METHOD FOR SUSPENSIONING A THIN LAYER OVER A CAVE WITH A STIFFENING EFFECT ACHIEVETED BY PRESSURE BUILDING IN THIS CAVE WITH IMPLANTED STYLES

Description

TECHNICAL FIELD

[0001] The field of the invention is that of the design of substrates containing buried cavities. The invention relates more particularly to the transfer of a thin film to suspend it over an initially open cavity and thus seal the cavity. PREVIOUS TECHNIQUE

[0002] The fabrication of buried cavities involves suspending a thin layer (from a few nanometers to a few microns thick) onto cavities of varying sizes and geometries, from a few microns for microelectromechanical resonator-type applications to a few hundred microns for assembly-type applications ( packaging ) microelectronics.

[0003] The most common method for creating this suspension is the thinning of a silicon substrate or a silicon-on-insulator (SOI) substrate. Other methods employ bonding technologies, substrate drilling, and chemical etching to machine the cavities. While these techniques are well-established, they present challenges in terms of cost (complete consumption of a silicon substrate or, worse, an SOI substrate), time (lengthy thinning process), and thin film thickness.

[0004] Smart Cut™ technology is a technique widely developed over approximately two decades for transferring thin films of semiconductor materials. According to this technique, ionic species such as hydrogen and / or helium are implanted into a donor substrate to create a plane of embrittlement. The donor substrate is then brought into contact with a support substrate, for example, by direct bonding. This technique then relies on the development of defects generated within the plane of embrittlement. This development requires an energy input, generally achieved through heat treatment at several hundred degrees, typically 500°C, for several tens of minutes. This leads to the formation of a confined layer of cavities and microcracks within which a fracture will initiate and propagate.This fracture separates the donor substrate along the weakening plane and a thin layer of the donor substrate is thus transferred onto the supporting substrate.

[0005] However, this method cannot be directly used to suspend a thin film over a cavity of any size. The publication by CH Yun and NW Cheung, "Fabrication of Silicon and Oxide Membranes Over Cavities Using Long-Cut Layer Transfer," Journal of Microelectromechanical Systems, vol. 9, no. 4, December 2000, demonstrates that a membrane of a given thickness can only be successfully transferred over cavities smaller than a certain size limit. This is because there is no stiffening effect within the cavity. Therefore, blistering can occur, leading to deformation of the membrane, which can then break (exfoliation), resulting in local failure of the transfer.

[0006] Because of this exfoliation phenomenon, the transfer onto cavities by Smart Cut ™ is therefore restricted in practice to cavities of a few microns, or even a few tens of microns, depending on various parameters and in particular the depth of implantation.

[0007] FR 3 074 358 A1 discloses a method for producing a thin-film sealed cavity.

[0008] WO 2015 / 119564 pressure control in cavities on a substrate.

[0009] EP 3 401 273 describes a method for making a MEMS device using several qualities, having different atmospheres.

[0010] US 2018 / 0346321 describes a MEMES device in a cavity of which pressure can be applied.

[0011] WO 2012 / 145485 describes the implantation of gaseous chemicals in cavities formed in intermediate dielectric layers for subsequent release by thermal diffusion. DESCRIPTION OF THE INVENTION

[0012] The invention aims to provide a technique for sealing a cavity by means of transferring a thin layer onto the cavity using Smart Cut™, a technique which would be applicable to sealing larger cavities.

[0013] A method is proposed for transferring a semiconductor layer from a donor substrate to a receiving substrate having an open cavity. The method comprises the steps of forming a weakened plane in the donor substrate and creating, by bringing the donor and receiving substrates into contact, an assembly in which the cavity is embedded. Prior to creating the assembly, the method includes a step of implanting gaseous species into either the donor or receiving substrate, and subsequent to the assembly, a step of diffusing these species into the cavity.

[0014] A method for transferring a semiconductor layer from a donor substrate to a recipient substrate with an open cavity is also proposed, comprising the steps of: formation of a weakening plane in the donor substrate; creation, by bringing the donor substrate and the recipient substrate into contact, of an assembly in which said cavity is buried; separation of the assembly by fracture along the weakening plane, said separation resulting in the transfer of the semiconductor layer to the recipient substrate and the sealing of the cavity by the semiconductor layer; including, prior to the assembly, a step of implantation in the donor substrate or in the receiving substrate of diffusing species and, subsequent to the assembly and prior to the separation of the assembly, a step of diffusion of said species in the cavity.

