Regeneration of a donor substrate for the fabrication of a POI structure

Abstract

The present invention relates to a method for regenerating a donor substrate for the fabrication of a piezoelectric structure on an insulator (POI), comprising supplying the donor substrate to be regenerated, comprising a support substrate and a first piezoelectric substrate formed on the support substrate comprising or consisting of one of lithium tantalate and lithium niobate, wherein the first piezoelectric substrate is a second piezoelectric substrate from which a piezoelectric layer has been transferred to a target substrate, and chemical-mechanical polishing (CMP) of the first piezoelectric substrate to obtain a regenerated donor substrate comprising a regenerated piezoelectric substrate, wherein the CMP comprises removing a layer of the first piezoelectric substrate with a thickness of at most 2 µm, in particular at most 1.2 µm. Figure for the abstract: Figure 1

Description

Title of the invention: Regeneration of a donor substrate for the manufacture of a POI structure

[0001] The present invention relates to the method for regenerating donor substrates for the manufacture of piezoelectric on insulator, POI, structures, in particular POI structures usable for the manufacture of microelectronic, micromechanical and photonic devices. Furthermore, the invention relates to the method for manufacturing POI structures using donor substrates thus regenerated.

[0002] In the field of microelectronics, micromechanics and photonics, POI structures are of increasing importance due to, for example, superior properties in terms of sensitivity and information propagation. For example, sensors such as surface acoustic wave (SAW) sensors, or bulk acoustic wave (BAW) sensors using the piezoelectric effect to convert an electrical signal into a mechanical / acoustic wave offer particularly advantageous options due to a wide variety of measurable ambient parameters, including for example temperature, pressure, strain and torque.

[0003] A typical POI structure comprises a layer of piezoelectric material, in particular a single-crystal material such as, for example, lithium niobate (LiNbO3) or lithium tantalate (LiTaO3), on a support substrate made of, for example, silicon. Various methods for forming a thin layer of piezoelectric material on the support substrate are known in the art. The application of Smart Cut™ technology has proven particularly advantageous. According to this technology (see, for example, WO 2020 / 200986 A1), light species are implanted into a piezoelectric substrate formed on a support substrate to form a weakened region in the piezoelectric substrate, and then the piezoelectric substrate is bonded to a target substrate. Via fracturing at the weakened region, it is possible to obtain the thin layer of piezoelectric material on the support substrate.The transferred piezoelectric material layer is subjected to an annealing process and, subsequently, to a polishing process, in particular, via chemical mechanical polishing, CMP, in an attempt to improve the crystal quality and achieve the desired thickness uniformity of a monodomain layer of piezoelectric material with substantially all dipole moments aligned parallel to each other in a given direction.

[0004] A big advantage of Smart Cut™ technology is that the expensive donor substrate can be reused multiple times for multiple subsequent transfer processes of thin piezoelectric layers to target substrates. Before reuse after transfer of a piezoelectric layer to a target substrate, the donor substrate must be regenerated / reconditioned. In particular, a rim / crown at the edge of the piezoelectric substrate after fracturing at the weakened region and transfer of the piezoelectric layer remains due to imperfect bonding.

[0005] It is an object of the present invention to provide a technique for regenerating a donor substrate after transferring a piezoelectric layer to a target substrate to provide a regenerated donor substrate comprising an upper portion with an exposed surface such that a highly uniform layer of piezoelectric material can be transferred from the regenerated donor substrate to another target substrate.

[0006] The present invention achieves this object by providing a method for regenerating (reconditioning) a donor substrate for manufacturing a piezoelectric-on-insulator, POI, structure. The method comprises providing the donor substrate to be regenerated comprising a support substrate and a first piezoelectric substrate formed on the support substrate comprising or consisting of one of lithium tantalate and lithium niobate, wherein the first piezoelectric substrate is a second piezoelectric substrate from which a piezoelectric layer has been transferred to a target substrate. In other words, the donor substrate to be regenerated has already been used to transfer a piezoelectric layer to a target substrate for manufacturing a POI structure, and the donor substrate needs to be regenerated for further transfer of a piezoelectric layer to another target substrate.A dielectric bonding layer, for example a photo(UV) polymer layer or a layer made of or comprising silicon oxide and / or silicon nitride may be present between the support substrate and the first piezoelectric substrate.

