JP2003185555A - Frequency detecting method and scanning probe microscope using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To speed up a probe microscope for detecting changes of resonance frequencies caused by a force generated when a vibrating probe is brought close to the surface of a sample and stabilize it by reducing noise. <P>SOLUTION: No phase lock loop (PLL) is used for detecting frequencies. Changes of frequencies are directly computed from a phase difference signal detected by a phase comparator, and used to control the distance between the probe and the surface of the sample. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention vibrates a probe having a mechanical resonance part such as a cantilever at its resonance frequency, and produces a force generated when the probe and a sample come close to each other. The present invention relates to a scanning probe microscope observing device for observing a physical quantity on the surface of a sample by detecting.

[0002]

2. Description of the Related Art In recent years, a scanning probe which allows a probe and a sample to come close to each other and directly observes an electronic structure on the surface of a substance or in the vicinity of the surface by utilizing a physical phenomenon (tunnel phenomenon, atomic force, etc.) generated at that time. A microscope (hereinafter abbreviated as SPM) has been developed, and it has become possible to measure real space images of various physical quantities with high resolution regardless of single crystal or amorphous.

Among them, a scanning atomic force microscope (hereinafter referred to as AFM)
Is called a weak acting force (atomic force) between the atom at the probe tip and the atom on the sample.
In order to detect irregularities on the surface of the sample by detecting, it does not require special properties such as conductivity and magnetism in the probe and sample, and it is effective for measuring the shape of insulators, especially organic substances these days. There is. The AFM is roughly classified into two types, one used when the atomic force is repulsive and the other used when the atomic force is attractive, and the former may be called a contact mode AFM and the latter a non-contact mode AFM.

The contact mode AFM measures the repulsive force between the object to be measured and the tip of the probe. In this case, the repulsive force changes greatly with respect to the change in the distance between the probe and the surface to be measured, and therefore the amount of change in the deflection of the probe that receives the force is large and the sensitivity is large, so the load on the measurement system can be small. . However, the probe and the measurement surface are very close to each other, and the force sometimes exerts more than elastic deformation on the measurement surface and the probe, and may damage the sample or the probe tip. It has a great influence on the measurement of the above-mentioned organic matter, especially a soft sample such as a biological material, and the probe deforms or destroys the measurement object, so that accurate observation cannot be performed.

On the other hand, the non-contact mode AFM measures the interatomic attractive force between the probe tip and the surface to be measured. The attractive force works because the distance between the probe tip and the surface to be measured is larger than in the contact mode. In addition, the influence on both the probe tip and the surface to be measured is very small. Therefore, the contact mode AFM does not have the above-mentioned drawbacks of the contact mode AFM, and is useful for measuring a soft sample.

However, a disadvantage of the non-contact mode is that the change in force is not very sensitive to the change in the distance between the probe tip and the sample surface.
Therefore, generally, a cantilever-shaped probe is slightly vibrated at the resonance frequency, and changes in the resonance state when a small attractive force acts between the probe tip and the sample surface, such as changes in amplitude, frequency, and phase, are monitored. By indirectly measuring.

Among them, in order to measure with high sensitivity, a method of exciting a cantilever-shaped probe at a resonance frequency and detecting a change in the resonance frequency or a phase change is adopted. In practice, a method using a self-excited oscillation system that constantly excites the probe at the resonance frequency by making the excitation frequency follow the change of the resonance frequency with an amplifier using an auto gain controller is generally used. .

Further, in the case of the above-mentioned structure,
Physical information on the sample surface is obtained as changes in resonance frequency and phase. That is, frequency modulation (F
M) A system for detecting the signal is required. FM such as radio
Various methods have been conventionally used for detection, and all of them can be used for frequency detection of non-contact mode AFM, but the main method is to use a phase-locked loop (hereinafter referred to as PLL). Has been adopted.

