JP3720270B2 - Method for producing oxide crystalline film - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a method for producing an oxide crystalline film.To the lawRelated.
[0002]
[Prior art]
  Ferroelectric thin films have many functions such as spontaneous polarization, high dielectric constant, electro-optic effect, piezoelectric effect, pyroelectric effect, etc., and thus are applied to a wide range of device development. For example, the pyroelectricity is used for an infrared linear array sensor, the piezoelectricity is used for an ultrasonic sensor, the electro-optic effect is used for a waveguide type optical modulator, and the high dielectric property is used. Are used in various fields, such as DRAM (dynamic random access memory) and MMIC (monolithic microwave integrated circuit) capacitors.
[0003]
  Among these developments of applied devices, the development of ferroelectric non-volatile memory (FRAM) that operates at high density and high speed in combination with semiconductor memory technology is prosperous with the progress of thin film formation technology in recent years. It is. Nonvolatile memories using ferroelectric thin films not only replace conventional nonvolatile memories but also SRAM (static random random numbers) due to their characteristics such as high-speed writing / reading, low-voltage operation, and high writing / reading resistance. As a memory that can be replaced with an access memory or a DRAM, research and development have been actively conducted for practical use.
[0004]
  For such device development, residual polarization (P_r) Large and coercive electric field (E_c) Is small, has a low leakage current, and requires a material having high resistance to repeated polarization inversion. In addition, when forming a thin film, it is preferable that the deposition time is short in order to improve throughput.
[0005]
  Conventionally, as a ferroelectric material used for these applications, PZT (lead zirconate titanate, Pb (Ti_xZr_1-x) O_ThreeThe mainstream is an oxide material having a perovskite structure represented by However, a material containing lead as its constituent element, such as PZT, has a high vapor pressure of lead or its oxide, so that lead evaporates during film formation, causing defects in the film, or severe cases. A pinhole has been formed. As a result, there is a drawback that a fatigue phenomenon occurs in which the magnitude of spontaneous polarization decreases when the leakage current increases or the polarization inversion is repeated.
[0006]
  In particular, regarding the fatigue phenomenon, considering the replacement of a ferroelectric random access memory (FRAM) with a ferroelectric nonvolatile memory, 10¹⁵Since it has to be ensured that there is no change in characteristics even after polarization reversal, the development of a ferroelectric thin film without fatigue has been desired.
[0007]
  On the other hand, in recent years, research and development of bismuth layered structure compound materials have been conducted as ferroelectric materials for FRAM. Bismuth layered structure compound material was discovered in 1959 by Smolenskii et al. (G. A. Smolenskii, V. A. Isupov and A. I. Agranovskaya, Soviet Phys. Solid State, 1, 149 (1959)), Subsequent studies were then made in detail by Subkarao (EC Subarao, J. Phys. Chem. Solids, 23,665 (1962)).
[0008]
  Recently, in International Publication No. WO93 / 10542, JP 7-502149, Carlos A. Paz de Araujo et al. Found that this bismuth layered structure compound thin film is suitable for application to ferroelectric and high dielectric integrated circuits.¹²It has reported excellent fatigue properties that the property does not change after polarization reversal more than once.
[0009]
  The bismuth layered structure compound has the following chemical formula
                        Bi₂A_m-1B_mO_{3m + 3}
      A: Selected from Na, K, Pb, Ca, Sr, Ba, Bi
      B: Selected from Fe, Ti, Nb, Ta, W, Mo
      m: natural number
Indicated by The crystal structure of the bismuth layered structure compound is (Bi₂O₂) 2+ layers and (A_m-1B_mO_{3m + 1}) It is a structure in which 2-layers are alternately stacked. That is, the basis of the crystal structure is (m−1) ABOs._ThreeThe upper and lower sides of the layered perovskite layer (Bi)₂O₂) A structure in which 2+ layers are sandwiched. In addition, what is selected as A and B here is not necessarily single.
[0010]
  As a typical example of such a bismuth layered structure compound material, SrBi₂(Ta_xNb_1-x)₂O₉(X = 0 to 1). This SrBi₂(Ta_xNb_1-x)₂O₉(X = 0 to 1) Films can be formed by physical methods such as vacuum vapor deposition, sputtering, laser ablation, etc., or by using an organic metal compound as a starting material, which is thermally decomposed and oxidized to produce strong oxide. A chemical method such as a sol-gel method for obtaining a dielectric, MOD (Metal Organic Decomposition) method, or MOCVD (Metal Organic Chemical Vapor Deposition) method is used.
