Low magnetic field inductor

FIELD OF THE INVENTION

The present invention relates to an 8 shaped inductor layout, which maximizes the Q- factor for a given inductor area and track density.

BACKGROUND OF THE INVENTION

IC inductors are essential to realize the voltage controlled oscillators needed in the many fully integrated transceiver chips serving a multitude of wireless communication protocols that are provided to the market today. The required inductance value is typically a few nH, and should be adjustable to the application, whereas the quality factor should be as high as possible.

An additional benefit could be a low net magnetic field, which has been the aim of the previous art such as in WO1998/005048(Al), WO2004/012213(Al), WO2005/096328(Al), and WO2006/105184(Al).

Typically achieving an acceptable quality factor requires a fairly large inductor size of hundreds of μm diameter and realizing the inductor in one or more thick top metal layers. To quantify the trade-offs in quality factor optimization in this invention the inventors use the normalized L/R ratio as a layout figure of merit. The (low frequency) inductance LDC and resistance RDC of a hypothetical multi-turn planar circular inductor with zero spacing between the windings are;

where;

Z ⁾ , + Z>

_{f =} D _≡L -D,

D _n ,,, + Z ⁾

and D _out and D _1n denote the inductor outer and inner diameter. The area A and track density d of this inductor are:

π

A = -D _o ² _ut = 4πr ² (l + f) ²

d = ^D ° ^ut , ^D ' ⁿ 4/ ²

[ I] Di, (1 + /) ⁵

The normalized L/R ratio is now calculated as:

A nice feature of this layout figure of merit is that is does not depend on process parameters like the metal sheet resistance R _s heet or the inductor area A but only on the fill factor f. To make the approach a bit more general instead of the fill factor f we will use the inductor track density d, which we can calculate for any inductor layout. Plots of the normalized L/R ratio versus inductor track density d for circular and square inductors are given in Fig. 1. As can be seen the normalized L/R ratio increases with inductor track density. For the same track density the circular inductor has a better normalized L/R ratio.

Taking the square inductor as a reference device we define the quality factor figure of merit (QFOM) for an arbitrary inductor layout as its normalized L/R ratio divided by that of a square inductor of the same track density d. For the inductor area we use the area enclosed by a minimum length path around the inductor. For a circular inductor this QFOM is about 1.2, with a minor dependency on inductor density. In the remainder of this invention this QFOM will be used to classify inductors of arbitrary shape and as a tool to select the most area efficient layout. This new concept should not be confused with the customary used inductor Q-factor, which is defined as the imaginary part of the complex inductor impedance divided by the real part of this impedance, which is generally highly frequency dependent, and which does not account for expenses paid, for instance in terms of inductor area to achieve a high value.

In WO2004/012213(Al) it is proposed to use a series connection of a clock- wise and a counter-clockwise, but otherwise identical inductors to obtain a low net magnetic field,

beneficial to minimize the inductive coupling between the different inductors of a circuit. To first order this series connection doubles inductance, resistance and occupied area, and therefore reduces the QFOM by a factor of V2. Fortunately, the mutual inductance between the 2 inductors enhances the total inductance to be somewhat more than the sum of the individual inductances, but this does not compensate for the loss in QFOM. In the preferred embodiment in this and other previous work, WO2005/096328(Al), the individual inductor eyes are distorted until their height is about halve their width, resulting in a square or somewhat circular foot-print for the resulting figure 8 shaped inductor.

Typically, due to design rules, inductors occupy a nearly square area. As a consequence, the loops of an 8-shaped inductor of the prior art are distorted.

Therefore, none of the cited prior art teaches how to optimize the QFOM of an 8- shaped inductor.

SUMMARY OF THE INVENTION

The present invention relates to an 8-shaped inductor having a width and a height, characterized in that a ratio between the height and the width is in the range of larger than 1 :1, and preferably smaller than 3:1.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect the invention relates to a semiconductor device comprising an 8- shaped inductor having a width and a height, comprising a first contact, a second contact, and tracks forming sections which sections being electrically connected to one another, comprising at least one loop in any first section, at least one loop in any second section, which at least one loop in the first section has substantially the same form as the form of the at least one loop in the second section, arranged such that an electrical current can run from the first contact to the second contact, wherein the electrical current in a first section runs in one direction, and wherein the electrical current in an adjacent second section runs in another direction, characterized in that a ratio between the height and the width is in the range of larger than 1 :1, preferably larger than 11:9, more preferably larger than 3 :2, such as 13:7, and preferably smaller than 3:1, more preferably smaller than 7:3.

