EP1109179A1 - Inductor structure on a semiconductor substrate - Google Patents

Abstract

The invention relates to an inductance structure arranged on a semiconductor substrate, comprising an inductor (1) and a conductive plane (10) disposed between the inductor and the substrate. The conductive plane (10) is formed by several disjointed conductive elements (20), the connection of which is made by conductive tracks connecting at least one conductive element (20) to a contact point M of the conductive plane. Each of the conductive tracks is arranged so that the result of the electromotive forces which are induced in said conductive track by the inductance is substantially zero. <IMAGE>

Description

The present invention relates to the field of circuits integrated, and more particularly an inductor formed above of a semiconductor substrate.

FIG. 1 represents an inductor 1, comprising a number of turns or turns, formed by a conductive element deposited on an insulating layer 2. The insulating layer 2, by example of silicon oxide, based on a semiconductor substrate 3, generally made of silicon, which, in the example shown, is connected to ground by its lower face 4.

A big disadvantage of the inductance of figure 1 is that it has high losses. So there is a capacity C relative to the substrate, the insulating layer 2 serving dielectric. Furthermore, the substrate 3 is resistive and it has a resistance R between its upper 5 and lower faces 4. Thus, when inductance 1 is crossed by a variable current, losses occur through capacity C and resistance R. These losses have the disadvantage greatly reduce the quality factor Q by inductance.

To remedy this drawback, the patent application European EP-A-0780853 proposes an inductance structure on a silicon substrate having a conductive plane located between inductance and substrate. This conductive plane, isolated from the substrate and inductance, is connected to ground or to a cold point of the circuit, in order to establish a "shield or electrostatic screen" between the inductance and the semiconductor substrate. To avoid a dissipation by creation of eddy currents in the plane conductor, said request proposes a division of the conductor plane.

A type of inductor with a cut inductor plane according to an example of the above mentioned request is illustrated in figure 2.

Figure 2 illustrates an inductor 1, an insulating layer 2 and a substrate 3 having an upper face 5 and a lower face 4 connected to earth. Figure 2 also illustrates, above the insulating layer 2, a conducting plane 10. The plane conductor 10 is cut into longitudinal strips 12 connected to a lateral strip 13, perpendicular to the strips 12 and having, in the middle, a point 11 connected to ground. The effect due to eddy currents is thus greatly decreased, but the structure of Figure 2 has drawbacks.

Thus, in FIG. 2, when the inductance 1 is traversed by a variable current, each of the bands 12 is the seat of an electromotive force e due to the existing inductive coupling between said bands and the inductance. Likewise, the side strip 13 is the seat of an electromotive force e 'due to the coupling inductive between strip 13 and inductance 1. These electromotive forces cause losses. Indeed, each of the points of bands 12 and 13 is at a nonzero potential with respect to the mass, due to the induced electromotive forces, and, therefore, losses occur through capacity due to layer 2 serving as dielectric and ohmic resistance of the substrate, these capacitance and ohmic resistance being quantities distributed and different at each point of the plane driver.

All these losses make the behavior of the structure of Figure 2 unsatisfactory and lower the coefficient of Q quality of the inductor.

The aforementioned patent application proposes other Ways to Cut the Conductor Plane (See Figures 7, 9 and 12 of this request). However, in all the examples offered of said request, including in its preferred embodiment, corresponding to FIG. 7, there remain portions of conductive plane in which an induced electromotive force high causes the side effect that has been described.

This is why an object of the present invention is to providing an inductance structure arranged on a semiconductor substrate which does not have the drawbacks described above.

Another object of the present invention is to provide an inductance structure arranged on a semiconductor substrate which minimizes the losses linked to the operation of the inductor.

Another object of the present invention is to provide an inductance structure which minimizes electromotive forces induced in the conductive plane.

To achieve these and other objects, this invention provides an inductance structure arranged on a semiconductor substrate, comprising an inductor and a plane conductor disposed between the inductor and the substrate. The plan conductor has several disjoint conductive elements and several conductive tracks, each conductive track connecting at least one conductive element at a contact point M on the plane driver. Each of the conductive tracks is arranged so that the result of the electromotive forces which are induced there by said inductance is substantially zero.

