JPH05319984A - Production of metallic epitaxial film - Google Patents

Abstract

PURPOSE:To improve the inferiority in productivity in producing a metallic epitaxial film according to a molecular beam epitaxial method by using the (110) face of GaAs or the (000) face of sapphire as a substrate. CONSTITUTION:The metallic epitaxial film is obtained by forming a copper thin layer having the (111) face on the (111) face of singe crystal silicon as a substrate, especially the clean surface having the (111) 7X7 superstructure according to the molecular beam epitaxial method and then forming a single crystal layer of a metal having a face-centered cubic structure or a metal having a hexagonal closed-packed structure or an alternate laminated layer of the metals on the (111) face of the copper single crystal thin layer according to the molecular beam epitaxial method. This metallic epitaxial film has an epitaxial relation among the silicon substrate, the copper thin layer and the metallic layer and further a flat surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a metal epitaxial film using a molecular beam epitaxy method.

[0002]

2. Description of the Related Art A metal epitaxial film having a flat metal crystal thin film formed on the surface of another crystal substrate with a fixed orientation relation is used for a magnetic recording medium, a magnetic sensor, an X-ray mirror, a neutron diffraction grating, an elastic wave. It is expected as a functional metal thin film applied to devices and the like.

Such a metal epitaxial film can be manufactured by a molecular beam epitaxy method, but when the surface orientation of the metal epitaxial film is (111), a (110) surface of gallium arsenide or sapphire () is used as a substrate. The use of (0001) surface of α-Al ₂ O ₃ ) was recommended.

The molecular beam epitaxy method referred to here is
Under ultra-high vacuum defined by “residual gas pressure below 10 ⁻⁹ Torr”, the raw material is thermally evaporated to form a molecular beam, and this molecular beam is irradiated onto a single crystal substrate. This is a method of obtaining a single crystal film by depositing and growing a crystal having an orientation having a certain relationship with the crystal orientation of the surface of the substrate, which is one kind of highly controlled special vacuum deposition method.

[0005]

When gallium arsenide is used as a substrate when a metal epitaxial film is produced by applying a molecular beam epitaxy method, (110)
There is a problem that it is difficult to form a large-area substrate on the surface.

Further, when a gallium arsenide substrate is subjected to heat treatment for surface flattening and cleaning, arsenic is selectively vaporized, so that this treatment must be carried out under the vapor pressure of toxic arsenic. ..

Further, since gallium arsenide and the metal easily react with each other, when the metal layer is thin, there is a problem that the purity is deteriorated by mixing gallium or arsenic into the metal layer.

For these reasons, it has been difficult to safely produce a large-area and highly-heavy metal epitaxial film by the method using gallium arsenide as the substrate.

On the other hand, when sapphire is used as the substrate, many metal films on the (0001) surface of sapphire do not grow epitaxially but become polycrystalline. Therefore, although annealing is performed at a temperature of 300 ° C. to 700 ° C. for several tens of minutes after the film formation, even if the annealing is performed, few metals become single crystals.

In the case of a metal that does not become a single crystal even when annealed, without directly forming the metal film on sapphire,
Although a silver underlayer was once formed, this underlayer was annealed to form a single crystal, and the metal single-crystal film was epitaxially grown on it. Was not easy.

The present invention is intended to solve such a problem, and an object of the present invention is to provide a new method for producing a metal epitaxial film having good crystallinity.

[0012]

According to the present invention, single crystal silicon is used as a substrate by a molecular beam epitaxy method, and its (111) surface, in particular, a clean surface of a (111) 7 × 7 superlattice structure, (111) After forming a thin layer of copper having a surface, a single-crystal layer of face-centered cubic structure metal or hexagonal close-packed structure metal is formed on this copper single-crystal thin layer (111) surface by molecular beam epitaxy. Alternatively, it is characterized by forming alternating layers of single crystal layers of face-centered cubic structure metal and hexagonal close-packed structure metal, whereby the silicon substrate, the copper thin layer and the metal layer have an epitaxy relationship and are flat. A metal epitaxial film having a smooth surface is obtained.

In the practice of the present invention, the thin copper layer formed on the silicon substrate should preferably have a thickness of 30 angstroms or more, and the temperature of the silicon substrate at that time should be kept at 0 ° C to 100 ° C. Good to do.

[0014]

[Function] As described above, the molecular beam epitaxy method applied in the present invention is to form a molecular beam by thermally evaporating a raw material under ultrahigh vacuum with a residual gas pressure of 10 ^-9 Torr or less. This is a method of obtaining a single crystal film having an epitaxial relationship by irradiating and depositing it on a single crystal substrate. The (111) 7 × 7 superlattice structure is a surface-specific structure having a period seven times as large as the atomic arrangement of the (111) crystal plane in the bulk state of silicon. It has been rearranged.

