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

Abstract

The invention relates to a method for preparing a single-domain thin film (4) of piezoelectric material, comprising finishing a first layer (8) transferred onto a substrate (2). The finishing process includes heat treatment of the free face (9) of the first layer, followed by its thinning to form the single-domain thin film (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 noble gas and a chlorinated gas. Figure 2

Description

Title of the invention: METHOD FOR PREPARING A THIN LAYER OF MATERIAL ION ETCHING PIEZOELECTRIC FIELD OF INVENTION

[0001] The present invention relates to a method for preparing a thin film of piezoelectric material. This method can, in particular, be used 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 of quantum or electro-optical technology. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Document WO2020200986A1 proposes a method for manufacturing 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 comprising a piezoelectric material onto a support, via an implantation step of so-called "light" species (typically hydrogen and / or helium) in accordance with 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 leading to the formation of the piezoelectric, single-crystal, single-domain thin film.

[0003] Document WO2023084164A1 proposes replacing, at least in part, the polishing step in the finishing sequence with ion etching, since this polishing step tends to degrade the uniformity characteristic of the thin film. This document specifically proposes performing this ion etching by exposing the free face of the thin film to argon or CHF3 ions. SUBJECT OF THE INVENTION

[0004] 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 to be carried out more quickly and without excessively degrading the uniformity of thickness of the thin layer after its transfer. BRIEF DESCRIPTION OF THE INVENTION

[0005] 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 piezoelectric donor substrate to form a weakening plane and define a first layer between the weakening plane and the first face of the donor substrate; - the assembly of the first face of the donor substrate to a support via an intercalated layer; - fracturing the donor substrate at the level of the weakening plane 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 including a heat treatment of its free face, followed by its thinning to form the single-domain thin film.

[0006] 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.

[0007] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: • the noble gas includes argon; • Chlorinated gas includes 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 and chlorine consists of a mixture of argon, chlorine and boron trichloride. • Light species are made up 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 (5) is LiTaO3 or LiNbO3. Brief description of the drawings

[0008] 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:

[0009] [Fig.1]

[0010] Fig. 1 represents a POI substrate that can be manufactured using a process according to the invention;

[0011] [Fig.2]

[0012] Fig. 2 represents a method for manufacturing the POI substrate of Fig. 1; DETAILED DESCRIPTION OF THE INVENTION

[0013] We briefly recall first the main manufacturing steps of a POI substrate according to an implementation method of the Smart Cut® technology.

[0014] With reference to figures 1 and 2, this process generally involves transferring a first crystalline piezoelectric layer 8, exfoliated from a donor substrate 5, onto a support 2, via an intercalated layer 3.

[0015] The crystalline piezoelectric material may 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 may be a bulk substrate made entirely of the piezoelectric material, as shown in [Fig. 2], or it may be a composite substrate consisting of a bulk portion, for example, silicon, on which rests a thick layer of piezoelectric material from which the first layer 8 is taken. The thick layer may be assembled and retained to the bulk portion by any possible 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.

[0016] In some embodiments, the support 2 consists of a solid conductive or semiconductor substrate. In other embodiments, the support 2 comprises a basic semiconductor substrate, generally having a high resistivity greater than 1000 ohms.cm, surmounted by a trapping layer. of surface electrical charges. 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.

[0017] 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.

[0018] In a subsequent step, as shown in Figure 2c, this front face 6 of the donor substrate is assembled to an exposed face 6' of the support 2, here via an interlayer 3. By way of example, the interlayer 3 may be formed of at least one dielectric material and comprise, or be composed of, silicon dioxide, silicon oxynitride, or silicon nitride. It may be formed on either or both of the donor substrate 5 and the support 2 prior to their assembly.

[0019] The donor substrate 5 is then fractured at the level of the embrittlement plane 7, for example by means of a moderate heat treatment and / or the application of a mechanical force. 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 in direct contact with the intercalated dielectric layer 3 of the support 2.

[0020] 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.

[0021] 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 thin "useful" 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. 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.

[0022] 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 layer thin single domain 4. This thinning may include ion etching of the first layer, possibly assisted by a polishing step.

[0023] 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 one containing oxygen, raised to a temperature between 300°C and the Curie temperature of the ferroelectric material composing the first layer 8, for a period of between 30 minutes and 10 hours. Following these manufacturing steps, the final substrate shown in [Fig. 1] is obtained.

[0024] In order to improve this prior art process, the Applicant has focused on the ion etching process included in the thinning phase of the first layer.

[0025] It should be noted that such a process is carried out 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 associated with a plasma source, for example, a capacitively coupled plasma (CCP) or an inductively coupled plasma (ICP) designed 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 density and energy of the ions. The equipment has a network of valves and regulators for introducing and mixing the gases ionized by the source in a controlled manner.The substrate is placed on a support arranged in the etching chamber to expose its free face to the ions projected onto it.

[0026] 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).

[0027] For each test, reported in a separate line of the table, the removal rate ("speed", in nm / min) and the degradation of uniformity of thickness of the layer ("Deg. of uniformity" in nm) were recorded, for the ions or the mixture of ions chosen.

