Heat treatment of a tantalum oxide film

A method of manufacturing a tantalum oxide film having a high dielectric constant suitable for use in a high density DRAM, and a small leakage current. After forming a tantalum oxide film in amorphous state which includes moisture and impurities such as organic substances, and before crystallizing the film, the tantalum oxide film in amorphous state is subjected to a first heat treatment at a temperature which is higher than the formation temperature of the tantalum oxide film in amorphous state and which is lower than the temperature of crystallization of the tantalum oxide film in amorphous state. As a result of the first heat treatment, the moisture and the impurities such as organic substances existing in the film are surely removed from the film. Accordingly, in a subsequent heat treatment for crystallization at a high temperature, there arises no phenomenon in which crystallization of the film proceeds while impurities in the film are being removed. In other words, the crystallized tantalum oxide film obtained is free from stresses and defects.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a manufacturing method of a tantalum oxide
 film, and more particularly to a semiconductor device employing a tantalum
 oxide film.
 2. Description of the Related Art
 Heretofore, a high dielectric constant material has been required as the
 capacitor insulating film.
 In particular, a tantalum oxide film has been considered to be a promising
 candidate for such a film, and has been studied widely. As the formation
 method of the tantalum oxide film, it is general to adopt a chemical vapor
 deposition (CVD) method using an organic tantalum. A tantalum oxide film
 formed by the CVD method is in amorphous state immediately after its
 formation, and includes moisture and impurities such as organic substances
 in the film.
 Subjecting a tantalum oxide film in amorphous state to a heat treatment in
 an inert atmosphere at 800.degree. C. has been disclosed in Unexamined
 Patent Applications Laid Open, No. Sho 58-134464. However, according to
 the manufacturing method in this invention, the tantalum oxide film in
 amorphous state is crystallized while a large amount of impurities
 existing in the film are removed through the heat treatment at 800.degree.
 C. Accordingly, in this manufacturing method, a large strain is generated
 in the crystallized tantalum oxide film, so it gives rise to a problem
 that many defects occur in the film.
 Moreover, in Unexamined Patent Applications Laid Open, No. Hei 10-223856
 there is disclosed a crystallization treatment of a tantalum oxide film in
 an active oxygen atmosphere. According to the technique disclosed in this
 invention, first, an amorphous tantalum oxide film including crystallized
 tantalum oxide in some parts is formed, then by subjecting it to a low
 temperature heat treatment in an active oxygen atmosphere, the amorphous
 tantalum oxide film including crystallized tantalum oxide in some parts is
 converted to a crystallized tantalum oxide film. However, according to
 this manufacturing method many defects are generated in the crystallized
 tantalum oxide film, analogous to the above-mentioned invention, and
 results in a problem that the defects become the cause of leakage current.
 Furthermore, in Unexamined Patent Applications Laid Open, No. Hei 2-283022,
 a method is disclosed in which after a tantalum oxide film is formed by a
 CVD method, a heat treatment in an ozone atmosphere at 300.degree. C. is
 given, then the product is subjected to a heat treatment for
 crystallization in an oxidizing atmosphere at 800.degree. C.
 Similarly, in Unexamined Patent Applications Laid Open, No. Hei 5-102422 is
 disclosed a method in which after formation of a tantalum oxide film, the
 tantalum oxide film is thoroughly oxidized by a heat treatment in an
 oxygen or ozone atmosphere, then the tantalum oxide film is further
 subjected to a heat treatment in an oxygen atmosphere at 800.degree. C.
 for crystallization.
 In both of the last two inventions, the amorphous tantalum oxide film is
 subjected to a heat treatment prior to the heat treatment for its
 crystallization at a high temperature. However, an oxidation treatment at
 a low temperature of about 300.degree. C. leaves moisture and impurities
 such as organic substances included in the film. Consequently, in the
 subsequent heat treatment of the amorphous tantalum oxide film for
 crystallization at a high temperature, the film is crystallized while a
 large amount of impurities are being removed, which gives rise to a
 problem that the crystallized tantalum oxide film contains many defects in
 it.
 In addition, Unexamined Patent Applications Laid Open, No. Hei 10-229080
 relates to the manufacture of an amorphous tantalum oxide film. In this
 invention is disclosed a method in which after the formation of a tantalum
 oxide film, the film is subjected to a heat treatment in an ozone
 atmosphere at a temperature in the range of 300 to 500.degree. C.
