JPH04215005A - Image processing method for scanning tunneling microscope - Google Patents

Abstract

PURPOSE:To obtain a scanning tunneling microscope image processing method enabling the accurate image representation of a sample having step differences on its surface. CONSTITUTION:At the time of scanning along the surface of a sample by a probe while controlling the distance between the sample and the probe to make a tunnel current flowing between the sample and the probe always constant so as to perform image processing to visualize the sample surface state on the basis of the obtained image data, the coordinates of at least three points out of the image data of a plane part formed of the same atom layer are inputted to correct the measured data of each coordinate with the plane formed by three points as a reference face.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is a scanning tunneling microscope that measures the state of a sample surface by measuring the tunneling current flowing between the surface of the sample to be observed and a probe facing the surface. The present invention relates to an image processing method.

0002

[Prior Art] A scanning tunneling microscope (hereinafter abbreviated as STM) applies a voltage between the sample and the probe, brings the probe close to the sample surface, and measures the tunneling current flowing between the two. This is used to measure the condition of the sample surface. The value of the tunneling current flowing between the two has a characteristic that it greatly depends on the distance between the two in an exponential manner, so this characteristic can be used to investigate very fine electronic states on the atomic size order within the sample plane. is measured and then image processed to make it visible.

This type of technology was developed in the 1960s by R. Yo
The principle invented by ung (Rev. Sic. Instr.
um 37 (1966) pp275), and its name is topografiner.
er). The components of this device are basically the same as those of today's STM, but differ in that electric current generated by electrolytic radiation is used as a means of measuring the surface shape of the sample. On the other hand, G. Binning and H. Rohere measured the surface shape of a sample by using a tunnel current as the measuring means. The contents are G. Binn
Phys. ing et al. Rev. Lett. 49(
1982) p57 and U. S Patent 43
43993. Below, such STM
will be explained with reference to the schematic configuration diagram shown in FIG.

In FIG. 4, a pedestal 3 is set up on a base 2 in a vacuum container 1, and a sample stage 4 is provided below the tip of the horizontal part of the pedestal 3. 5 is 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 in the vertical direction, are joined at one end of each so as to cross each other at right angles. The other ends of the piezoelectric element 7 are joined to frames 9 and 10 erected on the base 2, and the other end of the piezoelectric element 8 is joined to the base 2.

[0005] A probe 11 is supported on the intersection of each piezoelectric element 6 , 7 , 8 so as to face the sample 5 .
By driving the probe 11, the probe 11 is moved in two directions (raster scanning) perpendicular to the surface of the sample 5 in the horizontal plane (raster scanning), and by driving the piezoelectric element 8, the probe 11 is moved relative to the surface of the sample 5. It is moved in the Z direction, which is the vertical direction. That is, the probe 11 is moved in three dimensions of X, Y, and Z.

[0006] Each piezoelectric element 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 the tunnel current flowing therein is detected by the detection means 13. The control circuit 14 is
The drive of the piezoelectric element 8 is controlled by varying the voltage applied to the piezoelectric element 8 based on the detection result of the detection means 13 so that the value of the tunnel current flowing between the probe 11 and the sample 5 is constant.

In 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 image signal is a control signal applied to an actuator so that the tunnel current is always constant. There are two ways to do this. In the former case, the distance between the sample 5 and the probe 11 is very close, on the order of angstroms, so there is a high possibility that the sample 5 and the probe 11 will collide when scanning the probe 11 two-dimensionally, and the scanning range is extremely narrow. Limited to narrow cases. On the other hand, in the latter case to which the present invention is applied, the distance between the sample 5 and the probe 11 is always kept constant, so the probe 11 does not collide with the sample 5, and a wide range of images can be obtained. It is widely used in STM that aims to

[0008]

[Problem to be solved by the invention] The surface of the sample and the plane in which the actuator moves in two dimensions in the X and Y directions are not necessarily in a completely parallel state on the order of angstroms, but are often in a non-parallel state. Set. This means that, for example, when scanning the probe in the X direction, the distance between the sample and the probe differs between the starting point and the ending point, which has a very large effect on the tunneling current. Although STM attempts to observe changes in electron density on the sample surface as changes in tunneling current, the difference in the distance between the sample and the tip at the start and end points is due to a constant change unrelated to the signal component. A bias with a slope will be added. Therefore, in order to convert into an STM image, a means for subtracting this background is required in some way.

Examples of such methods include, for example, connecting the scanning start point and the vicinity of the end point with a straight line and subtracting the value of the straight line from the source signal, or expanding the bias component into a two-dimensional image to create a flat or multidimensional image. There is a way to treat it as a curved surface.

However, a problem arises when observing a single crystal surface with a resolution on the atomic order when the atomic layer on the surface has a terraced surface structure called a step. When the background is subtracted using the conventional method for a sample with a step difference like this, the sample surface and the background surface are not perfectly parallel, resulting in an unnatural effect where the contrast at the edge of the step is emphasized. There was a problem that an accurate image could not be obtained.

[0011] The present invention is intended to solve such conventional problems, and an object thereof is to provide an image processing method in STM that can accurately represent an image of a sample having steps on the sample surface. shall be.

0012

[Means for Solving the Problems] In order to achieve the above object, the present invention first creates a temporary image from which the background is subtracted using a relatively simple method, and then uses a mouse or the like while viewing this image. Using a coordinate input device, we selected three arbitrary points on a plane part made of the same atomic layer, avoiding step parts, calculated the plane passing through those three points as a reference plane, and subtracted the background from the measurement data. It is something.

