Abstract:
Inductive components, such as transformers, can be improved by the inclusion of a magnetic core. However the benefit of having a core is lost if the core enters magnetic saturation. One way to avoid saturation is to provide a bigger core, but this is costly in the context of integrated electronic circuits. The inventor realized that the flux magnetic flux density varies with position in a magnetic core within an integrated circuit, causing parts of the magnetic core to saturate earlier than other parts. This reduces the ultimate performance of the magnetic core. This disclosure provides structures that delay the onset of early saturation, enabling a transformer to handle more power.

Description:
FIELD 
       [0001]    The present disclosure relates to an improved inductor or improved transformer fabricated using microelectronic techniques, and to integrated circuits including such an inductive component. 
       BACKGROUND 
       [0002]    It is known that magnetic components, such as inductors and transformers have many uses. For example inductors may be used in the fabrication of filters and resonant circuits, or may be used in switched mode power converters to boost or reduce an input voltage for generation of a different output voltage. Transformers may be used in the transfer of power or signals from one circuit to another while providing high levels of galvanic isolation. 
         [0003]    Inductors and transformers can be fabricated within an integrated circuit environment. For example it is known that spaced apart conductors generally forming a spiral or an approximation of a spiral can be formed on or within a semiconductor substrate to form a coil as part of an inductor or a transformer. Such spaced apart spiral inductors can be placed side by side or in a stacked configuration. 
         [0004]    It is also possible to form a “coil” around a ferromagnetic core within an integrated circuit. However such an arrangement exhibits non-linearities in its behavior. It would be beneficial to provide an improved component within an integrated circuit. 
       SUMMARY 
       [0005]    According to a first aspect of the present disclosure there is provided an inductive component for use in an integrated circuit. The inductive component comprises: a magnetic core; a plurality of conductors arranged on a first side of the magnetic core; and a plurality of conductors arranged on a second side of the magnetic core. Each of the conductors on the first side and the second side of the magnetic core, which for simplicity may be regarded as being below and above the magnetic core, respectively, form sections of a coil that surrounds the core. A plurality of conductive connections connect conductors above the core to conductors below the core so as to form a first coil. The inductive component further comprises a compensation means, for example a compensation structure for compensating for saturation nonlinearity or non-uniformity. 
         [0006]    It is thus possible to provide a magnetic component on or as part of an integrated circuit where the magnetic core saturates more uniformly. This in turn gives rise to greater linearity and improved power transfer within an operating region where substantially none of the core has reached magnetic saturation. This can be achieved without incurring an increased footprint for the magnetic component on a substrate, such as a semiconductor, on which the magnetic component is carried. 
         [0007]    Advantageously, the plurality of conductors above and below the magnetic core are interconnected in such a way as to form first and second coils around the core in order to form a transformer. 
         [0008]    The compensation structure may comprise varying a parameter of the first coil. The parameter may be a turns density of the first coil, which may be achieved by varying a pitch of the conductors as they traverse from one side of the coil to the other; a spacing between the conductors; or a width of the conductors. Two or more of parameters may be varied in combination. Where the inductive component comprises a plurality of coils, for example because it is a transformer, then parameters of the second coil may also be varied as described above. 
         [0009]    Advantageously, in an embodiment of this disclosure, a conductor width of the conductors forming the first coil increases with increasing distance from an end of the magnetic core, and preferably from both ends of the magnetic core. This arrangement has the advantage of reducing the effective turns density of the coil around sections of the magnetic core which are located away from the ends of the core, while at the same time avoiding unnecessary increase in the resistance of the coil. 
         [0010]    Advantageously the magnetic core may be formed as a plurality of laminated sections of magnetically active material separated from one another by insulating regions. Advantageously the thickness and/or dielectric material provided between the plurality of layers of the magnetically functional material forming the core may be periodically or occasionally varied. 
         [0011]    The shape of the magnetic core may be varied, for example to depart from a simple rectangular shape to one which has end portions of reduced width compared to a central region. This spatial variation in the shape of the magnetic core may be used to modify the magnetic field distribution within the core such that magnetic flux density within the core is more evenly distributed. Where the core is a laminated core, the shape of individual ones of the laminations may be varied in order to modify the distribution of flux density within the magnetic core. 
         [0012]    Preferably the inductive component is formed on a substrate that carries other integrated circuit components. The substrate may be a semiconductor substrate, the most common example of which is silicon. However other substrates may be used and may be chosen for operation at high frequencies. Such a substrate may include glass, or other semiconductors such as germanium. 
         [0013]    According to a second aspect of the present disclosure there is provided a method of forming a magnetic component comprising depositing a first plurality of conductors on a substrate; forming an insulator between and above the plurality of conductors; forming a magnetic core above the insulator; forming an insulating layer above the magnetic core; forming a plurality of conductors above the insulating layer; and forming electrical interconnections between the first plurality of conductors and the second plurality of the conductors in an interconnect pattern so as to form a coil around the magnetic core. At least one of the magnetic core or the winding is non-uniform. The non-uniformity may be achieved by varying a width or thickness of the magnetic core or a winding/turns density of the coil along a coil axis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Embodiments of this disclosure will now be described, by way of non limiting example only, with reference to the accompanying drawings, in which: 
