# Patent

## Title
Ferroelectric semiconductor device and method of manufacturing the same

## Abstract

According to the present invention, contact plugs are formed by a CVD method without deteriorating the properties of the ferroelectric capacitor in a semiconductor device having a fine ferroelectric capacitor. Adhesive film is formed in a contact hole, which exposes an upper electrode of the ferroelectric capacitor after conducting heat treatment in an oxidizing atmosphere, and a W layer is deposited by the CVD method using such TiN adhesive film as a hydrogen barrier and the contact hole is filled.

## Background
BACKGROUND OF THE INVENTION
This application claims the benefit of a Japanese Patent Application No. 2001-350323 filed Nov. 15, 2001 in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to semiconductor devices, and more particularly to a semiconductor device having a ferroelectric film.
Semiconductor devices such as so-called Dynamic Random Access Memory (DRAM)s or Static Random Access Memory (SRAM)s are used extensively as a high-speed main memory device in various information processing apparatuses including computers. These semiconductor devices, however, have volatile memories and the information stored therein is lost when the power is turned OFF. Conventionally, nonvolatile magnetic disk units have been used as large-capacity secondary storage devices for storing programs and data.
However, magnetic disk units have disadvantages in that they are bulky and fragile, have large power consumption, and furthermore in that they have low access speed upon reading and writing information. Recently, Electrically Erasable Programmable Read Only Memory (EEPROM)s or flash-memory devices are frequently used for storing information in a floating-gate electrode in the form of electrical charges. Particularly, flash-memory devices tend to have a high integration density because they have cell construction similar to that of DRAM, and are expected to become the large-capacity storage device comparable to a magnetic disk unit.
In EEPROM and flash-memory devices, writing of information is achieved by injecting hot electrons through a tunneling insulation film into the floating gate electrode. Thus, a flash-memory device has drawbacks in that it inevitably takes a substantial time for writing and tunneling insulation film deteriorates after writing and erasing operations are repeated a number of times. When tunneling insulation film is deteriorated, operations of writing and erasing become unstable.
In view of the above, a ferroelectric memory device(designated hereinafter as FeRAM) for storing information in a ferroelectric film in the form of a spontaneous polarization is proposed. In such FeRAM, an individual memory cell transistor, consisting of a single MOSFET as in the case of DRAM, comprises a structure in which dielectric film in the memory cell capacitor is replaced by ferroelectric material such as PZT (Pb(Zr, Ti)O3) or PLZT(Pb(Zr,Ti,La)O3), and furthermore, SBT(SrBi2Ta2O3), or SBTN(SrBi2(Ta,Nb)2O3); and it becomes possible to integrate at high integration density. Moreover, FeRAM has an advantageous feature that because it controls the spontaneous polarization of the ferroelectric capacitor by applying an electric field, writing operation is achieved with a high speed, faster by a factor of 1000 or more than the EEPROM or flash-memory devices which carries out writing by injecting hot electrons, and the power consumption is reduced to about 1/10 compared to the EEPROM or flash-memory device. Furthermore, lifespan is longer because there is no need to use the tunneling oxide film; the FeRAM device can be erased 10 million times or more.
2. Description of the Related Art
FIG. 1 shows a structure of FeRAM 10 disclosed in a Japanese Laid-Open Patent Application No. 2000-156470.
Referring to FIG. 1, FeRAM 10 is formed on a Si substrate 11 in correspondence to an active region defined by a device isolating insulation film 12. FeRAM 10 includes gate electrode 13 formed on the Si substrate 11 via gate insulation film not shown in the Figure, and diffusion regions 11A and 11B formed on either side of the gate electrode 13 in the Si substrate 11.
Interlayer insulation film 14 is formed on the Si substrate 11 so as to cover the gate electrode 13. A ferroelectric capacitor in which a lower electrode 15, a ferroelectric film 16, and an upper electrode 17 are consecutively layered is formed on the interlayer insulation film.
The ferroelectric capacitor is covered with a separate interlayer insulation film 18 formed on the interlayer insulation film 14. Contact holes 18A, 18B that pass through the interlayer insulation film 14 and expose diffusion regions 11A and 11B, respectively, are formed in the interlayer insulation film 18. Additionally, contact hole 18C exposing the lower electrode 15, and contact hole 18D exposing the upper electrode 17 are formed in the interlayer insulation film 18.
In FeRAM 10 of FIG. 1, contact holes 18A through 18D are filled with contact plugs 19A through 19D, respectively, with a consecutively layered structure of TiN film, A—Cu film, TiN film and WSi film. Furthermore, interconnecting patterns contacting the contact plugs are formed on the interlayer insulation film 18.
