U.S. patent application number 09/848420 was filed with the patent office on 2001-09-27 for semiconductor memory device and manufacturing method thereof.
Invention is credited to Abe, Hisahiko, Horikoshi, Kazuhiko, Kato, Hisayuki, Ogata, Kiyoshi, Suenaga, Kazufumi, Tanaka, Jun, Yoshizumi, Keiichi.
Application Number | 20010023952 09/848420 |
Document ID | / |
Family ID | 14183637 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010023952 |
Kind Code |
A1 |
Suenaga, Kazufumi ; et
al. |
September 27, 2001 |
Semiconductor memory device and manufacturing method thereof
Abstract
The present invention is a high quality semiconductor memory
device using a ferroelectric thin film capacitor as a memory
capacitor at a high manufacturing yield, the ferroelectric thin
film of the capacitor is specified such that the relative standard
deviation of crystal grain sizes is 13% or less, to thereby ensure
a high remanent polarization value and a small film fatigue (large
rewritable number).
Inventors: |
Suenaga, Kazufumi;
(Yokohama-shi, JP) ; Ogata, Kiyoshi;
(Yokohama-shi, JP) ; Horikoshi, Kazuhiko;
(Kawasaki-shi, JP) ; Tanaka, Jun; (Chigasaki-shi,
JP) ; Kato, Hisayuki; (Kokubunji-shi, JP) ;
Yoshizumi, Keiichi; (Kokubunji-shi, JP) ; Abe,
Hisahiko; (Mito-shi, JP) |
Correspondence
Address: |
ANTONELLI TERRY STOUT AND KRAUS
SUITE 1800
1300 NORTH SEVENTEENTH STREET
ARLINGTON
VA
22209
|
Family ID: |
14183637 |
Appl. No.: |
09/848420 |
Filed: |
May 4, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09848420 |
May 4, 2001 |
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09288672 |
Apr 9, 1999 |
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6239457 |
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Current U.S.
Class: |
257/295 ;
257/306; 257/E21.009; 257/E21.272 |
Current CPC
Class: |
H01L 21/02197 20130101;
H01L 27/11502 20130101; H01L 28/55 20130101; H01L 21/31691
20130101; G11C 11/22 20130101 |
Class at
Publication: |
257/295 ;
257/306 |
International
Class: |
H01L 029/76 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 9, 1998 |
JP |
10-097117 |
Claims
We claim:
1. A semiconductor memory device using a ferroelectric thin film
capacitor as a memory capacitor, said capacitor comprising: a stack
structure having at least a lower electrode, a ferroelectric thin
film, and an upper electrode; wherein a relative standard deviation
of crystal grains within a plane having a normal line in the
thickness direction of said ferroelectric thin film is in a range
of 13% of less.
2. A semiconductor memory device according to claim 1, wherein said
crystal grains of said ferroelectric thin film have columnar shapes
elongated substantially in parallel to the film thickness
direction, and said columnar crystal grains have no grain boundary
in the film thickness direction.
3. A semiconductor memory device according to claim 1, wherein a
surface roughness of said ferroelectric thin film is specified such
that a difference between a maximum value and a minimum value with
respect to an average plane of surface irregularities of said
ferroelectric thin film is in a range of 40% or less of an average
thickness of said ferroelectric thin film.
4. A semiconductor memory device according to claim 1, wherein a
standard deviation of the surface roughness of said ferroelectric
thin film is in a range of 10 nm or less.
5. A semiconductor memory device according to claim 1, wherein said
ferroelectric thin film is made from an ABO.sub.3 type oxide having
a perovskite structure, and said ferroelectric thin film is formed
such that the (111) faces of said crystal grains are preferentially
oriented in a direction perpendicular to a substrate of said
semiconductor memory device.
6. A semiconductor memory device according to claim 5, wherein said
ferroelectric thin film is made from a material having a
composition at least part of which contains a crystalline ABO.sub.3
type oxide, an amorphous ABO.sub.3 type oxide, or a mixture
thereof, where A is at least one element selected from a group
consisting of Pb, La, Sr, Nd and Ba; B is at least one element
selected from a group consisting of Zr, Ti, Mn, Mg, Nb, Sn, Sb and
In; and O is oxygen.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a semiconductor memory
device using a ferroelectric thin film, which is most suitable for
a ferroelectric nonvolatile memory or high density DRAM, and to a
method of manufacturing the semiconductor memory device.
