Method for fabricating nonvolatile semiconductor memory device

An adhesion layer composed of a titanium film and a titanium nitride film is formed by CVD on the inner wall of a contact hole formed in a multilayer film composed of an interlayer insulating film, a silicon nitride film, and a silicon dioxide film. Then, a conductive film made of tungsten or polysilicon is filled by CVD in the contact hole and the respective portions of the conductive film and the adhesion layer which are located over the silicon dioxide film are removed by CMP. Subsequently, the silicon dioxide film is removed by an etch-back method or a CMP method so that the silicon nitride film is exposed. This can prevent the delamination of the adhesion layer from the silicon nitride film as a hydrogen barrier film and also prevent the formation of a scratch in the silicon nitride film.

CROSS-REFERENCE TO RELATED APPLICATIONS

The teachings of Japanese Patent Application JP 2005-177254, filed Jun. 17, 2005, are entirely incorporated herein by reference, inclusive of the claims, specification, and drawings.

BACKGROUND OF THE INVENTION

The present invention relates to a method for fabricating a nonvolatile semiconductor memory device having a capacitor element using a ferroelectric film or a high-dielectric-constant film.

A ferroelectric memory using a ferroelectric film as a capacitor insulating film is one of nonvolatile memories. The most characteristic feature of the ferroelectric memory is that it continues to retain information once it is written, which is different from a DRAM as a typical common memory. The ferroelectric memory is also used as an embedded memory in a system LSI. The ferroelectric memory has performance advantages over a flash memory in terms of low-voltage and high-speed operation, the number of rewritable times can be rewritten, high reliability, and the like.

A description will be given herein below to a method for fabricating a semiconductor device having a conventional ferroelectric memory with reference toFIGS. 5A to 5E.FIGS. 5A to 5Eare cross-sectional views showing the process steps of the conventional method for fabricating the semiconductor device.

As shown inFIG. 5A, an interlayer insulating film1004composed of a silicon dioxide film and a hydrogen barrier film1005for preventing the diffusion of hydrogen are deposited on a semiconductor substrate1000formed with a transistor1001. Then, a contact hole1006reaching the semiconductor substrate1000is formed in each of the interlayer insulating film1004and the hydrogen barrier film1005.

Next, as shown inFIG. 5B, an adhesion layer1007composed of a multilayer film of a titanium film and a titanium nitride film is formed on the inner surface of the contact hole1006. Subsequently, as shown inFIG. 5C, a contact plug material1008is filled in the contact hole1006. Next, as shown inFIG. 5D, the portions of the contact plug material1008and the adhesion layer1007which are located over the upper surface of the hydrogen barrier film1005are removed by CMP, whereby a contact plug1009is formed (see, e.g., Japanese Laid-Open Patent Publication No. HEI 9-17787).

Then, as shown inFIG. 5E, a capacitor element comprising: a lower electrode1010connected to the contact plug1009; a capacitor insulating film1011composed a ferroelectric film; and an upper electrode1012is formed on the hydrogen barrier film1005.

Because the ferroelectric material is reduced by hydrogen generated in the LSI steps, the characteristics of the ferroelectric memory are thereby degraded. To counter this, the ferroelectric memory thus uses the structure in which the hydrogen barrier film1005is provided on the interlayer insulating film1004to prevent the diffusion of hydrogen in a direction from the substrate (see, e.g., Japanese Laid-Open Patent Publication No. 2001-7303).

SUMMARY OF THE INVENTION

The conventional method for fabricating the semiconductor device described above is different from a method for forming a contact plug for a typical LSI in that the contact plug1009is formed in the multilayer film of the interlayer insulating film1004composed of the silicon dioxide film and the hydrogen barrier film1005. As a result, when a silicon nitride is used as the material of the hydrogen barrier film1005mentioned above, the problem is encountered that the adhesion layer1007on the hydrogen barrier film1005delaminates therefrom in the step of filling tungsten or polysilicon, used as the contact plug material1008, in the contact hole1006having the adhesion layer1007formed by CVD on the inner surface thereof. Such a delaminated adhesion layer becomes the source of particles generated in a CVD system and thereby causes the problem of a reduced product yield.

In addition, since the contact plug material and the adhesion layer are polished by CMP till the hydrogen barrier film1005is exposed in the step of forming the contact plug1009shown inFIG. 5D, the surface of the hydrogen barrier film1005is also over-polished. At this time, a scratch is formed in the surface of the hydrogen barrier film1005by the over-polishing and a crack resulting from the scratch is formed in the subsequent steps, which leads to the problem of the degradation of the barrier property of the hydrogen barrier film.

