Abstract:
A semiconductor device comprises: an insulating film formed over a semiconductor substrate and having a first recess; a plurality of capacitor elements each of which is composed of a capacitor lower electrode formed on wall and bottom portions of the first recess and having a second recess, a capacitor insulating film of a dielectric film formed on wall and bottom portions of the second recess and having a third recess, and a capacitor upper electrode formed on wall and bottom portions of the third recess; and a conductive layer (referred hereinafter to as a low-resistance conductive layer) which is formed to cover at least portions of the respective capacitor upper electrodes constituting the plurality of capacitor elements and to extend across the plurality of capacitor elements and which has a lower resistance than the capacitor upper electrode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. § 119 on Patent Application No. 2005-185314 filed in Japan on Jun. 24, 2005, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   (a) Fields of the Invention 
   The present invention relates to semiconductor devices which are provided with capacitor elements of three-dimensionally stacked structures employing dielectric films of dielectric materials as capacitor insulating films, and to methods for fabricating such devices. 
   (b) Description of Related Art 
   In development of ferroelectric memory devices, those of planar structures having small capacities of 1 to 64 kbit start being produced in volume. Recently, the center of development of the ferroelectric memory devices has been shifting to those of stack structures having large capacities of 256 kbit to 4 Mbit. In the ferroelectric memory devices of stack structures, a contact plug electrically connected to a semiconductor substrate is arranged immediately below a capacitor lower electrode, which reduces their cell sizes to improve their packing densities. 
   The trend in the ferroelectric memory devices toward miniaturization will advance in the future. With this advancement, it becomes difficult for flat-type capacitor elements to secure the quantity of electric charge necessary for memory operation, so that capacitor elements of three-dimensionally stacked structures are required which include so-called three-dimensional capacitor elements. In order to realize the capacitor elements of three-dimensionally stacked structures, it is necessary to form a dielectric film and a capacitor upper electrode with good coverage on a capacitor lower electrode of an increased surface area having the shape of a step. Conventionally, the capacitor lower electrode, the dielectric film, and the capacitor upper electrode are formed in a concave hole using a CVD method, thereby realizing the capacitor element of a three-dimensionally stacked structure (see, for example, Japanese Unexamined Patent Publication No. 2003-7859 (page 8 and FIG. 5)). 
   Hereinafter, the structure of a conventional semiconductor device of a three-dimensionally stacked structure having a capacitor element will be described with reference to  FIG. 10 . 
   Referring to  FIG. 10 , on a semiconductor substrate  100 , a first interlayer insulating film  103  is formed which is composed of an oxide film  101  and a first anti-reflection film  102  of a nitride film (a SiON film). A polysilicon film  104  and first and second barrier metal films  105  and  106  are disposed in the first interlayer insulating film  103 . The polysilicon film  104  is formed in a lower portion of a storage contact hole reaching an active region (not shown) of the semiconductor substrate  100 , and the first and second barrier metal films  105  and  106  are formed on the polysilicon film  104  and in an upper portion of the storage contact hole. The polysilicon film  104  is formed by a chemical vapor deposition method (a CVD method). During a high-temperature thermal treatment in an oxygen atmosphere, oxygen diffuses through a storage electrode to induce oxidation of polysilicon at the interface between a polysilicon plug of the polysilicon film  104  and the storage electrode. The first and second barrier metal films  105  and  106  serve to prevent this induction. 
   On the first interlayer insulating film  103 , a second interlayer insulating film  110  is formed which is made of an etch stop film  107 , an oxide film  108 , and a second anti-reflection film  109 . A capacitor lower electrode  111 , a first BST thin film  112 , a second BST thin film  113 , and a capacitor upper electrode  114  are sequentially disposed in the second interlayer insulating film  110 . The capacitor lower electrode  111  with a thickness of 5 to 50 nm is formed in a storage node hole by a CVD method. The first BST thin film  112  is formed by an ALD (atomic layer deposition) method. The second BST thin film  113  is formed by a CVD method. The capacitor upper electrode  114  is formed by a CVD method or a sputtering method. Note that the capacitor upper electrode  114  and the capacitor lower electrode  111  constitute a storage electrode. 
   As shown above, the conventional semiconductor device includes the capacitor element having the three-dimensionally stacked structure of a concave-shaped three-dimensional configuration, thereby realizing a miniaturized, highly-integrated dielectric memory device. 
   SUMMARY OF THE INVENTION 
   In the conventional example described above, when a thermal treatment is performed for crystallization of the dielectric film forming the first and second BST thin films  112  and  113 , a problem arises that in a portion of the capacitor upper electrode  114  located around the bottom of the concave hole and having the worst step coverage, the upper electrode  114  becomes broken depending on the selected material thereof. In addition, the capacitor upper electrode  114  is made of a platinum film for the reason that the platinum film has a good compatibility with a dielectric film such as the first and second BST films  112  and  113 , and a high ductility of the capacitor upper electrode  114  made of a platinum film in turn facilitates stress migration. From this, it is obvious that such a thermal stress migration causes a high incidence of breaks in the capacitor upper electrode  114 . 
