JP2919997B2 - Scanning tunnel microscope measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning tunneling microscope, and more particularly to a scanning tunneling microscope capable of performing high-speed and wide-area measurements. The measurement method.

[0002]

2. Description of the Related Art A scanning tunneling microscope (hereinafter referred to as STM)
) Is described in, for example,
-220260, JP-A-61-206148, or Physical Review Letter, 49 (1982) pp. 57-61.

In order to increase the speed of measurement and increase the area over the STM, the scanning movement of the probe on the measurement surface of the sample should be separated by a sufficiently large distance compared to the irregularities on the sample surface. Measurement locations for performing measurements on the surface are discretely determined, and these measurement locations are moved almost linearly.At the time of measurement, the probe is moved toward the sample surface at each measurement location, and the tunnel is moved. There is an STM configured to retreat to the position of the scanning movement path after measuring the current (Japanese Patent Laid-Open No. 1-169304,
JP-A-2-5340).

[0004]

The STM disclosed in the latter two documents, which aims to increase the speed of measurement and increase the area of the measurement area by accelerating the scanning movement of the probe, employs a probe between measurement locations. Is moved at a location on the scanning path away from the sample surface, which necessarily includes an operation of moving between the scanning path and a location near the sample surface. Therefore, it is necessary to speed up the movement of the probe approaching the sample surface in order to speed up the entire measurement.

An object of the present invention is to provide a method of measuring an STM in which the probe is moved at a position distant from the sample surface in a scanning manner so that the probe approaches the sample surface at high speed. It is in.

It is a further object of the present invention to provide a method for measuring an STM in which vibration is not generated when a probe approaching a sample surface at high speed is stopped.

[0007]

According to the STM measuring method of the present invention, a plurality of measuring locations are discretely determined on a sample surface,
Scan the probe linearly between the measurement locations at a position sufficiently far away from the unevenness of the sample surface, and move the probe to the sample surface at the position on the scanning path corresponding to each of the measurement locations. Approach, stop after measuring the tunnel current, then retreat to a position on the scanning path, perform scanning again, in the measuring method of the scanning tunneling microscope to perform the approaching operation for measurement at each measurement location, The probe approaches the sample surface at the measurement location at a high speed when there is no or small detection current, and approaches with a proportional control so that the speed decreases as the detection current increases. And switches to integral control to approach and finally stop.

[0008]

In the STM measuring method according to the present invention, when the probe performs an approaching operation on the sample surface at each of a plurality of discretely set measurement locations at a predetermined distance, a high-speed approach is performed in an initial stage. In the middle part, a proportional approach proportional to the distance between the probe and the sample is performed, and an integral approach is performed in the final stage.
It is a high-speed approach as a whole and can stably stop at a predetermined position with respect to the sample surface without causing vibration,
Enables high-speed access at each measurement location. Therefore, S
In the measurement by TM, the speed can be further increased.

[0009]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a diagram showing the movement trajectory of a probe according to the STM measurement method according to the present invention. FIG. 2 is a diagram showing the approach operation of the probe to the sample surface, the relationship between the probe position, the probe movement speed, and the tunnel current. FIG. 3 is a diagram showing a main part of a device configuration for controlling the approach operation of the probe, and FIG. 4 is a flowchart showing a control procedure of the approach operation.

In FIG. 1, reference numeral 1 denotes a part of the surface of a sample to be measured, and the sample surface 1 has irregularities. This uneven shape is an object to be measured by the STM. In the STM, the irregular shape of the sample surface 1 is measured by using a tunnel current flowing between the probe and the sample when the probe approaches the sample surface 1 by a predetermined distance. Reference numeral 2 denotes an STM probe, and the illustration of other general structures of the STM is omitted since it is known to those skilled in the art.

Here, the configuration and operation of a typical STM will be outlined for reference. The STM includes a probe, a movement mechanism for fine movement that supports the probe and moves the probe in an approaching / retreating direction (Z direction) with respect to the sample and in a scanning direction (X, Y directions) along the sample surface. A power supply for generating a bias voltage for causing a tunnel current to flow through the probe when approaching a predetermined distance to the probe, a tunnel current detection unit for detecting the tunnel current, and converting the detected tunnel current into a predetermined electric signal. And a servo controller for driving the Z-direction actuator of the moving mechanism based on the electric signal to control the distance between the probe and the sample surface to be constant.
A scanning control unit for controlling the scanning movement of the probe by driving an actuator for the Y direction; taking in control signals of the servo control unit and the scanning control unit; processing data relating to the position of the probe; And a signal processing unit that generates image data by calculating the image data, and a display unit that displays an image of the sample surface based on the image data of the signal processing unit. The tunnel current detected by the tunnel current detection unit is set to a constant value, so that the tunnel current detected when scanning the probe along the sample surface is held at the constant value,
The actuator for the Z direction is controlled. Thus, when the probe is scanned and moved along the irregularities on the sample surface, the probe moves while maintaining a distance corresponding to a constant value of tunnel current with respect to the sample surface. This state is shown by the movement trajectory 4 in FIG. Therefore, by periodically sampling the position data from the Z-direction position data of the probe 2 continuously moving along the sample surface 1, it is possible to obtain the unevenness information of the sample. In FIG. 5, a plurality of points 5 indicate measurement locations where sampling is performed.

