Method for preparing single-domain thin layers composed of lithium-containing ferroelectric materials

Abstract

The present invention relates to a method for preparing a thin single-domain layer (4) made of a lithium-containing ferroelectric material. The method includes providing a first single-domain layer (8) made of a lithium-containing ferroelectric material, the layer being bonded to a carrier (2), the first layer (8) having a lithium-rich surface thickness (11). The preparation method includes a first step of wet-cleaning the free side (9) of the first layer (8), which can remove the lithium-rich surface layer. It then includes a second step aimed at removing or preventing the appearance of lithium-rich and hydrogen-rich dendrites (12) that tend to nucleate on the free side (9) of the first layer (8) when the first layer (8) lacks the lithium-rich surface layer.

Description

[Technical Field]

[0001] The present invention relates to piezoelectric-on-insulator (POI) type structures. Such structures are particularly useful in the fields of microelectronics, microsystems, photonics, etc. In particular, they can be used to form or build filters or resonators based on radio frequency (RF) components, especially acoustic wave components such as surface acoustic waves. [Background technology]

[0002] 1a and 1b, which show state-of-the-art POI structures, the POI structure is typically formed by a thin piezoelectric layer 4 applied to a first surface of a carrier 2. A dielectric interlayer 3 is disposed between and in contact with the carrier 2 and the thin layer 4.

[0003] The thin layer 4 is composed of a single-crystal piezoelectric material, such as lithium tantalate or lithium niobate. These materials also exhibit ferroelectric properties. It should be noted that a ferroelectric material is one that has spontaneous electric polarization in its natural state. The thin layer 4 of the POI structure must be uniformly polarized, i.e., all dipole moments must be aligned parallel to each other in a given direction.

[0004] The carrier 2 is preferably selected to be made of silicon. The substrate may be a monocrystalline silicon-based substrate with a resistivity of more than 1000 Ωcm. Alternatively, as shown in FIG. 1b, the carrier 2 may be formed by a base substrate 2a on which a charge trapping layer 2b is arranged. In this alternative, a dielectric intermediate layer 3 is arranged in contact with the trapping layer 2b.

[0005] WO 2020 / 200986 proposes a method for producing a POI substrate that preserves the single-domain properties of the thin layer. This document, based on the principles of Smart Cut® technology, provides for the transfer of a layer obtained from a donor substrate containing piezoelectric material onto a carrier 2 via a light seed implantation process. Following this transfer, the removed layer is subjected to a finishing sequence including a heat treatment followed by a polishing step, which results in the formation of a single-domain, single-crystal piezoelectric thin layer 4. It was observed that during this sequence, the heat treatment results in the formation of a multi-domain surface portion on the sampled layer, which is then removed by a subsequent polishing process, resulting in a thin layer 4 with the required single-domain properties. Summary of the Invention [Problem to be solved by the invention]

[0006] However, under certain conditions, particularly when light species are implanted at high doses and / or high currents to increase production rates, defects have been observed in thin layer 4. The first type of defect observed, referring to FIG. 2, consists in the presence of depressions D1, or in some cases bumps, on the surface of the thin layer, which depressions / bumps D1 cause the thickness of thin layer 4 to be non-uniform. These defects D1, referred to for simplicity as "depression-type defects" and visible in the left-hand inset of FIG. 2, are generally circular or elliptical in shape, with dimensions (diameter or major axis) on the order of 1 micron to 100 microns, sometimes with a high aspect ratio. They typically have a depth or height of 1 to 30 nanometers relative to the exposed surface of thin layer 4.

[0007] The second type of defect observed is the presence of "triangular defects" D2. These defects take the form of ferroelectric domain-inverted bars with triangular cross-sections ranging from 0.1 microns to 10 microns on a side, as can be seen in the inset on the right side of Figure 2. These bars emerge from the surface of the thin layer 4 and extend into and, in some cases, through the thickness of the thin layer 4. They are oriented antiparallel to the spontaneous polarization direction Ps of the piezoelectric thin layer 4. These triangular defects extend over 10 microns on the exposed surface of the thin layer 4. 3 / cm 2 It may have a greater density.

