US 5,589,284 AGrant

Electrodes Comprising Conductive Perovskite-Seed Layers for Perovskite Dielectrics

Issue Date:1996-12-31

•13 Claims

•3 Drawing Sheets

Abstract

A preferred embodiment of this invention comprises a perovskite-seed layer (e.g. calcium ruthenate 40) between a conductive oxide layer (e.g. ruthenium oxide 36) and a perovskite dielectric material (e.g. barium strontium titanate 42), wherein the perovskite-seed layer and the conductive oxide layer each comprise the same metal. The metal should be conductive in its metallic state and should remain conductive when partially or fully oxidized. Generally, the perovskite-seed layer has a perovskite or perovskite-like crystal structure and lattice parameters which are similar to the perovskite dielectric layer formed thereon. At a given deposition temperature, the crystal quality and other properties of the perovskite dielectric will generally be enhanced by depositing it on a surface having a similar crystal structure. Undesirable crystal structure formation will generally be minimized and lower processing temperatures may be used to deposit the perovskite dielectric layer. Another benefit of this electrode system is that the perovskite-seed layer should do little or no reduction of the perovskite dielectric layer.

Metadata

Assignee

Texas Instruments Incorporated

Inventors

Scott R. Summerfelt
Howard R. Beratan

Application Information

Application Number:US 4748034

Filing Date:1995-06-07

Priority Date:1994-08-01

Art Unit:135

Classifications

IPC:

B32B 1800

Field of Search:

428365252437505688;689;697;699;700;701;702145520;521;518;519201701;702;703;776;778;781

Patent Drawings (3 sheets)

Description

Cross-References to Related Applications

The following related applications were filed concurrently with the instant application:

Field of Title Invention

This invention generally relates to improving electrical connections perovskite materials, such as in the construction of pyroelectric detectors.

Background of the Invention

Without limiting the scope of the invention, its background is described in connection with current methods of forming electrical connections to perovskite materials, as an example.

The increasing density of integrated circuits (e.g. DRAMs) is increasing the need for materials with high-dielectric-constants to be used in electrical devices such as capacitors. Generally, capacitance is directly related to the surface area of the electrode in contact with the capacitor dielectric, but is not significantly affected by the electrode volume. The current method generally utilized to achieve higher capacitance per unit area is to increase the surface area unit area by increasing she topography, such as in trench and stack capacitors using SiO.sub.2 or SiO.sub.2 /Si.sub.3 N.sub.4 as the dielectric. This approach becomes very difficult in terms of manufacturability for devices such as the 256 Mbit and 1 Gbit DRAMs.

An alternative approach is to use a high permittivity dielectric material. Many perovskite, pyroelectric, ferroelectric, or high-dielectric-constant (hereafter abbreviated HDC) materials such as (Ba,Sr)TiO.sub.3 (BST) usually have much larger capacitance densities than standard SiO.sub.2 --Si.sub.3 N.sub.4 --SiO.sub.2 capacitors. Various metals and metallic compounds, and typically noble metals such as Pt and conductive oxides such as RuO.sub.2, have been proposed as the electrodes for these perovskite dielectric materials. To be useful in electronic devices, however, reliable electrical connections should generally be constructed which do not diminish the beneficial properties of these perovskite dielectric materials.

Summary of the Invention

As used herein, the term "high-dielectric-constant" means a dielectric constant greater than about 50 at device operating temperature. As used herein the term "perovskite" means a material with a perovskite or perovskite-like crystal structure. As used herein the term "dielectric", when used in reference to a perovskite, means a non-conductive perovskite, pyroelectric, ferroelectric, or high-dielectric-constant oxide material. The deposition of a perovskite dielectric usually occurs at high temperature (generally greater than about 500.degree. C.) in an oxygen containing atmosphere. The lower electrode structure should be stable during this deposition, and both the lower and upper electrode structures should be stable after this deposition.

It is herein recognized that there are several problems with the materials thus far chosen for the lower electrode in thin-film (generally less than 5 um) applications; many of these problems are related to semiconductor process integration. For example, Pt has several problems as a lower electrode which hinder it being used alone. Pt generally allows oxygen to diffuse through it and hence typically allows neighboring materials to oxidize. Pt also does not normally stick very well to traditional dielectrics such as SiO.sub.2 or SiN.sub.4, and Pt can rapidly form a silicide at low temperatures. A Ta layer has been used as a sticking or buffer layer under the Pt electrode, however during BST deposition, oxygen can diffuse through the Pt and oxidize the Ta and make the Ta less conductive. This may possibly be acceptable for structures in which contact is made directly to the Pt layer instead of to the Ta layer, but there are other associated problems as described hereinbelow. For example, Pt has a radioactive isotope, Pt-190, that, even though it has a relatively long half-life and makes up a small percentage of the total number of Pt atoms, could create a substantial number of detrimental alpha-particles when used in a standard thin-film structure.

