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    • 专利摘要:
    The invention relates to a surface elastic wave device comprising a stack comprising: a thin film made of a first piezoelectric material; a substrate made of a second material; excitation means for generating at least one acoustic surface wave propagation mode at the level of said piezoelectric film; characterized in that - the first material is a monocrystalline material and the second material is a crystalline material; the first material and the second material having viscoelastic coefficients less than or equal to those of quartz for the propagation mode induced by the excitation means.
    公开号:FR3033462A1
    申请号:FR1551803
    申请日:2015-03-04
    公开日:2016-09-09
    发明作者:Sebastien Grousset；Sylvain Ballandras
    申请人:Commissariat a lEnergie Atomique CEA；Commissariat a lEnergie Atomique et aux Energies Alternatives CEA；
    IPC主号:
    专利说明:

    [0001] FIELD OF THE INVENTION The field of the invention is that of surface-wave-elastic devices and more particularly that of surface-wave-elastic resonators. BACKGROUND OF THE INVENTION Such resonators have high quality coefficients (> 10000) at frequencies of a few hundred MHz to a few GHz. Their use is particularly advantageous in oscillation loops intended for the synthesis of frequencies or stable and compact time references for embedded applications. In general, components using surface waves commonly referred to as SAW components for Surface Acoustic Wave are well known and present in many applications.
    [0002] Among these, mention may be made of signal processing, and in particular band filtering. These components may also constitute radio frequency identification devices (SAW-tags or surface-wave tags). They can also be used to make sensors of different physical sizes, such as temperature or pressure, for example. The SAW devices are generally made by deposition of metal electrodes in the form of a network of interdigitated combs on a piezoelectric material. This generally consists of a monocrystalline substrate such as quartz (SiO2), lithium niobate (LiNbO3), lithium tantalate (LiTaO3) or more recently langasite (La3Ga5Si014). More marginally, they can also be made on thin layers deposited by different routes, such as aluminum nitride (AIN) or zinc oxide (ZnO). Monocrystalline piezoelectric substrates are generally available as 100 mm diameter ("4 inch") wafers oriented at particular angles to optimize surface wave properties. For quartz, the most common directions are AT, ST, SC, BT etc. The sections thus named correspond to reticular planes defined by angular rotations around the X, Y, Z axes making it possible to describe the state of the plate with respect to the IEEE Standard Std-176, version 1949 (Std-176 IRE). In this standard, the cut AT corresponds to a plate called YX 3033462 2 (the Y axis is orthogonal to the plane of the plate - collinear to its thickness - and the X axis is defined along the length of the plate) turned 36 ° around the X axis. It is designated by the nomenclature (YXI) / 35,25 °. The section ST corresponds to the nomenclature (YXI) / 42.75 °, the section BT to (YXI) / -49 °, as to the section SC, it proceeds from two successive rotations, one of 21.9 ° around the Z axis and the second 34 ° around the X axis according to the nomenclature (YXwl) / 21.9 ° / 34 °. Each section corresponds to particular elastic propagation conditions which directly impact the characteristics of the devices.
    [0003] In particular, the choice of a substrate and its crystalline orientation for producing a SAW device is dictated by a certain number of criteria, the following two being the main ones considered for filtering applications: the electromechanical coupling coefficient ( 0% <k2 <100%), which represents the efficiency of conversion of electrical energy into acoustic energy and vice versa and which characterizes the maximum relative band that is known to perform for given losses; The temperature coefficient of the frequency (CTF expressed in ppm / K), which reflects the thermal stability of the resonator. For frequency source resonator applications, the following are also taken into account: the energy flux angle, which represents the difference between the propagation direction of the surface wave (wave vector) and the direction of propagation of the energy of this wave (Poynting vector) (William STEICHEN, Sylvain BALLANDRAS: Acoustic components used for filtering: Review of different technologies, Engineering techniques, E 2000 - 1- 30 28), it is preferably zero to transmit a maximum of surface energy to the output transducer and thus avoid shifting the transducers to catch up with this energy flow angle and maximize the operation of the transducers, but this deviation is potentially geometrically compensable; - the unique "obstacle" reflection coefficient of the electrode type, etched groove or any elementary structure whose characteristic period corresponds to the so-called Bragg condition, ie a half-wavelength which conditions the efficiency of the 5 networks; Bragg for the confinement of acoustoelectric energy in the plane, this coefficient plays an important role in the optimization of the quality coefficient of the SAW resonators; the speed of propagation which essentially conditions the dimensions of the resonator and the manufacturing capacity at a given frequency; the directivity of the transducer that one seeks to minimize or maximize according to whether one desires a pure mode or a natural directional effect (in particular useful for certain filters).
