JP2763745B2 - Tip-coated atom transfer method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a probe-coated atom transfer method. More specifically, the present invention
Probe-coated atom transfer particularly useful for creating quantum wells, quantum dots, quantum wires, etc., which are the core parts of new electronic devices using nanometer-scale functional materials such as single-electron memories and atomic switches, and quantum effects utilizing quantum effects. It is about the method.

[0002]

2. Description of the Related Art In recent years, in particular, an STM (scanning tunneling microscope) has been used as a method for producing a new material such as an electronic device, a single-electron memory, or an atomic switch using an entirely new operating principle utilizing the quantum effect. The method is getting attention. Conventionally, a method of arranging atomic scales on the atomic scale or nanometer scale on the surface using the STM probe has been described by using an X
Spraying e-atoms and then manipulating the tip to arrange them in the desired shape [Eigler and schwizer, NATURE, VOL. 344.
1990] and a method in which a high electric field is applied to the probe to cause the probe atoms to fly to the surface [Mamin, Guethner, Ruger, Phys. Rev. Letter,
VOL. 65, 1990], a method in which a probe is wetted with liquid metal Ga and Ga atoms are ejected to the surface [Hosoki, Hasegawa, Beckman, K
ohno, Workshop on Atomic order measurement and con
trol, April 26-27, 1993, ABSTRACT]. Each of these methods is a technology that enables the construction of electronic devices and / or various functional materials on the atomic or nanometer scale on a metal or semiconductor surface.

However, the above-mentioned conventional Eigler and
According to Schwizer's method, in order to manipulate atoms on the substrate, the probe must be more strongly bonded to the probe when approaching the probe, and more strongly to the substrate surface when the probe is separated. There is a problem that the combinations of atoms on the substrate, atoms on the substrate surface, and probe atoms that enable the above are extremely limited.
Mamin, Guethner, and Ruger's method of flying the probe atoms themselves, except in special cases such as Au, causes a defect in the structure of the probe after the atoms have popped out due to the electric field effect, resulting in the atom manipulation. Has a problem that it does not function properly as a probe for STM. Hosoki, Hase
The inventions of gawa, Beckman and Kohno have the problem that the types of metals that are stable as liquids during the fabrication of electronic devices are very limited.

[0004] The present invention has been made in view of the above-mentioned circumstances, and solves the drawbacks of the prior art and relates to a technique for producing a nanometer-scale device using a scanning tunneling microscope (STM). It is an object of the present invention to provide a method for stably supplying atoms serving as raw materials to the tip of an STM probe.

[0005]

SUMMARY OF THE INVENTION The present invention provides a scanning tunneling microscope (STM) .
In an atomic operation method for manipulating atoms using a fine probe to construct an atomic-scale electronic device or various functional materials on a semiconductor or metal surface, a material for these electronic devices or various functional materials is used for a STM fine probe. Evaporated
Coating, then atomizing the coated material with the tip of the microtip
After the step of evaporating the electric field with the fine probe as the positive electrode from the part
In addition, the raw material covered with the tip exposed by electric field evaporation
A voltage is applied to the fine probe from which the probe
Moving the atoms of the coated material to the exposed tip of the fine probe by the action of a current flowing through the fine probe.
Probe coating characterized that you allow the next field evaporation Te
Provide an atom transfer method. In other words, the field evaporation-coated
Loss of material atoms, exposure of the tip of the fine probe-exposed tip
Process of moving coated material atoms to
Provided is a method for moving a probe-coated atom. Further, according to the present invention, as the aspect, the probe current is obtained by emitting tunnel electrons from the tip of the probe,
In the combination of the coating material and the probe material, when the probe material is W, Mo or Si, the coating material is Pd,
At least one kind of Au, Ni, Ge or Al is used. Further, in the combination of the raw material and the probe material, a probe-coated atom transfer method using a substance that causes electromigration of the raw material on the probe is also used. provide.

[0006]

According to the present invention, since the probe current is large enough to move the atoms, the atoms of the probe coating material can approach the tip of the probe. In this case, as a method for obtaining a sufficiently large probe current, a tunnel electron current that applies a negative voltage to the probe and emits tunnel electrons into vacuum from near the tip of the probe can be used.

Here, the fact that the current flowing through the probe at the time of the emission of the tunnel current is large enough to cause electromigration is determined by using the tunneling electron current flowing through the field emission microscope to remove atoms on the probe. It will be described by showing that it can be moved. That is, first, as shown in FIG. 1, for example, a molybdenum probe having a radius of curvature れ た placed in a vacuum is placed on a flat positive electrode 10 cm away from a positive electrode.
When multiplied by the negative voltage of 000V, the intensity of the electric field on the probe surface, as shown in FIG. 2 as a function of distance (x) from the apex of the tip, 10 ⁷ V / cm in the probe tip near The size of the table. Under such a high voltage, the electrons in the probe protrude from the solid due to the tunnel effect and become a field emission current. It is also known that the Fowler-Nordheim formula for the field emission current density (j) holds well under the external voltage and the probe diameter as described below.

