JP2000260955A - Capacitor - Google Patents

Abstract

(57) [Problem] It is difficult to reduce the thickness of a conventional lead zirconate titanate-based ferroelectric thin film, and it has been difficult to reduce the voltage in memory operation. Further, the characteristics after exposure to a reducing atmosphere are deteriorated as compared with those after forming the capacitor, and it cannot be said that changing the electrode material is sufficient. A thin film made of a thermally and chemically stable material is formed between a ferroelectric thin film and an upper electrode and between the ferroelectric thin film and a lower electrode. Moreover,
In the film thickness direction of the ferroelectric thin film 25 sandwiched between the upper electrode 26 and the lower electrode 23, the ferroelectric thin film and the thin film 24 made of a thermally and chemically stable material are alternately laminated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a capacitor for a ferroelectric memory, and more particularly, to a structure of a capacitor using a lead zirconate titanate-based ferroelectric thin film.

[0002]

2. Description of the Related Art Ferroelectric memories have attracted attention as non-volatile memories which are excellent in high speed, low power consumption and high integration, and have been vigorously developed in recent years. As ferroelectric materials for capacitors, lead zirconate titanate (PZT) and bismuth layered perovskite such as SrBi ₂ Ta ₂ O ₉ called Y1 have been developed. However, since Y1 has many problems such as low remanent polarization, high crystallization temperature, and difficulty in obtaining good morphology, PZT-based materials are considered to be more promising at present.

[0003] PZT-based materials have the greatest technical problem in the practical use of deterioration of rewriting characteristics, so-called “fatigue”.
cs11, 161 (1995),
By changing the electrode material from Pt / Ti to an oxide electrode such as IrO ₂ , oxygen deficiency and diffusion of lead, which is a main component, to the electrode are prevented, and the fatigue is improved. In addition, the effect of the Ir-based electrode on the integration process with the semiconductor, that is, the reduction process is also shown. This has greatly advanced the practical use of ferroelectric memories.

[0004]

However, the PZT-based ferroelectric thin film used for the capacitor of the conventional ferroelectric memory still has the following problems.

First, the PZT thin film has a problem that the polarization saturation voltage is higher than that of the Y1 thin film. That is, it is difficult to lower the voltage in the memory operation. To make low voltage polarization reversal possible, the film may be thinned. However, when the thickness of the PZT-based thin film is 200 nm or less, the amount of polarization is remarkably reduced, and it is difficult to reduce the thickness. Although the cause has not been fully elucidated yet, it is considered that the influence of the electrode interface extends to the inside of the thin film.

As described above, in the reduction process, the use of an Ir electrode or the like also improves the deterioration of characteristics as compared with the case of a Pt / Ti electrode, but does not eliminate oxygen deficiency in the thin film. The amount of polarization after that is lower than that after the formation of the capacitor, and the prevention of reduction is not sufficient. This not only affects fating, but also affects imprint characteristics, which is another indicator of reliability, and causes erroneous reading. Therefore, a further margin for the amount of polarization is required.

The present invention has been made to solve the above-mentioned problems, and enables thinning for low-voltage polarization reversal, further suppresses oxygen deficiency during the reduction process, and increases the amount of polarization. It is an object of the present invention to provide a capacitor using a ferroelectric thin film, particularly a capacitor using a PZT-based ferroelectric thin film.

[0008]

SUMMARY OF THE INVENTION The capacitor of the present invention solves the above-mentioned problems. In a capacitor of a ferroelectric memory in which a ferroelectric thin film is sandwiched between an upper electrode and a lower electrode, a ferroelectric thin film and an upper capacitor are provided. A thin film made of a thermally and chemically stable material was formed between the electrodes and between the ferroelectric thin film and the lower electrode, and the thickness of the thin film made of the thermally and chemically stable material was 50
Angstroms or less.

