JP3121619B2 - Image processing method for scanning tunneling microscope - 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 for measuring a state of a sample surface by measuring a tunnel current flowing between a surface of the sample to be observed and a probe facing the surface. In the image processing method.

[0002]

2. Description of the Related Art A scanning tunneling microscope (hereinafter abbreviated as STM) measures a tunnel current flowing between a sample and a probe by applying a voltage between the sample and the probe to approach the sample surface. Thus, the state of the sample surface is measured. Since the value of the tunnel current flowing between the two has a feature that depends exponentially greatly on the distance between the two, the very small electronic state of the atomic size order within the sample surface is exploited by using this feature. Is measured, and image processing is performed so that this can be visually recognized.

[0003] This type of technology was introduced in the 1960s by R. Young.
Is based on the principle (Rev. Sic. Instrum 37 (1966) pp275), whose name is topografiner.
iner). The components of this device are basically the same as those of today's STM, but differ in that a current by electrolytic radiation is used as a means for measuring the surface shape of a sample. On the other hand, G. Binning and H. Rohere measured the surface shape of the sample by using a tunnel current as the measuring means. The contents are Phys.Re by G. Binning et al.
v. Lett. 49 (1982) p57 and US Patent 4343993. Hereinafter, such an STM will be described with reference to the schematic configuration diagram shown in FIG.

In FIG. 4, a gantry 3 is erected on a base 2 in a vacuum vessel 1, and a sample table 4 is provided below a front end of a horizontal portion of the gantry 3. 5 are supported. The piezoelectric elements 6 and 7 which are actuators arranged at right angles to each other in the horizontal direction and the piezoelectric element 8 which is an actuator arranged at right angles to each other are joined at one ends thereof so as to intersect at right angles to each other. The other end of each of the piezoelectric elements 7 is joined to pedestals 9 and 10 erected on the base 2, and the other end of the piezoelectric element 8 is joined to the base 2.

A probe 11 is supported on the intersection of each of the piezoelectric elements 6, 7 and 8 so as to face the sample 5, and each of the piezoelectric elements 6, 7 and 8 is supported.
The probe 11 is moved (raster scan) in two directions of X and Y orthogonal to the surface of the sample 5 in a horizontal plane with respect to the surface of the sample 5, and the probe 11 is moved with respect to the surface of the sample 5 by driving the piezoelectric element 8. It is moved in the vertical Z direction. That is,
The probe 11 is moved in three dimensions of X, Y, and Z.

Each of the piezoelectric elements 6, 7, 8 is driven by applying a voltage from a drive circuit 12. Further, a voltage is also applied between the probe 11 and the sample 5, and a tunnel current flowing therethrough is detected by the detecting means 13. The control circuit 14
The driving of the piezoelectric element 8 is controlled by varying the voltage applied to the piezoelectric element 8 based on the detection result of the detecting means 13 so that the value of the tunnel current flowing between the probe 11 and the sample 5 becomes constant.

In the STM, the value of the tunnel current flowing between the sample 5 and the probe 11 is directly used as an image signal, and the control signal applied to the actuator so that the tunnel current is always constant is used as the image signal. There are two cases. In the former case, since the distance between the sample 5 and the probe 11 is very close to the angstrom order, there is a high possibility that the sample 5 and the probe 11 will collide when the probe 11 is two-dimensionally scanned, and the scanning range is extremely large. Limited to narrow cases. On the other hand, in the latter case to which the present invention is applied, the interval between the sample 5 and the probe 11 is always kept constant, so that the probe 11 does not hit the sample 5 and an image of a wide range can be obtained. It is widely used in STM.

[0008]

The plane on which the actuator that scans two-dimensionally in the X and Y directions with the sample surface moves is not necessarily completely parallel in the order of Angstroms, but rather is in a non-parallel state. Set. This means that, for example, when the probe is scanned in the X direction, the distance between the sample and the probe differs between the start point and the end point, which has a very large effect on the tunnel current. In the STM, despite the fact that the change in electron density on the sample surface is to be observed as a change in tunnel current, the difference in distance between the sample and the probe at the start point and the end point is a constant value independent of the signal component. A bias with a slope will be applied. Therefore, in order to convert the background into an STM image, means for subtracting the background by some method is required.

As such means, for example, a method of connecting the vicinity of the starting point and the end point of scanning with a straight line and subtracting the value of the straight line from the source signal, or enlarging the bias component into a two-dimensional image to obtain a plane or multidimensional image There is a way to treat it as a curved surface.

However, the problem here is that when observing the single crystal surface at a resolution of the atomic order, the atomic layer on the surface has a surface structure such as a stepped field called a step. If the background of such a sample with steps on the step is subtracted by the conventional method, the sample surface and the background surface are not completely parallel, and the contrast at the end of the step is unnatural. There is a problem that the image becomes an image and an accurate image cannot be obtained.

An object of the present invention is to solve such a conventional problem, and an object of the present invention is to provide an image processing method in an STM capable of accurately expressing an image of a sample having a stepped step on the surface of the sample. And

[0012]

According to the present invention, in order to achieve the above object, first, a temporary image is created by subtracting the background by a relatively simple method. Using a coordinate input device, three arbitrary portions of a plane portion made of the same atomic layer except for the step portion were selected, a plane passing through the three points was calculated as a reference plane, and the background was subtracted from the measured data. Things.

