VIALESS INTEGRATED INDUCTIVE ELEMENTS FOR ELECTROMAGNETIC APPLICATIONS

Abstract: An inductor suitable for integration in VLSI circuits and devices is fabricated by means of a post foundry procedure involving the deposition and patterning of layers of conductor material (32), isolation material (34), core material (50), isolation material (52) and conductor material (54) to produce conductive coils (60, 62) wrapped around a magnetic core without the need for vias. The fabrication steps utilize a combination of microelectronic and micromachining technologies.

Description: VIALESS INTEGRATED INDUCTIVE ELEMENTS FOR ELECTRO MAGNETIC APPLICATIONS BACKGROUND OF THE INVENTION Technical Field: This invention relates to microelectronics. It is specifically directed to inductive devices which are batch fabricated by the application of micromachining technology. State of the Art: Devices of various types have been batch fabricated using integrated circuit processing technology. Certain passive devices, notably resistors and capacitors, have been successfully incorporated in standard "very large scale integration" (VLSI) circuit design, and are routinely batch fabricated in accordance with conventional integrated circuit technology. The techniques developed for integrating other passive devices are inappropriate for the fabrication of inductors, however. The fabrication of the coils needed for inductors represents a special technical problem. Moreover, the microelectronic batch processes needed for obtaining a low reluctance magnetic core are undeveloped. Inductors having a maximum linear turns density of approximately 15 turns/mm have been fabricated for high flux applications. Vias have been provided through plasma etching procedures, and conductors have been provided by electroplating. Inductors have also been fabricated by means of thick film technology. These previous efforts are disclosed in the literature, for example, by the papers: CH. Ahn, "Micromachined Components As Integrated Inductors and Magnetic Microactuators " , Ph.D. Dissertation, Georgia Institute of Technology, May, 1993, Chapter 2; B. Lochel, A. Maciossek, M. Rothe, and W. Windbracke, "Micro Coils Fabricated By UV Depth Lithography and Galvanoplating", Proceedings of the 8th International Conference on Solid-State Sensors and Actuators, and Eurosensors IX, Stockholm, Sweden, June 25-29, 1995, pp. 264-267; N. Yamada, Y. Yokoyama, and H. Tanaka, "Fabrication of Wrapped Micro Coils Wound Around A Magnetic Core" , Proceedings of the 8th International Conference on Solid-State Sensors and Actuators, and Eurosensors IX, Stockholm, Sweden, June 25-29, 1995, pp. 272-275; Y. Watanabe, M. Edo, H. Nakazawa, and E. Yonezawa, "A New Fabrication Process of a Planar Coil Using Photosensitive Polyimide and Electroplating", Proceedings of the 8th International Conference on Solid-State Sensors and Actuators, and Eurosensors IX, Stockholm, Sweden, June 25-29, 1995, pp. 268-271. U.S. Patent 3,614,554 discloses the fabrication of inductor coils with feed through holes (vias) through the use of thin film technology. The metallic layers deposited by this means are necessarily of very thin cross section, and thus the current carrying coils are characterized by high resistance, while the magnetic cores are characterized by high reluctance. The presence of vias contributes to both conductor resistance per turn and power loss. There remains a need for a process for the batch fabrication of inductors which is compatible with integrated circuit processing technology and which minimizes the number of additional processing steps necessary to integrate the inductor into a circuit or micro system. Ideally, the inductors resulting from this process should be capable of producing a relatively large electromotive force. Such a device would find application in various sensing applications. DISCLOSURE OF THE INVENTION This invention provides a novel inductive element, which may be constructed through batch fabrication techniques. A typical fabrication process for realization of the inductor utilizes standard microelectronics materials and equipment. The inductor may be fabricated by means of a post process utilizing micromachining technologies, rather than the thin film technologies conventional to integrated circuit fabrication. Presently preferred fabrication procedures utilize relatively few, typically four, masks, and are completely compatible with, although distinguishable from, current integrated circuit fabrication technology. The manufacturing process used to realize the integrated inductive components of the invention can be implemented to enhance or supplement foundry produced integrated circuits. The process requires no high temperature processing steps or specialized equipment or materials. The inductor of this invention is generic in character; variations on the manufacturing procedure in accordance with specific design requirements produce