Method for correcting thickness of piezoelectric layer

Abstract

The present invention relates to a method for correcting the thickness of a piezoelectric layer (3) disposed on a piezoelectric-on-insulator substrate, the method comprising the steps of: - measuring the thickness of at least one intermediate layer (2) arranged between the piezoelectric layer (3) and the carrier substrate (1); Measuring the thickness of the piezoelectric layer (3); calculating a thickness correction for the piezoelectric layer (3) based on said measurements of the thicknesses 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) depending on a plurality of pairs of thicknesses of the piezoelectric layer (3) and the thickness of the at least one intermediate layer (2) to obtain a target value of each property; and applying the thickness correction to the piezoelectric layer (3) using a milling process in a locally discriminatory manner. Includes.

Description

[Technical field]

[0001] The present invention relates to a method for compensating for the thickness of a piezoelectric layer and to a piezoelectric-on-insulator substrate having a thickness compensated using said method. The present invention is particularly applicable to the manufacture of high frequency devices such as resonators or filters. [Background technology]

[0002] It is a known technique to fabricate radio frequency (RF) devices, such as resonators or filters, on a substrate which comprises, in order from the bottom to the top surface of the substrate, a carrier substrate, typically made of a semiconductor 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 piezoelectric material (e.g. obtained by slicing an ingot) onto a carrier substrate, for example using the Smart Cut™ process. The carrier substrate is typically a silicon substrate, optionally comprising one or more layers of one or more other materials.

[0004] Transfer of the piezoelectric layer requires bonding a thick piezoelectric substrate to a carrier substrate and then thinning the thick piezoelectric substrate such that only a thin piezoelectric layer of the desired thickness remains on the carrier substrate for RF device fabrication.

[0005] To obtain good adhesion between the piezoelectric substrate and the carrier substrate, a layer of oxide (eg silicon oxide SiO2) is generally deposited on each of the two substrates, said substrates being bonded via said oxide layer.

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

[0007] To improve the thickness uniformity of the piezoelectric layer, it is known to locally adjust the thickness of the piezoelectric layer by scanning the surface of the layer, e.g., milling it with an ion beam of argon ions, a process called trimming.

[0008] However, because the piezoelectric layer is very thin, irregularities in the thickness of one or more layers disposed below the piezoelectric layer can result in significant variations in the properties of the piezoelectric layer. Summary of the Invention

[0009] One object of the invention is to provide a method which makes it possible to obtain a uniform distribution of at least one of the parameters of the electromechanical coupling coefficient, the propagation velocity of the acoustic waves and the temperature coefficient of frequency of the piezoelectric layer. To this end, the present invention provides a method for correcting the thickness of a piezoelectric layer disposed on a piezoelectric-on-insulator substrate, the method comprising: measuring the thickness of at least one intermediate layer arranged between the piezoelectric layer and the carrier substrate; Measuring the thickness of the piezoelectric layer; - calculating a thickness correction for the piezoelectric layer based on said measurements of the thickness of the at least one intermediate layer and the piezoelectric layer and on a numerical model of at least one property of the piezoelectric layer depending on a plurality of pairs of thicknesses of the piezoelectric layer and the thickness of the at least one intermediate layer in order to obtain a target value for each property; applying thickness correction to the piezoelectric layer using a milling process in a locally discriminative manner; Includes.

[0010] Preferably, said property of the piezoelectric layer is selected from electromechanical coupling coefficient, wave propagation velocity and / or temperature coefficient of frequency.

[0011] One or more properties of the piezoelectric layer may be selected depending on the application for which the substrate is intended. A single property may be selected to obtain a very uniform distribution of this property across the substrate, or a compromise between two or three parameters may be selected such that each parameter is as uniform as possible without introducing significant non-uniformity into each of the other parameters.

[0012] Preferably, the thickness of at least one of the intermediate layer and the piezoelectric layer is locally measured 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 selected depending on the measurement technique and the desired accuracy.

