WO2008149097A2

WO2008149097A2 - Method for forming conductive material layers in the manufacture of solid state capacitors

Info

Publication number: WO2008149097A2
Application number: PCT/GB2008/001929
Authority: WO
Inventors: Brian W. Glasper
Original assignee: Avx Limited
Priority date: 2007-06-06
Filing date: 2008-06-06
Publication date: 2008-12-11
Also published as: GB2449885A; GB0710850D0; WO2008149097A3

Abstract

The present invention concerns the field of solid state capacitors and the laying down of layers of material in the manufacture of such components. In particular aspects, the invention to a novel method for forming cathode material and protective barrier layers in solid state capacitors. According to one aspect of the invention there is provided a method of applying layers of material in the manufacture of electronic components, each layer being applied as a liquid precursor and then caused or allowed to solidify to form a solid layer, the method including the steps of: providing dispensing means for delivering precursor liquid, the dispensing means comprising a delivery head adapted to deliver precursor liquid from the dispensing means, providing mechanical transport means for moving the delivery head relative to a liquid delivery site, and for aligning the delivery head with the liquid delivery site, delivering one or more doses of liquid to the targeted delivery site in amounts suitable to form a layer of precursor material in a controlled amount at the desired location. Thus, in contrast to coating by dipping in a bulk liquid, components, or elements of components, may be coated in a manner which is targeted, selective and needs no more coating liquid than in necessary. By using the methods of the invention capacitors of improved performance may be obtained.

Description

Method for forming conductive material layers in the manufacture of capacitors

The present invention concerns the field of electronic components and the laying down of layers of material in the manufacture of solid state capacitors. In a particular aspect, the invention concerns a novel method for forming cathode material layers in solid state capacitors. The invention finds particular application in methods of manufacturing multiple capacitors simultaneously on a substrate wafer, such as those disclosed in the following EP patent publications of AVX Corporation (inventor Salisbury): EP-A-O, 688, 030, EP-A- 0,893,808, EP-A-I, 047, 087 and PCT applications of AVX Limited (inventor Huntington) : PCT/GB02/02466, PCT/GBOO/02466, PCT/GBOO/02630, PCT/GBOO/02629, PCT/GBOO/04355 and

PCT/GBOO/03558, amongst others. However, as will be evident to the skilled addressee, the invention is not limited to these particular examples and finds broad application, including for example the manufacture of individual capacitors piecemeal or in series.

Valve action materials are used to form the anodes of solid state capacitors, typically by powder metallurgical routes, such as sintering. These methods form highly porous bodies having a high internal surface area which is suitable for use as the anodes of capacitors. Examples of suitable valve action materials are metals including tantalum and niobium, and the conductive sub-oxides of niobium, in particular niobium monoxide .

It is important for solid state capacitors to achieve a low equivalent series resistance (ESR) and low DC leakage (DCL) performance. Traditional tantalum capacitors have an inorganic conductive cathode layer, typically formed by applying a liquid manganese nitrate layer to a porous tantalum capacitor anode pre-formed with a dielectric layer. The manganese nitrate is heated to form a conductive layer of manganese dioxide. Such a tantalum anode capacitor, manufactured for example according to the method of Salisbury, will typically provides a DCL of 0.2 to 0.3μA and an ESR range of 2.5 to 4.5 Ohms, for the common type LlOMlO (i.e. one which has a 10V rating for a lOμF part) .

Capacitors which use a conductive polymer layer instead of manganese dioxide are known. However, these typically provide a DCL of ten times that expected for an equivalent MnO₂ product. Furthermore, it has not been possible to date to form capacitors by a micro-scale method of the type disclosed by Salisbury (the micro-chip process) in which multiple capacitors are formed on a substrate wafer. This is due to problems with forming suitable polymer layers on individual capacitor anodes.

In order to coat anode bodies, touch dipping may be employed, and this is the current preferred method of application. However, with this method there is a degree of wastage of coating material. Additionally, touch dipping results in some degree of harm to the dielectric film on top of the anodes during application, due to contact damage. This can affect capacitor performance.

It is a general object of the present invention to provide an improved method of applying liquid conductive layer precursor to form conductive layers during the manufacture of capacitors .

It is an object of one aspect of the present invention to provide an improved method of forming cathode layers on solid state capacitors .

In particular it is an object of the present invention to provide a method of forming conductive polymer layers on capacitor anodes which avoids waste the conductive polymer, thereby providing economic and environmental benefits. It is an object of another aspect of the invention to provide such a method which is suitable for use in the simultaneous manufacture of multiple micro-scale capacitors on a substrate, such as by the Salisbury method and related methods.

It is an object of yet another aspect of the present invention to provide a solid state capacitor formed with a conductive polymer cathode layer which has improved DCL and ESR performance.

It is an object of a further aspect of the invention to provide a method by which multiple micro-scale solid state capacitors may be formed with a conductive polymer cathode layer.

In addition, it is yet another object of the present invention to improve the ESR and/or DCL performance of solid state capacitors having a conductive polymer cathode layer.

The foregoing and other objects may be achieved by the aspects and embodiments of the inventions set out below or in the claims hereinafter.

According to one aspect of the invention there is provided a method of applying layers of material in the manufacture of solid state capacitors, each layer being applied as a liquid precursor and then caused or allowed to solidify to form a solid layer, the method including the steps of: providing dispensing means for delivering precursor liquid, the dispensing means comprising a delivery head adapted to deliver precursor liquid from the dispensing means, providing mechanical transport means for moving the delivery head relative to a liquid delivery site, and for aligning the delivery head with the liquid delivery site, delivering one or more doses of liquid to the targeted delivery site in amounts suitable to form a layer of precursor material in a controlled amount at the desired location.

