Method of fabricating capacitor element in super-LSI

A capacitor element of a semiconductor device used for a super-LSI is formed by the steps including (a) removing a natural oxide film on a surface of a lower electrode of polysilicon, (b) forming on the surface of the lower electrode an impurity-doped tantalum oxide film, and (c) forming an upper electrode with at least a bottom thereof constituted by titanium nitride. The steps may further include (d) nitriding the surface of the lower electrode after the removal of the natural oxide film, and (e) densifying the tantalum oxide film by way of a high temperature heat treatment after the formation of the tantalum oxide film. In this way, it is possible to reduce thickness of a capacitive insulating film and to form the capacitor element in which the leakage current characteristics are improved.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a method of fabricating semiconductor 
devices, and more particularly to a method of forming a capacitor element 
in super-LSIs such as DRAMs (Dynamic Random Access Memories). 
(2) Description of the Related Art 
With respect to the capacitor of super-LSI memory devices over 256 MB 
(megabytes) DRAM, researches and investigations have been made for the 
adoption of high dielectric capacitive insulating films which permit 
increasing the capacitance per unit area. Among the high dielectric 
capacitive insulating films that are being studied, the tantalum oxide 
film formed by a chemical vapor deposition (CVD) process has high specific 
dielectric constant .epsilon..sub.r of 25 to 30 and an excellent step 
coverage characteristic. Further, its film formation process is extremely 
easy compared to that in other high dielectric capacitive insulating 
films. For these reasons, extensive researches are being carried out in 
this field of technology. 
FIGS. 1A, 1B and 1C show, in sectional views, successive steps of The 
conventional process of forming a capacitor element using a tantalum film. 
First, as seen in FIG. 1A, polysilicon is deposited by the CVD process on a 
p-type silicon substrate 1, which has an n-type diffusion layer la at its 
surface region, and on which an element isolation region 2 having an 
opening reaching the n-type diffusion layer la is formed. Then, phosphorus 
(P) is thermally diffused, and then a polysilicon inner or lower electrode 
3 is formed by a usual lithographic technique. In this stage, a natural 
oxide film 4 is formed on the surface of the polysilicon electrode 3. 
Subsequently, as seen in FIG. 1B, a tantalum oxide film 7 is formed on the 
polysilicon lower electrode 3 by a low-pressure chemical vapor deposition 
(LPCVD) process using ethoxytantalum (Ta(OC.sub.2 H.sub.5).sub.5) as a 
source gas. The wafer is then subjected to a high temperature heat 
treatment in oxygen atmosphere at 600.degree. to 1000.degree. C. to reduce 
leakage current, thereby improving the leakage current characteristics in 
the tantalum oxide film 7. At this time, the natural oxide film 4 becomes 
a SiO.sub.2 film 4a. Subsequently, as shown in FIG. 1C, an outer or upper 
electrode 6 is formed. For the upper electrode 6, tungsten (W) is 
generally used. Through the above steps, the formation of the capacitor is 
completed. 
The above prior art capacitor structure has the following problems. In the 
prior art capacitor formation process, the capacitor that is formed by 
forming the tantalum oxide film 7 on the polysilicon as the lower 
electrode 3, followed by the high temperature heat treatment in oxygen 
atmosphere to improve the leakage current characteristics, has a 
capacitance of only up to about 3 nm in thickness (Cs=12 pF/.mu.m.sup.2) 
in terms of the equivalent thickness converted into SiO.sub.2 film 
(specific dielectric constant .epsilon..sub.r =3.9). This is so because, 
due to the high temperature heat treatment performed in oxygen atmosphere 
to improve the leakage current characteristics of the tantalum oxide film, 
the natural oxide film 4 present at the interface between the tantalum 
oxide film 7 and the polysilicon electrode 3 is increased in thickness and 
becomes the SiO.sub.2 film 4a. Where this capacitive insulating film is 
used in a capacitor over 256 MB DRAM, a sufficient capacitance cannot be 
obtained. Another problem is that the capacitor element formed in the 
prior art has a leakage current characteristic (10.sup.-8 A/cm.sup.2) with 
a low voltage of about 0.7 V as can be seen in FIG. 6. The capacitor 
element having such leakage current characteristics cannot be applied to 
any practical device. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to overcome the 
problems existing in the prior art method and to provide a method of 
fabricating a capacitor element of a semiconductor device adapted to be 
used for a super-LSI such as a DRAM in which a capacitive insulating film 
is made thin thereby enabling to improve leakage current characteristics. 
