Abstract:
This invention discloses an integrated inductor structure, including a first metal trace, a second metal trace, and a connecting metal trace. Tow terminals of the connecting metal trace are respectively connected to the first metal trace and the second metal trace through at least a connecting structure. The connected first metal trace, the connecting metal trace and the second metal trace together form an inductor structure. The connecting structure is connected to a connecting area of the first metal trace. The connecting area of the first metal trace has a first width. A smallest width of the first metal trace is a second width. The second width is smaller than the first width.

Description:
BACKGROUND 
       [0001]    1. Field of the Disclosure 
         [0002]    This invention relates to integrated inductor structures, especially to highly compact integrated inductor structures. 
         [0003]    2. Description of Related Art 
         [0004]    Due to manufacturing restrictions, on-chip inductors are often in the design of planner structures.  FIG. 1  illustrates a prior-art planner spiral inductor. The planner spiral inductor  100  includes a spiral first metal trace  110  (light color), and a second metal trace  120  (dark color). The first metal trace  110  and the second metal trace  120  are deployed in different layers of a semiconductor structure. In  FIG. 1  the first metal trace  110  is above the second metal trace  120 , but it is also possible that the first metal trace  110  is under the second metal trace  120 . The first metal trace  110  and the second metal trace  120  are connected through a connecting structure  130 . The first metal trace  110  includes a coil of 3 turns. When there is a need to increase the inductance of the planner spiral inductor  100 , the number of turns of the coil of the first metal trace  110  must be increased. The increase in the number of turns not only causes an increase in the area of the planner spiral inductor  100 , but also causes an increase in the parasitic series resistance and the parasitic capacitance of the planner spiral inductor  100 , which in turn causes the self-resonant frequency and the quality factor Q of the planner spiral inductor  100  to decrease. In addition, metal loss and substrate loss are also key factors to the quality factor Q. The metal loss is caused by the resistance of the metal itself. The substrate loss arises from two reasons. One is that when the inductor operates, a time-varying electric displacement happens between a metal coil of the inductor and a substrate, which cause a displacement current between the metal coil and the substrate. When the displacement current penetrates into the low resistance substrate, energy losses occur. The displacement current is related to an area of the coil, the larger the area, the greater the displacement current. The other reason is that a time-varying electromagnetic field of the inductor penetrates a dielectric layer, which causes a magnetically induced eddy current on the substrate. Energy losses occur due to the opposite directions of the induced current and the current of the inductor. 
         [0005]    When an inductor operates in low frequencies, the current distributes evenly in the metal coil and the metal loss at the time is from the series resistance in the metal coil. When the inductor operates in high frequencies, greater magnetic fields are induced at inner turns of the metal coil than at outer turns. Intense magnetic fields induce eddy currents at inner turns of the metal coil. The eddy currents causes uneven distribution of currents, most currents being pushed to the surface of the metal coil, which is known as a skin effect. Under the skin effect, the cross section of the metal through which the current flows becomes smaller, and hence the current experiences a larger resistance, which results in a decreased quality factor Q.  FIG. 2  shows another prior-art planner spiral inductor. This tapered spiral inductor alleviates the skin effect since the inner turns of the metal coil of the inductor suffer the most severe skin effect. Further, the tapered spiral structure reduces the area of the inductor and decreases the parasitic capacitance so the quality factor Q and the self-resonant frequency of the inductor can be improved. Because of its asymmetric structure, a position of the inductor&#39;s center tap is hard to decide. Moreover, the positions of an inductive center, a capacitive center, and a resistive center of this spiral inductor are different from one another, which makes this spiral inductor improper for passive components in a differential circuit. 
         [0006]    To address the problem, a symmetric spiral inductor is introduced.  FIG. 3  shows a structure of a prior-art symmetric spiral inductor. The symmetric spiral inductor  300  includes a plurality of metal traces  310  ( 310   a - 310   d ), a plurality of connecting traces  320  ( 320   a - 320   c ), and a plurality of connecting structures  330 . A connecting trace  320  is also referred to as a bridge. All the metal traces  310   a - 310   d  are deployed on the same layer of a semiconductor structure (light color), and all the connecting traces  320   a - 320   c  are made of metal and are deployed on a different layer (dark color) from the metal traces  310   a - 310   d.  As an example, the connecting traces  320   a - 320   c  are deployed under the metal traces  310   a - 310   d  in  FIG. 3 . 
