JPH0792461B2 - Scanning tunneling microscope - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a scanning tunneling microscope that obtains a scanning image of a sample surface by detecting a tunnel current by scanning with a probe close to the sample surface.

[Conventional technology]

1n where the atom at the tip of the probe and the electron cloud of the atom of the sample overlap
The probe is brought close to the sample to about m or less, and in this state, a tunnel current for applying a voltage flows between the probe and the sample. This tunnel current is about 1 to 10 nA when the voltage is several mV to several V, and changes depending on the distance between the sample and the probe. Therefore, by measuring the magnitude of the tunnel current, the distance between the sample and the probe can be measured ultra-precision, and the surface shape of the sample can be obtained.

A scanning tunneling microscope (STM) controls the height of the probe so that the tunnel current is constant, and when the probe is moved in the horizontal direction, the surface shape of the sample is determined by the height trajectory of the probe. This is a device to be observed and is attracting attention in analyzing surface atomic arrangements. When observing the surface shape (concavo-convex image) of the sample by the tunnel current, first move the probe closer to the sample to about 0.1 μm by coarse movement, and then move the probe closer to the sample from 0.1 μm to 1 nm by fine movement.
Å The order is controlled.

FIG. 12 is a diagram showing a schematic configuration of a scanning tunneling microscope. In the figure, 42 is a sample, 43 is a probe, 44 is a head, 46 and 48
Reference numeral 49 is a piezoelectric element, 45 and 47 are insulating plates, 51 is an electrode, 52 is an XY scanning circuit, 53 is a servo circuit, 54 is a tunnel current amplifier, 55 is a bias power supply, and 56 is a display device.

Referring to FIG. 12, in the STM unit, a probe 43 is attached to a head 44, and the head 44 includes the insulating plates 45 and 47 and the piezoelectric elements 46 and 4.
Supported by 8 and 49. The piezoelectric elements 46, 48, 49 are
A three-dimensional actuator composed of an X axis, a Y axis, and a Z axis is configured, and the piezoelectric element 46 is the Z axis, the piezoelectric element 48 is the X axis, and the piezoelectric element 49.
Drive the Y axis. A driving voltage is applied to the electrodes 51 arranged on both sides of each of the piezoelectric elements 46, 48 and 49 which form the three-dimensional actuator. In the control of the three-dimensional actuator, the XY scanning circuit 52 sweeps the applied voltage to the piezoelectric elements 48 and 49 in the X-axis and Y-axis directions to move the probe 43 in the X-axis and Y-axis directions to perform scanning. While maintaining a constant tunnel current, the servo circuit 53
To control the voltage applied to the Z-axis direction piezoelectric element 46.

Therefore, by displaying this control voltage value on the display device 56, the surface shape (concavo-convex image) of the sample 42 can be observed.

[Problems to be Solved by the Invention]

13 and 14 are diagrams for explaining an example of a conventional scanning method.

As described above, in a tunnel microscope, a piezoelectric element is generally used for scanning a sample or a probe.However, since the piezoelectric element has hysteresis, when it is driven by a triangular wave, the same voltage can be used for the return and return positions. There is a problem that it is out of alignment.

Therefore, conventionally, as shown in FIG.
No image of the return portion was displayed by directional scanning or Y-direction scanning. That is, when the probe goes over the sample in the direction of the solid line and scans as shown in FIG. 14 (a), the image is displayed in synchronization with this scanning as shown in FIG. 14 (b), and the dotted line It erases when scanning back in the direction. Moreover, since the servo circuit for controlling the Z-direction drive of the search is always operating, the return scanning speed cannot be increased because the probe is not hit on the sample surface. Therefore, the return scan is a dead time.

An object of the present invention is to solve the above-mentioned problems, and an object thereof is to provide a scanning tunneling microscope which, when driven by a triangular wave, captures an image not only on the way but also on the way back and effectively uses it.

[Means for Solving the Problems]

Therefore, the present invention is a scanning tunneling microscope that obtains a scanning image of the sample surface by detecting the tunnel current by scanning the probe or the sample by bringing the probe close to the sample surface, and the video signal of the forward scan and the scan of the backward scan. It is characterized in that a scanning image of the sample is separately obtained based on the video signal.

[Action]

In the scanning tunneling microscope of the present invention, since the scanning image is obtained in each direction of the reciprocating scanning, it is possible to obtain the comparison information between the image obtained by the forward scanning and the image obtained by the returning scanning, and the returning scanning is effective without waste. Can be used for.

