BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a structure of an integrated inductor, especially to a structure of an integrated inductor that provides a high quality factor Q, a large bandwidth, and good symmetry. 
         [0003]    2.Description of Related Art 
         [0004]    An on-chip inductor is a kind of integrated inductor structure, which is usually of a spiral shape.  FIG. 1  illustrates a conventional asymmetric spiral inductor. The asymmetric spiral inductor  100  includes a spiral-shaped metal segment  110  (in light gray color) and a metal segment  120  (in dark gray color). The metal segment  110  and the metal segment  120  are disposed on different layers in a semiconductor structure; for example the metal segment  110  is on the upper layer and the metal segment  120  is on the lower layer, as shown in  FIG. 1 . The metal segment  110  and the metal segment  120  are connected via a connecting structure  130 , which can be a via structure in a semiconductor manufacturing process.  FIG. 2  is a cross section of the asymmetric spiral inductor  100  in  FIG. 1 . The lowermost layer is a substrate  210  and on top of the substrate  210  is an oxide layer  220 . The metal segment  120  is contained in the oxide layer  220  while the metal segment  110  is on top of the oxide layer  220 . The connecting structure  130 , which is made up of a via array, forms a plurality of via holes on the surface of the oxide layer  220  and connect the metal segment  110  and the metal segment  120 . In general, the metal segment  120  is made on an ultra-thick metal (UTM) layer, which is usually made of copper and is the upmost metal layer of the oxide layer  220 , whereas the metal segment  110  is made on the re-distribution layer (RDL), which is usually made of aluminum-copper alloy and is on top of the oxide layer  220 . Specifically, the oxide layer  220  is a protection layer formed in a passivation process of semiconductor manufacture and is usually made of SiO2 or SiN3. 
         [0005]    The number of turns of the metal segment  110  is 3, and can be increased to enhance the inductance of the asymmetric spiral inductor  100 . The increase in the number of turns results in an increase in the area of the asymmetric spiral inductor  100 , and in an increase in the parasitic series resistance and the parasitic capacitance of the asymmetric spiral inductor  100  as well, which decrease the self-resonant frequency and the quality factor Q. In addition, metal loss and substrate loss are also key factors to the quality factor Q. The metal loss arises from resistance of the metal itself while the substrate loss arises from two situations. One is caused by a time-varying electric displacement between the metal coil of the inductor and the substrate as the inductor is functioning. The time-varying electric displacement causes a displacement current between the metal coil and the substrate that penetrates into the low-impedance substrate and in turn causes energy loss. The magnitude of the displacement current is related to the area of the inductor; the bigger the area, the higher the displacement current. The other is caused by a time-varying electromagnetic field of the inductor that penetrates through a dielectric layer and causes a magnetically induced eddy current in the substrate, which flows in a direction opposite to the current direction in the inductor and thus causes energy loss. 
         [0006]    A center tap of the inductor is hard to decide because of the asymmetric structure of the asymmetric spiral inductor. Moreover, the asymmetric spiral inductor is impractical for being used as a passive component in a differential circuit because positions of the inductive center, the capacitive center and the resistive center are different.  FIG. 3  shows a conventional symmetric spiral inductor. The symmetric spiral inductor  300  can be roughly divided into an outer part and an inner part. The metal segment  310  includes the left portion of the outer part and the entire inner part; the metal segment  330  includes the right portion of the outer part. The metal segment  310  and the metal segment  330  belong to the same metal layer in the structure (in dark gray color) and are connected by a bridging metal segment  320  of another metal layer (in light gray color). The center of the inner part is connected to a center tap  340 , which is on a layer different from the metal segments  310  and  330  and the bridging metal segment  320 . A connecting structure  350 , a connecting structure  360  and a connecting structure  370  respectively connects the metal segment  310  and the bridging metal segment  320 , the bridging metal segment  320  and the metal segment  330 , and the metal segment  310  and the center tap  340 . The connecting structures can be implemented by vias. Since the symmetric spiral inductor  300  is symmetric in structure, its center tap  340  is easy to decide. Two inductors are respectively defined by the terminal  342  of the center tap  340  and the terminal  312  of the metal segment  310  as well as by the terminal  342  of the center tap  340  and the terminal  332  of the metal segment  330 . Ideally, these two inductors have similar inductance, but a practical analysis of the current path of each inductor renders an unideal consequence. A current from the terminal  332  to the center tap  340  (dashed line) flows sequentially through the right portion of the outer part (i.e., the metal segment  330 ), the connecting structure  360 , the bridging metal segment  320 , the connecting structure  350  and the left portion of the inner part; on the other hand, the current from the terminal  312  to the center tap  340  flows through only the left portion of the outer part and the right portion of the inner part. Generally, resistances of different metal layers are not the same and the connecting structure also increases the resistance, which accounts for differences in the inductances of the two inductors. When the two inductors are being used as the inductor  410  and the inductor  420  of the VCO (voltage controlled oscillator) in  FIG. 4 , asymmetric inductances may cause common mode phenomenon in this differential circuit, which affects the stability of the circuit. 