[0013]

The STM expresses a tunnel current flowing when a sample and a probe are brought extremely close to each other by applying a voltage to the sample and the probe in an image of unevenness. The probe is scanned along the surface of the sample while the distance between the sample and the probe is controlled by the actuator in the Z direction so that is always constant. The scanning method returns to the original position after moving a certain distance in the X direction from the starting point. Next, after moving by a small distance in the Y direction, it similarly moves by a certain distance in the X direction and returns to the starting point. By repeating such operations, data on the sample surface is obtained.

In general, the plane formed by the movement of the actuator in the XY directions and the sample plane are not parallel but slightly inclined, and the distance between the start point and the end point of the probe gradually differs. For this reason, in addition to a signal originally due to the electronic state of the sample surface, a bias having a constant inclination due to the above-described inclination is applied to the actuator in the Z direction. This bias needs to be subtracted as a background when expressing an image.

In the present invention, a tentative STM image is created by the same simple method as in the prior art, and a portion of the same atomic layer is searched for while viewing this image, and coordinates of at least three points are input using a coordinate input device such as a mouse. And obtain the plane formed by the same atomic layer. The obtained plane is not only a plane composed of the same atomic layer but also a plane parallel to the sample surface. Therefore, by calculating this plane as a reference plane and subtracting this from all measurement data, the same data as when the probe is scanned in parallel with the sample surface can be derived. By such an operation, a faithful STM image can be obtained even with a sample having steps.

[0016]

An embodiment of the present invention will be described below with reference to the drawings. First, the silicon (1) is shown in FIG.
11 shows an example of measurement data obtained by observing a normal surface of the surface by STM. This shows a change in voltage applied to the actuator in the Z direction when scanning in the X direction. This voltage is a signal voltage controlled so that the tunnel current always has a constant value. The small pulsating flow shown in the figure indicates a change in the electronic state of the surface atoms, and the sudden rise in the middle of the step is a so-called step in which the atomic layers are displaced from layer to layer. There are two steps. Since the sample and the probe have non-parallel slopes as a whole, the voltage is applied by gradually changing the distance as the scanning is performed.

Next, an image processing method for obtaining a correct STM image based on such measurement data will be described. First, as shown in FIG. 2 (A), the measured data of the starting point and the ending point are connected with a straight line to determine the background level, and then, as shown in FIG. Subtract background level. Conventionally, after performing such an operation for all the scanning lines, the lowest level and the highest level are obtained from the obtained data group, the interval between the lowest and the highest is divided into 256 equally, and 256 gradations are obtained. Had been converted to an image. However, in such a conventional method, images having different heights from the same atomic layer can be obtained.

In this embodiment, the background is subtracted from the measurement data obtained for all the scanning lines as described above to obtain image data as shown in FIG. Three points P1, P2, and P3 composed of the same atomic layer are input using a mouse as a coordinate input device. In this case, it is possible to obtain a reference plane with less error by selecting three points as far apart as possible within the same atomic layer.

Now, one scanning line will be described for easy understanding. In FIG. 1 (A), assuming that points designated by a mouse are x1 and x2, a straight line connecting these two points represents the background. You can see the same as the inclination. Next, when the inclined portion representing the background is subtracted from the measurement data, the obtained data shows a clear step shape as shown in FIG. 1 (B). Therefore, in the three-dimensional coordinates, there are three measurement points, and it can be seen that a plane passing through these three points corresponds to the reference plane of the background.

In this manner, a correct surface state of the sample can be observed by creating a grayscale image in the same manner as described above based on the data from which the background has been subtracted.

[0021]

As described above, according to the present invention, even in a sample in which the surface of a sample is not composed of only the same atomic layer and there are so-called steps in which the atomic layer is displaced in a stepwise manner, even a temporary image can be obtained. And using a mouse or the like which is a coordinate input device,
A correct image can be obtained by inputting the coordinates of three points on a plane composed of the same atomic layer, finding a reference plane passing through these three points, and correcting the background caused by the inclination of the sample plane. The STM image obtained in this way is
Since the quality of the STM image and the reliability of the data are improved without distorting the image, the effect of the present invention is extremely large.

[Brief description of the drawings]

FIG. 1A is a graph showing a method for determining measurement data of a scanning tunneling microscope and a background inclination in one embodiment of the present invention. FIG. 1B is a graph showing data obtained by subtracting a background from measurement data. Graph

2A is a graph showing a method of determining the inclination of the measurement data and the background in the step before FIG. 1; and FIG. 2B is a graph after the background is subtracted from the measurement data in the step before FIG. Graph showing data

FIG. 3 is a graph showing image data obtained for all scanning lines in the embodiment.

FIG. 4 is a schematic configuration diagram showing a conventional scanning tunnel microscope.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Vacuum container 2 Base 3 Mount 4 Sample stand 5 Sample 6, 7, 8 Piezoelectric element 11 Probe 12 Drive circuit 13 Detecting means 14 Control circuit

Continued on the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01N 13/10-13/24 G01B 7/34 G01B 21/30 H01J 37/28 G06F 15/62-15/62 360 JICST File (JOIS)

Claims (1)

(57) [Claims] 1. A probe is scanned along a surface of a sample while controlling a distance between the sample and the probe so that a tunnel current flowing between the sample and the probe is always constant, and a voltage change of the probe is measured. <br/> to perform image processing based on the obtained measurement data
To visualize the state of the sample surface as image data.
At least three points are selected from the image data of the plane portion composed of the same atomic layer except the step portion in the image data.
An image processing method for a scanning tunneling microscope, wherein the measurement data is corrected using a plane formed by the three points as a reference plane.