[0008] Both types of defects, pit defects and triangular defects, have a significant impact on the performance of devices such as acoustic filters formed on and in POI substrates. [Means for solving the problem]

[0009] It is an object of the present invention to at least partially address this problem. More specifically, it is an object of the present invention to provide a piezoelectric-on-insulator structure in which the thin piezoelectric layer does not contain dimple-type defects or triangular defects, or at least has a lower density of defects than the thin piezoelectric layer obtained by related art methods.

[0010] To this end, the subject of the present invention is a method for preparing thin single-domain layers of lithium-containing ferroelectric materials, comprising the steps of: providing a first single-domain layer of a lithium-containing ferroelectric material, the layer being bonded to a carrier, the first layer having a lithium-rich surface thickness; finishing the first layer, the finishing comprising heat treating a free side of the first layer followed by thinning the first layer to form a single-domain lamina.

[0011] The preparation method includes, before the heat treatment, a first step of wet cleaning the free side of the first layer, the cleaning step being capable of removing the lithium-rich surface layer, a second step of preparing the first layer to eliminate, prevent, or limit the appearance of lithium-rich and hydrogen-rich dendrites that tend to nucleate on the free side of the first layer when the first layer lacks a lithium-rich surface layer.

[0012] According to other advantageous, non-limiting features of the present invention, taken alone or in any technically feasible combination, Providing the first layer is implanting a light species into a first surface of a lithium-containing ferroelectric donor substrate to form an embrittlement plane and define a first layer between the embrittlement plane and the first surface of the donor substrate; Assembling the first side of the donor substrate to the carrier by means of a dielectric interlayer; transferring the first layer onto a support substrate and exposing the free side of the first layer to the atmosphere, which exposure results in the formation of a lithium-rich surface thickness; and fracturing the donor substrate at an embrittlement plane to expose the first layer. the first cleaning step includes brushing and spraying deionized water onto the free side of the first layer; The second preparation step is the wet cleaning of the free side of the first layer (8), the second preparation step includes brushing and spraying deionized water onto the free side of the first layer; a second preparation step designed to remove dendrites and applied at least 50 hours, preferably at least 75 hours, after application of the first cleaning step; an intermediate step between the first preparation step and the second preparation step, the intermediate step including exposing the first layer to a temperature higher than ambient temperature; the second preparation step is designed to prevent or limit the appearance of dendrites and is applied less than 50 hours, preferably less than 10 hours, after the application of the first cleaning step; the second preparation step includes exposing the free side of the first layer to a plasma; Plasma is O2 plasma, N2 plasma, and SF6 or C x H y Fz a fluorine-based plasma such as a plasma, or a combination of these plasmas; The support is formed by a single conductive or semiconductive substrate, the support includes a base substrate and a trapping layer, the trapping layer being disposed between the dielectric intermediate layer and the base substrate; the first layer and the thin layer are made of a single crystal piezoelectric material such as lithium tantalate or lithium niobate; the first layer and the thin layer are made of lithium niobate; The dielectric interlayer comprises at least one layer of silicon oxide, silicon oxynitride, or silicon nitride. [Brief explanation of the drawings]

[0013] Other features and advantages of the present invention will become apparent from the following detailed description of the invention which refers to the accompanying drawings.

[0014] [Figure 1a] Shows the latest POI structure. [Figure 1b] Shows the latest POI structure. [Figure 2] Defects present in the thin layers of the POI structure are shown. [Figure 3] The manufacturing process of the POI structure is shown. [Figure 4a] 1 shows the state of the first layer and lamina of the POI structure during various stages of manufacture. [Figure 4b] 1 shows the state of the first layer and lamina of the POI structure during various stages of manufacture. [Figure 4c] 1 shows the state of the first layer and lamina of the POI structure during various stages of manufacture. [Figure 4d] 1 shows the state of the first layer and lamina of the POI structure during various stages of manufacture. [Figure 5] 1 illustrates a method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] We begin by recalling the steps of the method for manufacturing the POI1 structure as presented in the introduction of this application and as shown in Figures 1a and 1b.