[0002] (1) A conventional ferroelectric thin film capacitor has, as
described in "Ferroelectric Thin Film Memory" (published by Science
Forum, 1995), page 227, a stacked structure of Pt upper
electrode/ferroelectric layer (PZT)/Pt lower electrode.
[0003] (2) Based on a surface observation photograph by a scanning
electron microscope for a PZT ferroelectric thin film crystallized
on a lower electrode, described in Integrated Ferroelectrics, 1995,
Vol. 10, pp. 145-154, an average crystal grain size is about 180 nm
and a relative standard deviation of crystal grain sizes is about
15%.
[0004] (3) In a method of forming a thin film described in Japanese
Patent Laid-open No. Hei 7-142600, a compound of BaTiO.sub.3 is
formed on a Pt thin film, whereby orientation of a ferroelectric
thin film is controlled by allowing crystal orientation of the
ferroelectric thin film to follow that of the Pt thin film, to
thereby ensure remanent polarization.
[0005] (4) In an oriented ferroelectric thin film described in
Japanese Patent Laid-open No. Hei 6-151601, an epitaxial or
oriented buffer layer having a two-layer structure on a
semiconductor single crystal substrate and an epitaxial or oriented
perovskite ABO.sub.3 type ferroelectric substance is formed
thereon, to obtain a highly oriented ferroelectric thin film.
[0006] In the above references, description is made of the
nonvolatile memories using a ferroelectric substance as a
capacitor. The problems to be examined, however, are also present
in DRAMs using a ferroelectric substance as a capacitor.
[0007] (5) For example, as described in "Ferroelectric Thin Film
Integration Technology" (published by Science Forum, 1992), pages
13-16, for a 256 Mb DRAM or the like, an attempt has been made to
use a crystal thin film made from a high dielectric constant
material such as BaSrTiO.sub.3 or the like for a capacitor.
[0008] In the above-described references (1) and (2), it is
difficult to control the crystal grain size and orientation of the
ferroelectric thin film. When such a ferroelectric thin film is
patterned to form a memory capacitor, a variation in
characteristics between memory cells becomes large because of a
large variation in crystal grain size, a larger variation in
crystal orientation, and a larger surface roughness of each of the
ferroelectric thin film and an electrode. This makes it difficult
for all of the memory cells to equally obtain sufficient
characteristics, giving rise to a problem in exerting adverse
effect on the stability in manufacturing yield.
[0009] In the above-described reference (3), a variation in
orientation is reduced; however, since a variation in grain size of
crystal grains in a memory cell is large, a leakage current occurs,
an effective voltage between capacitors is reduced because of
concentration of an electric field at a grain boundary portion
present in the thin film in the film thickness direction, or
remanent polarization becomes uneven, which results in degradation
of the performance of the memory cell.
SUMMARY OF THE INVENTION
[0010] An object of the present invention is to solve the
above-described problems, and to provide a ferroelectric thin film
capacitor capable of reducing a variation in characteristics
between memory cells, thereby realizing a highly integrated
ferroelectric memory having an enhanced performance at an improved
manufacturing yield.
[0011] To solve the above problems, according to the present
invention, there is provided a semiconductor memory device using,
as a memory capacitor, a ferroelectric thin film capacitor composed
of a stacked structure having at least a lower electrode, a
ferroelectric thin film and an upper electrode formed on a
substrate, wherein a relative standard deviation of crystal grain
sizes of crystal grains of the ferroelectric thin film is
controlled at a value of 13% or less; the crystal grains are formed
into columnar shapes elongated in the film thickness direction; and
the columnar crystal grains have no grain boundary in the film
thickness direction. With this configuration, it is possible to
prevent occurrence of a leakage current and also to prevent a
reduction in effective voltage applied between capacitors due to
concentration of an electric field in the ferroelectric thin film
or at an interface between the ferroelectric thin film and an
electrode.
[0012] The lower electrode of the above capacitor may be configured
as a Pt electrode or a Pt alloy electrode, and the lower electrode
may be formed such that the (111) faces of crystal grains are
preferentially oriented in the direction perpendicular to a
substrate plane. This makes it possible to improve the orientation
of a ferroelectric thin film formed on the lower electrode, and
hence to further enhance the uniformity between memory cells. The
lower electrode may be also made from a compound containing a
material such as Ru, Ir, an oxide thereof or Pt, and an element
contained in the ferroelectric thin film. In this case, the same
effect as that described above can be achieved.
