JP2016023952A - Scanning probe microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scanning microscope capable of performing stable measurement even when a mode is switched during scanning of a sample.SOLUTION: The scanning probe microscope has a signal conversion unit 24 which is arranged between a feedback control part 15 and a displacement amount detection part 9, and which includes: an offset circuit 11 for making a reference value for keeping a distance between a cantilever and a sample constant when a tapping mode is selected, coincide with a reference value for keeping the distance between the cantilever and the sample constant when a contact mode is selected; a branching point 27 where a signal outputted from the displacement amount detection part 9 is divided into a first signal and a second signal; a first signal path 26 through which the first signal passes; a second signal path 28 through which the second signal passes; a switch 12 for connecting the first signal path 26 to the feedback control part 15 when the contact mode is selected, or for connecting the second signal path 28 to the feedback control part 15 when the tapping mode is selected; and a DC conversion part 14, arranged in the second signal path 28, for converting an AC signal into a signal proportional to an amplitude of the AC signal.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a scanning probe microscope that can observe surface shape and physical property information of a sample by scanning a probe on the surface of the sample.

  The scanning probe microscope can measure physical properties to acquire physical property information simultaneously with the surface shape of the sample.

  The scanning probe microscope can acquire physical property information simultaneously with the surface shape.

  In the scanning probe microscope apparatus described in Patent Document 1, the tapping mode and the contact mode are switched in 10 to 20 ms, information on the surface shape of the sample is acquired in the tapping mode, and information on the conductivity of the sample is acquired in the contact mode. Thus, the conductivity of the sample, which is difficult to measure in the contact mode, is evaluated.

JP 2004-85321 A

  However, in the scanning probe microscope apparatus described in Patent Document 1, if the distance between the probe suitable for the tapping mode and the sample is controlled due to the difference in the signal between the tapping mode and the contact mode, the appropriate probe is used in the contact mode. There is a problem that the distance between the sample and the sample cannot be controlled, and measurement cannot be performed in any measurement mode by setting a desired pressing pressure.

Therefore, in the present invention, a cantilever having a probe that scans the surface of the sample, a displacement amount detection unit that measures the displacement amount of the cantilever, and the cantilever of the cantilever in order to keep the distance between the cantilever and the sample constant. A feedback control unit that adjusts the height of the sample based on the amount of displacement; and a contact mode in which the sample is scanned by bringing the probe into contact with the sample; and the sample is scanned by vibrating the cantilever. In a scanning probe microscope that switches the tapping mode to perform measurement of the sample,
Between the feedback control unit and the displacement amount detection unit,
A reference value for keeping the distance between the cantilever and the sample constant when the tapping mode is selected and a reference for keeping the distance between the cantilever and the specimen constant when the contact mode is selected An offset circuit for matching the values;
A branching unit for branching a signal output from the displacement amount detection unit into a first signal and a second signal;
A first signal path through which the first signal passes; and a second signal path through which the second signal passes and different from the first signal path;
A switch for connecting the first signal path and the feedback control unit when the contact mode is selected, and a switch for connecting the second signal path and the feedback control unit when the tapping mode is selected;
Provided is a scanning probe microscope comprising: a signal conversion unit having a DC conversion unit that converts an AC signal present in the second signal path into a signal proportional to the amplitude of the AC signal. .

  By using the scanning microscope and the scanning method of the present invention, it is possible to set an appropriate probe and sample pressing pressure in both the tapping mode and the contact mode. Measurement can be performed stably even when the contact mode is switched. As a result, it is possible to evaluate the physical properties of a sample whose shape is difficult to evaluate in the contact mode.

It is the schematic which shows the structure of the scanning probe microscope apparatus of 1st embodiment. It is the schematic which shows the structure of a general scanning probe microscope apparatus. (A) It is the surface shape image which measured the surface shape of CNT (carbon nanotube) measured by the comparative example. (B) CNT current image measured in a comparative example. It is a figure which shows the output from each part in the switching signal of a measurement mode and a feedback signal control unit in an Example. (A) It is the surface shape image which measured the surface shape of CNT (carbon nanotube) measured in the Example. (B) It is the electric current image of CNT measured in the Example. (A) It is the schematic which shows the structure of the scanning probe microscope of 2nd embodiment. (B) It is the schematic which shows the structure of the scanning probe microscope of 3rd embodiment. (C) It is the schematic which shows the structure of the scanning probe microscope of 4th embodiment.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

(Assumption)
First, a general scanning probe microscope (which may be displayed as SPM in the following description) will be described with reference to FIG. In the description, the vertical direction of the sample 6 is Z, and the in-plane directions of the sample are X and Y.

