JPH06151762A - Ferroelectric material and ferroelectric memory element formed thereof - Google Patents

Abstract

PURPOSE:To obtain a ferroelectric material film small in fatigue by a method wherein ferroelectric material of ilmenite or perovskite structure is specified in composition. CONSTITUTION:The electrical properties of a ferroelectric body are represented by a P-E hysteresis loop. A ferroelectric body grows more excellent in memory properties when it is larger in both self-polarization Ps and residual polarization Pr and smaller in read electrical field En. PbTiO3 and PZT of perovskite structure and LiNbO3 and LiTO3 of ilmenite structure are large in self-polarizability and desirable. But, oxygen-shorted ABOx [wherein A and B are elements which compose ferroelectric material, X denotes a numerical value smaller than 3] is excellent, and stable in actual ferroelectric properties. A ferroelectric memory element of this design is formed through a laser ablation method. By this setup, ABOx of ferroelectric material can be controlled in oxygen content, so that a thin film excellent in ferroelectric properties can be obtained.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to the field of a ferroelectric memory that stores information by utilizing polarization of a ferroelectric substance.

[0002]

2. Description of the Related Art PZT [PbyZrx] is used as a material for a ferroelectric memory that stores information by utilizing polarization of a ferroelectric.
TiO ₃ (y = 1−x)] is the mainstream, but recently, PLZT containing lanthanum (La) has appeared. The manufacturing method is a sputtering method, a sol-gel method, or MOCVD.
Although a thin film of the dielectric memory material is formed by these methods, the process temperature of these methods is around 200 to 250 ° C., and the thin film formed at this temperature has an amorphous structure. Therefore, in order to obtain a thin film having ferroelectric properties, these thin films are further annealed at 500 to 700 ° C. to improve their crystallinity. Therefore, a high melting point metal such as Pt is used as the electrode material of the memory. Further, in order to protect the transistor existing in the lower layer of the memory from the thermal damage due to the high temperature heat treatment as described above, a Ti layer is buffered under the Pt electrode layer in consideration of the adhesion with the underlying SiO _2. It may be provided as a layer. However, the PZT [PbyZrxTiO ₃
(Y = 1-x)] and PLZT also start film fatigue after about 10 ⁶ times and deteriorate the polarization characteristics. Further, the thin film forming method also has the following problems. Sputtering method → Damage to underlying transistor due to ion bombardment MOCVD method → Insufficient control of material composition Sol gel method → There is a problem in film denseness, causing short between electrodes. Poor controllability of film thickness. Further, Pt is currently mainly used as an electrode material, but Pt has a high cost, and a process for processing it has not been sufficiently established.

[0003]

An object of the present invention is to provide a ferroelectric material with less film fatigue, and a novel method for manufacturing and processing the material.

[0004]

[Structure] The electrical characteristics of the ferroelectric substance are P as shown in FIG.
-E Can be expressed by a hysteresis loop. It can be said that the ferroelectric substance has excellent memory characteristics as the spontaneous polarization Ps and the remanent polarization Pr are large and the read electric field En is small. The spontaneous polarizability is a perovskite structure.
PbTiO ₃ , PZT, and LiNbO ₃ , LiTaO ₃ having an illuminite structure show great values and are promising. Table 1 shows the values of spontaneous polarization. This value is sufficient for use as a ferroelectric memory. (Below margin)

[Table 1] However, although the composition of ABO ₃ shows a perfect stoichiometric composition, the actual electrical characteristics are that oxygen-deficient A
BOx exhibits good and stable ferroelectric properties. Especially 2.
5 <x <3 is desirable. In particular, the illuminite structure has good memory characteristics, and LiNbO ₃ , LiTaO ₃ and its mixed crystal LiNbxTazO ₃ (z = 1-x) (0 ≦ x ≦
1) is effective. However, conventionally, LiNb
Materials such as O ₃ and LiTaO ₃ are sputtered and MOCVD method e
Even if a ferroelectric thin film is formed by tc, in the sputtering method, high-energy ions are likely to deteriorate the crystallinity,
Since the MOCVD method has a high temperature process of 800 ° C. or higher, the characteristics are adversely affected, and a ferroelectric memory having good characteristics cannot be obtained.