         [0007]    The connecting traces  320  are to connect different metal traces  310 . For example, the connecting trace  320   a  connects the metal trace  310   a  and the metal trace  310   b,  the connecting trace  320   b  connects the metal trace  310   b  and the metal trace  310   c,  and the connecting trace  320   c  connects the metal trace  310   c  and the metal trace  310   d.  The two ends of a connecting trace  320  connect the metal traces  310  through the connecting structures  330 . The connecting structure  330  can be a via structure of a semiconductor manufacturing process that connects components at different layers. It is easy to find the position of the center tap of the symmetric spiral inductor  300  for its symmetric structure; however the connecting traces  320  are frequently used. When an inductor with large inductance is required, the turns of the symmetric spiral inductor  300  must be increased, which in turn increases the number of the connecting traces  320 . If the sheet resistance of the connecting trace  320  is larger than that of the metal trace  310 , then the quality factor Q of the symmetric spiral inductor  300  is dominated by the resistance of the connecting traces  320  and the parasitic resistance of the connecting structures  330 . 
         [0008]      FIGS. 4A and 4B  illustrate a structure of another prior-art symmetric spiral inductor and its partial enlargement. The symmetric spiral inductor  400  includes a plurality of metal traces  410  ( 410   a·   410   d ), a plurality of connecting traces  420  ( 420   a˜   420   c ), and a plurality of connecting structures  430 . Similar to  FIG. 3 , the metal traces  410  are connected by the connecting traces  420 . The two ends of a connecting trace  420  connect the metal traces  410  through a connecting structure  430 , respectively. In contrast to the structure in  FIG. 3  which is rectangular, the structure of the inductor in  FIG. 4  is octagonal and therefore has better inductive effects.  FIG. 4B  shows a partial enlargement (corresponding to the area enclosed by a dotted line in  FIG. 4A ) of the symmetric spiral inductor  400 . Generally a cross section of the connecting structure  430  is designed to be a rectangle. Due to restrictions of the IC design rule, the width of the connecting structure  430  must be greater than D, and a distance to the edge of the metal trace  410   b  must be greater than h. In other words, if the width of the connecting structure  430  is designed to be D, the width W of a connecting area  440  (depicted by stripped lines) of the metal trace  410   b  through which the connecting structure  430  connects the metal trace  410   b  must be greater than D+2h. For example, if D is 3 μm and h is 0.5 μm, then W must be greater than 4 μm. Likewise, the connecting trace  420   b  has a connecting area on the corresponding position through which the connecting structure  430  connects the connecting trace  420   b,  and the width of this connecting area must be greater than W as well. In general, the width of the connecting trace  420   b  is designed to be W. Because the widths of the metal traces  410  and the connecting traces  420  of the symmetric spiral inductor  400  are designed to be W, the flexibility of designing the entire area of the symmetric spiral inductor  400  is restricted; in particular, when the turns of the symmetric spiral inductor  400  are increased to enhance the inductance, the increase in the entire area of the inductor causes an increase in the parasitic capacitance and a decrease in the self-resonant frequency. 
       SUMMARY 
       [0009]    In consideration of the imperfections of the prior art, an object of the present disclosure is to provide integrated inductor structures, so as to make an improvement to the prior art. 
         [0010]    The present invention discloses an integrated inductor structure, comprising: a first metal trace; a second metal trace; and a connecting metal trace with its two ends respectively connecting the first metal trace and the second metal trace through at least a connecting structure. The connected first metal trace, the connecting metal trace, and the second metal trace form an inductor structure. The connecting structure connects a connecting area of the first trace, and the connecting area of the first metal trace has a first width. A smallest width of the first metal trace, except for the connecting area, is a second width, and the second width is smaller than the first width. 
         [0011]    The present invention also discloses an integrated inductor structure, comprising: a first metal trace; a second metal trace; and a connecting metal trace, for connecting the first metal trace and the second metal trace through a connecting structure. The connected first metal trace, the connecting metal trace, and the second metal trace form an inductor structure. The connecting structure connects a connecting area of the connecting metal trace, and the connecting area of the connecting metal trace has a first width. A smallest width of the connecting metal trace, except for the connecting area, is a second width, and the second width is larger than the first width. 
         [0012]    An integrated inductor structure of the present invention reduces the inductor&#39;s overall area by reducing the widths of metal lines to decrease the parasitic capacitance. In comparison with the prior art, for a given inductance, the integrated inductor structure of the present invention has improved self-resonant frequency and quality factor Q. The integrated inductor of the present invention provides better quality factor Q and more stable inductance than the prior-art inductor when operating at the same frequency. 
         [0013]    These and other objectives of the present disclosure will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the embodiments that are illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  illustrates a prior-art planner spiral inductor. 
