JPH05164511A - Scanning type tunnel microscope - Google Patents

Abstract

PURPOSE:To shorten a measuring time and to improve measuring precision by a method wherein time speed of scanning of a probe on the surface of a sample is determined automatically in accordance with the state of indentation of the sample surface. CONSTITUTION:When a probe 1 approaches the surface of a sample 2 at a distance of an atomic level, a tunnel current flows and a tunnel current detector 5 detects this current and gives it to a servo control device 6. The device 6 makes the probe l follow the indentation of the surface of the sample, while a control for scanning an area to be measured is conducted by a scanning device 7. The device 6 gives a data processing device 9 the control data used for a control on the position of the probe 1 in the direction of the axis Z, as positional data. Meanwhile, the scanning device 7 gives the device 9 the control data for a control on scanning of the probe 1 in each direction of the axes X and Y, as the positional data. Using these data, the device 9 makes a data display device 10 display an image of the sample 2. A servo state detecting- deciding device 8 receives the data of the detector 5 as an input and controls operations of the devices 6 and 7.

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, it detects an optimum servo control state based on a detected value of a tunnel current so that an optimum scanning can be performed according to the unevenness of a sample surface. The present invention relates to a scanning tunneling microscope in which the speed is automatically set and the measurement time is shortened as a whole.

[0002]

2. Description of the Related Art The basic structure and operation of a scanning tunneling microscope are as follows. The conductive probe is brought close to the surface of the sample to be observed at an atomic level distance (on the order of nanometers) by using a moving mechanism including a coarse moving mechanism and a fine moving mechanism. The probe is stationary when a predetermined amount of tunnel current flows between the probe and the sample surface. Next, with the above-mentioned predetermined amount of tunnel current flowing,
The scanning device is operated to perform a two-dimensional planar scanning operation using the fine movement mechanism along the measurement surface. At this time,
The tunnel current flowing through the probe is controlled so as to be set to a predetermined constant amount. Since the value of the tunnel current corresponds to the distance between the probe and the sample surface, in order to keep the tunnel current constant, it is necessary to keep the distance between the sample surface and the probe constant. Therefore, the tunnel current detected from the probe is taken in, and when the tunnel current increases or decreases, it is necessary to adjust the distance between the probe and the sample surface so that the current value is maintained at the set constant value. For this purpose, a servo control mechanism is provided. When scanning the probe along the measurement surface of the sample, the probe
The position is controlled in the axial direction of the probe so as to maintain a constant distance from the sample surface. Therefore, the probe moves in a scanning manner so as to follow the irregularities on the sample surface. Therefore, in order to follow the sample surface, if the position data obtained by finely moving the probe in its axial direction is collected and stored in the signal processing system and data processing is performed, atomic level unevenness information on the sample surface can be obtained. .. If an image is created using this unevenness information, it is possible to create an image of the unevenness at the atomic level on the sample surface.

By the scanning movement of the probe on the sample surface, a plurality of points are discretely set on the sample surface at fixed intervals. Therefore, the probe linearly moves a fixed distance between the measurement points, stops at the measurement points, and approaches the sample surface. The scanning movement is performed while repeating such operations.
At each measurement point, the probe is controlled in the axial direction in order to keep the probe and the sample surface at a constant distance. At this time, a tunnel current flows through the probe, and servo control is applied based on the detected tunnel current. The operation of approaching or retracting the probe by this servo control is attempted at a predetermined number of times at each measurement location regardless of the unevenness of the measurement surface. Therefore, in such scanning movement of the probe, the stop time at each measurement point and the movement time between each measurement point were constant regardless of the unevenness of the sample surface. That is, the scanning time (measurement time) was constant regardless of the unevenness of the sample surface. The optimum constant scanning time for a specific sample has heretofore been determined by trial and error by repeating preliminary rough measurements.

