JP2001210794A - Ferroelectric memory material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferroelectrics memory wherein repeated recording characteristic in 100 deg.C environment is excellent with a high curie temperature while film-forming is allowed at such low temperature order as 600 deg.C for matching the semiconductor process. SOLUTION: Relating to a material constituting a ferroelectrics layer of a ferroelectrics memory, a transiting metal oxide comprises a transition metal as a main component, wherein a difference between the energy level in d-state of a valence band of the transition metal in an atomic state constituting the transition metal oxide and the energy level in 2p state of oxygen constituting the transition metal oxide is 0.33 Ry or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a ferroelectric memory material comprising a transition metal oxide and constituting a ferroelectric layer.

[0002]

2. Description of the Related Art In recent years, PbZr _1-x Ti _x O ₃ (hereinafter referred to as PZT) and SrBi ₂
A ferroelectric material composed of a transition metal oxide represented by Ta ₂ O ₉ (hereinafter, referred to as SBT) is about to be put to practical use as a nonvolatile memory material that has realized large capacity and low power consumption. For example, an application example to a non-contact type IC card which does not require a battery has been reported. Also, these ferroelectric memories are taking advantage of the advantages of high-speed writing and low-voltage driving, and are being incorporated into logic-memory mixed type integrated circuits for notebook computers. For this reason, ferroelectric memory materials are required to have two points: high environmental reliability and low process temperature.

[0003]

However, PZT has a problem in environmental reliability in memory operation, especially in durability of repeated recording. In particular, when using ferroelectric memories in integrated circuits exposed to high temperatures of around 50 ° C to 100 ° C, to ensure the durability of repetitive recording, curable materials higher than PZT and SBT as ferroelectric materials are used. Temperature is required. It is required that the temperature be at least higher than the typical Curie temperature of PZT of 400 ° C. SBT has a Curie temperature of 335
° C, which is problematic in environmental reliability.

When an integrated circuit in which a memory unit and a logic unit are mixed is manufactured, it is important to ensure consistency between the process temperature of the memory unit and the logic unit. Therefore, it is desired that the temperature at which the ferroelectric of the memory portion is formed be 650 ° C. or less. However, SBT has a disadvantage that a high film forming temperature of about 800 ° C. is required. This is because the atomic structure of the unit cell in the SBT crystal is complicated, and a desired crystal structure cannot be obtained unless a high process temperature is applied in any of the film forming methods. On the other hand, PZT is 650 ℃
Although the following film formation is possible, it cannot be said that the environmental reliability is sufficiently cleared as described above.

Accordingly, the present invention is to provide a ferroelectric memory material for forming a ferroelectric layer which has a high Curie temperature, has high environmental reliability, and can be formed at a low temperature. Aim.

[0006]

In order to solve the above problems, a ferroelectric memory material according to the present invention is a material comprising a transition metal oxide constituting a ferroelectric layer, wherein the transition metal oxide comprises The difference between the energy level of the d state of the valence band in the atomic state of the transition metal constituting the substance and the energy level of the 2p state of oxygen constituting the transition metal oxide is 0.
It is characterized by containing a transition metal within 33 Ry as a main component.

[0007] The ferroelectric memory material is preferably
It is characterized by being composed of a transition metal oxide containing bismuth and iron as main components. Further, it is preferable that the transition metal oxide containing bismuth and iron as main components has BiFeO ₃ as a main component. Further, in the transition metal oxide containing BiFeO ₃ as a main component, Ba, S
r, Ca, K, Na, Y, La, Ti, V, Cr, Mn, Co, Ni, Cu, Z
It is preferable to add at least one of n, Ru, Rh, Pd, Ag, Ir, Pt, and Au.

