Patent Publication Number: US-2002011615-A1

Title: Ferroelectric memory device and method for producing the same

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a ferroelectric memory device and a method for producing the same. In particular, the present invention relates to a ferroelectric memory device including a selection transistor, a capacitor having upper and lower electrodes and a ferroelectric film, and a conductive plug for electrically interconnecting the selection transistor and the capacitor, as well as a method for producing the same.  
       [0003] 2. Description of the Related Art  
       [0004] Ferroelectric materials are utilized as component elements in various devices in a wide range of fields because these materials provide many functions, e.g., spontaneous polarization, high dielectric constant, electrooptical effects, piezoelectric effects, and pyroelectric effects. For example, the pyroelectric properties of ferroelectric materials are utilized in infrared linear array sensor applications; their piezoelectric properties are utilized in ultrasonic wave sensor applications; their electrooptical effects are utilized in waveguide type optical modifier applications; their ferroelectric properties are utilized in capacitors for use in DRAMs (dynamic random-access memories) and MMICs (microwave monolithic integrated circuits), and so on. Above all, the recent development in thin film formation technologies, taken in conjunction with semiconductor memory technologies, has led to vigorous research and development activities for non-volatile memories (FRAMs: ferroelectric random access memories) which are capable of high-density implementation and high-speed operation.  
       [0005] FRAMs provide advantages such as high-speed writing/reading, low voltage operation, and high endurance through repetitive writing/reading. Therefore, research and development efforts have been made to implement FRAMs which can replace not only conventional EPROMs (erasable and programmable read only memories) and EEPROMs (electrically erasable and programmable read only memories), and flash memories, but potentially SRAMs (static random access memories) and/or DRAMs also.  
       [0006] Conventionally, oxide ferroelectric materials, e.g., Pb(Zr 1-x Ti x )O 3 , SrBi 2 Ta 2 O 9 , Bi 4 Ti 3 O 12 , etc.) have been studied as ferroelectric materials for use in ferroelectric capacitors in FRAMs. Among others, SrBi 2 Ta 2 O 9  is the most suitable material for FRAMs because it is capable of low-voltage operation at 3 V or less, and has excellent reliability (e.g., fatigue characteristics, imprint characteristics, temperature characteristics, and the like) to ensure stable operation of a ferroelectric memory. As a lower electrode of such a ferroelectric capacitor, electrodes formed of a precious metal material (e.g., Pt, Pt/Ta, Pt/Ti), or complex electrodes composed of a precious metal material layer and a closely contacting film, have been used to study the characteristics of thin ferroelectric films.  
       [0007] A ferroelectric film must be subjected to a high-temperature heat treatment at about 650° C. to about 800° C. in an oxygen atmosphere for imparting the film with satisfactory ferroelectric characteristics. On the other hand, it is generally essential to employ a stacked structure in order to achieve a high density integration, e.g. , 1 Mbit or above, by utilizing such a ferroelectric capacitor and ferroelectric-capacitor manufacturing processes.  
       [0008]FIG. 1 illustrates the structure of a conventional ferroelectric memory device having a stacked structure. In this structure, a selection transistor  20  and a ferroelectric capacitor  40  are electrically interconnected via a conductive plug  30  (of polysilicon or the like)., The selection transistor  20  includes a gate electrode  22  formed on a silicon substrate  10 , and source/drain regions  24   a  and  24   b  interposing the gate electrode  22 . The conductive plug  30  is buried in a through hole in a first interlayer insulation film  52 . The ferroelectric capacitor  40  includes a lower electrode  44  which is directly coupled to the conductive plug  30 , a thin ferroelectric film  46 , and an upper electrode  48 . Furthermore, a second interlayer insulation film  54  is formed on the ferroelectric capacitor  40  and the first interlayer insulation film  52 . The upper electrode  48  is coupled to a take-out electrode  62  via a contact hole in the overlying second interlayer insulation film  54 . The source region  24   a  of the selection transistor  20  is coupled to a take-out electrode  64  via a contact hole in the first and second interlayer insulation films  52  and  54 . A LOCOS film  51  is provided as a device separation region.  
       [0009] In the above-described stacked structure, the lower electrode  44  (of Pt or the like ) is formed directly on the conductive plug  30  (which is made of polysilicon). Therefore, a silicidation reaction may occur between Pt and polysilicon during a heat treatment in the ferroelectric capacitor processing, thereby deteriorating the ferroelectric characteristics.  
       [0010] Conventionally, a diffusion barrier film (e.g., TiN) is employed between the lower electrode  44  and the polysilicon (conductive plug)  30 , so as to prevent the aforementioned silicidation reaction. However, the TiN layer in the Pt/TiN structure may be oxidized by gaseous oxygen which has permeated the Pt film grain boundaries during the crystallization heat treatment process for the ferroelectric material, as reported in “Proceedings of 43rd Applied Physics Association Lecture Meeting, 1996 Spring, 28p-V-6 (p. 500)”. Furthermore, as reported in “Proceedings of 43rd Applied Physics Association Lecture Meeting, 1996 Spring, 28p-V-7 (p. 500)”, oxidation of TiN, if it occurs, may cause peeling at the Pt/TiN interface or an upward formation of Pt hillocks due to stress associated with the volume expansion resulting from the oxidation of TiN. This presents a considerable problem.  
       [0011] In the case of a Pt/polysilicon structure or a Pt/TiN/polysilicon structure, the electrical contact between the lower electrode and the polysilicon may become insufficient due to Pt silicidation or TiN oxidation resulting in hillocks or the like. Thus, it has been difficult to realize a stacked structure.  
       [0012] On the other hand, the use of not only Pt but also Ir, PtRh, Ru or oxides thereof (e.g., IrO 2 , PtRhO x , or RuO 2 ) for a lower electrode has begun to be studied because of their excellent barrier properties and excellent matching with any overlying oxide dielectric material. Among others, the use of Ir and IrO 2  can greatly improve the fatigue characteristics of PZT formed upon an Ir/IrO 2 /polysilicon electrode structure or a Pt/lrO 2 /polysilicon electrode structure, as reported in Appl. Phys. Lett., vol. 65 (1994), pp. 1522-1524 and Jpn. J. Appl. Phys., vol. 33 (1994), pp. 5207-5210. Such improvement is ascribed to the barrier properties of the IrO 2  film against the elements (e.g., Pb) composing the ferroelectric film.  
       [0013] However again, such a structure is susceptible to the problem of insufficient contact due to oxidation of the polysilicon at the IrO 2 /polysilicon interface during the IrO 2  film formation and the ferroelectric film formation.  
