Surface planarization process
The selective ion bombardment and mechanochemical polishing method addresses the issues of amorphous layer formation and non-uniformity in piezoelectric layer planarization, ensuring high surface uniformity and maintaining device performance for microelectronic devices.
Patent Information
- Authority / Receiving Office
- FR · FR
- Patent Type
- Applications
- Current Assignee / Owner
- COMMISSARIAT A LENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES
- Filing Date
- 2024-12-06
- Publication Date
- 2026-06-12
AI Technical Summary
Existing surface planarization methods for thin piezoelectric layers in microelectronics, such as localized etching and chemical-mechanical polishing, result in the formation of amorphous material that degrades device performance and cause non-uniformity, while heat treatment risks losing critical elements like lithium, potassium, or sodium.
A method involving selective ion bombardment followed by mechanochemical polishing, where the etching step creates a specific topography and the polishing step removes amorphous layers, achieving a flat surface with less than 1% uniformity and non-uniformity, using species like argon, nitrogen trifluoride, or trifluoromethane, and heat treatment to maintain critical elements.
The method achieves a flat surface with high uniformity and removes amorphous layers without degrading performance, maintaining critical elements, suitable for manufacturing microelectronic devices like acoustic resonators and filters.
Smart Images
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Abstract
Description
Title of the invention: Method for planarizing a surface Technical field of the invention
[0001] The present invention relates to the field of microelectronics, and more particularly to surface planarization processes. State of the art
[0002] In the field of microelectronics, the performance of fabricated micrometric devices can depend on certain geometric parameters of their constituent elements. For example, in the case of resonators or acoustic filters, performance depends on the thicknesses of the layers used, and in particular on the piezoelectric material layer. Indeed, its thickness, or variations in the layer's thickness, can affect the resonant frequency (fr), the coupling coefficient (k2), the bandwidth, or its quality factor (Q).
[0003] Consequently, it is crucial to be able to control the flatness of the piezoelectric material layer. This is made particularly difficult by the fact that such a layer is very thin. It is therefore not possible to implement certain etching processes that would thin this thin layer too much, or even degrade its performance.
[0004] More generally, it is common to want to control the thickness of a layer, and to guarantee its flatness.
[0005] To achieve these objectives, it is known in the prior art to planarize a surface using a localized etching (or trimming) method implemented after a measurement step. For example, implementing localized etching by ion bombardment makes it possible to locally correct a thickness defect to flatten the surface. Although such a method is satisfactory in that it makes the surface flatter and more uniform, it results in the formation of a layer of amorphous material on the surface of the layer that subsequently needs to be removed. This layer of amorphous material can degrade the performance of the manufactured micrometric devices.
[0006] To remove this layer of amorphous material, it is known to implement either a heat treatment at a temperature between 200°C and 500°C, or to carry out a mechano-chemical polishing (or Chemical Mechanical Polishing - CMP) on the surface, or a combination of the two.
[0007] Although these solutions are satisfactory in that they allow the removal of the amorphous material layer, they have drawbacks. Implementation Heat treatment can cause the loss of certain lightweight elements in the material, such as lithium (Li), potassium (K), or sodium (Na), which are sometimes included in piezoelectric materials. Chemical-mechanical polishing, on the other hand, sometimes results in non-uniformity across the planarized surface.
[0008] There is therefore a need to find a planarization process that makes a surface sufficiently flat without generating the formation of a layer of amorphous material on the surface.
[0009] Object of the invention
[0010] The present invention aims to provide a solution that addresses all or part of the aforementioned problems.
[0011] This goal can be achieved through the implementation of a surface planarization process, the process comprising successively: - a step of providing a support substrate comprising a thin layer, said thin layer having a surface extending to a peripheral edge; - an etching step, in which the thin film is selectively etched by localized bombardment of ions, such that the thickness of said thin film varies progressively from a center of the thin film towards the peripheral edge; and - a planarization step, in which the thin layer is thinned by mechanochemical polishing, so that at the end of the planarization step, the thin layer has a substantially flat surface extending to the peripheral edge.