Thus, in contrast to coating by dipping in a bulk liquid, components, or elements of components, may be coated in a manner which is targeted, selective and needs no more coating liquid than in necessary.

The mechanical transport means may comprise a mechanism by which a delivery head may be moved along X or Y axes over a substrate. Alternatively, the substrate may be moved relative to the delivery head.

In one embodiment of the invention, the liquid dispenser means comprises a piezoelectric droplet delivery apparatus, which ejects droplets by means of piezoelectric pressure.

In another embodiment, the liquid dispenser means comprises a thermal bubble jet apparatus in which volumes of liquid are ejected by heat-induced vaporisation of liquid.

In a further embodiment, the liquid dispenser means comprises a needle dispenser apparatus in which at least one elongate hollow needle member is provided in liquid communication with a precursor material precursor reservoir.

The liquid may be dispensed as droplets formed and ejected by a pneumatic pressure.

The applied material may be the precursor of a conductive material which sets or reacts to form a solid conductive layer. The material may be a polymer or polymer precursor.

The method has particular application in the manufacture of solid state capacitors, and the provision of cathode layers on porous anode bodies. The cathode layers may be organic (such as a conductive polymer) or inorganic (e.g. manganese nitrate, which is heated to form manganese dioxide) . Thus the site upon which the material is applied may be the anode portion of a capacitor.

In a particular aspect of the invention there is a plurality of components, distributed so that each is spaced apart from the others. The doses may delivered to each component one after the other, or simultaneously in groups.

Thus, in one aspect, he doses are delivered to a first series of components simultaneously, and thereafter simultaneously to a second series of components.

To facilitate massed production, the components may be provided as an array supported on a substrate. The array of components may be an array of anode portions of solid state electronic capacitors distributed as islands on the substrate. The liquid applied may be the precursor of a conductive material, and each dose of liquid may be applied to a surface portion of each anode. In a preferred embodiment, the conductive material is a conductive polymer.

In another aspect of the invention, wherein the liquid applied may be a protective barrier material which is applied to a substrate surface portion in between the anode islands. The barrier material may be a polymer, such as polyamideimide, polyurethane or polyethylene terephthalate. In the case of a capacitor, the barrier material is applied to cover underlying cathode material, and protect it against mechanical damage or chemical attack. Generally, the protective barrier polymer should not be an unsaturated or free radical containing type, so as to be compatible with the underlying cathode material conductive polymer layer.

The precursor liquid may be a liquid which reacts, sets or cures to form a solid layer. The precursor material may be dissolved in a solvent and then dry after application. In some applications the liquid will comprises a mixture or solution of two or more liquid reactants.

The reactants may be mixed immediately before the liquid is dispensed. Alternatively two or more different precursor liquid reactants are dispensed and one liquid reacts with the other to form a solid layer in situ. A typical reaction is a polymerisation or co-polymerisation.

The liquid may be applied in multiple doses sequentially on each component to build up the layer thickness, coat by coat.

In a preferred aspect of the invention, after the application of one dose, the following dose is applied before the precursor material of the previous coat has completely set, reacted or cured. This permits inter-dose (coat) curing, setting or reacting. This enhances the integrity of the final layer and prevents laminar heterogeneity between coats. It also speeds the build up of a coat because time is not spent waiting for each layer to completely, cure, set or react.

In another, preferred aspect of the invention, there is provided a method for forming multiple solid state capacitors state capacitors comprising: forming an array of porous anodes of valve action material, forming a dielectric layer on surfaces of the porous anode, forming, on each anode, a cathode layer in electrical contact with the dielectric layer, wherein the cathode layer is applied as liquid precursor material by a method of applying layers of material as hereinbefore described. In another aspect of the invention there is provided a method for forming multiple solid state capacitors state capacitors comprising: forming an array of porous anodes of valve action material spaced apart on a substrate, forming a dielectric layer on surfaces of the porous anode, forming, on each anode, a cathode layer in electrical contact with the dielectric layer, wherein, after application of the cathode layer, a barrier layer is applied to substrate surfaces between anodes, which barrier layer is chemically inert with respect to the cathode layer.

In a preferred embodiment the barrier layer is formed from a liquid precursor material that is applied by a method of applying layers of material as hereinbefore described.

In yet another aspect of the invention, there is provided a method of forming a solid state capacitor comprising: forming a porous anode of valve action material, forming a dielectric layer on the surface provided by the porous anode, forming a cathode layer in electrical contact with the dielectric layer, characterised in that the cathode layer is applied to the anode as a liquid precursor which reacts or sets in situ to form a solid layer, and in that the cathode layer is built up on each anode by the application of a sequentially applied series of cumulative layers.

The next layer in the sequence is preferably applied before the underlying layer is cured or reacted so that cross-layer reaction or curing occurs so as to improve the cohesion and homogeneity of the cathode layer. Each of the cumulative layers may be applied by a method of applying layers of material as hereinbefore described.

In a particular aspect of the invention, there is provided a method of forming a solid state capacitor comprising: forming a porous anode of valve action material in electrical, forming a dielectric layer on the surface provided by the porous anode, forming a cathode layer in electrical contact with the dielectric layer, which layer is formed of an electrically conductive polymer material, providing an anode terminal in electrically conductive contact with the anode body and a cathode terminal in electrically conductive contact with the cathode layer, characterised in that, after formation of the cathode layer, a barrier layer is on an exposed portion of the cathode layer between the anode terminal and the cathode terminal, thereby to protect the cathode layer against environmental or mechanical or chemical damage during processing.