         [0007]    In addition, a metal loss of an inductor operating in a low frequency arises from the series resistance of the metal coil when the current in the metal coil has a uniform distribution. When the inductor operates at a high frequency, the inner metal coil generates a high magnetic field, which induces an eddy current inside the metal coil that causes the skin effect phenomenon. Under the skin effect phenomenon, most current is pushed to the surface of the metal coil by the eddy current, which results in uneven current distribution and in turn degrades the quality factor Q because the current encounters a greater resistance as flowing through a smaller cross section of the metal. 
       SUMMARY OF THE INVENTION 
       [0008]    In view of the problems of the prior art, an object of the present invention is to provide a spiral integrated inductor structure that provides a high quality factor Q, a large bandwidth, and good symmetry, so as to make an improvement to the prior art. 
         [0009]    The present invention discloses an integrated inductor structure, which comprises an outer metal segment, an inner metal segment arranged in an area surrounded by the outer metal segment, at least a bridging metal segment for connecting the outer metal segment and the inner metal segment, and at least a connecting structure for connecting the bridging metal segment and the outer metal segment or the inner metal segment. The outer metal segment and the inner metal segment are on different metal layers of a semiconductor structure. 
         [0010]    The present invention also discloses an integrated inductor structure, which comprises an outer metal segment, an inner metal segment, and at least a connecting structure. The outer metal segment comprises a first metal sub-segment and a second metal sub-segment. The inner metal segment is arranged in an area surrounded by the outer metal segment and comprises a third metal sub-segment and a fourth metal sub-segment. The connecting structure connects the outer metal segment and the inner metal segment. The first metal sub-segment and the third metal sub-segment correspond to each other and belong to different metal layers of a semiconductor structure, and the second metal sub-segment and the fourth metal sub-segment correspond to each other and belong to different metal layers of the semiconductor structure. 
         [0011]    The present invention further discloses an integrated inductor structure, which comprises a first spiral inductor and a second spiral inductor connected to the first spiral inductor. Corresponding metal sub-segments of an outer metal segment and an inner metal segment in the first spiral inductor or the second spiral inductor belong to different metal layers in a semiconductor structure. 
         [0012]    The integrated inductor structure of this invention effectively reduces parasitic capacitances among metal segments as well as those between the metal segments and the substrate to improve the quality factor Q and the bandwidth of the integrated inductor. The effects of this invention can be better appreciated as the manufacture becomes more advanced with smaller chip sizes. Furthermore, this invention also enhances the degree of symmetry of the symmetric spiral inductor so that the symmetric spiral inductor of this invention is more suitable for passive components in a differential circuit. 
         [0013]    These and other objectives of the present invention no doubt becomes obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiments that are illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  illustrates a conventional asymmetric spiral inductor. 
           [0015]      FIG. 2  illustrates a cross section of the asymmetric spiral inductor  100  in  FIG. 1 . 
           [0016]      FIG. 3  illustrates a conventional symmetric spiral inductor. 
           [0017]      FIG. 4  illustrates a conventional VCO. 
           [0018]      FIG. 5  illustrates a symmetric spiral inductor  500  according to an embodiment of the present invention. 
           [0019]      FIG. 6  illustrates a cross section of a conventional symmetric spiral inductor  300 . 
           [0020]      FIG. 7  illustrates a cross section of a symmetric spiral inductor  500  of the present invention. 
           [0021]      FIG. 8  illustrates a relationship between the quality factor Q and the frequency when an outer metal segment and an inner metal segment of a 2-turn symmetric spiral inductor are implemented on the same or different metal layer(s). 
