Patent Publication Number: US-2021175006-A1

Title: Asymmetric spiral inductor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to integrated inductors, and, more particularly, to asymmetric spiral integrated inductors. 
     2. Description of Related Art 
       FIG. 1  and  FIG. 2  respectively show an asymmetric spiral inductor and a symmetric spiral inductor of conventional types. The asymmetric spiral inductor  100  and the symmetric spiral inductor  200  are planar structures. The symmetric spiral inductor  200  is mainly made up of conductor segments in two conductor layers, which are respectively represented in gray and black. The conductor segments in different conductor layers are connected by through structures  105 , such as a via structure or a via array in a semiconductor process. In general, the symmetric spiral inductor  200  is suitable for differential signals because it is symmetric in structure, while the asymmetric spiral inductor  100  is suitable for single-ended signals. 
     One of the approaches to increase the inductance value of the asymmetric spiral inductor  100  and the symmetric spiral inductor  200  is to increase their numbers of turns. In addition to an increase in the area of the asymmetric spiral inductor  100  and the symmetric spiral inductor  200 , the increase in the numbers of turns results in increases in parasitic series resistance and parasitic capacitance as well. High parasitic series resistance and parasitic capacitance cause the self-resonant frequency and the quality factor Q of the asymmetric spiral inductor  100  and the symmetric spiral inductor  200  to decrease. In addition, metal loss and substrate loss are also key factors that affect the quality factor Q. Metal loss is due to the resistance of the metal itself. There are two reasons for substrate loss. The first reason is that when the inductor is in operation, a time-varying electric displacement between the metal coil of the inductor and the substrate is generated; this electric displacement results in a displacement current between the metal coil and the substrate, and this displacement current penetrates into the substrate of low impedance, thereby causing energy loss. The displacement current is associated with the coil area of the inductor. The larger the area, the larger the displacement current. The second reason is that the time-varying electromagnetic field of the inductor penetrates through the dielectric and generates a magnetically induced eddy current on the substrate. The magnetically induced eddy current and the inductor current are opposite in directions, resulting in energy loss. 
     When the inductor is operated at low frequencies, the current in the metal coil is evenly distributed. In this case, the metal loss at low frequencies is due to the series resistance of the metal coil. When the inductor is operated at high frequencies, the metal coil closer to the inner turns generates stronger magnetic field; a strong magnetic field induces eddy currents in the inner turns of the metal coil. The eddy currents cause uneven distribution of currents—most of the currents are pushed to the surface of the metal coil; this phenomenon is known as the skin effect. Because the currents pass through a smaller metal cross section in the skin effect, the currents encounter a greater resistance, thereby resulting in decrease in the quality factor Q. 
     Therefore, it is important in the art to improve the quality factor Q and the inductance value of the inductor without increasing the inductor area. 
     SUMMARY OF THE INVENTION 
     In view of the issues of the prior art, an object of the present invention is to provide asymmetric spiral inductors, so as to make an improvement to the prior art. 
     An asymmetric spiral inductor is provided. The asymmetric spiral inductor includes a first winding, a second winding and a third winding. The first winding has a first end and a second end and is implemented in an ultra-thick metal (UTM) layer of a semiconductor structure. The second winding, which has a third end and a fourth end, is implemented in a re-distribution layer (RDL) of the semiconductor structure and has a first maximum trace width. The third winding, which has a fifth end and a sixth end, is implemented in the UTM layer of the semiconductor structure and has a second maximum trace width smaller than the first maximum trace width. The second end and the third end are connected through a first through structure, the fourth end and the fifth end are connected through a second through structure, and the first end and the sixth end are two ends of the asymmetric spiral inductor. 
