JPH03122506A - Scanning tunneling microscope - Google Patents

Abstract

PURPOSE:To keep a servo system in a proper state at all times irrespective of the state of the surface of a sample by scanning a probe while adjusting a distance between the probe and the sample on the basis of a detection signal of a tunnel current. CONSTITUTION:A Z stage 14 is moved downward under the control of a CPU 1a and a tube scanner 15 is made to extend near a reference length and to reach a tunnel area. While a Z servo voltage from a servo circuit 19 is read by the CPU 1a through an A/D conversion element 7, thereafter, a Z D/A output of a D/A converter 31 is controlled to approach 0V. At a time point when the Z serve voltage becomes 0V, the Z D/A output of the converter 31 is fixed. As the result, a state brought about by impression of a voltage obtained by amplifying 20 an addition output of the Z D/A output and the Z servo voltage is held. In this way, a deviation of the impressed voltage based on the resolution of a rough motion from a reference voltage can be corrected (correction of the Z servo voltage) by adjusting the Z D/A output.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a scanning tunneling microscope capable of observing, for example, the surface of a sample in units of atoms.

[Prior Art] Recently, a scanning tunneling microscope (STM) that can observe the surface of a sample on an atomic basis has been developed.

Generally a metal probe (or probe needle) with a sharp tip
When the probes are placed as close as lrv to the surface of a conductive sample and a predetermined voltage is applied between these probes and the sample,
It is known that a tunnel current flows between the two.

This tunneling current sensitively responds to changes in the distance between the probe and the sample and changes significantly. The so-called STM uses this property of tunneling current to observe the surface of a sample. In other words, when the probe is attached to an actuator that can move in three dimensions and the probe is scanned so that the tunneling current remains constant, the probe traces the irregularities on the sample surface while maintaining a constant distance. Become. By outputting the position of the probe at this time as a three-dimensional image, changes in the shape of the sample surface in atomic units can be observed as an image.

Normally, a servo circuit is used to adjust the distance between the probe and the sample in such an STM. This servo circuit detects a tunnel current flowing between the probe and the sample, and based on this detects the tunnel current flowing between the probe and the sample, controls the drive of the actuator so that the distance between the probe and the sample is constant. It is.

[Problem 8 to be Solved by the Invention] As described above, in the conventional STM, adjustment of the distance between the probe and the sample is controlled only by the servo circuit. Therefore, if the surface condition of the sample is poor, there is a drawback that the intended control cannot be performed. In other words, if the servo output based on the tunnel current is displayed on a CRT as the output of the surface condition when the surface of the sample is inclined, undulating, or has holes in some areas, the CR7 screen will be displayed. Not only can it not be used effectively, but the S7M image will be off the screen. In such cases, it is inappropriate to use a servo system to control the distance between the probe and the sample, and the resulting S7M image may only be displayed at low resolution, or the dynamic range may vary from the center. There were times when it would shift.

SUMMARY OF THE INVENTION An object of the present invention is to provide a scanning tunneling microscope that can always maintain an appropriate servo system that controls the distance between the probe and the sample, regardless of the surface condition of the sample.

[Means for Solving the Problems] In order to achieve the above object, the scanning tunneling microscope of the present invention detects a tunneling current flowing by applying a voltage between a probe and a sample, Based on the detection signal of this tunnel current, the distance between the probe and the sample is adjusted by a distance adjusting means (2) The surface state of the sample is observed by scanning the probe. In the above, the distance adjusting means for adjusting the distance between the probe and the sample includes a piezoelectric drive body that supports the probe or the sample, and a piezoelectric drive body that can expand and contract the piezoelectric drive body. a servo circuit that controls the tunnel current detection signal to be constant; a digital/analog converter that generates a correction voltage to set a predetermined reference position at the servo output; and an output of the servo circuit. an addition circuit that adds the output of the digital/analog converter; an analog/digital converter that converts the output of the servo circuit from analog to digital; and an analog/digital converter that performs the digital/analog conversion based on the output of the analog/digital converter. and a control circuit that controls the adder circuit.