       [0014] An IrO 2 /Ir/TiN/Ti lower electrode, incorporating a TiN film as a barrier metal for an oxide electrode (IrO 2 ), has been reported in “Proceedings of 43rd Applied Physics Association Lecture Meeting, 1996 Spring, 28p-V-4 (p. 499)” as a solution to the problem associated with reactions between Ir or IrO 2  and polysilicon. The report describes that a ferroelectric SrTiO 3  film was actually produced, and in accordance with this structure, ohmic contact was realized between the ferroelectric SrTiO 3  film and a silicon substrate whose resistance had been lowered by ion injection, and that ferroelectric characteristics similar to those of Pt were obtained.  
       [0015] In the case of employing a relatively low temperature process in the range of about 200° C. to about 450° C. (which is applicable to ferroelectric materials such as SrTiO 3 ), an lrO 2 /Ir/TiN/Ti lower electrode structure might seem as promising for realizing a stacked structure incorporating a ferroelectric capacitor, because deterioration in the electric characteristics of the capacitor due to hillock generation or reduced flatness does not occur.  
       [0016] However, in the case of ferroelectric materials such as PZT, a ferroelectric crystallization process in an oxygen atmosphere at about 600° C. or more is required. For SrBi 2 Ta 2 O 9 , a high temperature heat treatment in the range of about 650° C. to about 800° C. is required. At such a high temperature, the use of a Pt/TiN lower electrode structure may result in the generation of Pt hillocks due to changes in the film stress caused by TiN oxidation.  
       SUMMARY OF THE INVENTION  
       [0017] A ferroelectric memory device according to the present invention includes: a capacitor having an upper electrode, a ferroelectric film, and a lower electrode; a conductive plug disposed under the lower electrode for electrically connecting the lower electrode to a selection transistor; and a diffusion fusion barrier film formed between the conductive plug and the lower electrode for or preventing a diffusion reaction between the conductive plug and the lower electrode, wherein a silicide layer is formed between the conductive plug and the diffusion barrier film, the silicide layer containing a first metal element.  
       [0018] In one embodiment of the invention, the first metal element is selected from IV-A group elements (Ti, Zr, and Hf), V-A group elements (V, Nb, and Ta), VI-A group elements (Cr, Mo, and W), and VIII group elements (Ru, Os, Co, Rh, Ir, Ni, Pd and Pt).  
       [0019] In another embodiment of the invention, the diffusion barrier film is composed essentially of one of A x Si 1-x N y , A x Al 1-x N y , and BN z  (where 0.2≦x≦ 1 ;  0 ≦y≦1; 0 ≦z≦1), wherein A is an element which is selected from a group including Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt; and B is an element which is selected from a group including Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt.  
       [0020] In still another embodiment of the invention, the lower electrode is composed essentially of at least one of an Ir film and a multilayer film including an Ir layer and an IrO 2  layer.  
       [0021] In still another embodiment of the invention, the silicide layer has a thickness of about 2 to about 60 nm.  
       [0022] In still another embodiment of the invention, the conductive plug is composed essentially of polysilicon.  
       [0023] A method for producing a ferroelectric memory device including a capacitor having an upper electrode, a ferroelectric film, and a lower electrode; a conductive plug disposed under the lower electrode for electrically coupling the lower electrode to a selection transistor; and a diffusion barrier film formed between the conductive plug and the lower electrode for preventing a diffusion reaction between the conductive plug and the lower electrode includes a step of forming a silicide layer between the conductive plug and the diffusion barrier film, the silicide layer contains a first metal element.  
       [0024] In one embodiment of the invention, the first metal element is selected from IV-A group elements (Ti, Zr, and Hf), V-A group elements (V, Nb, and Ta), VI-A group elements (Cr, Mo, and W), and VIII group elements (Ru, Os, Co, Rh, Ir, Ni, Pd and Pt).  
       [0025] In another embodiment of the invention, the conductive plug is composed essentially of silicon, and the step of forming the silicide layer includes: forming a metal layer on the conductive plug, the metal layer containing the first metal element; and performing a heat treatment in an inert gas atmosphere to convert the metal layer into the silicide layer.  
       [0026] In still another embodiment of the invention, the step of forming the silicide layer includes: forming a first layer on the conductive plug, the first layer containing the first metal element and Si; and performing a heat treatment in an inert gas atmosphere to convert the first layer into the silicide layer.  
       [0027] In still another embodiment of the invention, the conductive plug is composed essentially of silicon, and the step of forming the silicide layer includes: forming a metal layer on the conductive plug, the metal layer on the metal layer, the first layer containing the first metal element and Si; and performing a heat treatment in an inert gas atmosphere to convert the metal layer and the first layer into the silicide layer.  
       [0028] In still another embodiment of the invention, the conductive plug is composed essentially of silicon, and the step of forming the suicide layer includes: forming a first metal layer on the conductive plug, the first metal layer containing the first metal element; performing a heat treatment in an inert gas atmosphere to convert the first metal layer into the silicide layer; and forming a second metal layer on the silicide layer, the second metal layer containing the first metal element.  
       [0029] In still another embodiment of the invention, the inert gas is composed essentially of nitrogen gas.  
       [0030] In still another embodiment of the invention, the heat treatment is performed before ore forming the diffusion barrier film, the heat treatment being performed at a temperature ranging from about 500° C. to about 950° C.  
       [0031] In still another embodiment of the invention, the heat treatment is performed after forming the diffusion barrier film, the heat treatment being performed at a temperature ranging from about 500° C. to about 800° C.  
       [0032] In still another embodiment of the invention, the diffusion barrier film is composed essentially of one of A x Si 1-x N y , A x Al 1-x N y , and BN 2  (where 0.2≦x≦ 1 ;  0 ≦y≦1; 0 ≦z&lt;1), wherein A is an element which is selected from a group including Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt; and B is an element which is selected from a group including Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt.  
       [0033] Thus, the invention described herein makes possible the advantages of (1) providing a ferroelectric memory device of a stacked structure including a lower electrode structure which has high resistance against a heat treatment conducted in an oxygen atmosphere at about 600° C. or more and exhibits excellent contact characteristics with respect to a conductive plug; and (2) providing a method for producing the same.  
       [0034] These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0035]FIG. 1 is a cross-sectional view illustrating a conventional ferroelectric memory device.  
     [0036]FIG. 2 is a cross-sectional view illustrating a ferroelectric memory device according to the present invention.  
     [0037]FIGS. 3A to  3 D are cross-sectional views illustrating respective steps of a method for producing a ferroelectric memory device according to the present invention.  
     [0038]FIG. 4 is a hysteresis diagram of a ferroelectric material.  
     [0039]FIGS. 5A to  5 H are cross-sectional views ( 5 A to  5 G) and a plan view ( 5 H) illustrating a process for preparing a Kelvin pattern.  
     [0040]FIG. 6 is a graph illustrating the contact diameter dependency of contact resistance, measured by a four-terminal method using a Kelvin pattern.  
