JP2003179210A - Semiconductor storage device - Google Patents

Abstract

(57) [Problem] To prevent the occurrence of oxygen vacancy in a ferroelectric layer due to voltage stress. SOLUTION: An Si plug 10 connected to one main electrode of a transistor; a first barrier layer 21 having the same area as an upper end surface of the Si plug 10 and in contact with the Si plug 10;
The first barrier layer 21 having the same area as the upper end surface of the plug 10
A second barrier layer 22 in contact with the first barrier layer 21; a lower electrode 24 made of a conductive perovskite oxide in contact with the second barrier layer 22; Dielectric layer 25; this dielectric layer 2
5 in contact with the upper electrode 26. The lower electrode 24, the upper electrode 26, and the ferroelectric layer 25 that insulates the lower electrode 24 and the upper electrode 26 form a thin-film capacitor, and the a-axis length Ab of the barrier layer is Chief Ao
And the a-axis length Ae of the lower electrode 24 is smaller than Ao.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor memory device using a thin film capacitor using a ferroelectric as a capacitor insulating film.

[0002]

2. Description of the Related Art In recent years, a semiconductor memory device (ferroelectric memory) using a thin film capacitor (ferroelectric capacitor) made of a ferroelectric thin film has been developed, and some have already been put to practical use. There is. Ferroelectric memory is non-volatile and does not require a power supply for memory retention. Moreover, when the ferroelectric thin film is sufficiently thin, the spontaneous polarization can be inverted rapidly, and writing and reading can be performed at a high speed like DRAM. In addition, a 1-bit memory cell can be manufactured with one transistor and one ferroelectric capacitor, which is suitable for large capacity.

A ferroelectric thin film suitable for a ferroelectric memory has a large remanent polarization, a small coercive electric field, a small temperature dependence of the remanent polarization, and a long-term retention of the remanent polarization. (Retention) etc. are required.
Currently, lead zirconate titanate (hereinafter abbreviated as PZT) is mainly used as the ferroelectric material.
PZT is a solid solution of lead zirconate and lead titanate, but a solid solution having a molar ratio of about 1: 1 has a large spontaneous polarization and can be inverted even in a low electric field and is excellent as a capacitor insulating film. It is believed that. Moreover, the transition temperature (Curie temperature) of the ferroelectric and paraelectric is 300 ° C.
Since the temperature is relatively high as described above, there is little concern that the stored contents will be lost due to heat in the temperature range (120 ° C. or less) in which a normal electronic circuit is used.

However, it is known that it is difficult to produce a high quality PZT thin film. First, Pb, which is the main component of PZT, easily evaporates at 500 ° C. or higher, which makes it difficult to accurately control the composition. Secondly, PZT exhibits ferroelectricity only when a perovskite type crystal structure is formed. However, it is difficult to obtain PZT having this perovskite type crystal, and a crystal structure called pyrochlore is more easily obtained. is there. Further, when applied to a silicon device, it is difficult to prevent diffusion of Pb, which is the main component, into silicon.

Other than PZT, barium titanate (BaT
iO _3: hereinafter referred to as BTO) has been known as a typical ferroelectric. It is known that BTO has a perovskite type crystal structure like PZT, and the Curie temperature is about 120 ° C. Since Ba is less likely to evaporate than Pb, it is easy to control the composition when forming a thin film of BTO. Further, when BTO is crystallized, it rarely takes a crystal structure other than the perovskite type.

Despite these advantages, the reason why the BTO thin-film capacitor has not been studied as a storage medium for a ferroelectric memory is that the residual polarization is smaller than that of PZT and that the residual polarization has a large temperature dependence. It can be mentioned. The cause of this is that the Curie temperature of BTO is low (120 ° C.). Therefore, when a ferroelectric memory is manufactured, there is a possibility that stored contents may be lost when exposed to a high temperature of 100 ° C. or higher. The reason is that the temperature dependence of the remanent polarization is large even in the temperature range (85 ° C. or less) where an electronic circuit is usually used, and the operation is unstable.
Therefore, it has been considered that the thin film capacitor using the ferroelectric thin film made of BTO is not suitable for use as a storage medium of a ferroelectric memory.

As a new ferroelectric thin film, the present inventors have made a dielectric material (for example, B, which has a relatively large lattice constant relatively close to that of the lower electrode (for example, SrRuO ₃ )).
a _x Sr _1-x TiO ₃ (hereinafter abbreviated as BST) is selected, and a film forming method called RF magnetron sputtering is used in which a misfit transition is relatively unlikely to occur and a single crystal substrate is formed. By the epitaxial growth, it is possible to keep the lattice constant in the in-plane direction (a-axis) smaller than the original lattice constant of the dielectric by the epitaxial effect, and to keep the lattice constant in the in-plane direction (a-axis) contracted. It was found (Patent Publication No. 2878986, registration date January 22, 1999). As a result, it has been confirmed that a ferroelectric thin film capable of shifting the ferroelectric Curie temperature to the high temperature side and maintaining a large remanent polarization in the room temperature region can be realized.

