JPH04320902A - Evaluating method for probe of scanning tunneling microscope - Google Patents

Abstract

PURPOSE:To obtain a true STM image by evaluating the shape of the fore end of a probe regularly. CONSTITUTION:A reference sample 62 for which 1-to-1 correspondence of an STM image to a probe 5 is attained beforehand is provided in the vicinity of a sample 61 to be measured, the reference sample 62 is measured before and after STM measurement of the sample 61 to be measured, and constitution is so made that the shape of the fore end of the probe 5 can be evaluated on the basis of two STM images thereof obtained before and after the above measurement. Since evaluation of the shape of the fore end of the probe 5 can be conducted, a correct STM image of the sample 61 to be measured can be obtained.

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 method for evaluating a probe.

0002

BACKGROUND OF THE INVENTION In recent years, scanning tunneling microscopes (STM) have been widely used as a device for observing the extreme surfaces of materials for the purpose of materials research. It is possible to observe everything from atomic images to three-dimensional measurements of surface irregularities, and can be observed in the atmosphere, vacuum, and liquid. Therefore, its range of applications is considered limitless. The principles of this scanning tunneling microscope are described in the IBM Journal of Research and Development (IBM Journal of Research and Development).
J. RES. DEVELOP. VOL. 30 NO.
4 P355-P369 JULY 1986). For information on how to approach the probe and sample and how to select the field of view, please refer to Applied Physics Letters, Vol. 40 (1982), pp. 178 to 180 (A
ppl. Phys. Lett. 40 (1982) p17
8 to p180).

0003

[Problems to be Solved by the Invention] The above-mentioned conventional technology uses constant servo control (Z-axis The probe is scanned two-dimensionally (X, Y-axis piezo) over the sample surface to obtain physical information about the sample. Generally, probes are made by electropolishing tungsten or platinum in a liquid to sharpen them. The tip polar diameter of the probe thus sharpened is approximately 0.1 μm or less. Since these probes are scanned two-dimensionally over the sample surface, they become contaminated with residual gas and sample contaminants. Furthermore, the tip of the probe often breaks due to poor servo control or the like. These changes in the tip shape of the probe create various STM images. The STM image obtained with a scanning tunneling microscope is thus highly dependent on the tip shape of the probe. Therefore, it is important to constantly observe the tip shape of the probe. However, the polar diameter of the tip of the probe is as small as 0.1 μm, and cannot be easily observed. Therefore, unless the shape of the tip of the probe is evaluated using some method, an incorrect STM image will be recognized. The techniques for evaluating the tip shape of these probes are not specifically described in the above-mentioned documents.

An object of the present invention is to provide a scanning tunneling microscope that can obtain true STM images by evaluating changes in the tip shape of a probe.

[0005]

[Means for Solving the Problems] In order to achieve the above object, a configuration is adopted in which a reference sample can be placed on the same plane as the sample to be measured. This reference sample has been measured in advance by some means, such as a high-resolution SEM or TEM.
In addition, a probe with a one-to-one correspondence between a normal probe shape and an STM image is used.

[0006]

[Operation] Before and after measuring the STM image of the sample to be measured, a reference sample is measured, and the shape of the tip of the probe is evaluated from the STM image of the reference sample. This allows the STM of the sample to be measured to be
You can evaluate whether the image is correct or incorrect.

[0007]

[Embodiment] An embodiment of the present invention will be explained below with reference to FIG.

