Patent Publication Number: US-2007096189-A1

Title: Semiconductor device

Description:
This application is a divisional application of U.S. Application Ser. No. 10/479,703, filed Dec. 5, 2003, which is a National Stage of International Application No. PCT/JP02/05478, filed Jun. 4, 2002, the entire contents of which are incorporated herein by reference 
    
    
     TECHNICAL FIELD  
      The present invention relates to a semiconductor device.  
     BACKGROUND ART  
      Recently, the area of a memory capacitor has been reduced and the absolute value of capacitance has been also reduced in accordance with miniaturization of a semiconductor device. The capacitance C, for example in the case of a parallel-plate capacitor structure, is determined by C=ε·S/d, wherein ε means the dielectric constant of a capacitor insulating film, S means the area of an electrode, and d means the film thickness (distance between electrodes) of a dielectric. In order to assure a capacitance value without increasing the area S of an electrode used for a capacitor element for information accumulation, it is necessary to use a capacitor insulating film material having a high dielectric constant ε or to decrease the film thickness d of a capacitor insulating film.  
      Heretofore, a silicon oxide film has been used as a capacitor insulating film, and high integration has been carried out by decreasing the thickness of this film. However, in a highly integrated memory of not less than 256 megabits, reduction of the film thickness has been put to the limit, and, therefore, there has been introduced a capacitor insulating film material such as tantalum oxide that has a higher dielectric constant ε than silicon oxide. Furthermore, in a DRAM (Dynamic Random Access Memory) of not less than 1 Gbit, there has been considered use of a high dielectric constant material of barium strontium titanate (Ba x Sr y Ti s O t :BST) as disclosed in, for example, JP-A-9-186299. The similar problem is applicable to not only a highly integrated memory but, also, with regard to a condenser used for various electronic circuits requiring miniaturization. For example, as disclosed in JP-A-10-41467, there has been considered use of titanium oxide having a high dielectric constant as a capacitor insulating film material for a condenser.  
     SUMMARY OF THE INVENTION  
      However, when titanium oxide was used as a capacitor insulating film and platinum was used as an electrode material as disclosed in JP-A-10-41467, the dielectric constant was found not to be stable in some cases. In consideration of various contributing factors including such cases, there is desired a semiconductor device having high reliability.  
      Thus, an objective of the present invention resides in providing a semiconductor device having high reliability.  
      In order to resolve the above problem, semiconductor devices according to the present invention include a construction in accordance with the following discussion.  
      Specific examples are described below.  
      In the first place, by providing a thin film capacitor having high reliability, there can be provided a system-in-package having high reliability. Thereby, there can be provided a semiconductor device having high reliability.  
      That is, said problem can be resolved by a thin film capacitor, a system-in-package, and a semiconductor device having the following constitutions.  
      (1) A thin film capacitor comprising a first capacitor electrode, a capacitor insulating film formed in contact with the first capacitor electrode, and a second capacitor electrode formed in contact with the capacitor insulating film, wherein said capacitor insulating film is comprised of mainly titanium oxide, and said first capacitor electrode and said second capacitor electrode use conductive oxide films comprising mainly ruthenium oxide or iridium oxide.  
      In this connection, it is desirable that said capacitor insulating film and said conductive oxide film have a film thickness of not less than 0.9 nm and that said titanium oxide consists of crystals of rutile structure.  
      (2) A system-in-package comprising a substrate and a circuit in which a LSI (Large Scale Integration Device), a condenser and a resistance are connected by wiring divided with an insulating layer on one main surface side of said substrate, wherein said condenser comprises a first capacitor electrode, a capacitor insulating film formed in contact with the first capacitor electrode, and a second capacitor electrode formed in contact with the capacitor insulating film, and wherein said capacitor insulating film comprises mainly titanium oxide and said first capacitor electrode and second capacitor electrode use conductive oxide films comprising mainly ruthenium oxide or iridium oxide.  
      In this connection, it is desirable that said capacitor insulating film and said conductive oxide film have a film thickness of not less than 0.9 nm and that said titanium oxide consists of crystals of rutile structure.  
      (3) A semiconductor device comprising a semiconductor substrate, a first capacitor electrode formed on one main surface side of said semiconductor substrate, a capacitor insulating film formed in contact with the first capacitor electrode, and a second capacitor electrode formed in contact with the capacitor insulating film, wherein said capacitor insulating film comprises mainly titanium oxide, and said first capacitor electrode and said second capacitor electrode use conductive oxide films comprising mainly ruthenium oxide or iridium oxide.  
      In this connection, it is desirable that said capacitor insulating film and said conductive oxide film have a film thickness of not less than 0.9 nm and that said titanium oxide consists of crystals of rutile structure. Herein, “comprise mainly” (or “consist mainly of”) means “contain at least 70 at. %”.  
      Secondly, by providing a semiconductor device having improved gate electrode structure such as a gate insulating film that can suppress leakage current effectively, there can be provided a semiconductor device having high reliability.  
      Recently, in a semiconductor device provided with plural MOS (Metal Oxide Semiconductor) transistors each of which has a gate insulating film present between a semiconductor substrate and a gate electrode, reduction in thickness of the gate insulating film has been required and an oxide film of not more than 3.0 nm in thickness has been used in accordance with miniaturization of a semiconductor device. When thickness of the insulating film is reduced to 3.0 nm or less, there arises a problem in which direct tunnel current (hereinafter referred to as DT current) becomes too large to be ignored, leakage current increases, and electric power consumption increases. Thus, by using as a gate insulating film titanium oxide or the like, having a higher dielectric constant than that of silicon oxide, which leads to improvement in the dielectric characteristics, and, at the same time, increase the thickness of the gate insulating film, a tendency for increase in DT current can be suppressed. For example, when relative dielectric constants of titanium oxide and silicon oxide are respectively 60 and 4.0, it follows that a titanium oxide film of 30 nm in thickness has dielectric characteristics equivalent to those of a silicon oxide film of 2 nm in thickness. In such case, the titanium oxide film of 30 nm in thickness is said to have a silicon-oxide equivalent thickness of 2 nm. On the other hand, the factual thickness of 30 nm is called a physical film thickness or factual film thickness.  
      On the other hand, when a titanium oxide film is formed on a silicon substrate, in some cases oxygen atoms in the titanium oxide film diffuse to the silicon substrate side to form silicon oxide at the interface between the titanium oxide film and the silicon substrate. The formation of silicon oxide increases the equivalent thickness of a gate insulating film. For example, when silicon oxide is formed at said interface in a thickness of not less than 1 nm, it becomes impossible to have the equivalent thickness of a gate insulating film in the range of not more than 1 nm.  
      Thus, in order to prevent formation of silicon oxide at said interface, there is contrived a method of forming a silicon nitride film between a titanium oxide film and a silicon substrate (for example, see JP-A-2000-58831). Formation of silicon oxide at said interface can be suppressed by forming a silicon nitride film between a titanium oxide film and a silicon substrate. However, silicon nitride has only a relative dielectric constant of about 7.8, and when a gate insulating film is allowed to have a silicon-oxide equivalent thickness of not more than 1 nm, the factual thickness thereof is decreased and leakage current by direct tunnel is increased. Hence, there is a possibility that leakage current would surpass the acceptable value.  
      The possibility that leakage current is increased and would surpass the acceptable value reduces yield of products and causes reduction in reliability of the products.  
      Thus, in order to supply a semiconductor device having high reliability, there is produced a semiconductor device provided with plural MOS transistors each of which has a gate insulating film constituted so as to contain a titanium oxide film, wherein formation of silicon oxide is suppressed at the interface between the titanium oxide film and a silicon substrate, and wherein the gate insulating film is allowed to have a silicon-oxide equivalent thickness of not more than 1 nm.  
      Alternatively, there is produced a semiconductor device wherein leakage current flowing through the gate insulating film can be suppressed to a low extent.  
      Furthermore, there is produced a semiconductor device having a high yield.  
      The inventors carried out experiments and calculations with various materials and, as a result, have found that when a gate insulating film is constituted by forming a titanium silicate film on the surface of a silicon substrate and forming thereon a titanium oxide film, diffusion of oxygen atoms from the titanium oxide film to the silicon substrate can be prevented, and leakage current can be reduced effectively because the relative dielectric constant of titanium silicate is larger than that of silicon nitride.  
      (4) The present invention resolving the above problems provides for a semiconductor device comprising plural MOS transistors each of which has a gate insulating film disposed between a semiconductor substrate and a gate electrode, characterized by the fact that said gate insulating film has a laminated structure containing a titanium silicate film formed on the semiconductor substrate side and a titanium oxide film formed on the gate electrode side.  
