JP4427151B2 - Method for forming ferroelectric thin film - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming a ferroelectric thin film.
[0002]
[Prior art]
At present, a ferroelectric memory (FeRAM) in which a DRAM MOS capacitor is replaced with a ferroelectric capacitor, or a ferroelectric memory having a structure in which a ferroelectric capacitor is connected to a gate portion of a MOS type or MIS type field effect transistor (FET). Is being actively developed. These ferroelectric memories are characterized by non-destructive reading of data and non-destructive reading of data such that written data does not disappear even when the power is turned off.
[0003]
A ferroelectric capacitor, which is an important element constituting a ferroelectric memory, has a structure in which a ferroelectric thin film is sandwiched between metal electrodes. Currently, lead-based oxide ferroelectrics such as PbTiO ₃ and Bi-layered oxide ferroelectrics such as SrBi ₂ Ta ₂ O ₉ (SBT) are being studied as ferroelectric materials for ferroelectric memories. . When producing a capacitor structure using these oxide ferroelectrics, it is necessary to deposit a crystallized thin film on an electrode such as Pt. Ferroelectric thin film deposition methods include solution coating methods such as sol-gel methods, physical deposition methods such as sputtering, and chemical deposition methods such as CVD. A process of performing crystallization by heat treatment after the deposition is common.
[0004]
[Problems to be solved by the invention]
Usually, high temperature heat treatment in oxygen is performed for crystallization of the oxide ferroelectric thin film. For example, in the case of an SBT thin film, a crystallization temperature of around 750 ° C. is necessary. However, in the temperature region where crystallization is performed, compositional disturbance due to evaporation of constituent elements from the film tends to occur at the same time as crystallization, so that the polarization characteristics of the ferroelectric thin film tend to deteriorate.
[0005]
As a countermeasure against this, a method of forming an amorphous film containing excessive elements that easily volatilizes has been carried out, but it is not sufficient to prevent the disorder of the composition due to evaporation of constituent elements from the film surface. The polarization characteristics cannot be improved sufficiently.
Accordingly, an object of the present invention is to provide a method for manufacturing a ferroelectric thin film that solves the problem that the composition is disturbed during the heat treatment for crystallization as described above.
[0006]
[Means for Solving the Problems]
The present invention provides the following to achieve the above object.
(1) In a method of depositing a ferroelectric thin film on a substrate and then heat-treating it to form a ferroelectric crystal thin film, at least two substrates are arranged with the ferroelectric thin films facing each other. A method for forming a ferroelectric thin film, comprising performing a heat treatment.
[0007]
(2) The method for forming a ferroelectric thin film according to (1), wherein the ferroelectric thin film is made of a ferroelectric material containing Bi or Pb.
(3) The method for forming a ferroelectric thin film according to (1) or (2), wherein an interval between the ferroelectric thin films is set within a range of 0 to 600 μm.
In the present invention, the elements evaporated from the film surface by the heat treatment compensate each other by the elements evaporated from the thin film surfaces facing each other, so that the disorder of the composition in the film is eliminated, so that the polarization characteristics can be improved.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, a ferroelectric thin film deposited on a substrate, usually a ferroelectric thin film that is amorphous but may be crystallized to some extent, is subjected to a heat treatment for crystallizing or improving the quality of the crystal. The method is characterized in that at least two substrates are used and the ferroelectric thin films are arranged face to face.
[0009]
Ferroelectric materials include oxide-based and non-oxide-based materials. The method of the present invention can be applied to both, but is mainly directed to oxide-based materials. Examples of the oxide-based ferroelectric include Pb (Zr, Ti) O ₃ [PZT], (Pb, La) (Zr, Ti) O ₃ [PLZT], SrBi ₂ Ta ₂ O ₉ [SBT], Bi ₄ Ti. ₃ O ₁₂ , YMnO ₃ , Sr ₂ (Ta, Nb) ₂ O ₇ , (Ba, Sr) TiO ₃ , Pb ₅ Ge ₃ O _{11 and the} like. PZT and SBT are suitable ferroelectrics.
[0010]
The present invention is a particularly effective method when a change in composition is likely to occur when an element that easily evaporates during heat treatment is contained. Of the elements constituting the ferroelectric, Pb, Bi, etc. are typical as elements having a particularly high vapor pressure at the heat treatment temperature. Therefore, the present invention is particularly useful for film formation of PZT, PLZT, SBT, etc., which are ferroelectric materials containing these elements.