Further, according to the present invention, there is provided a capacitor for a ferroelectric memory in which a ferroelectric thin film is sandwiched between an upper electrode and a lower electrode. At least five or more thin films made of chemically stable materials are alternately stacked, and the thin films made of thermally and chemically stable materials are formed so as to be in contact with the upper electrode and the lower electrode. The film thicknesses of the thin films made of chemically and chemically stable materials are all the same and are not more than 50 angstroms, and the film thicknesses of the ferroelectric thin films are all the same.

Further, the capacitor of the present invention is characterized in that the thin film made of a thermally and chemically stable material has a substantial lattice constant smaller than that of the ferroelectric thin film, and that the thermally and chemically stable thin film has a substantial lattice constant. The lattice mismatch between the ferroelectric thin film, the upper electrode material, and the lower electrode material of the thin film made of the material is within 10%, and the ferroelectric thin film material is a lead zirconate titanate-based material. And a thin film made of a chemically stable material is cerium oxide,
Alternatively, it is made of zirconium oxide or yttria-stabilized zirconia, and the upper electrode and the lower electrode contain at least an oxide.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to embodiments.

[0012]

(Embodiment 1) FIG. 1 is a view showing a sectional structure of a capacitor according to Embodiment 1 of the present invention. It comprises a substrate 1, a buffer layer 2, a lower electrode 3, a thin film 4 made of a thermally and chemically stable material, a ferroelectric thin film 5, and an upper electrode 6.

The specific configuration is as follows. Substrate 1
Is generally used Si (100).
The ferroelectric thin film 5 includes lead zirconate titanate (PZT), which is considered to be a promising practical application of ferroelectric memory.
Is used. In order to obtain good capacitor characteristics, it is desirable that at least the ferroelectric thin film 5 is an epitaxial film having a uniform crystal orientation and of high quality. To this end, the lower electrode 3 also desirably has a crystal structure similar to that of PZT, that is, a perovskite structure, and naturally needs to be an epitaxial film. Therefore, here, a SrRuO ₃ thin film is used for the lower electrode 3. Other than the perovskite structure, a rutile structure of IrO ₂ or RuO ₂ may be used as long as the crystal orientation can be controlled. Now, SrR directly on Si substrate
When an attempt is made to produce a uO ₃ thin film, amorphous silicon oxide is formed at the interface, so that high quality SrRuO 3
_{3 No} thin film is obtained. Therefore, it is necessary to form the buffer layer 2. Here, as the buffer layer 2, T
A double buffer of i _0.7 Al _0.3 N / Pt was used. Ti _0.7 Al _0.3 N is a material having excellent oxidation resistance, and is used for preventing oxidation of Si. However,
Since Ti _0.7 Al _0.3 N and SrRuO ₃ have a large lattice mismatch, Pt is further formed between them. T
The lattice mismatch between i _0.7 Al _0.3 N and Pt is 6.6.
%, It is possible to epitaxially grow Pt at such a value. On the other hand, Pt and SrRuO ₃
Shows a very good lattice mismatch of 0.43%, and high-quality SrRuO ₃ thin film growth can be expected. Here, cerium oxide (CeO ₂ ) is used as the thin film 4 made of a thermally and chemically stable material. P
In order to epitaxially grow a ZT thin film, it must also be an epi-grown film. CeO ₂ is a cubic having a CaF ₂ structure and is considered to be compatible with both SrRuO ₃ and PZT. PZT thin film on CuO ₂ is cube-on-cube, ie, CeO ₂ <100> // PZT <
100>, CeO ₂
Is larger in lattice constant and lattice mismatch, so it is considered that such epi growth does not occur. CeO
45 ° rotation of the PZT on _2, i.e. _CeO 2 <100
>> // PZT <110>, the actual lattice constant is P
CeO ₂ is smaller than ZT, and the lattice mismatch shows a small value of 2% or less. That is, epi growth of the PZT thin film can be expected. Further, the material of the upper electrode 6 was SrRuO ₃ which was the same as the material of the lower electrode 3.

