JPH03118403A - Probe of scanning tunneling microscope - Google Patents

Abstract

PURPOSE:To correctly measure the roughness of a sample by forming a conductive thin film at least at a part of an end of a probe to allow a tunnelling current to run between the probe and the surface of the sample. CONSTITUTION:A thin protrusion 2 is formed at an end of a base 1 of a probe. The protrusion is formed by using a scanning electron microscope and left for 10-20 minutes while the focus of the microscope is met. At this time, the beam size is made minimum and the magnification of the microscope is set to be 20-50K. Since the thin electron beam is continuously irradiated to one point at the end of the probe, the protrusion 2 is formed on the base 1. Thereafter, a conductive film (Au film) 3 is coated on the base 1 and protrusion 2 through sputtering. By using the probe with this structure, a tunnelling current flowing between a specimen and the end of the probe when a bias voltage is impressed to the probe can be detected through the conductive thin film. The surface of the specimen can be monitored by controlling the value of the tunnelling current.

Description

Detailed Description of the Invention [Purpose of the Invention (Industrial Application Field) This invention relates to the improvement of a probe for a scanning tunneling microscope, and in particular to a scanning tunneling microscope for observing the surface shape or surface roughness of a sample. Concerning the probe of a microscope.

(Prior art) As is well known, a scanning tunneling microscope (3 cannin
g Tunnel M tcroscope,
(hereinafter abbreviated as rsTMJ), the probe is placed on the sample surface once.
When the sample surface is scanned while approaching the tip to about 0 angstroms and applying a bias voltage, the sample surface can be observed on an atomic scale by utilizing the phenomenon in which a tunnel current flows, which changes depending on the distance between the tip and the sample surface. This microscope has the configuration shown in Figure 9. For example, while applying a bias voltage B to the sample 8 using a drive voltage supplied from the drive voltage source 70, the probe 10 at the tip of the piezo fine movement element (which can be moved finely in the X, Y, and Z directions) is brought close to the sample surface.
When the distance to the sample surface reaches a certain value (approximately 10 angstroms), a certain tunnel current T flows. Therefore, by scanning the sample surface with the probe while keeping the tunneling current value constant, the shape of the sample surface due to the probe scanning is analyzed by the computer 80 and can be observed on the image display 90. In recent years, this STM has been developed as a microscope that can observe atomic images, and because it has a resolution on the angstrom order, it is also attracting attention as a device for measuring minute three-dimensional shapes. .

In addition, on the surface of optical disks that have been developed as large-capacity memories in recent years, multiple bits with a depth of about 01 μm, a bit width of 0.4 μm, and a constant pit length are formed in response to information signals. forming a track.

Despite the fact that a slight change in the bit width has a large effect on the reproduced signal, there has been no means to accurately measure the surface shape or cross-sectional shape of the bit.

However, as a result of the development of the above-mentioned STM, expectations are high for measuring the surface shape, cross-sectional shape, etc. of optical disks because of its performance with resolution on the angstrom order. Conventional STM probes usually have a radius of curvature of 5
Although it is smaller than 0 nm, it is not necessarily small and the apex angle is wide, making it impossible to accurately detect the shape of the bit on the surface of the optical disk with a probe. For example, when a sample surface 4 having a bit 5 of depth h on the optical disk surface is scanned with a probe 10 from left to right in the figure as shown in FIG.
A moving locus 6 of 0 indicates the cross-sectional shape 7 of the bit observed at this time. However, the tip 1 is smaller than the hole diameter of the bit 5.
Since the outer diameter of the bit 0 is large, it cannot fully penetrate into the bit 5 on the sample surface 4, and the true cross-sectional shape 7 of the bit 5 cannot be measured.

Further, as the STM probe, an electrolytically polished needle made of tungsten (W) material is used, and the tip of the probe has a diameter of about 0.1 μm and an apex angle of about 20 degrees.

(Problem to be solved by the invention) However, the above-mentioned electrolytic polishing needle made of tungsten material
It is difficult to always create needles with the same shape. Moreover, in the atmosphere, tungsten material tends to oxidize and lose its tip shape. For this purpose, platinum (
Noble metals such as Pt (Pt) are sometimes incorporated into STM probe materials, but electrolytic polishing is difficult.

