EP2024986A1 - Radiofrequency or hyperfrequency micro switch structure and method for producing one such structure - Google Patents

Abstract

The invention relates to a micro switch structure comprising: a substrate (1) covered by a passivation layer (2), a first signal line LS-IN and a second signal line LS-OUT being each located in the extension of the other and separated by a switching region (10); a control electrode (3) arranged in said region, a frequency-invariant dielectric material (4) with high relative permittivity being arranged on the control electrode such that the control electrode is wider on both sides and in the orthogonal direction, between the two signal lines, and the dielectric overlaps on both sides of the control electrode and lies on the passivation layer; and parallel mass lines arranged symmetrically either side of the signal lines and on a topological level separated from that of the signal lines by at least one insulating layer in a material different from that of the passivation layer.

Description

 RADIOFREQUENCY OR HYPERFREQUENCY MICRO-SWITCH STRUCTURE AND METHOD OF MANUFACTURING SUCH

STRUCTURE

The field of the invention is that of microsystem components also called MEMS (acronym for Micro Electro

Mechanical Systems) and more particularly microsystems radiofrequency or microwave. The main application areas are wireless telecommunications systems and radars.

In the following we use generically the term radio frequency or RF, to understand as covering both microwave and radio frequencies.

According to the state of the art of RF MEMS microswitches, the RF switching is obtained by varying the capacitance of a capacitor whose armatures consist on the one hand of a membrane and on the other hand of an electrode control, a dielectric being provided between the two frames generally on the electrode. The capacity then varies from a Con value to a Coff value. The dielectric used may be silicon nitride. In other embodiments, the dielectric is PZT or BZT, or other material with a high relative permittivity, which makes it possible in particular to increase the Con / Coff ratio, and thus improves the transmission and isolation qualities of the micro- switch, as well as its characteristic times of switching between the two states on and off. RF microswitches are increasingly used to improve the functionality of radio frequency circuits used especially in telecommunication systems. It is a question of obtaining better performances in terms of losses, noise, linearity, consumption. They are also used to obtain high component compactness and the lowest possible manufacturing costs.

There are two types of microswitches providing a radiofrequency signal switching function on a transmission line: the microswitches in series with the radiofrequency transmission line and the microswitches in parallel with the radiofrequency transmission line. When the microswitches are of the series type, the application of an activation voltage under the membrane changes it from a state of rest off, open, to state on, closed. The configuration of a microswitch in series with a radiofrequency line is as follows: the line is cut in the switching zone, above which one has the membrane. The membrane is isolated from the electrical mass. The membrane does not have to support the radiofrequency power over its entire surface, its role not being to short circuit the signal to ground but simply to connect two lines together by capacitive effect. When the microswitches are of parallel type, the application of a voltage on the control electrode moves it from a state of rest on, closed, to an open off state. The configuration of a microswitch in parallel with a radiofrequency line is as follows: the RF line is not cut in the switching zone, above which we have the membrane. The membrane is connected to the electrical earth and must be able to withstand the power of the radiofrequency signal.

The operation is as follows: in the rest position, the microswitch is in the on state, closed, which results in a very weak Coff capacity, which does not affect the radiofrequency signal transmission. When it is in the low state, under the effect of an activation voltage under the membrane, the capacity increases in a significant ratio, 100 for example. The capacitor then induces a sufficiently weak impedance between the line and the ground to shunt the radiofrequency line to the electrical ground: the radiofrequency signal then flows from the line bringing the RF signal to the electrical mass via the membrane. The two portions of lines, before and after the membrane, are then isolated: the microswitch is in the off state.

The main advantages of these types of microswitches series or parallel are essentially: • The realization techniques, which are derived from conventional technologies of manufacturing electronic integrated circuits. They simplify the realization and integration and therefore, to obtain low manufacturing costs compared to other technologies, while ensuring high reliability; • The very low electric powers consumed, some nanojoules being necessary for the activation;

• Clutter. A micro-switch is thus produced in a surface of the order of one tenth of a square millimeter, making it possible to achieve a high integration capacity;

• Performance in microwave use. This type of microswitch has very low insertion losses, of the order of a tenth of a decibel, much lower than those of devices providing the same functions. The search for higher switching speeds, larger RF power outages equal to or greater than ten watts, wideband operation of at least 18 GigaHertz, the lowest compactness possible and always at a lower cost, of duration increasingly important life (number of switches), of the order of 10 ¹¹ at least to respond to the evolution and the need of the market, including civil markets such as mobile telephony, leads to search for structures and processes optimized fabrication, the known structures of micro-switches imperfectly meet these needs.

