EP2614553A1

EP2614553A1 - Tunable high-frequency transmission line

Info

Publication number: EP2614553A1
Application number: EP11773052.3A
Authority: EP
Inventors: Philippe Ferrari; Gustavo Pamplona Rehder
Original assignee: Universite Joseph Fourier Grenoble 1
Current assignee: Universite Joseph Fourier Grenoble 1
Priority date: 2010-09-08
Filing date: 2011-09-08
Publication date: 2013-07-17
Also published as: FR2964499B1; WO2012032269A1; FR2964499A1; US20130249653A1; US9136573B2

Abstract

The invention relates to a high-frequency transmission line comprising a central conductor strip (6) associated with at least one conductor shielding plane (4). In said transmission line, at least some of the space between the conductor plane and the conductor strip comprises a ferroelectric material (10).

Description

TUNABLE HIGH FREQUENCY TRANSMISSION LINE

Field of the invention

The invention relates to a line of transmitted ^¬ sion radiofrequency (RF). Here radiofrequency refers to the field of millimeter or submillimeter waves, for example in a frequency range of 10 to 500 GHz.

Presentation of the prior art

The continuous development of silicon integrated circuits opens the possibility of very high frequency operation in the field of radio frequencies. The passive elements used include adapters, attenuators, power dividers and filters. Trans lines ^¬ task connects these components, or the component, are an ele ^¬ base in an RF circuit. To exploit the technology on silicon, one needs transmission lines on chips with high factor of quality. Indeed, the quality factor is an essential parameter because it represents the insertion losses of a transmission line for a given phase shift. In addition, these lines must provide a determined phase shift and have a characteristic impedance determined for the frequency used.

In general, these transmission lines consist of a conductive ribbon having lateral dimensions of 10 to 50 μm and a thickness of about 1 μm (0.5 to 5 μm). depending on the technology used). This conductive ribbon is surrounded by one or more side conductors, upper or lower constituting ground planes for forming with the conductive ribbon a waveguide type structure. In technologies consistent with the achievement of Internal circuits ^¬ electronic sandstone, the conductive strip and the ground planes are made of metallization levels of elements formed over a semiconductor substrate.

FIG. 1 shows a transmission line formed on an insulating substrate 100. This line comprises a central conductor ribbon 102 constituting the transmission line proper, surrounded by lateral coplanar mass ribbons 104, 106. This structure is commonly called a guide coplanar wave and designated by the acronym CPW (according to the terms CoPlanar Waveguide).

Various documents, including US Pat. No. 6,498,549, have proposed making this line CPW tunable by placing below the line a layer 108 of a ferroelectric material such as TSB. However, this solution is not very effective. Thus, Figure 12 of U.S. Patent 6,498,549 shows that at frequencies of 7 9 GHz, and for very high voltages (Supé ^¬ EXTERIORFEATURES 200 V), it is obtained that phase shifts of a few tens of degrees, while it would be desirable to obtain significant phase shifts (eg 180 ° to 60 GHz for a line length of about one millimeter) for voltages compatible with the field of integrated circuits, that is to say say voltages of 1 to 5 volts. This limitation seems to be due in particular to the fact that it is not practically possible to deposit layers of BST with low dielectric losses to a thickness greater than 1 μm (400 nm in US Pat. No. 6,498,549).

A type of transmission line particularly ^per¬¬ forming is described in US Patent No. 6,950,590 of which Figure 4a is reproduced in Figure 2 attached. On a silicon substrate 128 coated with metal levels separated by an insulator 127, a lower shield plane 136 is formed. divided into parallel strips of small width, for example of the order of 0.1 to 3 μm. In a higher level of metallization is formed a central conductive strip 122 constituting the transmission line itself, surrounded by coplanar lateral mass ribbons 124, 126.

Advantages and characteristics of such a line are described in detail in U.S. Patent No. 6,950,590. As stated in this patent, the structure constitutes a slow wave guide commonly referred to as S-CPW for use as a guide. slow wave coplanar wave (Slow wave CoPlanar Waveguide).

