JP2007287836A - Method for forming SBT thin film - Google Patents

Abstract

To enable crystallization of SBT at a lower temperature.
An amorphous SBT thin film 104 made of strontium (Sr), bismuth (Bi), tantalum (Ta), and oxygen (O) is formed by a platinum layer without heating the substrate by ECR sputtering. It is assumed that it is deposited on 103. Next, the amorphous SBT thin film 104 (substrate 101) is heated to about 650 ° C. in an atmosphere of oxygen gas 105. By this heat treatment, a state is obtained in which the SBT thin film 106 having the Aurillius structure oriented in the <115> direction is formed on the substrate 101.
[Selection] Figure 1

Description

  The present invention relates to a method for forming an SBT thin film comprising strontium, bismuth, titanium, and oxygen.

SBT, which is a layered bismuth compound, is used as a capacitor material for a ferroelectric memory because it has excellent characteristics with little fatigue due to polarization reversal. Up to now, for the production of SBT thin films, an organometallic compound decomposing method in which an organometallic compound containing each element is applied on a substrate and thermally decomposed at a high temperature to crystallize has been mainly used (non-contained) Patent Document 1).
In the organometallic compound decomposition method, first, an organometallic compound is coated on a substrate by a sol-gel method to form a coating film, which is dried. Next, the dried coating film is annealed in an oxygen atmosphere to convert it into an Aurivillus phase having a perovskite structure, and to show a state of ferroelectricity.

  However, as the memory cells become finer, the required level of film uniformity and the like increases, so that it is possible to obtain the film thickness uniformity necessary for uniformly forming a finer element over the entire area of the substrate. The organometallic compound decomposition method is considered to have limitations.

  As a manufacturing method other than the organometallic compound decomposition method, there is a method of forming a film on a substrate by sputtering a constituent element from a solid target. For example, there are a general sputtering method (see Non-Patent Document 2), a laser ablation method, and the like, which are also applied as a method for forming an SBT thin film. Recently, an organic metal compound vapor phase growth method (MOCVD method) has been widely studied as a method with excellent productivity.

  The SBT thin film deposited by these methods is in an amorphous state or is composed of crystal grains having a fluorite structure, and usually does not exhibit ferroelectricity. For this reason, the deposited thin film is heated to approximately 700 to 800 ° C. to be changed to the Aurivirius phase.

K. Amamma, T. Hase, and Y. Miyasaka, "Preparation and ferroelectric properties of SrBi2Ta2O9 thin filmes", Appl.Phys.Lett., Vol.66, No.2, pp.221-223, (1995). TKSong, J.-K.Lee, and HJJung, "Structural and ferroelectric properties of the c-axis oriented SrBi2Ta2O9 thin films deposited by tha radio-frequency magnetron sputtering", Appl.Phys.Lett., Vol.69, No .25, pp.3839-3841, (1996). C.D.Gutleben, "THE EVOLUTION OF SrBi2Ta2O9 FILMS FOR FERROELECTRIC MEMORYS", Mat.Res.Soc.Symp.Proc., Vol.433, pp.109-118, 1996. M. Takahashi, M. Noda, and M. Okuyama, "X-ray Photoelectron and UV Photoyield Spectroscopic Studies on Structural and Electronic Properties of SrxBiyTa2O9 Films", Mat.Res.Soc.Symp.Proc., Vol.748, pp. 311-316, 2003.

  As described above, in any of the methods, a heat treatment is performed to convert the Auuribilius layer having a perovskite structure exhibiting ferroelectricity. The temperature required for this heat treatment depends on various parameters such as the elemental composition of the thin film, the film formation history, the film thickness, the structure of the heating furnace, the heating rate during annealing, and the atmospheric gas. 750 ° C. is the lower limit temperature.