      In this case, it is desirable that the silicon-oxide equivalent thickness of said gate insulating film which is obtained from dielectric characteristics, is not more than 1.0 nm, and that said titanium silicate film has a factual thickness of not less than 1.0 nm but not more than 3.2 nm.  
      Actually, it is desirable that a factual thickness, T 2 , of said titanium silicate film is formed to fall within a range represented by 
 
1.0 (nm)≦ T   2 ≦5 T   eff −1.8 (nm) 
 
 wherein T 2  represents the factual thickness of said titanium silicate film and T eff  represents the silicon-oxide equivalent thickness of said gate insulating film. 
 
      (5) Furthermore, in preparing a semiconductor device comprising plural MOS transistors each of which has a gate insulating film present between a semiconductor substrate and a gate electrode, the gate insulating film is formed by a step including a procedure of forming a titanium silicate film on the semiconductor substrate and a procedure of forming a titanium oxide film on the titanium silicate film.  
      The titanium silicate film can be formed by either a method of forming a titanium film on the surface of a silicon substrate, heat-treating the titanium film into a titanium silicide film, and oxidizing the titanium silicide film into the titanium silicate film, or a method of forming a silicon oxide film on the surface of the silicon substrate, forming a titanium film superposed on the silicon oxide film, and reacting the two by heat treatment to form the titanium silicate film.  
      The semiconductor device of the present invention has a titanium silicate film at the interface between titanium oxide and a silicon substrate, and hence formation of a silicon oxide film having a low relative dielectric constant at said interface can be suppressed. Therefore, the silicon-oxide equivalent thickness of a gate insulating film can be reduced.  
      Furthermore, the present semiconductor device has, as the gate insulating film, titanium oxide which is a high dielectric constant material, and the titanium silicate film having a relatively large dielectric constant. Hence, the factual thickness of the gate insulating film can be increased and the silicon-oxide equivalent thickness thereof can be reduced. Therefore, leakage current can be reduced.  
      Moreover, because a semiconductor device wherein leakage current is difficult to flow can be obtained, there can be produced a semiconductor device having high reliability and, also, having a high yield.  
      Other objects, characteristics and advantages of the present invention will be clear from the following description of the working embodiments of the present invention relating to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is the sectional view of the main portion of a semiconductor device according to a first example of the present invention.  
       FIG. 2  shows diffusion constants at 300° C. of oxygen diffusing from a titanium oxide film of rutile structure having a thickness of 3 nm to capacitor electrodes of 3 nm in thickness with regard to the present invention.  
       FIG. 3  shows diffusion constants at 600° C. of oxygen diffusing from a titanium oxide film of rutile structure having a thickness of 3 nm to capacitor electrodes of 3 nm in thickness with regard to the present invention.  
       FIG. 4  shows diffusion constants at 300° C. of oxygen diffusing from a titanium oxide film of anatase structure having a thickness of 3 nm to capacitor electrodes of 3 nm in thickness with regard to the present invention.  
       FIG. 5  shows diffusion constants at 600° C. of oxygen diffusing from a titanium oxide film of anatase structure having a thickness of 3 nm to capacitor electrodes of 3 nm in thickness with regard to the present invention.  
       FIG. 6  shows diffusion constants at 300° C. of oxygen diffusing from a titanium oxide film of rutile structure having a thickness of 0.9 nm to capacitor electrodes of 0.9 nm in thickness with regard to the present invention.  
       FIG. 7  shows diffusion constants at 300° C. of oxygen diffusing from a titanium oxide film of anatase structure having a thickness of 0.9 nm to capacitor electrodes of 0.9 nm in thickness with regard to the present invention.  
       FIG. 8  shows diffusion constants at 300° C. of oxygen diffusing from a titanium oxide film of rutile structure having a thickness of 0.9 nm to capacitor electrodes of 0.8 nm in thickness with regard to the present invention.  
       FIG. 9  shows diffusion constants at 300° C. of oxygen diffusing from a titanium oxide film of rutile structure having a thickness of 0.8 nm to capacitor electrodes of 0.9 nm in thickness with regard to the present invention.  
       FIG. 10  is the sectional view of the main portion of a semiconductor device according to a second example of the present invention.  
       FIG. 11  is the sectional view of the main portion of a semiconductor device according to a third example of the present invention.  
       FIG. 12  is the sectional view of the main portion of a thin film capacitor according to a fourth example of the present invention.  
       FIG. 13  is the sectional view of the main portion of a thin film capacitor according to a fifth example of the present invention.  
       FIG. 14  is the sectional view of the main portion of a thin film capacitor according to a sixth example of the present invention.  
       FIG. 15  is the sectional view of the main portion of a thin film capacitor according to a seventh example of the present invention.  
       FIG. 16  is the sectional view of the main portion of a system-in-package according to an eighth example of the present invention.  
       FIG. 17  shows diffusion constants at 300° C. of oxygen diffusing from a titanium oxide film of rutile structure having a thickness of 30 nm to capacitor electrodes of 3 nm in thickness with regard to the present invention.  
       FIG. 18  shows diffusion constants at 300° C. of oxygen diffusing from a titanium oxide film of rutile structure having a thickness of 35 nm to capacitor electrodes of 3 nm in thickness with regard to the present invention.  
       FIG. 19  is the sectional view of the main portion of a semiconductor device according to a ninth example of the present invention.  
       FIG. 20  is the plan view showing the main portion of the semiconductor device of the example shown in  FIG. 19 .  
       FIG. 21  is the conceptual diagram showing energy bands of the gate electrode, titanium oxide, titanium silicate and silicon substrate in the example shown in  FIG. 19 .  
       FIG. 22  is the conceptual diagram showing energy bands of the gate electrode, titanium oxide, titanium silicate and silicon substrate when the electric voltage, V, was applied to the gate electrode in the example shown in  FIG. 19 .  
       FIG. 23  is the graph showing dependency of leakage current density on the thickness and equivalent thickness of titanium silicate film when the relative dielectric constant of titanium silicate is 15, the silicon-oxide equivalent thickness of the gate insulating film is 1.0 nm, and the electric voltage applied to the gate insulating film is 1.0 V in the ninth example of the present invention.  
       FIG. 24  is the graph showing dependency of leakage current density on the thickness and equivalent thickness of titanium silicate film when the relative dielectric constant of titanium silicate is 20, the silicon-oxide equivalent thickness of the gate insulating film is 1.0 nm, and the electric voltage applied to the gate insulating film is 1.0 V in the ninth example of the present invention.  
       FIG. 25  is the graph showing dependency of leakage current density on the thickness and equivalent thickness of titanium silicate film when the relative dielectric constant of titanium silicate is 25, the silicon-oxide equivalent thickness of the gate insulating film is 1.0 nm, and the electric voltage applied to the gate insulating film is 1.0 V in the ninth example of the present invention.  
       FIG. 26  is the graph showing dependency of leakage current density on the thickness and equivalent thickness of titanium silicate film when the relative dielectric constant of titanium silicate is 30, the silicon-oxide equivalent thickness of the gate insulating film is 1.0 nm, and the electric voltage applied to the gate insulating film is 1.0 V in the ninth example of the present invention.  
       FIG. 27  is the graph showing dependency of leakage current density on the thickness and equivalent thickness of titanium silicate film when the relative dielectric constant of titanium silicate is 15, the silicon-oxide equivalent thickness of the gate insulating film is 1.0 nm, and the electric voltage applied to the gate insulating film is 0.5, 0.7, and 1.0 V in the ninth example of the present invention.  
       FIG. 28  is the graph showing dependency of leakage current density on the thickness and equivalent thickness of titanium silicate film when the relative dielectric constant of titanium silicate is 15, the silicon-oxide equivalent thickness of the gate insulating film is 0.7 nm, and the electric voltage applied to the gate insulating film is 0.5, 0.7, and 1.0 V in the ninth example of the present invention.  
       FIG. 29  is the graph showing the desirable range of factual film thickness of titanium silicate when the silicon-oxide equivalent thickness of the gate insulating film is 0.7 to 1.0 nm in the ninth example of the present invention.  
       FIG. 30  is the sectional view showing an example wherein the gate electrode has a two layer structure of tungsten nitride film and tungsten film in the example shown in  FIG. 19 .  
      FIGS.  31 (A)- 31 (C) show sectional views for illustrating a phase in the process of producing the main portion of the semiconductor device shown in  FIG. 19 .  
      FIGS.  32 (A)- 32 (C) show sectional views for illustrating a phase in the process of producing the main portion of the semiconductor device shown in  FIG. 19 , which follow FIGS.  31 (A)- 31 (C).  
      FIGS.  33 (A)- 33 (C) show sectional views for illustrating a phase in the process of producing the main portion of the semiconductor device shown in  FIG. 19 , which follow that of FIGS.  32 (A)- 32 (C).  
