JP2008240040A - METHOD FOR PRODUCING SrRuO3 FILM, AND FILM OBTAINED THEREBY - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing an SrRuO<SB>3</SB>crystal film having low resistance equal to that of a bulk film by a sputtering process having excellent mass-productivity, and to provide a sputtered SrRuO<SB>3</SB>crystal film having low resistance equal to that of a bulk film. <P>SOLUTION: Regarding the method for producing an SrRuO<SB>3</SB>film, SrRuO<SB>3</SB>is deposited on a substrate in an oxygen-containing atmosphere under the pressure in the range of 8 to <300 Pa, preferably, in the range of 16 to 130 Pa by a sputtering process where a target and the substrate are confronted. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a method for producing a SrRuO film and a SrRuO ₃ film obtained, and particularly provides a method for forming a SrRuO ₃ film having a low resistivity equivalent to that of a bulk by sputtering.

  Substances having a perovskite structure have been reported to exhibit many functions such as dielectrics, magnetic substances, and conductors, and are a group of substances that can be expected to develop in the future.

Among the single crystals used in the basic research, regarding the electrodes, SrTiO ₃ doped with Nb or La is currently used as the conductive SrTiO ₃ . However, since the surface is made semiconductor by heat treatment, the usage is limited. Therefore, it is necessary to form a single crystal conductive layer on SrTiO ₃ .

SrRuO ₃ is a conductor having a perovskite structure, high thermal stability and chemical stability, and low resistivity (room temperature ρ = 280 μΩcm), so that it is a ferroelectric, magnetoresistive element, superconductor. It is expected as an electrode. SrRuO ₃ is an important electrode material for evaluating an epitaxial ferroelectric thin film. SrRuO ₃ is actually being used as an electrode of a ferroelectric memory used in an IC card such as Watermelon (trademark). Currently, SrRuO ₃ is the most promising candidate for electrodes of various functional materials having a perovskite structure.

As a film formation method of SrRuO ₃ , there are methods such as a pulse laser deposition method and a molecular beam epitaxy method in addition to the MOCVD method disclosed by the present inventors (Patent Document 1 and the like). The MOCVD method has a high reproducibility and is a promising film forming method, but has a problem that it is inferior in mass productivity including cost.

In consideration of industrial mass productivity, a sputtering method that is stable and easy to increase in area and reproducibility is desired. However, excluding the conventional sputtering methods with extremely slow deposition rates such as 90 ° off-axis sputtering method and high-pressure DC sputtering method, the SrRuO ₃ film deposited by the usual sputtering method is bulky. Compared to SrRuO ₃ , the resistivity was 5 times higher, the resistivity had a negative temperature dependence, and the unit cell volume of the crystal was large (Non-patent Document 1).
Japanese Patent Laying-Open No. 2005-079118 Jpn.J.Appl.Phys.Vol.41 (2002) pp5376-5380, Part.1, No.8, August 2002

Accordingly, the present invention is, intended to provide a low resistance of the sputtering SrRuO ₃ crystal film comparable to methods and bulk forming excellent sputtering mass production of SrRuO ₃ crystal film of a low resistivity comparable to bulk To do.

In order to achieve the above-mentioned object, the present inventor is a problem unique to plasma in order to obtain SrRuO ₃ with low resistance and high quality in the process of studying various sputtering methods and sputtering conditions. It is necessary to reduce the damage caused by the plasma, and to reduce the damage, it is effective to increase the deposition pressure to reduce the acceleration of the plasma particles. However, if the deposition pressure is too high, the deposition is performed. It has been found that the speed is lowered and the mass productivity is inferior, and it is confirmed that a low resistance SrRuO ₃ comparable to the bulk can be formed at a high film formation speed in the sputtering method by adopting a specific film formation pressure. Is completed.

Thus, the present invention provides the following.
(1) A method for producing a SrRuO ₃ film, characterized in that SrRuO ₃ is deposited on a substrate by sputtering with a target and a substrate facing each other under an oxygen-containing atmosphere and a pressure of 8 Pa or more and less than 300 Pa.
(2) The method for producing the SrRuO ₃ film according to (1) above, wherein the pressure is in a range of 13 Pa to 150 Pa.
(3) The method for producing the SrRuO ₃ film according to (1), wherein the pressure is 16 Pa to 130 Pa.
(4) Any one of the above (1) to (3), wherein the substrate is made of a material selected from a perovskite structure, a fluorite structure, a face-centered cubic structure, a simple cubic structure, a c-rare earth structure, and a NaCl structure. A method for producing the SrRuO ₃ film described in 1.
(5) The method for producing a SrRuO ₃ film according to any one of the above (1) to (4), wherein the deposition rate is 10 nm / hr or more.
(6) A SrRuO ₃ film obtained by the production method according to any one of (1) to (5) above.
(7) The SrRuO ₃ film according to (6), which has a perovskite structure having an in-plane lattice constant of 3.905 ± 0.2 ± and an out-of-plane lattice constant of 3.98 to 3.92３.

