EP2110881A1 - MEMS-based radio frequency circulator - Google Patents

Abstract

The circulator has condenser type electrostatic-actuation micro switches (MEMS1, MEMS2) formed on a substrate, and an antenna port (p2) and an outlet port (p3) that are arranged on a discontinuous radio frequency signal line (Ls). An inlet port (p1) is located on a continuous radiofrequency signal line (Lp). The switch (MEMS1) connects the signal lines by self-actuating membrane under an effect of input signal power. The switches are separated by a distance of an order of one quarter of length of wave corresponding to a frequency of the signal. The lines have an insulating material e.g. lead zirconate titanate, zirconium oxide silicon nitride, upper layer.

Description

The field of the invention is that of RF radio frequency circulators and their applications in radiofrequency or microwave telecommunication systems such as radar systems, or wireless telephony systems.

An RF circulator is a n port device, allowing an RF signal to flow in a single direction. Consider a circulator with three ports p1, p2, p3. A signal injected into a port p1 is transmitted to port p2 and isolated from port p3, while a signal entering via port p2 is transmitted to port p3 and isolated from port p1. There is thus a decoupling of the transmitted and received signals. A corresponding symbolic illustration of such a circulator whose port p2 is connected to an antenna is given on the Figures 1a and 1b . If the circulator receives on impedance port p1 a radiofrequency signal, there is a path with low insertion losses in the direction of clockwise and we observe high losses in the opposite direction. The power is directed almost without losses to the port p2 and radiated by the antenna. The same applies from port p2 to port p3, and port p3 to port p1. The circulator thus has the essential qualities of transmitting without losses in a given direction and of attenuating very strongly the reflected waves.

Circulators are particularly used in telecommunication systems or radars, according to the principle illustrated on the figure 2 . The figure 2 schematically an exemplary system for transmitting and receiving electromagnetic signals for applications including radar type, commonly referred to as the T / R module consisting essentially of three stages as described below.

The first stage, the heart of the CA system is used to manage and process the signals received and transmitted. The second stage is composed of the power amplifier elements. These elements are distributed in two functions, the high power amplifier commonly called HPA, 11 which is used to give power to the output signal of the first stage to be emitted by the antenna and the low noise power amplifier commonly called LNA , 16 which serves to amplify the power of the signal received by the antenna while limiting as much as possible the parasites. These two components are very sensitive to the power received by the antenna. The LNA to the extent that the power that enters the latter must not exceed a certain threshold otherwise the component is damaged and destroyed. The HPA which in so far as it is always connected with a feedback loop on the output must in no case receive power on its output if we do not want to degrade or destroy it. It is for this reason that there are in the third stage elements called limiters 12 and 15 which are electronic components whose function is to cut the microwave signal if the power thereof exceeds a certain threshold. There are also in this third stage elements called circulators 13 and 14. These are so-called active components that direct an incoming flow to an output specific to the input used. For example from port 1 to port 2, from port 2 to port 3 ... hence this name of circulator. This physically implies that regardless of the output circuit impedance, there is virtually no feedback on the circulator input. If there is reflection, the energy is considered as a flow entering through the first output and is therefore directed to the next output, isolating the input almost perfectly.

At present, this type of transmission / reception chains comprises circulators based on ferromagnetic materials and diode-based limiters.

• le coût élevé de ces composants qui ne sont pas facilement reproductibles car ils nécessitent une intervention humaine pour être réglés correctement ;
• les pertes engendrées par ces composants de l'ordre de 4 à 8 dB dans leur bande de fréquence elle même très étroite (0.2 à 2 GHz) pour les circulateurs et de l'ordre de 1 dB pour les limiteurs à base de diodes de puissance.

The existing defects are mainly:

• the high cost of these components that are not easily reproducible because they require human intervention to be properly adjusted;
• the losses generated by these components of the order of 4 to 8 dB in their frequency band itself very narrow (0.2 to 2 GHz) for circulators and of the order of 1 dB for limiters based on power diodes .

