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, comprising the following steps: measuring the thickness of at least one intermediate layer (2) located between the piezoelectric layer (3) and a support substrate (1), measuring the thickness of the piezoelectric layer (3), 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.Figure for the abbreviation: Fig 2E.

Description

Title of the invention: Method for correcting the thickness of a piezoelectric layer technical field

[0001] The present invention relates to a method for correcting the thickness of a piezoelectric layer, as well as to a piezoelectric-on-insulator substrate whose thickness is corrected by said method. The invention finds particular application in the manufacture of radio frequency devices, such as resonators or filters. Prior art

[0002] It is known to manufacture a radio frequency (RF) device, such as a resonator or filter, on a substrate comprising successively, from its base to its surface, a support substrate, generally in a semiconductor material such as silicon, one or more intermediate layers, and a piezoelectric layer.

[0003] 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.

[0004] The transfer of the piezoelectric layer involves bonding the thick piezoelectric substrate to the support substrate, followed by thinning the thick piezoelectric substrate, so as to leave on the support substrate only a thin piezoelectric layer, of the thickness desired for the manufacture of the RF device.

[0005] For good adhesion of the piezoelectric substrate to the support substrate, a layer of oxide (for example a silicon oxide SiO2) is generally deposited on each of the two substrates, and the said substrates are glued by means of said oxide layers.

[0006] The properties of the piezoelectric layer, such as the electromechanical coupling coefficient, the speed of propagation of acoustic waves and the temperature coefficient of the frequency, depend on the thickness of the piezoelectric layer.

[0007] It is known to locally adjust the thickness of the piezoelectric layer by abrasion with an ion beam, for example argon ions, sweeping the surface of the piezoelectric layer, in order to uniform this thickness. This adjustment process is called "trimming" in English.

[0008] However, since the piezoelectric layer is very thin, irregularities in the thickness of the layer or layers located under the piezoelectric layer can cause significant variations in these properties. Description of the invention

[0009] An object of the invention is to make available 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 frequency temperature coefficient.

[0010] To this end, the invention proposes a method for correcting the thickness of a piezoelectric layer arranged on a piezoelectric-on-insulator substrate, comprising the following steps: • the measurement of the thickness of at least one intermediate layer located between the piezoelectric layer and a supporting substrate, • measuring the thickness of the piezoelectric layer, • from said thickness measurements of at least one intermediate layer and of the piezoelectric layer and of 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, the calculation of a thickness correction of the piezoelectric layer to obtain a target value of each property, • the application of the correction of the thickness of the piezoelectric layer by an abrasion process in a topographically discriminated manner.

[0011] Advantageously, said property of the piezoelectric layer is chosen from an electromechanical coupling coefficient, a wave propagation speed and / or a frequency temperature coefficient.

[0012] The choice of the property or properties of the piezoelectric layer can be made according to the application for which the substrate is intended. One can either choose a single property to obtain a very homogeneous distribution of that property over the surface 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 in the other respective parameters.

[0013] Preferably, the thickness of at least one intermediate layer and the piezoelectric layer is measured locally 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 accuracy.

[0014] Advantageously, the abrasion process is an ion beam etching process. The process may include a step of scanning the ion beam along two axes of a principal plane of the piezoelectric layer, in which the duration irradiation by said ion beam at each position is adjusted according to the thickness of the piezoelectric layer to be obtained.

[0015] Advantageously, the intermediate layer comprises a dielectric layer, a stack of several dielectric layers, a metallic layer and / or an electric charge trapping layer.

[0016] Preferably, the thickness of the piezoelectric layer and / or the intermediate layer is measured by ellipsometry and / or reflectometry. The use of these techniques allows the simultaneous measurement of several superimposed layer thicknesses.

[0017] The invention also relates to a method for manufacturing a piezoelectric-on-insulator substrate, comprising the following steps: • supplying a support substrate, • the provision of a piezoelectric donor substrate, • the bonding of the donor substrate to the support substrate, with an intermediate layer arranged at the interface between the donor substrate and the support substrate, • the thinning of the donor substrate so as to transfer a piezoelectric layer of said donor substrate onto the support substrate, • the correction of the thickness of said piezoelectric layer by a process as described above.

