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  Method of fabricating thin dielectric film and thin film capacitor including the dielectric filmUSPTO Application #: 20060065349Title: Method of fabricating thin dielectric film and thin film capacitor including the dielectric film Abstract: A method of fabricating a thin dielectric film, a thin dielectric film formed according to the method, and a system including the thin dielectric film. The method includes: depositing a ceramic precursor material on a metal sheet, the ceramic precursor material including a mixture comprising ceramic particles and an organic carrier medium; heat treating the ceramic precursor material such that the organic carrier medium is substantially burnt off, and further such that a dielectric layer is formed including ceramic grains formed from the ceramic particles, and having grain sizes between about 100 nm and about 500 nm; depositing a CSD precursor material onto the dielectric layer; and heat treating the CSD precursor material such that organics in the CSD precursor material are substantially burnt off, and further such that a CSD medium is formed from the CSD precursor material including CSD grains substantially filling the voids between the ceramic grains (end of abstract) Agent: Blakely Sokoloff Taylor & Zafman - Los Angeles, CA, US Inventor: Cengiz A. Palanduz USPTO Applicaton #: 20060065349 - Class: 156089110 (USPTO) Related Patent Categories: Adhesive Bonding And Miscellaneous Chemical Manufacture, Methods, Surface Bonding And/or Assembly Therefor, With Vitrification Or Firing Ceramic Material The Patent Description & Claims data below is from USPTO Patent Application 20060065349. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD [0001] Embodiments of the present invention relate to thin film capacitors and to methods of fabricating same. BACKGROUND [0002] Creating thin films having a large capacitance density, that is, a capacitance density above about 1 .mu.F/cm.sup.2 on metal sheets presents a number of challenges. One way to achieve large capacitance density would be to achieve a large dielectric constant, given that capacitance density and dielectric constant are directly proportional to one another. It is well known that the dielectric constant of a material is among other things a function of the grain size of that material. In particular, as the grain size of a material increases, generally, so will its dielectric constant. However, growing thin films having large grain sizes, that is, thin films having grain sizes above about 50 nm to about 100 nm is a challenge. For example, growing a large grain microstructure requires an optimum combination of nucleation and grain growth. This is hard to achieve on a polycrystalline metal sheet. Typically, the multitude of random sites on the rough polycrystalline metal sheet acts as nucleation sites, resulting in a microstructure with very small grain size (about 10 nm to about 50 nm). Once the film microstructure is composed of a large number of small grains, further heating will not result in a large grain microstructure, because larger grains would grow at the expense of the smaller grains. However, a large number of similar-sized grains cannot grow into each other to form larger grains. [0003] As a result of the above, attempts at creating thin films having a large capacitance density has shifted toward reducing a thickness of the deposited thin film dielectric, while avoiding the problems noted above with respect to creating dielectrics of large grain size. Thus, the prior art typically focuses on small grain sized thin film technology (that is dielectric thin films having grain sizes in the range from about 10 nm to about 50 nm, with dielectric constants ranging from about 100 to about 450. To the extent that the capacitance density of a material is known to be inversely proportional to its thickness, the prior art has aimed at keeping the thickness of such dielectric films in the order of about 0.1 microns. However, disadvantageously, such films have tended to present serious shorting issues. First, a surface roughness of the metal sheet onto which the dielectric film has been deposited, to the extent that it is usually significant with respect to a thickness of the dielectric film, tends to present metal peak and valleys into the dielectric film which in turn can lead to a direct shorting between the electrodes of a capacitor that includes the dielectric film. In addition, again, since a thickness of the dielectric film is small, voids typically present in the film will allow metal from at least one of the capacitor electrodes to seep into the voids, leading to shorting and leakage between the electrodes. [0004] Voids in dielectric layers are disadvantageous for a number of other reasons. First, because of the presence of air pockets brought about as a result of the presence of voids, stress concentration points are typically created in the dielectric film, thus increasing the risk of crack propagation therein. In addition, to the extent that the dielectric constant of air is very small, the presence of air pockets results in an overall decrease in the dielectric constant of the dielectric layer. Thus, voids present disadvantages with respect to both the mechanical integrity and the electrical performance of a dielectric layer. The prior art proposes solving the problem of voids by exposing the dielectric layer to relatively long periods