[0011]
In the present invention, the ferroelectric film forming method is not particularly limited, and the ferroelectric substance can be deposited on the substrate by any known method. For example, physical deposition methods such as vacuum deposition, sputtering, ion plating, CVD, solution coating (sol-gel, organometallic decomposition, etc.), liquid phase epitaxy, thermal spraying, fine particle sintering There is a chemical film-forming method such as a melt rapid quenching method.
[0012]
The solution coating method is considered to be the most practical method because of the ease of control of the composition and the stability of the dielectric properties as a collection, but the sputtering method can form a relatively high quality film over a wide range. The CVD method is attracting attention in terms of excellent physical properties and high physical property controllability.
As a typical example, a film forming method of PZT by a solution coating method will be briefly described. For example, Pb (OOCCH ₃ ) ₂ .3H ₂ O, Zr (OCH ₂ CH ₂ CH ₃ ) ₄ , Ti {OCH (CH _{3 2} ) A raw material alkoxide such as ₄ } is made into a solution using 2-methoxyalcohol or the like as a solvent, and water is added thereto to cause hydrolysis to cause condensation polymerization to prepare a precursor solution. This is dropped on a substrate, spin-coated and dried. This is heat-treated and crystallized to obtain a PZT thin film. In the present invention, this final heat treatment step is performed by arranging the PZT thin films facing each other. In general, amorphous PZT is formed in the thin film before and after the heat treatment. In particular, a thin film without calcining after drying is in a precursor state, and the present invention is also effective in that case.
[0013]
When SBT is formed in place of PZT, a combination of alkoxides such as Bi (OC ₂ H ₅ ) ₃ , Sr (OC ₂ H ₅ ) ₂ , Ta (OC ₂ H ₅ ) ₅ is used as a starting material. Is used.
Further, in the CVD method (MOCVD method) using an organometallic compound, in the case of PZT, for example, Pb (C ₂ H ₅ ) ₄ , Zr (t-OC ₄ H ₉ ), Ti (i-OC ₃ H _7). ) ₄ can be used as a raw material, but when these are solid at room temperature, a gaseous raw material is prepared by an appropriate method such as spraying a heated carrier gas, and is transported onto a substrate together with oxygen gas. A ferroelectric thin film such as PZT can be deposited on the heated substrate. Although the ferroelectric thin film obtained by the MOCVD method may be crystallized, in order to improve the ferroelectric characteristics, the crystallinity is improved by further heat treatment. It can be used for the desired heat treatment.
[0014]
Further, in the sputtering method, a composite target of metal oxide such as PbO, ZrO ₂ and TiO ₂ is usually used for PZT, and a composite of metal oxide such as SrO ₃ , Bi ₂ O ₃ and Ta ₂ O _{5 is used for} SBT. Sputtering is performed using a target to deposit a target ferroelectric thin film on the substrate. This ferroelectric thin film is also heat-treated to improve the crystallinity.
[0015]
In the method of the present invention, the thickness of the ferroelectric thin film deposited on the substrate is not particularly limited, but the film thickness is preferably 50 nm or less for the purpose of maintaining the uniformity of the composition after the heat treatment. In this case, if a thinner film is required, it is desirable to repeat the cycle of deposition and heat treatment.
In the present invention, the substrate to be used is not particularly limited, but the ferroelectric thin film is often used by being sandwiched between electrodes, and in order to make the ferroelectric thin film have desired crystallinity and crystal orientation ( 111) Pt / Ti / SiO ₂ / Si, (111) Pt / Ta / SiO ₂ / Si, (111) Ir / IrO ₂ / SiO ₂ / Si, (111) Pt / IrO ₂ / SiO ₂ / Si (111 Pt / TiO ₂ / SiO ₂ / Si etc. are preferably used.
[0016]
When heat-treating the ferroelectric thin film deposited on the substrate as described above, according to the present invention, at least two substrates are arranged with the ferroelectric thin films facing each other. FIG. 1 schematically shows an arrangement method of the substrate. The ferroelectric thin film 2 deposited on the substrate 1 and the ferroelectric thin film 4 deposited on the substrate 3 are oriented to face each other. Two boards are arranged vertically or horizontally. The two substrates 1 and 3 may be in contact with each other or may be arranged with a slight gap therebetween. A jig can be used if necessary for the purpose of arranging two substrates in this way. Furthermore, in the heating furnace, it is needless to say that a plurality of sets of substrates arranged so as to face each other can be arranged.