The film deposited by the sputtering method of the present invention is a perovskite SrRuO ₃ crystal. FIG. 1 shows the crystal structure of SrRuO ₃ (RJBouchard et al., Mater. Res. Bull. 7 (1972) 873). The room temperature resistivity of SrRuO ₃ is ρ = 280 μΩcm.

Conventionally, a good SrRuO ₃ film has not been obtained by a normal sputtering method excellent in mass productivity. The SrRuO ₃ film formed by the usual sputtering method has a temperature dependency (semiconductor property) that is more than five times higher than the bulk SrRuO ₃ and has a negative resistivity, and has a unit cell volume of crystal. (Patent document 1 etc.). However, according to the present invention, when the pressure of the oxygen-containing atmosphere is set to a relatively high pressure of 8 Pa to less than 300 Pa in a normal sputtering method in which the target and the substrate are opposed to each other, SrRuO _{3 is} not dependent on other conditions. It was found that a crystalline film having the same unit cell volume and physical properties (resistivity, etc.) as the bulk was obtained, and the deposition rate was high. While the present inventor examined various sputtering methods and conditions that are excellent in mass productivity, there is no change in other methods or conditions, but a specific pressure brings about a desired result in a normal sputtering method. I found it. Until now, it was not known that a SrRuO ₃ crystal film having the same unit cell volume and physical properties as the bulk could be deposited by a normal sputtering method at a high deposition rate, so this finding is surprising, and Compared with a manufacturing method such as MOCVD, the ordinary sputtering method is clearly superior in industry, so that the usefulness of the present invention is extremely high.

  The sputtering method of the present invention is a sputtering method performed by arranging a substrate and a target so as to face each other. Unlike the 90 ° off-axis method in which the substrate and the target are arranged at 90 °, this sputtering method can obtain a high deposition rate. FIG. 2 schematically shows an apparatus for performing the magnetron sputtering method used in the present invention. A substrate 2 and a target 3 are disposed facing each other in the vacuum container 1, and the substrate 2 is attached to a heater 4 and connected to a power source 5. The target 3 is also connected to the power source 6. The power source may be either a high frequency power source or a DC power source. The vacuum vessel 1 is evacuated by a vacuum pump 7 which is a combination of a turbo molecular pump and a rotary pump, while atmospheric gas is introduced from a cylinder 8 or 9 (for example, oxygen 8 or argon 9) through a flow meter 10 to form a vacuum. The container 1 has an oxygen-containing gas atmosphere.

Typically, SrRuO ₃ powder to be deposited, a sintered body, or the like can be used as the target, but a composite target that provides SrRuO ₃ to be deposited may be used. For example, a composite target of SrCO ₃ and RuO ₂ can be used.

The substrate on which SrRuO ₃ is deposited is not limited, but a crystal substrate having a lattice constant close to SrRuO ₃ to be deposited is preferable. The substrate need not have a perovskite structure, but preferably has a perovskite structure. Typically, LaAlO ₃ (perovskite structure), (001) SrTiO ₃ (perovskite structure), (001) _{[(LaAlO 3) 0.3 - (SrAl} 0.5 Ta 0.5 O 3) 0.7 ] (LSAT; perovskite structure), MgO (NaCl structure), Pt (face centered cubic structure), LaNiO ₃ (perovskite structure), and the like.

If the sputtering film forming gas does not contain oxygen or a substance that gives oxygen to an inert gas such as argon or neon in the atmosphere, crystal defects such as oxygen defects may be generated in the deposited SrRuO ₃ . The mixing ratio of the inert gas and the oxygen-imparting substance is not particularly limited, but is generally in the range of 1: 1 to 10: 1, preferably in the range of 5: 4 to 10: 1. If oxygen is insufficient or too much, the resistivity will increase, and if there is too much oxygen, it will be difficult to generate plasma or the deposition rate will be slow. There is a possibility that the crystal quality is not stable even if the oxygen-providing substance is insufficient or too much.

The atmospheric pressure in the sputtering of the present invention is 8 Pa or more and less than 300 Pa. If the atmospheric pressure is less than 8 Pa, the unit cell volume of SrRuO _{3 in} the resulting film is larger than the bulk and the resistivity is undesirably low, which is not preferable. When the atmospheric pressure is 300 Pa or more, the deposition rate of the SrRuO ₃ film is slow, which is not practical and the resistivity also increases. The atmospheric pressure is preferably within a range of 13 Pa to 150 Pa, more preferably within a range of 16 Pa to 130 Pa, further preferably 20 to 130 Pa, and most preferably around 27 Pa.