           [0015]      FIG. 1  is a schematic illustration of a transformer formed within an integrated circuit; 
           [0016]      FIG. 2  is a plan view of a center-tapped transformer within an integrated circuit; 
           [0017]      FIG. 3  is a circuit diagram showing a circuit for measuring flux density as a function of coil current; 
           [0018]      FIG. 4  shows a graph of flux density versus coil current for a typical transformer on an integrated circuit; 
           [0019]      FIG. 5  is a graph of flux density versus coil current having straight line approximations to the response of the coil added thereto for the purposes of explaining the advantages of the present disclosure; 
           [0020]      FIG. 6  is a graph representing turns density as a function of position along a coil axis for a coil surrounding a rectangular magnetic core: 
           [0021]      FIG. 7  is a schematic view of the windings of a coil, suitable for use in an inductor or a transformer in accordance with the present disclosure; 
           [0022]      FIG. 8  is a schematic cross section through a laminated magnetic core; 
           [0023]      FIG. 9  is a schematic cross section through a device constituting an embodiment of this disclosure; and 
           [0024]      FIG. 10  shows a further variation in which the profile of the core is modified. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0025]      FIG. 1  schematically illustrates an example of a transformer  1  having a magnetic core, generally indicated by reference number  2 , formed above a portion of a substrate  4 . Advantageously the substrate  4  is a semiconductor substrate such that other components, such as drive circuitry and receiver circuitry associated with primary and secondary windings of the transformer  1 , may be formed on the substrate  4  or on physically separate substrates within the same integrated circuit package. However, in some applications non-semiconductor substrate materials may be used for their electrical properties, such as higher impedance. 
         [0026]    For the purposes of illustration, structures around the magnetic core  2  such as layers of insulating material, for example polyimide, have been omitted. Thus the only structures shown in  FIG. 1  are the substrate  4 , the magnetic core  2 , and conductive tracks that provide a plurality of conductors formed in first and second layers on either side of the magnetic core  2  and parallel to the plane of the substrate  4  (and also parallel to the plane of  FIG. 1 ). Thus the plurality of conductors exist above and below the magnetic core  2 . The second layer of conductors can be considered as being above the magnetic core  2  and hence closer to the viewer than the first layer, which lies between the magnetic core  2  and the substrate  4 . Conductors passing beneath the magnetic core  2  are shown in chain outline in  FIG. 1 , while conductors passing above the magnetic core  2  are shown in solid outline. 
         [0027]    A first coil or winding, for example a primary winding  10 , can be formed of linear track sections  12 ,  14 ,  16  and  18 , where sections  12  and  16  are formed in the second metallic layer above the core  2 , and sections  14  and  18  are formed in the first metallic layer below the core  2 , and are connected together by way of vias or equivalent interconnect regions,  20 ,  22  and  24 . A secondary winding  30  may be formed of planar track sections  32 ,  34 ,  36  and  38 , where sections  34  and  38  are formed in the second metallic layer above the core  2 , and sections  32  and  36  are formed in the first metallic layer below the core  2 , and the sections are connected together by way of vias or other suitable interconnection components  40 ,  42  and  44 . It can be seen that the primary and secondary coils are formed as structures that spiral around the magnetic core  2 . The primary and secondary coils are insulated from the core  2 , and are insulated from one another. Thus there is no galvanic path between the primary winding  10  and secondary winding  30 , and the primary mechanism coupling the coils together is a magnetic one. Minor parasitic capacitances may also form signal flow paths between the primary and secondary winding, but these are considerably less significant. The Y-direction in  FIG. 1  can also be considered the coil axis. 
         [0028]      FIG. 2  shows a more realistic plan view of a transformer of the type shown in  FIG. 1 , but as might actually be formed on an integrated circuit. It can be seen that the primary winding  10  and the secondary winding  30  spiral their way around the magnetic core  2 . Vias, generally designated  50 , connect the conductors of the first layer with conductors of the second layer so as to form a planar approximation of a coil or winding encircling the magnetic core. The windings may also be center tapped, as illustrated in  FIG. 2  by laterally extended tracks from the center of each coil. In the transformer shown in  FIG. 2  the width of each conductor forming a winding is uniform, as is the space between adjacent windings or conductors in either of the layers of conductors. Generally speaking, the space between adjacent conductors in a layer is substantially minimized, consistent with reducing the ohmic resistance of the coil, while giving sufficient spacing between adjacent conductors to achieve a desired breakdown voltage between the coils of the transformer and to avoid shorting between coils as a result of manufacturing defects. The illustrated uniform windings can maximize the number of turns for a given occupied real estate. 
         [0029]    Although a two winding transformer will be described, embodiments may have more than two windings. Also a single tapped winding may be used to form an autotransformer, or a single winding may be used to form an inductor. 
         [0030]    When forming a device, such as a transformer, the saturation current, being the maximum current which can be passed through the primary winding of the transformer before magnetic core saturation occurs, is a critical property of the transformer and its ferromagnetic core and is linked to the total power rating of the transformer. Therefore maximizing the saturation current and the power transfer of a given size transformer are highly desirable. 
         [0031]    It is known to the person skilled in the art that the magnetic flux density in the magnetic core of an ideal solenoid is determined by both the core material and the winding or core geometry since the inductance of a coil L is where 
         [0032]    μ 0 =permeability of free space=4π×10 −7  Hm −1    
         [0033]    μ r =core relative permeability 
         [0034]    N=number of turns of the coil 
         [0035]    t=core thickness (height) 
         [0036]    w=core width 
         [0037]    l=core length 
         [0038]    so t w (which may be expressed as t×w)=core cross sectional area. 
         [0039]    Magnetic flux density B=μ 0 μ r H 
         [0040]    where for ideal solenoid 
         [0000]    
       