Recently in particular, conductive oxide such as IrOx or SrRuOx is frequently used as the upper electrode of the ferroelectric capacitor used in FeRAM. Such conductive oxide is similar to ferroelectric film consisting of oxide in terms of chemical and crystallographic properties, and is convenient in optimizing the electrical property of ferroelectric film.
In the FeRAM 10 of FIG. 1, contact plugs 19A through 19D are formed by a sputtering method. However, in the sputtering method, if a semiconductor device is miniaturized, step coverage in contact holes 18A through 18D easily form defects and a problem of yield and reliability arises.
In a general semiconductor device, such contact plugs are frequently formed by depositing a W layer by means of a Chemical Vapor Deposition (CVD) method, and then removing the W layer on the insulation film by means of a Chemical Mechanical Planarization (CMP) method. It is possible to securely fill the contact holes with W plugs by the CVD method even in contact holes where the aspect ratio is large.
However, in semiconductors with ferroelectric film such as FeRAM, attempts to deposit a W layer by means of the CVD method develop a problem where H2 contained in the atmosphere during deposition acts on the ferroelectric film and reduces the ferroelectric film. When ferroelectric film is reduced, the electrical property with a characteristic of desired hysteresis is lost.
In an attempt to avoid this problem, steps of forming W plugs for diffusion regions, and then forming contact plugs for the ferroelectric capacitor can be considered.
In FeRAM 10 of FIG. 1, it may be considered to, for example, first form contact holes 18A and 18B, and then after filling the contact holes 18A, 18B with W plugs 19A and 19B, respectively, to form contact holes 18C, 18D. According to such staging, in the formation of W plugs 19A and 19B, a ferroelectric film 16 is sealed by interlayer insulation film 18 and the problem of ferroelectric film reduction can be avoided.
In such a method, however, it is necessary to form contact holes 18C and 18D in the ferroelectric capacitor by a dry etching method. This dry etching method involves, especially if upper electrode 17 is formed by conductive oxide film, the upper electrode 17 and ferroelectric film 16 underlying the upper electrode will be partially reduced, and a problem of oxygen defect arises.
For this reason, in the case where contact holes 18C and 18D are formed afterward, heat treatment in the oxidizing atmosphere to compensate for oxygen defect is necessary. However, when conducting heat treatment in the oxidizing atmosphere, previously formed W plugs 19A and 19B oxidize, and a problem concerning increase in contact resistance arises.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a novel and useful semiconductor device that solves the above discussed problems and a method of manufacturing the same.
More particularly, it is a specific object of the present invention to provide a method of manufacturing a semiconductor device having a ferroelectric capacitor, which method forms contact plugs that contact the upper electrode of the ferroelectric capacitor, by means of the CVD method without oxidizing the conductor contact plugs composed of a material such as W, while suppressing the reduction of ferroelectric film in the ferroelectric capacitor. A further object of the present invention is to provide a semiconductor device formed from such manufacturing method.
The above object is achieved by providing a semiconductor device having a semiconductor substrate; a transistor formed on the semiconductor substrate and including a diffusion region formed in the semiconductor substrate; a capacitor formed on the semiconductor substrate, and having a consecutively layered structure of a lower electrode, a ferroelectric film, and an upper electrode; an insulation film formed on the semiconductor substrate so as to cover the capacitor; a first contact hole formed in the insulation film so as to expose the upper electrode of the capacitor; a second contact hole formed in the insulation film so as to expose the diffusion region; a first conductive plug formed in the first contact hole; and a second conductive plug formed in the second contact hole; a conductive nitride film is formed between the first conductive plug and the upper electrode so as to contact a inner wall of the first contact hole and a surface of the upper electrode.
The present invention further achieves the above object by providing a method of manufacturing a semiconductor device comprising a semiconductor substrate, a transistor formed on the semiconductor substrate and including a diffusion region formed in the semiconductor substrate; a ferroelectric capacitor formed on the semiconductor substrate and having a consecutively layered structure of a lower electrode, a ferroelectric film, and an upper electrode; wherein the method comprises the steps of: (a) forming an insulation film on the semiconductor substrate so as to cover the ferroelectric capacitor; (b) forming a first contact hole and a second contact hole on the insulation film so as to expose the upper electrode and the lower electrode, respectively; (C) conducting heat treatment on the ferroelectric capacitor in an oxidizing atmosphere; (d) forming a third contact hole in the insulation film so as to expose the diffusion region; (e) forming a conductive nitride film on the insulation film so as to cover the first, second, and third contact holes; and (f) forming a conductive layer on the conductive nitride film so that the conductive layer fills the first, second, and third contact holes.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing a structure of a conventional FeRAM;
