- BACKGROUND OF THE INVENTION
This invention relates to capacitors, to methods of forming capacitors, and to DRAM cells.
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-dimensional 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 SiO2 and Si3N4 might not be suitable because of small dielectric constants.
Highly integrated memory devices, such as 256 Mbit DRAMs, are expected to require a very thin dielectric film for the 3-dimensional capacitor of cylindrically stacked or trench structures. To meet this requirement, the capacitor dielectric film thickness will be below 2.5 nm of SiO2 equivalent thickness. Chemical vapor deposited (CVD) Ta2O5 films are considered to- be very promising cell dielectric layers for this purpose, as the dielectric constant of Ta2O5 is approximately three times that of conventional Si3N4 capacitor dielectric layers. However, one drawback associated with Ta2O5 dielectric layers is undesired leakage current characteristics. Accordingly, although Ta2O5 material has inherently higher dielectric properties, as-deposited Ta2O5 typically produces unacceptable results due to leakage current.
Densification of Ta2O5 as deposited has been reported to significantly improve the leakage characteristics of such layers to acceptable levels. Prior art densification of such layers includes exposing the Ta2O5 layer to extreme annealing and oxidizing conditions. The anneal drives any carbon present out of the layer and advantageously injects additional oxygen into the layer such that the layer uniformly approaches a stoichiometry of five oxygen atoms for every two tantalum atoms. The oxygen anneal is commonly conducted at a temperature of from about 400° C. to about 1000° C. utilizing an ambient comprising an oxygen containing gas. The oxygen containing gas commonly comprises one or more of O3, NO, N2O and O2. The oxygen containing gas is typically flowed through a reactor at a rate of from about 0.5 slm to about 10 slm.
The Ta2O5 layer is typically from about 40 angstroms to about 150 angstroms thick and can be either amorphous or crystalline. Ta2O5 is generally amorphous if formed below 600° C. and will be crystalline if formed, or later processed, at or above 600° C. Typically, a Ta2O5 layer is deposited as an amorphous layer and the above-described oxygen anneal is conducted at a temperature of 600° C. or greater to convert the amorphous Ta2O5 layer to a crystalline layer. Undesirably, however, such has a tendency to form an SiO2 layer intermediate or between the polysilicon and Ta2O5. Further and regardless, a thin SiO2 layer will also typically inherently form during the Ta2O5 deposition due to the presence of oxygen at the polysilicon layer interface. It would be desirable to remove or eliminate this SiO2 layer intermediate the Ta2O5 and polysilicon layers, yet allow for such desired densification.
- SUMMARY OF THE INVENTION
One prior art technique reported includes exposing the polysilicon layer to rapid thermal nitridation prior to subsequent deposition of the Ta2O5 layer. Such are reported by Kamiyama et al., “Ultrathin Tantalum Oxide Capacitor Dielectric Layers Fabricated Using Rapid Thermal Nitridation prior to Low Pressure Chemical Vapor Deposition”, J. Electrochem. Soc., Vol. 140, No. 6, June 1993 and Kamiyama et al., “Highly Reliable 2.5 nm Ta2O5 Capacitor Process Technology for 256 Mbit DRAMs”, 830-IEDM 91, pp. 32.2.1-32.2.4. Such rapid thermal nitridation includes exposing the subject polysilicon layer to temperatures of from 800° C. to 1100° C. for sixty seconds in an ammonia atmosphere at atmospheric pressure. The nitride layer acts as a barrier layer to oxidation during Ta2O5 deposition and subsequent high temperature densification processes to prevent oxidation of the underlying polysilicon electrode. These processes do however have several drawbacks, including the undesired high temperature cycling and formation of a fairly thick native SiO2 on the nitride in series with the Ta2O5, all of which adversely effects the realization of high capacitance promised by inherent Ta2O5 layers.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention comprises capacitors, methods of forming capacitors and DRAM circuitry. In one implementation, a capacitor comprises a capacitor dielectric layer comprising Ta2O5 formed over a first capacitor electrode. A second capacitor electrode is formed over the Ta2O5 capacitor dielectric layer. Preferably, at least a portion of the second capacitor electrode is formed over and in contact with the Ta2O5 in an oxygen containing environment at a temperature of at least about 175° C. Chemical vapor deposition is one example forming method. The preferred second capacitor electrode comprises a conductive metal oxide. A more preferred second capacitor electrode comprises a conductive silicon comprising layer, over a conductive titanium comprising layer, over a conductive metal oxide layer. A preferred first capacitor electrode comprises a conductively doped Si—Ge alloy. Preferably, a Si3N4 layer is formed over the first capacitor electrode.