According to an embodiment of the present invention, each of the conductive tracks substantially follows an axis of inductance symmetry.

According to an embodiment of the present invention, the inductance has substantially the shape of a square and the tracks conductive are arranged along the diagonals and medians of said square.

According to an embodiment of the present invention, the inductance has substantially the shape of a circle and the tracks conductive are arranged along the radii of said circle.

According to an embodiment of the present invention, said conductive elements have an elongated shape and are arranged perpendicular to a portion of the turn under which they rest.

According to an embodiment of the present invention, said conductive elements are arranged under the turns of inductance only.

la figure 1, déjà décrite, représente une inductance déposée sur un substrat semiconducteur selon l'art antérieur ;

la figure 2, déjà décrite, représente une autre structure d'inductance déposée sur un substrat semiconducteur selon l'art antérieur ;

la figure 3A représente une structure d'inductance selon la présente invention ; et

la figure 3B représente un plan conducteur faisant partie de la structure de la figure 3A.

These objects, characteristics and advantages, as well as others of the present invention will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

Figure 1, already described, shows an inductor deposited on a semiconductor substrate according to the prior art;

FIG. 2, already described, shows another inductance structure deposited on a semiconductor substrate according to the prior art;

FIG. 3A represents an inductance structure according to the present invention; and

FIG. 3B represents a conducting plane forming part of the structure of FIG. 3A.

FIG. 3A represents an inductance structure according to the present invention. In FIG. 3A, an inductance 1 is formed by a three quarter turn, the turns being represented here in the form of straight turns. So the inductance 1 is formed by straight conductive turns AB, BC, CD, DE, EF, FG, and GH. FIG. 3A also represents, under inductance 1, a conducting plane 10.

According to the invention, the conducting plane 10 is formed separate conductive elements 20, isolated from each other. The conductive elements 20, which, in the example shown, have the shape of substantially rectangular bands can be produced in various ways, for example by etching a metallic layer or by diffusion of highly doped layers in the substrate. The fact that the conductive plane is made up of elements separate from each other has the advantage of allow great flexibility for their connection, flexibility used in the structure of the present invention. The elements 20 are connected to a contact point M by tracks conductive, further described in connection with Figure 3B. The contact point M allows the connection of the conductive plane, and is intended to be connected to earth or to a "cold" point any of the circuit.

For the sake of simplicity, FIG. 3A does not represent neither the necessary insulating layers, nor the substrate on which rests the inductance structure.

Figure 3B repeats Figure 3A, but inductance 1 has been removed to better show the elements 20 of the conductive plane 10, as well as the conductive tracks which connect elements 20 at the contact point M.

As noted above, the elements 20 have an elongated, substantially rectangular shape. They are willing perpendicular to or to the turn portions under which they rest. Their width is small so as to limit their surface to reduce eddy currents which, although weak, are not nonexistent. Preferably, the width is chosen as low as possible, while taking take care not to diminish the effectiveness of the elements 20 in their role of electrostatic screen. The length of the conductive elements 20 is enough to go on either side of the portion of turn considered, slightly exceeding the most turn internal and the outermost turn. The elements 20 are therefore more or less long, depending on the number of turns crossed. So, element 24 of FIG. 3B, placed under the central part of the portions of coil BC and FG is longer than element 25, placed under the central part of the DE turn portion, because the number of coils placed above is lower for the latter.

Under the central parts of the turns, all the elements adjacent 20 have the same length and the same width, in the example shown. Under the peaks of the inductance, by against, the elements 20, always perpendicular to the portion of coil under which they rest, are shorter because they meet the elements 20 on the adjacent side. So the elements conductors 26, 27 of FIG. 3B are shorter than the conductive elements 28, 29, of the same length as the elements 30, 31 located in the center of the turn portions AB and EF. We will note however that the representation of FIG. 3B is an example only, and that other ways of arranging the elements 20 conductors are possible without leaving the domain of the invention.

