JPH1012833A - Ferroelectric film covered base and its usage - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent asymmetry of the hysteresis loop, denseness and surface- flatening of a ferroelectric film, and preventing generation of leakage current, by forming a first ferroelectric film, an intermediate buffer layer and a second ferroelectric film of the same kind as the first ferroelectric film, on a substratum in this order. SOLUTION: An oxide silicon layer 12 as a thermal oxide film is formed on the surface of a base 11 of a silicon single crystal wafer, a bonding layer 13 of Ta and a lower electrode 14 of Pt are formed on the oxide silicon layer 12 in this order by using a sputtering method, and a substratum composed of Pt/Ta/SiO2 /Si is formed. A first ferroelectric film 15 of Bi4 Ti3 O12 is formed on the Pt electrode 14, and an intermediate buffer layer 16 of titanium oxide is formed on the first ferroelectric film 15. A second ferroelectric film 17 of Bi4 Ti3 O12 is formed on the titanium oxide 16, and a ferroelectric film covering substratum constituted of three layers is formed. By arranging the intermediate layer between the ferroelectric films, asymmetry of the hysteresis loop can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a ferroelectric film-coated substrate and its use. More specifically, the present invention relates to a ferroelectric film-coated substrate that can be used for a ferroelectric memory element, a pyroelectric sensor element, a piezoelectric element, and the like.

[0002]

2. Description of the Related Art Ferroelectric materials have many functions such as spontaneous polarization, high dielectric constant, electro-optic effect, piezoelectric effect and pyroelectric effect, and are therefore widely used in capacitors, oscillators, optical modulators or infrared sensors. Used in device development. However, in the past, single crystals or ceramics have been used for these applications.

On the other hand, with the development of thin film forming technology, high-quality ferroelectric films have been obtained, and applications that have not existed in the past are expected. In particular, recently, a ferroelectric memory (FRAM) that operates at high density and at high speed by being combined with a semiconductor memory such as a DRAM has been developed. A ferroelectric memory is a non-volatile memory that does not require a backup power supply and uses a ferroelectric characteristic (spontaneous polarization effect) of a ferroelectric. The development of such memories requires remanent polarization (P
It is necessary to use a ferroelectric material having characteristics such as a large r), a small coercive electric field (Ec), a low leakage current, and an excellent resistance to repetition of domain inversion. Further, to reduce the operating voltage and adapt to the semiconductor fine processing process,
It is desired to realize the above characteristics with a thin film having a thickness of 00 nm or less.

At present, Pb is used for application to FRAM and the like.
An oxide ferroelectric having a perovskite structure such as TiO ₃ , PZT, or PLZT is formed into a thin film by a sputtering method.
Attempts have been made by a thin film forming method such as a vapor deposition method, a sol-gel method, an organometallic decomposition (MOD) method, and an organometallic chemical vapor deposition (MOCVD) method. Of the above ferroelectric materials, P
ZT is currently the most intensively researched,
For example, the residual polarization Pr is 10 μC / cm ² to 26 μC /
Some have values as large as cm ² . However, since Pb having a high vapor pressure is contained, a change in the film composition is likely to occur during film formation or heat treatment, etc., generation of pinholes, generation of a low dielectric constant layer due to the reaction between the oxidation-resistant base electrode Pt and Pb. As a result, there is a problem in that the leakage current and the polarization inversion repetition resistance deteriorate as the film thickness decreases. Therefore, development of a material having excellent ferroelectric properties and polarization reversal resistance has been desired. In addition, in consideration of application to an integrated device, the thin film must have denseness and planar smoothness that can cope with fine processing.

On the other hand, as an oxide ferroelectric containing no Pb that adversely affects leakage current and polarization inversion resistance, bismuth titanate Bi ₄ Ti having a layered perovskite structure is used.
There is ₃ O ₁₂ . The ferroelectric properties are as follows: residual spontaneous polarization Pr = 50 μC / cm ² in the a-axis direction, coercive electric field Ec = 50 kV / c
The spontaneous polarization Pr = 4 μC / cm ² and the coercive electric field Ec = 4 kV / cm in the m and c axis directions show excellent characteristics.