The barrier layer may comprise a polymer that is chemically inert and stable with respect to the cathode layer polymer material. As previously mentioned the barrier layer may for example comprise a polyamideimide or a polyurethane or a polyethylene terephthalate. These are however are preferred but not limiting on the selection available.

The capacitor is typically encapsulated in a protective layer leaving the terminals exposed and covering the barrier layer and cathode layer.

In a further aspect of the invention, there is provided a solid state capacitor comprising a porous anode body formed with a dielectric surface layer, and a cathode layer on the dielectric surface layer, an anode terminal in electrical contact with the anode body and a cathode terminal in electrical contact with the cathode layer, characterised in that a barrier layer is provided on a portion of the cathode layer in between terminals, and preferably in a region between the anode terminal and the anode body sidewalls which has been coated with cathode layer material.

Surprisingly, it has been found that the presence of the barrier layer enhances the performance of capacitors, particularly in cases where the cathode layer is formed from conductive polymer.

The barrier layer may comprise a polymer that is chemically- inert and stable with respect to the cathode layer polymer material .

According to yet another aspect of the invention, there is provided a method of forming a conductive layer of material in a solid state electronic component, which layer is applied as a liquid precursor which reacts or sets to form a solid conductive layer, characterised in that the precursor layer is applied to the component by liquid dispenser means which is adapted to separate a predetermined volume of the precursor liquid from a liquid reservoir, and wherein the dispenser means is provided with transport means adapted to transfer the separated volume of precursor in a directed manner from the dispenser means to a surface region of the electronic component, thereby to form a precursor liquid layer on the component and wherein the liquid layer is thereafter caused or allowed to solidify.

This method typically finds application as a micro-dispensing method for laying down very small volumes consistent with microelectronic components, such as capacitors, diodes and transistors. The controlled volumes and targeted delivery prevent waste and. improve process efficiency, which is vital for the application of expensive conductive polymer precursor materials, for example. In a preferred arrangement the needle dispenser apparatus comprises a plurality of said needle members and the apparatus is adapted to dispense liquid precursor via the needle members sequentially or concurrently.

The transport means may comprise a spatial location mechanism which allows controlled movement of the liquid dispensing means from one location to another. For example the location mechanism may comprise a robotic arm, or a two dimensional location device such as are used in Cartesian plotters or printers .

In one embodiment the conductive material precursor comprises an inorganic liquid. For example, the precursor liquid may comprise manganese nitrate. The applied liquid would be heated to transform the nitrate to a manganese oxide layer, for example for use as the cathode layer in a capacitor.

In another embodiment the conductive layer is a conductive polymer layer and the precursor liquid comprises a liquid polymer or pre-polymer which dries, sets or cures to form a solid conductive layer.

The conductive polymer layer may be applied and formed in many ways using a liquid precursor. For example the polymer may be applied in solution and dried. The polymer may be reacted after application, for example by UV curing, or activation by a setting agent. Two or more precursor agents may be mixed before the liquids are applied so that polymerisation occurs during application. Reactive liquids may be applied sequentially so that monomers or co-monomers react on contact.

Thus the precursor liquid may comprise a mixture or solution of two or more liquid reactants . The reactants may be mixed immediately before the liquid is dispensed. Two or more precursor liquid reactants may be dispensed. Each liquid may be applied sequentially and one liquid reacts with the other to form a solid conductive layer. At least one of the liquid precursors may be a monomer and the reaction is a polymerisation reaction.

The conductive layer may be a conductive polymer formed by the reaction of two or more reactants placed in contact with each other on the anode. At least one of the precursor liquid reactants may be applied by the dispenser means before contacting with the other reactant.

Each of the two or more liquids reactants may be applied by the dispenser means so that the precursor liquids react in situ on the anode body after transfer of the precursor liquids .

In one arrangement there is a first liquid dispenser or first series of dispensers for the one reactant liquid, and a second liquid dispenser or second series of dispensers for the second reactant liquid. This allows for application of precursor to multiple components simultaneously.

The conductive layer may be built up on each anode by the application of a sequentially applied series of cumulative layers .

A next layer in the sequence may in one methodology be applied before the underlying layer is completely cured or reacted so that cross-reaction occurs between layers so as to improve the cohesion and homogeneity of the conductive layer.

The method may be a method for manufacturing multiple solid state capacitors in which an array of anodes is formed concurrently on a substrate, such as the method of Salisbury and other related methods developed by Huntington of AVX Ltd, UK. The dispenser means may be adapted to provide and transfer predetermined volumes of precursor liquid sequentially to each anode in the array.

Alternatively, or in addition, the dispenser means is adapted to provide and transfer predetermined volumes of precursor liquid concurrently to each of at least two anode bodies in the array.

In another aspect of the invention there is provided a method of forming solid state capacitors comprising: forming a porous anode of valve action material, forming a dielectric layer on the surface provided by the porous anode, forming a cathode layer in electrical contact with the dielectric layer, characterised in that the cathode layer is applied to the anode as a liquid precursor which reacts or sets in situ to form a solid layer, and in that the cathode layer is built up on each anode by the application of a sequentially applied series of cumulative layers.

