Method for correcting the thickness of a piezoelectric layer

Abstract

The invention relates to a method for correcting the thickness of a piezoelectric layer (3) arranged on a piezoelectric-on-insulator substrate, the method comprising the steps of: • measuring the thickness of at least one intermediate layer (2) located between the piezoelectric layer (3) and a carrier substrate (1); • measuring the thickness of the piezoelectric layer (3); • on the basis of the measurements of the thickness of the at least one intermediate layer (2) and of the piezoelectric layer (3) and of a numerical model of at least one property of the piezoelectric layer (3) according to a plurality of pairs of thicknesses of the piezoelectric layer (3) and of the at least one intermediate layer (2), calculating a correction for the thickness of the piezoelectric layer (3) to obtain a target value for each property, correcting the thickness of the piezoelectric layer (3) by means of a topographically discriminated abrasion process.

Description

[0001] METHOD FOR CORRECTING THE THICKNESS OF A PIEZOELECTRIC LAYER

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a system The present invention relates to a method for correcting the thickness of a piezoelectric layer, as well as a substrate of the piezoelectric on insulator type whose thickness is corrected by said method. The invention finds particular application in the manufacture of radiofrequency devices, such as resonators or filters.

[0004] STATE OF THE ART

[0005] It is known to manufacture a radiofrequency (RF) device, such as a resonator or filter, on a substrate successively comprising, from its base towards its surface, a support substrate, generally made of a semiconductor material such as silicon, one or more intermediate layers, and a piezoelectric layer.

[0006] The piezoelectric layer is typically obtained by transferring a thick substrate of a piezoelectric material (obtained for example by cutting an ingot) onto a support substrate, for example by a layer transfer process of the Smart Cut™ type. The support substrate is typically a silicon substrate optionally comprising one or more layers of one or more other materials.

[0007] Transferring the piezoelectric layer involves bonding the thick piezoelectric substrate to the support substrate, followed by thinning the thick piezoelectric substrate, so that only a thin piezoelectric layer of the desired thickness for RF device fabrication is left on the support substrate.

[0008] For good adhesion of the piezoelectric substrate to the support substrate, a layer of oxide (for example a silicon oxide SiC>2) is generally deposited on each of the two substrates, and said substrates are bonded via said oxide layers.

[0009] The properties of the piezoelectric layer, such as the electromechanical coupling coefficient, the propagation velocity of acoustic waves and the temperature coefficient of the frequency depend on the thickness of the piezoelectric layer.

[0010] It is known to locally adjust the thickness of the piezoelectric layer by abrasion with an ion beam, for example argon ions, scanning the surface of the piezoelectric layer, in order to standardize this thickness. This adjustment process is called "trimming" in English.

[0011] However, since the piezoelectric layer is very thin, irregularities in the thickness of the layer(s) located under the piezoelectric layer can cause significant variations in these properties. DISCLOSURE OF THE INVENTION

[0012] An aim of the invention is to provide a method for obtaining a homogeneous distribution of at least one of the following parameters of the piezoelectric layer: electromechanical coupling coefficient, acoustic wave propagation speed, and temperature coefficient of the frequency.

[0013] To this end, the invention proposes a method for correcting the thickness of a piezoelectric layer arranged on a substrate of the piezoelectric on insulator type, comprising the following steps: o measuring the thickness of at least one intermediate layer located between the piezoelectric layer and a support substrate, o measuring the thickness of the piezoelectric layer, o from said thickness measurements of the at least one intermediate layer and of the piezoelectric layer and a numerical model of at least one property of the piezoelectric layer as a function of a plurality of pairs of thicknesses of the piezoelectric layer and of said at least one intermediate layer, calculating a thickness correction of the piezoelectric layer to obtain a target value of each property, o applying the correction of the thickness of the piezoelectric layer by an abrasion method in a topographically discriminated manner.

[0014] Advantageously, said property of the piezoelectric layer is chosen from an electromechanical coupling coefficient, a wave propagation speed and / or a temperature coefficient of the frequency.

[0015] The choice of the property(ies) of the piezoelectric layer can be made according to the application for which the substrate is intended. One can either choose a single property so as to obtain a very homogeneous distribution of this property over the extent of the substrate, or choose a compromise between two or three parameters, so that each parameter is as homogeneous as possible without generating significant inhomogeneities for the other respective parameters.

