JPH05164510A - Scanning type tunnel microscope - Google Patents

Abstract

PURPOSE:To obtain a scanning type tunnel microscope wherein the quickness and precision of a control of the position in the axial direction of a probe at each measuring spot are increased. CONSTITUTION:A tunnel current between a probe 1 and a sample is detected by a tunnel current detecting part 6, converted into a signal showing a distance between the probe 1 and the sample 2 by a current-distance convertion part 7 and inputted to a servo circuit 8. Scanning of the probe 1 in each direction of the axes X and Y on the surface of the sample 2 is executed by a scanning part 9. The scanning part 9 gives a control signal to a piezoelectric element 3 for the direction of the axis X and a piezoelectric element 4 for the direction of the axis Y. With the movement of the probe 1 by motions of expansion and contraction of piezoelectric elements 3, 4 and 5 in the directions of the axes X, Y and Z at the time of a scanning movement, the amount of expansion and contraction of each piezoelectric element is stored as space coordinates in a storage part 10. The positional data on the probe 1 stored in the storage part 10 are taken out properly by a processing part 11, subjected to an image processing as to the shape of indentation of an observed surface of the sample 2 and displayed as information on the indentation of the surface of the sample 2 in a monitor part 12.

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 having improved swiftness and accuracy of axial position control of a probe at each measurement point.

[0002]

2. Description of the Related Art In a scanning tunneling microscope, a probe approaches a region of a sample surface to be measured at an extremely small distance such that a tunnel current flows, and scans the region of the sample surface in a certain range. So that the position is controlled. In the position control of the probe during scanning, position control is performed to move the probe parallel to the measurement surface, and the probe axis for holding the probe at a constant distance from the uneven sample surface during scanning movement. Directional position control is performed. When the tip of the probe approaches the measurement surface of the sample set on the sample stage at an atomic level distance, a tunnel current flows. The value of this tunnel current depends on the distance between the probe and the sample surface. Therefore, by using servo control to control the position of the probe so that the value of the tunnel current becomes constant, it becomes possible to measure the uneven state of the sample surface.

With respect to the spatial position coordinates of the probe, the axial direction from the base end to the tip of the probe is the Z axis, and the two directions are included in a plane perpendicular to the Z axis and are in a positional relationship orthogonal to each other. Are defined as the X axis and the Y axis, respectively, and are determined by the X, Y, and Z coordinate values. Using such spatial coordinates, a bird's-eye view of the measured sample surface is displayed on the CRT monitor, and a brightness modulation image for the Z coordinate is displayed. The image obtained by the scanning tunneling microscope thus obtained reflects the information of the uneven shape of the sample surface, and the fine shape at the atomic level on the sample surface can be measured and analyzed by this image.

[0004]

In the above scanning tunneling microscope, when the measurement surface is scanned with the probe being close to the sample surface so that a tunnel current flows through the probe, the probe is The position control of the probe in the Z-axis direction is performed at a plurality of measurement points set at a predetermined interval above. The position control in the Z-axis direction at each measurement point is repeated a plurality of times until a predetermined tunnel current value is detected. The number of repetitions is set to a fixed number in advance. As a result, since the same number of times of position control in the Z-axis direction is repeated at each measurement location, the measurement time becomes long as a whole.

In view of the above problems, the object of the present invention is to quickly and accurately control the position of the probe in the Z-axis direction at each measurement point, thereby shortening the measurement time at each measurement point, and overall. An object of the present invention is to provide a scanning tunneling microscope that shortens the measurement time.

[0006]

A scanning tunneling microscope according to the present invention includes a probe arranged on a sample, a first moving mechanism for changing a distance between the probe and the sample, and the probe on the sample surface. A second moving mechanism for moving along, a voltage applying means for applying a voltage for generating a tunnel current between the probe and the sample, a tunnel current detecting means for detecting the tunnel current, and a detected tunnel current For controlling the first moving mechanism to adjust the distance between the probe and the sample in order to maintain a constant value, and a scanning means for controlling the second moving mechanism to scan the sample surface with the probe. It is equipped with a data processing unit that records and processes the sample surface data obtained by the probe, and stops at the measurement points set at regular intervals on the measurement surface of the sample. Repeat position control It is configured to repeatedly detect a large tunnel current value, and a storage means for storing data relating to the unevenness information of the sample surface measured at a plurality of measurement points, and measurement data from at least one measurement point from now on. It is configured to include a prediction unit that predicts the unevenness state at the measurement location to be measured. The position control for detecting the tunnel current at each measurement point is performed by making a prediction based on the unevenness information obtained by the measurement at the previous measurement point, so that the position control is simplified. In the above-mentioned configuration, preferably, the measurement data for prediction is the measurement data of the measurement point near the measurement point to be measured. In the above configuration, preferably, when there are two measurement data used for prediction, the average value of them is used as the measurement data. In the above configuration, preferably, the aforementioned plurality of measurement points used for prediction are measurement points for one line existing before the measurement point to be measured. This one
The area corresponding to the line is defined based on the scanning area of the measuring surface.