The layers may each be applied by a dispensing method as hereinbefore described. However, the precursor liquid need not necessarily be applied by the said dispensing methods. For example, each of the cumulative layers may be applied by dipping of the pre-formed anode bodies into a bath of liquid polymer precursor.

In a preferred aspect the method is for manufacturing multiple solid state capacitors in which an array of anodes is formed concurrently on a substrate, such as the Salisbury and related methods for forming micro-scale capacitors.

The dispenser means may be adapted to provide and transfer droplets sequentially to each anode in the array. The dispenser means may be adapted to provide and transfer droplets concurrently to each of at least two anode bodies in the array.

In a preferred arrangement, the dispenser means comprises a needle dispenser apparatus in which at least one elongate hollow needle member is provided in liquid communication with a cathode material liquid precursor reservoir.

Needle dispensing apparatus such as that which relies upon a pneumatic piston delivery mechanism is known, but has not been applied in the field of solid state capacitor manufacture.

As mentioned in the foregoing, other liquid dispensing means may be used in the present invention. For example, mechanical auger feeds, piezoelectric actuators, bubble jet delivery and other means for dispensing controlled, quantifiable amounts of liquid to a selected location.

The needle dispenser apparatus may comprise a plurality of said needle members and the apparatus may be adapted to dispense liquid precursor via the needle members sequentially or concurrently.

In a further aspect of the invention, there is a first droplet dispenser or first series of dispensers for the one reactant liquid, and a second droplet dispenser or second series of dispensers for the second reactant liquid.

In certain embodiments the cathode layer is built up on each anode by the application of a sequentially applied series of cumulative layers. Preferably a following layer in the sequence is applied before the underlying preceding layer has cured or reacted so that cross-reaction occurs between layers so as to improve the cohesion and homogeneity of the cathode layer. This provides considerable time saving when building up a conductive cathode layer, as each layer does not need to be reacted, dried/cured, washed and reformed before the next is applied. The processing time saving results in significantly reduced unit costs.

The method of the invention allows the auto-dispensing of the conductive polymer on components such as capacitor anodes, transistors and diodes. This has the benefit permitting a precise meter and mix process for delivering cathode layer precursor polymer. In this the two reactive components of the conductive polymer are mixed at the dispensing head using only the volumes of material needed (theoretically 100% usage of material) , thereby avoiding avoids waste of cathode material by uncontrolled early polymerisation. Another main advantage of auto-dispensing is that it is a non-contact method, so dielectric damage is prevented. This improves reliability and yield of capacitors. Suitable meter and mix processes include miniature/low volume static mixers, nano-litre quantitative aspirate dispensers, chaotic electro-kinetic micro-flow mixers, micro TAS, micro-serpentine mixers. These methods are known and may readily be implemented by a person skilled in that field. Small volume mixers are commonly used in the field of microbiology and immunology research.

Conductive layer materials that may be applied according to the method of the invention include polyanaline, polypyrrole, PEDOT and conductive polymer dispersions like Bayer' s "Baytron Nano" and Ormecon's "Pani" dispersions.

Micro dispensing can also be used to facilitate the application of conductive polymer by sequential mixing of precursor solutions in situ (e.g. apply liquid monomer first followed by a second application containing the liquid oxidiser/polymer initiator solution) .

The application of the cathode by needle dispensing methods provides a number of benefits over the traditional application methods. Specific benefits are dependant on the type of cathode material applied to the anode. For example manganese nitrate, conductive polymer (such as polyethylene- dioxythiophene) applied by a sequential method and conductive polymer (such as polyethylene-dioxythiophene) applied by a mixed method.

Touch-dip application methods have associated with it some dielectric damage caused by physical contact of the anodes onto hard surfaces/plates. Needle dispensing of materials is non-contact therefore avoids any dielectric damage.

The following examples of illustrate the benefits of the invention to the skilled person.

(I) . Sequentially applied conductive polymer as the cathode.

The conductive polymer system is made up of two or more components. One component includes a monomeric material, and another component will contain an initiator/oxidiser for the polymerisation of the monomer. The components are applied to the anode separately in a sequential way so that the polymerisation reaction only occurs after application and in situ on the anode.

Once mixed together, polymerisation is quick. The main advantage of a sequential method over the application of a pre-mixed polymer liquid is the reduction of cross- contamination of the two reactive components. However, with touch dip and full immersion methods, there is still a degree of cross-contamination as the anodes are dipped in one then the other component. Additionally, the sequential method does not provide the optimum properties from the conductive polymer as no proper mixing of the two components is made and the mix ratio cannot be controlled.

The benefits afforded from dispensing conductive polymer in a sequential way include: a) no cross-contamination of reactive components. b) exact control of mix ratio of all the components by accurate volume control. c) consistency and reproducibility, giving improved quality standards .

(II) Application of mixed conductive polymer as the cathode.

The advantages of mixing the components of the conductive polymer before application to the anode over the sequential method include exact control of formulation (e.g. Mix ratio, solvent dilution and ratio of other additives, such as wetting agents) to achieve optimum and consistent polymer performance. However, once mixed, the conductive polymer is very reactive and has a short working life. This problem has been addressed in the art by extending the working life of a conductive polymer by the incorporation of blocking or retarding agents (such as cyclic ethers) .

