JP4441646B2 - target - Google Patents

Description

The present invention relates to a target used for forming a Bi ₄ Ti ₃ O ₁₂ thin film by sputtering.

  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 held 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, a ferroelectric memory (FeRAM: Ferroelectric RAM) using a polarization of a ferroelectric material, a ferromagnetic memory (MRAM: Magnetoresist RAM) using a magnetic resistance of a ferromagnetic material, etc. 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. In particular, Bi ₄ Ti ₃ O ₁₂ has been studied as a material for a ferroelectric memory because it has the largest ferroelectricity among non-lead-based ferroelectric materials. However, Bi ₄ Ti ₃ O ₁₂ has a large characteristic in polarization characteristics, the amount of polarization changes by about 10 times depending on the direction of the orientation axis, and there is less deterioration due to the number of times the polarization is reversed, and it is more fatigued than the Pb system. It has also been reported that it has excellent characteristics. However, Bi ₄ Ti ₃ O ₁₂ has a smaller amount of polarization than lead-based ferroelectrics, and there are many problems in both the film forming method and the processing method (see Non-Patent Document 1).

Meanwhile, in order to realize the device using a Bi ₄ Ti ₃ O ₁₂ is formed of Bi ₄ Ti ₃ O ₁₂ thin film onto the substrate is very important. As a thin film formation technique of a ferroelectric material such as Bi ₄ Ti ₃ O ₁₂ , various forming apparatuses and various thin film forming methods have been tried so far. For example, sol-gel method, MOCVD method, pulsed laser deposition (PLD), high frequency sputtering method (also called rf-sputtering, RF sputtering method or magnetron sputtering method), and ECR sputtering method (Electron cyclotron resonance sputtering) ) And the like.

  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 the inert gas and oxygen as the 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 to the substrate. 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 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, thereby causing 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).

Japanese Patent No. 2814416 Japanese Patent No. 2779997 Japanese Patent Laid-Open No. 10-152397 JP-A-10-152398 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).

By the way, when forming a Bi ₄ Ti ₃ O ₁₂ thin film by a sputtering method, an oxide target composed of bismuth, titanium, and oxygen is generally used for reasons such as easy manufacture. However, the above-described oxide target has a low sputter yield, and it is not easy to increase the deposition rate. In addition, when the oxide target described above is used, the sputtered particles from the target are oxides such as Bi-O and Ti-O, so that they adhere incompletely to the inner wall of the apparatus, and these fly onto the substrate. There is also a problem of becoming dust.

The present invention has been made to solve the above-described problems, and bismuth such as Bi ₄ Ti ₃ O ₁₂ and titanium are sputtered at a higher speed in a state where generation of dust and the like is suppressed. It is an object to provide a target capable of forming a thin film of a metal oxide containing

The target according to the present invention is composed of a sintered body in which a powder made of Bi fine particles with bismuth metal bonded and a powder made of a plurality of Ti fine particles with titanium metal bonded are compression-molded. Therefore, this target is in a state not containing oxygen. Here, the Bi fine particles and the Ti fine particles may be mixed so as to have a composition substantially equivalent to “Bi ₄ Ti ₃ ”. Further, it is sufficient that Bi fine particles and Ti fine particles are mixed and fired at a temperature lower than the melting point of Bi. Bi fine particles may be formed with a particle size of 10 to 50 μm, and Ti fine particles may be formed with a particle size of 10 to 50 μm.