Conductive oxides such as RuO.sub.2 have been proposed as the lower (and upper) electrode. Although Ru is generally not a standard integrated circuit manufacturing material, Ru/RuO.sub.2 can be used to provide an oxygen barrier between the underlying materials and the perovskite dielectric material. RuO.sub.2 will generally not reduce the perovskite dielectric material and can possibly be used as an oxygen source for the perovskite dielectric material.

Other structures which have been proposed include alloys of Pt, Pd, Rh as the electrode and oxides made of Re, Os, Rh and Ir as the sticking layer on single crystal Si or poly-Si. A problem with these electrodes is that these oxides are generally not stable next to Si and that these metals typically rapidly form silicides at low temperatures (generally less than about 450.degree. C.). In addition, elements such as Pt can normally diffuse quickly in Si and therefore can cause other problems.

One problem with these solutions is that an electrode surface with a crystal structure and lattice parameters different than that of a perovskite dielectric appears to degrade the properties of the perovskite dielectric. For example, the spontaneous polarization of a ferroelectric (e.g. lead zirconium titanate (PZT)) deposited on an RuO.sub.2 electrode is generally degraded compared to that of a Pt electrode. The reduced polarization may be caused by the different crystal structure and lattice parameters between PZT and RuO.sub.2 as compared to PZT and Pt.

As an example, PZT commonly forms an undesirable pyrochlore crystal structure prior to the formation of the perovskite crystal structure. To facilitate perovskite crystal formation, perovskite dielectrics such as PZT have been deposited on some conductive perovskite such as YBa.sub.2 Cu.sub.3 O.sub.7-x and (La,Sr)CoO.sub.3. Deposition of PZT on a substrate with a perovskite or perovskite-like crystal structure normally minimizes the formation of the pyrochlore phase and improves the properties of the perovskite dielectric. However, the materials used thus far for the deposition surface have several problems. For example, they typically involve new cations such as Cu and Co which are fairly reactive. These materials are also generally difficult to etch. In addition, these materials must be deposited in the stoichiometric ratio, and this deposition is generally difficult.

Generally, the present invention uses a conductive perovskite-seed layer between a conductive oxide layer and a perovskite dielectric material, wherein the perovskite-seed layer and the conductive oxide layer each comprise the same metal. The metal should be conductive in its metallic state and should remain conductive when partially or fully oxidized, and when in a perovskite. Generally, the perovskite-seed layer has a perovskite or perovskite-like crystal structure and lattice parameters which are similar to the perovskite dielectric layer formed thereon. At a given deposition temperature, the crystal quality and other properties of the perovskite dielectric will generally be enhanced by depositing it on a surface having a similar crystal structure. Undesirable crystal structure formation will generally be minimized and lower processing temperatures may be used to deposit the perovskite dielectric layer. Another benefit of this electrode system is that the perovskite-seed layer should do little or no reduction of the perovskite dielectric layer.

The disclosed structures generally provide electrical connection to perovskite dielectric materials while eliminating many of the disadvantages of the current structures. One embodiment of this invention comprises a conductive oxide layer comprising a first metal, a conductive perovskite-seed layer comprising the first metal overlaying the conductive oxide layer, and a perovskite dielectric layer overlaying the perovskite-seed layer. The similar crystal structure of the perovskite-seed layer improves the crystal quality of the perovskite dielectric layer and minimizes the formation of non-perovskite crystal structures. A method of forming an embodiment of this invention comprises forming a conductive oxide layer comprising a first metal, forming a conductive perovskite-seed layer comprising the first metal on the conductive oxide layer, and forming a perovskite dielectric layer on the perovskite-seed layer.

Another method of forming an embodiment of this invention comprises forming a conductive oxide layer having a top surface and side surfaces and comprising a first metal. A selectively reactive layer is formed on the conductive oxide layer and then the selectively reactive layer and the conductive oxide layer are heated, thereby causing the selectively reactive layer and the conductive oxide layer to react and form a perovskite seed layer on the conductive oxide layer. The perovskite seed layer comprises the first metal. Any unreacted portions of the selectively reactive layer are removed, and a perovskite dielectric layer is formed on the perovskite seed layer.