     [0041]FIG. 7A is a graph illustrating current-voltage characteristics when measuring contact resistance according to the present invention.  
     [0042]FIG. 7B is a graph illustrating current-voltage characteristics when measuring contact resistance under the prior art.  
     [0043]FIG. 8 is a graph illustrating the distributions of respective component elements of a ferroelectric capacitor, taken along the direction of the depth of the thin ferroelectric film. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0044] Hereinafter, fundamental features of the present invention will be described with reference to FIG. 2.  
     [0045]FIG. 2 is a cross-sectional view illustrating an essential portion of the fundamental structure of a ferroelectric memory device according to the present invention. The ferroelectric memory device includes a selection transistor  20  formed on a substrate  10 , a ferroelectric capacitor  140  formed on the selection transistor  20  with a first interlayer insulation film  52  interposed therebetween, and a conductive plug  30  buried in a contact hole through the first interlayer insulation film  52 . The conductive plug  30 , which is formed of polysilicon or the like, electrically interconnects the selection transistor  20  and the ferroelectric capacitor  140 .  
     [0046] The ferroelectric capacitor  140  includes a lower electrode  144 , a thin ferroelectric film  146 , and an upper electrode  148 . The lower electrode  144  of the ferroelectric capacitor  140  according to the present invention is formed of Ir or IrO 2 .  
     [0047] The ferroelectric capacitor  140  further includes a diffusion barrier film  143  and an underlying silicide layer  142  provided between the lower electrode  144  and the conductive plug  30 . The silicide layer  142  contains a first metal, which is defined as an element selected from IV-A group elements (Ti, Zr, and Hf), V-A group elements (V, Nb, and Ta), VI-A group elements (Cr, Mo, and W), and VIII group elements (Ru, Os, Co, Rh, Ir, Ni, Pd and Pt).  
     [0048] The diffusion barrier film  143 , which is formed of A x Si 1-x N y , A x Al 1-x N y , or BN z  (where 0.2≦x&lt;1; 0≦y≦1; 0 ≦z&lt;1), prevents the diffusion reaction between the conductive plug  30  and the lower electrode  144 . In the above chemical formulae, A is an element which is selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt; and B is an element which is selected from Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt.  
     [0049] Hereinafter, effects which are attained by the above structure will be described.  
     [0050] The lower electrode  144 , which is formed of an Ir or IrO 2 /Ir film, prevents oxygen from being diffused into the diffusion barrier film  143  (which in itself is formed of, e.g., a Ta x Si 1-x N y  film; hereinafter such a film will be referred to also as a “TaSiN film”) due to the high temperature at which a crystallization process is performed for the overlying ferroelectric film. As a result, the volume expansion of the Ta x Si 1-x N y  film and/or contact insufficiency due to oxidation can be prevented. The use of a lower electrode of an Ir or IrO 2  film, and the use of the aforementioned diffusion barrier film are partially described in Japanese Application No. 10-035639 (entitled “SEMICONDUCTOR MEMORY DEVICES AND METHOD FOR PRODUCING THE SAME), too.  
     [0051] The diffusion barrier film  143  (e.g., Ta x Si 1-x N y  film) functions as a diffusion barrier between the silicide layer  142  (e.g., titanium silicide) and the Ir or IrO 2 /Ir film formed on the Ta x Si 1-x N y  film. The Ta x Si 1-x N y  film  143  prevents elements such as silicon or Ti, from diffusing into the lower electrode  144 .  
     [0052] The use of the diffusion barrier film  143  and the lower electrode  144  as described above prevents the formation of hillocks, on the lower electrode  144  or between the lower electrode  144  and the diffusion barrier film  143 , during a ferroelectric crystallization process at about 700° C. for about 60 minutes. As a result, a ferroelectric memory device including a ferroelectric film having excellent ferroelectric characteristics can be provided.  
     [0053] Hereinafter, the significance of providing the silicide film  142  according to the present invention will be described.  
     [0054] It has been found through a detailed interfacial analysis by the inventors that if the Ta x Si 1-x N y  diffusion barrier film  143  is directly formed on polysilicon, a very thin reaction layer is formed at the interface between the Ta x Si 1-x N y  film and the polysilicon. Such a reaction layer, if formed, may increase the contact resistance between the lower electrode  144  and the conductive plug  30  after performing a ferroelectric crystallization process, possibly resulting in non-ohmic current-voltage characteristics. Such non-ohmic current-voltage characteristics are likely to cause problems such as unwanted delay during a high-speed operation and/or poor S/N ratios in the ferroelectric memory operation.  
     [0055] The generation of such a reaction layer can be prevented by providing a silicide layer. Furthermore, due to the presence of a silicide layer, the contact strength between the polysilicon and the Ta x Si 1-x N y  diffusion barrier film  143  is improved, thereby contributing to the reduction of hillocks. In accordance with this structure, problems such as peeling of the Ta x Si 1-x N y  diffusion barrier film  143  and hillock formation can be prevented even by performing a heat treatment at about 700° C. or more for 240 minutes, and yet excellent contact characteristics are attained between the lower electrode and the polysilicon plug  30 .  
     [0056] Hereinafter, the other constituent elements shown in FIG. 2 are described. It is commonplace to employ an oxide ferroelectric material, e.g., (lead-zirconate-titanate; PZT), SrBi 2 Ta 2 O 9  (SBT) or Bi 4 Ti 3 O 12  for the thin ferroelectric film  146  overlying the lower electrode  144 . In the case of an SBT material, any Bi type ferroelectric material having a laminar perovskite structure can be used, but especially preferable materials in this category include Bi 2 A m-1 B 3m+3  (where A represents Na, K, Pb, Ca, Sr, Ba or Bi; and B represents Fe, Ti, Nb, Ta, W, or Mo); it is even more preferable if m is a natural number. Examples of such preferable materials include Bi 4 Ti 3 O 12 , SrBi 2 (Ta x , Nb 1-x ) 2 O 9  (where 0≦x&lt;1), BaBi 2 Nb 2 O 9 , BaBi 2 Ta 2 O 9 , PbBi 2 Ta 2 O 9 , PbBi 2 Nb 2 O 9 , PbBi 4 Ti 3 O 15 , SrBi 4 Ti 4 O 15 , BaBi 4 Ti 4 O 15 , Sr 2 Bi 4 Ti 5 O 18 , Ba 2 Bi 4 Ti 5 O 18 , Pb 2 Bi 4 Ti 5 O 18 , Na 0.5 Bi 4.5 Ti 4 O 15 , K 0.5 Bi 4.5 Ti 4 O 15 , and the like. Alternatively, solid solutions such as SrBi 2 (Ta x , Nb 1-x ) 2 O 9  (where 0≦x&lt;1) and SrBi 2 (Ta x , Nb 1-x ) 2 O 9 ) 0.7  • (Bi 3 TiTaO 9 ) 0.3  (where 0≦x&lt;1) can also be used.  