[0018] Preferably, the thinning of the donor substrate includes, 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.

[0019] In some embodiments the measurement of the thickness of at least one intermediate layer is carried out after the transfer of the piezoelectric layer onto the supporting substrate.

[0020] In other embodiments, the thickness of at least one intermediate layer is measured before the donor substrate is bonded to the support substrate. This method also allows the thickness of opaque layers to be measured.

[0021] Advantageously, the at least one intermediate layer comprises: a metallic layer, a dielectric layer, a stack of several dielectric layers and / or an electric charge trapping layer.

[0022] Another object of the invention relates to a piezoelectric-on-insulator substrate, comprising successively a piezoelectric layer, an intermediate layer and a supporting substrate, the thickness of said piezoelectric layer being chosen as a function of the thickness of the intermediate layer according to 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, to obtain a target value of each property. Brief description of the figures

[0023] Other features and advantages of the invention will become apparent from the detailed description that follows, with reference to the accompanying drawings, in which:

[0024] [Fig.1] illustrates a piezoelectric-on-insulator (POI) type substrate comprising a base substrate, an intermediate layer and a piezoelectric layer.

[0025] Figures 2A to 2E illustrate steps in a process for manufacturing a POI-type substrate comprising adjusting the thickness of the piezoelectric layer according to the invention.

[0026] Figure 3 illustrates a grid of thickness measurement points for the interlayer median.

[0027] Figure 4 is a map of a POI-type substrate, representing the thickness of a layer of LiTaO3

[0028] Figure 5 is an example of reflectometry at two different angles on a substrate POI including a transparent intermediate layer.

[0029] Figure 6 shows the variation of the temperature coefficient of the frequency for POI type substrates exhibiting different thicknesses of an intermediate layer of LiTaO3.

[0030] Figure 7 shows the variation of the electromechanical coupling coefficient and the acoustic wave propagation speed for POI type substrates with different thicknesses of an intermediate layer of LiTaO3.

[0031] For the sake of clarity in 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 insulation may have been exaggerated. Detailed description of embodiments

[0032] 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.

[0033] By "on" we designate a relative position of the layers considering the layers from the back face (i.e. on the side of the supporting substrate) towards the front face (i.e. on the side of the piezoelectric layer) of the substrate.

[0034] Although the intermediate layer 2 and the piezoelectric layer 3 are shown with a constant thickness, said layers generally exhibit variations in thickness not shown in [Fig.1].

[0035] A method for manufacturing such a substrate comprises: • one or more substrate formation steps, which may involve layer deposition, bonding and / or transfer steps, • one or more steps for measuring the thickness of the intermediate layer(s), • a step involving the calculation of a thickness correction for the piezoelectric layer, and • the application of said correction to the piezoelectric layer by an abrasion process.

[0036] We will now describe each of these steps in detail. Substrate formation

[0037] The formation of the substrate generally involves the bonding of a support substrate and a piezoelectric donor substrate via at least one intermediate layer, followed by the transfer of a piezoelectric layer from the donor substrate onto the support substrate.

[0038] Before the bonding step, at least one intermediate layer is formed on the support substrate and / or on the donor substrate. This layer may be present on one or both of the substrates used, or be deposited during the manufacturing process of the piezoelectric substrate on insulator.

[0039] With reference to [Fig. 2A], an intermediate layer 2 is deposited on a support substrate 1. By way of illustration and not limitation, such an intermediate layer may be a dielectric layer such as an oxide layer. Although only one intermediate layer 2 is shown in [Fig. 2A], two or more intermediate layers may be deposited on the support substrate. In some 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 SiO2), at least one nitride layer (such as SiN), and / or at least one oxynitride layer (such as SiON). In some embodiments, either separate from or in combination with the aforementioned embodiments, at least one metallic intermediate layer and / or at least one electrically charge-trapping intermediate layer, for example, of polycrystalline silicon, are deposited.

[0040] Alternatively (not illustrated), 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.