Therefore, in order to apply the large spontaneous polarization of Bi ₄ Ti ₃ O _{12 to} a ferroelectric nonvolatile memory or the like, it is necessary to have many a-axis components of the crystal in the direction perpendicular to the substrate. desirable. The thinning of Bi ₄ Ti ₃ O ₁₂
Until now, attempts have been made by MOCVD or sol-gel methods, but most of them are c-axis oriented films with small spontaneous polarization, and at present, almost no a-axis oriented films have been obtained. In the conventional sol-gel method, a heat treatment at 650 ° C. or more is required to obtain good ferroelectric properties, and the film surface morphology is composed of crystal grains of about 0.5 μm, so high integration requiring fine processing is required. Difficult to apply to devices. On the other hand, Bi ₄
Ti ₃ O ₁₂ thin film is made of Pt / SiO
₂ / Made on Si substrates, these substrates are:
It cannot be directly applied to an actual device structure. That is, like a Pt / Ti / SiO ₂ / Si substrate, an adhesive layer such as a Ti film for securing the adhesive strength between the Pt electrode layer and the underlying SiO ₂ is required. However,
When a Bi ₄ Ti ₃ O ₁₂ thin film is formed by MOCVD on a substrate provided with such an adhesive layer, the surface morphology is composed of coarse crystal grains of about 0.5 μm and the paraelectric pyrochlore phase (Bi ₂ Ti ₂ O ₇ ) has been reported to occur easily (Jpn.
Appl. Phys. , 32,1993, pp. 408
6 and J.I. Ceramic Soc. Japan, 10
2, 1994, p. 512). If the film surface morphology is composed of coarse crystal grains, it cannot be applied to a highly integrated device requiring fine processing, and if the film thickness is small, it causes a pinhole and causes a leakage current. Therefore, in order to realize a ferroelectric film having good ferroelectric properties with a thin film thickness, a buffer layer is inserted between the base and the ferroelectric film to make the ferroelectric film dense and smooth. Is reported to be possible (Jp
n. J. Appl. Phys. , 32, 1993, p
p. 4086).

[0007]

However, in a capacitor having the above-mentioned ferroelectric thin film sandwiched between upper and lower electrodes by the same kind of electrode, a buffer layer is inserted between the lower electrode and the ferroelectric thin film layer, thereby making the upper and lower layers thin. , The asymmetry of the hysteresis loop due to the difference in the material in contact with the electrode is likely to occur. Such a phenomenon occurs when upper and lower electrode materials are different even when the buffer layer is not used.

As described above, the technique using the conventional buffer layer allows the ferroelectric thin film to be densified and the surface to be smoothed, but the asymmetry of the ferroelectric hysteresis loop caused by the multilayer structure of the film is possible. Have a problem.

[0009]

Thus, according to the present invention, a first ferroelectric film, an intermediate buffer layer and a second ferroelectric film of the same type as the first ferroelectric film are formed on a substrate in this order. A ferroelectric film-coated substrate characterized by comprising:
According to the present invention, the lower electrode, the first ferroelectric film, the intermediate buffer layer, the second ferroelectric film of the same type as the first ferroelectric film, and the upper electrode of the same type are formed on the base in this order. Is provided.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a substrate usable in the present invention includes a silicon substrate, a GaAs substrate and the like, and a substrate on which any underlayer such as a transistor and an interlayer insulating film is formed. . Next, a first ferroelectric film is formed on the base. As a material that can be used for the first ferroelectric film, any material that does not contain Pb may be used. Specifically, Bi ₄ Ti ₃ O ₁₂ , SrBi ₂ Ta ₂
O ₉ , SrBi ₂ Nb ₂ O ₉ , SrBi ₄ Ti ₄ O ₁₅ ,
BaBi ₂ Ta ₂ O ₉ , BaBi ₂ Nb ₂ O ₉ , BaB
i ₄ Ti ₄ O ₁₅ , Na _0.5 Bi _4.5 Ti ₄ O ₁₅ , K _0.5
Bi _4.5 Ti ₄ O ₁₅ , Sr ₂ Bi ₄ Ti ₅ O ₁₈ , Ba ₂
Bi-based layered compounds such as Bi ₄ Ti ₅ O ₁₈ , glycine sulfate (TGS), LiNbO ₃ , LiTaO ₃ , BaTi
O ₃ and SrTiO ₃ are exemplified. Among them, Bi ₄ T
i ₃ O ₁₂ is preferred. The thickness is preferably 10 to 200 nm. The method for forming the first ferroelectric film is not particularly limited, and any known method can be used. For example, sputtering, vapor deposition, CVD, MO
CVD, a sol-gel method, etc. are mentioned. Of these, MOC
The VD method is preferred.

