Abstract:
A planar inductive unit having at least one operating frequency is provided. The inductive unit comprises at least one inductor winding ( 120 ) having a first width ( 121 ) and a centre ( 122 ) and being arranged in a first plane. The inductive unit furthermore comprises at least one ground path ( 200 ) having a first section ( 205 ) extending in the first plane and at least a second section ( 210 ) with a second width ( 211 ) extending in at least a second plane.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a planar inductive unit and an electronic device comprising a planar inductive unit. 
     BACKGROUND OF THE INVENTION 
     When different circuits are coupled to each other, often the impedances thereof do not match. Therefore, an impedance matching network may be required to match the output impedance of a first unit to the input impedance of a second subsequent unit, i.e. the output impedance of a source is made equal to the output impedance of a load. Here, the first impedance can be e.g. an amplifier stage in a RF circuit and the second impedance may be an input impedance of an amplifier stage or an antenna. 
     WO 2004/055839 A1 discloses a planar inductive component which is arranged over a substrate. The substrate comprises a winding which is situated in a first plane and a patent ground shield for shielding the winding from the substrate. 
       FIGS. 1A and 1B  show a circuit diagram of Pi matching networks according to the prior art. The matching network according to  FIG. 1A  comprises an inductor L, two capacitors C, connecting a load with an input impedance Z 1  to a source with an output impedance. It should be noted that a non-zero ground inductance Lg can be present. Such a non-zero ground inductance is not desirable as it will have a negative influence on the behaviour of the matching network. 
       FIG. 2A  shows a basic representation of an inductor in an integrated circuit. In particular, the inductor has been placed in a two-port ground-signal-ground test configuration in order to test the performance of the IC inductor. In  FIG. 2A , a representation of an IC inductor is depicted which can for example be used in the matching networks according to  FIGS. 1   a  and  1   b . It should be noted that the inductor performance evaluation structure according to  FIG. 2A  comprises two very wide ground lines GL placed symmetrically around the inductor I. It should further be noted that the ground lines GL need to be very wide to minimize the inductance Lg. Furthermore, they should be placed with sufficient clearance from the inductor to ensure that the performance of the inductor I is not affected. If the IC inductor according to  FIG. 2A  is investigated, for example by means of simulations, the result will correspond to the circuit of  FIG. 1B  instead of the circuit according to  FIG. 1A . It should be noted that the huge area covered by the two ground lines can be a problem and is in particular not desirable. If the ground lines are removed or if their width is reduced, the ground inductance is increased significantly. 
       FIG. 2B  shows an 8 shaped inductor according to the prior art. The inductor according to  FIG. 2B  corresponds to the inductor as depicted in WO 2004/012213 A1. If several conductors are placed adjacent to each other, a crosstalk between the inductors may lead to undesired effects. According to WO 2004/012213 A1, two oppositely directed current loops  211 ,  212  in an 8 shaped inductor are advantageous with respect to the cancellation of the magnetic fields such that the crosstalk can be reduced. The eye  209  of the winding to which the supply lines lead to is smaller than the other eye  210 . This can be performed in order to compensate for the magnetic fields of the supply lines. The specific implementation of this requirement can be very tricky in particular as the geometry of the supply lines and the return path of the ground current should be known before the correction is performed. 
       FIG. 3  shows a three dimensional view of an inductor unit according to the prior art. Here, a ground path  200  and the inductor  100  is depicted. The inductor  100  comprises a number of turns  120  and is implemented as a planar inductor. The inductive component also comprises an underpass  110  which is used to couple one end of the inductor turns to one inductor terminal. It should be noted that the width as well as the clearance of the ground paths  200  have been reduced as compared to the inductor according to  FIG. 2A . This is performed in order to minimize the footprint of the device. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a planar inductive component with an improved ground path inductance. 
     This object is solved by a planar inductive component according to claim  1  or  5  and an electronic device according to claim  8 . 
     Therefore, a planar inductive unit with at least one operating frequency is provided. The planar inductive unit comprises at least one inductor winding having a first width and a centre. The at least one conductor winding is arranged in a first plane. The planar inductive unit furthermore comprises at least one ground path having a first section extending in the first plane and at least a second section with a second width extending in at least a second plane. 
     According to an aspect of the invention, the second width of the second section of the ground path and/or an offset of the second section of the ground path and/or an offset of the second section of the ground path from the centre of the at least one inductor winding is selected such that the mutual inductance of the at least one winding and the ground path equals a negative inductance of the ground path at the at least one operating frequency. 