[0013] Preferably, the milling process is an ion beam etching process, which may include scanning an ion beam along two axes of the main plane of the piezoelectric layer, in which the duration of irradiation by said ion beam at each position is adjusted depending on the thickness of the resulting piezoelectric layer.

[0014] Preferably, the intermediate layer comprises a dielectric layer, a stack of multiple dielectric layers, a metal layer, and / or a layer for trapping electric charges.

[0015] Preferably, the thickness of the piezoelectric layer and / or the intermediate layer is measured by ellipsometry and / or reflectometry, which techniques allow the thicknesses of multiple superimposed layers to be measured simultaneously. The present invention also relates to a method for manufacturing a piezoelectric-on-insulator substrate, the method comprising: providing a carrier substrate; Providing a piezoelectric donor substrate; - bonding the donor substrate to the carrier substrate by disposing an intermediate layer at an interface between the donor substrate and the carrier substrate; thinning a donor substrate for transferring a piezoelectric layer from the donor substrate to a carrier substrate; Correcting the thickness of the piezoelectric layer using the above process; Includes.

[0016] Preferably, the step of thinning the donor substrate comprises, prior to the bonding step, forming weakened areas to define the piezoelectric layer to be transferred, and, after the bonding step, cleaving the donor substrate along said weakened areas.

[0017] In certain embodiments, the thickness of the at least one intermediate layer is measured after the piezoelectric layer is transferred to a carrier substrate.

[0018] In another embodiment, the thickness of the at least one intermediate layer is measured before the donor substrate is bonded to the carrier substrate, and this method also allows for the thickness of the opaque layer to be measured.

[0019] Preferably, the at least one intermediate layer comprises a metal layer, a dielectric layer, a stack of multiple dielectric layers, and / or a layer for trapping electric charges.

[0020] Another subject of the present invention relates to a piezoelectric-on-insulator substrate comprising, in order, a piezoelectric layer, an intermediate layer and a carrier substrate, in which the thickness of the piezoelectric layer is selected as a function of the thickness of the intermediate layer according to a numerical model of at least one characteristic of the piezoelectric layer as a function of multiple pairs of thicknesses of the piezoelectric layer and the at least one intermediate layer in order to obtain a target value for each characteristic.

[0021] Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings. [Brief description of the drawings]

[0022] [Figure 1] 1 shows a piezoelectric-on-insulator (POI) substrate comprising a base substrate, an intermediate layer, and a piezoelectric layer. [Figure 2A]1 illustrates a step of a method for manufacturing a POI substrate including adjusting the thickness of a piezoelectric layer according to the present invention. [Figure 2B] Another step is shown. [Figure 2C] Further steps are shown. [Figure 2D] Further steps are shown. [Figure 2E] Further steps are shown. [Diagram 3] 1 shows a grid of interlayer thickness measurement points. [Figure 4] 1 is a map of the POI substrate showing the thickness of the LiTaO3 layer. [Diagram 5] 13 is an example of reflectance measurements at two different angles from a POI substrate with a transparent intermediate layer. [Figure 6] 4 shows the variation of the frequency temperature coefficient of the POI substrate with various thicknesses of the intermediate LiTaO 3 layer. [Figure 7] The variation of the electromechanical coupling coefficient and the propagation velocity of acoustic waves for POI substrates with various thicknesses of the intermediate LiTaO3 layer is shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] For ease of reading the drawings, the various elements are not necessarily drawn to scale, in particular the thickness variations of the various layers of the piezoelectric-on-insulator substrate may be exaggerated.

[0024] 1 shows a substrate for an RF device comprising a carrier substrate 1, typically made of a semiconductor such as silicon, at least one intermediate layer 2 arranged on the carrier substrate and a piezoelectric layer 3 arranged on the intermediate layer. In certain cases several intermediate layers are arranged between the carrier substrate and the piezoelectric layer.

[0025] The term "top" denotes the relative position of a layer considering the layer from the back side of the substrate (ie the free side of the carrier substrate) to the front side (ie the free side of the piezoelectric layer).