These components currently occupy 80% of the space in a telecommunication system and are an additional brake on miniaturization. Circulators commonly used being ferrite-based components are by nature active components and consume energy, they are also very bulky (about 70% mass versus volume of the T / R module) and because of their difficulty of reproducibility are very expensive.

As for the diodes, they are components with high costs and the losses generated by these components are of the order of 1 dB. In addition, diodes occupy an important part of the place in telecommunication systems and thus represent an additional brake on miniaturization.

It has already been proposed in the prior art to use microswitch microwaves also called RF MEMS switch. These are capacitive type micro-devices, functioning as switches, micro-devices that are called micro-switches in the following.

The capacitive type micro-switches are particularly appreciated in microwave applications, especially for their low response times combined with low control voltages ranging from a few volts to a few tens of volts. They are advantageously very small, of millimeter size (2 to 10 mm ² ), which is on average ten times smaller than a ferromagnetic circulator and much lighter. They consume very little. They are inexpensive to produce because they use the usual fabrication techniques in microelectronics, from a substrate generally silicon and are very easily reproducible. Their insertion losses are very low, generally of the order of 0.1 to 0.2 dB over a very wide frequency band, 18 to 19 GigaHertz. According to this solution it is more specifically proposed series-type microswitches: an input signal line and an output signal line in the extension of one another, separated by a switching zone, and isolated electrically, and above the switching zone, a flexible membrane, resting on pillars. The switching zone is covered with a dielectric. The membrane is either in the rest position, high, the capacity formed by the switching zone, the dielectric and the membrane having a low Coff value, so that the two signal lines are isolated, or in the low position so that the two line portions are capacitively coupled, the capacitance formed by the switching zone, the dielectric and the membrane having a high Con value, allowing the transmission of a radiofrequency or microwave signal. The membrane control is a voltage control suitably applied in the switching zone, the membrane being brought to a reference potential (electrical ground) by the pillars. The switching performance (transmission, isolation) depend in particular on the Con on Coff report which must be as high as possible.

The circulator includes at least first and second contact pads for applying on or off control voltages to at least one of the control electrode portions of the first micro-switch and the second micro-switch. The activation voltages are of the order of volts to a few tens of volts. The microswitches can be simultaneously controlled in the off state, or one in the on state and the other in the off state.

In order to further reduce the costs of this type of circulator, the present invention proposes a new type of circulator comprising self-actuated components.

• un premier et un second micro-commutateurs à actionnement électrostatique de type condensateur formés sur un même substrat et comportant chacun deux armatures dont la première est une membrane flexible et la seconde comporte au moins une zone d'une ligne signal, les deux armatures étant séparées par une épaisseur de vide ou de gaz ;
• les ports d'antenne et de sortie étant disposés sur une ligne signal principale, le port d'entrée étant situé sur une ligne signal secondaire ;
• le premier micro-commutateur étant disposé de manière à relier la ligne signal principale et la ligne signal secondaire par auto-actionnement de la membrane sous l'effet d'une puissance de signal d'entrée ;
• le second micro-commutateur ayant une membrane permettant de relier la ligne principale à des plans de masse par auto-actionnement de la membrane sous l'effet d'une puissance de signal d'entrée ;
• les micro-commutateurs étant séparés d'une distance de l'ordre du quart de la longueur d'onde correspondant à la fréquence du signal.

More specifically, the subject of the present invention is a circulator with at least three ports, an input port for receiving a radiofrequency signal to be transmitted to a port intended to be connected to a transmitting / receiving antenna called an antenna port, a output port adapted to be connected to a receiving device or a load, characterized in that it comprises:

• a first and a second capacitor-type electrostatic-actuated microswitches formed on the same substrate and each comprising two armatures, the first of which is a flexible membrane and the second comprises at least one zone of a signal line, the two armatures being separated by a thickness of vacuum or gas;
• the antenna and output ports being disposed on a main signal line, the input port being located on a secondary signal line;
• the first micro-switch being arranged to connect the main signal line and the secondary signal line by self-actuation of the membrane under the effect of an input signal power;
• the second microswitch having a membrane for connecting the main line to ground planes by self-actuation of the membrane under the effect of an input signal power;
• the microswitches being separated by a distance of the order of a quarter of the wavelength corresponding to the frequency of the signal.