[0016] Preferably, the measurement of the thickness of the at least one intermediate layer and of the piezoelectric layer is carried out in a localized manner at a plurality of measurement points, the method further comprising a step of linear interpolation of the thickness of each layer between at least two measurement points. The grid of measurement points can be chosen according to the measurement technique and the desired precision.

[0017] Advantageously, the abrasion method is an ion beam etching method. The method may comprise a step of scanning the ion beam along two axes of a main plane of the piezoelectric layer, in which the duration of irradiation by said ion beam on each position is adjusted according to the thickness of the piezoelectric layer to be obtained. Advantageously, the intermediate layer comprises a dielectric layer, a stack of several dielectric layers, a metal layer and / or an electric charge trapping layer.

[0018] Preferably, the measurement of the thickness of the piezoelectric layer and / or the intermediate layer is carried out by ellipsometry and / or by reflectometry. The use of these techniques allows the simultaneous measurement of several superimposed layer thicknesses.

[0019] The invention also relates to a method for manufacturing a piezoelectric-on-insulator type substrate, comprising the following steps: o providing a support substrate, o providing a piezoelectric donor substrate, o bonding the donor substrate to the support substrate, an intermediate layer being arranged at the interface between the donor substrate and the support substrate, o thinning the donor substrate so as to transfer a piezoelectric layer from said donor substrate to the support substrate, o correcting the thickness of said piezoelectric layer by a method as described above.

[0020] Preferably, the thinning of the donor substrate comprises, before bonding, the formation of a weakening zone so as to delimit a piezoelectric layer to be transferred, and, after bonding, the detachment of the donor substrate along said weakening zone.

[0021] In some embodiments, the measurement of the thickness of the at least one intermediate layer is carried out after the transfer of the piezoelectric layer onto the support substrate.

[0022] In other embodiments, the measurement of the thickness of the at least one intermediate layer is carried out before bonding the donor substrate to the support substrate. This method also makes it possible to measure the thickness of opaque layers.

[0023] Advantageously, the at least one intermediate layer comprises: a metal layer, a dielectric layer, a stack of several dielectric layers and / or an electric charge trapping layer.

[0024] Another subject of the invention relates to a substrate of the piezoelectric on insulator type, successively comprising a piezoelectric layer, an intermediate layer and a support substrate, characterized in that the local thickness of said piezoelectric layer is adjusted as a function of the local thickness of the intermediate layer by abrasion of the piezoelectric layer in a topographically discriminated manner from a digital model of at least one property of the piezoelectric layer as a function of a plurality of pairs of thicknesses of the piezoelectric layer and of said at least one intermediate layer, according to a method as described above. BRIEF DESCRIPTION OF THE FIGURES

[0025] Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings, in which:

[0026] Figure 1 illustrates a piezoelectric-on-insulator (POI) substrate comprising a base substrate, an intermediate layer, and a piezoelectric layer.

[0027] Figures 2A to 2E illustrate steps of a method for manufacturing a POI type substrate comprising adjusting the thickness of the piezoelectric layer according to the invention.

[0028] Figure 3 illustrates a grid of intermediate layer thickness measurement points.

[0029] Figure 4 is a map of a POI type substrate, representing the thickness of a LiTaOs layer.

[0030] Figure 5 is an example of reflectometry at two different angles on a POI substrate including a transparent interlayer.

[0031] Figure 6 shows the variation of the temperature coefficient of frequency for POI-type substrates with different thicknesses of a LiTaO3 interlayer.

[0032] Figure 7 shows the variation of the electromechanical coupling coefficient and the acoustic wave propagation speed for POI-type substrates with different thicknesses of a LiTaO3 interlayer.

[0033] For reasons of readability of the figures, the various elements have not necessarily been drawn to scale. In particular, the variations in thickness of the different layers of the piezoelectric substrate on insulator may have been exaggerated.