           [0015]      FIG. 2  illustrates another prior-art planner spiral inductor. 
           [0016]      FIG. 3  illustrates a structure of a prior-art symmetric spiral inductor. 
           [0017]      FIGS. 4A and 4B  illustrate a structure of another prior-art symmetric spiral inductor and its partial enlargement. 
           [0018]      FIGS. 5A to 5D  illustrate a structure of a symmetric spiral inductor and its partial enlargements according to an embodiment of the present invention. 
           [0019]      FIGS. 6A and 6B  illustrate relationships of the quality factor Q and the inductance with respect to the frequency of the presently disclosed integrated inductor and the prior-art integrated inductor. 
           [0020]      FIGS. 7A and 7B  illustrate a structure of a symmetric spiral inductor and its partial enlargement according to another embodiment of the present invention. 
           [0021]      FIGS. 8A and 8B  illustrate structures of the connecting trace of the symmetric spiral inductor according to other embodiments of the present invention. 
           [0022]      FIGS. 9A to 9D  illustrate structures of connecting traces of the prior art and the present invention. 
           [0023]      FIGS. 10A and 10B  illustrate a structure of a symmetric spiral inductor and its partial enlargement according to another embodiment of the present invention. 
           [0024]      FIG. 11  illustrates a cross section of a metal trace, a connecting trace and a connecting structure according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0025]    The following description is written by referring to terms of this technology field. If any term is defined in the specification, such term should be explained accordingly. Besides, the connection between objects or events in the following embodiments can be direct or indirect provided that these embodiments are still applicable under such connection. Said “indirect” means that an intermediate object or a physical space exists between the objects, or an intermediate event or a time interval exists between the events. Integrated inductor structures are disclosed, and the detail known in this field will be omitted if such detail has little to do with the features of the present disclosure. People of ordinary skill in the art can choose components or steps equivalent to those described in this specification to carry out the present disclosure, which means that the scope of this disclosure is not limited to the embodiments in the specification. 
         [0026]    Referring to  FIG. 4B  again, according to the IC design rule, the smallest widths of the metal trace  410   b  and the metal trace  410   c  (except for the connecting areas  440 ) can be smaller than D+2h. For example, if D+2h is 4 μm, the smallest width can be designed to be smaller than 4 μm, but there is still a minimum width of manufacturing restriction, such as 2 μm.  FIGS. 5A ,  5 B,  5 C, and  5 D show a structure of a symmetric spiral inductor and its partial enlargements according to an embodiment of the present invention. The symmetric spiral inductor  500  includes a plurality of metal traces  510  ( 510   a˜   510   d ), a plurality of connecting traces  520  ( 520   a˜   20   c ), and a plurality of connecting structures  530 . The connecting trace  520  is made of metal.  FIG. 5B  shows an enlargement of the area enclosed by the dotted line in  FIG. 5A . Taking the metal trace  510   b  and the metal trace  510   c  in  FIG. 5B  for example (the same design being applicable to the metal traces  510   a  and  510   d ), in this embodiment, the connecting parts (i.e., the connecting area of the metal trace  510 ), where the metal trace  510   b  and the metal trace  510   c  are connected to the connecting structure  530 , and the traces connecting thereto are designed to be the minimum width W (e.g., 4 μm) for connecting areas permitted by the IC design rule, and other parts are designed to have a width of W2, which is smaller than W. The smallest value of W2 can be as small as the minimum width permitted by the IC design rule (e.g., 2 μm). In another embodiment, the smallest widths of the metal trace  510   b  and the metal trace  510   c  can be different; for example, the smallest width of the metal trace  510   b  is Wb whereas the smallest width of the metal trace  510   c  is We (Wb≠Wc). The area of the inductor can be reduced as long as one of them is smaller than W and the other is not greater than W.  FIG. 5C  shows a structure of the metal trace  510   b.  The metal trace  510   b  includes a first segment  510   b - 0  (plain region), a second segment  510   b - 1 , and a third segment  510   b - 2 . In one embodiment, the first segment  510   b - 0  has a uniform width W2; in other embodiments, however, the width of the first segment  510   b - 0  can be inconsistent. As long as the segment  510   b - 0  has a partial segment whose width is smaller than W, the entire area of the inductor can be reduced. Referring to  FIG. 5D , the region that connects the third segment  510   b - 2  and the first segment  510   b - 0  has a width of W2 and the third segment  510   b - 2  includes the connecting area  540  (stripped region). The second segment  510   b - 1  has the same characteristics. The connecting area  540  is connected to the connecting structure  530  and has a width of W. The second segment  510   b - 1  and the third segment  