Also, the distance,! The Ii adjustment means further includes a variable amplifier provided between the servo circuit and the analog/digital converter.

Furthermore, the control circuit of the distance adjustment means includes a modulation pattern generation circuit for finely modulating the distance between the probe and the sample, and the output of the modulation pattern generation circuit causes the digital/analog conversion. The device is configured to control the device.

[Function] This invention enables servo control suitable for the surface condition of the sample because the distance adjustment means is operated by the above-described means in accordance with the surface condition obtained from the tunnel current. It is.

Furthermore, since the output of the servo circuit to the analog/digital converter can be amplified, it is possible to magnify and display minute changes in unevenness on the sample surface.

Furthermore, since the output of the digital/analog converter can be modulated at a higher speed than the time constant of the servo circuit, it can be used as a device for measuring the density of electronic states on the sample surface (STS). is possible.

・[Example] Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 13 shows the configuration of the STM (scanning tunneling microscope) of the present invention.

In FIG. 13, 1 is an 8-bit CPU (central processing unit) controller that controls the entire STM. This 8-bit CPU controller 1
An interface controller 2 is connected to the host computer 100 via an interface (CPIB).

The 8-bit CPU controller 1 also includes a pulse motor driver (P, M, D) 3 for moving the Y stage, P, M, D4 for moving the X stage, a scanning counter circuit 5, a Z
A 12-bit A/D converter 7 for A/D (analog/digital) conversion of P, M, D6, and Z electrode voltage signals for stage movement or tunnel current detection signals, and a 10-bit A/D converter 7 for applying bias voltage. A D/A converter 8 and a 16-bit D/A converter 31 for adding two voltages are each connected via an 8-bit data bus.

The 16-bit D/A converter 31 converts binary data supplied from the 8-bit CPU controller 1 via the 8-bit data bus into an analog signal (ZD/A output).

The Y stage movement P, M, and D3 move according to the drive signal (pulse data) from the 8-bit CPU controller 1.
The stage moving P, M, and 9 are driven to move the Y stage 10 in the Y direction (towards the front and back in the figure).

The X stage movement P, M, and D4 correspond to the drive signal from the 8-bit CPU controller 1.
, M, and 11 to move the X stage 12 in the X direction (horizontal direction in the figure).

The Z stage movement P, M, and D6 are
, M, 13 to move the Z stage 14 in the Z direction (vertical direction in the figure).

A tube scanner (piezoelectric drive body) 15 that constitutes an actuator that can be driven in three-dimensional directions is attached to the lower surface of the second stage 14.
A tunnel probe (probe) 16 as a metal probe having a sharp tip shape is supported on the lower surface of the probe 5 . This tunnel probe 16 has the above-mentioned 10-bit D/A.
A bias voltage (v) is applied by a converter 8.

On the other hand, a sample 17 is placed on the upper surface of the X stage 12 facing the two stages 14. A tunnel current CI) flows through the sample 17 by applying a predetermined bias voltage with the tunnel probe 16 brought close to the surface of the sample 17 to about 1 nI.

The tunnel current generated in the sample 17 is supplied to the servo circuit 19, the 12-bit A/D converter 7, and the approach detection instantaneous condensation circuit 32 via the tunnel current amplification preamplifier 18.

The servo circuit 19 generates a Z electrode voltage signal (Z servo voltage) that keeps the distance between the tunnel probe 16 and the sample 17 constant based on the tunnel current detection signal supplied via the preamplifier 18. is generated and outputted to the 12-bit A/D converter 7 or the Z voltage adder (addition circuit) 33. This servo circuit 19
is composed of a PI control circuit that generates a Z servo voltage (for example, -10V to +10V) and an analog switch circuit that switches the output light of these two servo voltages.