Hereinafter, a process for producing the capacitor of the present invention having the above-described structure will be specifically described. First, Si (10
0) to form a _Ti 0.7 _{Al 0.3} N on the substrate.

The formation of Ti _0.7 Al _0.3 N is performed by reactive electron beam evaporation in nitrogen plasma using each metal of Ti and Al as an evaporation source. Substrate temperature 600 ° C
(100) oriented Ti _0.7 Al
_{A 0.3} N epitaxial film was obtained. At this time, the film thickness was set to 1000 angstroms. Next, Pt was formed thereon by sputtering to a thickness of 500 angstroms to obtain a (100) oriented epitaxial film. After that, a SrRuO ₃ thin film is formed on Pt by SrR
It is produced by a laser ablation method using a single uO ₃ target. By starting irradiation of the oxygen plasma to the substrate after the start of the ablation, a (010) -oriented SrRuO ₃ epitaxial film was obtained at a substrate temperature of 650 ° C. According to the polar figure of the X-ray diffraction, the in-plane orientation was rotated by 45 ° with respect to Pt. That is, Pt <110> // SrRuO ₃ <100>. The film was much better in crystallinity and properties than when formed directly on a Si substrate. Then, on top of it, C
An eO ₂ film is similarly formed by using a laser ablation method. By irradiation with oxygen plasma, a (100) oriented CeO ₂ epitaxial film was obtained at a substrate temperature of 650 ° C. From the polar figure of X-ray diffraction, C on SrRuO ₃
eO ₂ was growing on a cube-on-cube. This is because SrRuO ₃ and CeO ₂ have similar lattice constants. That is, in this case, the actual lattice constant, in other words, the value described in the literature becomes the respective actual lattice constant, and the lattice mismatch is within 3%. At this time, the film thickness was less than 50 angstroms. The film thickness was controlled by RHEED vibration. CeO ₂ film thickness of 5
If the thickness is larger than 0 Å, the characteristics after forming the capacitor are adversely affected, which is not preferable. Next, Ce
PZ on O ₂ also by laser ablation method
A T film is formed. At this time, the composition ratio of the target was such that Pb was excessive and Zr: Ti = 30: 70. As a result, the film composition was changed from Pb _0.95 to _1.05 (Zr _0.3 Ti
_0.7) to obtain a _{O y.} By forming the film at a substrate temperature of 600 ° C. while irradiating with oxygen plasma, a (100) -oriented PZT epitaxial film was obtained. The film thickness is 1500
〜3000 Å. From the polar figure of the X-ray diffraction, the in-plane orientation was rotated by 45 ° with respect to CeO ₂ . That is, as described above, PZT <110> is
PZT to grow so as to overlap eO ₂ <100>
Is √2 times the literature value, which is about 3.5% larger than CeO ₂ . In other words, the lattice mismatch is about 3.5%. Further, CeO ₂ and SrRuO ₃ were successively formed thereon by the above-described method, thereby producing a capacitor as shown in FIG. The film thickness of CeO ₂ on PZT is also PZT
The thickness was set to 50 angstroms in order to prevent deterioration of the capacitor characteristics as in the case of the film thickness of CeO ₂ below. Here, it is described that evaluation by X-ray diffraction is performed each time after each thin film is formed. However, these are the results confirmed in advance at the stage of forming the film forming conditions, and in fact, the SrRuO _{3 of the} lower electrode 3 The layers up to SrRuO _{3 of the} upper electrode 6 are continuously formed in the same chamber.