[0009]

As described above, the non-contact mode AFM based on the frequency detection uses the AFM signal by detecting the change in the frequency. When this frequency change is detected, it can be detected with high accuracy because the fundamental frequency is stable. Therefore, it is common to vibrate at the resonance frequency of the cantilever used as a probe. In addition, it is necessary that this fundamental frequency is high for high-speed detection. That is, the probe used has a resonance frequency as high as possible. A typical resonance frequency of a cantilever-shaped probe used for AFM is about several 100 Hz to several 100 kHz.

On the other hand, the frequency change of the probe is, for example, 30
When a probe having a resonance frequency of about 0 kHz is used, the size is about 100 Hz, and at least about 0.1 Hz is required as the measurement resolution, which is extremely smaller than the fundamental frequency.

For example, conventionally, a phase locked loop (PLL) is used.
It used the frequency detection method according to
Frequency stability of the voltage controlled oscillator used internally,
That is, the influence of S / N of the control signal input to the oscillator cannot be ignored, and stable high-sensitivity frequency measurement cannot be performed. That is, even when using an oscillator having a relatively high stability using a crystal oscillator or the like as disclosed in Japanese Patent Laid-Open No. 2001-33465, the oscillation source is stable, but it is used as a voltage controlled oscillator. Therefore, the noise of the control voltage is mixed in not only the observation signal but also the feedback system. In order to suppress this noise, it is necessary to drop the control signal band, but in reality, the PLL
Since the phase-locked loop of is included in the feedback system (control of the distance between the probe tip and the sample surface), it is configured like double feedback, and when trying to increase the overall measurement speed, There was a problem that bands approached, interference was caused, and control became unstable.

[0012]

In order to solve the above problems, the present invention uses the following means to provide a non-contact mode AFM capable of highly sensitive and highly accurate frequency measurement without using a conventional PLL circuit. A cantilever-shaped AFM probe, sample holding means for holding a measurement sample facing the AFM probe, two-dimensional scanning means for relatively scanning the lever probe parallel to the measurement sample surface, Vibrating means for applying vibration to the lever probe, displacement detecting means for detecting the tip displacement of the lever probe, frequency measuring means for measuring the frequency of the signal from the displacement detector, the frequency measuring means is frequency adding means, Addition frequency signal generation means, reference signal generation means, phase comparison means for detecting the phase difference between the reference signal and the detection signal, phase comparison signal from the phase comparison means Error signal generating means for calculating a difference between a signal measured by the frequency measuring means and a preset frequency, and a probe based on the error signal from the error signal generating means. Distance control means for controlling the distance between the tip and the sample surface, the distance control means is a control amount calculating means for calculating a control amount from the error signal from the error signal generating means, and the distance between the probe tip and the sample surface is displaced. And driving means, and amplifier means for converting the control amount output from the control amount calculating means into a drive signal for the driving means.

[0013]

DETAILED DESCRIPTION OF THE INVENTION An embodiment of the present invention will be described with reference to FIG.

The XYZ stage 124 includes the probe 101.
The distance between the tip of the probe and the surface of the sample 102 is set, and the probe 101 is relatively two-dimensionally scanned in parallel with the surface of the sample 102. At this time, when the distance between the probe 101 and the surface of the sample 102 is a predetermined distance, the tip of the probe 101 and the sample 10
A physical force is generated between the two surfaces, and the cantilever-shaped probe 101 bends. This bending changes the angle of the reflected light generated when the laser beam emitted from the laser 104 hits the tip of the probe 101. This change in angle is detected by the four-division photodiode 105. That is, the magnitude of the force acting between the tip of the probe 101 and the surface of the sample 102 is detected as the amount of bending of the probe 101, and the amount of bending is converted into a change in angle. The position of the spot is changed, and the change in the position becomes a current output and is input to the preamplifier 106 at the next stage. Preamplifier 1
At 06, the current value is converted into a voltage, the output from each photodiode of the four-division photodiode 105 is calculated, and AF is performed.
An M signal (lever flexure amount) is generated. AFM generated
The signal is input to the auto gain control amplifier 107 and shaped into a constant amplitude.

The AFM signal having a constant amplitude is output to the phase shifter 108 and the mixer 110.