In view of the foregoing problems, it is therefore an object of the present invention to prevent the delamination of the adhesion layer for the contact plug from the hydrogen barrier film made of a silicon nitride, suppress the degradation of the barrier property of the hydrogen barrier film, and thereby provide a method for fabricating a nonvolatile semiconductor memory device with an improved productivity.

To solve the problems described above, a method for fabricating a nonvolatile semiconductor memory device according to the present invention comprises the steps of: forming, on a substrate, a multilayer film comprising an interlayer insulating film, a silicon nitride film as a hydrogen barrier film, and a silicon dioxide film; forming, in the multilayer film, a contact hole reaching the substrate; forming an adhesion layer on an inner surface of the contact hole and then filling a conductive film in the contact hole; and removing respective portions of the conductive film, the adhesion layer, and the silicon dioxide film which are located over the silicon nitride film and forming a contact plug in plug in the contact hole.

Since the contact hole is thus formed in the multilayer film comprising the interlayer insulating film, the silicon nitride film, and the silicon dioxide film, the portion of the adhesion layer which is located outside the contact hole is formed on the silicon dioxide film. This prevents the delamination of the adhesion layer from the silicon dioxide film during the filling of the conductive film in the step of forming the contact plug and thereby allows an improvement in yield.

Preferably, the respective portions of the conductive film and the adhesion layer which are located over the silicon dioxide film are removed by using a CMP method. Since the silicon nitride film is covered with the silicon dioxide film, the silicon nitride film is not polished in the step of removing the respective portions of the conductive film and the adhesion layer by using the CMP method. As a result, a scratch is not formed in the surface of the silicon nitride film and the degradation of the hydrogen barrier property of the silicon nitride film can be thereby prevented.

In the fabrication method described above, an etching selectivity of the silicon dioxide film to the silicon nitride film is preferably higher than 1. When the selectivity of the silicon dioxide film to the silicon nitride film is high, the conductive film can be left efficiently only within the contact hole. Since the selectivity of the silicon dioxide film to the silicon nitride film is high, an amount of the portion of the silicon nitride film reduced by a CMP process or an etch-back process can be reduced. This allows the hydrogen barrier property of the silicon nitride film to be retained and also allows the surface planarity of the substrate to be improved.

In the fabrication method described above, an end time of the etching of the silicon dioxide film is preferably controlled by detecting plasma light emission from the silicon nitride film or from the silicon dioxide film. By thus detecting the etching end time, the amount of the portion of the silicon nitride film reduced by the etching process can be reduced. This allows the hydrogen barrier property of the silicon nitride film to be retained.

In the fabrication method described above, when the respective portions of the conductive film and the adhesion layer which are located over the silicon dioxide film are removed, a recess from a surface of the silicon dioxide film is preferably formed over the contact plug has and a depth of the recess is preferably smaller than a thickness of the silicon dioxide film. By thus estimating the depth of the recess formed through the removal of the respective portions of the conductive film and the adhesion layer and setting the thickness of the silicon dioxide film such that it is equal to the depth of the recess, the recess can also be removed simultaneously with the removal of the silicon dioxide film. As a result, the upper surface of the contact plug is substantially flush with the upper surface of the silicon nitride film. This allows the formation of a stable capacitor element over the contact plug and the silicon nitride film.

Thus, the present invention forms the contact hole in the multilayer film including the silicon nitride film and the silicon dioxide film as the upper layer and fills the adhesion layer and the conductive film in the contact hole to form the contact plug. This can prevent the delamination of the adhesion layer from the silicon nitride film as the hydrogen barrier film and can also prevent the formation of a scratch in the silicon nitride film. Accordingly, it becomes possible to prevent the degradation of the barrier property of the silicon nitride film and improve the productivity.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, the embodiments of the present invention will be described herein below.

A description will be given to the first embodiment of the present embodiment with reference toFIGS. 1A to 1FandFIGS. 2A and 2B.FIGS. 1A to 1FandFIGS. 2A and 2Bare cross-sectional views showing the process steps of a method for fabricating a semiconductor device according to the first embodiment.

As shown inFIG. 1A, the method for fabricating a semiconductor device according to the first embodiment selectively forms an isolation layer101in a semiconductor substrate100and forms a transistor composed of impurity diffusion layers102and a gate electrode103. Then, as also shown inFIG. 1A, an interlayer insulating film104with a thickness of about 1000 nm and a silicon nitride film105with a thickness of about 150 nm are deposited successively in an ascending order over the entire surface of the semiconductor substrate100. The silicon nitride film105functions as a hydrogen barrier film for preventing the undesirable diffusion of hydrogen generated in the subsequent steps into a capacitance insulating film110.

