JP5939144B2 - Scanning probe microscope - Google Patents

Description

  The present invention relates to a scanning probe microscope, and more particularly to a scanning probe microscope that performs measurement by scanning a sample surface while vibrating a cantilever provided with a probe at a frequency near its resonance point.

  A scanning probe microscope (SPM) such as an atomic force microscope (AFM) is widely used for observing fine irregularities on a sample surface and measuring surface roughness. As one of the observation methods in this scanning probe microscope, dynamic scanning is performed along the sample surface while exciting a cantilever equipped with a probe at the free end in the direction of approaching / separating the sample surface in the vicinity of its mechanical resonance frequency. What is called a mode is known (see, for example, Patent Document 1). At the time of observation in the dynamic mode, it is necessary to measure the resonance frequency of the cantilever to be used and adjust the amplitude and frequency of the signal supplied to the actuator (piezoelectric element) that vibrates the cantilever before observation.

  As a method of measuring the resonance frequency of a cantilever, there is a conventional method of supplying a sine wave or rectangular wave signal having a constant amplitude to an actuator for exciting a cantilever and measuring the amplitude of the cantilever at each frequency while changing the frequency. . In this case, as shown in FIG. 4, the measured result is displayed in a graph with the amplitude value (vertical axis) for each frequency (horizontal axis) on the display, and the user resonates the frequency obtained from the graph. This is recognized as a frequency, and that frequency is set as the excitation frequency of the cantilever during observation.

JP 2008-122168 A

  By the way, there are various modes in the resonance frequency of the cantilever, and if the frequency range when measuring the resonance frequency is too wide, there is a possibility that a frequency that is not used in the observation by the scanning probe microscope is measured as the resonance frequency. is there.

  Therefore, in order to reliably measure the necessary resonance frequency, it is necessary to set a frequency range that matches the characteristics of the cantilever in advance and measure the amplitude of the cantilever while changing the frequency within the range. On the other hand, there are various types of cantilevers used in the scanning probe microscope, and their frequency characteristics are also various. Until now, it was necessary to grasp the information of the cantilever used by the user, and to set the frequency range appropriately using the content to measure the resonance frequency. However, when the information of the cantilever to be used is unknown, it is necessary to perform measurement while changing the frequency range for measuring the resonance frequency by trial and error.

  The present invention has been made in view of such circumstances, and provides a scanning probe microscope capable of always measuring a resonance frequency in an appropriate frequency range even when information on a cantilever to be used is unknown. It is an issue.

In order to solve the above problems, the scanning probe microscope of the present invention is equipped with a cantilever equipped with a probe, and is driven along the surface of the sample while vibrating the cantilever at a frequency near the resonance point by driving an actuator. In a scanning probe microscope that obtains information on a sample surface based on a change in vibration of the cantilever due to the interaction between the sample surface and the probe by scanning, an imaging unit that images the cantilever, and the imaging unit From the captured cantilever image, the length and width of the cantilever are read, the thickness of the cantilever is assumed to be a preset specified value, the material of the cantilever is assumed to be Si, and the resonance frequency of the cantilever is calculated by an approximate expression. It is characterized by comprising a resonance frequency estimating means for estimating (Claim 1).

  In the present invention, the amplitude at each frequency is measured while changing the excitation frequency of the cantilever by the actuator within a specified frequency range centered on the resonance frequency estimated by the resonance frequency estimation means. Further, it is possible to suitably employ a configuration (represented by claim 2) including a resonance frequency determining means for determining the frequency at which the maximum amplitude is obtained as the actual resonance frequency.

In the present invention, the resonance frequency estimation means, on the cantilever image captured by the image pickup means, by specifying the portion corresponding to the length and width of the cantilever, that read the length and width of the upper Symbol cantilever The configuration (claim 3) can be adopted.

Alternatively, in the present invention, the resonance frequency estimation means is employed by the image processing cantilever image captured by the image pickup means, determined Mel constituting the length and width of the cantilever (claim 4) You can also

  In the present invention, the frequency characteristics of a cantilever largely depend on its length and width, and in this type of scanning probe microscope, an optical microscope is usually attached as an auxiliary observation mechanism in many cases. The present invention intends to solve the problem by utilizing the ability to image a cantilever with this optical microscope.

That is, the resonance frequency of the cantilever can be expressed by a known general formula shown below.
f _o = (1 / 2π) · (1.875 / L) ² · √ (EI / ρA) (1)
In this formula (1), f _o is the resonance frequency, L is the length of the cantilever, E is the Young's modulus, I is the moment of inertia of the cross section, A is the cross sectional area of the cantilever, and ρ is the density.

  On the other hand, the optical microscope attached to the scanning probe microscope can usually image the cantilever from directly above, and therefore the length L and width of the cantilever are known from the imaging result of the cantilever by this optical microscope. be able to. However, the thickness cannot be known from the imaging result.

  In the equation (1), the parameters requiring the thickness are the cross-sectional secondary moment I and the cross-sectional area A. However, the cantilever used in this type of scanning probe microscope is not extremely different, although the thickness varies. Therefore, even if the cross-sectional secondary moment I and the cross-sectional area A are approximated assuming that the thickness is constant or the same as the width, for example, the obtained result does not become an extremely deviated value.