〔Example〕

 Hereinafter, embodiments will be described with reference to the drawings.

FIG. 1 is a diagram showing a configuration of an embodiment of a tunnel microscope according to the present invention, FIG. 2 is a waveform diagram showing an embodiment of an X scanning mode, and FIG. 3 is a waveform showing another embodiment of an X scanning mode. Figure,
FIG. 4 is a waveform diagram for explaining the Y scanning mode, and FIGS. 5 to 8 are diagrams showing examples of correspondence between scanning and display images.
1 is a Y scanning circuit, 2 is an X scanning circuit, 3 is a constant voltage generating circuit, 4 is a switching amplifier, 5 is an adding circuit, 6 is a Y driving amplifier, 7 is an X driving amplifier, 8 is an X display amplifier, and 9 is Y. A display amplifier, 10 is a display CRT, 11 is a blank circuit, 12 is a video amplifier, 13 is a Z control circuit, 14 is an X drive element, 15 is a Y drive element, and 16 is a Z drive element.

In FIG. 1, an X drive element 14, a Y drive element 15, and a Z drive element 16 are, for example, piezoelectric elements, which constitute a three-dimensional actuator. X scanning circuit 2, Y scanning circuit 1
Is a circuit for generating X and Y scanning signals,
The X drive amplifier 7 and the Y drive amplifier 6 control the voltage applied to the X drive element 14 and the Y drive element 15, respectively, based on the scanning signal. The Z control circuit 13 detects the tunnel current and controls the Z drive element so that the tunnel current becomes constant.
It controls the voltage applied to 16, and the signal becomes a video signal. The switching amplifier 4 switches the non-inversion mode in which the X scanning signal A is sent to the (B) adder circuit 5 as it is as shown in FIG. 3 and the inversion of the X scanning signal A in half cycle as shown in FIG. (B, + →-→ + ...) There is an inversion mode for sending. In the latter inversion mode, the inversion is switched by the timing signal x of the X scanning circuit 2 when switching from the going to the returning. The constant voltage generating circuit 3 generates a constant voltage C which is positive / negative inverted by the timing signal x every half cycle of the X scanning signal as shown in FIGS. 2 and 3. The adder circuit 5 adds the output B of the switching amplifier 4 and the output C of the constant voltage generation circuit 3 and supplies the display scanning signal D to the X display amplifier 8.

Next, the operation will be described.

First, the X scanning circuit 2 generates an X scanning signal having a triangular waveform shown in FIGS. 2A and 3A, amplifies this signal by the X driving AMP7, and supplies it to the X driving element 14 composed of a piezoelectric element. Moves the sample or probe in the X direction. Also in the Y scanning circuit 1, as shown in FIG. 4A, a Y scanning signal having a waveform having a slow slope corresponding to the required X scanning lines is generated, this signal is amplified by the Y driving AMP6, and the Y driving element is generated. The sample or the probe is moved in the Y direction by supplying the sample to the sample 15.

Therefore, when the switching amplifier 4 is in the inversion mode, X
Regarding the display, every time the direction of the X scan signal A changes, the output B of the switching amplifier 4 and the output C of the constant voltage generation circuit 3 are displayed.
Is changed as shown in FIG. 2 and added by the adder circuit 5 to halve. As a result, the waveform shown in FIG. 2D is obtained as the output of the adder circuit 5, and the signal D is amplified by the X display amplifier 8 and used for the X deflection of the display CRT 10. And the display amplifier 8
When the slope of the output D of 1 changes, the image is erased with the waveform shown in FIG. 2E. For Y display, the Y scanning signal is used as it is to perform Y deflection of the CRT. In this way, as shown in FIG. 5, the image obtained by the forward scanning and the image obtained by the backward scanning are displayed as symmetrical images.

Further, when the switching amplifier 4 is in the non-inversion mode, the output of the switching amplifier 4 is the X scan signal A as shown in FIG. 3B.
As shown in FIG. 6, the image obtained by the forward scanning and the image obtained by the backward scanning are displayed in parallel.

Furthermore, if the alternate long and short dash line portion in FIG. 1 (constant voltage generation circuit 3, switching amplifier 4, adder circuit 5) is added in front of the Y display amplifier 9 and both the X and Y displays are in the inversion mode of FIG.
As shown in the figure, four vertically and horizontally symmetrical images are obtained. Similarly, when the non-inversion mode shown in FIG.
As shown, four images in the same direction are obtained.

