WO2005078752A1 - Piezoelectric thin-film electromechanical microsystem device - Google Patents

Abstract

A piezoelectric thin-film electromechanical microsystem (MEMS) device (18) comprising an RF-MEMS switch (14) and a bulk acoustic wave piezoelectric thin-film RF resonator (10) mounted on a substrate (12) provided with an acoustic mirror (28). The resonator comprises a thin-film portion (16) between the two metal electrodes (20, 24). The switch (14) comprises a two-layer crossarm with an insulator layer (38) combined with a different portion (36) of the piezoelectric film (18) and having a movable metal contact (50) engageable with the stationary metal contacts (46, 48) when the crossarm is bent by a voltage across the control electrodes (40, 42).

Description

 The present invention relates generally to devices of the piezoelectric thin film electromechanical microsystem (MEMS) type.

The introduction of piezoelectric thin films in MEMS technology is fairly recent. In particular, patent US Pat. No. 6,504,118 B2 describes a bidirectional relay with electrostatic locking associated with thermal or piezoelectric actuation and patent application US 2003/0179535 A1 describes a capacitor with variable capacity by adjusting the distance between armatures using 'a plurality of piezoelectric actuators. In addition, the article by S.J. Gross et al. Entitled 'Lead-zirconate-titanate-based piezoelectric micromachined switch', Appl. Phys. Lett., Vol. 83, No. 1, pages 174-176 (July 7, 2003) describes a unimorphic switch with a deformable beam comprising a thin film of 0.23 μm of piezoelectric PZT activated by interdigitated electrodes (IDT). The incorporation into complex electronic circuits and / or arrangements of these known devices produced individually independently poses a certain number of difficulties, both in terms of their physical incorporation proper and in the production of metallic electrical interconnections (with sometimes the need of external welds) than at the level of electrical losses and other parasitic effects generated by these electrical connections, in particular of the resistive, inductive or capacitive type.

Document EP-A-0963000 shows a 'disengageable' resonator device obtained by a hybrid structure comprising a single beam of piezoelectric material usable on the one hand as an integral part of an acoustic wave resonator and on the other hand as the element mobile deformable by electrostatic effect of a micromechanical switch with electrostatic control mounted in series on the resonator.

Document US 2003/0205948-A1 shows an FBAR type acoustic wave resonator associated with a variable capacity for adjusting the resonant frequency of the resonator. According to a first particular embodiment illustrated in FIG. 4, the variable capacity of adjustment of the FBAR resonator 42 consisting of a piezoelectric layer disposed between the two electrodes is adjusted by the adequate dimensioning of the active surface of the electrodes forming the reinforcement of the capacity without recourse to the piezoelectric effect. According to a second embodiment illustrated in FIG. 10, the resonator's adjustment capacity does not include a piezoelectric layer, the adjustment being carried out by varying the spacing of the armatures of the capacity under the action of electrostatic forces.

The object of the invention is to propose a device of the electromechanical microsystem type with piezoelectric actuation capable of significantly reducing, or even eliminating, the difficulties of incorporating such a device and the associated parasitic effects.

To this end, the invention provides a device of the electromechanical microsystem type (or MEMS device) with piezoelectric actuation comprising at least one piezoelectric thin film characterized in that the thin film comprises at least one first portion of piezoelectric nature, participating in the actuation of said electromechanical microsystem and at least a second portion of piezoelectric nature integrated in an acoustic wave resonator.

Thus, the use of the same layer of piezoelectric film for the resonator and for the MEMS component integrating the first portion of the thin film, for example an RF switch, makes it possible to place the resonators (or filters) and the switches very close to the each other, considerably reducing the overall size of the assembly and, ultimately, relatively easily integrating this assembly into more complex electronic circuits, according to techniques already used for resonators and thin film acoustic wave filters. .

