate for holding the cantilever, and a substrate. And an elastic member having an elastic property with respect to the holding plate, and a second actuator for changing a holding force of the cantilever. The cantilever is placed so as to transmit to the cantilever, and the flat portion of the cantilever is brought into contact with an elastic member. , The other on the flat plate portion by interposing the second actuator, the scanning probe microscope characterized by pressing the cantilever resilient member in the second actuator. 2. The scanning probe microscope according to claim 1, wherein the first actuator has two functions of exciting the cantilever and pressing the first actuator against the elastic member. 3. 3. The scanning probe microscope according to claim 1, wherein a probe made of an optical fiber having a sharpened tip is used instead of the cantilever. While changing the pressing force of the pressing actuator, the cantilever or the probe is vibrated, the frequency characteristic is obtained by detecting the amplitude of the cantilever or the probe at that time, and the Q value is obtained from the obtained frequency characteristic. 4. The scanning probe microscope according to claim 1, wherein the Q value is optimized by calculation.

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