[0007] The method further comprises chemical mechanical polishing, CMP, of the first piezoelectric substrate to obtain a regenerated donor substrate comprising a regenerated piezoelectric substrate. The CMP comprises removing a layer of the first piezoelectric substrate with a thickness of at most 2 μm, in particular at most 1.2 μm (the thickness being measured in the corona region). The method may further comprise implanting a species (e.g., hydrogen, optionally supplemented with helium) into the regenerated piezoelectric substrate to obtain a weakened layer in the first piezoelectric substrate.

[0008] It has been observed that the rim / crown left on the (first) piezoelectric substrate of the donor substrate to be regenerated has a relatively high amount of hydrogen (e.g., about 1021 at / cm2) in the case of hydrogen implantation into the donor substrate before transfer of the piezoelectric layer to the target substrate. Due to this relative amount of hydrogen, the polishing rate of the corona is significantly higher than that of the central region of the upper part of the first piezoelectric substrate. The corona is thus removed relatively quickly during the chemical-mechanical polishing process, which helps to reduce the overall amount of polishing because the rest of the wafer is less impacted during the initial polishing treatment of the edge region. Therefore, it is possible to remove only a relatively thin layer of piezoelectric material in order to obtain a regenerated donor substrate ready for one or more subsequent transfers of piezoelectric layer(s) to one or more target substrates. The process can be repeated several times to regenerate the donor substrate after the respective transfer processes, and thus the donor substrate can advantageously be reused several times for the fabrication of high-quality POI structures.

[0009] The polishing recipes can be chosen in a conventional manner. Depending on the actual recipes and the material of the piezoelectric substrate as well as the thickness of the piezoelectric layer transferred from the second substrate, a layer of the first piezoelectric substrate can be removed during the CMP, with a thickness in the range of 2 pm to 1.2 pm, in particular in the range of 1.8 pm to 1.4 pm, or 1.6 pm to 1.4 pm.

[0010] The CMP is performed by means of a polishing pad. The polishing pad may comprise a secondary pad and a top pad (for contacting the piezoelectric material to be removed). According to one embodiment, a relatively fully hard CMP pad comprising a secondary pad with a hardness of more than 75 shore A and a conventional top pad (e.g. a rigid top pad made of polyurethane) is used. The shore A level may be defined / measured according to DIN ISO 7619-1 or ASTM D2240-10. In the prior art, secondary pads with a hardness of 53 shore A are commonly used.Depending on the actual application, according to one embodiment, a CMP pad comprising a secondary pad with a hardness of more than 80 shore A, in particular more than 90 shore A, or with a hardness in the range of 75 shore A to 95 shore A, in particular in a range of 85 shore A to 95 shore A, can be used. According to one example, the upper pad and the secondary pad may for example have a similar hardness in the range of 20 or 10 shore A. The use of such hard secondary pads is somewhat counterintuitive in view of the risk of generating scratches on the surface of the piezoelectric layer of the regenerated donor substrate to be transferred. However, it has been found that a uniform thickness of the piezoelectric layer of the regenerated donor substrate to be transferred can be achieved without causing scratches on the surface of this layer.

[0011] Other parameters of the entire process for regenerating the donor substrate to be regenerated and a process for manufacturing a POI structure comprising the process of regeneration can be chosen as is done in a usual manner known to those skilled in the art.

[0012] According to one embodiment, the secondary buffer has a density greater than 0.6 g / cm3, in particular greater than 0.7 g / cm3 or 0.8 g / cm3, or in the range of 0.6 g / cm3 to 0.9 g / cm3. These parameter ranges may prove advantageous in terms of thickness uniformity of the resulting polished piezoelectric layer of the regenerated piezoelectric substrate of the regenerated donor substrate, in particular in combination with the parameter ranges mentioned above.

[0013] According to another embodiment, the secondary pad has a compressive force deflection greater than 689.476 kPa (100 psi), in particular 758.423 kPa (110 psi) or 827.371 kPa (120 psi), or in the range of 689 kPa to 900 kPa. The compressive force deflection may be defined / measured according to ASTM D 3574. These parameter ranges may prove advantageous in terms of thickness uniformity of the resulting polished piezoelectric layer of the regenerated piezoelectric substrate of the regenerated donor substrate, in particular in combination with the parameter ranges mentioned above.