For example, a MgO single crystal substrate or S is used as the substrate.
Using a rTiO ₃ single crystal substrate, SrRu as a lower electrode
O ₃ (the lattice system is a pseudo-cubic system and the lattice constant a when converted to cubic system is a = 0.3930 nm) is used, and the composition region x of BST is x = 0.30 to 0.90 is used as the dielectric. As a result, ferroelectricity is exhibited even in a composition region (x ≦ 0.7) that should not exhibit ferroelectricity at room temperature, and in a composition region (x> 0.7) that originally exhibits ferroelectricity at room temperature. Have experimentally confirmed that it is possible to realize a practically preferable ferroelectric characteristic that the Curie temperature, which is originally above room temperature, further rises. That is, by using a BST ferroelectric capacitor in which the c-axis length is artificially extended, both a chemically and thermally stable BST process and a ferroelectric characteristic equal to or higher than PZT using Pb are achieved. It has become possible to

By the way, in order to manufacture the ultra-high-integration nonvolatile semiconductor memory by applying the above technique to the BTO film, a single crystal Si plug grown on or above the source / drain electrodes of the transistor is formed. It is necessary to form an oxide ferroelectric capacitor on top. At this time, a multilayer film such as a lower electrode and a barrier layer must be formed between the Si substrate and the BTO ferroelectric layer, but the multilayer film such as the lower electrode and the barrier layer satisfies the following conditions. (1) All are conductive; (2) Alleviate large lattice mismatch of 26% with lattice constant 0.543 nm of Si and in-plane lattice constant 0.399 nm of BTO; ) After the ferroelectric capacitor is formed, the strain introduced into the ferroelectric layer at the time of fine processing to the submicron level is not relaxed and the ferroelectric characteristics are not deteriorated; (4) Mutual diffusion of constituent elements occurs. (5) Not a material that is easily thermodynamically reduced such that an oxidation reaction at the interface proceeds due to the oxygen atmosphere during capacitor formation and the high temperature process after formation.

As an example of the lower electrode and barrier metal material satisfying the above requirements (1) to (5), the present inventors have
SrRuO ₃ / SrTiO ₃ : Nb / TiO ₂ : Nb /
A conductive film consisting of four layers of TiAlN was developed, a strained epitaxial BTO ferroelectric thin film was laminated on top of it, and good ferroelectric properties were confirmed for the solid film. In addition, since it is formed of an all-ceramic structure, it is also confirmed that the strain of BTO is not relaxed even when finely processed to the submicron level.

However, in this structure, since SrRuO ₃ having a weak bond with oxygen is used as the lower electrode, the oxygen desorbed from SrRuO ₃ diffuses downward when the temperature during film formation of the BTO film is increased. It is known that the TiAlN which is a barrier metal is oxidized, the contact resistance is significantly increased, and in the worst case, the nitrogen gas generated along with the progress of the oxidation reaction lowers the adhesion and causes the peeling of the film. In addition, since it is necessary to stack the BTO film in an oxygen atmosphere while suppressing the oxidation of the base, B
There is a problem that the process window for TO film formation is narrow and oxidation of the underlayer occurs due to slight changes in film formation conditions, which makes practical application difficult.

From the above-mentioned problems, an electrode material replacing SrRuO ₃ is desired, but the following four characteristics are required for the electrode: (A) The BTO film is sufficiently distorted. , In order to obtain good ferroelectric characteristics, the a-axis length is 0.5% or more shorter than the original a-axis length (0.399 nm) of BTO; (B) At the time of reading / writing during operation of the ferroelectric capacitor In order to prevent the generation of oxygen deficiency in the ferroelectric film due to the applied voltage stress, the electrode material should have a weak bond with oxygen; (C) the conductivity should be high with respect to the oxidizing atmosphere and the imperfections of crystals. Stable; (D) Stable in a reducing atmosphere.

As the lower electrode material satisfying the above conditions (B) to (D), not only the strain-induced ferroelectric capacitor but also many materials have been studied. Particularly in the case of a strain-induced ferroelectric capacitor, if a conductive perovskite material having the same crystal structure as that of the upper ferroelectric layer is used, high interfacial matching property can be obtained, so that improvement in ferroelectric characteristics is expected. However, it is not easy to find a material that satisfies all the conditions (A) to (D), and it is impossible to satisfy the conditions (A) to (D) using any conventionally known material.

As described above, Si has conventionally been used.
A ferroelectric capacitor formed directly on a substrate, especially a ferroelectric capacitor in which the ferroelectricity of BTO is enhanced by an epitaxial effect, is expected to be used in a non-volatile memory with an extremely high degree of integration. It was difficult to overcome the problems (A) to (D).

The present invention has been made in consideration of the above circumstances, and an object thereof is to form a lower electrode satisfying the above-mentioned specifications (A) to (D) and to obtain a dielectric property. Another object of the present invention is to provide a semiconductor memory device using the excellent and highly reliable ferroelectric capacitor (thin film capacitor).

[0015]

(Structure) In order to solve the above problems, a first feature of the present invention relates to a semiconductor memory device comprising a transistor and a thin film capacitor. That is,
The semiconductor memory device according to the first feature is (a) a Si plug connected to one main electrode of a switching transistor;
(B) A first barrier layer having the same area as the upper end surface of the Si plug and in contact with the Si plug; (c) A surface having the same area as the upper end surface of the Si plug and made of a material different from that of the first barrier layer. Cubic (100) plane, or tetragonal (00)
1) a second barrier layer having a surface which is in contact with the first barrier layer; (d) a lower electrode made of a conductive perovskite oxide in contact with the second barrier layer; (e) contacting with this lower electrode A dielectric layer; and (f) an upper electrode in contact with this dielectric layer. Then, a thin film capacitor is composed of a lower electrode, an upper electrode, and a ferroelectric layer that insulates between the lower electrode and the upper electrode, and the a-axis length Ab of the barrier layer is greater than the original a-axis length Ao of the dielectric layer. Small and a-axis length Ae of lower electrode
Is smaller than Ao.

