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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor memory device and a method
of manufacturing the same, and more particularly to a capacitive element.
2. Description of the Background Art
Conventionally, to enhance permittivity or obtain ferroelectricity of a
dielectric included in a capacitive element such as a capacitor, for
higher integration with capacitance value ensured, use of an oxide
dielectric film has been proposed. As an example of the oxide dielectric
film, an oxide dielectric of perovskite system can be taken.
In a capacitor using the oxide dielectric film, a precious metal electrode
is used. For the electrode is required not to be oxidized since an ambient
atmosphere needed for forming an oxide dielectric film has an oxidizing
power
There are two problems, however, in using the precious metal electrode as
follows. The first problem is that the precious metal electrode has a poor
adherence to an interlayer insulating film covering a capacitor and hence
the precious metal electrode is removed from the interlayer insulating
film in formation or after formation of the precious metal electrode.
The second problem is that the oxide dielectric film is reduced when a
hydrogen sintering is performed, where an aluminum interconnection line
connected to the capacitor is heat-treated in hydrogen. As the oxide
dielectric film is reduced, oxygen deficiency in crystal grains of the
oxide dielectric increases and a leak current flowing through deficiency
level based thereon disadvantageously increases.
Although techniques to solve the first and second problems, such as
interposing a so-called adhesion layer between the precious metal
electrode and the interlayer insulating film and interposing a block layer
having reduction species, have been proposed, the process becomes
complicated and some dedicated facilities are needed.
A technique for upper electrode of layered structure is disclosed in
Japanese Patent Application Laid Open Gazette No. 10-12844 as a technique
of removing adsorbed water in an interface between the upper electrode of
the capacitor and the interlayer insulating film, in Japanese Patent
Application Laid Open Gazette No. 8-274270 as a technique of suppressing
diffusion of, for example, lead from the oxide dielectric of perovskite
and Japanese Patent Application Laid Open Gazette No. 9-139476 as a
technique of suppressing out-of-composition of the dielectric film. A
technique for lower electrode of layered structure is disclosed in
Japanese Patent Application Laid Open Gazette Nos. 8-274270, 10-173149,
7-94680, 10-173138 and 9-139476.
SUMMARY OF THE INVENTION
The present invention is directed to a capacitive element. According to a
first aspect of the present invention, the capacitive element comprises: a
lower electrode; a dielectric layer formed on the lower electrode; and an
upper electrode having a first layer formed on the dielectric layer and a
second layer formed on the first layer and having a grain size larger than
the first layer.
According to a second aspect of the present invention, in the capacitive
element of the first aspect, the second layer is formed at higher
temperature than the first layer.
According to a third aspect of the present invention, in the capacitive
element of the first aspect, the second layer has higher concentration of
oxidizer than the first layer.
According to a fourth aspect of the present invention, in the capacitive
element of the third aspect, the second layer is formed in an atmosphere
with higher concentration of oxidizer than the first layer.
According to a fifth aspect of the present invention, in the capacitive
element of the first aspect, the upper electrode further has a third layer
formed between the first and second layers and having a grain size larger
than the second layer.
According to a sixth aspect of the present invention, the capacitive
element comprises: a lower electrode formed on a substrate; a dielectric
layer formed on the lower electrode and being thinner than the low
electrode; and an upper electrode formed on the dielectric layer. In the
capacitive element of the sixth aspect, unevenness in a surface of the
lower electrode on the side of the dielectric layer is not larger than a
tenth of the thickness of the dielectric layer and cycle of the unevenness
in a direction parallel with the substrate is not larger than a half of
the size of the lower electrode in the direction parallel with the
substrate.
According to a seventh aspect of the present invention, in the capacitive
element of the sixth aspect, the lower electrode has a first layer formed
on the substrate and a second layer formed on the first layer, and the
first layer is formed at higher temperature than the second layer.
According to an eighth aspect of the present invention, in the capacitive
element of the seventh aspect, the lower electrode further has a third
layer formed between the first and second layers and having a grain size
larger than the first layer.
According to a ninth aspect of the present invention, in the capacitive
element of the fifth or eighth aspect, the third layer is formed at higher
temperature than the first and second layers.
In the capacitive element of the first aspect of the present invention,
since the second layer of said upper electrode has less grain boundary and
is hard to connect to that of the first layer, with less grain boundary
diffusion caused, even if a hydrogen sintering is performed, reduction
species are unlikely to reach the dielectric layer through the grain
boundary. Further, since the surface area of the second layer increases,
deterioration of the dielectric layer is suppressed and the adherence
between the second layer and the interlayer insulating film if provided
thereon is improved.