a corresponding variety of integrated magnetic components. Generally, a planar substrate element serves as the structural base for the magnetic component of this invention. The substrate element may comprise any of the materials commonly used for that purpose by the microelectronic industry, including without limitation, silicon, gallium arsenide, indium phosphide, and ceramics. The substrate element may contain integrated circuit elements of systems with which a component fashioned in accordance with this invention is to be integrated. In normal practice, an insulating layer is deposited on the substrate, and is patterned to open contact pads for connection from the magnetic component to any underlying circuitry. A first (bottom) conductor is deposited and patterned in accordance with conventional photolithographic techniques. The conductor may comprise any or a combination of conductive materials, typically low resistance metals. These metals may be deposited by means of sputtering, electron beam evaporation, filament evaporation, electro-deposition or other suitable techniques. After patterning of the bottom conductor, an insulating layer (typically between about lμm to about 10μm thick) is deposited and patterned over the bottom conductors for isolation between the coil element and the magnetic core element(s). Conventional polymeric insulating materials, including polyimide and photoresist, are suitable for this layer. After patterning of the bottom isolation layer, a metallic seed layer (typically copper or gold) is deposited and patterned. The magnetic core is then deposited atop a patterned portion of the seed layer. This procedure may be accomplished by means of micro molding technology. The avoidance of sharp corners is desirable to reduce flux build-up (and saturation) at bends in the magnetic core of the devices. In general, any magnetic material that can be electroplated is a viable candidate for use in an inductor. The core material selected for any specific device depends upon the characteristics of interest (e.g. high permeability, low losses at high frequencies). The thickness of the electroplated metal is typically 1 μm to 50 μm. After electro-deposition of the core material, another coat of insulating material (usually within the range of about lμm to about 10μm thick) is typically deposited and patterned over the core. This procedure is followed by deposition and patterning of a second (top) conductor. The same materials are generally, but not necessarily, used for the top insulator and conductor as for the bottom insulator and conductor, respectively. The entire structure may be encapsulated for protection from moisture. For this step, a moisture resistant material is required. Bio compatible encapsulation may be required to render the inductors suitable for use in conductive body fluids, for example. Polyimide and parylene are examples of suitable encapsulating materials for most applications. There are many practical applications in which an integrated inductive component of this invention offers size and cost reduction over currently available macroscopic counterparts. The inductive elements of preferred embodiments provide a low reluctance core. In any case, inductors of this invention may be incorporated in a variety of practical devices, notably, integrated inductors, integrated transformers, position sensors, telemetry systems and micromotors. According to a preferred configuration, bar -type inductive elements are fashioned with metallic (ideally an electric grade aluminum alloy) conductors wrapped around a permalloy magnetic core. Connection between the upper and lower conductors of the device requires no vias. Therefore, losses due to series resistance through the wrapped conductor are minimized, and the heat generated by relatively large currents is also minimized. When an inductive component of this invention is used in a high frequency transformer application, the mutual flux produced by the primary (excitation) side should be maximized correspondingly to maximize the magnetization inductance of the transformer. With the magnetization inductance maximized, the behavior of the transformer more closely approximates that of an ideal transformer at high frequencies. The device may be embodied as an isolation transformer for use in communication applications. Typically, these applications require transmission of high frequency signals in the megahertz range, currents in the milliampere range and voltages less than 20 volts. Other devices capable of operating at much higher (100's of) volts are within contemplation. In general, an inductor capable of integration into solid state integrated circuits may be fabricated in accordance with this invention by first selecting an appropriate substrate from those otherwise useful in the fabrication of integrated circuits (an "integrated circuit component substrate".) A first conductive layer is then deposited