[0004]

The scanning speed in the conventional scanning tunneling microscope is set by the operator. The operator had to make many trial and error preliminary measurements until he found the optimal speed. In particular, the optimum scanning speed depends on the unevenness of the surface of the sample, so it has to be performed every time the type of sample changes.
On the other hand, even on the surface of one sample, the uneven state is not uniform depending on the location of the measurement region, and at that time, a relatively low scanning speed must be selected corresponding to the location where the unevenness is remarkable. It was Because, if the scanning speed is set high, the servo control cannot follow effectively and cannot be performed effectively, contact or collision between the probe and the sample occurs, and both are damaged,
Alternatively, the distance between the probe and the sample becomes too large, which causes a problem that measurement data cannot be obtained. Therefore, if the scanning speed is set low,
Since the scanning speed as a whole was low regardless of the unevenness of the sample surface, a large amount of time was required for one measurement.

An object of the present invention is to configure the scanning speed of the probe on the sample surface to be automatically determined according to the unevenness of the sample surface, thereby facilitating the operation of the operator and speeding up the scanning movement. Shortens one measurement time by
It is an object of the present invention to provide a scanning tunneling microscope which is intended to prevent damage to a probe or the like and improve measurement accuracy.

[0006]

A scanning tunneling microscope according to the present invention comprises a probe, a first moving mechanism for minutely changing the distance between the probe and the sample, and the probe moving along the surface of the sample. A second moving mechanism, a voltage applying means for applying a voltage to generate a tunnel current between the probe and the sample, a tunnel current detecting means for detecting the tunnel current, and a measured tunnel current held at a constant value. In order to achieve this, servo control means for controlling the first moving mechanism to adjust the distance between the probe and the sample, scanning means for controlling the second moving mechanism to scan the probe surface on the sample surface, and Data processing means for recording and processing the sampled surface of the sample, with the scanning movement stopped at the measurement points set at regular intervals on the measurement surface of the sample, the tunneling current with servo control by the servo control means. Of value In addition, the number of repetitions of the operation of inputting the detected value of the tunnel current and the servo control at each measurement point performed by the servo control means is set to the The response shape is automatically determined according to the intensity, and is changed according to the intensity of the response shape. In the above configuration, as a more specific configuration, preferably, the tunnel current value detected by the tunnel current detection means is input, the servo control state is detected / determined from this tunnel current value, and the tunnel current value becomes a predetermined constant value. When this happens, the servo control by the servo control means is stopped and the scanning means is provided with a servo state detecting / determining means for starting the scanning movement. In the above-mentioned configuration, more preferably, the time required for the detection operation of the tunnel current value is short at a measurement site with small unevenness and long at a measurement site with a large unevenness, and the overall measurement time is shortened. ..

[0007]

In the present invention, while the probe scans and moves along the measurement surface of the sample, it stops at each preset measurement point,
In the stopped state, the operation such as approaching is performed on the measurement surface under servo control until a predetermined tunnel current value is detected. In a measurement operation such as an approach for detecting a predetermined tunnel current value based on servo control, a fine movement mechanism (first moving mechanism) in the Z-axis direction repeats an approach or retract operation at predetermined time intervals to set a current value. When it falls within the range, the measurement operation is ended assuming that the predetermined tunnel current value is reached, and the process moves to the next measurement point. Therefore, when the unevenness of the measurement location is small, the predetermined tunnel current value is reached quickly and the measurement time is shortened. When the unevenness of the measurement location is large, the measurement time becomes relatively long. However, compared to the conventional measurement method in which a constant measurement time is always set at each measurement point regardless of the unevenness of the surface, the overall measurement time is
It is shortened.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An optimum embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a block diagram showing a system configuration of a scanning tunneling microscope according to the present invention, FIG. 2 is a perspective view for explaining a moving state of a probe with respect to a sample, and FIG. 3 is executed by a servo state detecting / determining means. It is a flowchart which shows a process.