[0007]

According to the present invention, when the uneven surface shape of each of a plurality of set measurement points is measured while scanning the measurement surface of the sample with the probe, the measurement point to be measured is traced back to the previous measurement point. For example, it is configured to store the data of the unevenness shape for one line, predict the unevenness state of the measurement point to be measured using these measurement data, and predict it before performing the probe position control of the measurement. Move the probe to the desired position quickly. When performing position control at each measurement location by such operation, the initial position of position control is
Set near the true position near the surface to be measured,
As a result, in the subsequent position control, the uneven shape can be measured quickly and accurately with high precision.

[0008]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a configuration diagram schematically showing an entire system of a scanning tunnel microscope according to the present invention. In particular, a configuration of a portion around a probe of a scanning tunnel microscope and a device configuration for performing position control of the probe, And the configuration of the measurement data detection / processing unit obtained by the probe. In FIG. 1, reference numeral 1 denotes a probe, and the tip of the probe 1 is sharply pointed and faces the measurement surface of the sample 2. The probe 1 is attached to a cross-fixing portion of three rod-shaped fine-movement piezoelectric elements 3, 4 and 5 arranged at right angles to each other in a tripod head (not shown). When changing the position of the probe 1,
The piezoelectric element 3 is an actuator involved in movement in the X-axis direction, the piezoelectric element 4 is an actuator involved in movement in the Y-axis direction, and the piezoelectric element 5 is an actuator involved in movement in the Z-axis direction. The piezoelectric elements 3, 4, 5 are moving mechanisms for moving the probe 1 minutely during measurement. Further, the tripod head including the piezoelectric elements 3, 4, and 5 is attached to the moving mechanism for coarse movement. The coarse movement mechanism is formed of an inchworm, a stepping motor, or the like, and is well known. The illustration is omitted in this figure. The coarse movement mechanism is mainly used to bring the probe 1 closer to the sample surface, for example, to a position where a tunnel current is detected.

The coarse movement moving mechanism described above is driven to drive the probe 1
When approaches the surface of the sample 2 and the distance between the probe 1 and the sample 2 becomes a predetermined minute distance at the atomic level, a tunnel current flows between them. However, in this case, in order for the tunnel current to flow, it is a condition that a predetermined voltage is applied between the probe 1 and the sample 2. Although the voltage application means is not shown in FIG. 1, it is usually provided in the tunnel current detection unit 6. The probe 1 stops its movement at a position where a predetermined tunnel current is detected. In this case, the operation of the coarse movement moving mechanism described above is basically kept in the final stopped state. The generated tunnel current is detected by the tunnel current detector 6, and the detected tunnel current is converted by the current / distance converter 7 into a signal representing the distance between the probe 1 and the sample 2. The scanning movement for the shape measurement of the sample surface to be performed next keeps the distance between the surfaces of the probe 1 and the sample 2 constant. Therefore, during scanning movement, the servo circuit 8 at the next stage receives the distance signal output from the current / distance converter 7 so that the distance detected by the current / distance converter 7 is maintained at a constant value. In addition, it has a function of controlling the expansion / contraction operation of the piezoelectric element 5 in the Z-axis direction.

The X axis and Y of the probe 1 on the surface of the sample 2
Scanning in each direction of the axis is performed by the scanning unit 9. The scanning unit 9 gives a control signal to the piezoelectric element 3 for the X-axis direction and the piezoelectric element 4 for the Y-axis direction, and these piezoelectric elements 3, 4
The two-dimensional scanning is performed on the measurement surface of the sample 2 by the stretching operation of.

The voltage applied to the piezoelectric elements 3, 4, and 5 as the probe 1 moves due to the expansion and contraction of the piezoelectric elements 3, 4, and 5 in the X, Y, and Z axis directions during scanning movement. That is, the expansion / contraction amount of each piezoelectric element is stored in the storage unit 10 as spatial coordinates. The position data of the probe 1 stored in the storage unit 10 is appropriately taken out by the processing unit 11, and the processing unit 11 performs image processing on the uneven shape of the observation surface of the sample 2, and the monitor unit 12 performs the image processing. It is displayed as unevenness information on the surface of the. The storage unit 10 and the processing unit 11 described above constitute an arithmetic processing unit 13 for image processing. In the example shown,
Although the arithmetic processing unit 13 has been described as an operation control system of the probe 1, the arithmetic processing unit 13 is actually formed by a microcomputer.

