Implantation wheel for forming embrittled surfaces on multiple donor wafers - Patents.com

Abstract

The invention relates to an implantation wheel (1) for forming embrittled surfaces of a plurality of donor wafers (3), the wheel (1) comprising a main disk (1a) and a plurality of wafer supports (2) arranged on one side of the main disk (1a), each wafer support (2) having a host surface on which the so-called "back" surface of the donor wafer (3) is arranged. According to a first embodiment, the host surface at least partially comprises a surface elastomer layer (2b) having dimensions at least equal to the dimensions of the back surface of the donor wafer (3). According to another embodiment, each host surface of the plurality of wafer supports (2) has a convex shape, which is selected to correspond to the shape of the donor wafer when it is subsequently deformed under the effect of temperature.

Description

[Technical field]

[0001] The present invention relates to the field of ion implantation, and more specifically to an ion implantation apparatus suitable for forming embrittled surfaces in multiple donor wafers as part of a thin layer transfer method implemented using Smart Cut® technology. [Background technology]

[0002] US Patent Application Publication No. 2020 / 0186117 proposes using the Smart Cut® technology to transfer a thin layer of ferroelectric material onto a substrate having a thermal expansion coefficient different from that of the ferroelectric material constituting the thin layer. In particular, the substrate may be a silicon substrate.

[0003] The Smart Cut® technology defines a thin layer by introducing light species into a donor substrate, which are concentrated at embrittled planes located at the average penetration depth of the species. The donor substrate is then bonded to a support and the layer is transferred by delaminating the thin layer at the embrittled planes. In the case of layers made of ferroelectric and / or piezoelectric materials and when the light species are selected from among hydrogen and helium ions, this delamination occurs by a gentle increase in temperature (approximately 170° C.) to which the assembly is subjected.

[0004] In order to limit the stresses that arise in the assembly when the temperature increases, especially during the aforementioned peeling stage, US 2020 / 0186117 proposes forming a donor substrate of the assembly from a thick layer formed of a ferroelectric material and a handling substrate, the handling substrate having a thermal expansion coefficient similar to that of the support substrate. Such a donor substrate is hereinafter referred to as a donor heterostructure.

[0005] The step of introducing light species into the donor heterostructure can be carried out using an ion implantation apparatus. Figures 1a and 1b show schematic diagrams of known configurations of such an apparatus.

[0006] The wheel 1 of the ion implanter comprises a disk 1a arranged in a plane at a slight inclined angle (typically 5°-10°) to the vertical. On one of its sides, the wheel carries a number of supports 2 for receiving a number of donor heterostructures. The disk rotates around a vertical axis R passing through its center at a speed of up to 1200 rpm. The support has a wedge 2a on which the edge of the donor heterostructure supports as the wheel rotates at high speed. During this rotation, the exposed surfaces of the donor heterostructures are sequentially placed against a source of ions S of light species (e.g. hydrogen or helium ions) accelerated to a given energy and shaped into an ion beam F that defines the implantation current. For a given implantation species, the implantation energy, the implantation current and the exposure time to the beam form the implantation conditions. The energy defines the average penetration depth of the ions and the current defines the implantation dose over a given exposure time. The beam F scans all exposed surfaces of the donor heterostructure.

[0007] Such an ion implantation apparatus is known, for example, from US 2007 / 0158583, US 4,832,781 or US 5,040,484.

[0008] The power generated by ions entering the donor heterostructure tends to heat the heterostructure. To prevent excessive heating, the main disk and wheel supports are fitted with ducts through which a cooling fluid, typically water, circulates. The substrates to which the donor heterostructures are clamped act as heat sinks, so it is important that the heterostructures are in intimate contact with those substrates to facilitate heat dissipation. These supports (which can be made of blocks of aluminum to help dissipate heat) are typically fitted with surface elastomer parts to ensure intimate contact.

[0009] The objective is generally to reduce the time of the implantation step by increasing the beam current, which tends to increase the power delivered to the donor heterostructures and thus their temperature.

[0010] When attempting to increase the injection current of donor heterostructures featuring thick ferroelectric layers, the applicant observed delamination of the thin layer during the injection step itself, an undesirable phenomenon that rendered the layer transfer step impossible, despite all the precautions taken to cool the injection disk support during this step. Summary of the Invention [Problem to be solved by the invention]

[0011] An object of the present invention is to propose an implantation wheel to solve this problem, more specifically to provide an implantation wheel that is particularly suitable for implanting donor heterostructures over a wide range of implantation conditions and that does not cause premature delamination of the donor heterostructure. [Means for solving the problem]

[0012] To achieve this objective, the object of the present invention is to propose an implantation wheel for forming embrittled surfaces in a number of donor wafers, the wheel comprising a main disk and a number of wafer supports arranged on one side of the main disk, the wheel comprising a cooling circuit for cooling the wafer supports and / or the main disk, each wafer support having a host surface on which the so-called "back" side of the donor wafer is placed.