  In the present invention, since it has been confirmed that conditions other than atmospheric pressure have little influence, there is no particular limitation, and general film forming conditions can be employed. For example, the substrate temperature is preferably in the range of about 450 to 650 ° C., more preferably in the range of about 500 to 600 ° C. The power source may be either AC power or DC power.

  Although it is desired that the film formation rate is industrially higher, it is preferably 1 nm / hr or more, more preferably 5 nm / hr or more, and further preferably 10 nm / hr or more.

In the present invention, the effect of the film forming pressure was experimentally confirmed for SrRuO ₃ . Considering from the experimental results, in the conventional normal sputtering method, oxygen vacancies, interstitial oxygen, anti-site of Sr, Ru, etc. due to damage to the film by ionized oxygen in the plasma, high speed particles knocked out of the target, etc. It is thought that this was the cause of increasing the crystal unit cell volume and increasing the resistivity, and at the film forming pressure of the present invention, plasma particles collided with other particles to reduce the impact on the film. The crystal defects can be eliminated, the unit cell volume can be reduced, and the resistivity can be lowered, but if the pressure is too high, the film is formed for a very long time and atoms such as Ti diffuse from the substrate, It is considered that Ru and the like are volatilized from the film surface, thereby lowering the crystal quality and increasing the resistivity.

According to the present invention, in a SrRuO ₃ film formed by sputtering, the unit cell volume is 61 Å ³ or less (more preferably, room temperature 60.7 ^{３ 3} or less) and resistivity compared to bulk SrRuO _3. A film having only a difference of 400 μΩcm or less at room temperature (more preferably 300 μΩcm or less at room temperature) can be obtained.

Using the sputtering apparatus shown in FIG. 2, SrRuO ₃ was deposited on the (001) SrTiO ₃ substrate under the following film forming conditions.

Target: SrRuO ₃ powder compact Target size: Disk shape with 2 inch (5.12 cm) diameter Substrate: (001) SrTiO ₃
Substrate temperature: 550 ° C
Atmospheric gas: Ar / O ₂ = 20: 5 (supply flow ratio)
Input power: RF50W (high frequency power)
Distance between substrate and target: 120mm
Deposition pressure: 1.3 Pa, 8.0 Pa, 13 Pa, 27 Pa, 53 Pa, 130 Pa
Deposited film thickness: 60nm

The obtained SrRuO ₃ film was analyzed for crystal structure with an X-ray diffractometer (Philips X'pert-MRD), and the room temperature resistivity (at room temperature 25 ° C.) was measured by a four-probe method. The temperature dependence of resistivity was measured by the method (van der Pauw method).

FIG. 3 shows an X-ray diffraction chart of the obtained film by enlarging only the region where θ is 40 ° to 50 °. As the film pressure increases from 1.3 Pa to 13 Pa, the peak position of (002) SrRuO ₃ shifts to the higher angle side (in which the lattice constant decreases), but from (27) SrRuO to 27 Pa to 130 Pa. The peak position of ₃ is almost constant.

When the lattice constant of the unit cell of the SrRuO ₃ film is obtained, as shown in FIGS. 4 and 5, in all the films, the substrate in-plane direction (both two axes) is the same as that of SrTiO ₃ as the substrate 3.905９０ However, the lattice constant in the direction perpendicular to the substrate was large on the low-pressure side, decreased as the pressure increased from below about 8 Pa, decreased, reached a minimum at 27 Pa, and remained constant thereafter. Therefore, the unit cell volume is proportional to the out-of-plane lattice constant. As shown in FIG. 5, the unit cell volume calculated from this lattice constant has the same shape as the lattice constant in the direction perpendicular to the substrate, but the unit of the SrRuO ₃ film obtained at a pressure of 27 Pa or more. The cell volume (3.905 Å x 3.905 Å x 3.965 Å = 6.046 x 10 ^-2 nm ³ ) is the unit cell volume of bulk SrRuO ₃ (3.93 Å x 3.93 Å x 3.93 Å = 6.93 Å). 05 × 10 ⁻² nm ³ ) (slightly smaller).

FIG. 6 shows the resistivity of the SrRuO ₃ film at room temperature as a function of the deposition pressure. The film formation pressure decreased to 27 Pa as the unit cell volume increased as the unit cell volume increased. However, when the film formation pressure increased beyond 27 Pa, the resistivity was not constant as the unit cell volume and slightly increased with the pressure. Rose to.

Table 1 shows the lattice constant, unit cell volume, and room temperature resistivity of the SrRuO ₃ film. For comparison, Patent Document 1 and bulk values are also shown.

FIG. 7 shows the temperature dependence of the resistivity of the SrRuO ₃ film as a function of the deposition pressure. On the low pressure side, the resistivity showed a negative temperature dependence, that is, a semiconductor behavior, whereas on the high pressure side, the resistivity showed a positive temperature dependence, that is, a metallic behavior. It goes without saying that a metal-like behavior is desirable.

