Method for separating a layer from a composite structure

Abstract

Method for separating a layer from a composite structure, wherein this composite structure comprises a composite stack formed from at least the following: a support substrate consisting of a material that is at least partially transparent at a certain wavelength; the layer to be separated; and a separating layer arranged between the support substrate and the layer to be separated, wherein the method comprises irradiating the separating layer through the support substrate using at least one incident light beam at the specified wavelength to cause attenuation or separation by flaking off the separating layer, wherein the method is characterized in that the incident light ray is inclined in such a way that there is no superposition of incident surfaces on an exposed surface of the support substrate, which are each formed by the incident light ray and the ray reflected at an interface between the support substrate and the separating layer on the exposed surface of the support substrate.

Description

Background of the invention

[0001] The invention relates to the field of fabricating composite (or multilayer) semiconductor structures and in particular to methods for separation by exfoliation, which make it possible to separate one or more layers of a composite structure, for example to transfer layers from a starting support to a final support. Examples of methods for separation by exfoliation are shown in US 2006 / 0 030 122 A1, WO 2013 / 011 372 A1 or US 10 220 603 B2.

[0002] In the field of composite fabrication, it is often useful to be able to join and / or separate layers, such as semiconductor or insulating layers. Such separations are particularly necessary for transferring a layer from a starting substrate to a final substrate. These transfers occur, for example, during the implementation of three-dimensional component technology, which involves the fabrication of electronic, photovoltaic, and / or optoelectronic components on both sides ("front" and "back") of the same layer (3D integration). Layer transfers are also performed to transfer circuits in the fabrication of back-illuminated imaging devices.The transfer of layers is also useful to change the substrate on which one or more layers are formed, so that the new substrate meets requirements regarding cost, physical properties (cell size, thermal stability, etc.).

[0003] A method for thin-film transfer is described, for example, in patent EP 0 858 110 B1. This method specifically involves separating a layer using a peeling technique, which in particular requires irradiating a composite structure through a transparent substrate.

[0004] With reference to Fig. 1 An embodiment of a method for producing a composite structure (steps S1 and S2) and a method for separation by peeling (steps S3 and S4) is now described.

[0005] First, a so-called separating layer 10 (or optical absorption layer) is bonded onto one side of a support substrate 5 (step S1). The support substrate 5 is at least partially transparent at a predetermined wavelength.

[0006] A layer 15 (also called “layer to be separated”) is then applied by bonding to the side of layer 10 opposite the side that is in contact with the support substrate 5 in order to obtain a composite structure 25 (step S2).

[0007] It should be noted that the joining of layers 5, 10 and 15 in steps S1 and S2 can be carried out using a suitable joining technique, such as a technique of bonding by molecular adhesion or by including a bonding intermediate layer.

[0008] Furthermore, layers 10 and 15 are not necessarily joined together by bonding to form the composite structure 25. As an alternative, at least one of layers 10 and 15 can be formed using a suitable deposition technique. The separating layer 10 can, for example, be formed by PECVD (plasma-enhanced chemical vapor deposition) or LPCVD (low-pressure CVD).

[0009] Once the composite structure 25 has been formed, the separation layer 10 can be separated by peeling. This process allows the layer 15 to be detached from the support substrate 5.

[0010] For this purpose, the separating layer 10 is irradiated with electromagnetic radiation 20 through the support substrate 5 (step S3). The radiation 20 has a wavelength for which the support substrate 5 is at least partially transparent. "Partially transparent" here refers to a substrate whose transmittance at the wavelength in question is at least 10% and preferably equal to or greater than 50%. As shown below, the required degree of transmittance varies depending on the energy of the electromagnetic beam 20 received by the separating layer 10.

[0011] During this irradiation step S3, the separating layer 10 absorbs the incident light that passes through the interface 8 between the support substrate 5 and the separating layer 10. This irradiation leads to a reduction or elimination of the adhesive forces between atoms or molecules in the material of the separating layer 10. This is because, under the influence of the radiation 20, the material forming the separating layer 10 is subjected to photochemical and / or thermal excitation, which leads to the rupture of a chain of atoms or molecules. These ruptures lead to the separation of the separating layer 10 by flaking, either at the actual thickness of the layer 10 (so-called "internal flaking") or at the interface (between the layer 10 and the support substrate 5 or at the interface 12 between the layer 10 and the layer 15 to be separated ("interface flaking").This peeling phenomenon can also involve one or more gases that escape from the material of the separating layer 10 under the influence of radiation 20.