atop the substrate, the first conductive layer being patterned to provide a first set of spaced conductor elements. A first isolation layer is then deposited atop the first conductive layer, the first isolation layer being patterned to expose opposite ends of individual conductors of the first set of spaced conductor elements. A magnetic core element may then be deposited atop the first isolation layer within the boundaries of the first set of spaced conductor elements. A second isolation layer is deposited atop the magnetic core element, the second isolation layer being patterned to expose the opposite ends of individual conductors of the first set of spaced conductor elements. A second conductive layer is then deposited atop the second isolation layer in contact with the opposite ends of the first set of spaced conductors, the second conductive layer being patterned to provide a second set of spaced conductor elements. The conductors of the first and second sets of spaced conductor elements are inherently interconnected (as a consequence of patterning) to form primary and secondary coils around the core element. A moisture barrier coating may be applied to surround an entire assembly comprising the first conductive layer, the first isolation layer, the core, the second isolation layer and the second conductive layer. Significantly, the magnetic core is most practically fabricated through micromachining technology, including the steps of depositing a metallic seed layer atop the first isolation layer, depositing a layer of photoresist over the seed layer, etching the photoresist to create a mold, the bottom of which is composed of the seed layer, electroplating the core within the mold atop the seed layer, and removing the excess portion of the photoresist layer and the seed layer. By "excess" is meant all of the seed layer not covered by core material, and all of the photoresist layer not previously etched away during patterning. Ideally, the core is electroplated until it overflows the mold, whereby to achieve a shape characterized by an elliptical cross section. This elliptical section can be established as viewed from any or all coordinate axis directions. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which illustrate what is currently regarded as the best mode for carrying out the invention: FIGs. 1-4 are schematic illustrations of the steps of a fabrication process of the invention; FIG. 5 is a perspective view of an inductive component of the invention; FIG. 6 is a sectional view taken along the reference line 6-6 of FIG. 5; FIG. 7 is a perspective view of a magnetic position sensor of the invention; and FIG. 8 is a sectional view taken along the reference line 8-8 of FIG. 7. BEST MODES FOR CARRYING OUT THE INVENTION A vialess inductive component, designated generally 26 (FIG. 4), is fabricated atop a suitable substrate 30. A first conductive layer 32 is deposited on the substrate 30. (Although not shown, an isolation layer is often interposed between the substrate 30 and the first conductive layer.) A first isolation layer 34 is deposited atop the first conductive layer, leaving end portions 36, 38 exposed. A 0.1 μm thick layer of copper metal is deposited over the first isolation layer 34 to function as an electroplating seed layer 39. A 0.5 μm photoresist layer 40 is deposited over the seed layer and terminal ends (FIG. 2), leaving an open portion (mold) 42 of the photoresist layer 40 to expose the electroplating seed layer 39. A low reluctance magnetic core 50 is electroplated atop the isolation layer (FIG. 3). The photoresist 40 and seed 39 layers are then removed. As best shown by FIG. 3, the magnetic core 50 material is allowed to "overplate" out of the mold 42. This overplating allows for an elliptical cross-section, which is useful for obtaining coverage over the magnetic core 50 in subsequent steps. Moreover, overplating eliminates sharp corners in the magnetic core material. As best shown by FIG. 4, a second isolation layer 52 is deposited over the core 50, followed by the deposition of a second conductive layer 54. The entire assembly can then be encapsulated with suitable protective material 56. Referring to FIGs. 5 and 6, the basic inductive device is shown embodied as a transformer. The conductive layers 32, 54 are of aluminum and are patterned cooperatively as primary 60 and secondary 62 coils, characterized by high turns density. The isolation layers 34, 52 are of polyimide insulation material. The performance of a 1 : 1 transformer of similar construction has been characterized. The efficiency of the device tested was approximately 85 % . This efficiency can be increased by reducing the winding resistance of the current carrying coil. Such a reduction can be effected by either or both increasing the cross-sectional area of the conductor and decreasing the contact resistance between the upper and lower conductors of the device. The device has