  In Formula (1), the Young's modulus E and density ρ vary depending on the material of the cantilever, but most of the cantilevers used in this type of scanning probe microscope are Si. Therefore, even if the Young's modulus E and the density ρ are calculated by the equation (1) using Si, no significant error occurs in the obtained result except for the special one.

  Therefore, for example, by using Equation (1) as an approximate equation and obtaining and substituting the length and width obtained from the image of the cantilever attached to the apparatus, the resonance frequency of the cantilever can be roughly estimated.

  The estimated resonance frequency of the cantilever may be notified to the user, for example, by displaying it on a display. However, as in the invention according to claim 2, the apparatus side automatically executes a frequency counting operation. May be.

  That is, in the invention according to claim 2, the frequency is automatically changed within the specified frequency range around the estimated resonance frequency, and the frequency at which the maximum amplitude is obtained is determined as the resonance frequency.

  In the present invention, as a method for obtaining the length and width from the image of the cantilever, as in the invention according to claim 3, the actual size is obtained by designating a portion corresponding to the length and width on the cantilever image, Alternatively, any of the methods for obtaining the actual length and width dimensions by image processing as in the invention according to claim 4 can be adopted.

  According to the present invention, when a cantilever is attached and imaged with an imaging means (optical microscope), the resonance frequency of the cantilever is estimated based on the image of the cantilever, so a cantilever with unknown frequency characteristics is used. Even when doing so, it is possible to immediately measure the resonance frequency in an appropriate frequency range. As a result, it is not necessary to change the resonance frequency by mistake or to change the setting of the frequency range by trial and error, and the measurement time can be shortened.

The block diagram which concerns on embodiment of this invention. The figure which shows the example of a cantilever image imaged with the optical camera. The flowchart which shows the procedure of estimation of the resonant frequency of a cantilever and determination operation | movement in embodiment of this invention. The figure which shows the example of a graph display of the amplitude measurement result which concerns on the excitation frequency change of a cantilever.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a configuration diagram according to an embodiment of the present invention, and shows a schematic diagram showing a mechanical configuration and a block diagram showing a system configuration.

  A sample W to be observed is held on a sample table 2 provided on a drive mechanism (scanner) 1 made of a piezoelectric element or the like. The drive unit 1 can move the sample W in the x and y directions along the surface thereof, and at the same time finely move the sample W in the z direction orthogonal to the plane. The drive control signal of the drive mechanism 1 is supplied from a drive circuit 13 described later.

  Above the sample stage 2, a cantilever 3 having a probe 3 a provided at the tip (free end) portion is arranged. The cantilever 3 is attached to an excitation actuator 4 made of a piezoelectric element. The excitation actuator 4 is vibrated in the direction approaching / separating from the sample W, that is, in the z direction by application of an AC voltage from the excitation voltage generation circuit 5.

  The displacement of the cantilever 3 is detected by a known optical lever type displacement detection mechanism including a laser diode 6, a beam splitter 7, a mirror 8 and a photodetector 9. That is, the laser light output from the laser diode 6 is guided to the upper surface of the cantilever 3 by the beam splitter 7, and the reflected light enters the photodetector 9 through the mirror 8. In this photodetector 9, the light receiving surface is divided into two in the moving direction of the reflected light due to the movement of the cantilever 3 in the z direction, and the displacement of the cantilever 3 in the z direction due to the change in the amount of incident light on each light receiving surface. Can be requested. The output of the photodetector 9 is taken into the displacement detector 10 to generate a displacement signal, and the displacement signal is taken into the amplitude detector 11.

The amplitude detector 11 detects the amplitude value from the displacement signal. The detection output is taken into the control unit 12. When observing the sample W, the control unit 12 moves the drive mechanism 1 in the z direction through the drive circuit 13 so that the detection result of the amplitude value of the cantilever 3 from the amplitude detection unit 11 becomes a constant value. The amount of movement in the z direction becomes data representing the surface information of the sample W.
That is, when an attractive force or a repulsive force acts between the cantilever 3 that vibrates near the resonance frequency and the surface of the sample W, the vibration amplitude of the cantilever 3 changes. At the time of observation, the control unit 12 scans the drive mechanism 1 in the x and y directions through the drive circuit 13 and moves the drive mechanism 1 in the z direction through the drive circuit 13 so that the vibration amplitude of the cantilever 3 maintains a constant value at each position. Move to. The movement control signal in the z direction at each position in the x and y directions is taken into the personal computer 16 as surface information at each position in the x and y directions of the sample W and displayed on the display unit 17 as a surface information image of the sample W. Is done.

  An optical microscope 14 as an auxiliary observation mechanism is provided above the drive mechanism 1, and this optical microscope 14 has a built-in CCD 14 a, and its imaging output is sent to a personal computer 16 via an image data capturing circuit 15. Is taken in. The surface of the sample W can be supplementarily observed with the optical microscope 14, and the laser spot from the laser diode 6 is correctly positioned on the upper surface of the cantilever 3 while observing the upper surface of the cantilever 3 with the optical microscope 14. The optical system of the displacement detection mechanism can be adjusted.