[0012] It should be noted that the separation induced by the radiation 20 does not necessarily lead to detachment or actual separation in the separating layer 10 (or at one of the interfaces 8 and 12), but may also only cause a weakening of the material of the separating layer 10. In the latter case, the application of additional energy (for example, in the form of mechanical force) is necessary to achieve the actual separation between the support substrate 5 and the layer 15 (if such a separation is indeed desired).

[0013] Once substrate 5 and layer 15 are completely separated from each other (step S4), the support substrate 5 can be recycled to form, for example, a new composite structure.

[0014] Currently, those following the layout of Fig. 1. The composite structures produced generally consist of one of the following compositions: - GaN / Al2O3, which corresponds to a separating layer 10 consisting of GaN and a support substrate 5 consisting of sapphire; - Si3N4 / Al2O3, which corresponds to a separating layer 10 consisting of Si3N4 and a support substrate 5 consisting of sapphire.

[0015] With regard to these compositions, the results of separation by flaking are generally satisfactory. For example, when GaN layers deposited onto a sapphire substrate are separated, the application of radiation 20 (at a wavelength typically between 190 and 250 nm) is carried out under good conditions and the separation is achieved without major difficulties.

[0016] However, the applicant has observed that the results can deteriorate significantly when this separation method is applied to compositions other than the composite structure 25. For example, separation by flaking is much more difficult with a composite structure 25 of the SiO2 / Si type (i.e., silicon dioxide on silicon). The applicant observed large variations in the quality of separation by flaking in the entries examined, and generally, less uniform separations require longer irradiation.

[0017] Therefore, there is currently a need for a separation method by peeling that leads to better results in terms of quality, effectiveness and uniformity, both for conventional composite structures and for composite structures with unconventional composition. Purpose and Summary of the Invention

[0018] The present invention relates to a method for separating a layer from a composite structure according to claim 1, wherein the composite structure comprises a composite stack containing at least the following: - a support substrate consisting of a material that is at least partially transparent at a certain wavelength; - the layer to be separated; and - a separating layer that is arranged between the support substrate and the layer to be separated.

[0019] The inclination of the light beam according to the invention advantageously makes it possible to limit or avoid the optical interactions in the thickness of the support substrate between the incident rays on the one hand and the rays reflected from the interface between the support substrate and the separation layer on the other hand during the separation by peeling process.

[0020] The method according to the invention makes it possible to reduce or even prevent optical interference between incident and reflected rays in the support substrate. Reducing or eliminating this optical interference advantageously minimizes variations in the actual transmitted energy as a function of the support substrate thickness during the peeling process, since this interference can be destructive. The invention also makes it possible to maximize the amount of energy actually transmitted into the separation layer.

[0021] The quality and reproducibility of separation by flaking between the support substrate and the layer to be separated are thus greatly improved. In particular, the invention makes it possible to perform separation by flaking on composite structures whose compositions differ from those generally used and which are especially sensitive to optical interference.

[0022] In a specific embodiment, the angle of incidence θ is such that θ ≤ θ max , where θmax is defined as follows: θ max = tan -1 (n1 / n0). In other words, the maximum limit is equal to Brewster's angle.

[0023] The Brewster angle defines the maximum limit of the angle of incidence θ beyond which the entire light intensity of the incident beam is reflected at the exposed surface of the substrate.

[0024] As described in more detail below, this embodiment is only possible if θmax > θmin. This embodiment also makes it possible to maximize the amount of energy actually transmitted into the interface during the irradiation step.

[0025] Furthermore, the support substrate can consist of silicon and the separating layer of silicon dioxide. Brief description of the drawings

[0026] Further features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings, which illustrate an exemplary embodiment without implying any limitations. The figures are as follows: Fig. Figure 1 schematically shows a known method for producing a composite structure and a method applied to this structure for separation by flaking; Fig. Figure 2 schematically illustrates the mechanism by which optical interference arises in the support structure of a composite structure; and Detailed description of an exemplary embodiment

[0027] The present invention relates to a separation method that makes it possible to loosen or weaken a layer that is to be separated from a composite structure with conventional or unconventional composition by flaking.