These are apparently the first thin-film structures wherein an electrical connection to a perovskite dielectric material comprises a perovskite-seed layer between a conductive oxide layer and the perovskite dielectric, wherein the perovskite-seed layer and the conductive oxide layer each comprise the same metal. These structures may also be used for single or multilayer capacitors and other thin-film devices such as non-volatile memories, thin-film piezoelectric and thin-film electro-optic devices.

Brief Description of the Drawings

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to the detailed description which follows, read in conjunction with the accompanying drawings, wherein:

FIGS. 1-6 are cross-sectional views showing the progressive steps in the fabrication of a microelectronic structure in which a perovskite dielectric layer is deposited on an electrode comprising a perovskite-seed layer;

FIG. 7 is a cross-sectional view of a microelectronic structure comprising two metal layers that will be oxidized to form a perovskite-seed layer and a conductive oxide layer;

FIG. 8 is a cross-sectional view of a microelectronic structure comprising a metal-deficient layer overlaying a conductive oxide layer;

FIG. 9 is a cross-sectional view of a perovskite-seed layer overlaying a conductive oxide layer;

FIG. 10 is a cross-sectional view of a perovskite dielectric overlaying the structure of FIG. 9; and

FIGS. 11-13 are cross-sectional views of capacitors having a perovskite dielectric layer overlaying an electrode comprising a perovskite-seed layer.

Detailed Description of the Preferred Embodiments

With reference to FIGS. 1-6, there is shown a method of forming a preferred embodiment of this invention, a microelectronic structure comprising a perovskite-seed layer between a conductive oxide layer and a perovskite dielectric, wherein the perovskite-seed layer and the conductive oxide layer each comprise the same metal. FIG. 1 illustrates a silicon semiconductor substrate 30. FIG. 2 illustrates an SiO.sub.2 insulating layer 32 formed on the surface of the silicon substrate 30. FIG. 3 illustrates a patterned layer of ruthenium 34 deposited on the SiO.sub.2 layer 32. The ruthenium layer 34 is subsequently oxidized to form ruthenium dioxide layer 36, which will function as the base of the lower electrode, as shown in FIG. 4. Ruthenium is conductive from its unoxidized state through its partially oxidized state to its fully oxidized state. The thickness of ruthenium dioxide layer 36 can vary depending on the application; a typical range would be 50 to 100 nanometers (nm).

FIG. 4 further illustrates a thin layer of calcium oxide 38 deposited on the surface of ruthenium dioxide layer 36 and on the exposed surface of SiO.sub.2 layer 32. The structure is then heated, causing a solid state reaction between the CaO 38 and the surface of RuO.sub.2 layer 36 to form the CaRuO.sub.3 perovskite seed layer 40. CaRuO.sub.3 layer 40 would generally be less than 50 nm thick, typically less than 30 nm thick, and preferably 10 to 20 nm thick. The thickness and stoichiometry of CaRuO.sub.3 layer 40 is determined by how much CaO is deposited for layer 38 and by the processing temperature and time, and not by the deposition process. CaO layer 38 should be at least thick enough to form CaRuO.sub.3 layer 40 of the desired thickness. More CaO than is necessary can be deposited: if a partial reaction occurs, a mild wet etch such as hot deionized water can be used to dissolve the unreacted CaO on CaRuO.sub.3 layer 40, along with the portion of CaO layer 38 overlaying SiO.sub.2 layer 32, yielding the structure of FIG. 5.

Selective chemical reactivity is generally desirable for CaO layer 38, so that it reacts with RuO.sub.2 layer 36 and not with other materials which it contacts. To this end, a barrier layer such as Si.sub.3 N.sub.4 may be used between CaO layer 38 and SiO.sub.2 layer 32 to minimize the formation of silicates for example. Alternatively, a carbonate such as CaCO.sub.3 may be used instead of CaO since CaCO.sub.3 is generally less reactive.

These are apparently the first methods wherein an electrical connection to a perovskite dielectric material is formed by using a selectively reactive layer. This layer reacts with an underlying barrier layer to form a perovskite seed layer on the barrier layer, and then the excess portions of the selectively reactive layer are removed.