  There are a few research institutions that have obtained 650-670 ° C. as the lower limit of the crystallization temperature in the organometallic compound decomposition method by sufficiently optimizing all of the above conditions for the thin film and annealing. On the other hand, thin films formed by sputtering or laser ablation methods that do not use organometallic compounds are naturally free from contamination by organic compounds, so they should be better conditions than organometallic decomposition methods. It is said that crystallization will not occur unless the annealing temperature is at least 700 ° C. or higher.

Since high-temperature processing is required in this way, when an SBT thin film is used, for example, an aluminum wiring that is normally used in an integrated memory manufacturing process cannot be used. Pb (Zr, Ti) O ₃ (PZT), a ferroelectric material that has been put into practical use alongside SBT, is subject to polarization fatigue when repeated polarization inversion occurs in contrast to SBT that requires high-temperature treatment. Although there is a problem, it crystallizes at about 500 ° C. Thus, the fact that the temperature required for the heat treatment of SBT is 200 ° C. or more higher than that of PZT is the greatest obstacle to practical use of SBT as a material for integrated memories.

  The present invention has been made to solve the above problems, and an object thereof is to enable crystallization of SBT at a lower temperature.

  The method for forming an SBT thin film according to the present invention generates a plasma composed of an inert gas and an oxygen gas supplied at a predetermined composition ratio, and applies a negative vice to a target containing strontium, bismuth, and tantalum. By causing particles generated from the plasma to collide with the target to cause a sputtering phenomenon and depositing a material constituting the target on the substrate, an amorphous SBT thin film made of strontium, bismuth, tantalum, and oxygen is formed on the substrate. And at least a sputtering process for heating the SBT thin film in an oxygen gas atmosphere to form an auribilius structure, and the plasma is generated by electron cyclotron resonance and is moved by a divergent magnetic field. A co-electron cyclotron that is energized and supplied as a plasma stream in the direction of the substrate. The target is arranged in the middle of the flow of the plasma flow supplied in the direction of the substrate, and in the sputtering process, the atomic composition of oxygen atoms contained in the amorphous SBT thin film is changed to that of strontium in the SBT thin film. The oxygen gas is supplied so that the sum of the atomic composition and 1.5 times the atomic composition of bismuth is 5 or more. Therefore, in this method, the amorphous SBT thin film is heated to be crystallized into an auribilius structure.

  As described above, according to the present invention, since an amorphous SBT thin film is formed by ECR sputtering and then an auribilius structure is formed by heating, compared to the case of heating from a fluorite structure, auribilius is used. It is possible to lower the heating temperature for making the structure, and an excellent effect is obtained that SBT can be crystallized at a lower temperature than conventional.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1A to 1D are process diagrams showing an example of a method for forming an SBT thin film according to an embodiment of the present invention. Below, the case where SBT foil is formed on the platinum layer which is the material of the electrode used when using an SBT thin film for a capacitor is demonstrated. First, as shown in FIG. 1A, a substrate 101 made of single crystal silicon whose main surface is a (001) plane is prepared, and the surface of the substrate 101 is thermally oxidized to form a silicon oxide layer 102. And In addition, platinum is deposited by ECR (Electron Cyclotron Resonance) sputtering to form a platinum layer 103 having a thickness of about 200 nm on the silicon oxide layer 102. Note that ruthenium can be used as the electrode material, and a ruthenium layer having a thickness of about 20 nm may be formed instead of the platinum layer.

  Next, as shown in FIG. 1B, an amorphous material composed of strontium (Sr), bismuth (Bi), tantalum (Ta), and oxygen (O) without heating the substrate by ECR sputtering. Assume that the SBT thin film 104 in a state is deposited on the platinum layer 103. For example, the amorphous SBT thin film 104 is formed to a thickness of about 200 nm.