   The first and second BST films  112  and  113  that are high dielectric films have crystallization temperatures of 500 to 700° C., which fall within the group with a relatively low crystallization temperature. However, some ferroelectric films have crystallization temperatures reaching as high as 800° C., as typified by an SBT film as a ferroelectric film. From this fact, it is conceivable that if the rise in the crystallization temperature and the long-time consumption of the crystallization proceed, the probability of causing failures such as breaks extremely rises. 
   For an alternative electrode material, a study is made of the use of an oxide material such as IrO x  or RuO x  that is more resistant to stress migration than pure metal. Since such an oxide material is unreactive during crystallization of a high dielectric film or a ferroelectric film, it is desirable from this point to use the oxide material as an electrode material forming a capacitor provided within the concave hole. 
   However, the resistance of the oxide material is higher than that of a metal material. In particular, since the capacitor upper electrode functions as a cell plate of the dielectric memory device, the resistance of the material forming the capacitor upper electrode affects cell plate operation. That is to say, if a material of higher resistance is chosen as an oxide material for the capacitor upper electrode, interconnect delay in which start-up delays at the time of driving the cell plate occurs to hamper quick operation of a memory cell. 
   In view of the foregoing, an object of the present invention is to provide a highly-integrated, quickly operable dielectric memory device of a three-dimensionally stacked structure which prevents interconnect delay of a cell plate of a capacitor upper electrode without depending on the material for a dielectric material and the material for a capacitor upper electrode. 
   To attain the above object, a semiconductor device according to one aspect of the present invention comprises: an insulating film formed over a semiconductor substrate and having a first recess; a plurality of capacitor elements each of which is composed of a capacitor lower electrode, a capacitor insulating film, and a capacitor upper electrode, the capacitor lower electrode being formed on wall and bottom portions of the first recess and having a second recess, the capacitor insulating film of a dielectric film being formed on wall and bottom portions of the second recess and having a third recess, the capacitor upper electrode being formed on wall and bottom portions of the third recess; and a conductive layer (referred hereinafter to as a low-resistance conductive layer) which is formed to cover at least portions of the respective capacitor upper electrodes constituting the plurality of capacitor elements and to extend across the plurality of capacitor elements and which has a lower resistance than the capacitor upper electrode. 
   With the semiconductor device according to one aspect of the present invention, the low-resistance conductive layer is provided on the top of the capacitor upper electrode. Thus, even though the material of high resistivity such as an oxide is used as an electrode material for the capacitor upper electrode for the purpose of preventing breaks due to stress migration during a thermal treatment, start-up at the time of driving a cell plate can be prevented from delaying, thereby avoiding interconnect delay. Consequently, a capacitor element having a cell structure with no interconnect delay caused therein can be realized without depending on the material for the dielectric film and the material for the capacitor upper electrode, so that a semiconductor device capable of having a high packing density can be provided. 
   Preferably, in the semiconductor device according to one aspect of the present invention, the capacitor upper electrode is formed to be embedded within the third recess. 
   With this device, the capacitor upper electrode has no bending portion within the third recess. Therefore, not only the effects described above but also a highly-integrated semiconductor device in a concave shape can be provided in which no breaks occur in the capacitor upper electrode. In addition, a semiconductor device can be provided which has a structure capable of using a desired mask to form the low-resistance conductive layer without suffering any influences of the recess resulting from the concave structure, that is, without considering the thickness uniformity or the like of a photoresist film deposited around the recess in forming the low-resistance conductive layer. 
   Preferably, in the semiconductor device according to one aspect of the present invention, the capacitor upper electrode and the conductive layer have almost the same plan shapes. 
   With this device, the capacitor upper electrode and the low-resistance conductive layer can be patterned using the same mask. Therefore, unlike the case where they are patterned separately using different masks, the necessity to consider the alignment margin of masks is eliminated, so that a semiconductor device can be provided which can attain a further miniaturization of a cell. Furthermore, the number of times masks are used decreases, so that a semiconductor device having the structure superior in mass productivity can be fabricated. 
   Preferably, in the semiconductor device according to one aspect of the present invention, the capacitor upper electrode has a fourth recess, and the conductive layer is formed outside the fourth recess. 
   With this device, a semiconductor device can be provided which has a structure capable of using a desired mask to form the low-resistance conductive layer without suffering any influences of the recess resulting from the concave structure, that is, without considering the thickness uniformity or the like of a photoresist film deposited around the recess in forming the low-resistance conductive layer. 
