OPTIMIZED LAYOUT FOR LOW MAGNETIC STRAY-FIELD INDUCTOR

FIELD OF THE INVENTION

8-shaped and clover-shaped inductors layouts are described that minimize the amount of crossing points. Each crossing point increases resistance of inductor and capacitance between windings. Therefore, by keeping the number of crossings points at a minimum, the quality factor of an inductor increases as a consequence thereof.

BACKGROUND OF THE INVENTION

The present inventions relates to 8-shaped and clover-shaped inductors layouts, wherein the amount of crossing points is minimized. Many 8-shaped and clover- shaped have been described in the prior art. These have, however, typically been directed to reducing magnetic field effects thereof. Therein, symmetry is of relevance.

WO2006/105184 Al describes a method and apparatus for use in an integrated circuit or printed circuit board for reducing or minimizing interference. An inductance is formed using two or more inductors coupled together and configured such that current flows through the inductors in different directions, thus at least partially canceling magnetic fields. When designing a circuit, the configuration of the inductors, as well as the relative positions of portions of the circuit, can be tweaked to provide optimal interference or noise control.

WO0057437 Al describes a balanced inductor formed on lossy substrate material having adjacent strips leading current in opposite directions and being arranged in such a way that substrate currents relating to individual strips induced in the lossy substrate are balancing out one another leading to high Q-values. The inductor structure according to the invention can be implemented in MMIC devices using standard semiconductor substrates and do not require any special treatment of the substrate being needed.

Lossy substrate means that it is made from neither dielectric material nor ideal conductor. In other words, it conducts current but has not negligible resistance.

WO2005/096328 Al describes a method and system for reducing mutual EM coupling between VCO resonators and for implementing the same on a single semiconductor chip. The method and system involve using inductors that are substantially symmetrical

about their horizontal and/or their vertical axes and providing current to the inductors in a way so that the resulting magnetic field components tend to cancel each other by virtue of the symmetry. In addition, two such inductors may be placed near each other and oriented in a way so that the induced current in the second inductor due to the magnetic field originating from first inductor is significantly reduced. The inductors may be of various forms.

WO2007/006867 Al describes the inductance of a monolithic planar inductor, which is distributed into smaller inductor portions. The smaller inductor portions are provided in a cascade configuration in a manner that causes the inductor to function as a differential inductor device. The node (CM) between the immediate inductor portions (L21, L22 is a common-mode point of the inductor device, which is typically connected to the signal ground. Some of the inductor portions are arranged to be symmetrically by-passed or shortcut in relation to the common point in one or more steps for operation in one or more higher radio frequency band.

US2005/017836 Al describes an on-chip inductor including a main inductor portion configured to provide a main magnetic field of an on-chip inductor. An interconnect inductor portion is electrically coupled to the main inductor portion and is configured to provide an interconnect magnetic field that constructively combines with the main magnetic field.

The above document, however, does not relate to 8-shaped or clover-shaped inductors. Further, it does not relate to reducing capacitance and or reducing sheet resistance.

US2004/018823 Al describes an on-chip differential inductor includes a first interwound winding having a substantially octagonal shape, or rectangular octagonal shape, and a second interwound winding having a substantially octagonal shape, or rectangular octagonal shape, that is interwound with the first interwound winding. Both the first and second interwound windings are on the same layer of the integrated circuit. Each interwound winding includes two nodes; one of node of each winding is commonly coupled to a reference potential. The other node of each winding is operably coupled to receive a respective leg of a differential signal.

US2005/077992 Al describes a substantially symmetric inductor comprising a plurality of windings, at least one conductor crossover, and a peripheral conductor disposed at the periphery of the plurality of windings, the plurality of windings having a generally symmetric shape, each of the plurality of windings having a center and being of a different size from other ones of the plurality of windings, the peripheral conductor being generally symmetric and having a center, the plurality of windings and the peripheral conductor being

substantially concentric, the conductor crossovers being disposed such that the symmetry of the inductor in substantially preserved. A method of winding an inductor such that the inductor is substantially symmetric about a center of the inductor, whereby signal degradation due to asymmetry of the inductor is substantially minimized.