           [0022]      FIG. 9  illustrates a relationship between the quality factor Q and the frequency of the prior art and this invention in a 28 nm process with radius r=60 μm. 
           [0023]      FIG. 10  illustrates a relationship between the quality factor Q and the frequency of the prior art and this invention in a 28 nm process with radius r=45 μm. 
           [0024]      FIG. 11  illustrates a relationship between the quality factor Q and the frequency of the prior art and this invention in a 55 nm process with radius r=45 μm. 
           [0025]      FIG. 12  illustrates another embodiment of the symmetric spiral inductor of this invention. 
           [0026]      FIG. 13  illustrates a structure in which an integrated inductor is made on different metal layers according to an embodiment of this invention. 
           [0027]      FIG. 14  illustrates a structure in which an integrated inductor is made on different metal layers according to another embodiment of this invention. 
           [0028]      FIG. 15  illustrates a structure in which an integrated inductor is made on different metal layers according to another embodiment of this invention. 
           [0029]      FIG. 16  illustrates an asymmetric spiral inductor according to an embodiment of the present invention. 
           [0030]      FIG. 17  illustrates an asymmetric spiral inductor according to another embodiment of the present invention. 
           [0031]      FIG. 18  illustrates connected asymmetric spiral inductors according to an embodiment of this invention. 
           [0032]      FIG. 19  illustrates connected asymmetric spiral inductors according to another embodiment of this invention. 
           [0033]      FIG. 20  illustrates connected asymmetric spiral inductors according to another embodiment of this invention. 
           [0034]      FIG. 21  illustrates connected asymmetric spiral inductors according to another embodiment of this invention. 
           [0035]      FIG. 22  illustrates connected asymmetric spiral inductors according to another embodiment of this invention. 
           [0036]      FIG. 23  illustrates connected asymmetric spiral inductors according to another embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0037]    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 explained accordingly. In addition, the connection between objects or events in the below-described embodiments can be direct or indirect provided that these embodiments are practicable under such connection. Said “indirect” means that an intermediate object or a physical space exists between the objects. 
         [0038]      FIG. 5  shows a symmetric spiral inductor  500  according to an embodiment of the present invention. The symmetric spiral inductor  500  includes an outer metal segment made up of a metal segment  510  and a metal segment  530 , an inner metal segment made up of a metal segment  520 , and bridging metal segments  515  and  525 . The outer metal segment and the inner metal segment are respectively formed by a metal segment of an octagon shape, and the inner metal segment is disposed in an area surrounded by the outer metal segment. The bridging metal segment  515  and the bridging metal segment  525 , which connect the outer metal segment and the inner metal segment, belong to different metal layers in the semiconductor structure and are partially overlapped but not contact. A terminal of the bridging metal segment  515  connects a metal segment  510  on a different layer through a connecting structure  560 , and the other connects the metal segment  520  on the same layer directly. Under some circumstances, the bridging metal segment  515  can be deemed as a part of the metal segment  520 ; in this specification, however, the bridging metal segment  515  and the metal segment  520  are defined to be distinct from each other in order to describe this invention more explicitly. Likewise, a terminal of the bridging metal segment  525  connects a metal segment  520  on a different layer through a connecting structure  550 , and the other connects the metal segment  530  on the same layer directly. Under some circumstances, the bridging metal segment  525  can be deemed as a part of the metal segment  530 ; in the specification, however, the bridging metal segment  525  and the metal segment  530  are defined to be distinct from each other in order to describe this invention more explicitly. In addition, the symmetric spiral inductor  500  further includes a third connecting structure  570  that connects the metal segment  520  and the center tap  540 . As shown in  FIG. 5 , the current from the terminal  532  to the center tap  540  (dashed line) flows sequentially through the metal segment  530 , the bridging metal segment  525 , the connecting structure  550  and the left portion of the metal segment  520 ; in contrast, the current from the terminal  512  to the center tap  540  flows sequentially through the metal segment  510 , the connecting structure  560 , the bridging metal segment  515 , and the right portion of the metal segment  520 . It is apparent that the two inductors of the symmetric spiral inductor  500  are symmetric to each other and therefore can be used for differential circuits, such as being used as the inductor  410  and the inductor  420  of the VCO in  FIG. 4 , to prevent common mode and thus improves circuit stability. In one embodiment of this invention, the outer metal segment of the symmetric spiral inductor  500  (i.e., the metal segment  510  and the metal segment  530 ) is made of the RDL while the inner segment (i.e., the metal segments 520 ) is made of the UTM layer. The thickness of the UTM layer is generally greater than that of the RDL, which helps reduce the resistance of the inner metal segment when the inner metal segment is made of the thicker metal layer. As mentioned in the prior art, because the inner part of the integrated inductor structure is subject to the skin effect more easily than the outer part, the quality factor Q of the inductor can be improved by making the resistance of the metal segment  520  smaller than the resistances of the metal segment  510  and the metal segment  530 . 
