Patent Application
20050211135
The present invention relates to thin-film structures that comprise a ferro-electric material disposed over a non-amorphous conductive layer, and methods for making the same. The structures may comprise electrical, magnetic, electro-optic, electromagnetic, and electromechanical devices. The present invention more particularly relates to lanthanum-modified lead zirconium titanate (PLZT) disposed over metallic lanthanum nickel oxide (LaNiO3).
Several types of optical communication devices comprise optical waveguides, optical modulators, and optical switching structures made of electro-optic material. A substrate optical waveguide comprises a lower cladding layer formed on the substrate, a core layer having a higher index of refraction formed over the lower cladding layer, and usually an upper cladding layer formed over the core layer. An optical modulator, or an optical switching structure, may be formed in line with the optical waveguide by forming a body of electro-optic (E/O) material on the same level as the core material, with the electro-optic material usually sandwiched between upper and lower cladding layers. Two electrodes are formed on opposing surfaces of the body E/O material, and are used to apply an electric field to the E/O body. The electric field changes selected optical properties (e.g., refractive index, polarization) of the E/O material. The changes in optical properties may be used to achieve various types of modulating, switching, and filtering functions.
A coefficient may be used to relate the change in the optical property of the material with respect to the applied electric field (i.e., the applied voltage divided by the dimension of the material along which the voltage acts). Electro-optic materials are usually crystalline materials or highly ordered materials (as in the case of polymers). In both cases, the value of the electro-optic coefficient usually depends upon the direction of the electric field relative to the orientation of the material's crystal or highly-ordered structure. Because of this, the electro-optic property is usually specified as a matrix of coefficient values, each of which is measured along a different axis of the material's crystal or ordered structure. This matrix is often called the tensor matrix of the material's property.
In electro-optic devices used in large systems integrated on substrate carriers, the E/O material usually comprises an inorganic single crystalline material, such as lithium niobate, which is difficult to grow and pattern. However, such single crystalline materials have relatively low responses to the applied electric field compared to other inorganic crystalline materials, such as lanthanum-modified lead zirconium titanate (PLZT). But such crystalline materials cannot be easily formed on substrate carriers, and must be grown on top of a base crystalline substrate in order to cause the material to form a crystalline structure. This precludes using the crystalline materials in small-scale modulators and switching devices that have vertically oriented electrodes (top and bottom electrodes) since the material cannot be grown over the bottom electrode. Thus, the potential of using inorganic crystalline materials in these devices cannot be realized. The inventors have thought of, and considered, an approach of making large-sized optical modulators and switches that comprises the steps of: (1) forming 100 μm thin wafers of inorganic E/O material, (2) followed by coating top and bottom surfaces with metal, (3) dicing the substrate into small pieces, and (4) then bonding the bottom surfaces of the pieces to metal pads on the substrate carrier, which will hold all of the electro-optic devices. However, this approach is not currently able to construct small-sized electro-optic switches and modulators (less than 10 μm in thickness and width), and is not practical for large-scale integration of electro-optic devices. This approach is a conception of the inventors, and does not form a part of the prior art to the inventors' knowledge.
Accordingly, the inventors have recognized that electro-optic devices with higher modulation/switching rates could be made if the above problems were solved.
The inventors have recognized that there is a need in the art to provide for reliable integration of crystalline ferro-electric materials onto substrates, and more particularly onto conductive layers formed on the substrates. Such integration would enable the use of vertically-oriented electrodes (as opposed to conventional top electrodes), which in turn could exploit higher E/O coefficient values in the material's tensor matrix, and also provide shorter spacing distances between the electrodes that are to generate the electric field.