    [0004] Lithium tantalate and lithium niobate are generally used in the manufacture of SAW filters in the Radio Frequency (RF) range of relative bands of between 1 and 10% and for which electromechanical coupling greater than 1% is desirable. . Nevertheless, these substrates suffer from too much temperature drift for use as a frequency source. In contrast, quartz exhibits frequency stability with remarkable temperature and electromechanical coupling suitable for resonator-type narrow-band applications. It is very commonly used for this type of application and the frequency-stabilized oscillators with a surface elastic wave resonator on this type of substrate have the best compromise phase noise / short-term stability / compactness for embedded applications, and more particularly for frequency synthesis for radar detection. However, these oscillators could make it possible to reach detection levels higher than those obtained with those of the state of the art by gaining on the quality coefficients that are currently accessible. For information, the quality coefficient - frequency coefficient, noted Qf, achievable maximum in SAW with quartz alone is of the order of 1013, which corresponds to a quality coefficient of 20 000 to 500 MHz.
    [0005] In this context, a substrate capable of providing both a suitable electromechanical coupling, temperature characteristics allowing a frequency temperature coefficient (CTF) close to 0 or allowing temperature-temperature inversion temperatures. 5 positive and an improved quality coefficient compared to the previous values is of interest to improve the frequency sources and potentially extend the applications of SAW devices to new opportunities. US Patent 6933810 and the publication "Temperature Compensated LiTa03 / Sapphire Bonded SAW Substrate with Low Loss and High Coupling Factor Suitable for US-PCS Application" by M. Miura et al. published in 2004 in the proceedings of the IEEE Ultrasonics Symposium describe a substrate, for SAW type applications, made from an assembly of a lithium tantalate layer on a sapphire substrate that can be defined as a substrate hybrid. However, this type of device is in no way based on the study of elastic wave propagation modes in such a stack and has major drawbacks, namely: a limitation of the thermal expansion of tantalate by bonding on a sapphire substrate, a material with a low coefficient of thermal expansion (TEC for Thermal Expansion Coefficient) and a high Young's modulus (material with high elastic rigidity), which leads to the appearance of significant thermal stresses on a large surface of the substrate LiTaO3 / sapphire hybrid; Volume-wave reflections at the bonding interface which lead to the presence of spurious responses on the frequency characterisations of the SAW resonator; a strong temperature drift of the series resonance frequency (-15 ppm / K) and parallel resonance (-27 ppm / K) of the SAW resonators using this type of substrate. It has also been proposed in the publication: "High-Frequency Surface Acoustic Waves Excited on Thin-Oriented LiNbO3 Single-Crystal Layers Transferred Onto Silicon" by T. Pastureaud et al. published in 2007 IEEE Transactions on Ultrasonics, a structure in which a high coupling lithium niobate piezoelectric layer is reported by ion implantation / fracture on a silicon substrate having a high acoustic velocity for certain propagation modes. This approach makes it possible, in particular, to produce broadband filters operating at frequencies well in excess of GHz.
    [0006] However, such a configuration also has a certain number of disadvantages, for example: a very strong impact of the conductivity of the silicon substrate on the characteristics (electromechanical coupling, losses) of the shear mode; A limitation of the operation of the devices thus made, in particular because of the limit on the maximum thickness achievable with the implantation / fracture technology employed; too much temperature drift of the resonance frequency (down to -50 ppm / K) of the devices made for use in a "source" application. In this context, it therefore appears difficult to find monocrystalline piezoelectric materials offering a compromise between high intrinsic quality coefficient (higher than that of quartz, comparable to that of tantalate and lithium niobate) and stability of the frequency with temperature. . Moreover, very rarely resonators on materials with strong electromechanical coupling (LiTaO3, LiNbO3) simultaneously exhibiting coefficients of record quality (Qf »1013).
    [0007] In order to overcome the aforementioned drawbacks, the object of the present invention is a solution for obtaining excited surface wave devices which are guided and detected via a piezoelectric film carried on a substrate of high acoustic quality enabling propagation of waves. elastics without propagation losses and minimizing the phenomena of viscoelastic absorption. More specifically, the subject of the invention is a device with surface elastic waves, in particular for a resonator or filter, in particular with narrow transition bands of the order of a few%, comprising a stack comprising at least: a thin film produced in a first piezoelectric material; A substrate made of a second material; excitation means for generating at least one mode of propagation of surface elastic waves at said piezoelectric film; Characterized in that: - the first material is a monocrystalline material and the second material is a crystalline material; the first material and the second material have viscoelastic coefficients less than or equal to those of quartz for the propagation mode induced by the excitation means. In the present invention, the thin film assembly is defined as a first piezoelectric material on a second material substrate, such as a hybrid substrate.
    [0008] According to a variant of the invention, the second material is a monocrystalline material. The substrate may itself be on any support. According to a variant of the invention, the device comprises a first film intermediate between the substrate and the piezoelectric film, ensuring the bonding of said film on said substrate. According to a variant of the invention, the device comprises a second intermediate film of metallic material, located between the substrate and the piezoelectric film, the nature of the film and especially the thickness make it possible to adjust the temperature coefficient of the frequency of the propagation mode. According to a variant of the invention, the first material is quartz. According to a variant of the invention, the first material is one of the following materials: doped GaPO4 or SiO2, or langasite (LGS-30 La3Ga5Si014), langatate (LGT-La3Ga5.5Ta0.5014), langanite (LGN - La3Ga5.5N1D0.5014)), or Sr3NbGa3Si2O14 (SNGS), or Ca3NbGa3Si2014 (CNGS), or Ca3TaGa3Si2O14 (CTGS), or Sr3TaGa3Si2O14 (STGS) or Ca3TaAl3Si2O14 (CTAS)).