[0034] DETAILED DESCRIPTION OF EMBODIMENTS

[0035] Figure 1 illustrates a substrate for an RF device, comprising a support substrate 1, generally made of a semiconductor material such as silicon, at least one intermediate layer 2 arranged on the support substrate, and a piezoelectric layer 3 arranged on the intermediate layer. In some cases, several intermediate layers are arranged between the support substrate and the piezoelectric layer.

[0036] By "on" we mean a relative position of the layers by considering the layers from the rear face (i.e. on the side of the support substrate) towards the front face (i.e. on the side of the piezoelectric layer) of the substrate.

[0037] Although the intermediate layer 2 and the piezoelectric layer 3 are shown with a constant thickness, said layers generally have thickness variations not shown in Figure 1.

[0038] A method of manufacturing such a substrate comprises: o one or more steps of forming the substrate, which may involve steps of deposition, bonding and / or layer transfer, o one or more steps of measuring the thickness of the intermediate layer(s), o a step of calculating a thickness correction of the piezoelectric layer, and o applying said correction to the piezoelectric layer by an abrasion process.

[0039] We will now describe each of these steps in detail.

[0040] Substrate formation

[0041] Substrate formation generally involves bonding a support substrate and a piezoelectric donor substrate via at least one intermediate layer, followed by transferring a piezoelectric layer from the donor substrate to the support substrate.

[0042] Before the bonding step, at least one intermediate layer is formed on the support substrate and / or on the donor substrate. Said layer may be present on one and / or the other of the substrates used, or be deposited during the manufacturing process of the piezoelectric substrate on insulator.

[0043] With reference to Figure 2A, an intermediate layer 2 is deposited on a support substrate 1. In an illustrative and non-limiting manner, such an intermediate layer may be a dielectric layer such as an oxide layer. Although only one intermediate layer 2 is illustrated in Figure 2A, two or more intermediate layers may be deposited on the support substrate. In certain embodiments, these layers may be in the form of a stack of several superimposed dielectric layers, for example at least one oxide layer (such as SiC>2), at least one nitride layer (such as SiN) and / or at least one oxynitride layer (such as SiON). In certain embodiments, distinct from or combined with the aforementioned embodiments, at least one metallic intermediate layer and / or at least one intermediate layer for trapping electric charges, for example made of polycrystalline silicon, is deposited.

[0044] Alternatively (not shown), at least one intermediate layer as described above is deposited on the piezoelectric donor substrate. The deposition is carried out so that the intermediate layer is on the face of the donor substrate intended to be bonded to the support substrate.

[0045] In some embodiments (not shown), at least one first intermediate layer is deposited on the support substrate, and at least one second intermediate layer is deposited on the piezoelectric donor substrate, such that the first and second intermediate layers are located at the bonding interface of the donor substrate on the support substrate. Typically, the number of intermediate layers is between one and three, without limiting the invention.

[0046] As shown schematically in Figure 2A, each intermediate layer 2 has a certain variation in its thickness over the extent of the surface of the layer 2. In an illustrative and non-limiting manner, the variation in the thickness of each intermediate layer is between 5% and 30%.

[0047] The transfer of a piezoelectric layer onto the support substrate can advantageously be carried out by the Smart Cut™ process. For this purpose, with reference to FIG. 2B, a weakening zone 31 is formed in a donor substrate 30, so as to delimit the piezoelectric layer 3.

[0048] The weakening zone 31 is formed in the donor substrate 30 at a predetermined depth which corresponds substantially to the thickness of the piezoelectric layer 3 to be transferred. The piezoelectric layer 3 typically has a thickness of between 100 nm and 15 μm. Preferably, the weakening zone 31 is created by implanting hydrogen and / or helium atoms in the donor substrate 30.

[0049] An optional treatment of the donor substrate surface may be performed to prepare said surface for molecular bonding. This treatment may include, by way of illustrative and non-limiting example, chemical cleaning or plasma activation. In certain embodiments, the intermediate layer(s) are deposited on the donor substrate after the formation of the embrittlement zone and / or the optional treatment of the donor substrate surface.

[0050] With reference to Figure 2C, the donor substrate 30 is then bonded to the support substrate 1. The intermediate layer(s) 2 are thus arranged at the bonding interface between the support substrate 1 and the donor substrate 30.