This compactness also results in a reduction in the lengths of the metallic interconnections, or even the elimination of certain external welds. This results in a significant reduction in electrical losses and the effects of parasitic capacitors or chokes, which make it possible to use the devices according to the invention in very high frequency RF circuits (between 1 and 10 GHz), in particular for the new generation of UMTS mobile telephony.

According to a first embodiment, the device according to the invention comprises a switching sub-assembly with piezoelectric actuation, in whole or in part of a monomorphic or multimorphic structure. Advantageously, the switching sub-assembly comprises at least one deformable element of the beam and / or bridge type. and / or membrane and integrating all or part of said first portion of the piezoelectric thin film. Without departing from the scope of the invention, the switching subassembly is of the electrical contact and / or capacitive coupling / decoupling type (for example an RF switch of the deformable piezoelectric bridge type controlling the distance between armatures of a capacity disposed between two RF lines, in particular to connect the two RF lines or to divert a line carrying an RF signal to earth).

According to a second embodiment, the device according to the invention comprises at least one variable capacity with piezoelectric actuation, in whole or in part of a monomorphic or multimorphic structure. Advantageously, the variable capacity is of the type actuated by a deformable beam or membrane integrating all or part of said first portion of the piezoelectric thin film. Such a capacity can be easily integrated in various electronic microcircuits, in particular in oscillators.

In general, the devices according to the invention find their applications directly in integrated electronic circuits, in particular CMOS circuits, in particular for acoustic or other sensor and / or transducer systems or for transmission systems, RF or microwave transmission or reception. The devices according to the invention can in particular be used with profit in compact reconfigurable filter assemblies and duplexers. According to a first variant of the first embodiment, the second portion of piezoelectric nature is integrated into a volume acoustic wave resonator. Advantageously, the resonator is of the piezoelectric thin film type mounted flat on the substrate, preferably on a SMR ("Solidly Mounted Resonator") substrate with an acoustic mirror or of the FBAR ("Film Bulk Acoustic Resonator") membrane type, in particular a resonator. piezoelectric for active RF circuits between 1 and 10 GHz.

Among the piezoelectric materials used for the thin film, the following compounds may be mentioned: AIN, PZT, BaTiO ₃ , KNbO ₃ , PbNbO ₃ , PbZrO ₃ and ZnO, aluminum nitride (AIN), with its piezoelectric and elastic properties high performance, proving quite interesting for the production of a thin film usable in the context of the present invention. Preferably, the piezoelectric film is deposited by inert or reactive spraying in a single operation. Other characteristics and advantages of the present invention will appear on reading the description which follows, presented solely by way of nonlimiting example with reference to the attached drawing in which: FIG. 1 represents a schematic sectional view of 'A device according to the invention of the electromechanical microsystem (MEMS) type, comprising at least one thin piezoelectric film and integrating an RF switch and a volume acoustic wave resonator.

It should be noted, beforehand, that the term “switch” used in this presentation must be understood in its broadest sense, in particular as switching means in general, relays, switches, etc. even for the term “membrane” which covers any deformable thin layer of variable contour whatever its type of mounting, for example in a nonlimiting manner, mounting in beam embedded at one end, in bridge or with peripheral support.

The device according to the invention illustrated in FIG. 1 comprises a piezoelectric resonator with volume acoustic waves (BAW resonator), more particularly, a piezoelectric thin film resonator 10 of SMR type, mounted on a solid silicon substrate 12 (in species, intended to be used in particular in the field of RF telephony between 2 and 3 GHz) and associated with an RF-MEMS switch 14 with piezoelectric actuation.

The resonator 10 comprises a portion 16 of a thin piezoelectric film 18, for example made of aluminum nitride (AIN), disposed between two flat conductive metal electrodes 20 and 22, for example, of platinum. As illustrated in FIG. 1, the external electrode 20 defines the surface limits of the active volume of the resonator while the corresponding electrode 22 belongs to a metal layer 24 of larger surface extending towards the switch 14 and deposited on the upper planar face 26 of the substrate 12. The AIN piezoelectric material is deposited in a thin film in a single operation by reactive spraying so as to be able to exploit the piezoelectric component of first magnitude d ₃₃ of the layer 16, according to which the material is subjected to alternative efforts of longitudinal extension / compression. generating acoustic waves propagating parallel to the electric field generated between the electrodes 20 and 22, when the latter are respectively connected to the two terminals of an alternating voltage source RF (not shown). In the present case, the electric field lines and the longitudinal extension / compression forces are perpendicular to the upper face of the substrate 12.