Although silicon has a diamond structure,
On the (111) 7 × 7 superlattice structure washed surface of the single crystal,
When a thin copper layer is formed by molecular beam epitaxy,
It was found that a copper single crystal layer having an atomically flat (111) surface can be formed when the thickness of the copper layer is 30 Å or more.

In this way, (11
1) A copper single crystal having a surface is formed as a buffer layer, and then a metal film having a face-centered cubic structure or a metal film having a hexagonal close-packed structure is formed on the buffer layer by a molecular beam epitaxy method, or It was found that when two or more kinds of the metals are alternately laminated, a metal film having a flat surface with a silicon substrate, a copper buffer layer, and the metal having an epitaxy relationship can be formed.

The surface of the silicon single crystal substrate is (111) 7
A copper single crystal layer having a (111) surface is not formed except for a x7 superlattice structure. In this case, even if a face-centered cubic metal or a hexagonal close-packed metal is deposited on this, a polycrystalline It becomes a film and no epitaxy phenomenon appears.

Even if a silicon substrate having a (111) 7 × 7 superlattice structure surface is used, face-centered cubic metal or hexagonal metal other than copper is directly formed on the substrate without forming the copper buffer layer as described above. When the film of the closest packing metal is formed by the molecular beam epitaxy method, the metal film does not become a single crystal film but a polycrystal film having orientation. In contrast, when the copper buffer layer is provided, the metal has a (111) surface orientation when it has a face-centered cubic structure, and when the metal has a hexagonal close-packed structure (0001). ) A single crystal film having a surface orientation is formed.

To form a copper buffer layer on a silicon substrate, a molecular beam epitaxy method performed under ultrahigh vacuum is used. In methods other than the molecular beam epitaxy method, since the partial pressure of the active impurity gas is high, the gas molecules contaminate the substrate or mix in the formed film, so that the copper single crystal layer having a clean and (111) orientation is formed. Cannot be formed. The molecular beam epitaxy method does not have such a problem, and a copper single crystal layer having a (111) orientation can be formed.

The thickness of the copper buffer layer is preferably 30 Å or more. Below 30 Å, the copper layer forms an island structure on the silicon substrate, and the copper layer does not have a flat (111) surface, so the face-centered cubic metal or hexagonal close-packed metal formed on this also has a flat surface. Since it does not have a smooth surface, the crystallinity also decreases.

The type of single crystal metal formed on the copper buffer layer is preferably one having a face-centered cubic structure or a hexagonal close-packed structure. In the structures other than this, since the atomic arrangement on the (111) surface of copper, which is the buffer layer, does not match the three-fold symmetry, the rotation of the crystal orientation appears, and the metal film formed on the copper buffer layer becomes a single layer. It may become an oriented film instead of a crystal.

The temperature of the silicon substrate when forming the copper buffer layer is preferably in the range of 0 ° C to 100 ° C. If the substrate temperature is too low, surface diffusion slows down and the flatness of the copper buffer layer deteriorates, and an atomically flat surface cannot be obtained. Conversely, if the temperature is too high, copper reacts with silicon and the copper Does not grow epitaxially. Examples of the present invention will be described below based on tests conducted by the present inventors.

[0023]

[Example] As a metal layer formed on the copper buffer layer,
Gold, silver and platinum were selected for the face centered cubic (FCC) metal, and cobalt was selected for the hexagonal close packed (HCP) metal. The purity of raw metal is 99.9999% for copper and 99 for gold.
999%, silver 99.999%, platinum 99.99%, cobalt 99.9
It was 98%.

The metal film is formed at a base vacuum degree of 1 × 10 ^-11 T
It was carried out by a molecular beam epitaxy (MBE) device capable of obtaining an extremely high vacuum (XHV) of orr or less. In this MBE apparatus, a preparation chamber and a surface analysis chamber are connected to a film formation chamber, and a sample can freely move in each chamber without breaking an ultra high vacuum (UHV) condition.

As the molecular beam source, a Knudsen cell (K-cell) is used for copper, gold, and silver, and platinum and cobalt are used because of the temperature relationship that gives the equilibrium vapor pressure for obtaining the required molecular beam intensity. For the electron beam evaporation source (E-
Gun). In the case of using E-gun, in order to make the molecular beam intensity constant, feedback control was performed using electron impact emission spectroscopy (EIES). To monitor the film thickness, we used the most accurate quartz crystal film thickness meter commercially available today (resolution 0.01 Å / s).