[0028] The applicant has conducted an investigation in which the physical mechanism that causes the difficulties in carrying out a separation method by flaking was demonstrated. In particular, this investigation was able to show the role of optical interference that occurs in the support substrate during irradiation of the composite structure.

[0029] This mechanism is now referred to in relation to Fig. 2 described. The figure shows the composite structure 25, as described above with reference to Fig. 1 described.

[0030] In particular, it shows Fig.2 An incident light beam 22a reaches the exposed area 5a of the substrate 5 during irradiation step S3. As with any light beam striking a semi-absorbing medium, part (not shown) of the light beam 22a is reflected at the surface 5a of the substrate 5, while part 22b of the beam is transmitted into the substrate 5. On its path through the thickness of the substrate 5, part of the transmitted beam 22b is absorbed, and the remaining part reaches the interface 8 between the substrate 5 and the separating layer 10. The interface 8, in turn, acts as an optical diopter, so that part of the beam 22b is reflected (reflected beam 22d), and the remaining part 22c is transmitted into the separating layer 10. The reflected beam 22d hits the surface 5a and leads to new processes of internal reflection in the support substrate 5.

[0031] However, the various light rays (22b, 22d, 22e...) penetrating the thickness of the support substrate 5 interact with each other and, depending on their phase shift, generate waves with higher intensities (this is called constructive interference) or waves with lower intensities (this is called destructive interference). This interference phenomenon causes the strong variations and significant reductions that the applicant observed in the radiant energy transmitted from the support substrate to the separating layer.

[0032] More precisely, these investigations showed that the level of interference strongly depends on the jumps in the optical index (or refractive index) encountered by the radiation at the exposed surface 5a and the interface 8. Ideally, the difference in the optical indices of the various materials involved should be minimized. With regard to the compositions of conventional composite structures (namely GaN / Al₂O₃ or Si₃N₄ / Al₂O₃), the situation is particularly advantageous, since the index jumps are at most 0.87 for radiation with a wavelength between 150 and 300 nm (the optical indices of sapphire and Si₃N₄ are 1.87 and 2.27, respectively).

[0033] The situation is less favorable for a SiO2 / Si composition, where the optical index of silicon dioxide and silicon increases to 1.992 and 3.42, respectively, when exposed to radiation with a wavelength greater than 1.5 µm. When the exposed surface 5a comes into contact with air (with an index of 1), very large jumps in the optical index are achieved, for example, when exposed to radiation of 9.3 µm (on the order of 2.4 or 1.4 between the individual layers). These large jumps in the optical index contribute to the generation of very large variations in the light intensity transmitted to the interface 10 in step S3.

[0034] Furthermore, the sensitivity to interference relative to the thickness of the substrate is extremely high in the aforementioned case of the silicon dioxide separating layer and the silicon substrate. Specifically, a maximum transmitted intensity is observed at a silicon thickness of 1.35 µm. In other words, given a silicon substrate thickness L, at which the transmittance maximum is reached, a transmittance minimum is reached at a thickness L + 1.35 / 2 µm, and a new transmittance maximum is reached at a thickness L + 1.35 µm. This means that the thickness of the silicon substrate would have to be controlled with a much higher degree of precision than 0.675 µm to avoid significant variations in the transmittance of radiation into the separating layer.However, such a level of control is currently inconceivable for silicon substrates, whose thickness generally varies by a margin of 5 µm to 1.5 µm with a wafer diameter of 200 mm and 300 mm respectively (Total Thickness Variation or “TTV”).

[0035] The level of control over the thickness of substrates, such as those made of silicon, is therefore insufficient and contributes to the problem of fluctuations in transmitted energy explained above.

[0036] The problems with transmittance and optical interference have not yet been effectively solved, especially since they only have a limited impact on composite structures with traditional compositions. The applicant has therefore developed a new separation method that makes it possible to overcome the aforementioned disadvantages, even independently of the composition of the composite structure in question.

[0037] A separation process according to an embodiment of the invention is now described.

[0038] In this context, we consider a composite structure comprising the following composite stack: - a support substrate consisting of a material that is at least partially transparent at a specific wavelength, which is denoted by λ; - a layer to be separated; and - a separating layer that is arranged between the support substrate and the layer to be separated.