FIG. 2 is a diagram showing a structure of a FeRAM according to a first embodiment of the present invention;
FIGS. 3A-3I are diagrams showing the manufacturing steps of a FeRAM according to a second embodiment of the present invention;
FIGS. 4A-4C are diagrams showing the manufacturing steps of a FeRAM according to a third embodiment of the present invention;
FIGS. 5A-5H are diagrams showing the manufacturing steps of a FeRAM according to a fourth embodiment of the present invention;
FIGS. 6A-6B are diagrams showing a variant of the fourth embodiment according to the present invention.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
The inventor of the present invention has recognized through experiments constituting the basis of the present invention that conductive nitride film acts as an effective hydrogen barrier. In view of the above, the present invention thus proposes to form a conductive nitride film in contact holes exposing upper and lower electrodes of the ferroelectric capacitor after conducting heat treatment in the oxidizing atmosphere that accompanies the formation of contact holes. The problem of a hydrogen atmosphere acting on the ferroelectric capacitor via the contact holes and deteriorating the electric properties can be avoided during the subsequent CVD process that uses a hydrogen atmosphere by forming conductive nitride film in contact holes as described above. It is thus possible through the CVD method, a preferred method for step coverage, to form a low resistance metal plug that fills the contact hole or contact holes exposing diffusion regions, and is also possible to manufacture the semiconductor device without introducing a deterioration of properties.
First Embodiment
FIG. 2 shows a construction of FeRAM 20 according to a first embodiment of the present invention.
Referring to FIG. 2, FeRAM 20 is formed on a Si substrate 21 which may either be of p-type or of n-type, and which has a p-type well 21A and an n-type well 21B defined by field insulation film 22. A gate electrode 24A of polycide structure is formed on the p-type well 21A through gate insulation film 23A. Similarly, a gate electrode 24B of polycide structure is formed on the n-type well 21B through gate insulation film 23B. Furthermore, in the p-type well 21A, n-type diffusion regions 21a, 21b are formed on either side of the gate electrode 24A, and in the n-type well 21B, p-type diffusion regions 21c, 21d are formed on either side of the gate electrode 24B. The gate electrode 24A extends over the field oxide film 22 outside the active region and constitutes part of a word line (WL) of FeRAM.
Each gate electrode 24A, 24B has a sidewall insulation film, and is covered with a SiON cover film 25 formed on the Si substrate 21 by a CVD method with a thickness of about 200 nm so as to cover the field insulation film 22.
The cover film 25 is further covered with a SiO2 interlayer insulation film 26 with a thickness of about 1 μm and formed by a CVD method using TEOS gas as the precursor. The surface of the interlayer insulation film 26 is then planarized by a CMP method.
A ferroelectric capacitor having a consecutively layered structure is formed on the planarized surface of the interlayer insulation film 26, wherein this structure includes a lower electrode 27 in which a Ti film with a thickness of between 10 to 30 nm, preferably about 20 nm and a Pt film with a thickness of between 100 to 300 nm, preferably about 175 nm are consecutively layered; a ferroelectric capacitor insulation film 28 consisting of PZT(Pb(Zr,Ti)O3) or PZLT((Pb,La)(Zr,Ti)O3) with a thickness of between 100 to 300 nm, preferably about 240 nm; and an upper electrode 29 consisting of IrOx with a thickness of between 100 to 300 nm, preferably about 200 nm and formed on the ferroelectric capacitor insulation film 28. The Ti film and Pt film are formed typically by sputtering, and the ferroelectric capacitor insulation film 28 is crystallized typically by conducting rapid heat treatment for 20 seconds at 725° C. in an oxidizing atmosphere after sputtering. Preferably, ferroelectric film 28 is added with Ca and Sr, and can be formed by methods other than sputtering such as the spin-on method, sol-gel method, metal organic deposition (MOD) method or MOCVD method. It is also possible to use other films such as SBT(SrBi2(Ta,Nb)2O9)film, or BTO(Bi4Ti2O12)film for ferroelectric capacitor insulation film 28 instead of PZT or PLZT film. Furthermore, it is possible to form DRAM by using high dielectric film such as BST ((Ba,Sr)TiO3)film or STO(SrTiO3)film instead of ferroelectric capacitor insulation film 28. IrOx film constituting the upper electrode 29 is then formed typically by sputtering. It is still possible to use Pt film or SRO (SrRuO3) film for the upper electrode 29 instead of IrOx film.