   In the semiconductor device according to one aspect of the present invention, the conductive layer is a cell plate line. 
   A method for fabricating a semiconductor device according to one aspect of the present invention comprises the steps of: forming an insulating film with a recess on a semiconductor substrate; forming a plurality of capacitor elements each of which is made by forming, in the recess, a capacitor lower electrode, a capacitor insulating film of a dielectric film, and a capacitor upper electrode in this order; and forming a conductive layer to cover at least portions of the respective capacitor upper electrodes constituting the plurality of capacitor elements and to extend across the plurality of capacitor elements, the conductive layer having a lower resistance than the capacitor upper electrode. 
   With the method for fabricating a semiconductor device according to one aspect of the present invention, even though the material of high resistivity such as an oxide is used as an electrode material for the capacitor upper electrode for the purpose of preventing breaks due to stress migration during a thermal treatment, a semiconductor device can be fabricated which can prevent delay of start-up at the time of driving a cell plate to avoid interconnect delay. Consequently, a capacitor element having a cell structure with no interconnect delay caused therein can be realized without depending on the material for the dielectric film and the material for the capacitor upper electrode, so that a semiconductor device capable of being highly integrated can be provided. 
   As is apparent from the above, with the semiconductor device and its fabrication method according to the present invention, the conductive layer having a lower resistance than the capacitor upper electrode is formed on the top of the capacitor upper electrode. Thereby, even in the case where the material of high resistance is used for the capacitor upper electrode for the reason of the compatibility with the dielectric film, prevention of breaks during a thermal treatment, the gap-filling capability, and the like, a highly-integrated, quickly operable semiconductor device as a dielectric memory device can be provided in which no interconnect delay is caused at the time of driving the cell plate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a sectional view showing the structure of a semiconductor device according to a first embodiment of the present invention, and  FIG. 1B  is a plan view showing the structure of the semiconductor device according to the first embodiment of the present invention. 
       FIGS. 2A to 2D  are sectional views showing process steps of a method for fabricating a semiconductor device according to the first embodiment of the present invention. 
       FIGS. 3A to 3C  are sectional views showing process steps of the method for fabricating a semiconductor device according to the first embodiment of the present invention. 
       FIG. 4A  is a sectional view showing the structure of a semiconductor device according to a second embodiment of the present invention, and  FIG. 4B  is a plan view showing the structure of the semiconductor device according to the second embodiment of the present invention. 
       FIGS. 5A to 5D  are sectional views showing process steps of a method for fabricating a semiconductor device according to the second embodiment of the present invention. 
       FIGS. 6A to 6C  are sectional views showing process steps of the method for fabricating a semiconductor device according to the second embodiment of the present invention. 
       FIG. 7A  is a sectional view showing the structure of a semiconductor device according to a third embodiment of the present invention, and  FIG. 7B  is a plan view showing the structure of the semiconductor device according to the third embodiment of the present invention. 
       FIG. 8A  is a sectional view showing the structure of a semiconductor device according to a fourth embodiment of the present invention, and  FIG. 8B  is a plan view showing the structure of the semiconductor device according to the fourth embodiment of the present invention. 
       FIGS. 9A to 9F  are views showing respective plan arrangements of capacitor upper electrodes and low-resistance conductive layers in the structures of the semiconductor devices according to the first to fourth embodiments of the present invention. 
       FIG. 10  is a sectional view showing the structure of a conventional semiconductor device. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. 
   First Embodiment 
   The structure of a semiconductor device according to a first embodiment of the present invention will be described below with reference to  FIGS. 1A and 1B . 
     FIG. 1A  shows the structure of the semiconductor device according to the first embodiment of the present invention, and is a sectional view taken along the line Ia-Ia in  FIG. 1B .  FIG. 1B  is a plan view showing the structure of the semiconductor device according to the first embodiment of the present invention. 
   Referring to  FIG. 1A , a first interlayer insulating film  11  of a silicon oxide film having 300 to 800 nm is formed on a semiconductor substrate  10 . The first interlayer insulating film  11  is formed with a storage node contact  12  of a tungsten film or a polysilicon film. The storage node contact  12  penetrates the first interlayer insulating film  11  to reach an active region (not shown) of the semiconductor substrate  10 . On the first interlayer insulating film  11 , an oxygen barrier film  13  is formed which is connected to the upper end of the storage node contact  12 , has a thickness of 50 to 300 nm, and contains an iridium film, an iridium oxide film, or the like. The oxygen barrier film  13  serves to prevent oxidation of the storage node contact  12  in crystallizing a dielectric film formed above the oxygen barrier film  13 . 
