Method for forming a weakened zone in a semiconductor substrate

Abstract

The present disclosure relates to a method for forming a weakened zone (5) in a semiconductor substrate (6), the method comprising the following series of steps: a. forming a screening layer (4) on a first surface (61) of a substrate, the screening layer (4) exhibiting a controlled non-planar profile; b. implanting species through the screening layer and the first surface (61) of the substrate to form a weakened zone; and the geometry of the screening layer is selected to compensate for non-uniformities in the implantation depth of the seeds such that the weakened zone (5) is substantially contained in a plane parallel to the first surface (61).

Description

[Technical Field]

[0001] The present disclosure relates to a method for forming weakened zones in semiconductor substrates, which method is preferably used in the manufacture of semiconductor-on-insulator type substrates, the manufacture of such semiconductor components being particularly useful for the manufacture of waveguides. [Background technology]

[0002] The manufacture of components in the fields of microelectronics, optics and / or optoelectronics, and more particularly the manufacture of waveguides, makes use of semiconductor-on-insulator type substrates. Such substrates comprise, from the back surface to the front surface of the substrate, a support substrate, an electrically insulating layer and a monocrystalline semiconductor layer called the active layer. The active layer of such substrates is generally, but not exclusively, made from silicon, and the substrate is called a silicon-on-insulator (or SOI) substrate.

[0003] A method known from the prior art, called Smart Cut™, makes it possible to manufacture semiconductor-on-insulator substrates, which method comprises in particular a step of implanting species through the surface of a donor substrate in order to create weakened zones at a desired thickness in said donor substrate.

[0004] Conventionally, an electrically insulating layer is formed on the donor substrate, typically by oxidation, and species are implanted through the oxide layer into the donor substrate. The implanted species must then be able to penetrate the electrically insulating layer and to a desired depth in the donor substrate to form the active layer of the SOI substrate.

[0005] In practice, when the electrical insulating layer and / or the desired active layer are particularly thick, it can be difficult to provide the implanted species with enough energy to penetrate the insulating layer and the desired depth into the donor substrate. In this case, it is known to fabricate SOI substrates by "reverse" bonding, in which the insulating layer is formed not on the donor substrate itself but on a receiver substrate intended to receive the donor substrate by bonding, and the implantation occurs directly through the surface of the donor substrate without the need for the species to penetrate the insulating layer. After implantation, the implanted donor substrate is bonded to an insulating layer disposed on the receiver substrate, and then the donor substrate is detached along the weakened zone on the receiver substrate.

[0006] However, after the implantation step, the inventors have noticed that the uniformity of the implantation depth of the seeds is adversely affected, which manifests itself in a non-uniformity in the thickness of the active layer, which typically adopts a concave or convex profile, which is detrimental to the quality of the components subsequently formed in this active layer.

[0007] Certain prior art methods have attempted to correct the contours of the active layer after it has been transferred to the receiver substrate.

[0008] In particular, single-wafer cleaning (or SWC) is worth remembering. In this method, a semiconductor-on-insulator substrate is cleaned by dispensing a chemical etchant at its center, which is then dispersed by the centrifugal effect of the substrate's rotation about its central axis. This allows for a higher silicon etch rate at the substrate's center than at its edge, thus correcting convex active layer profiles. However, these correction methods are only effective for limited depths on the nanometer scale, and the use of several SWC steps to treat highly convex surfaces is very expensive. Furthermore, these methods do not allow for the correction of concave active layer profiles, i.e., profiles with a greater thickness in their peripheral regions than in their central regions.

[0009] Another post-implant correction method consists in forming a sacrificial electrically insulating layer on the active layer after it has been transferred to the receiver substrate. Such a method allows for the correction of concave or convex implant profiles, when oxide formation consumes the active layer at different rates at its center and its edges. However, this consumption rate is difficult to control, thereby compromising the accuracy of such methods. Furthermore, the formation of a sacrificial oxide layer often requires the substrate to be subjected to complex temperature variations, which hinders the implementation of such methods on an industrial scale. Summary of the Invention

[0010] It is therefore necessary to develop an implantation method that allows obtaining a uniform implantation depth over the entire surface of the substrate, or in other words, forming a flat weakened zone parallel to the main surface of the substrate, and that can be performed more easily than the methods described above. It is also necessary to develop an implantation method that allows accurate correction of any resulting implant profile depending on the implantation conditions used.