         
           
             H 
             = 
             
               NI 
               L 
             
           
         
       
     
         [0041]    Ultimately, for a long solenoid, the core magnetic flux density becomes: 
         [0000]      B=μnI
 
         [0000]    where n is the turns density (number of turns per unit distance) and I is current in the coil. A magnetic material can only take a certain magnetic flux before it becomes magnetically saturated and its relative permeability dramatically drops (if the material is fully saturated then its permeability drops to 1). Therefore the relative permeability in combination with turns density of the coil and the saturation flux density determine device saturation current. 
         [0042]    However, the magnetic field fringes towards the ends of the solenoid so the magnetic field strength H reduces near the ends. A further issue is the existence of a demagnetizing field. The demagnetizing field creates a magnetic field that is internal to the body of the core, and which acts in an opposite direction to the applied field from the solenoid. The demagnetizing field is strongest towards the ends of the core. The spatial variation of demagnetizing field can be described in terms of spatial variation of the relative permeability. Because the demagnetizing field gets stronger towards the ends of the core, the relative permeability drops towards the end and it takes higher current to magnetically saturate the ends of the core than the center of the core. 
         [0043]    In general terms, as a solenoid gets shorter, the demagnetizing field gets stronger. Also, the magnetic fields, both applied and demagnetizing, exist in three dimensions. Thus, although the magnetic core is essentially planar it experiences some fields at its ends which are out of the plane of the planar core. This gives rise to different internal field strengths as a function of position within the magnetic core. 
         [0044]    As a result of these factors, a ferromagnetic transformer core may suffer from early saturation of the central core area due to the uneven distribution of the magnetic flux density within the core. This onset of saturation, which grows in spatial extent as the bias current is increased, introduces early non-ideal behavior of the transformer and therefore limits the available saturation current. 
         [0045]      FIG. 3  shows an apparatus that can be used to measure the performance of the transformer. As shown, a DC current bias  52 , which could be a current source, is used to impose a DC current through the primary winding  10  of a transformer. An inductor  54  is typically included in series with the DC bias source  52  in order to present a high impedance to AC signals. An AC signal generator  56  in series with a DC blocking capacitor  58  is used to superimpose an AC signal onto the DC bias. The voltage appearing across the output of the secondary winding  30  is then measured, and then compared with the voltage provided by the AC excitation source  56 . This allows the instantaneous AC power transfer of the transformer to be measured as a function of the DC bias current. 
         [0046]    A graph illustrating measurement of this relationship is shown in  FIG. 4  for a transformer with uniform windings. It can be seen that, at relatively low bias currents the ratio of a V out  to V in , is relatively high, and can be regarded as operating the transformer in a region where its core is not saturated. Therefore the effective permeability to a small change in primary current is representative of a high value of the relative permeability μ r . Conversely, when the DC bias current becomes large and the core is fully saturated, the output 
         [0000]    
       