[0012] The above-described provisions allow for a planarization process that takes advantage of the localized etching during the etching step to create a specific topography of the thin film. The etching step thus prepares the surface of the thin film to improve the flatness obtained during the planarization step. Synergistically, the planarization step also removes any amorphous layer deposited incidentally during the etching step.
[0013] The planarization process may also have one or more of the following characteristics, taken alone or in combination.
[0014] According to one embodiment, the etching step is carried out by bombarding the surface of the thin film with ionic species, for example argon (Ar), nitrogen trifluoride (NHF3 / NF3), or trifluoromethane (CHF4), etc. For example, the ionic species bombardment is carried out at an energy between 0.05 and 3.0 MeV, with an ionic dose between 10¹⁰ and 10¹⁶ at / cm².
[0015] By "substantially flat" surface it is understood that at the end of the planarization step, the surface of the thin film has a surface uniformity of less than 1% and / or a percentage of non-uniformity of less than 1%.
[0016] For example, it is possible to characterize the surface funiformity in %, as being, for a set of points distributed on said surface, a ratio of the standard deviation to the mean of the thickness of the thin film at the level of each of these points, multiplied by 100: cfoStdD^ 100*f~
[0017] For example, it is possible to characterize the percentage of non-uniformity as being, for a set of points distributed over said surface, a ratio of the extent to twice the average thickness of the thin film at each of these points, multiplied by 100: %NonU = 100*^£-
[0018] According to one embodiment, during the provisioning step, the thin film has a thickness of less than 2 pm, and in particular less than 1 pm.
[0019] Thus, the planarization process is suitable for surface planarization for the production of microelectronic devices.
[0020] According to one embodiment, the etching step is implemented so that the surface of the thin layer etched at the end of the etching step has a concave shape.
[0021] The term "concave" in relation to the surface of the etched thin film means that said surface is curved and that the apex of the curvature passes through a plane from which the surface moves away, approaching the supporting substrate. Consequently, the thickness of the thin film decreases from the apex of the curvature to the peripheral edge.
[0022] Thus, the etched thin layer is prepared so that during the planarization step, the etched thin layer is polished starting from the center. Such preparation of the thin layer results in greater surface uniformity after polishing than when the thin layer is not selectively etched.
[0023] According to one embodiment, the etching step is implemented so that the surface of the thin layer etched at the end of the etching step has a convex shape.
[0024] The term "convex" in relation to the surface of the etched thin film means that said surface is curved and that the apex of the curvature passes through a plane from which the surface deviates, moving away from the supporting substrate. In other words, the thickness of the thin film increases from the apex of the curvature to the peripheral edge.
[0025] Thus, the etched thin layer is prepared so that during the planarization step, the etched thin layer is polished starting from the edge peripheral. Such preparation of the thin layer allows for greater surface uniformity after polishing than when the thin layer is not selectively etched.
[0026] According to one embodiment, the planarization process further includes a heat treatment step, implemented after the etching step, in which the thin layer undergoes heat treatment at an annealing temperature between 200°C and 500°C.
[0027] Thus, it is possible to remove, at least partially, a layer of amorphous material formed on the surface of the thin film during the etching step.
[0028] According to one embodiment, the thin film is a layer of pyroelectric, ferroelectric, or piezoelectric material; for example, during the provisioning step.
[0029] Thus, the planarization process is suitable for the manufacture of microelectronic devices or optical or acoustic microsystems.
[0030] In particular, the planarization process is suitable for the manufacture of acoustic resonators or filters, such as surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, or Lamb wave resonators.
[0031] For example, said piezoelectric material may be selected from the group comprising: LiNbO3, LiTaO3, LiNbi xTaxO3, quartz (SiO2), PbZri xTixO3, KNbO3, KTaO3, NaNbO3, KNbi xTaxO3, KTai xNbxO3, BaTiO3, SrTiO3, Bai xSrxTiO3, where 0 <x<l.
