JP3274087B2 - Scanning probe microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the field of scanning probe microscopes, and particularly to the field of scanning atomic force microscopes utilizing a force acting between a cantilever probe and a sample.

[0002]

2. Description of the Related Art The principle of a scanning probe microscope is to use a physical quantity acting on both a probe and a sample when approaching the sample. This type of microscope measures the influence of the physical quantity on the probe. This is a new type of microscope for observing the physical quantity of the sample surface.

A scanning atomic force microscope is a kind of a scanning probe microscope, and controls a distance between a cantilever probe and a sample by using an atomic force acting between the cantilever probe and the sample. is there. In a scanning atomic force microscope, while controlling the distance between the cantilever probe and the sample to be constant, in-plane scanning is performed in the in-plane direction of the sample by a fine movement mechanism such as a piezoelectric element, and the distance between the cantilever probe and the sample is adjusted. By imaging a control amount of a fine movement mechanism such as a piezoelectric element for controlling the distance of the sample, the shape or physical quantity of the sample surface can be observed.

There are two well-known scanning atomic force microscope measurement techniques. One is a direct current method, in which the cantilever tip is constantly in contact with the sample surface and the inside of the sample surface is scanned by a fine movement mechanism such as a piezoelectric element while keeping the distance between the cantilever tip and the sample constant. ,
This is a technique for imaging the control amount of a fine movement mechanism such as a piezoelectric element that controls the distance between the cantilever probe and the sample.

The other is an AC method in which the cantilever probe is separately excited at a frequency near its resonance frequency (from several tens of kHz to several hundreds of kHz), and the distance between the cantilever probe and the sample is changed by AC. The average distance between the cantilever tip and the sample can be controlled to a constant value, although it changes with time at the separately excited vibration frequency. Is a method of scanning with the fine movement mechanism and imaging the control amount of the fine movement mechanism such as a piezoelectric element for controlling the distance between the cantilever probe and the sample.

The principle of operation in the DC method will be described with reference to FIG. 2, and the principle of operation in the AC method will be described with reference to FIG.

In FIG. 2, reference numeral 1 denotes a cantilever, and reference numeral 2 denotes a probe formed on the cantilever 1. The cantilever 1 is deformed by an atomic force acting between the sample 6 and the probe 2.
The amount of the deformation is detected by the semiconductor laser 3, the photodetector 4 for detecting the reflected light of the laser light emitted from the semiconductor laser 3 to the cantilever 1, and the displacement detection unit 7 for amplifying the signal from the photodetector 4. The displacement signal detected by the displacement detector 7 is a DC signal,
After being sent to the Z control unit and compared with the displacement set value,
By driving the Z-direction control electrode of the piezoelectric element 5, the probe 2 and the sample 6
Control the distance between. Generally, the Z control unit 8 often employs proportional integral control.

In this state, the X and Y electrodes of the piezoelectric element 5 are driven via the XY in-plane scanning unit 9 by a command from the computer 10 to scan the surface of the sample 6 and scan the position. The output of the Z control unit 8 corresponding to
, It is possible to image unevenness information on the sample surface.

In FIG. 3, the cantilever 1 is attached to the cantilever excitation piezo 13, and the cantilever excitation piezo 13 is excited by the excitation frequency outputted from the oscillation section 14 near the resonance frequency of the cantilever 1, and the cantilever 1 itself is also excited. Is done. The state of the vibration amplitude of the cantilever 1 changes due to the atomic force acting between the sample 6 and the probe 2. The vibration amplitude is detected by the semiconductor laser 3, the photodetector 4 that detects the reflected light of the laser light emitted from the semiconductor laser 3 to the cantilever 1, and the displacement detector 7 that amplifies the signal from the photodetector 4.