In contrast to the above-described STM device configuration, the STM according to the present invention has basically the same device configuration. The difference is in the way the probe 1 moves. Therefore, the probe 1 is
The way of controlling the operation of the moving mechanism for moving in the direction or the XY direction is different. The manner of movement of the probe 1 according to the present invention is clear from the movement trajectory 3 of the probe 2 shown in FIG.

The movement locus 3 of the probe 2 shown in FIG. 1 shows a scanning movement locus 3a in the X direction and a locus 3b of approach / retreat movement in the Z direction for convenience. As is apparent from the movement trajectory 3 of the probe 2, the probe 2 moves for scanning in the X direction (or Y direction) at a position at a certain height sufficiently away from the unevenness of the sample surface 1. As described above, the position control is performed, and a plurality of measurement locations S1, S2,... Are discretely determined at predetermined fixed distance intervals in the scanning movement path. Therefore, in the scanning movement of the probe 2, the measurement locations S1, S
It moves linearly at high speed between 2,. This moving portion is the above-described scanning movement locus 3a. At each of the measurement locations S1, S2,..., The probe 2 moves toward the sample surface 1,
An approach operation is performed to detect a tunnel current having a predetermined value, and after the detection of the tunnel current, after stopping at the measurement point, moving away from the measurement point near the sample surface 1 and performing scanning movement. Retreat to a certain height on the road. Then, the scanning movement to the next measurement place is performed again. The approach operation of the probe 2 is controlled so that the tunnel current flowing between the probe 2 approaching the sample and the sample surface 1 becomes a constant value, so that the probe 2 approaches the sample surface 1 at each measurement location. The distance between the final approach position and the sample surface 1 is a constant value L1. Therefore, the final approach position of the probe 2 to the sample surface 1 is determined according to the uneven shape of the sample surface 1 as shown in FIG. Therefore, the data of the unevenness of the sample surface 1 can be obtained from the final approach position data of the probe 2 to the sample surface 1 at each measurement location. In the STM measurement method according to the present invention, the operation of approaching the probe 2 to the sample surface 1 at each measurement location is performed at high speed as described below.

The approach operation of the probe 2 at each measurement location is
This will be described with reference to FIGS. 2, the horizontal axis represents the position of the probe 2 approaching the sample with respect to the sample surface 1 (distance from the sample surface; Z), the left vertical axis represents the tunnel current (I), and the right vertical axis represents the probe 2 Represents the moving speed (V) of the object. Z _S is the position where the detection of the tunnel current is started, Z Z
_L is the reference position for changing the control method of the probe 2, Z _K represents a target position to stop the probe 2. The start position Z _S corresponds to the tunnel current I _S , the reference position Z _L corresponds to the tunnel current I _L , and the stop target position Z _K corresponds to the tunnel current I _K.
Corresponds. As shown in FIG. 3, the approach operation of the probe 2 is
The control is performed based on the control by the above-described tunnel current detection unit 6, signal conversion unit 7, servo control unit 8, memory 9 storing a control procedure for controlling the approaching operation of the probe 2, and the like, and an arithmetic processing unit 10. . Reference numeral 11 denotes an actuator for moving the probe 2 in the Z direction. The positions Z _S , Z _L , and Z _{K in} the Z direction are found by the detected tunnel current, and while checking the tunnel current value detected by the tunnel current detection unit 6, the respective set values Is,
I _L, when it is I _K, corresponding control processing unit 10
Performed by In FIG. 2, a graph 12 indicated by a solid line indicates a change characteristic of a tunnel current detected by the probe 2 when the probe 2 approaches the sample surface 1, and a graph 13 indicated by a broken line indicates 2 shows the change characteristics of the moving speed of the probe 2 when the probe approaches the sample surface 1.

