Method for transferring a thin film onto a supporting substrate - Patent application

Abstract

The present invention relates to a method for transferring a thin film onto a support substrate, comprising: providing a bonded structure comprising a donor substrate and a support substrate assembled by direct bonding at their respective front surfaces according to a bonding interface extending along a main plane, the donor substrate being substantially parallel to the main plane and comprising a buried brittle surface formed by implanting light species, the implanting light species comprising co-implantation of hydrogen ions at a first dose and a first implantation energy and helium ions at a second dose and a second implantation energy; and applying a thermal breakdown treatment to the bonded structure to induce spontaneous separation along said plane associated with the growth of microcracks in the buried brittle surface by thermal activation, the separation resulting in the transfer of the thin film from the donor substrate onto the support substrate. The implanting light species is notable in that it further comprises a localized implantation of hydrogen ions with a third dose and a third energy to form a localized overdosed region in the buried brittle surface. The third dose corresponds to more than three times the first dose, with the local area of ​​overdosing constituting the starting point of separation.

Description

[Technical field]

[0001] The present invention relates to the field of microelectronics and semiconductors. In particular, the present invention relates to a method for transferring a thin film onto a support substrate based on Smart Cut™ technology, where the thin film exhibits improved roughness after separation. In particular, the transfer method can be used to manufacture SOI structures. [Background technology]

[0002] The Smart Cut™ technology is well known for the manufacture of SOI (silicon-on-insulator) structures and, more generally, for thin film transfer. This technology is based on the formation of a buried brittle surface in a donor substrate by implanting said substrate with light chemical species, which together with the front surface of the donor substrate defines the thin film to be transferred. The donor substrate and the support substrate are then joined at their respective front surfaces to form a bonded structure. The assembly is advantageously carried out by direct bonding, by molecular adhesion, i.e. without the aid of an adhesive material, so that a bonded interface is established between the two assembled substrates. The growth of microcracks in the buried brittle surface by thermal activation can result in spontaneous separation along said surface, resulting in the transfer of the thin film onto the support substrate (forming a laminated structure, for example of the SOI type). The remaining donor substrate can be reused for subsequent film transfers. After separation, a finishing treatment is usually applied to the laminated structure to restore the crystal quality and surface roughness of the transferred thin film. These treatments are known to involve oxidizing or smoothing heat treatments (under neutral or reducing atmospheres), chemical cleaning and / or etching steps, and / or chemical-mechanical polishing steps. A variety of inspection tools are available to check the overall surface of the thin film.

[0003] If the separation at the embedded brittle surface is spontaneous, considerable variations in the surface roughness of the transferred thin film are observed, both in terms of high frequency (micro-roughness) and low frequency (rippling, localized areas of high roughness, mottle, etc.) These variations can be visualized and measured when checking the thin film in the final structure, in particular by the inspection tools mentioned above.

[0004] Recall that the surface roughness of the finished thin film can be mapped using a Surfscan™ inspection tool available from KLA-Tencor (Figure 1). The level of roughness and latent patterns (mottle, dense areas, etc.) is measured or revealed by measuring the diffuse background noise ("haze"), which corresponds to the intensity of light scattered by the surface of the thin film. The haze signal ranges from 0.1 to 10 μm. -1 The spatial frequency range of the haze varies linearly with the square of the RMS surface roughness. For details of such wide-area roughness inspection and evaluation techniques, see the paper by F. Holsteyns, "Seeing through the haze," Yield Management Solutions, Spring 2004, pp50-54.

[0005] The maps in FIG. 1 show the surface roughness of two thin layers transferred from two bonded structures that were identically processed to finish. Map (A) shows a peripheral area of ​​residual roughness known as the "dense zone" (ZD), while map (B) has none. More pronounced mottling (M) is also visible in map (A). The average and maximum roughness (expressed in ppm haze) also differ between the two maps (A) and (B). FIG. 1 shows the variation in the final quality and roughness of the thin films, mainly due to the variation in the surface roughness (high and low frequencies) after separation. Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, reducing the surface roughness of these layers after transfer (independent of spatial frequency) in the case of spontaneous detachment by thermal activation remains important to improve the final quality of the transferred thin films. [Means for solving the problem]

[0007] The present invention proposes a transfer method using a localized overdose of light chemical species at the buried brittle surface of the donor substrate to ensure early initiation of fracture and achieve improved roughness over the entire surface of the thin film after detachment, achieving excellent surface quality after the finishing stage of the laminated structure. The method is particularly advantageous for the production of SOI structures.

