Patent Application
20080123251
The present invention is related to improvements in ceramic capacitive couples and devices comprising capacitive couples. More specifically, the present invention is related to a method for forming ceramic capacitive couples comprising dielectric in the interstitial spaces of a porous anode and a method for improving the capacitance achievable in a given volume by increasing the coverage of the interstitial spaces.
The growth in electronic components has continued for many decades. One of the ongoing efforts is the continued push towards miniaturization of electronic circuitry and the components contained therein. This effort is often contradictory to the companion desire to increase the capabilities of the electronic components. For many of the components it is difficult to achieve further miniaturization without sacrificing electrical performance. This is particularly the case with capacitors or components comprising capacitive couples. There has been an ongoing effort to increase capacitive density to support the overall miniaturization of electronic components.
The capacitor industry is dominated by multilayer ceramic capacitors and valve metal capacitors. Multilayer ceramic capacitors are characterized by alternating layers of electrode and ceramic wherein the ceramic is the dielectric between the electrodes. Valve metal capacitors typically include a plug of a valve metal with an oxide of the valve metal forming the dielectric. A conductive layer is then applied over the dielectric as the cathode coating. Furthering the capacitive density of either multilayer ceramic capacitors or valve metal capacitors is getting increasingly more difficult due to the extensive amount of effort already applied over many years by many researchers. While there may still be advances forthcoming, the effort required to achieve these advances is becoming more difficult and further improvements may be approaching a plateau.
Efforts have been undertaken to incorporate the dielectric, and cathode, into the interior of a porous anode. This approach has been described with polymeric cathodes in U.S. Pat. Nos. 6,987,663 and 6,361,572 for example. Barium and strontium titanate has been described as the internal dielectric in U.S. Pat. No. 5,790,368. It is difficult to effectively introduce the dielectric, or polymeric cathode material, into the interior of the porous anode. As a result the interior is not completely covered, which leads to shorts and high leakage current in capacitors of this nature. Utilization of this technique has not been considered feasible on a large scale due to the high level of losses which occur with incomplete coverage of the interior of the porous anode body.
There is an industry wide need to transition from multilayer ceramic and valve metal capacitive structures towards a capacitive structure which can achieve higher capacitance volume than either current technique is expected to reasonably achieve. The present invention provides a novel structure and method of achieving such a structure.
It is an object of the present invention to provide a capacitive couple with a higher capacitive density and capacitors formed therewith.
It is another object of the present invention to provide a capacitive couple with the interior areas of the anode covered by ceramic dielectric and a method for insuring complete coverage of the interior.
It is another object of the present invention to provide a capacitive couple comprising a porous anode from a conductor such as a valve metal with a sufficient covering of dielectric on the interior surface of the porous body to eliminate electrical shorts and leakage current.
A particular feature of the present invention is the increase in capacitance as a function of volume without loss of electrical properties.
These and other advantages, as will be realized, are provided in a process for forming a capacitive couple. The process includes forming a highly porous conductive body, such as a valve metal, with interior struts and voids having a connective wire in electrical contact with the highly porous conductive body and extending beyond the highly porous body. A dielectric layer is formed in the voids on the struts with a material having a dielectric constant above 100. An insulating layer is formed on the struts not covered by the dielectric layer. A conductive layer is formed on the dielectric layer and on the insulating layer. The connective wire is connected to a first lead and a second lead is connected to the conductive layer.
Yet another embodiment is provided in a process for forming a capacitive couple. The process includes forming a porous body with at least one material selected from a valve metal, a valve metal alloy, a conductive valve metal oxide, valve metal nitride and valve metal carbide with interior struts and voids. A dielectric layer is formed in the voids on the struts with a material having a dielectric constant above 100. An insulating layer is formed on the struts not covered by the dielectric layer. A conductive layer is formed on the dielectric layer and on the insulating layer.