In a structure such as that of FIG. 2, the dimensions of the various elements are optimized to obtain, at a given frequency, given phase characteristics as well as a given characteristic impedance. It is not possible to modify these characteristics once the line has been completed. For example, it is not possible to make a phase shifter having a given identical phase shift for several different frequencies, or an impedance adapter for adapting various impedances.

Thus, it is found in the prior art or CPW lines tunable insensitively or requiring very high control voltages, or non-tunable S-CPW lines.

summary

An object of embodiments of the present invention is to provide a transmission line tunable substantially with control voltages of the order of a few volts.

For this, embodiments of the present invention combine the characteristics of ferroelectric layer-controlled CPW lines and S-CPW lines.

Thus, the present invention provides a coplanar waveguide type transmission line particularly suitable for integration on microelectronic integrated circuits. in which various parameters of the waveguide are adjustable to optimize the phase shift at a chosen frequency and for a selected characteristic impedance, and to modify the parameters of the line in order to adapt to a different operating frequency or to a different frequency. different characteristic impedance.

An embodiment of the present invention provides a high frequency transmission line comprising a conductive ribbon associated with at least one conductive shield plane, wherein at least a portion of the space between the shield plane and the ribbon conductor comprises a ferroelectric material.

According to one embodiment of the present invention, the line is of the slow wave coplanar waveguide type comprising two lateral ribbons extending on either side of the central ribbon.

According to one embodiment of the present invention, the ferroelectric material extends under all or part of the central ribbon and lateral ribbons.

According to one embodiment of the present invention, the line is associated with means for selective polarization (Vbias) of the central ribbon and / or lateral ribbons.

According to one embodiment of the present invention, the lateral strips have their central portions formed with recesses above and are associated with means of MOVE ^¬ lateral electrostatic cements.

Brief description of the drawings

These and other objects, features, and advantages will be set forth in detail in the following description of particular embodiments in a non-limitative manner with reference to the accompanying figures in which:

FIG. 1, previously described, represents a transmission line of the SCW type;

Figure 2, previously described, is a reproduction of Figure 4a of US Patent 6,950,590; Figures 3A, 3B and 3C are respectively a sectional view, a perspective view and a top view of a transmission line according to an embodiment of the present invention;

Fig. 4 is a sectional view of a transmission line according to another embodiment of the present invention; and

Figs. 5A and 5B are a top view and a sectional view of a transmission line according to an embodiment of the present invention.

It should be noted that, in general, as is customary in the representation of the microelectronic components, the elements of the various figures are not drawn to scale. detailed description

As illustrated in FIGS. 3A, 3B and 3C, on a substrate 1, for example a semiconductor substrate, for example made of silicon, metallization levels are formed separated by an insulating material 2. In a low level of metallization, is formed a shield plane divided into microstrip strips 4 similar to the structure 136 of FIG. 2. Above this level of metallization is formed a central transmission tape 6 similar to the ribbon 122 and, on either side of this central ribbon , are formed lateral ribbons of mass 8 and 9 similar to the mass ribbons 124 and 126 of Figure 2.

As illustrated in FIG. 3A, between the ribbons 6, 8,

9 and the shielding plane 4 is disposed a ferroelectric material 10 (the layer of ferroelectric material 10 is not shown in Figures 3B and 3C for simplicity of represen tation ^¬). A ferroelectric material generally has a high dielectric constant and said dielectric constant can go to values well above if an electric field is applied ^¬ continuous stick. For example, for BST (BaSrTi) with a thickness of 300 nm, the dielectric constant changes from 100 to 300 for polarization voltages ranging from 0 to 5 volts. It will be noted that the capacitive component between the central ribbon 6 and the transverse ribbons 8 and 9 is negligible if we look at the lateral faces facing the central ribbon 6 and lateral ribbons 8 and 9. Thus, the capacitive component between the central ribbon 6 and the side ribbons 8 and 9 essentially correspond to the capacitance between the central ribbon and the shielding plane 4 in series with the two capacitors (in parallel) between the shielding plane 4 and the lateral ribbons 8 and 9. these three capacities have the same value, Cw, the overall capacity will be equal to 2Cw / 3. Depending on the polarization voltage, this overall capacity may vary in a factor of 1 to 3.