[0011] To that end, the present disclosure proposes a method for forming a weakened zone in a semiconductor substrate, said method comprising the following series of steps: a. forming a screening layer on a first surface of a substrate, the screening layer exhibiting a controlled non-planar profile; b. implanting species through the screening layer and the first surface of the substrate to form a weakened zone; wherein the geometry of the screening layer is selected to compensate for non-uniformities in the implantation depth of the seeds such that the weakened zone is substantially contained in a plane parallel to the first surface.

[0012] The formation of a screening layer with controlled and non-planar contours makes it possible to compensate for variations in implantation depth that may occur on the implanted surface of the substrate, so as to obtain a substantially planar weakened zone along a plane parallel to the implanted surface, thereby resulting in a semiconductor-on-insulator substrate that exhibits a planar active layer and in particular makes it possible to manufacture components of better quality.

[0013] According to one embodiment of the method, the screening layer exhibits a convex or concave profile.

[0014] According to one embodiment of the method, the species implanted into the first surface of the substrate include hydrogen ions and helium ions.

[0015] According to one embodiment of the method, the distance between any two points on the free surface of the screening layer along an axis perpendicular to the first face is comprised between 0.2 nm and 5 nm.

[0016] According to one embodiment of the method, the substrate comprises silicon.

[0017] According to one embodiment of the method, the screening layer comprises an oxide of a semiconductor material, in particular silicon dioxide.

[0018] According to one embodiment of the method, the screening layer is obtained by thermal oxidation of the surface region of the semiconductor substrate.

[0019] According to one embodiment of the method, forming a screening layer by oxidation includes heating the substrate in a neutral atmosphere to a temperature higher than an oxidation temperature of the substrate, cooling the substrate to the oxidation temperature, and applying an oxidizing atmosphere to form the screening layer.

[0020] According to one embodiment, the method comprises, prior to thermal oxidation, selectively applying an oxidizing plasma to a zone on the first side of the substrate so as to form an oxide overthickness in said zone.

[0021] According to one embodiment, the method includes a step of calibrating the contour of the screening layer prior to the formation of the screening layer, the step of calibrating comprising the following steps: - implanting atoms through a first face of the calibration substrate so as to create a weakened zone; - separating the calibration substrate along the weakened zone into a first portion comprising the first surface and a second portion; - defining a target contour substantially identical in shape to the surface of the first portion exposed by the separating step; wherein the target contour is used to define the contour of the screening layer while the screening layer is being formed.

[0022] The present disclosure also relates to a method for making a semiconductor component of the semiconductor-on-insulator type, said method comprising the following series of steps: - providing a donor substrate and a receiver substrate made of a semiconductor material; - forming an electrically insulating layer on one side of a receiver substrate; - forming a weakened zone in a donor substrate by a forming method according to any one of the preceding claims, the weakened zone thus formed defining a semiconductor layer; - removing the screening layer; - bonding a first surface of a donor substrate to an electrically insulating layer of a receiver substrate; - separating the donor substrate at the weakened zones so as to transfer the semiconductor layer of the donor substrate to the receiver substrate; Includes:

[0023] According to one embodiment of the method for manufacturing a semiconductor component, the electrical insulating layer and the screening layer are formed by heat treatment in one and the same furnace.

[0024] According to one embodiment of the method for manufacturing a semiconductor component, the electrically insulating layer exhibits a thickness of at least 150 nanometers. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a diagram showing a conventional SOI substrate. [Figure 2]1 shows a method for manufacturing semiconductor-type components by reverse bonding as known from the prior art; [Figure 3] 1A-1C show a proposed method for forming a weakened zone in a semiconductor substrate. [Figure 4] 1A-1C illustrate a calibration performed as part of a method for forming a weakened zone in a semiconductor substrate. [Figure 5] 1A-1D show a proposed method for manufacturing a semiconductor-on-insulator type component. DETAILED DESCRIPTION OF THE INVENTION

[0026] 1 shows a conventional semiconductor-on-insulator (SOI) type substrate, which successively comprises a base substrate 11 made of a semiconductor material, generally silicon, an intermediate layer 2 made of an electrically insulating material, in particular silicon oxide, and a layer 13 made of a monocrystalline semiconductor material, generally also formed of silicon and called the "active layer". The thickness of the monocrystalline semiconductor layer is typically comprised between 50 nanometers and a few micrometers.