[0051] A detachment of the donor substrate is caused along the weakening zone 31, so as to transfer the piezoelectric layer 3 onto the support substrate 1, the intermediate layer(s) being arranged between the piezoelectric layer 3 and the support substrate 1 (see FIG. 2D). The intermediate layer(s) 2 and the piezoelectric layer 3 are arranged in direct contact over the extent of their interfaces.

[0052] As an alternative to the Smart Cut™ process, the transfer of the piezoelectric layer and / or one or more intermediate layers can be achieved by other techniques, without creating a weakened zone. For example, the piezoelectric layer can be transferred by thinning from the back side of the donor substrate.

[0053] The piezoelectric layer transferred to the support substrate has a thickness greater than the thickness of the piezoelectric layer desired for the intended application, in order to allow adjustment of the thickness by abrasion in a subsequent step as described below. Starting from a sufficiently thick layer of piezoelectric material, there is a margin to allow local optimization.

[0054] Thickness measurement

[0055] With reference to Figure 2E, a non-destructive measurement of the thickness of the intermediate layer(s) 2 and of the piezoelectric layer 3 is carried out to establish a map of the thickness of the intermediate layer(s) 2.

[0056] When the substrate comprises several intermediate layers 2A and 2B, with reference to FIG. 2F, it is possible to carry out successively or simultaneously a measurement of the thickness of a first intermediate layer 2A and a measurement of the thickness of the second intermediate layer 2B to establish a map of the thickness of each intermediate layer and of the piezoelectric layer 3. In other cases, only the thicknesses of the piezoelectric layer 3 and of the upper intermediate layer 2B are determined, without measuring the thickness of any lower intermediate layers 2A.

[0057] The measurement of the thickness of the intermediate layer(s) is advantageously carried out using an optical measuring device. Such a device is preferably an ellipsometry device or a reflectometry device. An advantage of these optical techniques is that they allow the simultaneous measurement of the thickness of several superimposed layers, as illustrated in Figure 2F. Such optical technologies are particularly suitable for piezoelectric layers such as LiTaOs and oxide intermediate layers such as SiO2, because these materials are optically transparent in the wavelength range conventionally used in the semiconductor field (e.g., 360 nm to 900 nm or 190 nm to 1700 nm). A stack of these materials further exhibits a large refractive index contrast between each respective layer, which facilitates the measurement of the layer stack by optical means.

[0058] However, the invention is not limited to these measurement techniques. The determination of the thickness of the intermediate layer can be carried out by any other device making it possible to measure the thickness of a layer arranged below the piezoelectric layer 3 in a non-destructive manner.

[0059] In some embodiments, the thickness measurement of one or more intermediate layers is performed before the transfer of the piezoelectric layer. This technique is particularly used in the case of opaque upper layers that do not allow thickness determination by optical means through the opaque layer.

[0060] For example, the thickness of an opaque layer or another layer below an opaque layer can be measured by ultrasonic analysis with picosecond resolution, or by wavelength dispersive X-ray fluorescence (WDXRF).

[0061] Advantageously, the measuring device is configured to perform a series of automated thickness measurements on a grid of measuring points which are distributed over the surface of the substrate. Such a grid of points is for example illustrated in Figure 3.

[0062] In such a grid, the measuring points 5 are typically located in a plane (X, Y) parallel to the surface of the substrate. Each measuring point is associated with a pair of coordinates X, Y in this plane. Illustratively and not limitingly, the measuring points are arranged along straight lines in order to facilitate the guidance of the measuring means. These lines may be radial relative to the center of the substrate. Alternatively, the measuring points may be arranged on a rectangular grid or be distributed uniformly over the surface of the substrate. If greater thickness variations are to be expected in a particular area, for example in the middle or, with reference to Figure 3, near the edge of the substrate, the measuring points 5 may be chosen to be denser in this area.

[0063] The grid, density and positioning of the measuring points can be chosen according to the measuring technique, the thickness variations in the intermediate layers and the piezoelectric layer, and depending on the desired accuracy.

[0064] Figure 4 illustrates a thickness mapping on a grid of measurement points, i.e., a spatial representation of the measured thicknesses on a grid of points as described above. Each measurement point is associated with a measurement value or, in the case of several superimposed intermediate layers, with a set of measurement values, each respective value of which corresponds to a respective intermediate layer. For example, each thickness range can be associated with a predefined color or shade on the mapping.