When forming a film by the MOCVD method, the substrate temperature is preferably 450 to 650 ° C. 450 ° C
If the temperature is lower, the ferroelectric film cannot obtain sufficient crystallinity, which is not preferable. If the temperature is higher than 650 ° C., the crystal constituting the ferroelectric film is undesirably coarse. For example, as a bismuth raw material that can be used in the MOCVD method for forming Bi ₄ Ti ₃ O ₁₂ , triortho tolyl bismuth, triphenyl bismuth, and the like can be mentioned. On the other hand, as titanium raw materials, titanium isopropoxide, titanium tetra Butoxide and the like. Actually, a film is formed by supplying a bismuth raw material, a titanium raw material and an oxygen gas into a film forming apparatus by using an inert gas (eg, argon, nitrogen or the like) as a carrier gas. Note that the pressure in the film forming apparatus is preferably 2 to 10 Torr. If the pressure is higher than 10 Torr, a gas phase reaction is likely to occur, and if the pressure is lower than 2 Torr, the film forming rate is undesirably reduced.

Next, an intermediate buffer layer is formed on the first ferroelectric film. Materials that can be used for the intermediate buffer layer include metal oxides such as titanium oxide, tantalum oxide, niobium oxide, strontium titanate, barium titanate, zircon oxide, aluminum oxide, bismuth oxide, yttrium oxide, and hafnium oxide. Of these, titanium oxide is preferred. The thickness is preferably in the range of 2 to 7 nm. If the film thickness is smaller than 2 nm, it is not preferable because the entire surface of the first ferroelectric film cannot be covered, and if it is larger than 7 nm, the applied voltage is less likely to be applied to the first ferroelectric film. The method for forming the intermediate buffer layer is not particularly limited, and any known method can be used. For example, sputtering, vapor deposition, CVD, MOCVD and the like can be mentioned. Of these, the MOCVD method is preferable. For example, the film can be formed by performing the same operation under the same conditions as in the above-described method of manufacturing the first ferroelectric film except that no bismuth raw material is supplied.

The ferroelectric film-coated substrate of the present invention can be formed by further forming a second ferroelectric film on the intermediate buffer layer. Here, the second ferroelectric film is made of the same material as the first ferroelectric film. The film thickness is 50-30.
It is preferably 0 nm. Further, a lower electrode described below may be provided between the base and the first ferroelectric film, an insulating film may be provided between the base and the lower electrode, and an adhesion layer may be provided between the insulating film and the lower electrode. The insulating film is made of silicon oxide, silicon nitride, or a stacked film thereof having a thickness of 50 to 300 nm, and can be formed by a thermal oxidation method, a CVD method, a sputtering method, or the like. Examples of the adhesion layer include Ti, Ta, and the like.

The substrate coated with a ferroelectric film of the present invention can be used for any device utilizing a ferroelectric effect, a piezoelectric effect, a pyroelectric effect, an electro-optical effect and the like.
Examples of such a device include a semiconductor device, an optical modulator, an ultrasonic sensor, and an infrared linear array sensor. Further, according to the present invention, the lower electrode, the first ferroelectric film,
A capacitor is provided, comprising an intermediate buffer layer, a second ferroelectric film of the same type as the first ferroelectric film, and an upper electrode of the same type on a base in this order.

As the substrate, the same substrate as the ferroelectric film-coated substrate can be used. Materials that can be used for the lower electrode include Pt, Al, Cu, RuO ₂ , Ir, IrO _{2 and the} like. Of these, Pt is preferred. The film thickness is 5
It is preferably from 0 to 300 nm. The method for forming the lower electrode is not particularly limited, and any known method can be used. For example, sputtering, evaporation, MO
CVD method and the like can be mentioned.

Next, a first ferroelectric film, an intermediate buffer layer and a second ferroelectric film are formed on the lower electrode in this order. The raw materials, film thicknesses, forming methods, and the like that can be used are the same as those described for the ferroelectric film-coated substrate. Furthermore, the capacitor of the present invention can be formed by forming an upper electrode on the second ferroelectric film. Here, the upper electrode is made of the same material as the lower electrode. The film thickness is 50 to 300 n.
m is preferable. Note that the same method as that for forming the lower electrode can be used.

Incidentally, an adhesion layer made of Ti, Ta or the like for improving the adhesion may be provided between the base and the lower electrode. As an example of the use of the capacitor, there is a nonvolatile memory as shown in FIG. 1, for example. In this memory, a transistor 3 comprising a word line 2 is provided on a substrate between bit lines 1 provided on a surface layer of the substrate, and a lower electrode 5 is provided on the word line 2 via an insulating film 4.
A capacitor 8 including a ferroelectric film 6 and an upper electrode 7 is arranged (an intermediate buffer layer is omitted in this figure). The capacitor has a structure connected to one bit line 1 by a wiring layer 9. In this figure, the base is composed of the insulating film 4 located below the lower electrode 5, the transistor 3, and the substrate 1.