[0016] The method generally provides for transferring a first ferroelectric layer 8 onto a carrier 2, the first layer 8 being taken from a single-domain ferroelectric donor substrate 5 by a transfer technique based on the implantation of light species, such as hydrogen and / or helium species. In the present disclosure, the ferroelectric material of the donor substrate 5 comprises lithium. For example, it may be lithium tantalate or lithium niobate. In addition to ferroelectric properties, the donor substrate material also has piezoelectric properties. The ferroelectric material advantageously has a crystallographic orientation between 30°RY and 60°RY. The donor substrate 5 may be a homogeneous substrate entirely composed of ferroelectric and piezoelectric materials, as shown in FIG. 3, or it may be a composite substrate composed of a homogeneous portion composed of, for example, silicon, on which a thick layer of ferroelectric and piezoelectric materials is placed, from which the first layer is taken.

[0017] In some embodiments, the carrier 2 is a single conductive or semiconductive substrate. In other embodiments, the carrier 2 comprises a base substrate 2a having a surface charge trapping layer 2b. This trapping layer 2b is disposed on a first side of the carrier 2, which is intended to receive the thin layer 4. In these illustrated embodiments, a dielectric interlayer 3 is in contact with the trapping layer 2b and the thin layer 4.

[0018] According to the light species implantation based transfer technique, referring to Figure 3b, hydrogen and / or helium are implanted into the first surface 6 of the donor substrate 5 to form a buried embrittlement plane 7. The implantation dose is 8 x 10 16 at / cm 2and / or the injection current is greater than 20 mA, thereby achieving the conditions leading to the appearance of defects as presented in the introduction of this application. In this way, a first layer 8 is defined between the embrittlement plane 7 and the first side 6 of the donor substrate 1. This first side 6 of the donor substrate is then bonded to the exposed side 6' of the carrier 2, here via a dielectric interlayer 3, as shown in FIG. 3c. By way of example, the dielectric interlayer 3 may comprise or consist of silicon oxide, silicon oxynitride, or silicon nitride.

[0019] The donor substrate 5 is then fractured at the embrittlement plane 7, for example using a moderate heat treatment and / or the application of mechanical force. The first layer 8 is then released from the donor substrate 5, exposing a free side 9 of the first layer 8, with the other surface 4 of the first layer 8 in direct contact with the dielectric interlayer 3 of the carrier 2.

[0020] The remaining portion 5' of the donor substrate 5, after removal of the first layer 8, can be reconditioned to remove a new layer in a removal cycle similar to that described above.

[0021] To form a "useful" thin layer 4, it is generally necessary to provide a finishing of the first transfer layer 8 bonded onto the carrier 2. These steps generally aim to improve the crystalline quality of the first layer 8 and its surface condition (e.g. its roughness).

[0022] As mentioned in the introduction to this application, this finishing includes a step of heat treating the free side 9 of the first layer 8, followed by a step of thinning the first layer 8 to form a single-domain thin layer 4.

[0023] The step of heat treating the free side 9 of the first layer 8 may involve exposing this layer for 30 minutes to 10 hours in a neutral or oxygen-containing atmosphere heated to a temperature between 300°C and the Curie temperature of the ferroelectric material comprising the first layer 8. It should be noted that this step of heat treating the free side 9 of the layer 8 is separate from the step that fractured the donor substrate 5. Indeed, this heat treatment cannot treat the free side, since the layer 3 has not yet been fully released during the fracture anneal.

[0024] The thinning process can be performed by mechanical / chemical polishing.

