Abstract:
The present disclosure relates to composite inductor structures for use in integrated circuits. There is provided a composite inductor structure comprising a first inductor coil and a second inductor coil. The second inductor coil comprises a multi-turn loop that surrounds the first inductor coil. The first inductor coil comprises two multi-turn loops which are connected in a figure-of-eight configuration about a central terminal so as to cause a current flowing in a first loop of the multi-turn loops to circulate around the first loop in a first rotational direction, and a current flowing in a second loop of the multi-turn loops to circulate around the second loop in a second rotational direction opposite the rotational direction of current flow in the first loop, said direction of current flow in the first and second loops being mirror images of each other.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to inductor structures found in RF designs such as low noise amplifiers (LNA), power amplifiers (PA), and/or Voltage Controlled Oscillators (VCO). The present invention is particularly applicable to dual wide-band VCOs. 
         [0002]    BACKGROUND 
         [0003]    Inductors are often used in integrated circuits, such as the voltage controlled oscillator  100  shown in  FIG. 1 . When multiple inductors L 1  and L 2  are present in such circuits, or in separate circuits on the same IC substrate, there is a risk that the inductors magnetically couple with each other, which, in turn, may affect the operation of the integrated circuit as the resulting currents induced in the components can cause unwanted changes in their behavioural characteristics. The location and proximity of these components is a factor in the degree of magnetic coupling present. To mitigate this problem, integrated circuits are often designed such that inductors are physically separated as far as is practical. However, such design topologies occupy a large area on chip and it is desirable to minimise the chip area required for an integrated circuit. Furthermore, it is desirable to conserve chip area without compromising the performance of the integrated circuit. 
         [0004]    It has been proposed to reduce the area required by a circuit comprising more than one inductor by embedding an inductor within another. A design of an integrated inductor and transformer known in the art is illustrated in  FIG. 2  (from US 2011/0248809 and US2012/0326826) where the inductor structure  200  includes a first inductor  201  and a transformer  202  comprising a second inductor  203  and a third inductor  204 , in which the first inductor  201  is embedded within the transformer  202 , such that the magnetic effect of a current flowing through the inductor  201  cancels that of the outer inductors  203  and  204  such that no magnetic coupling of these coils occurs. This cancellation of magnetic effect in  FIG. 2  is due to the figure-of-eight configuration of the first inductor  201  such that the magnetic component generated by the current flowing in outer inductors  203  and  204  is removed, while at the same time said outer inductors  203  and  204  are interleaved to form the transformer  202 . 
         [0005]    A low noise amplifier (LNA) circuit  300  using the inductor structure  200  is shown in  FIG. 3 . The circuit  300  comprises several elements and includes inductor elements  201 ,  203  and  204 . The circuit  300  shows how the first inductor  201  and outer inductors  203  and  204  of inductor structure  200  can be connected. It is clear from  FIG. 3  that while the inductor structure  200  of  FIG. 2  economises on chip area, the device does not have the functionality to operate all inductors as discrete isolated inductors that can be configured to operate independently or together as coils L 2  and L 3  cannot be decoupled from transformer  202  and used separately. Further, the outer inductors  203  and  204  (represented by coils L 2  and L 3 ) and the inner inductor  201  (represented by coil L 1 ) do not share a common ground connection which balances the whole structure in which a node common to all of the coils is forced to specific potential where the common mode current through coils L 1  to L 3  can be controlled. 
         [0006]    There is therefore a need for an improved integrated inductor structure that can be configured to operate as independent inductors, or as a composite inductor, as required by an integrated circuit, while minimising the occupied chip area and ensuring mutual isolation between the independent devices. 
       SUMMARY OF THE INVENTION 
       [0007]    There is provided herewith a composite inductor structure comprising a first inductor coil and a second inductor coil, the second inductor coil comprising a multi-turn loop that surrounds the first inductor coil, and the first inductor coil comprising two mirror imaged multi-turn loops which are connected in a figure-of-eight configuration about a central terminal so as to cause current flowing in a first loop of the two mirror imaged multi-turn loops to circulate around the first loop in a first rotational direction, and a current flowing in a second loop of the two mirror imaged multi-turn loops to circulate around the second loop in a second rotational direction opposite to the rotational direction of current flow in the first loop, wherein the central terminal connects the first inductor coil to the second inductor coil such that the size of the first loop of the first inductor coil connected to the central terminal is equal to the size of the second loop of the first inductor coil connected to the central terminal 
         [0008]    In embodiments, the central terminal equally divides the loop of the second inductor coil. 