The invention thus relates to a structure and a method of manufacturing a micro-switch that can meet all of these different needs. It applies to both a serial and parallel microswitch.

As characterized by the invention therefore concerns the structure of a capacitor-type radio frequency or microwave microswitch with a first armature comprising a voltage control electrode arranged in a switching zone between a first conductive line called an input signal line. and a second conductive line called an output signal line arranged in the extension of one another, separated by the switching zone, a second armature being a flexible membrane disposed above said switching zone, the two armatures being separated by a vacuum or gas thickness and at least one layer of a dielectric material, two parallel ground lines being arranged symmetrically with respect to the signal lines, said structure being formed on an insulating substrate covered with a layer of passivation. According to the invention, the structure is such that: Said control electrode is formed on said passivation layer,

Said layer of dielectric material has a high relative permittivity greater than one hundred, and it is deposited on said control electrode, so that in the direction of the input and output lines, said dielectric material rests only on said electrode in the orthogonal direction, said dielectric material overflows on each side and comes into contact with said passivation layer of the substrate,

The flexible membrane is conductive and comprises at least one layer of a conductive material.

At least one layer of insulator in a material different from that of the passivation layer separates the level of the ground lines from the level of the signal lines.

Said high-permittivity material is preferably PZT (Lead Zirconium Titanate, PbZrTiO 3).

According to one embodiment of the invention, the metal membrane comprises a lower layer of a resistive material, typically a titanium-tungsten alloy and a low-resistivity layer, of a material capable of withstanding mechanical stress, selected from Al, Au, Cu, preferably Al.

For use as a variable capacitor, in which it seeks to control the movement of the membrane between the rest position and a maximum position between the dielectric and the rest position, the membrane is formed of a single layer of aluminum.

The invention also relates to a method of manufacturing a radio frequency microswitch or microwave frequency of such a micro-switch on an insulating substrate covered with a passivation layer, characterized in that it comprises at least the following sequence of steps : at). Formation of the control electrode; b). Formation of the dielectric on said control electrode c). Deposition on the entire surface of a first resistive conductive layer and a second non-resistive conductive layer, and etching the second layer, to form the input / output signal lines and contact pads, d). Deposition on the entire surface of an insulating layer, in a material different from that of the passivation layer, then opening on the signal lines, contact pads and on the dielectric, e). Deposition on the entire surface of a first resistive conductive layer and a second non-resistive conductive layer, and etching of the second layer, to form the ground lines, f). Deposition of a determined thickness of resin over the entire surface and localized charging up to the resin height of the material of said second conductive layer with low resistive signal and ground lines, g). Localized etching under the location of the membrane of the first conductive layer deposited in step e). h). Formation of the membrane; i). Release of the membrane, by removing the resin layer of step f).

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 thanks to the appended figures among which:

FIGS. 1a to 1c illustrate a series switch structure according to the invention; FIGS. 2a to 2c illustrate a parallel switch structure according to the invention;

FIG. 3a and the following figures up to FIG. 12 illustrate steps of a method according to the invention.

A radiofrequency or microwave microswitch structure according to the invention is illustrated in FIGS. 1a to 1d, for a series-type microswitch or in FIGS. 2a to 2c for a parallel-type microswitch.

A structure according to the invention comprises on a substrate 1 covered with a passivation layer 2, a first signal line LS-IN and a second signal line LS-OUT arranged in the extension of one another, separated by a switching zone 10; a control electrode 3 in said zone, a dielectric material 4 with a high relative invariant frequency permittivity, arranged on the control electrode so that between the two signal lines, the control electrode is wider on both sides and in the orthogonal direction, the dielectric overflows both sides of the control electrode, and rests on the passivation layer; parallel ground lines arranged symmetrically on either side of the signal lines and made on a topological level separated from that of the signal lines by at least one insulating layer 6 in a material different from that of the passivation,

The insulating material is advantageously silicon nitride Si ₃ N ₄ .