     An asymmetric spiral inductor is provided. The asymmetric spiral inductor includes a spiral coil, a first trace and a second trace. The spiral coil has a first end and a second end and is implemented in a first conductor layer of a semiconductor structure. The first trace has a third end and a fourth end and is implemented in a second conductor layer of the semiconductor structure. The first conductor layer is different from the second conductor layer, and a length of the first trace is less than one turn of the spiral coil. The second trace has a fifth end and a sixth end and is implemented in the first conductor layer of the semiconductor structure. The second end and the third end are connected through a first through structure, the fourth end and the fifth end are connected through a second through structure, and the first end and the sixth end are two ends of the asymmetric spiral inductor. 
     The asymmetric spiral inductors of the present invention are implemented in two different conductor layers in a semiconductor structure. In comparison with the traditional technology, the present invention can increase the inductance value of the asymmetric spiral inductors without increasing the inductor area, and, therefore, the quality factor Q is improved. 
     These and other objectives of the present invention no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiments with reference to the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conventional asymmetric spiral inductor. 
         FIG. 2  is a conventional symmetric spiral inductor. 
         FIG. 3A  shows an illustrative four-turn asymmetric spiral inductor. 
         FIG. 3B  shows another illustrative four-turn asymmetric spiral inductor. 
         FIG. 4A  shows the winding  410  of  FIG. 3B . 
         FIG. 4B  shows the winding  420  of  FIG. 3B . 
         FIG. 4C  shows the winding  430  of  FIG. 3B . 
         FIG. 5  shows the cross-sectional side view A-A of  FIG. 3B  according to an embodiment. 
         FIG. 6  shows the cross-sectional side view A-A of  FIG. 3B  according to another embodiment. 
         FIG. 7  shows the cross-sectional side view A-A of  FIG. 3B  according to another embodiment. 
         FIG. 8A  shows an illustrative asymmetric spiral inductor according to another embodiment of the present invention. 
         FIG. 8B  is a cross-sectional side view B-B of  FIG. 8A . 
         FIG. 9A  is an illustrative asymmetric spiral inductor according to another embodiment of the present invention. 
         FIG. 9B  is the cross-sectional side view C-C of  FIG. 9A . 
         FIG. 10A  is an illustrative asymmetric spiral inductor according to another embodiment of the present invention. 
         FIG. 10B  is the cross-sectional side view D-D of  FIG. 10A . 
         FIG. 11  shows the quality factor Q of the asymmetric spiral inductor  300  and the asymmetric spiral inductor  400 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following description is written by referring to terms of this technical field. If any term is defined in this specification, such term should be interpreted accordingly. 
     The disclosure herein includes asymmetric spiral inductors. On account of that some or all elements of the asymmetric spiral inductors could be known, the detail of such elements is omitted provided that such detail has little to do with the features of this disclosure, and that this omission nowhere dissatisfies the specification and enablement requirements. A person having ordinary skill in the art can choose components equivalent to those described in this specification to carry out the present invention, which means that the scope of this invention is not limited to the embodiments in the specification. 
       FIGS. 3A and 3B  each show a top view or a bottom view of a four-turn asymmetric spiral inductor. The asymmetric spiral inductor  300  of  FIG. 3A  is implemented in the first conductor layer or the second conductor layer of the semiconductor structure, and the asymmetric spiral inductor  400  of  FIG. 3B  is implemented in the first conductor layer and the second conductor layer of the semiconductor structure. The first conductor layer and the second conductor layer can be any two different conductor layers of the semiconductor structure. For example, the first conductor layer can be one of the ultra-thick metal (UTM) layer and the re-distribution layer (RDL), and the second conductor layer is the other. 
     As shown in  FIG. 3A , the asymmetric spiral inductor  300  is formed by a single winding  310 . In other words, the winding  310  itself is the asymmetric spiral inductor  300 . The winding  310  can be regarded as being formed by or made up of a single trace. 
