Method for preparing a thin layer of piezoelectric material by ion beam etching

Abstract

The invention relates to a method for preparing a single-domain thin film (4) of piezoelectric material, the method comprising the finishing of a first layer (8) transferred to a support (2). The finishing comprises a heat treatment of the free face (9) of the first layer, followed by thinning of said layer to form the single-domain thin layer (4). According to the invention, the thinning of the first layer (8) comprises reactive ion etching using a plasma prepared by a mixture of a rare gas and a chlorinated gas.

Description

METHOD FOR PREPARING A THIN LAYER OF PIEZOELECTRIC MATERIAL BY ION ETCHING FIELD OF INVENTION

[0001] The present invention relates to a method for preparing a thin film of piezoelectric material. This method can be used, in particular, to form a piezoelectric-on-insulator (POI) structure, comprising the thin film transferred to a substrate via an interlayer. Such a structure finds application in the fields of microelectronics, microsystems, and photonics. It can be used to form radio frequency (RF) components or to construct such components, in particular filters or resonators based on elastic wave components, for example, surface elastic wave components. It can also be used to form optical modulators, integrated photonic components, and components for quantum or electro-optical technology. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Document WO2020200986A1 proposes a manufacturing process for such a POI substrate that preserves the single-domain nature of the thin film. This document describes transferring a layer taken from a donor substrate containing a piezoelectric material onto a support, via an implantation step of so-called "light" species (typically hydrogen and / or helium) according to the principles of Smart Cut® technology. Following this transfer, the extracted layer is processed in a finishing sequence comprising a heat treatment followed by a polishing step, this finishing sequence resulting in the formation of the piezoelectric, single-crystal, single-domain thin film.

[0003] Document WO2023084164A1 proposes replacing, at least partially, the polishing step in the finishing sequence with ion etching, as this polishing step tends to degrade the uniformity of the thin film. This document specifically proposes performing this ion etching by exposing the free side of the thin film to argon or CHF3 ions.

[0004] The CN116721966 document also proposes a finishing sequence including a polishing step and an ion beam etching step. SUBJECT OF THE INVENTION

[0005] The present invention aims to improve this prior art. In particular, it aims to provide an ion etching step for a finishing sequence of a piezoelectric thin film deposited on a substrate, this ion etching step being able to be carried out more quickly and without excessively degrading the thickness uniformity of the thin film after its transfer. BRIEF DESCRIPTION OF THE INVENTION

[0006] With a view to achieving one of these goals, the object of the invention proposes a method for preparing a single-domain thin film of piezoelectric material comprising the following steps: – the implantation of so-called “light” species in a first face of a donor substrate comprising the piezoelectric material to form a plane of embrittlement and define a first layer between the plane of embrittlement and the first face of the donor substrate; – the assembly of the first face of the donor substrate to a support by means of an intercalated layer; – the fracturing of the donor substrate at the level of the plane of embrittlement to transfer the first layer onto the support and expose a free face of the first layer; – the finishing of the first layer, this finishing comprising a heat treatment of its free face, followed by its thinning to form the single-domain thin film.

[0007] According to the invention, the thinning of the first layer includes a reactive ion etching using a plasma prepared by a mixture of a rare gas and a chlorinated gas.

[0008] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: the noble gas comprises argon; the chlorinated gas comprises chlorine or boron trichloride; the mixture of a noble gas and a chlorinated gas consists of a mixture of argon and chlorine; the mixture of a noble gas and a chlorinated gas consists of a mixture of argon and boron trichloride; the mixture of a noble gas and a chlorinated gas consists of a mixture of argon, chlorine and boron trichloride.The light species consist of hydrogen and / or helium ions; reactive ion etching is followed by mechanochemical polishing of the free face of the first layer; the thinning consists solely of reactive ion etching and mechanochemical polishing; ion etching is preceded by mechanochemical polishing; mechanochemical polishing leads to a thinning of the first layer to a thickness of less than 100 nm; the heat treatment of the free face of the first layer is carried out at a temperature between 300°C and the Curie temperature of the piezoelectric material composing the thin film, and for a duration between 30 minutes and 10 hours in a determined gaseous atmosphere; the interlayer is dielectric, for example comprising a silicon oxide, a silicon oxynitride or a silicon nitride; the piezoelectric material of the thin film is LiTaO3 or LiNbO3.

[0009] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures in which:

[0010]

[0011] Lare represents a POI substrate that can be manufactured using a process according to the invention;

[0012]

[0013] Lare represents a manufacturing process for the POI substrate of the ; DETAILED DESCRIPTION OF THE INVENTION

[0014] We briefly recall first the main manufacturing steps of a POI substrate according to an implementation method of the Smart Cut® technology.