The dielectric material is advantageously PZT whose relative permittivity is equal to 150, and is independent of the frequency, which contributes to increasing the width of the operating frequency band of the micro-switch. In addition, the PZT which has a monocrystalline structure resists well to significant RF powers.

A series-type microswitch structure will be described in more detail. FIG. 1a represents a view from above of such a microswitch and FIGS. 1b and 1c are views in section respectively along AA and following BB.

This structure is made by superposition of layers on a base substrate 1, typically a highly resistive silicon substrate, covered with a passivation layer 2, typically silicon oxide SiO ₂ .

It comprises two signal lines LS-IN and LS-OUT arranged coplanar in the extension of one another, separated by a switching zone 10. In the switching zone, a control electrode 3 is formed between the two lines signal, in two parts electrically isolated: each part contacts a signal line. A dielectric 4 with a high relative permittivity greater than one hundred and invariant with the frequency is deposited on the control electrode 3. It has a shape such that in the direction along the signal lines, the control electrode is wider on both sides, and in the orthogonal direction, it overflows each side of the control electrode 3, on the passivation layer 2.

The dielectric 4 must make it possible to respond to the constraints of high radiofrequency or microwave powers: in on-state transmission, passing (membrane in downwardly bent position, in contact with the dielectric), and in isolation in the off or open state. (membrane in initial high position).

The dielectric 4 is preferably PZT, which combines the advantages of having a high relative permittivity greater than one hundred (typically 150), invariant with the frequency, of being able to work in microwave, up to 100 GigaHertz, and to support the power, because of its monocrystalline nature.

In practice, the gap separating the two parts of the control electrode has a width g of the order of 10 microns. The cut between the two parts may be straight section. It is advantageously such that the two parts are interdigitated. In known manner, such a shape makes it possible to significantly increase the dielectric capacitance of the capacitor formed by the membrane m, the control electrode 3 and the dielectric 4. Preferably, the control electrode is made of a platinum titanium alloy surmounted a Platinum / Gold layer for technological needs.

The membrane m rests at each end, on a conductive pillar 5a, 5b. It is also possible to consider only one conductive pillar on the two that support the membrane.

LM 1 and LM 2 ground lines are formed on the same side of the substrate as the LS-IN and LS-OUT signal lines. These coplanar ground lines are made on a topological level separated from the level of the input / output signal lines by an insulating layer 6, in a material different from that used for the passivation layer. In this way, it is certain that there will not be a short circuit between a signal line and a ground line, via the substrate. This has the technical effect that the micro-switch structure according to the invention can rise very high in frequency, typically up to at least 100 GigaHertz. The insulation used is advantageously silicon nitride. The pillars, the signal lines and the ground lines typically comprise a first resistive hung conductive layer, shown in thick black in FIGS. 4b and 4c and a second, weakly resistive layer, typically gold. The first layer is sufficiently resistive to prevent the propagation of a radiofrequency or microwave signal. It is typically a layer of tungsten titanium, preferably 80% titanium and 20% tungsten to 1 or 2%, by which the best radiofrequency and microwave performance are obtained. The titanium-tungsten layer 7 of the signal lines and pillars also serves for the realization of connection lines through which an activation voltage of the microswitch can be applied in the switching zone. In practice, at least one contact pad (not shown in FIGS. 4a to 4c) is produced in the same way as the signal line and the pillars, on the same topological levels, and a connection line is formed between this pad and the pad. least one signal line. Preferably the contact pad is connected to the two signal lines LS-IN and LS-ouτ, so that the voltage is found on the two parts of the control electrode 3. The arrangement in interdigitated fingers allows to have a part substantially in the middle under the membrane. These two combined characteristics make it possible to obtain a maximum electrostatic field substantially in the middle of the membrane, which ensures optimum on and off switching times. The conductive membrane comprises: - a conductive layer of suspended, resistive, typically titanium-tungsten, facing the switching zone. This layer is sufficiently resistive to prevent the propagation of a radiofrequency or microwave signal. The tungsten titanium preferably has a proportion of 80% of titanium and 20% of tungsten to 1 or 2%, as indicated previously; and a highly conductive layer. It can be a dielectric. Preferably, a metal material selected from Al, Cu and Au is chosen for their low electrical resistivity and their ability to withstand a mechanical stress greater than 30 megapascals: the membrane must be able to deform in order to come into contact with the dielectric 4 without breaking (state on), and return to its initial state (off state). Preferably, it is aluminum which is used, whereby the best results are obtained in terms of switching speed and resistance to mechanical stress.