Preferred embodiments of the present invention include the following: (1) Each a-axis length Ab, Ao, Ae is Ab / Ao≤0.9.
95Ae / Ao ≦ 0.995 is satisfied; (2) Some of the constituent elements of the lower electrode are Ca, Ba, Ti,
Substituted by at least one element selected from V, Mo, Cr and Ru, or lacking some of the constituent elements; (3) The barrier layer is made of conductive perovskite ABO ₃ (A is alkaline earth) Group, at least one selected from rare earths, B is at least one selected from Ti, V, Nb, Mo, Cr) (4) The barrier layer is formed of a metal film containing at least one of Ir and Rh (5) The lower part of the barrier layer is substantially TiN, Ti _1-x Al _x
(6) Ferroelectric thin film has c-axis length Ce after epitaxial growth.
And the original tetragonal c-axis length or cubic a-axis length Co before epitaxial growth corresponding to this c-axis length Ce.
Satisfy Ce / Co ≧ 1.02; (7) The lower electrode is Sr _1-x NbO _3-d (0 ≦ x ≦ 0.
5,8 ≦ d ≦ 1); (8) The barrier layer contains Ir, Rh, or a part of Ir or Rh from Re, Ru, Os, Pt, Pd, Ir, Rh. Fc substituted with at least one selected metal
(9) The ferroelectric layer has a perovskite structure represented by the chemical formula of ABO ₃ , A is at least one selected from Ba, Sr, and Ca, and B is Ti, Zr, H
At least one selected from f, Sn, and Nb; (10) SrNbO ₃ as the lower electrode is formed by RF magnetron sputtering in an oxygen-free inert gas (for example, Ar) atmosphere. . Alternatively, it is formed by RF magnetron sputtering in an atmosphere containing a reducing gas (for example, H ₂ ).

A second feature of the present invention relates to a semiconductor memory device including a transistor and a thin film capacitor. That is,
The semiconductor memory device according to the second feature is (a) a Si plug connected to one main electrode of a switching transistor;
(B) It has the same area as the upper end surface of the Si plug and is electrically coupled to the Si plug, and a cubic (100) plane or a tetragonal (001) plane appears on the surface, and Ir, Rh, or Ir
Alternatively, a part of Rh is Re, Ru, Os, Pt, Pd,
A barrier layer which is an alloy substituted with at least one kind of metal selected from Ir and Rh; (c) a lower electrode made of a conductive perovskite oxide in contact with this barrier layer;
(D) A dielectric layer in contact with the lower electrode; and (e) an upper electrode in contact with the dielectric layer. And the lower electrode,
A thin film capacitor is composed of an upper electrode, a lower electrode, and a ferroelectric layer that insulates the upper electrode from each other. The a-axis length Ab of the barrier layer is smaller than the original a-axis length Ao of the dielectric layer, and the lower electrode The gist is that the a-axis length Ae of is smaller than Ao.

(Function) In the present invention, in a semiconductor memory device having a capacitor structure in which a barrier layer, a lower electrode, a ferroelectric layer and an upper electrode are laminated, a perovskite containing BTO as a main component in the ferroelectric layer and a lower electrode. To SrNbO ₃
Is used as a main component, the a-axis length Ab of the barrier layer is smaller than the original a-axis length Ao of the ferroelectric layer, and the a-axis length A of the lower electrode after epitaxial growth is A.
e is made smaller than Ao. This makes it possible to form a lower electrode that satisfies the specifications (A) to (D) described above.

Therefore, according to the present invention, a capacitor using a ferroelectric thin film utilizing strain introduced during epitaxial growth is formed on Si with a good film quality, and is used in the process of forming the capacitor and in the process after forming the capacitor. It is possible to maintain good ferroelectric characteristics without deteriorating the interface between the lower electrode and the barrier layer. Then, by highly integrating the ferroelectric capacitor and the transistor of the present invention on the Si substrate, it becomes possible to manufacture a highly reliable semiconductor memory device which is highly integrated.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the embodiments, the basic principle of the present invention will be described.

In order to achieve the above-mentioned object, the present inventors have studied various combinations of conductive films. As a result, they have found that it is essential to use a conductive perovskite SrNbO ₃ layer epitaxially grown on a barrier layer having a small lattice constant to make the a-axis length shorter than the original value. Hereinafter, this technique will be described in detail.

First, the electrode material of the strain-induced ferroelectric capacitor will be examined. In a capacitor using a perovskite-based oxide ferroelectric (BTO in the present invention), from the viewpoint of preventing deterioration of ferroelectric characteristics due to generation of oxygen defects inside the ferroelectric when an operating voltage is repeatedly applied, The electrodes are preferably oxides. In particular, a conductive perovskite oxide having the same crystal structure as that of the ferroelectric substance is preferable because it has good interface matching.