[0065] Optical reflectometry consists of measuring the variation in the intensity of a beam of light reflected on a surface or an interface, compared to the intensity of an incident beam (this ratio is called reflectivity) as a function of the wavelength of the beam.

[0066] Referring to Figure 5, reflectometry measurements at different incidence angles provide reflection intensity spectra as a percentage of the incident intensity as a function of the wavelength X of the incident beam in nanometers. The spectrum represented by a solid line corresponds to a reflection angle of 70°, and the spectrum represented by a dotted line corresponds to a reflection angle of 6.5°. Other incidence angles and / or a greater number of different angles can be used.

[0067] The reflected intensity depends on the wavelength of the light and the thickness of each layer traversed by the incident and reflected beams. The intensity also depends on the optical properties of each layer, which are known for the materials used. For each reflection angle, the variation of intensity with wavelength is different. Each spectrum recorded at a different reflection angle can thus provide additional information on the thickness of each layer in a stack of several superimposed layers.

[0068] The thickness of a set of n layers can be calculated from n reflectometry spectra at different angles, where n is an integer. The roughness of the layers can be used as an additional fitting parameter, or considered constant.

[0069] Ellipsometry is a characterization technique based on the change in the polarization state of light, by reflection of light on a surface or interface. An ellipsometry spectrum (not shown) therefore shows the change in polarization as a function of the wavelength of an incident beam. The change in polarization also depends on the thickness of each layer crossed by the incident beam and the reflected beam, and on the angle of reflection of the beam. Similar to a set of reflectometry spectra, the respective thicknesses of a set of n layers can be calculated from n ellipsometry spectra at different angles, where n is an integer.

[0070] Similarly, one can combine ellipsometry and reflectometry spectra to calculate the respective thicknesses of a set of intermediate layers at each point of the chosen grid.

[0071] A thickness interpolation step is then carried out between the measurement points in order to obtain a thickness map over the extent of the substrate. Advantageously, a linear interpolation is carried out which is quick and easy to implement.

[0072] Digital model

[0073] The piezoelectric layer has several parameters that depend on the thickness of the piezoelectric layer and the thickness of the intermediate layer(s). These parameters are, for example, the electromechanical coupling coefficient, the acoustic wave propagation speed and the temperature coefficient of the frequency.

[0074] Each of these parameters may depend on the thicknesses of each intermediate layer in different ways, depending on the mechanical, electrical and / or thermal properties of each layer.

[0075] When the intermediate layer(s) exhibit variations in thickness, a variation in each of these parameters is obtained as a function of the respective thickness of each intermediate layer.

[0076] After obtaining the mapping of the piezoelectric layer and at least one intermediate layer, the local thicknesses and their location on the substrate are taken into account in a numerical model of at least one property of a piezoelectric layer. Such a numerical model comprises a data matrix for one or more parameters of the piezoelectric layer. In this matrix, the model associates each value of the respective parameter with all combinations of thicknesses of superimposed layers in the substrate stack and vice versa.

[0077] Starting from a target value for such a parameter and the thickness of each intermediate layer considered, we can therefore determine a target thickness of the piezoelectric layer.

[0078] To homogenize such a parameter over the extent of the substrate, a target value of said parameter can be chosen corresponding to an indicative thickness for each respective intermediate layer present in the substrate. Since the thickness of the intermediate layer(s) is not modified during the process, a target value is advantageously chosen which remains compatible with all the thicknesses of the intermediate layers present in the substrate.

[0079] In some cases, a target value is chosen that corresponds to an indicative thickness close to the average thickness of each respective intermediate layer. In other cases, it may be necessary to use a maximum or minimum indicative thickness, so that the target value can be achieved for all thicknesses of the intermediate layer(s) present in the substrate.

[0080] A target value for the thickness of the piezoelectric layer is then calculated for each position over the extent of the substrate, based on the thickness of the respective intermediate layers below the piezoelectric layer corresponding to the same position on the substrate. A target thickness of the piezoelectric layer is obtained at each point on the substrate, such that the chosen parameter has the target value corresponding to the previously chosen indicative value, independently of the actual value of the intermediate layer(s) at each respective position. This allows a parameter of the piezoelectric layer to be chosen which will be very homogeneous over the extent of the substrate after obtaining the target thickness at each respective location of the piezoelectric layer.