[0026] Although the intermediate layer 2 and the piezoelectric layer 3 are shown with constant thickness, said layers will typically exhibit thickness variations that are not shown in FIG.

[0027] A method for producing such a substrate comprises: one or more steps of forming a substrate, which may include steps of deposition, bonding and / or transfer of a layer; one or more steps of measuring a thickness of one or more intermediate layers; calculating a thickness correction for the piezoelectric layer; applying said correction to the piezoelectric layer using a milling process; Includes.

[0028] Each of these steps will now be described in detail.

[0029] Substrate formation Formation of the substrate typically involves bonding a carrier substrate and a piezoelectric donor substrate via at least one intermediate layer, followed by transferring the piezoelectric layer from the donor substrate to the carrier substrate.

[0030] Prior to the bonding step, at least one intermediate layer is formed on the carrier substrate and / or on the donor substrate, said layer may be present in either or both of the substrates used or may be deposited during the process used to manufacture the piezoelectric-on-insulator substrate.

[0031] With reference to FIG. 2A, an intermediate layer 2 is deposited on a carrier substrate 1. By way of example and without limitation, such an intermediate layer may be a dielectric layer, such as an oxide layer. Although a single intermediate layer 2 is shown in FIG. 2A, two or more intermediate layers may be deposited on the carrier substrate. In certain embodiments, these layers may take the form of a stack of multiple superimposed dielectric layers, for example at least one layer of an oxide (such as SiO2), at least one layer of a nitride (such as SiN), and / or at least one layer of an oxynitride (such as SiON). In certain embodiments, which may be different from or combined with the above-mentioned embodiments, at least one metal intermediate layer and / or at least one intermediate layer for trapping charges (for example made of polysilicon) is deposited.

[0032] Alternatively (not shown), at least one intermediate layer as described above is deposited on the piezoelectric donor substrate, the deposition being carried out such that the intermediate layer is on the side of the donor substrate that is intended to be bonded to the carrier substrate.

[0033] In a particular embodiment (not shown), at least a first intermediate layer is deposited on a carrier substrate and at least a second intermediate layer is deposited on a piezoelectric donor substrate, such that the first and second intermediate layers are positioned at the bonding interface of the donor substrate on the carrier substrate.

[0034] Typically, the number of intermediate layers is comprised between 1 and 3, but this is not a limitation of the present invention.

[0035] 2A, each intermediate layer 2 exhibits some variation in thickness across the area of ​​the layer 2. By way of example and not limitation, the variation in thickness of each intermediate layer may be comprised between 5% and 30%.

[0036] The piezoelectric layer may be transferred to a carrier substrate, preferably using the Smart Cut™ process. To this end, referring to FIG. 2B, weakened areas 31 are formed in a donor substrate 30 so as to define the piezoelectric layer 3.

[0037] The weakened zones 31 are formed in the donor substrate 30 at a predetermined depth that substantially corresponds to the thickness of the piezoelectric layer 3 to be transferred. The piezoelectric layer 3 typically has a thickness comprised between 100 nm and 15 μm. Preferably, the weakened zones 31 are formed by implanting hydrogen and / or helium atoms into the donor substrate 30.

[0038] Optionally, a treatment can be performed on the surface of the donor substrate to prepare said surface for direct bonding. This treatment can include, as illustrative and non-limiting examples, chemical cleaning or plasma activation. In certain embodiments, one or more intermediate layers are deposited on the donor substrate after the formation of the weakened regions and / or optional treatment of the surface of the donor substrate.

[0039] 2C, the donor substrate 30 is then bonded to the carrier substrate 1. One or more intermediate layers 2 are thus disposed at the bonding interface between the carrier substrate 1 and the donor substrate 30.

[0040] The donor substrate is cleaved along the weakened regions 31 (see FIG. 2D) to transfer the piezoelectric layer 3 to the carrier substrate 1 and to position the one or more intermediate layers between the piezoelectric layer 3 and the carrier substrate 1. The one or more intermediate layers 2 and the piezoelectric layer 3 are positioned in direct contact across the entire interface of the one or more intermediate layers 2 and the piezoelectric layer 3.