[0027] For the production of SOI substrates with particularly thick active layers 13 and / or particularly thick intermediate layers 2, i.e. layers exhibiting a thickness of 200 nm or more, it is known to use the reverse bonding Smart-Cut™ method shown in FIG. 2, which method comprises the following steps: - forming an electrical insulating layer 2 on the receiver substrate 7; - implanting species into a donor substrate 6 so as to form a weakened zone 5 in this substrate; - bonding a donor substrate 6 to a receiver substrate 7, with an electrical insulating layer 2 being placed between these two substrates; - separating the donor substrate along the weakened zone so as to transfer the active layer 13 from the donor substrate 6 to the receiver substrate 7; Includes:

[0028] The active layer 13 thus obtained often exhibits a non-uniform thickness over its entire length, i.e., the weakened zone 5 is not flat. Typically, thickness differences of a few nanometers are observed between different zones of the active layer 13. Thickness is understood to mean the direction perpendicular to the plane formed by the first surface 61 of the substrate 6 through which the species are implanted. Figure 2 shows a convex implant profile, which results in a convex transferred active layer 13, although other non-flat implant profiles can be obtained depending on the implementation of the reverse bonding method.

[0029] The present disclosure proposes a method for forming weakened zones 5 that allows improving the thickness uniformity of the active layer 13. In its general embodiment, shown in Figure 3, the method comprises the following steps: a formation step 102 for forming a screening layer 4 exhibiting a controlled non-flat profile on a first face 61 of a substrate intended to form a donor substrate 6; a seed implantation step 103 for implanting seeds through the screening layer 4 and the first side 61 of the substrate 6 to form a weakened zone 5; Includes:

[0030] The method optionally includes a removal step 104 for removing the screening layer 4 before the donor substrate is bonded to the receiver substrate, which may be achieved, for example, by chemical etching using an etchant suitable for etching the screening layer without attacking the material of the donor substrate.

[0031] A "controlled" profile is understood to mean that the profile of the screening layer 4 thus formed exhibits a profile of a predefined thickness so as to make it possible to compensate for any non-uniformities in the seed implantation depth and to obtain a weakened zone 5 that is substantially contained in a plane parallel to the first face 61. In the following text, an "uncompensated" profile refers to the profile of a weakened zone 5 that results from the seed implantation without compensation.

[0032] One advantage of the proposed method is that it compensates for the non-flat contour of the weakened zone 5 without the need to consume the semiconductor active layer after it has been transferred. Specifically, known methods for forming weakened zones, such as the single-wafer cleaning or the formation of a sacrificial oxide layer described above, involve consumption of the transferred active layer after the bonding and layer transfer steps. In comparison, the proposed method allows compensation to be performed during the seed implantation step, i.e., before the active layer is transferred, to obtain a substantially flat active layer contour without the need to consume the active layer. Specifically, the volume ratio between silicon and silicon dioxide, 0.44, must be taken into account; i.e., the formation of a silicon oxide layer with a thickness of 0.1 nm consumes 0.044 nm of silicon in the transferred active layer. Therefore, to make the thickness of the transferred active layer, which is convex and represents an overthickness, determined at the center relative to the edge, more uniform after the bonding and transfer steps, it is necessary to form an oxide layer with a thickness substantially twice the thickness of the overthickness to be removed. In contrast, with the proposed method, the injection contour coincides with the contour of the screening layer, so that a convex screening layer presenting a thickness that corresponds substantially to the overthickness to be avoided is sufficient.

[0033] The active layer obtained by the reverse bonding method can exhibit different irregular thickness profiles, but the uncompensated profile is often concave (the active layer is thicker at its edges than at its center) or convex (the active layer is thicker at its center than at its edges).

[0034] As a result, according to one embodiment of the method, the screening layer 4 exhibits a convex or concave profile to accurately compensate for the convex or concave uncompensated profile produced by the seed implantation. Specifically, a concave screening layer 4 (i.e., thicker at the edge than at the center) allows for compensation of an implantation that would result in a concave active layer 13 without compensation, and a convex screening layer 4 (i.e., thicker at the center than at the edge) allows for compensation of an implantation that would result in a convex active layer 13 without compensation, the latter case being shown in FIGS. 2 and 3.

