Patent Grant
9023186
The present invention relates generally to energy storage devices, and more specifically to titania capacitors and scalable fabrication methods thereof.
Capacitors store electrical energy in an electric field that forms between two electrodes separated by a dielectric material. The electrodes are most commonly configured as parallel plates with fixed separation. The capacitance, C, in Farads, is directly proportional to the electrode surface area, A, usable for charge separation and inversely proportional to the distance, d, between the two parallel electrodes, as shown in equation (1), where ∈ is the permittivity of the dielectric.
The power rating, P, in Watts, of a capacitor cell is given by equation (2), where V is the potential difference between the two plates in volts and ESR is the equivalent series resistance in ohms.
The energy rating, E, in Joules, of a capacitor cell is given by equation (3).
To increase the capacitance, energy and power performance of a capacitor, one has to increase ‘∈’, ‘A’, ‘V’ and/or decrease ‘d’. However the value of ‘d’ is largely determined by the intrinsic properties of the dielectric material and device working voltage and cannot be varied significantly. (The thickness of the dielectric film is determined by the required working voltage and the strength of the dielectric material, measured in volts per meter. The dielectric strength is a measure of the maximum electric field that can exist in a dielectric without an electrical breakdown.) Therefore, when high capacitance, high power and energy performance are desired, it is necessary to combine the mutual benefits achieved with a high permittivity dielectric material, an increased effective surface area, and an enhanced dielectric strength.
Capacitors may use a bulk dielectric made from an insulating material. Alternatively, capacitors may have a microscopic dielectric layer, such as a metal oxide layer. Compared to bulk dielectric capacitors, this very thin dielectric allows for much more capacitance in the same unit volume.
In electrolytic capacitors, an electrolyte and a cathode foil together form the cathode, the second “plate” of the capacitor. Most commercial electrolytic capacitors are made with aluminum or tantalum anodes. There are also ceramic-based electrolytic capacitors, as described below.
Aluminum electrolytic capacitors are used as power supplies for automobiles, aircraft, space vehicles, computers, monitors, motherboards of personal computers and other electronics. There are two types of tantalum capacitors commercially available in the market; wet electrolytic capacitors which use sulfuric acid as the electrolyte and solid electrolytic capacitors which use MnO2 as the solid electrolyte.
A typical aluminum electrolytic capacitor includes an anode foil and a cathode foil. Usually, the dielectric film is fabricated by anodizing high purity Al foil for high voltage applications in boric acid solutions. Anode, cathode and separator sheets are cut to a specific size, as per the design specification. A laminate is made up of the anode foil, the cathode foil which is opposed to the dielectric film of the anode foil, and a separator interposed between the anode and cathode foils. The laminate is wound to provide an element. The wound element is then immersed in an electrolyte, to saturate the separator, and housed in a metallic sheathed package with a cylindrical format. Here the electrolyte and the cathode foil together form the cathode. Ta and Al electrolytic capacitors fabricated using this general type of process are produced with capacitances up to 10 μF.
The capacitances of both Ta and Al based electrolytic capacitors are fairly similar. Al-based capacitors are cheaper than Ta-based capacitors on a $/farad basis, but Al-based capacitors produce current-spike noise in certain applications. On the other hand, tantalum-based capacitors are superior to Al-based capacitors in temperature and frequency characteristics and are preferred for circuits which need high stability characteristics. However, Ta metal is relatively rare and is subject to supply constraints and highly fluctuating prices. Clearly there is a need for electrolytic capacitors with performance comparable to Ta-based capacitors without having to rely on a metal that is subject to supply constraints and fluctuating prices.
Ceramic capacitors are based on the high dielectric constant rather than the electrode area. A ceramic capacitor is a capacitor constructed of alternating layers of metal and ceramic, with the ceramic material acting as the dielectric. Multilayer ceramic capacitors (MLCs) typically consist of approximately 100 alternating layers of electrode and ceramic sandwiched between two ceramic cover layers. MLCs are fabricated by screen-printing of electrode layers on ceramic layers and co-sintering of the laminate. Conventionally, the electrode material is Ag—Pd and the ceramic is BaTiO3. MLCs are produced with capacitances up to tens of μF. MLCs are well suited for high frequency applications. However, MLCs have a complicated manufacturing process and that is relatively expensive.
Referring to equation (1), it may be appreciated that improvements in the performance of electrolytic capacitors are achieved by increasing the effective surface area, A, of the anode. For example, for an aluminum electrolytic capacitor this can be achieved by electrolytic etching of the aluminum substrate before anodization to form the dielectric layer at the aluminum anode surface. Further improvements in performance may be achieved by increasing ∈ by using composite dielectric layers comprising relatively large ∈ value compounds. For example, tantalum electrolytic capacitors with Ta metal anodes, polypyrrole cathodes and Ta2O5 dielectric layers have been fabricated. See M. Satoh, H. Ishikawa, K. Amane, E. Hassegawa, K. Yoshino, Syn. Metals, 71 (1995) 2259. Titania-polypyrrole nanocomposites may also be used to improve E. See J. Lin et al., Appl. Phys. Lett. 74, 2370 (1999).
Others have tried using alternate dielectric materials. For example, Chung in U.S. Pat. No. 7,144,768 describes the use of titanium and titanium alloy anodes with advantages in energy density, cost and material density when compared with tantalum. Chung states that the insulating and dielectric behavior of the titanium anode film—as measured by the leakage current and capacitance, for example—are uncertain and inconsistent and as a result, titanium and titanium alloys have generally not been used in capacitors. To overcome this limitation, Chung describes methods for controlling leakage current and capacitance in capacitors using titanium and titanium alloy anodes. Chung describes the following methods: (i) mechanical treatment, such as shot peening, of the surface to enhance the density of active sites; (ii) thermal treatment, such as quenching, to give the Ti anode an amorphous structure; and (iii) chemical treatment, such as doping the Ti or etching the Ti surface.
Having identified titanium and titanium alloys as potentially useful anode materials for capacitors there remains a need to develop a cost effective method for high volume manufacturing of titania capacitors. Furthermore, there remains a need to identify a titania capacitor structure compatible with such a cost effective method.
In general, embodiments of the present invention include methods to produce titania-based electrodes for incorporation into devices such as electrolytic capacitors with high capacitance, high power, high operating voltage, high energy density and long life. Embodiments of the present invention include high volume manufacturing methods for producing high quality dielectric films in a reel-to-reel process with favorable process speeds, uniformity and control. Electrodes compatible with these high volume manufacturing processes may comprise an aluminum foil covered on at least one surface with a titania dielectric. The electrodes may be used in capacitors, electrolytic capacitors, and other such short-duration energy storage devices.
According to aspects of this invention, a first embodiment of a method of manufacturing an electrode includes: providing a metal foil; etching the metal foil to increase the surface area; depositing titanium metal on the etched metal foil; pretreating the Ti/metal foil; and anodizing the Ti/metal foil so as to produce a titania dielectric on the surface of the anode. Wherein the pretreatment includes one or more of the following: (i) mechanical treatment, such as shot peening, of the Ti metal surface; (ii) thermal treatment, such as quenching; and (iii) chemical treatment, such as doping the Ti or etching the Ti surface; and wherein the pretreatment improves the dielectric strength of the titania film subsequently formed by anodization. The process may be limited to only one surface of the metal foil. Alternatively, the metal foil may be processed on both sides. The metal foil may be an aluminum foil.
According to further aspects of the invention, a second embodiment of a method of manufacturing an electrode includes: providing a metal foil; depositing titanium metal on the metal foil; masking the titanium metal surface to control the density of sites where anodization will occur; and anodizing the Ti/metal foil so as to produce a nano-porous titania dielectric on the surface of the anode. The process may be limited to only one surface of the metal foil. Alternatively, the metal foil may be processed on both sides. The metal foil may be an aluminum foil. The porous titania dielectric may comprise titania nanotubes. Further embodiments may also include pretreating the Ti metal for improving the dielectric strength of the titania film subsequently formed by anodization.
According to embodiments of the invention, a linear process tool for reel-to-reel processing of a metal foil to form an electrode structure may include: a foil etching station for increasing the surface area of the metal foil; a titanium deposition station for depositing a uniform thin film of titanium on the surface of the etched metal foil; a pretreatment station; and an anodization station. Wherein the pretreatment station provides one or more of the following: (i) mechanical treatment, such as shot peening, of the Ti metal surface; (ii) thermal treatment, such as quenching; and (iii) chemical treatment, such as doping the Ti or etching the Ti surface; and wherein the pretreatment improves the dielectric strength of the titania film subsequently formed at the anodization station.
According to further embodiments of the invention, a linear process tool for reel-to-reel processing of a metal foil to form an electrode structure may include: a titanium deposition station for depositing a uniform thin film of titanium on the surface of the metal foil; a masking station for modifying the titanium surface to control the density of sites where anodization will occur; and an anodization station for transforming the Ti thin film into a porous titania dielectric film. The porous titania thin film may comprise titania nanotubes. Further embodiments may also include stations for pretreating the Ti metal prior to anodization, mask stripping and/or anodization of the stripped Ti metal surface.
The present invention includes the electrode structures corresponding to the aforementioned first and second methods of manufacturing, and electrolytic capacitor structures incorporating these electrode structures.
These and other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein:
a to 4c illustrate a capacitor anode fabrication process, according to the first embodiment of the present invention;
a to 6b illustrate a capacitor anode fabrication process, according to the second embodiment of the present invention;
a to 7d illustrate a capacitor anode fabrication process, according to a third embodiment of the present invention; and
The present invention will now be described in detail with reference to the drawings, which are provided as illustrative examples of the invention so as to enable those skilled in the art to practice the invention. Notably, the figures and examples below are not meant to limit the scope of the present invention to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present invention will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the invention. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the invention is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present invention encompasses present and future known equivalents to the known components referred to herein by way of illustration.
In general, embodiments of the present invention include methods to produce titania-based electrodes, for use in devices such as capacitors with high operating voltage, high capacitance, high energy density and long life. Embodiments of the present invention include high volume manufacturing methods for producing high quality dielectric films in a reel-to-reel process with favorable process speeds, uniformity and control. The present invention includes electrode structures fabricated using these methods. Some embodiments of these titania-based electrodes may be integrated into electrolytic capacitors suitable to serve capacitor markets currently served by tantalum and aluminum electrolytic capacitors. The examples provided herein are directed to manufacturing electrolytic capacitors; however, the concepts are applicable to electrodes for other devices, for example anodes for capacitors (other than electrolytic), ultracapacitors (also referred as supercapacitors), Li-ion batteries.
The aluminum foil may be provided with an etched surface, in which foil etching 120 is nor required. Also, the etching step may be skipped, in which case Ti metal is deposited onto a metal foil with a planar surface. The Ti deposition 130 can be a sputtering/physical vapor deposition process, chemical vapor deposition (CVD), or other deposition process. The Ti layer uniformity, thickness, phase, composition and microstructure are controlled using the known art applicable to PVD, sputtering, webtools, etc. The Ti deposition may be conducted in a high speed roll to roll process tool, referred to as a web tool, such as web tools manufactured and sold by Applied Materials. The pretreatment may be one or more of (i) mechanical treatment, such as shot peening, of the Ti metal surface; (ii) thermal treatment, such as quenching; and (iii) chemical treatment, such as doping the Ti or etching the Ti surface; wherein the pretreatment improves the dielectric strength of the titania film subsequently formed by anodization. The anodization process may be conducted in a tool such as Applied Material's Desica™ tool.
An electrode was fabricated following the process of
The Ti deposition 230 can be a sputtering/physical vapor deposition process, CVD, or other deposition process. The Ti layer uniformity, thickness, phase, composition and microstructure are controlled using the known art applicable to PVD, sputtering, webtools, etc. The Ti deposition may be conducted in a high speed roll to roll process tool, referred to as a web tool, such as web tools manufactured and sold by Applied Materials. The titanium coated Al foil is anodized to form a porous titania dielectric layer using methods known to those skilled in the art. For example, see Woo-Jin Lee et. al., Journal of The Electrochemical Society, 153, 11, B499 [2006] and H. E. Prakasam et. al., Journal of Phys Chem C 111, 7235 [2007]. Applied Material's Desica™ tool platform may be used as a basis for a tool to form the porous titania dielectric layer following the aforementioned anodization processes.
The masking process (245) and the anodization process (250) are known processes which have been adapted for this particular application, as described below. Processes for the formation of titania nanotube structures by anodization have been reported in the literature. See D. Gong, et.al., Titanium Oxide Nanotube Arrays Prepared by Anodic Oxidation, J. Mater. Res., Vol. 16, No. 12, December 2001. However, the nanotube arrays so formed will have limited active interfacial area between the conductor (Ti) and Dielectric (TiO2) layers. In some embodiments of the current invention, titania tubular structures with high interfacial area between Ti and TiO2 are beneficial in enhancing the capacitance and energy ratings of the dielectric layers and the capacitors, as follows from Eq. 1 and Eq. 3. Fabrication of high Ti/TiO2 interface area titania tubular structures, as shown in
An electrode was fabricated using the process of
In both the first and second embodiments of methods of the present invention the goal is to grow defect-free titania thin films of controllable thickness—typically a few hundred nanometers thick—with properties which may include high dielectric constant, high break down potential and low direct current leakage.
The foil etch tool 320 is an etch tool suitable for this process that will be familiar to those skilled in the art. The Ti deposition tool 330 can be a sputtering/physical vapor deposition tool, a CVD tool, or other deposition tool. The Ti layer uniformity, thickness, phase, composition and microstructure are controlled using the known art applicable to PVD, sputtering, webtools, etc. The Ti deposition may be conducted in a high speed roll to roll process tool, referred to as a web tool, such as web tools manufactured and sold by Applied Materials. For example the Multimet™ production metallization tool. The pretreatment tool 340 may be a tool configured to provide one or more of (i) mechanical treatment, such as shot peening, of the Ti metal surface; (ii) thermal treatment, such as quenching; and (iii) chemical treatment, such as doping the Ti or etching the Ti surface; wherein the pretreatment improves the dielectric strength of the titania film subsequently formed by anodization. The masking tool 345 is a masking tool suitable for processing the substrates according to the masking process described above. The anodization process may be conducted in an anodization tool 350 such as a tool based on Applied Material's Desica™ tool. The stripping tool 355 may be a standard chemical or mechanical stripping tool. The anodization tool 360 may be configured very similarly to the anodization tool 350.
a through 4c show cross-sections of an anode structure according to embodiments of the present invention, wherein the anode has been processed on only one side according to the process steps 110, 120 and 130 of the process flow of
a and 6b show cross-sections of an anode structure according to embodiments of the present invention, wherein the anode has been processed on only one side according to the process steps of the process flow of
According to further embodiments of the present invention, a similar process to that described above could be applied to BaSrTiO3 capacitors. This may be achieved by replacing the Ti with Ba, Sr, and Ti in the PVD/CVD deposition step and modify the anodization chemistry accordingly.
According to a third embodiment of the present invention, pretreatment of the Ti films may be incorporated into the process flow of
The process of
Although the present invention has been particularly described with reference to embodiments thereof, it should be readily apparent to those of ordinary skill in the art that changes and modifications in the form and details may be made without departing from the spirit and scope of the invention.
This application claims the benefit of U.S. Appl. No. 61/220,761 filed Jun. 26, 2009.
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| Number | Date | Country | |
|---|---|---|---|
| 61220761 | Jun 2009 | US |