[0015] Some preferred, but not exhaustive, aspects of this process are as follows: the diffusion stage of said species in the cavity includes annealing of the assembly; said annealing is continued until separation of the assembly by fracture along the plane of embrittlement; said species are implanted in the receiving substrate at a depth less than the depth of the plane of embrittlement in the donor substrate; diffusing species are not implanted through the bottom of the cavity; diffusing species are implanted in the donor substrate at a depth less than the depth of the plane of embrittlement. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Other aspects, objectives, advantages, and features of the invention will become clearer upon reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the accompanying drawings in which: [ Fig. 1 ] is a diagram illustrating microcrack-type defects developing in the presence of a stiffener; [ Fig. 2 ] is a diagram illustrating blister-type defects developing in the absence of a stiffener; [ Fig. 3 ] is a diagram illustrating the different stages of a process according to the invention; [ Fig. 4 ] is a diagram illustrating the phenomenon of vertical growth of defects in the cavity in the absence of implementation of the invention; [ Fig. 5 ] is a diagram illustrating the phenomenon of lateral growth of defects by back pressure of the cavity due to the implementation of the invention. DESCRIPTION OF IMPLEMENTATION METHODS

[0017] During the fracture annealing of the Smart Cut™ process, the species previously introduced by ion implantation to form the embrittlement plane reacts and forms defects at a certain depth in the donor substrate. These defects are pressurized, and their evolution depends on the stiffness of the system (stiffening effect).

[0018] As depicted on the figure 1 In the presence of a stiffening effect, provided by a substrate glued to the donor substrate or by a thick deposit on it, defects develop in the form of microcracks which propagate preferentially laterally and can reach sizes of a few hundred microns.

[0019] As depicted on the figure 2In the absence of a stiffening effect, with the substrate surface left free, defects develop in the form of blisters which spread horizontally and vertically, their growth being limited by the exfoliation phenomenon corresponding to their decapsulation.

[0020] The maximum size of the blisters, and therefore the size of the exfoliations, is critical in the cavity transfer process because this growth pattern is present in cavities during fracture annealing. Thus, if exfoliation occurs before the fracture passes through, the cavity will invariably be perforated.

[0021] With reference to the figure 3The invention relates to a method for transferring a semiconductor layer 2 from a donor substrate 1 to a recipient substrate 3. The recipient substrate has at least one open cavity C, for example, one created by etching, and the semiconductor layer transfer aims to seal this cavity. The invention is applicable to the fabrication of any type of device based on the use of a suspended membrane, such as capacitive micromachined ultrasonic transducers (cMUTs).

[0022] The receiving substrate may have multiple open cavities, which may potentially have different sizes and / or geometries. The transferred thin film then covers all or part of the cavities in the receiving substrate.

[0023] The donor substrate may comprise a multilayer structure, for example a multi-material multilayer structure (e.g. a silicon oxide or nitride deposit, thin metallic deposits, or other functional deposits). These deposits may be made before ion implantation (while remaining compatible with it) or after ion implantation (but in this case, they must not use thermal budgets that could lead to the maturation of defects in the implanted area; they are thus typically made at a temperature below 400°C, advantageously less than or equal to 350°C).

[0024] The process according to the invention comprises a step (A) of forming a weakening plane F within the thickness of the donor substrate 1 by means of implanting ionic species, for example hydrogen and / or helium. This weakening plane delimits a superficial portion of the donor substrate which will form the thin layer to be transferred 2 from a bulk portion of the donor substrate.

[0025] The process includes a step (B) of implanting gaseous species into the receiving substrate, which are called diffusing because they are able to diffuse into the material of the receiving substrate under the effect of, in particular, a heat treatment.

