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U.S. Patent Dec. 31, 1996 Sheet 2 of 3 5,589,284
The increasing density of integrated circuits (e.g. DRAMs) is increasing the need for materials with highdielectric-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 Si02 or Si02/Si3N4 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)Ti03 (BST) usually have much larger capacitance densities than standard Si02—Si3N4— Si02 capacitors. Various metals and metallic compounds, and typically noble metals such as Pt and conductive oxides such as Ru02, 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° 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 Si02 or SiN4, and Pt can rapidly form a silicide at low temperatures. ATa 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 Ru02 have been proposed as the lower (and upper) electrode. Although Ru is generally not a standard integrated circuit manufacturing material,
Ru/Ru02 can be used to provide an oxygen barrier between the underlying materials and the perovskite dielectric material. Ru02 will generally not reduce the perovskite dielectric material and can possibly be used as an oxygen source for the perovskite dielectric material. 5
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 10 rapidly form silicides at low temperatures (generally less than about 450° 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 15 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 Ru02 electrode is 20 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 Ru02 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 3Q YBa2Cu307_;(. and (La,Sr)Co03. 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 3S 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. 4Q
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 45 and should remain conductive when partially or fully oxidized, and when in a perovskite. Generally, the perovskiteseed 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 depo- 50 sition 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 55 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 60 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 65 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
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
Si02 insulating layer 32 formed on the surface of the silicon substrate 30. FIG. 3 illustrates a patterned layer of ruthenium 34 deposited on the Si02 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 5 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 Si02 layer 32. The structure is then heated, causing a solid state reaction between the CaO 38 and the surface of Ru02 layer 36 to form the CaRu03 perovskite seed layer 40. CaRu03 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 CaRu03 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 20 should be at least thick enough to form CaRu03 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 CaRu03 layer 40, along with the portion of CaO 25 layer 38 overlaying Si02 layer 32, yielding the structure of FIG. 5.
Selective chemical reactivity is generally desirable for CaO layer 38, so that it reacts with RuOz layer 36 and not with other materials which it contacts. To this end, a barrier 30 layer such as Si3N4 may be used between CaO layer 38 and Si02 layer 32 to minimize the formation of silicates for example. Alternatively, a carbonate such as CaC03 may be used instead of CaO since CaC03 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 45 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 5Q 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 55 the selectively reactive layer can be removed with a selective, but unpatterned etch.
FIG. 6 illustrates a perovskite dielectric, BST layer 42, deposited on CaRu03 layer 40. At a given deposition temperature, the crystal quality and other properties of BST go layer 42 will generally be enhanced by depositing it on the surface of CaRu03 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. 65
There are many alternative ways to form the CaRu03 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 CaRu03. 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 CaRu1-;cO,. 46 is then sputtered on the structure. CaRU[_xOz 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 CaRu03 as the Ru deficient CaRu^O., 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 CaRu03 40 overlaying a layer of Ru02 36, and FIG. 10 illustrates a layer of BST 42 deposited on CaRu03 layer 40. The crystal quality and other properties of BST layer 42 are enhanced by depositing it on the surface of CaRuOa 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 CaRu03 perovskite-seed layer 40, which in turn overlays RuOz 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 CaRu03 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 Si02 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 CaRu03 perovskite-seed layer 40 is again formed on Ru02 layer 36. However, in FIG. 12, Ru02 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, Ru02 layer 36 is used for electrical contact. However, in FIG. 13, Ru02 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.