    [0009] According to a variant of the invention, the second material is sapphire. Advantageously, its crystalline orientation may be of the C-plane, or R-plane, or M-plane or A-plane type. Type C corresponds to a plate with Z axis or optical axis C 5 normal to the surface also defined with Miller indices (001) defined as a section (ZX) or (YXI) / 90 ° in IEEE standard Std- 176, the type R with a Y cut rotated 33 ° around the X axis correq.Donding to the indices (102) noted (YX /) / 33 ° according to the Std-176 IEEE standard), the type M to a plate section (YX) corresponding to indices (010) denoted (YX) in Std-176, type A corresponds to an orientation (120). According to a variant of the invention, the second material is lithium niobate or lithium tantalate. According to a variant of the invention, the lithium niobate substrate has a section chosen from the following: cutting (YX), cutting (YX /) / ± 41 °, cutting (YX /) / + 64 °, cutting ( YX /) / + 128 ° with a tolerance of ± 5 ° around these crystalline orientations, defined according to the IEEE Std-176 standard. According to a variant of the invention, the lithium tantalate substrate has a section selected from the following: cutting (XY), cutting (YX /) / + 36 °, cutting (YX /) / + 42 °, cutting (YX /) / ± 0 with 0 between 30 ° and 50 °. According to a variant of the invention, the thin film has a thickness of between about 1 μm and about 20 μm. The invention also relates to a filter or a resonator comprising a device according to the invention.
    [0010] The invention further relates to a method for manufacturing an elastic wave device according to the invention, characterized in that it comprises: - the transfer of a first substrate comprising said first piezoelectric material to a second substrate comprising Said second material; an operation of thinning said first substrate to define the film of first piezoelectric material; the realization of means for exciting said first piezoelectric material on said film of first material.
    [0011] According to one variant of the invention, the transfer step comprises a direct hydrophilic bonding step by means of a dielectric layer that may be made of SiO 2 deposited on the first and / or second substrate (s). .
    [0012] According to a variant of the invention, the transfer step comprises a step of bonding by direct hydrophilic bonding or thermocompression-assisted bonding, by means of a metal layer, which may be gold, deposited on the first and / or second substrate (s).
    [0013] The invention will be better understood and other advantages will become apparent on reading the following description, which is given in a nonlimiting manner and by virtue of the appended figures in which: FIGS. 1a and 1b illustrate the cutting angles defined according to IEEE 49 (Std-176 revision 1949) and the direction of elastic wave propagation; FIGS. 2a to 2f illustrate the steps of the method of realization according to the invention of a device SAW on a hybrid substrate obtained by gluing / thinning according to the known art; FIG. 3 schematizes a constituent stack of a SAW device 20 according to the invention; FIG. 4 illustrates the dispersion of the shear mode (STW waves) in the case of a thin quartz layer (YXIt) / 37 ° ± 5 ° / 90 Ω 2 ° (in IEEE notation std-176 revision 1949) on sapphire substrate (C-plane) as a function of the product (frequency) (quartz thickness) expressed in GHz.pm (equivalent to km.s-1); FIG. 5 illustrates the evolution of the coupling K2 (%) and the coefficient CTF1 (ppm / K) as a function of the quartz thickness (pm) in the case of a thin layer of quartz (YX / t) / 37 ° ± 5 ° / 90 ° ± 2 ° (in IEEE Std-176 revision 1949 notation) on sapphire (C-plane) substrate as a function of quartz film thickness (μm); FIG. 6 illustrates the evolution of coupling K2 (%) and losses (dB / λ) as a function of quartz thickness (μm) in the case of a thin layer of quartz (YXIt) / 37 ° ± 5 ° / 90 ° ± 2 ° (in IEEE notation std-176 revision 1949) on sapphire substrate (C-plane); FIG. 7 illustrates the dispersion of the shear mode (STW waves) in the case of a thin layer of quartz (YX / t) / 37 ° ± 5 ° / 90 ° ± 2 ° (in IEEE std-176 notation). revision 1949) on a sapphire (C-plane) substrate comprising a SiO 2 layer at the 200 nm thick quartz / sapphire interface; FIG. 8 illustrates the evolution of the coupling coefficient K 2 (%) and the coefficient CTF 1 (ppm / K) as a function of quartz thickness (μm) in the case of a thin layer of quartz (YX / t) / 37 ° ± 5 ° / 90 ° ± 2 ° (in IEEE notation std-176 revision 1949) on a sapphire (C-plane) substrate comprising a SiO 2 layer at the 200 nm quartz / sapphire interface thickness ; FIG. 9 illustrates the dispersion of the shear mode (STW waves) in the case of a quartz (YX / t) / 37 ° ± 5 ° / 90 ° ± 2 ° thin film (in IEEE std-176 notation). revision 1949) on a sapphire (C-plane) substrate comprising a gold layer at the quartz / sapphire interface of 200 nm thick; FIG. 10 illustrates the evolution of the coupling K2 (%) and the coefficient CTF1 (ppm / K) as a function of the quartz thickness (pm) in the case of a thin layer of quartz (YX / t) / 37 ° ± 5 ° / 90 ° ± 2 ° (in IEEE notation std-176 revision 1949) on a sapphire (C-plane) substrate comprising a gold layer at the 200 nm thick quartz / sapphire interface .