US2005/024178 Al describes a switchable inductance that can be formed in an integrated circuit, including a spiral interrupted between two first points connected to two terminals via two metallizations running one above the other, one of the two metallizations being deformable; a hollowing between the two metallizations; and a switching device capable of deforming the deformable metallization to separate or to put in contact said two metallizations.

In the publication "A cloverleaf-like structure extending the operational bandwidth of integrated inductors", by Minerva, V.; Politi, M.; and Cavalieri d'Oro, S., in 34th European Microwave Conference, 2004. Volume 3, Issue, 11-15 Oct. 2004 Page(s): 1369 - 1372, a structure is proposed for realization of integrated inductors of relatively high inductance, capable of significantly increasing the self-resonance frequency in comparison with standard structures.

Thus, substantially symmetrical inductors, with eight or clover shaped structures, are known in the art, as can be seen in WO 2006/105184 Al, WO 00/57437 Al, WO 2005/096328 Al and possibly the Minerva document. For circular inductors influence of the crossing points is also known to be an area of interest in obtaining desired performance in such inductor arrangements (see disclosures in US 2005/0017836 Al, for example). In single turn arrangements, in such inductors, the number of crossing points may be proportional to the number of turns. US 2005/0017836 Aldescribes a possible design of a crossing of two turns of an inductor. These types of crossings make it possible for turns to have positive mutual inductance at the cost of increased resistance. Furthermore, there are some pictures of octagonal inductors with multiple turns and a number of crossings that is proportional to the number of turns. However, almost on the contrary, providing a layout of 8-shaped or clover- shaped inductors, with symmetrical layout, and a number of crossings that is proportional to the number of turns, is not trivial at all, and clearly does not follow from the this patent. Therefore, it is regarded utmost as background for the present invention. It is noted that a disclosed single turn 8-shaped inductor per se is regarded as being according to the prior art.

Thus, for the above it is known for race track or 8-shaped symmetrical inductors with multiple turns to have crossing points which are proportional to the number of turns, however, typically quadratic proportional. However, there is no clear way how to

combine the teaching in the documents cited above with one and another. Combining such symmetrical inductors can become rather complex with regard to the crossing point arrangements, and therefore not straightforward.

There are no clear disclosures in the above documents of symmetrical 8- shaped or clover-shaped inductor structures where the number of crossing points is proportional to the number of turns used in the inductor, where the number of turns is 2 or more. Further, no particular details relating to the track width and the inner diameter of an eye of the inductor was noted during the search.

IC inductors are essential to realize the voltage-controlled oscillators needed in many fully integrated transceiver chips serving a multitude of wireless communication protocols. These are being provided to the market today. In the prior art WO1998005048(Al), WO2004012213(Al), WO2005096328(Al) and WO2006105184(Al) conductors are described that indeed have a lower magnetic coupling. However, as a consequence, the price that must be paid is a smaller quality factor of such an inductor.

One of the reasons of a reduced quality factor is an increased number of crossing points. Such a crossing point can be identified as a geometrical point, situated in a 2- D plane parallel to the main plane of an inductor, where one conducting track of an inductor crosses another line of the inductor, wherein at the crossing point the two conducting tracks are not in the same plane. In other words, conducting tracks are running in 2 parallel planes, whereas at the crossing point each track is using only one plane, so no shortcut is created (see e.g. figures 2-4). As one can see on this drawing, each conducting track is terminated on one plane and an electrical current flows using the other plane.

An inductor at low frequency could be represented by equivalent network

represented in Figure Ia. The quality factor of such a network would be Q ~ ( CR)(ύ .

R

That is why making parasitic resistance and capacitance as small as possible is vital for getting a high quality factor.

It is noted that for e.g. 8-shaped and clover-shaped inductors, as those provided by the prior art, each crossing point gives an additional contribution to the resistance of an inductor. In order to make a crossing point one should use at least two conducting layers. Through the whole length of an inductor all used conducting layers are connected in parallel. This reduces the sheet resistance of the conducting material, which follows from basic physics. If two or more resistive materials are connected in parallel than the total resistance of the construction gets smaller than the resistance of each parallel-

connected component. However, in order to make a crossing point between two turns, each turn should preferably use only one layer. This leads to a dramatic increase of sheet resistance for the parts of an inductor where such crossing is implemented. As a result total resistance of inductor increases.