Under these conditions, the total thickness of the piezoelectric layer coated with its two electrodes corresponds to a half wavelength of the acoustic waves of the mechanical resonance frequency of the resonator, a wavelength which depends on the nature of the piezoelectric material. , more particularly, of the speed of propagation of these acoustic waves in the piezoelectric thin film. By way of nonlimiting example, a mechanical resonance frequency of the order of 2 GHz is obtained with an AIN film thickness of 1.6 μm and platinum electrodes of 0.1 μm thickness.

To avoid acoustic losses in the substrate 12 so as to increase the quality factor Q of the resonator and eliminate unwanted parasitic reflections and / or vibrations and the risks of interference which would result therefrom with the original waves, the upper part of the silicon substrate 12 includes an acoustic reflector 28 (analogous to a Bragg mirror) formed by a stack of so-called quarter-wave layers 30 of two materials with very different elastic properties (for example, layers of AIN or SiO ₂ ) , the thickness of the layers 30 and 32 and their numbers depending on the desired degree of insulation but also on the ratio of the respective acoustic impedances of the materials used for these layers.

The switch 14 is mainly constituted by a deformable bi-layer beam (or membrane) 34 embedded at one end (or on one side) consisting of a second portion 36 of the layer of the piezoelectric film 18 and of a layer of insulating material 38 (chosen for example from SiO ₂ , ZrO ₂ and Si ₃ N ₄ ) with mechanical properties which are substantially different from the piezoelectric material AIN, the two layers 36, 38 being intimately bonded or welded to each other. The device of actuation or actuator of the switch is produced by two control electrodes 40 and 42 made of metal (for example platinum) deposited facing each other on each face of the bi-layer beam 34 near the embedded end 35 thereof this. Finally, the switching part (in this case of the ohmic type) is produced by two fixed metal contacts 46, 48 mounted spaced directly on the upper layer of the Bragg mirror, which also serves as an insulator (these fixed contacts being in the species obtained from the metal layer 24 in which the electrode 22 of the resonator is formed) and a movable metal contact 50 mounted on the insulating layer 38 of the beam 34 near the free end 37 of the latter ci to take advantage of the maximum deflection and so as to overlap the spacing 47 between the fixed contacts 46, 48.

As illustrated in FIG. 1 which shows the switch 14 at rest (corresponding to a zero voltage at the terminals of the electrodes 40-42), the beam 34 extends parallel to the substrate 12 at a short distance from the metal layer 24 to leave a race from the movable contact 50 to the fixed contacts 46, 48 of one to 5 microns, the corresponding space 52 between the beam 34 and the metal layer 24 being vented. The switch is activated by a DC voltage source DC (not shown) of a few tens of volts at most suitably connected to the electrodes 40 and 42 (in this case, ten volts is sufficient for the beam 34 comprising a piezo layer 36 of 1.6 μm AIN). This voltage develops by reverse piezoelectric effect a piezoelectric extension or expansion constraint of this layer 36 perpendicular to its thickness without affecting, at least substantially, the insulating layer 38. By the play of the resulting differential expansion, the beam 34 has tendency to close towards the insulating layer 38, itself weakly or not expanded, and to bend towards the substrate 12 until bringing the movable contact 50 in abutment on the two fixed contacts 46-48 and thus closing the switch or switch 14.