[0025] In order to understand the origin of the defects in the thin layer 4 described in the introduction, the Applicant carried out high-precision analyses of the first layer 8 obtained at the end of the method shown in Figure 3, i.e. before finishing this first layer 8 to prepare the single-domain thin layer 4. These analyses were carried out on the first lithium tantalate layer 8.

[0026] Referring to FIG. 4a, Applicant observed the presence of a lithium-rich surface layer 11 on the first layer 8 obtained immediately after the fracture process. This surface layer 11 is composed of Li2CO3. Its formation appears to be promoted by the specific conditions under which the fracture process is carried out. The presence of light species, i.e., hydrogen and / or helium, and the moderate temperature at which fracture occurs appear to make the lithium in the first layer 8 particularly mobile and the surface of this layer 8 particularly reactive. When the remaining portion 5' of the donor substrate is removed and the free side 9 of the first layer 8 is exposed to the atmosphere, this surface reacts with carbon dioxide, hydrocarbons, and oxygen naturally present in the atmosphere to form the Li2CO3 surface layer 11. The thickness of this surface layer overlying the first layer 8 is on the order of nanometers or more. It is stable over time, i.e., it does not change in consistency or thickness when the first layer remains exposed to the atmosphere.

[0027] However, this surface layer 11 is relatively fragile and the applicant has observed that the first layer 8 can be removed by wet cleaning.

[0028] Applicant also observed that the first layer 8 remains particularly reactive, even without its lithium-rich surface layer 11. By leaving the free side 9 of this first layer 8 exposed to the atmosphere for an extended period of time, amorphous dendrites 12 rich in lithium and hydrogen (and other species present in the atmosphere, such as chlorine or fluorine) nucleate and grow on the free side 9 of the first layer 8. This development is particularly noticeable after 50 to 75 hours. As illustrated in Figure 4b, these dendrites 12 are distributed unevenly on the surface of the first layer 8. They densely accumulate in certain zones on the surface of the layer 8, particularly in certain topologies of this surface, such as localized roughness or topologies caused by the appearance of dislocations, while other zones are entirely devoid of them.

[0029] Applicant has applied a final heat treatment step to the first layer 8, which comprises these dendrite dense zones 12 and other zones which are dendrite free.

[0030] At the end of this heat treatment, the first layer 8 (FIG. 4c) exhibited a multi-domain surface layer 13, as documented in the references cited in the introduction to this application. The dendrites 12 had disappeared from the first layer 8, presumably having dissolved during the heat treatment. However, in areas of the first layer 8 where the dendrites 12 were initially densely packed, the multi-domain surface layer had a variant morphology 14 that differed from the morphology of this multi-domain surface layer 13 in areas where the dendrites 12 were initially absent. This variant morphology 14 was characterized by a thinner multi-domain surface layer, as if the presence of dendrites 12 in the dense zone limited the formation of this layer 13. Furthermore, the applicant has demonstrated that the morphology of the multi-domain surface layer 13 is less than 10^4 / cm 2 It was observed that triangular defects 15 exist in the first layer at a density exceeding 1.

[0031] Applicant then applied a chemical-mechanical polishing thinning step to the first layer 8 obtained after the heat treatment to provide the thin layer 4 shown in Figure 4d. It should be noted that triangular defects 15 are made visible on the surface of the thin layer 4 after the first layer 8 has been treated by chemical-mechanical polishing to remove the multi-domain surface layer 13. However, these defects 15 were present in the first layer 8 before the thinning step.

[0032] The resulting thin layer 4 exhibited pit-type 16 defects in the regions of the heteromorphism 14 of the multi-domain surface layer 13 .

[0033] Based on these results and observations, applicant has established a method for preparing a thin single-domain layer 4 of lithium-containing ferroelectric material. The method is applied to a first single-domain layer 8 of lithium-containing ferroelectric material bound to a carrier 2, as shown with reference to FIG. 3 of the present application. This is shown in FIG. 5. Thus, under the manufacturing conditions of this layer, the first layer has a lithium-rich surface thickness 11, typically Li2CO3.

