JP3935350B2 - Distance control method and scanning probe microscope using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention oscillates a probe having a mechanical resonance part such as a cantilever at the resonance frequency, and detects the amount of change in the resonance frequency caused by the force generated when the probe and the sample come close to each other. The present invention relates to a scanning probe microscope observing apparatus for observing the physical quantity.
[0002]
[Prior art]
In recent years, a scanning probe microscope (hereinafter referred to as SPM) that can directly observe the electronic structure of a material surface and the vicinity of the surface by using a physical phenomenon (tunnel phenomenon, atomic force, etc.) generated by bringing the probe close to the sample. (Abbreviated) has been developed, and real space images of various physical quantities can be measured with high resolution regardless of single crystal or amorphous.
[0003]
Among them, the scanning atomic force microscope (hereinafter referred to as AFM) measures the unevenness of the sample surface by detecting a weak acting force (atomic force) between the atoms at the tip of the probe and the atoms on the sample. Therefore, the probe and the sample do not require special properties such as conductivity and magnetism, and are effective for measuring the shape of an insulator, particularly an organic substance. AFM can be broadly divided into two types, one used when the interatomic force is in the repulsive state and the other used in the attractive state. The former is sometimes referred to as contact mode AFM, and the latter is referred to as non-contact mode AFM.
[0004]
The contact mode AFM measures the repulsive force between the measurement object and the probe tip. 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. Therefore, the amount of change in the probe bending due to the force is large and the sensitivity is high, so the load on the measurement system can be reduced. . However, the probe and the measurement surface are very close to each other, and the force sometimes affects the measurement surface and the probe more than elastic deformation, and may damage the sample and the probe tip. The influence on the measurement of the above-mentioned organic matter, in particular, a soft sample such as a biological material, is great, and the probe deforms or destroys the measurement target, so that accurate observation cannot be performed.
[0005]
On the other hand, the non-contact mode AFM measures the interatomic attractive force between the probe tip and the surface to be measured. However, since the attractive force acts from a state where the distance between the probe tip and the surface to be measured is larger than the contact mode, The effect on both the tip and the surface to be measured is very small. Therefore, the contact mode AFM does not have the disadvantages of the contact mode AFM described above, and is useful for measuring soft samples.
[0006]
However, a disadvantage of the non-contact mode is that the change in force is not very sensitive to changes in the distance between the probe tip and the sample surface. Therefore, in general, a cantilever-like probe is microvibrated at a 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, phase, etc., are monitored. Measure indirectly.
[0007]
In particular, in order to perform measurement with high sensitivity, a method is adopted in which a cantilever-like probe is excited at a resonance frequency and a change in the resonance frequency or a phase change is detected. In practice, a method using a self-excited oscillation system in which a probe is continuously vibrated at a resonance frequency by causing the excitation frequency to follow a change in the resonance frequency by an amplifier using an auto gain controller is generally performed. .
[0008]
In the case of the configuration as described above, physical information on the sample surface is obtained as a change in resonance frequency or phase. That is, for detecting physical information, a system for detecting a frequency modulation (FM) signal is required. Various methods have been conventionally used for FM detection such as radio, and all of them can be used for frequency detection in non-contact mode AFM, but the main method is a phase-locked loop (hereinafter referred to as PLL). Is called).
[0009]
[Problems to be solved by the invention]
As described above, the non-contact mode AFM by frequency detection is used as an AFM signal by detecting a change in 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 a cantilever used as a probe. In addition, when the detection is performed at a high speed, the fundamental frequency needs to be large. In other words, a probe having a resonance frequency as high as possible is used. A typical resonance frequency of a cantilever-like probe used for AFM is about several hundred Hz to several hundred kHz.
[0010]
On the other hand, the frequency change of the probe is, for example, about ˜100 Hz when a probe having a resonance frequency of about 300 kHz is used, and a measurement resolution of at least about 0.1 Hz is required. Very small compared to
[0011]
For example, in the past, a frequency detection method using a phase-locked loop (PLL) was used. According to this method, however, the frequency stability of a voltage-controlled oscillator used therein, that is, the S / S of a control signal input to the oscillator is used. The influence of N cannot be ignored, and stable high-sensitivity frequency measurement could not be performed. That is, even when a relatively stable oscillator using a crystal oscillator or the like is used as disclosed in JP-A-2001-33465, the oscillation source is stable, but this is used as a voltage controlled oscillator. Therefore, the noise of the control voltage is mixed not only in the observation signal but also in the feedback system. In order to suppress this noise, it is necessary to reduce the control signal band. However, in reality, since the PLL phase-locked loop is included in the feedback system (distance control between the probe tip and the sample surface), it looks like double feedback. When the entire measurement speed is to be increased, the two feedback system bands approach each other, causing interference, and the control becomes unstable.