[0014] According to another embodiment, the CMP is carried out by means of a CMP suspension consisting of an aqueous suspension of amorphous silicon with a weight percentage of the amorphous silicon of less than 20, in particular in the range of 10 to 18 or 12.5 to 17.5, in contrast to the parameter range of a conventionally used CMP suspension given by 25 to 35 wt%. The parameter ranges according to the embodiment may prove advantageous in terms of thickness uniformity of the resulting polished piezoelectric layer of the regenerated piezoelectric substrate of the regenerated donor substrate, in particular in combination with the parameter ranges mentioned above. The amorphous silicon used for the CMP suspension may comprise or consist of precipitated amorphous silicon particles with diameters in the range of 40 to 60 nm.

[0015] For the polishing process, the donor substrate to be regenerated is positioned on a rotating head and brought into contact with a rotating polishing pad. According to particular embodiments, the head is rotated 30 to 110 revolutions per minute (rpm), in particular 40 to 105 revolutions per minute, and the polishing pad is rotated in the same direction as the head at a different speed compared to the head in the range of 50 to 90 revolutions per minute, in particular 60 to 80 revolutions per minute (stage speed). The parameter ranges according to the embodiment may prove advantageous in terms of thickness uniformity of the resulting polished piezoelectric layer of the regenerated piezoelectric substrate of the regenerated piezoelectric substrate of the regenerated donor substrate, in particular in combination with the parameter ranges mentioned above.

[0016] According to another embodiment, the wafer pressure applied to the donor substrate to be regenerated to press it against a polishing pad is in the range of 44.8 kPa (6.5 psi) to 58.6 kPa (8.5 psi), and the retainer pressure in the range of 37.9 kPa (5.5 psi) to 44.8 kPa (6.5 psi). These parameter ranges may prove advantageous in terms of thickness uniformity of the resulting polished piezoelectric layer of the regenerated piezoelectric substrate of the regenerated donor substrate, particularly in combination with the parameter ranges mentioned above.

[0017] Further, a method of manufacturing a piezoelectric-on-insulator, POI, structure is provided, comprising performing the steps of the method of regenerating a donor substrate according to one of the examples described above and transferring a piezoelectric layer of the regenerated piezoelectric substrate from the regenerated donor substrate to another target substrate, comprising bonding the regenerated donor substrate to the target substrate on the side of the regenerated piezoelectric substrate and fracturing the regenerated piezoelectric substrate at the weakened layer in an annealing process.

[0018] Thus, a uniform piezoelectric layer can be reliably transferred to the other target substrate without excessively heavy defects. Additional post-processing (annealing and polishing) may, however, still be necessary to provide a piezoelectric layer suitable for particular applications. It should be noted that naturally occurring silicon oxide may be present between the transferred piezoelectric layer and the other target substrate. In addition, a dielectric assembly layer may be formed on or above a surface of the other target substrate prior to transfer of the piezoelectric layer to that surface. This dielectric assembly layer may be made of or comprise silicon oxide and / or silicon nitride or a stack of layers composed of these materials.Further, depending on the actual application, a charge trapping layer may be formed on or above a surface of the other target substrate prior to transfer of the piezoelectric layer to that surface. The charge trapping layer may be made of or comprise polycrystalline silicon.

[0019] As mentioned above, post-processing of the transferred piezoelectric layer onto the other target substrate may be performed. The post-processing may include a thermal annealing step to increase the crystal quality of the transferred piezoelectric layer and consolidate the bonding of the transferred piezoelectric layer to the other target substrate. The post-processing may further include CMP of the transferred piezoelectric layer after the thermal annealing to increase thickness uniformity and surface quality and to eliminate multi-domain regions with different polarities caused by the annealing.

[0020] Further, a POI structure is provided, comprising a piezoelectric layer formed on or above a target substrate and obtainable by the method according to some of the examples described above, wherein the piezoelectric layer may have a thickness uniformity (thickness range across the diameter of the layer) of less than 50 nm, in particular less than 20 nm. Further, a microelectronic, micromechanical or photonic device is provided, or a microelectromechanical system (MEMS) comprising such a POI structure.