[0032] According to one embodiment, the thin film is a single-crystal layer.
[0033] According to one embodiment, the thin film comprises LiNbO3 or LiTaO3
[0034] Thus, the manufacturing process is particularly suitable for the manufacture of acoustic resonators.
[0035] According to one embodiment, the planarization process further includes a measurement step, implemented before the etching step, in which a measurement at different locations of a thickness of the thin film is carried out by ellipsometry or reflectometry.
[0036] Thus, it is possible to adapt the engraving step according to the result of the measurement step. This makes it possible to form the engraved surface while minimizing material removal during the engraving step.
[0037] According to one embodiment, the measurement step is carried out by ellipsometry, in particular in the case where an average thickness of the thin film is less than or equal to 1 pm.
[0038] According to one embodiment, the measurement step is carried out by reflectometry, in particular in the case where an average thickness of the thin film is strictly greater than 1 pm.
[0039] For example, the measurement step can be implemented so as to measure a thickness of the thin film at 9 to 121 distinct points chosen on the surface of the thin film.
[0040] According to one embodiment, the provisioning step comprises the following steps: - a step of supplying a donor substrate comprising a thick layer, the donor substrate having a main face on the side of the thick layer; - an implantation stage, in which light species are implanted in the thick layer to generate a weakening plane and thus define the thin layer between the weakening plane and the main face of the donor substrate; - an assembly step, in which the main face of the donor substrate is brought into contact with a receiving face of the support substrate; - a detachment step, in which the thin layer is formed by detaching part of the thick layer at the level of the embrittlement plane, by applying a heat treatment.
[0041] The steps described above make it possible to extract a thin layer from a massive substrate by fracture, using Smart Cut™ technology.
[0042] According to one embodiment, during the etching step, the variation in the thickness of the thin film between the center of the thin film and the peripheral edge is between 50 nm and 200 nm.
[0043] The thickness ranges described above allow for good surface preparation prior to the planarization step, limiting the thinning of the thin film during the planarization process. This makes it possible to maintain significant control over the thin film thickness, which is a particularly critical parameter in the fabrication of micrometric acoustic resonators.
[0044] Brief description of the drawings
[0045] Other aspects, objectives, advantages and features of the invention will become clearer upon reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the accompanying drawings in which:
[0046] [Fig-1] Fig. 1 is a schematic view of a provisioning step according to a method of embodiment of the invention.
[0047] [Fig.2] Fig.2 is a schematic view of a provisioning step according to a another embodiment of the invention, using SmartCut™ technology.
[0048] [Fig.3] The [Fig.3] is a schematic view of a measurement step according to one embodiment of the invention.
[0049] [Fig.4] The [Fig.4] is a schematic view of the last steps of the planarization process according to a first embodiment of the invention.
[0050] [Fig.5] The [Fig.5] is a schematic view of the last steps of the planarization process according to a second embodiment of the invention. Detailed description
[0051] In the figures and throughout the description, the same reference numerals represent identical or similar elements. Furthermore, the various elements are not drawn to scale in order to enhance the clarity of the figures. Moreover, the different embodiments and variants are not mutually exclusive and may be combined.
[0052] As illustrated in Figures 1 to 5, the invention relates to a method for planarizing a surface s10. "Planarization" refers to a method for making said surface s10 flat. The flatness of a surface is a determining parameter for many applications in the field of microelectronics. The method can therefore be adapted for various devices for which surface flatness is required.
[0053] The process first comprises a step E0 of providing a support substrate 5 comprising a thin layer 10, said thin layer 10 having a surface slO extending to a peripheral edge blO. Generally, the thin layer 10 has a thickness elO of less than 2 pm, and in particular less than 1 pm. Figures 1 and 2 illustrate two different embodiments of the provision step E0.