[0081] In the case where the chosen parameter has maxima and / or minima depending on the thickness of the piezoelectric layer, several target thicknesses may be possible. In such a case, one can either choose a target thickness that minimizes the thickness variations on the substrate overall, or choose the target thickness based on other parameters to be made as homogeneous as possible over the extent of the substrate.

[0082] Alternatively, one can choose two or three different parameters of the piezoelectric layer, and calculate the target values ​​of the piezoelectric layer for each of the two or three respective parameters. In general, these target values ​​will not be the same for each respective parameter for the different locations across the substrate. Therefore, for each position on the substrate, an average value is calculated that represents a compromise for the two or three parameters, so that each parameter is as homogeneous as possible across the substrate, without generating excessive inhomogeneities for the other respective parameters. The thickness of the piezoelectric layer is chosen based on the thicknesses of the intermediate layers for each location on the substrate surface.

[0083] The thickness of the piezoelectric layer can thus be adjusted by taking into account the influence of the thickness of the intermediate layer on a specific parameter, or for a compromise of a set of parameters. The choice of the parameter(s) to be optimized is typically made according to the intended application of the substrate.

[0084] The most relevant parameters of the piezoelectric layer are the electromechanical coupling coefficient, the acoustic wave propagation velocity and the temperature coefficient of the frequency. However, other parameters of the piezoelectric layer can be adjusted with the method of the invention.

[0085] Commercially available software uses models based on the Fresnel coefficient of thin film multilayers using the transfer matrix method. Such models allow thicknesses to be determined quickly and reliably.

[0086] Figure 6 shows the propagation speeds of acoustic waves vi or Vf on a free surface, the propagation speeds of acoustic waves v m on metallized surface in m / s, and the square of the coupling coefficient k s in %. These parameters are represented as a function of the product d*f of the thickness d of a piezoelectric layer of lithium tantalate (LiTaOs) and the frequency f in m GHz or km / s.

[0087] Typically, the application frequency is between 500 and 3000 MHz. The representation of the parameters as a function of the thickness-frequency product makes it easy to evaluate the necessary thickness of the piezoelectric layer for a planned application frequency.

[0088] The parameters were measured for different thicknesses (100 nm, 500 nm and 900 nm) of a silicon oxide (SiC>2) interlayer. Lithium tantalate and silicon oxide are optically transparent and their refractive indices show a high contrast between the respective layers, which facilitates optical measurements on the layer stack.

[0089] If, for this configuration of a piezoelectric layer in LiTaOs and an intermediate layer in SiO2, we wish to optimize the electromechanical coupling coefficient, we can choose either the value of an intermediate layer of 500 nm for a thickness-frequency product greater than or equal to 1 km / s, or an intermediate layer of 100 nm for small values ​​of the thickness-frequency product. Starting from the 500 nm layer at 2 km / s, we obtain a square of the electromechanical coupling coefficient of approximately 8.5%. The same value can be obtained for a 100 nm layer at 2.3 km / s and for a 900 nm layer at 1.8 km / s (dotted lines in Figure 6).

[0090] To homogenize, for the same configuration, the wave propagation speed v m on a metallized surface at 1 km / s of product of thickness of the piezoelectric layer times the frequency, we can choose as target value the value for a thickness of 500 nm which is approximately 4100 m / s.

[0091] In areas where the SiC>2 layer thickness is thinner, 100 nm, the piezoelectric layer thickness times the frequency should be adjusted to about 3 km / s. In areas where the SiC>2 layer thickness is 900 nm, this product should either be increased to 3 to obtain the same wave propagation speed, or reduced to about 0.4 km / s. If we wish to simultaneously maintain a certain homogeneity of the electromechanical coupling coefficient, we should instead choose a value of 3 km / s, because the coupling coefficient shows significant variations for a thickness-frequency product of 0.4 km / s.

[0092] The maximum of the coupling coefficient k shifts towards lower frequencies with the increase of the dielectric layer. The thickness distribution will therefore have to be chosen according to the frequency used in the substrate application.

