Process for producing bismuth compounds, and bismuth compounds

Abstract: The formation of an electrically conductive phase in an dielectric or ferroelectric composed of a bismuth compound is inhibited. Described is a process for producing a bismuth compound, which comprises introducing a gas of starting materials in an atmosphere under a pressure of 0.01 to 50 torr, depositing a precursor of a bismuth compound on a substrate, and thermally treating it in an oxidizing atmosphere.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for producing a useful bismuth compound used in electric devices such as ferroelectric memory etc. as well as to a dielectric ferroelectric of a bismuth compound. Description of the Related Art The bismuth-layered compound forms a group of industrially extremely important compounds such as bismuth series superconducting oxides with a critical temperature of 110 K, or materials for ferromagnetic memory etc. For application of these compounds to electric devices, it is essential to develop a thin film process. The structure of this bismuth-layered compound is found in e.g. Bi 2 PbNb 2 O 9 and is represented by a repeating structure in a pseudo-tetragonal system having a layer of bismuth oxide and a layer of an oxide of another element laminated repeatedly in a definite order and extended in column shape along the c axis direction (see G.A. SMOLENSKII et al., SOVIET PHYSICS-SOLID STATE, pp. 651-655 (1961)) and E.C. SUBBARAO, J. Phys. Chem. Solids Pergamon Press, Vol. 23, pp. 665-676 (1962)). In this repeating structure, the number of bismuth oxide layers in 1 unit structure and the length of 1 unit structure vary depending on an individual bismuth-layered compound. At present, the application of this bismuth-layered compound to electric devices is attempted, and in such an attempt, a thin layer of a bismuth-layered compound excellent in ferroelectric properties is obtained by spin coating methods including metal organic decomposition (MOD) etc. P.C. Van Buskirk et. al. (Integrated Ferroelectrics, 1995, Vol. 10, pp. 9-22) and US-A-5,426,075 describe the preparation of ferroelectric bismuth layered materials, such as strontium bismuth tantalate. OBJECT AND SUMMARY OF THE INVENTION The phenomenon of short-circuiting can occur in capacitors using this bismuth-layered compound. This phenomenon is considered to be caused for various reasons including a hillock on the bottom platinum electrode, and one of the possible reasons is the presence of an electrically conductive phase resulting from inappropriate composition. Hence, the present inventors found that it becomes necessary to develop a production process for inhibiting the formation of this electrically conductive phase. Particularly in a production process where oxidation and crystallization are carried out by post-annealing after preparation of thin film, it is important to control the composition at the time of film forming. To solve the above-described problem, the object of the present invention is to provide a process for producing a capacitor composed of a bismuth-layered compound with a low degree of the appearance of the phenomenon of short-circuiting (low degree of short-circuiting) by controlling the composition in the preparing process to inhibit the formation of the electrically conductive phase. The present invention is a process for producing a bismuth compound, which comprises a first step of introducing a gas of starting materials in an atmosphere under a pressure of 1.33 Pa to 6.650 kPa (0.01 to 50 torr) to prepare a precursor of a bismuth compound on a substrate, and a second step of thermally treating the precursor in an oxidizing atmosphere. Further, the present invention is a dielectric ferroelectric of a bismuth compound, which comprises a bismuth-layered compound as a main phase and a composite oxide of pyrochlore structure as a secondary phase. According to the constitution of the present invention as described above, a gas of starting materials is introduced in an oxidizing atmosphere whereby a precursor of a bismuth compound can be deposited on a substrate, and it is thermally treated in the oxidizing atmosphere whereby a highly insulating bismuth compound containing a bismuth-layered compound as a main phase and a highly insulating by-product such as a compound of pyrochlore structure can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the relationship between the partial pressure of oxygen in the range of 20 to 70 % and the composition of the resulting thin film; FIG. 2 shows an X ray diffraction pattern of a thin film obtained under 38.9 % oxygen partial