     [0057] Such a ferroelectric film can be formed by a known method, e.g., a spin-on method, a reactive vapor deposition method, an electron beam (EB) vapor deposition method, a sputtering method, a laser ablation method, or a metal-organic chemical vapor deposition (MOCVD) method. For example, a spin-on method can be performed as follows. All or some elements which compose the thin ferroelectric film  146  are dispersed in a solvent and applied on a substrate by spin coating, and dried. Next, carbon components which are present in the film are burnt through sintering (preliminary sintering). Thereafter, the resultant composite is subjected to sintering in a gas containing oxygen or an oxygen compound so as to be converted into a crystal having a perovskite structure. As a result, the ferroelectric film  146  is formed on the substrate.  
     [0058] The upper electrode  148  provided on the ferroelectric film  146  can be formed so as to have a single layer film structure of Pt or the like, or alternatively formed from a material similar to the material of the lower electrode  144  by using a similar method.  
     [0059] The selection transistor  20  has a gate electrode  22  and source/drain regions  24   a  and  24   b  formed on the silicon substrate  10 , the source/drain regions  24   a  and  24   b  interposing the gate electrode  22 . In the structure shown in FIG. 2, a second interlayer insulation film  54  is formed on the ferroelectric capacitor  140  and the first interlayer insulation film  52 . The upper electrode  148  is coupled to a take-out electrode  62  via a contact hole in the overlying second interlayer insulation film  54 . The source region  24   a  of the selection transistor  20  is coupled to a take-out electrode  64  via a contact hole in the first and second interlayer insulation films  52  and  54 . A LOCOS film  51  is provided as a device separation region.  
     [0060] Any substrate that can be used as a substrate for usual semiconductor devices or integrated circuits can be used as the substrate of the ferroelectric memory device according to the present invention, but a silicon substrate is preferable.  
     [0061] The ferroelectric memory device according to the present invention incorporating a ferroelectric material can be mounted on a wafer for use in integrated circuits in such a manner that the incorporated ferroelectric material serves as a component element of a given ferroelectric device or semiconductor device of an integrated circuit. For example, the ferroelectric element can be utilized as a capacitor section of a non-volatile memory or a gate electrode of an FET. With the addition of a gate insulation film, source/drain regions and the like, MFMIS-FETs (metal ferroelectric metal insulator semiconductor FETs), MFS-FETs (metal ferroelectric semiconductor FETs), and the like can be conveniently realized.  
     [0062] Hereinafter, the present invention will be described by way of examples, with particular reference to a method for forming a silicide layer.  
     EXAMPLE 1  
     [0063] With reference to FIGS. 3A to  3 D, a method for producing a ferroelectric memory device according to Example 1 of the present invention will be described. In the present example, a metal film (Ti film) formed on a polysilicon conductive plug is subjected to a heat treatment, thereby forming a silicide layer.  
     [0064] First, as shown in FIG. 3A, a LOCOS (local oxidation of silicon) film  51  (thickness: about 50 nm) is formed on the surface of a silicon substrate  10  so as to form a device separation region. Then, by using a known method, a selection transistor is formed having a gate electrode  22 , source/drain regions  24   a  and  24   b.  Thereafter, a first silicon oxide film  52  is formed as an interlayer insulation film with a thickness of about 500 nm by a vapor deposition (CVD) method. Then, a contact hole  30   a  having a diameter of about 0.6 μm is formed.  
     [0065] Next, as shown in FIG. 3B, polysilicon is buried in the contact hole  30   a  by CVD, and thereafter the surface of the polysilicon is flattened by a chemical mechanical polishing (CMP) method, thereby forming the polysilicon conductive plug  30 . Next, the surface of the polysilicon is subjected to a wet process with hydrofluoric acid in order to prevent any spontaneous oxidation films from being formed on the polysilicon, as a pretreatment before forming a Ti film on the polysilicon conductive plug  30 .  
     [0066] Thereafter, a Ti film  142   a  is formed on the polysilicon conductive plug  30  and the first silicon oxide film  52  by DC magnetron sputtering so as to have a thickness of about 1 to about 30 nm (preferably about 5 nm to about 25 nm). If the thickness of the Ti film  142   a  is less than about 1 nm, it becomes difficult to obtain satisfactory contact resistance. If the thickness of the Ti film  142   a  exceeds about 30 nm, the Ti film surface may become coarse after performing an annealing for ferroelectric crystallization.  
     [0067] Thereafter, a Ta x Si 1-x N y  diffusion barrier film  143  (where 0.2≦x≦1 and 0≦y≦1) is formed on the Ti film  142   a  so as to have a thickness of about 50 to about 150 nm (preferably about 80 to about 120 nm) by a DC reactive magnetron sputtering method. If the thickness of the diffusion barrier film  143  is less than about 50 nm, it becomes difficult for the diffusion barrier film  143  to function as a diffusion barrier layer. If the thickness of the diffusion barrier film  143  exceeds about 150 nm, the film thickness of the entire capacitor portion is increased, leading to a poorer fine processing accuracy. The film formation conditions for forming the Ta x Si 1-x N y  film  143  according to the present example are as follows: a target whose Ta/Si molar ratio is about 10/3 is used; the substrate temperature is about 500° C.; a sputter power of about 2 kW is used; a sputter gas pressure of about 0.7 Pa is used; and an Ar/N 2  flow ratio of about 3/2 is used.  
     [0068] After the Ta x Si 1-x N y  film  143  is formed, a heat treatment is conducted in a pure nitrogen atmosphere at about 500° C. to about 800° C. (preferably about 600° C.) for 1 hour, as a result of which the Ti film  142  reacts with the polysilicon to form the silicide layer  142  (thickness: about 2 to about 60 nm). If the treatment temperature is about 500° C. or less, it becomes difficult to ensure sufficient formation of silicide. If the heat treatment temperature exceeds about 800° C., the reaction between the Ti film and the polysilicon may proceed too far during the 1-hour heat treatment, thereby resulting in a somewhat rough silicide surface. Furthermore, if the heat treatment temperature exceeds about 800° C., the Ta x Si 1-x N y  film  143  may itself be unfavorably affected. Instead of pure nitrogen, the heat treatment can employ any other inert gas, e.g., Ar, Kr, or He for similar effects.  
     [0069] An X-ray diffraction analysis was performed to confirm that The Ta x Si 1-x N y  diffusion barrier film  143  had an amorphous structure. Furthermore, the composition of the Ta x Si 1-x N y  was confirmed to be Ta 0.85 Si 0.15 N 0.41  through an Auger electron spectroscopy. The resistivity of the Ta x Si 1-x N y  film  143  after a heat treatment in a pure nitrogen atmosphere was measured to be in the range of about 100 to about 2000 μΩ cm. If the value of x in Ta x Si 1-x N y  is smaller than about 0.2, the Si component becomes excessive, resulting in an extremely high resistivity which is not suitable for the device. Accordingly, the range between about 0.2≦x≦ about 1 is considered suitable.  