The adder 33 adds the ZD/A output from the 16-bit D/A converter 31 and the two servo voltage outputs from the one servo circuit, and adds this to the approach detection instantaneous condensation circuit 3.
This is what is output to 2.

The approach detection instantaneous condensing circuit 32 selects one from the addition output of the adder 33 or the two signals of the most condensed voltage preset so that the tube scanner 15 is most condensed, and applies this to the Z electrode. High voltage amplifier (HoV, Z)
20. This approach detection instantaneous condenser circuit 32 includes an analog switch circuit for selecting one signal and an analog switch circuit for selecting one signal, and an analog It consists of a flip-flop circuit that instantaneously switches the switch circuit.

H, V, Z20 are the approach detection instantaneous condensation circuit 3
The output from the tube scanner 15 is amplified by a factor of 10, for example, and applied to the tube scanner 15. This causes the tube scanner 15 to detect the tunnel probe 16 and the sample 17.
It is expanded and contracted to change the spacing distance between the two. In this case, the tube scanner 15 applies a signal (V
The reference length is set when z) is OV, and when -100V is applied, the length is shrunk by 1 μm, and when +100V is applied, it is extended by 1 μm.

The 12-bit A/D converter 7 is an A/D converter that converts an input signal into digital (binary) data and outputs it to the 8-bit CPU controller 1, and converts the digital data converted by this A/D converter into real-time data. It is composed of a real-time line memory circuit and the like for displaying a tomographic image on a monitor TV34E.

The scan counter circuit 5 counts up and down in accordance with a scan start signal supplied from the 8-bit CPU controller 1 via the 8-bit data bus.
It generates a scanning signal in the X direction and a scanning signal in the Y direction.

X-scan 16-bit D/A to which X-direction scanning signals are supplied
The converter 21 generates an analog voltage output (X applied voltage) corresponding to the input, and applies this to the −X electrode via a high voltage amplifier (H, V, 10X) 22 for applying the +X electrode and an inverting circuit 23. It is output to the high voltage amplification amplifier (H, V, -X) 24 for use. And these +X applied voltages, -
X applied voltage is H, V, 10X22. By applying voltage to the tube scanner 15 through H, V, and -X24, the tube scanner 15 is deformed so that the tip of the tunnel probe 16 scans the surface of the sample 17 in the X direction.

Y-scan 16-bit) D/A to which a Y-direction scanning signal is supplied
The converter 25 generates an analog voltage output (Y applied voltage) corresponding to the input, and converts this into a −
This signal is output to a high voltage amplification amplifier (H, V, -Y) 28 for applying voltage to the Y electrode. And these +Y applied voltages, -Y
By applying the applied voltage 7>(H, V, +Y26, H, V, -Y28 to the tube scanner 15, respectively, the tube scanner 15 detects that the needle tip of the tunnel probe 16 is on the surface of the sample 17. is deformed to scan in the Y direction.

FIG. 1 shows a Z-direction scanning system as a distance l&adjustment means in a first embodiment of the present invention.

In this figure, 1a is a CPU serving as a control circuit constituting the 8-bit CPU controller 1. As shown in FIG. Further, 7a is an A/D converter, and 7b is a real-time line memory circuit, and these A/D converter 7a and real-time line memory circuit 7b are used to convert the 12-bit A/D
A conversion unit 7 is configured.

Next, an automatic approach method in such a configuration will be explained.

First, under the control of CPola, the ZD/A output of the 16-bit D/A converter 31 is set to OV. Further, the analog switch circuit of the servo circuit 19 is turned off to prevent the two servo voltages from being output to the adder 33. Furthermore, the approach detection instantaneous condensation circuit 32 is set to select the addition output from the adder 33. As a result, H,
The voltage (Vz) applied to the tube scanner 151 via V, 220 is OV. Therefore, the tube scanner 15 remains at the standard length as shown in FIG. 2(a).