According to a variant of the invention, the main signal line is a discontinuous line.

According to a variant of the invention, the secondary signal line is a continuous line.

According to a variant of the invention, the secondary line comprises a ground element separated by a distance of the order of a quarter of the wavelength corresponding to the frequency of the signal.

According to a variant of the invention, the main and secondary lines are made of gold and / or copper and / or titanium / tungsten alloy.

According to a variant of the invention, the main and secondary lines further comprise an upper layer of insulating material at the level of refined parts located under the membranes.

According to a variant of the invention, the insulating material is PZT or ZrO 2 or Si 3 N 4 or any other dielectric whose relative permittivity will be adapted to the working frequency of the element

The subject of the invention is also a module for transmitting / receiving microwave signals comprising an antenna, a first stage for processing the microwave signals transmitted and received, a second stage for amplifying said signals and an intermediate stage comprising at least one circulator according to the invention.

According to a variant of the invention, the intermediate stage further comprises at least one power limiter.

According to a variant of the invention, the transmission / reception module comprises a power limiter on the output port towards the receiver or the load and / or a second power limiter on the antenna port.

According to a variant of the invention, the power limiter or limiters comprise a main line with an input for receiving an incident power and an output, their main line comprising a condenser-type electrostatic actuation microswitch comprising two armatures, the first of which is a flexible membrane and the second comprises at least one zone of the main line, the two armatures being separated by a thickness of vacuum or gas, said micro-switch further comprising two ground planes connected by said membrane.

According to one variant of the invention, the microswitch of the limiter or limiters is self-actuating by an incident power greater than a threshold value so as to bring the said zone of the main line into contact with the two ground planes and thus block the microwave signal.

• les figures 1a et 1b illustrent les modes de fonctionnement classique d'un circulateur ;
• la figure 2 illustre un exemple de système d'émission et de réception de signaux électromagnétiques pour applications notamment de type radars selon l'art connu ;
• la figure 3 illustre un exemple de circulateur selon l'invention ;
• les figures 4a et 4b illustrent respectivement l'évolution du gap séparant la membrane de la zone de ligne coplanaire en fonction de la tension d'actionnement de ladite membrane et le cycle pouvant être effectué par la membrane sous l'action d'une tension de commande;
• les figures 5a, 5b, 5c et 5d illustrent différentes vues d'un micro-commutateur utilisé dans un circulateur selon l'invention ;
• la figure 6 illustre l'étage intermédiaire d'une chaîne de transmission/réception intégrant un circulateur selon l'invention.

The invention will be better understood and other advantages will become apparent on reading the description which follows given by way of non-limiting example and by virtue of the appended figures among which:

• the Figures 1a and 1b illustrate the conventional modes of operation of a circulator;
• the figure 2 illustrates an example of a system for transmitting and receiving electromagnetic signals for applications such as radar type according to the prior art;
• the figure 3 illustrates an example of a circulator according to the invention;
• the Figures 4a and 4b respectively illustrate the evolution of the gap separating the membrane from the coplanar line zone as a function of the operating voltage of said membrane and the cycle that can be performed by the membrane under the action of a control voltage;
• the Figures 5a, 5b, 5c and 5d illustrate different views of a microswitch used in a circulator according to the invention;
• the figure 6 illustrates the intermediate stage of a transmission / reception chain incorporating a circulator according to the invention.

In general, the circulator which is the subject of the present invention comprises two microswitches based on electrostatically activated RF MEMS components.