The ferroelectric capacitor formed in this manner is covered with an encapsulating layer 330A of Al2O3 with a thickness of about 50 nm, and formed by the sputtering method under ordinary temperature. This encapsulating layer 330A is covered with a separate encapsulating layer 330 of Al2O3 with a thickness of about 20 nm, formed on the interlayer insulation film 26 by sputtering.
SiO2 interlayer insulation film 30 whose thickness is about 400 nm from the top of the upper electrode is formed on the encapsulating layer 330 by a CVD method, preferably the plasma CVD method which uses polysilane compound such as SiH4, Si2F6, Si3F8, Si2F3Cl or SiF4, or TEOS as precursors. Contact holes 30A, 30B exposing the upper electrode 29 and the lower electrode 27, respectively, are formed in the interlayer insulation film 30. Contact holes 30C, 30D, 30E and 30F exposing diffusion regions 21a, 21b, 21c, and 21d, respectively, and extending through the interlayer insulation film 26 are also formed. Furthermore, a contact hole 30G exposing a word line pattern WL formed on the element isolating film 22 is formed through the interlayer insulation film 30.
In this embodiment, an adhesive film 31A or 31B with a thickness of about 50 nm and consisting of conductive nitride such as TiN is formed in each contact hole 30A and 30B so as to directly contact the inner wall planes of the respective contact holes and to directly contact the surface of the upper electrode 29 or the lower electrode 27. In contact hole 30A, a conducting plug 32A consisting of W is formed on the TiN adhesive film 31A, and in contact hole 30B, a conducting plug 32B consisting of W is formed on the TiN adhesive film 31B, both through CVD method using mixed gas of WF6, Ar, and H2. In this embodiment, TiN adhesive films 31C through 31G are formed on the inner wall plane of contact holes 30C through 30G, respectively, in the same manner, and W plugs 32C through 32G are formed on the TiN adhesive films 31C through 31G, respectively, to fill the respective contact holes.
Interconnect patterns 33A through 33F consisting of Al are formed on the interlayer insulation film 30 in correspondence to each W plug 32A through 32G, and the interconnect patterns 33A through 33F are covered with a next interlayer insulation film 34 consisting of SiO2 formed by plasma CVD method. Similar to the interlayer insulation film 30, the interlayer insulation film 34 can be formed using polysilane compound such as SiH4, Si2F6, Si3F8, or SiF3Cl, or TEOS as precursors.
Furthermore, a protective insulation film 35 consisting of SiO2 with a thickness of more than 100 nm is formed on the interlayer insulation film 34 by CVD method. The protective insulation film 35 formed in this way covers the slit (cavity) exposed by planarization stage (CMP), which follows the formation of interlayer insulation film 34.
Contact holes 35A, 35B, which pass through the protective insulation film 35 and expose interconnect patterns 33A and 33F, are formed in the interlayer insulation film 34. Furthermore, W plugs 37A, 37B are formed on the inner wall plane of contact holes 35A, 35B through TiN adhesive films 36A and 36B.
Furthermore, interconnect patterns 38A, 38B consisting of Al or Al alloy, which contact the W plugs 37A, 37B are formed on the protective insulation film 35. TiN adhesive films 36A, 36B covering the inner wall planes of the contact holes 35A, 35B then extend between the interconnect patterns 38A or 38B and the protective insulation film 35.
The interconnect patterns 38A, 38B are covered with an interlayer insulation film 39 formed in a similar way as the interlayer insulation film 30 or 34. The interlayer insulation film 39 is covered with a protective insulation film 40 similar to the protective insulation film 35, and interconnect patterns 41A through 41E including a bit line (BL) are formed on the protective insulation film 40.
The inventor of the present invention has recognized through experiments constituting the basis of the present invention, that upon forming W plugs 32A, 32B for contact holes 30A, 30B respectively, by covering the inner wall planes and the bottom planes of contact holes 30A, 30B continuously with respective TiN films 31A and 31B, entering of hydrogen into the upper electrode 29 and the ferroelectric film 28 can be effectively suppressed, and furthermore deterioration in electrical properties of the ferroelectric film 28 can be effectively suppressed when W plugs 32A, 32B are formed through CVD method using mixed gas of WF6, Ar and H2. On the other hand, when using a film with a conventional layered structure of Ti film and TiN film for the adhesive film covering the inner wall planes and the bottom planes of the related contact holes, particularly contact hole 30A exposing the upper electrode 29, it can be observed that contact resistance increases significantly. This is shown in the table below.












