Rf mems switch with a flexible and free switch membrane

RF MEMS SWITCH WITH A FLEXIBLE AND FREE SWITCH MEMBRANE

מפסק תדרי רדיו למערכות מיקרו-אלקטרומכניות עם מפסק ממברנה גמיש חופשי

Pearl Cohen Zedek Latzer

P-70250-IL

PCT7EP2006/002076

RF MEMS SWITCH WITH A FLEXIBLE AND FREE SWITCH MEMBRANE

Technical field

The present invention relates to the technical field of Radio Frequency (RF) switches implemented by Micro Electromechanical Systems (MEMS).

Prior Art

Micro Electromechanical Systems (MEMS) switches are now widely used in Radio Frequency (RF) communication systems such as for example phased array antennas, phase shifters, switchable tuning components, etc.

RF MEMS switches are essentially miniature devices that use a mechanical movement which is electrically actuated in order to achieve a short circuit or an open circuit in a RF transmission line. A RF MEMS switch thus essentially comprises two distinct means:

- micromechanical means that will be generally referred as “switching means” and that can be moved into two different positions : an offposition corresponding to the off-state of the switch, i.e. a state wherein the transmission line is opened and can not be used for transmitting a RF signal, and an on-position corresponding to the onstate of the switch, i.e. a state wherein the transmission line is “closed and can be used for transmitting a RF signal

- electric means that will be generally referred as “actuation means”; said electric actuation means are generally used for generating a force that is applied onto the mechanical switching means in order to move the latter into their on-position and/or into their off-position.

MEMS RF switches can be classified into several categories based on the electric actuations means used i.e. electrostatic, electromagnetic, piezoelectric, or electrothermal actuation means. Electrostatic actuation is the prevalent technique in use to date, since it enables to reach shorter switching times (typically less than 200ps) and a virtually zero power consumption. Furthermore, in RF MEMS switch designs, different actuation techniques can be combined (for example an electrostatic voltage hold can be coupled with a thermal actuation).

MEMS RF switches can also be classified into two categories based on the contact used for switching the transmission line, i.e. “metal to metal 10 contact switch״ or a “capacitive contact switch”. So-called metal to metal contact switches (also referred as “ohmic contact switches״) are typically used for switching signal from DC to 60GHz. Capacitive switches are more particularly used for switching RF signals between 6GHz-120GHz.

To date, MEMS RF switches can also be classified into two main 1.5 categories based on the structure of their micromechanical switching means.

A first main category comprises RF MEMS switches whose micromechanical switching means comprise a flexible membrane that is anchored onto a substrate of the switch. A second category comprises RF MEMS switches whose micromechanical switching means compnse a 2 0 stress-free rigid beam that is freely mounted onto the substrate of the switch.

RF MEMS having a flexible anchored membrane ׳

In a first configuration, the flexible membrane is anchored onto the substrate at both extremities and is thus forming a bridge. MEMS switches using a flexible bridge as switching element are disclosed for example in the 25 following publications: US patent application No 2004/0091203, US patent No 6,621,387, European patent application EP 1 343 189, PCT application WO 2004/076341.

In a second configuration, the flexible membrane is anchored onto the substrate only at one extremity, and is thus forming a cantilever. MEMS 30 switches using a flexible cantilever membrane as switching element are disclosed for example in US patent No 5,638,946.

US 2004/050674 relates to a micromechanical switch formed on the top of a fabrication substrate, by deposition of a stressed material which flexes either toward or away from the substrate surface in order to relieve the stress in the material.

The use of RF MEMS switches having a flexible anchored membrane (bridge or cantilever) as switch element encounters the main following drawbacks. These switches are very sensible to temperature variations and to mechanical and/or thermal deformation of the substrate (first main drawback). During actuation, when the anchored switch membrane is deformed under the force generated by the actuation means, the said membrane is subjected to high mechanical stress, which in turn dramatically decreases the lifetime of the RF MEMS switch (second main drawback).

RF MEMS switches having a stress-free and rigid beam

RF MEMS switches having a stress-free and rigid beam are for example disclosed in European patent application EP 1489 639. In this publication, the switch element is a rigid beam that can freely move between both on-position and off-position in a plane that is parallel to the substrate. In another variant, the rigid beam can be a floating beam that can freely move between both on-position and off-position in a direction that is perpendicular to the substrate.

US 2005/001701 teaches a switch including an actuator with a supporting column on a substrate, and a cap plate provided on the supporting column. The supporting column pivotally supports the cap plate. At ends of the cap plate, a plurality of beams are provided, respectively. The plurality of beams are subjected to electrostatic force of absorbing electrodes. Tilting directions of the cap plate (beams) can be set freely.