[0054] As can be seen in [Fig. 1], a supply step E01 can be implemented, in which a donor substrate 1 comprising a thin film 10 is supplied. This donor substrate 1 has a principal face fp3 on the side of the thin film 10. For example, the donor substrate 1 also includes a primary bonding layer disposed on the thin film 10 and has a surface opposite the thin film 10 that forms the principal face fp3. This donor substrate 1 can be inverted and brought onto a secondary bonding layer 4 disposed on the support substrate 5, during an assembly step E03. In other words, during this assembly step E03, the principal face fp3 of the donor substrate 1 is brought into contact with a receiving face fr5 of the support substrate 5, said receiving face fr5 being formed on the secondary bonding layer 4.The resulting stacking then comprises the superposition in this order of the support substrate 5, the primary and secondary bonding layers 2, 4, and the thin layer 10 to be planarized.
[0055] Figure 2 illustrates another implementation of the E0 delivery step known as SmartCut™. In summary, this embodiment may include the following steps.
[0056] In the same way as before, a supply step E01 is implemented, in which a donor substrate 1 is supplied. This donor substrate 1 comprises a thick layer 3, and optionally a primary bonding layer disposed on the thin layer. The donor substrate 1 thus presents a principal face fp3 on the side of the thick layer 3, and in particular on the side of the primary bonding layer 2.
[0057] An implantation step E02 can then be implemented, in which light chemical species are implanted in the thick layer 3 to generate a weakening plane P3. This weakening plane P3 will subsequently delimit the thin layer 10 with the main face fp3.
[0058] An assembly step E03 is then implemented by bringing the main face fp3 of the donor substrate 1 into contact with a receiving face fr5 of the support substrate 5. As before, it may be advantageous for the receiving face fr5 of the support substrate to be formed on a secondary bonding layer 4. A bond is thus achieved between the main face fp3 and the receiving face fr5 via the primary and secondary bonding layers 2, 4.
[0059] Finally, a detachment step E04 can be implemented. During this step, the thin layer 10 is formed by detaching a portion 6 of the thick layer 3 at the level of the embrittlement plane P3, through the application of a heat treatment. It is therefore understood that the thin layer 10 is the portion of the thick layer 3 that remains after the detachment of portion 6 of the thick layer. The steps described above make it possible to extract a thin layer 10 from a solid substrate by fracture, using Smart Cut™ technology.
[0060] Such a technology is frequently used for the formation of thin films of certain materials, such as pyroelectric, ferroelectric, or piezoelectric materials. It is also possible that the thin film 10 to be planarized according to the invention is a layer of one of these types of materials. It is also possible that the thin film 10 is a single-crystal layer. Thus, the planarization process is suitable for the fabrication of optical or acoustic microelectronic devices. In particular, the planarization process is suitable for the fabrication of acoustic resonators or filters, such as surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, or Lamb wave resonators.
[0061] For example, said piezoelectric material may be selected from the group comprising: LiNbO3, LiTaO3, LiNbi xTaxO3, quartz (SiO2), PbZri xTixO3, KNbO3, KTaO3, NaNbO3, KNbi xTaxO3, KTai xNbxO3, BaTiO3, SrTiO3, Bai xSrxTiO3, where 0 <x<l. Plus précisément, la couche mince 10 peut comprendre ou être constituée de LiNbO3 , ou de LiTaO3. Ainsi, le procédé de fabrication est particulièrement adapté pour la fabrication de résonateurs acoustiques.
[0062] With reference to [Fig. 3], the planarization process may advantageously include a measurement step El, in which the thickness is measured at various locations on the surface s 10 of the thin film 10 by ellipsometry or reflectometry. Thus, an etching step E2 (which will be described later) can be adapted based on the result of the measurement step El. This allows the etched surface s 10 to be formed while minimizing material removal during the etching step E2.
[0063] More specifically, the measurement step El can be carried out by ellipsometry, in particular in the case where an average thickness elO of the thin film 10 is less than or equal to 1 pm, and by reflectometry, in particular in the case where an average thickness elO of the thin film 10 is strictly greater than 1 pm.