The signal of the predetermined amplitude input to the phase shifter 108 is adjusted in phase and is applied to the piezoelectric vibrator 103 through the driver amplifier 109. This shaker 10
Since the probe 101 is installed on the 3
The probe 101 is vibrated by 03. In the initial state, the probe 101 vibrates slightly at the resonance point due to thermal vibration or the like.
Since this is detected by the M signal detection system, the feedback system starts self-oscillation by adjusting the phase shifter 108.

Now, the AF input to the mixer 110 on the other hand
M signal is a local oscillator 1 having a crystal oscillator 1 (114)
The signal having the predetermined frequency oscillated from (113) is mixed (multiplied) and output. After that, only a signal in a predetermined band is extracted by the bandpass filter 111 and sent to the phase comparator 112 in the next stage.

The phase comparator 112 combines the AFM signal to which the frequency of the signal oscillated from the local oscillator 1 is added and the reference signal output from the local oscillator 2 (115) including the crystal oscillator 2 (116). The phases are compared and a signal indicating the phase difference is output. The phase difference signal is sent to the next phase signal processing 117.

A PLL (Phase Locked Loop) is often used as the FM signal detection system of the conventional non-contact mode AFM. This is the configuration as shown in FIG. First, the input signal of the AFM is input to the phase comparator 401. The phase comparator 401 detects and outputs a phase difference between the reference signal output from the voltage controlled oscillator 403 and the input signal. The phase difference signal passes through the filter and becomes the input of the voltage controlled oscillator 403. That is, when the phase difference between the input signal and the reference signal is deviated, a control voltage corresponding to the deviation amount is sent to the voltage controlled oscillator to raise or lower the frequency of the reference signal. By this feedback, the frequency and phase of the voltage controlled oscillator 403 are controlled so that they match the input signal exactly. The output signal corresponding to the AFM signal is the control voltage of the voltage controlled oscillator 403.

If the phase locked loop shown in FIG. 4 is applied to the block diagram shown in FIG. 1, the phase comparator 112 is used.
It is necessary to control the oscillation frequency of the local oscillator 2 (115) from the phase difference signal from. However, when such control is performed, the detection speed decreases due to the time constant (time until the frequency locks) associated with the phase locked loop. This time constant is determined by the value of the filter shown in FIG. 4, but in order to reduce the noise of the output frequency fluctuation of the voltage controlled oscillator 403, the time constant of the filter must be increased, so that the detection speed is improved and the noise is reduced. Are contradictory.

Therefore, in the present invention, the phase shift itself is detected without forming the phase locked loop, and the synchronization is not applied. The phase comparator 112 has a configuration shown in FIG.
Detects the phase difference between two signals (measurement signal and reference signal).
In this case, the measurement signal that has passed through the bandpass filter 111 and the reference signal that is sent from the local oscillator 2 (115) are binarized in the phase comparator 112 and converted into a rectangular wave train as shown in FIG. Become. The phase comparator 112 detects the rising portion of the two signals and calculates a signal having a pulse width representing the phase difference as shown in FIG. Next, this phase difference signal is output from the phase comparator 112 to the next phase signal processing 117.
Sent to.

The phase signal processing 117 outputs the difference in frequency by the configuration shown in FIG. When the signal shown in FIG. 3B is input, it is integrated by the integrating circuit 201.
The local oscillator 2 (11
It receives the signal from 5) and operates to reset the integration at the same time as the rising of the reference signal. As a result, Fig. 3
A sawtooth waveform whose crest value changes as shown in (c) is output. The waveform is output to the peak hold circuit 202 at the next stage. The peak hold circuit 202 is configured to sample and hold the peak portion of the input sawtooth wave using a timing signal. As a result, the peak hold circuit 202 shown in FIG.
A stepwise waveform as shown in (d) is output. Since the slope of this step-like waveform is proportional to the frequency difference between the input signal and the reference signal, the slope is obtained by the filter 203 and the differentiating circuit 204 that follow. Filter 203
As a result, a gentle signal having no step is formed as shown in FIG. 3E, and then the frequency difference signal as shown in FIG. 3F is output by the differentiating circuit.