    [0014] In general, the subject of the present invention is a surface-wave device having an innovative stack for producing a so-called "hybrid" substrate enabling the production of SAW technology devices on said hybrid substrate, comprising at least two elements. elements: a piezoelectric film and a high acoustic quality substrate. This type of hybrid substrate makes it possible to operate at frequencies of between about one hundred MHz and a few GHz without major implementation difficulties and can be optimized for a particular application by seeking to maximize the quality coefficients, favoring a coupling coefficient between 0.1 and 1% and having an effective insensitivity to temperature effects (ie allowing a compensation of the thermoelastic effects within the device to the desired working temperature, which can be classically in the range 20/120 ° VS).
    [0015] The devices in SAW technology, thus produced on such a hybrid substrate, can advantageously be of the resonator type. The judicious choice of the angular configuration, the polarization of the wave and the metallization also makes it possible to access structures of reduced size compared to the solutions of the state of the art.
    [0016] The material of the piezoelectric film may advantageously be quartz in view of the existence of compensated crystalline orientations of the temperature effects for Rayleigh waves and surface shear (STW) or having a slightly positive thermal drift for these types. waveform (typically between 0 and 10 ppm.K-1).
    [0017] The substrate may advantageously consist of monocrystalline sapphire because of the low viscoelastic losses of this type of substrate which generally lead to obtaining very high quality coefficients (Qf = - .. 1014). The surface elastic waves are excited and detected on the piezoelectric film by InterDigitated Transducer (IDD) transducers, made at the top surface of the structure by conventional photolithography, deposition and machining technologies. (of type lift-off for example) of metal layers (usually aluminum).
    [0018] It should be noted that advantageous characteristics of the resonators can be obtained on this type of substrate, for a certain range of thicknesses and for a range of specific crystalline orientations of the quartz piezoelectric film. Similarly, the sapphire substrate is also selected with a particular crystalline orientation. The crystalline sections are defined in the present invention according to the IEEE 49 standard (Std-176, revision 1949, in particular for a coherent definition of the signs of rotation angles of the quartz compared to the practice in force for the man of the art). More specifically, a crystal section is defined by two angles of rotation. The angle (I) defines the rotation 3033462 11 about the optical axis Z and the angle 0 the rotation about the X axis. FIGS. 1 a and 1 b illustrate how these angles are defined in the IEEE 1949 standards. For the surface waves, a third angle defines the direction of propagation of the wave, the angle noted ip, about the axis normal thereto, denoted Y "(the initial Y axis having undergone the two previous rotations (I) and 0. Also called: - single rotation cut any cut such that 1) = 0 and 0 # 0 - double rotation cut any cut such that ca # 0 and 0 # 0 10 A cut is defined in the IEEE Std-176 standard by the axes carried by the thickness and the length of the plate (example Figure 1a, cut YX) and the angles of rotation around the axes carrying the width (w), the length (I) and the thickness of the plate (t), w as "width" represents the width of the plate and designates the Z axis, I as "length" the X axis and of course the Y axis , these xes being understood by principle after rotation, that is to say that for a section with three rotations is carried out first rotation (I) about the Z axis (w) and then the rotation 0 along the X axis turned noted X '(I) and finally the rotation lp around the Y axis shot noted Y "(t) which will therefore undergo the first two rotations.
    [0019] In general, combinations of materials with low viscoelastic losses can be comprehensively identified (sapphire, SiC silicon carbide, FZ-zone silicon, monocrystalline or nano-crystalline diamond carbon, garnets-type garnet-type wafers). YAG and YAG aluminum, YIG iron and iron garnet, etc. - niobate and lithium tantalate), the cuts of these materials being chosen so that the phase velocity of the waves propagating on their surface exceeds that of the waves propagating on the quartz sections enumerated above, ie at least 3300 ms-1 for Rayleigh waves and 5100 ms-1 for transverse surface waves (STW).
    [0020] In the present invention, a material is considered to have relatively low viscoelastic losses and when its viscoelastic constants are less than or equal to those of quartz indicated below.