[0064] For example, the measurement step El can be implemented so as to measure a thickness elO of the thin film 10 at 9 to 121 distinct points chosen on the surface s 10 of the thin film 10. Figure 3 illustrates a variant in which the measurement step El is carried out by measuring the thickness of the thin film 10 at 21 points distributed on the surface s 10 in the plane formed by the X and Y axes of an area of the surface s 10 (see graph labeled A). It is thus possible to obtain a distribution of the thickness elO of the thin film 10 on the surface s 10 (see graph labeled B).
[0065] This measurement step El advantageously allows for the characterization of surface uniformity %StdD and / or the percentage of non-uniformity %NonU. Table C presents the values obtained for this measurement step. The surface uniformity, denoted "%StdD" in %), is calculated to be, for all 21 points distributed over the surface slO, a ratio of the standard deviation, denoted "StdD", to the mean, denoted "Mean", of the thickness elO of the thin layer 10 at each of these points, multiplied by 100: %StdD = 100 * ...
[0066] Figures 4 and 5 then illustrate the remaining steps of the planarization process, and in particular the etching step E2, in which the thin layer 10 is etched selectively by localized ion bombardment, such that a thickness elO of said thin film 10 varies progressively from a center CIO of the thin film 10 towards the peripheral edge blO. For example, the etching step E2 is implemented by bombarding ionic species onto the surface s 10 of the thin film 10. For example, the ionic species bombardment includes bombardment with argon (Ar), nitrogen trifluoride (NHF3 / NF3), or trifluoromethane (CHF4) at an energy between 0.05 and 3.0 MeV, with an ionic dose between 1010 and 1016 at / cm2.
[0067] Generally, during the etching step E2, the thickness elO of the thin film 10 varies between the center CIO of the thin film 10 and the peripheral edge b 10 from 50 nm to 200 nm. The previously described elO thickness ranges allow for good surface preparation of s 10 prior to the implementation of a planarization step E3 (which will be described later), by limiting the thinning of the thin film 10 during the planarization process. It is thus possible to maintain significant control over the thickness elO of the thin film 10, which is a particularly critical parameter in the fabrication of micrometric acoustic resonators.
[0068] Fig. 4 illustrates a first variant in which the etching step E2 is implemented so that the surface s10 of the thin layer 10 etched at the end of the etching step E2 has a convex shape. The term "convex" in relation to the surface s10 of the etched thin film 10 means that said surface s10 is curved and that the apex of the curvature passes through a plane from which the surface s10 moves away, away from the supporting substrate 5. In other words, the thickness e10 of the thin film 10 increases from the apex of the curvature to the peripheral edge b10. Thus, the etched thin film 10 is prepared so that during the planarization step E3, the etched thin film 10 is polished starting from the peripheral edge b10. Such preparation of the thin film 10 results in greater surface uniformity after polishing than when the thin film 10 is not selectively etched.
[0069] Alternatively, and as shown in [Fig. 5], the etching step E2 is implemented such that the surface s10 of the thin film 10 etched at the end of the etching step E2 has a concave shape. The term "concave" in relation to the surface s10 of the etched thin film 10 means that said surface s10 is curved and that the apex of the curvature passes through a plane from which the surface s10 moves away, approaching the support substrate 5. Consequently, the thickness s10 of the thin film 10 decreases from the apex of the curvature to the peripheral edge b10. Thus, the etched thin film 10 is prepared so that during the planarization step In E3, the etched thin layer 10 is polished starting from the center CIO. This preparation of the thin layer 10 results in greater surface uniformity after polishing than when the thin layer 10 is not selectively etched.
[0070] The planarization process then includes a planarization step E3, in which the thin film 10 is thinned by mechanochemical polishing, so that after the planarization step E3, the thin film 10 has a substantially flat surface slO extending to the peripheral edge blO. By substantially flat surface slO, it is understood that after the planarization step E3, the surface s 10 of the thin film 10 has a surface uniformity %StdD of less than 1% and / or a percentage of non-uniformity %NonU of less than 1%. Such a planarization step E3 can, for example, be carried out using a polishing solution based on particles, such as silica (SiO2) or alumina (Al2O3) particles; or in a basic solution (ammonia, potash, etc.).Polishing can be carried out with a pressure between 1 psi and 7 psi, where 1 psi is approximately equal to 6894.76 Pa; and with rotational speeds of the polishing head and the plate between 30 RPM and 150 RPM, where 1 RPM is one revolution per minute.