[0034] The preparation method is applied before finishing the first layer 8, ie before applying a heat treatment step to its free side 9, and before the subsequent thinning step.

[0035] The preparation method includes a first step of wet cleaning the free side 9 of the first layer 8. This cleaning step removes the lithium-rich surface layer, typically formed from Li2CO3. It may include or consist of spraying deionized water onto the free side of the first layer while brushing the free side. Experiments performed by the applicant have shown that this cleaning process is perfectly suitable for removing the Li2CO3 surface layer 11.

[0036] The preparation method further comprises, after the first step, a second step of preparing the first layer 8. This second step aims to eliminate dendrites or prevent / limit their appearance. When the first layer 8 lacks a lithium-rich surface layer, these amorphous lithium-rich and hydrogen-rich dendrites 12 tend to nucleate on the free side of the first layer 8.

[0037] This second step can be carried out in several different ways.

[0038] In a first variant, the dendrites 12 are left to grow and stabilize on the surface of the first layer 8. The second preparation step then aims to remove these dendrites 12. It is therefore applied at room temperature for at least 50 hours, preferably at least 75 hours, after the application of the first cleaning step, so as to effectively stabilize the growth of the dendrites 12. This minimum waiting time of 50 hours can be reduced by storing the substrate at a temperature higher than room temperature.

[0039] Thus, in one variant, the present invention provides an intermediate treatment that includes exposing the first layer 8 to a temperature above room temperature to promote dendrite growth. In this way, the waiting time can be reduced to less than 50 hours. This may involve, for example, placing one or more substrates in an oven at 50°C or in the temperature range of 27°C to 80°C. More generally, this intermediate treatment may include any treatment that accelerates the formation of these dendrites, such as annealing the first layer 8.

[0040] The second preparation step can in fact be carried out by wet cleaning the free side, for example by the same type of cleaning step carried out to remove the Li2CO3 surface thickness 11. It can therefore comprise or consist of spraying deionized water onto the free side 9 of the first layer 8 while brushing this side.

[0041] In another variant, the free side of the first layer is treated to prevent the appearance of dendrites 12. This second preparation step is therefore applied less than 50 hours, preferably less than 10 hours, after the application of the first cleaning step. It may involve exposing the free side of the first layer to a plasma, for example a plasma selected from the list formed by an O2 plasma, an N2 plasma, and a fluorine-based plasma such as an SF6 or a CxHyFz plasma, or a combination of these plasmas.

[0042] A 30 second RF plasma (13.55 MHz) of nitrogen (N2) at a power of 150 W and a pressure of 50 mT in a nitrogen flow of 75 SCCM was found to be particularly effective.

[0043] Similarly, a 30 second RF plasma sequence (13.55 MHz) at a power of 150 W and a pressure of 50 mT in a flow of 75 SCCM oxygen and 3 SCCM SF6 was also found to be effective in treating the free side 9 of the first layer 8 and preventing the appearance of dendrites 12.

[0044] The SCCM unit of measurement ("standard cubic centimeters per minute") is the number of square centimeters per minute (cm) at a density defined by standard temperature and pressure conditions. 3 It is the physical unit of gas mass flow rate in units of / min.

[0045] The inventors have unexpectedly observed that application of such a plasma prevents / limits the growth and emergence of dendrites 12 .

[0046] [Table 1]

[0047] After applying the first cleaning step and the second step to eliminate or prevent / limit the appearance of dendrites 12, the method for manufacturing the POI structure can continue. As already mentioned, this involves applying a finishing sequence to the first layer 8 to form the single-domain thin layer 4 of the final POI structure. This finishing sequence begins with a heat treatment of the free side of the first layer, followed by thinning.

