WO2003088482A1

WO2003088482A1 - Module comprising acoustic wave interface components

Info

Publication number: WO2003088482A1
Application number: PCT/FR2003/000988
Authority: WO
Inventors: Marc Solal; Vincent Laude; Sylvain Ballandras; Serge Camou
Original assignee: Thales
Priority date: 2002-04-12
Filing date: 2003-03-28
Publication date: 2003-10-23
Also published as: FR2838577A1; FR2838577B1; AU2003236876A1

Abstract

The invention relates to acoustic wave interface devices. The subject of the invention is a hybrid module comprising electronic components (F), which are disposed on a base (E), and acoustic wave interface components. Each acoustic wave interface component consists of a substrate (S), an interface area (Z) comprising at least electro-acoustic transduction devices and the part of the base located below the interface area.Said areas can be used to carry out all the functions conventionally attributed to acoustic surface wave components such as delay lines, band filters, resonators, dispersive filters and phase code devices. Said modules can also be used as remote access sensors in order to measure pressure, temperature, acceleration or as remote access coded devices. They can be used for the main mobile telephone standards in order to ensure radiofrequency signal input/output functions.

Description

MODULE COMPRISING INTERFACE ACOUSTIC WAVE COMPONENTS

The field of the invention is that of hybrid modules comprising interface acoustic wave devices produced at the interface of two substrates.

It is known to produce surface acoustic wave devices which use the propagation of waves of wavelength λ on the surface of a piezoelectric substrate. In the general case, the generation and the reception of the waves are ensured by transducers with interdigital combs composed of interlaced electrodes between which a potential difference is imposed. FIG. 1a represents the classic diagram of a device of this type. It comprises a piezoelectric substrate A of thickness ΘA large compared to the acoustic wavelength of use and an interface zone Z in contact with the ambient air. Said zone is generally a laminated structure of thickness e ₂ and which comprises at least the electro-acoustic transduction devices D.

These devices have two main drawbacks:

On the one hand, for the surface waves to propagate correctly on the surface of the interface zone, this surface must remain free. This condition is obtained by encapsulation technologies making it possible to obtain a cavity.

On the other hand, the pitch of the electrodes making up the interdigital combs is often small, of the order of a few hundred nanometers. Also, conductive particles of very small dimensions present inside the housing can short-circuit a transducer and disturb the normal operation of the device. To overcome this drawback, it is necessary either to make the housings of the components hermetic, or to deposit on the transducers a thin layer of insulating dielectric material. This operation, called passivation, removes the sensitivity to conductive particles. However, it does not eliminate the encapsulation operation which is an expensive operation to perform. With regard to conventional electronic circuits, numerous works have aimed to optimize their integration. The integration of passive components (inductors, capacitors and resistors) which are used in the field of radio frequency bands is part of recent technological developments. This work resulted in two different types of technology.

On the one hand, by using multilayer deposits on ceramic type materials, passive elements (inductances, capacitors and resistances) are integrated into the ceramic layers. This technology called LTCC (Low Temperature Cofired Ceramic) allows the integration of a large number of layers. The resolutions accessible to conductors by this technology are of the order of a hundred microns.

Competitively, another technology for integrating passive components (known as Thin Film or IPD for Integrated Passive Device) is based on photolithography of conductors on insulating materials, often of the glass or Gallium Arsenide type. The number of possible layers is lower than for LTCC technology, but the accessible resolution of the conductors, of the order of a few microns, allows greater integration. Also, this technology is currently frequently used.

When we want to integrate surface acoustic wave devices on a module of the LTCC or Thin Film type, used for example for the realization of "Front End modules" for transmission and reception applications of radio sensors , the entire production process therefore requires a large number of operations. The production of such a hybrid module notably includes the following steps:

• Realization of the module base;

• Integration in this base of the various electronic circuits and components;

• Installation of conventional electronic components and any measuring devices on the surface of the base;

• Creation of substrates for surface acoustic components; • Realization of the interface zones including the transduction devices on these substrates;

• Passivation and encapsulation;

• Transfer of the final components to the initial base.

The multiplication of operations necessarily generates additional costs and reduces the reliability of the final device.

It is possible to replace surface acoustic wave devices with interface acoustic wave devices. In this case, the propagation of the acoustic waves on the surface of the substrate is no longer used but at the interface between two substrates. In general, an interface acoustic wave device consists of two substrates denoted A and B of thicknesses ΘA and are large in front of the acoustic wavelength of use, at least one of which is piezoelectric and of an interface zone Z situated between these two substrates as indicated in FIG. 1 b. The acoustic wave propagates mainly in the interface zone and the amplitudes of the acoustic displacements are evanescent in the substrates A and B. The interface zone Z is a structure of thickness e _z which can be either small compared to the length use acoustic wave or of a few wavelengths (typically less than ten times the wavelength), and which includes at least the electro-acoustic transduction devices. In the general case, the interface zone is a laminated structure comprising several layers of dielectric materials. When the interface zone comprises only the electro-acoustic transduction devices, these are necessarily included in one of the two substrates. Electrical interconnections coupled to said devices allow the transmission and transmission of signals.