contact metal for




upper electrode
S/D contact
contact resistance


and S/D
resistance
of upper electrode

















TiN (50 nm)/Ti (20 nm)
7.7
61.1


TiN (50 nm)
8.3
8.0









Referring to Table 1, it can be seen that when adhesive film with a layered structure of Ti film with a thickness of 20 nm and TiN film with a thickness of 50 nm was used, the contact resistance in contact hole 30C or 30D exposing diffusion region 21a or 21b was 7.7 Ω/via, whereas the contact resistance in contact hole 30A exposing upper electrode 29 was increased to 61.1Ω/via.
As opposed to this, when Tin film with a thickness of 50 nm was used, contact resistance was 8.0-8.3Ω/via in both contact holes, and increase of contact resistance in contact hole 30A was not observed.
It is possible to consider that when layered film of Ti film and TiN film is used, the reaction

Ti+IrOx→TiOx+Ir

takes place when Ti film contacts the IrOx upper electrode 29, and the TiOx film formed as a result causes the contact resistance to increase. To the contrary, if only TiN film is used for the adhesive film, such reaction will not take place and thus the contact resistance will not increase.

TiN film has a composition represented by TiNx, and ideally has a stoichiometric composition represented by Ti3N4. Accordingly, it is considered that suppression effect against increase in contact resistance appear significantly as the composition of TiN adhesive film becomes closer to the above stoichiometric composition. Furthermore, the suppression effect against increase in contact resistance that accompany the use of TiN adhesive film is not limited to the case in which the upper electrode 29 is an IrOx film, but is considered also effective with conductive oxide film such as SrRuOx film.
Second Embodiment
Here, the manufacturing steps of FeRAM of FIG. 2 are explained as a second embodiment of the present invention with reference to FIG. 3A through FIG. 3I. In the figures, components corresponding to those described earlier have identical reference numbers and thus their explanations are omitted.
Referring to FIG. 3A, SiO2 interlayer insulation film 26 is formed with a thickness of about 1 μm on a Si substrate 21, which is formed with diffusion regions 21a through 21d and having polycide gate electrodes 24A, 24B, by plasma CVD method using TEOS as the precursor so as to cover the gate electrodes 24A, 24B. The interlayer insulation film 26 is planarized by CMP method. Ti film and Pt film with a thickness of 20 nm and 175 nm respectively are consecutively deposited, and as previously mentioned, ferroelectric film with a thickness of 240 nm added preferably with Ca and Sr is formed thereon by sputtering. The PLZT film formed in this way is crystallized by conducting rapid heat treatment for 20 seconds at 725° C. with a temperature rise of 125° C./sec. in an oxidizing atmosphere.
After the ferroelectric film is crystallized, IrOx film with a thickness of 200 nm is formed on the ferroelectric film by sputtering.
IrOx formed in this way is patterned by a resist process, and the upper electrode 29 is formed. After the resist process, the ferroelectric film is subjected to heat treatment for 60 minutes at 650° C. in the oxidizing atmosphere again, and defects produced in the ferroelectric film during the sputtering stage and the patterning stage of IrOx film are compensated for.
Next, a resist pattern including the upper electrode 29 is formed and the ferroelectric film is patterned using such resist pattern as the mask. The ferroelectric capacitor insulation film 28 is then formed. After the ferroelectric capacitor insulation film 28 is formed, the interlayer insulation film 26 is dehydrated by further conducting heat treatment in a nitrogen atmosphere.
Furthermore, encapsulating layer 330A for protecting ferroelectric capacitor insulation film 28 from H2 is formed by sputtering Al2O3 film at ordinary temperature so as to cover the ferroelectric capacitor insulation film 28 and the upper electrode 29. It is also possible to deposit PZT film, PLZT film or TiOx film instead of Al2O3 film for the encapsulating layer film 330A. After encapsulating layer 330A is formed, heat treatment is conducted for 60 minutes at 550° C. in an oxidizing atmosphere enhancing the film property of strong encapsulating layer 330A.
Furthermore, a resist pattern is formed on the encapsulating layer 330A, and Pt/Ti layer is patterned using such resist pattern as the mask and the lower electrode 27 is formed.
The resist pattern used during patterning of lower electrode 27 is removed, and the lower electrode is subjected to heat treatment for 30 minutes at 350° C. The encapsulating layer 330 is formed on the interlayer insulation layer 26 by sputtering Al2O3 in a manner so that encapsulating layer 330 covers the underlying encapsulating layer 330A.
In the step shown in FIG. 3A, after forming the encapsulating layer 330, heat treatment is conducted for 30 minutes at 650° C. in the oxidizing atmosphere, compensating for the damage caused in the ferroelectric capacitor insulation film 28. Furthermore, as previously described, interlayer insulation film 30 with a thickness of about 1200 nm is formed on the encapsulating layer 330 through a plasma CVD method using a polysilane compound such as SiH4, Si2F6, Si3F8 or Si2F3Cl, or SiF4 as precursors. It is also possible to form interlayer insulation film 30 using TEOS as the precursor. Moreover, the thermal excitation CVD method and laser excitation CVD method may be used besides plasma CVD method. After the interlayer insulation film 30 is formed, it is polished until the thickness measured from the surface of the upper electrode 29 amounts to about 400 nm, and then planarized.