[0035] According to one embodiment of the method, the implanted species comprise hydrogen ions and / or helium ions, the co-implantation of hydrogen ions and helium ions in particular making it possible to improve the quality of the separation obtained during the step of separating the active layer.

[0036] To ensure a variation in the thickness of the screening layer that makes it possible to compensate for non-uniformities in the implantation depth, the distance between any two points on the free surface 41 of the screening layer 4 along an axis perpendicular to the first face 61 may be comprised between 0.2 and 5 nanometers.

[0037] The step of forming the screening layer 4 can be performed in several ways. Preferably, the donor substrate 6 comprises a semiconductor material and the screening layer 4 is an oxide layer of this material obtained by thermal oxidation of the surface region of the donor substrate 6. Preferably, the donor substrate 6 comprises silicon and the screening layer 4 comprises silicon dioxide obtained by thermal oxidation of the surface region of the substrate 6.

[0038] Thermal oxidation is typically carried out in a furnace in an oxidizing atmosphere. According to one embodiment of the method, oxidation, which allows for the formation of a screening layer 4 with a controlled convex profile, is carried out in multiple steps. First, the substrate 6 is heated in a neutral, i.e., non-oxidizing, atmosphere to a temperature slightly higher than the substrate's oxidation temperature, typically a few degrees higher than the oxidation temperature, e.g., 5-20°C above the oxidation temperature. The substrate is then cooled in the oxidizing atmosphere until the oxidation temperature is reached, and then the substrate is maintained at the substrate's oxidation temperature, still in the oxidizing atmosphere. Therefore, due to the thermal inertia of the substrate, the substrate cools more rapidly at its edges than at its center, so that the depth of the oxidized surface of the substrate is greater at its center than at its edges. This therefore contributes to obtaining a convex screening layer. It should be noted that the purpose of the step of cooling the substrate in an oxidizing atmosphere is to control the unevenness of the screening layer 4, while the step of maintaining the substrate at its oxidation temperature allows for the thickness of the screening layer 4 to be controlled.

[0039] Other embodiments of the formation of the screening layer 4 can be envisaged.

[0040] For example, it is possible to form the oxide screening layer 4 by selectively applying an oxidizing plasma to one or more zones of the first side 61 of the donor substrate 6. Thus, it is possible to form thicker oxide layers in these zones, for example by appropriately confining the plasma, before performing conventional thermal oxidation.

[0041] According to another embodiment, a uniformly thick screening layer 4 can be formed by thermal oxidation of the donor substrate 6 before processing the screening layer 4 to form the desired thickness profile. For example, the screening layer 4 can be etched by applying hydrofluoric acid or according to a cleaning method known from the prior art called "RCA cleaning", which consists in using a solution of water, hydrogen peroxide and ammonia to etch the oxide layer.

[0042] The various embodiments described above may be optionally combined.

[0043] Thermal oxidation with a controlled temperature profile is particularly advantageous in that it requires only one piece of equipment and a single type of process to form the controlled contour screening layer.

[0044] Before the step of forming the screening layer 4, the method may include calibrating the geometry of the screening layer 4 to allow accurate compensation of the implantation geometry.

[0045] Referring to FIG. 4 , a calibration substrate 600 made of the same material as the donor substrate is provided, on which an implantation 701 of atomic or ionic species is performed through a first surface 601 of the calibration substrate 600 to create a weakened zone 500. The calibration then includes a separation step 702 for separating the calibration substrate into two parts along the weakened zone 500: a first part 602 including the first surface 601 and a second part 603. Furthermore, a step of bonding the calibration substrate 600 to a support substrate is preferentially performed. The support substrate serves as a mechanical stiffener intended to make it easier to separate the layer 621 from the calibration substrate 600. The first part 602 then exhibits the same contour as the active layer 13 obtained as part of the reverse bonding manufacturing method, without the proposed compensation. Finally, the calibration includes a definition step 703 for defining a target contour 400, substantially defined by the shape of the surface 621 of the first part 602 along the weakened zone 500, exposed by the separation.