  The cantilever 1 holds a probe 2 that is a needle for scanning the sample 6 and is disposed so as to face the surface of the sample 6. The cantilever 1 is fixed to a holder (not shown) provided with an actuator 3, and a tapping mode signal or a contact mode signal selected in advance as a measurement mode is input from the oscillator 25 to the actuator 3 to detect the probe 2. Scans sample 6.

  Specifically, when the tapping mode is selected as the measurement mode in advance, an AC voltage having an appropriate frequency and a constant amplitude is output from the oscillator 25, and the cantilever is vibrated with a constant amplitude by the output AC voltage. Then, the sample 6 is scanned. On the other hand, when the contact mode is selected as the measurement mode in advance, a constant voltage is output from the oscillator 25, and the sample 6 is scanned by contacting the cantilever without vibrating with the constant voltage.

  The amount of deflection of the cantilever 1 while scanning the sample 6 is such that the light generated from the light source 4 such as a laser is irradiated onto the cantilever 1, the reflected light is detected by the light detection unit 5, and the detected reflected light signal is displaced. It is measured by inputting to the detection unit 9 and converting it into a displacement amount. Specifically, when a quadrant photodiode is used for the light detection unit 5, by comparing the light intensity detected in each channel of the photodiode and calculating the displacement amount by the displacement amount detection unit 9, An electric signal proportional to the amount of deflection of the cantilever 1 is output by the displacement amount detection unit 9.

  The signal output from the displacement detection unit 9 is input to the SPM control unit 17 including the DC conversion unit 14, the reference signal generation unit 16, and the feedback control unit 15, and the distance between the probe 2 and the sample 6. The position of the sample stage 7 is adjusted by the piezo scanner 8 so that is constant.

  When the tapping mode is selected as the measurement mode in advance, an AC signal resulting from the vibration of the cantilever 1 is output from the displacement amount detection unit 9, and the AC signal output from the displacement amount detection unit 9 is the DC conversion unit 14. Is converted into an amplitude signal proportional to the amplitude of the AC signal, and the signal is input to the feedback control unit 15. The amplitude signal input to the feedback control unit 15 and the preset reference value input from the reference signal generation unit 16 to the feedback control unit 15 are compared by the feedback control unit 15, and the input amplitude signal is The Z direction of the sample stage 7 is adjusted by the piezo scanner 8 so as to coincide with the reference value. The reference value described here is a target value for keeping the distance between the cantilever and the sample constant.

  On the other hand, when the contact mode is selected as the measurement mode in advance, the signal output from the displacement amount detection unit 9 is a signal resulting from the deflection amount of the cantilever 1 (hereinafter referred to as “deflection amount signal” in the following description). May be output). The deflection amount signal input to the DC converter 14 is output as the unconverted deflection amount signal and is input to the feedback controller 15. Then, the deflection amount signal input to the feedback control unit 15 and the preset reference value input from the reference signal generation unit 16 to the feedback control unit 15 are compared by the feedback control unit 15, and the deflection amount signal and the reference are compared. The Z direction of the sample stage 7 is adjusted by the piezo scanner 8 so that the values match.

  When the probe 2 is scanned, signals related to the positions in the X and Y directions are obtained by scanning the probe in the X and Y directions simultaneously with the control in the Z direction. Thereby, the information regarding the surface shape of the sample 6 can also be obtained by taking the X, Y, and Z positions of the sample 6 into the SPM control computer 18 such as a personal computer.

  When acquiring physical property information of the sample 6, the SPM control computer 18 controls the physical property measurement electrical signal generator 19, and the physical property measurement electrical signal is output to the sample stage 7, so that the physical property measurement electrical signal is applied to the sample 6. A signal is given. Then, a signal responding to the electrical signal for measuring physical properties generated by applying the electrical signal for measuring physical properties to the sample 6 is input to the physical property signal measuring unit 20 and taken into the SPM control computer 18. For example, when obtaining the conductivity information of the sample 6, a conductive cantilever is used as the cantilever 1, a voltage is applied from the electrical signal generator 19 for measuring physical properties to the sample stage 7, and the current flowing through the grounded cantilever 1. Is detected by the physical property signal measuring unit 20. Thereby, the information regarding the electrical conductivity which is the physical property information of the sample 6 can be acquired.