[0012]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention uses the following means to construct a non-contact mode AFM capable of highly sensitive and highly accurate frequency measurement without using a conventional PLL circuit. A probe, sample holding means for holding the measurement sample opposite to the AFM probe, two-dimensional scanning means for relatively scanning the lever probe parallel to the measurement sample surface, and the lever probe and the measurement sample surface Distance displacement means for displacing the distance, vibration means for applying vibration to the lever probe, displacement detection means for detecting the tip displacement of the lever probe, addition frequency signal generation means, addition frequency from the addition frequency signal generation means A frequency adding means for adding the frequency of the signal to the frequency of the displacement signal output from the displacement detecting means; and a signal from the frequency adding means. First filter means for filtering the reference signal generating means for generating a reference signal of constant frequency, a phase comparator for detecting a phase difference between the output signal from the reference signal and the first filter means from said reference signal generating means Means, second filter means for filtering and integrating the signal from the phase comparison means, control amount calculation means for receiving a signal from the second filter means and calculating a control amount for driving the distance displacement means And driving amplifier means.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will be described with reference to FIG.
[0014]
The XYZ stage 123 sets the distance between the tip of the probe 101 and the surface of the sample 102 and causes the probe 101 to relatively two-dimensionally scan parallel to 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, a physical force is generated between the tip of the probe 101 and the surface of the sample 102, and the cantilever-shaped probe 101 is bent. This bending changes the angle of the reflected light generated when the laser beam irradiated from the laser 104 hits the tip of the probe 101. This change in angle is detected by the quadrant 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 deflection of the probe 101, and the amount of deflection 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. The preamplifier 106 converts a current value into a voltage, calculates an output from each photodiode of the four-division photodiode 105, and generates an AFM signal (lever deflection amount). The generated AFM signal is input to the auto gain control amplifier 107 and shaped into a constant amplitude.
[0015]
The AFM signal having a constant amplitude is output to the phase shifter 108 and the mixer 110.
[0016]
A signal having a predetermined amplitude input to the phase shifter 108 is adjusted in phase and applied to the piezo vibrator 103 through the driver amplifier 109. Since the probe 101 is installed on the vibrator 103, the probe 101 is vibrated by the vibrator 103. In the initial state, the probe 101 vibrates minutely at the resonance point due to thermal vibration or the like, but this is detected by adjusting the phase shifter 108 in order to detect this by the AFM signal detection system 104-106. Starts self-oscillation.
[0017]
Meanwhile, the AFM signal input to the mixer 110 is mixed (multiplied) with a signal of a predetermined frequency oscillated from the local oscillator 1 (113) having the crystal oscillator 1 (114) and output. Thereafter, only a signal in a predetermined band is extracted by the band pass filter 111 and sent to the phase comparator 112 at the next stage.
[0018]
The phase comparator 112 compares the phases of the AFM signal obtained by adding the frequencies of the signals oscillated from the local oscillator 1 and the reference signal output from the local oscillator 2 (115) including the crystal oscillator 2 (116). Then, a signal indicating the phase difference is output. The phase difference signal is sent to the filter 118 at the next stage. The filter 118 filters and integrates the phase difference signal, and sends the generated signal to the control amount calculation 119 in the next stage. In the control amount calculation 119, phase correction, gain adjustment, etc. are performed, a Z direction drive signal is sent to the Z direction drive amplifier 121 of the stage 123, and the distance between the probe 101 and the surface of the sample 102 is changed.
[0019]
As an FM signal detection system of a conventional non-contact mode AFM, a PLL (phase locked loop) is often used. This is a configuration as shown in FIG. First, a detection signal whose frequency is to be obtained is input to the phase comparator 201b. The phase comparator 201b detects and outputs the phase difference between the reference signal output from the voltage controlled oscillator 203b and the input signal. The phase difference signal passes through the filter and becomes the input of the voltage controlled oscillator 203b. That is, when the phase difference between the input signal and the reference signal is shifted, a control voltage corresponding to the shift amount is sent to the voltage controlled oscillator to increase or decrease the frequency of the reference signal. By this feedback, the frequency and phase of the voltage controlled oscillator 403 are controlled so as to match the input signal. An output signal corresponding to the AFM signal is a control voltage of the voltage controlled oscillator 403.