[0021] Additional features and advantages of the present invention will be described with reference to the drawings. In the description, reference is made to the accompanying drawings, which are intended to illustrate preferred embodiments of the invention. It should be understood that such embodiments do not represent the full scope of the invention.

[0022] [Fig.l] illustrates the steps of a method of manufacturing a POI structure according to an embodiment of the present invention.

[0023] [Fig.2] illustrates the effect of a variable stage speed used for CMP of a piezoelectric substrate of a donor substrate to be regenerated on the resulting corona width.

[0024] [Fig.3] illustrates the effect of varying retainer pressure used for CMP of a piezoelectric substrate of a donor substrate to be regenerated on the resulting corona width.

[0025] [Fig.4] illustrates the effect of a variable stage speed used for CMP of a piezoelectric substrate of a donor substrate to be regenerated on the resulting bright spot defects.

[0026] [Fig.5] illustrates the effect of a variable holding pressure used for CMP of a piezoelectric substrate of a donor substrate to be regenerated on the resulting light spot defects.

[0027] A method of regenerating a donor substrate is provided herein, comprising a piezoelectric substrate for manufacturing a POI structure. Regenerating the donor substrate that has been previously used to transfer a piezoelectric layer to a target substrate comprises CMP of the piezoelectric substrate to obtain a regenerated piezoelectric substrate, a layer of which can be transferred to a target substrate. According to the invention, only a relatively thin layer of piezoelectric material needs to be removed in order to obtain the regenerated piezoelectric substrate providing a piezoelectric layer to be transferred to a target substrate. The piezoelectric layer to be transformed to a target substrate has a high thickness uniformity resulting from the polishing process which is in particular carried out with a relatively hard CMP polishing pad. The method may comprise a method of manufacturing a POI structure according to Smart Cut™ technology.

[0028] [Fig.l] illustrates steps of a method of manufacturing a POI structure according to an embodiment of the present invention. The method is similar to a method described in WO 2020 / 200986 A1, but differs therefrom by the inventive method of regenerating the donor substrate.

[0029] As shown in step i) of [Fig.l], a donor substrate (pseudo-donor) 1 is provided, which comprises a piezoelectric substrate 1a formed on a support substrate (manipulator) 1b. The piezoelectric substrate 1a is made of lithium tantalate (LiTaO3) or lithium niobate (LiNbO3), and may have a thickness of about 20 μm. The support substrate 1b may be made of a material (or a plurality of materials) having a coefficient of thermal expansion close to that exhibited by a target substrate 7, i.e. the coefficient of thermal expansion of the support substrate 1b differs from that of the target substrate 7 by less than the difference in the coefficient of thermal expansion of the piezoelectric substrate 1a and that of the target substrate 7. The support substrate 1b and the target substrate 7 may have identical coefficients of thermal expansion and both substrates may, for example, be made of or comprise silicon.Additionally, both substrates can have similar thicknesses.

[0030] In order to obtain the donor substrate 1, a solid block of piezoelectric material may be attached to the support substrate 1b, for example using a molecular adhesion bonding technique. The bonding may be promoted by a dielectric bonding (adhesion) layer (not shown in [Fig.l]), for example a photo (UV) polymer layer or a layer made of or comprising silicon oxide and / or silicon nitride. The bonding method may comprise the application of a low temperature heat treatment (for example at a temperature between 50 and 300°C, typically 100°C) to sufficiently enhance the bonding energy to enable the following thinning step.

[0031] Next, the piezoelectric substrate 1a is formed by thinning, for example, by chemical mechanical polishing (CMP). The thinning step is carried out in such a way that the piezoelectric substrate 1a has a sufficiently low thickness so that the stresses generated during the heat treatment applied in a subsequent processing step are reduced. On the other hand, the thickness must be sufficiently high to provide the piezoelectric layer 3 which is to be transferred to the target substrate 7 or to provide a plurality of such layers which are to be transferred one after the other in multiple transfer steps (after respective regeneration of the donor substrate 1) to respective target substrates. The thickness of the piezoelectric substrate 1a may for example be between 5 and 400 μm, for example 20 μm, or 100 μm, or 200 μm.