In the above device configuration, the probe 1 and the sample 2
When a tunnel current flows between the probe 1 and the sample 2
The distance is usually about 1 nm at the atomic level, and it is necessary to control the operation of the piezoelectric element 5 so as to keep this distance constant in order to detect the uneven state of the sample surface. The tunnel current is sensitive to a change in the distance between the probe 1 and the sample 2, and thus a high resolution can be obtained as a microscope.

With the scanning tunneling microscope having the above-mentioned structure, it is possible to obtain information on the uneven shape of the measurement surface of the sample 2 by a series of operations. In order to obtain the information on the uneven shape, a plurality of measurement points are set in advance on the track on which the scanning is performed on the measurement surface, and the movement of the probe 1 for scanning is stopped at each of these measurement points, Needle 1
Servo control for maintaining a constant distance between the surface of the sample 2 and the surface of the sample 2 is repeated a plurality of times by the servo circuit 8. In the case of the conventional device, the number of repetitions was previously set to a fixed number. Therefore, in the conventional control method in which the position control is mechanically repeated a plurality of times, there is a tendency that the position control for the measurement at each measurement location requires time and the scanning time becomes long.

On the other hand, in this embodiment, a line data memory 14 is additionally provided for the servo circuit 8. This line data memory is provided inside the servo circuit 8 in the actual device configuration. In the line data memory 14, in the scanning operation for measuring the probe 1, the measurement data of, for example, one line worth of measurement data is stored retroactively from the measurement position. This "1 line" is an area defined corresponding to the measurement area. When the position control for measurement is performed at the measurement point to be measured, the data to be measured is predicted based on the data of the measurement point for one line stored in the line data memory 14. Then, after moving the probe 1 to the position indicated by the prediction data, position control is performed so that the distance between the probe 1 and the surface of the sample 2 is set to a predetermined constant distance. As a result, the measurement operation at each measurement point is accelerated, the measurement time is shortened, and the measurement time is shortened as a whole.

Next, the position control at each measurement point will be described with reference to FIGS. 1, 2 and 3 described above. FIG. 2 shows an example of the measurement surface of the sample 2 scanned by the probe 1. Reference numeral 15 is a rectangle representing an area of the measurement surface, and in this example, a rectangular area is assumed for simplification of description. Z _{i, j} in the scanning region 15 is the predicted value of the unevenness data in the Z-axis direction at the position to be measured existing in the i-th row and the j-th column, and Z _{i-1, j} is the i−1-th row and the j-th column. The measured value of the uneven shape at the measurement point, Z _{i, j−1,} is the measured value at the i-th row and the j−1-th column. As is clear from the figure, the rectangular scanning area 1
5, the X-axis direction is defined in the horizontal direction and the Y-axis direction is defined in the vertical direction.
The axial direction is defined. Therefore, "i" is the coordinate on the X axis and "j" is the coordinate on the Y axis. The range in the X-axis direction is from the origin O to NX-1, and the range in the Y-axis direction is from the origin O to NY-1.
Is. Where NX is the number of measurement points in the X-axis direction,
NY is the number of measurement points in the Y-axis direction.

Further, in the figure, a shaded area 16 indicates an area of unevenness data corresponding to one line stored in the line data memory 14, and the unevenness data of the measurement points included in this area is stored in the line data memory 14. It is stored. One line in the area 16 is one line in the X-axis direction.

In this embodiment _, the value of the position Z _{i, j in} the Z-axis direction where the first probe 1 is set at the measurement point where the uneven shape is to be measured is the value Z _i already obtained by the measurement. _{-1, j} , Z _{i, j-1} is used for prediction.

Next, with reference to FIG. 3, a flow of control processing of the probe 1 in the Z-axis direction will be described with respect to the operation of scanning and measuring the area 15 which is the measurement surface with the origin O as a starting point.
The symbols used in this flowchart will be described. i, j, Z _{i, j} are as described above, and are used as variables in the flowchart. M _i represents each i-th data in the line data memory 14, NX represents the number of data points in the X-axis direction, and NY represents the number of data points in the Y-axis direction.

