JP2007042784A - Metal oxide element and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a metal oxide element capable of obtaining stable operation, for example holding a more stable state, by using a material composed of a metal oxide. <P>SOLUTION: An insulating layer 102, a lower electrode layer 103 provided commonly, a plurality of metal oxide layers 104 that are approximately 30-200 nm thick composed of Bi, Ti, and O, and an upper electrode 105 provided for each metal oxide layer 104 are provided on a substrate 101 made of single-crystal silicon. Adjacent oxide layers 104 are separated by an insulating separation layer 106 made of tantalum pentoxide. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a metal oxide device using a metal oxide thin film having characteristics such as having ferroelectric characteristics.

  Conventionally, many semiconductor devices have been used for memories. As one of these, DRAM (Dynamic Random Access Memory) is widely used. A unit storage element (hereinafter referred to as a memory cell) of a DRAM is composed of one capacitor and one MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor), and the state of charge stored in the capacitor of the selected memory cell The memory operation is performed by reading the voltage change corresponding to 1 as “0” or “1” of the digital signal.

  However, the DRAM has a disadvantage that the data stored in the capacitor must be retained while being energized because the charge stored in the capacitor decreases with time. Further, in the DRAM, the state of the capacitor charge changes every time data is read out, so rewriting is required. These problems are major limitations in developing a memory device that operates at high speed with low power consumption, which is necessary in the ubiquitous service society.

  At present, as a high-speed and non-volatile memory, there are a ferroelectric memory (FeRAM: Ferroelectric RAM) using a polarization of a ferroelectric, a ferromagnetic memory (MRAM: Magnetoresist RAM) using a magnetic resistance of a ferromagnetic material, and the like. It has attracted attention and is actively studied. Among them, FeRAM has already been put into practical use, and if various problems can be solved, it is expected that flash memory and logic DRAM can be replaced.

Among ferroelectric materials, FeRAM mainly uses oxide ferroelectrics. Oxide ferroelectrics include perovskite structures such as BaTiO ₃ and PbTiO ₃ , pseudo-ilmenite structures such as LiNbO ₃ and LiTaO ₃ , tungsten such as PbNb ₂ O ₆ and Ba ₂ NaNb ₅ O _15. -Bismuth layer-structure ferroelectric (BLSF) such as Bronze (TB) structure (Tumgsten-bronze), SrBi ₂ Ta ₂ O ₉ , Bi ₄ Ti ₃ O ₁₂ , Pyrochlore such as Pb ₂ Nd ₂ O ₇ Classified into structure (Pyrochlore).

Among these, lead-based ferroelectrics represented by Pb (Zr, Ti) O ₃ (PZT) have become mainstream in practical use. However, lead-containing materials and lead oxides are materials regulated by the Occupational Safety and Health Act, and there are concerns about impacts on ecology and an increase in environmental burden. For this reason, in Europe and the United States, it is becoming an object of regulation from the viewpoint of ecological knowledge and pollution prevention.

  Due to the necessity of reducing environmental impact in recent years, ferroelectric materials that are lead-free (lead-free) and comparable to the performance of lead-based ferroelectrics are attracting worldwide attention. Among these, lead-free perovskite ferroelectrics and bismuth layered layers Structural ferroelectrics (BLSF) are promising. Bismuth layered structure ferroelectrics have a large characteristic in polarization characteristics, the amount of polarization changes by about 10 times depending on the direction of the orientation axis, and the deterioration due to the number of times the polarization is inverted is less, and the fatigue characteristics are better than those of the Pb system. There are reports that it is excellent. However, it is also true that the bismuth layer structure ferroelectric has a smaller amount of polarization than lead-based ferroelectrics and has many problems in both the film forming method and the processing method (see Non-Patent Document 1).

  FeRAM expected as a substitute for flash memory is mainly classified into a stack type and an FET type. The stack type is also called a one-transistor one-capacitor FeRAM, and there are a stack type capacitor, a planar type capacitor, and a three-dimensional type capacitor. In these structures, “0” and “1” of the memory are read using the fact that the amount of current flowing through the transistor changes depending on the direction of polarization of the ferroelectric in the capacitor. In addition, since the polarization of the ferroelectric can be maintained without energization, FeRAM is also non-volatile. However, FeRAM has a drawback in that when data is read, polarization reversal may be accompanied, resulting in a destructive read operation. In addition, since FeRAM has a large area occupied by one memory cell, high integration is not easy.

  In contrast to the above-described stack type FeRAM, the FET type FeRAM is expected as the next-generation FeRAM. The FET type FeRAM is also called a one-transistor type FeRAM. From this structure, an MFS (Metal-ferroelectric-semiconductor) type FeRAM, MOSFET in which a ferroelectric film is arranged in place of the gate electrode of the MOSFET and the gate insulating film in the channel region. Metal-ferroelectric-metal-insulator-semiconductor (MFMIS) type FeRAM in which a ferroelectric film is disposed on the gate electrode of MFIS, and MFIS (Metal in which a ferroelectric film is disposed between the gate electrode of the MOSFET and the gate insulating film) There is a one-transistor FeRAM such as a -ferroelectric-insulator-semiconductor) type FeRAM (see Non-Patent Document 2).

  These FeRAMs are obtained by applying ferroelectric polarization to the operation of the MOSFET. Depending on the state of polarization, there are cases where a channel is formed on the semiconductor surface immediately below the gate insulating film and when it is not formed. The memory operation is realized by reading out the current value between the source and the drain at this time and taking it out as an electric digital signal “0” or “1”.

  The FET-type FeRAM is expected to operate at high speed because non-destructive reading is possible because the amount of polarization of the ferroelectric material does not change even when data is read from the principle of operation. Further, since the occupied area can be reduced as compared with the one-transistor one-capacitor type FeRAM, it is advantageous for high integration.

  However, in the configuration described above, the ferroelectric layer is formed on the semiconductor. However, as is well known, it is very difficult to form the ferroelectric layer on the semiconductor. For example, when a semiconductor substrate such as Si is used, the sol-gel method or metal-organic chemical vapor deposition (MOCVD) method, which is often used for ferroelectric film formation, forms a film at a high temperature. Therefore, the surface of the semiconductor is oxidized or altered. As a result, an unnecessary oxide film or defect is formed at the interface, which causes the memory characteristics to be greatly deteriorated.

  Actually, the oxide film at the interface generates a depolarization electric field that prevents the polarization of the ferroelectric material from being held, so that the retention characteristics of the memory are remarkably deteriorated. In addition, the formation of defects increases the leakage current from the gate to the channel, thereby degrading the ON / OFF ratio of the transistor. In order to solve such problems, a structure in which an insulating film having a high dielectric constant is sandwiched between a ferroelectric and a semiconductor has been proposed, but the influence of a depolarizing electric field cannot be ignored, and a long-term There are many reports that polarization maintenance is very difficult.

  As is apparent from the above, formation of a ferroelectric thin film on a substrate is very important in order to realize an FeRAM that is attracting attention as a next-generation memory. To date, various forming apparatuses and various thin film forming methods have been tried. For example, in addition to the sol-gel method and MOCVD method described above, pulsed laser deposition (PLD), high-frequency sputtering (also called rf-sputtering, RF sputtering, or magnetron sputtering), ECR sputtering (Electron) cyclotron resonance sputtering).

  In the chemical solution deposition method such as the sol-gel method, a ferroelectric substrate is dissolved in an organic solvent and applied to a substrate, and this coating film is repeatedly sintered to form a ferroelectric layer having a predetermined thickness. It is a method of forming. The sol-gel method is simple and can form a film in a relatively large area, but there are many problems such as wettability with the substrate to be applied and contamination due to the solvent remaining in the formed film. Has a drawback.

  The MOCVD method has attracted much attention as a ferroelectric film forming method that can form a film with good crystallinity over a large area and has excellent step coverage characteristics. However, since an organic solvent is used to supply the source gas, contamination by carbon atoms in the film becomes a serious problem. The handling of the gas to be used is not easy, and the apparatus becomes very large.