In the switch 14 described here, the closing state of the switch is ensured by maintaining the DC voltage between the electrodes 40-42 of the piezo layer 36. However, it is possible, without departing from the scope of the invention, to add an electrostatic locking of the closed position of the switch, which makes it possible to very significantly reduce the electrical energy necessary for this operation, the DC voltage across the electrodes 40-42 then being reduced or canceled. In practice, this locking (not shown) can be achieved by a device forming a capacitor with parallel metal armatures separated by a layer of dielectric material and suitably connected to a DC voltage source (which can be the DC voltage source), the first metal reinforcement being placed on the insulating layer 38 between the electrode 42 and the contact 50 and the second metal reinforcement being produced by a small surface of the metal layer 24 suitably insulated (by elimination of this same layer around its periphery) and covered with the layer of dielectric material.

It should be noted that the structure, proposed for the device according to the invention with a common piezoelectric layer, does not in any way alter the proper functioning of each of its two major components, namely the resonator, on the one hand (here the resonator 12), and the MEMS component used, on the other hand (here the switch 14), the acoustic waves hardly extending beyond the active piezoelectric surface of the resonator (here the edges of the electrode 20 of the resonator 12) . It is thus possible to place the two components, resonator and switch, very close to each other and to obtain a high compactness of the assembly. Of course, the same piezoelectric layer can be used to integrate a plurality of resonators and a plurality of switches (or other MEMS components).

From a practical point of view, the configuration and the dimensioning of the switch 14 are carried out after the thickness of the piezoelectric film layer 18 has been determined, once the piezo material used and the mechanical resonance frequency of the resonator 14 have been chosen. By modeling, we can then choose or determine the other parameters of the beam (in particular the nature and dimensions of the insulating layer 38, the length of the beam 34, the surface of the control electrodes, the DC voltage, the stroke mobile contact 50, etc.).

Obviously, it is possible, without departing from the scope of the invention, to make modifications in the very design of the device according to the invention described with reference to FIG. 1, for example, to separate the portions 16 and 36 of the piezoelectric layer during deposition or after deposition, to separate the fixed contact 46 from the surface metal layer 24 or to modify or eliminate the acoustic mirror 28 of the substrate in line with the switch 14, etc. It will be noted that the quality requirement required for the manufacture of the resonator 10, in particular for RF use, in particular in terms of the thickness of the deposited layers and / or the uniformity of the deposition of the piezoelectric thin film, also has repercussions. on all the manufacturing operations of the switch 14, consequently improving the performance of the latter. In addition, the devices according to the invention are compatible with the technologies commonly encountered in the electronics of integrated circuits, in particular, CMOS circuits. It is also possible, still within the framework of the invention, to use other thin film resonator structures, for example surface acoustic wave resonators (SAW resonators) using thin films or other BAW resonators in particular. resonators with a so-called FBAR structure with acoustic insulation of the piezoelectric vibrating layer with air, either by the vibrating suspension bridge technique over a substrate with an interposed vacuum obtained by elimination of a sacrificial layer, either by the vibrating membrane technique stretched over a cavity made in a silicon substrate. Likewise, the switch 14 can be replaced by another component.

MEMS, for example by a line switch RF with capacitive coupling / decoupling or by a variable capacity with piezoelectric actuation, in particular, of the type with armatures displaceable relative to each other using a device of actuation integrating all or part of a portion of the common piezoelectric thin film with the resonator, or even by a microphone or a sensor with a deformable membrane.

Finally, an example, still given without limitation, of the method of manufacturing the device according to the invention described with reference to FIG. 1 associating the resonator 10 and the switch 14 is presented below.