[0093] Figure 7 shows the temperature coefficient of the frequency CTF as a function of the product d*f of the thickness d of a lithium tantalate (LiTaOs) piezoelectric layer and the frequency f in m GHz or km / s. The temperature coefficient of the frequency increases with the thickness d of the dielectric layer. Simultaneously, the maximum of this coefficient shifts towards higher frequencies for thicker piezoelectric layers. When one wishes to homogenize this parameter over the extent of a substrate, it is also advisable to choose the desired thickness according to the frequency of the intended application.

[0094] This parameter can be adjusted, for example, starting from a value of about 9 ppm / K for a 500 nm layer and a thickness-frequency product of 1 km / s. Thickness-frequency products of about 0.6 km / s will be obtained for a 100 nm piezoelectric layer, about 0.8 km / s for a 200 nm layer, 1.1 km / s for a 700 nm layer and 1.2 km / s for a 900 nm thick layer.

[0095] Abrasion of the piezoelectric layer

[0096] Based on the target value of the piezoelectric layer thickness determined by the calculation described above, local abrasion of the piezoelectric layer is then carried out. With reference to Figure 2G, this abrasion makes it possible to adjust the thickness of the piezoelectric layer according to the parameters taken into account during the calculation.

[0097] The abrasion process is typically an etching process using an ion beam, typically an argon ion beam. The ions strike the surface of the sample at very high speed, tearing off material in the target area. By way of illustration and not limitation, the ion beam is scanned along two axes of a principal plane of the piezoelectric layer in order to etch the piezoelectric layer over the entire extent of the substrate. Such a process makes it possible to precisely adjust the thickness of the piezoelectric layer at each position over the extent of the substrate, with a continuous scanning of the surface. Thus, the variation in thickness is continuously adapted to the desired parameters over the entire surface. The abrasion process may further comprise etching using a chemical etching agent, typically a reactive gas.

[0098] The local thickness removed is determined by the beam dwell time at each location on the surface of the piezoelectric layer. This dwell time is calculated by an algorithm to adapt the scanning process to the desired thickness uniformity.

[0099] In some abrasion tools, the beam can be adjusted to the material hardness by varying the beam energy and current to achieve a suitable flow rate without excessively affecting the final surface roughness.

[0100] The nature of the ionic species used for the beam can also cause a chemical reaction with the etched material which can either accelerate the etching process or smooth the surface of the piezoelectric layer.

[0101] Intermediate layer thickness correction

[0102] The same approach can be used for the correction of the thickness of the intermediate layer (e.g., a dielectric layer such as SiC>2, SiON or Si N ) arranged under the piezoelectric layer, in a step after the deposition of the intermediate layer on the donor substrate or the support substrate and before the transfer of the piezoelectric layer to the support substrate. This can advantageously lead to a very uniform dielectric layer, which also has a beneficial impact on the uniformity of the electromechanical coupling coefficient over the extent of the substrate. The steps to be carried out to obtain a substrate having a correction of the thickness of the dielectric layer as described above will now be described.

[0103] One begins by depositing a layer of SiO2 on a base substrate which is preferably made of Si. The base substrate may comprise a stack of one or more layers on its surface which typically comprises a layer of polycrystalline silicon which is rich in traps for electric charge carriers. The stack may further comprise a layer of silicon oxide, silicon oxynitride, silicon nitride, aluminium oxide, tantalate nitride or a combination of said layers, other layers of dielectric materials or stacks of layers.

[0104] A mapping of the local distribution of the thickness of the dielectric layer is then carried out by ellipsometry and / or reflectometry of at least the SiO2 dielectric material and the underlying layer, on a grid of measurement points, in the same way as described above for the intermediate layers below the piezoelectric layer.

[0105] Subsequently, the map of the local thickness of the respective layers is injected into a numerical model comprising data for a parameter to be optimized as a function of the thickness. Preferably, the map is injected in the form of values ​​associated with X, Y coordinates on the measuring points used. An interpolation is also carried out between the measuring points, preferably a linear interpolation.

[0106] The thickness of the dielectric layer is adjusted locally by an abrasion process, for example by an ion beam. The thickness value after abrasion is based on the local target value determined above.