In the capacitive element of the second aspect of the present invention,
crystal grains formed in the second layer are larger than those formed in
the first layer
In the capacitive element of the third aspect of the present invention,
since the oxidizer blocks transmission of reduction species in the second
layer, the reduction species are unlikely to reach the dielectric layer.
In the capacitive element of the fourth aspect of the present invention,
since an oxide is likely to be adsorbed to the first layer in formation of
the second layer and inhibits movement of atoms constituting the second
layer, granular growth is caused in the second layer to increase the
surface area thereof.
In the capacitive element of the fifth aspect of the present invention,
providing the third layer improves the effect of blocking transmission of
reduction species while an increase of crystal grains in the second layer
farthest from the electric layer is suppressed so that a good contact to
the second layer may be obtained.
In the capacitive element of the sixth aspect of the present invention,
since the unevenness in the surface of the lower electrode on the side of
the dielectric layer, it is possible to suppress electric field
concentration in the dielectric layer.
In the capacitive element of the seventh aspect of the present invention,
the first layer and the lower electrode cause a chemical reaction to
enhance adherence, while growth of crystal grains is suppressed in the
second layer, to achieve evenness in the surface.
In the capacitive element of the eighth aspect of the present invention,
providing the third layer improves the effect of the seventh aspect.
In the capacitive element of the ninth aspect of the present invention,
crystal grains formed in the third layer are larger than those formed in
the first and second layers.
An object of the present invention is to enhance adherence between an
electrode and an interlayer insulating film without providing additional
adhesion layer or block layer, that is, without additional facility or
complicated process. Another object of the present invention is to
suppress diffusion of reducing gas and prevent an increase of leak current
in a dielectric.
These and other objects, features, aspects and advantages of the present
invention will become more apparent from the following detailed
description of the present invention when taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 to 16 are cross sections showing a method of manufacturing a
semiconductor device in accordance with a first preferred embodiment of
the present invention step by step;
FIGS. 17 to 22 are cross sections showing a method of manufacturing a
semiconductor device in accordance with a third preferred embodiment of
the present invention step by step;
FIG. 23 is a cross section showing a semiconductor device in accordance
with a fourth preferred embodiment of the present invention; and
FIG. 24 is a cross section showing a variation of the semiconductor device
in accordance with the fourth preferred embodiment of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The First Preferred Embodiment
FIGS. 1 to 16 are cross sections showing a method of manufacturing a
semiconductor device (e.g., DRAM) in accordance with the first preferred
embodiment of the present invention step by step.
In a surface of a semiconductor substrate 1 using, e.g., a silicon, an
oxide film 101 is formed by, e.g., trench isolation or LOCOS isolation, to
obtain a structure of FIG. 1. Subsequently, a layered structure of a gate
oxide film 102 having a thickness of, e.g., several tens .ANG. and a gate
electrode 103 is formed on the surface of the semiconductor substrate 1
away from the oxide film 101, to obtain a structure of FIG. 2. A drain
region 2 and a source region 104 are formed by lithography technique and
ion implantation at a dose of, e.g., 1.times.10.sup.15 cm.sup.2 in the
surface of the semiconductor substrate 1 so as to sandwich the gate
electrode 103, to obtain a structure of FIG. 3. A bit line 105 using,
e.g., tungsten is formed above the gate electrode 103. Further, an
interlayer insulating film 3 is formed around the bit line 105 and over
the structure of FIG. 3 (see FIG. 4). Then, a plug 4 is formed of, e.g.,
doped polysilicon so as to penetrate the interlayer insulating film 3 to
come into contact with the drain region 2, to obtain a structure of FIG.
5.
On the structure of FIG. 5, a barrier metal of a lower electrode is formed,
e.g., titanium nitride, to obtain a structure of FIG. 6. In FIGS. 6 to 16,
only a region corresponding to a region P of FIG. 5 indicated by a broken
line is shown. As the barrier metal 5, for example, a layered film of
titanium nitride (TiN) of 1000 .ANG./titan (Ti) of 100 .ANG. is used. On
the barrier metal 5, a lower-electrode base metal 6 having a thickness of
1000 .ANG. is formed of a metal, especially, a precious metal, e.g.,
platinum (Pt), to obtain a structure of FIG. 7. The barrier metal 5 and
the lower-electrode base metal 6 are reshaped by photolithography and
etching so as to be left above the plug 4 and its periphery by, e.g., 0.2
.mu.m in width, to obtain a structure of FIG. 8.