[0041] In certain embodiments (not illustrated), at least one first intermediate layer is deposited on the support substrate, and at least one second intermediate layer on the piezoelectric donor substrate, so that the first and second intermediate layers should be located at the interface of bonding of the donor substrate to the support substrate.

[0042] Typically, the number of intermediate layers is between one and three, without limiting the invention.

[0043] As schematically shown in [Fig.2A], each intermediate layer 2 exhibits a certain variation in its thickness over the extent of the surface of the layer 2. By way of illustration and not limitation, the variation in the thickness of each intermediate layer is between 5% and 30%.

[0044] 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 weakened zone 31 is formed in a donor substrate 30, so as to delimit the piezoelectric layer 3.

[0045] The embrittlement zone 31 is formed in the donor substrate 30 at a predetermined depth that corresponds substantially to the thickness of the piezoelectric layer 3 to be transferred. The piezoelectric layer 3 typically has a thickness between 100 nm and 15 pm. Preferably, the embrittlement zone 31 is created by implanting hydrogen and / or helium atoms into the donor substrate 30.

[0046] An optional treatment of the donor substrate surface can be performed to prepare said surface for bonding by molecular adhesion. This treatment may include, by way of illustrative and non-limiting example, chemical cleaning or plasma activation. In some embodiments, the intermediate layer(s) are deposited onto the donor substrate after the formation of the embrittlement zone and / or the optional treatment of the donor substrate surface.

[0047] With reference to [Fig.2C], the donor substrate 30 is then glued onto 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.

[0048] The donor substrate is detached along the embrittlement 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.

[0049] As an alternative to the Smart Cut™ process, the transfer of the piezoelectric layer and / or one or more intermediate layers can be carried out by other techniques, without creating a zone of embrittlement. For example, the piezoelectric layer can be transferred by thinning through the back face of the donor substrate.

[0050] The piezoelectric layer transferred onto the substrate has a thickness greater than the thickness of the piezoelectric layer desired for the application This approach is designed to allow for thickness adjustment by abrasion in a later step, as described below. Starting with a sufficiently thick layer of piezoelectric material provides some leeway for local optimization. Thickness measurement

[0051] With reference to [Fig.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.

[0052] When the substrate comprises several intermediate layers 2A and 2B, with reference to [Fig.2F], a thickness measurement of a first intermediate layer 2A and a thickness measurement of the second intermediate layer 2B can be carried out successively or simultaneously to establish a thickness map of each intermediate layer and of the piezoelectric layer 3. In other cases, only the thicknesses of the piezoelectric layer 3 and the upper intermediate layer 2B are determined, without measuring the thickness of any lower intermediate layers 2A.

[0053] The thickness of the intermediate layer(s) is advantageously measured 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 [Fig. 2F]. Such optical technologies are particularly well-suited for piezoelectric layers such as LiTaO3 and oxide intermediate layers such as SiO2, since these materials are optically transparent in the wavelength range commonly used in the semiconductor field (e.g., 360 nm to 900 nm or 190 nm to 1700 nm). A stack of these materials also exhibits a large refractive index contrast between each respective layer, which facilitates the measurement of the layer stack by optical means.

[0054] 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 enabling the non-destructive measurement of the thickness of a layer arranged below the piezoelectric layer 3.

[0055] In some embodiments, the thickness of one or more intermediate layers is measured before the transfer of the piezoelectric layer. This technique is particularly useful in the case of opaque top layers, preventing thickness determination by optical means through the opaque layer.

[0056] For example, the thickness of an opaque layer or another layer located in- The thickness beneath an opaque layer can be measured by ultrasonic analysis with picosecond resolution, or by wavelength dispersive X-ray fluorescence (WDXRF).

[0057] Advantageously, the measuring device is configured to perform a series of automated thickness measurements on a grid of measuring points distributed over the surface of the substrate. Such a grid of points is illustrated, for example, in [Fig. 3].

[0058] In such a grid, the measurement points 5 are typically located in an (X, Y) plane parallel to the substrate surface. Each measurement point is associated with a pair of X, Y coordinates in this plane. By way of illustration and without limitation, the measurement points are arranged along straight lines to facilitate the guidance of the measuring instruments. These lines may be radial with respect to the center of the substrate. Alternatively, the measurement points may be arranged on a rectangular grid or distributed uniformly over the substrate surface. If greater thickness variations are expected in a particular area, for example in the middle or, with reference to [Fig. 3], near the edge of the substrate, the measurement points 5 may be chosen to be denser in that area.