The displacement signal detected by the displacement detection unit 7 is an AC signal, becomes a DC signal via the RMS-DC conversion unit 12, is sent to the Z control unit 8, and is compared with a displacement set value. Driving the Z-direction control electrode of the piezoelectric element 5,
The distance between the probe 2 and the sample 6 is controlled. In this state, the scanning unit 9 in the XY plane is controlled by a command from the computer 10.
By driving the X and Y electrodes of the piezoelectric element 5 through the interface, the surface of the sample 6 is scanned, and the output of the Z control unit 8 corresponding to the scanning position is displayed on the display unit 11, thereby displaying the sample surface. Can be imaged.

[0011]

The typical relationship between the displacement detector 7 and the drive voltage of the piezoelectric element 5 for driving the sample in the Z direction in the scanning atomic force microscope having the configuration shown in FIG. It is known that the result is as shown in FIG. Even after the cantilever probe is once in contact with the sample, even if the Z drive voltage of the piezoelectric element 5 is applied in a direction to increase the distance between the probe 2 and the sample, the cantilever probe traps in the adsorption layer on the sample surface. Has been interpreted to have such properties.

In this state, when the surface of the sample 6 having the surface shape as shown in FIG. 8 is measured, since the probe 2 is trapped on the surface of the sample 6, the signal of the displacement detector 7 is It is relatively easy to generate a signal reflecting the shape of the sample surface and measure the accurate surface shape. However, in the case of the measurement as shown in FIG. 2, since the probe 2 is trapped in the adsorption layer on the surface of the sample 6, when the sample surface is soft due to the surface tension of the adsorption layer, the measurement is performed while damaging the sample surface. It is often known as a drawback.

FIG. 3 shows a configuration implemented as a measuring method for compensating for the above-mentioned drawback. In this case, the RMS-to-drive voltage of the piezoelectric element 5 for driving the sample in the Z direction is measured.
It is known that a typical relationship between the DC converters 12 is as shown in FIG. In the measurement in FIG. 3, the probe 2 is not trapped on the surface of the sample 6, and even if the sample surface is soft, the sample surface is hardly damaged, and a faithful sample surface shape can be measured. Further, the more the set operating point is set to an operating point closer to the side A in FIG. 9, the smaller the force acting between the probe 2 and the sample 6, and the more faithful the shape of the sample surface can be measured.

However, when the probe scans the concave portion with respect to the concave and convex shape of the sample surface, that is, when the relative distance between the probe 2 and the sample 6 increases, it also depends on the responsiveness of the entire control system. , The signal of the RMS-DC converter 12 saturates at the value of point A in FIG. 9, and as a result, the response speed of the entire control system is slow, and it is necessary to perform measurement while performing in-plane scanning slowly. It was a limitation of improvement.

Conventionally, in order to solve such a problem, the stiffness of the piezoelectric element 5 itself and the stiffness of the structure holding the sample 6 and the cantilever 1 have been improved to improve the responsiveness. The AC measurement method shown in FIG. 3 inherently has the properties shown in FIG.

An object of the present invention is to solve the above-mentioned problems and to improve the throughput in the AC measurement method shown in FIG. In other words, it is an object to set the force acting between the probe 2 and the sample 6 as small as possible, and to improve the measurement throughput while keeping the sample surface damage at the time of measuring the sample surface small.

[0017]

As described above, the force acting between the probe 2 and the sample 6 is set as small as possible, which hinders the improvement of the measurement throughput when measuring the sample surface. The factor is that the distance between the cantilever probe and the sample is relatively small when the output of the sensor signal for detecting the distance between the cantilever probe and the sample is viewed from a desired operating point (a value closer to the value of point A in FIG. 9). This is because the effective detection range is narrower on the side that is farther away than on the side that approaches closer.

In the present invention, when the sensor signal outputs the value at point A (or a value close to point A) in FIG. 9, this signal is used as it is as a feedback signal for the distance between the cantilever probe and the sample. Instead, while the value at point A (or a value close to point A) in FIG. 9 is being output, a value modified by a certain time function (such as an integral operation) is generated as a virtual sensor signal, and the value is generated. Used for a feedback signal that controls the distance between the cantilever tip and the sample.