         [0039]    The symmetric spiral inductor  500  also improves the bandwidth of the inductor by reducing the parasitic capacitance. Referring to  FIGS. 3 and 6 , the conventional symmetric spiral inductor  300  and its cross section are illustrated. In  FIG. 6 , only the metal segments and the substrate are shown and other parts, such as the oxide layer, are omitted for brevity. Because the metal segment  310  and the metal segment  330  of the conventional symmetric spiral inductor  300  are implemented on the same metal layer, the cross section corresponding to the dashed line A-A′ shows that the metal segment  310  and he metal segment  330  have the same distance to the substrate  305 . Because all the metal segments are on the same layer, the parasitic capacitance C tends to exist between metal segments. In addition, there is also parasitic capacitance between a metal segment and the substrate  305 . Note that although only the parasitic capacitance between the metal segment  330  and the substrate  305  is shown in  FIG. 6 , the parasitic capacitance also exits between the metal segment  310  and the substrate  305 . In comparison with the metal segments on the RDL, a parasitic capacitance is more easily generated between the metal segments on the UTM layer and the substrate because the UTM layer is closer to the substrate than the RDL. Referring to  FIGS. 5 and 7 , the symmetric spiral inductor  500  of the present invention and its cross section are illustrated. The metal segments of the symmetric spiral inductor  500  are distributed on at least two metal layers rather than being implemented on the same layer. The cross section in  FIG. 7  corresponds to the dashed line A-A′ in  FIG. 5 . It is obvious that the metal segment  510  and metal segment  530  are on the metal layer far from the substrate  505 , whereas the metal segment  520  is on the metal layer close to the substrate  505 . Under the premise that the horizontal distance between two adjacent metal segments remains unchanged, the parasitic capacitance between these two metal segments can be reduced due to an increased linear distance between the two metal segments as being implanted on different metal layers. The distance between the metal segment  520  and the substrate  505  is d 1 , and the vertical distance between the metal layer (with thickness h 1 ) where the metal segment  520  resides and the metal layer (with thickness h 2 ) where the metal segment  510  and the metal segment  530  reside is d 2 . For example, in a 28 nm process, the metal segment  520  implemented on the UTM layer has a thickness h 1  of 3.4 nm and the distance d 1  between it and the substrate  505  is 1.5 μm; the metal segment  510  implemented on the RDL has a thickness h 2  of 2.8 μm and the distance d 2  between it and the UTM layer is 0.8 nm. Namely, the distance d 1 =1.5 μm between the metal segment  520  and the substrate  505  is so small that a relatively large parasitic capacitance is expected; however, the distance between the metal segment  510  or the metal segment  530  and the substrate  505  is 5.7 μm (d 1 +hl+d 2 ), which is 3.8 times the distance between the metal segment  520  and the substrate  505 , so the parasitic capacitance between the metal segment  510  or the metal segment  530  and the substrate  505  is greatly reduced. In other words, by implementing some metal segments of the symmetric spiral inductor  500  on a different metal layer from other metal segments, such as in the aforementioned embodiment where the outer metal segments (i.e., the metal segment  510  and the metal segment  530 ) are implemented on the UDL while the inner metal segment (i.e., the metal segment  520 ) is implemented on the UTM layer, the overall parasitic capacitance of the integrated inductor can be reduced. Low parasitic capacitance is good for improving the quality factor Q and the bandwidth. 