  Regarding the purity and composition of the thin film to be formed, the PLD method is an effective film forming method. In this method, a thin film is formed by depositing atoms, ions, and clusters emitted on a substrate by ablating a target of a ferroelectric material with a powerful laser light source such as an excimer laser. In the PLD method, since a thin film having relatively good crystallinity can be formed, there is great interest. However, since the area irradiated with the laser is small, a large in-plane distribution is generated in the thin film formed on the substrate, and it is not easy to form a film with a large area. Therefore, from an industrial viewpoint such as mass production, the current PLD method is a very disadvantageous method.

  In contrast to the various film forming methods described above, a sputtering method (also simply referred to as a sputtering method) has attracted attention as a method for forming a ferroelectric film. Sputtering has become one of the promising film forming apparatuses and methods because the surface irregularity (surface morphology) of the deposited film is relatively good without using high-risk gas or toxic gas. Yes.

  In the RF sputtering method conventionally used, an oxide ferroelectric is deposited using a sintered body of a target compound as a target. However, when sputtering is performed using argon as an inert gas and oxygen as a reactive gas, oxygen is not sufficiently taken into the ferroelectric thin film formed on the substrate, and a ferroelectric thin film with good film quality is obtained. There was a problem that it was not possible. For this reason, the sputtering method described above has required annealing in oxygen after the film is formed.

  On the other hand, as a method for improving the quality of the sputtered film, plasma is generated by electron cyclotron resonance (ECR), and a plasma flow created by using the divergent magnetic field of the plasma is irradiated on the substrate, and at the same time, between the target and the ground. There is an ECR sputtering method in which a film is deposited on a substrate by applying a high-frequency or negative DC voltage, attracting and colliding ions in a plasma flow generated by ECR with a target, and performing sputtering.

  Plasma using ECR has excellent characteristics such as discharge at a low gas pressure (about 0.01 Pa), control of ion energy in a low energy (about several tens of eV) region, and a high ionization rate. The ions in the ECR plasma sputter the target material due to the negative charge applied to the target, and give appropriate energy to the raw material particles sputtered and flying on the substrate to cause a binding reaction between the raw material particles and oxygen. It is considered that the quality of the deposited film is improved. Therefore, the ECR sputtering method is characterized in that a high-quality film can be formed at a low substrate temperature, and the surface morphology is extremely excellent. This effectiveness is particularly demonstrated in the formation of a gate insulating film (see Patent Document 1 and Patent Document 2).

In addition, some studies on the formation of a ferroelectric thin film using the ECR sputtering method have been reported (see Patent Documents 3 and 4). They report on the production of ferroelectrics containing barium or strontium. A method for producing Bi ₄ Ti ₃ O ₁₂ by ECR sputtering has also been reported (see Non-Patent Document 2).

  In contrast to the situation surrounding the memory as described above, the memory is not realized by the effect of changing the state of the semiconductor (forming a channel) by the amount of polarization of the ferroelectric material, but formed directly on the semiconductor substrate. A technique for changing the resistance value of the ferroelectric layer and, as a result, realizing a memory function has been proposed (see Patent Document 5). The resistance value of the ferroelectric layer is controlled by applying a voltage between the electrodes.

Japanese Patent No. 2814416 Japanese Patent No. 2779997 Japanese Patent Laid-Open No. 10-152397 JP-A-10-152398 JP-A-7-263646 Supervised by Tadashi Shiogama, “Development and Application of Ferroelectric Materials”, CM Publishing Masumoto et al., Applied Physics Letter, 58, 243, 1991 (Appl. Phys. Lett., 58, 243, (1991).

  However, the structure proposed in Patent Document 5 has a structure in which a ferroelectric layer is provided on a semiconductor in the same manner as immediately below the gate electrode of the MFS type FeRAM. Therefore, in the conventional device, not only is it difficult to form a high-quality ferroelectric layer on the semiconductor, which is the biggest problem in the manufacturing process of the MFS type FeRAM, but also semiconductor oxidation between the semiconductor and the ferroelectric layer is difficult. As a result, the generation of a depolarizing electric field and the occurrence of many defects greatly affect the characteristics, and it is expected that long-term data retention is impossible. Actually, in the conventional element, only a holding time of about 2 minutes is achieved, and rewriting of data is forced in about 1 minute. Further, the ON / OFF ratio as a memory is about 3, which is not sufficient.

  Further, the current-voltage hysteresis observed in the above-described element is supposed to occur because electrons or holes are captured (trapped) by a defect generated at the interface between the semiconductor substrate and the ferroelectric layer. For this reason, in Patent Document 5, it is preferable that the material in contact with the ferroelectric is not a metal but a semiconductor substrate with few carriers. In the case where there are a large number of carriers such as metal, the electrical conduction becomes dominant, and the trapping effect at the interface becomes inconspicuous. In order to prevent this, the semiconductor substrate plays a role of controlling the number of carriers, and is an indispensable element in the structure of Patent Document 5. However, when the trapping phenomenon at the interface causes the hysteresis of the current-voltage characteristic, the retention time of the memory is about the dielectric relaxation time, so that a long-term memory retention cannot be expected in principle.

  The present invention has been made to solve the above-described problems, and a metal that can be stably operated using a material composed of a metal oxide, such as a more stable state retention. An object is to provide an oxide device.

  A metal oxide element according to the present invention includes a plurality of elements formed on a substrate having at least a metal oxide layer and a first electrode and a second electrode connected to the metal oxide layer, and adjacent elements. A portion provided between the metal oxide layers and in contact with the metal oxide layer at least includes an insulating separation layer made of tantalum oxide such as tantalum pentoxide, and the metal oxide layer includes at least two metals It is what is. Accordingly, reaction at the interface between the metal oxide layer and the insulating separation layer is suppressed.

  In the metal oxide element, the resistance value of the metal oxide layer is changed by an applied electric signal, and the metal oxide layer has a first resistance value when a voltage higher than the first voltage value is applied. The first state is a second state having a second resistance value lower than the first resistance value by applying a voltage equal to or lower than a second voltage value having a polarity different from that of the first voltage. Here, the metal oxide layer includes a base layer composed of at least a first metal and oxygen, and crystals having a stoichiometric composition of the first metal, the second metal, and oxygen, and is dispersed in the base layer. What is necessary is just to have at least a plurality of fine crystal grains. The base layer may be composed of a first metal, a second metal, and oxygen, and the composition ratio of the second metal may be smaller than the stoichiometric composition. The base layer may include a first metal, a second metal, and oxygen columnar crystals. The metal oxide layer may be disposed in contact with the base layer, and may include a metal oxide single layer that is composed of at least a first metal and oxygen and is at least one of columnar crystals and amorphous. In this case, the metal oxide single layer is composed of the first metal, the second metal, and oxygen, and the composition ratio of the second metal is smaller than the stoichiometric composition. In addition, the metal oxide single layer does not include microcrystalline grains. Note that the first metal is titanium, the second metal is bismuth, and the base layer may be in an amorphous state including a layer containing excess titanium as compared with the stoichiometric composition.

  In the metal oxide element, the insulating separation layer may be made of tantalum oxide such as tantalum pentoxide. The insulating separation layer is composed of a protective layer made of tantalum oxide such as tantalum pentoxide provided in contact with the metal oxide layer, and an insulating layer provided between adjacent elements through the protective layer. May be.

  The method for manufacturing a metal oxide element according to the present invention includes a step of forming a metal oxide layer on a substrate, and a first electrode formed in contact with one surface of the metal oxide layer. And connecting to the metal oxide layer and the metal oxide layer on the substrate by the step of forming the second electrode by contacting the other surface side of the metal oxide layer and the step of forming the second electrode A method of manufacturing a metal oxide element in which a plurality of elements including at least a first electrode and a second electrode are formed, the metal oxide layer being provided between metal oxide layers of adjacent elements A step in which an insulating separation layer composed of tantalum oxide, such as tantalum pentoxide, is formed, and the metal oxide layer includes an inert gas and an oxygen gas supplied at a predetermined composition ratio. A first plasma comprising: a first metal and a second gold A negative vice is applied to the target composed of the following, particles generated from the first plasma collide with the target to cause a sputtering phenomenon, and a material constituting the target is deposited on the substrate, so that at least the first A state in which at least a base layer composed of metal and oxygen and a plurality of microcrystalline grains made of crystals of a stoichiometric composition of the first metal, the second metal, and oxygen are dispersed in the base layer The first plasma is an electron cyclotron resonance plasma generated by electron cyclotron resonance and given kinetic energy by a diverging magnetic field, and the substrate is heated to a predetermined temperature.