[0011]
  Among the film formation methods described above, the sputtering method is a film formation method that has a proven track record in mass production for various materials, and it has been developed by various research institutes because it enables large-area film formation and high reproducibility. Yes. Further, the MOCVD method is excellent in step coverage and has a possibility of low-temperature film formation. Therefore, the MOCVD method is promising particularly in the case of achieving high integration of FRAM, and research and development have recently become active. On the other hand, the chemical solution deposition method such as the sol-gel method or the MOD method can use a raw material solution that can be homogeneously mixed at the atomic level. Therefore, the composition control is easy and the reproducibility is excellent. It is widely used because it can form a film with a large area at normal pressure without necessity, and is industrially inexpensive. In particular, the above SrBi₂(Ta_xNb_1-x)₂O₉The MOD method is used as a suitable method for forming the (x = 0 to 1) film.
[0012]
  Conventionally, as a method for manufacturing a ferroelectric thin film using the MOD method, there is one described in Japanese Patent Publication No. 2658878, for example. In the method for manufacturing a ferroelectric thin film disclosed in Japanese Patent Publication No. 2658878, a ferroelectric thin film or a dielectric thin film is manufactured by performing the following steps (1) to (3).
[0013]
  (1) Film formation process
  A solution using an organic metal salt as a raw material is applied and formed on a substrate by spin coating.
[0014]
  (2) First heat treatment step
  In order to remove the solvent, the alcohol produced by the reaction in the step (1) and the residual moisture from the film, the film is heated and dried at 250 ° C. for 10 minutes to form an amorphous film.
[0015]
  (3) Second heat treatment step
  Heat treatment is performed at a high temperature of 600 to 850 ° C. for about 10 minutes in order to thermally decompose and remove the organic components in the film.
[0016]
  A desired film thickness is obtained by repeating the steps (1) to (3) described above.
[0017]
  In the manufacturing method of the ferroelectric thin film of the above-mentioned Japanese Patent Publication No. 2658878, the film thickness after drying of (2) is set to 20 to 80 nm, so that there is no crack and the (105) axis-oriented epitaxial film. Is stated to be obtained. In the embodiment, the film thickness after drying obtained in the step (2) is set to about 30 nm, and the steps (1) to (3) are repeated six times to highly align SrBi with a total film thickness of about 200 nm.₂(Ta_xNb_1-x)₂O₉(X = 0 to 1) A film is obtained.
[0018]
  Also in the sputtering method and the MOCVD method, there is widely used a method of crystallizing an amorphous film by heat treatment in an oxygen atmosphere without crystallizing it during vapor phase growth. In particular, in the chemical solution deposition method, a ferroelectric thin film manufacturing method in which the thickness of each layer before crystallization is reduced in order to obtain a high quality crystalline film is a lecture at the 60th JSAP Scientific Lecture. Proposal No. 3p-A-1 is proposed on page 454 of the proceedings.
[0019]
  Next, the leakage current characteristics of the ferroelectric thin film element will be described. A ferroelectric memory composed of a ferroelectric thin film element has a characteristic non-volatility when the power is turned off. When this is applied to an NVDRAM (nonvolatile dynamic random access memory) that operates as a DRAM during normal operation, if the leak current is large, the refresh time is shortened. At this time, if the leakage current can be reduced by several orders of magnitude while maintaining the accumulated charge amount or remanent polarization at a good value, the refresh time during DRAM operation can be increased, and the device characteristics can be greatly improved. Further, when the leakage current increases, the electric field applied to the ferroelectric thin film becomes small, and polarization inversion does not occur sufficiently.
[0020]
[Problems to be solved by the invention]
  By the way, the ferroelectric thin film having the bismuth layered structure has a complicated crystal structure and strong anisotropy of the crystal grain growth rate, so that large crystal grains are likely to grow and it is difficult to obtain a good morphology. Therefore, the following problems occur in the ferroelectric thin film manufacturing method disclosed in Japanese Patent Publication No. 2658878 and the ferroelectric thin film manufacturing method of the 60th Japan Society of Applied Physics.
[0021]
  In the method for manufacturing a ferroelectric thin film disclosed in Japanese Patent No. 2658878, the film thickness in (1) is set to about 80 nm, and the steps (1) to (3) are repeated twice to obtain SrBi having a thickness of 160 nm.₂Ta₂O₉Create a film and its SrBi₂Ta₂O₉The film was observed with SEM (scanning electron microscope). As a result, the above SrBi₂Ta₂O₉In the film, it was found that two layers of grain boundaries are arranged in the film thickness direction and the cross-sectional shape is rough. In addition, the above SrBi₂Ta₂O₉There is a problem that the polarization characteristics and leakage current characteristics of the film are low.