been operated up to 10 MHz with minimal changes in characteristics due to high frequency losses. This performance is attributable to: 1) the correct choice of magnetic core materials, 2) the large cross sectional area of the core, and 3) the elimination of sharp corners due to the elliptical design of the core. Presently, the maximum turns density of the devices is 100 turns/mm. The resistance of the coils is less than 1.0 ohms per turn at 500 mA of current. The inductive component 26 may alternatively be embodied as a magnetic position sensor, as illustrated by FIGs. 7 and 8. The structure is similar to that depicted by FIGs. 5 and 6, except that the core 70 is shorter than the tunnel 72 provided by the isolation layers 34, 52. Moreover, the core 70 is reciprocally mounted within the tunnel between blocks 74, 76 of non-magnetic shielding material. Longitudinal movement of the core 70 within the tunnel 72 causes detectable changes in the current and/or voltage detected in the secondary coil 62. Reference in this disclosure to details of the illustrated and other preferred embodiments is not intended to limit the scope of the appended claims which themselves recite those features regarded as important to the invention. Those skilled in the art will recognize variations and modifications to the specific teachings of this disclosure which are, nevertheless, consistent with its teachings and objectives.

Claims: CLATMS What is claimed is: 1. An inductor capable of integration into solid state integrated circuits, comprising: a. a substrate; b. a first conductive layer deposited atop said substrate, said first conductive layer being patterned to provide a first set of spaced conductor elements; c. a first isolation layer deposited atop said first conductive layer, said first isolation layer being patterned to expose opposite ends of individual conductors of said first set of spaced conductor elements; d. A magnetic core element, between about 1 and 50μm in thickness, deposited atop said first isolation layer within boundaries of said first set of spaced conductor elements; e. a second isolation layer deposited atop said magnetic core element, said second isolation layer being patterned to expose said opposite ends of said individual conductors of said first set of spaced conductor elements; and f. a second conductive layer deposited atop said second isolation layer in contact with said opposite ends of said individual conductors of said first set of spaced conductor elements said second conductive layer being patterned to provide a second set of spaced conductor elements, the conductors of said first and second sets of spaced conductor elements being interconnected to form primary and secondary coils around said magnetic core element. 2. An inductor according to Claim 1, further comprising a moisture barrier coating surrounding an assembly comprising said elements a. through f.. 3. An inductor according to Claim 1, further including a metallic seed layer between said elements c. and d.. 4. An inductor according to Claim 3, wherein said magnetic core element is fabricated through micromachining technology, including the steps of: depositing a layer of photoresist over said seed layer; etching said layer of photoresist to create a mold, a bottom of which is composed of said seed layer; electroplating said magnetic core within said mold atop said seed layer; and removing an excess portion of said photoresist layer and said seed layer. 5. An inductor according to Claim 4, wherein said magnetic core element is electroplated until it overflows said mold, whereby to achieve a shape characterized by an elliptical cross section. 6. An inductor, comprising: a. an integrated circuit component substrate; b. a first conductive layer deposited atop said substrate, said first conductive layer being patterned to provide a first set of spaced conductor elements; c. a first isolation layer deposited atop said first conductive layer, said first isolation layer being patterned to expose opposite ends of individual conductors of said first set of spaced conductor elements; d. A magnetic core element electroplated atop said first isolation layer within boundaries of said first set of spaced conductor elements; e. a second isolation layer deposited atop said magnetic core element, said second isolation layer being patterned to expose said opposite ends of said individual conductors of said first set of spaced conductor elements; and f. a second conductive layer deposited atop said second isolation layer in contact with said opposite ends of said individual conductors of said first set of spaced conductor elements, said second conductive layer being patterned to provide a second set of spaced conductor elements, the conductors of said first and second sets of spaced conductor elements being interconnected to form primary and secondary coils around said magnetic core element. 