[0015] With reference to Figures 1 and 2, this process generally involves transferring a first crystalline piezoelectric layer 8, exfoliated from a donor substrate 5 comprising a crystalline piezoelectric material, onto a support 2, via an intercalated layer 3.

[0016] The crystalline piezoelectric material can be, for example, lithium tantalate or lithium niobate. The piezoelectric material has any crystal direction, for example, between 15° and 70°RY. The donor substrate 5 can be a bulk substrate made entirely of the piezoelectric material, as shown in Figure 1, or it can be a composite substrate consisting of a bulk portion, for example, of silicon, sapphire, crystalline or polycrystalline silicon carbide, on which rests a thick layer of piezoelectric material from which the first layer 8 is taken. The thick layer can be bonded to and retained by any technique, for example, by molecular adhesion or by means of an adhesive layer, for example, a polymer adhesive. The advantages of a donor substrate thus constituted are presented in document US2020186117.

[0017] In some embodiments, the support 2 consists of a solid conductive or semiconducting substrate. In other embodiments, the support 2 comprises a basic semiconductor substrate, generally exhibiting a high resistivity greater than 1000 ohms·cm, surmounted by a surface charge-trapping layer. This trapping layer is located on the side of the first face of the support 2, which is intended to receive the thin film 4. The trapping layer can be made of polycrystalline silicon, as is customary, or of any material with sufficient trap density. In these embodiments, the interlayer 3 is in contact with both the trapping layer and the thin film 4.

[0018] According to the transfer technique based on the implantation of light species, and with reference to Figure 2b, light species, typically hydrogen and / or helium in ionic form, are implanted in a front face 6 of the donor substrate 5 to form a buried weakening plane 7. The first layer 8 is thus defined between the weakening plane 7 and the first face 6 of the donor substrate 1.

[0019] In a subsequent step, as shown in Figure 2c, this front face 6 of the donor substrate is joined to an exposed face 6' of the support 2, here via an interlayer 3. By way of example, the interlayer 3 can be formed of at least one dielectric material and comprise, or be composed of, silicon dioxide, silicon oxynitride, or silicon nitride. It can be formed on either or both of the donor substrate 5 and the support 2 prior to their joining.

[0020] The donor substrate 5 is then fractured at the embrittlement plane 7, for example by means of moderate heat treatment and / or the application of mechanical stress. The first layer 8 of the donor substrate 5 is then released to expose a free face 9 of this first layer 8, the other face 6 being bonded to the support 2 via the intercalated dielectric layer 3, and more specifically in direct contact with the intercalated dielectric layer 3.

[0021] A remaining portion 5' of the donor substrate 5, after the removal of the first layer 8, can be reconditioned in order to remove a new layer, in a removal cycle similar to that which has just been described.

[0022] As stated in the introduction to this application, it is necessary to provide for the finishing of the first layer 8 transferred and depositioned onto the support 2, to form a "useful" thin layer 4. These steps generally aim to improve the crystalline quality of the first layer 8 and its surface finish (e.g., its roughness) and, where appropriate, adjust its thickness to a target thickness, typically less than 1.1 µm. It is also sought to maintain the thickness of the first layer 8 as uniformly as possible over its entire extent, bearing in mind, for reference, that the thickness of the first layer immediately after the fracture step typically varies within a range of approximately 5 nm.

[0023] The finishing of the first layer 8 may include a heat treatment step, followed by a thinning step of this layer 8 to form the single-domain thin layer 4. This thinning may include ion etching of the first layer, possibly assisted by a polishing step.

[0024] The heat treatment step of the first layer 8 may consist of exposing the free face 9 of the first layer 8 to a neutral atmosphere, for example an atmosphere of nitrogen or argon, or containing oxygen, at a temperature between 300°C and the Curie temperature of the ferroelectric material composing the first layer 8, for a duration of between 30 minutes and 10 hours. Following these manufacturing steps, the final substrate shown in Figure 8 is obtained.

[0025] In order to improve this prior art process, the Applicant has focused on the ion etching process included in the first layer thinning phase.

[0026] It should be noted that such a process is conducted in equipment comprising an etching chamber designed to maintain a controlled subatmospheric pressure (typically between 2 mTorr - 1.33 Pa and 100 mTorr - 13.33 Pa). The etching chamber is connected to a plasma source, for example, a capacitively coupled plasma (CCP) or an inductively coupled plasma (ICP), to activate the etching ions and chemical radicals (referred to as "ions" in the remainder of this description). A radio frequency (RF) source provides the energy to accelerate the ions to the substrate surface. The typical frequency used is 13.56 MHz. Some systems use a dual RF source to separately control the ion density and energy. The equipment has a network of valves and regulators to introduce and mix the ionized gases in a controlled manner.The substrate is placed on a support arranged in the etching chamber to expose its free face to the ions that are projected onto it.