[0022]
  In addition, in the manufacturing method of the ferroelectric thin film at the 60th JSAP Scientific Lecture, the film thickness in (1) is 20 nm as proposed in the lecture number 3p-A-1 on page 454 of the lecture proceedings. The process from (1) to (3) was repeated 8 times, and SrBi with a thickness of 160 nm was made thin.₂Ta₂O₉A membrane was created. This SrBi₂Ta₂O₉Although the film has a columnar structure in the film thickness direction, there is a problem that the leakage current characteristic is low because the film is a non-uniform film having a particle size of 80 to 400 nm in the surface shape and becomes a film with severe irregularities.
[0023]
  The present invention has been made to solve the above problems, and its object is to produce an oxide crystalline film capable of improving polarization characteristics and leakage current characteristics.The lawIt is to provide.
[0024]
[Means for Solving the Problems]
  The present inventor considered the above problem as follows.
[0025]
  (A) When the film for each layer is thickened, it is difficult to grow into a columnar structure in which initial nuclei are generated from all over the film in the film thickness direction and the lower crystal grains are dragged as they are. Conceivable. That is, since a plurality of crystal grains are stacked in the film thickness direction, it is considered that the presence of the grain boundary has an adverse effect on the polarization characteristics.
[0026]
  (B) In addition, when the thickness of each layer is reduced, the initial nucleus generation rate per unit area is reduced, and as a result, although the crystal grains grow in a columnar shape, there are many that have a large crystal grain size. And it is thought that it becomes non-uniform.
[0027]
  (C) The leakage current characteristic of the ferroelectric thin film is strongly influenced by its own shape. For example, if there is a drop in film thickness, a strong electric field strength is applied to the thin part, dielectric breakdown is likely to occur, and a leak path is likely to occur.
[0028]
[0029]
[0030]
  Based on the above consideration, the method for producing an oxide crystalline film of the present invention is as follows.
  In a method for producing an oxide crystalline film, comprising: a step of forming an amorphous film on a substrate; and a step of performing a heat treatment for crystallizing the amorphous film to form an oxide crystalline film.
  After laminating at least two layers of the amorphous film,650 ° C or lessA heat treatment is performed to form a single oxide crystalline film,
  On the one-layer oxide crystalline film,Amorphous filmAt least one layerAfter the lamination, the laminated amorphous film is subjected to a heat treatment at 650 ° C. or lower to form at least one acid layer.Forming a crystalline crystalline film,
  The thickness of the oxide crystalline film closest to the substrate among the oxide crystalline films of a plurality of layers is made thicker than the thickness of other oxide crystalline films,
  The total thickness of the multiple oxide crystalline films is 100 nm or less.ToIt is characterized by that.
[0031]
  According to the method for manufacturing an oxide crystalline film having the above structure, at least two layers of the amorphous film are stacked, and then heat treatment is performed to form a single oxide crystalline film. Then, at least one oxide crystalline film is formed on the one oxide crystalline film. At this time, by setting the thickness of the oxide crystalline film closest to the substrate among the plurality of oxide crystalline films to be larger than the thickness of the other oxide crystalline films, the temperature of the heat treatment is lowered. However, dense crystal grains having a columnar structure can be obtained in the oxide crystalline film. Therefore, the polarization characteristics and leakage current characteristics of the oxide crystalline film can be improved.
[0032]
  In addition, since the first heat treatment is performed in a state where at least two layers of the amorphous film are stacked, the heat treatment time can be shortened as compared with the case where the amorphous film is heat treated for each layer.
[0033]
  In one embodiment of the method for producing an oxide crystalline film, the oxide crystalline film is a ferroelectric thin film having a bismuth layered perovskite structure.
[0034]
  According to the method for producing an oxide crystalline film of the one embodiment, since the oxide crystalline film is a ferroelectric thin film having a bismuth layered perovskite structure, good leakage current characteristics can be obtained.
[0035]
[0036]
[0037]
[0038]
[0039]
DETAILED DESCRIPTION OF THE INVENTION
  Hereinafter, a method for producing an oxide crystalline film of the present inventionThe lawThis will be described in detail with reference to the illustrated embodiment.
[0040]
  (Reference example)
  FIG. 1 is a schematic cross-sectional view of a main part of a semiconductor device according to a reference example of the present invention. As shown in FIG. 1, the semiconductor device includes a lower electrode 4 and SrBi sequentially stacked on a silicon single crystal substrate 1.₂Ta₂O₉A ferroelectric capacitor including a ferroelectric thin film 5 and an upper electrode 6 is provided. The ferroelectric thin film 5 is composed of a plurality of oxide crystalline films and has a bismuth layered perovskite structure. A silicon oxide film 2 having a thickness of 200 nm and a titanium oxide film 3 having a thickness of about 20 nm formed on the silicon oxide film 2 are formed between the silicon single crystal substrate 1 and the lower electrode 4. are doing. The titanium oxide film 3 serves as an adhesive layer between the thermal oxide film 2 and the lower electrode 4. The lower electrode 4 and the upper electrode 6 are made of Pt. The film thickness of the lower electrode 4 is 200 nm.