Next, on the structure of FIG. 8, an oxide dielectric film 7 having a
thickness of, e.g., 300 to 600 .ANG. is formed of, e.g., barium strontium
titanate ((Ba, Sr)TiO.sub.3), to obtain a structure of FIG. 9. On the
oxide dielectric film 7, a first layer 81 is formed of a metal,
especially, a precious metal, e.g., platinum (Pt) by, e.g., sputtering, to
obtain a structure of FIG. 10. Deposition is performed at a low
temperature not lower than a room temperature to not higher than
250.degree. C. to form a film thickness of 250 to 500 .ANG.. Such a
setting of the temperature in deposition makes it possible to form a wavy
pattern in a cycle gentler as compared with the size of a storage node,
i.e., depth, width and height (e.g., each about 0.2 .mu.m) of the barrier
metal 5 and the lower electrode 6 as a whole. Further, on the first layer
81, a second layer 82 is formed of a metal, especially, a precious metal,
e.g., platinum (Pt) by, e.g., sputtering, to obtain a structure of FIG.
11. Deposition is performed at a high temperature in a range from 250 to
500.degree. C. to form a film thickness of 250 to 500 .ANG.. Such a
setting of the temperature in deposition allows growth of smaller crystal
grains as compared with the size of the storage node to increase surface
area. Thus, the first and second layers 81 and 82 constitute an upper
electrode 8 of the capacitor.
Next, on the second layer 82, an interlayer insulating film 10 is formed
of, e.g., a silicon oxide film, to obtain a structure of FIG. 12. The
interlayer insulating film 10 is opened by lithography technique and
etching above the lower-electrode base metal 6 and the barrier metal 5, to
form an opening 201 of FIG. 13. Then, the opening 201 is filled with a
plug 202, to obtain a structure of FIG. 14. After that, an aluminum
interconnection layer 11 is formed, coming into contact with the plug 202,
to obtain a structure of FIG. 15. Further, an SiN is formed by, e.g.,
plasma CVD, or a passivation film 203 is provided by using polyimide, to
obtain a structure of FIG. 16. Furthermore, there may be a case where the
process of FIG. 14 is omitted and the opening 201 is filled with the
aluminum interconnection layer 11 when the layer 11 is formed.
In the upper electrode 8 formed as above, since the first layer 81 has less
grain boundary and is hard to connect to that of the second layer 82, with
less grain boundary diffusion caused, even if a hydrogen sintering of the
aluminum interconnection layer 11 is performed, reduction species are
unlikely to reach the oxide dielectric film 7 through the grain boundary.
That suppresses deterioration of the oxide dielectric film 7. Moreover,
since the surface area of the second layer 82 increases, the adherence
between the second layer 82 and the interlayer insulating film 10 provided
thereon is improved. Therefore, it is possible to solve the first and
second problems.
The Second Preferred Embodiment
Though the upper electrode is constituted of a layered structure of the
first and second layers 81 and 82 which are deposited at different
temperatures in the first preferred embodiment, the same effect can be
produced by forming the first and second layers 81 and 82 which are
deposited in different ambient atmospheres. Specifically, an atmosphere
for formation of the second layer 82 includes more oxidizer than that for
formation of the first layer 81. Therefore, the second layer 82 includes
more oxidizer than the first layer 81.
For example, when a sputtering is performed, the first layer 81 is formed
in the atmosphere where oxygen is added into argon by about 0.0001 to 1%
while the second layer 82 is formed in the atmosphere where oxygen is
added into argon by about 0.1 to 5% (more oxygen is set than in the
atmosphere for formation of the first layer 81). It is natural that no
oxygen may be added in the atmosphere for formation of the first layer 81
or oxidizing gas, instead of oxygen, such as N.sub.2 O and H.sub.2 O may
be used.
It is known that the oxidizing gas is generally easy to adsorb and inhibits
movement of atoms in a film surface where the oxidizing gas is adsorbed.
Then, if the oxidizing gas is supplied by the degree as above in formation
of the electrode, granular growth is caused in the second layer 82 to
increase surface area and adherence. Moreover, with oxidation species
included in the second layer 82, a reducing gas becomes hard to transmit.
Therefore, it is possible to solve the first and second problems.
The Third Preferred Embodiment
FIGS. 17 to 22 are cross sections showing a method of manufacturing a
semiconductor device in accordance with the third preferred embodiment of
the present invention step by step. In these figures, only the region
corresponding to the region P of FIG. 5 is shown.
After obtaining the structure of FIG. 6 through the process shown in the
first preferred embodiment, a first layer 61 is formed of a metal,
especially, a precious metal, e.g., platinum on the barrier metal 5 by,
e.g., sputtering to have a thickness of 250 to 750 .ANG., to obtain a
structure of FIG. 17. At this time, it is desirable that the sputtering
pressure should be about 0.2 Pa and the power should be lowered as
possible, not so as to deteriorate productivity. For example, in an RF
sputtering, the power is set at about 1 kW. The temperature in deposition
is set high at about 250 to 400.degree. C. Through this formation at such
a high temperature, a chemical reaction is caused between the first layer
61 and the barrier metal 5. A compound of barrier metal and platinum is
created in a boundary J therebetween.