For this reason, although the tip is shaped by cutting, the shape of the tip becomes asymmetrical, making it difficult to create an STM probe with a sharp tip such as one made of tungsten material.

On the other hand, as a method for producing an STM needle with a sharp tip shape, there is a method of growing the probe by electron beam irradiation. This is because by irradiating a single point with an electron beam in a vacuum, contaminants such as carbon present in the vacuum chamber are concentrated at one point due to the effects of heat and charging, and over time they grow in the irradiated area. It was used.

According to this method, when using a sufficiently narrowed charged particle beam, such as an electron beam, the probe diameter is less than 0°1μ.
It is possible to make a cylindrical probe with a tip radius of curvature of 0.01 μm or less and a length of several μm.

However, with the STM probe created in this way, if the resistance of the base material itself is large, it becomes difficult to detect the tunneling current between the probe and the sample, making it difficult to observe the shape of the sample surface. It tends to be difficult to do accurately.

In view of the above circumstances, it is an object of the present invention to provide an STM probe that can eliminate the difficulties in producing an STM probe and accurately measure the roughness of a three-dimensional shape of a sample.

[Structure of the Invention] (Means for Solving the Problems) In order to solve the above problems, the basic structure of the STM probe of the present invention is as shown in FIGS. 6 to 8. At least a part of the tip of the probe and the sample surface 4
It is characterized in that a conductive WJ film 3 is formed so that a tunnel current flows through the film.

(Function) Since the STM probe of the present invention is configured as described above, when a bias voltage is applied to the probe, the tunnel current flowing between the test sample and the tip of the probe is caused by the conductivity. It can be detected through a thin film, and the sample surface can be observed by controlling the tunnel current value.

(Example) Next, an example of the STM probe according to the present invention will be described with reference to the drawings.

Figure 1 shows a scanning electron microscope (hereinafter abbreviated as rsEMJ) used to fabricate the probe of the scanning tunneling microscope of the present invention.
FIG. In the figure, 100 is a chamber, and an electron optical lens barrel 20 is installed above this chamber 100. A sample stage 11 that can move and tilt in the X and Y directions is arranged in the chamber 100, and a probe base material (needle body) 2 is attached to this sample stage 11. Electron optical lens barrel 2
0 is the electron gun 21. Condenser lens 22. It consists of a deflection coil 23, an objective lens 24, etc., and a lens barrel 2.
The electron beam 25 focused by 0 is irradiated onto the surface of the probe base material 1.

An electron detector 13 is installed in the chamber 100 to detect secondary electrons from the target.
The detection signal of 3 is supplied to the CRTl 5 via an amplifier 14. On the other hand, current is supplied to the deflection coil 23 from a scanning power source 18 via a resistor 17 for varying magnification. This current is also supplied to the deflection coil 16 of the CRT15. The secondary electron information obtained by the beam scanning is displayed on the CRT 15, and the display is imaged by the camera 19.

The basic configuration shown in FIG. 1 is the same as that of a well-known SEM, but in this device, gas is introduced into the side wall of the chamber 100, so a gas inlet 10a is provided. A raw material gas for producing a probe is introduced through the gas inlet 10a, and the gas inside the chamber 100 is exhausted through the exhaust port 10b.

Next, a method for manufacturing an STM probe using such an apparatus will be described. First, a mechanically polished or electropolished thin wire is used as the probe base material 1, and this is mounted on the sample stage 11 of the SEM at an angle of 45 degrees, for example, and the top of the tip of the probe base material 1 is searched for while observing with the SEM. The sample stage 11 is tilted to align the axis of the probe base material 1 and the electron beam, and the diameter of the electron beam 25 is appropriately narrowed down to focus and irradiate. As a result, a linear needle-like substance (EBD probe) 30 having a diameter substantially equivalent to the diameter of the electron beam 25 is formed at the tip of the probe base material 1, as shown in FIG. EBD probe 30
The shape of the probe base material 1 does not depend on the type of the probe base material 1 (for example, a conductive material or an insulator), but is determined by the diameter of the electron beam. The growth of the EBD probe 30 increases as the electron beam irradiation time increases, but the growth rate is not proportional to the irradiation time. For example, when comparing an EBD probe that is continuously irradiated for 15 minutes and an EBD probe that is irradiated at 5-minute intervals for a total of 15 minutes, the growth rate is higher with divided irradiation because thermal problems can be resolved.

