Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
1999-09-24
2002-04-23
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C438S660000, C438S686000
Reexamination Certificate
active
06376090
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
This application is related to Japanese patent application No. HEI 10(1998)-270399 filed on Sep. 25, 1998 whose priority is claimed under 35 USC §119, the disclosure of which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing a substrate with an oxide ferroelectric thin film formed thereon and a substrate with a oxide ferroelectric thin film formed thereon. More particularly it relates to a method for manufacturing a substrate with an oxide ferroelectric thin film formed thereon and a substrate with a oxide ferroelectric thin film formed thereon that can be suitably applied to a memory element, a pyroelectric element, a piezoelectric element, an optical device and the like.
2. Description of the Related Art
Among many oxide materials, some have various properties such as a ferroelectric property, a high dielectric property, a piezoelectric property, a pyroelectric property, an electro-optical effect and the like, and are generally referred to as oxide ferroelectric materials. These oxide ferroelectric materials include, for example, Pb(Zr
1−x
Ti
x
)O
3
(hereafter referred to as PZT), BaTiO
3
, Bi
4
Ti
3
O
12
, LiNbO
3
and the like. Development of many devices such as a capacitor, a pressure sensor, an infrared sensor, an oscillator, a frequency filter, an optical switch and the like has been carried out by utilizing these excellent properties of the oxide ferroelectric materials.
Especially, in accordance with recent development of a thin film forming technique, scale reduction and process simplification of devices are attempted by applying a high dielectric property of the oxide ferroelectric materials to a capacitor of a semiconductor device such as a DRAM. Also, development of a device having a novel function such as a non-volatile memory (ferroelectric non-volatile memory) having a high density and operating at a high speed is carried out by applying the ferroelectric property to a memory of a semiconductor device such as a DRAM.
A ferroelectric non-volatile memory realizes a memory that eliminates the need for a backup power supply by utilizing a ferroelectric property (hysteresis effect) of a ferroelectric substance. For the development of such devices, it is necessary to use a material having a large residual polarization and a small coercive field.
At present, regarding destructive read-out type ferroelectric memories referred to as 1T1C, small-capacitance products of 256 kbit or less are becoming commercialized using a ferroelectric thin film having a residual polarization value of about 10 to 20 &mgr;C/cm
2
. However, it is essential to develop a ferroelectric thin film having a residual polarization value of 20 &mgr;C/cm
2
or more in order to develop a ferroelectric memory having a capacitance of 1 Mbit or more.
Further, in order to obtain good electric properties, it is necessary to use a material having a low leakage current and a high endurance against repetition of polarization inversion. For this purpose, control of a surface morphology after forming a film is an importance problem. Further, for reduction of an operation voltage and adaptation to fine semiconductor processes, it is desired to realize the above-mentioned properties with a thin film having a thickness of less than several hundred nanometers.
Among the above-mentioned oxide ferroelectric materials, PZT is the most extensively studied material, and a thin film using PZT is fabricated by the sputtering method or the sol-gel method. PZT is a solid solution of PbZrO
3
and PbTiO
3
, and has a Zr/Ti ratio of 1 to 1.5. PbTiO
3
is a ferroelectric substance having a perovskite structure that belongs to a tetragonal system and has a spontaneous polarization along a c-axis direction. Although PbZrO
3
is an anti-ferroelectric substance having a perovskite structure that belongs to an orthorhombic system, PbZrO
3
forms a solid solution with PbTiO
3
to increase a Ti content and is transformed into a ferroelectric substance.
An oxide ferroelectric substance Bi
4
Ti
3
O
12
is a ferroelectric substance having a perovskite structure that belongs to an orthorhombic system. The spontaneous polarization of Bi
4
Ti
3
O
12
has two components, one along an a-axis direction and the other along a c-axis direction. Especially, since the coercive field along the c-axis direction is small, application of Bi
4
Ti
3
O
12
to a ferroelectric non-volatile memory operable at a low voltage is expected.
In order to achieve practical application of the above-mentioned oxide ferroelectric material to these devices, a thin film forming technique is important and carried out by the sol-gel method, the sputtering method, the MOCVD method, the laser abrasion method or the like.
A substrate for forming the above-mentioned oxide ferroelectric material thereon by using one of these film forming methods may be a substrate having an electrode made of Pt(111), Ir(111), an oxide superconductive material or the like.
In the case of PZT, although the ferroelectric property of PZT depends largely on the composition x, the film composition is liable to change at the time of forming the film or carrying out a thermal treatment because PZT contains Pb having a high vapor pressure. Therefore, it is difficult to find a factor that determines the orientation or the crystallinity (morphology). As a result, leakage current and deterioration in endurance against polarization inversion are liable to occur in accordance with the reduction of the film thickness due to generation of pinholes, generation of a low dielectric layer caused by reaction of the underlayer electrode Pt and Pb, or the like.
On the other hand, in the case of Bi
4
Ti
3
O
12
, it is necessary to perform a thermal treatment of 650° C. or more in order to obtain a good ferroelectric property by means of the conventional sol-gel method, so that the surface directions to be obtained are limited and it is difficult to control the orientation. In the case of forming the film by the MOCVD method, it is reported that a pyrochlore phase (Bi
2
Ti
2
O
7
) is liable to be generated (See Jan. J. Appl. Phys., 32, 1993, p. 4086, and J. Ceramic Soc. Japan, 102, 1994, p. 512).
In any of the above-mentioned film-forming methods, the obtained surface morphology generally consists of gross crystal particles of 0.5 &mgr;m or more, so that the thin film cannot be applied to a highly integrated semiconductor device that requires a fine processing. Moreover, if the thin film has a thickness of several hundred nanometers or less, pinholes are generated and it is difficult to suppress leakage current. Also, it is not easy to obtain a residual polarization and a coercive field as desired by controlling the orientation direction.
Japanese Unexamined Patent Publication No. HEI 08(1996)-222711 and Japanese Unexamined Patent Publication No. HEI 10(1998)-50960 disclose that suppression of leakage current and endurance against repetition of polarization inversion are greatly improved by inserting a TiO
x
thin layer at an interface between a PZT thin film and a lower electrode made of Pt or Ir, and at an interface of laminated PZT thin film layers to provide nucleation for the PZT thin film in forming the PZT film by the sol-gel method. However, the obtained ferroelectric property gives a residual polarization value of 20 &mgr;C/cm
2
or less, which does not exceed a conventionally obtained range.
As described above, if an oxide ferroelectric thin film such as PZT or Bi
4
Ti
3
O
12
is to be formed on a metal electrode such as Pt or Ir by using a film forming technique such as the sol-gel method, the sputtering method or the MOCVD method according to the above-mentioned technique, it is difficult to control the orientation property, the orientation direction or the morphology of the oxide ferroelectric thin film because the ferroelectric thin film must be exposed to a high temperature for a long period of time at the time of forming the film or carryi
Alexander John B.
Conlin David G.
Edwards & Angell LLP
Sharp Kabushiki Kaisha
Zimmerman John J.
LandOfFree
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