Patent Application
20060001068
Embodiments of the invention relate generally to capacitors and in particular, but not exclusively, to multi-layer capacitors including dielectric layers having different compositions.
Multi-layer capacitors are used in many different types of power delivery applications, from computer motherboards and packages to automotive applications. Multi-Layer Ceramic Capacitors (MLCCs) are a widely-used type of multi-layer capacitor that includes several layers of a single ceramic dielectric material separated from each other by layers of a conductive material. In many applications it is necessary or desirable for capacitors to have a uniform capacitance over a wide temperature range that could cover −55° C. to 125° C., or even larger. A uniform capacitance simplifies application design because the temperature need not be taken into account, and also improves performance of the application. The capacitance C of MLCCs, however, exhibits a very strong capacitance variation with temperature because the capacitance of ceramic dielectric materials used in ceramic capacitors—or, more accurately, their dielectric constant ε—varies significantly with temperature. Thus, the capacitance of an MLCC is not constant with temperature, but rather is a function of temperature C(T).
To reduce the variation of an MLCC's capacitance with temperature, ceramic capacitor suppliers use dopants. The exact effect of the dopant depends on the particular dopant used and the amount of dopant mixed with the base dielectric, although two results are predominant. Some formulations result in smaller variation of capacitance with temperature, but with significant loss in capacitance because dopants substantially reduce the dielectric constant of the material. Other formulations maintain the high capacitance values and strong temperature dependence, but shift the temperature where the capacitance is a maximum.
Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
Embodiments of a multi-layer capacitor including dielectric layers with different variations of capacitance with temperature are described herein. In the following description, numerous specific details are described to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in this specification do not necessarily all refer to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
Although the illustrated embodiment shows the conductive layers 108 with substantially the same thickness as the alternating dielectric layers 102 and 104, in other embodiments the conductive layers may be thinner or thicker than the dielectric layers. Similarly, in the illustrated embodiment the first dielectric layers 102 are shown with the same thickness as the second dielectric layers 104, but in other embodiments the first dielectric layers 102 may have greater or smaller thicknesses than the second dielectric layers 104. Finally, although in the illustrated embodiment the number of first dielectric layers 102 and second dielectric layers 104 is equal, in other embodiments there need not be equal numbers of first and second dielectric layers.
The conductive layers 108 sandwiched between each pair of first and second dielectric layers can be made of any kind of conductive material. In one embodiment, the conductive layers are made of a metal such as gold (Au), silver (Ag), aluminum (Al), nickel (Ni), platinum (Pt) or palladium (Pd), or alloys or combinations of these metals, such as palladium/silver (Pd/Ag). In other embodiments, other metals not listed and their combinations or alloys can be used. In still other embodiments, the conductive layers 108 can be made of conductive non-metals.
The first dielectric layers 102 and the second dielectric layers 104 are made of dielectric materials having different variations of capacitance with temperature and, therefore, different compositions. In other words, each first dielectric layer 102 will have a first composition, while each second dielectric layer 104 will have a second composition. The first composition will be different than the second composition, such that the first and second dielectric layers have different C(T) distributions. No particular composition is required for the dielectric layers 102 or 104, as long as the chosen compositions, when stacked together as shown, provide the desired C(T) distribution for the capacitor 100. In one embodiment, the first and second compositions may comprise the same base dielectric (e.g., Barium Titanate, BaTiO3) but include different dopants, thus creating different compositions. For example, the first composition can include a base dielectric of Barium Titanate doped with zirconium (Zr), while the second composition can include the same Barium Titanate base dielectric doped with calcium (Ca) instead of zirconium. In other embodiments, the different first and second compositions can include the same base substrate, but with different concentrations of the same dopant or dopants; different base substrates, but with the same dopants; different base substrates with different dopants; or different base substrates with no dopants at all.
The capacitor 100 can be made in a variety of ways. In one embodiment of a process for making the capacitor, batches of the first and second compositions are prepared to create two separate slurries, one for each composition. The first slurry (i.e., the slurry of the first composition) includes solvents mixed with a base dielectric and any dopants, while the second slurry (i.e., the slurry of the second composition) similarly includes solvents mixed with a base dielectric and any dopants. The first slurry is spread into a layer on a sheet and allowed to dry. After the first slurry layer dries, a conductive layer is deposited on the first slurry layer, and then a second slurry layer (i.e., a layer of the second slurry) is deposited onto the conductive layer and also allowed to dry. Another conductive layer is deposited on the second slurry layer, and the process is repeated again until the desired number of layers has been stacked. The result is a sheet of many capacitors composed of stacked dielectric layers separated by conductive layers.
Once completed, the flexible sheet must cured, diced into individual capacitors and fired. Curing involves raising the temperature of the sheet to evaporate the slurry solvents. After curing, the sheet is “diced” into individual capacitors, which are then fired by heating to a high temperature; the exact temperature of firing will depend on the dielectric compositions used. Firing the capacitors hardens the dielectric layers and crystallizes grains in the dielectric layers, perfecting their dielectric properties.—After the individual capacitors have been fired, terminals are added to the exterior of the capacitor so that voltage can be applied to the internal conductive layers. The process described above for making the capacitor is only one potential process; in other embodiments, other processes having more, less, or different operations can be used.
The dielectric materials stacked between the cover layers comprises a set of first dielectric layers 402 separated by conducting layers 408 and set of second dielectric layers 404 separated from each other by conductive layers 208. The first and second sets are also separated from each other by a conductive layer 208. The capacitor 400 includes a pair of terminals 410 and 412 on opposite sides of the exterior of the capacitor. The terminals 410 and 412 provide the means through which voltages can be applied to or more of the internal conductive layers 408. The terminals 410 and 412 are connected to alternating conductive layers 408; in other words, terminal 410 is connected to one conductive layer 408, terminal 412 to the next one in the stack, terminal 410 to the next, and so forth.
The above description of illustrated embodiments of the invention, including what is described in the abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. These modifications can be made to the invention in light of the above detailed description.
The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.