[0017]
It is desirable to place the ferroelectric thin films on the two-plate substrates in contact with each other from the viewpoint of controlling the evaporation component and mutual compensation. However, depending on the type of thin film and the heat treatment conditions, the ferroelectric thin films may be arranged. In some cases, adhesion or physical damage due to friction may be applied to the thin film. In such a case, it is desirable to provide a slight gap between the ferroelectric thin films. This interval is desirably as small as possible, generally 600 μm or less, more preferably 200 μm or less. In order to control such an interval, an appropriate jig (holding tool) is used. When a space is provided between the substrates, some compositional disturbance appears in the periphery of the substrate depending on the size of the space, but if the distance is within the above range, the region of disturbance is small and the center of the substrate The disorder of the composition of the part is suppressed.
[0018]
In the method of the present invention, the conditions for the heat treatment may be the same as in the case where the ferroelectric thin films are not arranged face to face, and are not particularly limited. Typically, PZT performs heat treatment for crystallization at a substrate temperature of 500 ° C. to 650 ° C. in an oxygen gas atmosphere, and SBT performs heat treatment for crystallization at a substrate temperature of 650 ° C. to 800 ° C. in an oxygen gas atmosphere. I do.
[0019]
However, the present inventors have found that polarization is improved when ozone (O ₃ ) treatment is additionally performed during the heat treatment. After the heat treatment for crystallization, if the substrate temperature is lowered to a temperature at which ozone is not decomposed (about 400 ° C.), and the substrate temperature is kept for about 30 minutes in oxygen gas added with about 6% of ozone, the polarization amount Can be further improved by 10% to 20%.
[0020]
【Example】
In order to confirm the effect of the present invention, the following examples were performed on a ferroelectric SrBi ₂ Ta ₂ O ₉ (hereinafter abbreviated as SBT) thin film deposited by a solution coating method. This will be described below.
1. The experimental method <br/> _{example, Sr, Bi (OC 4 H} 9) as a raw material for Bi and _{Ta 3, Sr (OC 3 H} 7) using _{2, Ta (OC 2 H 5} ) 5, and these solvents A sol-gel solution (manufacturer: Toshima Seisakusho Co., Ltd.) diluted with (1-methoxy-2-propanol) was used. The raw material composition in the solution is adjusted to a composition (Sr _0.8 Bi _2.2 Ta ₂ O ₉ ) that maximizes the remanent polarization. In the experiment, a solution having a concentration of 0.33 mol / kg was used.
[0021]
The ferroelectric thin film was deposited on a Si substrate having a Pt thin film formed on the surface. FIG. 2A shows the cross-sectional structure of the substrate used in the experiment. First, an oxide film (SiO ₂ ) 12 having a thickness of 100 nm is formed on the surface of the Si (100) single crystal substrate 11 by steam oxidation at a substrate temperature of 950 ° C., and then the thickness is formed on the SiO ₂ by sputtering. A 20 nm Ti thin film 13 and a 200 nm thick Pt thin film 14 are deposited.
[0022]
The substrate 11 is cut into dimensions of 10 mm × 10 mm (substrate A) and 10 mm × 20 mm (substrate B), and several mL of the sol-gel solution is dropped on the Pt thin film 14 of those substrates using a dropper or the like, and then a spin coater is used. A coating film having a uniform thickness was formed on the substrate. The rotation speed of the spin coater at this time was 2500 rpm and the time was 30 seconds. Finally, in order to dry the coating film formed on the substrate, the substrate was left on a hot plate heated to about 140 ° C. in the atmosphere for about 3 minutes to evaporate the solvent in the coating film (FIG. 2 ( I)). The thickness of the film 15 deposited by this method was about 44 nm per coating after firing the thin film. The crystallinity of the coating film 15 after drying was confirmed to be amorphous from the evaluation by the X-ray diffraction method.
[0023]
Next, the dried coating film was crystallized. An infrared gold image furnace manufactured by Vacuum Riko Co., Ltd. was used as the heating device. Since this apparatus heats by condensing the light of an infrared lamp on the sample support stand (susceptor) 16 made of silicon carbide (SiC), the sample temperature can be raised in a short time. The arrangement of the samples A and B on the susceptor 16 is shown in FIG. First, place the substrate B on which the dry coating film is deposited on the susceptor with the thin film side facing up, and then place the substrate A on the substrate A so that the thin film side of the substrate A faces down and overlaps the thin film side of the substrate B. Placed on B. At this time, a heavy object such as a weight was not placed on the substrate A. Since the substrate B is larger than the substrate A, the thin film on the substrate B has a region (region 1) that is not opposed to the region (region 1) that is opposed to the thin film side of the substrate A. Arise.