pressure; FIG. 3 shows an X ray diffraction pattern of a thin film obtained under 22.2 % oxygen partial pressure; FIG. 4 shows the dependence of the ratio of the bismuth-layered compound to the secondarily phase with pyrochlore structure as the formed phases on the partial pressure of oxygen; and Fig. 5 shows the relationship among the partial pressure of oxygen, the remnant polarization and the degree of short-circuiting. DESCRIPTION OF THE PREFERRED EMBODIMENT The process for producing a bismuth compound according to the present invention involves introducing a gas of starting materials in an atmosphere under a pressure of 1.33 Pa to 6.650 kPa (0.01 to 50 torr) depositing a precursor of a bismuth compound on a substrate, and thermally treating it in an oxidizing atmosphere. In the production of the bismuth compound of the present invention, a thin film of a precursor is first formed by metal organic chemical vapor deposition (MOCVD) method using a dipivaloylmethanate (DPM) etc. as a source or flash chemical vapor deposition (CVD) method where a source is transferrred and mixed in a liquid phase and then evaporated under rapidly reduced pressure in a reactor. In the process for producing a bismuth compound according to the present invention, examples of desired bismuth compounds are bismuth-layered compounds having the formula Bi 2 (Sr, Ba, Ca) (Ta, Nb) 2 O 9 used as ferroelectrics. For the production of these bismuth-layered compounds, it is important for the composition of a thin film of a precursor to contain tantalum or niobium in slight excess relative to the composition of the desired bismuth-layered compounds. Contrary to this, alkaline earths such as strontium etc. are made slightly short. In the film forming methods described above, film forming conditions are controlled so as to attain such desired composition. Film forming conditions for a thin film as a precursor are preferably as follows. Reaction temperature: 400 to 700°C Reaction gas pressure: 1.33 Pa to 6.650 kPa (0.01 to 50 torr) Reaction gas: in an oxidizing gas containing at least 5 % oxygen Annealing conditions of thin film as a precursor are preferably as follows. Annealing temperature: from a temperature for film forming to 850°C Atmosphere: oxidizing atmosphere The capacitor material prepared under such conditions contains small amounts of secondary phase such as composite oxides with pyrochlore structure besides the desired bismuth-layered compound. Composite oxides with pyrochlore structure of the formula (Bi, (Sr, Ba, Ca)) 2-x (Ta, Nb) 2 O 7±δ are formed as by-products where the main phase is a bismuth-layered compound of the formula Bi 2 (Sr, Ba, Ca) (Ta, Nb) 2 O 9 . Dielectric or ferroelectric capacitors prepared using such capacitor material have a low degree of short-circuiting because the material does not contain any electrically conductive phase. Hereinafter, the process for producing a bismuth compound according to the present invention is illustrated by reference to the production of a bismuth-layered compound of the formula Bi 2 SrTa 2 O 9 among the above bismuth-layered compounds. Among the above-mentioned film forming methods, the flash CVD method will be illustrated for film forming. The substrate used is Sio 2 having Ti and Pt deposited sequentially on it by sputtering. The CVD source is suitably selected from BiPh 3 (triphenylbismuth), Bi(O-Tol) 3 etc. as Bi source, Sr(DPM)2 (dipivaloyl metanate-strontium), Sr(Me 5 C 5 ) 2 .2THF etc. as Sr source, and Ta(OCH 3 ) 5 , Ta(O-iPr) 5 etc. as Ta source. These source materials are dissolved in an organic solvent etc. and transferred in liquid form and introduced with an argon (Ar) carrier gas into a reactor. In this step, the source solution is preferably vapored in a vaporization unit under a rapidly reduced pressure of about 13.3 Pa to 1.33 kPa (0.1 to 10 torr) and introduced in a vapor phase into the reactor where the vapor is deposited on a substrate in the reactor. When pure oxygen is used as an oxidizing gas, the flow is fed to the reactor while the partial pressure of oxygen is controlled in the range of 20 to 50 % or thereabout relative to the total pressure. If the partial pressure of oxygen is less than 20 %, none of the bismuth-layered compound will result with the composite oxide with pyrochlore structure forming as the main phase, while if the partial pressure of oxygen exceeds 50 %, it is disadvantageous that none of the oxide of pyrochlore structure will be formed as secondary phase. A suitable mixing ratio for the respective sources is determined preferably in the following manner. A thin film is experimentally