     [0070] Thereafter, an Ir lower electrode film  144  is formed on the diffusion barrier film  143  so as to have a thickness of about 50 to about 300 nm (preferably about 100 to about 200 nm) by a DC magnetron sputtering method. If the thickness of the Ir film  144  is less than about 50 nm, the oxygen in the atmosphere may permeate the Ir film  144  during the annealing for ferroelectric crystallization, thereby allowing hillocks to be formed due to volumetric expansion of the Ta x Si 1-x N y  film  143 . If the thickness of the Ir film  144  exceeds about 300 nm, the thickness of the entire capacitor portion is increased, thereby resulting in a poorer fine processing accuracy and/or difficulty to perform processing with a conventional resist film thickness. The film formation conditions for forming the Ir film  144  according to the present example are as follows: a DC power of about 0.5 kW a substrate temperature of about 500° C.; and a gas pressure of about 0.6 Pa.  
     [0071] Next, an SrBi 2 Ta 2 O 9 (SBT) film  146  is formed on the Ir film  144  by a spin-on method. Specifically, the SBT film can be formed as follows: First, a precursor solution is prepared which contains elements for composing the SBT dispersed in a solvent. The precursor solution is applied on a substrate by using a spinner whose rotation rate is set at about 3000 rpm; then the SBT film is dried at about 150° C. in the atmosphere for about 10 minutes; then the SBT film is subjected to a preliminary sintering in the atmosphere at about 400° C. for about 30 minutes; then a crystallization process is performed at about 675° C. for about 60 minutes. After repeating these steps four times, the SBT film has a thickness of about 120 nm. The resultant SBT film does not include hillocks and does not present peeling problems. As a result of a cross-sectional SEM (scanning electron microscopy) observation, it was confirmed that no reaction occurred in the respective layers.  
     [0072] Next, a Pt upper electrode  148  is formed on the SBT film  146  so as to have a thickness of about 100 nm by a DC magnetron sputtering method.  
     [0073] Thereafter, as shown in FIG. 3C, the upper electrode  148  is patterned into areas having a size of about 1 μm×1 μm to about 3 μm×3 μm by a dry etching using Cl 2 . The underlying SBT film  146  is patterned into a predetermined shape by a dry etching using C 2 F 6  and Ar. Then, the Ir lower electrode  144 , the Ta x Si 1-x N y  diffusion barrier film  143 , and the silicide layer  142  are processed so as to have a predetermined shape by a dry etching using Cl 2  and C 2 F 6 .  
     [0074] Thereafter, as shown in FIG. 3D, a second silicon oxide film  54  is formed as an interlayer insulation film by CVD. Then, a contact hole is formed in the upper Pt electrode  148 . An Al take-out electrode  62  is formed by a DC magnetron sputtering method; this Al take-out electrode  62  is to be coupled to the upper Pt electrode  148  of the ferroelectric capacitor. Next, a contact hole is formed in the first silicon oxide film  52  and the second silicon oxide film  54  overlying the source region  24   a,  and an Al take-out electrode  64  is formed. Thus, the ferroelectric memory device as shown in FIG. 3D is accomplished.  
     [0075]FIG. 4 shows a hysteresis curve which is obtained by applying a voltage having a triangular waveform between the Al take-out electrode  62  from the Pt upper electrode  148  and the Al take-out electrode  64  from the source region  24   a  which have been formed by the above-described procedure (it is assumed that the applied triangular-waveform voltage has a field intensity of about 150 kV/cm and a frequency of about 75 Hz). The ferroelectric characteristics were as follows: Pr=about 6 μC/cm 2 ; Ec=about 35 kV/cm; a leak current density with a +3 V applied voltage of about 8×10 −8  A/cm 2 ; and an insulation withstand voltage exceeding about 10 V.  
     [0076] As seen from the results shown in FIG. 4, the above-described method provides satisfactory ferroelectric characteristics although the device is employed as a ferroelectric capacitor. Since the symmetry of the hysteresis curve shown in FIG. 4 is maintained, it can be seen that a satisfactory contact is attained between the silicon substrate  10  and the Ir lower electrode  144 .  
     [0077] The contact characteristics between the polysilicon conductive plug  30  and the lower electrode  144  and the diffusion barrier film  143  were evaluated by using a Kelvin pattern. Hereinafter, a procedure for preparing a Kelvin pattern will be described with reference to FIGS. 5A to  5 H:  
     [0078] First, as shown in FIG. 5A, a LOCOS film  851  is formed on a p-substrate  810 . Next, as shown in FIG. 5B, an n-region  823  is formed by injecting phosphorous into the substrate  810 . Then, after an interlayer insulation film  852  is formed on the entire substrate surface as shown in FIG. 5C, an n-polysilicon plug  830  is buried for electrical conduction with the n-region  823  as shown in FIG. 5D. After the substrate surface is flattened by CMP, a Ti film  842   a,  a TaSiN film  843 , an Ir film  844 , and an SBT film  846  are sequentially formed by the above-described method, as shown in FIG. 5F. Next, the SBT film  846  is subjected to a usual anneal process for crystallization. As a result of performing these steps, the Ti film  842   a  reacts with the polysilicon of the n-polysilicon plug  830  so as to be converted into a Ti silicide layer  842 . Thereafter, as shown in FIG. 5F, the SBT film  846  is removed through dry etching. Next, as shown in FIG. 5G, a hole is formed through the Ti silicide layer  842 , the TaSiN film  843 , and the Ir film  844  so as to reach the interlayer insulation film  852 . Thus, a Kelvin pattern has been produced.  
     [0079]FIG. 5H is a plan view of the structure shown in FIG. 5G. An n-implanted region  880  shown in FIG. 5H for electrically coupling the discrete sections  844 ( a ) to  844 ( c ) of the Ir film  844  is connected to the respective sections  844 ( a ) to  844 ( c ) via a contact section  882 . By using this Kelvin pattern, a constant current is allowed to flow from the Ir electrode pad  844 ( a ) to the pad  844 ( b ), and a decrease in the voltage at the contact section  882  is measured between the Ir electrode pad  844 ( b ) and the pad  844 ( c ). Since the wiring resistance is completely negligible, it is possible to measure only the resistance component at the contact section  882 .  
