Method for separating a layer from a composite structure

Abstract

The disclosure relates to a method for separating a layer from a composite structure, the structure comprising a composite stack formed from at least a support substrate, which is partially transparent at a determined wavelength, the layer to be separated and a separation layer interposed between the support substrate and the layer to be separated, the method comprising irradiation of the separation layer through the support substrate by means of incident light ray at the determined wavelength in order to induce weakening or separation by exfoliation of the separation layer, the light ray being inclined so as to form an angle of incidence such that >.sub.min, where .sub.min=sin.sub.1((.sup.n1/n.sub.0)sin(tan.sup.1(s/2h))), n1 and n0, respectively, being the refractive index of the support substrate and the refractive index of the external medium in contact with the support substrate, from which the ray comes, S being the width of the ray and h being the thickness of the support substrate.

Claims

1. A method for separating a layer from a composite structure, the method comprising: providing a composite structure including a support substrate comprising a material at least partially transparent to a ray of electromagnetic radiation having a determined wavelength, the layer to be separated, and a separation layer interposed between the support substrate and the layer to be separated; providing an external medium in contact with an upper surface of the support substrate; determining a refractive index (n.sub.1) of the support substrate; determining a refractive index (n.sub.0) of the external medium; determining a thickness (h) of the support substrate; using the equation =sin.sup.1((n.sub.1/n.sub.0) sin(tan.sup.1(tan.sup.1(s/2.sub.h))), to determine an angle of incidence in which S is a width of an at least one incident ray of electromagnetic radiation to be directed at the composite structure; orienting the at least one incident ray of electromagnetic radiation at the separation layer and inclining the at least one incident ray of electromagnetic radiation at the angle of incidence relative to the upper surface of the support substrate; irradiating the separation layer through the support substrate with the at least one incident ray of electromagnetic radiation having the determined wavelength; and causing the separation layer to weaken or separate by exfoliation.

2. The method of claim 1, wherein the support substrate comprises sapphire and the separation layer comprises at least one of silicon dioxide and silicon nitride.

3. The method of claim 2, wherein the support substrate consists essentially of sapphire and the separation layer consists essentially of at least one of silicon dioxide and silicon nitride.

4. The method of claim 2, wherein the separation layer comprises silicon dioxide.

5. The method of claim 4, wherein the support substrate consists essentially of sapphire and the separation layer consists essentially of silicon dioxide.

6. The method of claim 1, wherein the angle of incidence is less than a Brewster angle .sub.max wherein the Brewster angle is a maximum angle limit above which all of the at least one incident rays of electromagnetic radiation are reflected by a surface of the support substrate such that there is no penetration of the at least one incident ray into the support substrate; and the Brewster angle is determined using the equation .sub.max=tan.sup.1(n.sub.1/n.sub.0).

7. The method of claim 1, wherein the width of the at least one incident ray of electromagnetic radiation is 120 m, the angle of incidence of the at least one incident ray of electromagnetic radiation is approximately 60, the external medium is air, the separation layer comprises silicon dioxide, the support substrate comprises sapphire, and the support substrate has an optical index of 1.87 and a thickness of 430 m.

8. The method of claim 1, wherein a width of the at least one incident ray at the surface of the support substrate does not substantially overlap with a width of a reflected ray at the surface of the support substrate.

9. The method of claim 1, wherein the width of the at least one incident ray at an interface between the support substrate and the separation layer does not substantially overlap with a width of a reflected ray at the interface between the support substrate and the separation layer.