[0013] In accordance with the invention, each host surface of the plurality of wafer supports has a convex shape that is selected to correspond to the shape of the donor wafer when it is subsequently deformed under the influence of temperature.

[0014] According to other advantageous, non-limiting features of this aspect of the invention, taken alone or in any technically feasible combination, each host surface comprises a surface elastomer layer on which the back surface of the donor wafer rests, the surface elastomer layer having dimensions at least equal to the dimensions of the back surface of the donor wafer so as to enable the full extent of the back surface of the donor wafer to be in contact with the wafer support; the surface elastomer layer has a variable thickness, which defines the convex shape of the wafer support; - The convex shape can be either a concave dimple or a convex dome. The concave depression or convex dome has a height of 0.1 mm-5 mm. the implantation wheel has an axis of symmetry defining an axis of rotation, and each host surface of the plurality of wafer supports is oriented toward the axis of rotation such that, during rotation of the wheel, a component of centrifugal force perpendicular to the host surface is applied to the donor wafer, pressing the donor wafer against the host surface. Each wafer support is provided with a wedge for holding the donor wafer by opposing a component of the centrifugal force that is applied to the donor wafer flush with the host surface during rotation of the wheel. The highest elevation of the host surface of each wafer support is a wedge. Each wafer support is fitted with a retractable pin. Each wafer support is fitted with at least one clip for positioning and holding the wafer relative to the wedge. The host surface of each wafer support has a height, the height of the host surface relative to the base of the support being highest at the wedge. [Brief description of the drawings]

[0015] Other features and advantages of the present invention will become apparent from the following detailed description of the invention which refers to the accompanying drawings.

[0016] [Figure 1a] 1 shows a schematic diagram of an ion implantation apparatus configuration according to the prior art and the present invention; [Figure 1b] 1 shows a schematic front view of an injection wheel according to the prior art and the present invention; [Figure 2a]1A and 1B show a donor wafer disposed on a prior art wafer support before and during an ion implantation step, respectively. [Figure 2b] 1A and 1B show a donor wafer disposed on a prior art wafer support before and during an ion implantation step, respectively. [Figure 3a] 1 shows a wafer support for an implantation wheel according to the present invention. [Figure 3b] 1 shows a wafer support for an implantation wheel according to the present invention. [Figure 3c] 1 shows a wafer support for an implantation wheel according to the present invention. [Figure 3d] 1 shows a wafer support for an implantation wheel according to the present invention. [Figure 3e] 1 shows a wafer support for an implantation wheel according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, the expression "coefficient of thermal expansion" used in relation to a layer or substrate refers to the coefficient of expansion in a direction defined by the main plane defining this layer or substrate. If the material is anisotropic, the retained value of the modulus is the value with the largest amplitude. The values ​​of the modulus are those measured at room temperature.

[0018] (Preliminary experiment) To understand the cause of the delamination phenomenon described above, applicants conducted a series of investigative experiments. In these experiments, the spatial temperature profile of the donor heterostructure that occurs in the heterostructure during the implantation step under conditions designed to form an embrittled surface using Smart Cut® technology was measured. Specifically, using prior art ion implantation equipment, 3.0×10 16 at / cm 2of hydrogen dosing was introduced. The donor heterostructure consisted of a layer of piezoelectric material, 15-20 microns thick, bonded to a silicon wafer. The temperature of the heterostructure was measured by distributing on its front side, i.e. the side exposed to the ion beam, several heat-sensitive adhesive patches covered with a Kapton® film. Each patch has a visual indicator of the highest temperature to which it is exposed. By placing these patches on one side of the wafer and recording the visual indicator of each patch after the end of the experiment, it is possible to estimate the spatial temperature profile that occurred within the wafer during the implantation step.

[0019] The same experiments were reproduced under the same implantation conditions, this time applying the implantation step to a solid silicon wafer, identical to the wafer that forms the handling substrate of the donor heterostructure. The table below shows the results of these experiments.

[0020] [Table 1]

[0021] The results of these experiments show that the temperature rise in the donor heterostructure is surprisingly higher than that in a simple silicon wafer. It should be noted that this temperature can reach and even exceed 160° C., approaching and even reaching the temperatures that cause delamination of the ferroelectric thin layer in the application of the Smart Cut® technology mentioned above. It is therefore easy to understand why delamination of the thin layer is observed during the implantation step itself.