Next, in order to obtain good ferroelectric characteristics when BTO is epitaxially grown on the top, BTO
It is theoretically shown that it is necessary to compress the in-plane lattice constant of 0.5% or more. FIG. 4 is a diagram (theoretical calculation value) showing the relationship between the a-axis length and the c-axis length of BTO, and the c-axis length decreases as the a-axis length increases. Where BT
It is known that O has good ferroelectricity when the c-axis length is 0.41 nm or more, and the a-axis length at this time is 0.397 or less. The original a-axis length of BTO is 0.3
Since it is 99 nm, good ferroelectric characteristics can be obtained by compressing the a-axis length by 0.5% or more.

As described above, the a-axis length of BTO needs to be compressed to 0.397 nm or less after the epitaxial growth, and even when there is no strain relaxation of the dielectric film, the a-axis of the lower electrode is
The axial length needs to be 0.397 nm or less. SrRu
Many conductive perovskites using Sr as the A site, including O ₃ , satisfy this condition: SrVO ₃ (0.3842 nm); SrCrO ₃ (0.3818 nm); SrFeO ₃ (0.3850 nm); SrCoO ₃ (0.385 nm); SrRuO ₃ (0,393 nm); SrMoO ₃ (0.3975 nm); SrNbO ₃ (0.402 nm) However, including the above conductive perovskite,
Most of the conductive perovskites having a transition metal at the B site cannot be said to have a strong bond with oxygen, and oxygen desorbed in the reducing atmosphere or in the thermal process after electrode formation diffuses downward to form an underlying nitride barrier. It may oxidize the metal layer. Oxidation of the underlying barrier metal layer results in a significant increase in contact resistance and, in severe cases, nitrogen gas is generated as a product of the oxidation reaction, causing the film to peel.
Therefore, it is desirable to use a conductive oxide having a strong bond with oxygen for the electrode.

Further, it is important that the conductivity is not easily lost in the process after forming the electrodes. For example, SrTiO ₃ which is an insulator is doped with Nb, La or the like,
If oxygen deficiency is introduced, it becomes a candidate for a conductive oxide material that is thermodynamically stable and has a strong bond with oxygen. However, the material in which the base material that is an insulator is made into a conductor by oxygen deficiency or element substitution, the conductivity disappears even if the crystallinity of the film is slightly deteriorated, and the amount of defects and the amount of substitution are slightly different. However, there is a problem in that the conductivity is greatly changed and it is difficult to control the conductivity.

As a result of a comparative study of various conductive oxides, the present inventors used a conductive perovskite SrNbO ₃ in which the a-axis length was compressed by utilizing the lattice mismatch with the base in order to satisfy all the above conditions. Has been found to be optimal. That is, (i) SrNbO ₃ has a strong bond with oxygen and its stability is comparable to that of SrTiO ₃ ; (ii) SrNbO ₃ exhibits conductivity not only in a single crystal but also in a polycrystal, and the crystallinity of the film is high. (Iii) SrNbO ₃ is different from BTO, which is a ferroelectric substance, in that it cannot be expected to obtain energy due to polarization even if it is compressed in-plane and expanded in the film thickness direction. Although it is difficult to control the a-axis length by the method, the a-axis length is distorted to 0.397 nm or less if the film forming method and the film forming conditions are optimized and epitaxially grown on the base film having a small lattice constant; iV) Since compounds that consist of constituent elements do not contain unstable substances, they do not cause contamination in the film deposition equipment, dust generation, etc., and they have good process compatibility because they are easy to process. The reason It is a reason.

As described above, the lower electrode containing SrNbO ₃ as a main component is epitaxially grown on the barrier layer having a small lattice constant to compress the in-plane lattice constant.
For the first time, it becomes possible to satisfy all of the above conditions (A) to (D), and it becomes possible to fabricate a strain-induced epitaxial ferroelectric capacitor that is optimal as a semiconductor memory.

Next, the present inventors conducted a detailed study on the composition of SrNbO ₃ . SrNb as lower electrode
Some of the constituent elements of O ₃ are Ca, Ba, Ti, V, M
At least one element selected from o, Cr, and Ru was substituted, or a defect was provided in a part of the constituent elements. In this way, by adding an additive to each of the A and B sites of SrNbO ₃ , or by adding an A site deficiency and an oxygen deficiency, the characteristics of SrNbO ₃ , specifically, the conductivity and the lattice constant can be more accurately measured. It becomes controllable.

It is known that SrNbO ₃ can reduce the lattice constant by Sr deficiency, substitution of Ca at the A site, substitution of Ti at the B site, or the like. As described above, it is possible to control the in-plane lattice constant by compressing the unit cell of SrNbO ₃ in-plane by utilizing the difference in lattice constant from the underlayer, but especially the underlayer barrier layer ( The second barrier layer is an Ir-based metal (a = 0.384n) having a small lattice constant.
In the case of m), the amount of lattice mismatch becomes large, and it is difficult to form an SrNbO ₃ epitaxial film having good crystallinity. Therefore, it is necessary to reduce the original lattice constant by Sr deficiency, Ti substitution of B site, and Ca substitution of A site.

However, the substitution of the constituent elements as described above brings about an increase in the specific resistance value, and is not preferable in view of its use as an electrode. Therefore, in order to compensate for the decrease in conductivity, a dopant that creates a level may be added just below the conduction band composed of the 4d orbital of Nb. Specific examples thereof include Ru, Mo, and V. However, since all of these dopant substances form an oxide having a weak bond with oxygen, the amount of the dopant is preferably 30% or less.

