Capacitor for integrated circuit and its fabrication method

Abstract: A method for fabricating a capacitor for an integrated circuit comprises the steps of forming a titanium film for an adhesion layer over a substrate, forming a titanium dioxide film for a diffusion barrier layer by annealing the titanium film after ion-implantation of oxygen ion into a surface region of the titanium film so as to change titanium in the surface region to titanium dioxide, and forming a high dielectric constant capacitor on the titanium dioxide film.

Description: BACKGROUND OF THE INVENTION This invention relates to a capacitor for an integrated circuit and its fabrication method. A technology for applying a high dielectric constant capacitor, such as a ferroelectric capacitor for a Random Access Memory (RAM), has been developed. In a dielectric film of the capacitor, metal oxides such as Lead Zirconate Titanate (PZT), Barium Strontium Titanate (BST) and other high dielectric constant oxides are used. In the bottom and top electrodes of the capacitor, platinum, which is hard to react with the metal oxides, is used. Between the bottom electrode and a silicon substrate, an adhesion layer of a titanium film to improve the adhesive strength between both, and a diffusion barrier layer of a titanium nitride film to prevent the adhesion layer from reacting with the bottom electrode, are formed. A ferroelectric capacitor is generally fabricated by a method described below. First, a titanium film, a titanium nitride film and a platinum film for a bottom electrode are deposited in layers on an insulating surface of a silicon substrate by a sputtering method using a titanium target, a titanium nitride target and a platinum target, respectively. Next, a ferroelectric metal oxide film is deposited on the platinum film by a sputtering method or a sol-gel method, and a platinum film for a top electrode is deposited on the metal oxide film. Then, these films are finished with a predetermined shape and size of the capacitor by etching techniques to remove unnecessary portions of the films, after heat-treatment for improving the dielectric constant of the ferroelectric metal oxide film. In the fabricated capacitor, some peelings and cracks induced by the heat-treatment are often observed between the substrate and the bottom electrode. This seems to arise because the adhesive strength between the titanium film and the titanium nitride film is not strong, because the titanium film and the titanium nitride film are not formed contineously, but are formed with separate steps. Another capacitor, which includes a titanium dioxide film formed between the substrate and the bottom electrode in stead of the titanium film and the titanium nitride film, is known in the art. The titanium dioxide film acts as both an adhesion layer and a diffusion barrier layer. The fabricating process of this capacitor is that a titanium film, a platinum film for bottom electrode, a ferroelectric metal oxide film and a platinum film for a top electrode are deposited in layers on a substrate, and the resulting structure are heated in an oxygen gas atmosphere. As a result, the titanium film becomes a titanium dioxide film. However, in this capacitor, the adhesive strength between the substrate and the titanium dioxide film is not strong, too. The document EP-A-0 404 295 describes an integrated circuit capacitor according to the preamble of the appended independent claims. According to this document, it is possible to oxidise a titanium adhesion layer to create a titanium-oxide diffusion barrier layer to improve the adhesion of the platinum electrode and to lower the film stress. Further, the document EP-A-0 574 275 describes a method of fabricating a dielectric capacitor on an IC containing other semiconductor devices. By using a specific heat treatment, deterioration of the other devices may be avoided. This document further shows how to etch the different layers to obtain a reliable capacitor and integrated circuit. Also, the document JP-A-01 030 252 describes a semiconductor device such as a capacitor using a titanium-oxide film as the dielectric layer. A titanium thin film may be converted into the titanium-oxide film by thermal oxidation or lamp annealing or oxidising ion implantation. Document EP-A-609 081 shows a capacitor comprising a metal sticking layer deposited on the surface of a semiconductor body. A conductive oxygen gettering layer comprising oxide particles is formed on the metal sticking layer. The oxygen gettering layer is used as an oxygen diffusion barrier layer during the formation of a high-dielectric-constant material on a noble metal layer and the underlying oxygen gettering layer. SUMMARY OF THE INVENTION A method of fabricating a capacitor for an integrated circuit is defined in claim 1 and comprises a step of forming an adhesion layer of a metal over a substrate, a step of forming a diffusion barrier layer of a compound of the metal and a non-metal on the adhesion layer by annealing the adhesion layer after ion implantation of the non-metal into a surface region of the adhesion layer so as to change the metal in the surface region to the compound, a step of forming a bottom electrode