The formation of the amorphous SBT thin film 104 will be described in more detail. First, 8 sccm of xenon (Xe) gas and 10 sccm of oxygen gas are supplied to the plasma generation chamber as inert gases, and the total pressure is 6.65. × 10 ⁻² Pa. Here, a 2.45 GHz microwave (about 500 W) and a 0.0875 T magnetic field are supplied to establish an electron cyclotron resonance condition, whereby an ECR plasma is generated in the plasma generation chamber. The partial pressure of oxygen gas at this time is 1.3 × 10 ⁻² Pa. Sccm is a unit of flow rate, and indicates that a fluid at 0 ° C. and 1 atm flows 1 cm ³ per minute. T (Tesla) is a unit of magnetic flux density, and 1T = 10000 gauss. The plasma thus generated is emitted from the plasma generation chamber to the film formation chamber side by the divergent magnetic field of the magnetic coil.

In the state where ECR plasma is generated as described above, high frequency power (for example, 500 W) of 13.56 MHz is supplied from the high frequency power source to the SBT target disposed at the outlet of the plasma generation chamber. As a result, a sputtering phenomenon occurs in the SBT target made of a sintered body (oxide sintered body) of Sr, Bi, Ta, and oxygen, and particles (atoms) constituting the target jump out. Here, the SBT target is composed of a sintered body having a composition in which the atomic composition is given by Sr: Bi: Ta = 1: 2.1: 2. Each particle jumping out from the SBT target reaches the surface of the platinum layer 103 together with the plasma released from the plasma generation chamber and the oxygen gas introduced and activated by the plasma, and is oxidized by the activated oxygen to be SBT. It becomes a thin film. As described above, when the partial pressure of oxygen gas is 10 ⁻² Pa or more, a sufficient amount of oxygen atoms are taken into the formed thin film.

  Even if the substrate is not heated, the surface temperature of the substrate 101 (platinum layer 103) rises to about 150 ° C. by the plasma flow irradiation, but is completely amorphous on the platinum layer 103. The SBT thin film 104 is formed. When the amorphous SBT thin film 104 formed as described above was analyzed by ICP emission analysis, the composition of each element was Sr: Bi: Ta = 0.82: 2.29: 2. Note that since the sputtering efficiency and the spatial spread of the sputtered particles differ from element to element, the composition of the thin film formed does not necessarily match the composition of the target.

  Next, as shown in FIG. 1C, the amorphous SBT thin film 104 (substrate 101) is heated to about 650 ° C. in the atmosphere of the oxygen gas 105. By this heat treatment, as shown in FIG. 1 (d), a state is obtained in which the SBT thin film 106 having an Aurillius structure oriented in the <115> direction is formed on the substrate 101. As described above, according to the above-described method, the SBT thin film 106 having an auribilius structure exhibiting ferroelectricity can be obtained by heat treatment at a temperature lower by about 50 ° C. than the conventional method.

  Incidentally, the ECR sputtering apparatus illustrated in FIG. 2 may be used to form the amorphous SBT thin film 104 described above. FIG. 2 is a configuration diagram showing a configuration example of the ECR sputtering apparatus, and shows a schematic cross section. The ECR sputtering apparatus shown in FIG. 2 will be described. First, a processing chamber 201 and a plasma generation chamber 202 communicating with the processing chamber 201 are provided. The processing chamber 201 communicates with an evacuation device (for example, a turbo molecular pump) (not shown), and the inside of the processing chamber 201 is evacuated together with the plasma generation chamber 202 by the evacuation device. The processing chamber 201 is provided with a substrate holder 204 to which the film formation target substrate 101 is fixed. The substrate holder 204 is tilted at a desired angle (for example, 25 °) by a tilt rotation mechanism (not shown) and is rotatable. By tilting and rotating the substrate holder 204, it is possible to improve the in-plane uniformity of the film and the step coverage with the material to be deposited.

  Further, a ring-shaped target 205 is provided so as to surround the opening region in the opening region into which the plasma from the plasma generation chamber 202 in the processing chamber 201 is introduced. The target 205 is placed in a container 205 a made of an insulator, and the inner surface is exposed in the processing chamber 201. Further, a high frequency power source 222 is connected to the target 205 via a matching unit 221 so that, for example, a high frequency of 13.56 MHz can be applied. When the target 205 is a conductive material, a negative DC voltage may be applied. Note that the target 205 may be not only a circular shape but also a polygonal state as viewed from above.

