Patent Publication Number: US-2021193367-A1

Title: Integrated stack transformer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to integrated stack transformers. 
     2. Description of the Prior Art 
     In a combination circuit of an integrated stack transformer and a power amplifier, a common mode inductance of the integrated stack transformer is associated with third-order intermodulation distortion (IMD3), i.e. the greater the common mode inductance, the worse the IMD3. Thus, for the purpose of improving signal quality, design of an integrated stack transformer with a lower common mode inductance is an important issue. 
     SUMMARY OF THE INVENTION 
     This in mind, an objective of the present invention is to provide an integrated stack transformer with a lower common mode inductance, to solve the problems mentioned in the related art. 
     In an embodiment of the present invention, an integrated stack transformer is provided. The integrated stack transformer comprises a first winding, a second winding and a third winding implemented by a first metal layer, and a fourth winding and a fifth winding implemented by a second metal layer. The second winding is positioned between the first winding and the third winding. The fourth winding substantially overlaps the first winding. The fifth winding substantially overlaps the third winding. A distance between the fifth winding and the fourth winding is less than a distance between the third winding and the first winding. In addition, the first winding, the third winding, the fourth winding and the fifth winding form a part of one of a primary inductor and a secondary inductor of the integrated stack transformer, and the second winding is a part of the other of the primary inductor and the secondary inductor. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a circuit according to an embodiment of the present invention. 
         FIG. 2A  is a diagram illustrating a first metal layer of an integrated stack transformer according to an embodiment of the present invention. 
         FIG. 2B  is a diagram illustrating a second metal layer of an integrated stack transformer according to an embodiment of the present invention. 
         FIG. 2C  is a diagram illustrating a bridge metal layer of an integrated stack transformer according to an embodiment of the present invention. 
         FIG. 2D  is a top view of an integrated stack transformer according to an embodiment of the present invention. 
         FIG. 3  illustrates a cross section of an integrated stack transformer according to an embodiment of the present invention. 
         FIG. 4  illustrates a cross section of an integrated stack transformer according to another embodiment of the present invention. 
         FIG. 5  illustrates a cross section of an integrated stack transformer according to another embodiment of the present invention. 
         FIG. 6  illustrates a cross section of an integrated stack transformer according to another embodiment of the present invention. 
         FIG. 7  illustrates a cross section of an integrated stack transformer according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagram illustrating a circuit  100  according to an embodiment of the present invention. As shown in  FIG. 1 , the circuit  100  comprises an integrated stack transformer  110  and a power amplifier  120 , wherein the integrated stack transformer  110  comprises a primary inductor  112  and a secondary inductor  114  that are electrically isolated from each other. The primary inductor  112  comprises two input nodes Nin 1  and Nin 2  of the integrated stack transformer  110 , and the secondary inductor  114  comprises two output nodes Nout 1  and Nout 2  of the integrated stack transformer  110 . In this embodiment, the integrated stack transformer  110  is a balanced to unbalanced (balun) transformer, but the present invention is not limited thereto. 
     Refer to  FIG. 2A , which is a diagram illustrating a first metal layer of the integrated stack transformer  110 . As shown in  FIG. 2A , the first metal layer comprises an outermost winding  210 , a first winding  211 , a second winding  212 , a third winding  213  and an innermost winding  214 . In this embodiment, the first winding  211  and the third winding  213  act as a part of the primary inductor  112 , and the outermost winding  210 , the second winding  212  and the innermost winding  214  act as a part of the secondary inductor  114 . 
     Refer to  FIG. 2B , which is a diagram illustrating a second metal layer of the integrated stack transformer  110 . As shown in  FIG. 2A , the second metal layer comprises a fourth winding  224 , a fifth winding  225  and bridge wires  228  and  229 . The fourth winding  224  substantially overlaps the first winding  211  of the first metal layer, a width of the fourth winding  224  is greater than that of the first winding  211 , and the fourth winding  224  partially overlaps the second winding  212 . The fifth winding  225  substantially overlaps the third winding  213  of the first metal layer, a width of the fifth winding  225  is greater than that of the third winding  213 , and the fifth winding  225  partially overlaps the second winding  212 . In this embodiment, the fourth winding  224  and the fifth winding  225  are electrically connected to the first winding  211  and the third winding  213  through vias, respectively, i.e. the fourth winding  224  and the fifth winding  225  also act as a part of the primary inductor  112 . 
       FIG. 2C  illustrates a bridge metal layer, which is configured to connect the outermost winding  210  and the second winding  212  of the first metal layer. In addition,  FIG. 2D  further illustrates a top view of the integrated stack transformer  110 . 
     In this embodiment, the first metal layer is an Ultra-Thick Metal (UTM) layer, the second metal layer is an Aluminum Re-Distribution Layer (RDL), and the bridge metal layer may be any suitable metal layer, but the present invention is not limited thereto. In this embodiment, the first winding  211 , the third winding  213 , the fourth winding  224  and the fifth winding  225  act as a part of the primary inductor  112  of the integrated stack transformer  110 , and the outermost winding  210 , the second winding  121  and the innermost winding  214  act as a part of the secondary inductor  114 . In other embodiments, however, the first winding  211 , the third winding  213 , the fourth winding  224  and the fifth winding  225  may act as a part of the secondary inductor  114 , and the outermost winding  210 , the second winding  212  and the innermost winding  214  may act as a part of the primary inductor  112 , but the present invention is not limited thereto. 