The benefits afforded from needle dispensing conductive polymer mixed prior to application include: a) no requirement for a retarding/blocking agents because the materials can be metered and mixed at the dispensing head immediately before application. b) no scrap or waste of spent material as only the material required to fill the anode is mixed and applied. c) exact control of mix ratio of all the components by accurate volume control. d) lowest possible raw material and operational costs as very small quantities of material are being handled, again with minimal or no material waste, i.e. no baths or reservoirs of material to be kept warm or chilled) . e) Consistency and reproducibility giving improved quality standards.

(III) Application of Manganese nitrate as the cathode. The benefits of the needle dispensing invention are not restricted to the application of conductive polymers. The method may be used for the dispensing of manganese nitrate, giving: a) reduced operating cost as volumes handled are greatly reduced. No requirements to heat large baths of material, b) reduced health and safety risks as volumes handled are small and contained within the dispensing equipment (i.e. no large open baths of hot material) . c) exact control of volumes applied to each anode providing improved reproducibility. d) automated control of manganese nitrate' s specific gravity by meter/mixing with water removes need to make up large volumes of solutions at different specific gravities.

In general, the needle or droplet dispensing application of cathode precursor material gives the following potential benefits :

a) exact control of volumes applied to each anode. b) exact control of formulation being dispensed. c) clean process with reduced chemical handling. d) reduced emissions to the environment (small volumes and closed system) . e) reduced health and safety risks (small volumes and closed system) . f) lowest possible operational costs (material heating and/or cooling, minimum volumes of scrap material etc) . g) on some anode designs (e.g. Salisbury microchip type) dispensing of small volumes of cathode material, as a drop, that then seeps through the anode from top-to- bottom by gravity displacing air on route may provide better anode pore fill/dielectric coating than other application methods that may restrict air evacuation, such as dipping. Following is a description with reference to the figures of the drawings of modes for putting the present invention into effect. In the drawings :-

Figure 1 is plan view of a tantalum anode array on a substrate used in the manufacture of multiple solid state capacitors.

Figure 2 is a sectional side view through the substrate array of figure 1, showing a dispenser for liquid used in the method of one aspect of the present invention.

Figure 3 is a photographic view of a cross sectional side view through a capacitor which has been processed according to the invention.

Figure 4 is a schematic version of figure 3, provided to clarify the internal structure of the capacitor.

In the following examples capacitors are manufactured generally in accordance with the method of Salisbury disclosed in EP-O, 688,030. With reference to figure 1, this involves the formation of a plurality of porous tantalum anode bodies 16 on a substrate 10 by powder metallurgical methods. The anode bodies are disposed in an orthogonal two dimensional array of bodies, separated by channels formed as machined rows 14 and columns 15. The exposed surfaces of the anodes are subjected to an anodisation process by which a dielectric oxide layer of tantalum pentoxide is built up. Subsequently a cathode layer is applied as a liquid precursor to upper surfaces of each anode body to form cathode layers 27. The precursor liquid is dispensed in individual drops 12 of predetermined volume via a needle dispenser 20. The needle dispenser comprises a vertically aligned needle element 21. The needle element is carried in a collar 22. The collar is provided with a transport mechanism (not shown) which permits location of the needle along X and Y axes of the substrate. A vertical trim mechanism (not shown) is also provided to permit fine spacing between a lower end of the needle and the upper end of each anode body. A flexible tubular conduit 23 leads from an upper end of the needle to a precursor liquid reservoir 24. The reservoir is generally cylindrical and provided at one side thereof with a plunger 25. Movement of the plunger permits displacement of the precursor liquid (shown as hatching in the drawing) and hydraulic delivery of the liquid as droplets 12 at the needle element tip. These fall from the needle under gravity to the underlying anode top surfaces to wet them. In situ the coatings cure or dry to form a solid layer. Each layer may be built up by the application of several droplets so as to gradually build up the coat.

Each anode is then provided with a carbon and silver coating at an upper end thereof (not shown) . A conductive adhesive is applied and a lid layer (not shown) is disposed to cover sandwich the anodes between the substrate. This forms a mould in which an encapsulation material (such as epoxy resin) is then infiltrated to fill the channels 14, 15 between anodes. The sandwich is then sawed into individual capacitors in which a lid portion forms the cathode terminal and the substrate portion forms the anode terminal.

The delivery of liquid is not limited to the cathode layer precursor liquid, or to the anode upper face locations. In other aspects of the invention a barrier layer may be applied along the channels 14 and 15, and around the bottom sides of each anode body. This protects cathode material against chemical contamination or mechanical damage, as will de described in further detail below.

The present examples concern the manner in which layers of material may be laid down on the anode bodies and substrate.

Example 1 Needle Dispensing:

This example concerns the application of a conductive polymer cathode layer onto capacitor anodes by needle dispensing. The example was conducted using a needle dispensing apparatus provided by EFD Inc. of East Providence, RI 02914, USA

Results are presented from two trials, each applying mixed conductive polymer of the type PEDOT (Trade name Baytron) . The polymer was applied to anodes of the type used to make capacitors of the specification TACLlOMlO {lOuF 10V} . These are porous tantalum anodes formed by sintering of anode bodies on a tantalum substrate wafer, and which form capacitors having lOuF capacitance and a rated voltage of 10V. The anodes were formed and laid out on the wafer substrate as an orthogonal array of rectilinear bodies, each having a flat upper surface, see figures 1 and 2 for example.

Two trials were carried out in which polymer was applied to an upper surface of each capacitor. The multiple anodes are prepared on a solid tantalum wafer substrate, followed by formation of an insulating dielectric by electrolytic deposition. A pre-coat is applied to the anode bodies (for example of shellac or the like) . This aids bonding of the subsequent conductive polymer to the anode.