  In the metal oxide element manufacturing method, the insulating mask layer made of an insulating material is formed on the substrate, and the insulating mask layer is provided at a predetermined interval. Through the process of making a resist pattern with an opening formed and through the insulating mask layer by isotropic etching using the resist pattern as a mask, the area of the resist pattern is wider than the opening of the resist pattern. A step of forming a hole in the insulator mask layer, a step of forming a metal oxide layer inside the through hole, and after the metal oxide layer is formed, the insulator mask layer and You may make it provide at least the process made into the state in which the resist pattern was formed.

  As described above, according to the present invention, the insulating separation layer disposed between the elements made of the metal oxide layer is configured such that the portion in contact with the metal oxide layer is made of tantalum oxide such as tantalum pentoxide. As a result, it is possible to obtain an excellent effect of obtaining a metal oxide element capable of obtaining a stable operation using a material composed of a metal oxide, such as a more stable state retention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view schematically showing a configuration example of a metal oxide element in an embodiment of the present invention. The element shown in FIG. 1 has, for example, an insulating layer 102 on a substrate 101 made of single crystal silicon, a lower electrode layer 103 provided in common, and a film thickness of about 30 to 200 nm composed of Bi, Ti and O. A plurality of metal oxide layers 104 and an upper electrode 105 provided for each metal oxide layer 104 are provided. Further, in the configuration example shown in FIG. 1, elements are separated between adjacent metal oxide layers 104 by an insulating isolation layer 106 made of tantalum oxide such as tantalum pentoxide. In FIG. 1, three elements including a lower electrode layer 103, a metal oxide layer 104, and an upper electrode 105 are shown.

  The substrate 101 may be made of any conductive material such as a semiconductor, an insulator, or a metal. When the substrate 101 is made of an insulating material, the insulating layer 102 is not necessary. Further, when the substrate 101 is made of a conductive material, the insulating layer 102 and the lower electrode layer 103 may not be provided. In this case, the substrate 101 made of the conductive material serves as a lower electrode.

The lower electrode layer 103 and the upper electrode 105 may be made of transition metal including noble metals such as platinum (Pt), ruthenium (Ru), gold (Au), and silver (Ag). The lower electrode layer 103 and the upper electrode 105 are made of titanium nitride (TiN), hafnium nitride (HfN), strontium ruthenate (SrRuO ₂ ), zinc oxide (ZnO), tin lead (ITO), lanthanum fluoride (LaF). ₃ ) transition metal nitrides, compounds such as oxides and fluorides, and a composite film in which these are laminated.

  A specific example of the configuration of the element shown in FIG. 1 will be described. For example, the lower electrode layer 103 is a ruthenium film having a thickness of 10 nm, and the metal oxide layer 104 is a metal made of Bi and Ti having a thickness of 40 nm. It is an oxide film, and the upper electrode 105 is made of gold. Note that, as described above, the structures of the substrate 101 and the insulating layer 102 are not limited to this, and other materials can be appropriately selected as long as the electrical characteristics are not affected.

Next, the metal oxide layer 104 composed of the metal oxide thin film according to the present invention will be described in more detail. Metal oxide layer 104, Bi ₄ Ti ₃ compared to the stoichiometric composition of O ₁₂ in the base layer comprising a layer containing excess titanium, particle size 3 comprising a crystal of Bi ₄ Ti ₃ O ₁₂ A plurality of fine crystal grains of about ˜15 nm are dispersed. This has been confirmed by observation with a transmission electron microscope. The base layer may be TiO _x with a bismuth composition of approximately zero. In other words, the base layer is a layer in which any metal is less than the stoichiometric composition in a metal oxide composed of two metals.

  According to the element using such a metal oxide layer 104, a functional element in which two states are maintained can be realized as described below. The characteristics of the functional element shown in FIG. 1 will be described. This characteristic has been investigated by applying a voltage between the lower electrode layer 103 and the upper electrode 105. When a voltage was applied between the lower electrode layer 103 and the upper electrode 105 by a power source, and the current when the voltage was applied was observed with an ammeter, the result shown in FIG. 2 was obtained. In FIG. 2, the vertical axis represents the current density obtained by dividing the current value by the area.

  Hereinafter, FIG. 2 will be described, and the operation principle of the device using the metal oxide layer 104 of the present invention will be described. However, the voltage values and current values described here are those observed with actual elements. Therefore, this phenomenon is not limited to the following numerical values. Other numerical values may be observed depending on the material and thickness of the film actually used for the element and other conditions.

  FIG. 2 shows the current flowing through the metal oxide layer 104 when the voltage applied to the upper electrode is increased from zero to positive and then returned to zero, further decreased in the negative direction, and finally returned to zero. It represents the hysteresis characteristic drawn by the value. First, when a voltage is gradually applied to the upper electrode 105 in the positive direction from 0 V, the positive current flowing through the metal oxide layer 104 is relatively small (about 0.1 μA at 0.1 V).

  However, when the voltage exceeds 0.5 V, the positive current value starts to increase rapidly. When the voltage is further increased to about 1V and then the positive voltage is decreased, the positive current value further increases from 1V to about 0.4V despite the decrease in the voltage value. When the voltage value is about 0.4 V or less, the current value also starts to decrease, but the positive current at this time is more likely to flow than before, and the current value is about 0.1 μA at 0.1 V. (About 10 times the previous). When the applied voltage is returned to zero, the current value also becomes zero.

  Next, a negative voltage is applied to the upper electrode 105. In this state, when the negative voltage is small, the previous history is taken over and a relatively large negative current flows. However, when a negative voltage is applied to about -0.5V, the negative current begins to decrease suddenly. Thereafter, even if a negative voltage is applied to about -1V, the negative current value continues to decrease. Finally, when the negative voltage applied from -1V to 0V is decreased, both the negative current values are further decreased and return to zero. In this case, a negative current hardly flows and is about 0.1 μA at −0.1V.

It can be interpreted that the hysteresis of the current flowing in the metal oxide layer 104 as described above appears due to the resistance value of the metal oxide layer 104 being changed by the voltage applied to the upper electrode 105. By applying a positive voltage V _W1 having a certain magnitude or more, the metal oxide layer 104 transitions to a “low resistance state” (data “1”) in which a current hardly flows. On the other hand, by applying a negative voltage V _W0 having a certain magnitude, the metal oxide layer 104 is considered to transition to a “high resistance state” (data “0”) in which current does not easily flow.

The metal oxide layer 104 has two stable states, a low resistance state and a high resistance state, and each state is maintained unless a positive or negative voltage exceeding a certain level is applied. To do. The value of V _W1 is about + 1V, the value of V _W0 is about −1V, and the resistance ratio between the high resistance state and the low resistance state is about 10-100. By using the phenomenon in which the resistance of the metal oxide layer 104 is switched by a voltage as described above, a memory element that is non-volatile and capable of nondestructive read operation can be realized by the functional element shown in FIG.

When the element shown in FIG. 1 using the metal oxide layer 104 is used as a memory element, when a DC voltage is used, the memory operation is performed as follows. First, a positive voltage greater than or equal to V _W1 is applied to cause the metal oxide layer 104 to transition to a low resistance state. This corresponds to writing data “1” as a memory. This data “1” can be read by observing the current value J _R1 at the read voltage V _R. As V _R , it is important to select a value that is as small as possible so that the state does not change and that a resistance ratio sufficiently appears (in the above example, about 0.1 V is appropriate). Thereby, it becomes possible to read many times without destroying the low resistance state, that is, the data “1”.

On the other hand, by applying a negative voltage having a magnitude greater than or equal to V _W0 , the metal oxide layer 104 can transition to the high resistance state, and data “0” can be written. The reading in this state can be performed by observing the current value J _R0 at the reading voltage V _R (J _R1 / J _R0 ≈10-100). Further, in a state where no current is applied between the electrodes, the metal oxide layer 104 has a non-volatility in order to maintain each state, and it is not necessary to apply a voltage except during writing and reading. This element can also be used as a switch element for controlling current.