       FIG. 3  is a cross-sectional diagram illustrating an A-A′ cross section of  FIG. 2D . As shown in  FIG. 3 , by increasing the width of the fourth winding  224  to make the fourth winding  224  partially overlap the second winding  212 , and by increasing the width of the fifth winding  225  to make the fifth winding  225  partially overlap the second winding  212 , the distance between the fourth winding  224  and the fifth winding  225  may be reduced to thereby further increase a mutual inductance of the primary inductor  112 . As the mutual inductance of the primary inductor  112  is increased, a common mode inductance of the primary inductor  112  may be reduced, and third-order intermodulation distortion (IMD3) of the circuit  100  can be improved to enhance signal quality. In addition, in  FIG. 3 , the first winding  211 , the third winding  213 , the fourth winding  224  and the fifth winding  225  belonging to the primary inductor  112  are annotated with “P”, and the outermost winding  210 , the second winding  212  and the innermost winding  214  belonging to the second inductor  114  are annotated with “S”. 
     In one embodiment, for the purpose of increasing the mutual inductance of the primary inductor  112  as much as possible, the distance between the fourth winding  224  and the fifth winding  225  may be reduced as much as possible, e.g. the distance between the fourth winding  224  and the fifth winding  225  is the minimum spacing allowed to be used in a process for the second metal layer. For example, assuming that the second metal layer is an Aluminum RDL, the distance between the fourth winding  224  and the fifth winding  225  may be approximately 2 micrometers (um). 
     It should be noted that the distance between the fourth winding  224  and the fifth winding  225  of the embodiments shown in  FIG. 2A  to  FIG. 2D  are for illustrative purposes only, and are not limitations of the present invention. In other embodiments, as long as the distance between the fourth winding  224  and the fifth winding  225  is less than a distance between the third winding  213  and the first winding  211 , the fourth winding  224  does not have to overlap the second winding  212 , and/or the fifth winding  225  does not have to overlap the second winding  212 . In addition, the width of the fourth winding  224  and the width of the fifth winding  225  do not have to be the same, e.g. widths of two windings implemented by the second metal layer shown in  FIG. 4  are different. These alternative designs also belong to the scope of the present invention. 
     In the embodiments shown in  FIG. 2A  to  FIG. 2D , the fourth winding  224  and the fifth winding  225  implemented by the second metal layer are configured to increase the mutual inductance of the primary inductor  112 . In other embodiments, multiple windings implemented by the second metal layer may be configured to increase a mutual inductance of the secondary inductor  114 ; for example, those shown in the cross-sectional diagram of  FIG. 5 . As those skilled in this art can understand the implementation shown in  FIG. 5  according to the embodiments of  FIG. 2A  to  FIG. 2D , related details are omitted for brevity. 
     In the embodiments shown in  FIG. 2A  to  FIG. 2D , the second metal layer is implemented above the first metal layer. In some embodiments, the second metal layer may be implemented below the first metal layer. For example, the fourth winding  224  and the fifth winding  225  shown in  FIG. 2A  to  FIG. 2D  may be implemented by a third metal layer shown in  FIG. 6 , i.e. the fourth winding  224  and the fifth winding  225  belonging to the primary inductor  112  may be replaced with a fourth winding  624  and a fifth winding  625  shown in  FIG. 6 , where the third metal layer may be any metal layer implemented below the UTM layer (i.e. a process time of the third metal layer is earlier than a process time of the UTM layer). 
     In another embodiment of the present invention, the embodiments shown in  FIG. 2A  to  FIG. 2D  and  FIG. 6  may be combined to utilize the second metal layer and the third metal layer to implement multiple windings of the primary inductor  112  as shown in  FIG. 7 , and the mutual inductance of the primary inductor  112  may be further enhanced. In detail, in the embodiment of the present invention, two windings implemented by the third metal layer may be regarded as a sixth winding  736  and a seventh winding  737  of the primary inductor  112 , and a distance between the sixth winding  736  and the seventh winding  737  is less than the distance between the first winding  211  and the third winding  213 . In another embodiment, the distance between the sixth winding  736  and the seventh winding  737  may be further designed to be the minimum spacing allowed to be used in a process for the third metal layer. 
     Briefly summarized, the integrated stack transformer of the present invention implements multiple windings with small spacing by utilizing the second metal layer. This can effectively increase the mutual inductance of the primary/secondary inductor and reduce the common mode inductance, to thereby improve the IMD3 of the circuit  100  and enhance the signal quality. In addition, implementations of the embodiments of the present invention will not require additional chip area of the integrated stack transformer, so the signal quality can be improved without increasing manufacturing costs. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.