The use of waves propagating at the interface of two materials naturally makes it possible to obtain a passive component no longer requiring cavity production.

The interface waves can be used to make passive components. In general, any type of device obtained using waves propagating on the surface of a crystal can be produced using interface waves. In particular, it is possible to reflect the waves interface using networks of metal electrodes with a period equal to half a wavelength placed at the interface. A resonator is thus first produced by placing an interdigitated transducer between two reflective arrays and then a filter by coupling resonators together by electrical or acoustic means. The directivity of a transducer is improved by inserting reflectors therein. All the applications of surface wave components are then accessible, in particular delay lines, band filters, resonators, dispersive filters, so-called phase-coded devices whose electrode distribution is as we know associate a particular code with a given component.

Interface wave devices can also be used as remote searchable sensors for measuring pressure, temperature, acceleration or as coded identification devices searchable from a distance. Remote interrogable devices are fully passive components that can be interrogated remotely by radio wave. In general, a phase code is used when the wave is transmitted. The wave is captured by an antenna connected to the input of the component. Conventionally, the transducer converts the signal into a mechanical wave. The wave propagates to the output transducer where it is then reconverted into an electrical signal and re-emitted. The received signal is analyzed and the component which transformed the signal is thus identified.

The two materials do not have to be piezoelectric, allowing the device to be integrated into an electronic module. It then becomes possible to produce hybrid modules comprising conventional components and components with interface acoustic waves.

More specifically, the subject of the invention is a module comprising a base comprising at least one electronic circuit, characterized in that said module comprises:

At least on one of the faces of the base a plane acoustic interface zone interconnected to the electronic circuit, said zone comprising at least one electro-acoustic transduction device, • at least one substrate comprising a plane face opposite said interface zone; each assembly consisting of the base area located under an interface area, said interface area, and said substrate located on said interface area, constituting an interface acoustic wave component.

Advantageously, said substrate is made of piezoelectric material.

Advantageously, the circuit includes passive or active electronic components, said components being able to be either integrated on or inside the base, or attached to said base.

Advantageously, the configuration chosen makes it possible to guide the acoustic energy generated by the transduction device mainly in the interface zone, that is to say that the mode of propagation used by the device is such that the acoustic waves are evanescent in the substrate and in the base. Advantageously, if the module operates around an acoustic wavelength λ, the thicknesses of the base and of the substrate are large in front of said wavelength so that the acoustic waves remain confined inside the module without disturbances possible exterior. Depending on the nature of the materials, the thickness of the interface zone may be:

• low compared to the wavelength so as to slightly disturb the propagation of the acoustic waves in the substrate.

• of the order of a few wavelengths. Advantageously, the area of the base located under the interface area is free of any component over a depth greater than the acoustic wavelength. This avoids any disturbance in the propagation of the acoustic waves inside the base under the interface zone. Advantageously, the interface zone includes layers of materials intended to facilitate the transfer and to improve the operation of the interface acoustic wave component. It is thus possible to improve the guidance of the waves and to reduce the losses by propagation of the acoustic waves. In the case where it is not possible to obtain a propagation of the waves at the interface between the two materials alone, for example if the substrate is not piezoelectric, the presence of one or more intermediate layers having as characteristic a propagation speed of the acoustic waves lower than the propagation speeds in the substrate and the material of the base, makes it possible to obtain guidance and makes it possible to produce the device.

Advantageously, the base material is made of an insulating material so as to electrically isolate the various components. In the case where the material of said base is not electrically insulating, in particular if the latter is a semiconductor material, the interface zone comprises layers of materials intended to electrically insulate the transducers from the base.

The interface zone can be produced: • either on the base before integration of the substrate on said zone,

• either on the substrate before integration of the substrate on the base.

The choice of the best possible process essentially depends on the complexity of the interface area to be produced.

It is important that the contact between the interface zone and the substrate is the best possible so that the propagation of the waves is not disturbed. Also, the planarization of the face of the substrate in contact with the interface zone or the planarization of the interface zone itself is advantageously carried out by polishing. The interface zone transducer is made in particular of aluminum or copper. Advantageously, the connections between the piezoelectric substrate, the interface zone and the base are established by molecular bonding, which ensures the most perfect adhesion possible. These connections can also be made by anodic welding.

Finally, a protective resin can be deposited on the entire module after integration of the substrates and electronic components.