The characteristics of the capacitor manufactured by the above method were evaluated. First, the value of the remanent polarization Pr is expressed as PZT
Showed little change when the film thickness was 1500 to 3000 angstroms, and showed a similar value. That is, thinning, which has been an issue to date, has become possible. As a result, the polarization inversion voltage was lower than that of the related art, and was almost the same as that of the Y1-based material. Fatigue was hardly observed until the number of switching was 10 ¹⁵ . Further, even after the wiring process and after the exposure to the reducing atmosphere at the time of forming the final protective film SiN, the reduction of the switching charge was improved to within 5% after the formation of the capacitor as compared with the conventional case. They are,
It is considered that the thin film 4 made of a thermally and chemically stable material exists at the interface between the lower electrode 3 and the ferroelectric film 5 and at the interface between the ferroelectric film 5 and the upper electrode 6. That is,
Compared with the case of using only the conventional electrode material such as SrRuO ₃ , the C of the thin film 4 made of a thermally and chemically stable material is
It is considered that eO ₂ plays a more effective role in diffusion of Pb and prevention of oxygen deficiency, and further, it has become possible to reduce the thickness of the thin film in order to prevent the influence of the electrode interface from reaching the inside of the thin film. At this time, it should be noted that forming the thin film 4 made of a thermally and chemically stable material thickly deteriorates the characteristics. However, the above results were obtained only when a multilayer epitaxial growth technique became possible. Lattice mismatch is an important point in epi growth,
In particular, the lattice mismatch of the thin film 4 made of a thermally and chemically stable material to the ferroelectric thin film 5, the material of the upper electrode 6, and the material of the lower electrode 3 must be within 10%. Above that, epi-growth of the ferroelectric thin film 5 is difficult.

Here, CeO ₂ is used for the thin film 4 made of a thermally and chemically stable material, but the same effect can be obtained with zirconium oxide ZrO ₂ or yttria stabilized zirconia YSZ. However, the lattice mismatch between the ferroelectric thin film 5, the material of the upper electrode 6, and the material of the lower electrode 3 at that time must also be within 10%. Ti _0.7 Al is used as the buffer layer 2.
_{Although a 0.3} N / Pt double buffer layer was used, other materials may be used. The method of forming each material may be any method other than the above as long as an epitaxial film can be obtained.

(Embodiment 2) FIG. 2 is a view showing a sectional structure of a capacitor according to Embodiment 2 of the present invention. Substrate 2
1, a buffer layer 22, a lower electrode 23, a thin film 24 made of a thermally and chemically stable material, a ferroelectric thin film 25,
And the upper electrode 26.

The specific configuration is the same as in the first embodiment.
That is, Si (100) and buffer layer 2
2 is a double buffer of Ti _0.7 Al _0.3 N / Pt, SrRuO _{3 is used for} the lower electrode 23 and the upper electrode 26,
PZT was used for the ferroelectric thin film 5. However, the thin film 24 made of a thermally and chemically stable material is Y here.
SZ was used.

The manufacturing process of the capacitor of the present invention having the above-described structure will be specifically described. Buffer layer 22Ti
The manufacturing method of _0.7 Al _0.3 N / Pt and the lower electrode 23 and the upper electrode 26 SrRuO ₃ is the same as that of the first embodiment. A method of manufacturing a laminated structure of a thin film 24YSZ and a ferroelectric thin film 25PZT made of a thermally and chemically stable material between the lower electrode 23 and the upper electrode 26 will be described below. YS
The Z and PZT films are continuously formed in the same chamber as the SrRuO ₃ thin film using a laser ablation method. The YSZ used here has a substitution amount of yttria of 9
It has a cubic structure in mol%. YSZ on SrRuO ₃ has a (100) orientation, and the in-plane orientation is rotated by 45 ° with respect to SrRuO ₃ from the polar figure. That is, SrRuO ₃ <100> // YSZ <110>. Therefore, the lattice mismatch is 3% or less, in other words, the actual lattice constant of SrRuO ₃ is about 3% larger than that of YSZ. The lattice constant in the YSZ literature is 5.13.
If the film thickness is as thin as 50 Å or less, a tensile stress is applied under the influence of SrRuO ₃ , so that the a and b axis lengths are long and the c axis length is short. On the other hand, PZT on YSZ is also (100) -oriented, and the in-plane orientation is rotated by 45 ° with respect to YSZ from the polar figure. That is, YSZ <100> // PZT <110
>. Therefore, the lattice constant in the literature is larger in YSZ than in PZT, but the actual lattice constant is Y in comparison with PZT.
SZ is smaller and the lattice mismatch is on the order of a few percent. Thus, PZT on YSZ / SrRuO ₃ is affected by the underlayer up to a film thickness of 200 Å,
A compressive stress is applied, and the a- and b-axis lengths are short, and the c-axis length is long. After confirming the above in advance, continuous film formation of the following laminated structure was performed.