That is, when the output of the sensor for detecting the distance between the cantilever probe and the sample is equal to or less than the value at point A (or a value close to point A) in FIG.
If the value at point A (or a value close to point A) is output, a virtual sensor signal is generated by a modification operation using a time function, and this signal is used as a feedback signal, so that the signal between the cantilever probe and the sample can be obtained. Responsiveness of the feedback system that controls the distance of the measurement can be improved, and the measurement throughput can be improved.

[0020]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a scanning probe microscope according to the present invention. The signal output from the displacement detection unit 7 is input to the modification operation unit 15. The modification operation unit 15 determines the value of the output value of the displacement detection unit 7 and determines whether to output the value as it is as the output of the modification operation unit 15 or to output the value subjected to the modification operation.

FIG. 5 is a block diagram of another embodiment.
The output of the RMS-DC converter 12 is the RM of the cantilever vibration amplitude when the cantilever 1 is vibrated by the cantilever excitation piezo 13 near the resonance frequency of the cantilever.
This is the output after S-DC conversion. FIG. 9 shows a typical relationship between the RMS-DC converter 12 and the drive voltage of the piezoelectric element 5 for driving the sample in the Z direction.

That is, the closer the cantilever tip approaches the sample, the smaller the vibration amplitude of the cantilever becomes due to the atomic force acting on both. When the value at point A (or a value close to point A) in FIG. 9 is output, a virtual sensor signal is generated by the modification operation unit and used as a feedback signal for controlling the distance between the cantilever probe and the sample. The operation process of the modification operation unit will be described below. 1) When the output signal value of the displacement detection unit 7 is smaller than the value at point A (or a value close to point A), the output of the modification operation unit 15 outputs the output value of the displacement detection unit 7 as it is. 2) When the output signal value of the displacement detection unit 7 is the value at the point A (or a value close to the point A) and the state is maintained for the time T, the output of the modification operation unit 15 is, for example, Kdt. Modification operation and output.

Here, K is a constant for determining the contribution ratio of the modification operation. The responsiveness of the feedback system that controls the distance between the cantilever probe and the sample is improved by generating the virtual sensor signal from the modification operation unit 15 by employing the algorithm described in the above 1) and 2).

FIG. 6 is a block diagram of another embodiment.
The output of the phase detection unit 16 detects the phase difference between the cantilever vibration amplitude and the signal driving the cantilever excitation piezo 13 when the cantilever 1 is vibrated by the cantilever excitation piezo 13 near the resonance frequency of the cantilever. is there. FIG. 10 shows a typical relationship between the phase detectors 16 with respect to the drive voltage of the piezoelectric element 5 for driving the sample in the Z direction.

That is, as the cantilever tip approaches the sample, the phase difference between the signal driving the cantilever excitation piezo 13 and the signal output from the photodetector 4 increases due to the atomic force acting on both. Is shown. When the value at point A in FIG. 10 is output, the modification operation unit generates a virtual sensor signal and uses it as a feedback signal for controlling the distance between the cantilever probe and the sample. The operation process of the modification operation unit is the same as that of the second embodiment. 1) When the output signal value of the phase detection unit 16 is smaller than the value at the point A (or a value close to the point A), As the output, the output value of the displacement detection unit 7 is output as it is. 2) When the output signal value of the phase detection unit 16 is the value at the point A (or a value close to the point A) and the state is maintained for the time T. The output of the modification operation unit 15 is, for example, Kdt. Modification operation and output.

Here, K is a constant for determining the contribution ratio of the modification operation.

FIG. 7 is a block diagram of another embodiment.
The cantilever 1 includes a cantilever excitation piezo 13, a semiconductor laser 3 for detecting displacement of the cantilever, a photodetector 4 for detecting reflected light of laser light emitted from the semiconductor laser 3 onto the cantilever 1, and a displacement detection for detecting displacement of the cantilever. A self-excited oscillating circuit comprising a unit 7, a phase shifter 18 for adjusting a phase of an output signal of the displacement detecting unit 7, and an attenuator 19 for adjusting a vibration amplitude of a cantilever is configured. Therefore, the cantilever 1 vibrates at its own resonance frequency.
The FV converter 20 is a frequency-voltage conversion circuit for detecting the self-excited oscillation frequency of the cantilever 1.