  In addition, when acquiring the physical property information of the sample 6, it can be performed simultaneously with the scanning in the XY directions. In such a case, the shape information of the sample 6 is acquired and the physical property information of the sample 6 is also acquired. can do.

  Next, the scanning microscope according to the first embodiment, which is a preferred embodiment of the present invention, will be described with reference to FIG. FIG. 1 is a schematic view showing a scanning probe microscope according to the first embodiment.

<First embodiment>
The scanning probe microscope of the present embodiment includes a feedback signal control unit 13 between the displacement amount detection unit 9 and the SPM control unit 17. In FIG. 1, the same reference numerals as those in FIG. 2 is different from the scanning probe microscope of FIG. 1 in that it has the feedback signal control unit 13 and the other parts are the same as those in FIG.

  The feedback signal control unit 13 controls the signal conversion unit 24 that controls the signal input to the SPM control unit 17, the oscillator control unit 22 that controls the vibration of the cantilever 1, and the piezo scanner 8 that adjusts the position of the sample stage 7. A sample stage fine adjustment unit 23, a signal conversion unit 24, an oscillator control unit 22, and a feedback signal control unit internal control unit 21 that controls the operation of the sample stage fine adjustment unit 23. The signal conversion unit 24 includes the RMS-DC converter 10, the offset circuit 11, and the switch 12.

  The feedback signal control unit internal control unit 21 is a control unit capable of controlling the switch 12, the oscillator control unit 22, and the sample stage fine adjustment unit 23. The feedback signal control unit internal control unit 21 is, for example, a PC. Since the feedback signal control unit internal control unit 21 is a PC, the measurement mode can be easily switched.

  When the tapping mode is selected in accordance with an instruction from the feedback signal control unit control unit 21, an AC voltage having an appropriate frequency and a constant amplitude is output from the oscillator control unit 22, and the probe 2 vibrates with a constant amplitude. To do. On the other hand, when the contact mode is selected in accordance with the instruction from the control unit 21 in the feedback signal control unit, a constant voltage is output from the oscillator control unit 22 to stop the vibration of the probe 2, and the probe 2 is moved to the sample. 6 is measured. That is, the cantilever 1 is vibrated or stopped in synchronization with the switching of the measurement mode of the scanning probe microscope instructed from the control unit 21 in the feedback signal control unit.

  The sample stage fine adjustment unit 23 outputs different voltages in the tapping mode and the contact mode in accordance with an instruction from the control unit 21 in the feedback signal control unit, and inputs the voltage to the piezo scanner 8 as a signal. A signal input from the sample stage fine adjustment unit 23 to the piezo scanner 8 is generated by changing the position of the piezo scanner 8 in the Z direction set by the SPM control unit 17 from the tapping mode to the contact mode or vice versa. 8 is for adjusting the position of the piezo scanner 8 in the Z direction by the amount of movement corresponding to the voltage difference input to 8. This is because the height of the sample stage 7 is changed between the tapping mode and the contact mode when different voltages are input to the piezo scanner 8 in the tapping mode and the contact mode. Since the distance between the probe 2 and the sample 6 needs to be longer in the tapping mode than in the contact mode, the sample table is longer when the tapping mode is selected than when the contact mode is selected. The piezo scanner 8 is controlled by the sample stage fine adjustment unit 23 so that 7 is lowered. By performing this in synchronization with measurement mode switching (that is, switching from tapping mode to contact mode or vice versa), the amount in the Z direction adjusted by feedback control unit 15 at the time of measurement mode switching decreases, so SPM control The time for adjusting Z by the unit 17 can be shortened.

  A signal input from the displacement detection unit 9 to the signal conversion unit 24 is divided into two signals, a first signal and a second signal, by a branching unit 27 that divides the signal into a first signal and a second signal. . Since the signal output from the displacement amount detection unit 9 is usually a voltage, the first signal and the second signal have the same signal value.

  Then, the first signal passes through the first signal path 26 and reaches the switch 12 directly.