Thus, by using SrNbO ₃ whose in-plane lattice constant is reduced by controlling the composition, I
It is possible to epitaxially grow a SrNbO ₃ conductive perovskite excellent in both crystallinity and flatness on a film having a large lattice mismatch with SrNbO ₃ such as r.

Next, the present inventors examined the barrier layer which is the base of the SrNbO ₃ lower electrode. Since the barrier layer is formed on the non-oxide underlying barrier metal layer, it must be thermodynamically stable. Therefore, it is preferable to use, as a barrier layer, a conductive SrTiO ₃ film in which conductivity is exhibited by substituting a part of SrTiO ₃ which is an insulator, which is extremely stable thermodynamically, with Nb or La. Unlike the case of forming a dielectric film, SrNbO ₃
Since the sputtering atmosphere during the epitaxial growth of the lower electrode is an oxygen-free atmosphere, even if SrTiO ₃ made into a conductor by introducing oxygen deficiency is used for the barrier layer, there is no fear of losing the conductivity during the formation of the lower electrode. .

The a-axis length of SrTiO ₃ is 0.390.
Since the thickness is nm and the a-axis length is compressed in conformity with the underlying layer when SrNbO ₃ is epitaxially grown on the upper portion, it can be used as the lower electrode of the strain-induced ferroelectric substance. Furthermore, when using this structure, since the ceramic material is used from the Si substrate to the upper ferroelectric film,
The strain is not relaxed by the microfabrication and the ferroelectric characteristics are not deteriorated, and the process compatibility of the ferroelectric capacitor in the case of ultra-high integration is high.

From this point of view, the conductive perovskite ABO ₃ (A
Is at least one selected from alkaline earth and rare earth,
B is preferably at least one selected from Ti, V, Nb, Mo and Cr.

Further, as the barrier layer, a small amount of Ir and Rh is contained.
It is desirable to use a metal film containing at least one. Ir and
And Rh have lattice constants of 0.384 nm and 0.38, respectively.
It is as small as 0 nm, and unlike soft metals such as Pt
Because it is hard, it is said that strain relaxation of the ferroelectric film does not occur.
Have merit. In addition, since oxides also show conductivity,
Even under the process like
There will be no rise. However, SrNbO
₃Has a large lattice mismatch with S, so crystallinity is good.
rNbO₃To grow the film epitaxially,
As described above, Sr deficiency is introduced and the constituent element is Ti.
SrNbO by substituting ₃The lattice constant of
It is preferable to shorten it.

The details of the present invention will be described below with reference to the illustrated embodiments and comparative examples.

(First Embodiment) FIG. 1 shows a first embodiment of the present invention.
3 is a cross-sectional view showing an element structure of a thin film capacitor according to the embodiment of the present invention, particularly a thin film capacitor used in a ferroelectric memory. FIG.

The first embodiment of the present invention is a thin film capacitor.
Strained SrNbO as lower electrode ₃Specially used
In the figure, 1 is a Si substrate, 2 is element isolation insulation
Film, 3 is a gate insulating film, 4 is a gate electrode (word line),
5 is an n-type diffusion layer (source / drain region), 6, 8 and 9
Is an interlayer insulating film, 7 is a bit line, 10 is a Si plug, 11
Is a TiAlN barrier layer, 12 is TiNb (TiO 2₂ : N
b) film, 13 is SrTiNbO₃Membrane, 14 is SrNbO
₃Lower electrode, 15 is BaTiO 3 as a ferroelectric film
₃(BTO), 16 is SrRuO₃Shows the upper electrode
It

First, a helicon sputtering apparatus having an ultrahigh vacuum chamber is used as a first barrier layer (Ti, Ti) on a substrate having Si plugs 10 made of single crystal Si ((100) orientation) and completed. Al) N film 11 of 10n
m deposited. Furthermore, a TiNb film 12 having a thickness of 8 nm was deposited as a second barrier layer on the upper portion of the film using a DC sputtering apparatus. (Ti, Al) N is sputtered in a Ar / N ₂ atmosphere using a TiAl alloy target, and TiNb is sputtered in a Ar atmosphere using a TiNb alloy target.

On top of this, a Sr (Ti, Nb) O ₃ film 1 was formed as a thermodynamically stable conductive perovskite barrier layer.
3 was deposited to 30 nm. The film forming atmosphere at this time is Ar:
Although it is 0.1 Pa, it has been confirmed by X-ray diffraction that the TiNb film 12 is oxidized by oxygen from the oxide target and a conductive TiO ₂ : Nb single layer having an anatase structure is formed.

The Sr (Ti, Nb) O ₃ barrier layer 1
If the Nb substitution amount of 3 is too large, the a-axis length becomes large, and the a-axis length of SrNbO ₃ as the lower electrode also becomes large in conformity with the base, so that the BTO film as the ferroelectric film has a good strength. Does not show dielectric properties. On the contrary, if the Nb substitution amount is too small, stable conductivity cannot be obtained. Therefore, Nb
The substitution amount is preferably 0.1% to 60%, and more preferably 1% to 20%.

A SrNbO ₃ film 14 as a lower electrode was deposited on the conductive perovskite barrier layer 13 to a thickness of 30 nm by RF magnetron sputtering. A BTO film 15 having a thickness of 20 nm as a ferroelectric substance and a SrRuO ₃ film 16 having a thickness of 100 nm as an upper electrode are further deposited thereon.
A ferroelectric capacitor was produced. The film forming atmosphere at this time is Ar atmosphere for the SrNbO ₃ film 14 and Ar / O ₂ atmosphere for the BTO film 15, and the temperature is 600 ° C.