  The plasma generation chamber 202 communicates with the vacuum waveguide 206, and the vacuum waveguide 206 is connected to the waveguide 208 through a quartz window 207. The waveguide 208 communicates with a microwave generation unit (not shown). In addition, a magnetic coil (magnetic field forming means) 210 is provided around the plasma generation chamber 202 and at the top of the plasma generation chamber 202. These microwave generator, waveguide 208, quartz window 207, and vacuum waveguide 206 constitute a microwave supply means. There is a configuration in which a mode converter is provided in the middle of the waveguide 208.

The operation example of the ECR sputtering apparatus of FIG. 2 will be described. First, after the processing chamber 201 and the plasma generation chamber 202 are evacuated, Xe gas, which is an inert gas, is introduced from the inert gas introduction unit 211. In addition, oxygen gas is introduced from an oxygen gas introduction part 212 arranged on the downstream side of the plasma flow of the target 205 to bring the inside of the plasma generation chamber 202 to a predetermined pressure (for example, 1.3 × 10 ⁻² Pa). In this state, a magnetic field of 0.0875 T is generated in the plasma generation chamber 202 from the magnetic coil 210, and then a 2.45 GHz microwave is introduced into the plasma generation chamber 202 through the waveguide 208 and the quartz window 207. Then, an electron cyclotron resonance (ECR) plasma is generated.

  The ECR plasma forms a plasma flow in the direction of the substrate holder 204 by the divergent magnetic field from the magnetic coil 210. In such a state that plasma (plasma flow) is generated and supplied in the direction of the substrate holder 204, high frequency power (for example, 500 W) is supplied from the high frequency power source 222 to the target 205 disposed at the outlet of the plasma generation chamber 202. Supply. As a result, when the RF voltage is negative, Xe ions generated by the plasma are attracted to and collide with the target 205, causing a sputtering phenomenon, and each particle constituting the target 205 jumps out of the target 205.

  Further, in the generated ECR plasma, electrons pass through the target 205 by the divergent magnetic field formed by the magnetic coil 210 and are extracted to the substrate 101 side and irradiated onto the surface of the silicon oxide layer 102. At the same time, the positive ions in the ECR plasma are drawn to the substrate 101 side so as to neutralize the negative charges due to electrons, that is, to weaken the electric field, and the surface of the layer (platinum layer 103) on which the film is formed Is irradiated. Thus, while each particle is irradiated, some of the positive ions are combined with electrons to become neutral particles. In addition, by irradiating with plasma in this manner, the temperature of the substrate 101 rises to about 100 to 150 ° C. without heating by the heating mechanism.

  Here, since the target 205 has a ring shape (cylindrical shape), Xe ions attracted to the target 205 at a high speed enter the ring-shaped inner surface almost perpendicularly, and sputter and reflect the atoms of the target 205. To do. The direction of this reflection is towards the other inner surface of the ring shape. Thus, Xe particles having an energy of several hundreds eV incident at high speed are reflected in the inner direction of the ring shape, and this reflection is repeated inside the ring shape of the target 205. Further, some of the ions reflected in the high energy state are combined with electrons in the ECR plasma to become neutral particles in the high energy state.

  On the other hand, the constituent atoms that have jumped out of the target 205 have an energy of about 10 to 20 eV, and some of them fly outside from the inner region of the ring-shaped target 205, and these are generated on the substrate 101 (platinum layer 103). Will be reached. As described above, in the ECR sputtering apparatus, constituent atoms in a low energy state to be deposited reach the substrate 101, but high-speed (high energy state) ions and neutral particles are deposited on the substrate 101 during deposition. It is possible to make it not enter.