     According to a further aspect of the invention, the inductor winding is arranged in a first metal layer in the first plane and the second section of the ground path is arranged in a second layer in the second plane. 
     According to a further aspect of the invention, the planar inductive unit comprises a ground shield unit which is arranged in a third plane and which is used for shielding the at least one winding from a substrate. 
     The invention also relates to a planar inductive unit having at least one operating frequency. The planar inductive unit comprises at least one 8 shaped inductor having at least one first and at least one second eye. The inductor is arranged in a first plane and has at least one first width. The inductive unit furthermore comprises at least one ground path having a second width and extending in the first plane. The ground path is arranged between the first and second eye of the at least one inductor. 
     According to an aspect of the invention, the inductor comprises at least one underpass for coupling the first and second eye. The underpass is arranged in a second plane. 
     According to a further aspect of the invention, the distance between the first and second eye and the ground path is selected such that the mutual inductance of the first and second eye and the ground path equals a negative inductance of the ground path at the at least one operating frequency. 
     The invention also relates to an electronic device which comprises at least one planar inductive unit as described above. 
     The invention relates to the idea to use the ground path as a part of an impedance matching inductor or an inductive unit. Furthermore, instead of minimizing the ground inductance by minimizing the length of the ground path, the adverse effect of the ground impedance can optionally be compensated by a mutual inductance between the signal current and the ground current. It should be noted that the ground inductance relates to the development of a ground lift voltage Vg at the load impedance. The signal voltage corresponds to V s =jω(L s I s +M sg I g ) and the ground voltage corresponds to V g =jω(L g I g +M sg I s ). Is and Ig correspond to the signal currents and the ground currents. If the impedances are matched, the signal and ground currents are equal, i.e. the ground lift voltage Vg can be minimized by providing a ground path such that Msg equals −Lg at the operating frequency of the matching network. 
     The invention also relates to the idea to place a ground path between a first and second eye of an 8 shaped inductor, wherein the ground path is arranged in the same plane as the inductor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments and advantages of the present application will be described in more detail with reference to the Figures. 
         FIGS. 1A and 1B  show a circuit diagram of Pi matching networks according to the prior art, 
         FIG. 2A  shows a basic representation of an inductor in an integrated circuit according to the prior art, 
         FIG. 2B  shows an 8 shaped inductor according to the prior art, 
         FIG. 3  shows a three dimensional view of an inductor unit according to the prior art, 
         FIG. 4  shows a three dimensional view of an inductive unit according to a first embodiment, 
         FIG. 5  shows a graph of the quality factor versus the frequency of an inductor component according to the first embodiment, 
         FIG. 6  shows a graph depicting the ground inductance versus the frequency of an inductor according to the prior art as compared to an inductive component according to the first embodiment, 
         FIG. 7  shows a graph depicting a coupling between two straight lines running in parallel in close proximity, 
         FIG. 8  shows a representation of an inductive unit according to the second embodiment, 
         FIG. 9  shows a three dimensional representation of an inductive unit according to the third embodiment, 
         FIG. 10  shows a graph depicting the quality factor versus the frequency of an inductive component according to the second embodiment, 
         FIG. 11  shows a graph depicting the ground impedance versus frequency of the inductive components according to the second and third embodiment, and 
         FIG. 12  shows a three dimensional representation of parallel symmetric impedance matching inductors according to a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 4  shows a three dimensional view of an inductive unit according to a first embodiment. The inductive component comprises an inductor  100  with a first width  105  and several inductive turns  120  as well as an underpass  100  for coupling one terminal  106  of the inductor to the end of the inductor turns  121 . Furthermore, a ground path  200  with a second width  211  and an underpass  210  and a ground shield  300  is depicted. 
     It should be noted that the footprint of the inductive component according to the first embodiment as compared to the footprint of the inductive component according to the prior art as depicted in  FIG. 3  is reduced by a factor of 2 from for example 0.23 mm 2  down to 0.11 mm 2 . The turns  120  of the inductor are for example implemented by 3 μm aluminium top metal layer which can be manufacture in an IC manufacturing process. The underpass  110 ,  210  can be implemented by a 1 μm thick semiconductor metal layer. The ground shield  300  can be made of a 0.3 μm bottom metal layer. The separation between the metal layers can for example be 3 μm. The resistivity of the substrate is for example 10 ohm/cm which can be manufactured by a typical IC process. Optionally, the ratio between the width of the turns  120  of the conductor to the width of the underpass is approx. 3:1. 