 According to this technique, an amorphous tantalum oxide film not yet
 crystallized, formed in the manufacturing method as described in the
 above, is used as an insulating film for a capacitor.
 Moreover, Unexamined Patent Applications Laid Open, No. Hei 8-69998 relates
 to the manufacture of an amorphous tantalum oxide film. This invention
 discloses a method in which a tantalum oxide film is subjected to a heat
 treatment in a low temperature ozone or oxygen plasma at 400.degree. C.
 According to this technique an amorphous tantalum oxide film not yet
 crystallized, formed in the manufacturing methods as described in the
 above, is used as an insulating film for a capacitor.
 Moreover, Unexamined Patent Applications Laid Open, No. Hei 6-163519 also
 relates to the manufacture of an amorphous tantalum oxide film. This
 invention discloses a method of manufacturing an amorphous tantalum oxide
 film by subjecting a tantalum oxide film to a heat treatment in an
 oxidizing atmosphere at a temperature in the range of 450 to 600.degree.
 C.
 SUMMARY OF THE INVENTION
 It is the object of the present invention to provide a method of
 manufacturing a tantalum oxide film having a high dielectric constant
 suitable for use in a high density DRAM, few defects in the film, an
 excellent breakdown strength, and little leakage current, by improving the
 drawbacks in the conventional techniques as described above, as well as to
 provide a semiconductor device employing the tantalum oxide film thus
 formed.
 In the conventional techniques in the above, the amorphous tantalum oxide
 film is subjected to an oxidation treatment at a low temperature prior to
 a heat treatment at a high temperature for crystallizing the film.
 However, even when the amorphous tantalum oxide film is subjected to a
 heat treatment at a low temperature, it is not possible to remove moisture
 and impurities such as organic substances existing in the film, so they
 are left intact in the film. Accordingly, when the amorphous tantalum film
 including impurities is subjected subsequently to a heat treatment for
 crystallization at a high temperature, the crystallization proceeds while
 the large amount of impurities in the film being removed. As a result,
 there arises a problem that the crystallized tantalum oxide film includes
 a large number of defects. The present method of manufacturing a tantalum
 oxide film is characterized in that after the formation of a tantalum
 oxide film in amorphous state, it is subjected to a heat treatment at a
 temperature higher than the temperature for film formation and is lower
 than the temperature for crystallization of the amorphous tantalum oxide
 film in amorphous state. Following this, the tantalum oxide film in
 amorphous state is crystallized.
 In this invention, a tantalum oxide film in amorphous state including
 moisture and impurities such as organic substances is subjected, prior to
 its crystallization, to a heat treatment at a temperature which is higher
 than the film formation temperature of the tantalum oxide film in
 amorphous state and is lower than the crystallization temperature of the
 tantalum oxide film in amorphous state. As a result of this heat
 treatment, moisture and impurities such as organic substances existing in
 the film are removed completely from the film. Accordingly, in the
 subsequent high temperature for crystallization, crystallization while
 impurities being removed from the film will not take place. In other
 words, the crystallized tantalum oxide film formed is free from strains
 and defects in the film.
 Moreover, it is also preferable to subject the amorphous tantalum oxide
 film to a first heat treatment at a temperature which is higher than the
 temperature for film formation of the tantalum oxide film in amorphous
 state and is lower than the temperature for crystallization of the
 tantalum oxide film in amorphous state, then subjecting the film to a
 second heat treatment in an active oxygen atmosphere at a low temperature
 before finally subjecting the film to the heat treatment for
 crystallization. In this method, the heat treatment for crystallization is
 carried out after supplementing oxygen in the second heat treatment to the
 portions from which impurities in the film are removed by the first heat
 treatment. Accordingly, at the heat treatment for crystallization, oxygen
 is supplemented to the portions of the film from which impurities are
 removed. As a result, in the heat treatment for crystallization,
 crystallization will not proceed in a manner in which impurities are
 removed from the film while it goes on. In other words, the crystallized
 tantalum oxide film formed is free from strains and defects in the film.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS
 The present invention basically adopts a technical constitution as
 described in the following.
 Namely, the manufacturing method of a tantalum oxide film according to this
 invention is characterized in that it includes a step of forming a
 tantalum oxide film in amorphous state, a step of heat treating the
 tantalum oxide film in amorphous state at a temperature which is higher
 than the temperature for formation of the tantalum oxide film in amorphous
 state and is lower than the temperature for crystallization of the
 tantalum oxide film in amorphous state, and a step of crystallizing the
 tantalum oxide film in amorphous state.