When the frequency difference signal is output from the phase signal processing 117, it is compared with a preset reference frequency by the error signal generation 119 in the next stage to generate an error signal. In the non-contact AFM, when the probe is vibrated at the resonance frequency of the free vibration of the probe and the probe is brought close to the measurement sample surface and a force is generated between the probe tip and the sample surface, the binding force is added to the free vibration to cause resonance. The frequency shifts according to the distance between the probe tip and the sample surface. Therefore, in order to apply the control with the force kept constant, it is necessary to perform the distance control so as to keep the shift amount constant. The frequency corresponding to this shift amount is used to generate the error signal 11
It is necessary to set in advance in 9. When the frequency difference signal is larger or smaller than this shift amount, an error signal is output from the error signal generation 119, and the value is sent to the control amount calculation 120 of the next stage.

The control amount calculation 120 outputs a drive signal for driving the stage 124 in the direction between the probe and the sample surface from the error signal. Generally these stages 124
Is composed of a mechanism using a piezo actuator. In such a case, it is often composed of a control amount calculation section by PID control and an addition section for integrating it for piezo drive. The amplifier 122 is for optimizing the drive signal for the piezo actuator.

As described above, the stage 124 is constructed so that it can be driven in the Z-direction element which is driven in the direction between the probe and the sample surface and the direction perpendicular thereto, that is, in the XY directions which are the inward directions of the measured sample surface. An XY scanning control 121 and an amplifier 123 are prepared as a drive signal generating mechanism.

Finally, the data processing 116 takes in the phase difference signal output from the phase signal processing 117 and the control amount output from the control amount calculation 120 and processes it as AFM data. Specifically, it includes visualization as an AFM image, analysis by image processing, and the like.

A non-contact AFM is realized by the above configuration and the operation of each part.

The non-contact AFM measurement was specifically carried out by the apparatus constructed according to the above-mentioned mode.

The probe has a length of 125 μm and a width of 25.
A Si cantilever having a thickness of 5 μm and a thickness of 5 μm was used. The resonance frequency of this probe is f _R
Was ˜321 kHz. The vibrator 103 uses a single-layer piezo element, and the output amplitude of the auto gain control amplifier is adjusted so that the displacement of the AFM becomes 5 nm.

Further, the local oscillator 1 uses a crystal oscillator having a center frequency of f _{OCS1 to} 4.7 MHz. The local oscillator 2 (frequency f _OCS2 ) also uses a crystal oscillator, but the frequency can be changed to some extent by an external element. Since a constant frequency is sufficient, even if a VCO using a crystal oscillator is used, for example, it is not necessary to superimpose a signal component on the control voltage, so that it can be configured to be strong against noise. In this embodiment, the VCO
Was used. The oscillation frequency is about 5.0 MHz at the center. When the probe and the sample are sufficiently separated from each other, the output from the mixer shows both the sum and difference of the frequency of the local oscillator 1 and the resonance frequency of the probe. In the present embodiment, the sum frequency is changed by the bandpass filter. Using. That is, f
_OCS1 a + _{f R.}

Let f _{OCS2 to} f _OSC1 + f _R. It can be obtained by adjusting the frequency of the local oscillator 2 so that the output from the phase signal processing 117 becomes zero when the probe is not brought close to the measurement sample (during free vibration).

The reference value set in the error signal generator 119 was set to a value corresponding to 50 Hz as the frequency shift amount.

When HOPG (highly oriented graphite) was observed as a measurement sample with the above settings, a stable AFM image with little noise was obtained with atomic level resolution. Moreover, the conventional PLL is used as the measurement band.
The speed was higher than that of the Z-actuator, and a speed of a little less than 20 kHz, which is the mechanical resonance of the Z actuator (cylindrical piezo in this embodiment) used for the stage, was obtained.