The present invention provides thin-film structures and methods for making the same, with the structures comprising a non-amorphous conductive layer, which in turn can support and interface with a layer of a ferro-electric material. The non-amorphous bottom conductive layer may have a crystalline structure, or a semi-crystalline structure (e.g., a body of large grains of crystalline material). A preferred exemplary structure comprises metallic lanthanum nickel oxide (LaNiO3) as the non-amorphous bottom conductive layer, which is advantageous for interfacing to a lanthanum-modified lead zirconium titanate (PLZT) layer disposed over it. In making their invention, the inventors have recognized that a structure can be configured to enable the construction of vertically-oriented electrodes of high quality if one is able to provide a bottom electrode that has a crystal lattice that substantially matches the crystal lattice of the ferro-electric material (e.g., a ferro-electric oxide, PLZT), that also has a smooth surface morphology to assist the formation of subsequent layers, and that has suitable chemical compatibility to enhance adhesion to the substrate. The present invention is directed to methods for enabling the fabrication of such electrodes, and to the resulting devices.
One preferred aspect of the present invention comprises a method of forming a LaNiO3 layer. The method comprises coating a substrate with a liquid solution that comprises a solvent and a chelated complex having lanthanum atoms, nickel atoms, and one or more diol compounds, to form a material layer. As used herein and in the claims, the term “diol” has its ordinary and customary meaning in the chemical art, that of any alcohol molecule having two hydroxyl groups. As used herein, a diol compound comprises a diol molecule absent one or both hydrogen atoms of the hydroxyl groups. Thereafter, the material layer is heated to a temperature above room temperature (20° C.) to evaporate off the solvent. Next, the material layer is heated to a temperature of 500° C. or more in an oxygen atmosphere to decompose the diol compound and to form a layer of LaNiO3. This step is called the firing stage. A ferro-electric material may then be formed over the LaNiO3 layer by any available method. In this preferred aspect, the diol of the diol compound comprises 1,3-propanediol.
Another preferred aspect of the present invention comprises methods of forming a solution that includes lanthanum atoms, nickel atoms and a diol compound, such as a diol compound of 1,3-propanediol. An exemplary method comprises dissolving lanthanum acetylacetonate in a diol, such as 1,3-propanediol, and reacting the components to form a first solution having molecules that include lanthanum atoms and diol compounds, with at least some of the diol compounds being bonded to the coordinate sites of the lanthanum. The method further comprises dissolving nickel acetate in a diol, such as 1,3-propanediol, and 2,4-pentanedione, and reacting the components to form a second solution having molecules that include nickel atoms and diol compounds, with at least some of the diol compounds being bonded to the coordinate sites of the nickel. The method further includes combining the first and second solutions.
The inventors have found that the complexes formed between the metal atoms and the diol compounds comprise linear chains. The linear chain structure provides for an advantageous distribution of lanthanum and nickel atoms on the substrate surface prior to the firing stage, which leads to formation of crystal grains having improved electrical conductivity. In comparison to epitaxial deposition approaches, the present invention enables a lower-cost method of making the bottom electrodes.
Accordingly, it is an object of the present invention to provide improved methods of forming non-amorphous conductive films suitable for interfacing with ferro-electric materials.
It is another object of the present invention to enable these non-amorphous conductive films to be manufactured with improved control over composition and stoichiometry.
It is another object of the present invention to enable these non-amorphous conductive films to be manufactured with reduced electrical conductivity.
It is yet another object of the present invention to enable simple and low-cost methods of constructing these non-amorphous conducting films.
It is a further object of the present invention to enable the construction of electrical, optical, magnetic, and/or micro-mechanical devices on substrates, and to enable the construction of such devices with vertically-oriented electrodes.
These and other objects of the present invention will become apparent to one of ordinary skill in the art from the present specification and drawings.
Electro-optical (E/O) thin-film planar waveguides comprising transparent ferro-electric materials, such as lanthanum-modified lead zirconium titanate (PLZT), could potentially have substantially better performance than other electro-optical materials because of the material's large electro-optic coefficients. However, this material cannot be grown over conventional electrode materials, and so the alternative has been to use lateral electrodes (the type shown in
In order to produce an electro-optical device that can utilize different components of the PLZT's coefficient tensor and that can have closely spaced electrodes, the inventors have recognized that a structure with vertically-oriented electrodes should be used. When constructing such a structure, the inventors have further recognized that the bottom electrode should have a crystal lattice that matches to the substrate beneath, and further matches to the ferro-electric material disposed above. Moreover, the inventors have recognized that the bottom electrode should have a smooth surface morphology to maintain the quality of subsequent coatings and suitable chemical compatibility to enhance the adhesion of the layers. Thus, preparing the bottom electrode is a critical step in constructing E/O devices when vertically-oriented electrodes and non-amorphous materials are used.
