Method of forming a Ta2O5 dielectric layer, method of forming a capacitor having a Ta2O5 dielectric layer, and capacitor construction

[0002] This application is a continuation application of Ser. No. 08/664,305, which was filed on Jun. 11, 1996, now U.S. Pat. No. 5,814,852, and a con't of Ser. No. 08/444,853 filed May 19, 1995, U.S. Pat. No. 5,663,088.

[0003] This invention relates to methods of forming a capacitor having a Ta₂O₅dielectric layer, to methods of forming a Ta₂O₅dielectric layer, and to capacitor constructions.

[0004] As DRAMs increase in memory cell density, there is a continuing challenge to maintain sufficiently high storage capacitance despite decreasing cell area. Additionally, there is a continuing goal to further decrease cell area. One principal way of increasing cell capacitance is through cell structure techniques. Such techniques include three-dimension cell capacitors, such as trenched or stacked capacitors. Yet as feature size continues to become smaller and smaller, development of improved materials for cell dielectrics as well as the cell structure are important. The feature size of 256 Mb DRAMs will be on the order of 0.25 micron, and conventional dielectrics such as SiO₂and Si₃N₄might not be suitable because of small dielectric constants.

[0005] Chemical vapor deposited (CVD) Ta₂O₅films are considered to be very promising cell dielectrics layers, as the dielectric constant of Ta₂O₅is approximately three times that of Si₃N₄. Proposed prior art capacitor constructions include the use of Ta₂O₅as a capacitor dielectric layer, in combination with an overlying predominately crystalline TiN electrode or other layer. However, diffusion relative to the tantalum layer is problematic in the resultant capacitor construction. For example, tantalum from the Ta₂O₅tends to undesirably out-diffuse from the dielectric layer. Further, materials from the adjacent conductive capacitor plates can diffuse into the tantalum layer. In either event, the dielectric properties of the Ta₂O₅layer are adversely affected in a less than predictable or an uncontrollable manner.

[0006] Preferred embodiments of the invention are described below with reference to the following accompanying drawings.

[0007] FIG. 1 is a diagrammatic sectional view of a semiconductor wafer fragment in accordance with the invention.

[0008] FIG. 2 is a diagrammatic sectional view of an alternate embodiment semiconductor wafer fragment in accordance with the invention.

[0009] FIG. 3 is a diagrammatic sectional view of yet another alternate embodiment semiconductor wafer fragment in accordance with the invention.

[0010] This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8).

[0011] In accordance with one aspect of the invention, a method of forming a capacitor includes the following steps:

[0012] providing a node to which electrical connection to a capacitor is to be made;

[0013] providing a first electrically conductive capacitor plate over the node;

[0014] chemical vapor depositing a capacitor dielectric layer of Ta₂O₅over the first electrically conductive capacitor plate; and

[0015] providing a predominately amorphous diffusion barrier layer over the Ta₂O₅dielectric layer.

[0016] In accordance with another aspect of the invention, a method of forming a dielectric layer comprises the following steps:

[0017] chemical vapor depositing a dielectric layer of Ta₂O₅atop a substrate; and

[0018] providing a predominately amorphous diffusion barrier layer over the Ta₂O₅dielectric layer.

[0019] In accordance with still a further aspect of the invention, a capacitor comprises:

[0020] a first electrically conductive capacitor plate;

[0021] a capacitor dielectric layer adjacent the first electrically conductive capacitor plate, the capacitor dielectric layer comprising Ta₂O₅;

[0022] a predominately amorphous diffusion barrier adjacent the Ta₂O₅dielectric layer; and

[0023] a second electrically conductive capacitor plate, the Ta₂O₅dielectric layer being positioned between the first and second electrically conductive plates.

[0024] More particularly and first with reference to FIG. 1, a semiconductor wafer fragment is indicated generally with reference numeral 10. Such comprises a bulk silicon substrate 12 having a conductive diffusion area 14 formed therein. In insulating layer 16, typically borophosphosilicate glass (BPSG), is provided over substrate 12, with a contact opening 18 having been previously provided therein to diffusion area 14. A conductive material 20 fills contact opening 18, with material 20 and oxide layer 16 having been planarized as shown. Material 20 might be any suitable conductive material, such as tungsten or conductively doped polysilicon. A capacitor construction 25 is provided atop layer 16 and plug 20, with conductive plug 20 constituting a node to which electrical connection to capacitor 25 is made.

[0025] Capacitor 25 comprises a first electrically conductive capacitor plate 26 which has been provided and patterned over node 20. An example and preferred material is conductively doped polysilicon, provided to a thickness of 1000 Angstroms for 256 Mb density. A capacitor dielectric layer 28 of Ta₂O₅is provided over first electrically conductive capacitor plate 26. An example process for depositing layer 28 is by low pressure chemical vapor deposition at 450° C. using Ta(OC₂H₅)₅and oxygen as precursors. Ta(OC₂H₅)₅can be vaporized at 170° C., and introduced into a reactor chamber using argon or another suitable carrier gas. Subsequently, densification by rapid thermal annealing in a dry oxygen atmosphere at a temperature ranging from 700° C. to 900° C. is utilized. Preferably, the polysilicon surface of lower capacitor plate 26 is cleaned by an in situ HF dip prior to provision of the Ta₂O₅. Rapid thermal nitrogen treatment can also be carried out immediately prior to Ta₂O₅deposition, such as at 900° C. for 60 seconds in NH₃. An example and preferred thickness for layer 28 in accordance with 256 Mb integration is 100 Angstroms.