         [0040]      FIG. 8  shows a relationship between the quality factor Q and the frequency when an outer metal segment and an inner metal segment of a 2-turn symmetric spiral inductor are implemented on the same or different metal layer(s). This figure is a 28 nm process, the width of a metal segment is 22 μm, and the radius r of the integrated inductor (i.e., the distance from a center of the inductor to the inner side of the inner metal segment as shown in  FIG. 5 ) is 45 μm. In the prior art, a higher quality factor Q can be achieved by implementing both the outer and inner metal segments on the UTM layer than implementing them on the RDL, but the bandwidths of both cases are small. In contrast, although the peak of the quality factor Q of implementing the inner/outer metal segment on the RDL/UTM respectively is slightly smaller than implementing both the inner and the outer metal segments on the UTM layer, the bandwidth is higher and the quality factor Q at high frequencies (&gt;10 GHz) is better. Moreover, by implanting the inner/outer metal segment on the UTM/RDL respectively, better quality factor Q and greater bandwidth can be obtained; besides, a wide range of frequencies is achieved for quality factors Q greater than 20. Therefore, the integrated inductor of the present invention makes a great improvement in bandwidth in comparison with a conventional integrated inductor; further, the quality factor Q is greatly improved as well when the inner metal segment is implemented on the UTM layer while the outer metal segment on the RDL. 
         [0041]    A comparison is made between an embodiment in the prior art (both the inner and outer metal segments on the UTM layer) and an embodiment of the present invention (the inner/outer metal segments on the UTM/RDL respectively) under different process parameters and radiuses, where the width of the metal segment is set to be 22 μm and the number of turns of the integrated inductor is  2 . FIG.  9  illustrates a relationship between the quality factor Q and the frequency of the prior art and this invention in a 28 nm process with radius r=60 μm. It is observed that the peak of the quality factor Q of this invention is close to that of the prior art, but this invention obviously has a wider bandwidth (quality factors Q greater than 20 spreading over approximately 11 GHz).  FIG. 10  illustrates a relationship between the quality factor Q and the frequency of the prior art and this invention in a 28 nm process with radius r=45 μm. It is obviously that the integrated inductor of this invention has better quality factor Q and bandwidth (quality factors Q greater than 20 spreading over approximately 14.2 GHz). In comparison with  FIG. 9 , the advantages of this invention can be better appreciated in integrated inductors with smaller radius. After several experiments, it is found that this invention has good performance when the radius is smaller than 50 μm. In other words, this invention comes up with a more practical implementation as the size of the integrated inductor becomes smaller, making the integrated inductors more adapted to miniaturized components operating in high frequencies.  FIG. 11  illustrates a relationship between the quality factor Q and the frequency of the prior art and this invention in a 55 nm process with radius r=45 μm. In comparison with the prior art, the integrated inductor of this invention has greater quality factor Q and bandwidth (quality factors Q greater than 20 spreading over approximately 15.1 GHz). Although this embodiment has a slightly better bandwidth than that in  FIG. 10 , the quality factor Q is not as good as that in  FIG. 10 . In addition,  FIG. 10  shows a better improvement to the prior art. In summary, the advantages of this invention are not easily perceived until the radius of the integrated inductor becomes smaller and the integrated inductor is manufactured in a more advanced process. 
         [0042]    The principle of this invention can be extended to integrated inductors with more than 2 turns.  FIG. 12  shows another embodiment of the symmetric spiral inductor of this invention. The symmetric spiral inductor  600  includes metal segments  610 ,  620 ,  630 ,  640  and  650 , bridging metal segments  615 ,  625 ,  635  and  645 , and connecting structures  660 ,  670 ,  680  and  690 . The symmetric spiral inductor  600  has 3 turns and is of a rectangular shape while the embodiment of  FIG. 5  is of an octagon shape; however, this invention is not limited to these two shapes. The outer part of the symmetric spiral inductor  600  includes the metal segment  610  and the metal segment  650 , the middle part includes the metal segment  620  and the metal segment  640 , and the inner part includes the metal segment  630 . The metal segment  610  and the metal segment  620  are connected by the bridging metal segment  615 . A terminal of the bridging metal segment  615  connects the metal segment  610  of a different layer via the connecting structure  660  and the other terminal connects the metal segment  620 . Under some circumstances, the bridging metal segment  615  can be deemed as a part of the metal segment  620 ; in the specification, however, the bridging metal segment  615  and the metal segment  620  are defined to be distinct from each other in order to describe this invention more explicitly. The metal segment  630  connects the metal segment  620  and the metal segment  640  via the bridging metal segment  625  and the bridging metal segment  635 , respectively. A terminal of the bridging metal