 Moreover, it is also preferable to subject the tantalum oxide film in
 amorphous state to a step of heat treating at a temperature below
 500.degree. C., after a step of heat treating the film at a temperature
 which is higher than the temperature for formation of the tantalum oxide
 film in amorphous state and is lower than the temperature for
 crystallization of the tantalum oxide film in amorphous state, then
 subjecting the film to the heat treatment for crystallization.
 Furthermore, it is also preferable to subject the tantalum oxide film in
 amorphous state to a step of heat treating in an active oxygen atmosphere,
 after a step of heat treating the film at a temperature which is higher
 than the temperature for formation of the tantalum oxide film in amorphous
 state and is lower than the temperature for crystallization of the
 tantalum oxide film in amorphous state, then subjecting the film to the
 heat treatment for crystallization.
 In more detail, this invention is characterized in that following the
 formation of a tantalum oxide film by a CVD method using an organic
 tantalum as a material, and prior to the crystallization of the film, the
 film is subjected to a heat treatment in two stages.
 That is, the tantalum oxide film in amorphous state is subjected first to a
 heat treatment at a first temperature, then is subjected consecutively to
 a heat treatment in an ozone atmosphere at a temperature lower than the
 first temperature.
 More specifically, the first heat treatment is for removing moisture and
 impurities such as organic substances included in the tantalum oxide film.
 The following heat treatment in an ozone atmosphere, namely, the second
 heat treatment, is aimed at complementing oxygen for the portions of the
 film from which impurities are removed.
 The first heat treatment is carried out at a temperature higher than that,
 of the formation of the tantalum oxide film, namely 400.degree. C. Since
 the effect of removal of the impurities from the film is more conspicuous
 at higher temperatures, it is preferable to carry it out at a temperature
 higher than 500.degree. C. However, the temperature of this heat treatment
 must not exceed the crystallization temperature (above 650.degree. C.) of
 the tantalum oxide film.
 Here, the atmosphere for the first heat treatment needs not necessarily
 contain oxygen, but it is more effective when the heat treatment is done
 under reduced pressure. A heat treatment under reduced pressure is more
 effective for removal of the impurities from the film than is done under
 normal pressure. More specifically, a pressure less than 10 Torr is
 preferable. The first heat treatment needs be continued for a period by
 which the impurities in the film are removed completely, and it is
 preferable that it is continued especially for over two minutes.
 On the other hand, the second heat treatment is preferable that it is done
 at a temperature below 400.degree. C. The second heat treatment is done in
 an ozone atmosphere, and the lifetime of ozone is reduced rapidly at
 temperatures above 400.degree. C. Accordingly, the effect of oxidizing the
 tantalum oxide film is reduced if the second heat treatment is carried out
 at a temperature above 400.degree. C.
 Furthermore, in this invention, care needs be taken for not exposing the
 tantalum oxide film to the open air to the extent possible, between the
 first heat treatment at a high temperature and the subsequent heat
 treatment in the ozone atmosphere. More specifically, it is preferable
 that each heat treatment is executed while maintaining the heat treatment
 device in sealed conditions without taking out the film into the open air.
 In other words, the condition of the film after the first heat treatment is
 such that impurities in the film are removed but the portions from which
 the impurities are removed are not supplemented by oxygen or the like.
 Accordingly, if the tantalum oxide film after the first heat treatment is
 exposed to the open air, the portions of the film from which the
 impurities are removed absorb moisture in the outer atmosphere. As a
 result, the effect of the heat treatment at a high temperature will be
 self-defeatedly lost.
 For this reason, after the formation of the tantalum oxide film, the
 annealing processing is executed in two stages, and the heat treatment for
 crystallization is carried out thereafter. More specifically, the first
 heat treatment is carried out at a high temperature, and the next heat
 treatment is carried out at a low temperature in an atmosphere containing
 active oxygen. In this way, the tantalum oxide film includes fewer
 defects, and as a result, the leakage current is reduced.
 In this invention, it is preferable that the tantalum oxide film is formed
 with pentaethoxy tantalum Ta(C.sub.2 H.sub.5 OH).sub.5 as the main
 constituent element.
 Moreover, in this invention, it is preferable that the temperature of the
 first heat treatment does not exceed the crystallization temperature of
 the tantalum oxide film, and is higher than the temperature of formation
 of the tantalum oxide film. Furthermore, it is preferable that the
 temperature of the first heat treatment lies in the range of 400 to
 650.degree. C.
 Moreover, in this invention, the first heat treatment process may be
 executed in a nitrogen atmosphere. Furthermore, it is preferable that the
 first heat treatment is executed under a reduced pressure.
 It is preferable that the temperature of the second heat treatment in this
 invention is less than 400.degree. C, and the second heat treatment is
 executed in an active oxygen atmosphere. Moreover, it is also preferable
 that the second heat treatment is executed in an ozone atmosphere.
 Besides, it is preferable that the transfer of the tantalum oxide film,
 between the first heat treatment process and the second heat treatment
 process of this invention, does not include processes which will expose
 the tantalum oxide film to the open air.
 In the following, referring to the drawings, the method of manufacturing
 the tantalum oxide film according to this invention and a specific example
 of a semiconductor device using the tantalum oxide film will be described
 in detail.
 Namely, FIG. 1(A) to FIG. 1(G) are sectional views showing main steps of
 the method of manufacture a specific example of the tantalum oxide film
 according to this invention. These figures illustrate a manufacturing
 method of a tantalum oxide film in which following the formation of a
 tantalum oxide film, first, the film is subjected to a heat treatment at a
 first temperature, then the tantalum oxide film is subjected to a heat
 treatment at a second tempera-Lure which is lower than the first
 temperature in an atmosphere including active oxygen.
 In other words, FIG. 1(A) to FIG. 1(G) show schematic sectional views of
 respective major processes in the manufacturing method of the tantalum
 oxide film according to the present invention.
 First, as shown in FIG. 1(A), an insulating oxide film, for example,
 silicon oxide film 102, is formed on a silicon substrate 101, then an
 appropriate opening is formed in the insulating film. Then, a
 phosphorus-doped polysilicon film is formed in the opening and on the
 silicon insulating film 102, and a lower electrode pattern 103 for a
 capacitor is formed by patterning the polysilicon film.
 Next, as shown in FIG. (B), a silicon nitride film 104 is formed by
 nitriding the surface of the electrode 103 by the rapid thermal nitriding
 (RTN) method. In this case, the conditions of the RTN processing are set
 so as to obtain a thickness of 2 nm for the silicon nitride film.
 Next, as shown in FIG. 1(C), a tantalum oxide film 105 of thickness 10 nm
 is formed on the surface of the substrate and the electrode 103 by a low
 pressure CVD method. For the formation of the tantalum oxide film 105 use
 is made of pentaethoxy tantalum Ta(C.sub.2 H.sub.5 OH).sub.5 and oxygen as
 the raw materials. The flow rates of the raw materials are set to 0.1
 ml/min for pentaethoxy tantalum and 2 SLM (standard liter/min) for oxygen,
 respectively. Besides, the pressure of the gas and the temperature of the
 substrate at the formation of the film are set to 1 Torr and 450.degree.
 C., respectively.
 In this example, the tantalum oxide film is formed using a low pressure CVD
 method, but this invention will not be affected even if a tantalum oxide
 film formed by another method and under other conditions is used.
 Next, following the formation of the tantalum oxide film, as shown in FIG.
 1(D), the film is subjected to a heat treatment, namely, a first heat
 treatment, in a nitrogen atmosphere at 550.degree. C. for 30 min, to form
 a tantalum oxide film 106 which is heat treated in a nitrogen atmosphere.
 In succession to the foregoing, as shown in FIG. 1(E), the film is
 subjected to a heat treatment, namely a second heat treatment, in an ozone
 atmosphere at 300.degree. C. for 10 min, to form a tantalum oxide film 107
 which is heat treated in an ozone atmosphere.
 During the first heat treatment shown in FIG. 1(D) and the second heat
 treatment shown in FIG. 1(E), the wafer is transferred in an atmosphere
 filled with nitrogen in order not to expose the film 106 to the outer air.
 After the above heat treatments, the tantalum oxide film 107 is subjected
 to a furnace annealing in an oxygen atmosphere at 800.degree. C. for 10
 min to crystallize the tantalum oxide film 107, obtaining a semiconductor
 device having a crystallized tantalum oxide film 108 as shown in FIG.
 1(F).