According to the above-described material, when the ferroelectric layer is formed, the covalent bond between the transition metal and oxygen surrounding the transition metal in an octahedral manner is increased,
A high Curie temperature sufficient for a memory can be provided. Therefore, even in a temperature environment of 100 ° C, 10 ¹²
The durability of repeated recording more than once can be guaranteed. At the same time, since a unit cell of an oxide crystal containing bismuth and iron as a main component has a simple cubic structure, a high temperature is not required in a crystal forming process at the time of film formation. A simple cubic is defined as a crystal structure that slightly modulates the perovskite structure with cubic symmetry (Landolt
Bernstein p.21 by Springer-Verlag 1981). Also BiF
In transition metal oxides mainly composed of eO ₃ , B
a, Sr, Ca, K, Na, Y, La, Ti, V, Cr, Mn, Co, Ni, C
By adding at least one of u, Zn, Ru, Rh, Pd, Ag, Ir, Pt, and Au, the coercive electric field and the film stress of the ferroelectric layer can be adjusted without impairing a high Curie temperature. it can.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings along with examples.

(Embodiment 1) In this embodiment, the energy level E _{TM_d} of the d orbital of the outer shell of the transition metal atom and the energy level E _{TM_d} of the oxygen atom
We _{show that} the difference D _dp of the energy level E _{O_2p} of the 2p orbit is closely related to the Curie temperature, and show that the use of BiFeO ₃ as the ferroelectric layer is beneficial in raising the Curie temperature.

FIG. 1 is a diagram showing a difference D _dp between the energy level E _{TM —} d of the d orbital of the outer shell of the transition metal atom and the energy level E _{O —} 2p of the 2p orbital of the oxygen atom in the embodiment of the present invention. The horizontal axis is the shell s orbital and d in the transition metal atom
It is the sum n of the number of electrons in orbit. Table 1 shows the correspondence between n and the element.

[0012]

[Table 1]

These energy levels were obtained by performing calculations on atoms floating in vacuum within the range of local density approximation based on the density functional theory. The electron configuration of the transition metal is assumed to have two electrons in the s orbital in order to make the calculation conditions uniform. Actually, both the d orbital of the transition metal atom and the 2p orbital of oxygen undergo spin orbit splitting, and two energy levels appear. Here, the 2p orbital of oxygen and the transition metal
The center of gravity weighted by the number of occupied electrons for the d orbitals is E _{O_2p} ,
E _Set to _{TM_d} .

[0014] As FIG. 1 shows, n E _{TM_d} decreases for the Coulomb potential becomes deep with electrons feel d orbital of larger as the transition metal atom, approaches relatively E _{O_2p.} The closer the energy levels of these two orbitals are, the stronger the hybrid of the electron orbitals of the transition metal atom and the oxygen atom when the crystal forms a perovskite structure. Therefore, by obtaining orbital energy, the transition metal atom is easily displaced with respect to the oxygen octahedron. That is, a state where the distance between the transition metal atom and the specific oxygen atom is short is realized. At this time, the potential surface felt by the transition metal atoms becomes deeper, and an increase in the Curie temperature of the system is observed. The line 101 in FIG. 1 shows the value of D _dp necessary for obtaining a suitable Curie temperature of 400 ° C., which is expected from FIG. 2 described in the next section. If D _dp is below the line 101, a higher Curie temperature than conventional PZT and KNbO ₃ can be developed. From these facts, the transition metals particularly promising as the composition of the ferroelectric memory composed of the transition metal oxide from FIG. 1 are Fe, Co, Ni, Cu, and Zn in the 3d transition metal. For 4d transition metals, Ru,
Rh, Pd, Ag. Further, for 5d transition metals, Ir, P
t, Au are promising. Hg is excluded because it is difficult to handle in production. Of these, Fe can form an insulator in BiFeO ₃ under relatively loose manufacturing conditions and exhibit ferroelectricity.

Next, the relationship between the intensity of orbital hybridization and the Curie temperature will be described in detail. FIG. 2 is a diagram showing a relationship between an index W _cov indicating the intensity of hybridization of electron orbitals of transition metal atoms and oxygen atoms and Tc of each substance obtained from experiments. Here, a simple cubic unit cell is shown. In this experiment, each material was configured as a 200 nm thin film sandwiched between metal electrodes, and the Curie temperature was determined by obtaining a CV curve while changing the temperature. The metal electrode located under the ferroelectric layer has a great influence on the physical properties of the ferroelectric layer laminated thereon through lattice matching, particularly the Curie temperature, the polarization moment, and the piezoelectric constant. Here, Pt was used for both the upper and lower electrodes. W _cov is called covalency, WA Har
by rison