segment  625  connects the metal segment  620  via the connecting structure  690  and the other terminal connects the metal segment  630 . A terminal of the bridging metal segment  635  connects the metal segment  630  via the connecting structure  680 , and the other connects the metal segment  640 . The bridging metal segment  645  connects the metal segment  640  and the metal segment  650 , with one terminal connecting the metal segment  640  via the connecting structure  670  and the other connects the metal segment  650  directly. In this embodiment, metal segments of two adjacent parts are implemented on different metal layers to reduce parasitic capacitance between metal segments. This embodiment can also be modified by implementing only the inner part on the UTM layer while implementing others on the RDL; as a result, the parasitic capacitance of the overall integrated inductor is reduced since the inner part which is sensitive to resistance is implemented on a thicker layer and most of the other metal segments are away from the substrate. As the embodiments of  FIGS. 5 and 12  show, the metal segments of an integrated inductor structure with multiple turns are disposed on at least 2 different metal layers. On the contrary, the conventional integrated inductor structure in  FIG. 3  has most of its metal segments, including the inner and outer metal segments, disposed on the same metal layer, except for the bridging metal segment  320 . Moreover, although the integrated inductor structure of this invention can be implemented in even turns and odd turns, such as the 2-turn symmetric spiral inductor  500  and the 3-turn spiral inductor  600 , integrated inductor structures with even turns can better show the advantages of symmetry of the present invention, in contrast to the conventional symmetric spiral inductor  300 . It is because that in the conventional approach, a symmetric spiral inductor with odd turns has an even number of bridging metal segments, and therefore two inductors of the symmetric spiral inductor are able to improve their symmetry by containing the same number of connecting structures; however, this approach is not applicable to even turns since a symmetric spiral inductor with even turns has an odd number of bridging metal segments. In summary, in comparison with a conventional symmetric spiral inductor, the prevent invention presents an even better improvement in symmetry for an integrated inductor structure with even turns. However, whether the number of turns is even or odd, the integrated inductor structure of this invention makes its two inductors include the same number of connecting structures, which is important for the symmetry performance. 
         [0043]    The metal layers used in this invention are not limited to the UTM layer and the RDL, which are adjacent; on the contrary, this invention can be implemented in a more complex semiconductor structure.  FIG. 13  illustrates a structure in which an integrated inductor is made on different metal layers according to an embodiment of this invention.  FIG. 13  shows a structure of a flip chip, including two dies  1310  and  1320  face to face. The die  1310  includes a substrate  1312 , an oxide layer  1314  and a redistribution layer  1316 . The oxide layer  1314  includes the ultra-thick metal  1315 . The die  1320  includes a substrate  1322 , an oxide layer  1324  and a redistribution layer  1326 . The oxide layer  1324  includes the ultra-thick metal  1325 . The oxide layers  1314  and  1324  may include other metal layers (not shown). In this embodiment, two metal layers of an integrated inductor can be respectively implemented on the redistribution layer  1316  and the redistribution layer  1326 , and the connecting structure that connects different metal layers can be implemented by the micro-bump  1330 . The surroundings of the micro-bump  1330  are filled with underfill to strengthen the structure of the flip chip. As the manufacturing process advances with smaller semiconductor components, the distance between the die  1310  and the die  1320  becomes smaller, which improves the stability of the integrated inductor structure. 
         [0044]      FIG. 14  illustrates a structure in which an integrated inductor is made on different metal layers according to another embodiment of this invention. The semiconductor structure includes a die  1410 , which includes a substrate  1412 , an oxide layer  1414  and a redistribution layer  1416 . A substrate  1422  and an oxide layer  1424  are stacked on top of the die  1410 . The oxide layer  1414  and the oxide layer  1424  respectively include a metal layer  1415  and a metal layer  1425 . In this embodiment, the two metal layers of an integrated inductor can be implemented respectively on the metal layer  1415  and the metal layer  1425 , and the connecting structure that connects different metal layers can be implemented by a TSV (through-silicon via)  1430 , which is a via penetrating through a silicon layer and can be a single TSV or an array of TSVs. The metal layer  1415  and the metal layer  1425  can be made by the UTM layer or by a structure of other stacked metal layers in the oxide layer such as the metal layers M1 , M2 and M3 connected in parallel. The number of metal layers of this kind is different in various manufacturing processes and is usually 3 to 11 layers. Metal layers of this kind are usually thin, such as 0.09 μm as opposed to 3.4 μm of the UTM layer in a 28 nm process; therefore these metal layers are connected in parallel to reduce resistance. 