As the substrate, a silicon single crystal plate having a (111) -oriented mechanochemical mirror-polished surface and a silicon single-crystal plate having a (100) -oriented mechanochemical mirror-polished surface were used for comparison.

The deposition rate for film formation is 0.05 to 0.2 ML (where ML is a monoatomic layer), and during the film formation, reflection high energy electron diffraction (RHEED) (25 to 30 Ke)
Simultaneous observation by V) and videotape recording were performed and used for later structural analysis. The produced metal film was subjected to surface chemical analysis under Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) under UHV conditions.

First, clean the surface of the substrate with (111) 7 ×
In order to obtain a 7 superlattice structure, a silicon single crystal plate having the (111) surface orientation described above was used for UHV of 3 × 10 ^-11 Torr.
In the preparatory chamber which reached the temperature, rapid heating at 1200 ° C. for 1 minute was continuously performed twice. Similarly, with respect to a silicon single crystal plate having a (100) surface orientation, rapid heating at 1200 ° C. for 1 minute was continuously performed twice to provide a comparative material. For comparison, a silicon single crystal plate having a (111) surface orientation was not heated, 700 ° C. × 30 minutes heat treatment, and 700 ° C.
The thing which heat-processed for 1 hour was also prepared.

RHEED observation of the substrate surface under UHV conditions after this substrate heat treatment revealed that when a silicon substrate with a (111) surface orientation was heated to 1200 ° C., it was an atomically flat 7 × 7 surface superlattice. A pattern having a structure appeared clearly. When a substrate having a (100) surface orientation was heated to 1200 ° C., a pattern showing an atomically flat 2 × 1 surface superlattice structure appeared. In contrast, when a silicon substrate having a (111) surface orientation was heated to 700 ° C. and not heated, a pattern showing a diamond structure appeared, but the pattern was unclear.

When these silicon substrates were transported to the surface analysis chamber without breaking the UHV and subjected to surface chemical analysis by AES and XPS, only silicon was found in the (111) 7 × 7 substrate and (110) 2 × 1 substrate. No impurities were detected. On the other hand, oxygen was detected in addition to silicon on the substrate heated to 700 ° C, and it was found that silicon dioxide was formed by XPS. Oxygen and carbon were detected in addition to silicon on the substrate not subjected to heat treatment, and two types of oxygen were found by XPS: one forming silicon dioxide and one chemically adsorbed. From the above, the Syringo substrate can have a clean surface only when it is heat-treated at a high temperature of 1200 ° C.

(111) 7 × 7 obtained by performing the above-mentioned processing
Copper was deposited on a silicon substrate having a surface under the above-mentioned conditions. The substrate temperature was set to 0 ° C, 50 ° C or 100 ° C.
At any substrate temperature, when the thickness of the copper layer was less than about 30 Å, the RHEED pattern of the copper layer was a diffuse spot pattern and the silicon pattern was partially overlapped. However, when the thickness of the copper layer was 30 angstroms or more, a clear streak pattern based only on the copper layer was formed, and when the substrate was rotated around the normal line of the substrate surface, six-fold symmetry was exhibited.

This means that when the thickness of the copper layer is less than about 30 Å, copper grows in an island structure on the (111) 7 × 7 silicon surface, and the electron beam penetrates the island portion to form a bulk film. This means that diffraction is occurring. On the other hand, when the thickness of the copper layer is 30 angstroms or more, it means that the copper has the FCC structure and the surface orientation is (111), and it is atomically flat and has only one atomic layer step. ..

Further, the crystal orientation of silicon and the crystal orientation of the copper layer had the epitaxy relationship shown in "Chemical formula 1".

[0034]

[Chemical 1]

Copper was deposited in the same manner as above except that the substrate temperature was changed. First, when copper was deposited at a substrate temperature of 200 ° C. on a silicon substrate having a (111) 7 × 7 surface, the RHEED pattern showed a halo pattern. This suggests that at this substrate temperature, copper reacts with silicon without growing epitaxially on silicon. When the substrate temperature is set to -50 ° C, copper RHEE
In the D pattern, the spot pattern, which is a contribution of bulk diffraction, continued to overlap with the streak, and did not show a perfect streak pattern. This indicates that, because the substrate temperature is low, copper atoms cannot be sufficiently diffused on the surface after deposition, and a flat surface cannot be formed.

