JPH0293348A - Apparatus for evaluating stm probe - Google Patents

Abstract

PURPOSE:To evaluate the electron state of an STM probe by measuring the energy distribution of the element ion subjected to electric field ionization from a needle-shaped sample. CONSTITUTION:A probe 1 is set to a probe holder 2 and a vacuum tank 7 is evacuated by a high vacuum pump 18 to introduce helium 9 into said tank 7. Next, when high voltage is applied to the probe 1, the electric field ionization of helium is started in the vicinity of the surface of the probe 1 and the image of the high electric field part on the surface of the probe 1 is projected on a screen 11. This image is observed from an observation window 13 to investi gate the presence of the atomic cluster 19 contributing to the tunnel current of STM. Subsequently, the azimuth of the probe 1 is changed by a manipulator and the image of the cluster 19 is allowed to coincide with a probe hole 14. At this time, the helium ion 19 subjected to electric field ionization passes through the hole 14 to be detected by an energy analyser 15. The energy spec trum of this ion is measured to know the local state density of the atomic cluster on the surface of the probe 1.

Description

[Detailed description of the invention] [Purpose of the invention] (Industrial application field) The present invention relates to STM, and particularly to a probe evaluation device.

(Prior Art) STM is a device that measures the undulations of a sample surface with high resolution based on changes in the tunneling current flowing between a metal probe and a sample when the two are brought close to each other within about 1 nm. The magnitude of the tunneling current depends on the electronic state (more precisely, the local state density) of the sample surface and the direction of the probe, and is theoretically expressed as the following equation.

is the local density of states of the sample surface and the tip, and EF is the Fermi city level.As is clear from the above equation, in order to obtain information only about the sample surface from IT, two lenses (r, E) are required. E
It is necessary to be constant for . In other words, STM
In addition to the small radius of curvature of the tip (which is necessary to obtain high resolution), the necessary conditions for the probe are that the local density ρ is constant. It has become. It is known that if ρ7 is not constant and has a large peak with respect to E, the image obtained by STM does not necessarily reflect the undulations of the surface.

Therefore, in order to evaluate the STM tip, it is necessary to
It is essential to evaluate the ding.

However, the region in which tunneling current actually flows in an STM probe is a few atoms at the tip of the probe, and it is not easy to measure nine atoms in this region). Electron spectroscopy, which has traditionally been used to measure the electronic states of bulk and surfaces of solids, measures the average electronic state within a macroscopic region, and only measures the local electronic states of clusters of several atoms. It is impossible to know.

One of the methods for evaluating a probe is FEM (field emission microscope), and it is also possible to know the electronic state of the probe from the intensity distribution of field emission current from the tip of the probe. However, F.E.M.
However, the resolution is insufficient to identify atoms on the probe surface, making it insufficient as a method for evaluating the nine STM probes.

(Problem to be solved by the invention) As mentioned above, in the conventional technology, due to lack of resolution,
It has been difficult to capture the probe surface atoms that contribute to the STM tunneling current and measure their electronic states.
An object of the present invention is to evaluate the electronic state of an STM probe by measuring nine STM probes with truly atomic-level spatial resolution.

[Structure of the invention]

(Means for Solving the Problems) The gist of the present invention is an FIM (i.e.
Nine atomic clusters on the probe surface are measured by imaging the atoms on the probe surface using a t-field ion microscope and simultaneously measuring the energy distribution of field-ionized ions. The present invention is comprised of an FIM section for imaging surface atoms, a probe hole for selecting atomic clusters, and an energy analyzer for measuring the energy of ions passing through the probe hole.

(Function) Field ionization is the core of the present invention.
-ization) will be explained first.

Field ionization is a phenomenon in which atoms spontaneously ionize under a high electric field. When a voltage is applied to a sharp metal needle like an STM probe, the electric field strength at the tip of the probe increases, causing electric field ionization of surrounding gas atoms near the tip.

At this time, the electrons of the gas atoms transfer to the probe through tunneling, and the gas atoms lose electrons and become ions. Which electronic state of the tip the gas atom electron tunnels into depends on the distance from the surface of the atom when tunneling occurs; the larger the distance, the more likely the electron state will be higher than the Fermi level. The electrons tunnel, while the generated ions are accelerated by the electric field and move away from the tip, but the energy of the ions far from the tip also depends on the location where ionization occurs. In this case, the ion energy decreases as the ionization position moves away from the surface. Therefore, there is a one-to-one correspondence between the energy of ions generated by electric field ionization and the electronic level of the probe above the Fermi level. In particular, when the density of states has a peak, the probability of electron tunneling to the peak state, that is, electric field ionization, increases, and the generated ions form a peak in the energy distribution. From this, by measuring the ion energy distribution, it is possible to determine whether the density of states of the probe is constant or has a peak at a certain energy.