         
           
             
               V 
               out 
             
             
               V 
               in 
             
           
         
       
     
         [0000]    reduces to a smaller value, which is more akin to that of an air core transformer as the ferromagnetic core can no longer provide enhancement of the flux density as a result of a small change in the current. 
         [0047]      FIG. 5  re-plots the data of  FIG. 4  to show the saturated and non-saturated regions more clearly, and also to apply straight line approximations to sections of the graph. 
         [0048]    Between the non-saturated region and the fully saturated region is a transition region, generally designated  60  where the permeability transitions from the non-saturated to the fully saturated values. 
         [0049]    Mathematical modelling indicates that the flux density B within the ferromagnetic core is non-uniform and is weaker at the edges or ends of the core, and more intense towards the center of the core. As a result, as the DC bias current increases the central portion of the core starts to saturate, indicated in  FIG. 5  by the point at which the ratio 
         [0000]    
       
         
           
             
               V 
               out 
             
             
               V 
               in 
             
           
         
       
     
         [0000]    starts to degrade around the area of the graph generally designated  62 . The area of saturation then continues to grow from the middle to the ends until the core becomes fully saturated. 
         [0050]    Ideally, the core transition to saturated state would start with higher bias current and it would transition more abruptly from non-saturated operation to saturated operation. This would enable a given size of magnetic core to handle more power and current before saturation occurs, although its performance would then degrade much more rapidly. 
         [0051]    The inventor realized that steps could be taken to reduce the tendency of the central section of the magnetic core to saturate earlier than the end sections of the magnetic core. This can be achieved by a structural feature of the magnetic component, and in an embodiment this is achieved by varying the turns density of the coil as a function of distance along the coil axis. 
         [0052]      FIG. 6  is a graph schematically illustrating variation of turns density as a function of distance along a core having a length of one arbitrary unit L c . It can be seen that the turns density can be increased towards the ends of the core, as represented by values of x=0 and x=1, and decreased towards the center of the core, in order to reduce the tendency for early saturation of the central section. 
         [0053]    The dimensions of a coil around a magnetic core within an integrated circuit are quite compact, and it is therefore unlikely that the turns can be modified in a smoothly varying manner represented by the optimized curve in  FIG. 6 , but a step wise approximation is also possible as also shown in  FIG. 6 . As a result of applying a step wise approximation to the turns density, a winding density as shown in  FIG. 7  may be achieved where the coil may be comprised of spaced apart conductors, of which only the uppermost layer is shown, but a corresponding pattern is formed on the lowermost layer beneath the core  2 . The conductor strips are arranged to give a coil having a relatively low winding density, designated density  1 , towards a central portion of the coil, and an intermediate winding density, designated density  2 , on either side of the area at the center of the coil. Either end of the coil has a higher winding density, designated density  3 , compared to the central and intermediate densities. In the illustrated embodiment, differing densities are achieved by varying the conductor widths at different sections of the coil. The first section of the coil comprises relatively wide strips of conducting material designated  100 ,  102  and  104  having a width w 1  and an inter-conductor gap distance g 1 . The intermediate areas of coil density, density  2 , are comprised of conductors  90 ,  92  and  94  having a conductor width w 2  and an inter conductor gap spacing g 2 , and similarly for conductors  110 ,  112  and  114 . The end portions having the highest winding density, density  3 , are comprised of conductors  80 ,  82  and  84 , and similarly conductors  120 ,  122  and  124 , having a width to w 3  and an inter conductor spacing g 3 . 
         [0054]    It would be possible to vary the gap between the conductors, and keep the conductor width the same such that w 1 =w 2 =w 3  and g 3 &gt;g 2 &gt;g 1 . However this arrangement, while giving generally desirable magnetic properties, can give rise to a undesirable increase in resistance of the coil compared that which could be obtained by keeping the gap between the adjacent conductors the same, such that g 1 =g 2 =g 3 , and then varying the relative width of the conductive elements w 1 , w 2  and w 3  such that w 1 &gt;w 2 &gt;w 3 . Varying the widths of the conductors forming the coils, rather than varying the dielectric gaps, maximizes the amount of conductor (for a given thickness of conductor) involved in carrying the current through the coil, and thereby reduces resistance. 