In this state, when the two stage movement P and MD6 are controlled by the CPU 1a, P and M.

13 is driven, the Z stage 14 is moved downward. Then, as the two stages 14 move, the tube scanner 15 is lowered, thereby bringing the tunnel probe 16 closer to the sample 17. In this case, as shown in FIG. 2(b), the tunnel probe 16 is connected to the above-mentioned points P and M.

13 (eg, 0.1 μm).

On the other hand, assume that during this approach, a tunnel current is detected in response to the application of the bias voltage. Then, P, M,
13 is stopped, and the analog switch circuit in the approach detection instantaneous compression circuit 32 is switched to the forging voltage (-10V). As a result, a voltage (Vz--100V) amplified ten times by H, V, and z20 is applied to the tube scanner 15. As a result, the tube scanner 15 is instantly retracted, as shown in FIG. 2(c). However, PlM, 13
Due to the resolution of , the second stage 14 is stopped at a position lowered by Δ.

Also, at this time, the analog switch circuit of the servo circuit 19 is turned on, and the Z servo voltage is sent to the adder 33. Then, under the control of the CPU 1a, the 2D/A output of the 16-bit D/A converter 31 is set to 110V. As a result, the tube scanner 15 has 1
ZD/A output from 6-bit D/A converter 31 (-10
V) and Z servo voltage from servo circuit 19 (+9 V
) is applied, and a voltage Vz (-1QV) is applied, which is obtained by amplifying the summed output by 10 times with H, V, and 220. As a result, as shown in FIG. 2(d), the tube scanner 15 is extended to nearly the reference length and reaches the tunnel region.

However, due to the resolution of P, M, and 13, the length of the tube scanner 15 is about 0 to 0.1 μm shorter than the reference length.

Thereafter, while the two servo voltages from the servo circuit 19 are read by the CPU 1a via the 12-bit A/D converter 7, the ZD/A output of the 16-bit D/A converter 31 is controlled so as to approach Ov. Ru.

Then, when the two servo voltages become OV (reference voltage), the ZD/A output of the 16-bit D/A converter 31 is fixed. As a result, as shown in FIG. 2(e), 16
ZD/A output (-1V) from bit D/A converter 31
and the two servo voltages (Ov) from the servo circuit 19 are added to H, V, 2201. : Voltage V amplified by 10 times
The state caused by the application of z (-10V) is maintained.

2 When the servo voltage is OV, if the input dynamic range of the A/D converter 7a is 10V to +IOV, it becomes possible to capture unevenness data centered on Ov during scanning. Further, in this case, as the obtained real-time tomographic image, an image based on the central horizontal line will be displayed on the monitor TV 34.

In this way, the tunnel probe 16 is brought closer to the sample 17 using the coarse movement mechanism that moves in steps such as P, M, 13, and the tube scanner 15 is instantly retracted when entering the tunnel area. Along with stopping coarse movement,
At this time, when the application of 2 servo voltages is started and the tube scanner 15 is extended almost to the reference length to enter the servo state, the deviation of the applied voltage Vz from the reference voltage due to the coarse movement resolution is calculated as the ZD/A output. Correction (two servo voltage adjustments) can be made by adjusting.

Furthermore, according to this method, it is possible to start the STM operation at a higher speed than when approaching by repeating coarse and fine movements.

FIG. 3 shows a two-direction scanning system as a distance adjusting means in a second embodiment of the invention.

In this figure, 7a is an A/D converter, 7b is a real-time line memory circuit, and 7c is an amplification switching circuit as a variable amplifier.These A/D converter 7a and real-time line memory circuit 7b.

The 12-bit A/D converter 7 is constituted by the amplification switching circuit 7C.

The amplification switching circuit 7C includes, for example, an analog multiplexer 71 that selects several types of resistors, as shown in FIG.
and an inverting amplifier 72, and converts the Z servo voltage supplied from the servo circuit 19 to 2n (n-0 to
4) It amplifies the voltage twice. Selection switching of the analog multiplexer 71 is performed by a selection signal from CPola.