The internal conductive polymer layers are then applied as liquid droplets by the needle dispensing apparatus according to the parameters below:

Trial* : 1 2

Dispensing equipment: ϋltra-325TT Ultra-325TT

(ex EFD Inc. )

Dispense method: Pneumatic Pneumatic

Needle gauge: 25 25

Syringe barrel volume: 3 ml 3 ml

Number of dispensing cycles per anode: 10 6

Needle to anode gap: 120 urn 120 urn

Application rate: 0. lsec/anode 0. lsec/anode The process is very sensitive to environmental conditions. In particular it has been established that temperature and humidity controls are very important to obtain an effective coating of polymer cathode layer. Ideal conditions are 18-24°C with relative humidity (RH) within range of 40-65%. The optimum conditions are 20-22⁰C, RH 50-60%) .

After the final internal application cycle, the product is allowed to further cure under the application conditions for 30 minutes. This is followed by a 10 minute dry stage at a somewhat raised temperature of 40⁰C.

The coated anodes are then reformed to ensure that the integrity of the dielectric layer is repaired if damaged by the previous processing. This would typically be an electrolytic oxide layer formation process, well know in the art.

A conductive cathode layer coating is then applied by a conventional touch-dip method. This is allowed to cure/react on the anodes for 30 minutes and then dried at 40⁰C for 10 minutes. The conductive layer material in these trials was a single coat of mixed conductive polymer of the PEDOT type. However the coating could equally have been other conductive polymer dispersions like Baytron K type (trade name) .

The anodes are then dipped into carbon and then silver pastes by known and established techniques, i.e. touch dipping to controlled depths. At this stage, the capacitors' electrical properties were tested.

After carbon/silver coating a protective barrier coating is applied by needle dispensing as previously described. The barrier coat is based on PAI (polyamide-imide) and has been developed to ensure that the incompatible chemistries are kept isolated. The barrier coat material is applied by needle dispensing between the anodes prepared on the tantalum substrate. The barrier coat is then cured so that the capacitors can be assembled in the normal way using existing assembly materials.

The performance data is presented below as average values for each of the trials.

Results: Trial #1 Trial #2

Average DCL: 0.201 uA 0.058 uA Average Cap: 9.1 uF 9.3 uF

Average DF: 6.9% 7.7%

Average ESR: 1.8 Ohms 1.8 Ohms

These results are for an unfinished capacitor which has not been provided with terminals and encapsulated. For finished capacitors the capacitance drops to about 7uF and the ESR increases to circa 10 Ohms or more.

In practice, the multiple capacitor elements on the tantalum substrate are assembled to produce the final product. Assembly involves the application of a silver coated iron/nickel top plate (or lid) glued to the capacitor by a conductive adhesive, followed by liquid encapsulation or transfer moulding. The capacitors are then diced into individual units, any shards remaining from the dicing process are eroded off. Each end of the individual units is silver terminated and nickel/tin plated. The structure is shown in figures 3 and 4. It has been found that some of the moulding materials used as standard contain curing agents that, when placed in contact with the conductive polymer, chemically interact producing a significant fall off in performance compared to pre-assembly . This is evident from the trial data below. The protective barrier layer helps retain pre-assembly performance.

Trial#: 1 2

Results pre-assembly (but post carbon & slivering) : Average Capacitance: 8.7μF 9.5μF Average DF: 14 . 8 % 4 . 1%

Average ESR: 1670 mOhms 545 mOhms

BARRIER COAT APPLIED: NO YES

Results post-assembly: Average Capacitance: 5.6μF (-36%) 9.8μF(+3%) Average DF: 44.5% 2.1% Average ESR: 9043mOhms 846mOhms

For trial#2, a slight increase in Cap and ESR is likely due to capacitor to capacitor variability for the units selected and tested. However, change in cap also occurs depending on how "wet" the capacitor is from natural environmental humidity that gets absorbed into the units with time.

Further Example: Sequential deposition to form the conductive polymer cathode layer.

Product TACLlOMOIHTA.

Multiple anodes are prepared on a tantalum substrate according to the method of Salisbury. The anodes have a dielectric layer grown thereon by electrochemical deposition in a conventional manner. A pre-coat layer of "shellac" or the like is then applied to improve subsequent bonding of conductive polymer.

Liquid conductive polymer, in particular pre-formulated and mixed PEDOT, is applied by a needle dispensing method.

The conductive polymer precursor formulation contains a fluorinated flow agent and is also thinned with IMS to achieve optimum anode impregnation and wetting.

Unlike all other known conductive polymer processes, the precursor polymer is applied over typically eight or more repeat cycles of application, dry, cure and wash. After each application cycle, the last applied layer is not allowed to dry fully, but the next layer is applied while the underlying layer is still drying/curing. The final layer is allowed to dry fully.

This methodology has at least two benefits. The first is that this provides a very quick processing route (less than lhour to apply the internal conductive polymer coats) . The second is that there is a reduced risk of building up discrete laminate layers within the conductive polymer film, because the layers tend to cure together. In contrast, the formation of discrete laminate layers may result in the formation of inter-layer resistances, which affects capacitor performance.

After the cathode layer application, the anodes are "reformed" to ensure a coherent dielectric layer. At this stage a conductive polymer, such as mixed Baytron M&C or Baytron K types, may be applied as an outer coat. The anodes are then carbon and silvered over an upper end of each anode.