    [0021] 3033462 12 1111 1133 1112 1113 1144 1166 1114 Quartz 1.37 (0) 0.96 (9) 0.73 (0) 0.71 (5) 0.36 (4) 0.302 0.01 (2) Table 1.1 Quartz viscoelastic friction coefficients The figures in parentheses represent the decimal point for which the measurement uncertainty does not make it possible to freeze the value definitively, it is given for information only and makes it possible to fix an idea of the degree precision of these constants. This table gives the coefficients of viscoelastic friction equivalent (viscoelastic friction in the sense of the standard modeling of the mechanics of continuous media, it is an analogy with acoustics in fluids for which we speak of absolute viscosity) for quartz (r) ii in Ns / m2) measured at 450 MHz in waves of volume. Data are from Lamb & Richter: J. Lamb and J. Richter, "Anisotropic Acoustic Attenuation With New Measurements for Quartz at Room Temperature," Proc. R. Soc. London, Ser. A293, pp. 479492, 1966, for quartz. These constants give rise to a coefficient of quality product per frequency Qf of 1013 max for Rayleigh waves: T.E. Parker, J. A. Greer, "SAW Oscillators with Low Vibration Sensitivity", Proc. of the 45th CBSA, 1991, pp.321-329 and D. Andres, T. E. Parker, "Designing smaller SAW oscillators for low vibration sensitivity", Proc. of the IEEE IFCS, 1994, pp.352-358 and 1.5 x 1013 max for quartz-guided surface shear waves: Avramov, ID, "Low voltage, high performance, GHz range STW clocks with BAW crystal stability , "Frequency Control Symposium and 25 Exposition, 2005. Proceedings of the 2005 IEEE International, vol., No., Pp.880,885, 29-31 Aug. 2005, doi: 10.1109 / FREQ.2005.1574049. Concerning the sapphire orientations, the orientations of interest can be of C-plane, R-plane, M-plane and A-plane type. Among the orientations of interest of lithium niobate (LiNbO3) and of lithium tantalate (LiTaO3), the following sections can be mentioned: 3033462 13 Cut crystal - LiNbO3 propagation Y - ZXXX Y + 41 ° - Y + 64 ° - Y + 128 ° - LiTaO3 X - Y + 112 ° Y + 36 ° - X Y + 42 ° - X r + e X - (30 ° <e <scn Elliptical waves (Rayleigh wave type) and shear waves must be taken into account and a method of predicting the quartz thickness and its crystalline orientation for a given CTF coefficient, must be added.The quartz may be replaced by one of its isomorphs (GaPO4, doped Si02, etc.) or by a material of the same crystalline class (langasite (LGS - La3Ga5Si014), langatate (LGT - La3Ga5,5Tao, 5014), langanite (LGN - 10 La3Ga5,5Nb0,5014), and generally any material of this same family without omitting the new complex materials recently proposed by FOMOS such as Sr3NbGa3Si2O14 (SNGS), Ca3NbGa3Si2O14 (CNGS), Ca3TaGa3Si2014 (CTGS), Sr3TaGa 3Si2014 (STGS) and Ca3TaAl3Si2014 (CTAS)).
    [0022] The structure of this hybrid substrate can be optimized to increase the quality factor and the coupling coefficient of the resonator while minimizing or even compensating for the effects of temperature on the resonator's natural frequency by digital simulation by varying the various parameters, in particular the nature of the materials, the crystalline orientation of these materials, the thickness of the first material film. In general, the device of the present invention thus consists in the stacking of two crystalline materials, one of which is at least one monocrystalline, requiring suitable manufacturing methods for producing the hybrid substrate. Advantageously, the processes used use the technique of transfer by gluing / thinning to maintain the monocrystalline character of a chosen initial substrate. The main steps of this type of process are illustrated by FIGS. 2a to 2f. A first step illustrated in FIG. 2a shows the use of two substrates: a first substrate 10 made of a first monocrystalline piezoelectric material 10 and a second substrate 20 made of a second crystalline material covered with a bonding layer 30, with preparation surfaces to be facing. According to a second step, illustrated in FIG. 2b, the two substrates are bonded directly via the bonding layer 30.
    [0023] According to a third step illustrated in FIG. 2c, a step of thinning of the piezoelectric single crystal substrate 10 is carried out in order to define a piezoelectric monocrystalline thin film whose thickness is between 1 and 20 μm as a function of the working frequency (between 100 MHz and 3 GHz) and target operating points in terms of electromechanical coupling coefficients and CTF frequency temperature, by a mechanical honing operation, followed by a polishing step. According to a fourth step illustrated in FIG. 2d, a layer of resin 40 is deposited, exposed to its insolation and then developed 25 in order to define a resin mask. According to a fifth step illustrated in FIG. 2e, a metal layer 50 is deposited for the purpose of producing the excitation means in the form of electrodes. According to a sixth step illustrated in FIG. 2f, the resin mask is removed, making it possible to obtain electrodes Ei on the surface of the monocrystalline piezoelectric material film, on the surface of the crystalline substrate. FIG. 3 illustrates an exemplary configuration of electrodes made on the surface of the hybrid substrate, showing two sets of interdigitated electrodes Ei1 and Ei2 (for example the first set of electrodes 3033462 comprises an electrode comb Ei11 and a comb of electrodes Ei12 interdigitated). The glue / thinning technology provides easy access to the targetable quartz thickness range of the present invention (1 μm <20 μm). A more detailed description of the gluing / thinning approach is described in the publication by Grousset, S .; Augendre, E .; Benaissa, L .; Signamarcheix, T .; Baron, T .; Courjon, E .; Ballandras, S., "SAW pressure sensor based on single-crystal quartz layer 10 transferred on Silicon," European Frequency and Time Forum and International Frequency Control Symposium (EFTF / IFC), 2013 Joint, vol., No., Pp.980,983 As a practical matter, it is possible to transfer the quartz substrate to the sapphire host substrate through a silicon oxide layer, SiO 2 of about 100 nm, located at the collage interface. This variant is produced by direct hydrophilic bonding. It is also possible to transfer the quartz substrate onto the sapphire host substrate via a gold layer of thickness between 40 nm and 500 nm by metal bonding (direct or assisted by thermo-compression). . The method described above thus makes it possible to define the following stack: a quartz piezoelectric layer obtained by mechanical thinning and mechano-chemical polishing of the solid monocrystalline quartz substrate; - a bonding layer compatible with direct bonding or metal bonding; a substrate made of a material with a high acoustic quality is included in all materials such as sapphire, LiNbO 3, LiTaO 3, YAG, which are known for their viscoelastic coefficients 5 to 10 times lower than those of quartz (see Table 1.1) Advantageously, the structure may be composed of a SiO 2 or gold-type bonding layer which modifies only very little the characteristics of the dispersion curves obtained. The bonding thickness is preferably less than one tenth of the wavelength. In the case of metal bonding, a passivation layer may advantageously be deposited on the wafer to be thinned on the bonding side in order to minimize the impact of the gold layer on the final properties of the stack.