Yet another embodiment is provided in a capacitive element. The capacitive element has a porous conductor, such as a valve metal, anode with struts and voids between the struts. A first dielectric is in the voids coated on the struts wherein the first dielectric has a dielectric constant of at least 100. A second dielectric is in the voids and coated on the struts at locations where the first dielectric does not coat the struts. A conductive layer is in the voids and coating the first dielectric and the second dielectric. External termination is in electrical contact with the porous anode and second external termination is in electrical contact with the conductive layer.
The invention will be described with reference to the figures forming an integral part of the instant application.
A capacitor of the present invention is illustrated in cross-sectional view in
The capacitor illustrated in
A close-up view of the anode of
The process for forming the inventive capacitor is described with reference to
The formed porous body is optionally heat treated, at 102, to achieve a substrate microstructure with adequate strength. Care must be taken during the heating operation to maintain as much surface area and open porosity as possible. In cases were the porous body is a foil formed by etching, or similar techniques, the heating may not be necessary. The heating step is primarily stage 2 sintering at a temperature of about 900 to 1,700° C. since this achieves the desired mechanical strength without compromising the microstructure or porosity.
The porous body is impregnated with a high dielectric constant material, or precursor material at 104. The dielectric material has a dielectric constant typically above 100 and is preferably a ferroelectric or relaxor material or a combination thereof. Particularly preferred dielectric materials comprises at least 60 wt % of at least one of barium titanate, barium strontium titanate, strontium titanate, barium neodymium titanate, barium zirconium titanate, lead titanate, lead zirconium titanate, lead magnesium niobate, lead zinc niobate, or precursors thereof. The dielectric may also be doped as is typical in the art. The method of impregnation is selected based on the desire to coat as much of the surface area of the internal struts of the microstructure as possible. The method of impregnation is preferably selected from dipping, wicking, vacuum impregnation, spin coating, centrifugal coating, spraying, pressure coating, pressure impregnation, freeze drying, chemical vapor deposition and electrophoretic deposition. The dielectric may be preformed and deposited or it may be formed in situ by impregnating the porous substrate with components which are then treated to react to form the dielectric. As would be realized the dielectric is impregnated as a component of a carrier matrix comprising solvents, rheology modifiers, wetting adjuvants, counter ions and the like. After impregnation the carrier solution, comprising dielectric or dielectric precursor must be converted to remove all non-ceramic components of the carrier matrix. The carrier matrix may contain a ceramic or the carrier matrix may contain precursors materials, such as salts, which form the ceramic upon heating.
The dielectric, or dielectric precursor, is then converted, at 106, to form a dense, semi-continuous insulating dielectric film on the internal struts of the porous substrate microstructure. Conversion removes all solvents, rheology modifiers, wetting adjuvants, counter ions and the like and, if necessary, sintering the ceramic or ceramic precursors to form the ceramic leaving only a dielectric ceramic coated on the interior struts of the porous anode body. The method of conversion is not particularly limiting. Particularly preferred methods of conversion include drying, firing and sintering by rapid thermal annealing, vacuum firing, microwave heat treatment or induction heating. The conversion can be done under ambient atmosphere, under reduced pressure, in a reducing or partially reducing atmosphere or in neutral atmosphere.
After conversion it is preferred that the dielectric layer have a thickness of at least 0.5 nm to no more than 2000 nm. The deposition and conversion may need to be repeated as necessary to achieve an adequate thickness and dielectric coverage. The dielectric layer may be a particulate coating, a dense film or coating, an interconnected lacy film or coating or have localized coverage such as with islands.
The dielectric layer is then completed by formation of a continuous dielectric insulating film at 108 with a dielectric having a dielectric constant of less than about 100. The continuous insulating film removes any shorts or high leakage areas remaining after the conversion. The completion can be accomplished by anodization, thermal oxidation, or a secondary impregnation. The insulating material is preferably introduced into the voids by dipping, wicking, vacuum impregnation, spin coating, centrifugal coating, spraying, pressure coating, pressure impregnation, freeze drying, chemical vapor deposition, electrophoretic deposition or similar processes. The thickness of the insulating layer is preferably at least about 0.5 nm to no more than about 5000 nm. More preferably the thickness of the insulating layer is no more than about 3000 nm. The completion step is a necessity owing to the incomplete coverage of the interior struts of the porous substrate during the impregnation of high dielectric constant materials. In standard manufacturing processes it is preferred to add a continuous insulating film to increase the manufacturing yields resulting from some, albeit small, percentage of incomplete dielectric layers. It is preferred to minimize the coverage of the insulating film as the relative dielectric constant is low compared to the resultant dielectric insulating film previously deposited.