As shown in FIG. 1, a scanning tunneling microscope generally has a Z-axis in the direction perpendicular to the sample surface, and an X-axis and a Y-axis in the horizontal direction to the sample surface, respectively. The gap between the sample to be measured 61 and the probe 5 is determined by first operating the Z-axis piezo for coarse movement by the Z-axis piezo control circuit 11 for coarse movement to bring the probe closer to the sample to the tunnel region. Here, by applying an arbitrary voltage between the sample and the probe, a tunnel current of several nA flows between the sample and the probe. Since this tunnel current is a function of the gap between the sample and the probe, if feedback control is performed on the fine movement Z-axis piezo 2 so that the tunnel current remains constant, the fine movement Z-axis
Information on the unevenness of the sample can be obtained from the displacement of the axial piezo. Therefore, by scanning the probe two-dimensionally using the X-axis piezo 3 and Y-axis piezo 4, three-dimensional information on the sample surface can be obtained. Based on the above principle, the tunnel current detection circuit 7 is a circuit that detects a minute tunnel current, and the servo control circuit 8 is a control circuit that includes P (proportional), I (integral), and D (differential) elements. This is a circuit that feeds back an arbitrary output signal to the fine movement Z-axis piezo control circuit 9 so that the tunnel current is constant. In response to the output signal, the fine movement Z-axis piezo control circuit expands and contracts the fine movement Z-axis piezo to keep the tunnel current constant. Therefore, the output signal of the servo control circuit corresponds to the displacement of the fine movement Z-axis piezo. This signal is digitized and sent to the E. W. Transfer to S12,
By displaying the information on the display, it becomes possible to observe the unevenness information on the sample surface. Here, X, Y scanning circuit 1
0 is a circuit for scanning the probe two-dimensionally. Furthermore, all these circuits are controlled by an engineering workstation EWS12.

In the configuration of the scanning tunneling microscope described above, the present invention includes a sample to be measured 61 and a reference sample 62 placed on a sample stage 6, as shown in FIG. In this example, a thin layer of gold or platinum deposited on HOPG was used as a reference sample. It is known that the sizes of these vapor deposited particles range from several nanometers to several tens of nanometers. Therefore, the deposited particles are first transferred to the ST of the normal probe.
M is measured, and the obtained STM image is used as a reference STM image. Next, the sample to be measured is measured by STM to obtain an STM image. Finally, obtain another STM image of the reference sample. This STM image is compared with the first STM image obtained, and if the size distribution of the deposited particles is similar and there is no change in the quality of the STM image, the STM image of the sample to be measured obtained between The image can be evaluated as correct. This means that there is no change in the tip shape of the probe during measurement. In this way, if this method is used every time the sample to be measured changes or every time the measurement position changes, it is possible to accurately evaluate the STM image. This is nothing but evaluating the tip shape of the probe. If there is a significant change in the two STM images of the reference sample, it is assumed that some change has occurred in the shape of the tip of the probe, and it is necessary to replace it with a new probe and perform the measurement again.

[0010] In the above explanation, graphite with thinly vapor-deposited gold or platinum was used as the reference sample, but the same result can be achieved even if a Si wafer with thinly vapor-deposited gold or platinum is used. Particles of various shapes can be obtained. Moreover, it is clear that without being particular about these vapor-deposited particles, it is also possible to use as a reference sample a material whose three-dimensional shape is already known by some method.

[0011] Through the above operations, the shape of the tip of the probe can be evaluated, so that the STM image can be correctly interpreted.

0012

According to the present invention, the shape of the probe tip of a scanning tunneling microscope can be evaluated, thereby providing a scanning tunneling microscope that can obtain accurate STM images.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram of an embodiment of the present invention.

[Explanation of symbols] 1...Z-axis piezo for coarse movement, 2...X-axis piezo for fine movement, 3...X
Axis piezo, 4... Y-axis piezo, 5... probe, 6... sample, 7...
Tunnel current detection circuit, 8... Servo control circuit, 9... Z-axis piezo control circuit for fine movement, 10... X, Y scanning circuit, 11...
Z-axis piezo control circuit for coarse movement, 12...EWS, 13...CP
U bus.

Claims (4)

[Claims] Claim 1: At least one of the probe and the sample is supported by a plurality of moving means, the relative positions of the probe and the sample are brought close to each other on the nanometer order, a tunnel current is generated, and the sample surface is moved with the probe. In a so-called scanning tunneling microscope that scans the top two-dimensionally to observe three-dimensional information of a sample, a scanning tunneling microscope is characterized in that it is configured so that a reference sample for probe evaluation can be installed near the measurement sample. Microscope tip evaluation method. 2. The scanning tunneling microscope according to claim 1, wherein the reference sample for tip evaluation is measured before and after the measurement of the measurement sample, and the probe is evaluated from the STM image thereof. A method for evaluating the tip of a scanning tunneling microscope. 3. The scanning tunneling microscope according to claim 1, wherein gold or platinum particles on graphite are used as the reference sample for the probe evaluation. Method. 4. A scanning tunneling microscope probe according to claim 1, wherein gold or platinum particles on a Si wafer are used as a reference sample for evaluating the probe. Evaluation method.