         [0045]      FIG. 15  illustrates a structure in which an integrated inductor is made on different metal layers according to another embodiment of this invention. This semiconductor structure includes a die  1510 , which includes an oxide layer  1514 , a substrate  1512  and an oxide layer  1516 . The oxide layer  1514  and the oxide layer  1516  respectively include the metal layer  1515  and the metal layer  1517 , and are respectively the front side and the back side of the die  1510 . In this embodiment, the two metal layers of an integrated inductor can be implemented respectively on the metal layer  1515  and the metal layer  1517 , and the connecting structure that connects different metal layers can be implemented by a TSV  1520 . Likewise, the metal layer  1515  and the metal layer  1517  can be made by the UTM layer or by a structure of other stacked metal layers in the oxide layer connected in parallel. The through-silicon via  1520  can be a single TSV or an array of TSVs. In addition to connecting the outer metal segments and the inner metal segments, the through-silicon via can also be used as a connecting structure that connect the outer/inner metal segments with the center tap. 
         [0046]    In addition to symmetric spiral inductor, this invention can also be applied to asymmetric spiral inductor.  FIG. 16  illustrates an asymmetric spiral inductor according to an embodiment of the present invention. This embodiment only shows a spiral inductor with 2 turns as an example, but this invention can be applied to a spiral inductor with more turns. An asymmetric spiral inductor  1600  includes metal segments  1612 ,  1614 ,  1616 ,  1622 ,  1624  and  1626 . The metal segments  1612 ,  1614  and  1616  form an outer part of the asymmetric spiral inductor  1600  and the metal segments  1622 ,  1624  and  1626  form an inner part. Specifically, because the metal segments  1614  has a turning point in a rectangular angle (can be different angles in other embodiments), the metal segments  1614  can be regarded as a combination of two metal sub-segments. Metal segments at the corresponding positions of the outer part and the inner part are disposed on different metal layers to reduce the parasitic capacitance between adjacent metal segments; to be specific, the corresponding metal segments  1612  and  1622  are on different layers, the corresponding metal segments  1614  and  1624  are on different layers, and the corresponding metal segments  1616  and  1626  are on different layers. If both the outer part and the inner part are circular, only one segment is included in each part, and the metal segment in the outer part and the metal segment in the inner part are corresponding metal segments and are therefore disposed on different metal layers according to this invention. In addition, different metal segments are connected by connecting structures; for example, the connecting structure  1630  connects the metal segment  1612  and the metal segment  1614 , the connecting structure  1632  connects the metal segment  1614  and the metal segment  1616 , the connecting structure  1634  connects the metal segment  1616  and the metal segment  1622 , the connecting structure  1636  connects the metal segment  1622  and the metal segment  1624 , and the connecting structure  1638  connects the metal segment  1624  and the metal segment  1626 . 
         [0047]      FIG. 17  illustrates an asymmetric spiral inductor according to another embodiment of the present invention. This embodiment shows a spiral inductor with only 2 turns as an example, but this invention can be applied to a spiral inductor with more turns. The asymmetric spiral inductor  1700  includes a metal segment  1710  (in light gray color) and metal segment  1720  (in dark gray color). The metal segment  1710  constitutes an outer part of the asymmetric spiral inductor  1700  and the metal segment  1720  constitutes an inner part of the asymmetric spiral inductor  1700 . In other words, metal segments of two adjacent turns are disposed on different metal layers. Specifically, because the metal segment  1710  has 3 turning points in rectangular angles (can be different angles in other embodiments), the metal segment  1710  can be regarded as a combination of 4 metal sub-segments  1710 - 1 ˜ 4710 - 4 . Likewise, the metal segment  1720  can be regarded as a combination of  4  metal sub-segments  1720 - 1 ˜ 1720 - 4 . The metal segment  1710 - 1  ( 1710 - 2 ,  1710 - 3 ,  1710 - 4 ) corresponds to the metal segment  1720 - 1  ( 1720 - 2 ,  1720 - 3 ,  1720 - 4 ) and are disposed on different metal layers. In one embodiment, the thickness of the metal segment  1710  is smaller than that of the metal segment  1720  so that the metal segment  1720 , namely the inner part of the asymmetric spiral inductor  1700 , has a smaller resistance; for example, the metal segment  1710  is implemented on the RDL while the metal segment  1720  is implemented on the UTM layer. If, however, there is a third metal segment in the area surrounded by the metal segment  1720 , the third metal segment is not on the same layer as the metal segment  1720 ; for example, it can be implemented on the same layer as the metal segment  1710  or on a third metal layer. The metal segment  1710  and the metal segment  1720  are connected by the connecting structure  1730 . In this embodiment, the parasitic capacitance between two adjacent metal segments is reduced by implementing metal segments of two adjacent turns on different metal layers and thus the quality factor Q and the bandwidth of an integrated inductor can be improved. 