  On the other hand, the second signal is input to the RMS-DC converter 10 which is a DC converter that converts an AC signal into a DC signal. The second signal input to the RMS-DC converter 10 is converted into an amplitude signal proportional to the amplitude of the AC signal. Therefore, when the second signal input to the RMS-DC converter 10 is an AC voltage, the signal output from the RMS-DC converter 10 is a DC voltage.

  Then, the amplitude signal is input to the offset circuit 11, is output with a signal offset described later, and reaches the switch 12.

  Here, in the present embodiment, the path from the branch unit 27 through which the first signal passes to the switch 12 is defined as the first signal path 26, and the RMS-DC converter 10 and the offset from the branch unit 27 through which the second signal passes. A path that reaches the switch 12 via the circuit 11 is a second signal path 28. As shown in FIG. 1, the second signal path 28 is a different path from the first signal path 26.

  The switch 12 connects the first signal path 26 and the SPM control unit 17 and connects the first signal to the SPM control unit when the contact mode is selected according to the instruction from the control unit 21 in the feedback control unit. 17 to output. When the tapping mode is selected according to the instruction from the feedback control unit internal control unit 21, the second signal path 28 and the SPM control unit 17 are connected, and the RMS-DC converter 10 and the offset circuit 11 are connected. The second signal that has passed through is output to the SPM control unit 17.

  Note that, when the SPM control unit 17 is set to the contact mode and used, the signal input to the DC conversion unit 14 is output as it is and input to the feedback signal control unit 15.

  Next, the signal offset applied by the offset circuit 11 will be described. Since the signal output from the displacement detection unit 9 is a normal voltage, the signal used from the signal input to the RMS-DC converter 10 to the feedback control unit 15 is expressed in voltage below. The control signal of the present invention may be other signals.

  Based on the signal detected by the light detector 5, the displacement detector 9 outputs a voltage proportional to the amount of deflection of the cantilever 1.

  The RMS-DC converter 10 performs AC coupling on the signal input from the displacement detection unit 9, and when the maximum amplitude of the AC voltage signal input to the RMS-DC converter 10 is a [mV], G · a [mV ] And output the voltage. Further, the offset circuit 11 outputs the input voltage by applying an appropriate offset voltage Voff.

  In the following description, an appropriate offset voltage will be described using the z-axis with the origin being a certain position when the cantilever 1 is not bent and the direction from the sample 2 toward the probe 6 being positive. The output from the variable position detection unit 9 when the position of the cantilever 1 is z = 0 is b [mV], and the deflection amount when the cantilever 1 is bent in the Z direction is associated with the output voltage from the displacement amount detection unit 9. When the proportional constant (DIF sensitivity) is Sd [mV / nm] and the position (deflection amount) of the cantilever 1 when scanning in the contact mode is set to z = δ [nm], the SPM control unit 17 in the contact mode is set. The set voltage of the reference value of the first signal input to is (δ · Sd + b) [mV].

On the other hand, when the amplitude of the cantilever 1 when scanning in the tapping mode is set to A [nm], the set voltage of the reference value of the second signal that has passed through the RMS-DC converter 10 and the offset circuit 11 is (G · A · | Sd | −Voff) [mV]. Therefore, an appropriate signal offset Voff for making the reference value of the tapping mode and the contact mode the same is expressed by the following formula (Sd, G, A, δ, b) as variables according to the configuration of the signal control unit 24 of FIG. 1).
Voff = G · A · | Sd | −δ · Sd−b (1)

  That is, when the voltage output from the RMS-DC converter 10 is lowered by the offset voltage Voff [mV], the reference values for the tapping mode and the contact mode are the same.

  By making the reference value of the tapping mode and the contact mode the same with an appropriate signal offset, the pressing pressure between the probe 2 and the sample 6 becomes constant even when the measurement mode is switched during scanning. Measurement can be performed stably.

  Further, by switching between the contact mode and the tapping mode during scanning, the surface shape information of the sample 6 is acquired in the tapping mode, and the physical property value of the sample 6 is measured in the contact mode. Even for a sample that is difficult to measure, it is possible to acquire surface shape information and physical property information.

  Of the parameters of equation (1), Sd, G, A, δ, and b, Sd, G, and b are values determined by the apparatus, and A and δ are optimal for each measurement in the tapping mode and contact mode. The value determined by the measurer to be

  In the scanning probe microscope of the present embodiment, the switching of the switch 12, the oscillator control unit 22, and the sample stage fine adjustment unit 23 is synchronized with the switching of the measurement mode. However, in the scanning probe microscope of the present invention, each switching is performed. An appropriate delay time may be provided.