In FIG. 1, 1 is a probe and 2 is a sample. The probe 1 is arranged so that the tip of the probe 1 faces the measurement surface of the sample 2 at a substantially right angle. The probe 1 is made of a conductive member. In the illustrated example, the probe 1 is placed vertically and the sample 2 is placed horizontally. But,
This arrangement posture can be arbitrarily selected.

The probe 1 is attached to the fine movement mechanism 3. The fine movement mechanism 3 moves the probe 1 only in the vertical direction in the figure. For this reason, the fine movement mechanism 3 is called a vertical fine movement mechanism. The vertical fine movement mechanism 3 changes the distance between the probe 1 and the sample 2. Reference numeral 4 is a horizontal fine movement mechanism. The horizontal fine movement mechanism 4 is a fine movement mechanism for moving the probe 1 two-dimensionally, and is used when performing a scanning operation of the probe 1.
The vertical fine movement mechanism 3 and the horizontal fine movement mechanism 4 are integrated. The structure in which the two fine movement mechanisms are integrated is known as, for example, a tripod fine movement element. For convenience of explanation, illustration of a specific structure of the fine movement mechanism is omitted.

The tripod fine movement element will be briefly described.
This fine movement element includes piezoelectric elements in a direction perpendicular to the surface of the sample 2 (referred to as Z-axis direction) and in each direction of the X axis and the Y axis perpendicular to the direction and horizontal to the sample surface. Therefore, a total of three piezoelectric elements are provided. The distance between the surface of the sample 2 and the probe 1 is changed by the expansion / contraction operation of the Z-axis direction piezoelectric element. That is, the probe is moved toward and away from the sample (or moved forward and backward). Further, the measurement surface of the sample is scanned by the expansion and contraction operation of the piezoelectric element in each direction of the X axis and the Y axis.
The X-, Y-, and Z-axis directions are shown in FIG. Fine movement mechanism 3,
The piezoelectric element related to the displacement in each axial direction of No. 4 is 0.
It is a fine-motion piezoelectric element for displacing from 01 nm to several μm. The vertical fine movement mechanism 3 and the horizontal fine movement mechanism 4 described above are attached to the coarse movement mechanism 4 that enables movement of a larger movement amount. The coarse movement mechanism 4 can set the positional relationship between the probe 1 and the sample 2. The coarse movement mechanism will not be described in detail.

A tunnel current detector 5 is arranged between the probe 1 and the sample 2. The tunnel current detector 5 includes a power supply for passing a tunnel current inside. This power source applies a voltage of about 0.01 to several volts between the probe 1 and the sample 2. When the probe 1 approaches the surface of the sample 2 at an atomic level distance, a tunnel current flows. The tunnel current detector 5 detects the tunnel current, converts it into a required electric signal, and supplies it to the servo control device 6 in the next stage. The servo control device 6 gives a command signal for controlling the vertical fine movement of the probe 1 to the vertical fine movement mechanism 3. In the vertical direction fine movement mechanism 3 including the piezoelectric element in the Z-axis direction, the servo circuit included in the servo control device 6 controls the tunnel current so that the tunnel current always becomes a preset constant value (reference value).
Therefore, servo control is performed to keep the distance between the probe 1 and the measurement surface of the sample 2 constant. Thus, the servo control device 6
Causes the probe 1 to follow the unevenness of the sample surface. Further, the horizontal fine movement mechanism 4 including the piezoelectric elements in the respective directions of the X axis and the Y axis is controlled by the scanning device 7 so as to scan the region where the measurement is required.

In the above, the scanning operation executed by the scanning device 7 is predetermined by a program.
In the scanning operation, as indicated by the mark X in FIG.
The measurement points are discretely set at intervals of nm. The probe 1 is position-controlled so that the distance between the probe 1 and the measurement surface of the sample is constant (L) in a stopped state at each measurement location. In the position control in the Z-axis direction by the servo control device 6 at each measurement location, the approaching and retracting movements are repeated a plurality of times so that the distance between the probe and the measurement surface is controlled to a set constant value. It When moving between measurement points,
It moves at a predetermined speed without controlling the distance.