[0059] The grid, density and positioning of the measurement points can be chosen according to the measurement technique, the variations in thickness in the intermediate layers and the piezoelectric layer, and according to the desired accuracy.

[0060] Figure 4 illustrates a thickness map on a grid of measurement points, that is, a spatial representation of the thicknesses measured 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 corresponding to a respective intermediate layer. For example, each thickness range can be associated with a predefined color or shade on the map.

[0061] Optical reflectometry consists of measuring the variation of the intensity of a beam of light reflected on a surface or an interface, with respect to the intensity of an incident beam (this ratio is called reflectivity) as a function of the wavelength of the beam.

[0062] With reference to [Fig. 5], reflectometry measurements at different angles of incidence provide reflectance intensity spectra as a percentage of the incident intensity as a function of the wavelength 1 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 dashed line corresponds to a reflection angle of 6.5°. Other angles of incidence and / or a larger number Different angles can be used.

[0063] The reflected intensity depends on the wavelength of the light and the thickness of each layer through which the incident and reflected beams pass. The intensity also depends on the optical properties of each layer, which are known for the materials used. For each angle of reflection, the variation of the intensity with wavelength is different. Each spectrum recorded at a different angle of reflection can thus provide additional information on the thickness of each layer in a stack of several superimposed layers.

[0064] 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 adjustment parameter, or considered constant.

[0065] Ellipsometry is a characterization technique based on the change in the polarization state of light, due to 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 traversed by the incident and reflected beams, 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.

[0066] Similarly, ellipsometry and reflectometry spectra can be combined to calculate the respective thicknesses of a set of intermediate layers on each point of the chosen grid.

[0067] A thickness interpolation step is then performed between the measurement points to obtain a thickness map over the extent of the substrate. Advantageously, linear interpolation is performed, which is fast and easy to implement. Digital model

[0068] 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 include, for example, the electromechanical coupling coefficient, the acoustic wave propagation speed, and the temperature coefficient of the frequency.

[0069] Each of these parameters can depend on the thicknesses of each interlayer median in different ways, depending on the mechanical, electrical and / or thermal properties of each layer.

[0070] When the intermediate layer(s) exhibit variations in thickness, a variation of each of these parameters is obtained as a function of the respective thickness of each intermediate layer.

[0071] After obtaining the map 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 includes 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 layer thicknesses stacked in the substrate and vice versa.

[0072] Starting from a target value for such a parameter and the thickness of each intermediate layer considered, a target thickness of the piezoelectric layer can therefore be determined.

[0073] To homogenize such a parameter over the entire substrate, a target value for 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, it is advantageous to choose a target value that remains compatible with all the thicknesses of the intermediate layers present in the substrate.

[0074] In some cases, a target value is chosen corresponding 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 as to be able to achieve the target value for all the thicknesses of the intermediate layer(s) present in the substrate.

[0075] A target thickness value for the piezoelectric layer is then calculated for each position on 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, regardless of the actual value of the intermediate layer(s) at each respective position. It is thus possible to choose a piezoelectric layer parameter that will be very homogeneous across the substrate after obtaining the target thickness at each respective location of the piezoelectric layer.

[0076] In cases where the chosen parameter exhibits 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 thickness variations across the substrate globally, or choose the target thickness based on other parameters to be made as homogeneous as possible over the substrate.

[0077] Alternatively, two or three different parameters of the piezoelectric layer can be chosen, and target values ​​for the piezoelectric layer can be calculated for each of the two or three respective parameters. In general, these target values ​​will not be the same for each respective parameter at different locations across the substrate. Therefore, for each position on the substrate, an average value is calculated that represents a compromise between 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 at each location on the substrate surface.

[0078] 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 by finding a compromise between a set of parameters. The choice of the parameter(s) to be optimized is typically made according to the intended application of the substrate.