  In the thin film forming apparatus of FIG. 2, microwave power supplied from a microwave generation unit (not shown) is once branched in the waveguide 208, and plasma is supplied to the vacuum waveguide 206 above the plasma generation chamber 202. The generation chamber 202 is coupled from the side through a quartz window 207. By doing so, it becomes possible to prevent the scattered particles from adhering to the quartz window 207 from the target 205, and the running time can be greatly improved. In addition, a shutter or the like may be provided between the substrate 101 to be processed and the target 205 to control the arrival of the raw material with respect to the silicon oxide layer 102.

  Next, the low temperature crystallization of the SBT thin film will be described in more detail. First, crystallization heating will be described. The temperature required for crystallization mainly depends on the SBT thin film itself and annealing (heating) conditions. Parameters related to the SBT thin film itself are the film thickness, composition, oxygen content, impurity amount, etc. of the SBT thin film. Parameters related to annealing conditions include the type of gas atmosphere during annealing, the gas flow rate, the temperature rise state (control), the structure of the annealing furnace, and the like.

In order for the SBT thin film to exhibit the characteristics as a ferroelectric, the conditions of 0.7 ≦ x ≦ 1 and 2.0 ≦ y ≦ 2.4 are first satisfied with respect to the metal element composition of Sr _x Bi _y Ta ₂ O _z. It is necessary to be satisfied. Solid-phase crystallization is a phenomenon that occurs due to the movement of the interface between crystal grains and amorphous. If there are many defects in the thin film, the progress of crystallization tends to stop midway. Furthermore, in a state where the number of oxygen atoms contained in the thin film is not sufficient, that is, in a thin film having many oxygen vacancies, the crystallization temperature becomes high. In fact, annealing in an oxygen gas atmosphere at atmospheric pressure or higher during annealing can be interpreted as terminating such oxygen defects with oxygen. When the laser ablation method or the RF sputtering method is used for film formation, the oxygen gas excitation efficiency is not sufficient, so that an oxygen-deficient thin film is easily formed, and it is not easy to set the crystallization temperature to 700 ° C. or lower.

In order to solve these problems, the first feature of the present invention is to apply the ECR sputtering method in the formation of the SBT thin film by the sputtering method. Since the ECR plasma can excite the gas with high density, the oxygen gas introduced at a low gas pressure into the processing chamber is efficiently excited. As a result, a large amount of oxygen radicals and ions are generated, and the generated oxygen radicals and ions are sufficiently taken into the SBT thin film. SBT is a composite oxide containing SrO, Bi ₂ O ₃ , and Ta ₂ O ₅ containing a metal atom alone. Each metal atom is bonded at a ratio of 1 atom of oxygen per 1 atom of Sr, 1.5 atoms of oxygen per 1 atom of Bi, and 2.5 atoms of oxygen per 1 atom of Ta. Then, the maximum oxidation state is reached. Therefore, when the atomic composition ratio of the SBT thin film is given by Sr _x Bi _y Ta ₂ O _z , the maximum oxidation state is obtained when the oxygen composition z is (x + 1.5y + 5).

  If the oxygen flow rate during film formation is sufficiently high, oxygen atoms that are not bonded to metal atoms but are weakly trapped in the thin film also exist. Therefore, in practice, a state in which oxygen is contained in the film can be realized to the extent that the inequality z ≧ x + 1.5y + 5 is satisfied. In other words, the atomic composition of oxygen atoms contained in the amorphous SBT thin film can be more than the sum of the atomic composition of strontium and 1.5 times the atomic composition of bismuth plus 5 It is. By setting the amorphous SBT thin film in such a state, oxygen vacancies can be suppressed and the crystallization temperature can be lowered. As an additional feature of the ECR sputtering system, the high-speed ions and neutral particles do not directly enter the substrate during film formation, so there is little damage to the thin film and there is no vacancy. Can be obtained.