[0041]
  For the formation of the ferroelectric thin film 5, a MOD method including a film forming process using a precursor raw material solution of a ferroelectric material is used. In the synthesis of the precursor raw material solution of this MOD method, tantalum ethoxide (Ta (OC₂H_Five)_Five), Bismuth 2-ethylhexanate (Bi (C₇H₁₅COO)₂) And strontium 2-ethylhexanate (Sr (C₇H₁₅COO)₂).
[0042]
  Hereinafter, the synthesis of the precursor raw material solution used for forming the ferroelectric thin film 5 will be described.
[0043]
  First, in order to dissolve the weighed tantalum ethoxide in 2-ethyl hexanate and promote the reaction between tantalum ethoxide and 2-ethyl hexanate, the mixture was stirred while heating from 100 ° C. to the maximum temperature of 120 ° C. for 30 minutes. React. Thereafter, strontium 2-ethylhexanate dissolved in 20 ml to 30 ml of xylene is added to the ethanol and water solution removed at 120 ° C., and the mixture is heated and stirred from 125 ° C. to a maximum temperature of 140 ° C. for 30 minutes. Thereafter, bismuth 2-ethylhexanate dissolved in 10 ml of xylene is added to the solution that has been heated and stirred, and the mixture is heated and stirred at a maximum temperature of 150 ° C. from 130 ° C. for 10 hours.
[0044]
  Next, in order to remove low molecular weight alcohol, water, and xylene used as a solvent from the solution after stirring for 10 hours, distillation is performed at a temperature of 130 ° C. to 150 ° C. for 5 hours. And in order to remove dust from a solution, a solution is filtered with a 0.45 micrometer diameter filter. Then, SrBi of the solution₂Ta₂O₉Is adjusted to 0.1 mol / l, and this is used as a precursor raw material solution. In the above process, the strontium: bismuth: tantalum ratio is adjusted to 8:22:20. Further, these raw materials are not limited to those described above, and any solvent may be used as long as the above starting materials are sufficiently dissolved.
[0045]
  Hereinafter, a film forming method by a general chemical solution deposition method using the precursor raw material solution thus synthesized will be described.
[0046]
  First, as shown in FIG. 2, in step S21, a precursor material solution of a ferroelectric thin film is spin-coated on a silicon single crystal substrate on which a silicon thermal oxide film, a titanium oxide film, and a lower electrode are sequentially stacked. Apply to form a coating film.
[0047]
  Next, in step S22, the coating film is heat-treated on a hot plate (HP) set to a temperature lower than the crystallization temperature (about 600 ° C.) in order to remove carbon in the coating film. The attached film is dried to form an amorphous film (first heat treatment step).
[0048]
  In step S23, the amorphous film is crystallized and fired using an RTA (Rapid Thermal Annealing) method. Specifically, the amorphous film is crystallized by performing heat treatment at a crystallization temperature (about 600 ° C.) or higher in an oxygen atmosphere to form an oxide crystalline film (first 2 heat treatment step).
[0049]
  Thereafter, steps S21 to S23 are repeated a plurality of times until a desired film thickness is obtained.
[0050]
  Hereinafter, as a preliminary experiment of the film forming method according to the present invention, an attempt is made to change the film thickness of the coating film by changing the spin speed in step S21. By the method, steps S21 to S23 were performed using the precursor raw material solution to form a single oxide crystalline film. At this time, the heat treatment in step S22 is performed in two steps of a heat treatment at 150 ° C. and a heat treatment at 400 ° C., and the heat treatment temperature in step S23 is 800 ° C. The surface shape of the oxide crystalline film thus obtained is observed with a scanning electron microscope, and the particle sizes from the maximum to the fifth are measured in the range of 2.5 × 2.0 μm. Such a measurement is performed at two locations on the surface of the oxide crystalline film, and the relationship between the average particle size and the film thickness of the oxide crystalline film is shown in the graph of FIG. From the graph of FIG. 3, it can be seen that the crystal grain size is reduced as the thickness of the oxide crystalline film is increased. In particular, it can be said that a relatively dense grain structure with a film thickness of 35 nm or more and a particle diameter of 300 nm or less is obtained. That is, in order to obtain a film having a small particle diameter, it is understood that the thickness of one oxide crystalline film is better. This suggests that the crystal nucleus generation density increases with increasing film thickness. However, for example, a sample formed by laminating a plurality of oxide crystalline films having a thickness of about 50 nm so as to have a thickness of about 150 nm is porous, and good electrical characteristics cannot be obtained. This is presumably because, since the thickness of the stacked layers is large, crystal nuclei are generated in the stacked films, and the resulting film is difficult to form dense crystal grains having a columnar structure.