This type of switch advantageously overcomes the aforesaid

3a drawbacks of RF MEMS switches having an anchored switching membrane. In return, these RF MEMS switches with a stress-free and rigid switch beam have a longer switching time (i.e. time that is necessary for moving the switch beam between on-position and off-position). Furthermore they are more sensible to mechanical shocks or vibrations.

Objectives of the invention

A main objective of the invention is to propose a novel structure for a RF MEMS switch.

A further objective of the invention is to propose a novel RF MEMS 10 switch that overcomes the aforesaid drawbacks of the RF MEMS switches using an anchored switch membrane.

A further objective of the invention is to propose a novel RF MEMS switch that has a short switching time, in comparison with the aforesaid RF

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MEMS switches having a stress-free and rigid switch beam.

A further objective of the invention is to propose a novel RF MEMS switch that is less sensible to mechanical shocks or vibrations in comparison with the aforesaid RF MEMS switches having a stress-free and rigid switch beam.

Summary of the invention

At least the above main objective is achieved by the RF MEMS switch of claim 1.

The RF MEMS switch of the invention comprises: , ־ micromechanical switching means that can be actuated between two positions: a first position (off state) and a second position (on state), and

- actuation means for actuating the position of the switching means.

According to one main novel feature of the invention, the 15 micromechanical switching means comprise a flexible membrane which is freely supported by support means, which is bendable under the action of the actuation means, and which can freely slide relatively to the support means (3) during its bending movement.

The wording “freely supported' used therein (in the description and 2 0 in the claims) means that the switch membrane can freely slide relatively to the supports means during the switching movement of the membrane between the off-position and on-position.

Short description of the drawings

Other characteristics and advantages of the invention will appear 25 more clearly on reading the following detailed description which is made by way of non-exhaustive and non-limiting example, and with reference to the accompanying drawings on which:

- figure 1 is section view (in plane l-l of figure 3) of a capacitive RF MEMS switch of the invention, the switch being in the off-state,

- figure 2 is a section view of the switch of figure 1, the switch being in the on-state,

- figure 3 is a top view of the capacitive RF MEMS switch of figure and,

- figure 4 is a section view of the switch during the manufacturing process, and just before the final releasing step.

Detailed description

Figures 1 to 3 show a capacitive RF MEMS switch that is made according to preferred embodiment of the invention. For sake of clarity, it must however be underlined that the scope of the invention is not limited to a capacitive RF MEMS switch but encompasses also ohmic contact RF MEMS switches. The capacitive RF MEMS switch of figures 1 to 3 has a 10 novel structure that is now going to be detailed, and can be manufactured by using conventional surface micromachining technologies.

Referring to figure 1, the RF MEMS switch comprises a wafer 1 (for example made of silicium) forming the substrate of the switch. A thin dielectric layer 2 is deposited onto the surface of said wafer 1. On the 15 dielectric layer 2, the switch comprises:

- two spaced-apart and parallel lateral supporting members 3, that extend in the transverse direction of figure 1 (see figure 3 - direction Y).

- one central supporting member 4 that extends in a direction 20 substantially parallel to the main direction of lateral supporting members 3 (i.e. in the transverse direction of figure 1 - see figure 3), the said central supporting member 4 being disposed between the two lateral supporting members 3, and preferably at the centre between lateral supporting members 3.

In contrast with the lateral supporting members 3, the top surface of the central supporting member 4 is covered by a thin dielectric layer 5.

The two lateral supporting members 3 and the dielectric layer 2 form a Coplanar Waveguide (CPW), the two lateral supporting members 3 corresponding to the ground lines. The central supporting member 4 forms 30 the signal line for the transmission of the RF electric signal within the coplanar wave guide (CPW).

PCI7EP2006/002076

The lateral and central supporting members 3, 4 are for example made of a metal such as gold. The dielectric material for layers 2 and 5 can be any material, and notably polymer, with a very low electrical conductivity. For example, the dielectric layers 2 and 5 are made of silicon nitride.

The RF MEMS switch further comprises a capacitive switch element which is constituted by a thin flexible membrane 6 made of a metal for example such as aluminium, gold, or any conductive alloy. The flexible switch membrane 6 is freely supported at least by the lateral supporting members 3.

Referring to figure 3, the flexible switch membrane 6 has a main central part 6a and two opposite extremities in the form of plates 6b. In the particular embodiment of the figures, the central part 6a is rectangular, and extends in the lateral direction (X) above the lateral supporting members 3. The plates 6b have a dimension (E) taken in the longitudinal direction (Y) of 15 the supporting members 3 that is. greater than the width (e) of the rectangular part 6a. The shape of the flexible membrane 6 is not important for the invention.

Each supporting members 3 further includes on its top part a bridge part 3a forming a passage 3b trough which the central part 6a of the 20 membrane is freely positioned. The plates 6b of the membranes in combination of the bridge parts 3a are used as fastening means for maintaining the membrane on the supporting member 3 but without hindering the membrane 6 from freely moving relatively to the supporting members 3 during normal use of the switch.