First, a YSZ thin film is deposited on SrRuO ₃ by RH.
46 Å is formed while observing the EED oscillation.
After waiting for the intensity of the RHEED vibration to recover, PZT is formed to about 200 angstroms while watching the RHEED vibration in the same manner. Further, YSZ is deposited thereon in a similar manner by 46 angstroms, and PZT is further formed thereon by about 200 angstroms. This is repeated, and the uppermost layer is YSZ. What is important here is that the thickness of each YSZ is the same, and the thickness of each PZT is the same. If they are not the same, the charge will be biased after the production of the capacitor, and the characteristics will be degraded.
Finally, SrRuO ₃ as the upper electrode 26 is formed to obtain the capacitor shown in FIG.

The PZT 5 layer and YSZ prepared by the above method
The characteristics of the six-layer capacitor were evaluated. Where PZT
The total film thickness of the five layers is about 1000 Å. First, regarding the value Pr of remanent polarization, although the PZT film thickness is as thin as 1000 Å as a whole, it is increased by about 10% from the case where the PZT film thickness of Example 1 is 1500 Å. That is, it is possible to further reduce the thickness. This is because BaTiO ₃ / SrTi
It is considered that, as in the case of the O ₃ strained superlattice, compressive stress was applied to PZT and the relative dielectric constant increased. In addition, the polarization inversion voltage was further lowered by about 5% as compared with Example 1. Fatigue was hardly observed until the number of switching times was 10 ¹⁸ . Further, even after the wiring step and after the exposure to the reducing atmosphere at the time of forming the final protective film SiN, the switching charge amount was hardly reduced. These results are even better than those of Example 1. These are considered to be due to the effect of the thin film 24 made of a thermally and chemically stable material laminated between the ferroelectric films 25 as well as the electrode interface. That is, not only at the electrode interface but also in the film, the YSZ of the thin film 24 made of a thermally and chemically stable material plays a more effective role in diffusion of Pb and prevention of oxygen deficiency. It is considered that the film can be made thinner in order to prevent the inside of the thin film from reaching the inside. At this time, it should be noted that if the thin film 24 made of a thermally and chemically stable material is formed thickly, the characteristics will be deteriorated. However, the above results were obtained only when a multilayer epitaxial growth technique became possible. Lattice mismatch is an important point in epi growth. In particular, the mismatch between the lattice of the ferroelectric thin film 25 and the material of the upper electrode 26 and the material of the lower electrode 23 of the thin film 24 made of a thermally and chemically stable material is as follows. Must be within 10%. Above that, it is difficult to epitaxially grow the ferroelectric thin film 25. In order to apply a compressive stress to the ferroelectric thin film 25, it is important that the substantial lattice constant of the thin film 24 made of a thermally and chemically stable material is smaller than that of the ferroelectric thin film 25.

Here, the PZT and YSZ have a total of 11 layers, but the present invention is not limited to this as long as the total is 5 layers or more. The thickness of each layer of YSZ may be any value as long as it is 50 Å or less. The thickness of each layer of PZT may be 400 Å or less. YSZ is applied to the thin film 24 made of a thermally and chemically stable material.
Was used, but similar effects can be obtained with CeO ₂ or ZrO ₂ . However, the lattice mismatch between the ferroelectric thin film 25, the material of the upper electrode 26, and the material of the lower electrode 23 at that time must also be within 10%. Further, Ti _0.7 Al _0.3 N / Pt is used as the buffer layer 2.
Although a double buffer layer was used, other materials may be used. The method of forming each material may be any method other than the above as long as an epitaxial film can be obtained.