[0048] Advantageously, particularly in the case of the first variant of the preparation method, the time between the second step of preparing the first layer and the first washing step is controlled to be less than 50 hours.

[0049] Naturally, the invention is not limited to the described embodiments, and variant embodiments can be added thereto without departing from the scope of the invention as defined by the claims.

Claims

1. A method for preparing a single-domain thin layer (4) of a lithium-containing ferroelectric material, - To provide a first single-domain layer (8) of a lithium-containing ferroelectric material, wherein the layer is bonded to a carrier (2), and the first layer (8) has a lithium-rich surface thickness (11), -Finishing the first layer (8), the finishing comprising a heat treatment of the free side (9) of the first layer (8), and a subsequent step of thinning the first layer to form the single-domain thin layer (4), Equipped with, The above preparation method, before heat treatment, - A first step of wet cleaning the free side (9) of the first layer (8), wherein the cleaning step can remove the lithium-rich surface layer, - A second step of preparing the first layer (8) in order to eliminate, prevent, or limit the appearance of lithium-rich and hydrogen-rich dendrites (12) that are prone to nucleation on the free side (9) of the first layer (8) when the first layer (8) lacks the lithium-rich surface layer, A preparation method characterized by including the following.

2. Providing the first layer (8) is, - A light material is injected into the first surface (6) of the lithium-containing ferroelectric donor substrate (5) to form an embrittlement plane (7), and the first layer (8) is defined between the embrittlement plane (7) and the first surface (6) of the donor substrate (5), - The dielectric intermediate layer (3) assembles the first surface (6) of the donor substrate (5) onto the carrier (2), - In order to transfer the first layer (8) onto the support substrate (2), the donor substrate (5) is fractured at the embrittlement plane (7), exposing the free side (9) of the first layer (8) to the atmosphere, thereby forming the lithium-rich surface thickness (11). The preparation method according to claim 1, comprising:

3. The preparation method according to claim 1 or 2, wherein the first washing step comprises brushing and spraying deionized water onto the free side (9) of the first layer (8).

4. The preparation method according to claim 1, wherein the second preparation step is wet washing of the free side (9) of the first layer (8).

5. The preparation method according to claim 4, wherein the second washing step includes brushing and spraying deionized water onto the free side (9) of the first layer (8).

6. The preparation method according to claim 1, wherein the second preparation step is designed to remove dendrites (12) and is applied for at least 50 hours, preferably at least 75 hours, after the application of the first washing step.

7. The preparation method according to claim 1, comprising an intermediate step between the first preparation step and the second preparation step, wherein the intermediate step exposes the first layer (8) to a temperature higher than the ambient temperature.

8. The preparation method according to claim 1, wherein the second preparation step is designed to prevent or limit the appearance of dendrites (12) and is applied for less than 50 hours, preferably less than 10 hours, after the application of the first washing step.

9. The preparation method according to claim 8, wherein the second preparation step involves exposing the free side (9) of the first layer (8) to plasma.

10. The aforementioned plasma is O ２ Plasma, N ２ Plasma, and SF ６ Or C ｘ H ｙ F ｚ The preparation method according to claim 9, which is selected from a list formed by a fluorine-based plasma such as plasma, or a combination thereof.

11. The preparation method according to claim 1, wherein the support (2) is formed from a single conductive or semiconducting substrate.

12. The preparation method according to claim 2, wherein the support (2) includes a base substrate (2a) and a trap layer (2b), and the trap layer (2b) is disposed between the dielectric intermediate layer (3) and the base substrate (2a).

13. The preparation method according to claim 1, wherein the first layer (8) and the thin layer (4) are composed of a single-crystal piezoelectric material such as lithium tantalate or lithium niobate.

14. The preparation method according to claim 13, wherein the first layer (8) and the thin layer (4) are composed of lithium niobate.

15. The preparation method according to claim 2, wherein the dielectric intermediate layer (3) includes at least one layer of silicon oxide, silicon oxynitride, or silicon nitride.

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