The membrane is made in the form of a grid, that is to say that it has holes passing through it from one side to the other. This configuration contributes to facilitating the production of the membrane, as will be seen in connection with the manufacturing process, because it facilitates the passage of solvents or gases to remove the sacrificial resin layer which serves as a plane support for producing the membrane. This configuration also contributes to improving the flexibility of the membrane. Finally, the grid shape is well known in the form of a performant in the radio frequency and microwave domain.

In a preferred embodiment, the serial micro-switch has the following sizing characteristics:

The section of the signal lines has a width Is of 80 microns, and the distance d between each side of the signal line of the ground line is 120 microns.

The gold layer e9 signal lines and pillars has a thickness of about 3 microns. The control electrode has a thickness of about 0.7 microns. The thickness of the ground lines is not an important parameter. It is substantially equal to that of the signal lines, from 0.2 to 0.4 microns, the negligible difference arising from the technological process. The layer 4 of PZT has a thickness e4 less than one micron, for example 0.4 micron. The width of the overhang on each side of the dielectric is of the order of 20 microns. The mobile part of the membrane, that is to say off pillars, is part of a rectangular parallelepiped, whose dimensions are advantageously: a width Im of 100 microns, in the direction of the signal lines, and a length wm between the two pillars, of the order of 280 microns. The total thickness e em of the membrane is of the order of 0.7 microns, the first layer of tungsten titanium being of less thickness than the second layer. In one example, the tungsten titanium layer has a thickness of 0.2 microns.

The structure of a parallel microswitch according to the invention is illustrated in FIGS. 2a (top view), 2b and 2c (sectional views following respectively AA and BB), which use the same references as in Figures 1a to 1c. The structure is substantially identical to that of the serial micro-switch. The differences are due to the specificities of the parallel type compared to the series type. In particular, the membrane resting directly on the ground lines, there are no pillars 5a, 5b, and the control electrode has a continuous shape and contacts each side a signal line. For these reasons, the foregoing description made in connection with FIGS. 1a to 1c applies in the same way, with the reservations that have just been made. The preferred dimensioning characteristics of a parallel microswitch according to the invention are identical to those indicated above for the series structure.

In a variant applicable both to the series and parallel mode, the membrane is made of a single aluminum layer, preferably with a thickness of the order of 2.5 microns, making it possible to produce a capacitor with variable capacitance, the voltage activation then defining the value of the capacitance, as a function of the displacement imposed on the membrane.

The series and parallel microswitches according to the invention have good radio frequency and microwave performance, in particular for the transmission of radiofrequency or microwave significant power signals of the order of about ten watts or more.

A method of manufacturing a micro-switch advantageously used in the invention, as described in connection with Figures 3a to 3c will now be described. It is illustrated by Figures 10a and following, which show different stages.

Beforehand, each conductive element: signal lines, ground lines, contact pads, and membrane are made by a first highly resistive conductive layer and a second non-resistive conductive layer.

Preferably, the first layer is a tungsten titanium alloy in a proportion of 80% of titanium and 20% of tungsten to 1 or 2%, and the second layer is gold, this choice making it possible to obtain the better performance. In the description of the process steps, for simplicity, we speak directly of tungsten titanium and gold, but others materials such as copper and aluminum, for example, could be used without departing from the scope of the invention.

Step 1, Figures 3a (top view) and 3b (section along X). We have a substrate 100, preferably highly resistive silicon (or GaAs, GaN ...). This substrate 100 is deposited on a SiO ₂ silicon oxide passivation layer (relative permittivity 4). The control electrode 102 is formed, with its shape in two isolated parts a, b, preferably as illustrated, interdigitated. The width g of the gap between the two parts is typically 10 microns. The control electrode is for example made of a titanium / platinum alloy surmounted by a gold / platinum layer.