As described above, by disposing the intermediate buffer layer between the ferroelectric films, the hysteresis caused by the asymmetry of the structure when the buffer layer is disposed between the conventional substrate and the ferroelectric film. Loop asymmetry can be prevented.
Further, the surface of the ferroelectric film can be densified and flattened, and the generation of a leak current due to the morphology of the ferroelectric film can be prevented.

[0019]

【Example】

Example 1 A capacitor as shown in FIG. 2 was manufactured by the following steps. First, a thermal oxide film (silicon oxide layer) 12 having a thickness of 200 nm is formed on the surface of a silicon single crystal wafer (substrate) 11, and a Ta layer (adhesive layer) 13 having a thickness of 30 nm and a Pt electrode (200 nm) are formed thereon. A lower electrode 14 is formed in this order by a sputtering method, and Pt / Ta / SiO ₂ / S
A substrate made of i was prepared.

Next, on the Pt electrode 14, a 20 nm Bi
_{A 4} Ti ₃ O ₁₂ film (first ferroelectric film) 15 was formed by MOCVD. The film forming conditions were a substrate temperature of 600 ° C. and a film forming time of 10 minutes. Subsequently, the above Bi ₄ Ti ₃ O ₁₂ film 1
On 5, a titanium oxide (intermediate buffer layer) 16 of 5 nm was formed at a substrate temperature of 400 ° C. and a film formation time of 2 minutes.

Further, on the titanium oxide 16, a substrate temperature of 600 ° C., a film formation time of 40 minutes, and a thickness of 80 nm of Bi ₄ Ti ₃
By forming the O ₁₂ film (second ferroelectric film) 17,
A ferroelectric film-coated substrate consisting of a three-layer film having a total thickness of 105 nm was formed. Table 1 shows other film forming conditions of the Bi ₄ Ti ₃ O ₁₂ film and the titanium oxide.

[0022]

[Table 1] The Bi ₄ Ti ₃ O ₁₂ film was formed as shown in Table 1 above.
Triortho tolyl bismuth (Bi)
_{(O-C 7 H 7)} 3 a), it was used as the titanium isopropoxide _{(Ti (i-OC 3 H} 7) 4) as a titanium raw material. Further, these raw materials were heated and vaporized at the temperatures shown in Table 1 (bismuth raw material 160 ° C., titanium raw material 50
° C), and supplied onto a substrate heated and held together with an argon (Ar) gas as a carrier gas and an oxygen (O ₂ ) gas as a reaction gas. Here, the argon gas flow rate was 200 sccm for the bismuth raw material and 50 s for the titanium raw material.
ccm and the oxygen gas flow rate was 1000 sccm.
Note that the pressure in the film forming chamber in the film forming step was 10 To
If it is more than rr, a gas phase reaction is likely to occur.
orr.

As for titanium oxide, only a titanium raw material and oxygen gas were supplied. Bi formed as above
FIG. 3 shows the result of observing the surface morphology of the ₄ Ti ₃ O ₁₂ film (second ferroelectric film) by SEM (scanning electron microscope). As can be seen from FIG. 3, the film of this example was made of grains having a particle size of about 0.15 μm, and was found to be dense and had a smooth surface. This is Bi ₄ Ti ₃
The O ₁₂ film (first ferroelectric film) has a large particle size because there is no buffer layer between the O ₁₂ film and the lower electrode. However, by forming titanium oxide (intermediate buffer layer), Bi ₄ Ti ₃ O ₁₂ It is thought that the particle diameter of the film (second ferroelectric film) was reduced, and as a result, the surface of the Bi ₄ Ti ₃ O ₁₂ film (second ferroelectric film) could be densified and flattened. Can be

FIG. 4 shows the result of evaluating the crystallinity of the Bi ₄ Ti ₃ O ₁₂ film (second ferroelectric film) by the X-ray diffraction method. The obtained film is made of Bi ₄ Ti having random orientation.
It is shown to be ₃ O ₁₂ . Next, the above Bi ₄ T
A Pt electrode (100 μmφ) 18 as an upper electrode was laminated on the i ₃ O ₁₂ film (second ferroelectric film) 17 by vacuum evaporation to form a capacitor. FIG. 5 shows current-voltage characteristics of this capacitor. In FIG. 5, when the applied voltage is 5 V, Il
A value of (leak current density) = 1 × 10 ⁻⁸ A / cm ² was obtained. FIG. 6 shows a hysteresis loop of this capacitor. Since a hysteresis loop with good vertical symmetry can be obtained and contains more a-axis components than the conventional c-axis alignment film, when the applied voltage is 5 V,
Remanent polarization Pr = 14 μC / cm ² , coercive electric field Ec = 130
It was possible to obtain a value of kV / cm higher than that of the ferroelectric film having the c-axis orientation.