         [0048]    A pair of integrated inductors that can be used as the inductor  410  and the inductor  420  of the VCO in  FIG. 4  can be formed based on a combination of the asymmetric spiral inductors  1600  or the asymmetric spiral inductors  1700 . Taking the asymmetric spiral inductor  1600  as an example,  FIG. 18  illustrates connected asymmetric spiral inductors according to an embodiment of this invention. The asymmetric spiral inductor  1810  and the asymmetric spiral inductor  1820  are connected via the metal segment  1830 . Since the symmetry between the asymmetric spiral inductor  1810  and the asymmetric spiral inductor  1820  is very good, a center tap (not shown) can be connected with the metal segment  1830  so that the current in each asymmetric spiral inductor flows through equal number and length of metal segments and equal number of connecting structures; as a result, the connected spiral inductors shown in  FIG. 18  is quite suitable for the inductors in a differential circuit. Further,  FIG. 19  illustrates connected asymmetric spiral inductors according to another embodiment of this invention. The asymmetric spiral inductor  1910  and the asymmetric spiral inductor  1920  share one metal segment and the center tap (not shown) can be connected with the shared metal segment so that the current in each asymmetric spiral inductor flows through equal number and length of metal segments and equal number of connecting structures; as a result, the connected spiral inductors shown in  FIG. 19  is also quite suitable for the inductors in a differential circuit. However, whether the connected asymmetric spiral inductors of  FIG. 18  or the connected asymmetric spiral inductors of  FIG. 19 , the currents in the two connected asymmetric spiral inductors flow in the same direction (both clockwise or counterclockwise), which causes the radiation of magnetic field that affects other components in the circuit. The radiation of magnetic field can be greatly reduced by adjusting the connection of the asymmetric spiral inductors.  FIG. 20  illustrates connected asymmetric spiral inductors according to another embodiment of this invention. The asymmetric spiral inductor  2010  and the asymmetric spiral inductor  2020  are mirror structures and are connected by the metal segment  2030 . A center tap (not shown) can be connected to the metal segment  2030 . Most magnetic field, as indicated by dots and x&#39;s in the figure, is confined in the connected asymmetric spiral inductors because the current in the asymmetric spiral inductor  2010  flows clockwise whereas the current in the asymmetric spiral inductor  2020  flows counterclockwise. Therefore, components in a compact integrated circuit are free of interference from the radiation of magnetic field. 
         [0049]    In addition to the asymmetric spiral inductor  1600  in  FIG. 16 , the connected structure of the asymmetric spiral inductor in  FIGS. 18˜20  can be implemented by the asymmetric spiral inductor  1700  as well, as shown in  FIGS. 21˜23 . In addition, the asymmetric spiral inductors in  FIGS. 16 and 17  can also be implemented by the semiconductor structures in  FIGS. 13˜15 . 
         [0050]    The shape, size, and ratio of any element in the disclosed figures are just 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 present invention are all consequently viewed as being embraced by the scope of the present invention.
This invention discloses a structure of an integrated inductor, comprising: an outer metal segment which comprises a first metal sub-segment and a second metal sub-segment; an inner metal segment which is arranged inside an area surrounded by the outer metal segment and comprises a third metal sub-segment and a fourth metal sub-segment; and at least a connecting structure for connecting the outer metal segment and the inner metal segment. The first metal sub-segment corresponds to the third metal sub-segment, and the first metal sub-segment and the third metal sub-segment belong to different metal layers in a semiconductor structure. The second metal sub-segment corresponds to the fourth metal sub-segment, and the second metal sub-segment and the fourth metal sub-segment belong to different metal layers in a semiconductor structure.