It will also be noted that the elements 20 of the conductive plane 10 do not extend into the central region of inductance, in order to limit their surface to reduce the eddy current losses.

Note also that the shape and arrangement of the elements 20 of FIGS. 3A, 3B is only an example only, and that other forms and arrangements of the disjoint elements 20 are possible without departing from the scope of the invention.

Figure 3B also illustrates in detail the connections conductive elements 20 at the point of contact M. Point M is connected to a point O corresponding to the center of inductance 1 by a conductive track MO, which connects to the point M the elements 25, 25 'and 25 ". Various other conductive tracks pass through point O and connect a small number of elements 20. So the extension of the MO track, the ON track, connects the elements 24, 24 'and 24 ". An RS track connects elements 30, 31, 32, 33. From similarly, VW and TU tracks, diagonally in the figure 3B, connect the remaining elements 20, located under the vertices of inductance 1.

Preferably, these conductive tracks are of width minimum, compatible with the maximum tolerable resistance they can present.

Note that the RS, TU, MN and VW tracks are not limited not to straight segments defined by the above points, but that they are arranged to connect effectively the elements 20 for example as indicated in bold lines in Figure 3B. Thus, the ON track includes in addition to two segments NN ', NN ", perpendicular to ON, for connecting elements 24 'and 24 ".

Note also that point O is common to all tracks, which, because of the OM track, are all connected electrically at the point of contact M, and thus form, with the disjoint elements 20, the conducting plane 10. We will also note that in practice the OM track is wider than the other tracks, in order to be able to effectively drain currents if necessary residuals outside the conductor plane.

The arrangement of the conductive tracks connecting the elements 20 was chosen so that the result of the forces electromotors induced in the conductive tracks is substantially nothing.

To better understand the choice made, we will describe, in relation to FIGS. 3A and 3B, the behavior of the inductance structure according to the present invention when the inductance is traversed by a variable current i.

First, because the conductive plane is formed small disjointed conductive elements, the problem eddy currents, which nevertheless exist in each of conductive elements 20 (and not in the conductive tracks, of negligible surface), is practically resolved and only is consider the problem due to induced electromotive forces.

Generally speaking, the electromotive force induced in a first conductor, by a second conductor traversed by a variable current i, has the value e = -M.di / dt, M being the coefficient mutual inductance between the two conductors and di / dt the variation, as a function of time, of the current i traversing the second driver.

For two parallel straight conductors, the coefficient mutual inductance is a function of the length of conductors and their distance, M being all the higher as the length of the conductors is great and their distance is low. If the conductors are not parallel but form a certain angle, their mutual inductance coefficient M is proportional to the cosine of the angle formed by the two conductors. Finally, when two conductors are perpendicular (their angle is 90 °), their mutual inductance coefficient is no.

So to reduce the induced electromotive forces in the conductive tracks and consequently the losses suffered by inductance 1, three types of configurations.

According to a first configuration, a track or a section of conductive track is perpendicular to the portions of whorl, whence results a zero mutual inductance and a force electromotive induced null also.

According to a second configuration, a track or a section of conductive track is parallel to at least two portions of turn, and at an equal distance between them. Is equivalent to place these tracks in the center of the inductor and, like each coil of the inductor has portions traversed by a current with the same absolute value and in the opposite direction, the tracks considered have a mutual inductance made up of two components, one positive and the other negative, which are entrenched and cancel out exactly if the number of turns is the even on each side.

According to a third configuration, used for inductance peaks, a conductive track or section of track is arranged along the bisector of the angle formed by whorl portions. These portions of the turn, being traversed by currents of opposite directions (respectively directing and moving away from the top of the angle they form), the inductance resulting mutual insurance between these turns and the track or the section of track considered is also zero, as well as the resulting electromotive force induced in the runway or section of track considered.

Let us thus examine, in relation to FIGS. 3A and 3B, inductive coupling between the different conductive tracks of the present invention and each of the turns of inductance 1, and the electromotive forces that these portions of whorl occur in a track when the inductance is traversed by a variable current.