[0008] The present invention relates to a method for transferring a thin film onto a supporting substrate, the method comprising the steps of: providing a bonded structure including a donor substrate and a support substrate assembled by direct bonding at their respective front surfaces according to a bonding interface extending along a major plane, the donor substrate being substantially parallel to the major plane and including a buried brittle surface formed by implanting light species, the implanting light species including co-implantation of hydrogen ions at a first dose and a first implantation energy and helium ions at a second dose and a second implantation energy; applying a thermally adiabatic treatment to the bonded structure to induce spontaneous detachment along the buried brittle plane associated with the growth of microcracks in said plane by thermal activation, the detachment resulting in the transfer of the thin film from the donor substrate onto the support substrate; Includes.

[0009] The method is notable in that the step of implanting the light species further comprises localized implantation of hydrogen ions at a third dose and a third energy to form an over-dosed localized region at the buried brittle surface, the third dose being greater than three times the first dose, the over-dosed localized region constituting a separation initiation point.

[0010] Advantageous features of the present invention, taken alone or in any feasible combination, include the following: The third energy is lower than the first energy. The localized overdosing region is located in the central region of the donor substrate according to the main plane. First dose is 1E16 / cm 2 (1×10 16 / cm 2 ) ±40%, and the second dose is 1E16 / cm 2 (1×10 16 / cm 2 ) ± 40%, and the third dose is 3 times (not including the boundary value) to 7 times the first dose, preferably about 4 times the first dose. The local overdosage area is 10 μm on the main plane. 2 ~2cm 2 has a surface area of The donor substrate and / or the support substrate have an insulating layer on at least their respective front sides, which forms a buried insulating layer adjacent the bonding interface in the bonded structure. To form a stacked SOI structure, the thin film from the donor substrate is single crystal silicon and the support substrate comprises single crystal silicon.

[0011] Other features and advantages of the present invention will become apparent from the following detailed description of the invention with reference to the accompanying drawings. Some of the figures are schematic and are not drawn to scale for ease of reading. In particular, layer thicknesses along the z-axis are not drawn to scale relative to lateral dimensions along the x- and y-axes. The same reference numbers in the figures or description may be used for the same type of elements. [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 shows two representative surface roughness maps of two transferred thin layers from two bonded structures that were identically processed to finish using conventional methods, both maps obtained with a Surfscan™ inspection tool. [Diagram 2]2 is a graph showing the surface roughness of a thin film as a function of time to failure for several bonded structures (of a different type than the bonded structure shown with respect to FIG. 1) that have been identically processed to a finish by conventional methods. [Diagram 3] 1 shows a bonding structure used in an intermediate stage of the transfer method according to the invention; [Figure 4] FIG. 2 shows a laminate structure and remnants of a donor substrate obtained by a transfer method according to the invention. [Diagram 5] 1 is a table showing different localized hydrogen ion implantation tests and associated results. [Figure 6] FIG. 2 is a photograph of a laminated structure obtained by the transfer method according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present invention relates to a method for transferring a thin film onto a support substrate to form a laminated structure. As mentioned in the introduction, such a laminated structure may be of the SOI type, comprising a thin silicon surface layer, an intermediate insulating layer, and a silicon support substrate. Optionally, the support substrate may comprise other functional layers, such as, for example, a charge trapping layer for SOI structures designed for radio frequency applications. However, the transfer method described herein is not limited to the manufacture of SOI, but can be applied to many other laminated structures in the fields of microelectronics, microsystems and semiconductors.