[0039] The separating layer and the layer to be separated can be formed by a suitable coating technique (e.g., PECVD or low-pressure CVD) or any other suitable layer formation technique. Alternatively, at least one of these layers can be applied by bonding.

[0040] It is possible to separate the substrate layer by peeling off the separation layer. It should be noted that the composite structure can undergo 125 complementary technological steps before the peeling process is carried out. In particular, one or more layers can be formed or applied to the exposed side of the layer (for example, a final substrate), optionally after technological steps have been carried out on the reverse side of the layer (formation of components, etc.).

[0041] In the example described here, a final substrate is also formed on the exposed side of the layer to be separated (i.e., on the side opposite the one in contact with the separation layer) (for example, by bonding or vapor deposition).

[0042] According to the separation method of the invention, electromagnetic radiation in the form of a light beam is applied to the support substrate of the composite structure. The applicant was surprised to observe that the efficiency of a peeling method applied to the composite structure can be significantly improved by changing the inclination of the light beam projected onto the support substrate.

[0043] During the irradiation step, the incident rays strike the surface of the support substrate at an angle of incidence, each denoted by θ, which is defined with respect to the perpendicular to the surface.

[0044] The incident rays have a wavelength λ for which the substrate is at least partially transparent. At this wavelength, the substrate has a transmittance of at least 10% and preferably more than or equal to 50%. However, it is possible to compensate for a low transmittance by increasing the energy of the beam used in the irradiation step (for example, by focusing the beam).

[0045] It should be noted that the substrate and the individual layers forming the composite structure can consist of two or more sublayers. In particular, the separation layer can comprise a first so-called heating sublayer (consisting, for example, of silicon dioxide) and a second so-called peeling sublayer (consisting, for example, of Si3N4). The function of the heating sublayer is to generate heat under the influence of irradiation. The function of the peeling sublayer is to cause the separation layer to detach by peeling under the influence of the heat energy transferred by the heating sublayer (through thermal conduction). The peeling sublayer is designed to degrade or weaken under the influence of the heat generated by the heating sublayer during irradiation.

[0046] In one variant, the separating layer comprises at least one sublayer that simultaneously fulfills the functions of heating and peeling.

[0047] As with any light beam striking a semi-absorbing medium, part of the light beam is reflected from the exposed area of ​​the substrate, while part of the beam passes through the substrate. On its path through the thickness of the substrate, some of the transmitted beam is absorbed, and the remaining portion reaches the interface between the substrate and the separating layer. This interface acts as an optical diopter, causing part of the beam to be reflected. The reflected beam then strikes the surface and triggers further internal reflection processes within the substrate.

[0048] When a conventional peel separation method is used, the incident beam is applied to the surface at a very small angle of incidence θ with respect to the normal. As a general rule, the beam is projected perpendicularly onto the surface of the substrate. However, for optical interference to occur within the thickness of the substrate, the beam reflected from the interface must be able to interact with the incident beam transmitted into the substrate. The applicant has observed that by sufficiently tilting the incident beam with respect to the surface of the substrate, it is possible to reduce or completely eliminate the optical interference responsible for the problems of fluctuations and / or limitations in the energy actually transmitted to the separation layer during the irradiation step.

[0049] According to the invention, the angle of incidence θ, at which the incident beam strikes the support substrate, should be such that: θ≥θmin where the threshold θ min is defined as follows: θmin=sin−1((n1 / n0)sin(tan−1(S / 2h))) where n1 and n0 are the refractive index of the support substrate and the refractive index of the external medium that is in contact with the support substrate from which said beam comes, S is the width of the beam and h is the thickness of the support substrate.

[0050] In a first example, the angle of incidence θ1, at which the incident beam appears, is chosen such that θ1 < θmin. In this example, the transmitted and reflected beams overlap almost completely, allowing them to interact through the thickness of the substrate. Consequently, significant optical interference is expected to occur during the irradiation step of the separation process.

[0051] In another example, the incident ray is inclined, so that the angle of incidence θ2 satisfies the condition θ2 ≥ θ min This condition is met. In this case, there is no overlap of the incident surfaces formed by the rays on the illuminated surface. Similarly, there is no overlap of the incident surfaces formed by the rays on the interface.