      FIGS.  34 (A)- 34 (D) show sectional views for illustrating an alternative early phase in the process of producing the main portion of the semiconductor device shown in  FIG. 19 .  
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
      Hereinafter, the mode for carrying out the present invention will be described in detail. First, the sectional structure of the main portion of a DRAM (Dynamic Random Access Memory) memory cell which is the first example in the present invention, is shown in  FIG. 1 . As shown in  FIG. 1 , the semiconductor device of the present example is provided with MOS (Metal Oxide Semiconductor) type transistors  2  formed on a silicon substrate  1  which is a semiconductor, and a memory capacitor  3  disposed above the transistors. An insulating film  12  is a film for separating elements. In this connection, structures of the present figure and the other figures are schematically shown in order to help in the understanding of the present examples including the circuits thereof and which are mere examples.  
      The MOS transistor  2  in the memory cell is constituted by a gate electrode  5 , a gate insulating film  6  and diffusion layers  7 . The gate insulating film  6  consists of, for example, silicon oxide film, silicon nitride film or a high dielectric constant film or a laminated structure thereof. The gate electrode  5  consists of, for example, polycrystalline silicon film or metal thin film, conductive oxide film or metal silicide film or a laminated structure thereof. On the top and side walls of said gate electrode  5 , there is formed an insulating film  9  consisting of, for example, silicon oxide film. The one diffusion layer  7  of the MOS transistor for memory cell selection is connected to a bit-line  11  through a plug  10 . In the whole portion above the  
      MOS transistors, there is formed an insulating film  12  consisting of, for example, BPSG (Boron-doped Phospho Silicate Glass) film or SOG (Spin On Glass) film, or silicon oxide or nitride film formed by chemical vapor phase deposition method or sputtering method, or the like.  
      A memory capacitor  3  is formed on the insulating film  12  covering the MOS transistors. The memory capacitor  3  is connected to the other diffusion layer  8  of the MOS transistor for memory cell selection through a plug  13  consisting of, for example, polycrystalline silicon or tungsten or the like. The memory capacitor  3  is constituted by a laminated structure of a conductive barrier film  14 , a capacitor lower electrode  15 , a capacitor insulating film  16  consisting mainly of titanium oxide, and a capacitor upper electrode  17  in the order from the lowermost layer. This memory capacitor  3  is covered by an insulating film  18 . The conductive barrier film  14  consists of, for example, titanium, titanium nitride, tantalum, tantalum nitride or the like. In this connection, the conductive barrier film  14  may be absent, for example, in the case where adhesion between the capacitor lower electrode  15  and the plug  13  is good and, furthermore, their counter diffusion scarcely takes place.  
      The inventors have found that when as a material for the capacitor lower electrode  15  and the capacitor upper electrode  17  there is used polycrystalline silicon, tungsten, tungsten silicide, molybdenum, molybdenum silicide, ruthenium, iridium, platinum or the like, oxygen diffuses to the capacitor electrodes from the capacitor insulating film  16  comprising mainly titanium oxide, and oxygen deficit is caused in the capacitor insulating film  16 , and, furthermore, the inventors have found that the dielectric constant is not stable because of this oxygen deficit. Moreover, the inventors carried out intense research in order to obtain a means for suppressing diffusion of oxygen to a capacitor electrode from a capacitor insulating film comprising mainly titanium oxide, and, as a result, they have found that it is effective to use ruthenium oxide or iridium oxide as a capacitor electrode material which contacts with titanium oxide. Thus, in the present example, a conductive oxide film comprising mainly ruthenium oxide or iridium oxide is used for the capacitor lower electrode  15  and the capacitor upper electrode  17  so that oxygen hardly diffuses to the electrodes from the capacitor insulating film  16  comprising mainly titanium oxide. This conductive oxide film is formed by use of, for example, chemical vapor phase deposition method, sputtering method or the like.  
      With regard to diffusion of oxygen from titanium oxide to electrodes, by comparing ruthenium oxide and iridium oxide used in the present example with polycrystalline silicon, tungsten, tungsten silicide, molybdenum, molybdenum silicide, ruthenium, iridium, and platinum, which have been considered as a capacitor electrode material, the effect of the present example is illustrated as follows.  
      In order to explain the effect of the present example in detail, there is shown an analytical example based on molecular dynamics simulation. The molecular dynamics simulation is a method of calculating a force acting on each atom through interatomic potential, and calculating position of each atom at each time by resolving Newton&#39;s equation of motion on the basis of said force, as stated in, for example, Journal of Applied Physics, Vol. 54 (issued in 1983), pages 4864-4878. In this connection, in the present example, the below-mentioned relation could be obtained by taking into account charge-transfer in said molecular dynamics method and calculating the interaction between different elements.  
      The main effect of the present example is to suppress diffusion of oxygen from a capacitor insulating film to capacitor electrodes. Diffusion of other elements is also suppressed, but herein the effect of the present example is illustrated by calculating diffusion constants of oxygen, which diffuses to capacitor electrodes, and comparing the calculation results. The method for calculating diffusion constants by the molecular dynamics simulation is stated in, for example, Physical Review B, Vol. 29 (issued in 1984), pages 5363-5371.  
      First, the effect of the present example is shown by use of a calculation example in the case of using a laminated structure of a capacitor electrode having a film thickness of 3 nm and a capacitor insulating film having a thickness of 3 nm. As the capacitor insulating film, there was used a titanium oxide film of rutile structure or anatase structure, and as the capacitor electrode material there were used polycrystalline silicon, tungsten, tungsten silicide, molybdenum, molybdenum silicide, ruthenium, iridium, and platinum, which have been considered as a capacitor electrode, and ruthenium oxide and iridium oxide used in the present example.  
      Calculation results of diffusion constants, when oxygen diffuses to the electrodes from the titanium oxide film of rutile structure at 300° C., are shown in  FIG. 2 . Moreover, diffusion constants at 600° C. are shown in  FIG. 3 .  
      When diffusion constants at 300° C. are not less than 10 −20  m 2 /s, much oxygen deficit is formed in the capacitor insulating film. Hence, in order to ensure the reliability of a semiconductor device such as shown in  FIG. 1 , it is preferable that diffusion constants at 300° C. are less than 10 −20  m 2 /s.  
      From these figures it is seen that when ruthenium oxide or iridium oxide was used as an electrode in the cases of both 300° C. and 600° C., smaller diffusion constants are shown as compared with the other cases. That is, when ruthenium oxide or iridium oxide was used as an electrode, it can be said that oxygen hardly diffuses to the electrode and reliability is high.  FIG. 2  and  FIG. 3  show calculation results in the case of using titanium oxide of rutile structure, but calculation results of diffusion constants in the case of using titanium oxide having anatase structure are as shown in  FIG. 4  and  FIG. 5 .  FIG. 4  and  FIG. 5  show calculation results respectively at 300° C. and 600° C. Also in these figures, similarly to  FIG. 2  and  FIG. 3 , when ruthenium oxide or iridium oxide was used as an electrode, smaller diffusion constants are shown as compared with the other cases. When calculation results of  FIG. 4  and  FIG. 5  are compared with those of  FIG. 2  and  FIG. 3 , it is seen that diffusion constants in the case of using rutile structure are smaller than in the case of anatase structure. Therefore, it is more preferable to use titanium oxide of rutile structure as a capacitor insulating film and use ruthenium oxide or iridium oxide as a capacitor electrode. The titanium oxide of rutile structure is formed by a method of film formation at a high temperature or film formation at a low temperature and the subsequent heat treatment as stated in, for example, IBM Journal of Research and Development, Vol. 43, No. 3 (issued in May 1999), pages 383-391.  
       FIG. 2 ,  FIG. 3 ,  FIG. 4  and  FIG. 5  show calculation results when 3 nm was selected as the thickness of a capacitor electrode film and that of a capacitor insulating film, but in order to study dependency of diffusion constant on film thickness, results obtained by changing these thicknesses are shown hereinafter. When 0.9 nm was selected as both the thickness of a capacitor electrode film and that of a capacitor insulating film, calculation results at 300° C. for rutile structure and anatase structure are shown respectively in  FIG. 6  and  FIG. 7 .  
      From  FIG. 6  and  FIG. 7 , similarly to the case of 3 nm film thickness, even when the film thickness is reduced to 0.9 nm, it is seen that diffusion constants for ruthenium oxide and iridium oxide are remarkably small as compared with those for the other materials. Though not shown in the figures, also, in the case of 600° C., there was obtained the result that diffusion constants for ruthenium oxide and iridium oxide are remarkably small as compared with those for the other materials.  