[0013]

[Operation] STM is a method to express the tunnel current that flows when a voltage is applied to the sample and the probe and the sample and the probe are brought very close to each other as an uneven image.The measurement method is to measure the tunnel current that flows through the probe. The probe is scanned along the sample surface while controlling the distance between the sample and the probe using an actuator in the Z direction so that the distance is always constant. The scanning method is to move a certain distance from the starting point in the X direction and then return to the original position. Next, after moving a small distance in the Y direction, it similarly moves a certain distance in the X direction and returns to the starting point. By repeating such operations, data on the sample surface is obtained.

Generally, 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 them gradually differs between the starting and ending points of the probe. Therefore, in addition to the signal originally caused by the electronic state of the sample surface, a bias having a constant slope caused by the above-mentioned slope is applied to the Z-direction actuator. This bias needs to be subtracted as a background when representing an image.

In the present invention, a temporary STM image is created using a simple method similar to the conventional method, and then a portion of the same atomic layer is searched for while looking at this image, and the coordinates of at least three points are entered using a coordinate input device such as a mouse. Input , and find the plane formed by the same atomic layer. The obtained plane is a plane composed of the same atomic layer, and at the same time is a plane parallel to the sample surface. Therefore, by calculating this plane as a reference plane and subtracting it from all measurement data, it is possible to derive data similar to when the probe was scanned parallel to the sample surface. Through such operations, faithful STM images can be obtained even for samples with steps.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. First of all, Figure 3 shows silicon (1
11) An example of measurement data obtained by observing a normal surface using STM is shown. This shows the voltage change applied to the actuator in the Z direction when scanning in the X direction. This voltage is a signal voltage that is controlled so that the tunnel current always remains at a constant value. The small pulsating current shown in the figure indicates a change in the electronic state of the surface atoms, and the sudden rise in a stepwise manner in the middle is a so-called step where the atomic layers are shifted layer by layer. You can see two steps. The reason why the overall slope is upward to the right is because the sample and the probe are not parallel, so the voltage is applied as the distance gradually changes as the probe scans.

Next, an image processing method for obtaining a correct STM image based on such measurement data will be explained. First, as shown in Figure 2 (A), the background level is determined by connecting the measurement data at the start point and the end point with a straight line, and then, as shown in Figure 2 (B), the background level is determined from the measurement data using the same method as before. Subtract background level. Conventionally, after performing this operation on all scanning lines, the lowest level and highest level are found from the obtained data group, and the area between the lowest and highest level is divided into 256 equal parts, which has 256 gradations. It was converted to an image. However, with such conventional methods, images of the same atomic layer but with different heights are obtained.

Therefore, in this embodiment, after subtracting the background from the measurement data obtained for all the scanning lines as described above to obtain image data as shown in FIG. 3, while looking at this image data, Three points P1, P2, and P3 consisting of the same atomic layer portion are input using a mouse, which is a coordinate input device. At this time, it is better to select three input points as far apart as possible within the same atomic layer to obtain a reference plane with fewer errors.

Now, to make it easier to understand, we will explain one scanning line. In Fig. 1 (A), if the points specified with the mouse are x1 and x2, the straight line connecting these two points represents the background. The same thing can be seen with the slope. Next, when the slope portion representing this background is subtracted from the measured data, the obtained data exhibits a clear step-like shape, as shown in FIG. 1(B). Therefore, it can be seen that there are three measurement points in three-dimensional coordinates, and a plane passing through these three points corresponds to the background reference plane.

[0020] In this way, by creating a grayscale image in the same manner as described above based on the data from which the background has been subtracted, it is possible to observe the correct surface condition of the sample.

[0021]

As described above, the present invention can create a temporary image even in a sample where the sample surface is not composed of only the same atomic layers but has so-called steps where the atomic layers are shifted in a stepwise manner. Using a coordinate input device such as a mouse,
A correct image can be obtained by inputting the coordinates of three points on a plane made 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
The effects of the present invention are extremely significant because the quality of STM images and the reliability of data are improved without distorting the images.

[Brief explanation of the drawing]

[Figure 1] (A) is a graph showing measurement data of a scanning tunneling microscope in one embodiment of the present invention and a method for determining the slope of the background. (B) is a graph showing data after subtracting the background from the measurement data. graph

[Figure 2] (A) is a graph showing the measurement data in the previous step in Figure 1 and the method for determining the background slope (B) is Figure 1
A graph showing the data after subtracting the background from the measurement data in the previous process.

[Fig. 3] Graph showing image data obtained for all scanning lines in the same example.

[Figure 4] Schematic configuration diagram showing a conventional scanning tunneling microscope

[Explanation of symbols]

1 Vacuum container 2 Base 3 Mount 4 Sample stage 5 Sample 6,7,8 Piezoelectric element 11 Probe 12 Drive circuit 13 Detection means 14 Control circuit

Claims (1)

[Claims] Claim 1: The probe is scanned along the sample surface while controlling the distance between the sample and the probe so that the tunnel current flowing between the sample and the probe is always constant, and the obtained image data is also used. When performing image processing to visualize the state of the sample surface, input the coordinates of at least three points from the image data of a flat part made of the same atomic layer, and measure each coordinate using the plane formed by the three points as a reference plane. An image processing method for a scanning tunneling microscope characterized by correcting data.