The MO track is perpendicular to the turns DE, FG and BC. The mutual inductance coefficient between these portions of the turn and the MO track is therefore zero, and the force The electromotive motor created in MO by these portions of the turn is zero. Furthermore, the track MO is parallel to the portions of coil AB, CD, EF and GH, and located between these turns, at equal distance from them. The MO runway is the site of a first electromotive force induced by the AB and EF portions, but this electromotive force is compensated by a second force electromotive induced by the CD and GH turns, where it follows that the resultant of the electromotive forces created by the turns of AB, CD, EF and GH are zero. So the track MO is not the seat of any resulting electromotive force and the point O is at exactly the same potential as point M.

Similarly, the ON track, perpendicular to the portions of turns BC, FC and DE, and parallel to the portions of turns AB, CD, EF and GH, and at an equal distance between them, is the seat of a zero resulting electromotive force and point N is at the same potential as point M.

The OR track is perpendicular to the portions of turns AB, CD, EF and GH. No induced electromotive force will result therefore in the OR track due to these portions of turns. The OR track is also parallel to the turns of BC, DE and FG. As the OR track is located exactly in the middle of the DE and FG turns, the action of these turns is compensated noticeably. However, the influence of the BC turn portion is not compensated for by the fact that inductance 1 is not symmetrical and that it does not have a whole number of turns. The distance between the OR track and the portion of turn BC being fairly large, the electromotive force induced in the RO track remains low. Nevertheless, the point R is at a certain potential by relation to point O, therefore to point M, which generates losses. Similarly, the SO track is the seat of a force residual electromotive due to asymmetries in the inductance and point S is at a potential different from that of points O and Mr.

The TO runway, for its part, is substantially located on the bisector of the angle formed by the portions of coil AB and BC on the one hand, EF and FG on the other. We are in the case of one of the configurations described above and, the same current going towards points B and F and then moving away from it, the track TO is the seat of two electromotive forces which compensate and their resultant is zero. We can neglect in first approximation the action of the turns of CD, DE and GH, which partially offset each other and which are quite far from TO. The same can be said for the WO track, forming the bisector of the angle formed by the turns of BC and CD, respectively FG and GH.

On the other hand, the VO and UO tracks, still due to the asymmetries of the inductance, do not go exactly through the bisector of the angles formed by turns. The compensation is not perfect and it appears, at point V and at point U, a certain potential with respect to point O and, by next, from point M.

Thus, it appears that, thanks to the structure according to the present invention, the results of electromotive forces induced in the various conductive tracks connecting the various conductive elements 20 are zero or close to zero. In fact, if the inductance was perfectly symmetrical, the structure described above would allow perfect compensation of electromotive forces induced in the conductive tracks. Before describing a way to further reduce the forces electromotive residuals present in the tracks, we will describe the significant benefit of the fact that connection points of the elements 20 are at a potential substantially equal to that of contact point M.

Thus, returning to FIG. 2, the strip 13 and the two peripheral strips 12 are the seat of high electromotive forces e ′ and e, owing to the fact that they run over a considerable length of portions of the turn at short distance. The point 14, at one end of the strip 13 is thus at the potential e '/ 2 relative to the point 11 and the point 15, at the end of the peripheral strip 12, is at the potential e "= e + e '/ 2. Under points 14 and 15, there are capacitors, respectively C' and C ", located between the conductive plane and substrate, and resistors, respectively R 'and R", of the substrate. These capacitors C', C "and resistors R ', R" are distributed capacitors and resistances, the value of which depends on many parameters. In capacitors C' and C ", a current i 'and i" flows, the value of which is due to the fact that the resistances R 'and R "are low in practice in comparison with the impedance of the capacitances C' and C", respectively i '= C'.de' / dt and i "= C" .de "/ dt, de '/ dt and "/ dt respectively representing the variations of the voltages e 'and e" as a function of time. The currents i 'and i "cause losses in the substrate by Joule effect, equal respectively to R'.i' ² and R" .i " ² , all the higher as the voltages e 'and e" are high.