Although using very different materials and structures, this technique is similar to that used for the formation of platinum silicide or the formation of titanium nitride/titanium silicide/silicon ohmic contacts, wherein two layers of materials are caused to interact and form a third layer, with the unreacted material subsequently being removed. By using a layer that is selectively reactive to the conductive oxide layer, patterning is generally not needed to deposit the selectively reactive layer. Even though this process could possibly take more steps than a process in which the perovskite seed layer is deposited directly, it is generally easier and simpler to perform. Since the perovskite seed layer is self-aligned and no removal of portions of the perovskite seed layer is necessary. The unreacted portions of the selectively reactive layer can be removed with a selective, but unpatterned etch.

FIG. 6 illustrates a perovskite dielectric, BST layer 42, deposited on CaRuO.sub.3 layer 40. At a given deposition temperature, the crystal quality and other properties of BST layer 42 will generally be enhanced by depositing it on the surface of CaRuO.sub.3 layer 40 due to the similarity in crystal structure. As will be described in other alternate embodiments, an upper electrode may be deposited on BST layer 42.

There are many alternative ways to form the CaRuO.sub.3 perovskite-seed layer. In an alternate embodiment, FIG. 7 illustrates the structure of FIG. 3 but with a thin calcium layer 44 deposited on ruthenium layer 34. The calcium is then oxidized and in doing so reacts with the surface of ruthenium layer 34 to form a layer of CaRuO.sub.3. The excess calcium is then removed, yielding the structure of FIG. 5.

In another alternate embodiment, FIG. 8 illustrates a deposited ruthenium layer 34 with its surface oxidized to form ruthenium dioxide layer 36. A layer of Ru deficient CaRu.sub.1-x O.sub.z. 46 is then sputtered on the structure. CaRu.sub.1-x O.sub.z layer 46 may be deposited on the entire structure or on ruthenium dioxide layer 36 only, using a mask. The structure is then annealed in oxygen to form a near stoichiometric layer of CaRuO.sub.3 as the Ru deficient CaRu.sub.1-x O.sub.z reacts with the ruthenium dioxide. If necessary, excess material is removed, again yielding the structure of FIG. 5.

In another alternate embodiment, FIG. 9 illustrates a layer of CaRuO.sub.3 40 overlaying a layer of RuO.sub.2 36, and FIG. 10 illustrates a layer of BST 42 deposited on CaRuO.sub.3 layer 40. The crystal quality and other properties of BST layer 42 are enhanced by depositing it on the surface of CaRuO.sub.3 layer 40, which has a similar crystal structure.

In another alternate embodiment, FIG. 11 illustrates a perovskite HDC capacitor utilizing an electrode comprising a perovskite-seed layer. BST layer 42 overlays the CaRuO.sub.3 perovskite-seed layer 40, which in turn overlays RuO.sub.2 layer 36. A TiN upper electrode 50 overlays BST layer 42. TiN is generally a good sticking layer and diffusion barrier, in addition to being conductive. In this embodiment, conductive CaRuO.sub.3 layer 40 is connected to from above, via a conductive TiN plug 54. The TiN plug 54 makes electrical contact to the aluminum top metallization 56 through the second SiO.sub.2 insulating layer 52. The two other TiN plugs 54 make electrical contact from the aluminum top metallization layer 56 to the TiN upper electrode 50 and to the doped silicon region 48.

In another alternate embodiment, FIG. 12 illustrates a perovskite HDC capacitor utilizing an electrode comprising a perovskite-seed layer. As in FIG. 11, the CaRuO.sub.3 perovskite-seed layer 40 is again formed on RuO.sub.2 layer 36. However, in FIG. 12, RuO.sub.2 layer 36 provides electrical connection to doped silicon region 48 below it.

In yet another alternate embodiment, FIG. 13 illustrates a perovskite HDC capacitor utilizing an electrode comprising a perovskite-seed layer. As in FIG. 12, RuO.sub.2 layer 36 is used for electrical contact. However, in FIG. 13, RuO.sub.2 layer 36 connects to the doped silicon region 48 via a TiN plug 58. The sole Table, below, provides an overview of some embodiments and the drawings.

Many thin-film applications such as dynamic random access memories (DRAMs) require a small leakage-current-density in addition to a large capacitance per unit area. Specifically, it appears that the leakage current is controlled by Schottky barriers, thus the structures of the present invention may have a larger leakage current compared with Pt electrodes due to the smaller work function. Although the leakage current is important for DRAMs, it is generally not critical for other applications such as non-volatile memories and pyroelectric detectors.