         [0055]    The use of a ferromagnetic core with relatively high permeability ensures that magnetic flux generated by the primary winding  10  is efficiently coupled to the secondary winding  30 . 
         [0056]    However, as is experienced in macro-scale transformers, the magnetic flux generated around the primary winding  10  interacts with the magnetic core  2 , and can give rise to eddy currents flowing within the core  2 . These eddy currents flow through the resistive material of the core  2  and give rise to a loss mechanism. This reduces the efficiency of the magnetic component, and in the case of transformers may manifest itself as an apparent increase in the coil resistance of the primary and secondary windings as the excitation frequency of the primary winding increases. 
         [0057]    Drawing on the experience of macro-scale transformers, one way to address the eddy current problem is to segment the core into a plurality of sections which are insulated from one another. Within the context of an integrated circuit, it might be thought that the easiest approach would be to form a series of trenches in the magnetic core, with the longitudinal axis of the trenches running parallel to the direction of the magnetic field generated by the windings, in which case trenches would run from the top of  FIG. 1  to the bottom of  FIG. 1  (Y direction) so as to divide the core into a plurality of parallel “fingers”. In fact, in the micro scale environment of integrated circuits this approach would be highly disadvantageous as the thin fingers would then exhibit shape anisotropy which would cause the magnetically easy axis of the ferromagnetic material to extend along the Y direction of  FIG. 1 . This in turn would give rise to large hysteretic losses within the material and low values of saturation current, which could be avoided by having the magnetically easy direction extend along the X axis (horizontally) of  FIG. 1 . Such an arrangement would cause the “hard” direction to be parallel with the magnetic field and the Y axis, and this direction generally has a much smaller hysteresis loop and operates in the generally linear region of the hysteresis loop over a much wider range of applied magnetic fields. 
         [0058]    However the magnetically easy axis can be maintained along the “X” direction of  FIG. 1  if the magnetic core is segmented into a plurality of individual layers, each layer existing in the X-Y plane of  FIG. 1 . The easy axis can be defined during deposition of the layer of magnetic material. Several techniques are known to the person skilled in the art and need not be described here. 
         [0059]      FIG. 8  schematically illustrates a cross section through the magnetic core  2  of  FIG. 1 . While described below for convenience with respect to the schematic plan view of  FIG. 1  and orientations set forth therein, it will be understood that the magnetic core  2  of  FIG. 8  can be combined with the turns density variation of  FIG. 7  and/or the core dimension variation of  FIG. 10 . The cross section is perpendicular to the plane of  FIG. 1 , showing layers stacked in the Z direction working upwards from the substrate  4 .  FIG. 8  is not drawn to scale and the dimensions of the component layers within the magnetic core  2  are not shown to scale with respect to each other, and neither is the size of the magnetic core  2  shown correctly to scale with respect to the rest of the integrated circuit. 
         [0060]    As shown in  FIG. 8 , the substrate  4  may have one or more layers of material formed on it, generally designated  150 , between the substrate  4  and a base layer of the magnetic core  2 . The layer  150  may include metallic tracks forming part of the first metallic layer shown in  FIG. 1  and may also include one or more layers of insulating material, such as aluminum nitride or polyimide. 
         [0061]    The magnetic core  2  comprises a plurality of layers. In general, a first subsection, generally designated  160  of the core  2  comprises layers  170 ,  172 ,  174 ,  176 , and  178  of the first insulating material arranged in an alternating sequence with layers  180 ,  182 ,  184 ,  186  and  188  of magnetically functional material. In this example five layers of magnetically functional material sit above five layers of first insulating material in an alternating stack. It should be noted that fewer, or indeed more, layers of magnetically functional material and first insulating material may be used to form the first subsection  160 . 