[0052] The inclination of the light beam according to the invention advantageously makes it possible to limit or avoid the optical interactions in the thickness of the support substrate between the various optical beams involved (i.e. the beams and all resulting internal reflections) during the peeling process.

[0053] The method according to the invention makes it possible to reduce or even prevent all optical interference between incident and reflected rays in the support substrate. The reduction or elimination of this optical interference advantageously reduces the variations in the energy actually transferred to the separation layer as a function of the support substrate thickness during the separation by detachment process. The invention also makes it possible to maximize the amount of energy actually transmitted to the separation layer.

[0054] The quality and reproducibility of separation by flaking between the support substrate and the layer to be separated are thus greatly improved. In particular, the invention makes it possible to perform separation by flaking on composite structures whose compositions differ from those generally used and which are especially sensitive to optical interference.

[0055] As explained with reference to the composite structure 25, the separation caused by the beam does not necessarily lead to detachment or actual separation in the separating layer, but may only cause a weakening of the material of the separating layer, which requires the subsequent application of additional energy (for example in the form of mechanical force) to achieve the actual solution between the support substrate and the layer.

[0056] Once the substrate and the layer are completely separated, the support substrate can be recycled to form, for example, a new composite structure.

[0057] In a first example, it is assumed that: - the support substrate consists of silicon with an optical index n1 = 3.42 and a thickness h = 775 µm, - the separating layer consists of silicon dioxide, - the wavelength λ of the incident beam is 9.3 µm, - the external medium in which the composite structure is arranged is air, whose refractive index n0 is such that n0 = 1, and - the width S of the beam is such that S = 120 µm.

[0058] Then a threshold value θ is set. min = 17.9177° determined, i.e. in the order of 18°.

[0059] In this case, the separation method can be configured, for example, such that θ2 = 18°, 19° or 20°.

[0060] In a second example, it is assumed that: - the support substrate consists of sapphire with an optical index n1 = 1.87 and a thickness h = 430 µm, - the separating layer comprises a heating sublayer consisting of silicon dioxide and a peeling sublayer consisting of Si3N4, - the wavelength λ of the incident beam is 9.3 µm, - the external medium in which the composite structure is arranged is air, whose refractive index n0 is such that n0 = 1, and - the width of the incident beam is S = 120 µm.

[0061] Then a threshold value θ is set. min = 83° determined. However, if the angle of incidence θ2 of the incident ray meets the condition θ2 ≥ θ minIf this condition is met, the angle θ2 exceeds the angular limit known as the Brewster angle, i.e., the maximum angular limit at which the entire light intensity of the incident beam is reflected from the surface of the substrate. The Brewster angle is defined as follows: θB=tan−1(n1 / n0)

[0062] In the second example mentioned above (where the substrate is made of sapphire), the Brewster angle θ is B = 62°. This results in the following situation: θmin>θB.

[0063] Consequently, in this specific case it is not possible to define the angle of incidence θ such that θ ≥ θ min and θ < θ B In other words, optical inference cannot be overcome by changing the angle of incidence of the beam. Nevertheless, the angle of incidence can be set so that it is slightly smaller than θ. Bis (for example, on the order of 60°) to reduce the extent of optical interference in the substrate as much as possible.

[0064] In a specific embodiment of the invention, the angle of incidence θ of the optical ray therefore satisfies the following condition: θ < θ B This embodiment also makes it possible to maximize the amount of energy actually transmitted into the interface during the irradiation step. However, as explained above, this embodiment is only possible if θ min < θ B .

Claims

[1] Method for separating a layer from a composite structure, wherein this composite structure comprises a composite stack formed from at least the following: a support substrate consisting of a material that is at least partially transparent at a certain wavelength; the layer to be separated; and a separating layer arranged between the support substrate and the layer to be separated, wherein the method comprises irradiating the separating layer through the support substrate using at least one incident light beam at the specified wavelength to cause attenuation or separation by flaking off the separating layer, the procedure characterized byis that the incident light ray is inclined in such a way that there is no superposition of incident surfaces on an exposed surface of the substrate, which are formed on the exposed surface of the substrate by the incident light ray and the ray reflected at an interface between the substrate and the separating layer. [2] Separation method according to claim 1, wherein the support substrate consists of silicon and the separation layer consists of silicon dioxide.

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