[0046] The present disclosure also relates to a method for manufacturing a semiconductor-on-insulator type component 10 shown in Figure 5, which method comprises the following steps: - providing a donor substrate 6 and a receiver substrate 7 of semiconductor material, for example but not limited to silicon; forming an electrical insulating layer 2 on one face 71 of the receiver substrate, it being possible in particular to provide that this layer 2 exhibits a thickness of at least 150 nanometers, so as to guarantee optimal insulation of the active layer 13 relative to the base 11 of the substrate; - forming a weakened zone 5 in a donor substrate 6 by a method according to the present disclosure, said step comprising a step 102 of forming a screening layer 4 on a first face 61 of the donor substrate and a step 103 of implanting species through said first face, at the end of which step a flat weakened zone 5 is obtained that is substantially parallel to said first face 61; a step 104 of removing the screening layer 4 to expose the first face 61; a step 105 of bonding the donor substrate 6 to the receiver substrate 7, in which the first side 61 of the donor substrate is bonded to the receiver substrate 7 and an electrical insulating layer 2 is arranged between the two substrates; a step 106 of separating the donor substrate 6 at the weakened zones 5 so as to transfer a semiconductor layer of the donor substrate 6 to the receiver substrate 7, this semiconductor layer forming the active layer 13 of the SOI substrate;

Claims

1. A method for forming a weakened zone (5) in a semiconductor substrate (6), comprising the following sequence of steps: a. forming a screening layer (4) on a first surface (61) of said substrate, said screening layer (4) exhibiting a controlled non-planar profile; b. Implanting species through the screening layer and the first surface (61) of the substrate to form the weakened zone; wherein the geometry of the screening layer is selected to compensate for non-uniformities in the implantation depth of the seeds such that the weakened zone (5) is substantially contained in a plane parallel to the first surface (61).

2. 2. The method of claim 1, wherein the screening layer (4) exhibits a convex or concave profile.

3. The method of claim 1 or 2, wherein the species implanted into the first side (61) of the substrate include hydrogen ions and helium ions.

4. 4. The method according to claim 1, wherein the distance between any two points on the free surface (41) of the screening layer (4) along an axis perpendicular to the first face (61) is comprised between 0.2 nm and 5 nm.

5. The method according to any one of claims 1 to 4, wherein the substrate (6) comprises silicon.

6. The method according to any one of claims 1 to 5, wherein the screening layer (4) comprises an oxide of a semiconductor material, in particular silicon dioxide.

7. 7. The method of claim 6, wherein the screening layer (4) is obtained by thermal oxidation of a surface region of the semiconductor substrate (6).

8. 8. The method of claim 7, wherein the step of forming the screening layer (4) by oxidation comprises heating the substrate (6) in a neutral atmosphere to a temperature higher than an oxidation temperature of the substrate, cooling the substrate to the oxidation temperature, and applying an oxidizing atmosphere to form the screening layer (4).

9. 8. The method of claim 7, further comprising, prior to the thermal oxidation, selectively applying an oxidizing plasma to a zone of the first side (61) of the substrate (6) so as to form an oxide overthickness in said zone.

10. Before the step of forming the screening layer (4), a step of calibrating the outer shape of the screening layer (4) is included, and the step of calibrating includes the following steps: Implanting atoms (701) through a first surface (601) of a calibration substrate (600) to create a weakened zone (500); Separating (702) the calibration substrate along the weakened zone into a first portion (602) including the first surface (601) and a second portion (603); defining (703) a target outline (400) substantially identical in shape to the surface (621) of the first portion (602) exposed by the separating step; 10. The method of claim 1, wherein the target contour is used to define the contour of the screening layer while the screening layer is being formed.

11. A method for making a semiconductor component (10) of the semiconductor-on-insulator type, comprising the following sequence of steps: Providing a donor substrate (6) and a receiver substrate (7) made of semiconductor material; forming an electrically insulating layer (2) on one side (71) of the receiver substrate (101); - forming (102, 103) a weakened zone (5) in the donor substrate (6) by a method according to any one of claims 1 to 10, wherein the weakened zone (5) thus formed defines a semiconductor layer; removing (104) the screening layer (4); Bonding (105) the first surface (61) of the donor substrate (6) to the electrically insulating layer (2) of the receiver substrate (7); Separating (106) the donor substrate (6) at the weakened zone (5) so as to transfer the semiconductor layer of the donor substrate (6) to the receiver substrate (7); A method comprising:

12. 12. The method according to claim 11, wherein the electrical insulating layer (2) and the screening layer (4) are formed by heat treatment in one and the same furnace.

13. 13. The method according to claim 11 or 12, wherein said electrically insulating layer (2) exhibits a thickness of at least 150 nanometers.

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