In SEM observation after restraint scanning with STM using the EBD probe 30, no change in the shape of the END probe 30 is observed, which indicates that the adhesion between the EBD probe 30 and the probe base material 1 is high. has been confirmed. The components of the EBD probe 30 are normally (for example, room temperature, vacuum level of about 10' Torr)
In the case of , most of the content is carbon (C). Therefore, since it is conductive, it can be used as it is as an STM probe. Further, by providing a gas inlet 11a in the chamber 100 of the SEM and introducing a raw material gas, it becomes possible to make various EBD probes. Even when the probe base material or the EBD probe is made of an insulating material, it can be made conductive by vapor depositing a metal (for example, gold).

Moreover, in this case, adhesion can also be improved.

Here, when the electron beam 25 is focused on the surface of the probe base material 1, the charge density distribution near the probe base becomes as shown in FIG. In order to reduce the curvature of the tip of the EBD probe 30, it is sufficient to focus the electron beam on the tip of the EBD probe that is finally formed.

Furthermore, in order to create a thinner probe, the focus of the electron beam may be moved to match the tip of the EBD probe during growth. Note that instead of making the focus of the electron beam variable, a fourth
A sample holder as shown in the figure may also be used. This sample holder has a movable stage 43 provided on a base 41 via a piezoelectric element 42, and the sample stage 1 is mounted on the movable stage 43.
By driving the piezoelectric element 42 with the specimen 1 placed thereon, the specimen stage 11 can be moved up and down in minute steps.

On the other hand, in order for the EBD probe to function as an 87M probe, the tip of the EBD probe 30 must be the apex of the entire probe including the probe base material 1. for that purpose,
It is necessary to grow the EBD probe 30 at the true apex of the thin wire that will become the probe base material 1. However, when an ordinary mechanically polished probe is used as the probe base material 1, the tip has large undulations on a scale of 1 μm or less, and it is not necessarily easy to find the true apex by SEM observation. For this reason, it is preferable to make the tip of the thin wire serving as the probe base material 1 flat in terms of probe production.

In order to flatten the probe base material, it is possible to flatten it as shown in FIG. 5, for example. That is, a mechanically polished probe base material 1 with a sharp tip is attached to a probe holder 51, and a piezoelectric element (for fine movement) 52 and a micrometer head (for coarse movement) 52 are attached.
Lightly press the tip of the probe base material 1 against the opposing wall 54 using a tool or the like. The distance between the probe base material 1 and the opposing wall 54 is, for example, S
Detection is performed using a TM tunnel current detector 55, an SCaM capacitance detector 56, or the like. At this time, the surface of the opposing wall 54 is flat and covered with a material harder than the probe base material 1.

For example, when the probe base material 1 is made of platinum, a chromium vapor-deposited surface is used as the opposing wall 54. By using such a collision method, the tip of the probe base material 1 can be easily flattened.

FIG. 6 is a sectional view of essential parts showing the structure of an embodiment of the STM probe of the present invention. In Fig. 6, 1 is the probe base material;
Usually, a commercially available PT-1R cutting and polishing needle is used. The tip of this probe base material has a diameter of 0.5 to 1.0 μm. Therefore, the width of the optical disk surface is 0.4μ1. Depth 0.1μ■
It is not suitable for observing the uneven cross-sectional shape of the bit because the apex angle of the tip is too wide.

Therefore, a thin protrusion 2 is formed at the tip of the probe base material 1. Such a protrusion 2 is formed using a scanning electron microscope at the tip of the probe base material 1 with the beam size minimized.
Focus at 0-50x magnification and leave for 10-20 minutes. At this time, the finely focused electron beam continues to be irradiated onto one point at the tip of the probe, and as a result, the protrusion 2 grows on the probe base material 1. The size of the protrusion 2 varies depending on the size of the electron beam irradiated onto the probe base material 1 and the magnification, but can be adjusted to a diameter of 1001 mm. You can freely create up to a certain level. Moreover, the length can be controlled by the electron beam irradiation time, and can be grown into a columnar shape with a length of 1 to 2 μm in 10 minutes. This was taken out using a scanning electron microscope and covered with a conductive film (gold) over the entire probe base material 1 and protrusion 2.