A polyamide-imide barrier coat is then applied by dispensing the material down the streets defined between the array of anodes. The barrier layer is applied as a liquid and cured typically for 5 minutes at 200⁰C. The application method is preferably needle dispensing along X and Y axes of the substrate. This ensures accurate location of the barrier layer, and precise control of the amount of barrier coat applied. Sufficient barrier layer is applied to form a depth in the streets which covers exposed the cathode layer material which has not already been covered by the carbon/silver layers applied to the upper end of each anode.

The anodes can then be assembled as normal i.e. a lid plate is applied to the upper ends of the anodes, forming a sandwich with the underlying substrate. The remaining gaps between the anode body array are filled by an encapsulation material. This is applied by infilling into the substrate/ lid plate sandwich. The encapsulation material is an epoxy resin. The anodes are then diced into individual units, Harperised (rough edges removed by ball-milling) , silver terminated and Ni/Sn plated to form final anodes.

The present method has produced micro-chip derived conductive polymer cathode layer capacitors having DCLs typically of circa, 0. IuA with examples of less than 0. IuA possible.

Using conductive polymer in micro-chip capacitors produces low ESR performance typically between 0.5 and 1.0 Ohms. This is a major breakthrough as there are no conductive polymer capacitors presently in the market in small sizes of L-case (0603) or smaller performing at 10V for a lOμF part. Thus a conductive polymer capacitor formed by the present method, of the type LlO/10, is new and provides unique electrical properties .

Claims

1. A method of applying layers of material in the manufacture of solid state capacitors, each layer being applied as a liquid precursor and then caused or allowed to solidify to form a solid layer, the method including the steps of: providing dispensing means for delivering precursor liquid, the dispensing means comprising a delivery head adapted to deliver precursor liquid from the dispensing means, providing mechanical transport means for moving the delivery head relative to a liquid delivery site, and for aligning the delivery head with the liquid delivery site, delivering one or more doses of liquid to the targeted delivery site in amounts suitable to form a layer of precursor material in a controlled amount at the desired location.

2. A method as claimed in claim 1 wherein the applied material is the precursor of a conductive material which sets or reacts to form a solid conductive layer.

3. A method as claimed in claim 1 or claim 2 wherein the applied material is a polymer or polymer precursor.

4. A method as claimed in any preceding claim wherein the site upon which the material is applied is the anode portion of the capacitor, so that the layer forms a cathode layer in the capacitor.

5. A method as claimed in any preceding claim wherein there is a plurality of anode portions, distributed so that each is spaced apart from the others.

6. A method as claimed in claim 5 wherein the anode portions are provided as a generally planar array supported on a substrate.

7. A method as claimed in claim 6 wherein the doses are delivered to each anode portion one after the other.

8. A method as claimed in claim 6 or claim 7 wherein the doses are delivered to a first series of anode portions simultaneously, and thereafter simultaneously to a second series of anode portions.

9. A method as claimed in claim any preceding claim wherein the precursor liquid comprises a mixture or solution of two or more liquid reactants.

10. A method as claimed in claim 9 wherein the agents are mixed immediately before the liquid is dispensed.

11. A method as claimed in any of claims 1 to 8 wherein two or more different precursor liquid reactants are dispensed and one liquid reacts with the other to form a solid layer in situ on the anode portion.

13. A method as claimed any preceding claim wherein the liquid material is applied in multiple doses sequentially on each anode portion to build up each applied layer as a plurality of sub-layers .

14. A method as claimed in claim 13 wherein after the application of one dose, the following dose is applied before the precursor material of the previous sub-layer has completely set, reacted or cured, thereby to permit inter-dose curing, setting or reacting.

15. A method as claimed in any preceding claim, wherein the liquid dispenser means comprises a piezoelectric droplet delivery apparatus, which ejects droplets by means of piezoelectric pressure.

16. A method as claimed in any of claims 1 to 14 wherein the liquid dispenser means comprises a thermal bubble jet apparatus in which volumes of liquid are ejected by heat- induced vaporisation of liquid.

17. A method as claimed in any or claims 1 to 14 wherein the liquid dispenser means comprises a needle dispenser apparatus in which at least one elongate hollow needle member is provided in liquid communication with a precursor material precursor reservoir.

18. A method as claimed in claim 17 wherein the liquid is dispensed as droplets formed by pneumatic pressure.

19. A method for forming multiple solid state capacitors state capacitors comprising: forming an array of porous anodes of valve action material, forming a dielectric layer on surfaces of the porous anode, forming, on each anode, a cathode layer in electrical contact with the dielectric layer, wherein the cathode layer is applied to each anode as liquid precursor material by a method according to any of claims 1 to 18.

20. A method as claimed in claim 19 wherein after application of the cathode layer a barrier layer is applied on regions of the substrate between the anodes .

21. A method as claimed in claim 20 the barrier layer is applied up to a depth which corresponds to the barrier layer covering at least a portion of the cathode layer on the anode bodies .

22. A method as claimed in claim 20 or 21 wherein the barrier layer is chemically inert with respect to the cathode layer.

23. A method as claimed in any of claims 20 to 22 wherein an encapsulation material is applied over the barrier layer, up to a level corresponding to a top end of each anode.

24. A method as claimed in clam 23 wherein the substrate is divided to form individual capacitors comprising a porous anode body portion in electrical communication with an anode terminal at one end of the capacitor, a dielectric layer formed on the anode body, a cathode layer formed on the dielectric layer and a cathode terminal in electrical communication with the cathode layer, and encapsulation material which sleeves anode sidewall portions, wherein a portion of the sidewall portions located between the anode terminal and the encapsulation sleeve is provided by the barrier layer which is chemically inert to the cathode layer material .