   On the first interlayer insulating film  11 , a second interlayer insulating film  14  is formed which covers the side surface of the oxygen barrier film  13  and includes a first recess  15   a . The second interlayer insulating film  14  is made of a silicon oxide film with a thickness of 300 to 800 nm. The first recess  15   a  is formed to penetrate the second interlayer insulating film  14  and serves as an opening for forming a capacitor element (which will be described later) to be formed above each storage node contact. The first recess  15   a  has a hole shape. In this embodiment, the hole shape the first recess  15   a  has indicates an opening formed above each storage node contact  12  as shown in  FIG. 1B . This provides a highly-integrated semiconductor device in a concave shape with no breaks occurring in a capacitor upper electrode  18 A that will be described below. 
   On the wall and bottom of the first recess  15   a , a capacitor lower electrode  16  of iridium oxide is formed which has a thickness of 5 to 50 nm and includes a second recess  15   b . On the top of the capacitor lower electrode  16  and the wall and bottom of the second recess  15   b , a capacitor insulating film  17  of an SBT film as a dielectric film is formed which has a thickness of 5 to 100 nm and includes a third recess  15   c . On the top of the capacitor insulating film  17  and the wall and bottom of the third recess  15   c , a capacitor upper electrode  18 A of iridium oxide is formed. As shown above, the reason why the oxide material is used as the electrode material forming the capacitor upper electrode  18 A and the capacitor lower electrode  16  is that as compared to the case where an electrode of a precious metal material having a high ductility is employed as the capacitor upper electrode  18 A and the capacitor lower electrode  16 , the oxide material can prevent breaks due to thermal stress migration occurring intensively in the thinnest portion of the capacitor lower electrode  16  formed in the first recess  15   a  or in the thinnest portion of the capacitor upper electrode  18 A formed in the third recess  15   c.    
   On the top of the second interlayer insulating film  14 , the top of the capacitor upper electrode  18 A, and the bottom and wall of a fourth recess  15   d , a conductive layer  19 A (referred hereinafter to as a low-resistance conductive layer  19 A) of a platinum film is formed which has a lower resistance than the capacitor upper electrode  18 A. In this embodiment, since iridium oxide (112 Ω·m, 0° C.) is used for the capacitor upper electrode  18 A, platinum (9.81 Ω·m, 0° C.) having a lower resistance than iridium oxide is employed for the low-resistance conductive layer  19 A. If consideration is given to improvement of the adhesions to an interconnect and to an interlayer insulating film (SiO 2 ) to be formed on the top of the low-resistance conductive layer  19 A, it is also possible to use iridium (4.7 Ω·m, 0° C.) for the low-resistance conductive layer  19 A. Further, in the case where high level of thermal resistance is not demanded of this layer, for example, in the case where ferroelectric-film crystallization is completed before formation of the low-resistance conductive layer  19 A, tungsten (4.9 Ω·m, 0° C.), aluminum (2.5 Ω·m, 0° C.), or copper (1.55 Ω·m, 0° C.) may also be used therefor. This makes it possible to attain quicker operation of the memory device. 
   In the above-described structure of the semiconductor device according to the first embodiment, the capacitor insulating film  17  and the capacitor upper electrode  18 A are formed by patterning in the direction in which the cross section is taken (in the vertical direction when viewed in  FIG. 1B ) using the same mask. Alternatively, in consideration of the adhesion to a film as an underlying layer, the adhesion to a film as an overlying layer, unwanted residues created in the processing, and the like, the capacitor insulating film  17  and the capacitor upper electrode  18 A may be formed using different masks. 
   Although the capacitor upper electrode  18 A is formed in the horizontal direction when viewed in  FIG. 1B  and above each storage node contact  12 , it is also acceptable to form it to be shared among the storage node contacts  12 . Although the oxygen barrier film  13  is formed on the storage node contact  12 , the oxygen barrier film  13  does not necessarily have to be formed depending on the temperature (for example, a low temperature) or the atmosphere (for example, a nitrogen atmosphere) in crystallization of the dielectric film made of, other than the SBT-based material, a PZT-, BLT-, or BST-based metal oxide or the like. 
   As described above, with the semiconductor device according to the first embodiment of the present invention, the low-resistance conductive layer  19 A serving as a cell plate is formed on the top of the capacitor upper electrode  18 A. Thus, even though the material of high resistance is chosen as the material for the capacitor upper electrode  18 A for the reason of prevention of interconnect breaks during a thermal treatment, a highly-integrated, quickly operable semiconductor device as a dielectric memory device can be provided in which no interconnect delay is caused at the time of driving the cell plate. 
   Moreover, with the semiconductor device according to the first embodiment of the present invention, even though a thermal treatment at 800° C. is performed for crystallization of the capacitor insulating film  17 , breaks of the capacitor upper electrode  18 A can be prevented. 