Each crossing point contributes also to the capacitance between windings or coils due to potential difference between different layers of the crossing point.

Furthermore, in order to make an inductor to be used in common and differential mode, such an inductor must be made symmetrical. These modes are discussed in more detail in order to see why symmetry of the layout is so vital.

When difference of potential is applied to the signal pads of an inductor one says that it is used in the differential mode. That would roughly correspond to equivalent network of figure Ib.

If one could find a point that divides inductor in two parts, with the same resistance and self inductance, than the difference of potential could be applied to this point and signal taps in such a way, that an equivalent network would look as given in figure Ic.

This allows a circuit designer to use an inductor as a differential negative resistance oscillator. Such a way of using an inductor is called a common mode. Further, this explains why common mode requires a layout to be symmetrical.

Typically, 8-shaped and clover-shaped prior art inductors suffer from one or more of the above mentioned drawbacks.

Examples of symmetric inductors in case of prior art 8-shaped and clover- shaped layout are given in figure 2. The number of crossing points for 8-shaped and clover- shaped inductors are n ² + n — \ and 2n ² + n — \ , respectively. Here n is the number of turns of a coil, e.g. for an 8-shaped inductor, per definition having two coils per 8, and having n coils per layer. For example, the 8-shaped inductor of figure 4 has 3 turns (n=3) per coil, and as a consequence 11 crossing points. In both cases, i.e. for prior art 8-shaped and clover- shaped inductors, the number of crossing points increases quadratic with the number of turns per coil. This could be an obstacle for the use of a "large" (>1) number of turns per coil. Dashed regions show parts of the crossing points that use under layer(s) of the crossing (in Figure 2, left n=3, thus number of crossings is 11; each dashed crossing in the middle is counted three times. Coils typically use many, at least two, metal layers. Clearly at a crossing point conducting tracks cannot use parallel plains without creating an electrical shortcut. This is why each crossing point could significantly contribute to overall resistance of a coil. For

the example given this contribution was a relative increase of approximately 10%. The exact value depends on e.g. dimensions of a particular coil.

SUMMARY OF THE INVENTION

The present invention relates to a symmetrical inductor, preferably an 8- shaped or clover-shaped inductor, comprising two or four inductor eyes, respectively, each eye comprising 2 or more turns per coil, comprising a geometrical crossing line situated in a 2-D plane parallel to the main plane of the inductor, at which line a conducting track of the inductor crosses maximal one another conducting track of the inductor, wherein at the crossing point the two conducting tracks are not in the same plane, and devices comprising said inductor.

DETAILED DESCRIPTION OF THE INVENTION

In a first embodiment the invention relates to a symmetrical inductor, preferably an 8-shaped or clover-shaped inductor, comprising two or four inductor eyes, respectively, each eye comprising 2 or more turns per coil, comprising a geometrical crossing line situated in a 2-D plane parallel to the main plane of the inductor, at which line a conducting track of the inductor crosses maximal one another conducting track of the inductor, wherein at the crossing point the two conducting tracks are not in the same plane.

As can be seen from e.g. Figures 2-4 an inductor may comprise two or four inductor eyes, for 8-shaped or clover-shaped inductors, respectively. Each eye forms a coil, typically comprising 2 or more turns per coil, preferably 3 or more, more preferably 4 or more, such as 5 or more turns per coil.

As a consequence of multiple coils per eye, tracks need to cross each other frequently. The aim of the invention is to reduce the number of crossing points, as described above.

As two conducting tracks cross each other, the crossing point comprises more than one layer, in order to prevent the two tracks to be in electrical contact. Typical configurations of crossing points are indicated in figures 2-4, respectively.

In Figs. 2-4 a geometrical crossing line is indicated, which is running vertically. It is situated in a 2-D plane parallel to the main plane of the inductor. Typically this geometrical crossing line lies in a mirror plane of the inductor. The line itself is virtual,

and merely serves the purpose of identifying various components of the present inductor, specifically crossing points. Note that the blue line in figures 2-4 and the geometrical crossing line are situated above one and another. However, the blue line is not a "virtual" line at all. It is a center tap - metal strip that lies a number of metal layers under the metal layers where inductor itself is implemented. One end of the strip is connected (using vias) to the center of the inductor. In the figures 2-4 it is the upper end of the center tap that is connected to the inductor. In common mode potential difference on one side is applied to the signal pads of the inductor and on the other side second end of the center tap.