Next, a method of amplifying and displaying a real-time tomographic image in such a configuration will be described.

If the position of the unevenness you want to observe is determined in advance,
After the tunnel probe 16 is approached by the method described above, only one line is scanned in the X and Y directions. Then, the real-time tomographic image obtained is monitored on TV3.
4, and by observing this, it is possible to confirm whether or not the target uneven position has been correctly approached.

By the way, when the real-time tomographic image displayed on the monitor TV 34 has a slight change in unevenness as shown in FIG.
The amplification factor of c is reset. Then, one line scanning is performed again. As a result, as shown in FIG. 6, a real-time tomographic image is displayed on the monitor TV 34 in which the target unevenness is enlarged in two directions according to the set amplification factor of the amplification switching circuit 7c. In other words, when the unevenness is minute, for example when the change in the Z servo voltage is small, it is
After amplifying the voltage around v, the A/D converter 7a captures the voltage. This makes it possible to easily confirm whether or not the target uneven position has been correctly approached.

Furthermore, when the amplification factor of the amplification switching circuit 7C is set to x16, the A/D has a resolution of 16 bits.
It is possible to obtain unevenness data with the same resolution as when data is captured using a converter.

In this way, the Z servo voltage is set to 0 by the ZD/A output.
By correcting the offset so that
This makes it possible to acquire high-resolution data even from the sample 17 with minute irregularities without the offset being amplified and saturated.

Next, in such a configuration, sample 1 with a slope
A case will be described in which the scanning range is narrowed compared to 7, and the center position of the scanning is moved to perform re-scanning.

For example, as shown in FIG. 7, for a sample 17 with an inclination, move the tunnel probe 16 3.75 μm to the right in the figure from the center position of the opening by 10 μm, with the servo applied. It is assumed that the central 2.5 μm aperture is rescanned.

In this case, by controlling the scan counter circuit 5 by the 8-bit CPU controller 1, the deviation ff1 from the center position of the opening is calculated from the scan counter circuit 5 by 10 μm.
(3,75 μm), and rescan range (2,5 μm)
Scanning signals in the X and Y directions are respectively outputted.

Then, the tube scanner 15 is deformed by the application of voltages corresponding to these scanning signals, so that the tunnel probe 16 is moved while being servoed.

At this time, since the sample 17 has an inclination, the tube scanner 15 shown in FIG. 8 is shortened by 62 minutes. That is, ZD/A of the 16-bit D/A converter 31
The output is -IV, the second servo voltage of the servo circuit 19 is Ov, and the voltage Vz (-10V) amplified ten times by H, V, and Z20 is applied to the tube scanner 15 (eighth (See Figure 8(a)), the two servo voltages become 14V, and thereby the voltage Vz applied to the tube scanner 15 changes to 150V (see Figure 8(b).
)reference).

In this state, as explained in the auto approach above, the ZD/A output of the 16-bit D/A converter 31 is changed in the negative direction so as to set the Z servo voltage to Ov. Then, when the Z servo voltage becomes OV, the ZD/A output of the 16-bit D/A converter 31 is fixed. That is, as shown in Fig. 8(c), the two servo voltages are equal to -4
The ZD/A output is changed from V to Ov, and the ZD/A output is changed from -1v to -5v, respectively, but the tube scanner 15 has the same -5v.
A voltage Vz of 0V is applied.

As a result, even if the unevenness is minute as shown by a in FIG. 9, for example, by adjusting the Z servo voltage, the A/D converter 7 is
You can now import data using a. This makes it possible to obtain high-resolution data (real-time tomographic images) over a 2.5 μm aperture range, as shown by b in FIG.