Preferred embodiments of the invention are described below with reference to the following accompanying drawings.
FIG. 1 is a diagrammatic depiction of a capacitor stack in accordance with one aspect of the invention.
FIG. 2 is a diagrammatic depiction of another capacitor stack in accordance with one aspect of the invention.
FIG. 3 is a diagrammatic depiction of yet another capacitor stack in accordance with one aspect of the invention.
FIG. 4 is a diagrammatic depiction of still another capacitor stack in accordance with one aspect of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 5 is a view an alternate embodiment semiconductor wafer fragment in accordance with the invention.
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).
FIG. 1 diagrammatically depicts a capacitor stack 10 which would be formed over a substrate. To aid in interpretation of the claims that follow, the term “semiconductive substrate” is defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including but not limited to, the semiconductive substrates described above.
Capacitor 10 comprises a first capacitor electrode 12, a capacitor dielectric layer 14, and a second capacitor electrode 15. In the illustrated example, first capacitor electrode 12 comprises silicon material 16, such as elemental silicon in polycrystalline form which is suitably conductively doped with desired dopant impurities. Such can be formed, for example, by chemical vapor deposition using SiH4 as a precursor gas at a temperature of 535° C. and a pressure of 200 mTorr. Doping can occur during or after deposition. An example preferred thickness range for layer 12 is from 400 Angstroms to 1000 Angstroms. Electrode 12 could of course constitute some other conductive material. Further, electrode 12 could comprise a composite of conductive materials, such as by way of example only Ru or RuOx formed over conductively doped silicon.
Capacitor dielectric layer 14 is preferably formed over and in contact with electrode 12, and preferably comprises Ta2O5 material 18. Where electrode 12 comprises silicon, an intervening oxidation barrier layer (not shown) is ideally provided intermediate the Ta2O5 and silicon. Example conductive oxidation barrier layers include RuOx and Ru. Ta2O5 can be deposited by low pressure chemical vapor deposition utilizing Ta(C3H5)5, O2 and N2 as precursor gases. Example flow rates are 120 sccm; 2-5 slm; and 2-5 slm, respectively. An example temperature is 410° C., with an example pressure being from 200 to 400 mTorr. An example deposition thickness is from 60 to 90 Angstroms, with 70 Angstroms being preferred. The Ta2O5 layer is thereafter preferably subjected to a high temperature oxidation anneal by any one of the following processes, or other processes. In a first, rapid thermal processing is conducted over 40 seconds up to a temperature of 850° C. in a N2O ambient, with pressure being 660 Torr. Processing continues at 850° C. and 660 Torr for one minute. In a second process, furnace heating is conducted to 800° C. at a temperature increase rate of 7° C./min. in a N2O ambient, with pressure remaining at atmospheric and the wafer being maintained at 800° C. for 30 minutes. In a third, a higher pressure oxidation is conducted at 800° C. for 30 minutes in a N2O atmosphere at a pressure from 1 to 3 atmospheres, with the temperature being ramped to 800° C. at an approximate rate of 15° C./min.
Second capacitor electrode 15 is preferably formed over and in contact with Ta2O5 material 18 of capacitor dielectric layer 14. Such preferably is formed in an oxygen-containing environment at a temperature of at least about 175° C. Second capacitor electrode 14 preferably comprises a conductive metal oxide material 20 formed to a thickness of from about 400 Angstroms to about 1000 Angstroms. In the context of this document, a conductive metal oxide is any oxide having a resistance of less than or equal to about microohms.cm. Example materials include RuO2, IrO2, SnO2, In2O3:SnO2, VO3, CuO, Cu2O, and mixtures thereof. RuO2 and IrO2 are more preferred. An example process for forming such conductive metal oxide is by chemical vapor deposition. For RuOx, an example deposition process would be conducted at a pressure of 1 Torr and a temperature of 175° C., with precursor feeds of Ru(tricarbonyl cyclohexdienyl) at 300 sccm and O2 at 300 sccm.