[0022] Furthermore, the spatial temperature profile is inverted (the center rises to a lower temperature than the edge) when implanting a silicon wafer compared to when implanting a donor heterostructure (the edge rises to a lower temperature than the center).

[0023] In trying to interpret these results, the applicant observed that in the prior art implantation equipment used to carry out these experiments, the surface elastomer portion of the implantation wheel support, which ensures thermal contact between the support and the implanted wafer, did not extend entirely under the entire back surface of that wafer. As can be seen from FIG. 2a, which shows the donor heterostructure 3 placed on the wafer support before the start of the implantation step, this circular surface elastomer portion 2b, on which the circular wafer 3 is also centered, has a diameter smaller than that of the wafer. The peripheral ring C (2-3 millimeters wide) on the back surface of the wafer is therefore not in intimate contact with this surface elastomer layer 2b and is unable to dissipate the heat that accumulates in that layer during the implantation step. This heat therefore tends to diffuse towards the central part of the wafer 3, increasing its average temperature. In the case of the formed donor heterostructure 3, due to the thick layers of ferroelectric material and handling substrate, which have different expansion coefficients, as mentioned before, the increase in the average temperature tends to deform the donor heterostructure 3, taking on a convex shape to accommodate these stresses. This situation is illustrated in FIG. 2b, which shows a donor heterostructure 3 deformed by the temperature increase during the implantation step. As can be clearly seen in FIG. 2b, this convex shape tends to move the center of the heterostructure away from the implantation wheel support, maintaining contact only on the peripheral circle. The stresses generated by the difference in expansion coefficients are greater than the centrifugal forces that try to keep the heterostructure in intimate contact with the surface elastomeric part of the support. With this intimate contact lost, the heat that accumulates in the donor heterostructure 3 during implantation can no longer be evacuated with the same efficiency, which tends to increase the phenomenon of deformation of the heterostructure and also increases its temperature, through the effect of thermal runaway, until the heterostructure reaches at least locally the exfoliation temperature. This also explains why the center of the donor heterostructure 3, which is no longer in contact with the substrate 2, tends to reach a higher temperature than the edges that remain in contact with the substrate 2.

[0024] (Improved Injection Wheel) Hereinafter, an improved implantation wheel is proposed to prevent excessive heating of the donor wafer (particularly the donor heterostructure) when exposed to a beam of light species ions (typically hydrogen or helium) to form an embrittlement surface therein.

[0025] This wheel is particularly useful when the donor wafer is a donor heterostructure, consisting of a thick layer disposed on a handling substrate, the thermal expansion coefficients of the handling substrate and the thick layer differing from each other, e.g., by at least 5%, however, the wheel can be used with any type of donor wafer.

[0026] Similar to what has been described with respect to figures 1a and 1b, this wheel comprises a main disk 1a and a number of wafer supports 2 arranged, for example in an annular fashion, on one side of the main disk 1. Each wafer support 2 has a host surface designed to receive the "back" face of a donor wafer. The wafer support 2 is made up of a solid metal part, for example aluminium, that defines the host surface. This host surface is advantageously at least partially provided with a surface elastomer layer. This layer can be roughened in order to limit the slippage of the donor wafer, to maintain intimate contact with the back of the wafer and to allow an efficient evacuation of any heat that may accumulate during the ion implantation step. The material constituting this layer is therefore chosen to be a good thermal conductor in order to allow the heat to flow towards the wafer support. This material can be filled with thermally conductive particles, such as carbon or alumina, in order to increase the thermal conductivity of the layer. The surface elastomer layer can be deposited on the metal part of the wafer support, for example using a spin-coating process. Alternatively, the layer can be prepared as an adhesive film, cut to the dimensions of the metal part of the wafer support 2 and placed on the host surface.

[0027] The support 2 and / or the main disk 1 can be cooled by a cooling circuit to extract heat from the donor wafer throughout the implantation step.

[0028] The wheel 1 has an axis of symmetry R (perpendicular to the plane of the main disk constituting the wheel and passing through its centre) that defines the axis of rotation of the wheel, which the wheel 1 rotates around (completed by a scanning movement) in order to successively expose, during an implantation step, donor wafers held on their supports to a light species ion beam generated by a source of the implantation equipment.