The RF MEMS switch further comprises electrostatic actuating means that are used for bending the membrane 6 and that are formed by two lateral buried electrodes 7. In the preferred embodiment of figures 1 to 3, the buried electrodes are advantageously disposed outside the coplanar wave guide (CPW) under the two plates 6b of the switch membrane 6. The 3 0 top surface of each electrode 7 is covered by a dielectric layer 8 in order to avoid any ohmic contact between the membrane plates 6b and the electrodes 7. The dielectric layers 8 can be replaced by any other equivalent means that enable to avoid an ohmic contact between the membrane plates 6b and the electrodes 7.

Off-state

Figure 1 is showing the off-state configuration of the RF MEMS switch. In this off-state configuration, no electric actuation signal is applied on the electrodes 7.

i_n the off-state, the switch membrane 6 is at rest and is in contact with the dielectric layer 5. The signal line 4 is opened and can not transmit 10 any RF signal within the coplanar guide (CPW).

Preferably, a DC signal is applied on the central supporting member 4 in order to exert a small electrostatic force (F1) on the membrane 6 and maintain a perfect contact between the membrane 6 and the dielectric layer 5. The said off-state DC voltage can be advantageously very low (low 15 consumption).

This biasing DC signal is also useful in case there would be a very small initial gap between the electric layer 5 and the membrane 6 when the membrane is at rest. In such a case, the voltage of the biasing DC signal has to be sufficient in order to exert a contact force F1 (figure 1) on the central 20 part of the membrane 6 that is higher than the opposite rest force of the . membrane.

In the off-state position, the membrane 6 is advantageously supported in a stable position by the three supporting members 3,4, and is thus less sensible to mechanical vibrations or shocks in comparison with RF 25 MEMS switches that use a stress-free rigid switch beam (publication EP 1 489 639).

Preferably, when the membrane 6 is in the off-state position there is still a small gap between the plates 6b and the bridge parts 3a of the lateral supporting members 3 [figure 3 - dimensions (d)]. Hence, the supporting 30 members 3 only vertically supports the membrane 6 (direction Z of figure 1) and do not exert any mechanical force onto the membrane in plane (X,Y).

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There is thus no mechanical stress induced in the membrane 6 by the lateral supporting members 3 when the membrane 6 is in the off-state position.

On-state

Figure 2 is showing the on-state configuration of the RF MEMS 5 switch. In this on-state configuration, the membrane 6 is bended away from the substrate 1, and is no longer in contact with the dielectric layer 5 ; the RF signal line can be used for transmitting a RF signal.

In this bending state, the membrane stiffness in the out-of-plane direction is increased, which in turns increases the resistance of the switch 10 membrane 6 to vibrations or shocks.

From off-state to on-state

To achieve the on-state configuration, a DC signal is applied on the electrodes 7, in order to create electrostatic forces (F2) between the electrodes 7 and the membrane plates 6b. Said electrostatic forces F2 in 15 combination with the lateral supporting members 3 causes the bending of the membrane (figure 2) in such a way that the central part 6a of the membrane 6 is moved away from the dielectric layer 5 (closing of the RF signal line).

It has to be underlined that during this switching movement of the 20 membrane 6 (and also during the reverse switching movement from the onstate to the off-state), the membrane 6 is totally free to slide relatively to the supporting members 3, within the passages 3b of the said supporting members 3.

Thanks to this free movement of the membrane 6 during the 25 switching operations, there is less mechanical stress induced in the membrane 6 by the supporting members 3, in comparison with the RF switches of the prior art wherein the membrane (bridge or cantilever) is clamped onto a supporting structure. Mechanical fatigue and creep due to cyclic stresses in the extremities of the membrane 6 are avoided, and the 30 membrane life time is thus advantageously increased in comparison with RF MEMS switches of the prior art using a membrane that is clamped on the substrate.

Due to the fact that the membrane 6 is free to move relatively to the supporting members, when the RF MEMS switch is submitted to temperature variations, the expansion and relaxation of the switch 5 membrane 6 does not provoke an additional bending of the membrane. The RF MEMS switch of the invention is thus advantageously not temperaturedependant, in contrast with RF MEMS switches of the prior art using a membrane that is clamped on the substrate

Furthermore, thanks to the use of a totally free membrane 6, the 10 structure does not depend on the substrate (wafer) curvature. In particular small deformations of the substrate 1 can occur, for example under thermal variations or other mechanical constraints or during the fabrication process, without impairing the switching operations. The RF MEMS switch of the invention it thus advantageously less sensible to substrate deformation, in 15 comparison with RF MEMS switches of the prior art using a membrane that is clamped on the substrate.