After the acoustic mirror 28 has been produced on the silicon substrate 12, the following operations are carried out:

deposition of the metallic layer 24 in platinum followed by structuring by photolithography, in particular for the formation of fixed contacts 46, 48 and the spacing 47, - deposit, in particular above the contacts 46 and 48, of a sacrificial layer (SiO ₂ , amorphous silicon, polysilicon, resin or other) with configuration of the sacrificial layer to delimit the clearance space 52 of the switch 14 under the future beam 34, - deposition on the sacrificial layer of the metal layer forming the movable contact 50 and the metal layer forming the electrode 42 of the actuator (in one step if the metals used are identical or in two steps if the metals used are different),

- deposition and structuring of the insulating layer 38 (SiO ₂ , ZrO ₂ and Si ₃ N) forming the base of the beam 34 until it comes into contact with the metal layer 24,

- deposition, by reactive spraying, a CVD (Chemical Vapor Deposition) type method or another method, of a layer of aluminum nitride (AIN) and structuring of this layer of piezoelectric thin film 18 partly above layer 24 (resonator section 10) and partly above insulation layer 38 (switch section 14),

deposition and structuring of the metal electrode 20 of the resonator and of the control electrode 40 of the switch (in one or two steps if necessary), and

- Removal of the sacrificial layer by any known process suitable for the material thereof, for example, by dry etching (plasma) or wet etching with hydrofluoric acid HF (for SiO ₂ ).

Claims

CLAIMS: 1. Device of the piezoelectric actuation electromechanical microsystem type comprising at least one thin piezoelectric film (18) characterized in that the thin film comprises at least a first portion (36) of piezoelectric nature participating in the actuation of said electromechanical microsystem and at least a second portion (16) of piezoelectric nature integrated in an acoustic wave resonator (10). 2. Device according to claim 1, characterized in that it comprises a switching sub-assembly (14) with piezoelectric actuation in whole or in part of a monomorphic or multimorphic structure. 3. Device according to claim 2, characterized in that said switching sub-assembly (14) comprises at least one deformable element of the beam type (34) and / or bridge and / or membrane and integrating all or part of said first portion (36) of the piezoelectric thin film. 4. Device according to claim 3, characterized in that the switching sub-assembly (14) is of the type with electrical contact (46, 48, 50) and / or with capacitive coupling / decoupling. 5. Device according to claim 1, characterized in that it comprises at least one variable capacity with piezoelectric actuation in whole or part of a monomorphic or multimorphic structure. 6. Device according to claim 5, characterized in that said variable capacity is of the type with actuation by beam or deformable membrane integrating all or part of said first portion of the piezoelectric thin film. 7. Device according to claim 6, characterized in that said variable capacity is of the type with armatures displaceable relative to each other by means of an actuating device integrating all or part of said first portion * piezoelectric thin film. 8. Device according to one of the preceding claims, characterized in that it is used in a system for transmitting, transmitting or receiving RF radiofrequency or microwaves. 9. Device according to one of claims 1 to 8, characterized in that it is used in a sensor or acoustic transducer or other system. 10. Device according to one of claims 1 to 9, characterized in that said second portion (16) of piezoelectric nature is integrated in a resonator (10) with acoustic volume waves. 11. Device according to claim 10, characterized in that said volume acoustic wave resonator is of the type with thin piezoelectric film mounted flat on the substrate (12), preferably on an SMR substrate (12) with acoustic mirror (28) . 12. Device according to one of claims 10 and 11 wherein the thickness of the thin film (18) coated with its electrodes is substantially equal to or equivalent to half a wavelength of the acoustic waves of the mechanical resonance frequency of the resonator. 13. Device according to one of claims 10 to 12 in which the thin piezoelectric film (18) comprises at least one layer of piezoelectric material chosen from the group of AIN, PZT, BaTiO ₃ , KNbO ₃ , PbNbO ₃ , PbZrO ₃ and ZnO. 14. Device according to one of claims 10 to 13 wherein the thin piezoelectric film (18) is deposited by reactive sputtering or by CVD in a single operation. 15. Device according to one of the preceding claims, characterized in that it incorporates a microswitch (14) with piezoelectric actuator configured so as to use the thin film chosen for the resonator (10). 16. Device according to one of the preceding claims comprising at least one piezoelectric resonator (10) for active RF circuits between 1 and 10 GHz. 7. Device according to one of the preceding claims, characterized in that used in association with a microcircuit in particular of the CMOS type.