The electric field ionization probability is extremely sensitive to the electric field strength, and electric field ionization occurs only at locations on the probe surface where the electric field is high. Therefore, when ions generated by electric field ionization are applied to a distant screen, a portion of the probe surface where the electric field is high can be enlarged and projected onto the screen. This is FI
This is the principle of M. Since the magnification of FIM reaches 10 to a times and the spatial resolution is 1 Å or less, it is possible to easily identify atoms on the surface. Particularly in the case of atomic clusters on the surface, which are important in STM probes, the electric field around the clusters is particularly enhanced and the probability of electric field ionization increases, so that they can be easily found on the FIM screen. An aperture (this is called a probe hole) that selects only ions from the cluster found in this way is provided, and when the energy distribution of the ions that have passed through this probe hole is measured, the local density of states of the cluster, that is, the STM probe It becomes possible to measure ρT necessary for evaluation.

(Example) It is screwed to the router 2 and further attached to the manipulator 3. This manipulator has the function of rotating the direction of the probe. The probe is connected to a high voltage power source 6 via a lead wire 4 and a high voltage feedthrough 5.

These are placed in a vacuum chamber 7, into which a rare gas 9 can be introduced through a variable leak valve 8. Also inside the vacuum chamber are a microchannel plate 10 and a screen 11, both of which are perpendicular to the axis of the probe.
Also, they are placed so that their centers coincide with the axis.

There is a 45' mirror 12 behind the screen, and the image on the screen 11 can be observed through a viewing window 13. Microchannel plate 10. Screens 11 and 45 @ Mirror 12 has a hole in the center (~5++a diameter) 1
4, which serves as the probe hole. Further behind the mirror, an energy analyzer 15 is placed, which is connected to an analyzer controller and a measurement system 16. The vacuum chamber is evacuated through a valve 17 with a high vacuum pump (turbo molecular pump) 18.

Next, the operation of the above device will be explained. First, the probe 1 is set in the probe holder 2, and the vacuum chamber 7 attached to the manipulator 3 is evacuated using the high vacuum pump 18 to reduce the degree of vacuum to 10-' Torr or less. The valve 17 is closed, the variable leak valve 8 is opened, and a rare gas (helium) 9 is introduced into the vacuum chamber at a pressure of about 5X10-' to 1X10-'Torr. Next, when a high voltage (3 to 20 kV) is applied to the probe 1, electric field ionization of helium begins near the surface of the probe, and according to the above-mentioned FIM principle, the image of the portion of the probe surface where the electric field is high is transferred to the screen 11. projected on top. This image is observed through the viewing window 13, and it is investigated whether there is an atomic cluster 19 that contributes to the STM tunneling current. If it exists, it will be imaged as a collection of bright bright spots, so next use the manipulator 3 to change the orientation of the probe 1 so that the image of the cluster 19 exactly matches the probe hole 14. do. Then, the helium ions 19 ionized by the electric field just above the atomic cluster pass through the probe hole 14 and are detected by the energy analyzer 15. By measuring the energy spectrum of this ion, the local density of states of the atomic cluster on the probe surface can be determined. J
Since the ions 20 that are field-ionized outside the M-son cluster are blocked by the probe hole 14 and are not detected, the measured density of states is truly unique to the atomic cluster 19. If the density of states of the probe obtained in this way is almost constant with respect to energy, such a probe will be S
It is optimal as a TM probe.

〔Effect of the invention〕

The above-described STM probe evaluation device can evaluate the STM probe including its electronic state.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the STM probe evaluation device. 1... Probe, 14... Probe Hall Representative Patent Attorney Noriyuki Chika Yudo Mitsuyuki Matsuyama

Claims (4)

[Claims] (1) An STM probe evaluation device characterized by evaluating the electronic state of a needle by measuring the energy distribution of field-ionized ions from a needle-shaped sample. (2) Claim 1, characterized in that a FIM (field ion microscope) is used to image the surface of the sample.
The STM probe evaluation device described. (3) The STM probe evaluation device according to claim 1, further comprising a probe hole for selecting field-ionized ions from a specific region of the tip surface of the needle. (4) The STM probe evaluation device according to claim 1, characterized in that the direction of the needle is variable in order to align a region of the tip surface of the needle that is important in STM with the probe hole.