     [0080]FIG. 6 illustrates the contact diameter dependency of the contact resistance which has been measured by using a Kelvin pattern as described above. In FIG. 6, data  831  represents a resistance value obtained in a structure incorporating the Ti silicide layer  842  of the invention; data  832  represents a resistance value obtained in a structure not incorporating the Ti silicide layer  842 ; and data  833  represents a resistance value which is obtained prior to an anneal crystallization process in a structure not incorporating the Ti silicide layer  842 . Specifically, the data  833  is based on a Kelvin pattern which is prepared by omitting the step of forming an SBT film  846  and anneal crystallization between the step of FIG. 5E (formation of the Ir electrode  844 ) and the step of FIG. 5G.  
     [0081] As seen from FIG. 6, in accordance with the structure not incorporating a Ti silicide layer  842  (data  832 ), the contact resistance value obtained with a 0.6 μm contact diameter is increased about ten-fold after the anneal crystallization process from the value before the anneal. On the other hand, in accordance with the structure according to the present invention (data  831 ), the contact resistance value obtained with a 0.6 μm contact diameter is maintained at as low as about 80 Ω.  
     [0082] The current-voltage characteristics of the respective structures are shown in FIGS. 7A and 7B. As shown in FIG. 7B, the structure without a Ti silicide layer  842  shows a non-linear curve in the vicinity of 0 V, indicative of changing resistance. On the other hand, as shown in FIG. 7A, the structure according to the present invention exhibits a linear curve, indicative of constant resistance. Non-linear current-voltage characteristics are likely to cause problems such as unwanted delay during a high-speed operation and/or poor S/N ratios in the ferroelectric memory operation. Thus, the structure according to the present invention substantially completely solves such problems.  
     [0083]FIG. 8 shows an Auger electron spectroscopy analysis of element concentration, with respect to the depth direction of a device structure of the present invention incorporating a silicide layer, a diffusion barrier, a lower electrode, and an SBT film. The anneal crystallization for the SBT film was performed so that each layer received a 60-minute process in a pure oxygen atmosphere which was maintained at about 675° C., the process being repeated four times. As shown in FIG. 8, mutual diffusion fusion is observed neither between the Ir film and the Ta x Si 1-x N y  nor between the Ta x Si 1-x N y  and the polysilicon after the anneal crystallization process. Thus, it has been indicated that the Ir film functions as a diffusion barrier film. It can also be seen that the Ta x Si 1-x N y  film functions satisfactorily as a barrier film against mutual diffusion of Ir and polysilicon. None of the aforementioned mutual diffusion was observed in a cross-sectional SEM observation, either.  
     EXAMPLE 2  
     [0084] A method for producing a ferroelectric memory device according to Example 2 of the present invention will be described.  
     [0085] In Example 1, a metal silicide is formed by first forming a metal flm (Ti film) and a diffusion barrier film (Ta x Si 1-x N y  film) on a polysilicon plug, and then performing a heat treatment in a pure nitrogen atmosphere. In the present example, a metal film (Ti film) formed on a polysilicon plug is subjected to a heat treatment for silicidation before a diffusion barrier film is formed. Hereinafter, a method for producing a ferroelectric memory device according to the present example will be described with reference to FIGS. 3A to  3 D, which have previously been described in relation to Example 1.  
     [0086] First, as shown in FIG. 3A, a LOCOS film  51  (thickness: about 50 nm) is formed on the surface of a silicon substrate  10  so as to form a device separation region. Then, by using a known method, a selection transistor is formed having a gate electrode  22 , source/drain regions  24   a  and  24   b.  Thereafter, a first silicon oxide film  52  is formed by CVD as an interlayer insulation film with a thickness of about 500 nm. Then, a contact hole  30   a  having a diameter of about 0.6 μm is formed.  
     [0087] Next, as shown in FIG. 3B, polysilicon is buried in the contact hole  30   a  by CVD, and thereafter the surface of the polysilicon is flattened by a CMP method, thereby forming the polysilicon conductive plug  30 . Next, the surface of the polysilicon is subjected to a wet process with hydrofluoric acid in order to prevent any spontaneous oxidation films from being formed on the polysilicon, as a pretreatment before forming a Ti film on the polysilicon conductive plug  30 .  
     [0088] Thereafter, a Ti film  142   a  is formed on the polysilicon conductive plug  30  and the first silicon oxide film  52  by DC magnetron sputtering so as to have a thickness of about 1 to about 30 nm (preferably about 5 nm to about 25 nm).  
     [0089] Thereafter, a rapid thermal annealing (RTA) process is performed in a pure nitrogen atmosphere at about 500° C. to about 950° C. for about 5 to about 120 seconds (this process is preferably performed at about 850° C. for about 10 seconds). If the RTA process temperature is less than about 500° C., it becomes difficult to ensure sufficient formation of silicide. On the other hand, performing a heat treatment above about 950° C. may exert undesirable effects on the CMOS&#39;s (complementary metal oxide semiconductors) in the integrated circuit. As a result of this process, the polysilicon and the Ti film  242   a  react with each other so that a Ti silicide layer  142  (thickness: about 2 to about 60 nm) is formed.  
     [0090] Thereafter, a Ta x Si 1-x N y  diffusion barrier film  143  (where 0.2≦x≦1 and 0≦y≦1) is formed on the Ti film  142   a  so as to have a thickness of about 50 to about 150 nm (preferably about 80 to about 120 nm) by a DC reactive magnetron sputtering method. After the Ta x Si 1-x N y  film  143  is formed, a heat treatment is conducted in a pure nitrogen atmosphere at about 500° C. to about 800° C. (preferably about 600° C.) for 1 hour. The resistivity of the Ta x Si 1-x N y  film  143  after a heat treatment in a pure nitrogen atmosphere was measured to be in the range of about 100 to about 2000 μΩ cm.  
     [0091] The conditions for forming the Ti film  142  and the Ta x Si 1-x N y  diffusion barrier film  143  are identical with those employed in Example 1. The steps subsequent to the formation of the diffusion barrier film  143  (i.e., steps corresponding to FIGS. 3C and 3D) are identical with those employed in Example 1, and the description thereof is omitted.  
     [0092] A contact resistance evaluation was performed by using a Kelvin pattern in a similar manner to that described in Example 1. As a result, it was confirmed that the contact resistance value obtained with a 0.6 μm contact diameter according to the present invention is maintained at as low as about 80 Ω. The current-voltage characteristics of the structure according to the present invention also exhibited a linear curve, indicative of constant resistance. After the SBT film is formed according to the present invention, no hillocks or peeling was observed in the lower electrode.  
     [0093] Although a Ti film is employed as a metal film forming the silicide layer in Examples 1 and 2, similar effects can also be attained by employing a metal element selected from IV-A group elements (Zr and Hf), V-A group elements (V, Nb, and Ta), VI-A group elements (Cr, Mo, and W), and VIII group elements (Ru, Os, Co, Rh, Ir, Ni, Pd and Pt), instead of Ti.  
     EXAMPLE 3  
     [0094] A method for producing a ferroelectric memory device according to Example 3 of the present invention will be described.  