     As shown in  FIG. 3B , the asymmetric spiral inductor  400  is formed by three windings: the winding  410 , the winding  420  and the winding  430 . The winding  410  and the winding  430  are implemented in the first conductor layer, and the winding  420  is implemented in the second conductor layer. The winding  410 , the winding  420  and the winding  430  are connected via the through structure  401  and the through structure  402 . More specifically, the through structure  401  connects one end of the winding  420  with the winding  410 , and the through structure  402  connects the other end of the winding  420  with the winding  430 . The winding  420  extends along the edge of the asymmetric spiral inductor  400  or the winding  430 . As a result, the shape of the winding  420  is similar to a part of the contour of the asymmetric spiral inductor  400  and/or the winding  430 . 
     As shown in  FIGS. 3A and 3B , the asymmetric spiral inductor  300  and the asymmetric spiral inductor  400  are both four-turn spiral inductors, but differ in that the entire trace of the asymmetric spiral inductor  300  is implemented in the same conductor layer, whereas most of the traces of the asymmetric spiral inductor  400  are implemented in the first conductor layer and some of the traces of the asymmetric spiral inductor  400  (i.e., the winding  420 ) are implemented in the second conductor layer. In other words, the asymmetric spiral inductor  300  is a planar structure, and the asymmetric spiral inductor  400  is a three-dimensional structure. As a result, under the premise of having the same number of turns and the same outer diameter D 1  (i.e., under the premise that the areas of the two inductors are substantially the same), the inner diameter D 3  of the asymmetric spiral inductor  400  is larger than the inner diameter D 2  of the asymmetric spiral inductor  300 , causing the quality factor Q of the asymmetric spiral inductor  400  to be higher than the quality factor Q of the asymmetric spiral inductor  300 . 
       FIGS. 4A, 4B and 4C  show the winding  410 , the winding  420  and the winding  430 , respectively. The winding  410  is a trace or a coil whose length is approximately a half turn of the asymmetric spiral inductor  400 . The winding  420  is a trace or a coil whose length is approximately one turn of the asymmetric spiral inductor  400 . The winding  430  forms an asymmetric spiral coil. The two ends of the winding  410  are the end  411  and the end  412 , the two ends of the winding  420  are the end  421  and the end  422 , and the two ends of the winding  430  are the end  431  and the end  432 . The end  411  is one of the ends of the asymmetric spiral inductor  400 , the end  412  is connected to the end  421  via the through structure  401 , the end  422  is connected to the end  431  via the through structure  402 , and the end  432  is the other end of the asymmetric spiral inductor  400 . In this embodiment, the length of the trace of the winding  410  is approximately a half turn of the asymmetric spiral inductor  400  or the winding  430 , but, in other embodiments, the length can be longer (e.g., ¾ turn, one turn or multiple turns) or shorter (e.g., less than or equal to ¼ turn). In this embodiment, the length of the trace of the winding  420  is approximately one turn of the asymmetric spiral inductor  400  or the winding  430 , but, in other embodiments, the length can be longer (e.g.,  1 . 5  turns or more) or shorter (e.g., less than one turn). In this embodiment, the winding  430  is a multi-turn structure, preferably greater than or equal to one turn. 
       FIG. 5  shows the cross-sectional side view A-A of  FIG. 3B  according to one embodiment. In this embodiment, the widths of the traces of the windings  410 ,  420  and  430  are all W 1 . On the left side of the figure, a part of the trace of the winding  420  completely or partially overlaps with a part of the trace of the winding  430 , and on the right side of the figure, a part of the trace of the winding  420  partially overlaps with a part of the trace of the winding  410  and with a part of the trace of the winding  430 . There is mutual inductance Lm between the winding  420  and its adjacent winding  410  and/or winding  430 . 
       FIG. 6  shows the cross-sectional side view A-A of  FIG. 3B  according to another embodiment. In this embodiment, the widths of the traces of the winding  410  and the winding  430  are both W 1  (in other words, the maximum trace width of the winding  410  and the winding  430  is W 1 ), and the maximum trace width of the winding  420  is W 2  or W 3 . When W 2  is the same as W 3 , the trace of the winding  420  is uniform in width. When W 2  is different from W 3 , the width of the trace of the winding  420  is not uniform. Compared to  FIG. 5 , because the overlap between the winding  420  and the winding  410  and/or the winding  430  becomes larger (i.e., W 2  and/or W 3  being greater than W 1 ), the mutual inductance Lm′ between the winding  420  and the winding(s)  410  and/or  430  is greater than the mutual inductance Lm in the embodiment of  FIG. 5 . In other words, compared to the embodiment of  FIG. 5 , the inductor of  FIG. 6  has a higher inductance value. 