Here, data retention characteristics of the element shown in FIG. 1 will be described. For example, the positive voltage V _W1 is applied to the upper electrode to make a transition to the low resistance state (data “1”) shown in FIG. 2, and then the read voltage V _R is applied to observe the current value J _R1 . Next, the transition to the high resistance state by applying a negative voltage voltage V _W0 to the upper electrode, and a state of writing data "0", thereafter, the read voltage V _R to the upper electrode at predetermined time intervals applied Then, the current value J _R0 is observed. Although the observed ON / OFF ratio has a tendency to gradually decrease with time, it is in a range where data can be sufficiently discriminated. From this result, the ON / OFF ratio after 1000 minutes is expected to be about 21, and can be determined at this point. Thus, it can be seen that the element shown in FIG. 1 has a holding time of at least 1000 minutes. In the above embodiment, the applied voltage is a direct current. However, the same effect can be obtained by applying a pulse voltage having an appropriate width and strength.

  When a plurality of elements having the metal oxide layer 104 having the characteristics described above are integrated (arranged) as shown in FIG. 1, the elements are separated by an insulating material, and the leakage current between the elements is reduced. The stability of the element is increased. For this purpose, silicon oxide is generally used as an insulating material for separation. However, the metal contained in the metal oxide layer 104 reacts with silicon to form an interfacial oxide layer or an interfacial reaction layer, which may reduce the electric withstand voltage and increase the leakage current.

  FIG. 3 is a photograph showing the result of observing the state of the interface between the metal oxide layer formed on the silicon substrate and the silicon substrate with a transmission electron microscope. As shown in FIG. 3, an interface oxide layer 303 and an interface reaction layer 304 are observed at the interface between the silicon substrate 301 and the metal oxide layer 302. A similar state occurs in the element shown in FIG. 1. For example, when the metal oxide layer 104 is composed of Bi, Ti, and O, when the silicon oxide layer contacts the metal oxide layer 104, the interface In this case, an interface reaction layer in which titanium and silicon react is formed, which lowers the electric withstand voltage and causes an increase in leakage current.

  In contrast to the above, according to the structure for isolating the element shown in FIG. 1, the insulating isolation layer 106 made of tantalum pentoxide is used, and the metal oxide layer 104 is in contact with a layer of tantalum oxide such as tantalum pentoxide. It will be in the state. As a result, an interface layer in which silicon and metal react with each other is not formed at the interface with the insulating separation layer 106, so that a decrease in electric withstand voltage and an increase in leakage current are not caused.

Next, an example of a method for manufacturing the metal oxide element shown in FIG. 1 will be briefly described. First, as shown in FIG. 4 (a), a main surface resistivity plane orientation (100) is 1～2Omu ^- providing a substrate 101 made of cm p-type silicon, over the acid the surface of the substrate 101 Washing with a mixed solution of hydrogen oxide water, pure water and dilute hydrogen fluoride water, followed by drying. Next, the insulating layer 102 is formed on the cleaned and dried substrate 101. In the formation of the insulating layer 102, for example, an ECR sputtering apparatus is used, pure silicon (Si) is used as a target, and argon (Ar) and oxygen gas are used as plasma gases. In this state, the metal mode insulating layer 102 made of Si—O molecules is formed so as to cover the surface.

For example, Ar gas is introduced at a flow rate of about 20 sccm into a plasma generation chamber set to an internal pressure of the order of 10 ⁻⁵ Pa, and the internal pressure is set to about 10 ⁻³ to 10 ⁻² Pa. An Ar plasma is generated in the plasma generation chamber by supplying a magnetic field and a 2.45 GHz microwave (about 500 W) to satisfy the electron cyclotron resonance condition. 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 generated as described above is emitted from the plasma generation chamber to the processing chamber side by the divergent magnetic field of the magnetic coil. Moreover, high frequency power (for example, 500 W) of 13.56 MHz is supplied from a high frequency power source to a silicon target disposed at the outlet of the plasma generation chamber. As a result, Ar ions collide with the silicon target, causing a sputtering phenomenon, and Si particles pop out. The Si particles jumping out of the silicon target reach the surface of the substrate 101 made of silicon together with the plasma released from the plasma generation chamber and the oxygen gas introduced and activated by the plasma, and are oxidized by the activated oxygen. It becomes silicon dioxide. As described above, the insulating layer 102 made of silicon dioxide and having a thickness of, for example, about 100 nm can be formed on the substrate 101 (FIG. 4A).

  Note that the insulating layer 102 is designed to insulate so that the voltage does not leak to the substrate 101 when a voltage is applied to each electrode to be formed later, and the desired electrical characteristics are not affected. For example, a silicon oxide film formed by oxidizing the surface of a silicon substrate by a thermal oxidation method may be used as the insulating layer 102. The insulating layer 102 only needs to maintain insulating properties, and may be made of an insulating material other than silicon oxide. The thickness of the insulating layer 102 is not limited to 100 nm, and may be thinner or thicker. Also good. The insulating layer 102 is not heated to the substrate 101 in the above-described film formation by ECR sputtering, but the film may be formed while the substrate 101 is heated.

After forming the insulating layer 102 as described above, a lower electrode layer is formed by forming a ruthenium film on the insulating layer 102 by the same ECR sputtering method using pure ruthenium (Ru) as a target. 103 is formed. The formation of the Ru film will be described in detail. In an ECR sputtering apparatus using a target made of Ru, for example, first, a silicon substrate on which an insulating layer is formed is heated to 400 ° C., and in the plasma generation chamber, for example, at a flow rate of 7 sccm. Ar gas, which is a rare gas, is introduced. In addition, Xe gas is introduced at a flow rate of 5 sccm, for example, and the inside of the plasma generation chamber is set to a pressure of, for example, 10 ⁻² to 10 ⁻³ Pa.

  Next, a magnetic field under an electron cyclotron resonance condition is applied to the plasma generation chamber, and then a 2.45 GHz microwave (for example, 500 W) is introduced into the plasma generation chamber, and an Ar and Xe ECR plasma is generated in the plasma generation chamber. And The generated ECR plasma is emitted from the plasma generation chamber to the processing chamber side by the divergent magnetic field of the magnetic coil. Further, a high frequency power of 13.56 MHz (for example, 500 W) is supplied to the ruthenium target disposed at the outlet of the plasma generation chamber. As a result, a sputtering phenomenon occurs, and Ru particles are ejected from the ruthenium target. Ru particles that protrude from the ruthenium target reach the surface of the insulating layer 102 of the substrate 101 and are deposited.

  As a result, a state is obtained in which the lower electrode layer 103 having a thickness of, for example, about 10 nm is formed on the insulating layer 102 (FIG. 4A). The lower electrode layer 103 makes it possible to apply a voltage to the metal oxide layer 104 when a voltage is applied to the upper electrode 105 to be formed later. Therefore, the lower electrode layer 103 may be composed of other than ruthenium as long as it has conductivity. For example, the lower electrode layer 103 may be composed of platinum. However, it is known that when a platinum film is formed on silicon dioxide, it is easy to peel off. To prevent this, a laminated structure in which a platinum layer is formed via a titanium layer, a titanium nitride layer, or a ruthenium layer, and the like. do it. Further, the thickness of the lower electrode layer 103 is not limited to 10 nm, and may be thicker or thinner.

  By the way, when the Ru film is formed by the ECR sputtering method as described above, the substrate 101 is heated to 400 ° C., but it is not necessary to heat it. However, in the case where heating is not performed, the adhesion of ruthenium to silicon dioxide decreases, so that peeling may occur. In order to prevent this, it is preferable to form a film by heating the substrate.

  After the lower electrode layer 103 is formed as described above, the silicon oxide layer 401 is formed in the same manner as the formation of the insulating layer 102 described above. Next, a resist pattern 402 is formed on the silicon oxide layer 401. The resist pattern 402 is a mask pattern having an opening in a region where the pattern of the metal oxide layer 104 shown in FIG. 1 is arranged. Next, by using the resist pattern 402 as a mask, the silicon oxide layer 401 is isotropically etched, so that an undercut is formed in the resist pattern 402 as shown in FIG. The layer 401 is selectively etched away, and the opening 403 is formed. Accordingly, the end portion of the resist pattern 402 is in a state having a ridge extending to the region of the opening 403.