FIG. 10 shows a Z-direction scanning system as a distance adjusting means in a third embodiment of the present invention. Here, by modulating the distance between the tunnel probe 16 and the sample 1.7 and capturing the tunnel current, STM is converted into STS which measures the surface properties of the sample 17 (electronic state density on the sample surface). It is intended to be used as a

That is, in the case of this distance adjustment means, the 8-bit CPU controller 1 includes, in addition to the CPU 1a as a control circuit,
It is configured to include a two-modulation pattern data generator 1b as a modulation pattern generation circuit. Also, 12 bit A
The /D conversion unit 7 is composed of an A/D converter 7a, a real-time line memory circuit 7b, an amplification switching circuit 7cs, and a signal selection circuit 7d.

The Z modulation pattern boot generator 1b consists of up/down counter circuits from roooooJ to roooFHJ, rooIFHJ, and r003FHJ, and switches the up/down count width according to the modulation range of the tunnel probe 16, and outputs binary data from the counter circuit. By supplying the signal to the 16-bit D/A converter 31, the ZD/A output of the 16-bit D/A converter 31 is slightly modulated. The operation start, stop, and switching of the count width of the Karatan circuit in this two-modulation pattern data generator 1b are performed by the CP.
It is controlled by signals from ola.

The signal selection circuit 7d selects Z according to a signal from the CPU 1a.
One is selected from the servo voltage (unevenness data) or the tunnel current detection signal (voltage converted) and output to the amplification switching circuit 7C.

Further, the servo circuit 19 is provided with a switching circuit for only PI control/① control and a control gain switching circuit.

Next, a method for measuring the electronic state density on the sample surface in such a configuration will be described.

First, under the control of CPola, the servo circuit 19 is
At the same time, the control circuit (
Integral control) The gain is set so that the unevenness of the surface of the sample 17 can be sufficiently followed.

Next, the Z modulation pattern boot generator 1b is controlled to have a spatial frequency that is followed by the I control, that is, a frequency that is sufficiently higher than the changing roughness of the image, and thereby is faster than the time constant of the servo circuit 19. 16 bit D
The ZD/A output of the /A converter 31 is modulated.

After this, the Z servo voltage from the servo circuit 19 and the ZD/A output from the 16-bit D/A converter 31 are sent to the adder 33.
is added by Then, the added output of this adder 33 is amplified by HoV 220 and sent to the tube scanner 1.
5.

In this state, X-Y scanning is performed. Then, the tip of the tunnel probe 16 is moved over the surface of the sample 17, drawing a trajectory as shown in FIG.

At this time, the signal selection circuit 7d is controlled to switch the signal taken in by the A/D converter 7a from the Z servo voltage to the tunnel current detection signal. Then, by sampling the data of this signal, for example, at the point positions shown in FIG. 12, the electronic state density on the sample surface can be measured.

This needle tip modulation sequence is generally known as a method for measuring the density of electronic states on the surface of a sample, and the distance adjustment means of the present invention enables its operation.

In this way, by capturing the tunnel current detection signal in accordance with the modulation of the ZD/A output of the 16-bit D/A converter 31, it is possible to use it as an STS.

As described above, by operating the distance adjusting means in accordance with the surface condition obtained from the tunneling current, servo control suitable for the surface condition of the sample can be performed.

In other words, by applying a voltage that makes the servo voltage Ov, the deviation of the applied voltage in the Z direction from the reference voltage due to the resolution of the coarse movement mechanism when the tunnel probe approaches the surface of the sample is calculated. I am trying to correct it. This simplifies the approach operation and allows the distance of the tunnel probe to be servo-controlled in accordance with the surface condition of the sample. Therefore, during scanning, it is possible to capture unevenness data centered on the OV, so it is possible to prevent the resolution of the real-time tomographic image from being degraded and the dynamic range from shifting from the center.

Moreover, since it becomes possible to capture unevenness data centered on Ov, by amplifying it, it becomes possible to enlarge and display minute changes in unevenness on the sample surface.