As described above, according to the present invention, the target is composed of a sintered body obtained by compressively molding a powder made of Bi fine particles and a powder made of Ti fine particles. Therefore, Bi ₄ Ti is formed by sputtering. An excellent effect is obtained that a thin film of a metal oxide containing bismuth such as ₃ O ₁₂ and titanium can be formed at a higher deposition rate in a state where generation of dust and the like is suppressed.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view schematically showing a configuration example of the target 101 in the embodiment of the present invention. A target 101 shown in FIG. 1 is a sintered body composed of Bi fine particles 102 in which bismuth (Bi) is metal-bonded and Ti fine particles 103 in which titanium (Ti) is metal-bonded. A target 101 shown in FIG. 1 is composed of, for example, a powder made of Bi fine particles having a particle size of about 10 to 50 μm and a powder made of Ti fine particles having a particle size of about 10 to 50 μm with a composition of “Bi ₄ Ti ₃ ”. These are mixed so that they are filled into a predetermined container and molded into a predetermined shape. Air (oxygen) is mixed in and sealed in a state where bismuth and titanium are oxidized and no oxide is formed, and Bi dissolves. It can be formed by firing at a temperature (270 ° C. or less) and an appropriate pressure to such an extent that it does not occur. Bi particles 102 and Ti particles 103 need not have the same diameter. The composition of the target shown in FIG. 1 formed as described above was analyzed using an inductively coupled plasma emission spectrometer. The Bi content was 57 to 61 wt% and the Ti content was 39 to 43 wt%. It is confirmed that the composition is “Bi ₄ Ti ₃ ” (equal to).

  When the cross-sectional state of the target 101 shown in FIG. 1 is observed with a microscope, it is formed in a state where there are few voids and Bi and Ti are in close contact with each other, as shown in FIG. In the photograph of FIG. 2 (a), the portion that appears blackest is a void. As described above, according to the target 101 shown in FIG. 1, it is easy to form in a state where the gap is very small and the density is 98% or more. Moreover, since Bi and Ti in a pure metal state can be used as materials, according to the target 101 shown in FIG. 1, it is easy to improve the purity of the target. On the other hand, as shown in FIG. 2B, many voids are observed in the oxide target composed of bismuth, titanium, and oxygen that has been used conventionally, and Bi oxide and voids are Ti. It is covered with an oxide layer. In the photograph of FIG. 2B, the portion that appears blackest is the gap. Thus, in the target of the conventional configuration, there are many voids and the density is 50% or less. Moreover, since it is comprised from an oxide, it is difficult to improve the purity of the target which consists of a Bi and Ti oxide.

  The target shown in FIG. 1 can be used for an ECR sputtering apparatus as shown in FIG. 3, for example. The ECR sputtering apparatus shown in FIG. 3 will be described. First, a processing chamber 201 and a plasma generation chamber 202 communicating with the processing chamber 201 are provided. The processing chamber 201 communicates with an evacuation device (not shown), and the inside of the processing chamber 201 is evacuated together with the plasma generation chamber 202 by the evacuation device. The processing chamber 201 is provided with a substrate holder 204 to which a film formation target substrate is fixed. The substrate holder 204 can be rotated by a rotation mechanism 203. In the apparatus shown in FIG. 3, since the plasma generation chamber 202 is inclined with respect to the substrate holder 204, the substrate holder 204 is rotated with respect to the direction of the plasma flow drawn from the plasma generation chamber 202. Will do. With this configuration, it is possible to improve the in-plane uniformity of the film and the step coverage with the material to be deposited.

  Further, a ring-shaped target 205 is provided so as to surround the opening region in the opening region into which the plasma from the plasma generation chamber 202 in the processing chamber 201 is introduced. This target 205 is the target 101 having the configuration shown in FIG. The target 205 is placed in a container 205 a made of an insulator, and the inner surface is exposed in the processing chamber 201. Further, a high frequency power source 222 is connected to the target 205 via a matching unit 221 so that, for example, a high frequency of 13.56 MHz can be applied. When the target 205 is a conductive material, a negative DC voltage may be applied. Note that the target 205 may be not only a circular shape but also a polygonal state as viewed from above.

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

The operation example of the ECR sputtering apparatus of FIG. 3 will be described. First, the inside of the processing chamber 201 and the plasma generation chamber 202 is evacuated from 10 ⁻⁵ Pa to 10 ⁻⁴ Pa, and then an inert gas is introduced from the inert gas introduction unit 211. In addition, a reactive gas such as oxygen gas is introduced from the reactive gas introduction unit 212, and the inside of the plasma generation chamber 202 is brought to a pressure of 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 202 from the magnetic coil 210, and then a 2.45 GHz microwave is introduced into the plasma generation chamber 202 through the waveguide 208 and the quartz window 207. Then, an electron cyclotron resonance (ECR) plasma is generated.