      On the other hand, when 0.9 nm was retained as the thickness of a capacitor insulating film and  0 . 8  nm was selected as the thickness of a capacitor electrode film, calculation results for rutile structure at 300° C. are shown in  FIG. 8 . In this case, when compared with  FIG. 6  and  FIG. 7 , it is seen that diffusion constants for ruthenium oxide and iridium oxide are considerably greater than 10 −20  m 2 /s , in which the effect of the present example is reduced. Therefore, it is more preferable that the film thickness of ruthenium oxide or iridium oxide is not less than 0.9 nm. Next, when 0.9 nm was retained as the thickness of a capacitor electrode film and 0.8 nm was selected as the thickness of a capacitor insulating film, the calculation results for a rutile structure at 300° C. are shown in  FIG. 9 .  
      Also in this case, when compared with  FIG. 6  and  FIG. 7 , it is seen that diffusion constants for ruthenium oxide and iridium oxide are remarkably large and the effect of the present example is reduced. Therefore, it is more preferable, also, that the film thickness of titanium oxide is 0.9 nm or more.  
      The above phenomenon that diffusion constants become suddenly large when the thickness of an electrode or that of a capacitor insulating film is 0.8 nm or less is considered to occur because of the following reason. The diameter of atoms is approximately 0.1 to 0.3 nm, and 0.8 nm corresponds to the state wherein atoms stand in 4 to 8 lines. In this state, it is presumed that atoms in adjacent films commingle and would lead to a loss of film function.  
       FIG. 8  and  FIG. 9  show results for rutile structure. Similarly, also with regard to anatase structure, there was obtained the result that a film thickness of 0.9 nm or more is more preferable. The effect is reduced in a film thickness of 0.8 nm or less, because crystal structures of ruthenium oxide, iridium oxide and titanium oxide become slightly unstable. In addition, though not shown in  FIG. 2  to  FIG. 7 , when a material of low melting point such as gold or silver is used as an electrode material, diffusion constant of oxygen becomes larger than in the case of using silicon as an electrode. Therefore, it is not preferable either to use gold or silver as an electrode material which contacts with titanium oxide.  
      Next, the sectional structure of the main portion of a DRAM (Dynamic Random Access Memory) memory cell according to the second example of the present invention is shown in  FIG. 10 . The main difference from the first example resides in the point that the capacitor has not a parallel-plate structure but a rectangular structure. The use of a same number in  FIG. 10  as in  FIG. 1  means the same constituent component. This structure has the advantage that the capacitor has a larger effective area and a larger capacity. Also, in the present example, by use of a capacitor electrode consisting mainly of ruthenium oxide or iridium oxide, the suppression effect of oxygen diffusion can be obtained similarly to that in the first example. In addition, the capacitor may have a structure other than these structures.  
      Next, the sectional structure of the main portion of a DRAM (Dynamic Random Access Memory) memory cell according to the third example of the present invention is shown in  FIG. 11 . The main difference from the second example resides in the point that the capacitor electrode has a double structure. That is, the capacitor lower electrode is constituted by a conductive film  15  and a conductive film  19 , and the capacitor upper electrode is constituted by a conductive film  17  and a conductive film  20 . The use of a same number in  FIG. 11  as in  FIG. 10  means the same constituent component. In the case of  FIG. 11 , conductive films  15  and  17 , which are in direct contact with the capacitor insulating film  16 , consist of a film consisting mainly of ruthenium oxide or iridium oxide in order to suppress diffusion of oxygen from the capacitor insulating film. Electrode films  19  and  20  which do not directly contact with the capacitor insulating film, preferably consist of a material having a lower electric resistance than that of ruthenium oxide or iridium oxide such as ruthenium, iridium, platinum, osmium, rhodium, palladium, tungsten, molybdenum, gold, silver, or the alloys or silicide compounds thereof, or the like. The similar matter is applicable also to the electrodes of the following examples. In addition, an electrode structure is not limited to the structures shown herein, and other layers may be furthermore contained therein. Moreover, only the upper electrode may be formed by plural layers and the lower layer may comprise a single layer. On the contrary, only the lower electrode may be formed by plural layers and the upper layer may comprise a single layer. The similar matter is applicable also to the electrodes of the following examples.  
      The above examples relate to DRAM (Dynamic Random Access Memory), and for products having a thin film capacitor containing a capacitor insulating film comprising mainly titanium oxide there can be used electrodes comprising mainly ruthenium oxide or iridium oxide.  
      Next, a thin film capacitor according to the fourth example of the present invention is described by use of  FIG. 12 . In the present example, the thin film capacitor  102  is formed on a substrate  101  consisting of, for example, a semiconductor material, a resin, a glass or the like. This thin film capacitor  102  consists of a conductive barrier film  103 , a capacitor lower electrode  104 , a capacitor insulating film  105  comprising mainly titanium oxide, and a capacitor upper electrode  106  in the order from the lowermost layer. The conductive barrier film  103  consists of titanium, titanium nitride, tantalum, tantalum nitride or the like. As the main constituent material of the capacitor lower electrode  104  and the capacitor upper electrode  106 , there is used ruthenium oxide or iridium oxide to which oxygen hardly diffuses from the capacitor insulating film  105 . The thin film capacitor  102  is connected to a first layer wire  107  through a plug  108  consisting of, for example, copper, tungsten or the like.  
      A second layer wire  109  is connected to the first layer wire  107  through a plug  110 . In addition, though not shown in the figure, the second layer wire  109  and the capacitor upper electrode  106  are connected to another wire. Barrier films  111 ,  112 ,  113 ,  114 ,  115 ,  116 ,  117 , and  118  are adjacent to the first layer wire  107 , second layer wire  109 , plug  108  and plug  110 . When the first layer wire  107 , second layer wire  109 , and plug  108  have excellent adhesion with an insulating film  119  and, furthermore, hardly cause interdiffusion with the insulating film  119 , these barrier films may be absent.  120  and  121  are insulating films. The thin film capacitor  102  is used, for example, as a filter for flowing only alternating electric current of a specific range of frequency to the second layer wire.  
      Subsequently, a thin film capacitor according to the fifth example in the present invention is described by use of  FIG. 13 . The main difference between the present example and the fourth example resides in the point that, in the latter, the capacitor insulating film comprising mainly titanium oxide is used as an insulating film between the first layer wire and second layer wire, and the structure of the present example is simple. In addition, the use of a same number in  FIG. 13  as in  FIG. 12  means the same constituent component. According to the present example, the number of film formation steps can be reduced.  
      Furthermore, the layout of a thin film capacitor is not limited to that as above-described. For example, even the layout as shown in  FIG. 14  is usable. The main difference between the sixth example and the fourth example resides in the point that, in the latter, the thin film capacitor  102  is positioned at a layer between the second layer wire  109  and a third layer wire  125 . The use of a same number in  FIG. 14  as in  FIG. 12  means the same constituent components. In  FIG. 14 , further, reference numerals  123 ,  124 ,  126  and  127  are barrier films,  122  is a plug for interconnecting wires  109  and  125 , and reference numerals  128  and  129  are insulating films.  
      In addition, as a simpler example, there is cited the layout as shown in  FIG. 15 . The seventh example has the structure wherein a thin film capacitor  102  is formed on one main surface of a substrate  101  comprising, for example, silicon or the like. The thin film capacitor  102  comprises a conductive barrier film  103 , a capacitor lower electrode  104 , a capacitor insulating film  105  comprising mainly titanium oxide, and a capacitor upper electrode  106  in the order from the lowermost layer. The conductive barrier film  103  comprises, for example, titanium nitride, titanium or the like. In addition, though not shown in the figure, there may be present a single or plural other films between the barrier film  103  and the substrate  101 .  