An Au film) 3 is coated by sputtering.

FIG. 7 is a cross-sectional view of essential parts showing the structure of another embodiment of the STM probe of the present invention.

The STM probe shown in FIG. 7 is produced in the same manner as the STM probe shown in FIG. 6, except that only one side of the tip projection 2 of the probe base material 1 is coated with the conductive film III 3. 7th
In the STM probe shown in the figure, when the conductive thin film 3 (Au) is coated on only one side of the protrusion 2, the radius of curvature of the tip of the probe is less affected by the spread than when the entire protrusion 2 is coated. . Therefore, it is necessary to detect the tunnel current at the tip of the probe 10 through the conductive thin film 3.

The thickness of the conductive thin film 3 only needs to be conductive from the tip of the probe to the probe base 1, and a thickness of about 200 to 300 angstroms is usually sufficient.

FIG. 8 is an explanatory diagram, without showing the measurement method, when the surface of the optical disk (sample 4) is observed using the STM probe according to the present invention.

The tip of the protrusion 2 on the probe base material coated with conductive 11vi3 is formed to be sufficiently smaller than the bit 5 of the optical disk, so it is possible to scan inside the bit 5. The locus of the tip of the probe at this time can be observed as the cross-sectional shape line 6. This makes it possible to observe the true cross-sectional shape 7 of the bit almost accurately.

In the above embodiments, an example was explained in which a pt-Ir alloy was used as the probe base material 1, but in addition to this Pt-1r alloy, platinum (Pt), gold (Au), or tungsten (W) could be used.
, alloys of these metals, and non-ift diamond, plastic, etc. can also be used.

However, in this case, the tip of the probe must be sharp enough to allow observation of the surface shape, or the material must be able to grow thin protrusions without problems in adhering to the base material when irradiated with an electron beam. is necessary.

Furthermore, the conductive thin film as a coating material can be made of not only Al but also Pt and alloys thereof.

A conductive thin film of Cu or Fe may also be used. In addition to sputtering, film formation methods such as thermal evaporation, electron beam heating evaporation, and chemical vapor deposition are also possible. It can also be used.

[Effect of the invention] As is clear from the above explanation, the STM according to this invention
The probe has a conductive thin film formed on at least a portion of the tip of the probe so that a tunnel current flows between the probe and the surface of the sample to be tested, so there is no connection between the sample to be sampled and the tip of the probe. The tunnel current flowing through the conductive thin film can be detected through the conductive thin film. Further, by controlling the tunnel current value, it is possible to observe the sample surface using the scanning tunneling microscope 1.

[Brief explanation of drawings]

Figure 1 shows a schematic configuration diagram of the SEM used to manufacture the probe of the scanning microscope of the present invention, Figure 2 is a side view showing the state of manufacturing the probe, and Figure 3 shows the beam focus and electric v1 density. Fig. 4 is a schematic diagram showing the relationship with the distribution, Fig. 4 is a sectional view showing the schematic configuration of the sample holder, Fig. 5 is a schematic diagram showing the flattening device at the tip of the six buttons, and Fig. 6 is the STM probe of the present invention. FIG. 7 is a cross-sectional view of the main part showing the structure of one embodiment of the probe of the present invention, FIG. 8 is a cross-sectional view of the main part showing the structure of another embodiment of the STM probe of the present invention, and FIG. Fig. 9 is a schematic diagram of the STM configuration;
The figure is an explanatory diagram showing a mode of scanning measurement of a test sample surface using a conventional STM probe. 1... Probe base material 2... Protrusion on the probe base material 3... Conductive thin film 5... Pit hole on the surface of the optical disk 7... Bit cross-sectional shape 10... Probe (Tip base material and protrusion) 11... Sample stage

Claims (1)

[Claims] A probe for a scanning tunneling microscope, characterized in that a conductive thin film is formed on at least a portion of the tip of the probe so that a tunneling current flows between the probe and the surface of a sample to be examined.