  In the observation in the dynamic mode, the cantilever 3 is moved in the x and y directions along the surface of the sample W while vibrating the cantilever 3 in the z direction near the resonance point. Prior to the observation, It is necessary to know the resonance frequency of the cantilever 3 to be used and to set the oscillation frequency of the excitation voltage generating circuit 5 based on that frequency. The feature of this embodiment is that it has a function of determining the resonance frequency of the cantilever 3, and this function is realized by a program installed in the personal computer 16.

  The contents will be described below with reference to an example of a cantilever imaging screen on the display unit 17 shown in FIG. 2 and a flowchart showing a cantilever resonance frequency estimation and determination procedure shown in FIG.

  First, the cantilever 3 is imaged by the optical microscope 14 shown in FIG. 1, and the cantilever image is displayed on the display unit 17, and the cursor operation with the mouse or the like of the operation unit 18 of the personal computer 16 is performed as shown in FIG. Then, on the cantilever image Ic, line segments Cx and Cy are drawn at portions corresponding to the length and width, respectively. Thereby, the personal computer 16 reads the length L and the width B in the actual dimensions of the cantilever 3 from the number of pixels of the lengths of the line segments Cx and Cy and the known imaging magnification by the optical microscope 14.

Next, using the read length L and width B, for example, to estimate the resonance frequency f _o of the cantilever 3 by using the equation (1). At this time, although the thickness of the cantilever 3 is unknown, this is assumed to be a preset specified value, and the sectional secondary moment I and sectional area A in the equation (1) are calculated from the assumed value and the width B. determines, also, the material is on is unknown determining the Young's modulus E and density ρ in equation (1) this as Si, determine the resonant frequency f _o by the formula (1).

Then, around the estimated resonance frequency f _o, so as to actually excite the cantilever 3 in the defined frequency range, giving a drive instruction to the excitation voltage generating circuit 5. During the excitation, by reading the amplitude value at each vibration frequency from the amplitude detector 11, determines the frequency to obtain a maximum amplitude value as the actual resonant frequency f _r of the cantilever 3, it is displayed on the display 17 Notify users.

  Therefore, according to the embodiment of the present invention, the user images the cantilever 3 with the optical microscope 14, and draws the line segments Cx and Cy by the cursor operation on the length and width portions on the cantilever image Ic of the imaging screen. Thereafter, the resonance frequency of the cantilever 3 can be automatically known, and even if the frequency characteristics of the cantilever 3 are unknown, the resonance frequency can be quickly grasped and adjustment for observation can be performed.

  Here, in the above embodiment, an example is shown in which the actual dimensions are obtained from the line segment lengths by drawing the line segments Cx and Cy on the cantilever image Ic. However, the image of the cantilever 3 by the optical microscope 14 is shown. It is also possible to perform image processing, automatically recognize the length and width portions on the image, and convert them to actual dimensions.

Further, in the above embodiment, based on the estimated resonance frequency f _o, an example of determining the actual resonant frequency f _r performed automatically vibrated tested in the defined frequency range centered on this showed, the present invention may be in the estimated resonance frequency f _o just to inform the user.

DESCRIPTION OF SYMBOLS 1 Drive mechanism 2 Sample stand 3 Cantilever 4 Excitation actuator 5 Excitation voltage generation circuit 6 Laser diode 7 Beam splitter 8 Mirror 9 Photodetector 10 Displacement detection part 11 Amplitude detection part 12 Control part 13 Drive circuit 14 Optical microscope 14a CCD
15 Image Data Acquisition Circuit 16 Personal Computer 17 Display 18 Operation Unit W Sample

Claims (4)

By mounting a cantilever with a probe and scanning along the surface of the sample while vibrating the cantilever at a frequency near the resonance point by driving the actuator, the interaction between the sample surface and the probe In a scanning probe microscope that obtains information on the sample surface based on the change in vibration of the cantilever,
Imaging means for imaging the cantilever;
From the cantilever image picked up by the image pickup means, the length and width of the cantilever are read, the thickness of the cantilever is assumed to be a predetermined value set in advance, the material of the cantilever is assumed to be Si, A scanning probe microscope comprising resonance frequency estimating means for estimating a resonance frequency of a cantilever.   Within the specified frequency range centered on the resonance frequency estimated by the resonance frequency estimation means, the amplitude at each frequency was measured while changing the excitation frequency of the cantilever by the actuator, and the maximum amplitude was obtained. 2. The scanning probe microscope according to claim 1, further comprising resonance frequency determining means for determining the frequency as an actual resonance frequency. The resonance frequency estimation means, on the cantilever image captured by the image pickup means, by specifying the portion corresponding to the length and width of the cantilever, characterized by Rukoto reading the length and width of the upper Symbol cantilever The scanning probe microscope according to claim 1 or 2. The resonance frequency estimation means, by image processing of the cantilever image captured by the image pickup means, scanning probe according to claim 1 or 2, characterized in that the length and width of the cantilever Mel determined microscope.

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