Abstract:
Saturation of nonlinear ferromagnetic core material for on-chip inductors for high current applications is significantly reduced by providing a core design wherein magnetic flux does not form a closed loop, but rather splits into multiple sub-fluxes that are directed to cancel each other. The design enables high on-chip inductance for high current power applications.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to integrated circuit inductor structures and, in particular, to an on-chip inductor design for high current applications that significantly reduces saturation of nonlinear ferromagnetic core material. 
       DISCUSSION OF THE RELATED ART 
       [0002]    The ferromagnetic core elements of micro-fabricated on-chip inductors are currently designed such that the segmented laminations of the core elements provide a closed loop for magnetic flux. The advantage of this closed loop design is that it provides the highest possible inductance at low excitation current. The drawback of this commonly utilized approach is that magnetic flux quickly saturates the magnetic core, causing inductance to drop significantly as current increases. 
         [0003]    Many power electronics applications require inductors to carry high currents while also maintaining high inductance values. The core saturation problem becomes even more critical in the case of on-chip inductors because of strict area requirements and the complexity of the fabrication process for these structures. 
         [0004]    It would be highly beneficial to those attempting to incorporate inductors into integrated circuits, particularly circuits for hand-held devices such as cell phones and PDAS, to have available a technique for providing high on-chip inductance for high current applications. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention provides a magnetic core design for on-chip inductor structures in which the saturation of the nonlinear ferromagnetic core material is significantly reduced. This is accomplished by designing the core elements in such a way that the magnetic flux does not form a closed loop, but rather splits into multiple sub-fluxes that are directed to cancel each other. The core element design enables high on-chip inductance for high current applications. 
         [0006]    The features and advantages of the various aspects of the present invention will be more fully understood and appreciated upon consideration of the following detailed description of the invention and the accompanying drawings, which set forth illustrative embodiments in which the concepts of the invention are utilized. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]      FIGS. 1A and 1B  are cross section views illustrating two respective on-chip inductor structures in which the flux cancellation concepts of the present invention may be utilized. 
           [0008]      FIG. 2  is a top view illustrating a magnetic core element structure in accordance with the concepts of the present invention. 
           [0009]      FIGS. 3A-3C  are top views illustrating a bottom segmented magnetic core element, a conductive inductor coil and a top segmented magnetic core element, respectively, in accordance with the concepts of the present invention. 
           [0010]      FIG. 4  is a perspective drawing showing a simulated magnetic flux distribution in one L-shaped corner lamination of the  FIG. 2  magnetic core element structure under high current excitation. 
           [0011]      FIG. 5  shows an embodiment of alternate lamination design as a replacement for the standard closed loop laminations in the  FIG. 2  structure, in accordance with the concepts of the present invention. 
           [0012]      FIG. 6  provides saturation curves for a conventional closed loop four-turn square lamination inductor structure and for a four-turn square lamination inductor structure in accordance with the concepts of the present invention. 
           [0013]      FIG. 7  provides a top view of an embodiment of a lamination structure for a segmented magnetic core element in accordance with the concepts of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The present invention provides a design for the ferromagnetic core elements and conducting coil of an on-chip inductor. The magnetic core element design relies upon the principle of inducing magnetic flux in the core laminations to flow in different directions to further cancel each other in the meeting point. Since such a cancellation does not occur abruptly, but rather occupies non-zero volume where the magnitude of the magnetic induction vector decreases gradually, the material of this finite volume of core lamination is saturated at higher current than material in a conventional core lamination, which has a single direction of magnetic flux. The design trade-off for not using a closed loop for magnetic flux in the core material is lower inductance at very low current. 