[0013] An ABO.sub.3 type oxide having a perovskite structure may be
used as the ferroelectric material and the ferroelectric thin film
may be formed such that the (111) faces of crystal grains are
preferentially oriented in the direction perpendicular to the
substrate plane. With this configuration, it is possible to reduce
the non-uniformity in characteristics due to a variation in
orientation. By use of an ABO.sub.3 type ferroelectric substance
having a composition [A=Pb, B=(Zr.sub.1-x, Ti.sub.x)], there can be
obtained a ferroelectric thin film having a large remanent
polarization, which film is desirable for a nonvolatile memory.
Further, by use of an ABO.sub.3 type ferroelectric substance having
a composition [A=(Ba.sub.1-x, Sr.sub.x), B=Ti], there can be
obtained a ferroelectric thin film exhibiting no hysteresis at a
memory service temperature, which film is desirable for a capacitor
of a DRAM or the like. A erroelectric thin film can be made from a
material having a composition at least part of which contains a
crystalline ABO.sub.3 type oxide, an amorphous ABO.sub.3 type
oxide, or a mixture thereof, where A is at least one element
selected from a group consisting of Pb, La, Sr, Nd and Ba; B is at
least one element selected from a group consisting of Zr, Ti, Mn,
Mg, Nb, Sn, Sb and In; and O is oxygen.
[0014] According to the present invention, there is provided a
method of reducing the relative standard deviation of crystal grain
sizes of crystal grains of a ferroelectric thin film by forming
micro-nuclei necessary for growth of the crystal grains on the
lower electrode with less variation. The method includes the steps
of forming initial nuclei made from at least one or more of metals
contained in a ferroelectric thin film to be formed or an oxide or
compound containing the metals, or heat-treating the lower
electrode after formation thereof at a high temperature to
precipitate at least one or more of metals contained in an adhesive
layer (provided between the lower electrode and a CMOS substrate)
or an oxide or compound containing the metals on the surface of the
lower electrode, thereby forming initial nuclei necessary for
formation of micro-nuclei; and forming and crystallizing a
ferroelectric thin film on the initial nuclei layer to a thickness
required for a semiconductor device. With this configuration, there
can be obtained a ferroelectric capacitor in which the relative
standard deviation of the crystal grain sizes is small, the (111)
faces of the crystal grains are preferentially oriented in the
direction perpendicular to the substrate plane, and the surface
roughness is small.
[0015] Alternatively, the above initial nuclei layer to be formed
on the surface of the lower electrode may be made from an ABO3 type
oxide having a perovskite structure where A=Pb, B=(Zr.sub.1-x,
Ti.sub.x) or A=(Ba.sub.1-x, Sr.sub.x), B=Ti, or made from Ti,
TiO.sub.x, Sr or SrO.sub.x. The initial nuclei can be made from a
material having a composition at least part of which contains a
crystalline ABO3 type oxide, an amorphous ABO3 type oxide, or a
mixture thereof, where A is at least one element selected from a
group consisting of Pb, La, Sr, Nd and Ba; B is at least one
element selected from a group consisting of Zr, Ti, Mn, Mg, Nb, Sn,
Sb and In; and O is oxygen. With this configuration, there can be
obtained a ferroelectric thin film in which the crystal grain sizes
are small and the relative standard deviation of the crystal grains
is small. The ferroelectric thin film thus obtained is advantageous
in suppressing growth of crystal grains having the pyrochlore
structure and rosette-shaped ZrO.sub.x crystal grains causing
deterioration of ferroelectric characteristics, and ensuring
properties most suitable for a nonvolatile memory, that is, a large
remanent polarization value, a small leakage current, and a small
film fatigue (reduction in remanent polarization due to
rewriting).
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1A is a top view showing the schematic configuration of
a capacitor portion of a semiconductor memory device using a
ferroelectric thin film in which the relative standard deviation of
crystal grain sizes is 13% or less according to one embodiment of
the present invention;
[0017] FIG. 1B is a sectional view of FIG. 1A;
[0018] FIG. 2A is a top view illustrating a method of measuring
crystal grain sizes of a ferroelectric thin film in an observation
image (size: 1 .mu.m.times.1 .mu.m) obtained by an AFM according to
one embodiment of the present invention;
[0019] FIG. 2B is an essential portion enlarged view of FIG.