FIG. 11 shows a typical relationship between the FV converter 20 and the drive voltage of the piezoelectric element 5 for driving the sample in the Z direction. In other words, the closer the cantilever tip approaches the sample, the lower the resonance frequency of the cantilever becomes due to the interatomic force acting on both.
When the value at point A (or a value close to point A) in FIG. 11 is output, a virtual sensor signal is generated by the modification operation unit and used as a feedback signal for controlling the distance between the cantilever probe and the sample. The operation of the modification operation unit is the same as that of the second embodiment. 1) When the output signal value of the FV conversion unit 20 is smaller than the value at point A (or a value close to point A) The output 15 outputs the output value of the displacement detection unit 7 as it is. 2) When the output signal value of the FV conversion unit 20 is the value at the point A (or a value close to the point A) and the state is maintained for the time T, the output of the modification operation unit 15 is, for example, The result is modified by Kdt and output.

Here, K is a constant for determining the contribution rate of the modification operation.

[0031]

FIG. 12 is an embodiment of the block diagram shown in FIG. This will be described with reference to the drawings. The A / D converter 21 converts the output of the RMS-DC converter 12 into a digital value and inputs the digital value to a DSP (digital signal processor) 22. The DSP 22 includes an XY scanning D / A converter 24.
Scans the piezoelectric element 5 in the in-plane direction (XY directions) via the. The DSP 22 drives the piezoelectric element 5 in the Z direction via a Z control D / A converter 23 based on digital data from the A / D converter 21 to control the distance between the cantilever probe and the sample. . Here, a digital control operation is performed by a program in the DSP 22. At this time, the data from the A / D converter 21 is subjected to the modification operation described so far, so that the cantilever probe and the sample are sampled. The responsiveness of the feedback system that controls the distance between them can be improved.

[0032]

According to the present invention, even when the atomic force acting between the cantilever probe and the sample is as small as possible,
There is an effect that the shape of the unevenness on the sample surface can be measured without impairing the measurement throughput. In the prior art, in order to improve the measurement throughput, the atomic force acting between the cantilever tip and the sample must be set large, and when measuring a sample with a soft sample surface, the sample surface may be damaged. Sometimes I gave it.

When the present invention is not applied to FIG.
An example of the measurement data when applied to (b) is shown. This data is an example when a sample in which unevenness on the surface of the sample is continuously formed with a step of about 100 nm is scanned in the X direction. In this measurement, the scanning speed in the X direction is the same, and the atomic force acting between the cantilever probe and the sample is also set to the same.

As can be seen from the results, when the present invention is applied, the state of the unevenness on the surface of the sample is faithfully measured with the measurement throughput improved.

[Brief description of the drawings]

FIG. 1 is a block diagram of a scanning probe microscope according to the present invention.

FIG. 2 is a block diagram of a conventional scanning atomic force microscope configured by a DC method.

FIG. 3 is a block diagram of a conventional scanning atomic force microscope configured by an AC method.

FIG. 4 shows a typical relationship between the displacement detectors 7 with respect to the drive voltage of the piezoelectric element 5 for driving the sample in the Z direction in the configuration of FIG.

FIG. 5 is a block diagram of another embodiment.

FIG. 6 is a block diagram of another embodiment.

FIG. 7 is a block diagram of another embodiment.

FIG. 8 shows an example of a sample surface shape.

FIG. 9 shows a typical relationship between the RMS-DC converter 12 and the drive voltage of the piezoelectric element 5 for driving the sample in the Z direction in FIG.

FIG. 10 shows a typical relationship between the phase detectors 16 with respect to a drive voltage of the piezoelectric element 5 for driving the sample in the Z direction.

FIG. 11 shows a typical relationship between the FV converters 20 with respect to the drive voltage of the piezoelectric element 5 for driving the sample in the Z direction.

