Abstract:
A plurality of sequential electro-deposition, planarization and insulator deposition steps are performed over a patterned thick photoresist film to form a laminated ferromagnetic alloy core for micro-fabricated inductors and transformers. The use of a plurality of contiguous thin laminations within deep patterns on non-removable photoresist film provides sufficient volume of magnetic film in, for example, high frequency applications, and reduces eddy current loss at high frequency.

Description:
RELATED APPLICATION 
       [0001]    This application is related to co-pending and commonly-assigned U.S. patent application Ser. No. [Attorney Docket No. NSC1-N5000 [P07683]], filed on Sep. 12, 2011, and titled “A Method of Selectively Etching a Conductive Seed Layer in the Damascene Electroplating of Magnetic Alloy Laminations.” Application Ser. No. [Attorney Docket No NSC1-N5000 [P07682]]. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to integrated circuit technology and, in particular, to the formation of a ferromagnetic alloy core for high frequency micro-fabricated inductors and transformers utilizing multiple electro-deposition and chemical mechanical polishing (CMP) steps over a patterned thick photoresistive film. 
       BACKGROUND OF THE INVENTION 
       [0003]    Among the known methods for forming laminations in micro-fabricated high frequency inductors and transformers, the utilization of thick, non-removable photoresist (PR) is the most efficient since it provides a high volume of magnetic material while maintaining the small thickness of the film, which is important for minimizing eddy current losses. 
         [0004]      FIG. 1  shows a patterned layer of non-removable negative photoresist  100  formed on a substrate  102 , e.g., a semiconductor substrate such as crystalline silicon. The patterned photoresist layer  100  includes a number of vias  104  formed to expose upper surface regions of the substrate  102 . As shown in  FIG. 1 , each via  104  is lined on its bottom surface and sidewalls with copper seed material  106 . A magnetic alloy lamination  108  is formed on the copper seed material  106 . 
         [0005]    With the continuous increase in frequency of operation in many IC applications (e.g., switching frequency of a buck converter), skin effect contribution into the power loss becomes more pronounced. An obvious solution is plating thinner magnetic films. However, this has the undesirable effect of reducing the cross-sectional area of the magnetic core which, in turn, linearly reduces the inductance. 
       SUMMARY OF THE INVENTION 
       [0006]    In a disclosed embodiment, sequential electro-deposition, planarization and insulator deposition steps are performed over a patterned thick photoresist film to form a laminated ferromagnetic alloy core for micro-fabricated inductors and transformers. The use of a plurality of contiguous thin laminations within deep patterns in non-removable photoresist film provides sufficient volume of magnetic material in, for example, high frequency applications, and reduces eddy current loss at high frequency. 
         [0007]    The features and advantages of the various aspects of the subject matter disclosed herein will be more fully understood and appreciated upon consideration of the following detailed description and accompanying drawings, which set forth illustrative embodiments in which the concepts of the claimed subject matter are utilized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a cross section drawing illustrating a magnetic alloy core. 
           [0009]      FIGS. 2A-2D  are cross section drawings illustrating steps in a method of plating multiple thin magnetic film layers in a magnetic alloy core. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual implementation, numerous specific decisions must be made to achieve the designer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
         [0011]    The present subject matter will now be described with reference to the attached drawings. Various structures and methods are schematically depicted in the drawings for purposes of explanation only and so as not to obscure the present disclosure with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe illustrative embodiments of the present disclosure. The words and phrases utilized herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by those skilled in the art, such a special meaning will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. 
         [0012]    As stated above, generally, the present disclosure provides methods for the formation of a ferromagnetic alloy core for high frequency micro-fabricated inductors and transformers utilizing multiple electro-deposition and chemical mechanical polishing (CMP) steps over a patterned thick film photorestive film. With reference to  FIGS. 2A-2D , an illustrative embodiment will now be described in detail. 