      Subsequently, the main sectional view of a system-in-package according to the eighth example of the present invention is shown in  FIG. 16 . The system-in-package is the one wherein a LSI (Large Scale Integrated Circuit) and passive elements are integrated on a substrate, as stated in, for example, Nikkei Micro-device, March 2001, pages 114-123. In the present example, a LSI  218 , a thin film capacitor  202 , and a resistance  226  are formed on a substrate  201  consisting of, for example, a resin, an organic material, a glass, silicon or the like. The thin film capacitor  202  consists of a capacitor lower electrode  203 , a capacitor insulating film  204  comprising mainly titanium oxide, and a capacitor upper electrode  205  in the order from the lowermost layer. As the main constituent material of the capacitor lower electrode  203  and the capacitor upper electrode  205 , there is used ruthenium oxide or iridium oxide to which oxygen hardly diffuses from the capacitor insulating film  204 . The capacitor lower electrode  203  of this thin film capacitor  202  is connected to a wire  206  interposed between barrier films  207  and  208 . In this connection, when the wire  206  is comprised mainly of copper, it is preferable in the point of preventing the wire from peeling to use ruthenium or iridium as the main constituent material of the barrier film  208 . This is because the adhesion between copper and ruthenium and that between copper and iridium are excellent. Furthermore, ruthenium and iridium have good adhesion, also, with ruthenium oxide or iridium oxide, either of which may be used as the main constituent material of the capacitor lower electrode  203 , and hence there is obtained a structure which is hard to delaminate. This can similarly be said regarding the capacitor upper electrode  205 . That is, the capacitor upper electrode  205  is connected to a wire  209  interposed between barrier films  210  and  211 , and hence it is preferable in the point of preventing the wire from peeling to use ruthenium or iridium as the main constituent material of the barrier film  210 . In  FIG. 16 , reference numerals  212 ,  215 ,  219 ,  222 ,  227 ,  230 ,  233  and  236  show wires comprising mainly, for example, copper, and  213 ,  214 ,  216 ,  217 ,  220 ,  221 ,  223 ,  224 ,  228 ,  229 ,  231 ,  232 ,  234 ,  235 ,  237  and  238  show barrier films comprising mainly, for example ruthenium. In addition,  225  and  239  show an insulating layer consisting of, for example, a resin or the like. The thin film capacitor  202  is used, for example, as a filter for flowing only alternating electric current of a specific range of frequency to the resistance  226  and wire  227  among electric current flowing to the wire  222 . In place of this thin film capacitor  202 , there may be used a thin film capacitor formed on a substrate as shown in the seventh example.  
      In addition, though not shown in the figure, in some cases a memory chip such as ROM, RAM or the like may be provided in this system-in-package. There is the effect that since miniaturization of a capacitor is possible, flexibility in the layout of the capacitor is increased in designing complicated wiring.  
      Furthermore, in order to consider in more detail the dependency of effect on film thickness, similarly to that shown in  FIG. 2  to  FIG. 9 , diffusion constants of oxygen when the thickness of the capacitor insulating film was changed to 30 nm are shown in  FIG. 17 . Moreover, diffusion constants of oxygen when the thickness of the capacitor insulating film was changed to 35 nm, are shown in  FIG. 18 . According to  FIG. 17 , when the thickness of the capacitor insulating film was changed to 30 nm, in the case of using electrode materials other than ruthenium oxide and iridium oxide, the diffusion constants at 300° C. become 10 −20  m 2 /s or more. On the other hand, according to  FIG. 18 , when the thickness of the capacitor insulating film was changed to 35 nm, even in the case of using electrode materials other than ruthenium oxide and iridium oxide, the diffusion constants at 300° C. become smaller than 10 −20  m 2 /s. When diffusion constants at 300° C. are 10 −20  m 2 /s or more, much lack of oxygen is caused in the capacitor insulating film . Hence, in order to ensure reliability of a semiconductor device such as shown in  FIG. 1 , it is preferable that diffusion constants at 300° C. are smaller than 10 −20  m 2 /s. According to  FIG. 17  and  FIG. 18 , when the thickness of the capacitor insulating film is smaller than 35 nm, it is more important to use ruthenium oxide or iridium oxide as an electrode material. In addition, even when 3 nm was retained as the thickness of the capacitor insulating film and the thickness of the electrode films was increased to 30 nm or 35 nm, there was obtained almost the same result as in  FIG. 2 . That is, even when the thickness of the electrode films is increased, in the case of using electrode materials other than ruthenium oxide and iridium oxide, diffusion constants at 300° C. become 10 −20  m 2 /s or more.  
      According to the above-mentioned first to eighth examples, there can be provided a thin film capacitor having high reliability. Furthermore, there can be provided a system-in-package having high reliability.  
      Hereinafter, other examples of the present invention will be described in detail.  
      The plan layout of a semiconductor device according to the ninth working embodiment of the present invention is shown in  FIG. 20 .  FIG. 19  is the sectional view showing the sectional structure cut along the line XIX-XIX of the semiconductor device shown in  FIG. 20 . In the semiconductor device of the present working embodiment, as shown in  FIG. 19 , element-separating films  302  consisting of, for example, silicon oxide films are provided at a certain interval on the surface of a P type silicon substrate  301 , and an element-forming area  303  is formed between the element-separating films  302 . On the element-forming area  303 , there is provided a P channel MOS transistor.  
      The MOS transistor is constituted by containing a gate insulating film  1001  formed on the surface of the silicon substrate  301  and a gate electrode  306   a  facing the silicon substrate  301  through the gate insulating film  1001 . At both sides corresponding to said element-separating film sides of the gate electrode  306   a  and gate insulating film  1001 , there are formed side walls  307   a  consisting of, for example, silicon nitride. The gate insulating film  1001  is constituted by containing at least a two layer-laminated structure consisting of a titanium silicate film  304   a  at the silicon substrate side and a titanium oxide film  305   a  at the gate electrode film side. The gate electrode  306   a  consists of, for example, a polycrystalline silicon film, a metal thin film, a metal silicide film or the laminated structure thereof.  
      The MOS transistor shown in the figure has a P- type source-drain diffusion layer  308  formed in the state of self-aligning to the gate electrode  306   a  and a P+ type source-drain diffusion layer  309  formed in the state of self-aligning to the element-separating film  302  and gate electrode  306   a.    
      On the surface of this semiconductor device, there is formed an interlaminar insulating film  310 , and in this interlaminar insulating film  310  there is provided a contact hole  311  leading to the P+ type source-drain diffusion layer  309 .  
      In order to satisfy the demand for miniaturization of transistors, the factual thickness of the titanium silicate film  304   a  is the one which should give the silicon-oxide equivalent thickness of the gate insulating film  1001  of not more than 1 nm and which should prevent increase of leakage current. For example, when the silicon-oxide equivalent thickness of the gate insulating film  1001  is 1 nm and the electric voltage applied to the gate insulating film is 1 V, the factual thickness of the titanium silicate film  304   a  should be not less than 1.0 nm but not more than 3.2 nm. Thereby, there can be obtained a gate insulating film wherein leakage current is suppressed to a low level.  
      Next, a process for deriving the thickness of a titanium silicate film effective for suppressing increase of leakage current will be described.  
       FIG. 21  shows energy bands of the gate electrode, gate insulating film, and silicon substrate of the MOS transistor shown in  FIG. 19 . Herein, for example, the gate electrode consists of polycrystalline silicon doped with phosphorus. The gate insulating film consists of a two- layer structure of titanium oxide film of thickness T 1  and titanium silicate film of thickness T 2 , and the titanium silicate film is formed on the silicon substrate side. In addition, the silicon substrate is a P type substrate. Ev, Ec, and Ef in the figure mean, respectively, valence electron band, conduction band, and Fermi energy of silicon. Φ B1  and Φ B2  mean energy barriers of titanium oxide and titanium silicate.  
      When relative dielectric constants of silicon oxide, titanium oxide, and titanium silicate are respectively εSiO 2 , ε 1 , and ε 2 , silicon-oxide equivalent thicknesses T 1eff , T 2eff , and T eff  of said titanium oxide film, titanium silicate film, and the gate insulating film consisting of the two layer-structure there of are respectively represented by the following Expressi   
               T     1   ⁢           ⁢   eff       =         ɛ     S   ⁢           ⁢   i   ⁢           ⁢   O   ⁢           ⁢   2         ɛ   1       ⁢     T   1               (     Expression   ⁢           ⁢   1     )                 T     2   ⁢           ⁢   eff       =         ɛ     S   ⁢           ⁢   i   ⁢           ⁢   O   ⁢           ⁢   2         ɛ   2       ⁢     T   2               (     Expression   ⁢           ⁢   2     )                 T   eff     =         T     1   ⁢           ⁢   eff       +     T     2   ⁢           ⁢   eff         =       ɛ     S   ⁢           ⁢   i   ⁢           ⁢   O   ⁢           ⁢   2       ⁡     (         T   1       ɛ   1       +       T   2       ɛ   2         )                 (     Expression   ⁢           ⁢   3     )             
 
      For example, when relative dielectric constants of silicon oxide, titanium oxide, and titanium silicate are shown by ε Si02 =4, ε 1 =60, and ε 2 =15, and when the film thicknesses are shown by T 1 =15 nm, T 2 =3 nm, and T=18 nm, the equivalent thicknesses become as shown by T 1eff =1 nm, T 2eff =0.8 nm, and T eff =1.8 nm.  