  Thereby, it is possible to prevent the probe 2 from being strongly pressed against the sample 6 when the measurement mode is switched, and damage to the probe 2 can be reduced. For example, when switching from the contact mode to the tapping mode, the signal input from the oscillator control unit 22 to the oscillator 25 is switched about 1 ms after the switch 12 and the sample stage fine adjustment unit 23 are switched.

  In the scanning probe microscope of this embodiment, a PC is used for the feedback signal control unit control unit 21 that controls the switch 12, the oscillator control unit 22, and the sample stage fine adjustment unit 23. However, in the scanning probe microscope of the present invention, the control unit 21 in the feedback signal control unit is not limited to a PC, and for example, a function generator or a microcomputer is used to generate a signal for periodically switching the measurement mode and control is performed. You can do it.

  In the scanning probe microscope of the present embodiment, the RMS-DC converter 10 is used as a DC converter that converts the amplitude of an AC signal into an amplitude signal. However, in the scanning probe microscope of the present invention, it is proportional to the amplitude of a lock-in amplifier or the like. Other signals may be used as long as they can be converted into the above signals.

  The scanning probe microscope of the present embodiment has the sample stage fine adjustment unit 23. However, if it is not necessary to shorten the switching time between the tapping mode and the contact mode, the scanning probe microscope of the present invention has the sample stage fine adjustment unit 23. The fine adjustment unit 23 may not be provided.

  The scanning probe microscope of the present embodiment has a DC conversion unit 14 in the SPM control unit 17, and the second signal that has passed through the offset circuit 11 passes through the switch 12 and then passes through the DC conversion unit to be a feedback control unit. However, the scanning probe microscope of the present invention does not have the DC conversion unit 14 in the SPM control unit 17, and the second signal that has passed through the offset circuit 11 is directly input to the feedback control unit 15. It is also possible to use a configuration.

<Second Embodiment>
The scanning probe microscope of this embodiment is different from the scanning probe microscope of the first embodiment in that the offset circuit 11 includes a first signal path 26, a second signal path 27, a branching section 27 of 27, and a displacement. It is a point arrange | positioned between the quantity detection parts 9. FIG. The rest is the same as the first embodiment.

  As shown in FIG. 6A, the signal detected by the displacement amount detection unit 9 is input to the offset circuit 11 and converted into an offset signal. The first signal and the second signal are divided into two, and the first signal reaches the switch 12 directly through the first signal path 26, and the second signal is amplified by the RMS-DC converter 10. After being converted into a signal, the switch 12 is reached.

  Here, in the present embodiment, the first signal path 26 is a path from the branch part 27 through which the first signal passes to the switch 12, and the second signal path is from the branch part 27 through which the second signal passes. This is a path from the RMS-DC converter 10 to the switch 12.

When scanning in the contact mode, the reference voltage setting voltage is (δ · Sd + b−Voff) [mV] because it passes through the first signal path. On the other hand, when scanning in the tapping mode, since the signal is input to the RMS-DC converter by AC coupling, the reference voltage setting voltage is (G · A · | Sd |) [mV]. Therefore, an appropriate offset voltage in the present embodiment can be obtained by Expression (2).
Voff = δ · Sd + b−G · A · | Sd | (2)

<Third embodiment>
The scanning probe microscope of the present embodiment is different from the scanning probe microscope of the first embodiment in that the offset circuit 11 is arranged in the first signal path 26. The rest is the same as the first embodiment.

  As shown in FIG. 6 (b), the signal detected by the displacement detection unit 9 is divided into two signals, a first signal and a second signal, by the branching unit 27. After being converted into an offset signal applied by the offset circuit 11 existing in the first signal path shown, the signal reaches the switch 12, and the second signal is RMS-DC in the second signal path indicated by 28. After being converted into an amplitude signal by the converter 10, the switch 12 is reached.