         [0009]    In other embodiments, the first inductor coil is 180° rotationally symmetric about the central terminal. 
         [0010]    In other embodiments, the central terminal is connected to a DC supply with a de-coupling capacitor or to a ground terminal 
         [0011]    In other embodiments, the electromagnetic fields generated by the currents circulating in the first and second loops of the first inductor coil induce electromagnetic currents in the second inductor coil wherein the magnitude and direction of these induced currents are such that they effectively cancel out. 
         [0012]    In other embodiments, the first and second inductor coils may be operated independently, concurrently, or one at a time. 
         [0013]    In other embodiments, the first and second loops of the first inductor structure, and the loop of the second inductor structure, each have a plurality of windings. 
         [0014]    In other embodiments, the width of each of the windings of the first and second inductor coils is either varied or the same moving from the innermost winding to the outermost winding. 
         [0015]    In other embodiments, the turns of the windings in the first and second inductor coils are separated by a spacing. 
         [0016]    In other embodiments, the spacing is either varied or the same moving from the innermost winding to the outermost winding. 
         [0017]    In other embodiments, for use in a dual-band voltage controlled oscillator, Low Noise Amplifier (LNA), and Power Amplifier (PA). 
         [0018]    In other embodiments, the shape of the first and second inductor coils is one of: circular, octagonal or square. 
         [0019]    In other embodiments, the central terminal connects the first and second coil using on either the same metal layer, or on different metal layers using corresponding vias structures. 
         [0020]    In other embodiments, the first and second inductor coils are fabricated on the same layer within an integrated circuit. 
         [0021]    In other embodiments, the first and second inductor coils are fabricated on different layers within an integrated circuit and are connected using via structures. 
     
    
     
       FIGURES 
         [0022]      FIG. 1  illustrates a schematic circuit diagram of a voltage controlled oscillator known in the art; 
           [0023]      FIG. 2  depicts an integrated inductor and transformer structure known in the art; 
           [0024]      FIG. 3  illustrates an LNA circuit diagram using the integrated inductor and transformer structure of  FIG. 2 ; 
           [0025]      FIG. 4  depicts the dual-band integrated inductor structure according to an embodiment of the present invention; 
           [0026]      FIG. 5  illustrates the equivalent circuit diagram of the integrated inductor structure of  FIG. 4  according to an embodiment of the present invention; and 
           [0027]      FIG. 6A  illustrates the interconnections made between the integrated inductor structure of the present invention and two voltage controlled oscillators, while  FIG. 6B  depicts VCOs connected to two separate coils in a conventional arrangement. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]      FIG. 4  shows a schematic diagram of a dual-band IC inductor structure  400  according to an embodiment of the present invention for use in system-on-chip (SoC) integrated circuits such as Voltage Controlled Oscillators (VCO). The dual-band inductor  400  of  FIG. 4  is fabricated using known integrated circuit fabrication technology. 
         [0029]    The dual-band inductor structure  400  comprises a first inductor coil  401  and a second inductor coil  402 . The first inductor coil  401  comprises a first loop  403  and a second loop  404  connected in a figure-of-eight arrangement. The second inductor coil  402  comprises a loop that encloses the first inductor coil  401 . In the example of  FIG. 4  the first inductor coil  401  may take any shape. Preferably, the first  403  and second  404  loops of the first inductor coil  401 , and the second inductor coil  402 , are octagonal. Alternatively, the first  401  and second  402  inductor coils are either circular or square shaped. With this arrangement, the dual-band inductor structure  400  of  FIG. 4  utilises one coil space or area set by the size of the second inductor coil  402 . Hence the area occupied by the first inductor coil  401  is inherent and thus effectively ‘comes free’, thereby maximising utilisation of silicon area in an IC. 
         [0030]    The dual-band inductor structure  400  of the present invention can be utilised in multi-band VCOs. When used in a dual-band VCO, the size of the first inductor coil  401  determines the higher frequency band of the VCO while the size of the second inductor coil  402  determines the lower frequency band of the VCO. The lower frequency band (determined by inductor  402 ) of the VCO dictates the size of the dual-band inductor structure  400  and the coil  401  for the higher frequency band does not require additional area because it is enclosed inside the low frequency band coil  402 . Thus when designing the dual-band inductor structure  400  of the present invention, the coil  401  is designed first for the required inductance value and Q-factor. The coil  402  is then optimised independently for the required parameters and embedded around the coil  401 . The second coil  402  is then fine-tuned when it surrounds the first coil  401  to account for any changes in performance due to the first coil  401 . 