 After this, as shown in FIG. 1(G), a titanium nitride film 109 and a
 phosphorus-doped polysilicon film 110 are formed on the crystallized
 tantalum oxide film 108 to thicknesses of 20 and 200 nm, respectively, by
 a CVD method. In this way, an upper electrode 120 composed of polysilicon
 110 is formed.
 Finally, the crystallized tantalum oxide film 108, the titanium nitride
 film 109, and the polysilicon film 110 are patterned in a desired form by
 photolithography and a dry etching method. Thus, a semiconductor device
 100 having capacitor insulating film using the tantalum oxide film 108 is
 completed as shown in FIG. 2.
 Here, the properties of the tantalum oxide film formed by the manufacturing
 method according to this invention, and the properties of a tantalum oxide
 film manufactured by the conventional method will be described below.
 FIG. 3 and FIG. 4 show respectively the results of measurement for chemical
 substances with mass number 16 and 18 by thermal desorption spectroscopy
 (TDS), of the tantalum oxide film formed by the conventional manufacturing
 method and of the tantalum oxide film formed by this invention.
 FIG. 3 shows the dependence of the number of a substance with mass number
 16, namely, CH.sub.4 represented as the intensity, detected from the
 tantalum oxide film, on the change in the temperature.
 In addition, FIG. 4 shows the dependence of the number of a substance with
 mass number 18, namely, H.sub.2 O represented as the intensity, detected
 from the tantalum oxide film, on the change in the temperature.
 In FIG. 3 and FIG. 4, the curve (a) represents the intensity for the
 tantalum oxide film immediately after its formation by the low pressure
 CVD using an organic tantalum as the raw material. The curve (b)
 represents the intensity for the tantalum oxide film which is subjected to
 a heat treatment in an ozone atmosphere at 300.degree. C. after its
 formation by the low pressure CVD using an organic tantalum as the raw
 material. Further, the curve (c) represents the intensity for the tantalum
 oxide film subjected to the first and the second heat treatments of this
 invention, namely, the tantalum oxide film which was formed by the low
 pressure CVD using an organic tantalum as the raw material, and was
 subjected to the heat treatment in a nitrogen atmosphere at 550.degree.
 C., then subjected to the heat treatment in an ozone atmosphere at
 300.degree. C.
 As can be seen from the curves in FIG. 3 and FIG. 4, although the tantalum
 oxide film (b) subjected to the heat treatment in the ozone atmosphere at
 300.degree. C. has both CH.sub.4 intensity and H.sub.2 O intensity reduced
 compared with those of the tantalum oxide film (a) immediately after the
 formation of the tantalum oxide film, their degree of reduction is not
 sufficiently large.
 However, in the tantalum oxide film (c) which was subjected to the heat
 treatments of this invention, the reduction in the desorption amount of
 the respective impurities is very significant. Namely, when the first heat
 treatment and the second heat treatment according to this invention are
 applied to a tantalum oxide film formed, significant amounts of the
 impurities in the film are removed. Accordingly, the amounts of the
 impurities desorped from the tantalum oxide film after the heat treatments
 are very small.
 In FIG. 5 is shown the leakage current characteristic of the semiconductor
 device using the tantalum oxide film.
 The curve (a) in FIG. 5 shows the leakage current characteristic of the
 semiconductor device when use is made of the tantalum oxide film
 immediately after its formation by the low pressure CVD method using an
 organic tantalum as the raw material. The curve (b) shows the leakage
 current characteristic when use is made of the tantalum oxide film
 subjected to the heat treatment in the ozone atmosphere at 300.degree. C.
 after its formation by the low pressure CVD method using an organic
 tantalum as the raw material. Further, the curve (c) shows the leakage
 current characteristic when use is made of the tantalum oxide film
 subjected to the first heat treatment and the second heat treatment of
 this invention, namely, the tantalum oxide film subjected to the heat
 treatment in the nitrogen atmosphere at 550.degree. C., then to the heat
 treatment in the ozone atmosphere at 300.degree. C., after the film
 formation by the low pressure CVD method using an organic tantalum as the
 raw material.
 As can be seen from a comparison of the curves in FIG. 5, the leakage
 current (c) for the tantalum oxide film subjected to the oxidation and
 heat treatment of the manufacturing method of the tantalum oxide film
 according to this invention reveals a reduction of the leakage current of
 several orders of magnitude compared with the leakage current (b) for the
 tantalum oxide film subjected to the heat treatment in the ozone
 atmosphere at 300.degree. C. Accordingly, it is clear that the oxidation
 effect by the second heat treatment of this invention is very conspicuous.