[0041] As an alternative to the Smart Cut™ process, the piezoelectric layer and / or one or more intermediate layers can be transferred using other techniques without creating weakened regions, for example, the piezoelectric layer may be transferred by thinning from the back side of the donor substrate.

[0042] The piezoelectric layer transferred to the carrier substrate has a thickness greater than the desired piezoelectric layer thickness for the envisaged application to allow the thickness of the piezoelectric layer to be adjusted by milling in a subsequent step as described below. By providing a sufficiently thick layer of piezoelectric material initially, a margin is provided to allow for local optimization.

[0043] Thickness measurement Referring to FIG. 2E, non-destructive measurements of the thickness of the one or more intermediate layers 2 and the piezoelectric layer 3 are performed to generate a map of the thickness of the one or more intermediate layers 2 .

[0044] 2F, if the substrate comprises multiple intermediate layers 2A and 2B, the thickness measurement of the first intermediate layer 2A and the thickness measurement of the second intermediate layer 2B can be performed sequentially or simultaneously to generate a map of the thickness of each intermediate layer and the piezoelectric layer 3. In other cases, only the thickness of the piezoelectric layer 3 and the thickness of the upper intermediate layer 2B are determined, and the thickness of the lower intermediate layer 2A is not measured.

[0045] The thickness of one or more intermediate layers is preferably measured using an optical measurement device. Such devices are preferably ellipsometric or reflectometric devices. One advantage of these optical techniques is that they allow the measurement of thicknesses of multiple superimposed layers, as shown in FIG. 2F. Such optical techniques are particularly suitable for intermediate layers made of piezoelectric layers, such as LiTaO3, and oxides, such as SiO2, since these materials are optically transparent in the wavelength ranges traditionally used in the semiconductor field (e.g., 360 nm to 900 nm or 190 nm to 1700 nm). These material stacks also have a high refractive index contrast between the respective layers, which facilitates the measurement of the stack of layers by optical means.

[0046] However, the invention is not limited to these measurement techniques: the thickness of the intermediate layer can be determined by any other device that allows to measure non-destructively the thickness of a layer arranged below the piezoelectric layer 3.

[0047] In certain embodiments, the thickness of one or more intermediate layers is measured prior to transfer of the piezoelectric layer. This method is particularly useful when an opaque top layer prevents the thickness from being determined by optical means through the opaque top layer.

[0048] For example, the thickness of an opaque layer or another layer disposed below the opaque layer can be measured by picosecond ultrasound or wavelength dispersive X-ray fluorescence (WDXRF).

[0049] Preferably, the measurement device is configured to perform a series of automated thickness measurements on a grid of measurement points distributed on the surface of the substrate, such a grid of points being, for example, as shown in FIG.

[0050] In such a grid, the measurement points 5 are typically arranged in an (X,Y) plane parallel to the surface of the substrate. Each measurement point is associated with a pair of X,Y coordinates in this plane. By way of example and not of limitation, the measurement points are placed on straight lines to facilitate guiding the measuring means. These lines may be radial to the center of the substrate. Alternatively, the measurement points may be placed on a rectangular grid or evenly distributed across the surface of the substrate. If a larger thickness variation is expected in one particular area, for example the center, or, with reference to FIG. 3, close to the edge of the substrate, a higher density of measurement points 5 can be selected in this area.

[0051] The grid, density and location of the measurement points can be selected depending on the measurement technique, the thickness variations of the intermediate layer and the piezoelectric layer, and depending on the desired accuracy.

[0052] 4 shows a thickness map on a grid of measurement points, i.e. a spatial representation of thicknesses measured on a grid of points as described above. Each measurement point is associated with one measurement value, or, if multiple intermediate layers are superimposed, with a set of measurements where each value for each layer corresponds to one intermediate layer. For example, each thickness range may be associated with a predefined hue or color on the map.