       FIG. 22  shows energy bands when a positive electric voltage V is applied to the gate electrode. In this case, to the titanium oxide film and the titanium silicate film there are appl and E ox2                    V   1     =           ɛ   2     ⁢     T   1             ɛ   2     ⁢     T   1       +       ɛ   1     ⁢     T   2           ⁢   V             (     Expression   ⁢           ⁢   4     )                 V   2     =           ɛ   1     ⁢     T   2             ɛ   2     ⁢     T   1       +       ɛ   1     ⁢     T   2           ⁢   V             (     Expression   ⁢           ⁢   5     )                 E     o   ×   1       =       V   1     /     T   1               (     Expression   ⁢           ⁢   6     )                 E     o   ×   2       =       V   2     /     T   2               (     Expression   ⁢           ⁢   7     )               
      Tunnel electric current J flowing through the gate insulating film consisting of titanium oxide film and titanium silicate film shown above can be obtained by the following Expression  8  from the probability of electron&#39;s tunneling through the insulating film by use of WKB (Wentzel-Kramers-Brillouin) approach.  
                 J   ⁢     (       Φ   B     ,       T       o   ⁢           ⁢   x     ,       ⁢     E     o   ⁢           ⁢   x           )       =           n   v     ⁢     m   d     ⁢     k   B     ⁢   T       2   ⁢           ⁢     π   2     ⁢   ℏ       ⁢     ∫     T   *       T   WKB     ⁡     (       Φ     B   ⁢           ⁢   1       ,     Φ     B   ⁢           ⁢   2       ,     T   1     ,     T   2     ,     E     o   ⁢           ⁢   x   ⁢           ⁢   1       ,     E     o   ⁢           ⁢   x   ⁢           ⁢   2       ,   E     )       ⁢   I   ⁢           ⁢     n   ⁡     (     1   +     exp   ⁡     (         E   F     -   E         k   B     ⁢   T       )         )       ⁢   dE           ⁢     
     ⁢       T   *       T   WKB     ⁡     (       Φ     B   ⁢           ⁢   1       ,     Φ     B   ⁢           ⁢   2       ,     T   1     ,     T   2     ,     E     o   ⁢           ⁢   x   ⁢           ⁢   1       ,     E     o   ⁢           ⁢   x   ⁢           ⁢   2       ,   E     )         =     exp   ⁢     {         A   1     ⁡     (       E     n   ⁢           ⁢   1       -     E     n   ⁢           ⁢   2         )       +       A   2     ⁡     (       E     n   ⁢           ⁢   3       -     E     n   ⁢           ⁢   4         )         }         ⁢     
     ⁢       A   1     =       4   ⁢       2   ⁢           ⁢     m     i   ⁢           ⁢   n   ⁢           ⁢   s               3   ⁢           ⁢     h   _     ⁢           ⁢   q   ⁢           ⁢     E     o   ⁢           ⁢   x   ⁢           ⁢   1             ⁢     
     ⁢       A   2     =       4   ⁢       2   ⁢           ⁢     m     i   ⁢           ⁢   n   ⁢           ⁢   s               3   ⁢           ⁢     h   _     ⁢           ⁢   q   ⁢           ⁢     E     o   ⁢           ⁢   x   ⁢           ⁢   2             ⁢     
     ⁢       E     n   ⁢           ⁢   1       =     {                 {       Φ     B   ⁢           ⁢   1       -     (     E   -     E   F       )       }       3   /   2             E   &lt;       Φ     B   ⁢           ⁢   1       +     E   F                 0         E   ≥       Φ     B   ⁢           ⁢   1       +     E   F               ⁢     
     ⁢     E     n   ⁢           ⁢   2         =     {                 {       Φ     B   ⁢           ⁢   1       -     (     E   -     E   F       )     -     V   1       }       3   /   2             E   &lt;       Φ     B   ⁢           ⁢   1       +     E   F     -     V   1                 0         E   ≥       Φ     B   ⁢           ⁢   1       +     E   F     -     V   1               ⁢     
     ⁢     E     n   ⁢           ⁢   3         =     {                 {       Φ     B   ⁢           ⁢   2       -     (     E   -     E   F       )     -     V   1       }       3   /   2             E   &lt;       Φ     B   ⁢           ⁢   2       +     E   F     -     V   1                 0         E   ≥       Φ     B   ⁢           ⁢   2       +     E   F     -     V   1               ⁢     
     ⁢     E     n   ⁢           ⁢   4         =     {             {             Φ     B   ⁢           ⁢   2       -     (     E   -     E   F       )     -               (       V   1     +     V   2       )           }       3   /   2             E   &lt;       Φ     B   ⁢           ⁢   2       +     E   F     -     (       V   1     +     V   2       )                 0         E   ≥       Φ     B   ⁢           ⁢   2       +     E   F     -     (       V   1     +     V   2       )                                       (     Expression   ⁢           ⁢   8     )             
 
      In the above expressions, 
      n v : gate electrode electronic state degeneracy degree,     m d : gate electrode electron effective mass,     k B : Boltzmann constant,     T: temperature,     π: circular constant,     h: Planck constant (in the expressions—is added to h),     m ins : insulating film electron effective mass,     E: energy of electrons, and     E F : Fermi energy of gate electrode.    

       FIG. 23  shows dependency of leakage current density on the film thickness T 2  and equivalent thickness T 2eff  of titanium silicate, when the relative dielectric constant ε 2  of titanium silicate is 15, applied electric voltage is 1 V, temperature is 300 K, and the equivalent thickness T eff  of the gate insulating film is 1.0 nm. The figure shows results of calculation when the energy barrier Φ B2  of titanium silicate is 1.5, 2.0, 2.5, and 3.0 eV. The leakage current density is about 1.3×10 −8  A/cm 2  when the gate insulating film consists of only titanium oxide, that is, T 2 =0 nm, and the leakage current density decreases as the film thickness of titanium silicate increases. This is because a part of the electrons which can surpass the low energy barrier of titanium oxide cannot surpass the energy barrier of the silicate.  
      The leakage current density shows the minimum value when the equivalent thickness of titanium silicate is about 0.7 nm and the factual film thickness is about 2.5 nm, and the leakage current increases in accordance with increase in the film thickness of titanium silicate. This is because electrons permeate the energy barrier made by titanium silicate through direct tunnels and tunnel current flows.  
      From  FIG. 23 , it is seen that the leakage current density changes depending on the value of energy barrier of titanium silicate. However, when the electric voltage applied to the gate insulating film is 1 V and T 2  is 3.2 nm or less, it is seen that the leakage current can be suppressed to a lower value than the leakage current density in the case where the gate insulating film consists of only titanium oxide even when the value of energy barrier of titanium silicate changes.  
      Next,  FIG. 24  shows dependency of leakage current density on the film thickness T 2  and equivalent thickness T 2eff  of titanium silicate, when the relative dielectric constant ε 2  of titanium silicate is 20, the electric voltage applied to the gate insulating film is 1 V, and the equivalent thickness T eff  of the gate insulating film is 1.0 nm. Similarly to  FIG. 23 , the leakage current density decreases as the thickness of titanium silicate film increases from the point of T 2 =0 nm, and the leakage current density shows the minimum value when the equivalent thickness of titanium silicate film is about 0.8 nm and the factual film thickness is about 4.0 nm. Furthermore, if T 2  is 4.8 nm or less, it is seen that the leakage current can be suppressed to a lower value than the leakage current density in the case where the gate insulating film consists of only titanium oxide even when the value of energy barrier of titanium silicate changes.  
      On the basis of the similar calculations,  FIG. 25  and  FIG. 26  show dependency of leakage current density on the film thickness T 2  and equivalent thickness T 2eff  of titanium silicate, when the relative dielectric constant ε 2  of titanium silicate is 25 and 30. From the figures, if the condition of the equivalent thickness 1 nm of gate insulating film is satisfied by the film thickness of titanium silicate, it is seen that the leakage current can be suppressed to a lower value than the leakage current density in the case where the gate insulating film consists of only titanium oxide, even when the value of energy barrier of titanium silicate changes, by providing a titanium silicate film.  
      Furthermore, in order to give good dielectric characteristics to a titanium silicate film, at least one lattice of thickness is considered to be necessary, and hence factual thickness T 2  should be 1 nm or more.  
      From the above, even when the relative dielectric constant ε 2  of titanium silicate is changed in the range of 15 to 30 and the energy barrier Φ B2  thereof is changed in the range of 1.5 eV to 3.0 eV, the value of leakage current flowing through the gate insulating film can be suppressed to a low value, by forming a titanium silicate film in a factual thickness T 2  of 1.0 nm to 3.2 nm.  
      Hereinabove, there was described the case where the silicon-oxide equivalent thickness of gate insulating film was 1 nm, the electric voltage applied to gate insulating film was 1 V, and temperature was 300° K. Also in the case of the other silicon-oxide equivalent thickness, electric voltage and temperature, the film thickness of titanium silicate suitable for suppressing leakage current can be decided by the similar process.  
      Next, in the case where the electric voltage applied to gate is 0.5 to 1 V and the equivalent thickness is 0.7 to 1 nm, with regard to the film thickness of titanium silicate suitable for suppressing leakage current, descriptions are given by use of  FIG. 27 ,  FIG. 28  and  FIG. 29 .  