Next, the substrate temperature is 0 ° C., 50 ° C. or 100
For a copper buffer layer with a thickness of 50 Å formed on a (111) 7 × 7 silicon substrate at ℃,
On the copper buffer layer, a metal layer of gold, silver, platinum, or cobalt having a thickness of several hundred angstroms was formed at a substrate temperature of 0 ° C. to 100 ° C., respectively. Both are RHEED
Upon observation, a clear streak pattern similar to the streak pattern of the copper buffer layer appeared. Moreover, when the substrate was rotated about the normal line of the substrate surface as an axis, six-fold symmetry was exhibited.

This means that all of these metals are epitaxially grown on the copper single crystal buffer layer, and the epitaxy relation is "Chemical formula 2", and the surface of these metals is one atomic layer. It shows that it is atomically flat with only steps.

[0038]

[Chemical 2]

It is known that cobalt has an HCP structure in the bulk but has an FCC structure on the copper layer [for example, FJ Lamelas et al., Phys. Rev. B 40 (1
989) 5837], which is consistent with the results of this example.

On the same copper buffer layer formed on the silicon (111) 7 × 7 surface, gold and cobalt,
When alternate lamination of any combination of silver and cobalt, platinum and cobalt, or silver and gold (the thickness of each metal layer is several atomic layers or + several atomic layers), similar RHEE was obtained.
The D pattern was shown, the epitaxy relationship was "Chemical formula 3", and the surface was atomically flat and consisted of only one atomic layer step.

[0041]

[Chemical 3]

It is known that in such a single combination of M ₁ = Au and M ₂ = Co, Co has an HCP structure and the relationship is "Chemical formula 4" [eg CHLee e
t al., Phys. Rev. Lett. 62 (1989) 653].

[0043]

[Chemical 4]

For comparison, a (100) 2 × 1 surface silicon substrate, a (111) surface substrate heated at 700 ° C., and a (111) surface substrate not subjected to heat treatment were used. A copper layer was deposited on top. In this case, RH
The EED pattern showed a Debye ring in the stack. This means that the copper layer did not grow epitaxially on these substrates and became a fine polycrystal with random orientation.

When gold, silver, platinum and cobalt are deposited on this copper layer, the random crystal orientation of the copper layer is taken over,
All of these metals became fine polycrystals having a random orientation.

Further, for comparison, a test was conducted in which gold, silver or platinum was directly deposited on this (111) 7 × 7 superlattice structure silicon surface without forming a copper buffer layer on this silicon substrate. The RHEED patterns during deposition all showed a halo pattern in the initial stage, and became a spot pattern when the thickness of the metal layer exceeded several tens of angstroms, but the pattern did not change even if the substrate was rotated around the surface normal. It was unchanged.

This means that the metal layer has a structure close to an amorphous structure in the early stage of deposition and a specific crystallographic orientation is arranged perpendicular to the surface in the latter stage, but the orientation in the direction parallel to the surface is random. It means that it has become so-called fiber structure.

The above results are summarized in Tables 1 and 2.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

As described above, according to the present invention, the silicon substrate, the copper buffer layer and the FCC metal or HCP metal layer have an epitaxy relationship, and a metal epitaxial film having a flat surface can be obtained.

Claims (6)

[Claims] 1. A substrate made of single crystal silicon by a molecular beam epitaxy method and having a (111) surface made of copper (111)
Forming a thin single crystal layer having a surface and then forming a single crystal layer of a metal having a face-centered cubic structure or a hexagonal close-packed structure on the copper single crystal by a molecular beam epitaxy method. A method for producing a metal epitaxial film in which a substrate, a thin copper layer, and the metal layer have an epitaxy relationship and a flat surface. 2. The method for producing a metal epitaxial film according to claim 1, wherein the (111) surface of the single crystal silicon on which the single crystal thin layer of copper is formed is a clean surface having a 7 × 7 superlattice structure. 3. The method for producing a metal epitaxial film according to claim 1, wherein the copper single crystal thin layer formed on the single crystal silicon substrate has a thickness of 30 angstroms or more. 4. The metal layer having a face-centered cubic structure or a hexagonal close-packed structure formed on a copper single crystal thin layer is a layer of one kind of element or an alternate lamination of two or more kinds of elements. 2. The method for manufacturing a metal epitaxial film according to 2 or 3. 5. The surface orientation of a metal formed on a copper single crystal thin layer is (11) when the metal has a face-centered cubic structure.
The method for producing a metal epitaxial film according to claim 1, 2, 3 or 4, wherein the orientation is 1) and, when the metal has a hexagonal closest packing structure, the orientation is (0001). 6. The method according to claim 1, 2, 3, 4 or 5, wherein the formation of the copper single crystal thin layer by the molecular beam epitaxy method is carried out while maintaining the temperature of the silicon substrate at 0 ° C to 100 ° C. Manufacturing method of metal epitaxial film.