JPH06323843A - Ultrasonic oscillation detecting method and sample observing method for interatomic force microscope - Google Patents

Abstract

PURPOSE:To provide ultrasonic oscillation detecting method and sample observing method for interatomic force microscope in which the viscoelastic properties and the like can be measured at a frequency higher than 1MHz by generating ultrasonic oscillation of 1MHz or above in a sample and detecting the ultrasonic oscillation using a common cantilever. CONSTITUTION:A sample 8 is subjected to ultrasonic oscillation at a frequency sufficiently higher than the resonance frequency of a cantilever 11. Since the force functioning between a probe 4 and the sample 8 exhibits a nonlinear dependency on the distance, a displacement depending on the amplitude of oscillation is induced in the cantilever 11. Ultrasonic wave is detected by measuring the displacement.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention applies, in an atomic force microscope, a vibration of a frequency sufficiently higher than the resonance frequency of a cantilever to a sample, so that the force acting between the probe and the sample has a nonlinear dependence on the distance. The present invention relates to a technique for detecting not only the ultrasonic vibration itself but also its temporal envelope by utilizing what is shown, and a measurement and imaging technique using the signal. Such a technique can be used for material structure observation, cleanliness control, micro device evaluation, precision equipment failure analysis, medical examination diagnosis, biochemical examination.

[0002]

2. Description of the Related Art Atomic Force Microsc
ope; AFM) uses the displacement induced in the cantilever holding the probe by the force acting between the sample surface and the probe,
It is a new microscope that visualizes the unevenness of a minute area (Bi
nnig, Quate and Gerber, Phys. Rev. Lett. 12, 930,
1986.). Martin et al. Developed a method of applying longitudinal vibration to a cantilever to detect the attractive force of a sample from the change in resonance frequency (Y.Martin, CC Williams, HK Wick
ramasinghe: J. Appl. Phys., 61 (1987) 4723).
On the other hand, Maivald et al. And Radmacher et al. Developed a vibrating AFM and measured the viscoelasticity from the cantilever vibration response when the sample was longitudinally vibrated (P. Maivald, HJ Butt, S.
AC Gould, CB Prater, B. Drake, JA Gurley,
VB Elings, and PK Hansma: Nanotechnology 2
(1991) 103. and M. Radmacher, RW Tillmann, M. Fri.
tz, and HE Gaub: Science, 257 (1992) 1900).
Takada et al. Proposed a tunnel acoustic microscope that detects the vibration of the sample by applying the reverse process, that is, vibrating the probe (K. T.
akata, T. Hasegawa, Sumio Hosaka, Shigeyuki Hosok
u, Tsutomu Komoda: Appl. Phys. Lett. 55 (1989) 17). This was used by Cretin et al. To image internal defects (B. Cretin and F. Stahl Proc IEEE Ultrason.
ic Symposium, B5, 1992.). These vibrating AFM
In the method of detecting vibration occurrence in (1), there are various methods of vibration occurrence and detection, but they are common in that they are detected as signals of the same frequency as the frequency of the generated vibration.

[0003]

In the conventional vibration type AFM, the upper limit of the frequency of vibration used is the resonance frequency of the cantilever. Since the resonance frequency of a normal cantilever is about 100 KHz at maximum, the upper limit of the vibration frequency is 100 K.
It was about Hz. However, some synthetic and biopolymers have an interesting response when subjected to vibrations above 1MHz. However, it was difficult to measure such an object with the conventional vibration type AFM.

The reason is that in order to apply such high-frequency vibration to the sample, the device becomes large in scale, and the conventional vibration type AFM cannot be used as it is, and also such high frequency acts on the sample. This is because it is impossible for the conventional AFM to detect this.

The present invention solves the above problems of AFM,
AFM technology that makes it possible to measure viscoelastic properties in AFM at frequencies above 1 MHz by generating ultrasonic vibrations at frequencies above 1 MHz in AFM samples and detecting them using a normal cantilever. It is intended to provide.

[0006]

To solve this problem, the method for detecting ultrasonic vibrations in an atomic force microscope of the present invention provides an image of unevenness in a minute area due to the force acting between the sample surface and the probe. Force Microscope (Atomic Force M)
icroscope; AFM) is characterized by generating ultrasonic vibrations in the sample at a frequency sufficiently higher than the resonance frequency of the cantilever, inducing a displacement that depends on the amplitude of the ultrasonic vibration of the sample in the cantilever, and detecting this displacement. I am trying. In addition, the sample observation method in the atomic force microscope of the present invention is an atomic force microscope (Atomic Force Microscope) that measures irregularities in a minute region by the force acting between the sample surface and the probe.
A method of measuring the sample surface using scope; AFM)
Atomic Force Microscope (Atomic Force Microscope) that visualizes the unevenness of a minute area by the force acting on the sample between the sample surface and the probe.
Microscope; AFM) is characterized in that ultrasonic vibration of a frequency sufficiently higher than the resonance frequency of the cantilever is generated in the sample, a displacement that depends on the amplitude of the ultrasonic vibration of the sample is induced in the cantilever, and this displacement is detected. I am trying.