7. An inductor according to Claim 6, further including a metallic seed layer between said elements c. and d.. 8. An inductor according to Claim 7, wherein said magnetic core element is fabricated through micromachining technology, including the steps of: depositing a layer of photoresist over said seed layer; etching said layer of photoresist to create a mold, a bottom of which is composed of said seed layer; electroplating said magnetic core element within said mold atop said seed layer; and removing an excess portion of said photoresist layer and said seed layer. 9. An inductor according to Claim 8, wherein said magnetic core element is electroplated until it overflows said mold, whereby to achieve a shape characterized by an elliptical cross section. 10. An inductor according to Claim 9, further comprising a moisture barrier coating surrounding an assembly comprising said elements a. through f. 11. An inductor capable of integration into solid state integrated circuits, comprising: a. a substrate; b. a first conductive layer deposited atop said substrate, said first conductive layer being patterned to provide a first set of spaced conductor elements; c. a first polymeric isolation layer deposited atop said first conductive layer, said first isolation layer being patterned to expose opposite ends of individual conductors of said first set of spaced conductor elements; d. a magnetic core element greater than 1 μm in thickness deposited atop said first isolation layer within boundaries defined by said first set of spaced conductor elements; e. a second polymeric isolation layer deposited atop said magnetic core element, said second isolation layer being patterned to expose said opposite ends of said individual conductors of said first set of spaced conductor elements; and f. a second conductive layer deposited atop said second isolation layer in contact with said opposite ends of said individual conductors of said first set of spaced conductor elements, said second conductive layer being patterned to provide a second set of spaced conductor elements, the conductors of said first and second sets of spaced conductor elements being interconnected to form primary and secondary coils around said magnetic core element. 12. An inductor according to Claim 11, wherein said first and second isolation layers are at least about 1 μm thick. 13. An inductor according to Claim 11 , wherein first and second isolation layers are formed of polymeric photoresist or polyimide material. 14. An inductor according to Claim 11 , wherein said magnetic core element is between about 1 and about 50 μm in thickness, and said first and second isolation layers are between about 1 and about 10 μm in thickness. 15. An inductor according to Claim 14, wherein first and second isolation layers are formed of polymeric photoresist or polyimide material. 16. An inductor capable of integration into solid state integrated circuits, comprising: a. a substrate; b. a first conductive layer deposited atop said substrate, said first conductive layer being patterned to provide a first set of spaced conductor elements; c. a first polymeric isolation layer deposited atop said first conductive layer, said first isolation layer being patterned to expose opposite ends of individual conductors of said first set of spaced conductor elements; d. a magnetic core element electroplated atop said first isolation layer within boundaries defined by said first set of spaced conductor elements; e. a second polymeric isolation layer deposited atop said magnetic core element, said second isolation layer being patterned to expose said opposite ends of said individual conductors of said first set of spaced conductor elements; and f. a second conductive layer deposited atop said second isolation layer in contact with said opposite ends of said individual conductors of said first set of spaced conductor elements, said second conductive layer being patterned to provide a second set of spaced conductor elements, the conductors of said first and second sets of spaced conductor elements being interconnected to form primary and secondary coils around said magnetic core element. 17. An inductor according to Claim 16, wherein said first and second isolation layers are at least about 1 μm thick. 18. An inductor according to Claim 16, wherein first and second isolation layers are formed of polymeric photoresist or polyimide material. 19. An inductor according to Claim 16, wherein said magnetic core element is between about 1 and about 50 μm in thickness, and said first and second isolation layers are between about 1 and about 10 μm in thickness. 20. An inductor according to Claim 19, wherein first and second isolation layers are formed of polymeric photoresist or polyimide material.

Inventors: FRAZIER A BRUNO, FRAZIER, A., BRUNO
Applicants: UNIV UTAH RES FOUND, UNIVERSITY OF UTAH RESEARCH FOUNDATION
Application year: 1998
Main classification: H01L43
IPC codes: H01F17, H01L21, H01L27