Implant wheel for forming weak surfaces in plurality of donor wafers

Abstract

The invention relates to an implantation wheel (1) for forming weak faces in a plurality of donor wafers (3), the wheel (1) comprising a main disc (1a) and a plurality of wafer supports (2) arranged on one face of the main disc (1a), each wafer support (2) having a main body surface on which a so-called "back" face of the donor wafer (3) is placed. According to a first aspect, the body surface comprises, at least in part, a surface elastomeric layer (2b), the size of which is at least equal to the size of the back side of the donor wafer (3). According to another aspect, each body surface of the plurality of wafer supports (2) has a convex shape selected to correspond to the shape of a donor wafer when the donor wafer is deformed under the effect of temperature.

Description

Technical Field

[0001] The present invention belongs to the field of ion implantation, and more particularly to an ion implantation apparatus suitable for forming weak surfaces in a plurality of donor wafers, as a method for using Smart Part of the thin layer transfer method achieved by the technology. Background Art

[0002] US2020186117 proposes using Smart The thin layer of ferroelectric material is transferred to a substrate with a different thermal expansion coefficient than the ferroelectric material constituting the thin layer. Specifically, it can be a silicon substrate.

[0003] Smart The technology defines a thin layer by introducing a light substance into a donor substrate, wherein the light substance is concentrated in a weak plane located at the average penetration depth of the substance. The donor substrate is then bonded to a support and the layer is transferred by separating the thin layer at the weak plane. In the case of layers made of ferroelectric and / or piezoelectric materials, and when the light substance is chosen from hydrogen and helium ions, this separation is caused by a moderate increase in the temperature to which the assembly is subjected (approximately 170° C.).

[0004] In order to limit the stresses generated in the assembly when the temperature rises, in particular during the separation phase just mentioned, document US2020186117 proposes forming the donor substrate of the assembly from a thick layer formed of a ferroelectric material and a handle substrate, the thermal expansion coefficient of the handle substrate being similar to that of the support substrate. Such a donor substrate is referred to as a donor heterostructure in the rest of this description.

[0005] The step of introducing the light species into the donor heterostructure may be performed using an ion implantation apparatus. Figure 1a and Figure 1b A schematic diagram showing a known construction of such a device is shown.

[0006] The wheel 1 of the ion implantation device includes a disk 1a, which is arranged in a plane with a slight angle (usually between 5° and 10°) to the vertical direction. The wheel carries a plurality of supports 2 on one of its sides for receiving a plurality of donor heterostructures respectively. The disk rotates at a speed of up to 1200rpm around a vertical axis R passing through its center. The support has a wedge 2a, against which the edge of the donor heterostructure abuts when the wheel rotates at high speed. During this rotation, the exposed surface of the donor heterostructure is continuously placed opposite to a source S of light material ions (such as hydrogen ions or helium ions), which are accelerated to a given energy and shaped into an ion beam F that defines an injection current. For a given injection material, the injection energy, the injection current and the duration of exposure to the beam form the injection conditions. The energy defines the average penetration depth of the ions, and the current defines the injection dose for a given exposure time. The beam F scans all exposed surfaces of the donor heterostructure.

[0007] Such ion implantation devices are known, for example, from US20070158583A1, US4832781 or US5040484.

[0008] The power generated by the ions entering the donor heterostructure tends to heat the heterostructure. To prevent overheating, the master platter and wheel supports are equipped with pipes through which a cooling fluid (usually water) circulates. The substrate on which the donor heterostructure is clamped acts as a heat sink, so it is important that the heterostructure is in close contact with its substrate to promote heat dissipation. These supports, which can be made of aluminum blocks to help dissipate heat, are usually equipped with surface elastomeric portions to ensure close contact.

[0009] The goal is usually to reduce the duration of the implantation step by increasing the beam current, which tends to increase the power supplied to the donor heterostructures and thus raise their temperature.

[0010] When trying to increase the injection current of donor heterostructures featuring thick ferroelectric layers, the Applicant observed that the thin layers peel off during the implantation step itself. This undesirable phenomenon occurs, making the layer transfer step impossible, despite great care taken to cool the implantation disk support during this step.

[0011] Purpose of the Invention

[0012] An object of the present invention is to propose an implantation wheel for solving this problem. More specifically, an object of the present invention is to provide an implantation wheel that is particularly suitable for implanting a donor heterostructure under a wide range of implantation conditions and that does not cause premature debonding of the donor heterostructure. Summary of the invention

[0013] To achieve this objective, the present invention proposes an implantation wheel for forming weak surfaces in a plurality of donor wafers, the wheel comprising a main disk and a plurality of wafer supports, the plurality of wafer supports being arranged on one face 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 main surface on which the so-called "back" face of the donor wafer is placed.