of sintering in order to densify the layer. However, such a solution disadvantageously increases the thermal budget required for the fabrication of a dielectric film, increasing cost while not necessarily guaranteeing a satisfactory reduction in the number of voids. [0005] With respect to fabricating thin film capacitors, as noted above, a predominant prior art method involves chemical solution deposition (CSD). Referring to FIGS. 1A-1F, various stages of a prior art CSD method for creating a dielectric thin film are depicted. As seen in FIG. 1A, the shown CSD method involves the deposition of a CSD precursor film 10 onto a metal sheet or electrode 12. Deposition of the precursor may be achieved using well known spin-on deposition, spraying and dipping techniques. Thereafter, at FIG. 1B, the deposited precursor film 10 is shown as having been subjected to drying, burn-out of organics and annealing through heat treatment. As seen in FIG. 1B, heat treatment results in the decomposition of residual organics in the precursor and further in the growth of small-sized grains, which together contribute to form a first layer 14 of dielectric material. In FIGS. 1C and 1D, and in FIGS. 1E and 1F, fabrication stages similar to those in FIGS. 1A and 1B are respectively depicted. Thus, the deposition of a precursor film 10' and 10'' as seen in FIGS. 1C and 1E is followed by heat treatment as seen in FIGS. 1D and 1F to yield second and third layers 14' and 14'' of dielectric material, respectively. The resulting dielectric film 16 as shown in FIG. 1F disadvantageously contains grains of small size, in the order of about 10 nm to about 50 nm, thus exhibiting a low effective dielectric constant, typically in the range from about 100 to about 450. In addition, voids present in the dielectric film 16 tend to create shorts between the two electrodes of a capacitor formed from assembly 18 of FIG. 1F, as explained above. [0006] Conventional thin film dielectric fabrication methods thus do not allow the formation of a dielectric film that both exhibits a high capacitance density and substantially avoids shorting and/or leakage issues between electrodes in a capacitor including the film. BRIEF DESCRIPTION OF THE DRAWINGS [0007] Embodiments of the present invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings, in which the like references indicate similar elements and in which: [0008] FIG. 1A-1F illustrate stages in the fabrication of a dielectric film on a metal sheet according to the prior art; [0009] FIGS. 2A-2D illustrate stages in the fabrication of a dielectric film on a metal sheet according to an embodiment of the present invention; [0010] FIG. 3 illustrates a thin film capacitor fabricated according to an embodiment of the present invention; and [0011] FIG. 4 illustrates a system comprising a thin film capacitor fabricated according to embodiments of the present invention. DETAILED DESCRIPTION [0012] Embodiments of the present invention pertain to methods of creating a high dielectric constant thin film on a metal sheet, to a thin film capacitor fabricated from a combination of the thin film, the metal sheet, and to a system including the thin film capacitor. [0013] Various aspects of the illustrative embodiments will be described using terms commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. However, it will be apparent to those skilled in the art that the present invention may be practiced with only some of the described aspects. For purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the illustrative embodiments. However, it will be apparent to one skilled in the art that the present invention may be practiced without the specific details. In other instances, well-known features are omitted or simplified in order not to obscure the illustrative embodiments. [0014] Various operations will be described as multiple discrete operations, in turn, in a manner that is most helpful in understanding the present invention; however, the order of description should not be construed as to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation. [0015] The word "embodiment" is used repeatedly. The word generally does not refer to the same embodiment, however, it may. The terms "comprising", "having" and "including" are synonymous, unless the context dictates otherwise. [0016] The phrases "thin film" and "dielectric film" are used interchangeably in the instant description, and refer to a dielectric film adapted to be used in a thin film capacitor. In addition, "metal sheet" as used herein refers to a sheet of metal adapted to be used as an electrode in a thin film capacitor. [0017] Referring now to FIGS. 2A-2D, stages in the fabrication of a combination including a dielectric film on a metal sheet according to an embodiment of the present invention are shown. [0018] As seen in FIG. 2A, a method according to an embodiment of the present invention includes depositing a ceramic precursor material 100, such as, for example, a ceramic green sheet onto a metal sheet 110 to obtain a first combination 120. The metal sheet 110 may include, by way of example, Cu or Ni. The ceramic precursor material 100 may include a mixture comprising ceramic powder and an organic binder, a plasticizer, or an organic solvent. According to embodiments of the present invention, the ceramic powder may have a ceramic particle size between about 0.05 micron to about 0.5 micron. Ceramic green sheets are typically used in the multilayer ceramic capacitor art, where multiple layers of green sheets are stacked in between successive multiple layers of metal paste, then fired, singulated, termination metallization added, and surface mounted onto circuit boards, and are thus available on the market for that reason. Examples of ceramic green sheets encompassed by embodiments of the present invention may include ceramic green sheets available from ceramic capacitor manufacturers, such as Murata and TDK, which would eventually be used in processing capacitors with temperature ratings, such as, Z5U, X6S, X7R, X7S, etc. Ceramic green sheets according to embodiments of the present invention may be deposited onto the metal sheet by way of roller pressing, by way of a carrier tape adhesive provided on one side of the green sheets, or, in the alternative, they may be laminated onto the metal sheet by a press. [0019] Referring next to FIG. 2B, a method according to an embodiment of the present invention further includes heat treating the ceramic precursor material 100 after its deposition onto metal sheet 110 such that organics in the material are substantially burnt off, and further such that ceramic is sintered (densified) from the ceramic precursor material is densified, such as via sintering. During the sintering process, the driving force behind the densification of the ceramic is the tendency of the system to reduce its surface area, hence, its surface energy, by joining of the particles and elimination of voids in between. Process conditions for the heat treatment of ceramic precursor material 100 may, according to embodiments of the invention, comprise drying, burn-out of organics and annealing through heat treatment. During drying, the precursor material 100 may be exposed to temperatures between about 200 degrees Centigrade and about 300 degrees Centigrade for about 2 hours to about 5 hours to yield a dried deposit. During the subsequent burn-out stage, the dried deposit may be exposed to temperatures between about 400 degrees Centigrade and about 600 degrees Centigrade for about 3 hours to about 7 hours to yield an intermediate deposit. During annealing stage, the intermediate deposit may be exposed to temperatures between about 1000 degrees Centigrade and about 1400 degrees Centigrade for about 6 hours to about 24 hours. In order to avoid oxidizing the metal sheet 110 during heat treatment of the ceramic precursor material 100, such as, for example, when the metal sheet 110 is made of Cu or Ni, heat treatment may be performed in a reducing atmosphere. Heat treatment of ceramic precursor material 100 as shown in the embodiment of FIG. 2B results in the formation of a heat treated ceramic layer 130 above metal sheet 110, layer 130 including grains 140 having sizes between about 100 nm and about 500 nm in order to form a ceramic film with a thickness less than or equal to 1 micron. Typically, a ceramic green sheet tends to shrink by about 20% along its linear dimensions as a result of sintering. The above would suggest that the centers of adjacent ceramic grains would be closer to each other by 20% after heat treatment, as result of the atoms diffusing away from the inter-center bulk regions to voids, resulting in densification and shrinkage. Preferably, according to one embodiment, layer 130 may have a thickness between about 0.3 micron and about 1 micron, and preferably a thickness of about 0.5 micron. As seen in FIG. 2B, layer 130 may define voids 150 (or pinholes) between at least some of the grains 140. As noted above, voids in the dielectric layer present a number of disadvantages, such as, for example, shorting between the electrodes of a capacitor made from the dielectric layer, leakage within the capacitor, a reduction in the dielectric constant of the dielectric layer, and an increased risk of crack propagation within the dielectric layer. As will be explained further below with respect to FIGS. 2C and 2D, embodiments of the present invention pertain to a method of substantially eliminating voids in the dielectric layer, such as dielectric layer 130 shown in FIG. 2B, advantageously improving the electrical performance and mechanical integrity of a dielectric film made from the dielectric layer. [0020] Referring next to FIG. 2C, a method according to an embodiment of the present invention further includes filling voids 150 present in layer 130 with a CSD precursor material 160. As seen in FIG. 2C, the shown CSD method thus involves the deposition of CSD precursor material 160 onto dielectric layer 130. The CSD precursor material 160 may include an organic liquid solution of organic molecules with embedded metal atoms. The precursor material 160 may, by way of example, include either: (1) barium and strontium acetates, dissolved in acetic acid, mixed with titanium tetra-isopropoxide in isopropanol; (2) barium and strontium acetate dissolved in acetic acid mixed with titanium tetra n-butoxide stabilized with acetylacetone and diluted with 2-methoxyethanol; 3) barium and strontium propionates and titanium tetra n-butoxide stabilized with acetylacetone dissolved in a mixture of propionic acid and 1-butanol. Deposition of the CSD precursor material may be achieved using well known spin-on, spray or dipping techniques. The CSD precursor material thus deposited will substantially fill voids 150 by flowing through cracks in dielectric layer 130. Continue reading... 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