     Typically, the unit resistance value of the RDL is greater than that of the UTM layer. Thus, when the first conductor layer is the UTM layer and the second conductor layer is the RDL, the larger width(s) W 2  and/or W 3  (compared to W 1 ) can lead to a lower resistance value of the winding  420 . Therefore, despite being implemented in a conductor layer of a larger unit resistance value, the overall resistance value of the winding  420  may not become larger (compared to the resistance value when the winding  420  is implemented in the UTM layer and has a width of W 1 ) due to the increase in the trace width. 
       FIG. 7  shows another embodiment of the cross-sectional side view A-A of  FIG. 3B . Of the trace of the winding  430 , the part that overlaps or partially overlaps with the winding  420  may have a larger width than the part that does not overlap with the winding  420  (i.e., W 4 &gt;W 1 ); therefore, the mutual inductance Lm″ between the winding  420  and the winding  430  is greater than the mutual inductance Lm′ of the embodiment of  FIG. 6 . In some embodiments, the winding  420  and the winding  430  may overlap by more than one turn, for example, by two turns or more. In other words, compared to the embodiment of  FIG. 6 , the inductor of  FIG. 7  has a higher inductance value. The outer diameter of the inductor of  FIG. 7  is the same as the outer diameter of the inductor of  FIG. 6  (both D 1 ), but the inner diameter D 4  of the inductor of  FIG. 7  is smaller than the inner diameter D 3  of the inductor of  FIG. 6  because the trace of a part of the winding  430  in  FIG. 7  is wider. In the embodiment of  FIG. 7 , the width W 5  of a part of the trace of the winding  420  is greater than the sum of the width W 4  of a part of the trace of the winding  430  and the spacing D 5  between two adjacent turns of the winding  430 . 
       FIG. 8A  is a top view or a bottom view of an asymmetric spiral inductor according to an embodiment of the present invention, and  FIG. 8B  shows the cross-sectional side view B-B of  FIG. 8A . Like the asymmetric spiral inductor  400 , the asymmetric spiral inductor  800  is also a four-turn structure and includes a winding  810 , a winding  820  and a winding  830 . The winding  810  and the winding  830  are implemented in the first conductor layer, and the winding  820  is implemented in the second conductor layer. The winding  810 , the winding  820  and the winding  830  are connected via the through structure  801  and the through structure  802 . More specifically, the through structure  801  connects one end of the winding  820  with the winding  810 , and the through structure  802  connects the other end of the winding  820  with the winding  830 . The winding  820  extends along the edge of the asymmetric spiral inductor  800  or the winding  830 . As a result, the shape of the winding  820  is similar to the partial contour of the asymmetric spiral inductor  800  and/or the winding  830 . The end  811  and the end  832  are two ends of the asymmetric spiral inductor  800 . 
     As shown in  FIGS. 8A and 8B , a part of the winding  820  overlaps in part with one of the turns of the winding  830  (the outermost turn in this embodiment). There is mutual inductance Lm between the winding  820  and its adjacent winding  830 . Please refer to  FIGS. 5 to 7 , the widths of the traces of the windings  810 ,  820  and  830  are not limited to the example shown in  FIG. 8B . 