[0014] According to the invention, each body surface of the plurality of wafer supports has a convex shape selected to correspond to the shape of the donor wafer when the donor wafer deforms under the effect of temperature.

[0015] Other advantageous non-limiting features according to this aspect of the invention (alone or in any technically feasible combination):

[0016] - each body surface comprises a surface elastomeric layer on which the back side of the donor wafer rests, the surface elastomeric layer having dimensions at least equal to the dimensions of the back side of the donor wafer so as to enable the entire extent of the back side of the donor wafer to be in contact with the wafer support;

[0017] - the surface elastomer layer has a varying thickness, which defines the convex shape of the wafer support;

[0018] - the convex shape may be a concave recess or a convex dome;

[0019] - The height of the concave recess or convex dome is between 0.1 mm and 5 mm.

[0020] The implantation wheel has an axis of symmetry defining an axis of rotation, and each body surface of the plurality of wafer supports is oriented toward the axis of rotation such that a component of the centrifugal force perpendicular to the body surface is applied to the donor wafer during rotation of the wheel and presses the donor wafer against the body surface.

[0021] - Each wafer support is provided with a wedge for holding the donor wafer by opposing a component of the centrifugal force applied to the donor wafer during rotation of the wheel, coplanar with the body surface.

[0022] - the highest height of the main body surface of each wafer support is located at the wedge;

[0023] - Each wafer support is equipped with a retractable pin;

[0024] - Each wafer support is equipped with at least one jaw for positioning and holding the wafer against the wedge:

[0025] - The main body surface of each wafer support has a height, the height of the main body surface relative to the base of the support being highest at the wedge. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] 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, in which:

[0027] [ Figure 1a ]

[0028] [ Figure 1a ] shows a schematic diagram of the construction of an ion implantation device in the prior art and according to the present invention;

[0029] [ Figure 1b ]

[0030] [ Figure 1a ] shows a schematic front view of an injection wheel according to the prior art and according to the invention;

[0031] [ Figure 2a ]

[0032] [ Figure 2b ]

[0033] Figure 2a and Figure 2b A donor wafer is shown disposed on a prior art wafer support before and during an ion implantation step, respectively;

[0034] [ Figure 3a ]

[0035] [ Figure 3b ]

[0036] [ Figure 3c ]

[0037] [ Figure 3d ]

[0038] [ Figure 3e ]

[0039] Figure 3a , Figure 3b , Figure 3c , Figure 3d and Figure 3e A wafer support of an implant wheel according to the invention is shown. DETAILED DESCRIPTION

[0040] The expression "thermal expansion coefficient" used in the rest of this specification with respect to a layer or substrate refers to the expansion coefficient in a direction defined in a principal plane defining the layer or substrate. If the material is anisotropic, the retained value of the coefficient will be the value with the largest amplitude. The values ​​of the coefficient are those measured at room temperature.

[0041] Preliminary Experiments

[0042] In order to understand the origin of the exfoliation phenomenon described in the introduction of this application, the applicant has carried out a series of exploratory experiments. In these experiments, the spatial temperature distribution of the donor heterostructure was measured, which was obtained using Smart The donor heterostructure is formed during the implantation step under conditions designed to form a weak surface. Specifically, a dose of 3.0 10^16at / cm^2 of hydrogen is introduced using prior art ion implantation equipment at an injection current of 16mA and an energy of about 137keV. The donor heterostructure consists of a 15 to 20 micron thick layer of piezoelectric material bonded to a silicon wafer. The front side of the heterostructure (i.e., the side exposed to the ion beam) is covered with The temperature of the heterostructure is measured by placing multiple thermally sensitive adhesive patches of a film. Each patch has a visual indicator of the highest temperature it has been exposed to. By placing these patches on one side of the wafer and recording the visual indicator of each patch at the end of the experiment, we can estimate the spatial temperature distribution that has been formed in the wafer during the implantation step.

[0043] The same experiments were repeated under the same implantation conditions, this time applying the implantation step to a solid silicon wafer identical to the wafer on which the handle substrate of the donor heterostructure was formed. The table below shows the results of these experiments.