[0016]

(Equation 1)

[Electronic structure]
and the properties of solids p.22by Dover publicat
ions, INC. 1989). Here, V ₂ is called covalent energy, and in the case of this embodiment, is half the bandwidth determined from the partial density of states of oxygen 2p. In Fig. 2, 0.15Ry, which is a typical value for the transition metal oxide of perovskite, is used. (Partial density of states is within the range of local density approximation based on density functional theory. Method). V ₃ is a quantity called polar energy,
In the case of the present embodiment, the difference is given by one half of the difference D _dp between E _{TM — d} and E _{O — 2p} . From Equation 1, as the difference between E _{TM_d} and E _{O_2p} becomes smaller,
The covalency will increase. PbZrO3 and PbHfO3
Indicates antiferroelectricity, but FIG. 2 shows the transition temperature (Neel temperature) from the paraelectric phase as Tc. From FIG. 2, it can be seen that Tc increases as the covalency W _cov increases for each substance system. For example, when representing the composition of the simple cubic specific perovskite structure ABO _3, when A site is Pb, B site Hf, Zr, as changes and Ti, the Curie temperature increases. If site A is Ba, site B is Z
As it changes to r and Ti, the Curie temperature rises. Furthermore, when the A site is K, the Curie temperature increases as the B site changes to Ta and Nb. As for BiFeO3, Fe
Since the energy of the 3d orbital of the atom is close to the energy level E _{O_2p} of the 2p orbital of oxygen, it exhibits a high Curie temperature of 850 ° C despite its simple cubic structure. From the above, _if the difference D _dp between E _{TM_d} and E _{O_2p} is small,
It was shown that the covalency W _{cov of the} transition metal d-electrons and oxygen 2p-electrons increased, and the Curie temperature increased. In FIG. 2, reference numeral 201 denotes a line indicating 400 ° C., and it is considered that a Curie temperature higher than this temperature is preferable as a ferroelectric memory material. Reference numeral 202 denotes a W _cov obtained from the intersection of the KXO ₃ (X = Nb, Ta) system line and 201, which is a line indicating the value of W _cov required from the viewpoint of the Curie temperature. FIG. 2 shows that if W _cov is 0.47 or more, the Curie temperature required for the ferroelectric memory can be obtained. Pb
In the XO ₃ (X = Ti, Zr) system, the required W _cov may be 0.40 or more. But here, the more severe condition W _cov = 0.4
Adopt 7 If this W _cov = 0.47 is converted using Equation 1 and FIG. 1, a desired Curie temperature can be obtained if the difference D _dp between E _{TM — d} and E _{O — 2p} is 0.33 Ry or less.
Naturally, Fe in BiFeO ₃ is included in the range. Further, the ferroelectric made of the transition metal oxide in the first embodiment is not limited to a simple cubic structure.
For example, a Bi-based layered compound has the same effect.

Up to now, no report has been made on a ferroelectric memory containing BiFeO ₃ as a main component. This is because BiFeO ₃ may not exhibit ferroelectricity depending on the preparation conditions. However, if the ferroelectric material is thinned and the stress of BiFeO ₃ is controlled using lattice matching with the underlayer as in this embodiment, the ferroelectricity is stably exhibited.