FIG. 12 is an embodiment of the block diagram shown in FIG. 5;

FIG. 13 is an actual measurement example showing the effect of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Cantilever 2 Probe 3 Semiconductor laser 4 Photodetector 5 Piezoelectric element 6 Sample 7 Displacement detection part 8 Z control part 9 XY in-plane scanning part 10 Computer 11 Display part 12 RMS-DC conversion part 13 Cantilever excitation piezo 14 Oscillation part 15 Modification operation Unit 16 phase detection unit 17 interface 18 phase shifter 19 attenuator 20 FV conversion unit 21 A / D converter 22 DSP (digital signal processor) 23 Z control D / A converter 24 XY scanning D / A converter

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-8-201402 (JP, A) JP-A-7-270434 (JP, A) JP-A-7-325090 (JP, A) JP-A-6-2014 273158 (JP, A) JP-A-5-45111 (JP, A) JP-A-6-265343 (JP, A) JP-A 8-194975 (JP, A) WO 96/24026 (WO, A1) ( 58) Fields surveyed (Int.Cl. ⁷ , DB name) G01N 13/10 G01N 13/24 JICST file (JOIS)

Claims (4)

(57) [Claims] 1. A scanning probe microscope for detecting a physical quantity acting between a cantilever probe and a sample to measure the shape and physical information of the sample surface, wherein the distance between the cantilever probe and the sample and the physical quantity detection sensor Using the output signal of the above as a feedback signal, the control device for controlling the distance between the cantilever probe and the sample , the output value of the output signal to a predetermined saturated value for a certain time
It is in the case which produces a virtual sensor signal by modifying operations like integral operation using the output signal of the physical quantity detecting sensor, by the virtual sensor signal, especially to control the distance between the cantilever tip and the sample scanning probe microscope according to symptoms. 2. The distance between the cantilever probe and the sample when the cantilever probe approaches the sample in a state where the cantilever probe is separately excited at a frequency near its resonance frequency, and the cantilever probe. The relationship between the increase and decrease of the vibration amplitude of the object is detected by a physical quantity detection sensor, and the physical quantity detection sensor
The output signal from the use as a feedback signal for controlling the distance between the cantilever tip and the sample, the sample table
Scanning probe microscope to measure surface shape and physical information
In the above, the distance between the cantilever probe and the sample increases, and the output
When the output value of the signal has reached the specified saturation value for a certain period of time
Integral calculation of the vibration amplitude detection amount of the cantilever probe
By generating a virtual sensor signal modification operation on at etc., by controlling the distance between the cantilever tip and the sample
A scanning probe microscope characterized by the following . 3. The distance between the cantilever probe and the sample when the cantilever probe approaches the sample in a state where the cantilever probe is separately excited at a frequency near its resonance frequency, and the cantilever probe. The relationship between the excitation signal and the increase / decrease of the phase of the cantilever probe is detected by the physical quantity detection sensor
The output signal from the physical quantity detection sensor is used as a feedback signal for controlling the distance between the cantilever probe and the sample, and the shape and physical information of the sample surface are measured.
In the scanning probe microscope, the distance between the cantilever probe and the sample increases,
When the output value of the signal has reached the specified saturation value for a certain period of time
The, by generating a virtual sensor signal to <br/> modified calculates the phase detection of the cantilever tip integral calculation or the like,
JP that controls the distance between the cantilever tip and the sample
Scanning probe microscope according to symptoms. 4. The distance between the cantilever probe and the sample and the self-excited vibration of the cantilever probe when the cantilever probe and the sample are brought close to each other with the cantilever probe vibrating at the resonance frequency. Physical quantity detection of frequency increase / decrease relationship
Sensor to detect the output signal from the physical quantity detection sensor.
The issue, then used as a feedback signal for controlling the distance between the cantilever tip and the sample, the sample surface shape及
In a scanning probe microscope that measures physical and physical information, the distance between the cantilever probe and the sample increases and the output
When the output value of the signal has reached the specified saturation value for a certain period of time
Calculates the self-excited frequency detection amount of the cantilever tip
By generating a virtual sensor signal modification operation to the like, this for controlling the distance between the cantilever tip and the sample
And a scanning probe microscope.