The figure 3 represents a top view of an example of a circulator according to the invention. A RF main line, Lp, has the output port p3 to a receiver or load and the port antenna p2 towards a transmitting / receiving antenna. This main line, Lp, forms a cross with a discontinuous secondary line Rf Ls, said secondary line having the input port p1 capable of receiving a radio frequency signal. A first micro-switch MEMS1 is located at the intersection of the main and secondary lines, allowing when the membrane is lowered to bring into contact the two discontinuous elements of the secondary line. Furthermore, a second micro-switch MEMS2 is also positioned at the main line towards the port p3 and makes it possible to short-circuit said main line in the lowered position of the membrane of said second micro-switch. Indeed ground plans PM1 and PM2 are located on either side of the secondary line.

The two microswitches are separated by a distance equal to a quarter of the wavelength λ corresponding to the frequency of the operating signal of said circulator and the secondary line further comprises an EM mass element (for electromagnetic masses, similar to a mass for the direct current, these masses EM correspond to a reference potential for the central line) situated at a distance equal to also a quarter of the wavelength λ.

Thus, when the two microswitches are in state position 1, corresponding to undepressed membranes, a signal received from the antenna can propagate along the main line towards port p3 and not flow towards ports p1. and the EM mass element.

When both microswitches are in state position 2, corresponding to lowered membranes, a signal from the port p1, can flow in each of the four branches of the cross formed.

Indeed a portion of the signal transits in the direction of the port p3, nevertheless the MEMS2 is in position to short circuit the signal, the signal travels a go on a distance of λ / 4 and a return in phase opposition of said forward signal. Similarly, the signal portion towards the ground element EM also travels a distance to a distance of λ / 4 in phase opposition with a return signal on said same branch of the cross.

By summing the different signals in phase opposition, only a signal remains in the direction of the p2 port of the antenna.

• état 1 : les MEMS1 et MEMS2 sont en position haute, le circulateur fonctionne de p2 vers p3.
• état 2 : les MEMS1 et MEMS2 sont en position basse, le circulateur fonctionne de p1 vers p2.

We can thus summarize the two states of the circulator:

• state 1: MEMS1 and MEMS2 are in the up position, the circulator runs from p2 to p3.
• state 2: the MEMS1 and MEMS2 are in the low position, the circulator runs from p1 to p2.

According to the invention, the great advantage of this type of circulator is that it operates thanks to the presence of two micro-switches self-actuating and therefore without operating voltage to consume. Indeed, any radio frequency signal has an associated power which is equivalent to a voltage and an effective intensity. If the effective signal voltage exceeds a certain threshold, there is a phenomenon of self-activation of the membrane which bypasses the microwave signal to ground, protecting the downstream components.

The movement of the membrane of this RF MEMS switch during actuation follows the path illustrated in figure 4a which provides the distance of the membrane overlying the coplanar line area as a function of the operating voltage of said membrane called height.

où wW est la surface en regard, g ₀ le gap initial et k le coefficient de raideur de la membrane.The voltage Vp is the activation voltage (Vp for Voltage pull) determined by the following equation: $$V_p=\sqrt{\frac{8{\mathit{<figure-callout\; id="0"\; label="kg"\; filenames="imgf0003.png"\; state="\{\{state\}\}">kg</figure-callout>}}_0^3}{27{\mathrm{<figure-callout\; id="0"\; label="\epsilon "\; filenames="imgf0003.png"\; state="\{\{state\}\}">\epsilon </figure-callout>}}_0\mathit{wW}}}$$

where wW is the facing surface, g ₀ the initial gap and k the stiffness coefficient of the membrane.

où t_d est l'épaisseur de diélectrique séparant la ligne de la membrane et ε _r la permittivité du diélectrique.The release voltage V _r defined by the following formula is obtained: $$V_r=\sqrt{\frac{2k\left({\mathrm{boy\; Wut}}_0-t_d\right){\mathit{t}}_d^2}{\mathrm{\epsilon \text{'}}{\mathrm{<figure-callout\; id="0"\; label="\epsilon "\; filenames="imgf0003.png"\; state="\{\{state\}\}">\epsilon </figure-callout>}}_0\mathit{wW}{\epsilon }_{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="imgf0002.png,imgf0005.png"\; state="\{\{state\}\}">r</figure-callout>}}^2}}$$

where t _d is the dielectric thickness separating the line of the membrane and ε _r the permittivity of the dielectric.