However, even if E _{TM_d} and E _{O_2p} are close to each other, they do not exhibit ferroelectricity unless the system is an insulator. This is because in a ferroelectric composed of a transition metal oxide, the displacement of the transition metal atom lowers the Madelung potential based on long-range Coulomb interaction, which is a cause of ferroelectricity. However, in metals, this long-range Coulomb interaction is screened by free electrons and atomic displacement does not occur. So Figure 1
However, ferroelectricity will not be exhibited unless the A-site atom is properly selected or the B-site atom is made of a plurality of types so that the system becomes an insulator. For that purpose, two requirements are that the number of band electrons included in the unit cell be an even number and that the number of loads of the system becomes zero when considered with an ion model. In order to make the number of band electrons included in the unit cell even, if the simple cubic structure in ABO ₃ is used, the unit cell is periodically distorted from the cubic (this structure is called a simple cubic type), and the unit cell is replaced with the original unit. There is a method of making the size of the cell even number times. One example is BiFeO3. For BiFeO3, a total of 15 Bi atoms (10 5d electrons, 2 6s electrons, and 3 6p electrons), 8 Fe atoms (3d and 4s electrons combined), and a total of 4 (two 2s electrons,
2p electrons) constitute band electrons.
In the simple cubic structure, the total number of band electrons included in the unit cell is 45 (odd), and the system is a metal. Therefore, in BiFeO3, the total number of band electrons contained in a unit cell is 270 (even number) by incorporating six distorted cubics in one unit cell.
The system is an insulator. Here, the band electron is
An electron that has finite hopping between adjacent atoms.

According to the first embodiment, in the transition metal oxide constituting the ferroelectric layer, the difference between the energy level of the d-state of the valence band in the atomic state of the transition metal and the energy level of the 2p-state of oxygen in the atomic state of the transition metal. It has been found that if a transition metal whose is within 0.33Ry is the main component, it can have a sufficient Curie temperature as a memory material.

In the present embodiment, Ba respect BiFeO _3, S
r, Ca, K, Na, Y, La, Ti, V, Cr, Mn, Co, Ni, even if the coercive electric field and the film stress were adjusted to be suitable for memory by adding at least one of them, An effect equivalent to that of this embodiment is obtained without impairing the high Curie temperature.

(Embodiment 2) FIG. 3 is a sectional view of a cell of a memory element using the ferroelectric memory material described in Embodiment 1.

In FIG. 1, reference numeral 301 denotes a semiconductor substrate;
2 is a gate insulating film, 303 is a gate electrode, and 304 is a source / drain region. Cell transistors 301 to 304 form MOS transistors. As the semiconductor substrate, a silicon substrate having a plane orientation (100) is usually used. For the gate insulating film 302, a thermal oxide film SiO2 is used, and for the gate electrode 303, P-doped polycrystalline silicon or a silicon compound of metal or a high melting point metal such as W or Ta is used. In the case of an n-type MOS transistor, As is ion-implanted into the source / drain region 304 at about 10 ¹⁶ cm ² . 30
5 is a channel stopper, 306 is a field oxide film, and 307 is a first interlayer insulating film. In the case of the n-type MOS transistor, the channel stopper 305
B (boron) is ion-implanted. The field oxide film 306 is made of SiO ₂ by wet oxidation containing water vapor,
For the interlayer insulating film 307, SiO ₂ (PSG) containing P is usually used. 308 is a lower electrode of a ferroelectric capacitor, 309
Is a ferroelectric film, and 310 is an upper electrode of a ferroelectric capacitor. A ferroelectric capacitor is constituted by 308 to 310. For example, Pt, Ir or Ir oxide, Ti or Ti oxide is used for the lower electrode, and Pt, Ir or Ir oxide, Ti or Ti oxide, or the like is suitably used for the upper electrode 310. In this embodiment, IrO ₂ was used for the lower electrode and Ir was used for the upper electrode. Reference numeral 311 denotes a second interlayer insulating film, and PSG is preferably used similarly to the first interlayer insulating film 307. After forming the second interlayer insulating film 311, through holes are formed in the second interlayer insulating film 311 and the first interlayer insulating film 307, and then a metal wiring layer 312 is formed. Al for the metal wiring layer 312
Alternatively, Cu, Ag, or an alloy of Al, Cu, and Ag can be used. In this embodiment, Al was used. Metal wiring layer 312
It is desirable to form a thin film made of Ti oxide, Zr oxide, Hf oxide, or the like for preventing metal atom diffusion between the semiconductor device and the surrounding insulating film. In this example, a Ti oxide was used. In addition to the configuration shown in FIG.
The ferroelectric capacitor composed of the elements 310 to 310 can be realized directly above the MOS transistor composed of the elements 301 to 304, so that the density of the entire device can be increased.