10. A method for separating a layer from a composite structure, the method comprising: providing a composite structure including a support substrate comprising a material at least partially transparent to radiation having a determined wavelength, the layer to be separated, and a separation layer interposed between the support substrate and the layer to be separated; providing an external medium in contact with an upper surface of the support substrate; determining a refractive index (n.sub.1) of the support substrate; determining a refractive index (n.sub.0) of the external medium; determining a thickness (h) of the support substrate; using the equation .sub.min=sin.sup.1((n.sub.1/n.sub.0) sin(tan.sup.1(tan.sup.1(s/2.sub.h))), to determine a minimum angle of incidence (.sub.min) in which S is a width of an at least one incident ray to be directed at the composite structure; using the equation .sub.max=tan.sup.1(n.sub.1/n.sub.0); to determine a Brewster angle .sub.max, if the Brewster angle .sub.max is equal to or greater than the minimum angle of incidence .sub.min, then orienting the at least one incident light ray at the separation layer and inclining the at least one incident light ray at the minimum angle of incidence .sub.min relative to the upper surface of the support substrate; otherwise, if the Brewster angle .sub.max is less than the minimum angle of incidence .sub.min, then orienting the at least one incident light ray at the separation layer and inclining the at least one incident light ray at between 0 and 2 degrees less than the Brewster angle .sub.max relative to the upper surface of the support substrate; irradiating the separation layer through the support substrate with the at least one incident light ray having the determined wavelength; and causing the separation layer to weaken or separate by exfoliation.

11. The method of claim 10, wherein the support substrate comprises silicon and the separation layer comprises silicon dioxide.

12. The method of claim 11, wherein the support substrate consists essentially of silicon and the separation layer consists essentially of silicon dioxide.

13. The method of claim 10, wherein the minimum angle of incidence .sub.min of the at least one incident ray is between 18 and 20, the width of the at least at least one incident ray is 120 m, the external medium is air, the separation layer comprises silicon dioxide, and the support substrate comprises silicon that has an optical index of 3.42, and a thickness of 775 m.

14. The method of claim 10, wherein the width of the at least one incident ray at the surface of the support substrate does not overlap with a width of a reflected ray at the surface of the support substrate.

15. The method of claim 10, wherein the width of the at least one incident ray at an interface between the support substrate and the separation layer does not overlap with a width of a reflected ray at the interface between the support substrate and the separation layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the present disclosure will become apparent from the description given below with reference to the appended drawings, which illustrate an exemplary embodiment thereof implying no limitation. In the figures:

(2) FIG. 1 schematically represents a known method for fabricating a composite structure, as well as a method of separation by exfoliation applied to the said structure;

(3) FIG. 2 schematically illustrates the mechanism of the creation of optical interference in the support structure of a composite structure; and

(4) FIGS. 3A and 3B schematically represent a method of separation by exfoliation, according to a particular embodiment of the disclosure, applied to a composite structure.

DETAILED DESCRIPTION

(5) The present disclosure relates to a separation method making it possible to detach or weaken, by exfoliation, a layer to be separated from a composite structure of conventional or unconventional composition.

(6) The Applicant has carried out a study which has made it possible to demonstrate the physical mechanism giving rise to the difficulties encountered for carrying out a method of separation by exfoliation. The study has in particular demonstrated the role of the optical interference occurring in the support substrate during the irradiation of the composite structure.

(7) This mechanism will now be described with reference to FIG. 2. This figure represents the composite structure 25, as described above, with reference to FIG. 1.

(8) FIG. 2 represents in particular an incident light ray 22a reaching the exposed surface 5a of the support substrate 5 during the irradiation step S3. As for any light ray encountering a semi-absorbent medium, a part (not represented) of the light ray 22a is reflected at the upper surface 5a of the substrate 5 while a part of the ray 22b is transmitted into the support substrate 5. During its passage through the thickness of the support substrate 5, a part of the transmitted ray 22b is absorbed and the remaining part reaches the interface 8 between the support substrate 5 and the separation layer 10. The interface 8 again functions as an optical diopter so that the ray 22b is partially reflected (reflected ray 22d) and the remaining part 22c is transmitted into the separation layer 10. The reflected ray 22d again encounters the upper surface 5a and leads to new processes of internal reflection within the support substrate 5.

(9) However, the various light rays (22b, 22d, 22e . . . ) passing through the thickness of the support substrate 5 interact with one another in order to create, depending on their phase shift, waves with higher intensities (this is referred to as constructive interference) or waves with lower intensities (this is referred to as destructive interference). These interference phenomena give rise to the strong variations and significant reductions observed by the Applicant in relation to the radiative energy transmitted by the support substrate to the separation layer.