25. A method as claimed in any of claims 20 to 24 wherein the barrier layer is applied as a liquid precursor and then caused or allowed to solidify to form a solid layer, the method including the steps of: providing dispensing means for delivering precursor liquid, the dispensing means comprising a delivery head adapted to deliver precursor liquid from the dispensing means, providing transport means for moving the delivery head relative to a liquid delivery site between anodes, and for aligning the delivery head with the liquid delivery site, delivering one or more doses of liquid to the targeted delivery site in amounts suitable to form a layer of precursor barrier material in a controlled amount at the desired location.

26. A capacitor obtainable by the method of claim 24 or 25 and comprising a porous anode body portion in electrical communication with an anode terminal at one end of the capacitor, a dielectric layer formed on the anode body, a cathode layer formed on the dielectric layer and a cathode terminal in electrical communication with the cathode layer, and encapsulation material which sleeves anode sidewall portions, characterised in that a portion of the sidewall portions located between the anode terminal and the encapsulation sleeve is provided by the barrier layer which is chemically inert to the cathode layer material.

27. A method for forming multiple solid state capacitors state capacitors comprising: forming an array of porous anodes of valve action material spaced apart on a substrate, forming a dielectric layer on surfaces of the porous anode, forming, on each anode, a cathode layer in electrical contact with the dielectric layer, wherein, after application of the cathode layer, a barrier layer is applied to substrate surfaces between anodes.

28. A method as claimed in claim 27 the barrier layer is applied up to a depth which corresponds to the barrier layer covering at least a portion of the cathode layer on the anode bodies .

29. A method as claimed in claim 27 or 28 wherein the barrier layer is chemically inert with respect to the cathode layer.

30. A method as claimed in any of claims 27 to 29 wherein an encapsulation material is applied over the barrier layer, up to a level corresponding to a top end of each anode.

31. A method as claimed in clam 30 wherein the substrate is divided to form individual capacitors comprising a porous anode body portion in electrical communication with an anode terminal at one end of the capacitor, a dielectric layer formed on the anode body, a cathode layer formed on the dielectric layer and a cathode terminal in electrical communication with the cathode layer, and encapsulation material which sleeves anode sidewall portions, wherein a portion of the sidewall portions located between the anode terminal and the encapsulation sleeve is provided by the barrier layer which is chemically inert to the cathode layer material.

32. A method as claimed in any of claims 27 to 31 wherein the barrier layer is applied as a liquid precursor and then caused or allowed to solidify to form a solid layer, the method including the steps of: providing dispensing means for delivering precursor liquid, the dispensing means comprising a delivery head adapted to deliver precursor liquid from the dispensing means, providing transport means for moving the delivery head relative to a liquid delivery site between anodes, and for aligning the delivery head with the liquid delivery site, delivering one or more doses of liquid to the targeted delivery site in amounts suitable to form a layer of precursor barrier material in a controlled amount at the desired location.

33. A capacitor obtainable by the method of claim 31 or 32 and comprising a porous anode body portion in electrical communication with an anode terminal at one end of the capacitor, a dielectric layer formed on the anode body, a cathode layer formed on the dielectric layer and a cathode terminal in electrical communication with the cathode layer, and encapsulation material which sleeves anode sidewall portions, characterised in that a portion of the sidewall portions located between the anode terminal and the encapsulation sleeve is provided by the barrier layer which is chemically inert to the cathode layer material.

34. A capacitor as claimed in claim 33 wherein the barrier layer comprises a fillet or shoulder abutting interface between the anode body and the anode terminal .

35. A method of forming a solid state capacitor comprising: forming a porous anode of valve action material, forming a dielectric layer on the surface provided by the porous anode, forming a cathode layer in electrical contact with the dielectric layer, characterised in that the cathode layer is applied to the anode as a liquid precursor which reacts or sets in situ to form a solid layer, and in that the cathode layer is built up on each anode by the application of a sequentially applied series of cumulative layers.

36. A method as claimed in claim 35 wherein a next layer in the sequence is applied before the underlying layer is cured or reacted so that cross-layer reaction or curing occurs so as to improve the cohesion and homogeneity of the cathode layer.

37. A method as claimed in claim 35 or claim 36 wherein the each of the cumulative layers is applied by a method according to any of claims 1 to 18.

38. A method of forming a solid state capacitor comprising: forming a porous anode of valve action material, forming a dielectric layer on the surface provided by the porous anode, forming a cathode layer in electrical contact with the dielectric layer, which layer is formed of an electrically conductive polymer material, providing an anode terminal in electrically conductive contact with the anode body and a cathode terminal in electrically conductive contact with the cathode layer, characterised in that, after formation of the cathode layer, a barrier layer is applied an exposed portion of the cathode layer between the anode terminal and the cathode terminal, thereby to protect the cathode layer against environmental or mechanical or chemical damage during processing.

39. A method as claimed in claim 38 wherein the barrier layer comprises a polymer that is chemically inert and stable with respect to the cathode layer polymer material.

40. A method as claimed in claim 39 wherein the barrier layer comprises a polyamideimide or a polyurethane or a polyethylene terephthalate .

41. A method as claimed in any of claims 38 to 41 wherein the capacitor is encapsulated in a protective layer leaving the terminals exposed and covering at least a portion of the barrier layer and any exposed cathode layer.