[0008]

(Equation 1)

Here, the unit of j is Å / cm ² , F is the intensity of the electric field given in the unit of V / cm, and Φ is the work function in the unit of eV. Calculating the magnitude of the field emission current density on the probe surface using the Fowler-Nordheim formula yields as shown in FIG. 3 as a function of the distance (x) from the tip of the probe. By integrating at the surface, the total field emission current is obtained. This is equal to the total current flowing through the probe, and Table 1 shows the value.

[0010]

[Table 1]

As is apparent from FIG. 3, the electric field emission current density decreases rapidly as the distance from the tip of the probe increases. Here, the four orders of magnitude smaller portions than the maximum field emission current density of vertices, longer, as contribution negligible for all field emission current amount of which tip the point position P _f
It is assumed that electric field emission occurs in a region closer to the vertex (A portion in FIG. 1). State of P _f and field emission is shown schematically in Figure 1.

Meanwhile, over the probe shaft from the P _f (B portion in FIG. 1), the current is to distinguish the current flows through the probe and probe current and the named field emission current. The tip current density (j _Tip ) is a value obtained by dividing the total current value by the cross-sectional area of each tip, and the change in the magnitude near the tip of the tip is shown in FIG. The dependence of the probe current density on the distance (x) over the entire probe is as shown in FIG. 4 and is expressed by the power x of the distance from the tip of the probe as in Equation 2.

[0013]

(Equation 2)

Here, the unit of j _Tip is Å / cm ² , and the unit of x is Å. As described above, unlike the field emission current density, the probe current density is characterized by a gradual change in a wide region of B. If the tip is coated with a second metal,
It is considered that the coated metal atoms electromigrate in the current direction or in the opposite direction as a result of the interaction with the probe current. In fact, the results of conducting an electromigration experiment with a current flowing through a bilayer film of gold deposited on a molybdenum thin film surface have shown that electromigration does occur [A. Blech and E. Kinsbron. Thin SolidFilms,
VOL. 25, 1975, pp. 327]. It has been found that the speed of movement (v) of atoms carried by this electromigration is directly proportional to the current density (j) flowing through the gold-molybdenum bilayer film [A. Blech and E. Kinsbron. Thin S
olidFilms, VOL. 25, 1975, pp. 327]. And when the temperature is 430 ° C,

[0015]

(Equation 3)

## EQU1 ## Here, the unit of v is Å / sec,
The unit of j is Å / cm ² . By substituting the previously estimated tip current density j _Tip into Equation 3, the moving speed of the gold atom attached on the tip is obtained. The moving speed v at the position of P _f is also shown in Table 1.
It is shown in Table 1 also shows the number of atoms selected by electromigration when one layer of gold is coated.
It is shown in As is clear from this, the radius of curvature 50
At 0 ° and an external voltage of 3000 V, P _f is at 840 ° and the tip-coated metal atoms are carried at a rate of 560 ° / h. The number of mobile atoms at that time is 1.1 × 10
It has a sufficiently large value taking the construction of the ^two and the electronic device.

Here, the change in the P _f position when the probe material is changed will be described. As shown in Table 1, while the work functions of molybdenum and tungsten differ only by 4.8%, P _f differs. Changes by as much as 48%. Therefore, the value of the work function of the probe material is used in the present invention is an important parameter in determining the end position P _f of the coated metal atom. In the present invention, it is possible to approach the further probe tip end position of the coated metal atoms (Figure 1P _f).

FIG. 5 shows tungsten as a probe material,
Curvature change of the probe position P _f in the case of using gold as the coating metal atomic radius r = 500Å, r = 100Å, r = 5
It is shown as a function of the external voltage for the three cases of 0 °. Table 2 shows various numerical values with respect to the total electric field emission current amount, the position Pf, the moving speed, and the number of moving atoms when one layer is covered, based on drawing this figure.

[0019]

[Table 2]

As can be seen from FIG. 5, by decreasing the external voltage, P _f approaches the tip, and as can be seen from Table 2, a probe with a smaller radius of curvature maintains a higher moving speed (v). Mom P _f it is found that the closer to the tip. As described above, the operation of the present invention has been described assuming that the probe is treated as a continuum and that the Fowler-Nordheim formula holds the same work function on any surface. One important finding is that a single atom at the tip of the probe can emit electric field. Vu Thien Binh who did this research
(Vu Thien Binh, ST Purcell, N. Garica and J.
Doglioni, Phys. Rev. Letters vol. 69, pp2527, 199
According to 2), it is clearly recognized that electric field emission is generated from a single atom located at the tip of the probe. In this case, the total electric field emission current is 3 × 10 ^-13 to 4 × 10 ^-9 Å Although the area of the probe tip pedestal on which a single atom is mounted is extremely small, the probe current density is sufficiently large. As shown in Table 2, the moving speed calculated from the expression 3 is at most about 5
The number of mobile atoms when one layer is formed at × 10 ^-1 Å / h is an extremely large value of 10 ³ per second. Here, the tip position (P _f ) of the coated metal atom may be said to be located at the tip of the probe. In such a probe in which electric field emission is generated from one atom of the tip, the present invention is applied. According to this, it is possible to move the coating metal atom to this position from the probe shaft.