A conductive layer is deposited over the dielectric or insulating layer at 110. The conductive layer preferably comprises at least one of manganese dioxide, conductive polymer, organometallic, organometallic with metallic fillers, metallic glass, a metal or a metal powder in a suspension or a precursor thereof. The conductive layer may be deposited as a conductor or a conductive precursor can be deposited and converted. Silver nitrate, for example, may be coated after which the silver is reduced to elemental silver. The conductive layer is preferably 1 nm to 106 nm thick. The thickness is chosen to insure adequate coverage and electrical conductivity, increased thickness beyond that goal is of no benefit. The conductive film may require a post treatment to insure a conductive continuous film. The post treatment may include, but is not limited to, thermal treatment in ambient atmosphere, at a reduced pressure, or in an environment which is reducing, partially reducing or neutral. In a particularly preferred embodiment the conductive layer is self healing such as is the case with manganese dioxide to further improve insulation resistance to the device. A post cathode anodization, and/or reformation, may be used to further reduce leakage current.
The conductive polymer preferably comprises a heterocyclic five or six-membered ring compound. Non-limiting examples include pyrrole, thiophene, 3-alkylthiophene, isocyanaphthene, polyparaphenylene, polyaniline and polyparaphenylenevinylene. Doped polymers may be utilized but inherently conductive polymers are preferred. Particularly preferred conductive polymers include polyethylenedioxythiophene and derivatives thereof.
After formation of the conductive layer electrodes are formed, at 112, wherein one is in electrical contact with the conductive layer, typically referred to as the cathode, and the other is in electrical contact with the substrate material. The electrodes may be formed by traditional lead frame techniques or may be formed by deposition of conductive carbon thick film followed by silver paint and attachment of an external lead via conductive epoxy or the like. The electrodes form an electrical path from the interior to the exterior thereby forming a capacitor structure.
The capacitor is typically tested, burned-in and packaged as known in the art.
An embodiment of the present invention is illustrated in
Another embodiment of the present invention is illustrated in
Current conventional valve metal capacitive couples, for example with tantalum or niobium, may utilize about 150,000 μFV/gram materials, formed to a voltage of about 3 times the rated voltage (RV) with an associated dielectric constant of less than about 50 resulting in a dielectric thickness of about 2 to 2.5 nm per volt. Utilizing a dielectric constant of the dielectrics above about 1000 or greater and a 30 nm dielectric thickness with 100% high K dielectric coverage would be expected to achieve a capacitance increase of from 100 μF to about 2000 μF. or more. The achievable capacitance would decrease with increasing dielectric thickness. Doubling the dielectric thickness would reduce the achievable capacitance by a factor of about 2. With the present invention a continuous high K dielectric with a thickness of about 267 nm would be expected to provide a capacitance of about 227 or more than double the currently available capacitance for tantalum capacitors assuming full coverage of high K dielectric. With less than full coverage, the composite K of the high K and low K dielectric film would be expected to follow the perpendicular mixing rule. With 50% coverage at 30 nm thickness of the low K dielectric and a 267 nm dielectric thickness of high K dielectric the capacitance is expected to be about 167 which is still about 60% higher than current tantalum capacitors. The present invention can provide a significant increase in capacitance relative to current capacitors at approximately the same size.
The invention has been described with particular emphasis on the preferred embodiments. One of skill in the art would readily realize additional embodiments and alterations based on the description herein without departure from the scope of the invention which is more specifically set forth in the claims appended hereto.