FIG. 9 is a diagram showing another embodiment of the present invention using a frame memory, FIG. 10 is a diagram showing a configuration example of the frame memory,
FIG. 11 is a diagram for explaining the relationship between scanning and memory addresses, 21 is a controller, 22, 23 and 29 are D / A conversion circuits, 24 is an A / D conversion circuit, 25 is a frame memory, 26 Is Y
Drive amplifier, 27 X drive amplifier, 28 Z control circuit, 30 monitor CRT, 31 X drive element, 32 Y drive element, 33 Z
A drive element is shown.

In the example shown in FIG. 9, the controller 21 outputs data to the D / A conversion circuit 23 for X and the D / A conversion circuit 22 for Y at regular time intervals to respectively drive the X drive element 31 and Y drive. A voltage having the waveform shown in FIG. 2A is applied to both the elements 32.

Further, the output from the Z control circuit 28 for keeping the tunnel current constant is taken into the controller 21 through the A / D conversion circuit 24 at regular time intervals.

Then, for example, when the configuration of the frame memory 25 is set to 512 × 512 pixels as shown in FIG. 10, an address is generated on the frame memory 25 by an algorithm as shown in FIG. 11A for X and Y scanning. And store the data.

The data stored in the frame memory 25 is read at a speed that matches the cycle of the monitor CRT 30 and the D / A conversion circuit 29 reads it.
Converts, adds sync signal and sends to monitor CRT30.
By doing so, an image as shown in FIG. 7 can be displayed.

Also. When storing data in the frame memory 25, X, Y
When an adrel is generated by an algorithm as shown in FIG. 11B, an image as shown in FIG. 8 can be displayed.

The present invention is not limited to the above embodiment, and various modifications can be made. For example, in the above example
Although the CRT was used, an XY recorder may be used instead of the CRT. In that case, the Y-axis modulation eliminates the need for the luminance signal. In addition, brightness erase corresponds to pen-up. Further, the image of the return scanning period may be put in another frame memory to be displayed on another screen.

〔The invention's effect〕

As is clear from the above description, according to the present invention, since the respective images are obtained in response to the reciprocal scanning, it is possible to display the images side by side at the same time, or to display the images by switching the images.
The state of hysteresis of the Y and Y drive elements can be known. In addition, depending on the shape of the probe, the reciprocal images do not match, which can be utilized for evaluation of the probe. Furthermore, since the return (return) scanning period of the sample or the probe is also used, it is possible to eliminate wasteful time and obtain useful information.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of a tunnel microscope according to the present invention, FIG. 2 is a waveform diagram showing an embodiment of an X scanning mode, and FIG. 3 is a waveform showing another embodiment of an X scanning mode. Figure,
FIG. 4 is a waveform diagram for explaining the Y scanning mode, FIGS. 5 to 8 are diagrams showing examples of correspondence between scanning and display images, and FIG. 9 is another embodiment of the present invention using a frame memory. FIG. 10 is a diagram showing an example, FIG. 10 is a diagram showing a configuration example of a frame memory, FIG. 11 is a diagram for explaining the relationship between scanning and memory addresses,
FIG. 12 is a diagram showing a schematic configuration of a scanning tunneling microscope, FIG.
FIG. 14 and FIG. 14 are diagrams for explaining an example of a conventional scanning method. 1 ... Y scanning circuit, 2 ... X scanning circuit, 3 ... constant voltage generating circuit, 4 ... switching amplifier, 5 ... adding circuit, 6 ...
... Y drive amplifier, 7 ... X drive amplifier, 8 ... X display amplifier, 9 ... Y display amplifier, 10 ... Display CRT, 11 ... Blank circuit, 12 ... Video amplifier, 13 ... Z control circuit ,14
...... X drive element, 15 …… Y drive element, 16 …… Z drive element.

Claims (1)

[Claims] 1. A scanning tunneling microscope which obtains a scanning image of the sample surface by scanning the probe or the sample by bringing the probe close to the sample surface to obtain a scanning image of the sample surface, and an image signal of a forward scan and an image of a backward scan. A scanning tunneling microscope characterized in that a scanning image of a sample is separately obtained based on a signal.