  The ECR plasma forms a plasma flow in the direction of the substrate holder 204 by the divergent magnetic field from the magnetic coil 210. Among the generated ECR plasma, electrons penetrate through the target 205 by the divergent magnetic field formed by the magnetic coil 210 and are drawn to the substrate (substrate holder 204) side, and are irradiated onto the surface of the substrate. At the same time, positive ions in the ECR plasma are drawn out to the substrate side so as to neutralize negative charges due to electrons, that is, to 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. 3, microwave power supplied from a microwave generator (not shown) is once branched in the waveguide 208, and plasma is supplied to the vacuum waveguide 206 above the plasma generation chamber 202. The generation chamber 202 is coupled from the side through a quartz window 207. By doing so, it becomes possible to prevent the scattered particles from adhering to the quartz window 207 from the target 205, and the running time can be greatly improved.

Next, the relationship between the oxygen supply amount and the film formation rate by the ECR sputtering apparatus as shown in FIG. 3 using the target shown in FIG. 1 will be described. FIG. 4 shows the results when the substrate temperature is room temperature (about 20 ° C.) and when the substrate temperature is 420 ° C. When the flow rate of oxygen is higher than 6 sccm, the deposition rate is reduced. On the other hand, when the oxygen flow rate is between 4.5 sccm and 5.5 sccm, the deposition rate is higher than other oxygen flow rate conditions. Note that sccm is a unit of flow rate and indicates that a fluid at 0 ° C. and 1 atm flows 1 cm ³ per minute.

  Here, the relationship between the oxygen flow rate and the film formation rate described above will be considered. In the ECR sputtering method, since ions in the plasma flow are used for sputtering, sputtering can be performed without generating plasma with high frequency applied to the target itself. As is well known, since ECR plasma can be generated at a low pressure, the ECR sputtering method can form a film at a lower pressure (a region close to a molecular flow) than a general sputtering method. When a metal oxide thin film is formed by ECR sputtering having such characteristics, the deposition rate increases initially as the flow rate of the supplied oxygen gas increases, and the deposition rate decreases when a certain flow rate value is exceeded. .

  The reason why the deposition rate decreases when the oxygen gas flow rate exceeds a certain flow rate value is that the surface of the target is oxidized and sputtering is difficult to occur. Since the deposition rate increases as the sputtering rate increases, the deposition rate also decreases as the sputtering rate decreases as the surface of the target becomes difficult to sputter. As the oxygen gas flow rate increases, the rate at which the target surface is oxidized becomes larger than the rate at which the target surface is removed by sputtering, and thus the sputtering rate is considered to decrease. Details of this phenomenon are well-known phenomena in the reactive sputtering method. For details of this phenomenon, see, for example, Satoshi Kanehara, “Sputtering Phenomenon” (Tokyo University Press), pages 120-132.

  In the region where the oxygen gas flow rate is low, the target surface is not sufficiently oxidized, so titanium and bismuth are sputtered, and it is considered that particles rich in titanium and bismuth reach the substrate. However, in the case of the ECR sputtering method, since ions in the plasma flow of argon and oxygen gas are poured onto the substrate, the oxidation of titanium or bismuth particles is assisted or promoted on the substrate surface, so that the stoichiometric composition is achieved. It is believed that near metal oxides are deposited.

  As described above, a region where a large deposition rate can be obtained in a region where the oxygen flow rate is small, in other words, a region where the sputtering rate is large is a film formation region called a metal mode. In contrast, a region where the deposition rate is small in a region where the oxygen flow rate is large, in other words, a region where the sputtering rate is small is a film formation region called an oxide mode. A transition region is a region where the deposition rate changes greatly between the metal mode region and the oxide mode region. In this transition region, it can be said that the state is similar to a metal mode in which the oxidation of the target surface is not completely advanced. Therefore, if the film formation (film formation) mode is strictly defined, the region after the film formation rate is reduced in the region where the oxygen flow rate is large is the oxide mode, and the region where the oxygen flow rate below this is the transition region Including metal mode.

