Semiconductor device manufacturing: process – Having magnetic or ferroelectric component
Reexamination Certificate
1998-10-21
2001-02-27
Whitehead, Jr., Carl (Department: 2822)
Semiconductor device manufacturing: process
Having magnetic or ferroelectric component
C438S240000, C438S608000
Reexamination Certificate
active
06194228
ABSTRACT:
This application is based on Japanese Patent Application No.9-290079 filed on Oct. 22, 1997, and No.10-293698 filed on Oct. 15, 1998, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electronic device having perovskite-type oxide film, a process for producing the same, and a ferroelectric capacitor.
2. Description of the Related Art
Prevailing computers, regardless of their size, have the main memory composed of dynamic random access memory (DRAM) or static random access memory (SRAM) which are of volatile memory. Volatile memory holds data while it is supplied with electric power but loses stored data as soon as the supply of electric power is suspended. By contrast, non-volatile memory holds data even without power supply. One of such non-volatile memory attracting attention is FeRAM which is based on ferroelectrics. The advantage of FeRAM is non-volatility, low power consumption, and capability of high integration density. In addition, because of its greatly improved rewritability, FeRAM is expected to supersede the existing memory and to find use in new applications such as IC card.
The constituent element of FeRAM is a ferroelectric capacitor, which is constructed of a ferroelectric film and two electrodes of platinum (Pt) or the like holding it between them. The ferroelectric film is required to have a high degree of residual polarization, a hysteresis loop with good squareness ratio and low coercive field, and a good fatigue characteristic (or ability to retain polarization even after repeated applications of pulses more than 10
12
times). Its additional requirement is an ability to prevent leak current between capacitor electrodes when it forms a ferroelectric capacitor. Leak current aggravates hysteresis. Another requirement is good (low) imprint characteristics, which is an ability to keep the hysteresis unshifted when the ferroelectric capacitor is kept at a high temperature in its polarized state.
Among known ferroelectric materials suitable for ferroelectric capacitors is known lead titanate zirconate (Pb(Zr,Ti)O
3
) (abbreviated as PZT hereinafter). PZT is characterized by its comparatively high residual polarization even at room temperature and its sufficiently high Curie point compared to the operating temperature. When used for ferroelectric capacitor of layered structure (Pt/PZT/Pt), PZT suffers so-called fatigue (which is a phenomenon that residual polarization decreases when pulses are applied repeatedly more than 10
6
times). This is detrimental to reliability.
An FeRAM less subject to fatigue is attracting attention. It is provided with conductive oxide electrodes, such as SrRuO
3
(abbreviated as SRO), in place of platinum. These conductive oxide electrodes function as a barrier against diffusion of Pb and prevent the oxygen defects of ferroelectric materials. This is the reason whey they protect ferroelectric film from fatigue.
Ferroelectric capacitors with SRO electrodes have been reported by Argonne National Laboratory in USA and Texas Instruments Corp. (TI) in USA. The one reported by the former is not suitable for mass production because it uses SrTiO
3
single crystal as the substrate. The one reported by the latter has PZT film deposited by sol-gel method on the SRO film deposited by high-temperature sputtering. Its disadvantage is that residual polarization is as small as 14.3 &mgr;C/cm
2
when the applied voltage is 3V. Another disadvantage is that high-temperature sputtering to deposit SRO film tends to cause oxygen defects (probably due to operation under reduced pressure), which poses a problem with crystallinity, although it yields polycrystalline film (in which crystals are oriented) if the substrate is heated during sputtering.
Furthermore, it is anticipated that it would be difficult to uniformly heat the entire surface of the substrate of large diameter and it would be difficult to form SRO film of good quality on the entire surface of the substrate of large diameter.
Although SRO is a conductive oxide, it has a resistance of 350 &mgr;&OHgr;cm, which is slightly higher than that of Pt (11 &mgr;&OHgr;cm). This makes it difficult to establish a good ohmic contact with Ti
1−x
Al
x
N which forms an adhesion layer when plugs are made of Si or W. Therefore, there is a demand for SRO having bottom resistance.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an electronic device having a perovskite-type oxide film suitable for mass production.
It is another object of the present invention to provide a process for producing said electronic device.
According to one aspect of the present invention, there is provided a process for producing a layered structure having a PZT thin film, comprising the steps of: forming on the main surface of a base substrate an amorphous SrRuO
3
film by sputtering without directly heating the substrate, heating the amorphous SrRuO
3
film at a temperature high enough for SrRuO
3
to crystallize, thereby polycrystallizing the amorphous SrRuO
3
film, forming on the polycrystallized SrRuO
3
film a Pb(Zr,Ti)O
3
film by sol-gel method, and heating the Pb(Zr,Ti)O
3
film at a temperature high enough for Pb(Zr,Ti)O
3
to crystallize, thereby polycrystallizing the Pb(Zr,Ti)O
3
film.
When a top electrode is formed on the Pb(Zr, Ti)O
3
film, there is obtained a ferroelectric capacitor of layered structure composed of SrRuO
3
, Pb(Zr,Ti)O
3
, and top electrode. Moreover, this process can be applied to a substrate of large diameter and hence is suitable for mass production.
According to another aspect of the present invention, there is provided an electronic device having a ferroelectric capacitor comprising: a base substrate having a main surface of SiO
2
, an adhesion layer to promote adhesion between the main surface and its top layer, a Pt film formed on said adhesion layer, a bottom SrRuO
3
film formed on said Pt film, a Pb(Zr,Ti)O
3
film formed on said bottom SrRuO
3
film, and a top SrRuO
3
film formed on said Pb(Zr,Ti)O
3
film. The Pt film functions as a diffusion-preventing layer which blocks mutual reactions between SrRuO
3
and SiO
2
.
According to a further aspect of the present invention, there is provided a process for producing an electronic device comprising the steps of; (a) forming a first perovskite-type conductive oxide film in an atmosphere of reduced pressure on an underlying surface at a first temperature, and (b) performing heat treatment on said first conductive oxide film in an oxidizing atmosphere containing oxygen at a second temperature which is higher than said first temperature.
Oxygen defects that might occur when the perovskite-type conductive oxide film is formed in an atmosphere of reduced pressure will be decreased by the subsequent heat treatment at a higher temperature in an oxidizing atmosphere containing oxygen.
According to another aspect of the present invention, there is provided a capacitor comprising a bottom electrode formed of an Ru-containing perovskite-type conductive oxide, a first dielectric film formed of a Pb-containing perovskite-type dielectric oxide having a stoichiometric composition on said bottom electrode, a second dielectric film formed of said perovskite-type dielectric oxide containing excess Pb on said dielectric film, and a top electrode formed of an Ru-containing perovskite-type conductive oxide.
The above-mentioned two-step process enables formation of a perovskite-type conductive oxide film of good quality and a perovskite-type dielectric oxide film of good characteristics. In the case where an SrRuO
3
film is formed without directly heating the substrate, mass-productivity of a layered structure can be enhanced In which the SrRuO
3
film is used as an electrode and a (Pb,La)(Zr,Ti)O
3
film is formed thereon.
REFERENCES:
patent: 4316785 (1982-02-01), Suzuki et al.
patent: 5434102 (1995-07-01), Watanabe et al.
patent: 5440173 (1995-08-01), Evans, Jr. et al.
patent: 5489548 (1996-02-01), Nishioka et al.
patent
Cross Jeffrey S.
Fujiki Mitsushi
Tsukada Mineharu
Armstrong, Westerman Hattori, McLeland & Naughton
Fujitsu Limited
Jr. Carl Whitehead
Vockrodt Jeff
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