Examples of the present 8-shaped inductor are given in Figs. 3 and 4. Therein also the width and the height of one inductor are given.

The present inductor comprises a first and second contact, indicated at the bottom section of e.g. Figs. 3 and 4.

The inductors of e.g. Fig. 3 comprise a first section and a section, each section comprising one loop, whereas the section of Fig. 4 comprise 2 loops each, respectively. Due to the layout of an 8-shaped inductor the number of loop in the first section is equal to the number of loops in the second section. Preferably the at least one loop in the first section has substantially the same form as the form of the at least one loop in the second section, most preferably the two forms are substantially the same.

The current in the first section should run in another direction than in the second section, in order to compensate for the electro -magnetic field generated.

In a preferred embodiment the form of the loops, or equivalently called turns, 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. As can be deducted from Fig. 2, the more circular the loops are, the better the QFOM is. Therefore, a most preferred embodiment comprises circular loops. It may occur, that due to design rules, only horizontal and vertical sections are allowed. In such cases, a combination of horizontal and vertical sections, closely resembling a circle, may be preferred. Especially when the width of the tracks is relatively large enough, such a combination would form almost a perfect circle.

In Fig. 2 the Sonnet EM simulated QFOM's of Fig. 8 shaped inductors with various shapes and various height over width (H/W) ratios are compared. Examples of the simulated inductor layouts are depicted in Fig. 3. These figures clearly show that the best QFOM is obtained when the individual inductors are approximately circular. The full Fig. 8 layout with a height over width ratio which is only a fraction less than 2 has the highest QFOM of 0.94. This represents a 7% improvement over the case were same Fig. 8 shape inductor is squeezed into a square footprint. The latter inductor is a typical prior art inductor.

Due to the fixed course grid required for the Sonnet EM simulations to finish in a reasonable amount of time, it is likely that the optimal relative dimensions of the 45 degree angled segments have not yet been found, and that further layout optimization could improve the QFOM a bit more.

A feature of the low magnetic field inductor disclosed here compared to those known from WO2004012213(Al) and WO2005096328(Al) is that the individual inductor eyes are preferably approximately circular. This increases the L/R ratio for a given inductor area and track density, while furthermore known measures to improve inductor performance such as

increasing track width or adding additional inductor loops become more effective since in particular the inner inductor loops are less distorted. This indicates that the ratio between the height and the width is in the range of larger than 1 :1, preferably larger than 11:9, more preferably larger than 3:2, such as 7:13, and preferably smaller than 3:1, more preferably smaller than 7:3. These ratios represent the layouts of inductors chosen in Fig. 2, which all clearly have a much better performance in terms of QFOM than the inductor of the prior art.

In a preferred embodiment the width of the tracks is from 5-50 μm, preferably from 12-30 μm, even more preferably from 15-25 μm, such as 20 μm.

Building and using the invention should be fairly obvious given the previous discourse. To put things in perspective the gain in quality factor achieved with this invention is compared to the gain in quality factor which can be achieved by increasing the inductor track density using the layouts shown in Fig. 4.

Series connection of multiple turns is usually required to achieve sufficient inductance for a given application.

As can be seen from the Q-factor plots in Fig. 5, adding an extra 20 μm wide inductor track on the same inductor footprint approximately doubles resistance but more than doubles inductance, and increases the L/R ratio and the Q-factor at low frequencies in line with the trend shown in Fig. 1. The increased inductance and the increased capacitance, both to the substrate as well as between the windings, reduces the inductor resonance frequency significantly, and as a result the peak quality factor is reduced. This well-known single/multi turn inductance versus peak Q-factor design trade-off is less severe when individual inductor eyes are made approximately circular rather than when the entire component footprint is made approximately circular, because in the latter case in particular the inner inductor turns become highly distorted. When high inductance is not required parallel connection of multiple turns as described for standard inductors in WO2003015110(Al) could be attractive.