layer of a noble metal on the diffusion barrier layer, a step of forming a dielectric layer of a metal oxide on the bottom electrode layer, a step of forming a top electrode layer of a conductive metal on the dielectric layer, a step of etching the top electrode layer and dielectric layer selectively, a step of etching the bottom electrode layer selectively, and a step of etching the diffusion barrier layer and adhesion layer selectively. The present invention also provides a corresponding capacitor as defined in claim 5. As pointed out in greater detail below, capacitors fabricated according to the preferred process provide the important advantage of no or very little peelings at the interface of the adhesion layer and the diffusion barrier layer, because the composition of both layers has continuity, and the adhesive strength between both layers is improved. The invention itself, together with further advantages, will best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a sectional view of a semiconductor device having a capacitor of the present invention. FIGs. 2, 3, 4, 5, 6, 7 and 8 are sectional views for explaining the fabricating method of the capacitor. DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention is explained below, referring to FIGs. 1-8. As shown in FIG. 1, an isolation insulating layer 2 and a MOS transistor 5, comprising a diffusion region 3 and a gate electrode 4, are formed on predetermined regions of a semiconductor substrate 1 by using conventional techniques for constructing an integrated circuit. An interlayer insulating layer 6 coverring the layer 2 and MOS transistor 5 are formed on the substrate 1, and an adhesion layer 7 of titanium and a diffusion barrier layer 8 of titanium dioxide are formed in layers on the interlayer insulating layer 6. A capacitor, comprising a bottom electrode 9 of platinum, a di-electric layer 10 of PZT and a top electrode 11 of platinum is formed on diffusion barrier layer 8, and the capacitor and MOS transistor 5 are covered with a passivation layer 12. Contact holes 13a, reaching to diffusion region 3, are opened in interlayer insulating layer 6 and passivation layer 12, and contact holes 13b, reaching to the bottom electrode 9 and top electrode 11, are opened in the passivation layer 12. Aluminum interconnection 14 is connected to diffusion region 3 through contact holes 13a, and aluminum interconnection 15 is connected to the bottom and top electrodes 9, 11, respectively, through contact holes 13b, by using conventional wiring techniques. Further, a layer of titanium nitride or titanium-tungsten alloy (not shown) is disposed between platinum electrodes 9, 11 and aluminum interconnection 15 in order to improve the connection reliability, by preventing reaction of aluminum and platinum. A method for fabricating the capacitor is shown in FIGs. 2-8. A titanium film 7a for adhesion layer 7, having a thickness of 50 nm, is deposited on interlayer insulating layer 6 by a sputtering method using a titanium target, as shown in FIG. 2. In the next step shown in FIG. 3, oxygen ion is implanted into a surface region of the titanium film 7a by using an ion-implantation apparatus, and then the structure is annealed in an inert atmosphere of argon gas at a temperature of 650 °C. As a result, the surface region of the titanium film 7a changes to a titanium dioxide film 8a. At this time, the thickness of titanium dioxide film 8a increases by about two times, compared with the thickness of the surface region of titanium film 7a. In carrying out the oxygen ion-implantation step, the implantation conditions, such as the amount of the implanted oxygen ion, acceleration voltage and other factors are controlled, so that a titanium dioxide film 8a having a thickness of 60 nm is obtained. The thickness of the titanium film 7a after the annealing, and the thickness of the titanium dioxide film 8a are preferably in ranges of 15 - 25 nm and 50 - 70 nm, respectively. The annealing temperature is preferably in a range of 500 - 700 °C. As shown in FIGs. 4-6, a platinum film 9a on the titanium dioxide film 8a, a PZT film 10a on the platinum film 9a and a platinum film 11a on the PZT film 10a are deposited by sputtering. In depositing the platinum films 9a, 11a and the PZT film 10a, a platinum target and a PZT target are used, respectively. As shown in FIG. 7, the platinum film 11a and the PZT film 10a are etched selectively so as to form a top electrode 11 and dielectric layer 10, having predetermined shape and size. Then, as shown in FIG. 8, the platinum film 9a, titanium dioxide film 8a and titanium film 7a are etched selectively so as to form a bottom electrode 9, diffusion barrier layer 8 and adhesion layer 7, having a predetermined shape and size. These etching steps are performed by using conventional photolithographyand dry etching techniques. The formation of the diffusion barrier layer 8 by ion-implantation, provides several important advantages such as, the adhesive strength between the adhesion layer 7 and the diffusion barrier layer 8 is strong, resulting in little or no peelings between both layers. Further, the titanium dioxide layer 8a is formed accurately in thickness, because it is possible