[0032] In step ii) of the method illustrated in [Fig.l], hydrogen (optionally supplemented with helium) is implanted into the piezoelectric substrate 1a through the exposed surface 4 to generate a weakened layer 2 which marks the separation of the piezoelectric layer 3 with respect to the remaining part 5 of the donor substrate 1. The nature and dose of the implanted species and the implantation energy can be chosen according to the thickness of the piezoelectric layer 3 which is to be transferred to the target substrate 7 and the physicochemical properties of the piezoelectric substrate 1a. For example, for a lithium tantalate substrate, a dose of hydrogen ions between 1016 and 5« 1017 at / cm2 with an energy between 30 keV and 300 keV can be implanted to delimit the piezoelectric layer 3 with a thickness of 200 nm to 2000 nm, for example.

[0033] According to the method illustrated in [Fig.l], the implantation step ii) is followed by the step of attaching iii) the donor substrate 1 to the support substrate 7 on the side of the piezoelectric substrate 1a by molecular adhesion and / or electrostatic bonding. A dielectric assembly layer 7b may be provided between the piezoelectric substrate 1a of the donor substrate 1 and the target substrate 7. The dielectric assembly layer 7b may comprise an oxide and may be made of or comprise silicon oxide and / or silicon nitride or a stack of layers composed of these materials. Furthermore, a charge trapping layer, for example made of or comprising polycrystalline silicon, may be formed on or above the target substrate 7 in order to improve the electrical resistivity thereof if this is desired by an actual application.

[0034] The piezoelectric layer 3 is then detached from the remaining portion 5 of the donor substrate 1 to obtain iv) a POI structure 9 comprising the target substrate 7, the dielectric assembly layer 7b (if any) and the piezoelectric layer 3. Detachment at the weakened layer 2 is facilitated by heat treatment in a temperature range of about 100°C to 600°C to allow transfer of the piezoelectric layer 3 to the target substrate 7. Alternatively or additionally, detachment at the weakened layer 2 may be facilitated by the application of a blade or jet of gaseous or liquid fluid, or any other mechanical force applied to the weakened layer 2.

[0035] A post-treatment of the transferred piezoelectric layer 3 is necessary to obtain a transferred piezoelectric layer 3 having a satisfactory single-domain crystalline and surface quality (reduced roughness) and a thickness uniformity as required by real applications. The post-treatment comprises a heat treatment v) of the piezoelectric layer 3 for example at approximately 500°C in a neutral atmosphere or an atmosphere comprising oxygen. This heat treatment makes it possible to treat the crystalline defects present in the piezoelectric layer and consolidates the bonding between the piezoelectric layer 3 and the target substrate 7. However, the heat treatment causes a diffusion of the hydrogen contained in the piezoelectric layer 3, in particular in its upper part (with a thickness of approximately 50 nm or less, for example) and, consequently, the generation of a plurality of ferroelectric domains giving the upper part a multi-domain character. Indeed, the hydrogen implanted in the piezoelectric substrate 1a during the step of defining the piezoelectric layer 3 above the weakened layer 2 is distributed in this substrate according to a profile having a concentration peak at the weakening plane 2. After fracturing at the weakened layer 2, the piezoelectric layer 3 transferred to the target substrate 7 therefore has a significant concentration of hydrogen, and the heat treatment leads to the generation of multiple domains, i.e. a plurality of regions having different polarities. The performance of devices which are intended to be formed on / in the piezoelectric layer 3 would be strongly affected by such multiple domains.

[0036] In order to remove the multiple upper domains and increase the surface quality and thickness uniformity of the transferred piezoelectric layer 3, the post-processing comprises polishing the exposed surface of the piezoelectric layer 3 (see step vi) in [Fig.l]). For example, 100 to 300 nm of the upper portion of the piezoelectric layer 3 may be removed by the polishing process to achieve a predetermined target thickness, for example, about 600 nm.