Next, the situation advantageous for lowering the temperature will be considered from the viewpoint of the reaction path for crystallization. When the temperature of the amorphous SBT thin film is gradually raised from a low temperature, a fluorite structure is first formed. When the temperature is further increased, rearrangement of the constituent atoms occurs from the fluorite structure to the auribilius structure. FIG. 3 shows this state with the reaction coordinate on the horizontal axis and the energy of the system on the vertical axis. In order to switch from the amorphous state to the fluorite structure, the barrier given by the activation energy E _a must be exceeded. However, since the fluorite structure is a metastable state, it is amorphous in terms of energy. Located lower than. Furthermore, in order to change from the fluorite structure to the Aurivirius structure, the barrier given by the activation energy E _f must be exceeded. Here, without going through the fluorite structure, if it is possible to shift to the state of Auribiriusu structure directly from the amorphous state, activation barrier will be need at a lower E _b than E _f. That is, by directly changing from an amorphous state to an auribilius structure, crystallization can be performed by low-temperature annealing.

  Therefore, as a means for solving the problem of lowering the crystallization temperature, in the present invention, the SBT thin film is formed while keeping the substrate temperature at such a low temperature that the amorphous state is maintained. The second feature. In the ECR sputtering method, a plasma flow is irradiated on a substrate, and thus atoms on the substrate are electronically excited. As a result, the plasma flow provides a local heating effect beyond the increase in average substrate temperature, and the thin film may become nanocrystals even at fairly low temperatures.

  The magnitude of the effect of the plasma irradiation is unclear because it is related to the structure of the apparatus such as the density of the plasma and the distance to the substrate, but in order to maintain the amorphous state of the SBT thin film, it is necessary to cool the substrate. It is thought that it will come out when necessary. The state in which crystal nuclei are generated in the SBT thin film is stable in terms of energy, and is not desirable because the fluorite structure (fluorite phase) easily crystallizes using the crystal nuclei generated during annealing as nuclei. .

  Next, the characteristic investigation result of the formed amorphous SBT thin film 104 will be described. First, for comparison, a reference sample in which an SBT thin film is formed on a platinum layer formed in the same manner as described above is prepared by a standard sol-gel method. Briefly explaining this formation, a compound solution having an atomic composition ratio of Sr: Bi: Ta = 0.8: 2.2: 2 is spin-coated on a substrate (on a platinum layer), and then added at 150 ° C. for 2 minutes. And dry at 250 ° C. for 5 minutes. Further, the same spin coating and drying cycle is repeated, and a reference sample on which an SBT thin film having a film thickness of 200 nm is finally formed is manufactured.

  FIG. 4 shows an X-ray diffraction of an SBT crystal film obtained by annealing an amorphous SBT thin film formed on a platinum layer by ECR sputtering in an atmospheric pressure oxygen atmosphere for 3 hours (3 h). It is a pattern. Broad peaks at 2θ = 28.52 ° and 28.60 ° are observed from the SBT thin films having annealing temperatures of 600 ° C. and 620 ° C., respectively. This diffraction angle corresponds to <111> reflection of the fluorite structure. On the other hand, when the annealing temperature is increased to 650 ° C., the peak position is shifted to 29.00 °, which becomes sharper. This diffraction angle corresponds to the <115> reflection of the auribilius structure. Furthermore, in the SBT thin film heated to 650 ° C., peaks characteristic of the Aurillius structure such as <200>, <220>, <20-1-0>, <315> appear, and thus the heating at 650 ° C. It can be seen that the condition has been converted to an Aurivirius structure.

  FIG. 5 is an X-ray diffraction pattern of an SBT crystal film obtained by annealing a SBT thin film of a reference sample formed on a platinum layer by a sol-gel method in an oxygen atmosphere at atmospheric pressure for 3 hours. In annealing at 600 ° C., a very weak peak appears at 2θ = 28.48 °. When the annealing temperature is increased to 620 ° C., a broad peak centered at 2θ = 28.60 ° appears. Furthermore, at the annealing temperatures as high as 650 ° C. and 670 ° C., the peak positions are shifted to 28.80 ° (650 ° C.) and 29.00 ° (670 ° C.) to the high diffraction angle side, but around 28.6 °. The strength remains, indicating that it has been partially converted to an Aurivirius structure.