The inventors have recognized that lanthanum nickel oxide (LaNiO3), which is a perovskite-type metallic oxide having a pseudo-cubic structure with a lattice parameter of 3.84 A, may be useful as a bottom electrode for many ferro-electric materials. LaNiO3 can be formed by RF magnetron sputtering, pulsed laser deposition (PLD), metal-organic chemical vapor deposition (MOCVD), metal-organic decomposition (MOD) and chemical solution deposition (CSD). However, the inventors have found that these methods are expensive and/or currently lack the ability to control the stoichiometry of the deposited material to the degree needed for the above-described bottom electrode. The inventors have developed a different manufacturing approach. In this approach, lanthanum and nickel atoms are incorporated with diol compounds in a chelated complex to form a dispensing solution. The dispensing solution is disposed over a substrate, preferably pre-baked to remove (evaporate) solvents, and then fired at a high temperature under oxygen flow conditions. The pre-bake is preferably performed at temperatures between about 200° C. and about 400° C., inclusive of these values, for a time sufficient to remove the solvents (which generally depends upon layer thickness and solvent content). The pre-bake time is generally in the range of one minute to several minutes.
After E/O material layer 16 is initially formed, it is pattern-etched to form a mesa stripe 19 having a width W, a length L, and a height h. In use, a light beam will be conveyed through mesa stripe 19 along the length L. E/O material layer 16 of mesa stripe 19 has a refractive index that is higher than that of substrate 1, and higher than that of the air above mesa stripe 19 (in this case, the air effectively acts as an upper cladding layer). The higher refractive index provides vertical confinement of the light beam within the space below the top of mesa stripe 19. The step difference in height h causes the refractive index of the material layer 16 underneath mesa stripe 19 to be higher than that of the material layer 16 to the side of mesa stripe 19, which provides lateral confinement of the light beam within the space below the top of mesa stripe 19. After mesa stripe 19 is formed, an aperture 20 is formed through E/O material layer 16, to the side of mesa stripe 19, so as to expose a portion of bottom electrode 17. Finally, top electrode 18 is formed over the top surface of mesa stripe 19, and a conductive layer 21 is formed through via 20 so as to contact the exposed portion of bottom electrode 17. In this configuration, a voltage may be applied between top electrode 18 and bottom electrode 17 by providing the voltage to electrode 18 and conductive line 21.
The separation distance between electrodes 17 and 18 through mesa 19 is indicated as distance SV in
Bottom electrode 17 is preferably formed by coating substrate 1 with a viscous liquid that comprises equal amounts of lanthanum and nickel atoms in a mixture of chelated complexes and solvent(s), which will be described below in greater detail.
A first lanthanum nickel oxide sol-gel precursor was prepared by using 1,3-propanediol (diol). Lanthanum acetylacetonate hydrate (Aldrich) and nickel acetate tetrahydrate (98%, Aldrich), in a 1:1 molar ratio to one another, were separately dried at 75° C. for 8 hours under vacuum.
As a next step, the dried lanthanum acetylacetonate was dissolved in 1,3-propanediol (98%, Aldrich) at a molar ratio of 1 to 35 with 3 ml of nitric acid under reflux conditions at a temperature of around the boiling point of 1,3-propanediol (214.4 ° C.) for 30 minutes. The nitric acid, acting as a catalyst, facilitates the dissolving of the lanthanum acetylacetonate in 1,3-propanediol, as explained below. Without being held to any specific theory, the inventors currently believe that at least the following processes occur during the refluxing operation to produce an exemplary intermediate chelated complex shown in
Also as a next step, dried nickel acetate was refluxed with a mixture of 1:5 volume ratio of 1,3-propanediol and 2,4-pentanedione at a temperature of around the boiling point of 2,4-pentanedione (140.4° C.) for 30 minutes. Without being held to any specific theory, the inventors currently believe that at least the following processes occur during the refluxing operation to produce the intermediate chelated complex shown in
After the above two refluxing steps have been completed, which may be done in any order or simultaneously, two non-opaque solutions were obtained. The two solutions were then mixed and refluxed at a temperature of around the boiling point of 2,4-pentanedione (140.4° C.) for 60 minutes. Without being held to any specific theory, the inventors currently believe that at least the following processes occur during this third refluxing operation. First, initially referring to
After the second reflux operation, a dark green solution resulted. The solution had a concentration of approximately 0.18 M.