[0107] Starting with a sufficiently thick layer of dielectric material, there is sufficient margin to allow such local optimization.

[0108] A piezoelectric layer can then be deposited on the dielectric layer, and the thickness of the piezoelectric layer can be readjusted using a method according to the invention. This produces a piezoelectric layer whose thickness is finely adjusted to optimize one or more parameters of the piezoelectric layer.

Claims

CLAIMS A method for correcting the thickness of a piezoelectric layer (3) arranged on a piezoelectric-on-insulator substrate, comprising the following steps: o measuring the thickness of at least one intermediate layer (2) located between the piezoelectric layer (3) and a support substrate (1), o measuring the thickness of the piezoelectric layer (3), o from said thickness measurements of the at least one intermediate layer (2) and the piezoelectric layer (3) and a numerical model of at least one property of the piezoelectric layer (3) as a function of a plurality of pairs of thicknesses of the piezoelectric layer (3) and said at least one intermediate layer (2), calculating a thickness correction of the piezoelectric layer (3) to obtain a target value of each property, applying the thickness correction of the piezoelectric layer (3) by an abrasion process in a topographically discriminated manner.A method according to claim 1, wherein said property of the piezoelectric layer is selected from an electromechanical coupling coefficient, a wave propagation velocity, and / or a frequency temperature coefficient. A method according to claim 1 or claim 2, wherein the thickness of at least one intermediate layer (2) and of the piezoelectric layer is measured locally at a plurality of measurement points, the method further comprising a step of linear thickness interpolation of each layer between at least two measurement points. A method according to any one of the preceding claims, wherein the abrasion method is an ion beam etching method.A method according to claim 4, comprising a step of scanning the ion beam along two axes of a principal plane of the piezoelectric layer (3), wherein the irradiation time by said ion beam at each position is adjusted according to the thickness of the piezoelectric layer (3) to be obtained. A method according to any one of the preceding claims, wherein the intermediate layer comprises a dielectric layer, or a stack of several dielectric layers. dielectric layers, a metallic layer and / or an electrical charge trapping layer.

7. A method according to any one of the preceding claims, wherein the measurement of the thickness of the piezoelectric layer (3) and / or the intermediate layer (2) is carried out by ellipsometry and / or by reflectometry.

8. A method for manufacturing a piezoelectric-on-insulator type substrate, comprising the following steps: o providing a support substrate (1), o providing a piezoelectric donor substrate (30), o bonding the donor substrate (30) onto the support substrate (1), an intermediate layer (2) being arranged at the interface between the donor substrate and the support substrate, o thinning the donor substrate so as to transfer a piezoelectric layer (3) from said donor substrate onto the support substrate, o correcting the thickness of said piezoelectric layer (3) by a method according to any one of claims 1 to 7.

9. Method according to claim 8, wherein the thinning of the donor substrate (30) comprises, prior to bonding, the formation of a weakening zone (31) so as to delimit a piezoelectric layer (3) to be transferred, and, after bonding, the detachment of the donor substrate along said weakening zone (31).

10. A method according to any one of claims 8 or 9, wherein the measurement of the thickness of at least one intermediate layer (2) is carried out after the transfer of the piezoelectric layer (3) onto the support substrate (1).

11. A method according to any one of claims 8 or 9, wherein the thickness of at least one intermediate layer (2) is measured before the donor substrate (30) is bonded to the support substrate (1).

12. A method according to any one of claims 8 to 11, wherein at least one intermediate layer (2) comprises: a metallic layer, a dielectric layer, a stack of several dielectric layers and / or an electrical charge trapping layer. Piezoelectric-on-insulator substrate, comprising successively a piezoelectric layer (3), an intermediate layer (2) and a support substrate (1), characterized in that the local thickness of said piezoelectric layer (3) is adjusted as a function of the local thickness of the intermediate layer (2) by abrasion of the piezoelectric layer in a topographically discriminated manner from a numerical model of at least one property of the piezoelectric layer (3) as a function of a plurality of pairs of thicknesses of the piezoelectric layer (3) and of said at least one intermediate layer (2), according to a method according to any one of claims 1 to 12.

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