FIELD OF THE INVENTION

The present invention relates to an inductor.

BACKGROUND OF THE INVENTION Typically, an inductor comprises:

- a symmetry axis,

- one conductive path with two extremities wound to form at least two turns and bridges to connect the turns in series, the turns being symmetrically positioned on either side of the symmetry axis, the turn formed on one side of the symmetry axis being adapted to create a magnetic field in one direction, and the turn formed on the other side of the symmetry axis being adapted to create a magnetic field in the opposite direction, and

- two connection strips connected to respective extremities of the conductive path to supply current in the turns. The connection strips extend away from the inductor parallel to the symmetry axis. More precisely, one connection strip extends in one direction and the other connection strip extends in the opposite direction.

Such an inductor presents a low magnetic field outside the turns and therefore, a good magnetic coupling characteristic. Thus, such an inductor is particularly well-suited to be used in an integrated circuit.

An example of such an inductor is described in WO 01/37293. However, the design of these inductors can still be improved to obtain better performance.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide an inductor with improved characteristics and higher performance.

The invention provides an inductor wherein the connection strips are symmetrically positioned on either side of the symmetry axis

The axial symmetry of the above inductor is improved because the two connection strips are symmetrically positioned about the symmetry axis. Therefore, the magnetic coupling characteristic is further improved.

The features of claims 2, 3, 5 and 7 further enhance the magnetic coupling characteristic.

The features of claim 4 provide a compact multi-turn inductor.

The features of claim 6 provide an inductor that can be connected to an electronic circuitry only on one side so that it can be placed on the border of an integrated circuit.

The invention also relates to an integrated circuit including an inductor according to claim 1.

These and other aspects of the invention will be apparent from the following description, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig.l is a schematic diagram of an integrated circuit including an inductor, and Fig.2A is a schematic diagram of another embodiment of the inductor of

Fig.l and

Fig.2B is a schematic diagram of the background of the inductor of Fig.2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig.1 shows an integrated circuit 2 including an inductor 4 and an electronic circuitry 6 electrically connected to inductor 4 through connection strips 10 and 12.

The integrated circuit 2 is built using a multi- layer flat substrate 14. For example, substrate 14 is made of silicon.

Fig.1 shows only the details necessary to understand the invention. Inductor 4 is a "8" shaped flat coil. It has a symmetry axis 20 and a conductive path 22 wound around two central surfaces 24 and 26 to form a plurality of turns symmetrically positioned on either side of symmetry axis 20.

For example, on the right side of axis 20, path 22 forms four turns 30, 31, 32 and 33 from the innermost turn 30 that is in contact with surface 26, to the outermost turn 33.

On the left side of axis 20, path 22 forms four turns 36, 37, 38 and 39 from the innermost turn 36 in contact with surface 24 to the outermost turn 39.

Path 22 has two extremities 42 and 44 electrically connected to strips 12 and 10 respectively. Extremities 42 and 44 are placed inside central surfaces 24 and 26 respectively. In this embodiment, extremities 42 and 44 are symmetrically positioned about symmetry axis 20. Furthermore, extremities 42 and 44 coincide with connection points between strips 10, 12 and path 22.

Turns 30-33 are adapted to create a magnetic field Bl flowing through and perpendicular to surface 26 and turns 36-39 are adapted to create a magnetic field B2 flowing through and perpendicular to surface 24. The turns 30-33 and 36-39 are electrically connected in series. More precisely, from the innermost turn 30 to the outermost turn 33, the turns on the right side of axis 20 are connected in reverse order to turns 36-39, respectively. Inductor 4 has four conductive bridges 54-57 formed on the same substrate layer as the one used to form turns 30-33 and turns 36- 39. Bridges 54-57 electrically connect turn 30 to turn 39, turn 31 to turn 38, turn 32 to turn 37 and turn 33 to turn 36, respectively. Bridges 54-57 cross axis 20. Inductor 4 also includes three conductive bridges 60, 62 and 64 electrically connecting turn 33 to turn 37, turn 32 to turn 38, turn 31 to turn 39, respectively. Bridges 60, 62 and 64 are realized on a substrate layer under the substrate layer used for turns 30-33 and 36-39. In Fig.l, bridges 60, 62 and 64 are represented in dotted lines. As a result of the connection in reversed order of the turns on the right side of axis 20 to the turns on the left side of axis 20, the current flowing in turns 36 to 39 has a direction opposite to the direction of the current flowing in turns 30-33. Thus, magnetic fields Bl and B2 have opposite directions.

Dark arrows 50 and 52 show the direction of the current flowing in turns 33 and 39, respectively.

Conductive path 22 continuously extends from extremity 42 to extremity 44 through turns 30-33, 36-39 and bridges 54-57, 60, 62 and 64.

Connection strips 10 and 12 are symmetrically positioned about axis 20. Moreover, connection strips 10 and 12 extend parallel to axis 20 in the same direction. Strips 10 and 12 are formed on another substrate layer than the one used for turns 30-33 and 36-39. The free ends of connection strips 10 and 12, which extend outside the turns are connected to circuitry 6. For example, a current is fed through connection strip 12 to inductor 4 by circuitry 6 and the resulting current is output to circuitry 6 through connection strip 10.

For example, inductor 4 is 260 μm large and 190 μm long and its inductance value is about 4.23 nH.

Fig.2A shows a more compact design of an "8" shaped inductor 65 having the same inductance value as inductor 4.

In Fig.2A, the elements of inductor 65, which are identical to the elements of inductor 4, have the same references. In inductor 65, bridges 54-57 are replaced by bridges 68-71, and bridges 60,

62 and 64 are replaced by bridges 74, 76 and 78 respectively. Bridges 74, 76 and 78 are shown in Fig.2B. In this embodiment, more than 80% of the length of bridges 68- 71 and 74, 76 and 78 extend parallel to axis 20. Bridges 74, 76 and 78 are situated under bridges 68-71 and face bridges 68-71. Due to this superposition of bridges 74, 76 and 78 by bridges 68-71, the size of inductor 65 is smaller than the size of inductor 4. For instance, inductor 65 is 210 μm wide and 190 μm long. This structure of inductor 65 also improves the magnetic coupling characteristic of inductor 65.

In inductor 65, extremities 42 and 44 are disposed on either side of an axis 79 perpendicular to axis 20. The Axes 20 and 79 cross each other in a point P. The extremity 42 is the central symmetric point of extremity 44 about a point P.

Furthermore, in this embodiment the connection point between strips 10, 12 and path 22 do not coincide with extremities 42 and 44 so that extremities 42 and 44 are free. Therefore, no current flows in extremities 42, 44. The free extremities increase the symmetry of the electrical characteristics of inductor 65. Thus, a better performance, such as a better magnetic coupling characteristic, is obtained.

Finally, conductive plates 80 to 85 (Fig.2B) are etched under turns 37-39 and 31-33. These plates are rectangular. One extremity of plates 80-82 is electrically connected to bridge 74. One extremity of plates 83-85 is connected to bridge 78. The

other extremity of each plate is free, so that no current flows in these plates. These plates 80-85 increase the symmetry of the capacity charge partition in inductor 65 and improve the characteristic of inductor 65.

Many additional embodiments are possible. For example, plates 80-85 may be omitted.

Inductors 4 and 65 have been described in the special case of an inductor having 8 turns. However, the number of turns may be any even integer number greater than 2. In fact, the number of turns only depends on the inductance value to be obtained.