[0079] 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 using the method of the invention.

[0080] Commercially available software uses models based on the Fresnel coefficient of multilayer thin films using the transfer matrix method. Such models allow for the rapid and reliable determination of thicknesses.

[0081] Figure 6 shows the acoustic wave propagation velocities vt or vf on a free surface, the acoustic wave propagation velocities vm on a metallized surface in m / s, and the square of the coupling coefficient ksen %. These parameters are represented as a function of the product d*f of the thickness d of a piezoelectric lithium tantalate (LiTaO3) layer and the frequency f in m GHz or km / s.

[0082] Typically, the application frequency is between 500 and 3000 MHz. Representing the parameters as a function of the thickness-frequency product makes it easy to evaluate the required thickness of the piezoelectric layer for a given application frequency.

[0083] The parameters were measured for different thicknesses (100 nm, 500 nm and 900 nm) of an intermediate layer of silicon dioxide (SiO2). Lithium tantalate and silicon dioxide are optically transparent and their refractive indices exhibit significant contrast between the respective layers, which facilitates optical measurements of the layer stacking.

[0084] If, for this configuration of a piezoelectric layer in LiTaO3 and an intermediate layer in SiO2, it is desired to optimize the electro coupling coefficient Mechanically, one can choose either an intermediate layer thickness of 500 nm for a thickness-frequency product greater than or equal to 1 km / s, or an intermediate layer thickness of 100 nm for small thickness-frequency product values. Starting with the 500 nm layer at 2 km / s, the square of the electromechanical coupling coefficient is 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 (dashed lines in [Fig. 6]).

[0085] To homogenize, for the same configuration, the wave propagation speed vm on a metallized surface at 1 km / s of the product of the thickness of the piezoelectric layer times the frequency, we can choose as a target value the value for a thickness of 500 nm which is about 4100 m / s.

[0086] In areas where the SiO2 layer is thinner, at 100 nm, the thickness of the piezoelectric layer multiplied by the frequency should be adjusted to approximately 3 km / s. In areas where the SiO2 layer is 900 nm thick, this product should either be increased to 3 to obtain the same wave propagation speed, or reduced to approximately 0.4 km / s. If it is desired to simultaneously maintain a certain homogeneity of the electromechanical coupling coefficient, a value of 3 km / s should be chosen, since the coupling coefficient exhibits significant variations for a thickness-frequency product of 0.4 km / s.

[0087] The maximum of the coupling coefficient k shifts towards lower frequencies with increasing dielectric layer thickness. The thickness distribution should therefore be chosen according to the frequency used in the substrate application.

[0088] Figure 7 shows the temperature coefficient of the CTF frequency as a function of the product d*f of the thickness d of a lithium tantalate (LiTaO3) 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 it is desired to homogenize this parameter over the extent of a substrate, it is also necessary to choose the desired thickness according to the frequency of the intended application.

[0089] This parameter can be adjusted, for example starting from a value of approximately 9 ppm / K for a 500 nm layer and a thickness-frequency product of 1 km / s. We will obtain thickness-frequency products of approximately 0.6 km / s for a 100 nm piezoelectric layer, approximately 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. Abrasion of the piezoelectric layer

[0090] Based on the target value of the piezoelectric layer thickness determined by the calculation described above, a local abrasion of the piezoelectric layer is then carried out electrical. With reference to [Fig.2G], this abrasion allows the thickness of the piezoelectric layer to be adjusted according to the parameters taken into account during the calculation.

[0091] The abrasion process is typically an ion beam etching process, typically using an argon ion beam. The ions strike the sample surface at very high speed, removing material in the target area. By way of illustration, but not limitation, the ion beam is scanned along two axes of a principal plane of the piezoelectric layer to etch the piezoelectric layer over the entire surface of the substrate. Such a process allows the thickness of the piezoelectric layer to be precisely adjusted at each position on the substrate, with continuous scanning of the surface. Thus, the thickness variation is continuously adapted to the desired parameters across the entire surface. The abrasion process may further include etching with a chemical etching agent, typically a reactive gas.

[0092] 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 in order to adapt the scanning process to the desired thickness uniformity.

