Semiconductor device manufacturing: process – Having magnetic or ferroelectric component
Reexamination Certificate
2000-03-13
2002-03-12
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Having magnetic or ferroelectric component
C438S003000, C438S240000, C438S381000, C438S608000, C438S650000, C438S686000
Reexamination Certificate
active
06355492
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART
The present invention relates to an electrode for a capacitor suitable for use in a semiconductor memory cell in particular, said capacitor and a process for the manufacture of these.
A nonvolatile semiconductor memory cell using a ferroelectrics film (to be sometimes referred to as “FERAM” hereinafter) is a nonvolatile semiconductor memory cell which uses fast inversion of polarization of the ferroelectrics film and residual polarization thereof and permits rapid rewriting. A nonvolatile semiconductor memory cell having a ferroelectrics film under study at present can be classified into two memory cells, a memory cell based on a method of detecting a change in an amount of a stored charge on a ferroelectrics capacitor and a memory cell based on a method of detecting a change in resistance of a semiconductor caused by the spontaneous polarization of ferroelectrics. The semiconductor memory cell to be explained hereinafter in the present specification comes under the former, and it is, in principle, composed of a ferroelectrics capacitor and a selecting transistor and has a structure and constitution similar to those of DRAM.
In the nonvolatile semiconductor memory cell according to the method of detecting a change in the amount of the stored change on the ferroelectrics capacitor, data is written and read out by applying the P-E hysteresis loop of ferroelectrics shown in FIG.
11
. When an external electric field is applied to a ferroelectrics film and then removed, the ferroelectrics film exhibits spontaneous polarization. The residual polarization of the ferroelectrics film is +P
r
when an external electric field in a plus direction is applied, and it is −P
r
when an external electric field in a minus direction is applied. In this case, a state where the residual polarization is +P
r
(see “D” in
FIG. 11
) represents “0”, and a state where the residual. polarization is −P
r
(see “A” in
FIG. 11
) represents “1”.
For discriminating “1” or “0”,for example, an external electric field in a plus direction is applied to the ferroelectrics film. As a result, the polarization of the ferroelectrics film is brought into a state “C” in FIG.
11
. When data is “0” the polarization state of the ferroelectrics film changes from “D” to “C”. On the other hand, when data is “1”, the polarization state of the ferroelectrics film changes from “A” to “C” through “B”. When data is “0”, the ferroelectrics film causes no inversion of the polarization. On the other hand, when data is “1”, the ferroelectrics film causes an inversion of the polarization. As a result, a difference is caused in the amount of transferred charge depending upon a difference in the stored charge amount (polarization state) of the ferroelectrics capacitor. The stored charge is detected as a signal current by turning on the selecting transistor of a selected semiconductor memory cell. When the external electric field is changed to “0”, the polarization state of the ferroelectrics film is brought into a state “D” in
FIG. 11
even when the data is any one of “0” and “1”. When the data is “1”, therefore, the polarization is brought into a state “A” through “D” and “E” by applying an external electric field in a minus direction, to write data “1”.
The dielectric capacitor of the semiconductor memory cell is composed of two electrodes and a capacitor insulation layer formed of a dielectric film sandwiched between these two electrodes. In a conventional nonvolatile semiconductor memory cell (FERAM) using a ferroelectrics film, an upper electrode and a lower electrode are formed of platinum (Pt). Platinum is a stable material, while it has a defect that its processability is poor. It is therefore being studied to use Ru, RuO
2
, Ir, IrO
2
or the like as an electrode material-having excellent processability over platinum, for high-density integrated DRAM or FERAM.
For stably maintaining the electric properties of a dielectric capacitor for a long period of time, desirably, the electrode in the interface with a dielectric film is composed of RuO
2
or IrO
2
, as is discussed in “Improvement of Fatigue of PZT Capacitors by Optimizing Electrode Material and PZT Crystallinity”, K. Aoki, et al., PacRimFerro 3 in Kyoto, extended abstract, pp 87-90 (1996). However, it is known that when Ru is oxidized, volatile ruthenium oxides, RuO
3
and RuO
4
, are generally formed and that when Ir is oxidized at a high temperature, it is volatilized as IrO
2
. When these metals are used as a lower electrode in particular, the lower electrode is exposed to a high-temperature atmosphere for a long period of time and the flatness of the lower electrode surface is impaired by evaporation and/or re-oxidation of these materials. As a result, there is caused a problem that the dielectric capacitor is degraded in characteristics.
OBJECT AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an electrode for a capacitor, a dielectric capacitor of which is not degraded in characteristics when the electrode is exposed to a high-temperature atmosphere for a long period of time, said capacitor and a process for the manufacture of these.
According to the present invention, the above object of the present invention is achieved by an electrode for a capacitor composed of two electrodes and a capacitor insulation layer formed of a dielectric film sandwiched between the two electrodes,
at least one of the electrodes being formed of a metal layer and a metal oxide layer, and
said metal oxide layer being formed by oxidizing a surface of said metal layer on the basis of a diffusion-controlling reaction and being positioned in an interface to said capacitor insulation layer.
According to the present invention, the above object of the present invention is also achieved by a process for the manufacture of an electrode for a capacitor composed of two electrodes and a capacitor insulation layer formed of a dielectric film sandwiched between the two electrodes,
The process comprising forming a metal layer, and then oxidizing a surface of said metal layer on the basis of a diffusion-controlling reaction by heat-treating said metal layer in an oxidizing atmosphere, to form a metal oxide layer, thereby forming at least one of the electrodes composed of the metal layer and the, metal oxide layer positioned in an interface to the capacitor insulation layer.
Further, according to the present invention, the above object is achieved by a capacitor composed of two electrodes and a capacitor insulation layer formed of a dielectric film sandwiched between the two electrodes,
at least one of the electrodes being formed of a metal layer and a metal oxide layer,
said metal oxide layer being formed by oxidizing a surface of said metal layer on the basis of a diffusion-controlling reaction and being positioned in an interface to said capacitor insulation layer, and
the dielectric film being formed of a dielectric material having a perovskite structure, a pseudo perovskite structure or a layer structure.
According to the present invention, the above object of the present invention is also achieved by a process for the manufacture of a capacitor composed of two electrodes and a capacitor insulation layer formed of a dielectric film sandwiched between the two electrodes,
the process comprising forming a metal layer, and then oxidizing a surface of said metal layer on the basis of a diffusion-controlling reaction by heat-treating said metal layer in an oxidizing atmosphere, to form a metal oxide layer, thereby forming at least one of the electrodes composed of the metal layer and the metal oxide layer positioned in an interface to the capacitor insulation layer.
The present invention includes an embodiment in which the electrode has a two-layered structure of a metal layer and a metal oxide layer, or an embodiment in which the electrode is composed of an aggregate of metal particles and a metal oxide layer is formed on the entire surface, or on part of surface, of each particle. It depends u
Ami Miho
Tanaka Masahiro
Kananen Ronald P.
Rader & Fishman & Grauer, PLLC
Sarkar Asok Kumar
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