As shown in Fig. 5, this 2-parallel case provides a higher Q-factor at nearly all frequencies than the single turn (new) reference case. It is however also shown in Fig. 5 that a wide turn covering the same area plus the gap between the two parallel paths performs slightly better owing to its further enhanced track density. In many IC processes however, design rules limit the maximum track width to about 12 to 30 um, and the use of parallel paths is a good solution to further increase track density when the maximum track width has already been reached. In many IC processes the inductor performance will benefit from the use of a patterned ground shield as described in WO2004055839(Al), where again the ground bars are positioned orthogonal to the mirror axis of the inductor.

In a preferred embodiment the semiconductor device according to the present invention comprises loops which are in a same horizontal plane.

In a preferred embodiment the semiconductor device according to the present invention comprises at least two loops in each section.

In a preferred embodiment the semiconductor device according to the present invention further comprises a patterned ground shield.

In WO2005096328(Al) a clover shaped inductor is disclosed (Fig. 6). This clover shaped inductor has a QFOM of 0.7. Moreover due to the way the four eyes are connected the area enclosed by the clock- wise running currents is slightly larger than the area enclosed by the counter clock-wise running currents. It is however possible to correct for this by slightly changing the size of the individual eyes.

Fig. 7 shows the Sonnet EM model used for crosstalk evaluation. This model contains 17 victim single loop circular inductors placed at different distances and angles from the low magnetic field inductor under test. The (magnetic) coupling between the inductors is found from:

_Cχ

\ imag(Z _u )imag(Z _xx )

Where Z represents the Z parameter matrix simulated by the Sonnet EM model. The 8 shaped inductor according to this invention has two symmetry axes. Victim inductors 2 to 6 are on the symmetry axis where the magnetic field of the two inductor eyes is expected to cancel. We will refer to this as the line of cancellation. The clover shaped inductor of Fig. 6 also has two symmetry axes. Here the lines of cancellation make a 45 degree angle with these symmetry axis. Victim inductors 12 to 16 are positioned on this line of cancellation. As shown in Fig. 8 for victim inductors positioned on the line of cancellation typically 32 dB reduction in coupling coefficient is obtained for the 8-shaped inductor disclosed in this invention, whereas for the clover shaped inductor only 16 dB reduction in coupling coefficient is obtained since due to the fact that the area enclosed by the clock- wise running currents differs from the area enclosed by the counter clock- wise running currents the locations where the magnetic fields cancel have shifted. Also included in Fig. 6 is a result obtained for a clover shaped inductor where the size of the individual eyes is adjusted to ensure that a better cancellation of the magnetic fields is achieved. As can be seen this reduces the coupling coefficient further. When we are unable to optimally align any of these

inductor structures with respect to their victim inductor(s) much of the cancellation benefits are lost, and at 1.8 mm distance we may be left with only 9 dB improvement for the Fig. 8 shaped inductor to 13 dB improvement for the corrected clover inductor compared to the circular inductor.

In a second aspect the invention relates to a device comprising a semiconductor device according to any of claims 1-5, such as a low power fully integrated wireless transceiver chip, preferably serving a multitude of communication protocols, a power amplifier module, preferably delivering hundreds of watts, and preferably integrating only a few RF amplification stages.

In a third aspect the invention relates to a use of a semiconductor device according to the invention or a device according to the invention, for minimizing inductive crosstalk.

In a fourth aspect the invention relates to a use of a semiconductor device according to the invention or a device according to the invention, for maximizing inductor Q-factor,

In a fifth aspect the invention relates to a use of a semiconductor device according to the invention or a device according to the invention, for minimizing area needed for an inductor.

Minimizing inductive crosstalk while maximizing inductor Q-factor and minimizing the area needed for the inductors is important in many application fields of these devices, which range from low power fully integrated wireless transceiver chips serving a multitude of communication protocols, to power amplifier modules delivering hundreds of watts, but integrating only a few RF amplification stages. The invention teaches a way to achieve a 30 dB reduction in inductive coupling to a carefully positioned victim inductor while either sacrificing only 25 % of the inductor Q-factor or spending only 50 % additional inductor area.