In the approach operation of the probe 2 to the sample surface 1 according to the present invention at each measurement location, as is apparent from the speed characteristic 13 in FIG. 2, the position Z _S at which the tunnel current I _S is detected.
Section up is moving at a high speed constant velocity V _S, after detecting a tunnel current I _S is the section Z _S Z _L to the tunnel current I _L is detected, the probe 2 and the sample surface 1 Proportional control is performed to decrease the moving speed in proportion to the distance. After the detection of the tunnel current I _L , the control method is switched from the above-mentioned proportional control to the integral control, and the section Z _L
Z _K the integral control is performed, rapid probe 2 with large reduction
Stops at the target position Z _K. In this integration control, position integration is performed with respect to the detected tunnel current value, a deviation between a predetermined constant value and an integral value is obtained, and a speed corresponding to the deviation value is given as a moving speed of the probe 2. Done. Therefore the probe 2 at the target position Z _K, stops without vibration. Then, after stopping, the probe 2 returns to the position on the scanning movement path at which the approach started at high speed. In the above description, the high-speed constant speed V _S , proportional conditions such as a proportional coefficient determined by the proportional control, integration conditions for the integral control, and the like are determined in advance in accordance with the conditions of each measurement.

In the control of the approach operation of the probe 2 described above,
The proportional control is performed in the section Z _S Z _L , and the proportional control is switched to the integral control at the reference position Z _L. The setting of the reference position Z _L is appropriately set in consideration of a required displacement with respect to the target stop position Z _K. The mobile control after detecting the reference position Z _K, as described above integral control is executed. In this way, the probe 2 first approaches rapidly, gradually decreases in speed, the rate of decrease in speed from the reference position decreases, and stops at the target position in a short time. In particular, the acceleration can be set to change smoothly at the control switching point, and when stopping at the target position, the speed is gradually reduced to stop, so that generation of vibration can be suppressed.

FIG. 4 is a flowchart showing the control procedure of the approach operation described above. In the first step 21, various conditions for the approach operation are input. This input operation is performed by the operator using the input device 14 shown in FIG. Step 2
Processes 2 to 29 are approaching operations of the probe executed at each measurement location. In step 23, the servo control system is turned on, and the approach operation is started. In the first stage, the vehicle approaches at a high constant speed (step 23). This approach operation is continued until the tunnel current IS is detected (Step 24). After the detection of the tunnel current IS, the process proceeds to the above-described proportional control while maintaining the continuity of the speed (step 25). This proportional control is based on the tunnel current IL
Is continued until is detected (step 26). After the detection of the tunnel current IL, the above-described integration control is executed (step 27). Speed control is performed so that the deviation becomes zero. When the deviation is zero, the speed of the probe becomes zero, and the probe stops (step 28). Thereafter, the servo control system is turned off, the retreat operation is performed at a high speed, and the position returns to the first position on the scanning track separated from the sample surface.

In the above control, it is desirable that the servo control section 8 is constituted by a digital servo system.

When the probe 2 is brought closer to the sample surface 1 by the above-described control method, the time required for the approach in the approach operation under the same conditions can be shortened by, for example, about a quarter as compared with the conventional control method. Can be.

[0020]

As is apparent from the above description, according to the present invention, the object moves at a high speed at a place sufficiently distant from the surface of the sample, and approaches the surface of the sample at the measuring position to obtain information on the unevenness of the sample surface. In the STM configured as described above, high-speed approach can be performed even in the approach operation of the probe at each measurement location, so that high-speed measurement can be performed as a whole. Furthermore, even if such a high speed is executed by the approach operation of the probe, it can be stopped stably without causing vibration,
Reliable measurement can be performed.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a movement locus of a probe in an STM measuring method according to the present invention.

FIG. 2 is a diagram showing a change in speed and a change in tunnel current in an operation of approaching a probe to a sample surface.

FIG. 3 is a block diagram showing a main part of an apparatus configuration for executing the STM measurement method according to the present invention.

FIG. 4 is a flowchart showing a control procedure for executing the STM measurement method according to the present invention.

FIG. 5 is an explanatory diagram showing a movement locus of a probe in the case of a conventional STM measuring method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sample surface 2 Tip 3 Movement locus of a tip 6 Tunnel current detection part 8 Servo control part 10 Operation control part 11 Actuator in Z direction

Claims (2)

(57) [Claims] A plurality of measurement locations are discretely defined on a sample surface, and a scanning movement of a probe is linearly moved between the measurement locations at a position sufficiently distant from the unevenness of the sample surface, and the measurement location is determined. At a position on the scanning movement path corresponding to each of the above, the probe is brought close to the sample surface, stopped after measuring a predetermined tunnel current, then retracted to the position on the scanning movement path, and the scanning movement is performed again. Perform, in the measuring method of the scanning tunneling microscope performing an approaching operation for measurement at each measurement location, the approach of the probe to the sample surface at each measurement location is fast when there is no or small detection current. A scanning ton that approaches by speed, approaches by proportional control so that the speed decreases as the detected current increases, switches to the integral control at the reference position, approaches, and finally stops. How to measure with a tunneling microscope. 2. The measuring method for a scanning tunnel microscope according to claim 1, wherein the approaching operation of the probe is performed by a digital servo system.