[0044] Edge temperature Core temperature Silicon Wafer 54°C to 64°C 46°C to 49°C Heterogeneous structures 132℃ to 166℃ >166℃

[0045] The results of these experiments show that the temperature raised by the donor heterostructure is unexpectedly much higher than that raised by a simple silicon wafer. It should be noted that such temperatures of 160°C, which can be reached and even exceeded, approach or even reach the debonding temperature, which is close to that in Smart The temperature that causes the detachment of the ferroelectric lamellae in the application of the technique, as mentioned in the introduction, is therefore easily understood why the detachment of the lamellae can be observed during the implantation step itself.

[0046] Furthermore, the spatial temperature distribution is reversed when implanting a silicon wafer (where the center rises at a lower temperature than the edge) compared to when implanting a donor heterostructure (where the edge rises at a lower temperature than the center).

[0047] In trying to explain these results, the applicant observed that in the prior art implantation apparatus used to carry out these experiments, the surface elastomeric portion of the implantation wheel support (which ensures thermal contact between the support and the implanted wafer) does not extend completely under the entire back side of the wafer. Figure 2a], which shows a donor heterostructure 3 arranged on a wafer support before the start of the implantation step, with a circular surface elastomer portion 2b having a diameter smaller than that of the wafer, on which the wafer 3, also circular, is centered. Consequently, a peripheral ring C (a few millimeters wide) on the back side of the wafer is not in close contact with this surface elastomer layer 2b, and the heat accumulated there during the implantation step cannot escape. This heat therefore tends to diffuse towards the central part of the wafer 3, raising its average temperature. In the case of a donor heterostructure 3 formed by a thick layer of ferroelectric material with different expansion coefficients and a handle substrate as mentioned in the introduction, the increase in average temperature tends to deform the donor heterostructure 3, which assumes a convex shape in order to adapt to these stresses. This situation is shown in [ Figure 2b ], which shows a donor heterostructure 3 deformed due to its temperature increase during the implantation step. This convex shape tends to move the central part of the heterostructure away from the implantation wheel support and maintain contact only on the outer circumference, as shown in [ Figure 2b ] is clearly visible in the figure. The stresses generated by the difference in expansion coefficients are greater than the centrifugal forces that tend to keep the heterostructure in close contact with the surface elastomeric portion of the support. With the loss of this close contact, the heat accumulated in the donor heterostructure 3 during the implantation can no longer be discharged with the same efficiency, which tends to amplify the deformation phenomena of the heterostructure and to raise its temperature, by a thermal runaway effect, until it reaches, at least locally, the debonding temperature. This also explains why the central portion of the donor heterostructure 3, no longer in contact with the substrate 2, tends to reach a higher temperature than the edge that remains in contact with the substrate 2.

[0048] Improved injection wheel

[0049] This section of the specification proposes an improved implantation wheel for preventing excessive temperature rise of a donor wafer (particularly a donor heterostructure) when exposed to a beam of light species ions (typically hydrogen or helium) to form a plane of weakness therein.

[0050] The wheel is particularly useful in cases where the donor wafer is a donor heterostructure consisting of a thick layer arranged on a handle substrate, the handle substrate and the thick layer having coefficients of thermal expansion that differ from each other, for example by at least 5%. However, the wheel can be used for any type of donor wafer.

[0051] Similar to About Figure 1a and Figure 1bAs has been explained in the description, the wheel comprises a main disk 1a and a plurality of wafer supports 2 arranged, for example, in an annular manner on one side of the main disk 1. Each wafer support 2 has a main body surface designed to receive the "back" face of a donor wafer. The wafer support 2 consists of a solid metal part (e.g., aluminum) defining a main body surface. The main body surface is advantageously provided at least in part with a surface elastomer layer. This layer may be roughened to limit the sliding of the donor wafer, maintain close contact with the back of the wafer, and enable efficient discharge of any heat that may accumulate during the ion implantation step. Therefore, the material constituting the layer is selected to be a good thermal conductor so as to allow heat to flow toward the wafer support. This material may be loaded with thermally conductive particles (such as carbon or aluminum oxide) to increase the thermal conductivity of the layer. The surface elastomer layer may be deposited on the metal part of the wafer support, for example, using a spin coating process. Alternatively, the layer may be prepared as an adhesive film, cut to the size of the metal part of the wafer support 2, and placed on the main body surface.

[0052] The support 2 and / or the master 1 may be cooled by a cooling circuit in order to remove heat from the donor wafer during the entire implantation step.

[0053] The wheel 1 has an axis of symmetry R (perpendicular to the face of the main disk constituting the wheel and passing through its centre) defining the axis of rotation of the wheel. During the implantation step, the wheel 1 rotates about this axis R (accomplished by a scanning movement) so as to continuously expose the donor wafer held on its support to the beam of light ions generated by the source of the implantation device.