We get the cycle shown in figure 4b for the movement of the membrane according to the applied voltage.

• Premier cas : la tension Veq engendrée par le signal est supérieure à la tension Vp, on se trouve dans le cas de l'auto-actionnement pour un switch de cette sorte. Cela signifie que le simple fait de faire passer le signal à travers le MEMS RF provoque son actionnement.
• Second cas : la tension Veq est comprise entre les tensions Vr et Vp, on se trouve dans le cas de l'auto maintient. Cela signifie que le simple fait de faire passer le signal à travers le MEMS RF empêche la membrane de remonter après actionnement, auto-maintient, mais sans pour autant provoquer d'auto-actionnement.
• Troisième cas : la tension Veq est inférieure à la tension Vr, le MEMS fonctionne de manière simple, le signal ne perturbe pas le fonctionnement du MEMS RF.

At the power of the signal passing through the RF MEMS corresponds an average voltage Veq. Three cases of configurations are possible:

• First case: the voltage Veq generated by the signal is greater than the voltage Vp, it is in the case of self-actuation for a switch of this kind. This means that simply passing the signal through the RF MEMS causes it to be actuated.
• Second case: the voltage Veq is between the voltages Vr and Vp, it is in the case of self maintains. This means that simply passing the signal through the RF MEMS prevents the diaphragm from rising after actuation, self-maintaining, but without causing self-actuation.
• Third case: the voltage Veq is lower than the voltage Vr, the MEMS operates in a simple way, the signal does not disturb the operation of the RF MEMS.

• Les figures 5a, 5b, 5c et 5d illustrent respectivement une vue en perspective, une vue en coupe, une vue de dessus avant réalisation de la membrane et une vue de dessus après réalisation de la membrane. Sur un susbtrat de silicium 20, recouvert d'une couche 21 de diélectrique de type SiO₂, une ligne hyperfréquence 22 est réalisée typiquement en or, recouverte selon l'art connu d'une couche de diélectrique 23 de type PZT. Des piliers 24a et 24b permettent de relier des lignes de masse 24 et supportent la membrane constituée d'une couche de matériau conducteur 25. La figure 5c met en évidence la ligne hyperfréquence 22 recouverte d'une couche de matériau piézoélectrique 23, alors que la figure 5d met en évidence la membrane 25 surplombant ladite ligne hyperfréquence 22 recouverte de la couche diélectrique 23. Typiquement les épaisseurs des couches sont respectivement :
  • couche 21 : de l'ordre de 1 à 2 microns ;
  • couche 22 : de l'ordre de 0,5 à 0,7 microns dans la partie affinée, et d'environ 5 µm dans les parties non-affinées sous la membrane ;
  • couche 23 : de l'ordre de 0,2 à 0.4 microns ;
  • membrane 25 : de l'ordre de 500 à 700 nanomètres.

We will describe hereinafter in more detail, an example of a micro-switch used in a circulator according to the invention, and in particular the MEMS2 which in a lowered position allows to connect the central line to ground planes:

• The Figures 5a, 5b, 5c and 5d illustrate respectively a perspective view, a sectional view, a top view before the membrane is made and a view from above after the membrane has been made. On a silicon substrate 20, covered with a layer 21 of SiO ₂ type dielectric, a microwave line 22 is typically made of gold, covered in the known art with a dielectric layer 23 of the PZT type. Piers 24a and 24b make it possible to connect ground lines 24 and support the membrane consisting of a layer of conductive material 25. figure 5c highlights the microwave line 22 covered with a layer of piezoelectric material 23, while the figure 5d highlights the membrane 25 overlying said microwave line 22 covered with the dielectric layer 23. Typically the thicknesses of the layers are respectively:
  • layer 21: of the order of 1 to 2 microns;
  • layer 22: on the order of 0.5 to 0.7 microns in the refined part, and about 5 μm in the non-refined parts under the membrane;
  • layer 23: of the order of 0.2 to 0.4 microns;
  • membrane 25: of the order of 500 to 700 nanometers.

