Abstract:
A spiral inductor fabricated above a semiconductor substrate provides a large inductance while occupying only a small surface area. Including a layer of magnetic material above and below the inductor increases the inductance of the inductor. The magnetic material also acts as barrier that confines electronic noise generated in the spiral inductor to the area occupied by the spiral inductor. Inductance in a pair of stacked spiral inductors is increased by including a layer of magnetic material between the stacked spiral inductors.

Description:
This application is a Divisional of U.S. application Ser. No. 09/243,584 filed Feb. 3, 1999 in U.S. Pat. No. 6,191,468. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to inductors, and particularly to inductors used in integrated circuits. 
     BACKGROUND OF THE INVENTION 
     The telecommunications and computer industries are driving the demand for miniaturized analog and mixed signal circuits. Inductors are a critical component in the traditional discrete element circuits, such as impedence matching circuits, resonant tank circuits, linear filters, and power circuits, used in these industries. Since traditional inductors are bulky components, successful integration of the traditional discrete element circuits requires the development of miniaturized inductors. 
     One approach to miniaturizing an inductor is to use standard integrated circuit building blocks, such as resistors, capacitors, and active circuitry, such as operational amplifiers, to design an active inductor that simulates the electrical properties of a discrete inductor. Active inductors can be designed to have a high inductance and a high Q factor, but inductors fabricated using these designs consume a great deal of power and generate noise. 
     A second approach to miniaturizing an inductor is to fabricate a solenoid type inductor with a core using conventional integrated circuit manufacturing process technology. Unfortunately, conventional integrated circuit process steps do not lend themselves to precisely and inexpensively fabricating a helical structure with a core. So, integrated circuit process technology is only marginally compatible with manufacturing a solenoid type inductor. 
     A third approach, sometimes used in the fabrication of miniature inductors in gallium arsenide circuits, is to fabricate a spiral type inductor using conventional integrated circuit processes. Unfortunately, this approach has a high cost factor associated with it when applied to fabricating inductors for use in silicon integrated circuits. Silicon integrated circuits operate at lower frequencies than gallium arsenide circuits, and generally require inductors having a higher inductance than inductors used in gallium arsenide circuits. The higher inductance is realized in a spiral inductor occupying a large surface area on the silicon substrate. 
     For these and other reasons there is a need for the present invention. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the invention, an inductor comprises layers of material deposited on a silicon substrate. First, a layer of magnetic material is deposited on the silicon substrate. Next, an insulating layer is deposited on the magnetic material layer. An inductor pattern is deposited on the insulating layer and above the magnetic material layer. Finally, a second insulating layer is deposited on the inductor pattern, and a second magnetic material layer is deposited on the second insulating layer. The second magnetic material layer is deposited above the inductor pattern. 
     In an alternate embodiment, the inductor described above is coupled to another electronic device in an integrated circuit. 
     In still another embodiment, a plurality of sandwich structures are vertically stacked on an insulating layer that is deposited on a layer of magnetic material. The layer of magnetic material is deposited on a silicon substrate. The sandwich structures include an inductor pattern, an insulating layer deposited on the inductor pattern, a layer of magnetic material deposited on the insulating layer and above the inductor pattern, and an insulating layer deposited on the magnetic material layer. The structures also include a conducting path through the structures, such that each inductor pattern is serially connected to the inductor pattern above by the conducting path. The current flowing in the serially connected inductor patterns creates a reinforcing magnetic field in the magnetic material between adjacent inductor patterns. 
     In still another embodiment, a method of fabricating an inductor comprises a series of steps. First, a silicon substrate is selected, a layer of magnetic material is deposited on the substrate, and an insulating layer is deposited on the magnetic material layer. Next, a plurality of sandwich structures are stacked on the insulating layer. The method of fabricating the structures comprises the steps of depositing an inductor pattern on the insulating layer and above the magnetic material layer, depositing an insulating layer on the inductor pattern, depositing a layer of magnetic material on the insulating layer and above the inductor pattern, and depositing an insulating layer on the magnetic material layer. Finally, a conducting path is fabricated through the structures to connect each inductor pattern serially to the inductor pattern above, such that a current flowing in the serially connected inductor patterns creates a reinforcing magnetic field in the magnetic material between adjacent inductor patterns. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a cross-sectional view of one embodiment of a square spiral inductor embedded in a solid state structure. 
     FIG. 1B is a top view of one embodiment of a square spiral inductor pattern. 
     FIG. 2 is a cross-sectional view one embodiment of an inductor coupled to another electronic device in an integrated circuit. 
     FIG. 3A is a cross-sectional view of one embodiment of two vertically stacked inductors. 
     FIG. 3B is a cross-sectional view of one embodiment of two stacked and serially connected inductors showing the current in the inductors and the resulting magnetic field lines. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     Inductors intended for use in circuits fabricated on a silicon substrate usually operate at lower frequencies and require larger inductances than inductors intended for use in circuits fabricated on a gallium arsenide substrate. As mentioned above, a larger inductance is usually realized in silicon by having the inductor occupy a larger surface area. According to one embodiment of the present invention, rather than increasing the inductance by increasing the surface area occupied by the inductor, a larger inductance is achieved by adding a layer of magnetic material to the inductor. 
