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Dynamic mode afm apparatus UPDATE

外国特許コード F110004073
整理番号 N021-13WO
掲載日 2011年7月8日
出願国 欧州特許庁(EPO)
出願番号 09746445
公報番号 2275798
出願日 平成21年4月8日(2009.4.8)
公報発行日 平成23年1月19日(2011.1.19)
国際出願番号 JP2009057158
国際公開番号 WO2009139238
国際出願日 平成21年4月8日(2009.4.8)
国際公開日 平成21年11月19日(2009.11.19)
優先権データ
  • 特願2008-124891 (2008.5.12) JP
  • 2009JP57158 (2009.4.8) WO
発明の名称 (英語) Dynamic mode afm apparatus UPDATE
発明の概要(英語) There is provided a dynamic mode AFM apparatus that configures an automatic control system which can automatically obtain a probe-sample distance, and allows high-speed identification of atoms of the sample surface.The dynamic mode AFM apparatus comprises: a scanner 3 for performing three-dimensional relative scanning of a cantilever 2 and a sample 1; a means 8 for generating an AC signal of a resonance frequency in a mode with flexural vibration of the cantilever 2; a means 9 for exciting the flexural vibration of the cantilever 2 with the resonance frequency; a means 10 for generating an AC signal of a second frequency which is lower than the frequency of the flexural vibration; a means 11 for modulating a probe 2A-sample 1 distance of the cantilever 2 with the second frequency; a means 5 for detecting fluctuation of the resonance frequency; a means 4 for detecting vibration of the cantilever; and a means 6 for detecting a fluctuation component which is contained in a detected signal by the means 5 for detecting the resonance frequency fluctuation and synchronized with a modulation signal of the probe 2A-sample 1 distance, wherein an inclination of the resonance frequency against the probe 2A-sample 1 distance is obtained from the strength and polarity of the fluctuation component.
従来技術、競合技術の概要(英語) BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dynamic mode AFM apparatus.
2. Description of the Related Art
Firstly, an AFM (atomic force microscopy) will be described.
A contact mode AFM is a technique to image a "constant force surface" of a sample surface by detecting force (usually, repulsive force), which is exerted between a probe and a sample when a cantilever with the probe attached thereto is brought close to the sample surface, based on flexure of the cantilever, and by two-dimensionally scanning the sample with the probe while controlling a probe-sample distance so that the detected force is kept constant.This contact mode AFM gives substantial damage to the sample due to the strong force exerted between the probe and the sample, and the atomic resolution is difficult to achieve.
In contrast, a dynamic mode AFM is a technique to image a "constant force gradient surface" of the sample surface by bringing a cantilever with a probe attached thereto close to a sample surface, detecting change in a resonance frequency of the cantilever due to a differential (force gradient) of force exerted between the probe and the sample with respect to a probe-sample distance, and two-dimensionally scanning the sample with the probe while controlling the probe-sample distance so that the change in the resonance frequency is kept constant.
Fig. 1 shows an exemplary configuration in the area of a sample and cantilever of a conventional dynamic mode AFM apparatus.
In Fig. 1, reference numeral 201 denotes a sample, 202A denotes a probe of a cantilever 202, 202B denotes a base of the cantilever 202, 203 denotes an XYZ scanner, 204 denotes a cantilever excitation means, 205 denotes an optical position detector (detector with an optical lever) to detect the position of the cantilever 202 by irradiating a bottom face of the cantilever 202 with a laser beam 206, and 207 denotes a state of flexural vibration of the cantilever.
Fig. 1 shows X, Y, and Z directions because the XYZ coordinate will be used in the following description.Although the sample 201 is mounted on the XYZ scanner 203 in this example, there are other variations in which the cantilever 202 is attached to the XYZ scanner 203, or the sample 201 is attached to an XY scanner and the cantilever 202 is attached to a Z scanner.Moreover, although the figure illustrates the cantilever excitation means 204 similar to a piezoelectric element, it is also possible to utilize photothermal excitation or electromagnetic field.Furthermore, although the optical position detector 205 is used to detect the flexure of the cantilever 202 with the optical lever, it is also possible to apply speed detection by a laser Doppler vibrometer or displacement detection by an optical fiber interferometer.
Fig. 2 shows an exemplary relationship between the probe-sample distance and a force and force gradient acting on the cantilever, and Fig. 3 shows an exemplary relationship between the probe-sample distance and the resonance frequency of the cantilever.The reason why the resonance frequency of the cantilever varies due to the force gradient is that the force which varies dependent on the distance is equivalent to a spring and thus the force acted by the equivalent spring is added to that of a spring inherently provided for the cantilever.However, the equivalent spring will have a negative spring constant when the polarity of the force gradient is positive.When the negative spring constant is applied, the resonance frequency will decrease.