510   b - 2  are extensions of the first segment  510   b - 0 , and the width increases gradually from W2 to W. Likewise, in  FIG. 5A , the metal trace  510   a,  the metal trace  510   c,  and the metal trace  510   d  also have segments of different widths. The metal trace  510   a,  the metal trace  510   b,  the metal trace  510   c,  and the metal trace  510   d  have a wider width at their individual connecting region (i.e., the connecting area  540 ) that is connected to the connecting structure  530 , and have a width in other parts, which is smaller than W and can be consistent or not. The width W can be, for example, a minimum width for connecting areas permitted by the IC design rule. If W is 4 μm and W2 is 2 μm, the symmetric spiral inductor  500  in  FIG. 5A  saves about 30% area compared to the symmetric spiral inductor  400  in  FIG. 4A . An integrated inductor with a smaller area helps reduce the parasitic capacitance so the quality factor Q and the self-resonant frequency can be improved, which increases the range the inductor&#39;s operating frequency.  FIGS. 6A and 6B  show relationships of the quality factor Q and the inductance with respect to the frequency of the presently disclosed integrated inductor and the prior-art integrated inductor. The peak value of the quality factor Q of the present invention (around 8.5) is larger than that of the prior-art integrated inductor (around 7.8), and the self-resonant frequency of the present invention is higher than that of the prior-art integrated inductor. Thus, the range of the operating frequency of the integrated inductor of the present invention is increased. In addition, when the integrated inductor operates around the peak value of the quality factor Q, the variation in inductance with respect to the frequency is relatively small so a stable inductance can be obtained. 
         [0027]      FIGS. 7A and 7B  illustrate a structure of a symmetric spiral inductor and its partial enlargement according to another embodiment of the present invention. The symmetric spiral inductor  700  includes a plurality of metal traces  710  ( 710   a - 710   d ), a plurality of connecting traces  720  ( 720   a - 720   c ), and a plurality of connecting structures  730 . The connecting trace  720  is made of metal.  FIG. 7B  is a partial enlargement of the enclosed region in  FIG. 7A . When the metal trace  710  is made of the ultra thick metal (UTM) and the connecting trace  720  is made of the re-distribution layer (RDL), the large resistance of the connecting trace  720  may decrease the inductor&#39;s quality factor Q because the resistance of the RDL is higher than that of the UTM. In order to prevent the quality factor Q from decreasing, the area of the connecting trace  720  is increased in this embodiment to reduce its sheet resistance. The width at the connecting area  740  of the connecting trace  720  is maintained W, but the width of the connecting trace  720   b  is designed greater than W. Further, the width at the central part of the connecting trace  720   b  is W3, which is much larger than W, and thus the sheet resistance of the connecting trace  720   b  can be effectively reduced. In other embodiments, as shown in  FIG. 8A , the area of the connecting trace  720   b  can be increased to further reduce its sheet resistance. Alternatively, as shown in  FIG. 8B , the number of connecting structure  730  is increased to reduce the parasitic resistance of the connecting structure  730 , and the area of the connecting trace  720   b  is also increased to obtain a better result. 
         [0028]      FIGS. 9A to 9D  illustrate structures of connecting traces of the prior art and the present invention.  FIG. 9A  shows a structure of a prior-art connecting trace  420 . The connecting trace  420  includes two connecting areas  450  (stripped region) that are connected to the connecting structure  430 . The connecting area  450  of the connecting trace  420  is also restricted by the IC design rule, and its width should not be smaller than W=D+2h (assuming also that the permitted minimum width of the connecting structure  430  is D and the permitted minimum distance between the connecting structure  430  and the edges of the connecting trace  420  is h). According to different measuring manners, several widths can be obtained for the connecting trace  420  (except for the connecting area  450 ). These widths are a width W′ at the central segment, a longitudinal width W″ and a lateral width W″. The width W′ at the central segment is the smallest among the  3  widths. In the prior-art connecting trace  420 , the smallest width W′ is designed to be the lower limit W, which causes an increase to the sheet resistance. The connecting trace  720  (except for the connecting area  740 ) in  FIG. 9B  has  3  widths W′, W″ and W″, among which the width W′ is the smallest but is apparently much larger than W. In comparison with  FIG. 9A , the width W′ at the central segment of the connecting trace  720  is larger than the width at the central segment of the connecting trace  420 . An increase in the width of the connecting trace  720  reduces the sheet resistance, and therefore the integrated inductor can have a better quality factor Q. Likewise, in  FIGS. 9C and 9D , the widths W′ and W″ of the connecting trace  720  (except for the connecting area  740 ) are larger than W, and therefore the sheet resistance can be reduced. 