 That is, since the impurities in the film are removed effectively by the
 heat treatment at the high temperature in the nitrogen atmosphere, namely,
 the first heat treatment, it can be seen that the oxidation by the
 succeeding second heat treatment can be executed very efficiently.
 In the specific example in the above, the first heat treatment was executed
 in a nitrogen atmosphere, and the second heat treatment was executed in an
 ozone atmosphere. However, the heat treatments are not limited to these
 cases and different heat treatment may be applied to each heat treatment
 stage.
 What is important in such a case is that the first heat treatment be
 executed at a high temperature, and the second heat treatment be executed
 in an active oxygen atmosphere. As long as the heat treatments satisfy
 these conditions it is possible to adopt conditions different from those
 in the specific example.
 For example, it is promising to execute the first heat treatment under a
 reduced pressure, in which the removal of the impurities will proceed more
 effectively than under the normal pressure.
 Moreover, it is also effective to execute the first heat treatment in an
 atmosphere containing oxygen. An oxygen atmosphere has an enhanced effect
 for the removal of the carbon-based impurities. By adopting such a method,
 it is possible to carry out an effective removal of the carbon-based
 impurities in addition to the removal of the moisture.
 In the specific example, the second heat treatment is executed in an ozone
 atmosphere, but a higher effect can be expected if the heat treatment is
 carried out while irradiating ozone with an ultraviolet radiation because
 a larger amount of active oxygen will become available.
 Moreover, a similar effect can be expected by executing the heat treatment
 in an oxygen atmosphere while irradiating oxygen with an ultraviolet
 radiation. This is because, similar to the above case, active oxygen can
 be produced by the irradiation of oxygen with an ultraviolet radiation.
 Furthermore, a similar effect can be expected by carrying the heat
 treatment while supplying oxygen passed through a plasma atmosphere.
 Since the subsequent heat treatment is executed in an active oxygen
 atmosphere, it is possible to remove carbonbased impurities in the film.
 In the above example, it is recommended not to expose the film to the open
 air between the first heat treatment and the second heat treatment. If the
 film is exposed to the open air, the film will absorb the moisture in the
 air, diluting the effect of the first heat treatment.
 Accordingly, the transfer of the formed tantalum oxide film in a nitrogen
 atmosphere is desired, but the transfer of the film in a reduced pressure
 is also an effective method as an alternative.
 The semiconductor device constructed by using the tantalum oxide film
 formed by this invention has a structure as, for example, shown in FIG. 2.
 The feature of this semiconductor device 100 is as described below. On the
 surface of the electrode part 103 formed via the opening provided in the
 layer insulating film 102 on the substrate 101, the tantalum oxide film
 108 is formed via the silicon nitride film 104. The conductive film 120 is
 formed on the tantalum oxide film 108 via the titanium nitride film 109.
 This device has the property that the leakage current is small and the
 capacity is large.
 The properties of one example 100 of the semiconductor device according to
 this invention are shown in FIG. 6 and FIG. 7.
 Namely, FIG. 6 is a graph showing the dependence of the occurrence
 probability on the leakage current distribution in terms of the normal
 logarithmic distribution for measurement at 13 points, for the case when a
 voltage of 1.2V is applied to the upper electrode of the semiconductor
 device 100 or 200 or 300 employing the tantalum oxide film 108 or 208 or
 308 having a configuration as shown in FIG. 2 or FIG. 8 or FIG. 9. The
 ordinate represents the occurrence probability and the abscissa represents
 the leakage current density (A/cm.sup.2).
 Of the curves in FIG. 6, the curve drawn through the empty white circles
 shows the occurrence probability for the leakage current density, of the
 semiconductor device 100 employing the tantalum oxide film 108 subjected
 to the first heat treatment and the second heat treatment of the
 manufacturing method according to this invention, namely, the tantalum
 oxide film 108 obtained by subjecting the tantalum oxide film 105 formed
 by the low pressure CVD method using an organic tantalum as the raw
 material, to the heat treatment in a nitrogen atmosphere at 550.degree.