Methods to detect the change in the resonance frequency include: (1) a method in which the cantilever itself is used as a mechanical resonator to configure a self-excited oscillation circuit to detect the change in the oscillating frequency; and (2) a method in which the cantilever is forced to vibrate at a constant frequency near the resonance frequency to detect the change in the resonance frequency from a phase difference between a signal used for the vibration and the detected vibration.Assuming that the above methods (1) and (2) are referred to as the FM (frequency modulation) method and the PM (phase modulation) method, respectively, there is a third method (3) in which, while the forced vibration is used, the frequency for the forced vibration is controlled to follow the resonance frequency by utilizing the detected phase difference.Here, this method is referred to as the tracking separate-excited method.
Since any method above can detect information on a frequency axis with high sensitivity by narrowing a bandwidth to be observed, the dynamic mode AFM allows observation in a region where the probe-sample force is weak as compared to the contact mode AFM, resulting in less damage to the sample and thus the atomic resolution can be obtained more easily.
As described above, the dynamic mode AFM traces the "constant force gradient surface".The "constant force gradient surface" is generally considered to approximate a "constant height surface".Since the force gradient graph of Fig. 2 varies dependent on atomic species, however, the "constant force gradient surface" would be identical to the true "constant height surface" only in the case where the force gradient graph of Fig. 2 does not change while the sample consists of single-element atoms and the probe tip is placed right above an atom or between atoms.Therefore, for the sample consisting of atoms of plural elements, the "constant force gradient surface" is not identical to the true "constant height surface", and the observed atomic species cannot be estimated unless some information on constituent elements or crystal structures of the sample has been preliminarily provided.
Meanwhile, the literature has been published that describes the position of the minimum point (point B where the resonance frequency decreases most, i.e., the point where the force gradient of Fig. 2 is maximum) in the graph of Fig. 3 is characteristic of the atomic species, and thus the atomic species can be determined by obtaining the minimum point position (see Non-Patent Document 1 below).
According to this method, it is possible to color a topographic image (three-dimensional graphic representation of the "constant force gradient surface") of the sample observed by the conventional dynamic mode AFM based on the atomic species obtained from the minimum point position, so as to display the image as if each atomic species is differently colored.
Non-Patent Document 1: Yoshiaki Sugimoto et al., "Chemical identification of individual surface atoms by atomic force microscopy", Nature, Vol. 446, 2007, pp. 64-67
特許請求の範囲(英語) [claim1]
1. A dynamic mode AFM apparatus comprising:
(a) a scanner for performing three-dimensional relative scanning of a cantilever and a sample;
(b) a means for generating an AC signal of a resonance frequency in a mode with flexural vibration of the cantilever;
(c) a means for exciting the flexural vibration of the cantilever with the resonance frequency;
(d) a means for generating an AC signal of a second frequency which is lower than the frequency of the flexural vibration;
(e) a means for modulating a probe-sample distance of the cantilever with the second frequency;
(f) a means for detecting fluctuation of the resonance frequency;
(g) a means for detecting vibration of the cantilever; and
(h) a means for detecting a fluctuation component which is contained in a detected signal by the means for detecting the resonance frequency fluctuation and synchronized with a modulation signal of the probe-sample distance,
(i) wherein an inclination of the resonance frequency against the probe-sample distance is obtained from strength and polarity of the fluctuation component.
[claim2]
2. The dynamic mode AFM apparatus according to claim 1, wherein the probe-sample distance is automatically controlled so that the inclination of the resonance frequency against the probe-sample distance becomes zero.
[claim3]
3. The dynamic mode AFM apparatus according to claim 1 or 2, wherein a frequency in a mode with flexural vibration of a lower order is used as the second frequency, that is different from the frequency in the mode with flexural vibration.
[claim4]
4. The dynamic mode AFM apparatus according to claim 1, 2, or 3, wherein a self-excited oscillation circuit which oscillates at the resonance frequency in the mode is configured as the means for generating the AC signal of the resonance frequency in the mode with flexural vibration of the cantilever, and frequency detection is used as the means for detecting the fluctuation of the resonance frequency.
[claim5]
5. The dynamic mode AFM apparatus according to claim 1, 2, or 3, wherein a self-excited oscillation circuit which oscillates at the resonance frequency in the mode is configured as the means for generating the AC signal of the resonance frequency in the mode with flexural vibration of the cantilever, and phase detection is used as the means for detecting the fluctuation of the resonance frequency.
[claim6]
6. The dynamic mode AFM apparatus according to claim 1, 2, or 3, wherein a signal source to generate an AC signal of a frequency that is a constant frequency around the resonance frequency of the mode or that is controlled to slowly follow the resonance frequency of the mode is used as the means for generating the AC signal of the resonance frequency in the mode with flexural vibration of the cantilever, and the means for detecting the fluctuation of the resonance frequency is configured by detecting a phase of displacement or speed of the cantilever against the signal.
  • 出願人(英語)
  • JAPAN SCIENCE AND TECHNOLOGY AGENCY
  • 発明者(英語)
  • KAWAKATSU HIDEKI
  • KOBAYASHI DAI
国際特許分類(IPC)
参考情報 (研究プロジェクト等) CREST Nano Factory and Process Monitoring for Advanced Information Processing and Communication AREA
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