[0037] The remainder 5 of the donor substrate 1 represents a donor substrate 50 to be regenerated in step vii). The regeneration of the donor substrate 50 to be regenerated comprises the CMP of the piezoelectric substrate 10a which is the remainder of the piezoelectric substrate 1a resulting from the detachment of the piezoelectric layer 3. According to the invention, only a relatively thin upper layer of the piezoelectric material of the piezoelectric substrate 10a needs to be removed by CMP in order to provide a new piezoelectric layer of a regenerated piezoelectric substrate to be transferred to another target substrate. The thickness (measured in the corona region) of the relatively thin upper layer of piezoelectric material of the piezoelectric substrate 10a to be removed by CMP is at most 2 pm, in particular at most 1.2 pm and may be in the range of 2 pm to 1.2 pm, in particular in the range of 1.8 pm to 1.4 pm, or 1.6 pm to 1.4 pm.The thickness depends on the thickness of the transferred piezoelectric layer 3 and the actual hydrogen content of the corona of the donor substrate 50 to be regenerated which remains at the edge of the piezoelectric substrate 1a, 10a after the detachment of the piezoelectric layer 3. The corona can have a width of 2 to 3 mm and a thickness of 0.9 pm or 1.2 pm. For example, for a corona with a thickness of 0.9 pm, only a layer of piezoelectric material of the piezoelectric substrate 10a with a thickness of 1.4 pm needs to be removed, and for a corona with a thickness of 1.2 pm, only a layer of piezoelectric material of the piezoelectric substrate 10a with a thickness of 1.6 pm needs to be removed.

[0038] After completion of the CMP of the donor substrate 50, the polished donor substrate 50 to regenerate may be subjected to steps ii) to vii) described above and the overall procedure may be repeated until no sufficiently thick piezoelectric layer remains on top of the support substrate 1b.

[0039] In particular, hydrogen may be implanted into the polished donor substrate 50 to be regenerated (optionally supplemented with helium) to generate a weakened layer that marks the separation of the piezoelectric layer from the remaining portion of the regenerated donor substrate. The nature and dose of the implanted species and the implantation energy may be chosen as a function of the thickness of the piezoelectric layer that is to be transferred to a target substrate and the physicochemical properties of the regenerated piezoelectric substrate. For example, for a regenerated piezoelectric substrate made of lithium tantalate, a dose of hydrogen ions between 1016 and 5« 1017 at / cm2 with an energy between 30 keV and 300 keV can be implanted to delimit a piezoelectric layer to be transferred, with a thickness of 200 nm to 2000 nm, to a target substrate.

[0040] The CMP for regenerating the donor substrate 50 to be regenerated may be performed using a relatively hard secondary polishing pad. A polishing pad comprises a top pad for contacting the surface of the material to be polished and a secondary pad. While the top pad is chosen in a conventional manner (e.g., as a rigid top pad made of polyurethane), according to the invention, the secondary pad of the CMP pad used for polishing the top portion of the piezoelectric substrate and, therefore, the piezoelectric layer to be transferred to a target substrate in the method of manufacturing a POI structure, has a hardness of more than 75 shore A.Depending on the actual application, a material with a hardness of more than 80 shore A, in particular more than 590 shore A, or a hardness in the range of 75 shore A to 95 shore A, in particular in a range of 85 shore A to 95 shore A, may be chosen for the secondary pad of the CMP pad. According to an example, the upper pad and the secondary pad of the CMP pad may have a similar hardness in the range of 20 or 10 shore A.

[0041] By using such a relatively hard secondary buffer, bonding voids formed in the final POI structure after transferring a piezoelectric layer from the regenerated piezoelectric substrate can be significantly suppressed. Therefore, more reliable bonding of the transferred piezoelectric layer to a target substrate can be achieved.

[0042] According to various embodiments, the secondary buffer used for the CMP of the piezoelectric substrate of the donor substrate 50 to be regenerated has a density greater than 0.6 g / cm3, in particular greater than 0.7 g / cm3 or 0.8 g / cm3, or in the range of 0.6 g / cm3 to 0.9 g / cm3, and the secondary buffer may have a compressive force deflection greater than 689.476 kPa, in particular 758.423 kPa or 827.371 kPa, or in the range of 689 kPa to 900 kPa.

[0043] For example, the secondary pad may be made of thermoplastic polyurethane and have a hardness of 91 shore A, a density of 0.86 g / cm3, a compressive force deflection of 861.845 kPA (125 psi), and a thickness of 0.89 mm (35 mil) compared to a typical secondary pad commonly used for CMP of a piezoelectric layer of a donor substrate used in a Smart Cut™ process to obtain a POI structure made of polyurethane and having a hardness of 53 shore A, a density of 0.4 g / cm3, a compressive force deflection of 641.212 kPA (93 psi), and a thickness of 0.79 mm (31 mil).