Hereinafter, the present invention will be described in more detail with reference to Examples.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A 1 cm long single-layer extending in the <111> direction
Molybdenum is obtained by electropolishing crystalline molybdenum wire.
Created a probe. For electropolishing, use 0.5N KOH solution
Dip the 1 mm tip of this single crystal molybdenum wire
Applying a voltage of 5 to 15 V using a lithium wire as the positive electrode and applying a current
When it stops flowing,
Was. The obtained molybdenum probe is ultrasonically immersed in ethanol.
After washing, vacuum degree 10 ^-Tenin the torr vacuum chamber
I set it.

Without removing the probe from the vacuum chamber, after a heat treatment at about 1200 ° C. by an electron impact method, a high voltage of 8 Kv to 10 Kv was repeatedly applied using the probe as a positive electrode to perform field evaporation cleaning. The radius of curvature of the tip of the molybdenum probe was checked with a scanning electron microscope, and the
Then, the crystal structure of the tip of the molybdenum probe was examined with a field ion microscope. The tip is (11
1) It was confirmed that the surface was protruded and the molybdenum atom at the top was projected above the trimmer.

Thereafter, a gold vapor deposition source using a tungsten filament attached to a vacuum chamber is operated at a filament temperature of 1250 ° C. toward the probe for 2 minutes to convert gold atoms to a thickness theoretically estimated by gas molecule kinetics. 4 mm. According to the structure observation by a field ion microscope, the crystal structure of molybdenum at the tip was not recognized, so it was determined that gold had adhered to the tip.

Next, considering that the evaporation electric field of gold is 3.5 V / Å and the evaporation electric field of molybdenum is 4.6 V / Å,
The probe is used as a positive electrode with respect to a ground electrode 10 cm away so that the electric field strength at the tip of the probe corresponds to 3.55 V / ５５.
A voltage of KV was applied for one minute. When the tip structure was examined again by a field ion microscope, it was confirmed that the same molybdenum crystal structure as that before gold deposition was present, and that gold atoms were detached by field evaporation. Next, a voltage of 3 KV was applied for 30 seconds using the probe as a negative electrode. When the tip structure of the probe was examined using a field ion microscope, it was concluded that gold was covering the tip again because no crystal structure of molybdenum was observed.
Next, after applying the same 14 KV and confirming the electric field evaporation of gold, 1 KV was again applied for 30 seconds using the probe as a negative electrode. This time, observation with a field ion microscope showed no gold tip coating as at 3 KV. However, it was confirmed that the film was covered with gold when the voltage was increased to 2 KV and applied for 30 seconds.

From the above, it can be understood that gold can move the coated gold atom to the tip portion by applying a negative voltage to the probe, and the presence or absence of the movement depends on the magnitude of the applied voltage. confirmed.

[0027]

As described in detail above, according to the present invention, atoms of the coating material can be moved toward the tip of the STM probe. As the current for the movement of atoms, a tunnel current emitted from the tip of the probe can be used. When constructing a device using the STM, the probe can be used at any time without removing the probe from the vacuum chamber.
Material atoms for electronic devices or various functional materials can be supplied to the tip of the M probe.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an arrangement of a probe and a flat cathode.

FIG. 2 3000 molybdenum probe with a radius of curvature of 500 °
FIG. 4 is a relationship diagram showing a relationship between the strength of an electric field on the probe surface and the distance from the tip when a voltage of V is applied.

FIG. 3 is a relationship diagram showing a probe current density and a field emission current density as a function of a distance in an x-axis direction.

FIG. 4 is a relationship diagram showing the relationship between the movement amount of gold atoms and the probe current density when a molybdenum probe is coated with gold and a voltage of 3000 V is applied, as a function of the distance in the x-axis direction.

FIG. 5 is a relationship diagram showing a change in a tip position Pf with _{respect to} a voltage shown in FIG. 1 for three curvature radii.

Continuation of the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) H01J 37/30 H01J 37/28 G01N 37/00

Claims (4)

(57) [Claims] [Claim 1] with a fine probe of a scanning tunneling microscope operating the atom, the atomic operation method for constructing an electronic device or various functional materials atomic scale on the semiconductor or metal surface, these fine fine probe Vapor coating of materials for electronic devices or various functional materials , and then coating
The fine material tip from the tip of the fine probe is
After the step of evaporating the electric field, the tip
The fine probe where the exposed material atoms have disappeared
A voltage is applied as a pole to remove the remaining coated material atoms.
Probe covering atom is moved to the distal end of the fine <br/> probe was the exposed by the action of the current flowing through the fine probe in characterized <br/> Rukoto to allow the next field evaporation Moving method. 2. The method according to claim 1, wherein a current for atom transfer is obtained by emitting tunnel electrons from the tip of the probe. 3. The method according to claim 1, wherein the probe material is W, Mo or Si, and the coating material is at least one of Pd, Au, Ni, Ge and Al. 4. The method according to claim 1, wherein a coating material that causes electromigration on the probe is used.