To apply a transverse bias voltage to the ferroelectric material layer 10, it is possible, as illustrated in FIG. 3C, to apply a voltage Vbias to each of the lateral ribbons and to the central ribbon. In order to prevent this polarization connection from interfering with the RF signal applied to the central ribbon, an impedance between the bias voltage source and the central ribbon will be provided, preferably an inductance L but possibly also a high value resistor.

Moreover, in an S-CPW structure, it is possible to choose the dimensional parameters of the line so that it functions satisfactorily with a thickness of BST between the shield plane and the conductive ribbons having a value between 0.3 and 1 μm, which corresponds to the range of thicknesses in which one can in practice deposit a layer of BST.

According to simulations carried out by the inventors on structures of the type of that of FIGS. 3A-3C, in which the thickness of the BST layer was of the order of

400 nm, a phase shift of the order of 5 ° is obtained for a bias voltage of 5 V for a line of 60 ym long at a frequency of 60 GHz. Similar simulations on a CPW line of the type of that of FIG. 1 have shown that only a phase shift of the order of 0.15 ° is obtained. Thus, the setting sensitivity of the S-CPW line is at least 30 times higher than that of the CPW line. Can be at ^¬ tribuer this result, at least in part to the fact that, in the case of a CPW line, the field lines, indicated by arrows 109 in Figure 1, do not pass that game in the TSB, while in the case of an S-CPW line they pass only in the TSB and close again in the conductive shield plane.

In FIG. 3A, the ferroelectric layer 10 is represented as occupying the entire gap between the shielding plane and the conductive strips 6, 8 and 9. This embodiment is capable of numerous variants. For example, the ferroelectric layer does not necessarily go down to the lower shield plane and is optionally coated with an interface layer before the deposition of the metal strips 6, 8 and 9.

An alternative embodiment of the ferroélec ^¬ stick layer 10 is shown in Figure 4. In this embodiment, the ferroelectric layer 10, instead of being present in the set of conductive traces is present in portions only in a part of these conductive ribbons. More party ^¬ cularly as shown in the figure, a portion of ferroelectric material 10A is disposed under the strip 6 and the ferroelectric material portions 10B and 10C are formed in the strips 8 and 9.

According to other embodiments, it can be provided that the ferroelectric material is present only under the central ribbon or that under the lateral ribbons. This is likely to simplify the polarization control circuit because it will then be sufficient to apply a bias on the central ribbon or on the lateral ribbons.

The change in polarization between the ribbons or 6, 8, 9 and the shielding plane 4 has the main effect of modi ^¬ proud the equivalent capacitance C _e q of the transmission line. This causes a modification of the characteristic impedance Z = (L _ecr / C _ecr ) ^1/2 of the line, L _eCf being the equivalent inductance of the line. Correspondingly, the phase velocity of propagation signal, νφ = 1 / (L _e q. C _e q) ^1/2, will be modified and the result is a change in electrical length of the line, θ = 1 (ω / νφ), where 1 is the physical length of the transmission line and C is the angular frequency of the signal.

C _e q could be modified continuously by applying more or less significant differences in potential between the ribbon (s) 6, 8, 9 and the shielding plane 4. However, it may be preferable to act all or nothing by applying potentials such as the equivalent capacity takes one or more of several predetermined values.

As noted previously, the possibility of modifying ^¬ proud the equivalent capacitance C _e q results in a possibility of changing the characteristic impedance of the line and the phase velocity of a signal in the line. However, this does not allow to independently adjust these two parameters. To allow independent adjustment of the characteristic impedance and the phase velocity, an embodiment of the present invention provides that the lateral distance between the lateral ground strips and the central ribbon is adjustable, which has the essential effect of modify the equivalent inductance L _e q of the line.

One embodiment of a structure for obtaining this independent adjustment is illustrated in Figures 5A and 5B which are respectively a top view and a sectional view along the plane B-B of Figure 5A. Figures 5A and 5B will be described collectively hereinafter.