According to one aspect of the invention, there is provided a method of 
fabricating a semiconductor device, in which a capacitor element used for 
a super-LSI such as a DRAM is formed by a process comprising the steps of: 
removing a natural oxide film on a surface of a lower electrode of 
polysilicon; 
forming on the surface of the lower electrode a tantalum oxide film doped 
with impurities; and 
forming on the tantalum oxide film an upper electrode with at least a 
bottom of the upper electrode constituted by titanium nitride. 
In the method of forming the capacitor element according to the invention, 
a natural oxide film is removed on the surface of a polysilicon electrode 
as a lower electrode of the capacitor element, then an impurity-doped 
tantalum oxide film is formed as a capacitive insulating film, and then an 
upper electrode of titanium nitride is formed. It is possible to reduce 
the thickness of the capacitive insulating film and to form the capacitor 
element in which the leakage current characteristics are improved.

PREFERRED EMBODIMENTS OF THE INVENTION 
Now, the invention will be described with reference to the accompanying 
drawings. It should be noted that, throughout the following explanation, 
similar reference symbols or numerals refer to the same or similar 
elements in all the figures of the drawings. 
FIGS. 2A to 2D show the sequential steps of fabricating a capacitor element 
according to the first embodiment of the invention. 
FIG. 2A is first referred to. On a p-type silicon substrate 1 having on its 
surface region an n-type diffusion layer 1a, an element isolation region 2 
is formed by a LOCOS (Local Oxidation of Silicon) process. Then, a 
polysilicon film is deposited by the CVD process on the substrate, then 
doped with phosphorus (P) by thermal diffusion, and then patterned by a 
usual lithography and etching technique to form a polysilicon electrode 3. 
At this time, a natural oxide film 4 is formed on a surface of the 
polysilicon electrode 3. 
Then, as shown in FIG. 2B, the natural oxide film 4 on the polysilicon 
electrode 3 is removed using anhydrous hydrofluoric acid. 
Then, as shown in FIG. 2C, an impurity-doped tantalum oxide film 5 is 
deposited by the CVD process. 
Then, as shown in FIG. 2D, titanium nitride is formed as an upper electrode 
6. 
The impurity-doped tantalum oxide film 5 shown in FIG. 2C is formed by 
using an apparatus as shown in FIG. 3. Source gases for the tantalum oxide 
film formation are organic tantalum gas (tantalum penta-ethoxide 
(Ta(OC.sub.2 H.sub.5).sub.5)) and organic titanium gas (titanium 
tetra-butoxide (Ti(OC.sub.4 H.sub.9).sub.4)). These source materials are 
gasified in respective gasification chambers 12 and 13 to be led together 
with argon gas 9 as a carrier gas, to a reaction chamber 18. At the same 
time, oxygen gas is led through a valve 25 to the reaction chamber 18. The 
reaction chamber 18 is heated by a heater 16 to cause chemical vapor phase 
reaction of the introduced organic tantalum, organic titanium and oxygen 
gases. A tantalum oxide film doped with tantalum impurity is thus formed 
on the wafer 14. As suitable conditions for the film growth, the heating 
temperature in the gasification chamber 12 of the organic tantalum 
material is 30.degree. to 200.degree. C., the heating temperature in the 
gasification chamber 13 of the organic titanium material is also 
30.degree. to 200.degree. C., the growth temperature in the reaction 
chamber 18 heated by the heater 16 is 300 to 800.degree. C., the rate of 
flow of argon gas as the carrier gas is 10 to 1000 SCCM, and the flow rate 
and pressure of oxygen gas are 0.1 to 20.0 OSLM and 0.1 to 10 Torr, 
respectively. While this embodiment used the reaction chamber 18 for 
forming the titanium-doped tantalum oxide film 5, a similar film may be 
formed in the case where a reaction chamber 19 is used. Further, while in 
this embodiment, titanium was doped as impurity, it is possible to dope at 
least one element selected from the group consisting of silicon (Si), 
boron (B), phosphorus (P) and germanium (Ge). 
Further, while the above embodiment uses a titanium oxide monolayer as the 
upper electrode, the same effects are obtainable with tungsten or such a 
composite film as titanium oxide/tungsten, titanium nitride/molybdenum, 
titanium nitride/tungsten silicide. 
FIGS. 4A to 4D show, in sectional views, sequential steps for explaining a 
second embodiment of the present invention. 