[0007]

Function: In the atomic force microscope, vibration of ultrasonic frequency sufficiently higher than the resonance frequency of the cantilever is applied to the sample. Since the force acting between the probe and the sample has a non-linear dependence on distance, an ultrasonic envelope is formed. The ultrasonic wave is detected by detecting the envelope of this ultrasonic wave.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the drawings showing an embodiment. In FIG. 1, 1 is an atomic force microscope. The atomic force microscope 1 includes a sample table 2, a sample table driving device 3 for driving the sample table 2, a probe 4, and a cantilever measuring device 5.
The control device 6 and the display device 7 are provided. The sample table 2 can have a sample 8 mounted on its surface, and the sample table 2 is driven by a sample table driving device 3. Probe 4
Is located close to the sample 8 on the sample table 2 and is held at the tip of the cantilever 11. Cantilever measuring device 5
Is composed of a laser generator 12 and a photodetector 13, and the laser generator 12 emits a laser beam to the cantilever 1.
1, and the photodetector 13 detects the reflected light from the cantilever 11 and measures the position and posture of the cantilever 11. As the photodetector 13, a position sensitive photodetector (PSD) which is divided into four parts in the vertical and horizontal directions can be used.

As shown in FIG. 2, the ultrasonic vibrator 14 is attached to the sample stand 2 of the AFM, and the sample 8 is attached with an adhesive or grease 15. In the atomic force microscope having such a configuration, the following operation is performed when detecting ultrasonic waves acting on the sample. When an amplitude-modulated high-frequency signal is applied to the ultrasonic oscillator, the sample surface vibrates at the same frequency ω as the high-frequency signal when the high-frequency signal is on. At this time, the probe at the tip of the cantilever close to the sample receives a force F that changes at a frequency ω as a function of the distance z between the probe and the sample surface as shown in FIG. If much lower than ω, the cantilever cannot oscillate at the frequency ω. On the other hand, as shown in FIG. 3, the force F changes nonlinearly with respect to the distance z between the probe and the sample surface, so that a finite force remains when averaged over one period. This force raises the cantilever a certain amount. Then, when the high frequency signal is turned off, this displacement also disappears. As a result, the output signal of the cantilever measuring device 5 including a differential photodiode for detecting the displacement of the cantilever takes the same form as the modulation signal for modulating the high frequency signal.

By modifying the AFM, a device having the configuration shown in FIG. 2 was developed. A cantilever having a spring constant of 0.024 N / m and a resonance frequency of 33 KHz manufactured by Park Scientific was used. The displacement of the cantilever was detected by a differential photodiode. The voltage applied to the z-axis piezo was controlled by the z-axis piezo control circuit such that the force between the sample and the probe had a constant value F ₀ and the DC component of the photodiode signal had a constant value. When an intermittent high frequency signal (tone burst) is applied to the piezoelectric transducer in this state, the cantilever shows a displacement that changes with time. The time average of this displacement is due to the action of the z-axis piezo control circuit. It was held so that the mean value of the forces in between was F ₀ .

FIG. 4A shows the envelope of the intermittent high frequency signal (tone burst) applied to the piezoelectric transducer. Although not shown, there is a high-frequency signal inside this envelope. Here, the frequency of the envelope signal was 700 Hz, the frequency of the high frequency signal was 3.17 MHz, and the amplitude was +10 dbm. Figure 4
(B) shows the response of the cantilever when a silicon single crystal (100) plane is used as a sample. Here, the upward direction corresponds to the direction approaching the sample, and the downward direction corresponds to the direction away. When the high frequency vibration is turned on, the cantilever is shown in Fig. 3.
As shown in (b), it was displaced in the direction away from the sample.

FIG. 5A shows a high-frequency signal having a sawtooth wave as an envelope, and a waveform whose amplitude increases in proportion to time is repeated. The frequency of the envelope signal was 700Hz and the frequency of the high frequency signal was 3.17MHz. FIG. 5B is a response waveform of the cantilever displacement when a high frequency signal such as this signal is applied. The cantilever did not show any response while the amplitude of the high frequency signal was small, and it abruptly moved downward when the threshold value of a certain amplitude was exceeded. This is the direction in which the cantilever moves away from the sample. The reluctance force (+ 1.2N) (solid line) is higher than the average force F between the sample and the probe (dotted line) (dotted line). The value increased. This series of observations can be used as a new characterization tool for force interactions between the sample and the probe.