[0014] The transfer method according to the invention is based on the Smart Cut™ technology. If the separation at the embedded brittle plane is spontaneous, the fracture time (i.e. the time until separation occurs during thermal fracture annealing) can vary between identically processed bonded assemblies undergoing the same annealing in the same furnace. The fracture time (TF) depends on a number of parameters related to the formation of the embedded brittle plane, the fracture annealing, the nature of the bonded structure, etc. The applicant has noticed that for similarly prepared bonded structures undergoing the same fracture annealing, the separation occurring at a short fracture time (TFc) leads to a thin layer with a smaller high frequency surface roughness (micro-roughness) in the final laminated structure (i.e. after transfer and finishing) than the separation occurring at a longer fracture time (TF1), as shown in FIG. 2. Furthermore, a long fracture time induces localized zones of very large roughness (called dense zones ZD) at the edge of the thin film after fracture, which is not or is rare for short fracture times. As can be seen in map (A) of FIG. 1, such dense areas degrade the quality and roughness of the thin film, even after finishing.

[0015] The transfer method according to the invention therefore aims to initiate spontaneous separation at the embedded brittle surface in an early (short time to failure) and repeatable (low variability in time to failure between multiple similar bonded structures) manner in order to substantially improve the surface roughness of the transferred thin film.

[0016] For this purpose, the transfer method first comprises providing a bonded structure 100 (FIG. 3) comprising a donor substrate 1 and a support substrate 2 assembled by direct bonding at their respective front surfaces (1a, 2a) along a bonding interface 3.

[0017] The donor substrate 1 is preferentially in the form of a wafer with a diameter of 100 mm, 150 mm, 200 mm, 300 mm or even 450 mm and a thickness typically between 300 μm and 1 mm. The donor substrate 1 comprises a front face 1a and a rear face 1b. The surface roughness of the front face 1a is selected to be less than 1.0 nm RMS, preferentially even less than 0.5 nm RMS (measured by atomic force microscope (AFM), for example, on a 20 μm×20 μm scan). The donor substrate 1 may be made of silicon or any other semiconducting or insulating material of which thin film transfer may be of interest (for example, SiC, GaN, LiTaO 3 etc.)

[0018] It should also be noted that the donor substrate 1 may include one or more additional layers 12, such as an insulating layer, at least on its front surface 1a. This additional layer may be a few nanometers to a few hundred nanometers thick. As shown in Figure 3, this additional layer 12 becomes a buried intermediate layer in the bonded structure 100 after assembly of the donor substrate 1 and the support substrate 2.

[0019] The donor substrate 1 comprises a buried brittle surface 11 that defines the thin film 10 to be transferred. Such a buried brittle surface 11 can be formed by implanting light chemical species, as is well known for the Smart Cut™ technology. The light chemical species are implanted at a defined depth in the donor substrate 1, which corresponds to the thickness of the targeted thin film 10. These light chemical species form microcavities around the defined depth, distributed in the thin film, approximately parallel to the front surface 1a of the donor substrate 1 (i.e. parallel to the (x,y) plane in the figure). For the sake of simplicity, this thin film will be referred to as the buried brittle surface 11.

[0020] In particular, in the context of the present invention, the implantation step comprises co-implantation of hydrogen ions having a first dose and a first implantation energy and helium ions having a second dose and a second implantation energy.

[0021] The implantation energies of the light species are selected to reach a defined depth, for example hydrogen ions are implanted with a first energy between 10 keV and 180 keV and helium ions are implanted with a second energy between 20 keV and 210 keV to define a thin film 10 having a thickness on the order of 100 nm to 1200 nm.

[0022] The dose of implanted hydrogen ions (i.e., the first dose) is typically 1E16 / cm within the described range of first implant energies. 2 (1×10 16 / cm 2 ) ±40%. Also, the dose of implanted helium ions (i.e., the second dose) is within the stated range of second implant energies and is within the range of 1E16 / cm 2 (1×10 16 / cm 2 ) on the order of ±40%.

[0023] Advantageously, the helium ions are implanted before the hydrogen ions.

[0024] It is recalled that, prior to the ion implantation step, an additional layer may be deposited on the front surface 1a of the donor substrate 1. This additional layer may, for example, consist of a material such as silicon oxide or silicon nitride. It may be retained for the next assembly step (and form all or part of the intermediate layer of the bonded structure 100) or it may be removed.