[0029] Each host surface of the plurality of wafer supports 2 is oriented toward the axis of rotation so as to apply a component of centrifugal force perpendicular to the host surface to the donor wafer as the wheel 1 rotates. This force tends to press the donor wafer against the host surface. As can be clearly seen in Figure 3a, in order to orient the bearing surface of the support toward the axis of rotation of the wheel on which it rests, the support is thinner on the side facing the center of the wheel than on the side facing away from the center of the wheel.

[0030] Each wafer support 2 is fitted with a shim 2a, typically a lateral shim that matches the shape of a donor wafer and enables it to hold this wafer by counteracting the component of the centrifugal force applied to the wafer flush with the host surface as the wheel rotates.

[0031] Finally, as is well known, each wafer support is engaged with at least one clip (not shown) for accurately positioning the donor wafer on that support and holding the wafer against the wedge when the wheel is not rotating.

[0032] According to a first embodiment shown in Fig. 3a, the elastomeric layer 2b has dimensions at least equal to those of the rear surface of the donor wafer 3 so as to allow the entire area of ​​this rear surface to be brought into contact with the wafer support 2. Advantageously, the elastomeric layer 2b has dimensions strictly greater than those of the rear surface of the donor wafer 3.

[0033] In this way, it is prevented that during the implantation step, the peripheral ring C of the wafer accumulates excess heat and thus loses in intimate contact with the elastomer layer 2b, as was the case with the prior art configuration. This prevents or delays the occurrence of runaway phenomena, which would lead to a gradual increase in the average temperature of the donor wafer 3, leading to deformation of the wafer and loss of intimate contact between its back surface and the elastomer layer. It is therefore possible to carry out this implantation step without raising the average temperature of this plate above a critical temperature that would cause at least partial delamination of the thin layer. As a corollary, it is therefore possible to apply a wider range of implantation conditions (note that these are mainly defined as implantation current, implantation energy and ion beam exposure time) than was possible with the prior art equipment. In particular, it is possible to reduce the beam exposure time and therefore the duration of this implantation step by increasing the implantation current, without risking delamination phenomena.

[0034] According to a second embodiment, examples of which can be found in Figures 3b and 3c, each host surface of the multiple donor wafer supports 2 has a convex shape, which is selected to correspond to the convex or concave shape of the wafer when it is subsequently deformed under the effect of temperature.

[0035] This convex shape of the support 2, which tends to conform to the shape of the donor wafer after the increase in temperature during the implantation step, maintains intimate contact between the back surface of this wafer and the host surface of the support, ensuring that heat is dissipated to the support 2 by diffusion, thus limiting the increase in temperature during the implantation step.

[0036] It should be noted that when a donor wafer, flat and undeformed at room temperature, is initially placed on such a convex support, intimate contact is not achieved over the entire length of the wafer. However, the vertical component of the centrifugal force applied as soon as the wheel rotates tends to press the donor wafer 3 against the host surface and hold its back surface against this surface over its entire extent. This applied initial stress tends to be reduced as the temperature increases when the wafer is exposed to the ion beam, so long as the donor wafer 3 is at a relatively low temperature close to room temperature. This extended intimate contact is thus maintained even as the temperature increases, without the thermal runaway that occurs in prior art configurations.

[0037] FIG. 3d shows a variant of the second embodiment. In this variant, the height h of the support surface (measured at the base of the support, i.e. relative to the main plane of the disk 1) is highest at the wedge 2a. The convexity is therefore such that when the donor wafer 3 is first placed on the support 2, one side is in contact with the wedge 2a and a part of its back surface close to this side is in contact with the host surface. In this configuration, as the wheel rotates, the centrifugal force in a controlled manner deforms the donor wafer 3 against the wedge 2a. More precisely, a forced contact of the side of the wafer 3 against the wedge 2a is obtained, while the back surface close to this side is actually in contact with the host surface of the support 2. The donor wafer 3 can be deformed to gradually build up contact with the host surface over its entire back surface from the initial contact zone.

[0038] Using conventional clips, it may be difficult to hold the donor wafer 3 in the position shown in FIG. 3d when it is first placed on the support. Also advantageously, as shown in FIG. 3e, the support 2 may be fitted with at least one retractable pin 4. This pin 4 has a first end designed to contact the back surface of the donor wafer 3 to help hold it in place. This first end may be spherically shaped to ensure a point contact with the back surface and prevent damage. The pin is held in a raised position in cooperation with a spring 5 (or any other holding mechanism), the first end of the pin having a height that substantially corresponds to the height of the host surface at the wedge 2b. The stiffness of the spring 5 (or more generally of the holding mechanism) is selected to be sufficiently low so as not to resist deformation of the donor wafer 3 as the implantation wheel rotates. The deformation force applied to the donor wafer tends to retract the pins so that they become invisible as they are pressed into the support 2, thus bringing the back surface of the donor wafer 3 into intimate contact with the host surface of that support.

[0039] Depending on the nature of the material forming the thick layer and the handling substrate constituting the donor heterostructure, the latter may deform differently under the influence of temperature. In a first configuration, when the thermal expansion coefficient of the thick layer is higher than that of the handling substrate, this donor wafer tends to deform into a dome with its convex part (top) pointing away from the rear surface. Conversely, in a second configuration, when the thermal expansion coefficient of the thick layer is lower than that of the handling substrate, the donor wafer tends to deform by orienting its convex part (its top) towards the rear surface.

[0040] In this embodiment, the convex shape of the wafer support 2 is necessarily selected such that the support corresponds to the general shape of the wafer when it deforms under the effect of temperature, i.e. the support has a concave depression or a convex dome shape depending on the nature of the material forming the donor wafer. Thus, the wafer support 2 shown in Figures 3b, 3d and 3e is particularly suitable for the first configuration of the donor wafer, and the wafer support shown in Figure 3c is particularly suitable for the second configuration.

[0041] Naturally, the height of the apex of the convex shape depends on the nature of the material forming the donor wafer, its size and the temperature it can undergo during the implantation step without triggering the delamination phenomenon. This height can be around a millimeter, typically 0.1 mm to 5 mm.

[0042] In this embodiment, it is not absolutely necessary to provide an elastomeric layer, but in general it is very advantageous for this layer to be present. When present, this layer does not necessarily have dimensions such that it is in contact with the entire back surface of the donor wafer.

[0043] Of course, the two methods can be combined to provide a wafer support 2 with a convex shape and an elastomeric layer in contact with the entire back surface of the donor wafer.

[0044] The convex shape of the support can be obtained by machining, especially when the support is made of metal. When the support comprises a surface elastomer layer, this layer can have a variable thickness and thus define the convex shape of the substrate.

[0045] More generally, the invention is not limited to the embodiments described, but alternative embodiments may be added without departing from the scope of the invention as defined by the claims.

Claims

1. 1. An implantation wheel (1) for forming an embrittled surface of a plurality of donor wafers, said wheel comprising a main disk (1 a) and a plurality of wafer supports (2) arranged on one side of said main disk (1 a), said wheel comprising a cooling circuit for cooling said wafer supports (2) and / or said main disk (1 a), each wafer support (2) having a host surface for receiving a "back" side of a donor wafer (3), each host surface of said plurality of wafer supports (2) having a convex shape, said convex shape being selected to correspond to the shape of said donor wafer (3) when it is subsequently deformed under the effect of temperature.

2. 2. The implantation wheel (1) of claim 1, wherein each host surface comprises a surface elastomer layer (2b) on which the back surface of a donor wafer rests, the surface elastomer layer having dimensions at least equal to the dimensions of the back surface of the donor wafer (3) so as to allow the entire area of ​​the back surface of the donor wafer (3) to come into contact with the wafer support (2).

3. The implantation wheel (1) of claim 2, wherein the surface elastomer layer (2b) has a variable thickness, which defines the convex shape of the wafer support (2).

4. An injection wheel (1) according to any one of claims 1 to 3, wherein the convex shape is in the form of a concave depression or in the form of a convex dome.

5. The injection wheel (1) according to claim 4, wherein the concave depression or convex dome has a height of between 0.1 mm and 5 mm.

6. 6. The implantation wheel (1) according to claim 1, having an axis of symmetry defining a rotation axis (R), and each host surface of the plurality of wafer supports (2) is oriented towards the rotation axis (R) so as to apply a component of centrifugal force perpendicular to the host surface to the donor wafer (3) during rotation of the wheel, pressing the donor wafer against the host surface.

7. 7. An implantation wheel (1) according to any one of claims 1 to 6, wherein each wafer support (2) is provided with a wedge (2a) for holding the donor wafer (3) by counteracting a component of the centrifugal force coplanar with the host surface applied to the donor wafer (3) during rotation of the wheel.

8. The implantation wheel (1) according to claim 7, wherein the host surface of each wafer support (2) has a height, the height of the host surface relative to a base of the support being highest at the wedge (2a).

9. The implantation wheel (1) according to claim 8, wherein each wafer support (2) is provided with a retractable pin.

10. An implantation wheel (3) according to any one of claims 7 to 9, wherein each wafer support (2) is provided with at least one clip for positioning and holding a wafer relative to the wedge (2a).

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