When scanning in the contact mode, the reference voltage setting voltage is (δ · Sd + b−Voff) [mV] because it passes through the first signal path. On the other hand, when scanning in the tapping mode, since the signal is input to the RMS-DC converter by AC coupling, the reference voltage setting voltage is (G · A · | Sd |) [mV]. Therefore, an appropriate offset voltage in the present embodiment can be obtained by Expression (2) as in the second embodiment.
Voff = δ · Sd + b−G · A · | Sd | (2)

<Fourth embodiment>
The scanning probe microscope of the present embodiment is different from the scanning probe microscope of the first embodiment in that an offset circuit (first offset) is provided in the first signal path 26 in addition to the second signal path 28. Circuit) 29 is arranged. The rest is the same as the first embodiment.

  As shown in FIG. 6C, the signal detected by the displacement detection unit 9 is divided into two signals of the first signal and the second signal by the branching unit 27, and the first signal is the first signal. The signal is converted into a signal offset by the first offset circuit 29 existing in the signal path 26 and reaches the switch 12. Then, after the second signal is converted into an amplitude signal by the RMS-DC converter 10 in the second path 28, the second signal is input to the second offset circuit 30 to become a signal to which the second offset is applied, and the switch 12 is reached.

When the offset voltage (first offset voltage) of the first offset circuit is V1off and the offset voltage (second offset voltage) of the second offset circuit is V2off, the first signal path is used when scanning in the contact mode. Therefore, the reference voltage setting voltage is (δ · Sd + b−V1off) [mV]. On the other hand, when scanning in the tapping mode, since the signal is input to the RMS-DC converter by AC coupling, the reference voltage setting voltage is (G · A · | Sd | −V2off) [mV]. Therefore, an appropriate offset voltage in the present embodiment is that one of V1off and V2off is set to a certain value, and the other is calculated by Expression (3), and the first offset voltage and the second offset voltage are calculated. The voltage can be determined.
V1off−V2off = δ · Sd + b−G · A · | Sd | (3)

  Examples of the present invention are shown below.

(Example)
The sample was measured using the scanning probe microscope described in FIG.

  Sample 6 used was a carbon nanotube (CNT) spin-coated on a Si substrate. The physical property value was measured by applying a voltage of 0.5 V to the sample stage 7 and acquiring a current image. As the cantilever 1, an Rh-coated conductive cantilever was used.

  The results when measurement is performed using the feedback signal control unit 13 will be described. The control of the switch 12, the oscillator control unit 22 and the stage fine adjustment unit 23 is such that an L logic signal is output from the feedback signal control unit internal control unit 21 in the tapping mode, and an H logic signal is output in the contact mode. I went there. Switching between the tapping mode and the contact mode was performed every 26 ms.

The voltage of the offset circuit 11 of the feedback signal control unit 13 was set by the following procedure. The DIF sensitivity of the cantilever 1 attached to the apparatus is 7 [mV / nm], the output from the displacement detection unit 9 when the sample and the probe are sufficiently separated is 0 [mV], and the RMS-DC converter exchanges AC. The coefficient G for conversion to direct current was 0.7. The maximum amplitude when measuring in the tapping mode was 82 [nm], and the deflection of the cantilever when measuring in the contact mode was 14 [nm]. Therefore, the offset voltage of the offset circuit 11 is set to 303.8 mV according to the equation (1).
Voff = G · A · | Sd | −δ · Sd−b (1)

  FIG. 4 shows a logic signal at the time of mode switching and each output of the feedback control unit 13 when measurement is performed under the above conditions.

  FIG. 4A is a logic signal output by the feedback signal control unit internal control unit 21. When the value is low (L), it corresponds to the tapping mode, and when the value is high (H), the contact mode. In synchronization with the switching, the outputs from the oscillator control unit 22 and the stage fine adjustment unit 23 are as shown in FIGS. 4 (2) and 4 (3), respectively. That is, in the tapping mode, an AC voltage was applied to the actuator 3 to vibrate the cantilever 1 at a constant period. On the other hand, in the contact mode, the vibration of the cantilever 1 was stopped by connecting the actuator 3 to the ground. In addition, since the height at which stable scanning is possible is about 100 nm higher in the tapping mode than in the contact mode, a voltage is applied from the stage fine adjustment unit 23 to the piezo scanner 8 so as to lift the piezo scanner by 100 nm in the contact mode. I input it. Further, the signal input from the displacement amount detection unit 9 is controlled to output a signal that has passed through the RMS-DC converter 10 and the offset circuit 11 in the tapping mode, and to output the input signal as it is in the contact mode. did.