According to the invention, on the other hand, the tracks which connect elements of the conductive plane are arranged so that the resulting from the electromotive forces induced in these tracks is substantially zero. This means that the connection point between a conductive element 20 and the conductive track which connects is at a substantially zero potential. So although electromotive forces are induced in most elements conductors (only elements 24 and 25 are completely exempt in the example shown), the potential difference maximum between point M and each point of an element conductor 20 remains substantially limited by the value of the electromotive force induced in said element which is moreover weak because the conductive elements are perpendicular to the turns and their length is low. This therefore limits the current flowing through the capacity located under the elements 20 and consequently the losses ohmic in the substrate, and this in a significant way compared to the prior art, where the connection of the plane parts conductor is often made by the periphery.

However, as mentioned above, a residual electromotive force, mainly due to asymmetry of inductance, remains in certain conductive tracks connecting the different elements of the conductive plane, for example in the RO track, making the potential of the R point relative at point M is not entirely zero. It is possible to reduce this residual electromotive force. Indeed, there are electromagnetic simulation tools allowing, from various system parameters, calculate the coefficient of inductance mutual fund between inductance 1 as a whole and a particular track route. For example, for the RO track, a R'O track, located between the RO track and the VO track is more the influence of the turn portion DE and therefore is probably better, in terms of the result of electromotive forces induced, that the RO track. It is possible to use the electromagnetic simulation tool mentioned above for evaluate the performance of one or more R'O tracks offset by report to the RO track and choose the one for which the resulting electromotive forces induced therein is the most close to zero. This leads to a variant of the track pattern conductive and so it is possible to obtain many runway possibilities, which improve the driver plane of Figure 3B. However, they require a calculation quite complex, which is not always necessary. So in the case of FIG. 3A where the inductance 1 is almost symmetrical, the Figure 3B conductive plane provides satisfactorily the advantages described without requiring additional correction.

Of course, the inductance structure illustrated in Figures 3A and 3B is only an example and the present invention is susceptible to various variations and modifications which will appear to those skilled in the art. In particular, we note that the conductive elements 20 are arranged just below the location inductor 1 turns, and extend very little. However, this characteristic is not essential and the elements of the conductive plane could extend further under inductance without departing from the scope of the invention.

Furthermore, the example of FIGS. 3A and 3B represents a square inductor comprising a three-quarter turn. he goes of course the present invention can be applied whatever the number of turns of the inductance, and whatever or the form of the inductance.

So if the turns are rectangular, the elements of the conductive plane can be rectangles, as in the case present, and the conductive tracks will follow, unless corrected due to asymmetries in the inductance, medians and diagonals of the rectangle.

If the inductance turns are circular or in spiral and have a center O, we can keep a shape rectangular to the elements 20 of the conducting plane 10, but, of preferably, these will have either a trapezoidal shape, limited by rays of center O or either will be parts of sectors circulars from center O. Anyway, these elements will arranged radially, and their connection to the center will be by conductive tracks also arranged radially, such a structure, perfectly symmetrical, with minimal losses.

Claims (6)

Inductance structure arranged on a semiconductor substrate (3), comprising an inductor (1) and a plane conductor (10) disposed between the inductor (1) and the substrate, the conductive plane (10) comprising several conductive elements disjoint (20) and several conductive tracks, each track conductive connecting at least one conductive element (20) to a contact point M of the conductive plane, characterized in that each of the conductive tracks is arranged so that the resulting from the electromotive forces induced therein by said inductance is substantially zero. Inductance structure according to claim 1, in which each of the conductive tracks substantially matches an axis of symmetry of the inductance. Inductance structure according to claim 2, in which the inductance has substantially the shape of a square and the conductive tracks are arranged along the diagonals and the medians of said square. Inductance structure according to claim 2, in which the inductance has substantially the shape of a circle and the conductive tracks are arranged along the radii of said circle. Inductance structure according to claim 1, in which the conductive elements (20) have a shape elongated and are arranged perpendicular to a portion of turn under which they rest Inductance structure according to claim 1, in which the conductive elements (20) are arranged under inductor turns only.

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