[0051]
  Therefore, for example, when the ferroelectric thin film 5 having a film thickness of 150 nm is constituted by four layers of oxide crystalline films, the film thickness of the first layer among the four layers is set to 60 nm and above the first layer. The film thickness of the positioned second to fourth layers is set to 30 nm. That is, the thickness of the oxide crystalline film closest to the silicon single crystal substrate 1 among the four oxide crystalline films is set to 60 nm, and the oxide crystalline film other than the 60 nm oxide crystalline film is formed. The film thickness is set to 30 nm. When the ferroelectric thin film thus configured is observed with a scanning electron microscope, it can be seen that the ferroelectric thin film has a dense film structure showing columnar crystals with a crystal grain size of about 200 nm at maximum and 100 nm or less on average.
[0052]
  Thus, by making the film thickness of the oxide crystal film closest to the silicon single crystal substrate 1 out of the four layers of oxide crystal films larger than the film thickness of the other oxide crystal films, Since dense crystal grains having a columnar structure can be obtained in the oxide crystalline film, the polarization characteristics and leakage current characteristics of the oxide crystalline film can be improved.
[0053]
  And SrBi₂Ta₂O₉When the upper electrode 6 is formed on the ferroelectric thin film 5 made of, for example, by sputtering and patterning Pt into a desired shape, a capacitor structure is obtained.
[0054]
  Further, after the upper electrode 6 is formed, heat treatment is performed at 800 ° C. for 10 minutes in an oxygen atmosphere. This is to stabilize the interface between the ferroelectric and the electrode.
[0055]
  4 is a graph showing the relationship between the current density of the leakage current in the ferroelectric capacitor composed of the lower electrode 4, the ferroelectric thin film 5 and the upper electrode 6 and the applied voltage applied to the ferroelectric capacitor. Shown in From the graph of FIG.^-8A / cm²It turns out that the good leak current characteristic of the stand is obtained.
[0056]
  Next, when the characteristics of the ferroelectric thin film 5 according to the present invention were evaluated by the Soya tower method, a hysteresis curve as shown in FIG. 5 was obtained. In the ferroelectric thin film 5, a remanent polarization 2P of 5V_rIs 23 μC / cm²2E_cIs 100 kV / cm or less.
[0057]
  In the above reference example, the MOD method, which is a kind of chemical deposition method, has been described, but the present invention is not limited to this. In addition to the sol-gel method, which is the same chemical solution deposition method as the MOD method, an amorphous film may be formed by the MOCVD method, the sputtering method, or the like. Further, the heat treatment for crystallizing the amorphous film is not limited to this embodiment.
[0058]
  Further, although the spin coating method is used in step S21, the spin coating method in step S21 does not limit the present invention, and the film thickness of the solution such as dip coating method, LSMCD method (electrodeposition atomization method), etc. Any method can be used as long as it can be controlled and applied onto the substrate.
[0059]
  In addition, the heat treatment by the hot plate in step S22 does not have to be a single process. For example, the separation of the organic solvent and the thermal decomposition of the organic metal are separately performed at 150 ° C., 400 ° C. twice or more. Is preferred. Further, instead of the hot plate, for example, an electric furnace or the like may be used to slowly heat at a temperature lower than the crystallization temperature.
[0060]
  Further, the RTA method in step S23 may be performed in the atmosphere. In step S23, a method other than the RTA method may be used. For example, heat treatment for crystallization may be performed in an electric furnace or the like.
[0061]
  The number of oxide crystalline films constituting the ferroelectric thin film 5 may be plural.
[0062]
  The ferroelectric thin film 5 is made of SrBi.₂Ta₂O₉SrBi₂(Ta_xNb_1-x)₂O₉(X = 0-1) may be sufficient.
[0063]
  Further, although a silicon single crystal substrate is used as the substrate, the present invention is not limited to this.
[0064]
  (Embodiment)
  In the embodiment, an example in which the method for manufacturing an oxide crystalline film of the present invention is used for a low-temperature process with a film thickness of 100 nm or less and a heat treatment temperature of 650 ° C. or less will be described.
[0065]
  First, the necessity of reducing the heat treatment temperature to 650 ° C. or lower will be briefly described. A high-temperature heat treatment exceeding 650 ° C. is not so much a problem with a ferroelectric capacitor having a relatively low degree of integration, but in a stacked structure indispensable for highly integrating a ferroelectric memory, the lower electrode The polysilicon plug used for contact with the Pt, the Pt lower electrode, and a barrier metal such as TiN or TaSiN for preventing diffusion between the plugs are oxidized in a high temperature process.
[0066]
  When such polysilicon plug or barrier metal oxidation occurs, there is a problem that the plug and the lower electrode are not electrically connected, or the barrier metal expands and peels off, so the heat treatment temperature should be as low as possible. It is desirable to establish a technique for forming a ferroelectric thin film at a low temperature of 650 ° C. or lower.