[0054] Each body surface of the plurality of wafer supports 2 is oriented toward the axis of rotation so that when the wheel 1 rotates, a component of the centrifugal force perpendicular to the body surface is applied to the donor wafer. This force tends to press the donor wafer against the body surface. In order to orient the bearing surface of the supports toward the axis of rotation of the wheel on which they rest, the supports are thinner on the side facing the center of the wheel than on the side facing away from the center of the wheel, as in [ Figure 3a ] can be clearly seen in .

[0055] Each wafer support 2 is equipped with spacers 2a, typically lateral spacers matching the shape of the donor wafer, making it possible to hold the wafer by opposing the component coplanar with the body surface of the centrifugal force applied to the wafer when the wheel rotates.

[0056] Finally, as is known, each wafer support is equipped with at least one clamp (not shown) to precisely position the donor wafer on its support and to hold it against the wedge when the wheel is not rotating.

[0057] according to[ Figure 3a], the dimensions of the elastomer layer 2b are at least equal to those of the back side of the donor wafer 3, so as to enable the entire extent of this back side to be in contact with the wafer support 2. Advantageously, the dimensions of the elastomer layer 2b are strictly greater than those of the back side of the donor wafer 3.

[0058] In this way, it is prevented that during the implantation step, excessive heat is accumulated in the peripheral ring C of the wafer, which is therefore not in close contact with the elastomer layer 2b, as is the case in the prior art configuration. This prevents or delays the occurrence of runaway phenomena which, through a gradual increase in the average temperature of the donor wafer 3, lead to deformation of the wafer and loss of close contact between its back side and the elastomer layer. The implantation step can therefore be carried out without raising the average temperature of the plate above a critical temperature which leads to at least partial peeling of the thin layer. As a corollary, therefore, the implantation conditions (as a reminder, mainly defined by the injection current, the injection energy and the ion beam exposure time) can be applied within a wider range than in the prior art devices. In particular, the beam exposure time and therefore the duration of the implantation step can be reduced by increasing the injection current without the risk of peeling phenomena.

[0059] According to the second embodiment, an example can be found in Figure 3b and Figure 3c It is found that each main body surface of the plurality of donor wafer supports 2 has a convex shape which is selected to correspond to the convex or concave shape of the wafer when the wafer deforms under the effect of temperature.

[0060] When the temperature of the donor wafer increases during the implantation step, this convex shape of the support 2, which tends to match the shape of the donor wafer, maintains close contact between the back side of the wafer and the main surface of the support, ensuring that heat is dissipated by diffusion into the support 2 and thus limiting the temperature increase during the implantation step.

[0061] It should be noted that when a donor wafer, which is 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 normal component of the centrifugal force applied once the wheel rotates tends to press the donor wafer 3 against the body surface to hold its back side against the body surface over its entire extent. When the wafer is exposed to the ion beam, this initial stress applied as long as the donor wafer 3 is at a relatively low temperature close to room temperature tends to decrease with increasing temperature. Therefore, even when the temperature rises, this extended intimate contact is maintained without causing thermal runaway that occurs in prior art configurations.

[0062] [ Figure 3d] shows a variant of the second embodiment; in this variant, the height h of the support surface (obtained 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 initially placed on the support 2, one side of this donor wafer is in contact with the wedge 2a and a portion of its back side close to this side is in contact with the main surface. In this configuration, when the wheel rotates, the centrifugal force causes the donor wafer 3 to abut against the wedge 2a and to deform in a controlled manner. More precisely, a forced contact of this side with the wedge 2a is obtained while the back side close to the wafer 3 is indeed in contact with the main surface of the support 2. The donor wafer 3 can be deformed so as to gradually form, from the initial contact zone, a contact of its entire back side with the main surface.

[0063] Using conventional clips, when the donor wafer 3 is first placed on the support, the donor wafer is held in place. Figure 3d ] may be tricky. Additionally and advantageously, the support 2 may be equipped with at least one retractable pin 4, such as [ Figure 3e ] as shown. The pin 4 has a first end that is designed to contact the back side of the donor wafer 3 to help hold it in place. The first end may be shaped like a sphere to ensure point contact with the back side and prevent damage. The pin cooperates with a spring 5 (or any other holding mechanism) to be held in an elevated position, wherein the height of its first end substantially corresponds to the height of the main body surface at the wedge 2b. The stiffness of the spring 5 (or more generally, the holding mechanism) is selected to be low enough so as not to resist deformation of the donor wafer 3 as the implantation wheel rotates. The deforming force applied to the donor wafer tends to retract the pin so that it disappears when it is embedded in the support 2, thereby bringing the back side of the donor wafer 3 into close contact with the main body surface of the support.