1T / 1C (1 transistor / 1 capacitor) using the element of the ferroelectric memory represented by FIG.
A type integrated circuit A was created. The number of memories is 16k. Here, BiFe is used as the ferroelectric film 309 in the embodiment of the present invention.
Sample 1 was prepared using O ₃ . BiFeO ₃ production temperature is 650 ℃
It is. As a comparative example, Pb was used as the ferroelectric film 309.
Sample 2 was prepared by forming a film of (Zr _0.3 Ti _0.7 ) O ₃ at a forming temperature of 650 ° C.

Next, a recording durability test was repeatedly performed using these samples. The lower electrode 3 is set so that the polarization moment of the ferroelectric film 309 is inverted every recording step.
08 and the voltage of the upper electrode 310 were controlled. The environmental temperature at this time was set to 100 ° C. After the repetitive recording of 10 ¹² times, the bit error was measured at the same environmental temperature of 100 ° C. Although the bit errors of the sample 2, 2 × 10 ^-4 has appeared, the sample 1-bit error is 6 × 10 ^-
It was ¹² or less.

The above is the result in the case where BiFeO ₃ is used as the ferroelectric film 309. In the present embodiment, as long as the ferroelectric film 309 is an insulator, Ba and BiFeO ₃ are used.
Even if the coercive electric field and the film stress of the ferroelectric film 309 are adjusted by adding at least one of Sr, Ca, K, Na, Y, La, Ti, V, Cr, Mn, Co, Ni, and Cu. It is clear that the same effect as in the present embodiment is obtained without impairing the high Curie temperature.

[0027]

According to the present invention, a film can be formed at a low temperature which has a sufficient Curie temperature required as a memory material, has excellent repetitive recording characteristics, and is compatible with a semiconductor process. A ferroelectric memory can be provided.

[Brief description of the drawings]

FIG. 1 shows the outer shell of a transition metal atom in an embodiment of the present invention.
FIG. 4 is a diagram showing a difference D _dp between an energy level E _{TM_d} of a d orbital and an energy level E _{O_2p} of a 2p orbital of an oxygen atom.

FIG. 2 is a diagram showing a relationship between an index W _cov representing the intensity of hybridization of electron orbitals of a transition metal atom and an oxygen atom and Tc of each substance obtained from an experiment.

FIG. 3 is a sectional view showing a cell structure of a memory element using a ferroelectric memory material according to the first embodiment of the present invention.

[Explanation of symbols]

101 Line showing the value of D _dp expected from FIG. 2 necessary to obtain a suitable Curie temperature of 400 ° C. 201 Line showing a suitable Curie temperature of 400 ° C. 202 KXO ₃ (X = Nb, Ta) system A line _indicating W _cov obtained from the intersection of the line and 201 and indicating a value of W _cov required from the viewpoint of the Curie temperature 301 Semiconductor substrate 302 Gate insulating film 303 Gate electrode 304 Source / drain region 308 Lower electrode of ferroelectric 309 Ferroelectric film 310 Upper electrode of ferroelectric

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tenmitsu Higuchi 3-3-5 Yamato, Suwa-shi, Nagano F-term in Seiko Epson Corporation (reference) 4G002 AA06 AA07 AA08 AA09 AA10 5F083 AD21 AD49 GA21 JA13

Claims (4)

[Claims] 1. A material comprising a transition metal oxide constituting a ferroelectric layer in a ferroelectric memory, wherein energy of a d-state of a valence band in an atomic state of a transition metal constituting the transition metal oxide is provided. Level and the energy level of the 2p state of oxygen constituting the transition metal oxide is 0.
A ferroelectric memory material comprising a transition metal of 33 Ry or less as a main component. 2. The ferroelectric memory material according to claim 1, wherein the ferroelectric memory material is composed of a transition metal oxide containing bismuth and iron as main components. 3. The ferroelectric memory material according to claim 2, wherein the transition metal oxide containing bismuth and iron as main components mainly contains BiFeO ₃ . 4. The transition metal oxide containing BiFeO ₃ as a main component, wherein Ba, Sr, Ca, K, Na, Y, La, Ti,
V, Cr, Mn, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Ir, P
4. The ferroelectric memory material according to claim 3, wherein at least one of t and Au is added.