         [0031]    The first loop  403  and the second loop  404  of the first inductor coil  401 , and the loop of the second inductor coil  402  are connected to a central terminal  405  within the inductor structure. In an embodiment of the present invention, terminal  405  is the physical RF ground terminal of the inductor structure  400 . In the example of  FIG. 4 , excluding terminal  405 , the first inductor coil loops  403  and  404  are configured such that they are rotationally symmetric about point  406  as shown in  FIG. 4 . This symmetry ensures that the magnetic effects of these coils are matched and thus cancel out. 
         [0032]    In an embodiment of the present invention, inductor coils  401  and  402  are fabricated on the same layer within an integrated circuit structure. Alternatively, inductors  401  and  402  may be located on separate layers. Further, in the embodiment shown in  FIG. 4 , central terminal  405  is formed by an extension of the material used for the first loop  403  and the second loop  404  of the first inductor coil  401 . However it will be appreciated that this terminal may alternatively be implemented in other ways (for example, on a different layer to that containing the first  401  and second  402  inductor coils, connected to said coils with a via). 
         [0033]    Each of the turns  409  to  415  of the windings in the first  401  and second  402  inductor coils are separated from each other by spacing  408  for the first inductor  401  and spacing  407  for the second inductor  402 . In an embodiment of the present invention, the width of each of the windings  409  to  415  and the spacing  407  and  408  between said windings are the same. Alternatively, these dimensions  407  to  415  may vary within each of the inductors  401  and  402  to attain a target inductance and/or Q-factor to optimise performance. 
         [0034]    The dual-band inductor structure described herein before may (i) reduce the required area, (ii) allow the coils to operate independently, and concurrently as a single standalone inductor, and (iii) allow the use of an effective single S-parameter model where coupling between coils can be included and optimised. 
         [0035]    It is worth noting that the Q-factor is a representation of the losses in the coil due to the electromagnetic field distribution resulting from its unique structure. In the structure of  FIG. 4 , the first (inner) inductor coil  401  has a certain loss; when this inner coil  401  is surrounded by the second (outer) inductor coil  402 , the electromagnetic fields coupled from the outer coil  402  to the inner coil  401  will be subject to the losses of the inner coil  401 . Hence the lowest Q-factor inductor coil will dominate the structure performance. 
         [0036]    The coupling between the inner and outer coils can be reduced by pushing the outer coil  402  out and away from the inner coil  401 . Figure-of-eight coils have lower Q-factor than corresponding coils of standard design. Thus in the embodiment depicted in  FIG. 4 , the inner coil  401  has a lower Q-factor than the outer coil  402 . However, the reduction in Q-factor is more than compensated for by the reduction of area occupied by the integrated coils. 
         [0037]    In the foregoing description, the inductance of each coil can be set independently from each other while the Q-factor of either coil is set by the minimum Q-factor of either coil. 
         [0038]    In an exemplary embodiment of the present invention, the width of the windings  409  and  410  of the first inductor coil  401  increases outwards from 3p.m to 7p.m, and the widths of the windings  413  to  415  of the second inductor coil  402  are 8p.m, 9p.m and 6p.m, respectively, moving outwards. In both inductors, the respective windings are separated by a spacing  407  and  408  of 3p.m. In this exemplary embodiment, the coil  402  has a Q-factor of &gt;15 when no coil is embedded inside it and the coil  401  has Q-factor of &gt;13 when no coil is surrounding it. Combining both the coils in the dual-band inductor structure  400  of  FIG. 4 , a Q-factor of 13 for the coil  401  and 12.7 for the coil  402  is achieved. In one embodiment of the present invention, the coil  401  is designed to work optimally at 10 GHz and the coil  402  is designed to work optimally at 4 GHz. 
         [0039]    It should be noted that due to the skin depth effect, the current usually flows in the side walls of the coil trace. Hence the dimensions of features  407  to  415  will determine the self and mutual inductance inside the inductor structure  400 . Coils with wider turns have more electric field coupling (the capacitive effect); thus using this capacitive element, it is possible to tune and optimise the self resonance frequency of the inductor which, in turn, will shift the Q-factor peak and change the inductance value accordingly. 
         [0040]    Notably, in an embodiment of the present invention, the turns of the windings  413  to  415  and of the outer inductor coil  402  are discontinuous and are connected so as to form a continuous structure; this connection is facilitated by crossover sections  416  and  417  fabricated on a different layer to the windings and connected thereto by vias  418  to  421 . 