[0027] In a first series of experiments, the Applicant explored the use of various ion species (SF6, O2, Ar) as summarized in the following table, for identical etching conditions (pressure in the chamber, exposure time, source power).

[0028] For each test, reported in a separate line of the table, the removal rate ("speed", in nm / min) and the degradation of uniformity of layer thickness ("Deg. of uniformity" in nm) were recorded, for the ions or mixture of ions chosen.

[0029] Thickness uniformity is measured as the difference between the highest and lowest recorded thicknesses when the thickness measurement has been carried out over a plurality of locations in the layer (typically on the order of 20 to 50 measurement locations, by reflectometry or ellipsometry, these locations aiming to cover as much of the extent of the layer as possible).

[0030] Thickness uniformity degradation corresponds to the difference between the thickness uniformity of the layer before ion etching and the thickness uniformity of the layer after ion etching. A negative value therefore indicates a deterioration in thickness uniformity, and a positive value indicates an improvement in thickness uniformity.

[0031] # SF6 O2 Ar Velocity Degree of uniformity

[0032] 1 30% 0% 70% 39 9

[0033] 2 30% 70% 0% 35 13

[0034] 3 0% 0% 100% 91 -8

[0035] Firstly, these experimental results show that etching with argon alone achieves a removal rate nearly three times greater than that achieved with other ions. Further measurements revealed that when the ions used in ion etching contained fluorine (such as SF6 or F2), the removal rate tended to decrease. It appears that fluorine reacts chemically with the piezoelectric material. This chemical reaction tends to slow down the removal process because it leads to the formation of lithium fluoride (LiF), which generates a non-volatile layer on the material's surface.

[0036] It should be noted that simply removing the fluorine-containing ions to retain only the argon ions during ion etching is not a viable solution, as argon does not react chemically with the material and the thinning then results solely from the physical phenomenon of layer sputtering. This type of thinning can lead to unfavorable properties of the thinned layer, such as excessive roughness.

[0037] Therefore, the Applicant turned to other reactive ions that do not contain fluorine. In a second series of experiments, she compared the ion etching of the first layer using a mixture of argon and chlorine ions to the ion etching obtained with CHF3, which contains fluorine and is recommended in the prior art document presented in the introduction. The table below summarizes some of the results obtained during this second series of experiments. In addition to the nature of the ions and the degree of uniformity degradation, the table presents the target removal thickness ("Target Thickness," in nm) of the process and the actual thickness obtained ("Removed Thickness," in nm).

[0038] # Ions Ep.visée Ep. retiré Dég. d'uniforme

[0039] 1 Cl2+Ar 60 55 -14

[0040] 2 Cl2+Ar 60 55 -14

[0041] 3 Cl2+Ar 60 54 6

[0042] 4 Cl2+Ar 60 54 -9

[0043] 5 Cl2+Ar 60 54 3

[0044] 6 Cl2+Ar 60 54 -12

[0045] 7 Cl2+Ar 120 110 -13

[0046] 8 Cl2+Ar 120 109 -7

[0047] 9 CHF3 60 61 -89

[0048] 10 CHF 3 60 53 -62

[0049] 11 CHF 3 60 53 -63

[0050] 12 CHF3 120 106 -143

[0051] In addition, an average removal rate of 74 nm / min was observed during ion etching using a mixture of argon and chlorine ions, compared to 50 nm / min for ion etching using CHF3 ions, which confirms the detrimental effect of fluorine on this parameter.

[0052] The table shows that, while benefiting from a relatively fast etching speed, the mixture of argon and chlorine ions causes a degradation of thickness uniformity of the first layer that is much less significant than that caused by ionic etching using CHF3 ions, which nevertheless has a relatively slow etching speed.

[0053] Furthermore, the degradation of thickness uniformity in the case of the mixture of argon and chlorine ions seems to be little related to the thickness removed, which is not the case for the degradation of thickness uniformity obtained during ion etching using CHF3 ions.

[0054] Besides chlorine, similar results are expected for plasmas prepared with a mixture of argon and with other gases, particularly chlorinated gases such as boron trichloride (BCl3). Naturally, fluorine-based gases are excluded to avoid limiting the removal rate. In a preliminary experiment using BCl3-based etching, a good smoothing effect was observed without damaging the treated surface of the piezoelectric layer, for example, by forming an amorphous surface portion as occurs with some other gases.

[0055] Similarly, argon could be replaced by another rare gas, these gases being relatively unreactive and therefore producing an essentially spraying effect during etching.

[0056] The invention therefore takes advantage of these results to propose a process for preparing the thin film 4, the main steps of which have been presented previously.

[0057] In a very general way, in this preparation process, the thinning of the first layer 8 is carried out at least in part by reactive ion etching using a plasma prepared by a mixture of rare gas and a chlorinated gas.