[0026] This implantation (B) is preferably shallow. The implantation depth of the diffusing species in the receiving substrate is preferably less than the depth of the embrittlement plane F formed in the donor substrate. The implantation depth of the diffusing species can thus range from 5 nm to 200 nm. In particular, the implantation depth of the diffusing species is preferably less than the depth of the cavity. These depth criteria allow, during subsequent annealing, for maximizing the diffusion of the diffusing species into the cavity before the maturation of defects at the embrittlement plane of the donor substrate.

[0027] In a preferred embodiment, helium implantation is carried out during this step (B).

[0028] The implantation of diffusing species can be carried out through a mask to limit implantation to areas of the receiving substrate without cavities and thus not implant said diffusing species through the bottom of the cavity or cavities.

[0029] This implantation can also be carried out using the plasma immersion implantation technique in order to obtain a good distribution of the implanted species, particularly near the surface (between 5nm and 200nm depth).

[0030] In one embodiment, the diffusing species are implanted not in the receiving substrate, but in the donor substrate. The implantation depth of the diffusing species is less than the implantation depth of the ionic species leading to the formation of the embrittlement plane.

[0031] The process then includes a fabrication step (C), by bringing the donor substrate 1 and the recipient substrate 3 into contact, forming an assembly in which the cavity is embedded. The contact between the substrates 1 and 3 is thus achieved in such a way that the thin layer 2 seals the cavity. The assembly of the donor substrate 1 and the support substrate 3 can be achieved by means of a layer previously deposited on one or both of the substrates 1 and 3, such as, for example, a dielectric or metallic layer or a stress layer.

[0032] The process includes a step (D) of diffusing the diffusing species into the cavity. This diffusion step may include annealing the assembly (the temperature and duration of which may be those of fracture annealing). The gaseous species diffused into the cavity pressurize it. This pressure exerts a stiffening effect within the cavity and prevents or limits the development of blisters at the embrittlement plane directly above the cavity. This annealing may be continued until the assembly (E) separates by fracture along the embrittlement plane.

[0033] Thus, the process of the invention implements a counter-pressure within the cavity to force blister-type defects to propagate laterally. Under the effect of this counter-pressure, the exfoliation conditions are modified, and the growth of the defects approaches lateral growth, similar to that of microcracks. This is shown on the figure 4 A diagram illustrating the phenomenon of vertical growth of defects in the cavity in the absence of implementation of the invention. figure 5 illustrates the phenomenon of lateral growth of defects by counter-pressure of the cavity due to the implementation of the invention.

[0034] Thus, the transfer onto a cavity according to the invention is no longer limited by the exfoliation phenomenon and, with a fixed thickness of transferred film, it is possible to achieve the sealing of cavities of much larger sizes than in the state of the art.

[0035] The following description concerns a method for quantifying the required dose of diffusing species to be implanted to obtain the necessary back pressure. This method uses the example of embrittlement plane formation by hydrogen implantation in the donor substrate and helium implantation as a diffusing species in the recipient substrate.

[0036] To quantify the necessary helium dose, the amount of hydrogen present in the blisters before the fracture occurs must be determined. This requires describing the maximum size of the blisters that will be present based on the dimensions of the cavity. For example, consider a target cavity in the shape of a cylinder with height *a* and a base diameter *L*. The blister that will develop before fracture will therefore have a maximum radius *R* of *L / 2*.

[0037] To determine the helium dose X (denoted Dtot2) to be implanted, the pressure inside the blister must be compared with the pressure in the cavity. To determine the pressure inside the blister, the geometry of a blister is expressed according to the theory of elasticity of plates and blisters developed by Timoshenko.

[0038] The maximum blister height is given by h Max = 3 16 ∗ 1 − υ 2 Ee 3 P 1 R 4 where E is the Young's modulus of the material constituting the transferred thin film, and v is its Poisson's ratio. P 1 corresponds to the pressure within the blister, R to the radius of the blister, and e to the thickness of the transferred thin layer. The volume of the blister is then expressed as follows: V cloque = π 3 h Max R 2 .