[0025] Also, the support substrate 2 is preferably in the form of a wafer with a diameter of 100 mm, 150 mm, 200 mm, 300 mm or even 450 mm and a thickness typically between 300 μm and 1 mm. The support substrate 2 comprises a front face 2a and a rear face 2b. The surface roughness of the front face 2a is selected to be less than 1.0 nm RMS, preferentially even less than 0.5 nm RMS (measured by AFM, for example, on a 20 μm×20 μm scan). The support substrate 2 can be made of silicon or any other semiconducting or insulating material in which thin film transfer may be of interest. In the context of the present invention, due to the assembly of the donor substrate 1 and said support substrate 2, the material constituting the support substrate 2 must be compatible with the application of temperatures of 400° C. or more to the bonded structure 100.

[0026] It should also be noted that the support substrate 2 may include one or more additional layers, e.g. insulating and / or charge trapping layers, at least on its front surface 2a. The additional layers may have a thickness ranging from a few nanometers to a few micrometers. The one or more additional layers are embedded in the bonded structure 100 after assembly of the donor substrate 1 and the support substrate 2.

[0027] The assembly between the donor substrate 1 and the support substrate 2 is based on direct bonding by molecular adhesion. As is known per se, said bonding forms a bonding interface 3 without the need for adhesive materials, since an atomic-scale bond is established between the joining surfaces. In particular, there are several types of molecular adhesion bonds, which differ according to the respective temperature, pressure, atmospheric conditions or treatments before contacting the surfaces. We can mention bonding at room temperature with or without prior plasma activation of the surfaces to be assembled, atomic diffusion bonding (ADB), surface-activated bonding (SAB), etc.

[0028] The assembly process may include conventional chemical cleaning sequences (e.g. RCA cleaning), surface activation (e.g. oxygen or nitrogen plasma) or other surface treatments (such as cleaning by scrubbing) that can improve the quality of the bonding interface 3 (low defects, strong adhesion energy) before contacting the front surfaces 1a, 2a to be assembled.

[0029] Once the bonded structure 100 has been formed, the transfer method according to the invention involves applying a thermal break treatment to induce spontaneous detachment along the buried brittle plane 11. The detachment results in the transfer of the thin film 10 from the donor substrate 1 to the support substrate 2, forming the laminated structure 110 (FIG. 4). A donor substrate remnant 1′ is also obtained. For silicon-based bonded structures 100 in particular, the thermal treatment can typically be performed at temperatures between 200° C. and 400° C. in a horizontal furnace (where multiple bonded assemblies 100 can be processed in bulk).

[0030] As mentioned above, the step of implanting light chemical species applied to the donor substrate 1 to form the buried brittle surface 11 comprises co-implantation of hydrogen ions with a first dose and a first implantation energy and helium ions with a second dose and a second implantation energy. For example, starting from a silicon donor substrate 1 with a diameter of 300 mm, when a thin film 10 of 240 nm is harvested to form a FD-SOI (fully depleted SOI) stack structure 110, the co-implantation conditions are as follows: 40 keV-1E16 / cm 2 (1×10 16 / cm 2 ) and then helium ions at 25keV-1E16 / cm 2 (1×10 16 / cm 2 Introduction of hydrogen ions at 100 nm. An additional layer 12 of silicon oxide, for example having a thickness of about 100 nm, is applied to the donor substrate 1.

[0031] As explained with reference to FIG. 2, these co-implantation conditions applied to multiple structures can result in different outcomes regarding the post-separation (and post-finishing) surface roughness due to more or less short or long transfer times, which in either case are unpredictable.

[0032] Therefore, to address such transfer time reproducibility issues, the method according to the invention provides that the step of implanting light species includes a localized implantation of hydrogen ions with a third dose and a third energy, after or before the co-implantation of helium and hydrogen, which creates a localized overdosed region 11b in the buried brittle surface 11, which is intended as an initiation point for premature separation in the buried brittle surface 11. Such premature separation ensures a short break time and, as a result, a highly reproducible and excellent surface finish of the transferred thin film 10 during batch processing of multiple bonded structures 100.