[0067]
  In addition, a ferroelectric thin film of 100 nm or less is indispensable for formation of a three-dimensional capacitor in accordance with a comma several micron rule.
[0068]
  Hereinafter, a low temperature process with a film thickness of 100 nm or less and a heat treatment temperature of 650 ° C. or less will be described. Here again, the precursor solution described in the reference example is used.
[0069]
  The first heat treatment process in step S22 of FIG. 2 is performed twice at 150 ° C. and 400 ° C., and the temperature of the second heat treatment process in step S23 is set to 650 ° C. A crystalline film is formed. FIG. 6 shows a scanning electron micrograph of the surface shape of the oxide crystalline film having a thickness of 30 nm, and FIG. 7 shows a scanning electron micrograph of the surface shape of the oxide crystalline film having a thickness of 60 nm. In addition, the original photographs in FIGS. 6 and 7 are submitted in the property submission form. As apparent from FIGS. 6 and 7, the crystal growth proceeds in the oxide crystalline film having a thickness of 60 nm rather than in the oxide crystalline film having a thickness of 30 nm. This is suggested to be due to the result that the crystal nucleus generation density is increased by increasing the film thickness. However, for example, a sample formed by laminating a plurality of oxide crystalline films having a thickness of about 50 nm to have a thickness of about 100 nm is porous, and good electrical characteristics cannot be obtained. This is presumably because, since the thickness of the stacked layers is large, crystal nuclei are generated in the stacked films, and the resulting film is difficult to form dense crystal grains having a columnar structure.
[0070]
  Here, as in the reference example, the thickness of the oxide crystalline film closest to the silicon single crystal substrate 1 among the plurality of oxide crystalline films is larger than the thickness of the other oxide crystalline films. I tried to. However, the film quality of the ferroelectric thin film thus obtained has a high leakage current and a wide hysteresis curve. This is because the temperature of the second heat treatment step in step S23 is 650 ° C., which is 150 ° C. lower than that of the reference example, so that carbon is desorbed in the thick initial layer film that is the oxide crystalline film closest to the silicon single crystal. Seems to have been insufficient.
[0071]
  Therefore, a method of obtaining a crystalline film having a small grain size and a columnar structure in a short time by increasing the thickness of the first crystallization fired film and decreasing the thickness after the next lamination is used.
[0072]
  Specifically, as shown in FIG. 8, after setting the film thickness of the amorphous film formed in step S82 to 24 nm and performing steps S81 and S82 to form the first amorphous film, The process returns to step S81. Thereafter, Steps S81 to S83 are performed to obtain a first oxide crystalline film. Then, returning to step S81 again, steps S81 to S83 are repeated twice to finally obtain a ferroelectric thin film of about 96 nm. That is, after two layers of the amorphous film are stacked, a heat treatment (second heat treatment) is performed to form an oxide crystalline film having a thickness of 48 nm, and then the oxide crystalline film having a thickness of 48 nm. Two oxide crystalline films of 24 nm are formed on top.
[0073]
  As a result of observing with a scanning electron microscope the ferroelectric thin film comprising a plurality of oxide crystalline films thus obtained, a dense film structure having a crystal grain size of about 150 nm at maximum and 100 nm or less on average It was confirmed that.
[0074]
  Then, Pt is deposited as an upper electrode on the ferroelectric thin film by sputtering to form a capacitor structure. Thereafter, heat treatment is performed at 650 ° C. for 30 minutes in an oxygen atmosphere to stabilize the interface between the ferroelectric thin film and the electrode.
[0075]
  FIG. 9 shows the result of measuring the leakage current density of a ferroelectric capacitor using the ferroelectric thin film. From FIG. 9, it is 10 at an applied voltage of 5V.^-7A / cm²It can be seen that good leakage current characteristics of the table are obtained.
[0076]
  Next, when the ferroelectric characteristics of the ferroelectric capacitor were evaluated, a hysteresis curve as shown in FIG. 10 was obtained. In FIG. 10, the thickness of the ferroelectric thin film of the ferroelectric capacitor of the comparative example is also about 96 nm, and each of the plurality of oxide crystalline films constituting the ferroelectric thin film is an amorphous film. Each layer is obtained by heat treatment, and the thickness of each oxide crystalline film is 24 nm. Therefore, the comparative example has a longer crystallization heat history by one layer than the present example.
[0077]
  Compare this example with the comparative example in FIG._rThis example is 15 μC / cm²As described above, the comparative example is 12.5 μC / cm²That is the above, and the electric field 2E_cBoth of the inventive examples and the comparative examples were 120 kV / cm or less. This result shows that the crystallization time is shortened in this example.