Typically, the width of the tracks forming the coils is from 0.5-50 μm, preferably from 5-30 μm, even more preferably from 7-15 μm, such as 11 μm.

Further, typically the form of the coils is substantially square, hexagonal, octagonal, multigonal, oval, or substantially circular, such as horizontal and vertical sections forming a substantially circular loop or a circle, or combinations thereof, preferably substantially circular. Such a form provides the best quality factor.

The required inductance value is typically in the order of nH, such as from InH to 6OnH, and is adjustable to the application, e.g. an integrated circuit in question, whereas the quality factor is as high as possible, e.g. preferably more than 20. Depending on a process being used to fabricate a chip and on operating frequency, the quality factor may vary from 10 to 25. An additional benefit is a low net magnetic field, resulting in a lower magnetic coupling to other inductors.

Clearly the present inventor comprises further a first contact, and a second contact, and the above tracks form coils which coils are electrically connected to one another, arranged such that an electrical current can run from the first contact to the second contact, wherein the electrical current in a first eye runs in one direction, and wherein the electrical current in an adjacent eye section runs in another direction.

As a conductor a material is chosen with a low electrical resistivity, such as a metal or metal-like material, such as copper, aluminum, tungsten, or combinations thereof.

In a preferred embodiment the invention relates to a clover shaped symmetrical inductor, comprising a second geometrical crossing line situated in a 2-D plane parallel to the main plane of the inductor and being perpendicular to the first geometrical crossing line, at which second line a conducting track of the inductor crosses maximal one another conducting track of the inductor, wherein at the crossing point the two conducting tracks are not in the same plane.

A clover-shaped inductor benefits from minimizing the number of crossing points, as indicated above, also in a second dimension, in the present case a horizontal dimension.

In a further preferred embodiment the invention relates to an inductor, wherein the number of crossing points is equal to the product of the number of inductor eyes (E) and number of turns (n) per coil minus 1 (E*n-i).

In a further preferred embodiment the invention relates to an inductor, wherein the inductor is formed in two or more parallel layers.

In Fig. 3 one can see an example of implementations of 8-shaped and clover- shaped inductors, which have a number of crossings points being proportional to the number of turns. In both cases the number of crossing points is proportional to the number of turns. Dashed regions show parts of the crossing points that use under layer(s) of the crossing.

The number of crossing points for the new layouts (for example those of figure 4) of 8-shaped and clover-shaped inductors are 2n-l and 4n-l, respectively, wherein n is the number of turns. Thus the present invention is only advantageous for n>l . The following tables demonstrate the improvement.

# of turns Quadratic growth ( ) Linear growth ( )

1 1 1

2 5 3

3 11 5

4 19 7

5 29 9

Table 1. 8 shape layout comparison.

# of turns Quadratic growth ( ) Linear growth ( )

1 2 3

2 9 7

3 20 11

4 35 15

5 54 19

Table 2. Clover shape layout comparison.

In order to estimate a gain in quality factor we looked at reduction of dc resistance of an inductor that can be gained by using this invention. This reduction depends on number of turns and ratio between the width of inductor turn and inner diameter of an "eye" (both characteristics are demonstrated by figure 3). DC resistance of an inductor can be calculated by the formula. d N C ₁ + c ₂ — +c ₃ (*)

W W

Here d is an inner diameter of an "eye", w is a width of the track of the inductor and N is number of crossings points. Numbers C ₁ , C ₂ and C3 do not depend on d, w and N. These numbers depend on the sheet resistance of the inductor, distance between turns and other parameters of the layout. From the formula it follows that as smaller inner diameter d gets as more significant reduction of number of turns N is for the total resistance. The opposite is also true: the larger d becomes, the less significant JV becomes. This is why results in figures 5-8 are indicative. As such they were obtained for some specific dimensions of coils.

It is noted that in general, designers try to keep the inner diameter as small as possible in order to keep costs of a design as small as possible. Unfortunately, it is difficult to identify any optimal combination as a general rule of d and w. This is due to the fact that change of any of them has some benefits and some drawbacks.