   Hereinafter, a method for fabricating a semiconductor device according to the first embodiment of the present invention will be described with reference to  FIGS. 2A to 2D  and  FIGS. 3A to 3C . 
   Referring to  FIG. 2A , first, on the semiconductor substrate  10 , the first interlayer insulating film  11  of a silicon oxide film is formed which has a thickness of 300 to 800 nm. The first interlayer insulating film  11  is formed with a storage node contact hole exposing the surface of the active region (not shown) of the semiconductor substrate  10 , and then the storage node contact hole is filled with a tungsten film or a polysilicon film to form a storage node contact  12  penetrating the first interlayer insulating film  11  to reach the active region of the semiconductor substrate  10 . 
   Thereafter, on the first interlayer insulating film  11 , the oxygen barrier film  13  containing an iridium film, an iridium oxide film, or the like is formed which is connected to the upper end of the storage node contact  12  and which has a thickness of 50 to 300 nm. The oxygen barrier film  13  serves to prevent oxidation of the storage node contact  12  during crystallization of a dielectric film formed over the oxygen barrier film  13 . 
   On the first interlayer insulating film  11 , the second interlayer insulating film  14  of a silicon oxide film having a thickness of 300 to 800 nm is formed to cover the oxygen barrier film  13 . 
   The formed second interlayer insulating film  14  is patterned using a desired mask to form the first recess  15   a  penetrating the second interlayer insulating film  14  to provide a way to make an electrical connection to the oxygen barrier film  13  or the storage node contact  12 . In the first embodiment, the first recess  15   a  formed in the second interlayer insulating film  14  has a hole shape. Note that like the above description, the hole shape indicates an opening formed above each storage node contact  12  as shown in  FIG. 1B . 
   As shown in  FIG. 2B , on the top of the second interlayer insulating film  14  and the wall and bottom of the first recess  15   a , a first conductive film of iridium oxide is formed which has a thickness of 5 to 50 nm and includes a second recess  15   b . Thereafter, in order to electrically separate at least the storage node contacts  12 B from each other, the first conductive film is subjected to conduct patterning with a desired mask, a plating method, a combination of a sputtering method and a CMP method, or a self-alignment technique such as a sidewall formation method. Thereby, the capacitor lower electrode  16  with the second recess  15   b  is formed. 
   As shown in  FIG. 2C , using a CVD method, an SBT film  17   a  serving as a dielectric film is formed on the top of the second interlayer insulating film  14 , the top of the capacitor lower electrode  16 , and the wall and bottom of the second recess  15   b . The SBT film  17   a  includes the third recess  15   c , and has a thickness of 5 to 100 nm. 
   As shown in  FIG. 2D , a second conductive film  18   a  of iridium oxide is formed on the top of the SBT film  17   a  and the wall and bottom of the third recess  15   c . The second conductive film  18   a  includes the fourth recess  15   d , and has a thickness of 50 to 300 nm. 
   As shown in  FIG. 3A , the second conductive film  18   a  and the SBT film  17   a  are patterned using a desired mask to form the capacitor upper electrode  18 A of the second conductive film  18   a  and the capacitor insulating film  17  of the SBT film  17   a.    
   In this formation method, description has been made of the case where in forming the capacitor insulating film  17  and the capacitor upper electrode  18 , patterning thereof is conducted using the same mask. However, as mentioned previously, it is also acceptable not to use the same mask in consideration of the adhesion to a film as an underlying layer, the adhesion to a film as an overlying layer, unwanted residues created in the processing, and the like. 
   Although, like the above description, the capacitor upper electrode  18  is formed above each storage node contact  12 , it is also acceptable to form it to be shared among the storage node contacts  12 . Although the oxygen barrier film  13  is formed on the storage node contact  12 , the oxygen barrier film  13  does not necessarily have to be formed depending on the temperature (for example, a low temperature) or the atmosphere (for example, a nitrogen atmosphere) in crystallization of the dielectric film made of, other than the SBT-based material described above, a PZT-, BLT-, or BST-based metal oxide or the like. 
   Thereafter, as shown in  FIG. 3B , a third conductive film  19   a  of platinum is formed on the top of the second interlayer insulating film  14 , the top of the capacitor upper electrode  18 A, and the wall and bottom of the fourth recess  15   d.    
   As shown in  FIG. 3C , the third conductive film  19   a  is patterned using a desired mask to form a low-resistance conductive layer  19  of the third conductive film  19   a.    
   As described above, with the method for fabricating a semiconductor device according to the first embodiment of the present invention, the low-resistance conductive layer  19 A serving as a cell plate is formed on the top of the capacitor upper electrode  18 A. Thus, even though the material of high resistance is chosen as the material for the capacitor upper electrode  18 A for the reason of prevention of breaks during a thermal treatment, a highly-integrated, quickly operable semiconductor device as a dielectric memory device can be provided in which no interconnect delay is caused at the time of driving the cell plate. 