In the step shown in FIG. 3B, after the stage in which the interlayer insulation film 30 is dehydrated using N2 plasma or N2O plasma, contact holes 30A and 30B, which pass through the encapsulating layers 330 and 330A and expose the upper electrode 29 and lower electrode 27, respectively, are formed in the interlayer insulation film 30 by a resist process using mixed gas of CHF3, CF4, and Ar.
In the step shown in FIG. 3B, the structure formed is subjected to heat treatment for 60 minutes at 550° C. in the oxidizing atmosphere, and the deterioration in quality of the insulation film 28 of ferroelectric capacitor that accompanies the formation of contact holes 30A and 30B is recovered.
In the step shown in FIG. 3C, resist pattern R having openings corresponding to contact holes 30C through 30F are formed on the structure in FIG. 3B. Interlayer insulation films 30 and 26 as well as the encapsulating layer 330 interposed in between are patterned using resist pattern R as the mask, forming contact holes 30C through 30F, which expose diffusion regions 21a through 21d respectively. In FIG. 3C and in the following description, the formation of contact hole 30G shown in FIG. 2 is not shown for its formation can easily be understood.
In the step shown in FIG. 3D, the resist pattern is removed, and Tin film 31 with a thickness of about 50 nm is formed on the interlayer insulation film 30 after carrying out the preliminary treatment by Ar plasma etching, so that TiN film 30 continuously covers the inner wall plane and the bottom plane of contact hole 30A, as well as the inner wall plane and the bottom plane of contact hole 30B. Such TiN film contacts the exposed part of the upper electrode 29 at the bottom plane of contact hole 30A, and contacts the exposed part of the lower electrode 27 at the bottom plane of contact hole 30B. TiN film 31 also contacts the exposed diffusion regions 21a through 21d at contact holes 30C through 30F, respectively.
In the step shown in FIG. 3E, W layer 32 is deposited on the TiN film 31 in the structure in FIG. 3D so as to fill the respective contact holes 30C through 30F by a CVD method using WF6, Ar, and H2.
In the step shown in FIG. 3E, although H2 is used in the CVD stage of W layer, H2 will not reach the ferroelectric film 28 because the entire ferroelectric capacitor including the ferroelectric film 28 is continuously covered with TiN film 31 in the structure shown in FIG. 3E, and thus the problem of deterioration in the property of ferroelectric capacitor due to reduction is avoided.
In the step shown in FIG. 3F, the W layer 32 on the interlayer insulation film 30 is polished and removed. Consequently, W plugs 32A through 32F are formed from the remaining part of the W layer in the contact holes 30A through 30F, respectively. As a result of such CMP process, the TiN film 31 is planarized and TiN patterns 31A through 31F are formed in correspondence to contact holes 30A to 30F.
Among the W plugs 32A through 32F formed in this way, W plug 32A contacts the upper electrode 29 consisting of IrOx through the TiN pattern 31A. However, as explained above with reference to table 1, TiN pattern will not react with conductive oxide such as IrOx and thus will not increase the contact resistance.
In the step shown in FIG. 3G, a dehydration process and film quality improvement process with N2 is carried out for the interlayer insulation film 30. After the surface of each of contact holes 32A through 32F is cleaned by RF etching, conductive layer 33 with a layered structure of TiN film with a thickness of 50 nm, Al—Cu alloy film with a thickness of 5 nm, and TiN film with a thickness of 100 nm is formed on the interlayer insulation film 30 by sputtering.
In the step shown in FIG. 3H, the conductive layer 33 formed in this way is patterned to form interconnect patterns 33A through 33E in correspondence to contact plugs 32A through 32F.
In the step shown in FIG. 3I, cover film 33a consisting of Al2O3 is formed on the structure in FIG. 3H by sputtering.
Furthermore, the next interlayer insulation film is formed on the structure in FIG. 3I, and it is possible to form the interconnect layer of the upper layer by repeating the steps shown in FIG. 3G through FIG. 3I.
In this embodiment, after W layer 32 is formed, a heat treatment stage in an oxidizing atmosphere is not conducted and thus the problem concerning increase in contact resistance due to oxidation of W plugs 32A through 32F will not arise.
Third Embodiment
FIG. 4A through FIG. 4C show a method of manufacturing FeRAM according to a third embodiment of the present invention, wherein similar components have identical reference numbers and thus their explanations are omitted.
This embodiment is a variant of the previous embodiment and the step shown in FIG. 4A is carried out in succession to the step shown in FIG. 3E.
Regarding this embodiment, in the step shown in FIG. 4A, W layer 32 covering the TiN film 31 on the interlayer insulation film 30 is selectively removed by etch back using dry etching. In the step shown in FIG. 4A, TiN film 31 remains on the interlayer insulation film 30 in a continuously extending manner. The ferroelectric capacitor, therefore, is covered with TiN film 31 and thus the ferroelectric film 28 of ferroelectric capacitor will not be deteriorated even if the etch back stage is performed.