The results of dc resistance calculations of 8-shaped and clover-shaped coils are presented by figures 5 and 7, respectively. A corresponding estimate of an increase of quality factor is presented in figures 6 and 8. In order to calculate the actual gain of quality factor of an inductor more physical effects should be taken into account (for example a skin effect would play a big role). On the other hand figures 6 and 8 are well suited to indicate significance of the present invention.

This invention allows building inductors of low magnetic coupling without too big compromise in the quality factor. Invention is especially relevant for the multiple turn inductors. Proposed layouts have at least one axis of symmetry and therefore can be used not only in differential but also in the common mode.

In a second aspect the invention relates the use of an inductor, for reducing resistance of the inductor and/or for reducing parasitic capacitance, such as capacitance between turns of a coil.

As described by minimizing the number of crossing points leads to a dramatic increase of sheet resistance for the parts of an inductor where such crossing is implemented.

As a result total resistance of inductor increases. Further, each crossing point contributes also to the capacitance between windings or coils due to potential difference between different layers of the crossing point. Therefore, as is shown by experiments below, reducing the number of crossing points in an inductor is in this respect very important.

Further, as a consequence of the reduced number of crossing point, also the quality factor improves.

In a third aspect the invention relates to a semiconductor device comprising an inductor according to the invention.

In a fourth aspect the invention relates to an integrated transceiver chip comprising an inductor according to the invention and/or a semiconductor device according to the invention.

In a fifth aspect the invention relates to an device, such as mobile phone, Bluetooth transceiver, ww Ian, ww pan, ultra-wideband radio, TV tuner, and combinations thereof, comprising an inductor according to the invention and/or a semiconductor device according to the invention.

The present invention is further elucidated by the following figures and examples, which are not intended to limit the scope of the invention. The person skilled in the art will understand that various embodiments may be combined.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. la-c. a) An inductor at low frequency represented by an equivalent network, b) Difference of potential applied to signal pads of an inductor, used in differential mode, roughly corresponding to an equivalent network, c) A point dividing an inductor in two parts with the same resistance and self inductance.

Fig. 2. Example of symmetrical prior art 8-shaped and clover-shaped layouts.

Fig. 3. Example of symmetrical 8-shaped and clover-shaped layout.

Fig. 4. Inner radius of a coil is in fact inner radius of an "eye".

Fig. 5. The relative decrease of resistance of 8-shape inductor with 5 turns could reach 50% that is twice increasing of a quality factor.

Fig. 6. By using this invention gain of quality factor could vary from 15% to 95%.

Fig. 7. The relative decrease of dc resistance of clover shape inductor is very comparable with 8 shape layout although is a little bit smaller.

Fig. 8. Increase of quality factor for clover shape could also be very significant.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. la-c. a) An inductor at low frequency represented by an equivalent network, b) Difference of potential applied to signal pads of an inductor, used in differential mode, roughly corresponding to an equivalent network, c) A point dividing an inductor in two parts with the same resistance and self inductance. A difference of potential is applied to this point and signal taps in such a way giving an equivalent network.

Fig. 2. Example of symmetrical 8-shaped and clover-shaped layouts. In both cases the number of crossing points is proportional to the square of number of turns. Red regions show parts of the crossing points that use under layer(s) of the crossing.

Fig. 3. Example of symmetrical 8-shaped and clover-shaped layout. In both cases the number of crossing points is proportional to the number of turns. Red regions show parts of the crossing points that use under layer(s) of the crossing.

Fig. 4. Inner radius of a coil is in fact inner radius of an "eye".

Fig. 5. The relative decrease of resistance of 8-shape inductor with 5 turns could reach 50% that is twice increasing of a quality factor.

Fig. 6. By using this invention gain of quality factor could vary from 15% to 95%. This graphs should not be taken literally it is just an indication of the actual gain of the quality factor.

Fig. 7. The relative decrease of dc resistance of clover shape inductor is very comparable with 8 shape layout although is a little bit smaller.

Fig. 8. Increase of quality factor for a clover shape or 8-shape could also be very significant. Please note, that this graphs should not be taken literally it is just an indication of the actual gain of the quality factor.