The servo control device 6 gives the control data used for position control of the probe 1 in the Z-axis direction to the data processing device 9 as position data. On the other hand, the scanning device 7 gives control data for scanning control of the probe 1 in each of the X-axis and Y-axis directions to the data processing device 9 as position data. The data processing device 9 causes the data display device 10 to display an image relating to the measurement surface of the sample 2 using the given position data of the probe 1 in the X-, Y-, and Z-axis directions.

In FIG. 1, reference numeral 8 shows a servo state detecting / determining device. The servo state detecting / determining device 8 inputs data corresponding to the tunnel current value detected by the tunnel current detector 5, and controls the operations of the servo control device 6 and the scanning device 7 based on this data. Specifically, the servo state detection / judgment device 8 makes a predetermined calculation and a judgment of the calculation result, stops the servo control by the servo control device 6 according to the result of the judgment, and further permits the scanning device 7 to perform scanning movement. Command. By such control, the servo state detection / judgment device 8 shortens the measurement time required for each measurement point of the probe 1 in accordance with the uneven state of the measurement point, accelerates the scanning speed as a whole, and shortens the measurement time. .. The control contents executed by the servo control device 6, the scanning device 7, and the servo state detection / judgment device 7 are shown in the flowchart of FIG.

The control executed by the servo state detecting / judging device 8 will be described with reference to the flowchart of FIG. In this flowchart, when the predetermined tunnel current is detected at each measurement point, the scanning movement of the probe 1 by the scanning device 7 is completed, and it is executed on condition that the probe 1 is stopped at a specific measurement point. This is because, if the scanning device 7 moves before the predetermined tunnel current is detected, the probe 1 and the sample 2 come into contact with each other, and both are damaged, or the accuracy of the measurement data decreases. Since the vertical direction fine movement mechanism 3 accurately follows the surface shape of the sample, it must be violently expanded and contracted during the measurement. However, in reality, since the piezoelectric element as the actuator has a time constant, the response of the expansion / contraction operation is limited. The tunnel current is a current that flows under a minute distance of 1 nm, and if the distance between the probe and the sample is large, the tunnel current cannot be detected. In this way, the servo control by the servo control device 6 is continued until the tunnel current reaches a predetermined constant value while the scanning movement by the scanning device 7 is stopped at each measurement point.

In the flowchart, in step 31, a command to suspend the scanning operation of the scanning device 7 is issued.

In steps 38 and 40, the tunnel current value detected by the tunnel current detector 5 (actually, the average value of N times of the tunnel current value I _t as described later) is
This is a step of determining whether or not the tunnel current value has fallen within the set tunnel current value range. In step 38, the upper limit of the range is compared. When the tunnel current value is larger than the upper limit value, it is too close to the sample surface, and therefore step 39 is executed to move the probe 1 away from the surface of the sample 2. Then, the process returns to step 32, and steps 32 to 37 for measuring the tunnel current value are executed. In step 40, the lower limit of the range is compared. When the tunnel current value is smaller than the lower limit value, it is too far from the sample surface, so step 41 is executed to bring the probe 1 closer to the surface of the sample 2. Then, the process returns to step 32, and steps 32 to 37 for measuring the tunnel current value are executed.

When the measured tunnel current value falls within the set current value range, the process proceeds to step 42 and outputs a command for permitting the scanning device 7 to move to the next measurement position.

Steps 32-4 in the flow chart
The process executed in No. 1 is the servo control device 6 at the measurement point.
This is a process for position control of the probe 1 in the Z-axis direction by means of the servo control until the tunnel current of a predetermined value flows in the probe 1. Steps 32 to 37 are a flow for calculating the tunnel current as an average value. Step 3
In 2, the variable A is set and set to 0. Next, the number N of times of addition setting is set. At step 33-36, and stores the added value to the variable A are added to input N times the tunneling current I _t from the tunnel current detector 5. In step 37, the average of the added values A of the input values N times is calculated, and the average value is set to the variable A.
Stored in. Then, as described above, this value A and the current value range that determines the predetermined tunnel current value are set in step 3
The comparison is made at 8 and 40 to determine whether or not a predetermined tunnel current value is reached.

The reason for processing as described above is that the tunnel current value usually contains noise, and therefore an erroneous determination may occur if it is determined by an instantaneous value. Therefore, the averaging is used.

By controlling the measurement operation by the probe 1 at each measurement point by detecting and determining the servo control state as described above, the measurement time at each measurement point is automatically determined according to the unevenness of the measurement surface. Therefore, the scanning speed of the scanning device 7 is increased as a whole, and the overall measurement time is shortened. FIG. 5 shows an image regarding reduction of the scanning speed. The degree of the scanning speed is conceptually expressed by the length of the arrow. On the measurement surface of the sample 2, the scanning speed is high in the flat area 2a with small unevenness, and the scanning speed is low in the area 2b with large unevenness. However, in the past, even in a flat area, the scanning speed was set in accordance with an area having large unevenness, so that the control according to the present invention can increase the scanning speed as a whole.

[0023]

According to the present invention, since the optimum scanning speed is automatically determined according to the unevenness of the measurement surface, the measurement time can be shortened and the operator can select the scanning speed by trial and error. You can save the trouble of doing. Furthermore,
It is possible to prevent damage to the probe and improve the measurement accuracy.

[Brief description of drawings]

FIG. 1 is a block diagram showing a main configuration of a scanning tunneling microscope according to the present invention.

FIG. 2 is a perspective view for explaining a moving relationship between a probe and a surface of a sample.

FIG. 3 is a flowchart for explaining a procedure of servo control and servo state detection / judgment control.

FIG. 4 is an explanatory diagram showing a scanning operation and respective measurement points.

FIG. 5 is an explanatory diagram showing the scanning speed according to the unevenness of the measurement surface.

[Explanation of symbols]

 1 probe 2 sample 3 vertical fine movement mechanism 4 horizontal fine movement mechanism 5 tunnel current detector 6 servo control device 7 scanning device 8 servo state detection / judgment device 9 data processing device

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Sakae Hasaki 882 Igemo, Katsuta City, Ibaraki Prefecture Inside the Naka Factory of Hitachi, Ltd.

Claims (3)

[Claims] 1. A probe, a first moving mechanism for minutely changing the distance between the probe and the sample, a second moving mechanism for moving the probe along the sample surface, the probe and the sample. A voltage applying means for applying a voltage to generate a tunnel current between the first and second moving mechanisms, a tunnel current detecting means for detecting the tunnel current, and a first moving mechanism for holding the detected tunnel current at a constant value. Servo control means for controlling and adjusting the distance between the probe and the sample, scanning means for controlling the second moving mechanism to scan the probe on the sample surface, and a sample obtained by the probe Record surface data
A scan that includes data processing means for processing, and in which the tunneling current value detection operation is repeated while servo control is performed by the servo control means while the scanning movement is stopped at measurement points set at regular intervals on the measurement surface of the sample. In a scanning tunnel microscope, the number of repetitions of the tunnel current detection value input and servo control operation at each measurement location performed by the servo control means is automatically determined according to the unevenness state at each measurement location. A scanning tunneling microscope characterized by the above. 2. The scanning tunneling microscope according to claim 1, wherein the tunnel current value detected by said tunnel current detecting means is input, and the servo control state is detected / determined from this tunnel current value, and the tunnel current value is said. A scanning tunneling microscope comprising: a servo state detecting / judging means for instructing the servo control means to stop servo control and for instructing the scanning means to start scanning movement when a predetermined value is reached. 3. The scanning tunneling microscope according to claim 1, wherein the time required for the operation of detecting the tunnel current value is short at a measurement site with small unevenness and long at a measurement site with a large unevenness, and the total measurement time is Scanning tunneling microscope characterized by shortening.