       FIG. 9A  is a top view or a bottom view of an asymmetric spiral inductor according to another embodiment of the present invention, and  FIG. 9B  shows the cross-sectional side view C-C of  FIG. 9A . Like the asymmetric spiral inductor  400 , the asymmetric spiral inductor  900  is also a four-turn structure and includes a winding  910 , a winding  920  and a winding  930 . The winding  910  and the winding  920  are implemented in the first conductor layer, and the winding  930  is implemented in the second conductor layer. The winding  910 , the winding  920  and the winding  930  are connected via the through structure  901  and the through structure  902 . More specifically, the through structure  901  connects one end of the winding  920  with the winding  910 , and the through structure  902  connects the other end of the winding  920  with the winding  930 . The winding  920  extends along the edge of the asymmetric spiral inductor  900  or the winding  930 . As a result, the shape of the winding  920  is similar to the partial contour of the asymmetric spiral inductor  900  and/or the winding  930 . The end  911  and the end  932  are two ends of the asymmetric spiral inductor  900 . 
     As shown in  FIGS. 9A and 9B , the winding  920  does not overlap with the winding  910  and the winding  930 . The winding  920  is located at the outermost turn of the asymmetric spiral inductor  900 , and the length of the trace of the winding  920  is approximately a half turn of the asymmetric spiral inductor  900  or the winding  930 . Please refer to  FIGS. 5 to 7 , the widths of the traces of the windings  910 ,  920  and  930  are not limited to the example shown in  FIG. 9B . 
       FIG. 10A  is a top view or a bottom view of an asymmetric spiral inductor according to another embodiment of the present invention, and  FIG. 10B  shows the cross-sectional side view D-D of  FIG. 10A . Like the asymmetric spiral inductor  400 , the asymmetric spiral inductor  1000  is also a four-turn structure and includes a winding  1010 , a winding  1020 , a winding  1030  and a winding  1040 . The winding  1020  and the winding  1040  are implemented in the first conductor layer, and the winding  1010  and the winding  1030  are implemented in the second conductor layer. The winding  1010 , the winding  1020 , the winding  1030  and the winding  1040  are connected via the through structure  1001 , the through structure  1002  and the through structure  1003 . The winding  1010  and the winding  1030  extend along the edge of the asymmetric spiral inductor  1000  or the winding  1040 . As a result, the shape of the winding  1010  and the shape of the winding  1030  are similar to the partial contour of the asymmetric spiral inductor  1000  and/or the winding  1040 . The end  1011  and the end  1042  are two ends of the asymmetric spiral inductor  1000 . 
     As shown in  FIGS. 10A and 10B , the winding  1010  and the winding  1030  do not overlap with the winding  1040 . The winding  1010  and the winding  1030  are located at the outermost turn of the asymmetric spiral inductor  1000 , and the lengths of the trace of the winding  1010  and the trace of the winding  1030  are each approximately a half turn of the asymmetric spiral inductor  1000  or the winding  1040 . Please refer to  FIGS. 5 to 7 , the widths of the traces of the windings  1010 ,  1020 ,  1030  and  1040  are not limited to the example shown in  FIG. 10B . 
       FIG. 11  shows the quality factor Q of the asymmetric spiral inductor  300  and the asymmetric spiral inductor  400 . The curve  1110  represents the quality factor Q of the asymmetric spiral inductor  300 , and the curve  1120  represents the quality factor Q of the asymmetric spiral inductor  400 . Compared to the asymmetric spiral inductor  300 , the structure of the asymmetric spiral inductor  400  can improve the quality factor Q of the inductor. 
     Although the coils in the embodiments discussed above are of an octagonal shape, the inductors can also be of other polygonal shapes or a circular shape. The inductors of the present invention are not limited to four turns. 
     In summary, the present invention can increase the inductance value of the asymmetric spiral inductor without increasing the inductor area; therefore the quality factor Q is improved. 
     Please note that the shape, size, and ratio of any element in the disclosed figures are exemplary for understanding, not for limiting the scope of this invention. 
     The aforementioned descriptions represent merely the preferred embodiments of the present invention, without any intention to limit the scope of the present invention thereto. Various equivalent changes, alterations, or modifications based on the claims of the present invention are all consequently viewed as being embraced by the scope of the present invention.