(10) More particularly, the studies have shown that the level of the interference depends strongly on the jumps in the optical index (or refractive index), which are encountered by the radiation at the exposed surface 5a and at the interface 8. Ideally, the optical index difference between the various materials involved should be minimized. As regards, the compositions of conventional composite structures (namely, GaN/Al.sub.2O.sub.3 or Si.sub.3N.sub.4/Al.sub.2O.sub.3), the situation is particularly favorable since the index jumps are at most 0.87 for radiation with a wavelength of between 150 nm and 300 nm (the optical indices of sapphire and Si.sub.3N.sub.4 being 1.87 and 2.27, respectively).

(11) On the other hand, the situation is much less favorable for an SiO.sub.2/Si composition in which the optical index of silicon dioxide and silicon, respectively, rise to 1.992 and 3.42 for radiation having a wavelength of more than 1.5 m. If the exposed surface 5a is in contact with air (with an index equal to 1), then, for example, very large jumps in optical index are obtained (of the order of 2.4 or 1.4 between each of the layers) for radiation with a wavelength of 9.3 m. These large jumps in optical index contribute to generating very strong variations in the light intensity transmitted to the separation layer 10 during step S3.

(12) Furthermore, the level of sensitivity to interference in relation to the thickness of the support substrate is extreme in the above case of a separation layer consisting of silicon dioxide and a support substrate consisting of silicon. Specifically, a maximum transmitted intensity is observed with a silicon thickness period of 1.35 m. In other words, when considering a thickness L of the support substrate consisting of silicon for which a transmission maximum is reached, a transmission minimum will in principle be obtained for a thickness L+1.35/2 m and a new transmission maximum will be reached for a thickness L+1.35 m. This means that it would be necessary to control the thickness of the support substrate consisting of silicon with a level of precision much better than 0.675 m in order to avoid significant variations in transmission of the radiation into the separation layer. Such a level of control is not currently envisageable for support substrates consisting of silicon, the thickness of which generally varies with an amplitude of the order of 5 m and 1.5 m for a wafer diameter of 200 mm and 300 mm, respectively, (Total Thickness Variation or TTV).

(13) The level of control over the thickness for substrates such as those consisting of silicon is therefore insufficient, which contributes to the problem of the variations in transmitted energy as explained above.

(14) The problems of transmission and optical interference have not hitherto been dealt with effectively, in particular because they have a limited impact on composite structures of traditional composition. The Applicant has therefore developed a new separation method making it possible to overcome the aforementioned drawbacks, and to do so irrespective of the composition of the composite structure in question.

(15) A separation method, according to one embodiment of the disclosure, will now be described with reference to FIGS. 3A and 3B.

(16) More precisely, FIGS. 3A and 3B represent a composite structure 125 comprising the following composite stack: a support substrate 105 formed from a material which is at least partially transparent at a determined wavelength, denoted ; a layer 115 to be separated; and a separation layer 110 interposed between the support substrate 105 and the layer 115 to be separated.

(17) The separation layer 110 and the layer 115 to be separated may be produced by any suitable deposition technique (for example, by PECVD or LPCVD deposition) or any other suitable layer formation technique. It is also conceivable to assemble at least one of these layers by bonding.

(18) It is possible to detach the layer 115 from the support substrate 105 by separating the separation layer 110 by exfoliation. It will be noted that the composite structure 125 may be subjected to complementary technology steps before the method of separation by exfoliation is carried out. One or more layers may, in particular, be formed or assembled on the exposed face of the layer 115 (for example, a final substrate), optionally after technology steps have been carried out on the rear face of the layer 115 (formation of components, etc.).

(19) In the example described here, a final substrate 120 is furthermore formed (for example, by bonding or deposition) on the exposed face of the layer 115 to be separated (i.e., on the face on the opposite side from the one in contact with the separation layer 110).

(20) According to the separation method of the disclosure, electromagnetic radiation is applied in the form of a light beam onto the support substrate 105 of the composite structure 125. The Applicant has surprisingly observed that it is possible to substantially improve the effectiveness of a method of separation by exfoliation applied to the composite structure 125, by influencing the inclination of the light beam projected onto the support substrate 105.