         [0015]      FIGS. 1A and 1B  show cross section views of two on-chip inductor structures  100  and  110 , respectively, that are compatible with the concepts of the present invention. In the  FIG. 1A  structure  100 , a segmented top magnetic core element  102  and a segmented bottom magnetic core element  104  surround a conductive inductor coil  106  and touch each other. The inductor coil  106  is electrically insulated from both the top core element  102  and the bottom core element  104  by intervening dielectric material  108 . Large inductance can be made by the  FIG. 1A  configuration because reluctance is minimized. In the  FIG. 1B  inductor structure  110 , there is a finite gap (h) between the segmented top magnetic core element  112  and the segmented bottom magnetic core element  114  that surround the inductor coil  116 ; as in the case of the  FIG. 1A  structure, the coil  116  is insulated by dielectric material  118 . The magnetic path in this case is composed of the magnetic elements  112 ,  114  and the gap h. The total inductance can be adjusted in this case by changing the height h of the gap. Also, magnetic saturation due to high current levels can be controlled by the gap height h. In both the  FIG. 1A  and the  FIG. 1B  structures, the top and bottom core elements can be any ferromagnetic material (e.g., permalloy) and the conductive coil preferably comprises copper. 
         [0016]    As discussed above, in accordance with the present invention, the magnetic core elements of the inductor structures shown in  FIGS. 1A and 1B  are formed such that the magnetic flux in at least some of individual laminations of the segmented core elements flows in different directions to cancel each other in the meeting point.  FIG. 2  shows a four-turn square embodiment of a segmented ferromagnetic core element  200  in accordance with the concepts of the present invention shown. All L-shaped ferromagnetic laminations  202  in the four corners of the segmented core element  200  exploit the flux cancellation concepts of the present invention. The remaining laminations  204  provide a closed loop path for magnetic flux around the turns of the conducting coil (not shown). 
         [0017]      FIGS. 3A-3C  show top views of embodiments of segmented magnetic core elements and a conductive coil that are consistent with the inductor structures shown in  FIGS. 1A and 1B  and in accordance with the concepts of the present invention.  FIG. 3A  shows a top view of an embodiment of a bottom four-turn square magnetic core element  300  in accordance with the invention.  FIG. 3B  shows a top view of an embodiment of a conductive inductor coil  302 .  FIG. 3C  shows a top view of an embodiment of a top four-turn square magnetic core element  304  in accordance with the invention. 
         [0018]      FIG. 4  shows simulated magnetic flux distribution in an L-shaped corner lamination  400  under high current conditions. Those skilled in the art will appreciate that the top lamination  402  and the bottom lamination  404  are shown in  FIG. 4 , but the inductor coil is not. The dark shading (e.g. Point A) in  FIG. 4  means that the ferromagnetic core material is saturated (e.g., S{I }=1.00667c+00 to 1.0007c+00) at that particular point. The non-zero volume of the unsaturated (e.g., S{I}=1.4209c-01 to 1.0000c-02) core material is also shown by lighter shading (e.g., Point B). 
         [0019]    As shown in  FIG. 5 , the standard closed loop laminations  204  of the  FIG. 2  four-turn square core element structure  200  can be replaced by, for example, dual U-shaped ferromagnetic lamination structures  500  that take advantage of the flux cancellation concepts of the present invention. Those skilled in the art will appreciate that the non-zero volume of the unsaturated magnetic core material will occur in the region of the meeting point (Point C) of the laminations  500  in the  FIG. 5  embodiment. Those skilled in the art will also appreciate that other flux cancellation designs are also utilizable and within the scope of the present invention. 
         [0020]      FIG. 6  shows saturation curves for two different structures of a four-turn square inductor: one structure utilizes the conventional closed loop lamination design while the other structure utilizes flux cancellation laminations of the type discussed above in accordance with the invention. Both inductors use the same ferromagnetic core material and occupy the same area on a chip. As can be seen from  FIG. 6 , the inductance of the inductor that utilizes flux cancellations laminations in accordance with the concepts of the invention is larger at higher currents. 
         [0021]    Since the magnetic field is smaller in the vicinity of the cancellation area, the techniques of the present invention induce less eddy currents than the standard closed loop lamination, thereby improving the high frequency behavior of on-chip inductors that incorporate these concepts. 
         [0022]    A more advanced embodiment of a flux cancellation lamination structure in accordance with the invention is shown in  FIG. 7 , wherein a top view of the laminations is provided. A bottom view of the laminations is similar. 
         [0023]    It should be understood that the particular embodiments of the invention described above have been provided by way of example and that other modifications may occur to those skilled in the art without departing from the scope and spirit of the invention as expressed in the appended claims and their equivalents.