[0026] A predominately amorphous diffusion barrier layer 30 is provided over Ta₂O₅dielectric layer 28. Such layer is preferably electrically conductive, with an example and preferred thickness for 256 Mb integration being 200 Angstroms. A most preferred material for layer 30 is predominately amorphous TiC_xN_yO_zdeposited by metal organic chemical vapor deposition (MOCVD), where “x” is in the range of from 0.01 to 0.5, and “y” is in the range of from 0.99 to 0.5, and “z” is in the range of from 0 to 0.3, with the sum of “x”,“y” and “z” equalling about 1.0. A preferred metal organic precursor for such deposition is Ti(NR₂)₄, where R is selected from the group consisting of H and a carbon containing radical. Deposition conditions for producing such a predominately amorphous layer include from 200° C. to 600° C., and from 0.1 to 100 Torr. Most preferably, the amorphous diffusion barrier layer comprises a material which is a barrier to out-diffusion of tantalum from the Ta₂O₅layer. The preferred TiC_xN_yO_zmaterial provides this function.

[0027] Also most preferably, “z” will be equal to zero. Unfortunately however, oxygen can undesirably become incorporated in the deposited film when it is exposed to oxygen, even ambient air. This incorporated oxygen undesirably affects conductivity. Accordingly, exposure to oxygen is preferably minimized until the subject film is covered by subsequent layers which can effectively act as a barrier to oxygen incorporation.

[0028] Subsequently, a second electrically conductive capacitor plate 32 is provided outwardly of diffusion barrier layer 30, such that Ta₂O₅dielectric layer 28 is positioned between the first and second electrically conductive plates. Accordingly, second electrically conductive capacitor plate 32 constitutes a discrete, second electrically conductive capacitor plate provided over the amorphous diffusion barrier layer 30. An example and preferred material for second capacitor plate 32 is polysilicon, with 1000 Angstroms being an example preferred thickness for 256 Mb integration.

[0029] Prior art constructions and proposals for utilizing Ta₂O₅as a capacitor dielectric layers suffer from the drawback of undesired out-diffusion of tantalum atoms from such layer, or diffusion of undesired components into such layer. Provision of a thin predominately amorphous diffusion barrier layer as described above in the preferred embodiment facilitates maintaining integrity of the Ta₂O₅layer and, accordingly, its associated desired high dielectric constant in such a small application. It has been discovered in the context of the invention that amorphous materials are better diffusion barriers due to absence of crystal grain boundaries which can provide an easy path for diffusion in polycrystalline materials.

[0030] FIG. 2 illustrates an alternate embodiment capacitor construction and method in accordance with the invention. Like numerals from FIG. 1 have been utilized where appropriate, with differences being indicated by either different numerals, or like numerals with the suffix “a”. Wafer fragment 10acomprises a capacitor construction 25adiffering from the first described embodiment in provision of a predominately amorphous, electrically conductive second capacitor plate 32awhich functions as an inherent amorphous diffusion barrier layer. The preferred material is the previously described metal organic chemical vapor deposited TiC_xN_yO_zwhich is provided to a thickness which is effective to form the predominant portion (the entire portion, as shown) of second electrically conductive capacitor plate 32a. Alternately, both of plates 32 and 26 might be provided to constitute a conductive inherent amorphous diffusion barrier layer, such as the MOCVD TiC_xN_yO_z.

[0031] FIG. 3 illustrates yet another alternate embodiment capacitor construction and method. Again, like numerals from the first described embodiment are utilized where appropriate, with differences being indicated, by different numerals or with the suffix “b”. Wafer fragment 10bincludes a capacitor construction 25bhaving the same first and second capacitor plates 26 and 32, respectively, of the first described embodiment. However, a pair of predominately amorphous diffusion barrier layers 30 and 40 are provided to encapsulate Ta₂O₅dielectric layer 28 to prevent tantalum out-diffusion.

[0032] The invention was reduced to practice in a study of three types of rapid thermal nitrogen annealed polysilicon films having Ta₂O₅films thereatop, with different overlying titanium nitride or titanium carbonitride layers being deposited. The inventive constructions were deposited by metal organic chemical vapor deposition of a TiC_xN_yO_Zfilm deposited from tetrakisdimethylamido titanium (TDMAT) at 450° C., and 0.5 Torr in a cold wall chemical vapor deposition reactor. These wafers were compared with two non-inventive deposited titanium nitride layers. These two non-invention layers were deposited by chemical vapor deposition and by sputtering, respectively. The CVD TiN films were formed by reacting TiCl₄and NH₃at 600° C. (i.e., non-MOCVD) in a low pressure chemical vapor deposition reactor. The sputtered TiN films were formed by reactive sputtering of Ti in an Ar—N₂ambient at 0.14 Torr. The substrates were heated to 400° C. during deposition. On top of the CVD TiN film, and also on the inventive MOCVD films, an additional capping layer of sputtered TiN was deposited for protection against oxidation during plasma steps used to remove photoresist. The sputter deposited and CVD TiN films using TiCl₄and NH₃were crystalline, while the MOCVD films of the invention are substantially amorphous.

[0033] Four samples of each group were prepared, out of which three were annealed at 700° C., 800° C. and 900° C. for thirty minutes at 7×10⁻⁶Torr. With respect to the control CVD TiN films, out-diffusion of Ta into this crystalline TiN layer was observed, and to increase with increasing annealing temperature. Also with the crystalline sputtered TiN films, significant out-diffusion of tantalum into the TiN layer was observed after the 900° C. anneal.

[0034] With the amorphous MOCVD TiC_xN_yO_zfilms, no out-diffusion of tantalum was observed. Twenty atomic percent of carbon and oxygen were however detected in the film. Yet, the absence of interaction of the MOCVD TiC_xN_yO_zlayers with the dielectric Ta₂O₅up to 900° C. makes such composite layers attractive candidates for capacitor and other dielectric layers.

[0035] In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.