[0058] This can be a mixture of argon and chlorine, a mixture of argon and boron chloride (such as boron trichloride), or a mixture of argon, chlorine, and boron chloride (such as boron trichloride). As mentioned previously, the plasma prepared from these gases is composed of ions resulting from the ionic decomposition of these gases, chemical radicals, and other species.

[0059] Depending on the thickness of the first layer 8 transferred to the support and the target thickness for the thin layer 4 obtained at the end of the finishing sequence, the ion etching step can lead to the removal of a thickness of approximately 50nm to 900nm of piezoelectric material, and typically removes a thickness between 50nm and 400nm.

[0060] This thinning can be achieved at high removal speeds, strictly greater than 50nm / min, which promotes production rate.

[0061] Thickness uniformity can be less than 60nm as measured by reflectometry or ellipsometry, or even less than 30 nm and even reach 15 nm or less.

[0062] According to a particularly interesting embodiment, the preparation process of the invention comprises, between the finishing heat treatment step and the thinning step, a mechanochemical polishing of the first layer. To avoid degrading the uniformity of this layer, the removal during this polishing is limited to a thickness of less than 100 nm.

[0063] Whether or not this preliminary smoothing step by chemical polishing is implemented, the thin-film preparation process can be followed, after the ion etching step, by chemical polishing of the free face. This step prepares the free face 8 so that it ultimately exhibits (if this was not achieved after the thinning step) a low roughness, for example, less than 0.5 nm RMS 5x5 µm by atomic force measurement (AFM), without affecting the uniformity of this layer. Again, the material removal during this polishing will be limited to a thickness of less than 100 nm.

[0064] Preferably, to limit the number of polishing treatments, the thinning consists solely of reactive ion etching and, following this etching, mechanochemical polishing to eliminate residual excessive roughness.

[0065] The reactive ion etching described herein is distinguished by its ability to combine rapid etching with the preservation of essential thin-film properties, such as thickness uniformity. Compared to prior art, particularly processes using fluorinated ions (CHF3), this invention achieves a higher removal rate while minimizing uniformity degradation. Experimental results confirm the effectiveness of this approach, with significant gains in precision and productivity.

[0066] Of course the invention is not limited to the implementation methods described and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

A process for preparing a single-domain thin film (4) of piezoelectric material comprising the following steps: – the implantation of so-called "light" species in a first face (6) of a donor substrate (5) comprising the piezoelectric material to form a weakening plane (7) and define a first layer (8) between the weakening plane (7) and the first face (6) of the donor substrate (5); – the assembly of the first face (6) of the donor substrate (5) to a support (2) via an intercalated layer (3); – the fracturing of the donor substrate (5) at the level of the weakening plane (7) to transfer the first layer (8) onto the support (5) and expose a free face (9) of the first layer (8); – the finishing of the first layer (8), this finishing comprising a heat treatment of its free face (9), followed by its thinning to form the single-domain thin film (4);the process being characterized in that the thinning of the first layer (8) comprises a reactive ion etching using a plasma prepared by a mixture of a noble gas and a chlorinated gas.; A method according to the preceding claim, wherein the noble gas comprises argon. A process according to any one of the preceding claims, wherein the chlorinated gas comprises chlorine or boron trichloride. A process according to claim 1 wherein the mixture of a noble gas and a chlorinated gas consists of a mixture of argon and chlorine. A process according to claim 1 wherein the mixture of a noble gas and a chlorinated gas consists of a mixture of argon and boron trichloride. A process according to claim 1 wherein the mixture of a noble gas and a chlorinated gas consists of a mixture of argon, chlorine and boron trichloride. A process according to any one of the preceding claims, wherein the light species consist of hydrogen and / or helium ions. A method according to any one of the preceding claims in which reactive ion etching is followed by mechanochemical polishing of the free face (9) of the first layer (8). A method according to the preceding claim in which the thinning consists solely of reactive ion etching and mechanochemical polishing. A method according to any one of claims 1 to 8 in which ion etching is preceded by mechanochemical polishing. A method according to any one of the preceding claims in which mechanochemical polishing leads to a thinning of the first layer (8) to a thickness of less than 100nm. A method according to any one of the preceding claims wherein the heat treatment of the free face (9) of the first layer (8) is carried out at a temperature between 300°C and the Curie temperature of the piezoelectric material composing the thin layer (4), and for a duration between 30 minutes and 10 hours in a determined gaseous atmosphere. A method according to any one of the preceding claims wherein the interlayer is dielectric, for example comprising a silicon oxide, a silicon oxynitride or a silicon nitride. A method according to any one of the preceding claims wherein the piezoelectric material of the thin film (4) is LiTaO3 or LiNbO3.

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