[0093] In certain abrasion tools, the beam can be adjusted to the hardness of the material by varying the energy and current of the beam to obtain a suitable flow rate without excessively affecting the final roughness of the surface.

[0094] The nature of the ionic species used for the beam can also lead to a chemical reaction with the etched material which can either accelerate the etching process or smooth the surface of the piezoelectric layer. Intermediate layer thickness correction

[0095] The same approach can be used for correcting the thickness of the intermediate layer (e.g., a dielectric layer such as SiO2, SiON, or SiN) arranged beneath the piezoelectric layer, in a step occurring after the intermediate layer has been deposited on the donor or support substrate and before the piezoelectric layer has been transferred 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 substrate. The steps to be carried out to obtain a substrate with a dielectric layer thickness correction as described above will now be described.

[0096] The process begins by depositing a layer of SiO2 onto a base substrate, preferably made of Si. The base substrate may comprise a stack of one or more layers on its surface, typically including a layer of polycrystalline silicon, which is rich in traps for electric charge carriers. The stack may further include a layer of silicon oxide, silicon oxynitride, silicon nitride, aluminum oxide, or silicon nitride. tantalate or a combination of said layers, other layers of dielectric materials or stacks of layers.

[0097] A local distribution of the thickness of the dielectric layer is then mapped 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.

[0098] Subsequently, the local thickness map of the respective layers is injected into a numerical model comprising data for a parameter to be optimized as a function of thickness. Preferably, the map is injected as values ​​associated with X, Y coordinates on the measurement points used. Interpolation is also performed between the measurement points, preferably linear interpolation.

[0099] The thickness of the dielectric layer is locally adjusted by an abrasion process, for example by an ion beam. The thickness value after abrasion is based on the local target value determined above.

[0100] Starting from a sufficiently thick layer of dielectric material, there is sufficient margin to allow such local optimization.

[0101] A piezoelectric layer can then be deposited on the dielectric layer, and the thickness of the piezoelectric layer can be readjusted by a process according to the invention. This yields a piezoelectric layer whose thickness is finely adjusted to optimize one or more parameters of the piezoelectric layer.

Claims

Claims

1. Method for correcting the thickness of a piezoelectric layer (3) arranged on a substrate of the piezoelectric-on-insulator type, comprising the following steps: • measuring the thickness of at least one intermediate layer (2) located between the piezoelectric layer (3) and a support substrate (1), • measuring the thickness of the piezoelectric layer (3), • from said thickness measurements of the at least one intermediate layer (2) and of the piezoelectric layer (3) and 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), calculating a thickness correction of the piezoelectric layer (3) to obtain a target value of each property,applying the correction of the thickness of the piezoelectric layer (3) by an abrasion process in a topographically discriminated manner.,

2. The method of claim 1, wherein said property of the piezoelectric layer is selected from an electromechanical coupling coefficient, a wave propagation velocity and / or a temperature coefficient of the frequency.

3. Method according to claim 1 or claim 2, wherein the measurement of the thickness of the at least one intermediate layer (2) 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.

4. A method according to any preceding claim, wherein the abrasion method is an ion beam etching method.

5. A method according to claim 4, comprising a step of scanning the ion beam along two axes of a main 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.

6. Method according to one of the preceding claims, in which the intermediate layer comprises a dielectric layer, a stack of several dielectric layers, a metallic layer and / or an electric charge trapping layer.

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

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

9. A method according to claim 8, wherein the thinning of the donor substrate (30) comprises, before 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. Method according to one of claims 8 or 9, wherein the measurement of the thickness of the at least one intermediate layer (2) is carried out after the transfer of the piezoelectric layer (3) onto the support substrate (1).

11. Method according to one of claims 8 or 9, wherein the measurement of the thickness of the at least one intermediate layer (2) is carried out before bonding the donor substrate (30) to the support substrate (1).

12. Method according to one of claims 8 to 11, in which the at least one intermediate layer (2) comprises: a metal layer, a dielectric layer, a stack of several dielectric layers and / or an electric charge trapping layer.

13. Substrate of the piezoelectric on insulator type, successively comprising 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 digital 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.

Images