In the step shown in FIG. 4B, a conductive layer 33 is formed on the structure in FIG. 4A by depositing Al—Cy alloy film, Ti film, and TiN film with respective thickness of 500 nm, 5 nm, and 100 nm. In the step shown in FIG. 4C, the conductive layer 33 is patterned, and interconnect patterns 33A through 33E are formed. In the step shown in FIG. 4C, with the patterning of interconnect patterns 33A through 33E, TiN film 31 is also patterned, forming TiN patterns 31A through 31E corresponding to interconnect patterns 33A through 33E.
In this embodiment as well, deterioration will not occur after W plugs 32A through 32F are formed because the ferroelectric capacitor is covered with TiN film for preventing H2 from entering. Furthermore, heat treatment will not be conducted in the oxidizing atmosphere and the problem concerning increase in contact plug resistance will not arise.
Fourth Embodiment
FIG. 5A through FIG. 5H show a method of manufacturing FeRAM according to a fourth embodiment of the present invention. In the figures, components identical to those described earlier have identical reference numbers and thus their explanations are omitted.
FIG. 5A and FIG. 5B correspond to the steps shown in FIG. 3A and FIG. 3B. The ferroelectric capacitor formed on the interlayer insulation film 26 is covered with the interlayer insulation film 30, and the deterioration in the quality of the If ferroelectric film 28 is compensated for by conducting heat treatment in the oxidizing atmosphere after the contact hole 30A exposing upper electrode 29 and the contact hole 30B exposing the lower electrode 27, respectively, are formed.
In the step shown in FIG. 5C, TiN film 31 is formed on the structure in FIG. 5B by sputtering so as to cover the interlayer insulation film 30 continuously. In the step shown in FIG. 5D, TiN film 31 is patterned and TiN pattern 31a is formed on the ferroelectric capacitor.
In the step shown in FIG. 5E, contact holes 30C through 30F exposing diffusion regions 21a through 21d, respectively, are formed through the interlayer insulation film 30. In the step shown in FIG. 5F, Ti/TiN film 31b consecutively layered with a Ti film with a thickness of 20 nm and a TiN film with a thickness of 50 nm is formed on the structure in FIG. 5E to a thickness of about 70 nm(=20+50 nm) by sputtering so as to cover the contact holes 30C through 30F as well as the TiN pattern 31a. 
In the step shown in FIG. 5G, W layer 32 is formed on the Ti/TiN layer 31b by a CVD method to fill the contact holes 30A through 30F. In the step shown in FIG. 5H, W plugs 32A through 32F are formed in correspondence to contact holes 30A through 30F by removing W layer 32 on the interlayer insulation film 30 with a CMP method. Consequently, W plug 32A or 32B includes a consecutively layered layer system of TiN film, Ti film, TiN film and W film.
On the other hand, Ti/TiN patterns 31C′ through 31F′ are formed in contact holes 30C through 30F, respectively, so as to contact the inner wall plane and the bottom plane of each contact hole, and W plugs 32C through 32F contact the corresponding diffusion regions 21a through 21d through such TI/TiN patterns. In other words, in contact holes 30C through 30F, contact plugs have a layered structure of Ti/TiN film and W film.
Accordingly, by forming Ti patterns 31C′ through 31F′ in contact holes 30C through 30F, respectively, the adhesiveness of contact plugs 32C through 32F is enhanced, and as can be seen from table 1 the contact resistance will somewhat decrease.
In this embodiment as well, heat treatment in the oxidizing atmosphere is not conducted after the W layer 32 is formed, and the problem concerning increase in contact resistance due to oxidation of W plug will not arise.
In the step shown in FIG. 5D, TiN film 31 is patterned to facilitate the plasma formation when performing plasma cleaning on contact holes 30C through 30F in the step shown in FIG. 5E. If, however, such plasma cleaning is not necessary, or if cleaning is carried out in another method, there is no need to pattern such TiN film 31. In this case, the structure shown in FIG. 6A and FIG. 6B corresponding to FIG. 5F and FIG. 5G is obtained. In this case as well, if W layer 32 is removed from the interlayer insulation film 30 by means of CMP method, the same structure as that shown in FIG. 5H is obtained.
While the preferred form of the present invention has been described, it is to be understood that the present invention is not limited to these embodiments, and variations and modifications may be made without departing from the scope of the present invention.
According to the present invention, in a semiconductor device having a fine ferroelectric capacitor, it is now possible to prevent hydrogen atmosphere used in a post treatment stage from entering the ferroelectric capacitor by covering the ferroelectric capacitor with a conductive nitride film formed so as to contact the upper electrode through the contact hole. It thus becomes possible to fill the fine contact holes with W film formed by a CVD method and to form the W plugs. Since deterioration in the ferroelectric capacitor will not occur even if W plugs are formed, there is no necessity to conduct heat treatment in the oxidizing atmosphere after forming the W plugs and the problem concerning increase in contact resistance due to oxidation of W plugs will not arise.