Furthermore, the distance of the tunnel probe can be servo-controlled according to the surface state of the sample, so by modulating the distance between the tunnel probe and the sample, the electronic density of states on the sample surface can be measured. It will also be possible to use it as an STS.

In the above-described embodiments, the probe is supported by the piezoelectric driver, but the probe is not limited to this, and may be provided to support the sample, for example.

It goes without saying that various other modifications can be made without departing from the gist of the invention.

[Effects of the Invention] As detailed above, according to the present invention, the distance adjustment means is operated in accordance with the surface condition obtained from the tunneling current, so that the probe tip can be adjusted regardless of the surface condition of the sample. It is possible to provide a scanning tunneling microscope that can always maintain an appropriate servo system for controlling the distance between the sample and the sample.

[Brief explanation of drawings]

The drawings show an embodiment of the invention, and FIG. 1 is a block diagram of a distance adjusting means showing the first embodiment of the invention;
FIG. 2 is a diagram for explaining the auto-approach method in the first embodiment, FIG. 3 is a block diagram of the distance adjustment means in the second embodiment, and FIG. 4 is a configuration showing the amplification switching circuit. 5 and 6 are shown to explain the method of amplifying and displaying a real-time tomographic image in the second embodiment, and FIG. 5 is a diagram showing an example of displaying in a normal size. , FIG. 6 is a diagram showing an example of the case of amplification and display, and FIGS. 7 to 9 are shown to explain a method for rescanning a part of a tilted sample in the second embodiment. Fig. 7 is a diagram showing an example of rescanning operation, Fig. 8 is a diagram also showing a control example, and Fig. 9 is a diagram showing an example of rescanning operation.
10 is a block diagram of the distance adjustment means showing the third embodiment, and FIGS.
The figures are shown to explain the operation in the third embodiment, and FIG. 11 is a diagram showing the locus of the tip of the tunnel probe, FIG. 12 is a diagram showing data sampling points,
FIG. 13 is a block diagram schematically showing the configuration of the STM. 1...8-bit CPU controller, 1a...CP
U (control circuit), 1b...Z modulation pattern data generator (modulation pattern generation circuit), 7...12 bit A/
D conversion section, 7a... A/D converter, 7c... Amplification switching circuit (variable amplifier), 13... P for Z stage movement
, M, 14...Z stage, 15...tube scanner, 16...tunnel probe, 17...sample (specimen), 19...servo circuit, 31...16
Bit D/A converter, 32... Approach detection compression circuit, 33... Adder (addition circuit) 34... Monitor TV.

Claims (3)

[Claims] (1) Detecting a tunnel current flowing by applying a voltage between the probe and the sample, and adjusting the distance between the probe and the sample based on the detection signal of the tunnel current. In a scanning tunneling microscope that observes the surface condition of the sample by scanning the probe while adjusting the distance, the distance adjusting means for adjusting the distance between the probe and the sample adjusts the distance between the probe and the sample. A retractable piezoelectric drive body to be adjusted; a servo circuit that controls the expansion and contraction of the piezoelectric drive body so that the detection signal of the tunnel current is constant; a digital/analog converter for generating voltage; an adder circuit for adding the output of the servo circuit and the output of the digital/analog converter; and an analog/digital converter for converting the output of the servo circuit from analog to digital. A scanning tunneling microscope comprising: a control circuit that controls the digital/analog converter and the adder circuit based on the output of the analog/digital converter. (2) The scanning tunneling microscope according to claim 1, wherein the distance adjustment means further includes a variable amplifier between the servo circuit and the analog/digital conversion section. (3) The control circuit of the distance adjustment means includes a modulation pattern generation circuit for slightly modulating the distance between the probe and the sample, and the output of the modulation pattern generation circuit causes the digital/analog The scanning tunneling microscope according to claim 1, characterized in that the converting section is controlled.