[0044] According to one embodiment, the CMP for regenerating the donor substrate 50 to be regenerated is carried out by means of a CMP suspension consisting of an aqueous suspension of amorphous silicon with a weight percentage of the amorphous silicon of less than 20, in particular in the range of 10 to 18 or 12.5 to 17.5. The amorphous silicon used for the CMP suspension may comprise or consist of precipitated amorphous silicon particles with diameters in the range of 40 to 60 nm. For example, Klebosol 30HB50 having amorphous silicon for 25 to 35 wt% and an average diameter of the precipitated amorphous silicon particles of 50 nm may be suitably diluted with water, for example with a dilution of 1:1, in order to obtain the CMP suspension used for the CMP for regenerating the donor substrate 1.

[0045] Other parameters of the CMP process for regenerating the donor substrate 50 to be regenerated as well as the overall process for manufacturing the POI structure illustrated in [Fig.l] can be chosen in a usual manner. Optimization of the parameters for an actual application can be carried out easily by standard experiments.

[0046] For the polishing step vii) of the method illustrated in [Fig.l], the donor substrate 50 to be regenerated is positioned on a rotating head and brought into contact with a rotating polishing pad. According to particular embodiments, the head is rotated 30 to 110 revolutions per minute (rpm), in particular 40 to 105 revolutions per minute, and the polishing pad is rotated in the same direction as the head at a different speed compared to the head in the range of 50 to 90 revolutions per minute, in particular 60 to 80 revolutions per minute (platen speed).

[0047] The wafer pressure applied to the donor substrate 50 to be regenerated to press it against the polishing pad may be in the range of 44.8 kPa (6.5 psi) to 58.6 kPa (8.5 psi) and the retainer pressure applied to the retainer / ring to hold the donor substrate 50 to be regenerated may be in the range of 37.9 kPa (5.5 psi) to 44.8 kPa (6.5 psi).

[0048] For example, to regenerate a donor substrate 50 to be regenerated made of lithium tantalate, the following parameter results in few edge crack defects and voids edge bonding after bonding to a target substrate:

[0049] For a crown with a thickness of 0.9 μm, the head speed is chosen at about 40 rpm, and at about 105 rpm for a crown with a thickness of 1.2 μm. The platen speed is about 65 rpm. The pressure conditions may depend on the wafer size. For 150 mm, a wafer pressure of up to 55.2 kPa (8 psi) and for 200 mm, a wafer pressure of up to 48.3 kPa (7 psi) are chosen. The pressure used for the wafer and ring is 55.2 kPa (8 psi) and 41.4 kPa (6 psi), respectively, for a crown thickness of 0.9 pm, and 48.3 kPa (7 psi) and 41.4 kPa (6 psi), respectively, for a crown thickness of 1.2 pm and a 150 mm wafer and a crown thickness of 0.9 pm and a 200 mm wafer.

[0050] As an example, Figures 2 to 5 illustrate advantageous technical effects that can be provided by the CMP performed within the parameter ranges described above to regenerate the donor substrate 50 to be regenerated for a 200 mm wafer. A satisfactory reduction in crown widths can be achieved as illustrated in Figures 2 and 3, and a satisfactory reduction in light spot defects (LPD) representing the number of edge bonding voids and crown defects observed for a final POI structure including the piezoelectric layer before the post-processing CMP of the transferred piezoelectric layer can be achieved as illustrated in Figures 4 and 5. The abscissa of [Fig.2] indicates the stage speed in rpm and the ordinate indicates the crown width (CW) in mm (with 3H denoting the orientation for inspection, for a wafer including a land positioned at 6H, 3H is on the right side).Accordingly, the abscissa of [Fig.3] indicates the retainer pressure in psi and the ordinate indicates the crown width in mm. The abscissa of [Fig.4] indicates the platen speed in rpm and the ordinate indicates the LPD in units of 1 / cm2. The abscissa of [Fig.5] indicates the retainer pressure in psi and the ordinate indicates the LPD in units of 1 / cm2.

[0051] [Fig. 2] illustrates the effect of varying platen speed on the resulting crown length. The retainer and insert pressures are set at 41.4 kPa (6 psi) and 48.3 kPa (7 psi), respectively. Increasing the platen speed advantageously results in a decrease in the remaining crown length.