   Note that the structure may also be employed in which the low-resistance conductive layer  19 A is not used as a cell plate and the top of the low-resistance conductive layer  19 A is provided with a cell plate made of a conductive material with a lower resistance. 
   Second Embodiment 
   Hereinafter, the structure of a semiconductor device according to a second embodiment of the present invention will be described with reference to  FIGS. 4A and 4B . 
     FIG. 4A  shows the structure of the semiconductor device according to the second embodiment of the present invention, and is a sectional view taken along the line IVa-IVa in  FIG. 4B .  FIG. 4B  is a plan view showing the structure of the semiconductor device according to the second embodiment of the present invention. 
   A point of difference between the structure of the semiconductor device according to the second embodiment shown in  FIGS. 4A and 4B  and the structure of the semiconductor device according to the first embodiment shown in  FIGS. 1A and 1B  is that as shown in  FIG. 4A , a capacitor upper electrode  18 B is embedded to fully fill the inside of the third recess  15   c . In the second embodiment, as the electrode material used for the capacitor upper electrode  18 B, the material that is superior in the gap-filling capability is preferably employed, so that a platinum film is employed which can fully fill the inside of the third recess  15   c  by a plating technique. Thus, the capacitor upper electrode  18 B is made by fully filling the inside of the third recess  15   c  with a platinum film, which basically prevents breaks due to thermal stress migration occurring intensively in the thinnest portion of the electrode formed in the recess. 
   Moreover, on the top of the capacitor upper electrode  18 B, a conductive layer  19 B of iridium (referred hereinafter to as a low-resistance conductive layer  19 B) is formed which has a lower resistance than the capacitor upper electrode  18 B. Unlike the capacitor upper electrode  18 A of the first embodiment having the fourth recess  15   d , the capacitor upper electrode  18 B in the second embodiment has no recess. Therefore, when the low-resistance conductive layer  19 B is formed by patterning a conductive layer of low resistance (a third conductive film  19   b  that will be described later) formed on the capacitor upper electrode  18 B, the influence of the concave structure of the device is eliminated. Specifically, for the first embodiment, thickness unevenness of a photoresist film deposited in forming the low-resistance conductive layer  19 A arises around the fourth recess  15   d , which degrades the accuracy of dimension of the photoresist pattern. On the other hand, for the second embodiment, since no recess is formed in the capacitor upper electrode  18 B as mentioned above, the low-resistance conductive layer  19 B can be formed using a desired mask without considering degradation in the accuracy of dimension of the photoresist pattern. 
   As described above, with the semiconductor device according to the second embodiment of the present invention, the capacitor upper electrode  18 B is fully embedded within the third recess  15   c , which basically prevents the occurrence of breaks during a thermal treatment. Moreover, even though a thermal treatment at 800° C. is performed for crystallization of the capacitor insulating film  17 , such a structure can certainly prevent the occurrence of breaks in the capacitor upper electrode  18 B. Furthermore, the low-resistance conductive layer  19 B is formed on the top of the capacitor upper electrode  18 B. Thus, even though the material of high resistance is chosen as an electrode material forming the capacitor upper electrode  18 B in consideration of the gap-filling capability of that electrode material like the first embodiment, a highly-integrated, quickly operable semiconductor device as a dielectric memory device can be provided in which no interconnect delay is caused at the time of driving the cell plate. 
   Hereinafter, a method for fabricating a semiconductor device according to the second embodiment of the present invention will be described with reference to  FIGS. 5A to 5D  and  6 A to  6 C. 
   First, the steps shown in  FIGS. 5A to 5C  are identical to the steps previously described using  FIGS. 2A to 2C . 
   Next, as shown in  FIG. 5D , using a plating technique or the like, a second conductive film  18   b  of a platinum film having a thickness of 50 to 300 nm is formed on the SBT film  17   a  to fill the third recess  15   c.    
   As shown in  FIG. 6A , etch back or CMP is conducted to reduce the thickness of the second conductive film  18   b  to a desired value. 
   As shown in  FIG. 6B , the third conductive film  19   b  of iridium is formed on the second conductive film  18   b.    
   As shown in  FIG. 6C , the third conductive film  19   b , the second conductive film  18   b , and the SBT film  17   a  are patterned using a desired mask to form the low-resistance conductive layer  19 B of the third conductive film  19   b , the capacitor upper electrode  18 B of the second conductive film  18   b , and the capacitor insulating film  17  of the SBT film  17   a.    
   In the second embodiment, description has been made of the case where formation of the capacitor insulating film  17 , the capacitor upper electrode  18 B, and the low-resistance conductive layer  19 B is conducted by patterning with the same mask. Alternatively, in consideration of the adhesion to a film as an underlying layer, the adhesion to a film as an overlying layer, unwanted residues created in the processing, and the like, the same mask does not necessarily have to be used for this formation. 