Next, as shown inFIG. 1B, a silicon dioxide film106with a thickness of about 100 nm is formed on the silicon nitride film105. Subsequently, as shown inFIG. 1C, a contact hole107reaching one of the impurity diffusion layers is formed in a multilayer film composed of the interlayer insulating film104, the silicon nitride film105, and the silicon dioxide film106.

Next, as shown inFIG. 1D, an adhesion layer108is formed by CVD over the surface of the semiconductor substrate100including the inner wall of the contact hole107. The adhesion layer108is composed herein of a titanium film and a titanium nitride film formed successively in an ascending order.

Subsequently, as shown inFIG. 1E, a conductive film109amade of tungsten or polysilicon is formed by CVD in such a manner as to cover the entire surface of the semiconductor substrate100and fill in the contact hole107. Then, as shown in FIG1F, the respective portions of the conductive film109aand the adhesion layer108which are located over the silicon dioxide film106are removed by chemical mechanical polishing (hereinafter referred to as CMP) method.

Next, as shown inFIG. 2A, the silicon dioxide film106is removed by an etching (etch-back) method or a CMP method such that the silicon nitride film105is exposed, whereby a contact plug109reaching the semiconductor substrate100is formed.

In the case where the etch-back method is used to remove the silicon dioxide film106, the end time of etching is detected by using plasma light emission resulting from the silicon nitride film105or the silicon dioxide film106which occurs when etching is performed. This can reduce an amount of the portion of the silicon nitride film105reduced by etching and suppress the degradation of the hydrogen barrier characteristic of the silicon nitride film105.

Since the surface of the silicon nitride film105is thus exposed, a lower electrode can be formed on the silicon nitride film105in the subsequent steps without intervention of the silicon dioxide film106and the path of hydrogen diffused from below the capacitor element into the capacitor element can be cut off. This can prevent the situation in which the ferroelectric film is reduced by hydrogen generated in each of the steps and the ferroelectric property is thereby degraded and improve the reliability of the ferroelectric memory.

Subsequently, as shown inFIG. 2B, a lower electrode110connected to the contact plug110, a capacitor insulating film111composed of a ferroelectric film, and an upper electrode112are formed successively by etching on the silicon nitride film105so that a capacitor element composed of the lower electrode110, the capacitor insulating film111, and the upper electrode112is formed.

The lower electrode110has a multilayer structure in which the lowermost layer connected to the contact plug109is a conductive hydrogen barrier film composed of, e.g., a TiN film or a TiAlN film. On the other hand, the uppermost layer connected to the capacitor insulating film111is composed of, e.g., a Pt film or an IrO2film. The upper electrode112is composed of a film containing Pt or an IrO2.

The capacitor insulating film111is composed of a ferroelectric film made of any one of SrBi2(TaxNb1-x)2O9, Pb(ZrxTi1-x)O3, (BaxSr1-x)TiO3, and (BixLa1-x)4Ti3O12(in each of which x satisfies 0≦x≦1).

Thus, as shown inFIG. 1E, the present invention has formed the silicon dioxide film on the silicon nitride film and provided the contact hole in the multilayer film so that the adhesion layer is formed on the second silicon dioxide film. As a result, the delamination of the adhesion layer formed in the contact hole from the silicon nitride film does not occur. This is because the adherence between the titanium film composing the adhesion layer and the silicon dioxide film is more excellent than the adherence between the titanium film and the silicon nitride film. A description will be given herein below to the cause of the delamination. When tungsten is deposited by CVD in the contact hole in which the adhesion layer is formed directly on the silicon nitride film as in the conventional embodiment, a temperature increase during the deposition of tungsten causes a stress to be applied to the silicon nitride film so that the adhesion layer delaminates from the surface of the peripheral portion of the opening of the contact hole.

A description will be given next to the setting of the thickness of the silicon nitride film105and selectivity in the polishing of the silicon dioxide film106. In the step of forming the lower electrode110by etching described with reference toFIG. 2B, the silicon nitride film105is over-etched by an amount corresponding to a thickness of about 50 to 80 nm. Accordingly, the thickness of the nitride film105should be set in consideration of over-etching prior to the step of forming the lower electrode110. In addition, the silicon nitride film105should have a thickness of about 30 nm or more to sufficiently exert the function as the hydrogen barrier. Therefore, in consideration of the over-etching during the formation of the lower electrode110, a preferred thickness of the silicon nitride film105inFIG. 1Ais about 110 nm.