Step 2, Figures 4a and 4b. The PZT dielectric 103 is formed on the control electrode in the prescribed form, typically by a sol-gel or sputtering method: narrower in the Ds direction of the signal lines and wider on both sides in the orthogonal direction , coming to rest on the layer 101 of passivation.

Step 3, Figures 5a (top view) and 5b (section along YY '). Formation of the signal lines LS-IN and LS-OUT, contact pads Pc, and pillars P1, by deposition of a layer of titanium / tungsten 104, deposition and etching of a layer of gold 105. The layer The surface is then the layer 104. Step 4, FIGS. 6a and 6b: etching of the layer 104 of titanium / tungsten, to form connection lines, between a contact pad between one or both signal lines (to bring a voltage of activation on one or both parts of the control electrode), and a contact pad and a pillar, which makes it possible to put the membrane at a reference potential (an electrical mass, which is not the mass of the microswitch circuit). As a surface layer, apart from the elements made, the passivation layer 101 is found.

Step 5, Figures 7a and 7b. Deposition of the silicon nitride insulator layer Si ₃ N ₄ , then opening O on the signal lines, and the contact pads, the pillars and the dielectric 103, according to the dashed lines. The surface layer is this layer 106 of insulation.

Step 6, Figures 8a and 8b. Deposition of a layer 107 of titanium / tungsten and deposition and etching of a gold layer 109, to form the mass lines LM1 and LM2. The surface layer is titanium / tungsten layer 107. Step 7, Figures 9a and 9b. Localized removal of tungsten titanium in an area f under the location of the membrane.

Step 8, Figure 10. Localized refill of gold, by prior deposition of resin over the entire surface and by current injection via the contact pads and connection lines. The height of gold thus obtained is controlled by the resin thickness. In practice the thickness (or height) of gold signal lines and pillars reaches 3 microns. The thickness of the mass lines is substantially equal, with in practice a negligible difference of the order of 0.2 to 0.4 microns (less). The resin achieves the same level everywhere, which ensures the flatness of the membrane which is achieved in the next step.

Step 9, Figures 11a and 11b. Formation of the membrane by deposition of tungsten titanium and then deposition of aluminum (or gold, or copper), and etching of the membrane. Preferably, a tungsten titanium thickness of 0.2 microns and a gold thickness of 0.5 microns is used. For a micro-switch used as a variable capacitor as in the impedance matching circuit, this step 10 comprises the deposition of a single layer, aluminum, with a thickness of about 2.5 microns and etching.

Step 10, Figure 12: release of the membrane by removing the resin layer of step 8, for example by solvents. This operation is facilitated by a membrane which is pierced with holes. Such a membrane structure also has the effect of making the membrane less rigid, which contributes to improving the latency.

This method also applies to parallel-type switches which differ from serial microswitches simply in that there are no pillars, the membrane resting directly on the ground lines, and by the continuous form, without cutting off the control electrode between the two signal lines.

The succession of the steps of the method just described, leads to a micro-switch structure whose radio frequency performance in transmission, isolation, switching time, the service life, the width of the frequency band are substantially improved.