   Although, like the first embodiment, the capacitor upper electrode  18 B is formed above each storage node contact  12 , it is also acceptable to form it to be shared among the storage node contacts  12 . Although the oxygen barrier film  13  is formed on the storage node contact  12 , the oxygen barrier film  13  does not necessarily have to be formed depending on the temperature (for example, a low temperature) or the atmosphere (for example, a nitrogen atmosphere) in crystallization of the dielectric film made of, other than the SBT-based material mentioned above, a PZT-, BLT-, or BST-based metal oxide or the like. 
   As described above, with the method for fabricating a semiconductor device according to the second embodiment of the present invention, the low-resistance conductive layer  19 B is formed on the top of the capacitor upper electrode  18 B like the first embodiment. Therefore, a semiconductor device as a dielectric memory device can be provided which can carry out quick operation with no interconnect delay in the cell plate. Moreover, the capacitor upper electrode  18 B is buried to fill the inside of the third recess  15   c . This avoids the situation of local concentration of stress to reduce the influence of thermal stress migration, which basically prevents the occurrence of breaks in the capacitor upper electrode  18 B. Accordingly, a capacitor element with the cell structure capable of preventing breaks in the capacitor upper electrode  18 B can be realized without depending on the material for the capacitor insulating film  17  and the material for the capacitor upper electrode  18 B, so that a semiconductor device capable of being highly integrated can be provided. 
   Third Embodiment 
   The structure of a semiconductor device according to a third embodiment of the present invention will be described below with reference to  FIGS. 7A and 7B . 
     FIG. 7A  shows the structure of the semiconductor device according to the third embodiment of the present invention, and is a sectional view taken along the line VIIa-VIIa in  FIG. 7B .  FIG. 7B  is a plan view showing the structure of the semiconductor device according to the third embodiment of the present invention. 
   A point of difference between the structure of the semiconductor device according to the third embodiment shown in  FIGS. 7A and 7B  and the structure of the semiconductor device according to the first embodiment shown in  FIGS. 1A and 1B  is that as shown in  FIGS. 7A and 7B , a capacitor upper electrode  18 C and a low-resistance conductive layer  19 C of the third embodiment have almost the same shapes. 
   On the top of the capacitor insulating film  17  and the wall and bottom of the third recess  15   c , the capacitor upper electrode  18 C is formed which is made of iridium oxide. On the top of the capacitor upper electrode  18 C and the wall and bottom of the fourth recess  15   d , the low-resistance conductive layer  19 C of a platinum film is formed which has a lower resistance than the capacitor upper electrode  18 C. 
   In the structure of the semiconductor device according to the third embodiment of the present invention described above, the capacitor insulating film  17 , the capacitor upper electrode  18 C, and the low-resistance conductive layer  19 C are formed by patterning using the same mask, that is, formed to be shared among the storage node contacts  12 . 
   Thus, patterning using the same mask eliminates the necessity to consider the alignment margin of masks unlike the case where different masks are used for patterning. Therefore, a further miniaturization of a cell can be attained. Furthermore, since the number of times masks are used decreases, the structure of the third embodiment is superior in mass productivity. 
   As shown above, with the semiconductor device according to the third embodiment of the present invention, the low-resistance conductive layer  19 C is formed on the top of the capacitor upper electrode  18 C. Thus, even though the material of high resistance is chosen as the material of the capacitor upper electrode  18 C for the reason of prevention of breaks during a thermal treatment or the like as in the cases of the first and second embodiments, a highly-integrated, quickly operable semiconductor device as a dielectric memory device can be provided in which no interconnect delay is caused at the time of driving the cell plate. Moreover, since the capacitor insulating film  17 , the capacitor upper electrode  18 C, and the low-resistance conductive layer  19 C have almost the same plan shapes, a semiconductor device that excels in miniaturization and mass productivity can be provided. 
   Fourth Embodiment 
   Hereinafter, the structure of a semiconductor device according to a fourth embodiment of the present invention will be described with reference to  FIGS. 8A and 8B . 
     FIG. 8A  shows the structure of the semiconductor device according to the fourth embodiment of the present invention, and is a sectional view taken along the line VIIIa-VIIIa in  FIG. 8B .  FIG. 8B  is a plan view showing the structure of the semiconductor device according to the fourth embodiment of the present invention. 
   A point of difference between the structure of the semiconductor device according to the fourth embodiment shown in  FIGS. 8A and 8B  and the structure of the semiconductor device according to the first embodiment shown in  FIGS. 1A and 1B  is that as shown in  FIGS. 8A and 8B , a low-resistance conductive layer  19 D in the fourth embodiment is formed outside the fourth recess  15   d  of the capacitor upper electrode  18 A. 