    [0024] It should be noted that this thickness may be neglected in simulation operations (such as those described below) when it is less than one-hundredth of the wavelength. It is taken into account to correct substantially the optimal thicknesses in the opposite case. As a reminder, the wavelength is equal to the ratio of the speed of the elastic wave to the frequency of this wave: f = In order to optimize the electromechanical coupling coefficient and to reduce the losses associated with the wave propagation Thus, the invention is based on the use of elastic waveguides by producing a hybrid substrate. Applicants have performed simulations on various hybrid substrates of interest for the present invention.
    [0025] First example of hybrid substrate: The substrate consists of a quartz piezoelectric thin film on a monocrystalline sapphire substrate of several tens of acoustic wavelengths (greater than 30 wavelengths ideally for transverse surface waves STW at 15 wavelengths for elliptic polarization waves). In the case where the mode of propagation of the surface elastic waves generated in the film / substrate assembly occurs at speeds lower than the volume waves (of the same polarization) of the substrate alone, the energy can not be radiated towards the core of the substrate. the structure is guided by the substrate in the thin film. For an elliptically polarized wave, we speak of an inhomogeneous wave to describe the exponential attenuation modulated by a sinusoidal function of the wave in the substrate, whereas it is question of evanescent wave for the shear waves. In the first case, the wave is considered almost attenuated after two wavelengths while penetration of the shear waves can extend over several wavelengths. In both cases, propagating waves without radiation losses in the piezoelectric film are obtained under these conditions and much is said of guided elastic waves. FIG. 4 represents the results of numerical simulations of the behavior of transverse surface waves STW for a structure composed of a quartz film carried on a sapphire substrate. It may be noted that for frequency-thickness products greater than 2.5 GHz.pm, the mode is less and less sensitive to the presence of the substrate and behaves as on a quartz considered semi-final. The curve C41 is relative to the electromechanical coupling coefficient K2, the curve C42 relates to the propagation speed for a so-called free surface, that is to say without electrode deposition or any element that can hinder the propagation. , curve C43 is relative to the propagation speed for a fully metallized surface (without taking into account the mass effect induced by such a metallization). Conversely, for film thickness frequencies below 2.5 GHz.pm, the mode is very sensitive to the presence of the sapphire substrate.
    [0026] In particular, an electromechanical coupling maximum K2 of 0.3% corresponding to a quartz frequency-thickness product of about 2 GHz.pm appears. In practice, for a SAW resonator operating at 500 MHz this equates to an optimal quartz thickness of 4pm.
    [0027] This particularity makes it possible in particular to optimize the frequency of use of the device as well as the thickness of the quartz film in order to improve the electromechanical coupling with respect to that of the layer or of the substrate taken individually (in the case where the substrate would be piezoelectric).
    [0028] Figure 5 shows the coupling K2 of the shear mode (curve C51) as well as the temperature coefficient of the frequency (first order) (curve C52) as a function of the quartz thickness in the stack. These results show that for a quartz thickness of 4 .mu.m, corresponding to the maximum coupling that can be obtained, a relatively low temperature sensitivity (9 ppm / K) is obtained, close to the temperature compensation but above all allowing a frequency-temperature inversion temperature greater than 25 ° C, key parameter for "source" applications for which the oscillator is often thermostated at temperatures exceeding its operating thermal range 5 (-40 / + 85 ° VS). Remarkably, a plateau of CTF1 at 10 ppm / K appears for quartz thicknesses of up to 11 pm while maintaining a K2 coupling sufficient for the target applications, resonator type. Finally, FIG. 6 represents the evolution of the coupling K2 of the shear mode (curve C61) as well as the propagation losses, expressed in dB / λ (Curve C62). It is thus verified that for the optimum thickness (4 μm) of the quartz film corresponding to the maximum coupling as well as for greater thicknesses between 3 μm and 11 μm, corresponding to the level of the CTF1 observed, the propagation losses of the mode shears selected are negligible (well below 10-3 dB / Å). It is noted that the offset observed on all the curves shown corresponds to the propagation speed of the mode equal to that of the slow volume wave radiated by the surface, also called SSBW wave. Beyond this speed (of the order of 5800 m.s -1 for sapphire), the substrate no longer guides the wave in the piezoelectric layer. The innovative stacks constituting this hybrid substrate are made by direct bonding of the piezoelectric substrate to the substrate of high acoustic quality. Subsequently, the final thickness of 4 μm (or the thickness range of 3 to 11 μm) of the quartz film is achieved by mechanical thinning and then chemical mechanical polishing. The single-port surface wave resonators, which can be produced on this type of substrate, consist of a central transduction zone (with inter-digitized combs) acting as a resonant cavity and surrounded by two reflectors (mirrors). The transducer consists of an alternation of electrodes, which are repeated with a certain periodicity, called the metallization period, deposited on the hybrid substrate. Advantageously aluminum electrodes (which can be made by photolithography and so-called "lift-off" technique) have a thickness ranging from a few hundred angstroms typically to micron.