prepared and then analyzed for its composition in an electron probe microanalyzer (EPMA), fluorescence X ray spectroscopy etc., and the mixing ratio for the source materials is controlled according to the analysis result so that the best mixing ratio is determined. If the compounding ratio of the fed source materials is suitable, then a thin film of a precursor which is fluorite-structured crystals but partially amorphous can be obtained at a substrate temperature of 400 to 750 °C or thereabout. The precursor thus obtained is thermally treated preferably at 800 °C for 1 hour in an oxidizing atmosphere, preferably in a stream of oxygen under normal pressure, whereby a thin film containing the bismuth-layered compound Bi 2 SrTa 2 O 9 as a main phase and composite oxides (Bi, Sr) 2-x Ta 2 O 7±δ with pyrochlore structure as a secondary phase. A capacitor is formed by providing the resulting thin film with a film of platinum top electrode by e.g. sputtering to permit the determination of its electric characteristics. If the capacitor is free of the composite oxides (Bi, Sr) 2-x Ta 2 O 7± with pyrochlore structure, a high degree of short-circuiting results and it is conceivable that an electrically phase has occurred. As opposed to this, the degree of short-circuiting is lowered with an increasing content of composite oxides with pyrochlore structure, and the formation of the above-described electrically conductive phase is considered difficult in these compositions. Hence, the process for producing the bismuth compound of the present invention, according to which the degree of short-circuiting of a capacitor is lowered by secondary phase of composite oxides with pyrochlore structure, is useful. The composite oxides with pyrochlore structure secondarily formed in this step may have the pyrochlore structure in a cubic system, structures in a cubic system slightly altered due to lattice strain etc., or similar structures. In addition, the composite oxides with pyrochlore structure are relatively nonstoichiometric and may thus deviate from the pyrochlore structure in respect of their composition and structure in some cases. Hereinafter, the process for producing a bismuth compound according to the present invention is illustrated by reference to a specific example. The following example illustrates the preparation of a capacitor of a ferroelectric film composed of the bismuth-layered compound having the formula Bi 2 SrTa 2 O 9 . 100 nm each of Ti and Pt are subsequently accumulated by sputtering at room temperature on the (100) face of spontaneously oxidized silicon to form a substrate. Source materials in the CVD method, such as BiPh 3 , Sr(DPM) 2 , Ta(O-iPr) 5 , etc., are selected and these source materials are dissolved in organic solvent such as tetrahydrofuran (THF) etc. to prepare about 0.1 M/L of each solution. These solutions are mixed so as to attain the initial ratio of Bi/Sr/Ta (2/1/2) by atomic composition, then transferred and introduced to an vaporization unit. The vaporization unit has previously been heated such that the source materials are not deposited to be adhered onto its inside wall. The vapor from the solution is introduced with an Ar carrier gas into a reactor. The vaporization unit and the reactor have previously been reduced to 1.33 kPa (10 torr) or thereabout to evaporate the source solution. The vapor from the source solution is transferred in a gas phase to, and deposited on, the substrate. For deposition, the substrate has previously been heated at 600 °C or thereabout. Separately, an oxidizing gas such as pure oxygen etc. is introduced directly into the reactor without passing it through the vaporization unit. The Ar carrier gas and the oxidizing gas are fed to the reactor respectively under the control of a mass flow controller, preferably adjusting the total flow to 1800 sccm and the partial pressure of oxygen to about 20 to 50 %. In this manner, a thin film of a precursor whose main phase is fluorite-structured crystals but partially amorphous can be obtained. Subsequently, this thin film of the compound with fluorite structure is thermally treated at 800 °C for 1 hour in a stream of oxygen under normal pressure. As a result, the thin film containing the desired formula Bi 2 SrTa 2 O 9 as the main phase and the composite oxides (Bi, Sr) 2-x Ta 2 O 7±δ with pyrochlore structure as the secondary phase could be obtained. Thin films obtained by varying the partial pressure of oxygen in the range of 20 to 70 % were examined for their compositions. The temperature and pressure for film forming are 600 °C and 1.33 kPa (10 torr) respectively. The