   To be more specific, on the top of the capacitor insulating film  17  and the wall and bottom of the third recess  15   c , the capacitor upper electrode  18 A of iridium oxide is formed which has the fourth recess  15   d . On the top of the second interlayer insulating film  14  and part of the top flat portion of the capacitor upper electrode  18 A (with the inside of the fourth recess  15   d  not covered therewith), a conductive layer  19 D of a platinum film (referred hereinafter to as a low-resistance conductive layer  19 D) is formed which has a lower resistance than the capacitor upper electrode  18 A. Note that it is also acceptable that the capacitor upper electrode is formed to fill the third recess  15   c  like the second embodiment. 
   Such a structure eliminates the influence of the fourth recess  15   d  that has the shape of a step. That is to say, such a structure eliminates consideration of the thickness uniformity or the like of a photoresist film deposited around the fourth recess  15   d  in forming the low-resistance conductive layer  19 D, so that formation of the low-resistance conductive layer  19 D using a desired mask is enabled. 
   Moreover, since the low-resistance conductive layer  19 D is formed on the top flat portion of the second interlayer insulating film  14 , a cell plate with a stable resistance can be formed with no influence of level difference of the underlying layer. 
   As described above, with the semiconductor device according to the fourth embodiment of the present invention, the low-resistance conductive layer  19 D is formed on part of the top portion of the capacitor upper electrode  18 A. Thus, even though the material of high resistance is chosen as the material for the capacitor upper electrode  18 A for the reason of prevention of breaks during a thermal treatment or the like as in the cases of the first to third embodiments, a highly-integrated, quickly operable semiconductor device as a dielectric memory device can be provided in which no interconnect delay is caused at the time of driving the cell plate. 
   Herein,  FIGS. 9A to 9F  show views of respective plan arrangements of the capacitor upper electrodes  18  ( 18 A,  18 B, and  18 C) and the low-resistance conductive layers  19  ( 19 A to  19 D) of the semiconductor devices according to the first to fourth embodiments described above. In particular, these figures show possible arrangement variations in the case where a plurality of capacitor upper electrodes  18  are provided. 
     FIG. 9A  shows the structure in which a plurality of capacitor upper electrodes  18  are formed above the storage node contacts  12 , respectively, and the low-resistance conductive layer  19  is formed to entirely cover the plurality of capacitor upper electrodes  18 . The structure shown in  FIG. 9A  is suitable for setting of all or some of the cell plates at the same potential. Note that this structure is applicable to the first and second embodiments. 
     FIG. 9B  shows the structure in which a plurality of capacitor upper electrodes  18  are formed above the storage node contacts  12 , respectively, and the low-resistance conductive layer  19  is formed to cover each row of the plurality of capacitor upper electrodes  18 . The structure shown in  FIG. 9B  is suitable for setting of the cell plates aligned in the row direction at the same potential. Note that this structure is applicable to the first and second embodiments. 
     FIG. 9C  shows the structure in which the capacitor upper electrode  18  and the low-resistance conductive layer  19  are formed to be shared among a plurality of storage node contacts  12  and to have the same shape. The structure shown in  FIG. 9C  is suitable for the case where the number of times masks are used is reduced and the cell plates aligned in the row direction are set at the same potential. Note that this structure corresponds to the structure of the third embodiment. 
   The structures shown in  FIGS. 9D to 9F  show variations of the fourth embodiment.  FIG. 9D  shows the structure in which the low-resistance conductive layer  19  is formed on the capacitor upper electrodes  18  and outside the fourth recesses  15   d . This structure is suitable for setting of the cell plates aligned in the row direction at the same potential.  FIG. 9E  shows the structure in which the low-resistance conductive layer  19  is formed outside the fourth recesses  15   d  of the capacitor upper electrodes  18  to connect adjacent rows of the capacitor upper electrodes  18  for shared use. Thus, this structure is suitable for setting the cell plates aligned in the row direction at the same potential. Although this structure is identical to that in  FIG. 9A  in terms of potential, this structure can be constructed only by providing the low-resistance conductive layer between the adjacent rows to miniaturize the cell.  FIG. 9F  shows the structure in which the area of the low-resistance conductive layer  19  in contact with each capacitor upper electrode  18  is increased as compared with the structure in  FIG. 9D . This structure can reduce the contact resistance to prevent interconnect delay more effectively. 
   Accordingly, the semiconductor device and its fabrication method of the present invention can prevent interconnect delay of the cell plate by forming the low-resistance conductive layer on the capacitor upper electrode. Therefore, the semiconductor device and its fabrication method are useful for ferroelectric memory devices or high dielectric memory devices which have three-dimensionally stacked structures and require quick operations, and for fabrication methods of such devices.