[0078]
  Thus, by performing the first heat treatment for crystallization in a state where two layers of the amorphous film are stacked, the baking time for one layer of the amorphous film can be shortened.
[0079]
  Next, a case where the ferroelectric thin film formed according to the present embodiment is applied to a capacitor type nonvolatile memory which is a semiconductor device as shown in FIG.
[0080]
  First, a method for manufacturing a capacitor-type nonvolatile memory will be described below with reference to FIG.
[0081]
  As shown in FIG. 11, after a switching transistor is formed by a known MOSFET formation process using a substrate 100 and covered with an interlayer insulating film, only a portion corresponding to the impurity diffusion region 8 is dried by a known photolithography method and a dry process. The contact hole 9 is formed by using an etching method, and the polysilicon in which the impurity is diffused is buried, and then the surface is planarized by a known CMP (Chemical Mechanical Polishing) method to form the interlayer insulating film 10 and the polysilicon plug 11. To do.
[0082]
  Next, after depositing 200 nm of TaSiN, which is a material of the TaSiN barrier metal layer 12, by a known sputtering method, Ir, which is a material of the Ir film 13, is deposited by 200 nm by a known sputtering method. Then, SrBi as a ferroelectric thin film formed by the manufacturing method of the present embodiment on the Ir film.₂Ta₂O₉A film is formed. Next, the above SrBi₂Ta₂O₉100 nm of Ir is laminated on the film, and SrBi is used by using a known photolithography method and dry etching method₂Ta₂O₉The upper electrode 15 is formed by processing the Ir film on the film into a size of 1.7 μm square. Thereafter, heat treatment is performed for 30 minutes at a substrate temperature of 650 ° C. in an atmospheric pressure oxygen atmosphere for the purpose of stabilizing the ferroelectric characteristics by suppressing leakage current and supplementing oxygen vacancies. And SrBi₂Ta₂O₉By processing the film, the 200 nm Ir film, and the 200 nm TaSiN film into a 2.0 μm square size using a known photolithography method and dry etching method, SrBi₂Ta₂O₉A film 14, an Ir film 13, and a TaSiN barrier metal layer 12 are formed. The TaSiN barrier metal layer 12 and the Ir film 13 constitute a multilayer lower electrode. For the dry etching, an ECR etcher is used, and the gas used is SrBi.₂Ta₂O₉Ar and Cl for film 14₂And CF_FourFor mixed gas and Ir film 13₂F₆And CHF_ThreeAnd Cl₂For the TaSiN barrier metal layer 12₂It was.
[0083]
  Next, TiO with a film thickness of 30 nm₂A film is deposited using a known sputtering method, and then a 150 nm-thickness silicon oxide film is deposited by a known CVD method as an interlayer insulating film, and then a known photolithography method is formed on the upper electrode 15. Using a dry etching method, a 1.2 μm square contact hole is formed. As a result, TiO₂The barrier insulating film 16 and the silicon oxide film 17 are completed.
[0084]
  Then, an Al film having a film thickness of 400 nm is formed, and the Al film is processed into a plate line 18 by using a known photolithography method and dry etching method, and then at 400 ° C. for 30 minutes in a normal pressure nitrogen atmosphere. Heat treatment is performed to stabilize the electrode interface. Thereafter, the interlayer insulating film 19 is formed by laminating the material of the interlayer insulating film 19 using a CVD method and planarizing the material by a known planarization technique. Further, a contact hole 20 to the other impurity diffusion layer 28 of the switching transistor is formed in the interlayer insulating film 19 using a known photolithography method and dry etching method. Then, the bit line 21 is formed using a known Al wiring technique. Reference numeral 22 denotes a gate electrode of a switching transistor.
[0085]
  When the ferroelectric characteristics of the nonvolatile semiconductor memory element of the capacitor-type nonvolatile memory thus fabricated were measured, it was found that 2P_r= 13 μC / cm²2E_c= 120 kV / cm was obtained, and a sufficient operation as a ferroelectric capacitor was confirmed. Next, the leakage current density of the nonvolatile semiconductor memory element was measured. Leakage current density at applied current + 3V is 2 × 10^-7A / cm²In addition, since dielectric breakdown did not occur even at an applied voltage of 10 V, sufficient characteristics as a ferroelectric capacitor were confirmed.
[0086]
  The multilayer lower electrode of the ferroelectric capacitor manufactured as described above is Ir / TaSiN. However, the present invention is not limited to this, and Ir / IrO.₂/ Ir / TaSiN, IrO₂/ Ir / TaSiN or Pt / RuO₂/ Ru / TaSiN, Ru / RuO₂/ Ru / TaSiN, RuO₂/ Ru / TaSiN, Pt / IrO₂/ Ir / tiN, Ir / IrO₂/ Ir / TiN, IrO₂/ Ir / TiN, Pt / RuO₂/ Ru / TiN, Ru / RuO₂/ Ru / TiN, RuO₂/ Ru / TiN, Ir / TaSiN, Ir / TiN, Ru / TaSiN, Ru / TiN, etc. Any multilayer electrode having excellent heat resistance may be used.