## Claims

1. A semiconductor device comprising:
a semiconductor substrate;
a transistor formed on said semiconductor substrate and including a diffusion region formed in said semiconductor substrate;
a capacitor formed on said semiconductor substrate and having a consecutively layered structure of a lower electrode, a ferroelectric film, and an upper electrode;
an insulation film formed on said semiconductor substrate so as to cover said capacitor;
a first contact hole formed in said insulation film so as to expose said upper electrode of said capacitor;
a second contact hole formed in said insulation film so as to expose said diffusion region;
a first CVD-deposited conductive plug formed in said first contact hole; and
a second CVD-deposited conductive plug formed in said second contact hole,
wherein a first conductive nitride film is formed between said first conductive plug and said upper electrode so as to completely cover an inner wall of said first contact hole and a surface of said upper electrode directly, and
wherein there is provided a conductive film having a composition different from said first conductive nitride film between said diffusion region and said second conductive plug in said second contact hole,
said first conductive nitride film completely covering said inner wall of said first contact hole and said surface of said upper electrode directly being disconnected at a top edge of said first contact hole,
said conductive nitride film having a substantially stoichiometric composition.

2. A semiconductor device as claimed in claim 1, wherein a second conductive nitride film is formed on said conductive film in said second contact hole between said conductive film and said second conductive plug.

3. A semiconductor device as claimed in claim 2, wherein a layered structure including a second conductive film having the same composition as said conductive film of said second contact hole and a third conductive nitride film having the same composition as said second conductive nitride film of said second contact hole are interposed in said first contact hole between said first conductive nitride film making direct contact with said inner wall of said first contact hole and with said surface of said upper electrode and said first conductive plug.

4. A semiconductor device as claimed in claim 1, wherein a conductive pattern having a composition identical to said conductive film of said second contact hole is formed on said insulation film, and a fourth conductive nitride film having substantially the same composition as said first conductive nitride film of said first contact hole is interposed between said conductive pattern and said insulation film.

5. A semiconductor device as claimed in claim 1, wherein said first and second conductive plugs comprise W.

6. A semiconductor device as claimed in claim 1, wherein said first conductive nitride film comprises titanium nitride.

7. A semiconductor device as claimed in claim 1, wherein said upper electrode comprises a conductive oxide.

8. A semiconductor device as claimed in claim 1, wherein said device also comprises a third contact hole formed in said insulation film so as to expose said lower electrode, and a third conductive plug formed in said third contact hole; a conductive nitride film having the same composition as said conductive nitride film in said first contact hole is provided between said lower electrode and said third conductive plug in said third contact hole so as to contact an inner wall of said third contact hole and said lower electrode.

9. A semiconductor device as claimed in claim 1, wherein said conductive film provided in said second contact hole is a titanium film.

10. A semiconductor device as claimed in claim 9, wherein said titanium film carries thereon a titanium nitride film in contact with said second conductive plug.

11. A semiconductor device comprising:
a semiconductor substrate;
a transistor formed on said semiconductor substrate and including a diffusion region formed in said semiconductor substrate;
a capacitor formed on said semiconductor substrate and having a consecutively layered structure of a lower electrode, a ferroelectric film, and an upper electrode;
an insulation film formed on said semiconductor substrate so as to cover said capacitor;
a first contact hole formed in said insulation film so as to expose said upper electrode of said capacitor;
a second contact hole formed in said insulation film so as to expose said diffusion region;
a first conductive plug formed in said first contact hole; and
a second conductive plug formed in said second contact hole,
wherein a conductive nitride film of substantially stoichiometric composition is formed between said first conductive plug and said upper electrode so as to contact an inner wall of said first contact hole and a surface of said upper electrode.

12. A semiconductor device as claimed in claim 11, wherein said first and second conductive plugs being formed of tungsten, said conductive nitride film being formed of TiN, and there is further formed an Al interconnection layer over said conductive plug in electrical connection with said first and second conductive plugs.