    [0029] Thus, the choice of materials, their associations, their crystalline orientations, and the layer thickness offer a wide range of possibilities for optimizing the coupling coefficient and the temperature sensitivity of the propagation modes adapted to certain types of applications. .
    [0030] Second Example Hybrid Substrate: In one variant of the invention, the hybrid substrate structure may comprise a silica (SiO 2) layer or a gold layer at the interface between the quartz piezoelectric film and the high substrate. acoustical grade in sapphire. Indeed, in order to facilitate the transfer and for the purpose of monolithic integration of the quartz film on the sapphire substrate, it may be necessary to envisage the use of a SiO 2 or gold type bonding layer. This layer may advantageously be deposited, before bonding, onto the piezoelectric substrate or onto the high acoustic quality substrate or onto both substrates. In the case of the use of a SiO 2 layer, the two substrates are assembled by direct bonding at ambient temperature and atmospheric pressure. Bonding is made possible by an adequate surface preparation (chemical cleaning, chemical mechanical polishing, plasma activation) allowing direct bonding. It can be seen from Figs. 7 and 8 that the characteristics of the shear mode are almost unaffected by the presence of a SiO 2 layer of about 200 nm between the quartz piezoelectric film and the sapphire substrate. .
    [0031] The curve C71 relates to the evolution of the electromechanical coupling coefficient K2, the curve C72 relates to the propagation speed for a so-called free surface, the curve C73 relates to the propagation speed for a metallized surface, depending on the the thickness of the Quartz film.
    [0032] The curve C81 relates to the evolution of the evolution of the electromechanical coupling coefficient K2, the curve C82 relates to the temperature coefficient of the frequency (in the first order), as a function of the thickness of the film of Quartz.
    [0033] Third Example Hybrid Substrate: The use of a gold bonding layer is also possible. Gold is a particularly interesting material for this type of bonding, given its plastic properties and its sufficient mechanical strength to provide an acoustic connection between the quartz piezoelectric film and the high-quality acoustic sapphire substrate. Bonding between the two substrates can advantageously be achieved by thermocompression. From FIGS. 9 and 10, the mode characteristics are slightly less favorable in terms of coupling than in the case of a SiO 2 layer but are more interesting in terms of temperature sensitivity with compensation for temperature effects. (CTF = 0 ppm / K) obtained at the maximum of coupling K2 (;: e 0.23%) for a quartz thickness of about 3.5 μm. Overall, this approach is also favorable to the production of resonators on this type of substrate.
    [0034] The curve C91 relates to the electromechanical coupling coefficient K2, the curve C92 relates to the propagation speed for a so-called free surface, the curve C93 relates to the propagation speed for a metallized surface, depending on the thickness. quartz film.
    [0035] The curve C101 relates to the evolution of the evolution of the electromechanical coupling coefficient K2, the curve C102 is relative to the temperature coefficient of the frequency (first order), as a function of the thickness of the quartz film. .
    [0036] Thus, the use of resonators made on this type of hybrid substrate can advantageously be envisaged in oscillation loops intended for the synthesis of frequencies or ultra-stable time references. This type of hybrid substrate is generally advantageous for narrowband filters. 30
    权利要求:
    Claims (16)
    [0001]
    REVENDICATIONS1. A surface elastic wave device comprising a stack comprising at least: a thin film made of a first piezoelectric material; a substrate made of a second material; excitation means for generating at least one surface acoustic wave propagation mode at said piezoelectric film; characterized in that - the first material is a monocrystalline material and the second material is a crystalline material; the first material and the second material having viscoelastic coefficients less than or equal to those of quartz for the excitation mode induced by the excitation means.
    [0002]
    Surface-elastic-wave device according to claim 1, characterized in that the second material is a monocrystalline material.
    [0003]
    3. surface elastic wave device according to one of claims 1 or 2, characterized in that it comprises a first intermediate film between the substrate and the piezoelectric film, ensuring the bonding 25 of said film on said substrate.