  For the SBT thin film of the reference sample, the <115> peak shape similar to that at 650 ° C. in FIG. 4 was obtained for the first time at an annealing temperature of 700 ° C. Therefore, in comparison with FIG. 4, the crystallization temperature of the SBT thin film of the reference sample obtained by the sol-gel method is 700 ° C. These results show that if the film is formed using the ECR sputtering method, the process temperature can be reduced by about 50 ° C. as compared with the organometallic compound decomposition method currently used as a standard.

  FIG. 6 shows the time change of the X-ray diffraction pattern when the SBT thin film deposited by the ECR sputtering method is annealed at 650 ° C. in an oxygen gas atmosphere. A peak has already appeared at the position of the fluorite structure after 10 minutes (10 min) of annealing. There is no significant change in the peak shape until 1 hour (1h), but after 3h annealing, it becomes sharper and the peak position is also shifted. This means that the crystal grains do not grow to the size observed by X-ray diffraction if the annealing time does not elapse for 3 hours.

  Moreover, the same result as the case where the platinum layer was used was obtained also about the SBT thin film formed on the board | substrate which used the ruthenium layer instead of the platinum layer. FIG. 7 is an X-ray diffraction pattern of an SBT crystal film obtained by annealing an SBT thin film formed on a ruthenium layer in an oxygen atmosphere at atmospheric pressure. Under any annealing condition of 620 ° C. for 3 min, 650 ° C. for 3 min, and 650 ° C. for 10 min, the position and shape of the diffraction peak from the SBT thin film show a fluorite structure (fluorite phase), but at 650 ° C. for 3 h. After the annealing, a diffraction pattern showing the formation of an auribilius phase similar to the case of using the platinum layer is obtained.

  As described above, it can be understood that the SBT thin film formed by the film formation using the ECR sputtering method is crystallized at a lower temperature than the SBT thin film formed by the standard sol-gel method. In order to investigate this cause, the X-ray photoelectron spectrum (XPS) of the produced SBT thin film was measured. FIG. 8 shows the 4f spectrum of bismuth for the sample (a) before annealing and the sample annealed at 600 ° C. (b), 650 ° C. (c), and 750 ° C. (d) for 3 h.

As shown in FIG. 8, the 4f level is split into 4f _5/2 and 4f _7/2 by the spin-orbit interaction. 4f _5/2 has a binding energy of 164.3 eV and 4f _7/2 has a main peak (main peak) in the vicinity of the binding energy of 159 eV. Each peak has Bi ₂ O ₃ , Bi ₂ O ₂ , and By considering a chemical shift peculiar to Bi or the like constituting the perovskite block, it can be decomposed into elements reflecting the difference in chemical environment. Moreover, about the sample after annealing, a bulge (peak) is seen in the vicinity of 162 eV and 156.7 eV in addition to the main peak. These peaks are due to bismuth in the metal state.

  As shown in FIG. 8, the sample containing sufficient oxygen atoms formed by the ECR sputtering method does not contain bismuth atoms in a metal state that is not sufficiently oxidized, and some oxygen is desorbed by annealing. It can be seen that a small amount of bismuth atoms in the metal state is generated. Even in this case, as estimated from the peak area, the proportion of bismuth atoms in the metal state is only 4% or less of the entire bismuth atoms.

  For the SBT thin film formed by such an ECR sputtering method, FIG. 2 shows that the SBT thin film produced by the sol-gel method and the one annealed in an oxygen atmosphere both contain about 8% amount of bismuth atoms in the metal state. Also, FIG. 1 shows that the SBT thin film formed by the laser ablation method contains about 26% of bismuth atoms in a metal state.