The solution was then dispensed on test substrates according to the method outlined above. Two types of substrates were used to compare the resulting film properties: (1) lanthanum aluminum oxide (LaAlO3, <001> oriented, 2-side polished, 50 mm diameter, MTI Crystal), and (2) strontium titanium oxide (SrTiO3, <100> oriented, 2-side polished, 25 mm diameter, MTI Crystal). The La—Ni complex solution was filtered through a 0.45 μm-sized filter (polyvinylidene fluoride, PVDF, Whatman), and then dispensed on the test substrates. The substrates were spun at a speed of 1500 rpm for 60 seconds. The substrates were then subjected to heat treatment within a rapid thermal process furnace (RTP) with quartz chamber under oxygen flow. The heating profile has a one-minute pre-bake step at 350° C., and a 15-minute firing step at 850° C. All of the samples were coated and fired a second time under the above conditions.
The sheet resistance of the layers was measured by 4-point probe method at five different positions. The resistivity was calculated from film thickness, sheet resistance, and 4-point-probe geometrical factors based on sample shape and probe gap. Atomic force microscopy was used to observe the surface morphology, roughness Ra and thickness of the LaNiO3 films. X-ray diffractometry (XRD-Philips Norelco) for phase identification, preferred orientation and crystallite grain size (average diameter) was performed by a wide-angle powder diffractometer using Cu Kα radiation. The results of these measurements are provided in the first three columns of Table I. The crystal orientations of the LaNiO3 layers were highly oriented to the orientation of the underlaying substrate. The LaNiO3 layers have relatively large crystal grain sizes (average diameters) of 180 Å to 250 Å, and have lattice constants that are almost identical to the lattice constants of the substrates (3.905 Å for SrTiO3, and 3.84 Å for LaAlO3). The surface roughness Ra was measured to be 2.8 nm and 2.9 nm for LaNiO3 layers formed on SrTiO3 and LaAIO3 substrates, respectively, which are very good for building subsequent layers on. In addition, the resistivities were very good at 312 μΩ·cm and 237 μΩ·cm for LaNiO3 layers formed on SrTiO3 and LaAIO3 substrates, respectively (1 μΩ=1×10−6 Ω).
A second lanthanum-nickel-oxide precursor solution was prepared using 2-methoxyethanol (MOE), which only has a single OH group, instead of 1,3-propanediol (diol). MOE has the structure of CH3-O—CH2-CH2-OH. Lanthanum acetylacetonate hydrate (Aldrich) and nickel acetate tetrahydrate (98%, Aldrich), in a 1:1 molar ratio to one another, were separately dried at 75° C. for 8 hours under vacuum. The dried lanthanum acetylacetonate was refluxed with a mixture of 2,4-pentanedione (99+%, Aldrich) and 2-methoxyethanol (99.8%, anhydrous, Aldrich) at a 1:2.5 volume ratio, the refluxing being done at a temperature of around the boiling point of 2-methoxyethanol (125° C.) for 60 minutes. The dried nickel acetate was refluxed with a mixture of 2,4-pentanedione and 2-methoxyethanol at a 1:2 volume ratio for the same period of time as the lanthanum solution, and at the same approximate temperature. The lanthanum solution and the nickel solution were mixed together and then refluxed at a temperature of around the boiling point of 2-methoxyethanol (125° C.) for 2 hours. The resulting dark green solution had a concentration of 0.18M.