  When the oxygen flow rate is increased, the refractive index of the formed metal oxide thin film made of Bi and Ti changes as shown in FIG. In particular, when the substrate temperature is about room temperature, the refractive index decreases when the oxygen flow rate is greater than 5.5 sccm. In this region, a metal oxide thin film having a larger number of oxygen atoms than the stoichiometric composition is formed. On the other hand, if the substrate temperature is set to a high temperature of 420 ° C., the refractive index does not decrease even if the supply amount of oxygen is increased.

Next, the substrate temperature during film formation will be described. As shown in FIG. 6, the higher the substrate temperature, the lower the film formation rate, and the film formation rate is constant in a region exceeding 400 ° C. On the other hand, the refractive index becomes higher as the substrate temperature becomes higher at around 300 ° C., and from a region exceeding 400 ° C., the refractive index is about 2.5, which is considered to be a film of Bi ₄ Ti ₁₃ O _12. Stable to value. FIG. 6 shows the results when the supply amount of oxygen is 5.0 sccm.

Next, the difference in foreign matter generation state between the formation of a film by ECR sputtering using the target shown in FIG. 1 and the formation of a film using a conventional target will be described. The film formation conditions were a substrate temperature condition of 420 ° C., and a metal oxide thin film made of Bi and Ti was formed on a silicon substrate. As shown in FIG. 7A, when the target shown in FIG. 1 is used, almost no foreign matter is observed. On the other hand, when a conventional oxide target composed of bismuth, titanium, and oxygen is used, as shown in FIG. 7B, 241 particles / mm ² and many foreign matters are observed. Note that the observation with these microscopes is the result of observation in a dark field using epi-illumination from an oblique direction.

By the way, the target having the structure shown in FIG. 1 according to the present embodiment is a sintered body composed of a plurality of Bi fine particles 102 and a plurality of Ti fine particles 103 having a composition ratio of “Bi ₄ Ti ₃ ”. It is equivalent to an alloy having bismuth: titanium = 57: 42 (atom%). However, an alloy having the above composition ratio of bismuth and titanium cannot be obtained as shown below. As shown in the phase diagram of FIG. 8 (binary phase equilibrium diagram), in the solid phase in an equilibrium state where Bi is 57 atom%, it is separated into Bi and BiTi ₂ as (Bi) + BiTi _2. There is no condition for “Bi ₄ Ti ₃ ”. Thus, it turns out that the target which does not contain oxygen cannot be obtained from the alloy of bismuth and titanium.

It is typical sectional drawing which shows roughly the structural example of the target 101 in embodiment of this invention. It is the photograph (a) which shows the result of having observed the cross-sectional state of the target 101 shown in FIG. 1 with a microscope, and the photograph (b) which shows the result of having observed the cross-sectional state of the conventional oxide sintered compact target with a microscope. It is a block diagram which shows the structural example of the ECR sputtering apparatus which can apply the target shown in FIG. It is a characteristic view shown about the relationship between the oxygen supply amount by the ECR sputtering apparatus using the target shown in FIG. 1, and the film-forming speed | rate. It is a characteristic view shown about the relationship between the oxygen supply amount by the ECR sputtering apparatus using the target shown in FIG. 1, and the refractive index of the obtained film | membrane. It is a characteristic view shown about the relationship between the oxygen supply amount by the ECR sputtering apparatus using the target shown in FIG. 1, a film-forming speed | rate, and a refractive index. It is a photograph which shows the observation result of the difference in foreign matter generation by the difference in a target. It is a two-component phase equilibrium diagram of bismuth and titanium.

Explanation of symbols

101 ... target, 102 ... Bi fine particles, 103 ... Ti fine particles.

Claims (4)

  A target comprising: a sintered body obtained by compression-molding a powder composed of Bi fine particles to which bismuth is metal-bonded and a powder composed of a plurality of Ti fine particles to which titanium is metal-bonded. The target of claim 1, wherein
The target, wherein the Bi fine particles and the Ti fine particles are mixed so as to have a composition of Bi ₄ Ti ₃ . In the target according to claim 1 or 2,
A target characterized in that the Bi fine particles and Ti fine particles are mixed and fired at a temperature lower than the melting point of Bi. In the target according to any one of claims 1 to 3,
The Bi fine particles are formed to have a particle size of 10 to 50 μm, and the Ti fine particles are formed to have a particle size of 10 to 50 μm.