[0052] [Fig.3] illustrates the effect of varying retainer pressure on the resulting crown length. The platen speed is set at 80 rpm. The pad pressure is set at 7 psi. A retainer pressure of 6 appears to be preferable with higher reproducibility.

[0053] [Fig.4] illustrates the effect of varying stage speed on the resulting LPDs. The retainer and pad pressures are set at 41.4 kPa (6 psi) and 48.3 kPa (7 psi), respectively. The results are worse for stage speeds above 65 rpm.

[0054] [Fig.5] illustrates the effect of varying retainer pressure on the LPDs results. The platen speed is set at 80 rpm. The pad pressure is set at 7 psi. In principle, LPDs can be reduced with an increase in the retainer pressure.

Claims

Claims

1. A method of regenerating a donor substrate (50) for manufacturing a piezoelectric-on-insulator, POI, structure, comprising the steps of providing the donor substrate (50) to be regenerated comprising a support substrate (1b) and a first piezoelectric substrate (10a) formed on the support substrate (1b) comprising or consisting of one of lithium tantalate and lithium niobate, wherein the first piezoelectric substrate (10a) is a second piezoelectric substrate (1a) from which a piezoelectric layer (3) has been transferred to a target substrate (7); and performing chemical mechanical polishing, CMP, of the first piezoelectric substrate (10a) to obtain a regenerated donor substrate comprising a regenerated piezoelectric substrate, wherein the CMP comprises removing a layer of the first piezoelectric substrate (10a) with a thickness of at most 2 pm, in particular at most 1.2 pm.

2. The method according to claim 1, wherein a layer of the first piezoelectric substrate (10a) is removed, with a thickness in the range of 2 pm to 1.2 pm, particularly in the range of 1.8 pm to 1.4 pm, or 1.6 pm to 1.4 pm.

3. The method according to claim 1 or 2, wherein the CMP is carried out by means of a CMP pad comprising a secondary pad with a hardness of more than 75 shore A, in particular more than 80 shore A or 90 shore A, or with a hardness in the range of 75 shore A to 95 shore A, in particular in a range of 85 shore A to 95 shore A.

4. The method of claim 3, wherein the secondary pad has a density greater than 0.6 g / cm3, in particular greater than 0.7 g / cm3 or 0.8 g / cm3, or in the range of 0.6 g / cm3 to 0.9 g / cm3, and / or the secondary pad has a compressive force deflection greater than 689.476 kPa, in particular 758.423 kPa or 827.371 kPa, or in the range of 689 kPa to 900 kPa.

5. The method of any preceding claim, wherein the CMP is carried out using a CMP suspension consisting of an aqueous suspension of amorphous silicon with a weight percentage of amorphous silicon less than 20, particularly in the range of 10 to 18 or 12.5 to 17.

5.

6. The method of claim 5, wherein the amorphous silicon comprises or consists of precipitated amorphous silicon particles with diameters in the range of 40 to 60 nm.

7. The method according to any one of the preceding claims, wherein the CMP is carried out with a head speed in the range of 30 to 110 revolutions per minute, in particular 40 to 105 revolutions per minute, and a platen speed in the range of 50 to 90 revolutions per minute, in particular 60 to 80 revolutions per minute.

8. The method of any preceding claim, wherein the CMP comprises applying a wafer pressure to the donor substrate (50) to be regenerated to press it against a polishing pad in the range of 44.8 kPa to 58.6 kPa, and a retainer pressure in the range of 37.9 kPa to 44.8 kPa.

9. A method of manufacturing a piezoelectric-on-insulator, POI, structure, comprising the steps of performing the steps of any preceding claim; and transferring a piezoelectric layer from the regenerated piezoelectric substrate to another target substrate, comprising bonding the regenerated donor substrate to the other target substrate on the side of the regenerated piezoelectric substrate and fracturing the regenerated piezoelectric substrate at the weakened layer.

10. The method of claim 9, further comprising performing heat treatment and CMP of the transferred piezoelectric layer to the other target substrate.

11. A piezoelectric-on-insulator, POI, structure comprising a piezoelectric layer formed on or above a target substrate and obtainable by the method of any one of claims 9 and 10

12. IV. Microelectronic, micromechanical or photonic device or microelectromechanical system comprising the POI structure according to claim 11.