The capacitor plug may comprise multiple layers, with an upper portion overlying the first level insulator and underlying the first conductive oxide (or the first metallic layer if used). The capacitor plug, and especially the upper portion, may comprise still other materials than those listed in the table yet which may be preferred for various applications. For example, the lower electrode base may comprise ternary (or greater amorphous nitrides (e.g. Ta--Si--N, Ti--Si--N, Ta--B--N, Ti--B--N); exotic (non-standard Si processing) conductive nitrides (e.g. Zr nitride, Hf nitride, Y nitride, Sc nitride, La nitride and other rare earth nitrides, N deficient Al nitride, doped Al nitride, Mg nitride, Ca nitride, Sr nitride, Ba nitride); alloys of the above exotic conductive nitrides with common Si processing materials such as TiN, GaN, Ni nitride, Co nitride, Ta nitride, W nitride (e.g. Ta--Al--N, Ti--Al--N); or noble metal insulator alloys (e.g. Pd--Si--N, Pt--Si--N, Pd--Si--O, Pd--Si--O, Pd--B--(O,N), Pd--Al--N, Ru--Si--(O,N)).

A few preferred embodiments have been described in detail hereinabove. It is to be understood that the scope of the invention also comprehends embodiments different from those described, yet within the scope of the claims. With reference to the structures described, electrical connections to such structures can be ohmic, rectifying, capacitive, direct or indirect, via intervening circuits or otherwise. Implementation is contemplated in discrete components or fully integrated circuits in silicon, germanium, gallium arsenide, or other electronic materials families. In general the preferred or specific examples are preferred over the other alternate examples. For some drawing elements, the preferred or specific example and/or the other alternate examples may be used individually or combinations thereof may be used. The perovskite-seed layer may sometimes be referred to as being part of the electrode and may sometimes be referred to as being an interface to the electrode; the structures and methods of the present invention are substantially the same in either case. The conductive oxide layer (such as RuO.sub.2) can also contain unoxidized or partially oxidized forms of the metal (such as Ru or RuO).

While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.

Claims

What is claimed is:

1. A microelectronic structure comprising: a conductive oxide layer comprising a first metal; a conductive perovskite-seed layer comprising said first metal overlying said conductive oxide layer; and a perovskite dielectric layer overlying said perovskite-seed layer, whereby the similar crystal structure of the perovskite-seed layer improves the crystal quality of the perovskite dielectric layer and minimizes the formation of non-perovskite crystal structures.

2. The structure of claim 1, wherein said first metal is ruthenium and said conductive oxide is ruthenium dioxide.

3. The structure of claim 1, wherein said first metal is iridium and said conductive oxide is iridium oxide.

4. The structure of claim 1, wherein said perovskite-seed layer is calcium ruthenate.

5. The structure of claim 1, wherein said perovskite-seed layer is selected from the group consisting of: SrRuO.sub.3, BaRuO.sub.3, CaIrO.sub.3, SrIrO.sub.3, and BaIrO.sub.3.

6. The structure of claim 1, wherein said perovskite dielectric layer is barium strontium titanate.

7. The structure of claim 1, wherein said perovskite dielectric layer is selected from the group consisting of: (Ba,Sr,Pb)(Ti,Zr)O.sub.3, (Pb,La)(Zr,Ti)O.sub.3, bismuth titanate, potassium tantalate, lead scandium tantalate, lead niobate, potassium niobate, lead zinc niobate, and lead magnesium niobate.

8. The structure of claim 1, further comprising an upper electrode overlying said perovskite dielectric layer.

9. The structure of claim 8, wherein said upper electrode is titanium nitride.

10. The structure of claim 1, further comprising said conductive oxide layer overlying a first metallic layer composed of said first metal.

11. The structure of claim 1, wherein said conductive oxide layer further comprises unoxidized and partially oxidized forms of said first metal.

12. A microelectronic capacitor structure comprising: a ruthenium dioxide layer; a calcium ruthenate layer overlying said ruthenium dioxide layer; a barium strontium titanate layer overlying said calcium ruthenate layer; and an upper electrode overlying said barium strontium titanate layer.

13. A microelectronic capacitor structure comprising: an iridium oxide layer; a CaIrO.sub.3 layer overlying said iridium oxide layer; a barium strontium titanate layer overlying said CaIrO.sub.3 layer; and an upper electrode overlying said barium strontium titanate layer.

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