relationship between the partial pressure of oxygen and the composition of the resulting thin film is shown in FIG. 1. FIG. 1 shows the ratio of bismuth () to strontium (○) in each composition when the ratio of tantalum in the composition was normalized as 2. As can be seen from FIG. 1, tantalum becomes rich while bismuths and strontium become relatively poor with a decreasing partial pressure of oxygen. FIG. 2 shows an X ray diffraction pattern of a thin film obtained under 38.9 % oxygen partial pressure. In FIG. 2, the peaks designated Pt are attributable to Pt deposited for the bottom electrode on the substrate under the thin film, the peaks designated B are attributable to the bismuth-layered compounds as the object, the peaks P are attributable to the secondary phase with pyrochlore structure. As can be seen from FIG. 2, the bismuth-layered compounds as the object are formed as the main phase while small amounts with oxides with pyrochlore structure occur as the secondary phase. This secondary phase with pyrochlore structure can be separated into an almost single phase by further reducing the partial pressure of oxygen at the time of film forming. FIG. 3 shows an X ray diffraction pattern of a thin film obtained under 22.2 % oxygen partial pressure. The peaks are designated numerals indicating their crystal faces. Strong peaks in FIG. 3, which are attributable to reflection on the faces (111), (311), (222), (400), (440), (531), (622) and (444) respectively, well represent the characteristics of the pyrochlore structure. FIG. 4 shows the dependence of the ratio of the bismuth-layered compound to the secondary phase with pyrochlore structure as the formed phases on the partial pressure of oxygen. In this figure, the strength of the main peak (222) face was calculated from the peak of the secondly strong (400) face of the secondary phase with pyrochlore structure, and the ratio of this strength to the strength of the main peak of the bismuth-layered compound was determined for approximate estimation. It can be seen that the ratio of the secondary phase with pyrochlore structure as a phase in the composition is increased with a decreasing partial pressure of oxygen (FIG. 4) or with increasing tantalum (FIG. 3). To evaluate the characteristics of the thin film prepared in this example, an upper platinum electrode of about 200 nm was formed on the thin film by sputtering and the remnant polarization 2Pr of this thin film was determined. The remnant polarization 2Pr and the degree of short-circuiting of the electrode were determined with an applied voltage of 2 V. The relationship among the partial pressure of oxygen at the time of film forming, the remnant polarization and the degree of short-circuiting is shown in FIG. 5. In FIG. 5, the symbol  indicates the remnant polarization 2Pr, and the symbol ○ indicates the degree of short-circuiting. As can be seen from FIG. 5, the degree of short-circuiting is decreased as the partial pressure of oxygen is decreased in the range of 45 % or less, that is, as the secondary phase with pyrochlore structure is increased. Simultaneously, the ratio of the ferroelectric bismuth-layered compound as a phase is also decreased so that the remnant polarization 2Pr is decreased too. Practically, the construction of a ferroelectric capacitor from the bismuth-layered compound requires establishment of the composition of the bismuth-layered compound on the basis of the relationship between the remnant polarization and the degree of short-circuiting. Although in the aforementioned example, the bismuth-layered compound of the formula Bi 2 SrTa 2 O 9 is exemplified as the object, bismuth compounds cf other formulae can also be used to obtain thin films of the desired compositions. According to the present process for producing a bismuth compound, the occurrence of an electrically conductive secondary phase can be inhibited by secondary formation of a small amount of an insulating phase with pyrochlore structure in the bismuth-layered compound. Hence, when applied to materials such as capacitor, this bismuth compound can give a capacitor excellent in characteristics with a lower degree of short-circuiting. Further, the pyrochlore structure is nonstoichiometric to a certain degree though not so high as the fluorite structure and it is tolerant to strain in the composition, so there may be further effects such as inhibition of formation of other secondary phases. By virtue of its tolerance to strain in the composition, the bismuth compound is relative tolerant to various manufacturing conditions and can thus be produced more easily.