[0071] Finally, the planarization process may advantageously include a heat treatment step E4, implemented after the etching step E2, in which the thin film 10 undergoes heat treatment at an annealing temperature between 200°C and 500°C. Thus, it is possible to remove, at least partially, any layer of amorphous material formed on the surface s10 of the thin film 10 during the etching step E2.
[0072] All the arrangements described above allow us to propose a planarization process that takes advantage of the localized etching during the etching step E2 to create a specific topography of the thin film 10. The etching step E2 thus prepares the surface s 10 of the thin film 10 to improve the flatness obtained during the planarization step E3. Synergistically, the planarization step E3 also removes any amorphous layer deposited incidentally during the etching step E2.
Claims
Demands
1. A method for planarizing a surface (s 10), the method comprising successively: • a step of providing (E0) a support substrate (5) comprising a thin film (10), said thin film (10) having a surface (s10) extending to a peripheral edge (b 10); • an etching step (E2), in which the thin film (10) is selectively etched by localized bombardment of ions, so that a thickness (s10) of said thin film (10) varies progressively from a center (C10) of the thin film (10) towards the peripheral edge (b 10); and • a planarization step (E3), in which the thin layer (10) is thinned by mechanochemical polishing, so that at the end of the planarization step (E3), the thin layer (10) has a substantially flat surface (slO) extending to the peripheral edge (blO).
2. Planarization method according to claim 1, wherein during the provisioning step (E0), the thin film (10) has a thickness (elO) less than 2 pm, and in particular less than 1 pm.
3. Planarization method according to any one of claims 1 or 2, wherein the etching step (E2) is carried out so that the surface (slO) of the thin layer (10) etched at the end of the etching step (E2) has a concave shape.
4. Planarization method according to any one of claims 1 or 2, wherein the etching step (E2) is carried out so that the surface (slO) of the thin layer (10) etched at the end of the etching step (E2) has a convex shape.
5. Planarization process according to any one of claims 1 to 4, further comprising a heat treatment step (E4), carried out after the etching step (E2), in which the thin layer (10) undergoes heat treatment at an annealing temperature between 200°C and 500°C.
6. A planarization method according to any one of claims 1 to 5, wherein the thin layer (10) is a layer of pyroelectric, ferroelectric, or piezoelectric material.
7. Planarization method according to claim 6, wherein the thin film (10) comprises LiNbO3, or LiTaO3.
8. Planarization method according to any one of claims 1 to 7, further comprising a measurement step (E1), carried out before the etching step (E2), in which a measurement at different locations of a thickness of the thin film (10) is carried out by ellipsometry or reflectometry.
9. A planarization method according to any one of claims 1 to 8, wherein the provisioning step (E0) comprises the following steps: • a supply step (E01) of a donor substrate (1) comprising a thick layer (3), the donor substrate (1) having a principal face (fp3) on the side of the thick layer (3); • an implantation step (E02), wherein light species are implanted in the thick layer (3) to generate a weakening plane (P3) and thus define the thin layer (10) between the weakening plane (P3) and the principal face (fp3) of the donor substrate (1); • an assembly step (E03), wherein the principal face (fp3) of the donor substrate (1) is brought into contact with a receiving face (fr5) of the support substrate (5);• a detachment step (E04), in which the thin layer (10) is formed by detaching a part (6) of the thick layer (3) at the level of the embrittlement plane (P3), by the application of a heat treatment.;
10. Planarization method according to any one of claims 1 to 9, wherein during the etching step (E2), a variation in the thickness (elO) of the thin film (10) between the center (CIO) of the thin film (10) and the peripheral edge (b 10) is between 50 nm and 200 nm.