Claims

1. Structure of a capacitor type radiofrequency or microwave microswitch with a first armature comprising a voltage control electrode (3) arranged in a switching zone (10) between a first conductive line called an input signal line ( LS-IN) and a second conductive line called an output signal line (LS-ouτ) arranged in the extension of one another, separated by the switching zone (10), a second armature being a membrane (m) flexible arranged above said switching zone, the two armatures being separated by a vacuum or gas thickness and at least one layer of a dielectric material, two parallel ground lines (LM1, LM2) being arranged symmetrically with respect to the signal lines, said structure being formed on an insulating substrate (1) covered with a passivation layer (2), characterized in that: • said control electrode (3) is formed on said passivation layer (2), Said layer of dielectric material (4) has a high relative permittivity greater than one hundred, and it is deposited on said control electrode (3), so that in the direction of the input and output lines, said dielectric material rests only on said control electrode and in the orthogonal direction, said dielectric material protrudes on each side and comes into contact with said passivation layer (2) of the substrate, • the flexible membrane is conductive and comprises at least one layer of a conductive material, At least one insulating layer (8) in a material different from that of the passivation layer (2) separates the level of the ground lines from the level of the signal lines. 2. Structure according to claim 1, characterized in that said dielectric (4) is PZT. 3. Structure according to claim 1 or 2 characterized in that said insulator (8) is silicon nitride Si ₃ N ₄ . 4. micro-switch structure according to claim 1 or 2, characterized in that the thickness of the signal lines is of the order of 3 microns, and the thickness of the control electrode is of the order of 0.7 micron. 5. micro-switch structure according to claim 1 or 2, characterized in that the signal lines and the ground lines comprise a very resistive lower layer and a low resistive upper layer. Microswitch structure according to one of the preceding claims, characterized in that the membrane comprises a low resistive lower layer (m1) facing the tungsten titanium switching electrode, and an upper layer in a selected material. among Al, Cu, Au. 7. Structure according to any one of claims 1 to 5, characterized in that the membrane is formed of a single layer of aluminum with a thickness of the order of 2.5 microns, for use as a variable capacitor. 8. micro-switch structure according to claim 5 or 6, characterized in that said lower layer is a tungsten titanium layer being an alloy in an amount of 80% titanium and 20% tungsten to 1 or 2%. 9. micro-switch structure according to claim 6, characterized in that the total thickness of the membrane is of the order of 0.7 microns, the thickness of the upper layer being of the order of 0.5 microns. 10. Microswitch structure according to one of the preceding claims, characterized in that the dielectric thickness (4) is of the order of 0.4 microns. 11. micro-switch structure according to any one of the preceding claims, characterized in that the width (Is) of the signal lines is 80 microns, and the distance (d) to each line of mass of 120 microns. Serial-type microswitch structure according to one of the preceding claims, characterized in that said control electrode (3) comprises at least two separate parts, each in contact with one of the signal lines, a gap zone. between the two parts being located substantially in the middle of the switching zone and in that said membrane (m) rests at each end on a pillar (5a, 5b) disposed between the signal lines and a ground line. 13. Structure according to claim 12, characterized in that said parts of the control electrode are interdigitated, A parallel-type microswitch structure according to any one of claims 1 to 11, the membrane resting at each end on a ground line and the control electrode having a unitary shape connecting on each side the signal lines. . 15. Structure according to one of claims 1 1 to 13, characterized in that the portion of the membrane out of the pillars is substantially in a rectangular parallelepiped having a length between the pillars of the order of 280 microns, and a width of the order of 100 microns. 16. Structure according to one of claims 12 to 15, characterized in that the length of each edge of the dielectric (4) on the passivation layer (2) is of the order of 20 microns. 17. A method of manufacturing a radio frequency microswitch or microwave according to claim 1 on an insulating substrate (100) covered with a passivation layer (101), characterized in that it comprises at least the succession of steps. following: • at). Forming the control electrode (102); • b). Forming the dielectric (103) on said control electrode, • vs). Deposition on the entire surface of a first resistive conductive layer (104) and a second non-resistive conductive layer (105), and etching of the second layer, to form the input / output signal lines and contact pads , • d). Depositing the entire surface of an insulating layer (106) in a material different from that of the passivation layer (102), then opening on the signal lines, the contact pads (105) and the dielectric (103), • e). Deposition on the entire surface of a first resistive conductive layer (107) and a second non-resistive conductive layer (108), and etching of the second layer, to form the ground lines, F) .Deposit of a determined resin thickness over the entire surface and localized refill up to the resin height of the material (Au) of said second resistive conductive layer (105, 108) of the signal and ground lines, • g). Localized engraving under the location of the membrane of the first conductive layer (107) deposited in step e). • h). Formation of the membrane (m) • i). Release of the membrane, by removing the resin layer of step f). 18. The production method according to claim 17, characterized in that the layer of dielectric material is deposited by a sol-gel or sputtering process. 19. Production method according to claim 17, characterized in that the membrane is pierced with holes. 20. Production method according to claims 17 to 19 for a series-type microswitch, characterized in that in step c) the pillars are also formed, to which the localized step f), the membrane apply. formed in step i) resting at each end on said pillars.