Like the preceding first embodiment, after removal of the natural oxide 
film 4 with hydrofluoric acid, as shown in FIG. 4A, a silicon nitride film 
24 is formed on the polysilicon electrode 3 by a quick temperature raise 
heat treatment using ammonia gas (NH.sub.3). The temperature of this 
nitriding treatment is suitably 800.degree. to 1000.degree. C. 
Then, as shown in FIG. 4B, an impurity-doped tantalum oxide film 5 is 
deposited by the CVD process. This impurity-doped tantalum oxide film 5 is 
suitably formed under the same conditions as in the previous first 
embodiment. 
Further, as seen in FIG. 4C, the deposited titanium-doped tantalum oxide 
film 5 is converted into a densified tantalum oxide film 5a through a 
densifying treatment by a high temperature heat treatment. The densifying 
treatment is suitably carried out by using a high rate heating system 
using an electric furnace or lamp heating and in nitrogen, argon or like 
inert gas atmosphere at a temperature of 600.degree. to 1000.degree.. 
Subsequently, as shown in FIG. 4D, a titanium nitride film 6 is formed as 
an upper electrode 6. While this embodiment again used a titanium nitride 
monolayer as the upper electrode 6, the same effects are obtainable as 
well by using tungsten or such a composite film as titanium 
nitride/tungsten, titanium nitride/molybdenum, titanium nitride/tungsten. 
FIG. 5 shows changes in SiO.sub.2 converted equivalent film thicknesses 
with respect to tantalum oxide film thicknesses in devices fabricated on 
the basis of the above first and second embodiments of the invention. The 
graph shows results of measurement of the SiO.sub.2 converted equivalent 
film thickness plotted against the thickness of tantalum oxide film in 
devices fabricated on the basis of the first and second embodiments and 
the prior art. As can be seen from the graph, the SiO.sub.2 converted 
equivalent film thickness is smaller in the first and second embodiments 
compared to that in the device formed on the basis of the prior art. 
Further, it is smaller in the first embodiment than in the second 
embodiment. These results are obtained because, in the case of the prior 
art, a SiO.sub.2 film of about 2 nm is formed at the interface between the 
tantalum oxide film and the polysilicon electrode, whereas the film 
thickness of the SiO.sub.2 film is about 1 nm in the case of the second 
embodiment and is almost negligible in the case of the first embodiment. 
As an example, when a tantalum oxide film with a thickness of 10 nm is 
used, the equivalent thickness of the SiO.sub.2 converted film is about 
3.5 nm in the case of the prior art, whereas it is as small as about 1.5 
nm in the case of the first embodiment and about 2 nm in the case of the 
second embodiment. 
FIG. 6 shows leakage current characteristics of the tantalum oxide films in 
devices manufactured on the basis of the first and second embodiments of 
the invention. The graph shows the leakage current characteristics of 
tantalum oxide films formed on the basis of the first and second 
embodiments and the prior art. The leakage current characteristic of the 
tantalum oxide film formed on the basis of the first embodiment is 
superior to that on the basis of the prior art. This is so because the 
titanium impurity doped during the tantalum oxide film formation has an 
effect of burying dangling bonds in the tantalum oxide film, thus reducing 
electric trap sites. The leakage current characteristic of the tantalum 
oxide film obtained on the basis of the second embodiment is superior to 
that obtained on the basis of the first embodiment. This is so because the 
densifying treatment by the high temperature heat treatment carried out 
after the titanium-doped tantalum oxide film formation has an effect of 
externally diffusing carbon or water content in the tantalum oxide film. 
The leakage current characteristics of the tantalum oxide films formed on 
the basis of the first and second embodiments of the invention are 
satisfactory to practical devices. 
As has been described in the foregoing, according to the present invention, 
the capacitor used for super-LSIs such as DRAMs is formed by a process 
comprising the steps of (1) removing the natural oxide film on the surface 
of the lower or inner electrode of polysilicon, (2) forming the 
impurity-doped tantalum oxide film, and (3) forming an upper electrode 
with at least the bottom thereof constituted by titanium nitride, thus 
permitting the thickness reduction of the capacitive insulating film and 
the formation of a satisfactory capacitor device with improved leakage 
current characteristics. 
While the invention has been described in its preferred embodiments, it is 
to be understood that the words which have been used are words of 
description rather than limitation and that changes within the purview of 
the appended claims may be made without departing from the true scope and 
spirit of the invention in its broader aspects.