[0034]

As described above, since the conventional frequency measurement system does not use the phase-locked loop, high-speed observation and measurement of the non-contact AFM can be performed. Since it is no longer necessary to control the frequency of an oscillator such as a VCO by feedback, an AFM with very little noise can be obtained by using an oscillator with less fluctuation in output frequency.
It becomes possible to measure.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a non-contact AFM of the present invention.

FIG. 2 is a diagram illustrating phase signal processing used in the non-contact AFM of the present invention.

FIG. 3 is a signal waveform showing a detection process by phase signal processing.

FIG. 4 is a diagram for explaining a conventional phase locked loop used for frequency detection.

[Explanation of symbols]

101 AFM probe 102 measurement sample 103 shaker 104 laser 105 4-division photodiode sensor 109 Exciter drive amplifier 122, 123 XYZ stage drive amplifier

Claims (6)

[Claims] 1. When bringing a probe close to a measurement sample while vibrating the probe at a resonance frequency of mechanical resonance of the probe,
A probe having a cantilever shape, including the following in a scanning probe microscope for observing fine irregularities of the measurement sample by utilizing a change in the resonance frequency caused by a force generated between the probe and the measurement sample: Holding mechanism for holding the probe facing the probe, a two-dimensional scanning mechanism for relatively scanning the probe parallel to the surface of the measurement sample, a vibration mechanism for applying vibration to the probe, Displacement detection mechanism for detecting tip displacement, frequency measurement mechanism for measuring the frequency of the detection signal from the displacement detector, frequency measurement mechanism for adding a constant frequency to the detection signal, addition mechanism for generating a constant frequency signal Frequency signal generation mechanism, filter that filters the detection signal with a fixed frequency added in a predetermined band, and detects the phase difference between the filtered signal and the reference signal It has a phase comparison mechanism for outputting, a reference signal generation mechanism for generating the reference signal, and a phase signal processing mechanism for calculating the frequency from the phase comparison signal from the phase comparison mechanism. The signal measured by the frequency measurement mechanism and preset An error signal generation mechanism that calculates a difference from the frequency, a distance control mechanism that controls the distance between the probe tip and the sample surface based on an error signal from the error signal generation mechanism, and the distance control mechanism is an error from the error signal generation mechanism. Control amount calculating mechanism for calculating control amount from signal, drive mechanism for displacing distance between probe tip and sample surface, amplifier for converting control amount output from the control amount calculating mechanism to drive signal of the driving mechanism It consists of a mechanism. 2. The scanning probe microscope according to claim 1, wherein the phase signal processing mechanism has an integration mechanism, a peak hold mechanism, a filter mechanism, and a differentiation mechanism, and the integration mechanism and the peak hold mechanism are reference signals. It is characterized in that it operates by referring to a reference signal from the generation mechanism as a timing signal. 3. The scanning probe microscope according to claim 1, wherein the reference signal generating mechanism has a crystal oscillator, and the reference signal is generated using a frequency generated by the crystal oscillator. To do. 4. The scanning probe microscope according to claim 1, wherein the addition frequency signal generating mechanism has a crystal oscillator, and the reference signal is generated by using the frequency generated by the crystal oscillator. And 5. When the probe is brought close to a measurement sample while vibrating at a resonance frequency of mechanical resonance of the probe,
A method for detecting the frequency of tip displacement of a probe in a scanning probe microscope for observing fine irregularities of the measurement sample by utilizing a change in the resonance frequency caused by a force generated between the probe and the measurement sample. The step of adding a signal of a constant frequency to the detection signal output by the displacement detection mechanism that detects the displacement of the tip of the probe, the step of filtering the detection signal to which the constant frequency is added, the filtered signal and the reference signal. And outputting a signal corresponding to the phase difference, and calculating a frequency from the signal. 6. The frequency detecting method according to claim 5, wherein the step of calculating the frequency includes a step of integrating a signal corresponding to a phase difference, a step of detecting a maximum value of the integrated signal, and a maximum value signal. It is characterized in that it comprises a step of filtering, a step of calculating a derivative of the filtered signal, and the integrating step and the maximum value detecting step operate by referring to a reference signal as a timing signal.