to control the implantation amount and the implantation depth of oxygen ions accurately. Though the step of ion-implantation is performed before the step of depositing platinum film 9a for the bottom electrode 9 in the process described above for the first embodiment, the step of ion-implantation can be performed after the step of depositing the platinum film 9a. Namely, a titanium film 7a can first be deposited, and subsequently platinum film 9a deposited thereon. Then oxygen ions are implanted into the titanium film 7a through the platinum film 9a, and the structure is annealed in an inert atmosphere. As a result, the surface region of the titanium film 7a changes to titanium dioxide, and a titanium dioxide film 8a is formed between its titanium film 7a and the platinum film 9a. This fabricating method gives the same effect of preventing an occurrence of peelings between the adhesion layer 7 and the diffusion barrier layer 8, as the first embodiment. Further the method keeps the interface between the titanium film 7a and the platinum film 9a clean, because the titanium film 7a and the platinum film 9a are deposited in the same sputtering chamber, without taking out the structure from the chamber while sputtering. The above-mentioned methods for forming a diffusion barrier layer 8a of titanium dioxide by implanting oxygen ions are applicable to forming the layer 8a of other titanium compounds by implanting other non-metal ions. For example, in other preferable embodiments, titanium nitride film and titanium oxynitride film can be formed by implanting nitrogen ions and a mixture of oxygen ions and nitrogen ions, respectively. In these alternate cases, little or no peelings between the adhesion layer 7 and diffusion barrier layer 8 were observed. However, the titanium nitride film induces an oxygen defect to the dielectric metal oxide film, because oxygen in the dielectric film is taken in the titanium nitride film. As a result, the leakage current of the capacitor increases. Among titanium dioxide film, titanium nitride film and titanium oxynitride film, titanium dioxide film gives small leakage current and titanium nitride film gives a large leakage current. Therefore, titanium dioxide film is most preferrable as a diffusion barrier layer in the dielectric capacitor. In addition to the combination of titanium and titanium compound, combinations of other refractory metals such as tantalum, tungsten, molybdenum, etc. and compounds of these refractory metals are applicable to adhesion layer 7 and diffusion barrier layer 8. Especially, a combination of tantalum and tantalum compounds gives the same effect as the one of titanium and titanium compounds and is one of most preferrable combinations. Among tantalum compounds, tantalum oxide is preferrable as diffusion barrier layer 8. In addition to the sputtering method, an evaporation method and a CVD method are applicable to forming a metal adhesion layer 7. A second embodiment not forming part of the invention is described below. The second embodiment is different from the first embodiment with respect to the steps of forming titanium film 7a and titanium dioxide film 8a. In this embodiment, first, a titanium film 7a is deposited on an interlayer insulating layer 6 by sputtering in a sputtering chamber incorporating a titanium target. While sputtering, a predetermined amount of oxygen gas is induced into the chamber at the time that the thickness of the resulting deposited titanium film 7a reaches 20 nm. The sputtering with oxygen gas is continued until the thickness of the resulting deposited titanium dioxide film 8a formed by the reaction of titanium and oxygen reaches 60 nm. Thus a 20 nm thick titanium film 7a, and a 60 nm thick titanium dioxide film 8a deposited thereon, are formed by the contineous sputtering process. Next, in the same manner as the first embodiment, depositions of platinum film 9a, PZT film 10a and platinum film 11a and etching steps for finishing a capacitor are performed. In this case, the adhesive strength between the adhesion layer 7 and the diffusion barrier layer 8 is strong and therefore, peelings between both layers occures very little, because both layers are formed by a continuous sputtering process. On the other hand, when sputtering is performed while increasing the amount of oxygen gas with sputtering time, a titanium oxide film is deposited, having a gradient composition that increases along the perpendicular direction to the film surface as the oxygen content increases. By using titanium oxide, having the gradient composition as a diffusion barrier layer 8a, the degree of occurrence of separations and cracks becomes smaller, because stress induced by thermal expansion is relieved by the gradient composition. In addition to oxygen gas used in the second embodiment, other non-metal gases are applicable. In particular, nitrogen gas and a mixture of oxygen and nitrogen are preferable. Also, in addition to the titanium target, refractory metal targets such as tantalum, tungsten, molybdenum, etc. can be used. In particular, tantalum film is one of preferred as an adhesion layer 7. A third embodiment not forming part of the invention of which the step of forming titanium