  As a result, the signal input from the feedback signal control unit 13 to the SPM control unit 17 is as shown in FIG. 4 (4). Even when the tapping mode and the contact mode are switched, the voltage reaches a stable voltage after about 10 ms after switching. Therefore, stable measurement was possible even if the measurement mode was switched.

  The shape image and current image measured by this method are shown in FIG. Since the acquired image contains periodic noise due to switching between the tapping mode and the contact mode, the image processed by the low-pass filter is shown.

  As shown in FIG. 5, in this embodiment, both the shape image and the current image can be accurately measured.

(Comparative example)
In this comparative example, the same sample as the example was measured using the scanning probe microscope described in FIG. That is, the sample was measured using a scanning probe microscope without the feedback signal control unit of Example 1.

  As the cantilever 1, an Rh-coated conductive cantilever was used. A contact mode was used as a measurement mode of the scanning probe microscope, and a current image was obtained by applying a voltage of 0.5 V to the sample stage 7.

  The DIF sensitivity of the cantilever attached to the apparatus was 7 [mV / nm], and the output from the displacement detector 9 when the sample and the probe were sufficiently separated was 0 [mV]. FIG. 3 shows a shape image and a current image when the cantilever deflection amount at the time of measurement in the contact mode is set to 14 [nm]. As shown in FIG. 3, it was difficult to measure the current image in the contact mode in this comparative example.

DESCRIPTION OF SYMBOLS 1 Cantilever 2 Probe 3 Actuator 4 Light source 5 Light detection part 6 Sample 7 Sample stand 8 Piezo scanner 9 Displacement amount detection part 10 RMS-DC converter 11 Offset circuit 12 Switch 13 Feedback signal control unit 14 DC conversion part 15 Feedback control part 16 Reference signal generation unit 17 SPM control unit 18 SPM control computer 19 Physical property measurement electrical signal generation unit 20 Physical property signal measurement unit 21 Control unit in feedback signal control unit 22 Oscillator control unit 23 Sample stage fine adjustment unit 24 Signal conversion unit 25 Oscillator 26 First signal path 27 Branching section 28 Second signal path 29 First offset circuit 30 Second offset circuit

Claims (11)

A cantilever having a probe that scans the surface of the sample, a displacement detection unit that measures the displacement of the cantilever, and the displacement of the cantilever based on the displacement of the cantilever to keep the distance between the cantilever and the sample constant A feedback control unit that adjusts the height of the sample, and a contact mode that scans the sample by bringing the probe into contact with the sample, and a tapping mode that scans the sample by vibrating the cantilever. In a scanning probe microscope that switches and measures the sample,
Between the feedback control unit and the displacement amount detection unit,
A reference value for keeping the distance between the cantilever and the sample constant when the tapping mode is selected and a reference for keeping the distance between the cantilever and the specimen constant when the contact mode is selected An offset circuit for matching the values;
A branching unit for branching a signal output from the displacement amount detection unit into a first signal and a second signal;
A first signal path through which the first signal passes; and a second signal path through which the second signal passes and different from the first signal path;
A switch for connecting the first signal path and the feedback control unit when the contact mode is selected, and a switch for connecting the second signal path and the feedback control unit when the tapping mode is selected;
A scanning probe microscope comprising: a signal converter having a DC converter that converts an AC signal that is present in the second signal path into a signal proportional to the amplitude of the AC signal.   The scanning probe microscope according to claim 1, wherein the offset circuit is included in the second signal path.   The scanning probe microscope according to claim 1, wherein the offset circuit is provided between the displacement amount detection unit and the branch unit.   The scanning probe microscope according to claim 1, wherein the offset circuit is included in the first signal path.   The scanning probe microscope according to claim 1, wherein the offset circuit is provided in both the first signal path and the second signal path.   6. The scanning probe microscope according to claim 1, wherein switching between the contact mode and the tapping mode and switching of the switch are synchronized.   The scanning probe microscope according to claim 1, wherein the signal output from the displacement amount detection unit is a voltage.   The scanning probe microscope according to claim 1, wherein a signal output from the displacement amount detection unit when the tapping mode is selected is an AC voltage.   9. The scanning probe microscope according to claim 1, wherein a signal output from the displacement detection unit when the contact mode is selected is a DC voltage.   The scanning probe microscope according to claim 1, wherein the DC conversion unit is an RMS-DC converter.   The scanning probe microscope according to claim 1, wherein the displacement detection unit converts a light signal into an electrical signal.