1A;
[0020] FIG. 3 is an X-ray diffraction diagram showing the result of
X-ray diffraction of a ferroelectric capacitor in a semiconductor
memory device according to one embodiment of the present
invention;
[0021] FIGS. 4A and 4B are diagrams showing a correlation between
an average crystal grain size "a" and a remanent polarization value
P according to one embodiment of the present invention, and a
diagram showing a correlation between an average crystal grain size
"a" and a film fatigue according to one embodiment of the present
invention;
[0022] FIGS. 5A and 5B are diagrams showing a correlation between a
relative standard deviation .sigma. of crystal grain sizes of a
ferroelectric thin film and a remanent polarization value P
according to one embodiment of the present invention, and a
correlation between the relative standard deviation .sigma. and a
film fatigue according to one embodiment of the present
invention;
[0023] FIG. 6 is a schematic sectional view illustrating a method
of measuring surface irregularities of a ferroelectric thin film by
an AFM according to one embodiment of the present invention;
[0024] FIG. 7A and 7B are diagrams showing a correlation between a
surface roughness Rms and a remanent polarization value P according
to one embodiment of the present invention, and a diagram showing a
correlation between the surface roughness Rms and a film fatigue
according to one embodiment of the present invention;
[0025] FIGS. 8A, 8B, 8C and 8D are flow charts showing a method of
manufacturing a ferroelectric thin film according to one embodiment
of the present invention;
[0026] FIG. 9 is a top view showing one example of a film formation
apparatus for manufacturing a ferroelectric capacitor of a
semiconductor memory device according to one embodiment of the
present invention;
[0027] FIG. 10 is a sectional view showing the schematic
configuration of a capacitor portion of a semiconductor memory
device using a ferroelectric thin film according to one embodiment
of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] Hereinafter, embodiments of the present invention will be
described in detail.
[0029] (1) Semiconductor Memory Device on Which Ferroelectric Thin
Film is Mounted
[0030] FIG. 10 is a sectional view showing the schematic
configuration of a capacitor portion of a semiconductor memory
device using a ferroelectric thin film according to one embodiment
of the present invention. Symbol's number 98 is a Si substrate.
Symbol's number 101 is an underlayer LSI. Symbol's number 102 is an
insulating layer. Symbol's number 81 is an adhesive layer. Symbol's
number 11 is a lower electrode. Symbol's number 104 is a
ferroelectric layer. Symbol's number 105 is an upper electrode.
Symbol's number 103 is an interconnection layer (a wiring layer).
Symbol's number 106 is an interlayer insulating layer. Symbol's
number 107 is a protective layer. Symbol's number 108 is a sealing
resin. A CMOS, which will be taken as a transistor portion of a
memory cell, is formed on a Si substrate 98, and an insulating
layer 102 for planarization, insulation and protection of the CMOS.
In this embodiment, as the insulating layer 102, a SiO.sub.2 glass
film called a BPSG film is formed to a thickness of 300 nm. A
ferroelectric capacitor is formed on the SiO.sub.2 insulating layer
102. The ferroelectric capacitor has a stacked structure of a Ti
adhesive layer 81 (20 nm), a Pt lower electrode 11 (200 nm), an
inventive ferroelectric substance Pb.sub.(1+y)(Zr.sub.1-x,
Ti.sub.x)O.sub.3 layer 104 (250 nm) containing crystal grains in
which a relative standard deviation of crystal grain sizes is 13%
or less (not more than 13%), and an upper electrode 105 (10 nm). An
interlayer insulating layer 106 and an interconnection layer 103
for interconnecting the capacitor electrode 105 to the transistor
are stacked on the capacitor, and another interlayer insulating
layer 106 is formed on the interconnection layer 103. A protective
layer 107 made from SiO.sub.2 or the like is formed on the
interlayer insulating film 106. Finally, the entire structure is
packaged by a sealing resin 108.