         [0013]      FIGS. 2A-2D  illustrate a method of forming a laminated ferromagnetic core for use, for example, in high frequency inductors and transformers by sequentially plating a plurality of contiguous ferromagnetic laminations, separated by dielectric material, without depositing, exposing and developing new photoresist film for each ferromagnetic lamination. The method illustrated in  FIGS. 2A-2D  provides two ferromagnetic laminations. However, those skilled in the art will appreciate that the concept can be extended to a larger number of ferromagnetic laminations. 
         [0014]      FIG. 2A  shows the starting point for fabrication of the disclosed embodiment, which is similar to  FIG. 1  except that the deposited ferromagnetic lamination is thinner and the seam inside the lamination is wider so that further processing will not be limited by the aspect ratio. More specifically, a patterned layer of non-removable negative photoresist  200 , examples of which are well known to those skilled in the art (such as, for example, with the use of photo-imageable SU8 or BCB or polyimide or PBO epoxy systems) is formed on an underlying substrate  202 , of silicon, glass, metal or laminate board, e.g., a semiconductor substrate such as crystalline silicon. The patterned photoresist layer  200  has a plurality of vias  204  formed therein. In the illustrated embodiment, each via  204  extends from the upper surface of the photoresist layer  200  to the upper surface of the substrate  202 . A first conductive seed material layer (e.g. copper or Ti/Cu) is formed on the upper surface of the photoresist layer  200  and on the sidewalls and bottom surface of each via  204  by, for example, sputtering or atomic layer deposition (ALD). A first ferromagnetic material layer (e.g., NiFe, permalloy) is then formed on the conductive seed material layer by way of, for example, electrochemical deposition (ECD). As shown in  FIG. 2A , the first ferromagnetic material layer and the first conductive seed material layer are then planarized (for example, by chemical mechanical polishing (CMP)) to remove the first ferromagnetic material layer and the first conductive seed material layer from the upper surface of the photoresist layer  200  to define a first ferromagnetic lamination  206  in each of the vias, the first ferromagnetic lamination  206  including a conductive seed material layer  208  formed on the sidewalls and bottom surface of the via  204  and a ferromagnetic material layer  210  formed on the conductive seed material layer  208 . 
         [0015]    Next, as shown in  FIG. 2B , a dielectric material layer  212  (e.g. silicon oxide) is formed, utilizing well known techniques, on the exposed upper surface of the photoresist layer  200  and on the surface of each first ferromagnetic lamination  206 . A second conductive seed material layer  214  (e.g., copper or Ti/Cu) is then formed by, for example, sputtering or ALD, on the dielectric material layer  212 , as shown in  FIG. 2C . This is followed by the formation of a second ferromagnetic material layer (e.g., NiFe, permalloy) on the second conductive seed material layer  214 . 
         [0016]    Referring to  FIG. 2D , a planarization step (e.g., CMP) is then performed to remove the second ferromagnetic layer, the second conductive seed material layer  214  and the dielectric material layer  212  from the upper surface of the photoresist layer  200 , thereby defining a second ferromagnetic lamination  216  in each of the vias  204  that is contiguous with the first ferromagnetic lamination  206  and separated from the first ferromagnetic lamination by the dielectric layer  212 . As shown in  FIG. 2D , each second ferromagnetic lamination  216  includes a conductive seed material layer  214  and a second ferromagnetic material layer  218  formed on the second conductive seed material layer  214 . 
         [0017]    Those skilled in the art will appreciate that, as stated above, additional ferromagnetic laminations may be formed in each of the vias  204  by the sequential formation of a dielectric material layer, conductive material seed layer and ferromagnetic material layer and subsequent planarization thereof, as described in conjunction with  FIGS. 2A-2D . 
         [0018]    It should be understood that the particular embodiments described herein have been provided by way of example and that other modifications may occur to those skilled in the art without departing from the scope of the claimed subject matter as expressed in the appended claims and their equivalents.