         [0062]    A layer  200  of the second insulating material, which can be different from the first insulating material, is formed above the first subsection  160  of the magnetic core  2 . Alternatively a thicker layer of the first insulating material could be deposited. The layer  200  of second insulating material could be deposited directly onto the uppermost layer  88  of magnetically functional material in the first subsection  60 . Alternatively, a barrier layer may be formed between the layer  200  of the second insulating material and the uppermost layer  188  of magnetically functional material. Such a barrier layer  190  is illustrated in  FIG. 8 . For convenience, the barrier layer  190  may be formed of the first insulating material. A second subsection, generally designated  210 , of the magnetic core  2 , comprising alternating layers of magnetically functional material and the first insulating material as described hereinbefore, is formed above the layer  200 . A lowermost layer  220  of magnetically functional material could be deposited directly on to the layer  200  of the second insulating material. However, in an embodiment a layer  222  of the first insulating material is formed above the layer  200  of the second insulating material, and acts as a seed layer for the layer  220  of magnetically functional material. Thus, as shown in  FIG. 8  layer  200  of the second insulating material is bounded on its upper and lower faces by layers of the first insulating material. This can have the further advantage of, for example, stopping degradation of the magnetically active material in the layers  188  and  220  occurring when, for example, the layer  200  is made out of an oxide, such as silicon dioxide. 
         [0063]    The second subsection  210  comprises five layers of magnetically functional material  220 ,  224 ,  226 ,  228  and  230  with each layer of magnetically functional material being separated from an adjacent layer of magnetically functional material by a layer  232 ,  234 ,  236  and  238  of the first insulating material. 
         [0064]    The uppermost layer of magnetically functional material  230  of the second subsection  210  is bounded by a second layer  250  of the second insulating material. As before, the layer  250  of the second insulating material may be sandwiched between layers  252  and  254  of the first insulating material. As an alternative to depositing the layer of second insulating material, a layer of first insulating material having an increased thickness (compared to layers in the subsections) could be deposited. A third subsection  260  of the core  2  is formed above the second subsection  210 . This process can be continued until an uppermost portion of the magnetic core  2  is reached, where the final two layers may comprise a layer of magnetically functional material topped by a layer of the first insulating material. Thus, if the magnetic core is made of two subsections, only one layer of the second insulating material can be provided to separate the subsections. If the magnetic core is made of three subsections, then two layers of the insulating material can be provided to separate the subsections. In general it can be seen that if the magnetic core is made of N subsections, then N−1 layers of the second insulating material can be provided. 
         [0065]    In the example given each of the subsections comprises five layers of magnetically functional material. In general, each subsection does not have to be identical to the other subsections although such an arrangement has been described here. Similarly each subsection does not need to comprise five layers of magnetically functional material. In an embodiment of a core as shown in  FIG. 8 , the layers of the first insulating material may be aluminum nitride (although other insulating materials such as aluminum oxide may be used for some or all of the layers of first insulating material), and have thicknesses of approximately 10 nanometers, although other thicknesses can be used and it is envisaged that the first layers could typically have a thickness range of between 5 and 30 nanometers. The magnetically active layers can be formed of nickel iron, nickel cobalt or a composite of cobalt, iron, zirconium, niobium and boron and typically have a thickness of around 100 nanometers although thinner or thicker layers, for example in the range of 50 to 200 nanometers thick may be used. The second insulating material may be arranged such that capacitive coupling between the subsections is reduced compared to capacitive coupling between adjacent layers of magnetic material in a subsection, by virtue of one or both of an increased separation between the uppermost magnetically functional layer of one subsection, and the lowermost magnetically functional layer of the next subsection, and reduced permittivity of the second insulating material relative to the first insulating material. 
         [0066]    Aluminum nitride has a relative permittivity of about 8.5, whereas as silicon dioxide has a relative permittivity of about 3.9. Accordingly, in one embodiment the first insulating material is aluminum nitride and the second insulating material is silicon dioxide. 
         [0067]      FIG. 9  is a schematic cross section through an integrated circuit including a transformer having a magnetic core, generally indicated by reference numeral  2 , constituting an embodiment of the invention. The magnetic core  2  shown in  FIG. 8  is divided into six subsections  301  to  306  by intervening layers of the second insulating material. Each subsection is, as before, comprised of alternating layers of the first insulating material and magnetically functional material. 
         [0068]    As shown in  FIG. 9 , the integrated circuit comprises a substrate  4  which has a lowermost metallic layer  310  deposited thereon. After deposition, the metallic layer  310  is masked and etched so as to form conductive tracks, some of which act to form tracks  14 ,  18 ,  32  and  36  of  FIG. 1  which constitute part of the primary and secondary windings  10 ,  30 . As noted with respect to  FIG. 8 , while reference is made to the schematic plan view of  FIG. 1 , it will be understood that the structure of  FIG. 9  can be combined with the turns density variation described with respect to  FIG. 7  and/or the core profile variations described below with respect to  FIG. 10 . An insulating layer  320 , for example of polyimide, is then deposited above the metal layer  310  to insulate the magnetic core from the transformer windings. The transformer layers  301 - 306  are then deposited, for example by deposition across the entirety of the substrate. The structure is then masked and then etched so as to form isolated transformer core regions above the insulating layer  320 . Additional insulating material may then be deposited to fill in the gaps between adjacent transformer cores  2  on the substrate  4  and to overlie the cores to encapsulate them within a dielectric. Such an insulating layer is designated as  322  in  FIG. 9 . The insulating layer  322  may then be subject to planarizing in order to form a substantially flat upper surface of the integrated circuit. This surface may then be masked and etched in order to form depressions  340  in the insulating layer  322  and layer  320  which extend down to the lowermost metallic layer  310 . The upper surface may then have a metallic layer  350  deposited on it. The metal also deposits into the V shaped depressions  340  thereby forming interconnections between the lowermost metallic layer  310  and the uppermost metallic layer  350 . The layer  350  can then be masked and etched in order to form, amongst other things, the conductive tracks  12 ,  16 ,  34  and  38  shown in  FIG. 1  constituting parts of the primary and secondary windings  10 ,  30 . 
         [0069]    The lowermost metallic layer  310  may be formed over an insulating layer  360  for example of silicon dioxide, which may itself overlie various semiconductor devices (not shown) formed by implantation of donor or acceptor impurities into the substrate  4 . As known to the person skilled in the art, apertures may be formed in the insulating layer  360  prior to depositing the first metallic layer  310  in order to form device interconnections among the various circuit components. 
         [0070]    As well as varying the turns density within the transformer it is also possible to modify the flux density within the core by varying the shape of the core. These approaches can be used separately or in combination. Thus, as shown in  FIG. 10  the rectangular magnetic core  2  of  FIG. 1  can be modified to have tapered sections  400  and  402  at end portions of the magnetic core so as to reduce the width of the magnetic core at its ends. The relative diameter of the winding formed by the conductive elements may also vary to conform to that of the core, schematically represented by conductive tracks  410 ,  412  and  414  where tracks  410  is shorter than track  412 , and track  412  is shorter than track  414 . Profiling need not be performed to the entirety of the core where the core is formed of separate layers, as discussed with respect to  FIGS. 8 and 9 , but profiling may be performed on some layers and not the others. Furthermore, the spatial extent of the layers may also be varied such that the vertical height of the magnetic core may vary such that, for example, the center of the core has a greater vertical height than the ends of the core. This can be achieved by changing the relative spatial extents of some of the layers which are used to form the core when the core is provided as a laminated structure. 
         [0071]    It is thus possible to form an improved magnetic component, such as an inductor or a transformer within an integrated circuit. The substrate carrying the magnetic component and other components can be packaged in a chip scale (integrated circuit) package as known to the person skilled in the art. 
         [0072]    Although the claims presented here are in single dependency format for filing at the USPTO, it is to be understood that any claim may depend on any preceding claim of the same type except when that is clearly not technically feasible.