When the X-ray diffraction of the produced thin film capacitor was performed, the (Ti, Al) N film 11, Sr (Ti, N)
b) It was found that the O ₃ film 13, the SrNbO ₃ lower electrode 14, the BTO dielectric film 15, and the SrRuO ₃ upper electrode 16 were all epitaxially grown. Furthermore, when a cross-sectional electron microscope was observed, it was confirmed that the oxide layer (Ti,
No roughening of the interface between the Al) N film 11 and the Sr (Ti, Nb) O ₃ film 13 was found.

When the lattice constant was measured by a four-axis X-ray diffractometer, it was confirmed that the a-axis length of the SrNbO ₃ lower electrode 14 was 0.396 nm, which was matched with the base and was distorted by 0.5% or more. It was That is, the a-axis length could be made shorter than the original value by epitaxially growing the conductive perovskite SrNbO ₃ on the barrier layer having a small lattice constant under the above conditions. And this SrNbO ₃
The BTO film 15 on the lower electrode 14 has a c-axis length of 0.425n.
It was sufficiently distorted as m, and the rocking curve of the (002) diffraction peak had a half value width of 0.5 °, indicating good crystallinity.

When the ferroelectric characteristics of the capacitor were measured, the characteristics of remanent polarization of 0.52 C / cm ² and coercive voltage of 2.1 V were obtained, and the leakage current density when 2 V was applied was 2 × 10 ^−7. It was A / cm ² or less. Furthermore, 15
Dielectric breakdown did not occur even when a DC stress of V was applied.

Thus, Si / TiAlN / TiO
₂ : Nb / SrTiNbO ₃ / SrNbO ₃ / BTO /
In the capacitor having the SrRuO ₃ structure, since SrNbO ₃ which is thermodynamically stable is used as the lower electrode, the oxidation of the barrier metal layer can be suppressed. Moreover, since SrNbO ₃ as the lower electrode is aligned with the underlying barrier layer so that the a-axis length becomes shorter than the original value, BTO as the ferroelectric film can be sufficiently distorted, which is excellent. Capacitors with excellent ferroelectric characteristics can be manufactured.

(Second Embodiment) FIG. 2 shows a second embodiment of the present invention.
3 is a cross-sectional view showing the element structure of the thin film capacitor according to the embodiment of FIG. The same parts as those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted.

In the second embodiment of the present invention, as in the first embodiment, strain Sr is used as the lower electrode of the thin film capacitor.
NbO ₃ is used, and Ir is used as the barrier layer.

First, a helicon sputtering apparatus having an ultra-high vacuum chamber was used to form (Ti) as a first barrier layer on a substrate on which Si plugs 10 made of single crystal Si ((100) orientation) were completed. , Al) N film 21
0 nm was deposited. Further, an Ir film 22 as a second barrier layer was deposited to a thickness of 20 nm on the upper portion of the film using a DC sputtering apparatus. The (Ti, Al) N film 21 is formed by sputtering in a Ar / N ₂ atmosphere using a TiAl alloy target, and the Ir film 22 is formed by sputtering in an Ar atmosphere using an Ir target.

RF magnetron sputtering was used on the top of the Ir barrier layer 22 to form SrTi _0.25 Nb as a lower electrode.
_{A 0.75} O ₃ film 24 was deposited to 30 nm. Here, instead of SrNbO ₃ as the lower electrode 24, a part thereof is replaced with Ti in order to reduce the lattice mismatch between the lower electrode 24 and Ir as the second barrier layer 22 as described above. is there. The same applies when Rh is used as the second barrier layer 22 instead of Ir.

Then, a BTO film 25 as a ferroelectric film having a thickness of 20 nm was deposited on the lower electrode 24, and a SrRuO ₃ film 26 as an upper electrode was further deposited thereon for 100 m to fabricate a ferroelectric capacitor. The film forming atmosphere at this time is SrTi.
_{The 0.25} Nb _0.75 O ₃ film 24 is in an Ar atmosphere, the BTO film 25 is in an Ar / O ₂ atmosphere, and the temperature is 600 ° C.

When the X-ray diffraction of the produced thin film capacitor was performed, a (Ti, Al) N barrier layer 21, an Ir barrier layer 22, a SrTi _0.25 Nb _0.75 O ₃ lower electrode 24, and a BT
It was found that the O dielectric film 25 and the SrRuO ₃ upper electrode 26 were all epitaxially grown. Furthermore, when a cross-sectional electron microscope was observed, no surface morphology roughness was observed due to the formation of the oxide layer.

When the lattice constant was measured by a four-axis X-ray diffractometer, SrTi _0.25 N as the lower electrode 24 was measured.
It was confirmed that the a-axis length of b _0.75 O ₃ was 0.392 nm, which was distorted in conformity with the underlying layer. And the BT on top
The C-axis length of the O film 25 was 0.435 nm, which was sufficiently distorted, and the rocking curve of the (002) diffraction peak had a full width at half maximum of 0.4 ° and good crystallinity.

When the ferroelectric characteristics of the capacitor were measured, the characteristics of remanent polarization of 0.60 C / cm ² and coercive voltage of 1.9 V were obtained, and the leakage current density when applying 2 V was 1.5 × 10 5. ^-7 A / cm ² or less. Furthermore,
Dielectric breakdown did not occur even when a DC stress of 15 V was applied.

(Comparative Example 1) FIG. 3 is a sectional view showing an element structure of a thin film capacitor as a comparative example of the present invention. The same parts as those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted.

First, using a helicon sputtering apparatus having an ultrahigh vacuum chamber as a first barrier layer (Ti, Ti, on a substrate having Si plugs 10 made of single crystal Si ((100) orientation) completed). Al) N film 31 of 10n
m deposited. Further, a TiNb film 32 having a thickness of 8 nm was deposited on the upper portion of the film by using a DC sputtering device. (Ti, Al) N
The film 31 is formed by sputtering in a Ar / N ₂ atmosphere using a TiAl alloy target, and the TiNb film 32 is formed by performing sputtering in an Ar atmosphere using a TiNb alloy target.

On the top of the TiNb film 32, as a thermodynamically stable conductive perovskite barrier layer, Sr (Ti,
A 30 nm thick Nb) O ₃ film 33 was deposited. The film forming atmosphere at this time was Ar: 0.1 Pa, but the TiNb film 32 was oxidized by oxygen from the oxide target, and it was confirmed by X-ray that a conductive TiO ₂ : Nb single layer having an anatase structure was formed. Confirmed by diffraction.

On the conductive perovskite barrier layer 33,
Sr as a lower electrode using RF magnetron sputtering
A RuO ₃ film 34 having a thickness of 30 nm, a BTO film 35 having a thickness of 20 nm as a ferroelectric thin film, and a SrRuO ₃ film 36 having a thickness of 100 nm as an upper electrode were further deposited thereon to form a ferroelectric capacitor. The film forming atmosphere at this time is S
Both rRuO ₃ and BTO are in an Ar / O ₂ atmosphere and the temperature is 600 ° C.

When the X-ray diffraction of the produced thin film capacitor was performed, the (Ti, Al) N film 31, Sr (Ti, N)
b) It was found that the O ₃ film 33, the SrRuO ₃ lower electrode 34, the BTO dielectric film 35, and the SrRuO ₃ upper electrode 36 were all epitaxially grown.

However, when the surface was observed with a scanning electron microscope, swelling with a diameter of about 1 μm was found over the entire surface. Further, when a cross-sectional electron microscope observation was performed, film peeling occurred at the interface between the (Ti, Al) N film 31 and TiO ₂ : Nb. Next, when this laminated film was processed into a capacitor up to the Si plug by lithography and etching techniques and leakage characteristics were evaluated, 99% of the measured 400 capacitors could not be measured due to a short circuit.

Further, in order to suppress the oxidation of the (Ti, Al) N film 31, the temperature at the time of forming the SrRuO ₃ lower electrode is set to 50.
When a capacitor was formed by lowering the temperature to 0 ° C., the surface morphology roughness as described above was not observed, but X
As a result of line diffraction, the full width at half maximum of the rocking curve, which is an index of crystallinity, was 2.0 ° for the SrRuO ₃ (002) peak,
Regarding the BTO (002) peak, the crystallinity was poor at 1.8 °, and good ferroelectric characteristics could not be obtained.

Thus, SrRuO ₃ / BTO / Sr
RuO ₃ / SrTiO ₃ : Nb / TiO ₂ : Nb / Ti
In the AlN / Si structure capacitor, the lower electrode 3
Since SrRuO _3, which has a weak bond with oxygen, is used for No. 4, there is a problem that if the temperature at the time of film formation is increased to improve the film quality, the underlying nitride barrier metal layer is oxidized and the morphology becomes rough. Also, without using SrRuO ₃ , Sr
A method of using the (Ti, Nb) O ₃ layer as the lower electrode is also conceivable, but as described above, the conductivity is unstable with respect to the incomplete crystal and the oxygen atmosphere in the region where Nb is small. On the other hand, when the amount of Nb is large, the lattice constant becomes large, and when BTO is epitaxially grown on the upper side, it is not sufficiently distorted, so that there is a problem that good ferroelectric characteristics cannot be obtained.

On the other hand, in the first and second embodiments described above, SrNbO ₃ is used for the lower electrode, and the a-axis length is made shorter than the original value by optimizing the underlayer and manufacturing conditions. The a-axis length of the BTO ferroelectric film could be aligned with the lower electrode and distorted (compressed) by 0.5% or more, whereby good ferroelectricity and crystallinity could be obtained. Even when processed into a capacitor, a large remanent polarization and a coercive voltage of 2.1 V were obtained, the leak current could be reduced, and the resistance to DC stress could be sufficiently increased.

That is, by utilizing the lattice mismatch with the underlying barrier layer (second barrier layer), the in-plane lattice constant of the conductive perovskite SrNbO _{3 which} is thermodynamically extremely stable is shortened, A sufficiently strained BTO ferroelectric film can be epitaxially grown under more preferable film forming conditions, and a capacitor having good ferroelectric characteristics and high reliability can be obtained.

The present invention is not limited to the above embodiments. In the embodiment, the example of the barrier layer in which the cubic (100) plane appears on the surface has been described.
The barrier layer may have a tetragonal (001) plane. Further, the lower electrode is not limited to SrNbO ₃ , but some of its constituent elements may be Ca, Ba, T
It may be substituted with at least one element selected from i, V, Mo, Cr and Ru, or may be deficient in some of the constituent elements. Further, the barrier layer is made of conductive perovskite ABO ₃ (A is at least one selected from alkaline earth and rare earth, B is Ti, V, Nb, Mo,
At least one selected from Cr) may be used.

Further, the thin film capacitor of the present invention is not necessarily limited to the ferroelectric memory and can be used as an element device of various semiconductor circuits. Further, the method of manufacturing the barrier layer, the lower electrode, the ferroelectric film, etc. is not necessarily limited to sputtering, but any method that allows epitaxial growth may be used. In addition, various modifications can be made without departing from the scope of the present invention.

[0067]

As described in detail above, according to the present invention, in a capacitor structure in which a barrier layer, a lower electrode, a ferroelectric layer and an upper electrode are laminated, a perovskite containing BTO as a main component in the ferroelectric layer, A conductive perovskite containing SrNbO ₃ as a main component is used for the lower electrode, the a-axis length Ab of the barrier layer is smaller than the original a-axis length Ao of the ferroelectric layer, and the a-axis length Ae of the lower electrode is smaller than Ao. BTO ferroelectric thin film is formed on Si with a good film quality by setting a small value as well, and the interface between the lower electrode / barrier layer is not deteriorated during the process of forming the capacitor and the process after forming the capacitor, and the good strength is obtained. It becomes possible to maintain the dielectric properties.
Then, by highly integrating the ferroelectric capacitor and the transistor of the present invention on the Si substrate, it becomes possible to manufacture a highly reliable semiconductor memory device which is highly integrated.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a structure of a thin film capacitor section of a semiconductor memory device according to a first embodiment.

FIG. 2 is a cross-sectional view showing a structure of a thin film capacitor portion of a semiconductor memory device according to a second embodiment.

FIG. 3 is a sectional view showing a structure of a thin film capacitor portion of a semiconductor memory device according to a comparative example of the present invention.

FIG. 4 is a diagram showing a relationship between an a-axis length and a c-axis length in BTO.

[Explanation of symbols]

1 ... Si substrate 2 ... Element isolation insulating film 3 ... Gate insulating film 4 ... Gate electrode (word line) 5 ... N-type diffusion layer (source / drain region) 6, 8, 9 ... Interlayer insulating film 7 ... Bit line 10 ... Si plugs 11, 21 ... TiAlN film (first barrier layer) 12 ... TiNb (TiO ₂ : Nb) film (second barrier layer) 13 ... SrTiNbO ₃ film (barrier layer) 14, 24 ... SrNbO ₃ film (lower part) Electrodes 15 25 ... BaTiO ₃ (BTO) film (ferroelectric film) 16, 26 SrRuO ₃ film (upper electrode) 22 Ir film (second barrier layer)

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shin Fukushima             1st Komukai Toshiba-cho, Sachi-ku, Kawasaki-shi, Kanagawa             Inside the Toshiba Research and Development Center (72) Inventor Takashi Kawakubo             1st Komukai Toshiba-cho, Sachi-ku, Kawasaki-shi, Kanagawa             Inside the Toshiba Research and Development Center F-term (reference) 5F083 FR02 GA21 GA25 JA14 JA36                       JA39 JA40 JA43 JA45 MA06                       MA17 PR22 PR25

Claims (2)

[Claims] 1. A semiconductor memory device comprising a transistor and a thin film capacitor, wherein the Si plug connected to one of the main electrodes of the switching transistor has the same area as the upper end surface of the Si plug and is in contact with the Si plug. The first barrier layer has the same area as the upper end surface of the Si plug, is made of a material different from that of the first barrier layer, and has a cubic system (10
0) plane or tetragonal (001) plane appears, and the first
Second barrier layer in contact with the barrier layer, a lower electrode made of a conductive perovskite oxide in contact with the second barrier layer, a dielectric layer in contact with the lower electrode, and a dielectric layer in contact with the dielectric layer. And the lower electrode,
The thin-film capacitor is composed of an upper electrode, the lower electrode, and a ferroelectric layer that insulates the lower electrode from each other, and the a-axis length Ab of the barrier layer is smaller than the original a-axis length Ao of the dielectric layer. A semiconductor memory device characterized in that the a-axis length Ae of the lower electrode is smaller than Ao. 2. A semiconductor memory device comprising a transistor and a thin film capacitor, wherein the Si plug connected to one main electrode of the switching transistor has the same area as the upper end surface of the Si plug and is electrically connected to the Si plug. And a cubic (100) plane or a tetragonal (001) plane appears on the surface, and Ir, Rh, or I
Part of r or Rh is Re, Ru, Os, Pt, P
A barrier layer which is an alloy substituted with at least one kind of metal selected from d, Ir, and Rh, a lower electrode made of a conductive perovskite oxide which is in contact with the barrier layer, and a dielectric which is in contact with the lower electrode. A body layer and an upper electrode in contact with the dielectric layer, and the lower electrode,
The thin-film capacitor is composed of an upper electrode, the lower electrode, and a ferroelectric layer that insulates the lower electrode from each other, and the a-axis length Ab of the barrier layer is smaller than the original a-axis length Ao of the dielectric layer. A semiconductor memory device characterized in that the a-axis length Ae of the lower electrode is smaller than Ao.