[0064] Depending on the nature of the material constituting the thick layer and of the handle substrate constituting the donor heterostructure, the latter can deform differently under the effect of temperature. In a first configuration, when the thermal expansion coefficient of the thick layer is higher than that of the handle substrate, the donor wafer tends to deform into a dome with its convex face (its apex) pointing away from the back side. In contrast, in a second configuration, when the thermal expansion coefficient of the thick film is lower than that of the handle substrate, the donor wafer tends to deform by orienting its convex face (its apex) towards the back side.

[0065] In this embodiment, the convex shape of the wafer support 2 is naturally chosen so that it corresponds to the general shape of the wafer when it deforms under the effect of the temperature, that is to say, the support has a concave recess or a convex dome shape, depending on the nature of the material forming the donor wafer. Figure 3b , Figure 3d and Figure 3eThe wafer support 2 shown in FIG. 1 is particularly suitable for the first configuration of the donor wafer, and [ Figure 3c The wafer support shown in ] is particularly suitable for the second configuration.

[0066] The height of the apex of the convex shape naturally depends on the nature of the material constituting the donor wafer, its size and the temperature at which it can be received during the implantation step without triggering exfoliation phenomena. This height may be of the millimeter order, typically between 0.1 mm and 5 mm.

[0067] In this embodiment, it is not absolutely necessary to provide an elastomeric layer, although its presence is generally very advantageous. When present, this layer does not necessarily have to be of such a size that it is in contact with the entire back side of the donor wafer.

[0068] Of course, both approaches may be combined to provide a wafer support 2 having a convex shape and an elastomer layer in contact with the entire back side of the donor wafer.

[0069] The convex shape of the support may be obtained by machining, in particular when the support is made of metal. When it comprises a surface elastomeric layer, this layer may have a varying thickness, thereby defining the convex shape of the substrate.

[0070] More generally, the invention is not limited to the embodiments described and alternative embodiments can be added without departing from the scope of the invention as defined by the claims.

Claims

1. An implantation wheel (1) for forming weak surfaces in a plurality of donor wafers, the wheel comprising a main disk (1a) and a plurality of wafer supports (2), the plurality of wafer supports being arranged on one surface of the main disk (1a), the wheel comprising a cooling circuit for cooling the wafer supports (2) and / or the main disk (1a), each wafer support (2) having a main body surface for receiving the "back" surface of a donor wafer (3), each main body surface of the plurality of wafer supports (2) having a convex shape, the convex shape being selected to correspond to the shape of the donor wafer (3) when the donor wafer (3) is deformed under the action of temperature.

2. An injection wheel (1) according to the preceding claim, wherein: Each main body surface includes a surface elastomer layer (2b), on which the back side of the donor wafer rests, and the size of the surface elastomer layer is at least equal to the size of the back side of the donor wafer (3) so that the entire range of the back side of the donor wafer (3) can be in contact with the wafer support (2).

3. An injection wheel (1) according to the preceding claim, wherein: The surface elastomer layer (2b) has a varying thickness which defines the convex shape of the wafer support (2).

4. The injection wheel (1) according to one of the preceding claims, wherein: The convex shape is in the form of a concave recess or in the form of a convex dome.

5. An injection wheel (1) according to the preceding claim, wherein The height of the concave recess or convex dome is between 0.1 mm and 5 mm.

6. The injection wheel (1) according to one of the preceding claims, having an axis of symmetry defining an axis of rotation (R), and wherein: Each body surface of the plurality of wafer supports (2) is oriented toward the rotation axis (R) so that a component of the centrifugal force perpendicular to the body surface is applied to the donor wafer (3) during rotation of the wheel and presses the donor wafer against the body surface.

7. The injection wheel (1) according to one of the preceding claims, wherein: Each wafer support (2) is provided with a wedge (2a) for holding the donor wafer (3) by opposing a component of the centrifugal force applied to the donor wafer (3) during rotation of the wheel, coplanar with the body surface.

8. An injection wheel (1) according to the preceding claim, wherein: The main body surface of each wafer support (2) has a height, the height of the main body surface relative to the base of the support being highest at the wedge (2a).

9. An injection wheel (1) according to the preceding claim, wherein: Each wafer support (2) is provided with a retractable pin.

10. The injection wheel (3) according to one of claims 7 to 9, wherein: Each wafer support (2) is provided with at least one clamping piece for positioning and holding the wafer against the wedge (2a).

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