[0031] (2) Relative Standard Deviation of Crystal Grain Sizes of
Ferroelectric Thin Film
[0032] FIG. 1 is a view showing the schematic configuration view of
a capacitor portion of a semiconductor memory device using a
ferroelectric thin film 12 formed on a lower electrode 11 according
to one embodiment of the present invention, wherein the
ferroelectric thin film [Pb(Zr,Ti)O.sub.3(PZT)] 12 has crystal
grains 13 in which a relative standard deviation of crystal grain
sizes is 13% or less (not more than 13%). In the figure, FIG. 1A
shows the top view, and FIG. 1B shows the sectional view taken on a
cutting plane of the top view of FIG. 1A. Here, a variation in the
crystal grain sizes 14 of the crystal grains 13 is defined as a
relative standard deviation .sigma. (unit: %) expressed by
Numerical Formula 1. Depending on the large or small magnitude of
the relative standard deviation .sigma., it can be decided whether
or not the crystal grain sizes 14 are uniform. 1 = 100 .times. ( i
= 1 N ( a i - a i ( ave ) ) 2 N ) / a i ( ave ) ( 1 )
[0033] .sigma.: variation in crystal grain sizes (relative standard
deviation)
[0034] N.sub.i: number of crystal grains contained in scanning
line
[0035] L.sub.i: length of scanning line
[0036] a.sub.i: average crystal grain size in one scanning line
[0037] a.sub.i: L.sub.i/N.sub.i
[0038] N: number of scanning lines
[0039] a.sub.i(ave): average crystal grain size in scanning lines
of N pieces
[0040] a.sub.i(ave): a.sub.i/N
[0041] The relative standard deviation of crystal grain sizes was
analyzed by taking up a surface image or cross-sectional image of
the ferroelectric thin film using a scanning electron microscope
(SEM), an interatomic force microscope (AFM) or a cross-section
transmission electron microscope (TEM), measuring crystal grain
sizes of crystal grains within a plane having the normal line in
the thickness direction of the ferroelectric thin film, and
calculating the relative standard deviation .sigma. of the crystal
grain sizes. FIG. 2 shows a method of measuring crystal grain sizes
according to one embodiment. Straight lines (crystal grain size
calculating scanning lines 21) are set in the vertical and
horizontal directions on an observation image (size: 1
.mu.m.times.1 .mu.m) of the ferroelectric thin film obtained by an
AFM (atomic force microscope). Then, the number of crystal grains
is counted for each scanning line. The formula for calculating the
relative standard deviation of crystal grain sizes is shown in
Numerical Formula 1. By substituting the counted number of crystal
grains in Numerical Formula 1, an average crystal grain size and a
relative standard deviation are obtained. The AFM used for this
analysis is a scanning probe microscope (trade name: Nano Scope
III, produced by Digital Instrument Corporation in USA). The radius
of curvature of the tip of a probe of the AFM is 10 nm and the
taper angle of the tip of the probe is 35.degree.. In the case of
using the probe, when a gap between two adjacent ones of crystal
grains on the uppermost surface is 80 nm, the critical penetration
depth of the probe is 110 nm. In this embodiment, the AFM
measurement was performed in a tapping mode. The details of the
principle of the tapping mode are described in "Large-sized Sample
SPM Observation System Operation Guide" (April, 1996) published by
Touyo Technica.
[0042] FIG. 3 shows an X-ray diffraction pattern of a PZT
ferroelectric thin film of the present invention, wherein the
abscissa designates the diffraction angle 2 .theta. and the
ordinate designates the X-ray diffraction intensity (Log I). As a
measuring device, there was used a powder X-ray diffraction device
using an X-ray vessel having a Cu-target as an X-ray source. In
this measurement for the ferroelectric thin film, diffraction peaks
111 and 222 were measured, and other diffraction peaks 100, 110,
200, 201, 211, 202, and 301 were little measured. As a result, it
was revealed that the ferroelectric thin film of the present
invention is formed such that the (111) faces of crystal grains are
preferentially oriented in the direction perpendicular to the
substrate plane. In addition, diffraction peaks 111 and 222 of the
Pt electrode and a diffraction peak corresponding to the Ti
underlayer were observed.