[0053] Optical reflectance measurements consist in measuring the variation of the intensity of a light beam reflected from a surface or interface to the intensity of the incident beam (this ratio is called the reflectance) as a function of the wavelength of the beam.

[0054] Referring to Figure 5, reflectance measurements at various angles of incidence give a spectrum of reflected intensity in percent of incident intensity as a function of the wavelength λ of the incident beam in nanometers. The spectrum plotted with a solid line corresponds to a reflection angle of 70° and the spectrum plotted with a dotted line corresponds to a reflection angle of 6.5°. Other angles of incidence and / or a larger number of different angles may be used.

[0055] 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 further depends on the optical properties of each layer known for the material used. Each reflection angle has a different variation of intensity with wavelength. Thus, each spectrum recorded at a different reflection angle can provide additional information about the thickness of each layer in a stack of multiple superimposed layers.

[0056] It is possible to calculate the thickness of a set of n layers from n reflectometry spectra at various angles, where n is an integer. The roughness of the layers may be used as an additional fitting parameter or may be considered constant.

[0057] Ellipsometry is a characterization technique based on the change in the polarization state of light upon reflection from a surface or interface. Thus, an ellipsometry spectrum (not shown) represents the change in polarization as a function of the wavelength of the incident beam. The change in polarization also depends on the thickness of each layer through which the incident and reflected beams pass, and on the angle of reflection of the beam. As with a set of reflectometry spectra, it is possible to calculate the thickness of each of a set of n layers from n ellipsometry spectra at various angles, where n is an integer.

[0058] Similarly, it is possible to combine the ellipsometry and reflectometry spectra to calculate the thickness of each of a set of intermediate layers at each point on a selected grid.

[0059] A step of thickness interpolation between the measurement points is then performed to obtain a thickness map of the entire substrate. Preferably, linear interpolation is used as this type of interpolation is fast and easy to perform.

[0060] Numerical model The piezoelectric layer has several parameters that depend on the thickness of the piezoelectric layer and the thickness of the one or more intermediate layers, such as the electromechanical coupling coefficient, the propagation velocity of the acoustic waves, and the temperature coefficient of frequency.

[0061] Each of these parameters may depend differently on the thickness of each intermediate layer, and also on the mechanical, electrical and / or thermal properties of each layer.

[0062] If one or more intermediate layers exhibit thickness variations, then the respective variations of these parameters are obtained according to the respective thicknesses of each intermediate layer.

[0063] After the map of the at least one interlayer piezoelectric layer is obtained, the local thicknesses and their positions on the substrate are taken into account in a numerical model of at least one property of the piezoelectric layer, such a numerical model comprising a data matrix of one or more parameters of the piezoelectric layer, in which the model associates each value of the respective parameter with all combinations of thicknesses of layers superimposed in a stack of substrates and vice versa.

[0064] Thus, starting from the target values ​​of such parameters and the thickness of each intermediate layer in question, it is possible to determine the target thickness of the piezoelectric layer.

[0065] To increase the uniformity of such parameters across the substrate, target values ​​can be selected for said parameters that correspond to the indicated thicknesses of each interlayer present in the substrate, and since the thicknesses of one or more interlayers are not altered during the process, target values ​​are preferably selected that are compatible with all thicknesses of the interlayers present in the substrate.

[0066] In certain cases, a target value is selected that corresponds to an indicated thickness that is close to the average thickness of each interlayer, whereas in other cases, it may be necessary to use a maximum or minimum indicated thickness so that the target value can be achieved for all thicknesses of one or more interlayers present in the substrate.

[0067] Then, a target thickness value of the piezoelectric layer is calculated for each position across the substrate according to the thickness of each intermediate layer under the corresponding piezoelectric layer at the same position on the substrate. The target thickness of the piezoelectric layer at each point on the substrate is obtained such that the selected parameters have target values ​​corresponding to preselected designated values, regardless of the actual values ​​of the one or more intermediate layers at each position. Thus, after the target thickness is obtained at each position on the piezoelectric layer, it is possible to select parameters for the piezoelectric layer that are very uniform across the substrate.