     It should be noted that in contrast to the prior art inductor according to  FIG. 3 , the ground path is realized by an underpass  210  which can for example be implemented in a lower metal layer. The width and the offset of the ground underpass  210 ,  110  are chosen in order to realize the condition Msg=−Lg at the operating frequency of the matching network. It should be noted that Lg depends on the width  211  of the ground underpass  210  and that Lg is reduced if the width  211  of the underpass is increased. Msg increases with the offset of the ground underpass from the centre of the inductor until the underpass is immediately below the two outer most turns of the inductor  100 . 
     The opposite signs of the Lg and Msg can be realized by an offset as depicted in  FIG. 4 . Preferably, the ground path is not implemented in the same metal layer as the inductor  100 . Preferably, the inductor comprises more than a single turn. By the inductive component according to the first embodiment, a multi-turn impedance matching inductor can be realized which also enables a ground inductance cancellation. 
     Optionally, the inductive component according to the first embodiment also comprises a ground shield  300  which can be patterned and which can be realized in a further (third) metal or polysilicon layer. The ground shield is used in order to reduce losses which may arise from a capacitive coupling of the lossy substrate. 
     For the cases that the substrate resistivity is large (larger than 100 ohm/cm) or very low (less than 0.1 ohm/cm) such a substrate is less lossy for capacitive currents. Hence, in such a situation, the ground shield  300  can be omitted. 
       FIG. 5  shows a graph of the quality factor Q versus the frequency of an inductive unit/component according to the first embodiment. Here, a graph  3  depicting the quality factor versus the frequency of the prior art inductor and a graph  4  depicting the quality factor versus the frequency of the inductive component according to the first embodiment is depicted. The inductance of the inductor according to  FIG. 3  and the inductive component according to  FIG. 4  is both approx. 5 nH. It should be noted that the quality factor Q of the inductive component according to  FIG. 4  is reduced at low frequency but it has been improved at the operating frequency of 2 GHz. The reduction of the quality factor at low frequencies are due to the higher resistance of the ground path while the improvement at the operating frequency of 2 GHz is because of the patterned ground shield. 
       FIG. 6  shows a graph depicting the ground inductance versus the frequency of an inductor according to the prior art as compared to an inductive component according to the first embodiment. By means of the underpass as depicted in  FIG. 4 , a better or improved ground can be provided at the operating frequency of 2 GHz as compared to a large ground lead as depicted in the prior art inductor in  FIG. 3 . By positioning the underpass, the required cancellation of inductive effects can be realized. 
     The inductive component according to the first embodiment is advantageous as its footprint or area is reduced for example by up to 50% while the performance and the operating frequency can be improved. This can be achieved by exploiting a cancellation of inductive effects. 
     The inductive element according to the first embodiment can be used in almost all application fields like low power fully integrated wireless transceiver chips, power amplifier modules or RF amplification stages. 
       FIG. 7  shows a graph depicting an inductive coupling between two straight conductors running in parallel in close proximity. Here, the inductive coupling factor CF is depicted versus the length over width ratio l/w. A coupling between two inductor lines running in parallel over a sufficient length is approximately 0.5. According to the second embodiment, the ground lines are provided to pass through a centre point of symmetry signal lines with opposite currents in an 8 shaped inductor are placed sufficiently close to each side of the ground line to achieve a coupling factor with the ground of 0.5. By means of such an arrangement, the ground inductance can be completely cancelled. 
       FIG. 8  shows a representation of an inductive component according to the second embodiment. The inductive component comprises a ground path  200  with a width  201  and an inductor  100 , wherein two eyes  140 ,  150  of the inductor are provided in order to achieve an 8 shaped inductor. Here, the 8 shaped inductor is realized by two single turns. 
     The connection or coupling between the first eye  140  and the second eye  150  is implemented by an underpass  120 . Preferably, the underpass  120  has a hole  125  in its centre. The ground path  200  is provided in the same layer as the first and second eye  140 ,  150  while the underpass  120  is provided in a second (lower) layer. The inductive components furthermore comprise a ground shield  300  which can be arranged in a third (lower) layer. 
       FIG. 9  shows a three dimensional representation of an inductive component according to the third embodiment. The inductive component according to the third embodiment substantially corresponds to the inductive component according to the second embodiment. The difference is that the inductive components according to the second embodiment each comprise two turns. 