     [0095] In Examples 1 and 2, a metal film (Ti film) is formed on a polysilicon plug, and the metal film is subjected to a heat treatment to form a metal silicide layer. In the present example, a metal silicide layer is directly formed on a polysilicon plug.  
     [0096] First, the structure shown in FIG. 3A is formed by a method similar to those described in Examples 1 and 2, and a polysilicon plug  30  is further formed.  
     [0097] Next, a layer  142   b  containing Ti and Si is formed on the polysilicon film so as to have a thickness of about 1 to about 30 nm (preferably about 5 to about 25 nm) by a DC magnetron sputtering method. The film formation conditions for forming the layer  142   b  according to the present example are as follows: a mixture target whose Ti/Si molar ratio is about 10/3 is used; the substrate temperature is about 500° C.; a sputter power of about 2 kW is used; a sputter gas pressure of about 0.7 Pa is used; and an Ar sputter gas is used. Since the substrate temperature is low, the layer  142   b  has not been converted into a silicide film.  
     [0098] Thereafter, a Ta x Si 1-x N y  diffusion barrier film  143  (where 0.2≦x≦1 and 0≦y≦1) is formed on the layer  142   b  containing Ti and Si so as to have a thickness of about 50 to about 150 nm (preferably about 80 to about 120 nm) by a DC reactive magnetron sputtering method. Thereafter, a heat treatment is conducted in a pure nitrogen atmosphere at about 500° C. to about 800° C. (preferably about 600° C.) for 1 hour. This heat treatment causes the layer  142   b  containing Ti and Si to react with the polysilicion so as to be converted into a silicide layer  142  (thickness: about 2 to about 60 nm). The resistivity of the Ta x Si 1-x N y  film  143  after a heat treatment in a pure nitrogen atmosphere was measured to be in the range of about 100 to about 2000 μΩ cm.  
     [0099] The steps subsequent to the formation of the diffusion barrier film  143  (i.e., steps corresponding to FIGS. 3C and 3D) are identical with those employed in Example 1, and the description thereof is omitted.  
     [0100] Although the conductive plug  30  illustrated in the present example is formed of polysilicon, it may alternatively be formed of tungsten.  
     [0101] A contact resistance evaluation was performed by using a Kelvin pattern in a similar manner to that described in Example 1. As a result, it was confirmed that the contact resistance value obtained with a 0.6 μm contact diameter according to the present invention is maintained at as low as about 120 Ω. The current-voltage characteristics of the structure according to the present invention also exhibited a linear curve, indicative of constant resistance. After the SBT film is formed according to the present invention, no hillocks or peeling was observed in the lower electrode.  
     EXAMPLE 4  
     [0102] A method for producing a ferroelectric memory device according to Example 4 of the present invention will be described.  
     [0103] In Example 3, a layer  142   b  containing Ti and Si is formed on a polysilicon plug, and a diffusion barrier film  143  is formed, and thereafter the metal film is subjected to a heat treatment to convert the layer  142   b  into a silicide layer  142 . In the present example, a Ti silicide film  142  is first formed on polysilicon, and thereafter a diffusion film  143  is formed.  
     [0104] First, the structure shown in FIG. 3A is formed by a method similar to those described in Examples 1 and 2, and a polysilicon plug  30  is further formed.  
     [0105] Next, as shown in FIG. 3B, a layer  142   b  containing Ti and Si is formed on the polysilicon film so as to have a thickness of about 1 to about 30 nm (preferably about 5 to about 25 nm) by a DC magnetron sputtering method. The film formation conditions for forming the layer  142   b  according to the present example are as follows: a mixture target whose Ti/Si molar ratio is about 10/3 is used; the substrate temperature is about 500° C.; a sputter power of about 2 kW is used; a sputter gas pressure of about 0.7 Pa is used; and an Ar sputter gas is used.  
     [0106] Thereafter, an RTA process is performed in a pure nitrogen atmosphere at about 500° C. to about 950° C. for about 5 to about 120 seconds (this process is preferably performed at about 850° C. for about 10 seconds). As a result of this process, the layer  142   b  containing Ti and Si reacts with the polysilicon so as to be converted into a Ti silicide layer  142 .  
     [0107] Thereafter, a Ta x Si 1-x N y  diffusion barrier film  143  (where 0.2≦x≦1 and 0≦y≦1) is formed on the Ti silicide layer  142  so as to have a thickness of about 50 to about 150 nm (preferably about 80 to about 120 nm) by a DC reactive magnetron sputtering method. Thereafter, a heat treatment is conducted in a pure nitrogen atmosphere at about 500° C. to about 800° C. (preferably about 600° C.) for  1  hour. The resistivity of the Ta x Si 1-x N y  film  143  after a heat treatment in a pure nitrogen atmosphere was measured to be in the range of about 100 to about 2000 μΩ cm.  
     [0108] The steps subsequent to the formation of the diffusion barrier film  143  (i.e., steps corresponding to FIGS. 3C and 3D) are identical with those employed in Example 1, and the description thereof is omitted.  
     [0109] A contact resistance evaluation was performed by using a Kelvin pattern in a similar manner to that described in Example 1. As a result, it was confirmed that the contact resistance value obtained with a 0.6 μm contact diameter according to the present invention is maintained at as low as about 130 Ω. The current-voltage characteristics of the structure according to the present invention also exhibited a linear curve, indicative of constant resistance. After the SBT film is formed according to the present invention, no hillocks or peeling were observed in the lower electrode.  
     [0110] Although a Ti film is employed as a metal film forming the silicide layer  142  in Examples 3 and 4, similar effects can also be attained by employing a metal element selected from IV-A group elements (Zr and Hf), V-A group elements (V, Nb, and Ta), VI-A group elements (Cr, Mo, and W), and VIII group elements (Ru, Os, Co, Rh, Ir, Ni, Pd and Pt), instead of Ti.  
     EXAMPLE 5  
     [0111] A method for producing a ferroelectric memory device according to Example 5 of the present invention will be described. In the present example, a Ti film and a layer containing Ti and Si are sequentially formed on a polysilicon plug  30 , whereupon a Ta x Si 1-x N y  diffusion barrier film  143  is formed. Thereafter, a heat treatment is performed in a pure nitrogen atmosphere to form a Ti silicide layer.  
     [0112] First, the structure shown in FIG. 3A is formed by a method similar to those described in Examples 1 and 2, and a polysilicon plug  30  is further formed. Next, the surface of the polysilicon is subjected to a wet process with hydrofluoric acid in order to prevent any spontaneous oxidation films from being formed on the polysilicon, as a pretreatment before forming a Ti film on the polysilicon conductive plug  30 .  