In steps S1 and S2, initial values are set for variables i and j, and in step S3,
The piezoelectric elements 3 and 4 are operated on the basis of the scanning unit 9 to perform position control in the X and Y directions, corresponding to respective values of i and j (X,
The probe 1 is moved to the measurement position of Y). The contents executed in these steps S1, S2, and S3 mean that the measurement is started from the origin in FIG. 2, that is, the position of X = 0, Y = 0. In the subsequent measurements, scan from X = 0 to X = NX-1 in the X-axis direction,
Next, Y is sequentially incremented and Y = 0 to Y = NY-
Up to 1, the scanning operation of repeating scanning in the X-axis direction is performed. The actual X and Y coordinates are the values of i and j multiplied by the distance between the measurement points. Step S4 is a determination step, in which the state of the values of variables i and j is determined. Then, any one of steps S41 to S44 is executed according to the combination of the values of i and j.
By the combination of step S4 and steps S41 to S44, unevenness prediction is performed in position control in the Z-axis direction for measurement at each measurement location, and position data is given. However, in the case of i = 0 and j = 0, it is the first point where the measurement is started, and therefore, since there is no past measurement data, prediction cannot be performed. Then step S41
Then, the process for prediction is not performed. Other steps S4
In 2 to S44, the optimum data among the measurement data stored in the line data memory 12 according to each condition or the data obtained by averaging two data is used as the first position data for the position control of each measurement point. As Z _{i, j} .

First, in the case of i = 0 and j = 0, the prediction is impossible as described above. Therefore, step S45 is executed, and the coarse movement mechanism is used to search until a predetermined tunnel current is detected. The needle 1 is brought close to the sample surface. Next, in step S6, position control in the Z-axis direction is performed by the servo circuit 8 and the piezoelectric element 5 to obtain unevenness data, and Z _{i, j} is determined. The determined Z _{i, j} is the line data memory 1 in step S7.
4 is stored as M _i . Since j = 0 and i <NX, steps S8 and S9 are executed, i is set to 1, and the process returns to step S3.

In step S3, position control in each of the X and Y axis directions is performed, and the next measurement position (position of i = 1, j = 0)
A scan move to. Then, the following step 4 is executed. In this case, since i ≠ 0 and j = 0, step S42 is executed. In step S42, since the measurement data measured at the immediately preceding measurement location is stored in M _i-1 , this measurement data is used as the predicted position data Z _{i, j.}
Put in. In step S5, the position of the probe 1 is controlled by the servo circuit 8 using the data of Z _{i, j} to move it to the predicted position. When the probe 1 moves to the predicted position, step S
Z for measuring the uneven shape at the measurement point in 6
Axial position control is performed to determine Z _{i, j} . The determined measurement data Z _{i, j} is stored in M _i of the line data memory 14 in step S7. At this time, the line data memory 14 stores the measured data of M _i-1 and M _i . Next, i is set to 2 (step S8), and after step 9, the process returns to step S3. Then, until i becomes NX, the above steps S3, S4, S42, S5
S8 is repeated. When i becomes NX, the probe 1 jumps out of the scanning region with respect to the scanning movement in the X-axis direction.
Therefore, next, j is set to 1 in step S10, i is set to 0 again in step S2 after step 11, and the probe is moved to the position of i = 0 and j = 1 in the scanning region 15 (step S3).

In the next step S4, since it is determined that i = 0 and j = 1, step S43 is executed. In step S43, the measurement data M _i (i = 0) enters Z _{i, j} as predicted position data. This is i =
This means that the measurement data of the measurement position at the position i = 0, j = 0 directly below is used as the predicted position data of the measurement position at the position of 0, j = 1. Thereafter, similarly to the above, the probe 1 is quickly and accurately controlled in the Z-axis direction by the predicted position data Z _{i, j} and moved to the predicted position. Then, steps S6 to S9 are further performed. Step S
In 9, i becomes 1 again, so that the process proceeds to step S3. In step S6, the prediction data Z _{i, j} determined by the new measurement is stored in Mi of the line data memory 14. At this time, the old prediction data is discarded and new prediction data is stored. In the subsequent measurement, each data in the line data memory 14 is updated to new prediction data one after another.

Returning to step S3, the probe 1 moves to the next measurement position by scanning movement. This measurement point is i =
1, j = 1, i ≠ 0, j ≠ 0. Therefore, step 44 is executed. From the viewpoint of accumulating past prediction data in the line data memory 14, the situation at this measurement point is the same as the situation at the measurement point 17 shown in FIG. In such a case, the process proceeds to step S44, and the average value (M _i-1 + M _i ) / 2 of M _i-1 and M _i enters Z _{i, j} as the prediction data. After that, steps S5 to S7 are executed in the same manner as described above using this prediction data. Then, i is set to 2, and the process returns to step S3.