dioxide film 8a is different from the first embodiment, is explained below. First, a titanium film 7a with a thickness of 50 nm is deposited on an interlayer insulating layer 6 in a sputtering chamber using a titanium target. Subsequently, a predetermined amount of oxygen gas is induced into the sputtering chamber, and the surface of the titanium film 7a is annealed by halogen lamps incorporated in the sputtering chamber. As a result, the surface region of the titanium film 7a changes to titanium dioxide and a titanium dioxide film 8a with thickness of 60 nm is formed. Next, in the same manner as the first embodiment, depositions of platinum film 9a, PZT film 10a and platinum film 11a, and etching steps for finishing a capacitor are performed. In this case, occurrence of peelings and cracks between the adhesion layer 7 and the diffusion barrier layer 8 is suppressed. However, this lamp-annealing method is inferior to the ion-implantation method in controlling the accuracy of the thickness of diffusion barrier layer 8. But, the lamp-annealing method is superior to the ion-implantation method in view of cost, because a lamp-annealing apparatus is less expensive. In addition to the oxygen gas used in the third embodiment, other non-metal gases are applicable. In particular, nitrogen gas and a mixture of oxygen and nitrogen are preferable. Also, in addition to the titanium target, refractory metal targets such as tantalum, tungsten, molybdenum, etc. can be used. In particular, tantalum film is one of preferred as an adhesion layer 7. In addition to PZT, other dielectric or ferroelectric metal oxides with a high dielectric constant such as BST, PLZT, Bi 2 SrTa 2 O 9 , Bi 2 SrNb 2 O 9 , etc. can be used as the dielectric film of a capacitor, and also dielectric metal oxides such as a tantalum oxide, etc. may be used. A sol-gel method and a CVD method can be used to form of the dielectric layer in stead of sputtering. As an electrode material for the capacitor electrodes, a noble metal such as gold, palladium and rhodium, in addition to platinum, and a conductive metal such as aluminum and nickel, can be used. The position of the capacitor, including an adhesion layer and a diffusion barrier layer, is not limited to the insulating layer formed on a semiconductor substrate. The capacitor may be formed on other layers, such as an interconection layer, other barrier layer, etc. which are comprised of polysilicon, aluminum alloy, tungsten, tungsten silicide, titanium nitride and so on.

Claims: A method of fabricating a capacitor for an integrated circuit comprising the steps of: forming an adhesion layer (7) of a metal over a substrate (1); forming a diffusion barrier layer (8) on said adhesion layer (7); forming a bottom electrode layer (9) of a noble metal on said diffusion barrier layer; forming a dielectric layer (10) of a metal oxide on said bottom electrode layer; forming a top electrode layer (11) of a conductive metal on said dielectric layer; the method further comprises the steps of etching said top electrode layer (11) and dielectric layer (10) selectively; etching said bottom electrode layer (9) selectively; and etching said diffusion barrier layer (8) and adhesion layer (7) selectively, and characterized in that said diffusion barrier layer (8) comprises a compound of said metal composing the adhesion layer (7) and a non-metal obtained by annealing said adhesion layer after ion implantation of said non-metal into a surface region of said adhesion layer so as to change said metal in said surface region to said compound. The method of claim 1, wherein said step of forming a diffusion barrier layer (8) is performed after said step of forming a bottom electrode layer (9). The method of claim 1, wherein said non-metal comprises at least one of oxygen and nitrogen. The method of claim 1, wherein said adhesion layer (7) comprises one of titanium and tantalum. A capacitor for an integrated circuit comprising: a substrate (1); an adhesion layer (7) of a metal formed over said substrate; a diffusion barrier layer (8) formed on said adhesion layer; a bottom electrode (9) of a noble metal formed on said diffusion barrier layer, said bottom electrode (9) having a top surface facing away from said diffusion barrier layer (8); a dielectric layer (10) of a metal oxide formed on said bottom electrode; and a top electrode (11) of a conductive metal formed on said dielectric layer, characterised in that said diffusion barrier layer consists of an oxide of said metal composing the adhesion layer, and in that said top surface of said bottom electrode (9) receives electrical interconnections (15) so as to electrically connect said capacitor to other components. The capacitor of claim 5, wherein said adhesion layer (7) comprises one of titanium and tantalum.

Inventors: ARITA KOJI, AZUMA MASAMICHI, FUJII EIJI, INOUE ATSUO, IZUTSU YASUFUMI, NAGANO YOSHIHISA, SHIMADA YASUHIRO, UEMOTO YASUHIRO, ARITA, KOJI, AZUMA, MASAMICHI, FUJII, EIJI, INOUE, ATSUO, IZUTSU, YASUFUMI, NAGANO, YOSHIHISA, SHIMADA, YASUHIRO, UEMOTO, YASUHIRO, Shimada, Yasuhiro, Room No. 421
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1995
Main classification: H01L21
IPC codes: H01L21, H01L27