     [0113] Thereafter, a Ti film  142   a  is formed on the polysilicon conductive plug  30  and a first silicon oxide film  52  by DC magnetron sputtering so as to have a thickness of about 1 to about 30 nm (preferably about 5 nm to about 25 nm). Next, a layer containing Ti and Si is formed on the polysilicon film so as to have a thickness of about 1 to about 30 nm (preferably about 5 to about 25 nm) by a DC magnetron sputtering method. The film formation conditions for forming the layer containing Ti and Si according to the present example are as follows: a mixture target whose Ti/Si molar ratio is about 10/3 is used; the substrate temperature is about 500° C.; a sputter power of about 2 kW is used; a sputter gas pressure of about 0.7 Pa is used; and an Ar sputter gas is used. The structure including the Ti film and the Ti/Si-containing layer is referred to as a layer  142   c  (FIG. 3B).  
     [0114] Thereafter, a Ta x Si 1-x N y  diffusion barrier film  143  (where 0.2≦x≦1 and 0≦y≦1) is formed on the layer  142   c  so as to have a thickness of about 50 to about 150 nm (preferably about 80 to about 120 nm) by a DC reactive magnetron sputtering method.  
     [0115] After the formation of the Ta x Si 1-x N y  film  143 , a heat treatment is conducted in a pure nitrogen atmosphere at about 500° C. to about 800° C. (preferably about 600° C.) for 1 hour. As a result of this heat treatment, the Ti film within the layer  142   c  reacts with the underlying polysilicon so as to be converted into a Ti silicide layer. Also, the Ti/Si-containing layer within the layer  142   a  is converted into a Ti silicide layer. Thus, a Ti silicide layer  142  (having a total thickness of about 2 to about 60 nm) is formed. The resistivity of the Ta x Si 1-x N y  film  143  after a heat treatment in a pure nitrogen atmosphere was measured to be in the range of about 100 to about 2000 μΩ cm.  
     [0116] The steps subsequent to the formation of the diffusion barrier film  143  (i.e., steps corresponding to FIGS. 3C and 3D) are identical with those employed in Example 1, and the description thereof is omitted.  
     [0117] A contact resistance evaluation was performed by using a Kelvin pattern in a similar manner to that described in Example 1. As a result, it was confirmed that the contact resistance value obtained with a 0.6 μm contact diameter according to the present invention is maintained at as low as about 130 Ω. The current-voltage characteristics of the structure according to the present invention also exhibited a linear curve, indicative of constant resistance. After the SBT film is formed according to the present invention, no hillocks or peeling were observed in the lower electrode.  
     [0118] Similar effects can also be attained by employing, instead of Ti, any other metal element as exemplified in Examples 1 to 4 as a metal element composing the metal film (illustrated as a Ti film in the present example) and the other layer (illustrated as a layer containing Ti and Si in the present example).  
     EXAMPLE 6  
     [0119] A method for producing a ferroelectric memory device according to Example 6 of the present invention will be described. In the present example, a metal silicide layer is formed on a polysilicon plug  30 , whereupon a further metal layer is formed.  
     [0120] First, the structure shown in FIG. 3A is formed by a method similar to those described in Examples 1 and 2, and a polysilicon plug  30  is further formed. Next, the surface of the polysilicon is subjected to a wet process with hydrofluoric acid in order to prevent any spontaneous oxidation films from being formed on the polysilicon, as a pretreatment before forming a Ti film on the polysilicon conductive plug  30 .  
     [0121] Thereafter, a Ti film  142   a  is formed on the polysilicon conductive plug  30  and a first silicon oxide film  52  by DC magnetron sputtering so as to have a thickness of about 1 to about 30 nm (preferably about 5 nm to about 25 nm) (FIG. 3B).  
     [0122] Thereafter, an RTA process is performed in a pure nitrogen atmosphere at about 500° C. to about 950° C. for about 5 to about 120 seconds (this process is preferably performed at about 850° C. for about 10 seconds). As a result of this process, the Ti film  142   a  reacts with the polysilicon so as to be converted into a Ti silicide layer  142 . If the RTA process temperature is less than about 500° C., it becomes difficult to ensure sufficient formation of silicide. On the other hand, performing a heat treatment above about 950° C. may exert undesirable effects on the CMOS&#39;s in the integrated circuit.  
     [0123] Thereafter, a Ti film (not shown in FIG. 3B) is again formed on the Ti silicide layer  142  so as to have a thickness of about 1 to 30 nm (preferably about 5 to about 25 nm).  
     [0124] Thereafter, a Ta x Si 1-x N y  diffusion barrier film  143  (where 0.2≦x≦1 and 0≦y≦1) is formed on the Ti film so as to have a thickness of about 50 to about 150 nm (preferably about 80 to about 120 nm) by a DC reactive magnetron sputtering method. After the formation of the Ta x Si 1-x N y  film  143 , a heat treatment is conducted in a pure nitrogen atmosphere at about 500° C. to about 800° C. (preferably about 600° C.) for 1 hour. Even after this heat treatment, the Ti film formed on the Ti silicide layer  142  is not converted into a Ti silicide layer, but remains as a metal film between the Ti silicide layer  142  and the diffusion barrier film  143 . The resistivity of the Ta x Si 1-x N y  film after a heat treatment in a pure nitrogen atmosphere was measured to be in the range of about 100 to about 2000 μΩ cm.  
     [0125] The steps subsequent to the formation of the diffusion barrier film  143  (i.e., steps corresponding to FIGS. 3C and 3D) are identical with those employed in Example 1, and the description thereof is omitted.  
     [0126] According to the present example, the contact resistance is further reduced due to the presence of the Ti film between the Ti silicide layer  142  and the diffusion barrier film  143 .  
     [0127] A contact resistance evaluation was performed by using a Kelvin pattern in a similar manner to that described in Example 1. As a result, it was confirmed that the contact resistance value obtained with a 0.6 μm contact diameter according to the present invention is maintained at as low as about 80 Ω. The current-voltage characteristics of the structure according to the present invention also exhibited a linear curve, indicative of constant resistance. After the SBT film is formed according to the present invention, no hillocks or peeling was observed in the lower electrode.  
     [0128] Similar effects can also be attained by employing, instead of Ti, any other metal element as exemplified in Examples 1 to 4 as a metal element composing the silicide layer  142  and the metal film upon the silicide layer  142 .  
     [0129] According to the present invention, the following effects are provided.  
     [0130] Since a metal silicide layer is provided between a conductive plug and a diffusion barrier film, any increase in the resistance after the annealing for ferroelectric crystallization can be prevented. As a result, problems such as unwanted delay during a high-speed operation and/or poor S/N ratios in the ferroelectric memory operation can be prevented.  
     [0131] By combining a diffusion barrier film having a defined composition and a lower electrode having a defined material, it becomes possible to provide an electrode/diffusion barrier structure having sufficient stability (i.e., substantially free of hillocks and peeling).  
     [0132] Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.