[0087]
  In the above embodiment, the first heat treatment is performed in a state where two layers of the amorphous film are stacked. However, the first heat treatment may be performed in a state where two or more amorphous films are stacked. That is, an oxide crystalline film closest to the silicon single crystal substrate 1 among a plurality of oxide crystalline films may be formed by performing heat treatment after at least two layers of the amorphous film are stacked. . The oxide crystalline film formed on the oxide crystalline film closest to the silicon single crystal substrate 1 may be at least one layer.
[0088]
【The invention's effect】
[0089]
  As is clear from the above, in the method for producing an oxide crystalline film of the present invention, at least two amorphous films are laminated, and then heat treatment is performed to form a single oxide crystalline film. When forming at least one oxide crystalline film on the oxide crystalline film, the thickness of the oxide crystalline film closest to the substrate among the plurality of oxide crystalline films is changed. Therefore, even if the heat treatment temperature is low, dense crystal grains having a columnar structure are obtained in the oxide crystalline film, and the oxide crystalline film is polarized. Characteristics and leakage current characteristics can be improved.
[0090]
  In addition, since the heat treatment is performed in a state where at least two layers of the amorphous film are stacked, the heat treatment time can be shortened as compared with the case where the amorphous film is heat treated for each layer.
[0091]
  According to the oxide crystalline film manufacturing method of one embodiment, since the oxide crystalline film is a ferroelectric thin film having a bismuth layered perovskite structure, good leakage current characteristics can be obtained.
[0092]
[0093]
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of a main part of a semiconductor device according to a reference example of the present invention.
FIG. 2 is a flowchart for explaining a method of manufacturing an oxide crystalline film according to a reference example of the present invention.
FIG. 3 is a graph showing the relationship between the grain size of an oxide crystalline film of a ferroelectric thin film and the thickness of the oxide crystalline film in the semiconductor device.
FIG. 4 is a graph showing a relationship between a current density of a leakage current in a ferroelectric capacitor of the semiconductor device and a voltage applied to the ferroelectric capacitor.
FIG. 5 is a diagram showing a hysteresis curve of a ferroelectric thin film of the ferroelectric capacitor.
FIG. 6 is a scanning electron micrograph of the surface shape of an oxide crystalline film having a thickness of 30 nm.
FIG. 7 is a scanning electron micrograph of the surface shape of an oxide crystalline film having a thickness of 60 nm.
FIG. 8 is a flowchart for explaining a method of manufacturing an oxide crystalline film according to an embodiment of the present invention.
FIG. 9 is a diagram illustrating a current density of a leakage current in a ferroelectric capacitor having a ferroelectric thin film obtained by the method for manufacturing an oxide crystalline film according to the above embodiment, and a voltage applied to the ferroelectric capacitor. It is a graph which shows the relationship.
FIG. 10 is a diagram showing a hysteresis curve between a ferroelectric thin film obtained by the oxide crystalline film manufacturing method of the above embodiment and a ferroelectric thin film of a comparative example.
FIG. 11 is a schematic cross-sectional view of a main part of a capacitor-type nonvolatile memory having a ferroelectric thin film obtained by the oxide crystalline film manufacturing method of the above embodiment.
[Explanation of symbols]
1 Silicon single crystal substrate
2 Silicon thermal oxide film
3 Titanium oxide film
4 Lower electrode
5 Ferroelectric thin film
6 Upper electrode

Claims (2)

In a method for producing an oxide crystalline film, comprising: a step of forming an amorphous film on a substrate; and a step of performing a heat treatment for crystallizing the amorphous film to form an oxide crystalline film.
After laminating at least two layers of the amorphous film, heat treatment at 650 ° C. or lower is performed to form a single oxide crystalline film,
Over the oxide crystalline film of the first layer, after the amorphous film laminated at least one layer, the oxides crystalline film at least one layer subjected to heat treatment of 650 ° C. or less in an amorphous film that the laminated Forming,
The thickness of the oxide crystalline film closest to the substrate among the oxide crystalline films of a plurality of layers is made larger than the thickness of the other oxide crystalline films,
Method of manufacturing an oxide crystalline film, wherein to Rukoto the total thickness of the oxide crystalline film of the plurality of layers 100nm or less. In the manufacturing method of the oxide crystalline film of Claim 1,
The method for producing an oxide crystalline film, wherein the oxide crystalline film is a ferroelectric thin film having a bismuth layered perovskite structure .

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