If the thickness of the silicon dioxide film is 100 nm±15%, the selectivity of the silicon dioxide film to the silicon nitride film is preferably 1.2 or more. This allows a reduction in the amount of the portion of the silicon dioxide film105reduced by the over-etching or over-polishing of the silicon dioxide film in the step of forming the contact plug109.

Thus, when the silicon dioxide film is removed, the etch-back method is used preferably as a removal method which provides conditions including a higher selectivity of the silicon dioxide film to the silicon nitride film. The reason for this will be described herein below. If the surface planarity of the deposited silicon dioxide film106is poor inFIG. 1B, a larger part of the silicon nitride film105is removed through the removal of the silicon dioxide film106inFIG. 2Aso that the in-plane planarity of the interlayer insulating film104is degraded.

In addition, because the silicon dioxide film106underlying the conductive film109aand the adhesion layer108is also polished to a degree during the removal of the conductive film109aand the adhesion layer108, larger in-plane variations are observed in the thickness of the silicon dioxide film106after polishing than before polishing. Accordingly, if the silicon dioxide film106has a center thickness of 100 nm and in-plane variations of ±15% after the formation of the contact plug109, the difference between the maximum and minimum values of the film thickness is about 30 nm.

Therefore, if the selectivity of the silicon dioxide film to the silicon nitride film is assumed to be 1, the difference between the maximum and minimum values of the in-plane thickness of the silicon nitride film105is about 30 nm. That is, the in-plane variations in the thickness of the silicon dioxide film106are reflected directly on the in-plane variations in the thickness of the silicon nitride film105.

On the other hand, if the selectivity of the silicon dioxide film to the silicon nitride film is assumed to be less than 1, the difference between the maximum and minimum values of the thickness of the silicon nitride film is larger than 30 nm after the silicon dioxide film106has been removed. That is, variations not less than the order of the in-plane variations in the thickness of the silicon dioxide film106occur in the thickness of the silicon nitride film105.

Thus, when the selectivity of the silicon dioxide film to the silicon nitride film is larger than 1, the difference between the maximum and minimum values in the thickness of the silicon nitride film105after the removal of the silicon dioxide film106is smaller than 30 nm. Accordingly, a higher selectivity of the silicon dioxide film to the silicon nitride film is preferred. If the selectivity is assumed to be 10, the difference between the maximum and minimum values of the thickness of the silicon nitride film after the complete removal of the silicon dioxide film is 3 nm or less. This allows the interlayer insulating film104to retain the surface planarity.

As an etching method which provides a higher selectivity of the silicon dioxide film to the silicon nitride film, the use of C4F8, CF4, Ar, or O2for an etching gas is preferred.

A description will be given to the second embodiment of the present invention with reference toFIGS. 3A and 3B. As for the parts of the second embodiment which are the same as those of the first embodiment, the detailed description thereof will be omitted.FIGS. 3A and 3Bare cross-sectional views showing the process steps of a method for fabricating a semiconductor device according to the second embodiment. The description of the same components as shown inFIGS. 1 and 2will be omitted by retaining the same reference numerals.

The method for fabricating a semiconductor device according to the second embodiment prior to the process step shown inFIG. 3Ais the same as that according to the first embodiment shown inFIGS. 1A to 1E. Subsequently, as shown inFIG. 3A, the portions of the conductive film109aand the adhesion layer108which are located over the silicon dioxide film106are removed by CMP, whereby the adhesion layer108and the contact plug108are formed in the contact hole107. As shown inFIG. 3A, a recess113at a depth of about 30 to 40 nm is formed over the contact plug109by the polishing step using a CMP method described above.

Next, as shown inFIG. 3B, the portion of the silicon dioxide film106which corresponds to the depth of the recess113is removed by an etch-back method or a CMP method. The subsequent steps are the same as shown inFIGS. 2A and 2Bso that the description thereof will be omitted.

After the recess113has thus been removed, the lower electrode, the capacitor insulating film, and the upper electrode are formed on the silicon nitride film105with excellent planarity. This allows the prevention of the delamination of the lower electrode from the contact plug during the crystallization annealing of the capacitor insulating film.

A description will be given to the setting of the thickness of the silicon dioxide film according to the present embodiment with reference toFIG. 4.FIG. 4is a cross-sectional view of the principal portion of a recess in which the upper portion of the contact plug has been enlarged. InFIG. 4, the thickness b of the silicon dioxide film106is preferably set to be equal, to the depth a of the recess113. This allows simultaneously removal of the silicon dioxide film106and the recess113.

Thus, the method for fabricating a semiconductor device according to the present invention is useful for the process steps of forming a contact plug by using a CVD method.