    [0004]
    4. Surface-elastic wave device according to one of claims 1 to 3, characterized in that it comprises a second intermediate film of metallic material, situated between the substrate and the piezoelectric film, the thickness and nature of which of the film allow to adjust the temperature coefficient of the propagation mode frequency
    [0005]
    Surface-elastic wave device according to one of claims 1 to 4, characterized in that the first material is quartz. 3033462 22
    [0006]
    Surface-elastic-wave device according to one of Claims 1 to 4, characterized in that the first material is one of the following materials: doped GaPO4 or SiO2, or langasite (LGS - La3Ga5S10.14), of Langatate (LGT - La3Ga5.5Ta0.5014), Langanite (LGN - La Ga NID 0 11 or Sr N'In Si n (smr: ^ "_ _ _ _ _ 5.5, __ 0.5 - 14, Ca3NbGa3Si2014 (CNGS), or Ca3TaGa3Si2014 (CTGS), or Sr3TaGa3Si2014 (STGS) or Ca3TaAl3Si2014 (CTAS)).
    [0007]
    Surface-elastic wave device according to one of Claims 1 to 6, characterized in that the second material is sapphire.
    [0008]
    Surface-elastic-wave device according to claim 7, characterized in that the second material has a crystalline orientation of the C-plane, or R-plane, or M-plane or A-plane type. 15
    [0009]
    Surface-elastic wave device according to one of claims 1 to 5, characterized in that the second material is lithium niobate or lithium tantalate. 20
    [0010]
    Surface-elastic-wave device according to claim 9, characterized in that: the lithium niobate substrate has a section selected from the following: cutting (YX), cutting (YX /) / + 41 °, cutting (YX) /) / + 64 °, cut (YX /) / + 128 ° with a tolerance of ± 5 ° or; The lithium tantalate substrate has a section selected from the following: cutting (XY), cutting (YX /) / + 36 °, cutting (YX /) / + 42 °, cutting (YX /) / + 8 with 8 between 30 ° and 50 °.
    [0011]
    Surface-elastic-wave device according to one of the preceding claims, characterized in that the thin film has a thickness of between about 1 μm and about 20 μm.
    [0012]
    12. Resonator comprising a device according to one of the preceding claims. 35 3033462 23
    [0013]
    13. Frequency filter comprising a device according to one of claims 1 to 11. 5
    [0014]
    14. A method of manufacturing an elastic wave device according to one of claims 1 to 11, characterized in that it comprises: the transfer of a first substrate comprising said first piezoelectric material on a second substrate comprising said second material ; an operation of thinning said first substrate to define the film of first piezoelectric material; the realization of excitation means of said first piezoelectric material on said first material film.
    [0015]
    15. A method of manufacturing a surface elastic wave device according to claim 14, characterized in that the transfer step comprises a step of bonding by direct hydrophilic bonding via a dielectric layer that can be in S102. deposited on the first and / or second substrate (s).
    [0016]
    16. A method of manufacturing a surface elastic wave device according to claim 14, characterized in that the transfer step comprises a bonding step by direct hydrophilic bonding or thermocompression assisted bonding, via a metal layer, which may be gold, deposited on the first and / or second substrate (s).
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    同族专利:
    公开号 | 公开日
    FR3033462B1|2018-03-30|
    EP3113362A1|2017-01-04|
    US10270420B2|2019-04-23|
    US20160261248A1|2016-09-08|
    EP3113362B1|2018-01-03|
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    法律状态:
    2016-03-31| PLFP| Fee payment|Year of fee payment: 2 |
    2016-09-09| PLSC| Publication of the preliminary search report|Effective date: 20160909 |
    2017-03-31| PLFP| Fee payment|Year of fee payment: 3 |
    2018-03-29| PLFP| Fee payment|Year of fee payment: 4 |
    2018-12-07| TQ| Partial transmission of property|Owner name: COMMISSARIAT A L'ENERGIE ATOMIQUE ET AUX ENERG, FR Effective date: 20181031 Owner name: CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE, FR Effective date: 20181031 |
    2020-03-31| PLFP| Fee payment|Year of fee payment: 6 |
    2021-12-10| ST| Notification of lapse|Effective date: 20211105 |
    优先权:
    申请号 | 申请日 | 专利标题
    FR1551803|2015-03-04|
    FR1551803A|FR3033462B1|2015-03-04|2015-03-04|ELASTIC SURFACE WAVE DEVICE COMPRISING A SINGLE CRYSTALLINE PIEZOELECTRIC FILM AND A CRYSTALLINE SUBSTRATE WITH LOW VISCOELASTIC COEFFICIENTS|FR1551803A| FR3033462B1|2015-03-04|2015-03-04|ELASTIC SURFACE WAVE DEVICE COMPRISING A SINGLE CRYSTALLINE PIEZOELECTRIC FILM AND A CRYSTALLINE SUBSTRATE WITH LOW VISCOELASTIC COEFFICIENTS|
    US15/060,356| US10270420B2|2015-03-04|2016-03-03|Surface elastic wave device comprising a single-crystal piezoelectric film and a crystalline substrate with low visoelastic coefficients|
    EP16158390.1A| EP3113362B1|2015-03-04|2016-03-03|Device with elastic surface waves comprising a single-crystal piezoelectric film and a crystalline substrate, with low viscoelastic coefficients|
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