The figure 6 illustrates the intermediate stage of a transmission / reception chain comprising power limiters and a circulator according to the invention, such a chain can typically be that illustrated in FIG. figure 2 .

Thus, according to the invention, the third stage of this transmission / reception chain comprises, at the output of the amplifier of the transmission signal 11, a circulator composed of the two micro-switches MEMS1 and MEMS2 according to the invention, this third stage also comprises for the microwave reception signal a first limiter 12 between the circulator and the antenna port p2 and a second limiter 15 whose main line is connected at the input to the circulator comprising the two micro-switches MEMS1 and MEMS2 and output to an amplifier 16, LNA.

The interest of circulators and limiters based on RF MEMS switches is that they do not consume or very little energy in self-operating mode, they are very small so allow a gain of space and mass very important , circulator and limiters based on RF MEMS switches are also very easily reproducible and therefore very expensive.

Claims (13)

Circulator with at least three ports (p ₁ , p ₂ , p ₃ ), an input port (p ₁ ) for receiving a radiofrequency signal to be transmitted to a port (p ₂ ) intended to be connected to a transmitting antenna / reception (A) called antenna port, an output port (p ₃ ) adapted to be connected to a receiver device or a load, characterized in that it comprises: a first and a second microswitches (MEMS1, MEMS2) with electrostatic actuation of capacitor type formed on the same substrate and each comprising two armatures, the first of which is a flexible membrane and the second comprises at least one zone of a signal line the two armatures being separated by a thickness of vacuum or gas; the antenna and output ports being arranged on a main signal line, the input port being located on a secondary signal line; the first micro-switch being arranged to connect the main signal line and the secondary signal line by self-actuation of the membrane under the effect of an input signal power; the second microswitch having a membrane for connecting the main line to ground planes by self-actuation of the membrane under the effect of an input signal power; the microswitches being separated by a distance of the order of a quarter of the wavelength corresponding to the frequency of the signal. Circulator according to claim 1, characterized in that the main signal line is a broken line. Circulator according to one of claims 1 or 2, characterized in that the secondary signal line is a continuous line. Circulator according to one of claims 1 to 3, characterized in that the secondary line comprises a ground element separated by a distance of the order of a quarter of the wavelength corresponding to the frequency of the signal. Circulator according to one of claims 1 to 4, characterized in that the main and secondary lines are made of gold and / or copper and / or titanium / tungsten alloy. Circulator according to one of claims 1 to 5, characterized in that the main and secondary lines further comprise an upper layer of insulating material at the level of refined parts located under the membranes. Circulator according to one of claims 1 to 6, characterized in that the insulating material is PZT or ZrO2 or Si3N4. Radiofrequency signal transmission / reception module comprising an antenna, a radiofrequency signal processing stage transmitted and received, a second amplification stage of said signals and an intermediate stage comprising at least one circulator according to one of Claims 1 to 7. . Transmitting / receiving module according to claim 8, characterized in that the intermediate stage further comprises at least one power limiter. Transmission / reception module according to claim 9, characterized in that it comprises a power limiter on the output port towards the receiver or the load and / or a second power limiter on the antenna port. Transmitting / receiving module according to one of Claims 9 or 10, characterized in that the power limiter (s) comprise a main line with an input for receiving an incident power and an output, their main line comprising a microswitch to capacitor-type electrostatic actuation comprising two armatures, the first of which is a flexible membrane and the second comprises at least one zone of the main line, the two armatures being separated by a thickness of vacuum or gas, said micro-switch further comprising two ground planes connected by said membrane. Transmission / reception module according to claim 11, characterized in that said microswitch or limiter (s) is actuable by a voltage. Transmission / reception module according to Claim 11, characterized in that the microswitch of the limiter (s) is self-actuating by an incident power greater than a threshold value so as to bring the said zone of the main line into contact with the two ground plans and block the radiofrequency signal.

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