Comparative Example 1 Same as Example 1 except that the intermediate buffer layer was not formed, and the ferroelectric film of 105 nm was formed at 600 ° C. for 60 minutes without dividing the ferroelectric film into two layers. To form a capacitor. This capacitor had too large a leak current value to measure a hysteresis loop. Also, the surface of the ferroelectric film before forming the upper electrode was SEM
FIG. 7 shows the results of the observation. As can be seen from FIG. 7, the film of this comparative example is a film composed of coarse particles and having severe surface irregularities, and it was found that the increase in leak current was caused by the morphology of this surface.

Comparative Example 2 A capacitor was formed in the same manner as in Comparative Example 1, except that a buffer layer was provided between the ferroelectric film and the lower electrode. However, the thickness of the ferroelectric film was 100 nm. The buffer layer is made of titanium oxide having a thickness of 5 nm,
Formed by MOCVD for one minute.

FIG. 8 shows the result of SEM observation of the surface of the ferroelectric film before the formation of the upper electrode. As can be seen from FIG. 8, the film of this comparative example was made of grains having a particle size of about 0.1 μm, and the surface was dense and smooth. FIG. 9 shows the result of evaluating the crystallinity of the ferroelectric film by the X-ray diffraction method. The resulting film is shown to be Bi ₄ Ti ₃ O ₁₂ with random orientation.

FIG. 10 shows the current-voltage characteristics of this capacitor. In FIG. 10, when the applied voltage is 5 V, Il = 1 ×
A value of 10 ^-7 A / cm ² could be obtained. Further, FIG.
FIG. 1 shows a hysteresis loop of this capacitor.
A vertically asymmetric hysteresis loop was obtained. From these results, since the surface morphology of the ferroelectric film-coated substrate of this comparative example is dense and smooth, the leakage current due to the surface morphology is small. However, it can be seen that the asymmetry of the hysteresis loop occurs due to the different materials that contact the upper and lower electrodes.

[0029]

According to the present invention, by disposing the intermediate buffer layer between the ferroelectric films, it is possible to reduce the asymmetry of the structure when the buffer layer is disposed between the conventional substrate and the ferroelectric film. The resulting asymmetry of the hysteresis loop can be prevented. Further, the ferroelectric film can be densified and its surface can be flattened, and the generation of a leak current due to the morphology of the ferroelectric film can be prevented.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of an application example of a capacitor of the present invention.

FIG. 2 is a schematic sectional view of a capacitor according to the first embodiment.

FIG. 3 is an SEM photograph of the surface of a second ferroelectric film of Example 1.

FIG. 4 is an X-ray diffraction pattern of a second ferroelectric film of Example 1.

FIG. 5 is a diagram showing the applied voltage dependence of the leakage current density of the capacitor of Example 1.

FIG. 6 is a diagram illustrating a hysteresis curve of the capacitor according to the first embodiment.

FIG. 7 is an SEM photograph of the surface of a second ferroelectric film of Comparative Example 1.

FIG. 8 is an SEM photograph of the surface of a second ferroelectric film of Comparative Example 2.

FIG. 9 is an X-ray diffraction pattern of a second ferroelectric film of Comparative Example 2.

FIG. 10 is a diagram showing the applied voltage dependence of the leak current density of the capacitor of Comparative Example 2.

FIG. 11 is a diagram showing a hysteresis curve of the capacitor of Comparative Example 2.

[Explanation of symbols]

 Reference Signs List 1 bit line 2 word line 3 transistor 4 insulating film 5 lower electrode 6 ferroelectric film 7 upper electrode 8 capacitor 9 wiring layer 11 silicon substrate 12 silicon oxide layer 13 adhesive layer 14 lower electrode 15 first ferroelectric film 16 intermediate buffer Layer 17 Second ferroelectric film 18 Upper electrode

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical indication location H01L 21/8247 29/788 29/792

Claims (4)

[Claims] 1. A ferroelectric material comprising: a first ferroelectric film, an intermediate buffer layer, and a second ferroelectric film of the same type as the first ferroelectric film on a substrate in this order. Film-coated substrate. 2. The substrate according to claim 1, wherein the intermediate buffer layer is a metal oxide. 3. The substrate according to claim 1, wherein the intermediate buffer layer is titanium oxide, and the first and second ferroelectric films are Bi ₄ Ti ₃ O ₁₂ . 4. A substrate comprising a lower electrode, a first ferroelectric film, an intermediate buffer layer, a second ferroelectric film of the same type as the first ferroelectric film, and an upper electrode of the same type in this order. A capacitor, comprising: