The present invention is directed, in general, to integrated circuits and, more specifically, to a method of manufacturing an integrated circuit with a micromagnetic device employing a conformal mapping technique and an integrated circuit employing the same.
A magnetic device includes a magnetic core coupled to conductor windings such that magnetic flux flows in a closed path about the core. Magnetic devices are generally configured in an EE-type structure or a toroidal geometry. In the EE-type magnetic device, a first and second core-portion of the magnetic core surround the conductor windings. In the toroidal magnetic device, a first and second winding-portion of the conductor windings surround the magnetic core.
Micromagnetic devices (e.g., microinductors or microtransformers) are micron-scaled integrated circuit magnetic devices; the electromagnetic properties of the device are provided by the presence of the magnetic core and conductor windings. In the past, micromagnetic devices were only applicable to low-level signal applications (e.g., recording heads). With the advancement in production techniques for integrated circuits, it is now possible to fabricate micromagnetic devices for relatively large signal, power processing and high speed data transmission applications. For instance, micromagnetic devices may be employed in power systems for wireless communications equipment or in data transmission circuits.
While many power semiconductor devices (having ferrite cores, for instance) have been scaled down into integrated circuits, inductive elements at the present time remain discrete and physically large. Of course, there is a strong desire to miniaturize these inductive components as well. By extending thin-film processing techniques employed in power semiconductor devices to ferromagnetic materials, the size of the conventional discrete ferromagnetic-core inductive devices can be reduced significantly. Ferromagnetic materials such as alloys, however, have much higher saturation flux densities than ferrites (e.g., 10-20 kG verses 3 kG), thereby reducing the physical volume of the core for a given inductance and energy requirement. To limit the eddy current losses in the ferromagnetic materials, the materials must be fabricated in inordinately thin films. Processing thin-film ferromagnetic materials with traditional rolling and tape winding techniques proves to be very costly as the desired tape thicknesses drops below 0.001 inches (i.e., 25 xcexcm). It is thus advantageous to produce such thin films by other integrated circuit deposition techniques such as sputtering or electroplating.
Another germane consideration associated with manufacturing micromagnetic devices is securing the ferromagnetic material to a silicon substrate or the like. More specifically, forming an adequate bond between the ferromagnetic material and an insulator coupled to the substrate is an important consideration. Many factors (such as oxide formation, melting point temperature, interposed contamination, affinity between materials and mechanical stress at the interface) may influence the adhesion of a thin film to a substrate. For instance, one technique readily employed in thin film manufacturing processes is the formation of an oxide-metal bond at the interface between the substrate and the film. The oxide-metal bond may be formed by employing an oxygen-active metal (such as tungsten or chromium) on an oxygen-bearing substrate (such as glass or ceramic) in conjunction with a refractory metal (such as tantalum or tungsten). With regard to contaminants, it is advantageous to remove any impurities interposed on the substrate. Cleaning methods vary in effectiveness and the method selected depends on the ability of the deposition process to dislodge contaminant atoms. As an example, different cleaning techniques may be employed with sputtering or electroplating.
Of course, the ultimate consideration with regard to the adhesion properties depends on the materials employed. While others have attempted to address the adhesion of ferromagnetic materials to an insulator coupled to a substrate [e.g., Measured Performance of a High-Power-Density Microfabricated Transformer in a DC-DC Converter, by Charles R. Sullivan and Seth R. Sanders, IEEE Power Electronics Specialists Conference, p. 287-294 (July 1996), which is incorporated herein by reference], to date, the problem remains unresolved. The development of an adhesive material that simultaneously forms a bond with the insulator and the ferromagnetic material such that thin-film processing can be applied to inductive elements would provide a foundation for the introduction of power processing or data transmission micromagnetic integrated circuits.
Regarding the magnetic properties, current micromagnetic devices are typically isotropic in that their properties are the same when measured in different directions. Although anisotropic properties are generally known in the domain of magnetics, anisotropic properties have not been employed in the design of micromagnetic devices due, in part, to the limitations as addressed above regarding the fabrication of micromagnetic integrated circuits. Micromagnetic devices with the ability to induce a designed magnetic anisotropic property into the core, having a desired direction and characteristic, would be very useful.
A further problem arises with magnetic field fringing effects in the magnetic material such as near gaps of the core of the micromagnetic device. This problem has been addressed in the case of less complex magnetic structures through the use of conformal maps that map a curvilinear field from a z=x+iy domain to a uniform w=u+iv domain in which energy remains invariant under the transformation. The Schwartz-Christoffel transformation, T[w(z)], may be obtained for polygonal areas with any number of vertices including those at infinity. The present solutions, however, only apply to less complex magnetic structures.
W. J. Gibbs in Conformal Transformations in Electrical Engineering, Chapman and Hall Ltd., London 1958, which is incorporated herein by reference, addresses magnetic field fringing effects for magnetic devices having uniform pole faces. In this instance, the solution is possible by using symmetry and relatively large dimensions in non-critical directions, such as a small uniform gap and long uniform magnetic pole pieces. Then, a uniform faced pole may be modeled with one vertex at the corner and two others at infinity and the origin yielding three vertices altogether. (See also, Conformal Mapping: Methods and Applications, by R. Schinzinger and P. A. A. Laura, ElSevier Science Publications, Amsterdam, xc2xa77.3.1, 1991, which is also incorporated herein by reference). Other more complicated structures involving devices having parallel plates of finite thickness separated by a gap have been solved with an integral transformation with four vertices. (See A study of the field around magnetic heads of finite length, by I. E1Abd, IEEE trans, on Audio, AU-11, pp. 21-27, 1963 and xc2xa77.3.2. of Schinzinger, et al.). The solution for more complex magnetic structures, however, remains unsolved.
Accordingly, what is needed in the art is a conformal mapping technique that is applicable to micromagnetic devices that overcomes the deficiencies in the prior art and a method of manufacturing an integrated circuit having a micromagnetic device that employs conformal mapping to advantage.
To address the above-discussed deficiencies of the prior art, the present invention provides a method of manufacturing an integrated circuit and an integrated circuit employing the same. In one embodiment, the method of manufacturing the integrated circuit includes (1) conformally mapping a micromagnetic device, including a ferromagnetic core, to determine appropriate dimensions therefor, (2) depositing an adhesive over an insulator coupled to a substrate of the integrated circuit and (3) forming the ferromagnetic core of the appropriate dimensions over the adhesive.
The present invention introduces, in one aspect, the broad concept of providing a micromagnetic device that employs a conformal mapping technique to determine the appropriate size therefor. The conformal mapping technique may be employed to optimize the dimensions of the micromagnetic device to meet specific design criteria and exhibit desired magnetic properties. For instance, very fine features may be designed into the core of the micromagnetic device or windings may be formed about the core to better utilize the space thereabout. As a result of the aforementioned technique, the magnetic field of the device may be mapped and the core reshaped to accommodate magnetic flux variations associated with different geometries thereby creating micromagnetic devices with substantially uniform flux densities and improving the operational efficiencies thereof.
In one embodiment of the present invention, the act of conformally mapping includes transforming the appropriate dimensions to a hyperelliptical representation. In a related, but alternative embodiment, the act of conformally mapping further includes reducing the hyperelliptical representation to at least one elliptical integral. In still another related, but alternative embodiment, the act of conformally mapping further includes solving the at least one elliptical integral to render at least an approximation of a fringing magnetic field magnitude of the micromagnetic device. While the derivation of the aforementioned steps in relation to a specific core geometry will hereinafter be described in detail, other conformal mapping steps capable of mapping a micromagnetic device to achieve the desired magnetic properties are well within the broad scope of the present invention.
The foregoing has outlined, rather broadly, features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.