[0024]

As described above, according to the present invention, in a capacitor of a ferroelectric memory in which a ferroelectric thin film is interposed between an upper electrode and a lower electrode, the ferroelectric thin film and the upper electrode and the ferroelectric Forming a thin film made of a thermally and chemically stable material between the body thin film and the lower electrode, and further reducing the thickness of the thin film made of the thermally and chemically stable material to 50 angstroms or less; Also, since the lattice mismatch between the ferroelectric thin film and the upper electrode material and the lower electrode material of the thin film made of a thermally and chemically stable material is within 10%,
It has the effect of providing a capacitor using a lead zirconate titanate-based ferroelectric thin film that enables thinning of the ferroelectric thin film, further suppresses fatigue and suppresses deterioration of characteristics even in a reducing atmosphere. .

Further, according to the present invention, in a capacitor of a ferroelectric memory in which a ferroelectric thin film is sandwiched between an upper electrode and a lower electrode, the ferroelectric thin film and the ferroelectric thin film are thermally and thermally connected in the thickness direction of the ferroelectric thin film. Alternately laminating at least five or more thin films made of a chemically stable material, and forming the thin film made of the thermally and chemically stable material so as to be in contact with the upper electrode and the lower electrode; And the thickness of each thin film made of chemically stable material is the same,
And the thickness of each of the ferroelectric thin films is the same, and the substantial lattice constant of the thin film made of a thermally and chemically stable material is substantially the same as that of the ferroelectric thin film. The remanent polarization is increased by being smaller than the constant and having a lattice mismatch of 10% or less with respect to the ferroelectric thin film and the upper electrode material and the lower electrode material of the thin film made of a thermally and chemically stable material. At the same time, it is possible to provide a capacitor using a lead zirconate titanate-based ferroelectric thin film capable of further reducing the thickness of the ferroelectric thin film, and achieving further suppression of fatigue and prevention of deterioration of characteristics in a reducing atmosphere. It has the effect of.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a cross-sectional structure of a capacitor according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a cross-sectional structure of a capacitor according to a second embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 21 Substrate 2, 22 Buffer layer 3, 23 Lower electrode 4, 24 Thin film made of thermally and chemically stable material 5, 25 Ferroelectric thin film 6, 26 Upper electrode

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) H01L 21/8247 29/788 29/792

Claims (7)

[Claims] In a capacitor of a ferroelectric memory in which a ferroelectric thin film is sandwiched between an upper electrode and a lower electrode, thermal conduction is provided between the ferroelectric thin film and the upper electrode and between the ferroelectric thin film and the lower electrode. And a thin film formed of a chemically stable material. 2. The capacitor according to claim 1, wherein the thin film made of a thermally and chemically stable material has a thickness of 50 Å or less. 3. A ferroelectric memory capacitor comprising a ferroelectric thin film sandwiched between an upper electrode and a lower electrode, wherein the ferroelectric thin film is thermally and chemically stable in a thickness direction of the ferroelectric thin film. A thin film made of a thermally and chemically stable material is formed so as to be in contact with the upper electrode and the lower electrode. 4. The thin film made of a thermally and chemically stable material has the same thickness and is not more than 50 angstroms, and the ferroelectric thin film also has the same thickness. 4. The capacitor according to claim 3, wherein 5. The capacitor according to claim 3, wherein a substantial lattice constant of the thin film made of a thermally and chemically stable material is smaller than a substantial lattice constant of the ferroelectric thin film. 6. A lattice mismatch between a ferroelectric thin film, a thin film made of a thermally and chemically stable material, and an upper electrode material and a lower electrode material, respectively, is within 10%. 3. The capacitor according to 3. 7. The ferroelectric thin film material is a lead zirconate titanate-based material, and the thin film made of a thermally and chemically stable material is made of cerium oxide, zirconium oxide, or yttria-stabilized zirconia. The capacitor according to claim 1, wherein the electrode and the lower electrode include at least an oxide.