  Next, an ECR sputtering method using argon (Ar) and oxygen gas as a plasma gas using a target made of an oxide sintered body (Bi-Ti-O) with a ratio of Bi and Ti of 4: 3. As illustrated in FIG. 4C, the metal oxide layer 104 is formed on the lower electrode layer 103 inside the opening 403. Inside the opening 403, the sputtered particles that are flying in are difficult to reach the portion hidden by the wrinkles of the resist pattern 402, and the film forming speed is reduced. As a result, the metal oxide layer 104 having a trapezoidal cross-sectional pattern is formed inside the opening 403. At this time, the metal oxide film 104a is also deposited on the resist pattern 402, but is removed simultaneously with the removal of the resist pattern 402, as will be described later.

The formation of the metal oxide layer 104 will be described in detail. First, the substrate 101 is heated in a range of 300 ° C. to 700 ° C. In addition, Ar gas, which is a rare gas, is introduced into the plasma generation chamber at a flow rate of 20 sccm, for example, and set to a pressure of, for example, 10 ⁻³ Pa to 10 ⁻² Pa. In this state, a magnetic field having an electron cyclotron resonance condition is applied to the plasma generation chamber, and then a 2.45 GHz microwave (for example, 500 W) is introduced into the plasma generation chamber. By introducing this microwave, ECR is introduced into the plasma generation chamber. It is assumed that plasma is generated.

  The generated ECR plasma is emitted from the plasma generation chamber to the processing chamber side by the divergent magnetic field of the magnetic coil. Moreover, high frequency power (for example, 500 W) of 13.56 MHz is supplied to the sintered compact target arrange | positioned at the exit of the plasma generation chamber. As a result, Ar particles collide with the sintered compact target to cause a sputtering phenomenon, and Bi particles and Ti particles jump out.

Bi particles and Ti particles jumping out from the sintered compact target are formed on the surface of the heated lower electrode layer 103 together with the ECR plasma released from the plasma generation chamber and the oxygen gas activated by the released ECR plasma. It reaches and is oxidized by the activated oxygen. The oxygen (O ₂ ) gas as the reaction gas is introduced separately from the Ar gas, as will be described later. For example, it is introduced at a flow rate of 1 sccm, for example. Although the sintered compact target contains oxygen, oxygen deficiency in the deposited film can be prevented by supplying oxygen. By forming the film by the ECR sputtering method described above, for example, a state in which the metal oxide layer 104 having a film thickness of about 40 nm is formed is obtained (FIG. 4C).

  As described above, after the metal oxide layers 104 are formed in the openings 403, the resist pattern 402 and the silicon oxide layer 401 are removed, so that each of the metal oxide layers 104 is formed on the lower electrode layer 103. As a result, a state is obtained in which a plurality of metal oxide layers 104 are formed in a state of being separated from each other. As described above, the metal oxide film 104a on the resist pattern 402 is also removed at the same time as the resist pattern 402 is removed. According to the manufacturing method described above, first, the metal oxide layer 104 having a trapezoidal cross-sectional shape can be easily formed.

  In general, a metal oxide containing bismuth and titanium is not easy to be finely processed by dry etching. For this reason, conventionally, wet etching has been used, but shape controllability such as occurrence of underetching and undercutting is not good. Another problem is that notching is formed at the lower end of the pattern due to electrical non-uniformity at the interface with the lower layer. On the other hand, according to the method using the lift-off described above, it is easy to form a fine pattern. Further, since the above-described manufacturing method does not use plasma dry etching, problems such as plasma damage are also suppressed.

  Note that the formed metal oxide layer 104 may be irradiated with ECR plasma of an inert gas and a reactive gas to improve the film quality. The reactive gas is not limited to oxygen gas, and nitrogen gas, fluorine gas, and hydrogen gas can be used. This improvement in film quality is also applicable to the formation of the insulating layer 102. Further, after the metal oxide layer 104 is formed under a lower temperature condition of 300 ° C. or lower, the formed metal oxide layer 104 is annealed (heat treatment) in an appropriate gas atmosphere such as an oxygen atmosphere. The film quality characteristics may be greatly improved.

Next, as shown in FIG. 4D, a tantalum pentoxide layer 404 is formed in a state where each metal oxide layer 104 is covered by ECR sputtering using pure tantalum (Ta) as a target. And Note that the tantalum pentoxide layer 404 may be made of tantalum oxide. As described below, a metal mode film made of Ta—O molecules is formed to form a tantalum pentoxide layer 404. The formation of the metal mode film by Ta-O molecules will be described in detail. In the target ECR sputtering apparatus made of tantalum, first, Ar gas that is a rare gas is introduced into the plasma generation chamber, for example, at a flow rate of 25 sccm. Is set to a pressure on the order of 10 ⁻³ Pa, for example. In addition, a magnetic field of electron cyclotron resonance condition is applied to the plasma generation chamber by supplying a coil current of 28 A, for example, to the magnetic coil.

  In addition, for example, a 2.45 GHz microwave (for example, 500 W) is supplied from the microwave generation unit and introduced into the plasma generation chamber, and Ar plasma is generated in the plasma generation chamber by the introduction of the microwave. State. The generated plasma is emitted from the plasma generation chamber to the processing chamber side where the substrate is disposed by the divergent magnetic field of the magnetic coil. Further, high frequency power (for example, 500 W) is supplied from a high frequency electrode supply unit to a target disposed at the outlet of the plasma generation chamber.

  As a result, Ar particles collide with the target to cause a sputtering phenomenon, and Ta particles jump out of the target. Ta particles that have jumped out of the target reach the substrate 101 together with the plasma released from the plasma generation chamber and the oxygen gas introduced and activated by the plasma, and are oxidized by the activated oxygen and oxidized to tantalum oxide (pentapentoxide). Tantalum). As a result, as shown in FIG. 4D, a state in which the tantalum pentoxide layer 404 is formed is obtained.

  Next, by polishing and polishing the tantalum pentoxide layer 404 above the metal oxide layer 104 in a flattened state by, for example, a chemical mechanical polishing (CMP) method or the like, FIG. As shown, the upper surface of the metal oxide layer 104 is exposed, and the insulating separation layer 106 is formed in a state where the space between the metal oxide layers 104 is filled.

  After the metal oxide layer 104 is formed as described above, as shown in FIG. 4F, the upper electrode 105 made of Au having a predetermined area is formed on the metal oxide layer 104. Thus, the element shown in FIG. 1 is obtained. The upper electrode 105 can be formed by a well-known lift-off method and gold deposition by a resistance heating vacuum deposition method. The upper electrode 105 may be made of another metal material such as Ru, Pt, or TiN or a conductive material. Note that when Pt is used, there is a possibility of peeling because of poor adhesion, so that a structure such as Ti-Pt-Au is difficult to peel off, and a patterning process such as photolithography or lift-off process is performed on the structure. It is necessary to form an electrode having an area.

  The formation of each layer by ECR sputtering described above may be performed using an ECR sputtering apparatus as shown in FIG. The ECR sputtering apparatus shown in FIG. 5 will be described. First, a processing chamber 501 and a plasma generation chamber 502 communicating therewith are provided. The processing chamber 501 communicates with an evacuation device (not shown), and the inside of the processing chamber 501 is evacuated together with the plasma generation chamber 502 by the evacuation device. The processing chamber 501 is provided with a substrate holder 504 to which the film formation target substrate 101 is fixed. The substrate holder 504 is inclined at a desired angle by an inclined rotation mechanism (not shown) and is rotatable. By tilting and rotating the substrate holder 504, it is possible to improve the in-plane uniformity of the film and the step coverage with the material to be deposited.

  A ring-shaped target 505 is provided so as to surround the opening region in the opening region into which plasma from the plasma generation chamber 502 in the processing chamber 501 is introduced. The target 505 is placed in a container 505 a made of an insulator, and the inner surface is exposed in the processing chamber 501. In addition, a high frequency power source 522 is connected to the target 505 via a matching unit 521 so that, for example, a high frequency of 13.56 MHz can be applied. When the target 505 is a conductive material, a DC negative voltage may be applied. In addition, the target 505 may be not only a circular shape but also a polygonal state as viewed from above.

  The plasma generation chamber 502 communicates with the vacuum waveguide 506, and the vacuum waveguide 506 is connected to the waveguide 508 through a quartz window 507. The waveguide 508 communicates with a microwave generation unit (not shown). In addition, a magnetic coil (magnetic field forming means) 510 is provided around the plasma generation chamber 502 and at the top of the plasma generation chamber 502. These microwave generator, waveguide 508, quartz window 507, and vacuum waveguide 506 constitute microwave supply means. There is a configuration in which a mode converter is provided in the middle of the waveguide 508.

The operation example of the ECR sputtering apparatus in FIG. 5 will be described. First, the inside of the processing chamber 501 and the plasma generation chamber 502 is evacuated from 10 ⁻⁵ Pa to 10 ⁻⁴ Pa, and then an inert gas is introduced from an inert gas introduction unit 511. In addition, a reactive gas such as oxygen gas is introduced from the reactive gas introduction unit 512, and the pressure in the plasma generation chamber 502 is set to about 10 ⁻³ to 10 ⁻² Pa, for example. In this state, a magnetic field of 0.0872 T is generated in the plasma generation chamber 502 from the magnetic coil 510, and then a 2.45 GHz microwave is introduced into the plasma generation chamber 502 through the waveguide 508 and the quartz window 507. Then, an electron cyclotron resonance (ECR) plasma is generated.

  The ECR plasma forms a plasma flow in the direction of the substrate holder 504 by the divergent magnetic field from the magnetic coil 510. Among the generated ECR plasma, electrons penetrate through the target 505 by the divergent magnetic field formed by the magnetic coil 510 and are extracted toward the substrate 101 and are irradiated onto the surface of the substrate 101. At the same time, positive ions in the ECR plasma are drawn out to the substrate 101 side so as to neutralize the negative charge due to electrons, that is, weaken the electric field, and are irradiated onto the surface of the layer being formed. Thus, while each particle is irradiated, some of the positive ions are combined with electrons to become neutral particles.

  In the thin film forming apparatus of FIG. 5, the microwave power supplied from a microwave generation unit (not shown) is once branched in the waveguide 508, and is supplied to the vacuum waveguide 506 above the plasma generation chamber 502. The generation chamber 502 is coupled from the side through a quartz window 507. By doing so, it becomes possible to prevent the scattered particles from adhering to the quartz window 507 from the target 505, and the running time can be greatly improved.

Next, the characteristics of the Bi ₄ Ti ₃ O ₁₂ film formed by ECR sputtering will be described in more detail. The inventors have carefully observed the formation of the Bi ₄ Ti ₃ O ₁₂ film by using the ECR sputtering method, so that the composition of the formed Bi ₄ Ti ₃ O ₁₂ film is controlled depending on the temperature and the introduced oxygen flow rate. I found what I could do. In this sputtering film formation, an oxide sintered body target (Bi ₄ Ti ₃ O _x ) formed so that bismuth and titanium have a composition of 4: 3 is used. FIG. 6 is a characteristic diagram showing changes in the film formation rate with respect to the introduced oxygen flow rate when Bi ₄ Ti ₃ O ₁₂ is formed using the ECR sputtering method. Further, FIG. 6 shows the results under the condition where single crystal silicon is used for the substrate and the substrate temperature is 420 ° C.

  FIG. 6 shows that when the oxygen flow rate is as small as 0 to 0.5 sccm, the oxygen flow rate is 0.5 to 0.8 sccm, and the oxygen flow rate is divided into regions when the oxygen flow rate is 0.8 sccm or later. About this characteristic, the inductively coupled plasma emission (ICP) analysis and the cross-sectional observation of the transmission electron microscope were implemented, and the formed film | membrane was investigated in detail. As a result of the investigation, when the oxygen flow rate is as small as 0 to 0.5 sccm, Ti—O containing almost no Bi is used as the main component even though a Bi—Ti—O sintered target is used as the target. It was found that a crystalline film was formed. This oxygen region is referred to as oxygen region A.

Further, it was found that when the oxygen flow rate was about 0.8 to ₃ sccm, the film was formed of microcrystals or columnar crystals having a stoichiometric composition of Bi ₄ Ti ₃ O ₁₂ . This oxygen region is defined as oxygen region C. Further, it was found that when the oxygen flow rate is 3 sccm or more, the film has a high Bi ratio and deviates from the stoichiometric composition of Bi ₄ Ti ₃ O ₁₂ . This oxygen region is defined as oxygen region D. Furthermore, it has been found that when the oxygen flow rate is 0.5 to 0.8 sccm, the film formation is intermediate between the oxygen region A film and the oxygen region C. This oxygen region is defined as oxygen region B.

It has not been known so far that the composition of the supplied oxygen is changed into four regions, and Bi ₄ Ti ₃ O using a Bi—Ti—O sintered target by ECR sputtering. It can be said that this is a characteristic film formation characteristic when ₁₂ is formed. By grasping this area and controlling the film formation, a film having a desired composition and quality can be obtained. Further, from another strict measurement result, it was found that the film formation condition in which the obtained film clearly shows ferroelectricity is the oxygen region C in which the stoichiometric composition can be realized.

  Next, the state of the bismuth titanium oxide thin film produced under the oxygen flow rate condition of α in oxygen region A, β in oxygen region B, and γ in oxygen region C in FIG. 6 will be described with reference to FIG. . FIG. 7 shows a result of observing a cross section of the produced thin film (including bismuth, titanium, and oxygen) with a transmission electron microscope. In FIG. 7, (a), (b), (c), (d) are micrographs, and (a ′), (b ′), (c ′), (d ′) are the respective states. It is the schematic diagram which showed typically. First, under the condition α where the oxygen flow rate is 0, as shown in FIGS. 7A and 7A ′, the entire film is composed of columnar crystals. When the elemental composition state of the thin film produced under the condition α is analyzed by an EDS (energy dispersive X-ray spectroscopy) method, it is found that bismuth is not contained and this film is titanium oxide.

Next, under the condition β in which the oxygen flow rate is 0.5 sccm, as shown in FIGS. 7B and 7B ′, the produced thin film is separated into two layers, and Bi ₄ Ti ₃ O ₁₂ A metal oxide single layer 144 containing excess titanium compared to the stoichiometric composition of the base layer 141 and a base layer 141 containing excess titanium compared to the stoichiometric composition of Bi ₄ Ti ₃ O ₁₂ Thus, it is confirmed that a plurality of microcrystalline grains 142 having a grain diameter of about ₃ to 15 nm made of Bi ₄ Ti ₃ O ₁₂ crystals are dispersed in the base layer 141. The base layer 141 is in an amorphous state.

  Next, under the condition γ where the oxygen flow rate is 1 sccm, it is confirmed that the microcrystalline grains 142 are dispersed in the base layer 141 as shown in FIGS. 7C and 7C ′. . However, both the base layer 141 and the metal oxide single layer 144 are in a state where almost no bismuth is present. In the state described above, the temperature condition during film formation is 420 ° C. FIG. 7D and FIG. 7D ′ are observation results of the film manufactured under the condition that the oxygen flow rate is 1 sccm, but the temperature conditions during film formation are different as described below.

Bi ₄ Ti ₃ O ₁₂ film characteristics of which are formed by ECR sputtering also relates to a film formation temperature. FIG. 8 shows changes in the deposition rate and refractive index with respect to the substrate temperature. FIG. 8 shows changes in the deposition rate and refractive index of the oxygen flow rate corresponding to the oxygen region A, oxygen region C, and oxygen region D shown in FIG. As shown in FIG. 8, it can be seen that the deposition rate and the refractive index both change with temperature.

First, paying attention to the refractive index, it can be seen that the same behavior is exhibited for any of the oxygen region A, the oxygen region C, and the oxygen region D. Specifically, in a low temperature region up to about 250 ° C., the refractive index is as small as about 2, indicating amorphous characteristics. In the intermediate temperature range from 300 ° C to 600 ° C, the refractive index is about 2.6, which is close to the bulk reported in the paper, and Bi ₄ Ti ₃ O ₁₂ is being crystallized. I understand. Regarding these numerical values, see, for example, Japanese Journal Applied Physics, Yamaguchi et al., No. 37, page 5166, 1998 (Jpn. J. Appl. Phys., 37, 5166 (1998)). I want you to.

However, in a temperature region exceeding about 600 ° C., it is considered that the refractive index increases and the surface morphology (surface irregularities) increases, resulting in a change in crystallinity. This temperature is lower than 675 ° C., which is the Curie temperature of Bi ₄ Ti ₃ O ₁₂ , but energy is supplied by irradiating the surface of the substrate on which the film is formed with ECR plasma, and the substrate temperature rises to increase oxygen temperature. If deterioration of crystallinity such as defects occurs, it is considered that there is no contradiction in the above results. Looking at the temperature dependence of the deposition rate, it can be seen that each oxygen region exhibits the same behavior. Specifically, the film formation rate increases with temperature up to about 200 ° C. However, the film formation rate is drastically reduced in the range of about 200 ° C. to 300 ° C.

When the temperature reaches about 300 ° C., the deposition rate becomes constant up to 600 ° C. The film formation rate in each oxygen region at this time was about 1.5 nm / min for the oxygen region A, about 3 nm / min for the oxygen region C, and about 2.5 nm / min for the oxygen region D. From the above results, the temperature suitable for forming the crystal film of Bi ₄ Ti ₃ O ₁₂ is a region where the refractive index is close to the bulk and the film formation rate is constant. To 600 ° C.

The state of the metal oxide layer changes depending on the temperature conditions at the time of film formation described above, and when the film formation temperature condition is increased to 450 ° C. under the oxygen flow rate condition in the state shown in FIG. As shown in d) and FIG. 7 (d ′), the dimension is 3 to 15 nm in the plurality of columnar crystal parts 143 having a grain size of about 20 to 40 nm made of columnar crystals of Bi ₄ Ti ₃ O _12. A degree of fine crystal grains 142 is observed. In this state, the columnar crystal part 143 corresponds to the base layer 141 shown in FIGS. 7C and 7C ′. In any of the films shown in FIG. 7, the peak of the (117) axis of Bi ₄ Ti ₃ O ₁₂ is observed by XRD (X-ray diffraction method) measurement. Further, in the observation with the transmission electron microscope described above, it is confirmed by electron beam diffraction with respect to the fine crystal grains 142 that the fine crystal grains 142 have the (117) plane of Bi ₄ Ti ₃ O ₁₂ .

Generally, in a material exhibiting ferroelectricity, the crystallinity cannot be maintained at a temperature equal to or higher than the Curie temperature, and ferroelectricity is not exhibited. For example, the configured ferroelectric material from the Bi and Ti and oxygen, such as Bi ₄ Ti ₃ O _12, the Curie temperature is around 675 ° C.. For this reason, when the temperature is close to 600 ° C. or higher, energy given from the ECR plasma is also added, and oxygen deficiency is likely to occur, so that crystallinity is deteriorated and ferroelectricity is hardly exhibited.

Further, the analysis by X-ray diffraction revealed that the Bi ₄ Ti ₃ O ₁₂ film formed with the oxygen flow rate C in the above temperature range was a (117) oriented film. It was confirmed that the Bi ₄ Ti ₃ O ₁₂ film formed under such conditions exhibits a sufficient electric withstand voltage exceeding 2 MV / cm when the thickness is about 100 nm. As described above, by using ECR sputtering and forming the Bi ₄ Ti ₃ O ₁₂ film within the range shown in FIGS. 6 and 8, the composition and characteristics of the film can be controlled.

By the way, the state shown in FIG. 9 is also observed in the metal oxide layer 104. The metal oxide layer 104 shown in FIG. 9 includes a metal oxide single layer 144 containing excess titanium as compared with the stoichiometric composition of Bi ₄ Ti ₃ O ₁₂ and a plurality of microcrystalline grains 142 dispersed therein. It is a laminated structure with the base layer 141. The state shown in FIG. 9 is also confirmed by observation with a transmission electron microscope, similarly to the state shown in FIG. The state of each metal oxide layer described above varies depending on the state of the lower layer to be formed, the film formation temperature, and the oxygen flow rate during film formation. For example, the oxygen flow rate is shown in FIG. In the case of the β condition shown, it has been confirmed that the state shown in FIG.

  As described above, there are cases where the base layer is in an amorphous state and columnar crystals are observed within the range of film formation conditions where microcrystalline grains are observed. There is no change in the state, and the observed fine crystal grains have a size of about 3 to 15 nm. As described above with reference to FIG. 2, the metal oxide layer in the state where microcrystal grains are observed has two stable states, a low resistance state and a high resistance state. In the thin film in the state shown in FIG. 7 (a ′), the above two stable states cannot be obtained.

  Therefore, according to the metal oxide layer 104 in the state shown in FIGS. 7B to 7D ′ and FIG. 9, two states are maintained as described with reference to FIG. It is possible to realize an element. This characteristic is obtained in the film formed under the conditions of the oxygen regions B and C in FIG. 6 when the film is formed by ECR sputtering described above. Further, when paying attention to the film formation temperature condition shown in FIG. 8, the above characteristics are as described above under the temperature condition in which the film formation speed is decreased and stabilized, and the refractive index is increased and stabilized at about 2.6. A thin film having the above characteristics can be formed.

  In the above description, an oxide composed of a binary metal of bismuth and titanium has been described as an example. However, the characteristic that the two states are maintained is another metal composed of at least two metals and oxygen. It is considered that the oxide layer 104 can also be obtained. In a layer composed of at least two metals and oxygen, where one of the metals is less than the stoichiometric composition, a plurality of microcrystalline grains having a stoichiometric composition are present. If dispersed, it is considered that the characteristics described with reference to FIG. 2 appear.

For example, BaTiO ₃ , Pb (Zr, Ti) O ₃ , (Pb, La) (Zr, Ti) O ₃ , LiNbO ₃ , LiTaO ₃ , PbNb ₃ O ₆ , PbNaNb ₅ O ₁₅ , Cd ₂ Nb ₂ O ₇ , Even if the metal oxide layer 104 is composed of Pb ₂ Nb ₂ O ₇ , (Bi, La) ₄ Ti ₃ O ₁₂ , SrBi ₂ Ta ₂ O ₉ , any metal has a stoichiometric composition. As long as a plurality of microcrystalline grains having a stoichiometric composition are dispersed in a layer that is in a relatively small state, the same effect as the above-described embodiment can be obtained. Conceivable. For example, in the case of an oxide made of a binary metal of bismuth and titanium, lanthanum (La) or strontium (strontium) is added to the metal oxide layer 104 (La, Bi) TiO or (Sr, Bi) By making the state like TiO, it becomes possible to variably control the state of each resistance value.

  Next, another metal oxide element in the embodiment of the present invention will be described. FIG. 10 is a cross-sectional view schematically showing a configuration example of another metal oxide element in the embodiment of the present invention. The element shown in FIG. 10 has, for example, an insulating layer 102 on a substrate 101 made of single crystal silicon, a lower electrode layer 103 provided in common, and a film thickness of about 30 to 200 nm composed of Bi, Ti and O. A plurality of metal oxide layers 104 and an upper electrode 105 provided for each metal oxide layer 104 are provided. In the configuration example shown in FIG. 10, elements are separated between adjacent metal oxide layers 104 by a protective layer 161 made of tantalum oxide such as tantalum pentoxide and an insulating layer 107 made of silicon oxide. FIG. 10 shows three elements including the lower electrode layer 103, the metal oxide layer 104, and the upper electrode 105.

  In the element shown in FIG. 10, the side surface of each metal oxide layer 104 is covered with the protective layer 161, and the space between the metal oxide layers 104 covered with the protective layer 161 is filled with the insulating layer 107 to separate the elements. Has been. As shown in FIG. 10, higher insulating properties can be obtained by providing an insulating layer 107 made of silicon oxide in a portion for element isolation.

