Method of making integrated circuits

The invention provides an integrated circuit including a capacitor provided with a silicon nitride film formed on a lower electrode of a polycrystalline silicon film by a rapid thermal nitridation method, a BaTiO.sub.3 film formed on the silicon nitride film and an upper electrode. The above capacitor structure can prevent the formation of a silicon oxide layer at the interface between the polycrystalline silicon film and the BaTiO.sub.3, and thus has high capacitance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an integrated circuit and in more particular to 
structure of an insulating film of a capacitor which is required in a 
superhigh integrated circuit, e.g. a memory, logic or the like. 
2. Description of the Related Art 
In order that superhigh integrated circuit devices have sufficient soft 
error resistance, it is necessary that capacitors are capable of storing a 
large quantity of electric charges even if they are highly-integrated fine 
capacitors. Therefore, it is necessary to thin an insulating film enough 
to compensate for fineness of the capacitor. If a silicon oxide film is 
used as the insulating film, it is difficult to form a thin silicon oxide 
film having a uniform film thickness on a polycrystalline silicon film to 
constitute a lower electrode and the silicon oxide film having average 
thickness of about 10 nm is inside the limits in practical applications. 
If a silicon nitride film is used instead of the silicon oxide film, leak 
electric current increases when the thickness of the silicon nitride film 
is made to be about 4 nm or less if the thickness is expressed in terms of 
reduced thickness of the silicon oxide film and thus the resulting 
capacitor is not practical use and useful. Therefore, in order to attain 
even thinner film thickness in terms of reduced thickness of silicon oxide 
film, application of materials with high dielectric constants to the 
capacitor has been studied. 
As an example of the high-dielectric materials, there are used 
ferroelectric materials such as perovskite type oxides, e.g. BaTiO.sub.3 
and SrTiO.sub.3, and ilmenite type oxides, e.g. LiNbO.sub.3, which are 
represented by the following formula: ABO.sub.3. It is known that these 
oxides have the dielectric constants ranging from 100 to 10000 in the form 
of the single composition and solid solution composition as mentioned 
above. The thin film made from these materials is extremely advantageous 
to minuteness of the capacitor. In this respect, several studies have been 
performed very formerly. For instance, I. H. Pratt has reported that 
BaTiO.sub.3 films formed by sputtering and heat-treating had dielectric 
constants ranging from 16 to 1900 (refer to Proceedings of the IEEE, Vol. 
59, No. 10, October 1971, pp. 1440 to 1447). In experiments carried out by 
I. H. Pratt, metals were used as an electrode material. However, for the 
recent superhigh integrated circuits, polycrystalline silicon is widely 
used as the electrode material. Accordingly, if the better thin film of 
the above-mentioned ferroelectric materials can be formed on the 
polycrystalline silicon film, it is useful and advantageous to minuteness 
of the capacitors in the superhigh integrated circuit devices. 
However, for instance, W. B. Pennebaker has reported that a silicon oxide 
film about 10 nm thick was formed between a thin film of a high-dielectric 
material and a polycrystalline silicon film when the thin film of the 
high-dielectric material was formed on the polycrystalline silicon film 
(refer to "RF Sputtered Strontium Titanate Films", IBM Journal of Research 
and Development, November 1969, pp. 686 to 695, particularly pp. 687 to 
688). As can be seen from in this paper, an interfacial layer, i.e. a 
silicon oxide layer, had a low dielectric constant and as a result the 
thin film having high dielectric constant formed on the polycrystalline 
silicon film had increasingly lowered effective dielectric constant. Thus, 
the advantage of using the material with the high-dielectric constant is 
almost lost. As the other paper which is similar to the above-mentioned 
paper, there can be seen a paper concerning BaTiO.sub.3 of Janda K. G. 
Panitz et al., Journal of Vacuum Science and Technology, Vol. 16, No. 2, 
pp. 315 to 318 (1979), particularly p. 316. 
If the film of the ABO.sub.3 type material having high dielectric constant 
is formed directly on the polycrystalline silicon film, the 
polycrystalline silicon film is oxidized to form an oxide silicon film at 
the interface between the polycrystalline silicon film and the 
high-dielectric film. When the silicon oxide film having low dielectric 
constant is formed, effective capacitance of the capacitor is fixed by the 
silicon oxide film. Even if the film of any material having high 
dielectric constant is formed on the silicon oxide film, it is impossible 
to fabricate the capacitor having sufficient capacitance value. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an integrated circuit provided 
with a capacitor having high capacitance value and obtained by sandwiching 
a silicon nitride film between a polycrystalline silicon film and an 
ABO.sub.3 type composite oxide to thereby prevent oxidation of the 
polycrystalline silicon film. 