The structure of FIGS. 5A and 5B is a variant of that described with reference to FIGS. 3A to 3C. We find the shielding plane 4 and the central ribbon line 6 framed by the ground strips 8 and 9. An electrical material 10A is disposed only under the central ribbon 6 between this ribbon and the shielding plane 4. A recess 18, 19 is formed under each of the lateral ribbons 8 and 9 so that these tapes can be moved laterally under the effect of a voltage difference between them and external lateral electrodes 21, 22. The lateral ribbons 8 and 9 are connected to studs 23-1, 23-2 and 24-1, 24-2 respectively formed on the insulator 2 by lamellae 25-1. , 25-2 and 26-1, 26-2. The lamellae 25-1, 25-2 and 26-1, 26-2 form a spring and allow movement of the ground strips 8 and 9 when they are attracted by the external electrodes 21, 22.

Abutment systems may be provided for limiting the movement of the lateral strips and avoid a short circuit between these strips and the electrodes 21, 22 or the central conduc tor ^¬ 6. These stops may for example be formed of insulating layers deposited on the side faces of the various elements.

In operation of the structure of Figs. 5A and 5B, once a bias is applied, the side ribbons 8 and 9 move relative to the central ribbon. This has the main effect of modifying the equivalent inductance L _e q of the transmission line. We can thus independently adjust L _e q and C _e q and therefore Z and νφ.

The present invention is capable of many variations and modifications which will be apparent to those skilled in the art. Various means may be implemented to polarize the central ribbon and the lateral ribbons with respect to the shield plane, and to move the lateral ribbons relative to the central ribbon.

The invention has been described in the context of a particular example of its application to an S-CPW type structure. However, it will be understood that it applies generally to other types of tape transmission lines whose parameters depend on the distance or distances between this ribbon and various ground planes.

With regard to the lateral displacement, various variants may also be used. In particular, the attraction electrodes 21 and 22 and the mass ribbons 8, 9 may be coupled by interdigitated structures. Moreover, the leaf springs 25-1, 25-2, 26-1, 26-2 may have various configurations, for example meandering shapes.

One of the advantages of the structure described here is that it is well compatible with the usual techniques for forming metallization levels generally used for the realization of interconnections above a microelectronic integrated circuit.

Only by way of example, and to fix the ideas, one can choose for a transmission line intended to operate at frequencies close to 60 GHz the following dimensions:

width of the ribbons and distance between the ribbons 6, 8, 9: of the order of 7 to 15 μm,

vertical distance between the metallization levels: of the order of 0.5 to 1 μm,

distance between the ground strips 8 and 9 and the electrodes 21 and 22: of the order of 0.5 to 2 μm.

With these values it will be possible to control the electrostatic displacement of the various elements with voltages having values of the order of ten volts and to cause varia ^¬ tions of the capacitance and inductance values of a factor of the order of 1.5 to 3.

Various techniques may be implemented to achieve a transmission line as described herein. For example, with regard to FIG. 5B, the recesses 18, 19 in each of the lateral ribbons 8 and 9 can be made by forming on the surface of the structure a sacrificial layer before depositing the metallizations 6, 8 and 9 and removing the sacrificial layer after the metallization Réali ^¬ Sees. In addition, as regards the structure of Figure 5B, one could provide that the ferroelectric layer has a larger extent and it is covered with a sacrificed layer ^¬ cial before formation of conductors 6, 8, 9.

Claims

A high frequency transmission line comprising a central conductor tape (6) associated with at least one conductive shield plane (4), wherein at least a portion of the space between the conductive shield plane and the conductive tape comprises a ferroelectric material (10).

2. Transmission line according to claim 1 of the co-wave waveguide type slow wave comprising two lateral ribbons (8, 9) extending on either side of the central ribbon.

The transmission line of claim 2, wherein the ferroelectric material is BST.

The transmission line of claim 3, wherein the ferroelectric material has a thickness of 0.4 to 1 p.

A transmission line according to any one of claims 2 to 5, wherein the ferroelectric material extends under all or part of the central ribbon and side ribbons.

6. Transmission line according to any one of claims 2 to 5, associated with means of selective polarization (Vbias) of the central ribbon and / or lateral ribbons.

7. Transmission line according to any one of claims 2 to 6, wherein the side ribbons have their central portions formed above recesses and are associated with lateral electrostatic displacement means.