[0024]
The substrate A and the substrate B on which the dry coating film was deposited were placed on the susceptor 16 in the arrangement shown in FIG. 3, and then heat treatment for crystallization was performed. The heat treatment was first performed under atmospheric pressure in which oxygen gas (purity 99.9%) was continuously supplied into the furnace at a flow rate of 1 L per minute. The substrate temperature was raised to 750 ° C. at a rate of 40 ° C. per second, and then held at the substrate temperature of 750 ° C. for 30 minutes. Next, the substrate temperature is lowered to 400 ° C. at a rate of 35 ° C. per minute, and then, in order to remove residual hydrocarbon components in the thin film, a mixed gas of ozone gas and oxygen gas (ozone against oxygen) is removed in the furnace. A substrate temperature of 400 ° C. was maintained for 30 minutes under atmospheric pressure with a concentration of 6.2% by weight) kept flowing at a flow rate of 100 mL per minute. In this example, the series of steps from FIG. 2 to heat treatment was repeated four times to produce an SBT thin film 17 having a thickness of about 175 nm.
[0025]
After the thin film 17 was deposited, a capacitor structure was fabricated to evaluate the electrical characteristics. First, a 50 μm thick stainless steel plate (metal mask) with holes with a diameter of 200 μm drilled at intervals of 0.5 mm is kept in close contact with the SBT thin film. A circular Pt electrode (upper electrode) 18 was formed on the SBT thin film (FIG. 4). Finally, in order to improve the interface characteristics between the Pt upper electrode and the SBT thin film, heat treatment was performed using an infrared gold image furnace under the substrate arrangement of FIG. 3 and the above conditions.
2. Evaluation of polarization characteristics FIG. 5 shows the polarization characteristics of the SBT thin film fabricated on the substrate B. FIG. The magnitude of the remanent polarization in the region 2 where the substrate A is not superposed was the same value as that of the sample produced by the conventional method for producing a ferroelectric thin film. On the other hand, in the region 1 on which the substrate A was overlapped, a remanent polarization value that was twice or more larger than that in the region 2 was obtained with good reproducibility.
[0026]
The depth direction analysis of the constituent elements in the SBT thin film was performed using secondary ion mass spectrometry (SIMS method). The measurement results are shown in FIG. Comparing the measurement result of the region 2 where the substrate A is not superimposed (FIG. 6A) and the measurement result of the region 1 where the substrate A is superimposed (FIG. 6B), only the change in the secondary ion intensity of Bi is noticeable. Different results were obtained. The change in secondary ion intensity of about several nm from the surface is due to the measurement principle and does not reflect the element distribution in the film. In the region 2, the secondary ion intensity of Bi at a depth of about 40 nm from the surface is about half of the secondary ion intensity of the region 1, which means that the Bi composition in the region 2 is lower than that in the region 1. I mean. From the intensity ratio of Bi to Ta using the results of SIMS measurement of an SBT single crystal thin film having a stoichiometric composition (SrBi ₂ Ta ₂ O ₉ ) grown on a SrTiO ₃ (100) single crystal substrate at a substrate temperature of 750 ° C. The average composition of Bi from 30 nm to 140 nm from the surface was 2 to 2.5 in the region 1 and close to the raw material composition, whereas the average composition in the region 2 was about 1.4. It was low. It is known that when the Bi composition is low, a paraelectric pyrochlore phase is easily formed. Therefore, when the crystallinity was evaluated using the X-ray diffraction method, the diffraction intensities from the SBT (115) plane in the regions 1 and 2 were almost the same, and no difference in crystallinity was observed. From this, it is considered that the Bi composition decreases along the crystal grain boundary in the region 2 where the substrate A is not overlapped.
3. Effect of heat treatment with ozone / oxygen mixed gas In the above example, heat treatment in ozone / oxygen mixed gas was performed for the purpose of removing residual hydrocarbons. To confirm this effect, ozone / oxygen mixed gas was used. The polarization characteristics of the SBT thin film (thickness 116 nm) produced without performing the heat treatment step in the mixed gas were evaluated. As a result, a remanent polarization value about 20% larger in the region 1 where the substrate A was not overlapped than in the region 2 where the substrate A was not overlapped was obtained with good reproducibility (FIG. 7). However, comparing the results of FIG. 5B and FIG. 7B, it was found that the remanent polarization value of the SBT thin film subjected to the heat treatment in the ozone / oxygen mixed gas was about 20% larger.