 C., then subjecting the film to the heat treatment in an ozone atmosphere
 at 300.degree. C. The curve drawn through the solid black circles shows
 the occurrence probability for the leakage current density, of the
 semiconductor device 200 employing the tantalum oxide film 208 formed by
 the conventional manufacturing method, namely, the tantalum oxide film 208
 obtained by subjecting the tantalum oxide film 105 formed by the low
 pressure CVD method using an organic tantalum as the raw material, to the
 heat treatment in an ozone atmosphere under irradiation of an ultraviolet
 radiation at 400.degree. C. The curve drawn through the solid black
 triangles shows the occurrence probability for the leakage current
 density, of the semiconductor device 300 employing the tantalum oxide film
 308 formed by the conventional manufacturing method, namely, the tantalum
 oxide film 105 formed by the low pressure CVD method using an organic
 tantalum as the raw material, to a plain heat treatment.
 As is clear from these curves, the curve (drawn through the empty circles)
 for the semiconductor device according to this invention is at the extreme
 left of all the curves in FIG. 6 including the curve (drawn through the
 solid black circles) for the semiconductor device 200 and the curve (drawn
 through the solid black triangles) for the semiconductor device 300
 according to the conventional technique. This means that the semiconductor
 device according to this invention has the least occurrence probability
 for the leakage current of all the semiconductor devices including those
 according to the conventional technique.
 In FIG. 7 are shown the curves of the occurrence probability for the
 leakage current measured in the same method as in FIG. 6, for the case
 where a voltage of 1V is applied to the upper electrode 120 for the
 semiconductor device 100 or 200 or 300 having the configuration as shown
 in FIG. 2 or FIG. 8 or FIG. 9, and the thickness of the tantalum oxide
 film 108 or 208 or 308 is set to 80 .ANG., with the thickness of the
 tantalum oxide film 105 converted to the equivalent thickness of silicon
 oxide film.
 In other words, in the figure, the ordinate represents the occurrence
 probability, and the abscissa represents the thickness in nanometers (nm)
 of the tantalum oxide film 108, 208, and 308 converted to the equivalent
 thickness of silicon oxide film.
 In the figure, the curve drawn through the empty circles shows the
 occurrence probability for the leakage current density of the
 semiconductor device 100 employing the tantalum oxide film 108 subjected
 to the first heat treatment and the second heat treatment of the
 manufacturing method according to this invention, namely, the tantalum
 oxide film 108 obtained by subjecting the tantalum oxide film formed by
 the low pressure CVD method using an organic tantalum, to the heat
 treatment in a nitrogen atmosphere at 550.degree. C., then subjecting the
 film to the heat treatment in an ozone atmosphere at 300.degree. C. The
 curve drawn through the solid black circles shows the occurrence
 probability for the leakage current density of the semiconductor device
 200 employing the tantalum oxide film 208 formed by the conventional
 manufacturing technique, namely, the tantalum oxide film 208 obtained by
 subjecting the tantalum oxide film 105 formed by the low pressure CVD
 method using an organic tantalum as the raw material, to the heat
 treatment in an ozone atmosphere under irradiation of an ultraviolet
 radiation at 400.degree. C. The curve drawn through the solid black
 triangles shows the occurrence probability for the leakage current of the
 semiconductor device 300 employing the tantalum oxide film 308 formed
 according to the conventional technique, namely, the tantalum oxide film
 308 obtained by subjecting the tantalum oxide film 105 formed by the low
 pressure CVD method using an organic tantalum, to a plain heat treatment.
 As is clear from these curves, the curve (drawn through the empty circles)
 for the semiconductor device 100 according to this invention is found at
 the extreme left in FIG. 7 for all cases of the semiconductor devices
 including the curve (drawn through the solid black circles) for the
 semiconductor device 200 and the curve (drawn through the solid black
 triangles) for the semiconductor device 300 obtained according to the
 conventional technique. This means that the semiconductor device according
 to this invention has the smallest thickness of the silicon oxide film
 required for holding electrical charges when the same amount of charges
 are imparted, among all the semiconductor devices including those
 manufactured according to the conventional technique.
 These results show that the semiconductor device according to this
 invention has a higher capacity retaining performance than the device
 according to the conventional technique.
 Since the manufacturing method according to this invention adopts the
 technical constitution as described in the above, it is possible to
 manufacture a tantalum oxide film having a high dielectric constant
 suitable for application to a high density DRAM, and a smaller leakage
 current. At the same time, it has an accompanying effect which facilitates
 the manufacture of a semiconductor device adopting the tantalum oxide
 film.
 It is apparent that the present invention is not limited to the above
 embodiment, but may be modified and changed without departing from the
 scope and spirit of the invention.