Generally, in a capacitive RF MEMS switch, it happens frequently that the switch element sticks to the dielectric layer in the off-state, due to moisture or electrostatic charging of the dielectric layer. For the switch of the

0 invention, thanks to the use of an actuation force (electrostatic forces F2) for moving the membrane 6 towards the on-state position, the sticking problem of the membrane onto the dielectric layer 5 is solved.

From On-state to Off-state

When the DC on-state actuation signal on the electrodes 7 is lower 25 than a predetermined threshold (pull-out voltage), electrostatic actuation forces F2 are no longer applied onto the extremities (plates 6b) of the membrane 6, and the membrane 6 is bended back in the off-state position of figurel. The movement of the membrane 6 from the on-state position (figure 2) to the off-state position (figure 1) is caused by the restoring force of the

0 membrane 6, due to the natural stiffness of the membrane 6.

Thanks to the use of the natural stiffness of the membrane, the

PCI7EP2006/002076 movement from the on-state position to the off-state position is very fast and does not require high electric energy. The switching time (from the on-state to the off-state in case of the particular embodiment of figures 1 to 3) of the RF MEMS switch of the invention is thus very short and requires no electric energy in comparison with RF MEMS switches that use a stress-free rigid switch beam (publication EP 1 489 639).

Figure 4 / manufacturing process

The RF MEMS switch of figures 1 to 3 can be manufactured by using conventional surface micromachining technologies (i.e. by depositing 10 and patterning several layers onto a wafer).

Figure 4 shows the RF MEMS switch just after the fabrication process and before the releasing step. Three sacrificial 9,10 et 11 layers are being used. Theses sacrificial layers can be made of any material (metal, polymer, dielectric material).

One first sacrificial layer 9 is used for depositing the membrane 6 onto the dielectric layer 2. Once this sacrificial layer 9 is removed in the final releasing step, the two plates 6b of the membrane 6 and the part of the membrane 6 extending between the lateral supporting members 3 are released. The other sacrificial layers 10 and 11 are used for releasing the 20 membrane 6 from the dielectric layer 5, and from the lateral supporting members 3 (ground lines).

During the fabrication process, the distance between the membrane 6 and the dielectric layer 5 (i.e. thickness of the sacrificial layer 10) is very short. Typically, this distance is less than 0.1pm. This advantageously 2 5 involve that the profiles for the membrane 6 and the dielectric layer 5 are the same. Since in the off-state the membrane 6 is at rest and is not deformed, and the profile of the membrane 6 is the same than the profile of the dielectric layer 5, then a perfect surface-to-surface contact is obtained between the membrane and the dielectric layer 5 in the off-state position of the membrane 6.

The invention is not limited to the preferred embodiment of figure 1 to 3 but can be extend to all RF MEMS switches that comprise a flexible switch membrane that is freely supported onto substrate by supporting members or the like and that is bendable under the action of actuation means.

The actuation means are preferably but not necessarily electrostatic means.

The invention enable to make capacitive RF MEMS switch having very low actuation parameters, very fast switching and improved RF performances. The invention is more particularly and mainly interesting for 10 making capacitive RF MEMS switches that can be used at very high RF signal frequency and notably at RF frequencies over 25GHz.

The invention is however not limited to capacitive RF MEMS switches but can also be used for making ohmic contact RF MEMS switches (also commonly referred as “metal to metal contact RF MEMS switches”). In 15 an ohmic contact RF MEMS switch of the invention, the flexible and freely supported membrane 6 is for example designed in order to make a short circuit in the on-state position between one first metallic contact positioned between the lateral supporting members 3 and a second metallic contact which can be for example permanently in touch with the membrane 6. In the 20 off-state position, the membrane is no longer in touch with the said first metallic contact.

In case of a capacitive RF MEMS switch, the dielectric layer 5 can be carried by the membrane 6 and not necessarily by the signal line 4. Alternatively, dielectric layers can be carried both by the signal line 4 and the 25 membrane 6.

In the preferred embodiment of figure 1 to 3, the actuation electrodes 7 are positioned under the membrane 6 (i.e. between the dielectric layer 2 and the membrane 6) and outside the coplanar wave guide (CPW) formed by the lateral supporting members 3. This particular position 30 of the electrodes involves the following advantages. During the on-state configuration, there is advantageously no risk of interaction between the

WO 2006/099945 PCI7EP2006/002076 electrostatic forces (F2) used for actuating the position of the membrane 6 and the RF signal transmitted within the coplanar wave guide. The surface of the electrodes 7 can thus be large, and in turns the “on-state voltage״ applied to the electrode 7 can advantageously be very low. This particular position of the electrodes 7 is however only a preferred characteristics of the invention. In another variant of the invention, the RF MEMS switch could be for example designed in such a way that the actuation electrodes 7 are positioned above the membrane 6.