  Further, as shown in FIG. 11, an insulating layer 171 that fills between the metal oxide layers 104 is formed higher (thicker) than the metal oxide layer 104, and the upper electrodes 151 are separated by the insulating layer 171. You may make it. In this case, contact between the metal oxide layer 104 and the insulating layer 171 is suppressed by the protective layer 162 extending to the boundary portion between the upper surface of the metal oxide layer 104 and the upper surface of the upper electrode 151. In this configuration, as described below, the upper electrode 151 can be formed by forming a metal layer in the opening formed in the insulating layer 171.

  Hereinafter, an example of a method for manufacturing the metal oxide element having the configuration shown in FIG. 11 will be described. First, in the same manner as described with reference to FIGS. 4A to 4C, a plurality of pieces are separated from each other on the lower electrode layer 103 as shown in FIG. 12A. The metal oxide layer 104 is formed. Next, as shown in FIG. 12B, a protective layer 162 made of tantalum oxide such as tantalum pentoxide is formed on the lower electrode layer 103 including each metal oxide layer 104. The protective layer 162 may be formed in the same manner as the insulating separation layer 106 described above.

  Next, silicon oxide is deposited on the lower electrode layer 103 including each metal oxide layer 104 covered with the protective layer 162 by, for example, sputtering or CVD, as shown in FIG. The insulating layer 171 is formed. Next, as shown in FIG. 12D, a resist pattern 1201 having an opening in a region where the upper electrode 151 is formed is formed. The resist pattern 1201 can be formed by a known photolithography technique.

  Next, the insulating layer 171 and the protective layer 162 are selectively removed by dry etching using the formed resist pattern 1201 as a mask, and an opening is formed in this lower portion as shown in FIG. A part of the metal oxide layer 104 is exposed. Next, after the resist pattern 1201 was removed, an upper electrode 151 that was filled in the formed opening and was in contact with the upper surface of the metal oxide layer 104 was formed, as shown in FIG. In this state, a metal oxide element in which a separation structure is formed by the protective layer 162 and the insulating layer 171 similar to the structure shown in FIG. 11 can be obtained.

It is sectional drawing which shows typically the structural example of the metal oxide element in embodiment of this invention. 6 is a characteristic diagram showing a relationship between voltage and current in the metal oxide layer 104. FIG. It is a photograph which shows the result of having observed the state of the interface of the metal oxide layer and silicon substrate which were formed on the silicon substrate with the transmission electron microscope. It is process drawing explaining the example of a manufacturing method of the metal oxide element shown in FIG. It is a block diagram which shows the structural example of an ECR sputtering apparatus. The case of forming a Bi ₄ Ti ₃ O ₁₂ with an ECR sputtering is a characteristic diagram showing changes in deposition rate for the flow rate of oxygen introduced. It is explanatory drawing which shows the result of having observed the cross section of the layer of the metal oxide containing bismuth, titanium, and oxygen with the transmission electron microscope. It is the characteristic view which showed the film-forming speed | rate with respect to substrate temperature, and the change of refractive index. It is explanatory drawing which shows the result of having observed the cross section of the layer of the metal oxide containing bismuth, titanium, and oxygen with the transmission electron microscope. It is sectional drawing which shows typically the structural example of the other metal oxide element in embodiment of this invention. It is sectional drawing which shows typically the structural example of the other metal oxide element in embodiment of this invention. It is process drawing for demonstrating the example of a manufacturing method of the metal oxide element shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 ... Substrate, 102 ... Insulating layer, 103 ... Lower electrode layer, 104 ... Metal oxide layer, 105 ... Upper electrode, 106 ... Insulating separation layer.

Claims (16)

A plurality of elements formed on the substrate including at least a metal oxide layer and a first electrode and a second electrode connected to the metal oxide layer;
A portion provided between the metal oxide layers of the adjacent elements and in contact with the metal oxide layer at least includes an insulating separation layer made of tantalum oxide;
The metal oxide layer includes at least two metals. A metal oxide element, wherein: The metal oxide device according to claim 1, wherein
The metal oxide layer has a resistance value that changes according to an applied electric signal. The metal oxide device according to claim 2, wherein
The metal oxide layer is
When a voltage higher than the first voltage value is applied, the first state having the first resistance value is obtained.
The metal oxide element, wherein a second state having a second resistance value lower than the first resistance value is applied by applying a voltage equal to or lower than a second voltage value having a polarity different from that of the first voltage. In the metal oxide element according to any one of claims 1 to 3,
The metal oxide layer is
A base layer composed of at least a first metal and oxygen;
A metal oxide element comprising at least a plurality of microcrystalline grains made of a crystal having a stoichiometric composition of the first metal, the second metal, and oxygen, and dispersed in the base layer. The metal oxide device according to claim 4, wherein
The base layer is composed of the first metal, the second metal, and oxygen, and the composition ratio of the second metal is smaller than the stoichiometric composition. The metal oxide element. The metal oxide element according to claim 4 or 5,
The base layer includes a columnar crystal of the first metal, the second metal, and oxygen. In the metal oxide element according to any one of claims 4 to 6,
A metal oxide element, comprising: a metal oxide single layer which is disposed in contact with the base layer and is composed of at least the first metal and oxygen and which is at least one of a columnar crystal and an amorphous material. The metal oxide device according to claim 7, wherein
The metal oxide single layer is composed of the first metal, the second metal, and oxygen, and the composition ratio of the second metal is smaller than the stoichiometric composition. . The metal oxide element according to claim 7 or 8,
The metal oxide single layer does not contain the fine crystal grains. In the metal oxide element according to any one of claims 4 to 9,
The first metal is titanium, the second metal is bismuth, and the base layer is in an amorphous state composed of a layer containing excess titanium compared to the stoichiometric composition. Metal oxide element. In the metal oxide element according to any one of claims 1 to 10,
The insulating isolation layer is made of tantalum oxide. A metal oxide element, wherein: In the metal oxide element according to any one of claims 1 to 10,
The insulating separation layer is composed of a protective layer made of tantalum oxide provided in contact with the metal oxide layer, and an insulating layer provided between the elements adjacent to each other through the protective layer. There is provided a metal oxide element. In the metal oxide device according to any one of claims 1 to 12,
The metal oxide element, wherein the tantalum oxide is tantalum pentoxide. A step of forming a metal oxide layer on the substrate;
A step of bringing the first electrode into contact with one surface of the metal oxide layer; and
A step of forming a second electrode in contact with the other surface side of the metal oxide layer, the metal oxide layer on the substrate, a first electrode connected to the metal oxide layer, A method for producing a metal oxide element in which a plurality of elements each having at least two electrodes are formed,
A step of forming an insulating separation layer in which a portion provided between the metal oxide layers of the adjacent elements and contacting the metal oxide layer is formed of tantalum oxide;
The metal oxide layer is
From the first plasma, a first plasma composed of an inert gas and an oxygen gas supplied at a predetermined composition ratio is generated, and a negative bias is applied to a target composed of a first metal and a second metal. A base layer composed of at least the first metal and oxygen is formed by causing the generated particles to collide with the target to cause a sputtering phenomenon, and depositing a material constituting the target on the substrate, and the first metal , A second metal, and a crystal having a stoichiometric composition of oxygen, and formed in a state including at least a plurality of microcrystalline grains dispersed in the base layer,
The first plasma is an electron cyclotron resonance plasma generated by electron cyclotron resonance and given kinetic energy by a divergent magnetic field,
The method for producing a metal oxide element, wherein the substrate is heated to a predetermined temperature. In the manufacturing method of the metal oxide element according to claim 14,
A step of forming an insulator mask layer made of an insulating material on the substrate;
A step of forming a resist pattern having openings provided at predetermined intervals on the insulator mask layer; and
By etching the insulator mask layer by isotropic etching using the resist pattern as a mask, a through-hole having a larger area than the opening of the resist pattern is formed in the insulator mask layer. Process,
A step of forming the metal oxide layer inside the through hole;
And a step of forming the insulating mask layer and the resist pattern after the metal oxide layer is formed. The metal oxide element according to claim 14 or 15,
The said tantalum oxide is a tantalum pentoxide. The manufacturing method of the metal oxide element characterized by the above-mentioned.