[0043] FIGS. 4A and 4B show a correlation between an average
crystal grain size "a" and a remanent polarization value P and a
correlation between the average crystal grain size "a" and a film
fatigue, respectively. The film fatigue is defined as a percent
value (unit: %) obtained by dividing a remanent polarization value
after repeating writing 10.sup.8 times by the initial remanent
polarization value before writing. When the film fatigue is small,
the rewritable number is large, while when the film fatigue is
large, the rewritable number is small. It should be noted that the
physical meaning and definition of each of the remanent
polarization value P and film fatigue and further a
measuring/analyzing method thereof are described in "Ferroelectric
Thin Film Memory" (published by Science Forum, 1995) and various
textbooks associated with ferroelectric materials. As is apparent
from FIGS. 4A and 4B, for an average crystal grain size more than
80 nm, as the crystal grain size becomes smaller, the remanent
polarization value P becomes larger and the film fatigue becomes
smaller (that is, the rewritable number becomes larger), while for
an average crystal grain size equal to or less than 80 nm, the
remanent polarization value P is kept at a specific large value and
the film fatigue is kept at a specific small value (that is, the
rewritable number is kept at a specific large value).
[0044] FIGS. 5A and 5B show a correlation between a relative
standard deviation .sigma. of crystal grain sizes and a remanent
polarization value P and a correlation between the relative
standard deviation .sigma. of crystal grain sizes and a film
fatigue, respectively. In these figures, the abscissa designates
the relative standard deviation .sigma. (unit: nm) of crystal grain
sizes obtained by the above-described AFM on the basis of Numerical
Formula 1. As is apparent from FIGS. 5A and 5B, for a relative
standard deviation .sigma. of crystal grain sizes more than 13%, as
the relative standard deviation .sigma. becomes smaller, the
remanent polarization value P becomes larger and the film fatigue
becomes smaller (that is, the rewritable number becomes larger),
while for a relative standard deviation .sigma. of crystal grain
sizes equal to or less than 13%, the remanent polarization value P
is kept at a specific large value and the film fatigue is kept at a
specific small value (that is, the rewritable number is kept at a
specific large value).
[0045] (3) Surface Roughness of Ferroelectric Thin Film
[0046] FIG. 6 is a schematic sectional view illustrating a method
of measuring surface irregularities of a ferroelectric thin film
using the AFM. Symbol's number 63 is a CMOS substrate. Symbol's
number 11 is a lower electrode. Symbol's number 13 is a crystal
grain. Symbol's number 12 is a ferroelectric thin film. Symbol's
number 62 is an AFM probe. An AFM probe 62 is scanned on a
ferroelectric thin film 12 while being subjected to vibration
(tapping). At this time, on the surface of the ferroelectric thin
film, a recessed portion, that is, a grain boundary portion is
large vibrated, while a projecting portion, that is, a crystal
grain portion is small vibrated. The amplitude of the vibration is
converted into an electric signal, and the surface irregularities
are measured on the basis of the electric signals, to thus obtain a
surface roughness 61.
[0047] For the surface irregular shape (profile) of the
ferroelectric thin film obtained by the AFM, SEM or TEM described
in the first embodiment, the surface roughness of the ferroelectric
thin film was estimated in accordance with the following
manner.
[0048] FIGS. 7A and 7B show a correlation between a surface
roughness Rms and a remanent polarization value P and a correlation
between the surface roughness Rms and a film fatigue according to
one embodiment, respectively. In these figures, the abscissa
designates the surface roughness Rms (unit: nm) calculated from
surface irregularities measured by the AFM on the basis of
Numerical Formula 2. As is apparent from FIGS. 7A and 7B, for a
surface roughness Rms more than 10 nm, as the surface roughness Rms
becomes smaller, the remanent polarization value P becomes larger
and the film fatigue becomes smaller (that is, the rewritable
number becomes larger), while for a surface roughness Rms equal to
or less than 10 nm, the remanent polarization value P is kept at a
specific large value. 2 Rms = ( i = 1 N ( z i - z i ( ave ) ) 2 N )
( 2 )
[0049] Rms: surface roughness (standard deviation)
[0050] N: number of measured data
[0051] z.sub.i: height of measured point "i"
[0052] z.sub.i(ave): average value of z.sub.i
[0053] In this embodiment, the surface roughness was expressed by a
standard deviation of a difference between the maximum value and
minimum value of overall data of surface irregularities measured by
the AFM. In Numerical Formula 2, the surface roughness Rms is
expressed by the standard deviation (unit: nm). Alternatively, the
surface roughness is defined as a three-dimensional average surface