     Referring to FIG. 1A, a cross-sectional view of one embodiment of a square spiral inductor of the present invention is shown. Inductor  100  is formed on substrate  110  and comprises magnetic material layer  120 , insulating layer  130 , inductor pattern  140 , second insulating layer  150 , and second magnetic material layer  160 . Magnetic material layer  120  is deposited on substrate  110 , insulating layer  130  is deposited on magnetic material layer  120 , inductor pattern  140  is deposited on insulating layer  130 , second insulating layer  150  is deposited on inductor pattern  140 , and second magnetic material layer  160  is deposited on second insulating layer  150 . 
     Substrate  110 , in one embodiment, is a semiconductor, and even though the invention is not limited to a particular type of semiconductor, silicon is the preferred semiconductor substrate material. 
     Magnetic material layer  120 , in one embodiment, is deposited on the surface of substrate  110 . The particular magnetic material selected for use in a particular inductor design depends on the inductance requirement. In one embodiment, in which a large inductance in a small volume is desired, a high permeability ferromagnetic material, such as pure iron or a NiFe alloy is selected. An example of a high permeability NiFe alloy is an alloy of 81% Ni and 19% Fe. Electrically conducting films, such as an insulating magnetic oxide film, may also be suitable for use in the present invention. 
     Insulating layer  130  is deposited on magnetic material layer  120 . In one embodiment, insulating layer  130  is an inorganic silicon oxide film. In an alternate embodiment, insulating layer  130  is silicon dioxide. In still another embodiment, which is perhaps preferable in a low temperature processing environment, insulating layer  130  is an organic insulator, such as parylene and polyimide. 
     Inductor pattern  140  is deposited on insulating layer  130 . In one embodiment, inductor pattern  140  is a spiral. In an alternate embodiment, inductor pattern  140  is a circular spiral. In a second alternate embodiment, inductor pattern  140  is a polygonal spiral, where the polygonal spiral may be in the shape of a triangle, square, rectangle, octagon, or hexagon. A square spiral inductor pattern, which is shown as inductor pattern  140  in FIG. 1B, is preferred, since it is easy to manufacture. Inductor pattern  140  is fabricated from a high-conductivity material. In one embodiment the high-conductivity material is gold. In an alternate embodiment, the high-conductivity material is copper. 
     Referring to FIG. 1A, second insulating layer  150  is deposited on inductor pattern  140 , and is fabricated from the same materials as insulating layer  130 . 
     Second magnetic material layer  160  is deposited on second insulating layer  150 , and is fabricated from the same materials as magnetic material layer  120 . Second magnetic material layer  160  is preferably located above inductor pattern  140 , and second magnetic material layer  160  does not intersect the plane of magnetic material layer  160 . 
     Locating magnetic material layer  160  above inductor pattern  140  allows the contribution of the magnetic material to the inductance of the inductor to be precisely controlled during the manufacturing process. The thickness of the layer of magnetic material along with the magnetic properties of the material define the contribution of the layer to the inductance of the inductor. Once the properties of the material are established during the preparation of the material, the thickness of the layer, which can be precisely controlled in an integrated circuit manufacturing process, defines the contribution of the layer of magnetic material to the inductance. 
     In one embodiment, the inductor of the present invention is connected to other electronic devices in an integrated circuit. The inductor of the present invention is compatible with conventional silicon manufacturing processes. Structures for coupling passive devices, such as inductors, to other integrated circuit devices are known in the art. 
     Referring to FIG. 2, inductor  200  is coupled to device  210 . The coupling is accomplished by providing conducting path  220  from inductor pattern  230 , through vias  240 , to device  210 . 
     Referring to FIG. 3A, one embodiment of inductor structure  300 , which combines two inductors, is shown. Inductor structure  300  comprises base structure  305 , sandwich structure  310 , second sandwich structure  315 , and conducting path  320 . Base structure  305  includes substrate  325 , magnetic material layer  330 , and insulating layer  335 . Sandwich structure  310  includes inductor pattern  340 , insulating layer  345 , magnetic material layer  350 , and insulating layer  355 . Second sandwich structure  315  is stacked on sandwich structure  310 . Second sandwich structure  315  includes inductor pattern  360 , insulating layer  365 , magnetic material layer  370 , and insulating layer  375 . 
     Conducting path  320  couples sandwich structure  310  to second sandwich structure  315 , and serially connects inductor pattern  340  to inductor pattern  360 . A current flowing in the serially connected inductor patterns creates a reinforcing magnetic field in magnetic material layer  350 . Magnetic material layers  330  and  370  are located below inductor pattern  340  and above inductor pattern  360 , respectively. Magnetic material layers  330  and  370  confine the magnetic flux and noise radiated by a current flowing in inductor pattern  340  and inductor pattern  360  to the area bounded by the outer surfaces of magnetic material layers  330  and  370 . By stacking sandwich structures, in one embodiment, a large inductance can be created without increasing the surface area on a substrate occupied by the inductor. 
     Referring to FIG. 3B, a diagram showing the currents and the resulting reinforcing magnetic fields of the two inductor sandwich of FIG. 3A is shown. Current  375  flows in inductor pattern  380 , in conducting path  385 , and in inductor pattern  390 . The resulting magnetic field lines  395  are shown as reinforcing each other in magnetic material  398 , which corresponds to magnetic material layer  350  in FIG.  3 A. Magnetic field lines  395  are confined by magnetic material barrier layers  399 . 
     It is to be recognized that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.