         [0029]      FIGS. 10A and 10B  show a structure of a symmetric spiral inductor and its partial enlargement according to another embodiment of the present invention. The symmetric spiral inductor  1000  includes a plurality of metal traces  1010  ( 1010   a ˜ 1010   d ), a plurality of connecting traces  1020  ( 1020   a ˜ 1020   c ), and a plurality of connecting structures  1030 . The connecting trace  1020  is made of metal.  FIG. 10B  is a partial enlargement of the enclosed region in  FIG. 10A . The embodiment in  FIG. 10A  is a combination of the embodiments in  FIGS. 5A and 7A ; that is to say, any metal trace  1010  of the symmetric spiral inductor  1000  in  FIG. 10A  includes segments that are connected to the connecting structures  1030  and are designed to be wider to meet the restrictions of the IC design rule, and other segments that are designed to be narrower than the aforementioned segments to reduce the overall area of the symmetric spiral inductor  1000 . Further, the smallest width of the connecting trace  1020  (except for the connecting area) is larger than the width of the connecting area. As shown in  FIG. 10B , the widths of the segments of the metal trace  1010   b  and the metal trace  1010   c  that are connected to the connecting structures  1030  are W, and parts of the connecting trace  1020   b  that are connected to the connecting structure  1030  also have a width W. The widths of the segments of the metal trace  1010   b  and the metal trace  1010   c  that are not connected to the connecting structure  1030  are W2, which is smaller than W. A part of the connecting trace  1020   b  that is not connected to the connecting structure  1030  has a width larger than W; for example, the width at the central segment of the connecting trace  1020   b  is W3, which is larger than W. The symmetric spiral inductor  1000  has the advantages of the symmetric spiral inductor  500  and the symmetric spiral inductor  700  that the overall area of the integrated inductor is reduced to decrease the parasitic capacitance, and the sheet resistance of the connecting trace  1020  is also reduced. Referring to  FIGS. 6A and 6B  again, although the self-resonant frequency of the symmetric spiral inductor  1000  is close to that of the symmetric spiral inductor  500 , the peak value of the quality factor Q of the symmetric spiral inductor  1000  is higher than that of the symmetric spiral inductor  500 . Therefore, when the symmetric spiral inductor  1000  operates at a frequency where the inductance is relatively stable (around 3.5 GHz), a better quality factor Q can be assured. 
         [0030]    The connecting structure  530 , the connecting structure  730  and the connecting structure  1030  of this invention can be implemented by a via structure. Taking the metal trace  510   a,  the connecting trace  520   a,  and the connecting structure  530  in  FIG. 5  as an example, a cross section of one of the implemented structures is shown in  FIG. 11 . The structure in  FIG. 11  includes, from top to bottom, an oxide layer (or dielectric layer)  1110 , a substrate  1120 , and an oxide layer  1130 . The metal trace  510   a  is contained in the oxide layer  1110 , and the connecting trace  520   a  is contained in the oxide layer  1130 . The connecting structure  530  penetrates through the oxide layer  1110 , the substrate  1120 , and the oxide layer  1130  so that the metal trace  510   a  and the connecting trace  520   a,  which are deployed at different layers, are connected to each other. This connecting structure  530  is referred to as a through silicon via (TSV). 
         [0031]    The disclosed octagonal integrated inductor serves as merely an example; this invention can be applied to integrated inductors in other shapes. In addition, the connecting traces are arranged under the metal traces in this disclosure; however, the connecting traces can also be arranged above the metal traces. The shape, size, and ratio of any element in the disclosed figures are just exemplary for understanding, not for limiting the scope of this disclosure. Besides, each aforementioned embodiment may include one or more features; however, this doesn&#39;t mean that one carrying out the present disclosure should make use of all the features of one embodiment at the same time, or should only carry out different embodiments separately. In other words, if an implementation derived from one or more of the embodiments is applicable, a person of ordinary skill in the art can selectively make use of some or all of the features in one embodiment or selectively make use of the combination of some or all features in several embodiments to have the implementation come true, so as to increase the flexibility of carrying out the present disclosure. 
         [0032]    The aforementioned descriptions represent merely the embodiments of the present disclosure, without any intention to limit the scope of the present disclosure thereto. Various equivalent changes, alterations, or modifications based on the claims of present disclosure are all consequently viewed as being embraced by the scope of the present disclosure.