This solution, which we will call the “comparative solution,” was then dispensed on test substrates according to the method outlined above. Two types of substrates were used to compare the resulting film properties: (1) lanthanum aluminum oxide (LaAlO3, <001> oriented, 2-side polished, 50 mm diameter, MTI Crystal), and (2) strontium titanium oxide (SrTiO3, <100> oriented, 2-side polished, 25 mm diameter, MTI Crystal). The comparative solution was filtered through a 0.45 μm-sized filter (polyvinylidene fluoride, PVDF, Whatman), and then dispensed on the test substrates. The substrates were spun at a speed of 1500 rpm for 60 seconds. The substrates were then subjected to heat treatment within a rapid thermal process furnace (RTP) with quartz chamber under oxygen flow. The heating profile has a one-minute pre-bake step at 350° C., and a 15-minute firing step at 850° C. All of the samples were coated and fired a second time under the above conditions.
These layers of the comparative examples were analyzed in a manner similar to the analysis of the layers provided by the present invention. There was no substantive difference in the lattice constants between the Invention Examples and the Comparative Examples. In addition, the Comparative Examples had LaNiO3 layers that were also highly oriented to the orientations of the underlaying substrates. However, the comparative examples had smaller crystal sizes of 150 Å to 200 Å, and larger surface roughness Ra's of 3 nm and 4 nm for LaNiO3 layers formed on SrTiO3 and LaAIO3 substrates, respectively. More importantly, the resistivities of the comparative examples were significantly higher at 478 μΩ·cm and 301 μΩ·cm for LaNiO3 layers formed on SrTiO3 and LaAIO3 substrates, respectively, approximately 52% and 27% higher, respectively (1 μΩ=1×10−6 Ω).
The roughness of the comparative examples (MOE) was larger than the roughness of the invention examples (diol). Under Atomic Force Microscope (AFM), the surface of the LaNiO3 layers of the comparative examples, either on SrTiO3 or LaAlO3 substrates, showed round granular-particle stacks, as shown
The line profile analysis can depict this phenomenon in a qualitative way. In
The inventors believe that these difference in surface morphology can be explained by the molecule distributions of lanthanum and nickel in the solutions that are dispensed on the substrates. MOE has only one functional OH group that may react with the metal ions during precursor synthesis. The multiple coordination number in the transition metal ions provides the sites to open up the molecule structure and grow the molecule into large size. The inventors believe that the solvent molecule, 2-methoxyethanol, with its single functional group, tends to close up the open structure of the metal-organic molecule. Thus, the resulting precursor solution contains an agglomerate type of molecule clusters (i.e., round aggregated clusters) that is suspended and stabilized within the solution. On the other hand, the diol molecule used by the invention examples has two functional groups (OH) at both ends of the molecule, which provides the capability to link metal ions at both of its ends. Due to this characteristic, the resulting molecule within the precursor of the invention has long a chained structure. These long molecule chains are stabilized within the solution, and are totally different from the complex molecules formed in the comparative solutions. When these precursor solutions are coated on substrates and dried, the resulting pre-fired layers will form different morphologies, which are inherited from their suspension state. A conceptual drawing of this effect is illustrated in
While the present invention has been practiced with 1,3-propanediol, it may be appreciated that other diols may be used, such as 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol. As used herein and in the claims, the term “diol” has its ordinary and customary meaning in the chemical art, that of any alcohol molecule having two hydroxyl groups. At present, the inventors prefer to practice the invention with linear diols that have their hydroxyl groups positioned on the linear molecule as far a part from one another as possible. As used herein, linear diol has its ordinary and customary meaning in the chemical arts, and includes diols that comprise a linear backbone of carbon atoms. While the above examples have used the same diol for each of the initial dissolving processes, it may be appreciated that a first diol (or first mixture of diols) may be used for the initial dissolving of lanthanum acetylacetonate, and that a second and different diol (or second and different mixture of diols) may be used for the initial dissolving of nickel acetate.
Referring back to
While the present invention has been particularly described with respect to the illustrated embodiments, it will be appreciated that various alterations, modifications and adaptations may be made based on the present disclosure, and are intended to be within the scope of the present invention. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention is not limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.