The invention will be better understood and other advantages will appear on reading the description which follows given without limitation and thanks to the appended figures among which: - Figure 1a gives the general configuration shown schematically of a surface wave device according to the known art;

- Figure 1b gives the general configuration shown schematically of an interface wave device also according to the prior art; - Figures 2a and 2b show a sectional view of the module and a sectional view of a detailed part of the complete hybrid module;

- Figures 3a and 3b show two sectional views of part of a module according to the invention before assembly of the piezoelectric substrates S. In the first figure, the interface zone Z is integrated into the substrate S, in the second figure, it is integrated into the base E;

- Figure 4 shows a sectional view of part of a module according to the invention when the face carrying the components and the substrates is embedded in a protective resin.

In general, the production of the electronic module according to the invention may include the following steps:

• Realization of the base;

• Installation of piezoelectric substrates;

• Installation of electronic components.

The base is made of a material T. As indicated in FIG. 2, it comprises on one of its faces an electronic circuit C produced in the conventional manner. This comprises interconnection zones I making it possible to ensure the connection with electronic components F and with the interface zone or zones Z. FIGS. 2a and 2b represent a possible alternative embodiment of the interface zones when those these are integrated into the substrates before being placed on the base. The components F can be located either inside the base or on its surface. These can be passive or active. In a preferred embodiment, the material T is insulating, it comprises on its surface passive elements produced by the "thin film" technology. Active semiconductor or passive components F are transferred to its surface. In another preferred embodiment, the material T is semiconductor and contains integrated circuits. In this case, a layer electrically isolates the transducers from the semiconductor. Interconnection pads allow the module to be connected to the outside. An insulating layer G situated above the circuit C makes it possible to electrically isolate and protect the exterior surface of said circuit from the exterior in the zones not covered by the piezoelectric components or substrates.

Zones without components are arranged under each interface zone so as not to disturb the propagation of the acoustic waves.

There are two possible variants concerning the integration of the interface zone Z. In a first variant, the base comprises only connection pads I as indicated in FIG. 3a. The interface zone comprising in particular the interdigitated transducers is then produced on the piezoelectric material S.

It is possible, in a second variant, to integrate it into the base during the production of said base. Figure 3b illustrates this variant. In this case, the added substrate comprises only the piezoelectric material.

In both cases, the transducers are either integrated in the interface zone, or deposited in an intermediate layer located on said zone. The deposition in an intermediate layer is useful in the case where the material on which the transduction electrodes are deposited is difficult to etch, such as for example lithium tantalate.

The realization of integrated transducers goes through the following stages: • Engraving of the imprint of the transducers on the surface which will be used for the generation of the acoustic waves;

• Deposition of a layer of metal on said engraved surface;

• Planarization of the surface thus metallized so as to obtain a flat surface in which only the imprints remain metallized.

• Possible deposit of one or more additional layers making it possible to obtain a homogeneous bonding surface and / or to improve the guidance of the acoustic waves at the interface. The production of deposited transducers goes through the following stages:

• Deposition of an interlayer on the surface which will be used to generate the acoustic waves; • Engraving of the imprint of the electrodes in this layer;

• Deposit of a layer of metal on this engraved layer;

• Planarization of the metallized surface of the layer so as to obtain a flat surface in which only the imprints remain metallized. • Possible deposit of one or more additional layers to obtain a homogeneous bonding surface and / or to improve guidance.

In the latter case, it is possible to change the order of operations by first depositing the metal of the electrodes, then by etching the electrodes, then depositing the material of the intermediate layer, finally by planarizing the latter. The same end result is obtained. In all cases, the planarization operations can be carried out by polishing and the metal of the transducers is preferably copper or aluminum.

The substrates are placed either by molecular bonding or by anodic welding. In the case where the substrate carries the electrode combs, the connection between the electrode combs and the base is conventionally made at the level of interconnection areas I as indicated in FIG. 3a. This installation is carried out before the possible installation of components F on the surface of the base. It is indeed important to benefit from this installation of a surface which is as flat and clean as possible so as to ensure perfect adhesion and thus correct propagation of the waves. The material of the piezoelectric substrates is in particular either of

Lithium tantalate either lithium Niobate or quartz.

The transfer of any electronic components to the surface of the base is carried out in a conventional manner after the piezoelectric substrates have been placed. In the end, a module M is obtained, optionally comprising conventional electronic components F and components with interface acoustic waves.

To ensure the protection of the components, in particular when used in a polluted environment, a protective resin R can be deposited after integration of the substrates and the electronic components as shown in FIG. 4.

Examples of the realization of modules used for mobile telephony.

There are several major international standards for mobile telephony.

