Semiconductor device manufacturing: process – Having magnetic or ferroelectric component
Reexamination Certificate
1999-01-12
2001-12-25
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Having magnetic or ferroelectric component
C438S240000, C438S253000, C438S396000
Reexamination Certificate
active
06333201
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nonvolatile memory, and more particularly to a method for fabricating a nonvolatile ferroelectric memory which can prolong a life time of the memory.
2. Background of the Related Art
A ferroelectric memory, i.e., an FRAM(Ferroelectric Random Access Memory) is paid attention as a next generation memory which has a data processing rate as fast as a DRAM (Dynamic Random Access Memory) generally used as a semiconductor memory and retains the data even in turn off of power. The FRAM, having a structure almost identical to a DRAM, is a memory which employs a ferroelectric material as a capacitor material for utilizing a high residual dielectric polarization that is a characteristic of the ferroelectric material in retaining data even after removal of an electric field.
A related art method for fabricating a nonvolatile ferroelectric memory will be explained, with reference to the attached drawings.
FIGS. 1
a
~
1
c
illustrate sections showing the steps of a related art method for fabricating a capacitor of a ferroelectric material.
Referring to
FIG. 1
a,
the steps of a related art method for fabricating a capacitor of a ferroelectric material starts with forming an insulating film
2
on a semiconductor substrate
1
, and forming a bottom electrode
3
on the insulating film
2
. The bottom electrode
3
is formed of Pt (Platinum). As shown in
FIG. 1
b,
a PZT[Pb(Zr, Ti)O3]4, a ferroelectric film, is formed on the bottom electrode
3
. As shown in
FIG. 1
c,
a top electrode
5
is formed on the PZT
4
. The top electrode
5
is formed of Pt.
FIG. 2
illustrates a general hysteresis loop of a ferroelectric material, and
FIG. 3
illustrates P-E curves on an electric field cycle applied to the PZT used as the ferroelectric material. That is,
FIG. 2
illustrates hysteresis curves of ferroelectric materials, such as PZT used for the related art memory shown in
FIGS. 1
a
~
1
c
. The ferroelectric material of PZT has a spontaneous polarization and a polarization inversion by an electric field, which are essential properties in the ferroelectric material. As shown in
FIG. 2
, the polarization induced by the electric field does not disappear, but a certain amount of residual dielectric polarization(+Pr and −Pr) is kept due to the spontaneous polarization even after the electric field is removed. The states of +Pr and −Pr are corresponded to 0 and 1 respectively in utilizing as a memory, and different from the DRAM, data can be kept even after removal of the electric field, thereby implementing the nonvolatile memory. In operation of the FRAM, i.e., in reading and writing a data, the two states of +Pr and −Pr should be alternated as necessary, from +Pr to −Pr or vice versa, which is called as a polarization inversion, which causes a degradation.
That is, as shown in
FIG. 3
, when a bipolar field is applied in succession to a ferroelectric film, such as of PZT, for causing a polarization inversion, a P(Polarization)-V(voltage) characteristic shows a gradual decrease of residual dielectric polarization of +Pr and −Pr as applied voltage cycles are increased until the memory can not serve as a memory, finally.
FIG. 3
shows a sharp increase of a fatigue of the ferroelectric film when a number of the cycle is greater than 10
8
, which is caused by a mismatched of the ferroelectric film with an electrode material, for which researches on a new electrode material that has good match with the existing PZT is underway.
FIG. 4
illustrates a graph showing fatigue of the PZT ferroelectric film, wherein a number of applied field cycles vs. residual dielectric polarization is shown in a case when PZT is used as a ferroelectric film and Pt is used for the top and bottom electrodes, wherefrom it can be known that the fatigue sharply increases at around 10
8
switching cycles in the case of PZT.
However, the related art method for fabricating a nonvolatile ferroelectric memory has a problem in that the rapid fatigue of the ferroelectric film, such as PZT, started to occur when a number of re-writing which requires polarization inversion exceeds 10
8
times drops a reliability of the nonvolatile ferroelectric memory.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a method for fabricating a nonvolatile ferroelectric memory that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method for fabricating a nonvolatile ferroelectric memory which can prolong a life time of the nonvolatile ferroelectric memory.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the method for fabricating a nonvolatile ferroelectric memory includes the steps of forming an insulating film on a semiconductor substrate, forming a bottom electrode on the insulating film, forming a ferroelectric film on the bottom electrode, wherein the ferroelectric film is formed of a material containing zirconium oxide as a base composition, the material having an antiferroelectric phase which can not be induced to a ferroelectric phase by an electric field, and the induced ferroelectric phase exhibiting a hysteresis in polarization-electric field characteristic and unable to be induced to an antiferroelectric phase by an electric field, and forming a top electrode on the ferroelectric film.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
REFERENCES:
patent: 5610853 (1997-03-01), Akiyama et al.
Furuta et al., “Shape Memory Ceramics and Their Application to Latching Relays”, Sensors and Materials 3,4 (1992) pp. 205-215.*
Oh et al., “Piezoelectricity in the Field-Induced Ferroelectric Phase of Lead Zirconate-Based Antiferroelectrics” Journal of American Ceramic Soc., 75[4] Apr. 1992, pp. 795-799.*
Tatsumi Sumi et al., “FA 16.2: A 256kb Nonvolatile Ferroelectric Memory at 3V and 100 ns;” ICSCC94/Session 16/Technology Directions: Memory Packaging/Paper FA16.2; 1994 IEEE International Solid State Circuits Conference; pp. 268-269 and Slide Supplement pp. 209 & 315.
Oh Ki Young
Yoon Ki Hyun
Fleshner & Kim LLP
Hyundai Electronics Industries Co,. Ltd.
Kennedy Jennifer M.
Niebling John F.
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