In a fifth aspect the invention relates to a device, comprising one or more semiconductor devices according to the invention. Such a device has an improved QFOM, a reduced inductive crosstalk, and a minimized area per inductor. The device can be an integrated circuit, the use of an IC in a further application, such as a mobile phone, or an RFID, or a system in package, or a combination of functional elements on a chip, or combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows the normalized L/R ratio (eq. 2) versus inductor track density (eq.l) as calculated for a circular and a square inductor.

Fig. 2 shows a Sonnet EM simulated layout QFOM versus height over width ratio of different figure 8 shaped inductors. The inductor foot prints were changed from 440 x 360 μm to 200 x 600 μm. Track width was kept at 20 μm.

Fig. 3 shows from left to right: square, octagonal, and full figure 8 inductor layouts with the optimal width over height ratio.

Fig. 4 shows layouts with increased track density. From left to right: series connection of multiple turns, parallel connection of multiple turns and wide turns.

Fig. 5 shows the Q-factor versus frequency Sonnet EM simulated for various full Fig. 8 shaped inductors. For better comparison the Q-factor found for the known (red) square footprint inductor was corrected with the square root of the area ratio, to compensate for the area difference with the other four (slightly smaller) inductors.

Fig. 6 shows a clover shaped inductor which has a QFOM of 0.7. Moreover due to the way the four eyes are connected the area enclosed by the clock- wise running currents is slightly larger than the area enclosed by the counter clock-wise running currents.

Fig. 7 shows the Sonnet EM model used for crosstalk evaluation. This model contains 17 victim single loop circular inductors placed at different distances up to 1.8 mm and at 5 different angles from the low magnetic field inductor under test.

Fig. 8 shows coupling at a frequency of 1 GHz between victim inductor and DUT versus distance at the line of cancellation (left) and versus angle at 1.8 mm distance (right).

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 shows the normalized L/R ratio (eq.2) versus inductor track density (eq. 1) as calculated for a circular and a square inductor. Fig. 2 shows a Sonnet EM simulated layout QFOM versus height over width ratio of different Fig. 8 shaped inductors. The inductor foot prints were changed from 440 x 360 μm to 200 x 600 μm. Track width was kept at 20 μm. It is clearly visible that, going from left to right, the QFOM first increases to a maximum value, and than drops to a lower value again.

Fig. 3 shows from left to right: square, octagonal, and full figure 8 inductor layouts with the optimal width over height ratio. These layouts have been tested extensively in terms of QFOM versus width and height thereof.

Fig. 4 shows layouts with increased track density. From left to right: series connection of multiple turns, parallel connection of multiple turns and wide turns. Also these layouts have been tested extensively in terms of QFOM versus width and height thereof.

Fig. 5 shows the Q-factor versus frequency Sonnet EM simulated for various full Fig. 8 shaped inductors. For better comparison the Q-factor found for the known (red) square footprint inductor was corrected with the square root of the area ratio, to compensate for the area difference with the other four (slightly smaller) inductors. As can be seen the present inductor layouts perform better, in the chosen frequency region, in terms of Q-vector than the prior art inductors do. Figure 6 shows a clover shaped inductor which has a QFOM of 0.7. Moreover due to the way the four eyes are connected the area enclosed by the clock- wise running currents is slightly larger than the area enclosed by the counter clock-wise running currents. The QFOM of this clover shaped inductor is somewhat smaller than that of a comparable 8-shaped inductor according to the invention. Therefore, typically 8-shaped inductors are preferred in terms of QFOM.Fig. 7 shows the Sonnet EM model used for crosstalk evaluation. This model contains 17 victim single loop circular inductors placed at different distances up to 1.8 mm and at 5 different angles from the low magnetic field inductor under test. The results of this test layout are given in Fig. 8.

Fig. 8 shows coupling at a frequency of 1 GHz between victim inductor and DUT versus distance at the line of cancellation (left) and versus angle at 1.8 mm distance (right).

Again, it is clearly visible that the present inductor performs better, in the frequency region chosen, in terms of crosstalk and/or coupling, than similar inductors of the prior art.