     The eyes  140 ,  150  of the 8 shaped inductor according to the second and third embodiment are arranged such that the distance or separation between the eyes is increased such that a ground path  200  and an underpass  120  between the two eyes  140 ,  150  can be provided. The ground path  200  and the underpass  120  can be provided in a second, lower metal layer. The underpass  120  in the second layer may comprise a hole  125  such that optionally a ground shield  300  can be connected to the ground path  200  (through the hole  125 ). Furthermore, the capacitance between the underpass  120  and the ground return line as well as the substrate can be reduced by providing the second lower metal layer. In addition, the eddy current loss with may result from the inductor magnetic field in the underpass can be reduced. 
     It should be noted that the distance between the ground path  200  of the conductor to the ground current return line is chosen that Msg=−Lg in particular at the operating frequency of a matching network. It should be noted that Lg depends on the width of the ground return line and is reduced if its width is increased. Msg decreases with an increasing separation of the eyes. If the eyes are at a minimum distance from the ground return line, typically Msg&lt;−Lg such that a negative net ground inductance is achieved. A negative net ground inductance can be desirable in order to compensate a ground inductance encountered in the circuitry. 
     Optionally, a patterned ground shield  300  can be provided in a third metal layer or in a polysilicon layer. The patterned ground shield  300  is also used to reduced losses which may result from capacitive coupling to lossy substrates. However, if the substrate resistivity is very high (&gt;100 Ohm/cm) or very low (&lt;0.1 Ohm/cm), such a substrate is less lossy for capacitive currents such that the ground shield may be omitted. 
       FIG. 10  shows a graph depicting the quality factor Q versus the frequency. Here, a graph  8  depicting the quality factor Q of the inductive component according to  FIG. 8  and a graph  9  depicting the quality factor Q of an inductive component according to  FIG. 9  is depicted. 
       FIG. 11  shows a graph depicting the ground impedance versus frequency of the inductive components according to the second and third embodiment. In  FIG. 11 , a graph  8   a  depicting the ground impedance of the inductive component according to  FIG. 8  and a graph  9   a  depicting the inductive impedance of the inductive component according to  FIG. 9  is depicted. 
     It should be noted that by positioning the eyes of the inductor and the ground path, at least some of the inductive effects can be cancelled. Therefore, the ground line or ground path can provide a good ground at the operating frequency of 2 GHz. If the ground line is realized in a low resistivity top metal layer, the residual resistance at the cancellation frequency can be better than that of an inductive component according to  FIG. 4 . 
     The planar inductive unit according to the second and third embodiment is adapted to cancel net magnetic fields, to minimize the net inductance of the ground return path and to provide a beneficial inductive coupling for multiple units ins parallel. 
       FIG. 12  shows a three dimensional representation of parallel symmetric impedance matching inductors according to a fourth embodiment. Here, each symmetric impedance matching inductor is mirrored with respect to its neighbour. Neighbouring eyes of the inductors are placed at minimum space. The spacing between two eyes of the device can be optimised for minimal net ground inductance or for achieving a more compact layout with some degree of negative ground inductance. The impedance inductors according to the fourth embodiment substantially correspond to the inductive units according to the third embodiment. 
     Due to the close proximity of mirrored neighbours, the impedance of each unit can be improved from 4.3 nH to 5 nH. Furthermore, Msg is reduced and also allows a reduction of Lg which can be performed by doubling the ground path width. Such a doubling of the ground path width is advantageous with respect to the residual ground resistance per unit at the cancellation frequency which can may involve a factor of 2. 
     With the planar inductive units according to the above embodiments it is possible to design the inductor such that its terminals can extend to any direction, i.e. the terminals of the inductor can be implemented as depicted in the  FIG. 4 ,  8  or  9 , i.e. straight. Alternatively or additionally, the terminals may extend sideways e.g. with a certain angle, such as 90°, 270° and 120°. It should be noted that the above also applies for the ground path. Also combinations of terminals and groundpaths having a variety of angles are envisaged. 
     The planar inductive unit according to the above embodiments can be used in any electronic device or semiconductor device which requires an inductive component. By means of the invention the size of the inductor can be reduced by 50% while still improving the performance at its operating frequency. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 
     Furthermore, any reference signs in the claims shall not be constrained as limiting the scope of the claims.