  From the comparison with these results, in the SBT thin film deposited by the ECR sputtering method, since the excitation efficiency of oxygen gas is high, it is possible to contain a sufficient amount of oxygen in the thin film. From this result, It is considered that crystallization into the Auribilius structure can be induced by annealing at a low temperature of 650 ° C. In comparison with other sputtering methods, in the ECR sputtering apparatus, the target shape and the geometrical arrangement of the substrate are such that high-speed particles do not directly enter the substrate surface. The effect of resputtering is reduced. This also increases the atomic density of the film, and there is no problem that the progress of solid-phase crystallization stops at the defective part. The film formation by the ECR sputtering method contributes to lowering the crystallization temperature. It is thought that.

  As explained above, by preparing the precursor SBT thin film before annealing by depositing at a low temperature such as without heating using the ECR sputtering method, the standard organometallic compound decomposition method, sputtering method, laser ablation It was found that an SBT thin film having an auribilius structure can be formed by annealing at a lower temperature than by the method. Compared to the most widely used organic compound decomposition method, the fact that a crystallization temperature of about 50 ° C. is possible at a low temperature is advantageous in the process of ferroelectric memory.

It is process drawing which shows the example of the formation method of the SBT thin film in embodiment of this invention. It is a block diagram which shows the structural example of an ECR sputtering apparatus. It is explanatory drawing which shows a mode that it changes to an amorphous state, a fluorite structure, and an auribilius structure with a raise in temperature. A characteristic showing an X-ray diffraction pattern of an SBT crystal film obtained by annealing an amorphous SBT thin film formed on a platinum layer by ECR sputtering in an atmospheric pressure oxygen atmosphere for 3 hours (3 h). FIG. FIG. 4 is a characteristic diagram showing an X-ray diffraction pattern of an SBT crystal film obtained by annealing a SBT thin film of a reference sample formed on a platinum layer by a sol-gel method in an oxygen atmosphere at atmospheric pressure for 3 hours. It is a characteristic view showing the time change of the X-ray diffraction pattern when the SBT thin film deposited by the ECR sputtering method is annealed at 650 ° C. in an oxygen gas atmosphere. FIG. 6 is a characteristic diagram showing an X-ray diffraction pattern of an SBT crystal film obtained by annealing an SBT thin film formed on a ruthenium layer in an oxygen atmosphere at atmospheric pressure. It is a characteristic view which shows the X-ray photoelectron spectroscopy spectrum of a SBT thin film.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 101 ... Board | substrate, 102 ... Silicon oxide layer, 103 ... Platinum layer, 104 ... Amorphous SBT thin film, 105 ... Oxygen gas, 106 ... SBT thin film of Auribirius structure.

Claims (1)

A plasma composed of an inert gas and an oxygen gas supplied at a predetermined composition ratio is generated, and a negative vice is applied to a target containing strontium, bismuth, and tantalum, and particles generated from the plasma collide with the target. A state in which an amorphous SBT thin film made of strontium, bismuth, tantalum, and oxygen is formed on the substrate by causing a sputtering phenomenon and depositing the material constituting the target on the substrate. Sputtering process and
A heating step of heating the SBT thin film in an oxygen gas atmosphere to form an auribilius structure,
The plasma is an electron cyclotron resonance plasma generated by electron cyclotron resonance and given kinetic energy by a divergent magnetic field and supplied as a plasma flow in the direction of the substrate,
The target is disposed in the middle of the flow of the plasma flow supplied in the direction of the substrate,
In the sputtering step, the atomic composition of oxygen atoms contained in the amorphous SBT thin film is a sum of 5 plus the sum of the atomic composition of strontium and 1.5 times the atomic composition of bismuth in the SBT thin film. The oxygen gas is supplied so as to achieve the above state. A method for forming an SBT thin film, characterized in that:

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