Furthermore, a change in resistivity indicating a phase transition to a ferromagnetic phase was observed at about 150 K for the SrRuO ₃ film obtained at a deposition pressure of 27 Pa. This was consistent with the features found in bulk SrRuO ₃ .

From these results, it was confirmed that SrRuO ₃ crystals having substantially the same unit cell volume (crystal structure) and physical properties as the bulk were obtained in the SrRuO ₃ film formed at a pressure of 27 Pa. It was confirmed that an equivalent crystal film was obtained.

FIG. 8 shows the deposition rate of the SrRuO ₃ film as a function of the deposition pressure. The film formation speed decreases as the film formation pressure increases. The film formation speed is about 10 nm / hr at a film formation pressure of 130 Pa.

  From these results and other results, the film forming pressure in the present invention is preferably 8 Pa or more and less than 300 Pa, more preferably 13 Pa to 150 Pa, further preferably 16 Pa to 130 Pa, still more preferably 20 to 130 Pa, and most around 27 Pa. It is considered preferable.

(I) an example in which the substrate temperature was changed to 500 ° C. and 600 ° C. under the same conditions as above, and (ii) an example in which the film thickness was changed to 35 nm and 65 nm, and (iii) RF20W power, RF40W, examples of changing the DC40W, (iv) 70mm substrate and the distance of the target, an example for changing to 100 mm, the ratio of 33% (v) Ar / (Ar + O 2), 45%, 90% With respect to the changed examples, film formation experiments were performed, and the obtained SrRuO ₃ film was evaluated in the same manner as described above, but no substantial change was observed in the results.

Further, with respect to the SrRuO ₃ film obtained under the same conditions as described above and the film forming pressure is 8 Pa, the heat treatment temperatures are 300 ° C., 500 ° C., 650 ° C., 750 ° C., and 850 ° C. for 10 minutes in the oxygen atmosphere and the air. Post annealing was performed. When the heat treatment temperatures were 750 ° C. and 850 ° C., the unit cell volume was slightly reduced, but it did not recover to the bulk value. Room temperature resistivity decreased slightly, but was at the 900 μΩcm level and remained higher than the bulk value (280 μΩcm). In the above embodiment, the distance between the target and the substrate is 120 mm, but is not limited to this value.

The sputtering method of the present invention can be used for depositing SrRuO ₃ crystal material that exhibits many useful functions with physical properties such as dielectrics, magnetics, and conductors, and has high conductivity and high thermal stability. The present invention provides a method for industrially forming an electrode material having high chemical stability and easily lattice-matched with a perovskite material as an electrode material, and its industrial usefulness is high.

The perovskite crystal structure of SrRuO ₃ is shown. The schematic diagram of a sputtering device is shown. ₃ is an X-ray diffraction chart in which only a region where SrRuO _{3 formed} in Example 1 has a θ of 40 ° to 50 ° is enlarged. The unit cell (lattice constant) of SrRuO _{3 formed} into a film in Example 1 is shown. 4 is a graph showing the dependency of the lattice constant and unit cell volume of SrRuO ₃ deposited in Example 1 on the deposition pressure. The resistivity at room temperature of SrRuO ₃ was deposited in Example 1 is a graph showing as a function of formation pressure. 4 is a graph showing the temperature dependence of the resistivity of SrRuO _{3 formed} in Example 1. 4 is a graph showing the deposition rate of SrRuO ₃ deposited in Example 1 as a function of deposition pressure.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Vacuum container 2 Board | substrate 3 Target 4 Heater 5, 6 Power supply 7 Vacuum pump 8,9 Cylinder (oxygen, argon)
10 Flow meter

Claims (7)

A method for producing a SrRuO ₃ film, characterized in that SrRuO ₃ is deposited on a substrate by sputtering with a target and a substrate facing each other under an oxygen-containing atmosphere and a pressure of 8 Pa or more and less than 300 Pa. Is within the pressure 13Pa~150Pa, SrRuO ₃ film manufacturing method according to claim 1. A pressure 16Pa~130Pa, SrRuO ₃ film manufacturing method according to claim 1. The SrRuO ₃ film according to any one of claims 1 to 3, wherein the substrate is made of a material selected from a perovskite structure, a fluorite structure, a face-centered cubic structure, a simple cubic structure, a c-rare earth structure, and a NaCl structure. The manufacturing method. Deposition rate is 10 nm / hr or more, SrRuO ₃ film production method according to any one of claims 1 to 4. SrRuO ₃ film obtained by the process described in any one of claims 1 to 5. The SrRuO ₃ film according to claim 6, which has a perovskite structure having an in-plane lattice constant of 3.905 ± 0.2Å and an out-of-plane lattice constant of 3.98 to 3.92Å.

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