A second layer 62 is formed of a metal, especially, a precious metal, e.g.,
platinum on the first layer 61 by, e.g., sputtering to have a thickness of
250 to 750 .ANG., to obtain a structure of FIG. 8. At this time, it is
desirable that the sputtering pressure should be about 0.2 Pa and the
power should be enhanced as possible. For example, in an RF sputtering,
the power is set at about 2 kW. The temperature in deposition is set low,
not higher than about 250.degree. C.
Photolithography and etching are performed on the first and second layers
61 and 62, forming the lower electrode 6 of the capacitor, to obtain a
structure of FIG. 19. After that, the oxide dielectric film 7 is formed
like in the first preferred embodiment, to obtain a structure of FIG. 20.
On the oxide dielectric film 7, the upper electrode 8 is formed of, e.g.,
platinum by sputtering, to obtain a structure of FIG. 21. The interlayer
insulating film 10 is formed on the upper electrode 8, to obtain a
structure of FIG. 22. After that, the steps of FIGS. 13 to 16 in the first
preferred embodiment are executed.
The lower electrode 6 thus obtained can have an evenness in an interface
between the second layer 62 and the oxide dielectric film 7 and suppress
electric field concentration. Further, the high temperature in formation
of the first layer 61 causes a chemical reaction between the first layer
61 and the barrier metal 5, forming a compound of barrier metal and, e.g.,
platinum, to improve adherence therebetween.
Furthermore, to enhance the above effect, it is desirable that unevenness
of a surface of the second layer 62 (i.e., a surface of the lower
electrode 6) is smaller than the thickness of the oxide dielectric film 7.
More desirably, height difference from the top and bottom of the
unevenness should not be larger than a tenth of the thickness of the oxide
dielectric film 7. It is further desirable that a typical length in a
horizontal direction (cycle of unevenness) should not be larger than half
the size of storage node.
The Fourth Preferred Embodiment
FIG. 23 is a cross section showing a semiconductor device in accordance
with the fourth preferred embodiment of the present invention. In this
figure, only the region corresponding to the region P of FIG. 5 is shown.
In this preferred embodiment, the upper electrode 8 has a structure in
which a third layer 83 is formed between the first and second layers 81
and 82 shown in the first preferred embodiment. The third layer 83 is
formed at higher temperature than the second layer 82. Specifically, the
second layer 82 is formed at higher temperature than the first layer 81
and the third layer 83 is formed at higher temperature than the second
layer 82. For example, the third layer 83 is formed at not lower than
300.degree. C., higher temperature than the second layer 82.
The third layer 83 thus obtained is more crystallized than the second layer
82, to reduce grain boundary and increase grain size. Therefore, it is
possible to suppress transmission of reduction species.
Further, even if the third layer 83 is formed, it is desirable that the
second layer 82 should exist. For it is desirable that a top surface of
the upper electrode 8 should have smaller crystal grains than the opening
201 in order to avoid variation of contact resistance between the second
layer 82 and the aluminum interconnection layer 11 or plug 202 in contact
therewith (see FIG. 14 or 16). This is desirable in terms of increasing a
process margin.
Furthermore, like in the second preferred embodiment, the second layer 82
may have higher concentration of oxidizer than the first layer 81 and the
third layer 83 may have higher concentration of oxidizer than the second
layer 82. This is easily achieved by controlling the amount of oxidizer in
ambient atmosphere for film formation. This can still enhance the effect
of the second preferred embodiment.
Also in the third preferred embodiment, a third layer 63 may be formed
between the first and second layers 61 and 62 (see FIG. 24). The first
layer 61 is formed at higher temperature than the second layer 62 and the
third layer 63 is formed at higher temperature than the first layer 61.
Providing the third layer 63 improves the effect of the third preferred
embodiment.
VARIATION
The present invention can use materials other than those shown in the above
preferred embodiments. For example, as materials for the upper electrode 8
and the lower electrode 6, ruthenium (Ru), an alloy (PtIr) of platinum and
iridium, an alloy (PtRh) of platinum and rhodium or a conductive ceramic
such as strontium ruthenate (SrRuO.sub.3) may be used. As material of the
oxide dielectric film 7, PZT (lead zirconate titanate; Pb(Zr, Ti)O.sub.3)
may be used.
While the invention has been shown and described in detail, the foregoing
description is in all aspects illustrative and not restrictive. It is
therefore understood that numerous modifications and variations can be
devised without departing from the scope of the invention.
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