Claims: A process for producing a bismuth compound having ferroelectric properties comprising: a first step of introducing a gas of bismuth-containing starting materials in an atmosphere under a pressure of 1.33 Pa to 6.650 kPa (0.01 to 50 Torr) to deposit a precursor of a bismuth compound on a substrate; and a second step of thermally treating said precursor in an oxidizing atmosphere to obtain a bismuth compound containing a layered mixed metallic oxide comprising bismuth and a bismuth-containing composite oxide with pyrochlore structure. The process for producing a bismuth compound according to claim 1, wherein said layered mixed metallic oxide comprising bismuth is represented by the following formula: Bi 2 (Sr a Ba b Ca c ) (Ta d Nb e ) 2 O 9 in which a, b, c, d and e range respectively from 0 to 1 in atomic ratio; a + b + c = 1; and d + e = 1. The process for producing a bismuth compound according to claim 1, wherein said composite oxide with pyrochlore structure is represented by the following formula: (Bi, (Sr, Ba, Ca)) 2-x (Ta, Nb) 2 O 7±δ . The process for producing a bismuth compound according to claim 1, wherein the oxygen partial pressure in the first step ranges from 20 to 50 % of the total pressure. The process for producing a bismuth compound according to claim 1, wherein the substrate temperature in the first step ranges from 400 to 750°C. The process for producing a bismuth compound according to claim 1, wherein the precursor of said bismuth compound contains both a phase consisting of fluorite-structured crystals and an amorphous phase. The process for producing a bismuth compound according to claim 1, wherein the second step is carried out in an oxidizing atmosphere in the temperature range from the membrane manufacturing temperature of the precursor of said bismuth compound in the step 1 to 850°C. A process for producing a bismuth compound, comprising: a step of permitting a bismuth compound precursor containing both a phase consisting of fluorite-structured crystals and an amorphous phase to be deposited on a substrate; and a step of thermally treating said precursor in an oxidizing atmosphere, thereby forming bismuth oxide containing a layered mixed metallic oxide and a composite oxide with pyrochlore structure. A bismuth compound having ferroelectric properties, which comprises a layered bismuth-containing mixed metal oxide as a main phase and a bismuth containing composite oxide with pyrochlore structure as a secondary phase. The bismuth compound according to claim 9, wherein said layered bismuth-containing mixed metal oxide is represented by the following formula: Bi 2 (Sr a Ba b Ca c ) (Ta d Nb e ) 2 O 9 in which a, b, c, d and e range respectively from 0 to 1 in atomic ratio: a + b + c = 1; and d + e = 1. The bismuth compound according to claim 9, wherein said composite oxide with pyrochlore structure is represented by the following formula: (Bi, (Sr, Ba, Ca)) 2-x (Ta, Nb) 2 O 7±δ . The bismuth compound according to claim 9, wherein said bismuth-layered compound is represented by the following formula: Bi 2 SrTa 2 O 9 and said composite oxide with pyrochlore structure is represented by the following formula: (Bi, Sr) 2-x Ta 2 O 7±δ .

Inventors: AMI TAKAAKI, HIRONAKA KATSUYUKI, IKEDA YUJI, AMI, TAKAAKI, HIRONAKA, KATSUYUKI, IKEDA, YUJI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1996
Main classification: C23C16
IPC codes: H01G4, C01G29, C23C16, H01L21, C23C14, C04B35, C01G35, C01G33, H01L27