[0020]
If the phase-locked loop shown in FIG. 2B is applied to the block diagram shown in FIG. 1, it is necessary to control the oscillation frequency of the local oscillator 2 (115) from the phase difference signal from the phase comparator 112. 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. 2B, but the time constant of the filter must be increased in order to reduce the noise of the output frequency fluctuation of the voltage controlled oscillator 403, so that the detection speed is improved. And noise reduction is contradictory. In addition, a phase-locked loop is included in the distance control loop between the surface of the probe 101 and the sample 102 and a double loop is formed. Therefore, it is necessary to set the bandwidth of each loop strictly when attempting to increase the control speed. Not only that, but the feedback system itself becomes unstable due to mutual band interference.
[0021]
Therefore, in the present invention, the control loop and the phase locked loop are realized by the same loop. That is, the two loops are made to coincide by using the portion surrounded by the dotted line in FIG. 1 as the voltage controlled oscillator (VCO).
[0022]
The phase comparator 112 detects the phase difference between the two signals (upper: measurement signal, lower: reference signal) shown in FIG. In this case, the measurement signal that has passed through the bandpass filter 111 and the reference signal transmitted from the local oscillator 2 (115) are binarized in the phase comparator 112, and a rectangular wave train as shown in FIG. Become. The phase comparator 112 detects the rising edge of the two signals and calculates a signal having a pulse width representing a phase difference as shown in FIG. This signal is a signal output from the phase comparator output unit shown in FIG. 4. In FIG. 3 (b), the dotted line indicates that both MOS transistors are in the OFF state by the control signals A and B of the phase comparator output unit in FIG. That is, it shows that it is in a high impedance state. Therefore, the upward rectangular wave in FIG. 3B shows a state in which the upper MOS transistor in FIG. 4 is ON and the lower side is OFF, and the downward rectangular wave in FIG. 3B has the opposite relationship. . A phase comparator having such an output is mounted on, for example, a PLL device represented by 74 series 4046.
[0023]
Next, this phase difference signal is sent from the phase comparator 112 to the next-stage filter 118. The filter 118 is preferably a filter usually used in a PLL. For example, a lag lead type filter shown in FIG. 4 that can correct the phase is also useful. Alternatively, an integrator that integrates the current can be used. The output from the filter 118 is as shown in FIG.
[0024]
The phase difference signal that has passed through the filter 117 is actually a reference signal oscillated by the local oscillator 2 (115) and an output signal from the AFM (actually, the resonance frequency of the probe is the frequency of the local oscillator 1 (113)). The frequency difference from the offset). That is, as shown in FIG. 2 (a), a phase-locked loop is formed as a whole, but the entire system shown in FIG. It can be seen that the phase-locked loop shown in FIG. Therefore, using the fact that the resonance frequency of the probe fluctuates depending on the distance between the probe and the sample surface, it is used as an oscillator that can be controlled by the distance, and operates so as to control this frequency to match the reference frequency by the local oscillator. . As a result, the distance between the probe and the sample surface is controlled so that the force generated between them is constant, and the AFM measurement can be performed by monitoring the control signal by the data processing 117.
[0025]
In actual AFM measurement, the stage 123 receives a drive signal generated by the XY scanning control 120 via the drive amplifier 122 in order to measure physical quantities such as irregularities in a certain area of the sample surface, and thereby is applied to the surface of the sample 102. The probe and the sample can be relatively scanned in parallel directions.
[0026]
Finally, as an example of specific processing of the data processing 117, the frequency difference signal output from the filter 118 and the control amount output from the control amount calculation 119 are captured and stored, and visualized as an AFM image using them. And analysis by image processing or the like.
[0027]
The non-contact AFM is realized by the above configuration and the operation of each unit.
[0028]
The non-contact AFM measurement was specifically performed by the apparatus configured based on the above aspect.
[0029]
A Si cantilever having a length of 125 μm, a width of 25 μm, and a thickness of 5 μm was used as a probe. Resonance frequency of the probe was _f R _~321kHz case of oscillating free. The vibrator 103 uses a single-layer piezoelectric element, and the output amplitude of the auto gain control amplifier was adjusted so that the displacement of the AFM was 5 nm.