(21) During the irradiation step, the incident beams 122a and 124a strike the surface 104 of the support substrate 105 at an angle of incidence, respectively, denoted 1 and 2, defined with respect to the normal to the surface 104 (cf. FIGS. 3A and 3B, respectively).

(22) The incident beams 122a and 124a have a wavelength at which the support substrate 105 is at least partially transparent. At this wavelength, the substrate 105 has a transmittance of at least 10%, and preferably greater than or equal to 50%. It is, however, possible to compensate for a low transmittance by increasing the amount of energy of the beam applied during the irradiation step (for example, by focusing the beam).

(23) It will be noted that the support substrate and each of the layers forming the composite structure 125 may consist of two or more sublayers. In particular, the separation layer 110 may comprise a first so-called heating sublayer (for example, consisting of silicon dioxide) and a second so-called exfoliation sublayer (for example, consisting of Si.sub.3N.sub.4). The function of the heating sublayer is to induce heating under the effect of the irradiation. The function of the exfoliation sublayer is to lead to separation of the separation layer 110 by exfoliation under the effect of the heat energy transmitted from the heating sublayer (by thermal conduction). The exfoliation sublayer is adapted to decompose or weaken under the effect of the heating induced by the heating sublayer during the irradiation.

(24) In a variant, the separation layer 110 comprises at least one sublayer, which simultaneously fulfills the functions of heating and exfoliation.

(25) As for any light ray encountering a semi-absorbent medium, a part (not represented) of the light beam 122a (and 124a, respectively) is reflected by the exposed surface 104 of the support substrate 105, while a part of the beam 122b (and 124b, respectively) is transmitted into the support substrate 105. During its passage through the thickness of the support substrate 105, a part of the transmitted beam 122b (and 124b, respectively) is absorbed and the remaining part reaches the interface 106 between the support substrate 105 and the separation layer 110. The interface 8 again functions as an optical diopter so that the beam 122b (and 124b, respectively) is partially reflected in the form of a reflected beam 122c (and 124c, respectively). The reflected beam 122c (and 124c, respectively) again encounters the upper surface 104 and leads to new processes of internal reflection within the support substrate 105.

(26) Conventionally, when a method of separation by exfoliation is being carried out, the incident beam is applied onto the surface of the composite structure with a very small angle of incidence with respect to the normal to the surface. As a general rule, the beam is projected perpendicularly onto the surface of the support substrate. However, in order for there to be optical interference in the thickness of the support substrate 105, it is necessary for the beam reflected by the interface 106 to be able to interact with the incident beam transmitted into the substrate 105. The Applicant has observed that, by inclining the incident beam sufficiently with respect to the surface 104 of the support substrate 105, it is possible to reduce or fully eliminate the optical interference giving rise to the problems of variation and/or limitation of the energy, which is actually transmitted to the separation layer during the irradiation step.

(27) According to the disclosure, the angle of incidence at which the incident beam is applied onto the support substrate should be such that:
.sub.min,

(28) the threshold value .sub.min being defined in the following way:

(29) ${}_{\min }={\sin }^{-1}\left(\left(\frac{n_1}{n_0}\right)\sin \left({\tan }^{-1}\left(\frac{S}{2h}\right)\right)\right),$

(30) wherein n1 and n0 are, respectively, the refractive index of the support substrate 105 and the refractive index of the external medium 130 in contact with the support substrate, from which the said ray (or beam) comes, S being the width of the ray (or size of the spot) and h being the thickness of the support substrate.

(31) In the first example, represented in FIG. 3A, the angle of incidence 1 at which the incident beam 122a is applied is such that 1<min. In this example, the transmitted beam 122b and the reflected beam 122c overlap almost completely, so that they can interact throughout the thickness of the support substrate 105. Consequently, significant optical interference is liable to occur during the irradiation step of the separation method.