[0068] If the selected parameter exhibits maximum and / or minimum values ​​depending on the thickness of the piezoelectric layer, multiple target thicknesses may be possible, in which case a target thickness may be selected that allows minimizing thickness variations across the substrate, or may be selected depending on other parameters to make the other parameters as uniform as possible across the substrate.

[0069] Alternatively, two or three different piezoelectric layer parameters may be selected and a target piezoelectric layer value may be calculated for each of the two or three respective parameters. In general, these target values ​​are not the same for each parameter at various locations across the substrate. Therefore, for each location on the substrate, an average value is calculated that represents a compromise between the two or three parameters, such that each parameter is as uniform as possible across the substrate, but does not create too large a non-uniformity in each of the other parameters. The thickness of the piezoelectric layer is selected according to the thickness of the intermediate layer at each location on the surface of the substrate.

[0070] It is therefore possible to adjust the thickness of the piezoelectric layer in order to take into account the influence of the thickness of the intermediate layer on a particular parameter or to obtain a compromise for a set of parameters. The parameters to be optimized are typically selected depending on the envisaged application of the substrate.

[0071] The most relevant piezoelectric layer parameters are the electromechanical coupling coefficient, the propagation velocity of the acoustic waves and the temperature coefficient of frequency, but other piezoelectric layer parameters may also be adjusted by the method of the present invention.

[0072] Commercially available software packages use models based on the Fresnel coefficients of multiple layers of thin films and transfer matrix methods, allowing thickness to be determined quickly and reliably.

[0073] Figure 6 shows the propagation velocity of sound on a free surface, v I or v f and the propagation velocity of the sound wave on the metallized surface, v m is expressed in m / s, and the coupling coefficient k s These parameters are shown as a function of the product d*f (in m.GHz or km / s) of the thickness d of the piezoelectric layer made of lithium tantalate (LiTaO3) and the frequency f.

[0074] Typically, frequencies used in various applications are between 500 and 3000 MHz. By expressing the parameters as a function of thickness-frequency product, it becomes possible to easily determine the thickness of the piezoelectric layer required for a given application frequency.

[0075] The parameters were measured for various thicknesses of the intermediate layer made of silicon oxide (SiO2) (100 nm, 500 nm and 900 nm). Lithium tantalate and silicon oxide are optically transparent and there is a high contrast between the refractive index of each layer, which makes it easier to perform optical measurements on the stack of these layers.

[0076] If it is desired to optimize the electromechanical coupling coefficient for this configuration of a piezoelectric layer made of LiTaO3 and an intermediate layer made of SiO2, a value of 500 nm can be selected for the intermediate layer for thickness-frequency products of 1 km / s or more, or a value of 100 nm for the intermediate layer for lower values ​​of the thickness-frequency product. At 2 km / s, a squared electromechanical coupling coefficient of about 8.5% is obtained for a layer 500 nm thick. The same value can be obtained at 2.3 km / s with a 100 nm layer and at 1.8 km / s with a 900 nm layer (dashed lines in Figure 6).

[0077] In the same configuration, the wave propagation velocity v on the metallized surface for the product of the thickness of the piezoelectric layer and the frequency is equal to 1 km / s. m In order to improve the uniformity of the film, for a thickness of 500 nm, a value of about 4100 m / s can be selected as the target value.

[0078] In the thin region, where the SiO2 layer thickness is equal to 100 nm, the product of the thickness and frequency of the piezoelectric layer must be adjusted to about 3 km / s. In the region where the SiO2 layer thickness is 900 nm, this product must be increased to 3 or decreased to about 0.4 km / s to obtain the same wave propagation speed. If it is desired to simultaneously maintain a constant homogeneity of the electromechanical coupling coefficient, the value of 3 km / s is chosen instead, since the coupling coefficient shows a significant variation for a thickness-frequency product of 0.4 km / s.