Thereafter, similarly to the above, until the final measurement point (i = NX-1, NY-1) of the scanning region 15 is reached,
A scanning operation between measurement points and a measurement operation by position control in the Z-axis direction at each measurement point are performed.

In summary, the scanning area 15 which is the measurement surface
Then, first, in the first line defined by j = 0 and i = 0 to NX-1, i is incremented, and the uneven shape is measured by stopping at each measurement position while scanning and moving. that time,
The line data memory 14 stores the measurement data obtained at the measurement point for the purpose of using it as prediction data of the measurement point immediately after that. Therefore, at each measurement location, except for the first measurement location, the measurement data at the immediately preceding measurement location is used as the prediction data. Next, j = 1, i = 0 to NX-
The second line defined by 1 is similarly scanned and measured. In this way, each line is scanned and measured while incrementing j. At each measurement point, as described above, the predicted data defined corresponding to the position is used to first perform the position control at the predicted position, and then perform the highly accurate Z-axis direction position control. Measure the uneven shape of the spot.

During the above process, at the end of the process of step S6, the measurement data Z _{i, j} is transmitted to the storage unit 10 and the unevenness information of the entire scanning region 15 is stored.

In the above-mentioned embodiment, the predicted data of the measuring point to be measured is set to at least one measuring point included in one line measured before that, and the predicted data is to be measured. The nearest measurement point is used. However, the measurement data used is not limited to this, and any measurement data stored in the line data memory 14 can be used.

As means for executing the above flow chart, the arithmetic processing unit 13 may be provided with its function, or another dedicated central processing unit may be provided. .. As shown in FIG.
When the means for executing the flowchart of FIG. 3 is expressed as a functional means as a device, a storage means (line data memory 14) for storing predetermined past measurement data and an initial stage of the next measurement point based on the predetermined past measurement data. It comprises a prediction means for predicting the position.

[0029]

According to the present invention, in the scanning tunneling microscope, the position of each measurement point is controlled in the Z-axis direction while the plurality of measurement points arranged discretely on the measurement surface of the sample are scanned and moved. When measuring uneven shapes by using the past measurement data as predictive data by setting the starting point of the Z-axis position control at each measurement location, the position control of the probe in the Z-axis direction is quick and accurate. Therefore, highly accurate measurement data of the uneven shape can be obtained without extending the measurement time in the scanning region.

[Brief description of drawings]

FIG. 1 is a configuration diagram schematically showing a main part of an entire system of a scanning tunneling microscope according to the present invention.

FIG. 2 is a plan view showing an example of a scanning region which is a measurement surface of a sample.

FIG. 3 is a flowchart showing a processing flow of position control in the Z-axis direction at each measurement point according to the present invention.

[Explanation of symbols]

 1 Probe 2 Sample 3, 4, 5 Piezoelectric Element 6 Tunnel Current Detection Section 7 Current / Distance Conversion Section 8 Servo Circuit 9 Scan Section 10 Storage Section 11 Processing Section 12 Monitor Section 13 Calculation Processing Section 14 Line Data Memory 15 Scanning Area

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

Claims (4)

[Claims] 1. A probe arranged to face a sample, a first moving mechanism for changing a distance between the probe and the sample, and a second moving mechanism for moving the probe along a sample surface. ,
A voltage applying means for applying a voltage for generating a tunnel current between the probe and the sample, a tunnel current detecting means for detecting the tunnel current, and for holding the detected tunnel current at a constant value. 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 on the sample surface; The probe is provided with data processing means for recording and processing the data of the sample surface obtained by the needle, and in a state of being stopped at the measurement points set at a certain interval on the measurement surface of the sample, by the servo control by the servo control means. In a scanning tunneling microscope for detecting the tunnel current value by repeatedly performing the position control of the above, the storage means for storing the data on the unevenness information of the sample surface measured at the plurality of measurement points, From the measurement data of Kutomo one measurement location, the scanning tunneling microscope, characterized in that it comprises a prediction means for predicting the irregularity at the measurement point to be now measured. 2. The scanning tunneling microscope according to claim 1, wherein the measurement data for prediction is measurement data of a measurement point close to a measurement point to be measured from now on. .. 3. The scanning tunneling microscope according to claim 2, wherein when the number of the measurement data is two, an average value thereof is used as the measurement data for prediction. 4. The scanning tunneling microscope according to claim 1, wherein the plurality of measurement points are measurement points included in one line existing before reaching a measurement point to be measured. Scanning tunneling microscope.