Referring to FIG. 2, a second embodiment capacitor stack 10 a is shown. Like numerals from the first described embodiment are utilized where appropriate, with differences being indicated by the suffix “a” or with different numerals. Here, second capacitor electrode 15 a comprises a conductive silicon comprising layer 23 formed over and in contact with a conductive titanium comprising layer 22 (i.e., Ti or TiN), formed over and in contact with conductive metal oxide layer 20 a. Conductive metal oxide layer 20 a is also formed over and preferably in contact with Ta2O5 material 18 of capacitor dielectric layer 14. Accordingly, second capacitor electrode 15 a comprises both conductive silicon and a conductive metal oxide. A titanium comprising layer 22 is provided intermediate conductive metal oxide layer 20 a and conductive silicon layer 23. The preferred material for titanium-comprising layer 22 is TiN formed by chemical vapor deposition using an organic precursor. An example process utilizes precursors of ((CH3)2N)4Ti at 150 sccm and N2 at 80 sccm at a temperature of 420° C. and a pressure of 0.7 Torr. Silicon layer 23 preferably comprises conductively doped elemental polycrystalline silicon, with thus both the first and second capacitor electrodes comprising silicon. An example thickness for TiN layer 22 is from 150 Angstroms to 300 Angstroms. An example thickness for silicon layer 23 is from 400 Angstroms to 1000 Angstroms.
Yet another alternate embodiment capacitor stack 10 b is shown in FIG. 3. Like numerals from the first described embodiments are utilized where appropriate, with differences being indicated by the suffix “b” or with different numerals. Here, silicon material 16 b of first capacitor electrode 12 b is subjected to rapid thermal nitridation to form a silicon nitride layer 26 atop material 16 b and in contact with Ta2O5 material 18. Thereby, the capacitor dielectric layer 14 b essentially comprises a combination of the Ta2O5 and Si3N4. The nitridation is ideally conducted prior to formation of the Ta2O5, and functions as a diffusion restricting or barrier layer to formation of SiO2 during deposition of Ta2O5 material 18. Thickness of layer 26 is preferably from 30 Angstroms to 60 Angstroms. Exemplary rapid thermal nitridation conditions include exposing the substrate to a NH3 atmosphere at a flow rate of from about 10 to 20 sccm for 20 seconds at atmospheric pressure and 900-950° C. In the context of this document, rapid thermal nitridation is intended to define any process where a substrate is ramped to a temperature of at least 900° C. at a rate of 20° C./sec in a nitrogen containing environment.
Second capacitor electrode 15 b is formed to provide silicon both as polycrystalline silicon and as a silicide. Specifically, a silicide layer 24 is formed over silicon layer 23 b. Example techniques include a refractory metal deposition and conventional salicide process, or direct chemical vapor deposition of a silicide. Example precursor gases for chemical vapor depositing WSix include WF6 and WSH4. An example preferred thickness for silicide layer 24 is from about 300 Angstroms to 600 Angstroms.
Still a further alternate embodiment capacitor stack 10 c is described with reference to FIG. 4. Like numerals from the first described embodiments have been utilized where appropriate, with differences being indicated by the suffix “c” or with different numerals. Here, first capacitor electrode 12 c comprises a conductively doped silicon-germanium alloy material 16 c.
In each of the above described preferred embodiments where at least one of the first and second capacitor electrodes comprises titanium, no titanium-comprising material of either the first and second capacitor electrodes is formed in contact with the Ta2O5 material of the capacitor dielectric layer. Where the capacitor dielectric layer comprises Ta2O5, preferably such effectively gets heated in an oxygen rich atmosphere during top electrode deposition, which can minimize oxygen vacancy content in the Ta2O5. Where a conductive metal oxide is formed in contact with Ta2O5, reduction of the Ta2O5 can be avoided or at least reduced.
FIG. 5 depicts implementation of the invention in fabrication of DRAM circuitry. A wafer fragment 31 comprises two memory cells, with each comprising a capacitor 36 and a shared bit contact 46. Capacitors 36 electrically connect with substrate diffusion regions 34 through silicide regions 33. For simplicity, capacitors 36 are shown as comprising a first capacitor electrode 38, a capacitor dielectric layer 40, and a second capacitor electrode/cell plate 42. Such can be fabricated of materials described above, preferably to include silicon, barrier layers metal oxide, and a high K oxygen containing capacitor dielectric layers such as Ta2O5. Processing preferably occurs as described above. A dielectric layer 44 is formed over second capacitor plate 42. A bit line 47 is fabricated in electrical connection with bit contact 46. Word lines 48 are fabricated to enable selective gating of the capacitors relative to bit contact 47.
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.