[0028] Thickness uniformity is measured as the difference between the highest and lowest recorded thicknesses when the thickness measurement has been carried out on 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 the extent of the layer as best as possible).

[0029] The degradation of thickness uniformity 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 of thickness uniformity and a positive value an improvement in thickness uniformity. # SES Ai Speed ​​B tu ** & H-rh 0 H £ H' 1 30% 3¾ 70¾ 39 9 2 30% 7 n û % 3 RO 0% 0% icm- —8

[0030] It is noted firstly, based on these experimental results, that etching with argon alone achieves a removal rate nearly three times greater than the removal rates achieved with other species. Further measurements led to the conclusion that when the ions used in ionic etching included fluorine (such as SF6 or F2), the removal rate tended to decrease. It therefore appears that fluorine reacts chemically with the piezoelectric material, and this chemical reaction tends to limit the removal rate.

[0031] It should be noted that simply removing the fluorine-containing ions to retain only the argon ions during ion etching cannot be a viable solution, because 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, for example, excessive roughness.

[0032] The Applicant also turned to other reactive ions that do not contain fluorine. In a second series of experiments, it 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 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). 1? Ions Th. -13 012-Ax 123 10 S — 7 9 CHE3 5 G 61 “5 9 CHF3 60 53 “ 62 11 CHF3 An Su — 63 CHF3 120 106 -143

[0033] In addition, an average removal rate of 74 nm / min was observed during ion etching using a mixture of argon and chlorine ions against 50 nm / min for ion etching using CHF3 ions, which confirms the detrimental effect of fluorine on this parameter.

[0034] It can be seen from the table 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 much less significant than that caused by ionic etching using CHF3 ions, which nevertheless has a relatively slow etching speed.

[0035] 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.

[0036] In addition to chlorine, identical results are expected for plasmas prepared with a mixture of argon and with other gases, particularly chlorinated gases such as boron trichloride (BC13). It is of course excluded that this other gas should be fluorine-based, so as not to limit the removal rate. In particular, a preliminary experiment using BC13-based etching observed a good smoothing effect, without damaging the treated surface of the piezoelectric layer, for example by forming an amorphous surface portion as is the case when using certain other species.

[0037] Similarly, argon could be replaced by another rare gas, these gases being relatively unreactive and therefore producing an essentially spraying effect during etching.

[0038] The invention therefore takes advantage of these results to propose a method for preparing the thin film 4, the main steps of which have been presented previously.

[0039] 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.

[0040] This may 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 previously stated, the plasma prepared from these gases is composed of ions resulting from the ionic decomposition of these gases, chemical radicals, and other species.

[0041] Depending on the thickness of the first layer 8 transferred to the support and depending on 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 the order of 50nm to 900nm of piezoelectric material, and typically removes a thickness between 50nm and 400nm.

[0042] This thinning can be achieved at high removal speeds, strictly greater than 50nm / min, which promotes the production rate.

[0043] 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.

[0044] According to a particularly advantageous 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.

[0045] 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 exhibits (if this was not achieved after the thinning step) a low final roughness, for example less than 0.5 nm RMS 5x5 pm by atomic force measurement (AFM), without affecting the uniformity of this layer. Again, the removal during this polishing will be limited to a thickness of less than 100 nm.

[0046] Preferably, to limit the number of polishing treatments, the thinning consists solely of reactive ion etching and, at the end from this engraving, mechanochemical polishing to eliminate residual excessive roughness.

[0047] The reactive ion etching proposed in this description is distinguished by its ability to combine rapid etching with preservation of essential thin-film properties, such as thickness uniformity. Compared to the prior art, particularly processes using fluorinated ions (CHF3), the invention achieves a higher removal rate while limiting uniformity degradation. Experimental results confirm the effectiveness of this approach, with significant gains in terms of precision and productivity.

[0048] Of course the invention is not limited to the modes of implementation described and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

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8. Demands Method 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 piezoelectric donor substrate (5) 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 fracture 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 including a heat treatment of its free face (9), followed by its thinning to form the single-domain thin layer (4); The process is characterized in that the thinning of the first layer (8) comprises reactive ion etching using a plasma prepared by a mixture of a noble gas and a chlorinated gas. The process 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 method according to claim 1, wherein the mixture of a noble gas and a chlorinated gas consists of a mixture of argon and chlorine, or a mixture of argon, chlorine, and boron trichloride. A method 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, wherein reactive ion etching is followed by mechanochemical polishing of the free face (9) of the first layer (8).

9. A method according to the preceding claim in which the thinning consists solely of reactive ion etching and mechanochemical polishing.

10. A method according to any one of claims 1 to 8 wherein ion etching is preceded by mechanochemical polishing.

11. A method according to any one of the preceding claims in which mechanochemical polishing leads to a thinning of the first layer (8) of a thickness of less than 100nm.

12. 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 film (4), and for a duration between 30 minutes and 10 hours in a determined gaseous atmosphere.

13. 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.

14. A method according to any one of the preceding claims wherein the piezoelectric material of the thin film (5) is LiTaO3 or LiNbO3.

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