[0033] This localized injection is noteworthy in that the third dose corresponds to more than three times the first dose, which is very significant. In fact, the applicant has noticed that one, two or even three localized injections of the first dose of hydrogen are not sufficient to form reliable and reproducible initiation points of separation. If the third dose does not exceed three times the first dose, the localized overdosed area 11b will not repeatedly induce the initiation of separation, which will maintain a significant variability in terms of the time to fracture and thus an undesirable variation in the surface state of the transferred thin film 10. In fact, contrary to all expectations, a third dose of three times or less than the first dose is not sufficient to initiate fracture at the buried brittle surface 11 before other potential initiation points (i.e. localized bond defects at the bonded interface 3 or non-bonded peripheral areas of the bonded structure 100).

[0034] The table in Figure 5 shows helium and hydrogen at energies of 40 keV and 25 keV, respectively, and at 1E16 / cm 2 (1×10 16 / cm 2 ) and 1E16 / cm 2(1×10 16 / cm 2 1 shows the time to failure and post-separation surface finish results (ppm haze) for various localized hydrogen ion implantation tests where a buried brittle surface 11 is formed by co-implanting at a dose of 1000 ppm. The implantation energy of the hydrogen ions in the localized overdosed region 11b (i.e., the third energy) is 25 keV, the same as the first implantation energy. The separation anneal is performed at 350° C.

[0035] Contrary to what might have been expected, these results confirm that an overdose of up to 3 times the first dose (H) does not have the desired effect of initiating premature separation. The rupture times of structures 1-3 remain long and variable, and the surface finish is not improved over the normal value obtained without localized overdosing ("reference" structure, "haze" of about 26±2 ppm).

[0036] When the third hydrogen dose of the localized implant is equal to 5 times (Structure 4) or 7 times (Structures 5, 6) the first dose, the overdosed localized regions 11b effectively act as fracture initiation points, inducing shorter and more reproducible fracture times and improving the surface finish in terms of reproducibility and haze magnitude (12% to 25% reduction compared to structures without the overdosed localized regions 11b). Early fracture ensures little micro-roughness (high spatial frequency) as well as few, if any, localized regions of large roughness (also known as dense zones ZD).

[0037] It should also be noted that the local surface roughness of the thin film 10 at the level of the overdosed areas 11b is lower than in other areas of the layer 10 and therefore does not produce specific marks that may affect the quality of the final laminated structure 110. For example, for structures 5 and 6 shown in Figure 5, the haze value is of the order of 19 ppm (compared to 20.9 ppm or 20.7 ppm over the entire plate).

[0038] Preferentially, the first dose (H) is 1E16 / cm 2 (1×10 16 / cm 2 ) ±40%, the third dose is strictly greater than 3 times the first dose and not greater than 7 times the first dose, and even more preferentially the third dose is 4 to 5 times the first dose.

[0039] This particular choice of overdose was found to be highly effective in producing early and reproducible break initiation.

[0040] Above the upper limit of seven times the first dose, there is a significant risk of blisters appearing on the surface of the donor substrate 1. The presence of these blisters then leads to bonding defects at the bonding interface 3, degrading the quality of the bonded structure 100.

[0041] The overdosed region 11b may be located at the center (along the major plane (x,y)), at the periphery, or in an intermediate region between these two extremes of the donor substrate 1. If located at the center, it offers the advantage of propagation of the separation wave from the center to the edge of the bonded structure 100, which significantly limits the amplitude of the mottle M or other breaking waves (roughness and low frequency ripples) on the surface of the transferred thin film 10.

[0042] The localized overdosed region 11b is several tens of μm in the (x, y) major plane. 2 ~ A few centimeters 2 , typically 10 μm 2 ~2cm 2 It can occupy an area of.

[0043] Localized implantation can be performed through a mechanical mask featuring holes with a surface area equal to that of the targeted localized overdose region 11b, or alternatively, using masking techniques such as deposition of a screen layer, lithography and etching, or by controlled scanning of a hydrogen ion beam.