roughness (unit: nm) with respect to a center plane (the volume
formed by the plane and the surface shape projecting upwardly from
the plane is equal to that formed by the plane and the surface
shape projecting downwardly from the plane) as shown in Numerical
Formula 3. The details are described in "Large-sized Sample SPM
Observation System Operation Guide" (April, 1996) published by
Touyo Technica. 3 Ra = 1 LxLy 0 Ly 0 Lx f ( x , y ) x y ( 3 )
[0054] Ra: three-dimensional average surface roughness with respect
to center plane
[0055] Lx: dimension of surface in x-direction
[0056] Ly: dimension of surface in y-direction
[0057] f(x,y): roughness profile with respect to center plane
[0058] (4) Method of Manufacturing Ferroelectric Thin Film
[0059] FIGS. 8A to 8D shows a method of manufacturing a
ferroelectric thin film according to one embodiment of the present
invention. To obtain a ferroelectric thin film in which the
relative standard deviation of crystal grain sizes is 13% or less,
it is required to form initial nuclei necessary for growth of
crystal grains. Prior to formation of a ferroelectric thin film, an
extremely thin layer made from at least one or more of metals
contained in a ferroelectric material or an oxide or compound
containing the metals is provided by a sputtering method or sol-gel
method, followed by heat-treatment at a high temperature to form
initial nuclei 82 as shown in FIG. 8B. Alternatively, initial
nuclei 82 necessary for forming micro-nuclei are formed by
heat-treating a lower electrode at a high temperature after
formation of the lower electrode, to precipitate at least one or
more of metals contained in an adhesive layer 81 (disposed between
the lower electrode 11 and a CMOS substrate 64) or an oxide or
compound containing the metals on the surface of the lower
electrode 11 (see FIG. 8B). Here, the initial nuclei 82 are made
from an ABO.sub.3 type oxide having a perovskite structure
[composition: A=Pb, B=(Zr.sub.1-x, Ti.sub.x); or A=(Ba.sub.1-x,
Sr.sub.x), B=Ti], or made from Ti, TiO.sub.x, Sr, or SrO.sub.x. A
ferroelectric thin film (a ferroelectric thin film before
crystallization) 83 is then formed on the initial nuclei 82 to a
thickness required for a semiconductor memory device by the
sputtering method or sol-gel method (see FIG. 8C). Then, the
ferroelectric thin film 83 is subjected to rapid heat-treatment by
a lamp using a RTA (Rapid Thermal Annealing) apparatus with the
result that a ferroelectric thin film after crystallization 84 is
formed (see FIG. 8D). In this embodiment of the present invention,
the heat-treatment is performed in a 100% N.sub.2 atmosphere at
600.degree. C. for 30 sec. With this heat-treatment, in the
ferroelectric thin film 83, crystal grains have the perovskite
structure and the (111) faces of crystal grains are preferentially
oriented in the direction perpendicular to the substrate plane.
According to the above-described manufacturing method, it is
possible to obtain a ferroelectric thin film in which the (111)
faces of crystal grains are preferentially oriented, the average
crystal grain size is about 80 nm, the relative standard deviation
of crystal grain sizes is about 13%, and the standard deviation of
the surface roughness is about 10 nm. The ferroelectric thin film
thus obtained is advantageous in suppressing growth of crystal
grains having the pyrochlore structure and rosette-shaped ZrO.sub.x
crystal grains causing deterioration of ferroelectric
characteristics. As a result, a ferroelectric capacitor having a
large remanent polarization value and a small film fatigue (large
rewritable number) can be obtained by use of such a ferroelectric
thin film.
[0060] FIG. 9 shows a film formation apparatus for manufacturing a
ferroelectric thin film capacitor of a semiconductor memory device
of the present invention. A substrate 1 carried from a substrate
carrying-in chamber 99 into a substrate exchange chamber 94, and
carried among a film formation chamber (1) 91, film formation
chamber (2) 92, and film formation chamber (3) 93 in a high vacuum
atmosphere via the substrate exchange chamber 94 by a substrate
exchange arm 97. For film formation in the film formation chamber
(1) 91, a high frequency magnetron sputter system of a
multi-cathode type is adopted; and for film formation in each of
the film formation chamber (2) 92 and film formation chamber (3)
93, a DC magnetron sputter system of a single cathode type is
adopted. Symbol's number 95 is a gate valve. Symbol's number 96 is
a flat cathode. Symbol's number 98 is a Si substrate. In this
embodiment, the ferroelectric layer 83 is formed by the sputter
system of the multi-cathode type; however, it may be formed by the
sputter system of the single cathode type. In this case, a sintered
body of a mixture of a ferroelectric PZT and a lead oxide PbO.sub.x
may be used as a target. Further, the film formation may be
performed by the sol-gel method or combination of the sol-gel
method and the above-described sputtering.