[0030]
The local oscillator 1 uses a crystal oscillator and has a center frequency of f _{OCS1 to} 4.7 MHz. The local oscillator 2 similarly uses a crystal oscillator, but has a configuration in which the frequency can be changed to some extent by an external element. Since a constant frequency is sufficient, for example, even if a VCO using a crystal oscillator is used, it is not necessary to superimpose a signal component on the control voltage, so that it can be strongly configured against noise. In this example, a 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. Using. That is, f _OCS1 + f _R.
[0031]
The frequency of the reference signal output from the local oscillator 2 is set to f _{OCS2 to} f _OSC1 + f _R + frequency shift amount. As a result, the distance control system of the AFM is controlled so that the resonance frequency of the probe becomes f _OCS2 . That is, the distance between the probe and the sample surface is controlled so that a force corresponding to the applied frequency shift amount acts between the two.
[0032]
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. Further, the measurement band was faster than that using the conventional PLL, and a speed of less than 20 kHz, which is the mechanical resonance of the Z actuator (cylindrical piezo in this embodiment) used in the stage, was obtained.
[0033]
【The invention's effect】
As described above, since a frequency measurement system that does not use a conventional phase-locked loop is configured, high-speed non-contact AFM observation and measurement can be performed. Since it is not necessary to control the frequency of an oscillator such as a VCO by feedback, AFM measurement with very little noise can be performed by using an oscillator with little fluctuation in output frequency.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a non-contact AFM according to the present invention.
FIG. 2 is a diagram for explaining a feedback operation used in the non-contact AFM according to the present invention.
FIG. 3 is a signal waveform showing a frequency difference detection process.
FIG. 4 is a circuit diagram showing the relationship between the output of the phase detector and the next-stage filter in the phase-locked loop.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 101 AFM probe 102 Measurement sample 103 Exciter 104 Laser 105 Four division | segmentation photodiode sensor 109 Exciter drive amplifier 121,122 XYZ stage drive amplifier

Claims (4)

When the probe is brought close to the measurement sample while vibrating at the resonance frequency of the mechanical resonance of the probe, the change in the resonance frequency caused by the force generated between the probe and the measurement sample is used to make fine measurement of the measurement sample. a scanning probe microscope for observing the irregularities,
A probe with a cantilever shape,
A sample holding mechanism for holding the measurement sample facing the probe;
A two-dimensional scanning mechanism for relatively scanning the probe parallel to the measurement sample surface;
An excitation mechanism for applying vibration to the probe;
A displacement detection mechanism for detecting the tip displacement of the probe;
Addition frequency signal generation mechanism,
A frequency addition mechanism for adding the frequency of the addition frequency signal from the addition frequency signal generation mechanism to the frequency of the displacement signal output from the displacement detection mechanism;
A first filter for filtering the signal from the frequency summing mechanism;
A reference signal generation mechanism for generating a reference signal of a constant frequency ,
A phase comparison mechanism for detecting a phase difference between a reference signal from the reference signal generation mechanism and an output signal from the first filter;
Filtering the signal from the phase comparator, for driving a second filter for integrating, and the control amount calculating mechanism calculates a control amount for driving the distance displacement mechanism receives signals from the second filter Amplifier
A scanning probe microscope characterized by comprising: The scanning probe microscope according to claim 1, wherein the reference signal generation mechanism includes a crystal oscillator, and generates a reference signal using a frequency generated by the crystal oscillator. 2. The scanning probe microscope according to claim 1, wherein the addition frequency signal generating mechanism has a crystal oscillator, and generates an addition frequency signal using a frequency generated by the crystal oscillator. When the probe is brought close to the measurement sample while vibrating at the resonance frequency of the mechanical resonance of the probe, the change in the resonance frequency caused by the force generated between the probe and the measurement sample is used to make fine measurement of the measurement sample. A method for controlling the distance between a probe and a sample surface in a scanning probe microscope for observing irregularities,
A step for two-dimensional relative scanning of the probe parallel to the measurement sample surface;
A step for exciting at the resonant frequency of the probe;
Detecting the tip displacement of the probe;
Adding a signal having a constant frequency to the detection signal detected by detecting the tip displacement;
Filtering the detection signal to which the constant frequency is added;
Comparing the phase difference between the filtered signal and a constant frequency reference signal and outputting a signal according to the phase difference;
Filtering and integrating a signal corresponding to the phase difference, and displacing a distance between the probe and the measurement sample surface according to the filtered and integrated signal.
A distance control method comprising:

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