[0039] Assuming that a fraction α of the hydrogen dose (denoted D tot1) implanted in the donor substrate is found in the blister; a dose of hydrogen is found in the blister. D 1 = αD early 1 and a quantity of hydrogen N 1 = αD tot 1 π 4 L 2 . By applying the ideal gas law, we obtain a formula for pressure P 1 in the blister: P 1 = N 1 k B T V cloque Or k B denotes the Boltzmann constant, i.e. P 1 2 = αD tot 1 k B T 1 16 ∗ 1 − υ 2 Ee 3 L 2 4 .

[0040] To determine the dose D early2 of helium to be implanted in the receiving substrate, the geometry of the cavity must be expressed: V cavit é = a π 4 L 2 .

[0041] Assuming that the entire implanted helium dose will pressurize the cavity due to helium diffusion in the silicon, we obtain a quantity of helium in the cavity N 2 = D tot 2 π 4 L 2 . By applying the ideal gas law, we obtain a formula for the pressure P2 in the cavity: P 2 = D tot 2 k B T a .

[0042] To obtain sufficient pressure in the cavities to promote lateral blister propagation, P1 must at least equal P2. Equating the pressure formulas, we obtain: αD tot 1 k B T 1 16 ∗ 1 − υ 2 Ee 3 L 2 4 = D tot 2 k B T a 2

[0043] We can deduce a link between the dose of helium to be implanted as a function of the depth of the cavity (a), the diameter of the cavity ( L ), the properties of the thin film material ( E , v and the dose of hydrogen D tot 1 : D tot 2 = 16 a L 2 ∗ Ee 3 1 − ν 2 ∗ 1 k B T ∗ αD tot 1 .

[0044] To illustrate these formulas with a practical example, we consider the suspension of a 1.4µm silicon thin film using hydrogen implantation in the receiving substrate with a dose Δtt = 6 × 1016 at / cm2 and an energy of 160 keV. Taking the parameters E = 130 GPa, v = 0.28, L = 40 µm, a = 350 nm, T = 500°C, α = 30% we obtain as dose in helium to implant D tot1 =2.8*10 16< at / cm 2< .

Claims

1. A method for transferring a semiconductor layer from a donor substrate (1) to a receiver substrate (3) having an open cavity (C), comprising the steps of: - forming an embrittlement plane (F) in the donor substrate (1) by implanting ionic species; - making, by contacting the donor substrate (1) and the receiver substrate (3), an assembly in which said cavity is buried; - separating the assembly by fracturing along the embrittlement plane (F), said separation causing transfer of the semiconductor layer to the receiving substrate (3) and sealing of the cavity by the semiconductor layer; characterized in that it comprises: - prior to making the assembly, a step of implanting diffusing species in the donor substrate or in the receiver substrate, and, - subsequently to making the assembly and prior to separating the assembly, a step of diffusing said species in the cavity, thereby pressurizing the buried cavity and forming a stiffening effect in the cavity that prevents or limits development of blisters at the embrittlement plane (F) in vertical alignment with the cavity during the fracture separation step, forcing the blister-type defects to propagate laterally.

2. The method according to claim 1, wherein the step of diffusing said species into the cavity comprises annealing the assembly.

3. The method according to claim 2, wherein said annealing is continued until the assembly is separated by fracturing along the embrittlement plane.

4. The method according to one of claims 1 to 3, wherein the diffusing species are implanted in the receiver substrate at a depth less than the depth of the embrittlement plane in the donor substrate.

5. The method of claim 4, wherein the diffuser species are not implanted through the bottom of the cavity.

6. The method according to one of claims 1 to 3, wherein the diffusing species are implanted in the donor substrate at a depth less than the depth of the embrittlement plane.

7. The method according to any one of claims 1 to 6, wherein, during the step of implanting diffusing species, a dose Dtot2 of diffusing species to be implanted at least verifies the relationship: D tot 2 = 16 a L 2 ∗ Ee 3 1 − ν 2 ∗ 1 k B T ∗ αD tot 1 , where: - a and L are a depth and a diameter of the cavity; - E and v are the Young's modulus and the Poisson number of the material of the semiconductor layer transferred, and e is its thickness; - kB is the Boltzmann constant; - T is a temperature during the separation step; - Dtot1 is a dose of ionic species implanted in the donor substrate (1) during the embrittlement plane formation step (F), and α is a fraction of the dose of ionic species implanted Dtot1 that is found in the blister.