[0079] The maximum value of the coupling coefficient k shifts to lower frequencies with increasing dielectric layer. The thickness distribution is therefore selected according to the frequency used in the application of the substrate.

[0080] Figure 7 shows the frequency temperature coefficient FTC as a function of the product d*f (in m.GHz or km / s) of the thickness d and frequency f of a piezoelectric layer made of lithium tantalate (LiTaO3). The frequency temperature coefficient increases with the thickness d of the dielectric layer. At the same time, the maximum value of this coefficient shifts towards higher frequencies for thicker piezoelectric layers. If it is desired to increase the uniformity of this parameter across the substrate, it is also recommended to choose the desired thickness depending on the frequency of the targeted application.

[0081] 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, resulting in thickness-frequency products of about 0.6 km / s 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.

[0082] Milling of the piezoelectric layer Based on the target thickness of the piezoelectric layer determined by the above calculations, the piezoelectric layer is then locally milled, see Fig. 2G, which allows the thickness of the piezoelectric layer to be adjusted according to the parameters taken into account in the calculations.

[0083] The milling process is typically an etching process with an ion beam, typically composed of argon ions. The ions bombard the surface of the sample at very high speeds and peel off material from the target area. By way of example and not by way of limitation, the ion beam is scanned along two axes of the major plane of the piezoelectric layer to etch the piezoelectric layer across the entire extent of the substrate. Such a process allows the thickness of the piezoelectric layer to be precisely adjusted at each location across the substrate as the surface is continuously scanned. Thus, the thickness variation is matched to the desired parameters everywhere across the surface. The milling process may further include etching with a chemical etchant, typically a reactive gas.

[0084] The local thickness removed is determined by the dwell time of the beam at each location on the surface of the piezoelectric layer, which is calculated by an algorithm to adapt the scanning process to the desired thickness uniformity.

[0085] In a particular milling tool, the beam can be adjusted to the hardness of the material by varying the energy and current of the beam to obtain the appropriate flux without significantly affecting the final roughness of the surface.

[0086] Additionally, the nature of the ion species used in the beam may result in a chemical reaction with the material being etched, accelerating the etching process or enhancing the smoothing of the piezoelectric layer surface.

[0087] Interlayer thickness correction The same technique can be used to correct the thickness of an intermediate layer (e.g. a dielectric layer such as SiO2, SiON, SiN, etc.) located under the piezoelectric layer in a step included after deposition of the intermediate layer on the donor or carrier substrate and before transfer of the piezoelectric layer to the carrier substrate. This can advantageously result in a very uniform dielectric layer, which also has a beneficial effect on the uniformity of the electromechanical coupling coefficient across the substrate. The steps performed to obtain a substrate with a corrected thickness of the dielectric layer as described above are now described.

[0088] First, a SiO2 layer is deposited on a base substrate, preferably made of Si. The base substrate may comprise one or more stacks of layers on the surface of the base substrate, the stack typically comprising a layer of polysilicon rich in charge carrier traps. The stack may further comprise other layers or stacks of layers of dielectric materials, such as a layer of silicon oxide, a layer of silicon oxynitride, a layer of silicon nitride, a layer of aluminum oxide, a layer of tantalum nitride, or a combination of said layers.

[0089] A map of the local thickness distribution of the dielectric layer is then generated by ellipsometry and / or reflectivity measurements of at least the SiO2 dielectric material and its underlying layers over a grid of measurement points in the same way as described above for the intermediate layer below the piezoelectric layer.

[0090] The local thickness map of each layer is then introduced into the numerical model, which contains data on the parameters to be optimized as a function of thickness. Preferably, the map is introduced in the form of values ​​associated with the X, Y coordinates of the measurement points used. Furthermore, an interpolation, preferably a linear one, is performed between the measurement points.

[0091] The thickness of the dielectric layer is locally adjusted using a milling process, for example an ion beam milling process, with the post-milling thickness value being based on the local target value determined above.

[0092] By initially providing a sufficiently thick layer of dielectric material, a sufficient margin is provided to allow for such local optimization.