The United States uses the CDMA (Code Division Multiple Access) and WCDMA (Wide Code Division Multiple Access, the equivalent of CDMA on a wide band) standards based on signal multiplexing, a code being assigned to each speaker. A duplexer to separate the signals on transmission and reception is then necessary. Conventionally, a duplexer comprises two transmit and receive filters and a passive circuit making it possible to ensure the paralleling of the two filters. Typically, this passive circuit is an impedance inverter, consisting of a propagation line of a quarter wavelength or of elements with localized constants (an inductance and two capacitors). With technology using interface acoustic wave components, the passive circuit is produced in Thin Film technology on a T material and two piezoelectric substrates are transferred to the base so as to obtain the emission and reception filters. The transducers are made either on the base or on the piezoelectric material. However, their production on the piezoelectric material has several advantages from the manufacturing point of view:

• It is advantageous to make separately the embodiments of the transducers and the passive elements. Indeed, the transducers have small electrode widths and require means heavy technology that is less expensive to use on substrates with a high density of high resolution areas. • On the other hand, the piezoelectric substrates comprising the transducers are, before their bonding, components with surface acoustic waves (of reduced performance compared to the final component with interface waves). It is therefore possible to test them before the final bonding.

In Europe, we mainly use the GSM standard (Global

System for Mobile communication). According to this standard, the signals are time multiplexed (TDMA: Time Division Multiple Access). For GSM standards, there is never simultaneous transmission and reception and, therefore, a switch separates the transmitted and received signals. This function is carried out by a module called 'Front End'. Typically, a Front End module contains a switch, several low-pass filters (with discrete elements) intended to filter the harmonics of the transmitted signal and several band-pass filters often with surface waves. In the context of the invention, the bandpass filters are interface waves and transferred to the base. The low-pass filters are made in the base in Thin Film technology and the switch as well as the other components of the module are transferred to the base.

Claims

1. Module (M) comprising a base (E) comprising at least one electronic circuit (C) characterized in that said module comprises:

At least on one of the faces of the base a plane acoustic interface zone (Z) interconnected to the electronic circuit, said zone comprising at least one electroacoustic transduction device,

• at least one substrate (S) comprising a planar face opposite said interface zone; each assembly consisting of the base area located under an interface area, said interface area, and said substrate located on said interface area, constituting an interface acoustic wave component operating at a length of acoustic wave λ, the thicknesses of the base and the substrate being large compared to said wavelength and the thickness of the interface region being small compared to this wavelength.

2. Module according to claim 1, characterized in that said substrate (S) is made of piezoelectric material.

3. Module according to claims 1 or 2, characterized in that the circuit (C) comprises passive or active electronic components (F), said components can be integrated inside the base and / or located on said base .

4. Module according to claims 1 or 2, operating at an acoustic wavelength λ, characterized in that the thicknesses of the base and the substrate are large compared to said wavelength and the thickness of the area of interface is less than about ten times the acoustic wavelength.

5. Module according to one of claims 1 to 4, characterized in that the area of the base located under the interface area is free of all passive or active electronic component (F) at a depth greater than the acoustic wavelength.

6. Module according to claim 1, characterized in that the transducer of the interface area is made of copper or aluminum.

7. Module according to claim 1, characterized in that the interface zone comprises layers of materials intended to improve the operation of the interface acoustic wave component.

8. Module according to claim 7, characterized in that the interface zone comprise layers of materials intended to improve the guidance or to reduce the losses by propagation of the acoustic waves.

9. Module according to claim 8, characterized in that the interface zone comprises layers of materials intended to ensure the propagation of the acoustic waves when the propagation cannot be ensured directly between the material constituting the base and the substrate.

10. Module according to claim 9, characterized in that the propagation speeds of the acoustic waves in the materials of said layers are lower than the propagation speeds in the substrate (S) and the material (T) of the base.

11. Module according to claim 7, characterized in that the interface zone comprises layers of material insulating the transducers from the base in the case where the material of said base is not electrically insulating, the material of the base being semiconductor or conductive.

12. Method for producing the module according to one of the preceding claims, characterized in that the interface zone is produced on the base before integration of the substrate on said zone.

13. Method for producing the module according to one of the preceding claims, characterized in that the interface zone is produced on the substrate before integration of the substrate on the base.

14. A method of producing the module according to claims 12 or 13, characterized in that the planarization of the face of the substrate in contact with the interface area or the planarization of the interface area itself is carried out by polishing.

15. Method for producing the module according to one of the preceding claims, characterized in that the substrate is assembled to the interface zone by molecular bonding.

16. Method for producing the module according to one of the preceding claims, characterized in that the substrate is assembled to the interface zone by anodic welding.

17. Method for producing the module according to one of the preceding claims, characterized in that the interface zone is assembled to the base by molecular bonding.

18. Method for producing the module according to one of the preceding claims, characterized in that the interface zone is assembled to the base by anodic welding.

19. Module according to one of the preceding claims, characterized in that a protective resin is deposited on the entire module after integration of the substrates and the components.