The above object of the invention is accomplished by providing an 
integrated circuit comprising a lower electrode including a 
polycrystalline silicon film formed on a semiconductor substrate, a 
multilayered insulating film including silicon nitride and ABO.sub.3 (A is 
Sr, Ba or Li; B is Ti or Nb; and O is oxygen) type composite oxide films 
formed on the lower electrode, and an upper electrode formed on the 
multilayered insulating film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
According to this invention, a silicon nitride film is formed by directly 
nitriding a polycrystalline silicon film or is grown on the surface of the 
polycrystalline silicon film free of a natural oxide film according to a 
chemical vapor deposition (hereinafter referred to as CVD) method. The 
silicon nitride film thus obtained is quite different from a conventional 
silicon nitride film. The silicon nitride film formed by the 
above-mentioned methods has stoichiometric composition and is quite 
different from the conventional silicon nitride film as can be confirmed 
by X-ray photoelectron spectroscopy (XPS or ESCA). Chemical shift of 
silicon atoms in the conventional silicon nitride film always exhibits 
larger values than those of silicon atoms in the silicon nitride film in 
this invention because of being affected by the natural oxide film formed 
on the polycrystalline silicon film and silicon oxide formed when being 
introduced into a furnace. 
The silicon nitride film having the stoichiometric composition is very 
stable and thus serves as a stopper for diffusion of oxygen atoms. If the 
silicon nitride film is formed on the polycrystalline silicon film serving 
as a lower electrode, the polycrystalline silicon film is not oxidized 
when an ABO.sub.3 film is formed at the subsequent step. However, the 
silicon nitride film formed by a conventional CVD method does not have 
such action. The reason is caused by a growth mechanism of the silicon 
nitride film as mentioned below. 
When the silicon nitride film is formed by using ammonia and silane as 
source gases, silylene is first generated as a product in a CVD reactor. 
The silylene is adsorbed to the surface of the polycrystalline silicon 
film serving as a base by shift of electric charges and consequently the 
surface of the polycrystalline silicon film is terminated by hydrogen 
atoms. The silylene is reacted with the surface silicon-hydrogen bond so 
that it is inserted into the bond. After elimination reaction of the 
hydrogen molecule, the surface silicon atom having an sp.sup.2 -like 
composite orbit is left. The silicon atom is reacted with ammonia so that 
it is inserted into ammonia. By repeating such reactions, the silicon 
nitride film is grown. As mentioned above, the silicon nitride film is 
grown on the polycrystalline silicon film or the silicon nitride film. The 
detailed growth mechanism has been reported in a paper of A. Ishitani et 
al., Extended Abstract of the 22nd Conference on Solid State Devices and 
Materials, pp. 187 to 190, 1990, the disclosure of which is hereby 
incorporated by reference herein. 
In the practical fabrication method of superhigh integrated circuits, 
however, the natural oxide film is always formed on the surface of the 
polycrystalline silicon film. In addition, when a silicon wafer is 
introduced into the CVD reactor, the silicon oxide film 2 to 3 nm thick is 
formed on the surface of the polycrystalline silicon film directly before 
depositing the silicon nitride film by the air introduced into the reactor 
or the oxygen or water contained in the injected gases. According to the 
conventional CVD method, therefore, the silicon nitride film is formed on 
the silicon oxide film but not on the polycrystalline silicon film. In 
this case, the silicon nitride film is grown by first generating 
three-dimensional nucleus of silicon on the silicon oxide film and then 
forming silicon nitride on the surface of the silicon. Consequently, an 
insulating film of the capacitor which is formed on the polycrystalline 
silicon film comprises the natural oxide film on the polycrystalline 
silicon film, the silicon-rich silicon nitride film and the silicon 
nitride film from the point of view of the microscopic world. The natural 
oxide film and the silicon-rich silicon nitride film have the sum 
thickness of about 5 nm. Thus, in order to form the silicon nitride film 
serving as the stopper, in the multilayered insulating film formed by the 
conventional fabrication method, the sum film thickness of about 10 nm or 
more is needed. In this structure, even if a film of a material having 
high dielectric constant is formed on the insulating film, it is 
impossible to obtain the capacitor having satisfactory capacitance since 
the effective dielectric constant is fixed by the dielectric constant of 
the multilayered film. 
However, if the silicon nitride film having the stoichiometric composition 
according to this invention is used, the film serves as the stopper even 
if the thickness is of the order of 2 nm and further the natural oxide 
film does not exist between the silicon nitride film and the 
polycrystalline silicon film. Therefore, if the insulating film structure 
according this invention is used, the lower electrode comprising the 
polycrystalline silicon film is not oxidized and thus characteristics of 
the material having high dielectric constant can be utilized almost as 
they are. 
Next embodiments of this invention will be described with reference to 
drawings. 
FIG. 1 shows a cross-sectional view of a capacitor according to an 
embodiment of this invention, in which a capacitive insulating film 
comprising a thin film of an ABO.sub.3 type composite oxide and a silicon 
nitride film is used. As shown in FIG. 1, a field oxide film 2 and a lower 
electrode 3 comprising a polycrystalline silicon film are formed on a 
silicon substrate 1 and a capacitive insulating film comprising a silicon 
nitride film 4 and a film 5 of BaTiO.sub.3 which is of the ABO.sub.3 type 
composite oxide is formed thereon. And, an upper electrode 6 comprising a 
polycrystalline silicon film is formed thereon. For the lower electrode, a 
Ti-Pt film or a Pt-Ta film may be used. 
The fabrication method of the capacitor will be described in more detail 
with reference to the drawings. 
First, the field oxide film 2 about 600 nm thick is formed on the silicon 
substrate 1 having a (100) plane orientation in prescribed position by 
local oxidation of silicon (LOCOS). Then the polycrystalline silicon film 
is formed on the whole surface thereof by the CVD method. The 
polycrystalline silicon film in this embodiment has a thickness of about 
100 nm. The thickness is controlled corresponding to a stored amount of 
electric charge which is required in designing circuits. As the 
polycrystalline silicon film becomes thick, the stored amount of electric 
charge increases but exposure margin in photolithography is reduced. Then 
the polycrystalline film is doped with phosphorus of high concentration by 
an ion implantation process and then the polycrystalline silicon film with 
low resistance is patterned into a prescribed pattern to form a lower 
electrode 3 at an element region. 
The surface of the lower electrode 3 thus formed is covered by a natural 
oxide film. On the surface of the polycrystalline silicon film doped with 
phosphorus of a high level, a specially thicker natural oxide film is 
formed. When the natural oxide film exists, the amount of electric charge 
to be stored is reduced because of low dielectric constant. Incidentally, 
even if the natural oxide film is removed by any method and if a film of 
the ABO.sub.3 type composite oxide is formed directly on the 
polycrystalline silicon film, the polycrystalline silicon film is oxidized 
in the capacitor fabrication process to form a thick silicon oxide film at 
the interface between the polycrystalline silicon film and the composite 
oxide film. Therefore, by protecting the polycrystalline silicon film with 
the silicon nitride film 4, the formation of the natural oxide film must 
be prevented and also the oxidation of the polycrystalline silicon film 
must be prevented in the process for fabricating the capacitor by using 
the ABO.sub.3 type composite oxide film. 
For this end, in this embodiment, the silicon substrate 1 provided with the 
field oxide film and the polycrystalline silicon film is disposed on a 
holder 13 in a lamp annealing furnace wherein a quartz reactor 11 
surrounded with reflector banks 12A and 12B is heated with halogen lamps 
10 as shown in FIG. 2, and the lower electrode 3 on the substrate 1 is 
nitride in an atmosphere of ammonia at 850.degree. C. for 60 seconds. The 
silicon nitride film 4 about 2 nm thick is formed in the prescribed 
position between the natural oxide film and the lower electrode 3 by the 
above-mentioned operational procedures. Namely, according to the 
procedures, the silicon nitride film 4 is formed at the interface between 
the natural oxide film and the polycrystalline silicon film. An X-ray 
photoelectron spectroscopic spectrum of the silicon nitride film thus 
formed is shown in FIG. 3. 
The silicon nitride film formed by the conventional CVD method has chemical 
shift of 2.9 eV in ESCA, whereas the silicon nitride film in this 
embodiment has chemical shift of 2.5 eV in ESCA which is identical to that 
of silicon nitride having stoichiometric composition. The chemical shift 
varies corresponding to an amount of oxygen to be contaminated in the 
silicon nitride film. Unless oxygen is contaminated therein, the silicon 
nitride film exhibits chemical shift of 2.5 eV. 100% silicon oxide 
exhibits chemical shift of 3.2 eV. The dielectric constant is lowered as 
the amount of oxygen to be contaminated becomes large. In addition, oxygen 
is easily diffused into the film as the amount of oxygen to be 
contaminated becomes large, thus the polycrystalline silicon film as the 
lower electrode is oxidized in the process for fabricating the capacitor 
by using the ABO.sub.3 type composite oxide to thereby form the silicon 
oxide film at the interface. 
The relation between the the chemical shift corresponding to the amount of 
oxygen to be contaminated and the formation of the silicon oxide film at 
the interface was examined by changing the temperatures when the silicon 
nitride film is formed by lamp annealing. It was observed that when the 
chemical shift was up to 2.7 eV, the silicon oxide film was not formed at 
the interface in the capacitor fabrication process carried out at 
temperatures not more than 900.degree. C. In addition, the relation 
between the thickness of the silicon nitride film and the formation of the 
silicon oxide film at the interface was examined, with results that no 
silicon oxide film was formed at the interface if the silicon nitride film 
is about 0.5 nm thick in case of the chemical shift of 2.5 eV and if the 
silicon nitride film is about 3 nm thick in case of the chemical shift of 
2.7 eV. 
It is preferred that the chemical shift of the silicon nitride film ranges 
from 2.5 to 2.7 eV of the high energy side with a silicon atom in silicon 
single crystal as its center. 
Thereafter the natural oxide film on the surface of the silicon nitride 
film 4 was removed by wet etching to form the BaTiO.sub.3 film 5 as a thin 
film of the material having high dielectric constant. Of course, the thin 
film may be an SrTiO.sub.3 film or an LiNbO.sub.3 film in place of the 
BaTiO.sub.3 film. Then the upper electrode 6 comprising the 
polycrystalline silicon film was formed by the CVD method and ion 
implantation method. In the case that the BaTiO.sub.3 film 5 was used for 
the high-dielectric layer, the relation between a thickness of the 
BaTiO.sub.3 film 5 and the effective dielectric constant was examined, 
with results shown in FIG. 4. The BaTiO.sub.3 film was formed using a 
target having stoichiometric composition by a radio frequency magnetron 
sputtering method. The film was formed in an Ar--O.sub.2 mixed gas under 
gas pressure of 1.times.10.sup.-2 Torr at substrate temperatures ranging 
from 400.degree. to 500.degree. C. The BaTiO.sub.3 film thus obtained had 
effective dielectric constant keeping about 240 constant without depending 
on the film thickness. It is appreciated that the layer having low 
dielectric constant was not formed at the interface. 
In the above-mentioned embodiment, the silicon nitride film 4 free of 
oxygen was formed by the lamp annealing method. However, the silicon 
nitride film can be also formed by the CVD method using an apparatus shown 
in FIG. 5. The drawing shows a block diagram of a continuous growth 
apparatus which is another apparatus for fabricating the capacitor of the 
embodiment of this invention. 
A specimen, i.e. a substrate provided with a field oxide film and a 
polycrystalline silicon film, is introduced from an input/output port 
(I/O) 26 and fed to each of first to fifth chambers 21 to 25 in order by 
means of a carrier robot 27. In the first chamber 21, a natural oxide film 
formed on the surface of the polycrystalline silicon film is removed by 
using anhydrous hydrogen fluoride gas. Then the specimen is carried into 
the second chamber 22 without being exposed to the atmosphere and the 
silicon nitride film is formed by the CVD method. Thereafter, in the third 
chamber 23, the film of the material having high dielectric constant is 
formed thereon. Then, in the fourth chamber 24, the polycrystalline 
silicon film is deposited thereon. If the atmosphere during carrying is 
made to be under vacuum or an inactive gas to thereby prevent the 
formation of the natural oxide film, the capacitor having the structure 
shown in FIG. 1 can be obtained even if the silicon nitride film for the 
capacitor is formed by the CVD method. Furthermore, the fifth chamber 25 
is used to form a rapid thermal nitridation film (RTN film) and metal 
film. 
As discussed above, according to this invention, by using the multilayered 
insulating film comprising the thin ABO.sub.3 type composite oxide film 
and the silicon nitride film, it is possible to suppress the formation of 
the low-dielectric layer at the interface of the lower electrode 
comprising the polycrystalline silicon film. Therefore, it is possible to 
obtain integrated circuits provided with the capacitor having high 
effective dielectric constant and large capacitance. 
While this invention has been particularly described with reference to the 
preferred embodiments thereof, it will be understood by those skilled in 
the art that the foregoing and other changes may be made therein without 
departing from the spirit and scope of the invention.