[0061] (5) IC Card on Which Semiconductor Memory Including
Ferroelectric Thin Film is Mounted
[0062] IC cards uses various kinds of semiconductor memories
depending on the applications thereof. A semiconductor memory using
the ferroelectric thin film of the present invention is a
nonvolatile memory. An IC card of the present invention is
advantageous in terms of limitation of chip size, portableness, and
maintenance free because any battery is not required to be
contained for retention of data unlike a SRAM (Static Random Access
Memory). Since the semiconductor memory including the ferroelectric
thin film of the present invention can be manufactured at a high
yield, the IC card of the present invention can be obtained at a
low cost. Also since the rewritable number of the semiconductor
memory of the present invention is made larger than that of an
EEPROM (Electrically Erasable Programmable Read Only Memory)
(rewritable number: 10.sup.4 to 10.sup.5) which is one kind of the
nonvolatile memories, the service life of the IC card of the
present invention is improved, to thereby reduce the running cost.
One example of a simple system configuration of an IC card is
described in "The whole of Non-erasable IC Memory RAM" (edited by
Tomoji Kawai, published by Kougyou Chousa Kai, 1996) or Text of
Realize Corporation's Advanced Technical Lecture entitled "Advanced
Technology of Nonvolatile Ferroelectric Thin Film Memory and
Problem Associated with Process Technology" (Realize Corporation,
1996).
[0063] (6) Computer on Which Semiconductor Memory Including
Ferroelectric Thin Film is Mounted
[0064] A computer on which a conventional DRAM (Dynamic Random
Access Memory) is mounted cannot prevent erasing of working data
due to cutoff of a power supply. On the contrary, the semiconductor
memory using the ferroelectric thin film of the present invention
is a nonvolatile memory, and accordingly, even in case of power
failure, the computer of the present invention can retain the
working state until directly before the power failure. The computer
is not required to read the system or application every input of a
power supply, and therefore, it can start work directly after input
of the power supply. Further, since the computer is not required to
contain any uninterruptive power supply or battery, it is possible
to miniaturize the computer and to improve portableness of the
computer due to reduction in weight or achieve space-saving
thereof.
[0065] (7) Portable Information Terminal Apparatus on Which
Semiconductor Memory Including Ferroelectric Thin Film is
Mounted
[0066] With respect to a portable telephone representative of a
portable information terminal apparatus of the present invention,
the semiconductor memory of the present invention contained in the
portable telephone can be driven with a small power, and is not
required to be provided with a power supply for retention of data
because it is a nonvolatile memory. Accordingly, as compared with a
conventional portable information terminal apparatus on which a
DRAM, SRAM or EEPROM is mounted, the portable telephone of the
present invention is advantageous in reducing the weight of the
main body due to miniaturization of the integrated batter and
making longer the drive time required for driving the main body
without increasing the capacity of the battery.
[0067] (8) Video/Audio Apparatus on Which Semiconductor Memory
Including Ferroelectric Thin Film is Mounted
[0068] As compared with a conventional video camera containing a
semiconductor memory device such as a DRAM, SRAM or EEPROM for
recording video or audio information, a video camera representative
of a video/audio apparatus of the present invention is advantageous
in reducing the drive power for driving the integrated
semiconductor memory device, and eliminating the necessity of
provision of a power supply. This makes it possible to reduce the
weight of the main body due to miniaturization of the integrated
battery and to make longer the drive time required for driving the
main body without increasing the capacity of the battery.
[0069] As fully described above, according to the present
invention, it is possible to realize a ferroelectric capacitor
capable of reducing a variation in characteristics between memory
cells, and hence to obtain a high quality semiconductor memory
device using the ferroelectric capacitor at a high manufacturing
yield. The semiconductor memory device is a nonvolatile memory
which makes it possible to eliminate the necessity of provision of
a power supply for retention of data, save drive power, and improve
the rewritable number. Accordingly, a system on which the
semiconductor memory device of the present invention is mounted is
capable of reducing the capacity of the inner power supply or
eliminating the necessity of provision of the inner power supply,
thereby realizing miniaturization of the system, increasing the
service life thereof, and reducing the manufacturing cost
thereof.
* * * * *