(32) In the case of FIG. 3B, conversely, the incident beam 124a is inclined so that the angle of incidence 2 satisfies the condition 2min. In this case, the incidence surfaces 144a and 144c, respectively, formed by the beams 124a and 124c on the exposed surface 104 are not superimposed. Likewise, the incidence surfaces 144b and 144d formed by the beams 124b and 124d on the interface 106 are not superimposed.

(33) The inclination of the light ray, according to the disclosure, advantageously makes it possible, during the method of separation by exfoliation, to limit or avoid the optical interactions in the thickness of the support substrate between the various optical beams involved (i.e., the beams 124b, 124c, 124d and all the subsequent internal reflections).

(34) The method of the disclosure, thus, makes it possible to reduce, or even prevent, any optical interference between incident rays and reflected rays within the support substrate. The reduction or elimination of this optical interference advantageously makes it possible to reduce the variations in the energy actually transmitted to the separation layer 110 as a function of the thickness of the support substrate 105 during the method of separation by exfoliation. The disclosure, furthermore, makes it possible to maximize the amount of energy actually transmitted into the separation layer 110.

(35) The quality and reproducibility of the separation by exfoliation between the support substrate 105 and the layer 115 to be separated are thus greatly improved. The disclosure makes it possible, in particular, to carry out separations by exfoliation on composite structures, which have compositions different from those generally used and which are particularly sensitive to optical interference.

(36) As indicated with reference to the composite structure 25, the separation induced by the beam 124a does not necessarily lead to detachment or actual separation in the separation layer 110, but may simply lead to weakening of the material of the separation layer 110, requiring the subsequent application of additional energy (for example, in the form of mechanical forces) in order to obtain the actual detachment between the support substrate 105 and the layer 115.

(37) Once the substrate 105 and the layer 115 have been fully separated, the support substrate 105 may be recycled with a view to forming a new composite structure.

(38) In a first example, it is assumed that: the support substrate 105 consists of silicon with an optical index n1=3.42 and a thickness h=775 m, the separation layer 110 consists of silicon dioxide, the wavelength of the incident beam is 9.3 m, the external medium 130 in which the composite structure 125 is placed is air, the refractive index n0 of which is such that n0=1, and the width S of the beam is such that S=120 m.

(39) A threshold value min=17.9177 is then obtained, i.e., of the order of 18.

(40) In this case, the separation method may, for example, be configured so that 2=18, 19 or 20.

(41) In a second example, it is assumed that: the support substrate 105 consists of sapphire with an optical index n1=1.87 and a thickness h=430 m, the separation layer 110 comprises a heating sublayer consisting of silicon dioxide and an exfoliation sublayer consisting of Si.sub.3N.sub.4, the wavelength of the incident beam is 9.3 m, the external medium 130 in which the composite structure 125 is placed is air, the refractive index n0 of which is such that n0=1, and the width of the incident beam is S=120 m.

(42) A threshold value min=83 is then obtained. However, if the angle of incidence 2 of the incident beam 124a satisfies the condition 2min, the angle 2 exceeds the angle limit referred to as the Brewster angle, i.e., the maximum angle limit for which all the light intensity of the incident beam 124a is reflected by the surface 104 of the support substrate 105. The Brewster angle is defined in the following way:

(43) ${}_S={\tan }^{-1}\left(\frac{n_1}{n_0}\right)$

(44) In the second example above (support substrate consisting of sapphire), the Brewster .sub.B angle is 62. The following situation is therefore encountered:
min>.sub.B.

(45) Consequently, in this particular case it is not possible to set the angle of incidence so that .sub.min and <.sub.B. In other words, in this example it is not possible to overcome the optical interference by influencing the angle of incidence of the beam. It is nevertheless possible to set the angle of incidence so as to be slightly less than .sub.B (for example, of the order of 60) in order to reduce the level of optical interference in the support substrate as much as possible.

(46) In a particular embodiment of the disclosure, the angle of incidence of the optical beam therefore satisfies the following condition: <.sub.B. This embodiment makes it possible to maximize the amount of light energy actually transmitted to the separation layer during the irradiation step. As indicated above, however, this embodiment is feasible only if min<.sub.B.