[0013]

As described above, the lateral resolution of ultrasonic vibration detection can be dramatically improved. The ultrasonic vibration detection methods before the advent of atomic force microscopes are a method using a piezoelectric element and a method using an optical probe. However, with these methods, the spatial resolution, that is, the size of the place where vibration is measured is about 1 μm at the minimum, and the vibration distribution in a region smaller than this cannot be measured. With this method, the spatial resolution of vibration measurement can be improved to nanometers or single atoms, which is the same as the resolution of images. Next, compatibility with a normal atomic force microscope can be measured. The resonance frequency of a cantilever commonly used in atomic force microscopes is 100 KHz.
Since it is below, it does not respond to ultrasonic vibrations with a frequency of 1MHz or higher, and cannot be applied to the detection of ultrasonic vibrations. When detecting ultrasonic vibrations, a special cantilever must be designed and manufactured, but this manufacturing technique is difficult, and at the same time, special knowledge and skill are required for probe replacement and device adjustment.

On the other hand, according to the method of the present invention, the cantilever does not need to be changed at all while the ultrasonic vibrating element is incorporated in the sample holder for holding the sample, and at the same time, the signal processing circuit is added. You can easily add more while keeping the function of the atomic force microscope. For this reason,
It is possible to compare images of existing atomic force microscopes and images when ultrasonic vibration is applied to measurement data.

Furthermore, the new technique of detecting ultrasonic vibrations with a spatial resolution of atoms or nanometers has investigated the spatial distribution of viscoelasticity in the ultrasonic frequency domain, the possibility of visualizing internal structures on the order of nanometers, etc. With the birth of a microscope that will bring new insights that have never existed before, its generality and ripple effect will be extremely large.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a configuration of an atomic force microscope.

[Fig. 2] Experimental apparatus for ultrasonic AFM.

FIG. 3 is an explanatory view of the principle of the invention.

FIG. 4 is a response waveform of a cantilever when high frequency vibration having a rectangular wave envelope is applied.

FIG. 5 is a response waveform of a cantilever when high frequency vibration having a sawtooth wave envelope is applied.

[Explanation of symbols]

 1 atomic force microscope 2 sample stage 3 sample stage drive 4 probe 5 cantilever measuring device 6 control device 7 display device 8 sample 11 cantilever 12 laser generator 13 photodetector 14 ultrasonic transducer 15 grease or adhesive

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kazushi Yamanaka 1-2, Namiki, Tsukuba, Ibaraki Prefectural Institute of Industrial Science and Technology (72) Inventor Kazutoshi Watanabe 6-31-1 Kameido, Koto-ku, Tokyo Seiko Electronics Industry Co., Ltd.

Claims (4)

[Claims] 1. An atomic force microscope (At) for imaging unevenness in a minute region by a force acting between a sample surface and a probe.
omic force microscope (AFM), to generate ultrasonic vibration of a frequency sufficiently higher than the resonance frequency of the cantilever in the sample, induce a displacement in the cantilever that depends on the amplitude of the ultrasonic vibration of the sample, and detect this displacement. For detecting ultrasonic vibrations in an atomic force microscope. 2. The ultrasonic vibration is temporally amplitude-modulated by a low-frequency AC signal having a cantilever resonance frequency or less,
The ultrasonic vibration in the atomic force microscope according to claim 1, wherein a vibration having the same frequency as the low frequency modulation signal is excited in the cantilever by a force acting between the sample surface and the probe and detected. Detection method. 3. An atomic force microscope (Atomic F) for measuring irregularities in a minute area by a force acting between a sample surface and a probe.
or atomic force microscope (AFM), which is an atomic force microscope (AFM) that visualizes irregularities in a minute area by the force acting between the sample surface and the probe on the sample.
omic force microscope (AFM), to generate ultrasonic vibration of a frequency sufficiently higher than the resonance frequency of the cantilever in the sample, induce a displacement in the cantilever that depends on the amplitude of the ultrasonic vibration of the sample, and detect this displacement. A method for observing a sample with an atomic force microscope. 4. The ultrasonic vibration is temporally amplitude-modulated by a low-frequency AC signal having a cantilever resonance frequency or less,
The sample observing method in an atomic force microscope according to claim 3, wherein the cantilever excites vibration of the same frequency as the low frequency modulation signal by a force acting between the surface of the sample and the probe, and the vibration is detected. .