4). Influence of the distance between the substrates facing each other When a SBT thin film was prepared using a substrate (30 mm × 30 mm) having a size larger than that of the example, partial fusion between the films facing each other was confirmed. Therefore, the experiment was conducted by intentionally setting the distance between the thin films to about 600 μm. A substrate A and a substrate B having the same structure as FIG. 2 cut to a size of 26 mm × 26 mm are prepared, and a sol-gel solution containing 20% Nb on each substrate (raw material composition: Sr _0.8 Bi _2.2 Ta _1.6 Nb _0.4 O ₉ , Using a concentration of 0.25 mol / kg), a dry coating film of SBT (Nb 20%) was formed on the substrate by the same process as in the above example. Next, the thin film side of the substrate A and the thin film side of the substrate B were arranged on the susceptor of the infrared gold image furnace so as to face each other with an interval of 600 μm. FIG. 8 shows a specific arrangement method. In FIG. 8, the substrate A is arranged on the SiC susceptor 20, the spacer 21 is placed on the side, and the substrate B is placed on the spacer 21, thereby setting the distance 22 between the substrate A and the substrate B to 600 μm. The film was crystallized under the same conditions as in the example. Here, the process for removing the effect of the ozone / oxygen mixed gas is omitted. An SBT (Nb 20%) thin film having a film thickness of 139 nm was formed by repeating a series of steps four times. Finally, a Pt electrode was formed on the thin film to produce a capacitor structure.
[0027]
FIG. 9 shows the in-plane dependence of the remanent polarization value of the produced SBT (Nb 20%) thin film. The remanent polarization value at the central portion of the substrate was about 1.7 times larger than the remanent polarization value of the non-faced region indicated by the dotted line in the figure. Moreover, although the remanent polarization value decreased as it approached the periphery of the substrate, the remanent polarization value in the region of about 3 mm from the substrate edge shows a value about 30% larger than the remanent polarization value in the region where the substrates are not facing each other. It was. In the sample of FIG. 5, no significant in-plane dependence of the remanent polarization value was observed.
5. Effect of thick coating film thickness In the examples, the coating film thickness after drying was set to 50 nm or less. As a result, all the produced SBT thin films had a large leakage current, and the polarization characteristics could not be measured. Also, a conventional method for manufacturing a ferroelectric thin film, that is, a method in which the formation of a coating film and temporary baking at around 400 ° C. are repeated several times to produce a thick SBT thin film having a thickness of 100 nm or more and then the heat treatment of the embodiment is performed. When the substrate arrangement shown in FIG. 3 was performed, the remanent polarization value of the produced SBT thin film was comparable to that of the sample produced by the conventional method for producing a ferroelectric thin film. From these facts, crystallization of the substrate on which an amorphous ferroelectric thin film having a thickness of 50 nm or less is deposited in the arrangement shown in FIG. 3 under the heat treatment conditions of the embodiment gives a ferroelectric thin film having a large residual polarization. It was found to be effective.
6). Evaluation in PZT In the above examples, SBT was used, but it was confirmed that the same effect was exhibited in PZT.
[Brief description of the drawings]
FIG. 1 is a diagram showing the arrangement of substrates during heat treatment in the present invention.
FIG. 2 is a diagram illustrating film formation in an example.
FIG. 3 is a diagram showing an arrangement of substrates during heat treatment in an example.
FIG. 4 is a diagram showing a capacitor in an example.
FIG. 5 shows the polarization characteristics of region 1 and region 2 of the SBT thin film in the example.
FIG. 6 is a diagram showing the composition in the depth direction of regions 1 and 2 of the SBT thin film in the example.
FIG. 7 shows the polarization characteristics of an SBT thin film without ozone treatment in Examples.
FIG. 8 is a diagram showing an arrangement in which a space is provided between substrates in the embodiment.
FIG. 9 shows a distribution of polarization when a space is provided between substrates in an example.
[Explanation of symbols]
1,3 ... substrate 2,4 ... ferroelectric thin film 11 ... Si substrate 12 ... SiO ₂ film 13 ... Ti film 14 ... Pt film 15 ... SBT thin film 16 ... susceptor 17 ... SBT thin film 18 ... Pt electrodes 20 susceptor 21 ... Spacer 22 ... spacing

Claims (2)

In a method of depositing a ferroelectric thin film on a substrate and then performing a heat treatment to form a ferroelectric crystal thin film, at least two substrates are made of ferroelectric thin films made of a ferroelectric material containing Bi or Pb. A method of forming a ferroelectric thin film, wherein the heat treatment is performed in a state where the two are arranged face to face. 2. The method for forming a ferroelectric thin film according to claim 1, wherein an interval between the ferroelectric thin films is set within a range of 0 to 600 [mu] m.

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