[0044] Finally, advantageously, the localized implantation of hydrogen ions is performed at a third energy different from the first energy. Indeed, it has been shown that the thickness of the thin film 10 transferred to the area 10c corresponding to the localized overdosed area 11b is greater than the thickness of the thin film 10 everywhere else. The third implantation energy (related to the localized implantation of H) is therefore preferentially chosen to be lower than the first energy.

[0045] For illustration purposes, FIG. 6 shows a photograph of an SOI-type stack structure 110 (similar to structure 5 or 6 in FIG. 5). The visible surface is the free surface 10a of the thin film 10 after transfer. Region 10c (corresponding to the local overdosed region 11b) appears to be of a different color than the rest of the thin film 10 because the thickness of the layer 10 is different locally in region 10c. In this example, the difference in thickness of the thin film 10 between region 10c and the rest of the plate is of the order of 29 nm. In the range of implantation energies performed in these examples, it can be estimated that about 8-8.5 nm of transferred silicon thin film 10 is added per keV. Thus, in the example shown in FIG. 6, the third implantation energy (i.e. 36.5 keV) is preferentially set lower than the first implantation energy of 3.5 keV.

[0046] Tailoring the third implant energy further improves the surface finish of the transferred thin film 10 by avoiding local differences in thickness in region 10c.

[0047] The transfer process according to the invention provides an improved surface quality 10a of the transferred thin film 10 compared to SOI structures obtained from bonded structures processed by conventional processes, due to the presence of unique localized overdosed regions 11b, which act as initiation points for premature separation in an efficient and reproducible manner, since the surface 10a is completely or almost free of spots M or dense regions ZD. The level of micro-roughness ("haze") of the surface of the thin film 10 before or after smoothing is also lower than the level of roughness obtained by conventional methods.

[0048] Another important advantage is the repeatability of results from batch processing of multiple bonded structures 100 .

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

Claims

1. A transfer method for transferring a thin film (10) onto a support substrate (2), said transfer method comprising: providing a bonded structure (100) comprising a donor substrate (1) and a support substrate (2) assembled by direct bonding at their respective front faces (1 a, 2 a) according to a bonding interface (3) extending along a major plane (x, y), the donor substrate comprising a buried brittle plane (11) substantially parallel to the major plane and formed by implanting light species, the implanting light species comprising co-implantation of hydrogen ions at a first dose and a first implantation energy and helium ions at a second dose and a second implantation energy; applying a thermally desiccant treatment to the bonded structure (100) to induce spontaneous separation along the buried brittle planes (11) associated with the growth of microcracks in said buried brittle planes (11) by thermal activation, which separation results in the transfer of the thin film from the donor substrate (1) onto the support substrate (2); Including, The step of implanting the light species further includes localized implantation of hydrogen ions at a third dose and a third energy to form an over-dosed localized region at the buried brittle surface, the third dose being greater than three times the first dose, and the over-dosed localized region constituting a separation initiation point. A transfer method characterized by:

2. 2. The method of claim 1, wherein the third energy is less than the first energy.

3. 2. The transfer method according to claim 1, characterized in that the localized overdosed area (11b) is located in a central area of ​​the donor substrate (1) along a major plane (x, y).

4. The first dose was 1 x 10 16 / cm 2 + / - 40% and the second dose was 1 x 10 16 / cm 2 + / - 40%, and the third dose is greater than 3 times and not more than 7 times the first dose, preferentially of the order of 4 times the first dose.

5. The local overdose area is 10 μm in the major plane (x, y). 2 ~2cm 2 The method of claim 1 , wherein the surface area is

6. 2. The transfer method according to claim 1, characterized in that the donor substrate (1) and / or the support substrate (2) have an insulating layer (12) at least on their respective front sides (1 a, 2 a), which insulating layer (12) forms a buried insulating layer in the bonded structure (100) adjacent to the bonded interface (3).

7. 7. The transfer method according to claim 6, characterized in that the thin film (10) from the donor substrate (1) is made of monocrystalline silicon and the support substrate (2) comprises monocrystalline silicon, forming a stacked SOI structure (110).

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