       FIG. 27  shows dependency of leakage current density on the film thickness T 2  and equivalent thickness T 2eff  of titanium silicate, when the relative dielectric constant ε 2  of titanium silicate is 15, temperature is 300° K., and the equivalent thickness T eff  of the gate insulating film is 1.0 nm. The figure is based on calculations in the case where the applied electric voltage is 0.5 V, 0.7 V and 1 V and the energy barrier ε B2  of titanium silicate is 1.5 eV.  
      As shown in  FIG. 27 , the leakage current density decreases as the thickness of titanium silicate film increases from the case where the gate insulating film consists of only titanium oxide, that is, T 2 =0 nm. This is because a part of the electrons which can surpass the low energy barrier of titanium oxide cannot surpass the energy barrier made by titanium silicate.  
      Furthermore, it is seen that the leakage current density shows the minimum value when the equivalent thickness of titanium silicate is 0.7 nm and the factual film thickness is about 2.5 nm, and that the leakage current increases in accordance with increase in the film thickness of titanium silicate. This is because electrons permeate the energy barrier made by titanium silicate through direct tunnels and tunnel current flows.  
      From  FIG. 27 , it is seen that the leakage current density changes depending on the value of applied electric voltage. However, when the applied electric voltage is in the range of 0.5 to 1 V, if T 2  is 3.2 nm or less, it is seen that the leakage current can be suppressed to a lower value than the leakage current density in the case where the gate insulating film consists of only titanium oxide.  
      In  FIG. 27 , there is shown the case where the energy barrier Φ B2  of titanium silicate is 1.5 eV and the relative dielectric constant ε 2  is 15. As stated above with reference to  FIG. 23 - FIG. 26 , also in the case where the energy barrier Φ B2  is 1.5 to 3.0 eV and the relative dielectric constant ε 2  is 15 to 30, if T 2  is 3.2 nm or less, it can be shown that the leakage current can be suppressed to a lower value than the leakage current density in the case where the gate insulating film consists of only titanium oxide.  
       FIG. 28  shows dependency of leakage current density on the film thickness T 2  and equivalent thickness T 2eff  of titanium silicate, when the relative dielectric constant ε 2  of titanium silicate is 15, temperature is 300° K., and the equivalent thickness T eff  of the gate insulating film is 0.7 nm. The figure is based on calculations in the case where the applied electric voltage is 0.5 V, 0.7 V and 1 V and the energy barrier Φ B2  of titanium silicate is 1.5 eV.  
      From  FIG. 28 , it is seen that the leakage current density changes depending on the value of applied electric voltage. However, when the applied electric voltage is in the range of 0.5 to 1 V and the equivalent thickness T eff  of the gate insulating film is 0.7 nm, if the film thickness T 2  of titanium silicate is 1.7 nm or less, it is seen that the leakage current can be suppressed to a lower value than the leakage current density in the case where the gate insulating film consists of only titanium oxide.  
      In  FIG. 28 , there is shown the case where the energy barrier Φ B2  of titanium silicate is 1.5 eV, the silicon-oxide equivalent thickness T eff  of the gate insulating film is 0.7 nm and the relative dielectric constant ε 2  is 15. As stated above with reference to  FIG. 23 - FIG. 26 , also in the case where the energy barrier Φ B2  is 1.5 to 3.0 eV and the relative dielectric constant ε 2  is 15 to 30, if T 2  is 1.7 nm or less, it can be seen that the leakage current can be suppressed to a lower value than the leakage current density in the case where the gate insulating film consists of only titanium oxide.  
      For each thickness in the case where the silicon-oxide equivalent thickness of the gate insulating film is 0.7 to 1.0 nm, the range of factual film thickness of titanium silicate for suppressing increase of leakage current can be obtained by the similar process.  FIG. 29  summarizes said desirable range of factual film thickness of titanium silicate corresponding to the silicon-oxide equivalent thickness 0.7-1.0 nm of the gate insulating film in the case where electric voltage applied to the gate is 0.5-1.0 V. The desirable range of factual film thickness of titanium silicate shown in the figure corresponds to the case where the relative dielectric constant of titanium silicate is 15, and in the case where the relative dielectric constant of titanium silicate is higher, a broader range can be obtained.  
      In addition, in the figure the factual film thickness T 2  of titanium silicate is indicated as 1.0 nm or more, and this is because at least one lattice of thickness is necessary for giving titanium silicate good dielectric characteristics.  
      The range of the factual film thickness T 2  of titanium silicate shown in  FIG. 29  is represented by the following expression as a function of the silicon-oxide equivalent thickness T eff  of the gate insulating film: 
 
1.0 (nm)≦ T   2 ≦5 T   eff −1.8 (nm), 
 
 wherein 
 
0.7 (nm)≦ T   eff ≦1.0 (nm). 
 
      That is, a semiconductor device having a gate insulating film wherein increase of leakage current is suppressed can be obtained by forming, between titanium oxide and a silicon substrate, a titanium silicate film having a thickness in the range of factual film thickness shown in  FIG. 29  in response to the silicon-oxide equivalent thickness Teff of the gate insulating film required by the specification of the semiconductor device.  
      In the above working embodiment, there was described the case of a polycrystalline silicon film doped with phosphorus as a gate electrode. In addition to the polycrystalline silicon film, also in the case of a gate electrode consisting of a metal thin film such as tungsten, molybdenum or the like, a metal compound such as tungsten nitride or the like, or a metal silicide film such as tungsten silicide or the like, or the laminated structure thereof, the film thickness of titanium silicate adequate for suppressing leakage current can be similarly decided by such process.  
      Because depletion does not occur in a gate electrode film formed of a metal film such as tungsten, molybdenum or the like, the equivalent thickness of a gate insulating film can be decreased. Furthermore, tungsten is thermally stable, and the film quality thereof is scarcely changed in a high temperature process after formation of the gate electrode film. In addition, when tungsten is laminated in contact with titanium oxide, tungsten oxide is formed in some cases. Tungsten oxide has a smaller dielectric constant than titanium oxide, and formation of tungsten oxide leads to the increase in the equivalent thickness of a gate insulating film. Therefore, it is effective to use a tungsten nitride or tungsten silicide film having excellent oxidation resistance as compared with a tungsten film. Particularly in oxidation resistance, a tungsten nitride film is especially excellent. Moreover, when the tungsten nitride film is used for a gate electrode, by forming a gate electrode  314  of a two-layer structure, wherein tungsten nitride  312  is used as a layer in contact with titanium oxide and tungsten  313  having a lower resistance than tungsten nitride is used as the upper layer as shown in  FIG. 30 , a gate electrode having a low resistance can be obtained.  
      As stated above, according to the present working embodiment, titanium silicate film is present at the interface between titanium oxide film and silicon substrate . Hence, a silicon oxide film having a low relative dielectric constant can be prevented from being formed at said interface and, at the same time, the silicon-oxide equivalent thickness can be decreased as compared with the case of providing silicon nitride at said interface. Thus there can be provided a semiconductor device having a gate insulating film which can satisfy miniaturization.  
      Furthermore, according to the present working embodiment, a gate insulating film is constituted by a laminated structure of titanium oxide film as a high dielectric constant material and titanium silicate film having a relatively large dielectric constant. Hence, while the factual thickness of the gate insulating film can be made thick, the silicon-oxide equivalent thickness can be made thin, and leakage current can be reduced.  
      Moreover, according to the present working embodiment, because there can be obtained a semiconductor device wherein leakage current hardly flows, there can be obtained a semiconductor device having high reliability, and also there can be obtained a semiconductor device having a high yield.  
      The tenth example of the present invention is described by use of  FIG. 31  (including FIGS.  31 (A)- 31 (C)),  FIG. 32  (including FIGS.  32 (A)- 32 (C)) and  FIG. 33  (including FIGS.  33 (A)- 33 (C)).  FIG. 31 ,  FIG. 32  and  FIG. 33  show a process for preparing the semiconductor device having a gate insulating film consisting of titanium oxide film and titanium silicate film shown in  FIG. 19 . Herein, there is described the case wherein the factual thickness of titanium silicate film is 3 nm and the factual film thickness of titanium oxide is 3 nm.  
      First, plural grooves of 200-300 nm in depth are formed at a predetermined interval on the surface of a P type silicon substrate  301 , and silicon oxide films are embedded therein to form element-separating films  302  of shallow groove type ( FIG. 31  (A)).  