[0093] It is then possible to deposit a piezoelectric layer on the dielectric layer and readjust the thickness of the piezoelectric layer using the method according to the invention, thus obtaining a piezoelectric layer whose thickness has been fine-tuned to optimize one or more parameters of the piezoelectric layer.

Claims

1. A method for correcting the thickness of a piezoelectric layer (3) disposed on a piezoelectric-on-insulator substrate, comprising: measuring the thickness of at least one intermediate layer (2) arranged between the piezoelectric layer (3) and the carrier substrate (1); measuring the thickness of the piezoelectric layer (3); a step of calculating a thickness correction for the piezoelectric layer (3) to obtain a target value for each characteristic based on the step of measuring the thicknesses of the at least one intermediate layer (2) and the piezoelectric layer (3) and a numerical model of at least one characteristic of the piezoelectric layer (3) according to a plurality of pairs of the thickness of the piezoelectric layer (3) and the thickness of the at least one intermediate layer (2); applying said thickness correction to said piezoelectric layer (3) using a milling process in a locally discriminatory manner; A method comprising:

2. The method of claim 1 , wherein the properties of the piezoelectric layer are selected from electromechanical coupling coefficient, wave propagation velocity and / or temperature coefficient of frequency.

3. 3. The method according to claim 1, wherein the thicknesses of the at least one intermediate layer (2) and the piezoelectric layer are measured locally at a plurality of measurement points, and the method further comprises a step of linear interpolation of the thickness of each layer between at least two measurement points.

4. The method of claim 1 or 2, wherein the milling step is an ion beam milling step.

5. 5. The method of claim 4, further comprising a step of scanning the ion beam along two axes of the main plane of the piezoelectric layer (3), wherein the duration of irradiation by the ion beam at each position is adjusted depending on the thickness of the piezoelectric layer (3) to be obtained.

6. The method of claim 1 or 2, wherein the intermediate layer comprises a dielectric layer, a stack of multiple dielectric layers, a metal layer, and / or a layer for trapping charges.

7. 3. The method according to claim 1, wherein the thickness of the piezoelectric layer (3) and / or the intermediate layer (2) is measured by ellipsometry and / or reflectometry.

8. 1. A method for manufacturing a piezoelectric-on-insulator substrate, comprising: Providing a carrier substrate (1); Providing a piezoelectric donor substrate (30); bonding the donor substrate (30) to the carrier substrate (1) by disposing an intermediate layer (2) at the interface between the donor substrate and the carrier substrate; thinning the donor substrate to transfer a piezoelectric layer (3) from the donor substrate to the carrier substrate; Correcting the thickness of the piezoelectric layer (3) using the method according to claim 1 or 2; A method comprising:

9. 9. The method of claim 8, wherein the step of thinning the donor substrate (30) comprises, before the bonding step, forming weakened areas (31) to define the piezoelectric layer (3) to be transferred, and after the bonding step, cleaving the donor substrate along the weakened areas (31).

10. 9. The method of claim 8, wherein the thickness of the at least one intermediate layer (2) is measured after the piezoelectric layer (3) has been transferred to the carrier substrate (1).

11. 9. The method of claim 8, wherein the thickness of the at least one intermediate layer (2) is measured before the donor substrate (30) is bonded to the carrier substrate (1).

12. 9. The method of claim 8, wherein the at least one intermediate layer (2) comprises a metal layer, a dielectric layer, a stack of multiple dielectric layers, and / or a layer for trapping charges.

13. A piezoelectric-on-insulator substrate comprising, in order, a piezoelectric layer (3), an intermediate layer (2) and a carrier substrate (1), characterized in that the local thickness of the piezoelectric layer (3) is adjusted depending on the local thickness of the intermediate layer (2) by milling the piezoelectric layer in a locally distinguishable manner based on a numerical model of at least one characteristic of the piezoelectric layer (3) depending on multiple pairs of thicknesses of the piezoelectric layer (3) and the at least one intermediate layer (2), according to the method of claim 1 or 2.

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