In order to solve this, the present invention uses ABOx.
By adopting the laser ablation method as the manufacturing method of ABOx, the oxygen atoms of ABOx can be easily controlled, and a thin film of ABOx (x <3) having good ferroelectricity can be obtained. As one of the laser ablation methods, an oxide target of the metal element A and an oxide target of the metal element B are used as target base materials, and each is irradiated with a laser to form a layer of the A element oxide layer and the B element oxide layer. , Its controllability can be made very well, and
There is a method of directly making the produced ABO ₃ thin film. According to this method, when the laser beam is irradiated, the irradiated portion is evaporated (ablated) by light energy, and good processing becomes possible. Since the structure of ABO ₃ has a strong ionic bond, the bond is easily broken by the light having high photon energy of the excimer laser etc, and the film formation and processing as described above are possible. As another method of the laser ablation method, it is also possible to directly irradiate a laser having a composition of ABOy (y> 3) as a base material with a laser to form a film. In the laser ablation,
In order to control the amount of oxygen atoms in the film, O ₂ , O ₃ , N ₂ O e
Laser ablation in an oxidizing gas of tc enables better control of film quality. The ferroelectric film is
Considering the step difference of the underlying transistor, 500 to 3000
Å is preferred. As a laser, ArF (193n
m), KrF (248 nm), XeCl (308 nm)
Excimer lasers such as As laser irradiation conditions for the oxide target of the metal element A and the oxide target of the metal element B, an excimer laser having a wavelength of 350 nm or less is used, and a laser pulse for irradiating the oxide target once is 1 to 100 shots. It is controlled and irradiated.
As the laser irradiation conditions for the oxide target represented by ABOy, the same conditions as the above laser irradiation conditions can be adopted. Next, a cross section of a typical ferroelectric memory is shown in FIG. Above the gate of the transistor (Tr) which is the word line, Ti / P which is the buffer layer and the base electrode is formed.
After forming and processing the t layer, PZT, LiNbO ₃ , LiTaO ₃
Etc. are formed by sputtering or the like, and then dry etching is performed to form a predetermined shape. Further, Pt is formed as an upper electrode by sputtering, and a memory portion is formed by processing by dry etching. At this time, if heat treatment is performed at 500 to 700 ° C., orientation and polarization characteristics are improved. After that, an interlayer insulating film which is the dielectric 5 is formed, and then an Al electrode is formed to complete the process.

[0006]

【Example】

Example 1 Ti1 was deposited on the dielectric 1 of FIG. 1 by DC sputtering.
After forming 000Å and Pt500Å by sputtering, Ar100
The base electrode is processed by RIE of%. On top of that, Li
_Two kinds of targets, ₂ O and Nb ₂ O ₃ , are prepared, and ArF is used.
(Λ = 193 nm) Focus the excimer laser to 50 μm,
Irradiate the target with energy of 0.2 J / cm ² ,
Film formation was performed. The ablation rate is Li ₂
Both O and Nb ₂ O ₃ were 20Å / pulse. Computer control each target and repeat film formation for each target by 5 pulses, 100Å.
A film of iNbOx (x <3) was formed at 2000Å. Next, in addition to the necessary LiNbOx portion, the ArF excimer laser is similarly irradiated to perform ablation to remove unnecessary LiNbOx.
After removing Ox, it was annealed in O ₂ at 500 ° C., Pt was formed into a film by 1000 Å sputtering, and then etching processing was performed by the same method to complete a memory portion.

[Brief description of drawings]

FIG. 1 is a diagram showing a cross section of a typical ferroelectric memory.

FIG. 2 is a diagram showing a PE hysteresis loop in a ferroelectric.

FIG. 3 is a diagram showing the position of each element in the ABO ₃ structure. (A) It is a figure which shows the position of each element when polarity is +. (B) It is a figure which shows the position of each element when polarity is-.

[Explanation of symbols]

 1 Upper Electrode Layer (Pt) 2 Ferroelectric Layer (PZT, PLZT) 3 Lower Electrode Layer (Pt) 4 Al Wiring 5 Dielectric Layer II 6 Dielectric Layer I 7 Buffer Layer (Ti) 8 Wordline 9 Bitline Ps Spontaneous polarization Pr Remanent polarization En Readout electric field A A element BB element

Claims (7)

[Claims] 1. ABOx (wherein A and B are elements constituting a ferroelectric material, x
Is a composition of less than 3) and has an illuminite or perovskite structure. 2. A ferroelectric material represented by ABOx (wherein A, B and x are the same as above) is represented by the formula, LiNbOx (wherein x is the same as above) or the formula, LiTaOx The ferroelectric material according to claim 1, which is an oxide having a composition represented by the formula (wherein x is the same as above) or a mixed crystal thereof. 3. The ferroelectric material is formed by laser irradiation of an oxide target of metal element A (A is the same as above) and an oxide target of metal element B (B is the same as above). The ferroelectric material according to claim 1. 4. The ferroelectric material is formed by laser ablation of an oxide target represented by the following formula: ABOy (wherein A and B are the same as above, y is a numerical value exceeding 3). A ferroelectric material. 5. An oxide target of a metal element A (A is the same as above) and an oxide target of a metal element B (B is the same as above) are separately provided in the presence or absence of an oxidizing gas. Laser irradiation is carried out, The manufacturing method of the ferroelectric material of Claim 3 characterized by the above-mentioned. 6. Laser ablation is performed by irradiating an oxide target of ABOy (wherein A, B and y are the same as above) with a laser in the presence or absence of an oxidizing gas. 5. The method for manufacturing a ferroelectric material according to claim 4, wherein. 7. A ferroelectric memory device, wherein the ferroelectric thin film in the ferroelectric memory device is formed of the ferroelectric material according to claim 1, 2, 3 or 4.