      Next, a titanium film  1010  of about 1 nm in thickness is formed on the surface of the silicon substrate  301  by, for example, sputtering method ( FIG. 31  (B)). Next, the titanium film  1010  is subjected to heat treatment of 600° C. to form titanium silicide film  1011 . By this silicide reaction, the thickness of titanium silicide film  1011  becomes about 2 nm ( FIG. 31  (C)). In addition, at this time, the portions in contact with element-separating films  302  are left as such without being changed into the silicide.  
      Next, the titanium silicide film loll is oxidized to form titanium silicate film  304  ( FIG. 32  (A)). This oxidization reaction causes volume expansion, and the thickness of titanium silicate film  304  becomes about 3 nm. When the thickness of titanium silicate film  304  is larger than 3 nm, the titanium silicate film  304  is subjected to etching by sputtering method or the like to reduce the film thickness into a predetermined thickness.  
      Next, a titanium oxide film  305  of about 3 nm in thickness is formed on the surface of titanium silicate film  304  by, for example, CVD (Chemical Vapor Deposition) method.  
      Herein, when the equivalent thickness of a gate insulating film having titanium silicate film  304  and titanium oxide film  305  is larger than 1 nm, the titanium oxide film  305  is subjected to etching by sputtering method or the like to reduce the film thickness into a predetermined equivalent thickness.  
      Furthermore, a polycrystalline silicon film  306  containing impure phosphorus is formed on the surface of titanium oxide film  305  by CVD method or the like. The thickness of the polycrystalline silicon film  306  is, for example, about 200 nm ( FIG. 32  (B)).  
      Next, polycrystalline silicon film  306 , titanium oxide film  305  and titanium silicate film  304  are subjected to etching by use of a photoresist film as a mask. Thereby, gate insulating film  1001  and gate electrode  306   a  of a MOS transistor are formed. Herein, the gate insulating film  1001  has titanium silicate film  304   a  and titanium oxide film  305   a  ( FIG. 32  (C)).  
      Next, there are formed P- type source-drain regions  308  of the MOS transistor by ion implantation of boron. The P- type source-drain regions  308  are in the state of self-aligning to the gate electrode and gate insulating film ( FIG. 33  (A)).  
      Subsequently, a silicon nitride film  307  of 200 nm in thickness is deposited on the surface of the semiconductor substrate by sputtering method or CVD method ( FIG. 33  (B)), and the silicon nitride film  307  is subjected to etching to form side walls  307   a  covering the side walls of the gate electrode and gate insulating film on element-separating film  302  sides ( FIG. 33  (C)).  
      Next, P+ type source-drain diffusion layers  309  are formed on the silicon substrate  301  by ion implantation of boron with a mask of element-separating films  302 , gate electrode  306   a  and side walls  307   a . Subsequently, by CVD method there is formed an interlaminar insulating film  310  covering element-separating films  302 , gate electrode  306   a , side walls  307   a  and P+ type source-drain diffusion layers  309 , and in the resultant interlaminar insulating film  310  there are formed contact holes  311  leading to the surfaces of P+ type source-drain diffusion layers  309  from the surface thereof (see  FIG. 19 ).  
      As stated above, first a titanium silicide film is formed on the surface of a silicon substrate, and then the titanium silicide film is oxidized into a titanium silicate film, on which a titanium oxide film is formed. This is because if the titanium oxide film is formed directly on the surface of the silicon substrate, oxygen atoms in the titanium oxide film diffuse to the silicon substrate side, as stated previously, and silicon oxide having a low dielectric constant is sometimes formed at the interface between the titanium oxide film and the silicon substrate, which leads to a defective MOS transistor.  
      In the present working embodiment, a titanium silicide film is formed on the surface of a silicon substrate, and then the titanium silicide film is oxidized into a titanium silicate film, and hence the surface of the silicon substrate is not contacted with oxygen atmosphere and there is no fear of formation of silicon oxide at the interface of the substrate. Furthermore, the titanium silicate film is formed on the surface of the silicon substrate and a titanium oxide film is laminated on the silicate film, and hence oxygen atoms in the titanium oxide film are prevented from diffusing to the silicon substrate side. Moreover, the relative dielectric constant of titanium silicate is 15-40 whereas that of silicon nitride is about 7.8. Therefore, as compared with the case where a silicon nitride film is formed at the interface between the titanium oxide film and the silicon substrate, the factual thickness of titanium silicate film can be made larger than that of silicon nitride film, when the silicon-oxide equivalent thickness is the same. Therefore, the effect of suppressing leakage current is large.  
      The above-mentioned preparation process relates to the case of a P channel MOS transistor. This preparation process is also applicable to a N channel MOS transistor, and, furthermore, is applicable, also, to a CMOS transistor and a BiCMOS transistor.  
      The eleventh example of the present invention is described by use of  FIG. 34  (including FIGS.  34 (A)- 34 (D)).  FIG. 34  shows some steps of a process for preparing the semiconductor device having a gate insulating film consisting of titanium oxide film and titanium silicate film shown in  FIG. 19 . That is, FIGS.  34 (A)- 34 (D) show the main process steps leading to the formation of a titanium silicate film on a silicon substrate. Herein, there is described, for example, the case where the factual thickness of titanium silicate is 3 nm and that of titanium oxide is 3 nm.  
      First, plural grooves of 200-300 nm in depth are formed at a predetermined interval on the surface of a P type silicon substrate  301 , and silicon oxide films are embedded therein to form element-separating layers  302  of shallow groove type ( FIG. 34  (A)).  
      Next, a silicon oxide film  1020  of about 1.5 nm in thickness is formed on the surface of the silicon substrate  301  by, for example, thermal oxidation method ( FIG. 34  (B)).  
      Furthermore, on the above silicon oxide film, there is formed a titanium film  1021  of about 1.5 nm in thickness ( FIG. 34  (C)).  
      Next, the above silicon oxide film  1020  and titanium film  1021  are reacted by heat treatment of 400° C. to 500° C. In this heat treatment, the silicon oxide film  1020  disappears and a titanium silicate film  304  is formed by reduction reaction of titanium ( FIG. 34  (D)). The thickness of the above titanium silicate film  304  becomes about 3 nm, but when the thickness of titanium silicate film  304  is larger than 3 nm, the titanium silicate film  304  is subjected to etching by sputtering method or the like to reduce the film thickness and give a predetermined thickness. When the thickness is smaller, it is possible to give a predetermined film thickness by adjusting the thicknesses of the above silicon oxide film and titanium film.  
      In the subsequent steps, a gate insulating film, a gate electrode film and the like are formed to produce a MOS transistor similarly to  FIG. 32  (B) step and the subsequent steps of the above tenth example.  
      In the present working embodiment, a silicon oxide film  1020  is once formed on a silicon substrate  301 , but a titanium film  1021  is formed on the silicon oxide film  1020  and the two films are reacted by heat treatment into a titanium silicate film  304 . Hence, the silicon oxide film  1020  having a low dielectric constant disappears. Next, a titanium oxide film is formed thereon and, therefore, at the time of forming the titanium oxide film oxygen atoms in the titanium oxide film are prevented from diffusing to the silicon substrate side at the interface with the silicon substrate.  
      That is, also in the present working embodiment, there can be obtained effects similar to those of the above tenth example. In addition, as examples of further preferred embodiments, the constitution concerning the gate electrode structure containing the gate insulating film shown in the above ninth to eleventh example can be applied to the corresponding portions in the first to eighth example.  
      According to the above ninth to eleventh examples, a titanium silicate film is present at the interface between titanium oxide film and silicon substrate. Hence, silicon oxide film having a low relative dielectric constant can be prevented from being formed at said interface, and at the same time the silicon-oxide equivalent thickness can be decreased as compared with the case of providing silicon nitride at said interface, and thus there can be provided a semiconductor device having a gate insulating film which can satisfy the desire for miniaturization.  
      Furthermore, a gate insulating film is constituted by a laminated structure of titanium oxide film as a high dielectric constant material and titanium silicate film having a relatively large dielectric constant. Hence, the factual thickness of the gate insulating film can be made thick, the silicon-oxide equivalent thickness can be made thin, and leakage current can thereby be reduced.  
      Moreover, because there can be obtained a semiconductor device wherein leakage current hardly flows, there can be provided a semiconductor device having high reliability, and also there can be provided a semiconductor device having a high yield.  
      The above descriptions were disclosed with reference to the various examples given. The present invention, however, should not be construed as being limited thereto. It should also be obvious to those skilled in the art that various changes and modifications can be performed to the various disclosed and/or other alternative examples that are within the spirit and scope of the present invention and the attached claims.  
     INDUSTRIAL APPLICABILITY  
      The present invention relates to a semiconductor device and can be adapted to a semiconductor device having high reliability. Preferably, the present invention can be adapted to a semiconductor device provided with a thin film capacitor having high reliability or a semiconductor device having a gate structure wherein leakage current is suppressed.