         [0041]    In a further embodiment of the present invention, the first inductor coil  401  is continuous due to the loops  403  and  404  of its figure-of-eight structure. Alternatively, in a further embodiment, the first inductor coil  401  may have windings that are discontinuous similar to that of the second inductor coil  402 . In another embodiment, the second inductor coil  402  may have windings that are continuous similar to that of the first inductor coil  401  of the present invention. 
         [0042]    In the configuration of  FIG. 4 , inductor coils  401  and  402  can function as two independent inductor coils placed one within another that can be used independently and/or at the same time without affecting the electrical performance of either coil while sharing a common centre-tap point  405 . In one embodiment, this centre-tap point is an AC ground reference. 
         [0043]    The equivalent circuit  500  of the inductor structure  400  is shown in  FIG. 5 . The first inductor coil  401  is represented by identical equivalent inductors  510  and  520  and the second inductor coil  402  is represented by identical inductors  530  and  540 . Each of these inductors in the equivalent circuit  500  have one point connected to a common centre-tap point  550  and the other point connected to input terminals P 1 , P 2 , P 3  and P 4  in a star configuration, thereby effectively forming a five port passive device. The centre-tap point  550  in  FIG. 5  is physically realised by the central terminal  405  in  FIG. 4 . 
         [0044]    During use of the inductor structure  400 , alternating current flows in the first inductor  401  via terminal P 3  through to terminal P 4  as shown in  FIG. 5 . Due to the figure-of-eight structure of first inductor  401 , during a first cycle of AC operation, the current flows in a clockwise direction in loop  403  and an anti-clockwise direction in loop  404 ; these directions of current flow are also illustrated in  FIG. 5 . As loops  403  and  404  in the first inductor  401  are equal in size and shape, the electromagnetic fields generated by the currents travelling in each of loops  403  and  404  induce electromagnetic far field currents in the outer coil  402 ; these induced currents are equal in magnitude but opposite in direction, thus effectively cancelling each other out. Further, the current flowing in the inner coil  401  will induce current flow in the outer coil  402  due to electromagnetic near field coupling between these structures. Thus, an (induced) current will physically flow in the outer coil  402  and shortly get cancelled out at the mid-point  430  of outer coil  402  (labelled ‘X’ in  FIG. 5 ). Thus, when the coils  401  and  402  are used at the same time, no additional insulating means (such as active switches) are required to isolate the coils  401  and  402  from each other. The reverse would occur during a second, opposite, cycle of AC operation. 
         [0045]    The centre-tap point  550  enables the coils  401  and  402  to be utilised independently, concurrently or one at a time. 
         [0046]    The dual-band inductor coil  400  of the present invention can be utilised in two oscillators  601  and  602  using the connection terminals P 1  to P 4  of the dual-band inductor coil  400 , as shown in  FIG. 6A . This configuration removes the need for two separate coils  620  and  630 , each of which takes up area on a chip. Due to the isolation provided between the first inductor coil  401  and the second inductor coil  402  in the dual-band structure  400 , the proposed dual-band VCO  600  avoids interaction between the two coils during use. This is beneficial compared to using separate coils (such as coils  620  and  630  as shown in  FIG. 6B ) for each of VCO 1  and VCO 2 . Even if the coils are widely separated (thus consuming large areas), some mutual coupling occurs and has to be factored into the design (such as ensuring a minimum separation  640  between coils). 
         [0047]    The inductor structure  400  of the present invention allows VCO 1  and VCO 2  to be connected to coils  401  and  402  via terminals P 1  to P 4  as shown in  FIG. 4 , and can operate concurrently, independently or one at a time. As no active switches are required to isolate embedded coils  401  and  402 , a high Q-factor can be maintained for both VCO 1  and VCO 2 . Furthermore, the centre-tap point  550  provides a common DC feed and AC ground point which reduces supply routing on the IC should these oscillators be implemented separately. 
         [0048]    In a further embodiment, the embedded coil configuration of the present invention can be used for radio frequency (RF) circuits such as low noise amplifiers (LNA) in addition to the dual VCO application discussed above. 
         [0049]    In the foregoing, the term ‘size’ may take on the meaning of length. Thus in the context of the present inductor structure, the term ‘size’ would refer to the length of the metal conductor used to form the respective coil of the structure. 
         [0050]    It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention.