METHOD FOR PRODUCING A TRANSFERABLE THIN LAYER

Description

[0001] The present invention relates to the field of devices comprising one or more thin layers, also called multilayer materials, of electronic or optoelectronic devices, and in particular semiconductor devices, and especially photovoltaic cells. The invention also relates to the fields of nanotechnology, optoelectronics, and photovoltaics. State of the art

[0002] Various manufacturing processes for devices with thin films are known in the prior art. In particular, several methods for preparing semiconductor devices are known, especially for the fabrication of photovoltaic cells. Among these techniques is the preparation of thin films by epitaxy. Once the thin film has been epitaxially grown, it can be peeled off its substrate and transferred to another substrate of interest, or, for example, a less expensive one. Examples of such peeling or detachment methods include: SOITEC's Smartcut™ process enables the transfer of thin crystalline layers from a donor substrate to a support substrate. This technology is based on the combination of light ion implantation, particularly hydrogen ion implantation, and molecular adhesion to transfer ultrathin single-crystal layers from one substrate to another. Processes based on the use of porous silicon: at least two regions with different porosities are created beneath the substrate surface by electrochemical etching in HF solution, prior to epitaxy. A high-temperature annealing treatment breaks the porous zone to detach the desired portion. Sacrificial layer processes act as a crystalline buffer for epitaxial growth and can be etched or removed after deposition. This method is primarily used for epitaxial III-V semiconductors.

[0003] These methods, however, are either expensive, difficult to implement, or limited in their applications. The drawback of these processes is that they involve H+ ion implantation and / or high-temperature heat treatment (>700°C), which are complex and costly. High-temperature annealing also limits the processes to crystalline silicon substrates or refractory materials capable of withstanding the annealing temperature. Epitaxial growth has already proven to be a suitable route for the production of ultrathin single-crystal layers, particularly because it allows for precise control of the layer thickness and doping. Furthermore, growth can be carried out at low temperatures (<200°C), making epitaxy a low-cost process. However, in this type of process, transferring the epitaxially grown layers to a low-cost substrate is a crucial step.Thus, there is a need to develop innovative processes to address existing technical problems. Objectives of the invention

[0004] The present invention aims to solve the technical problem of providing a method for manufacturing a crystalline semiconductor material comprising at least one fully single-crystal semiconductor layer that is easily detachable from the substrate on which this semiconductor layer or layers are prepared, particularly for the purpose of transferring the semiconductor layer(s) onto another substrate, preferably one that is low-cost or possesses desirable properties. In particular, it is desirable to use a second substrate that is less expensive than the first substrate or, for example, a flexible substrate. The present invention also aims to solve the technical problem of providing a method in which the first substrate is recyclable.

[0005] The present invention also aims to solve the technical problem of providing a method for manufacturing an inexpensive multilayer semiconductor material.

[0006] The present invention also aims to solve the technical problem of providing an easy-to-implement method for manufacturing a multilayer semiconductor material, particularly for considering various industrial applications.

[0007] The present invention also aims to solve the technical problem of providing a method for preparing photovoltaic cells.

[0008] The present invention also aims to solve the technical problem of providing new multilayer semiconductor materials, for example for applications in photovoltaic cells, in particular for photovoltaic cells whose substrate is not the growth substrate.

[0009] The present invention also aims to solve the technical problem of providing materials comprising one or more semiconducting layers deposited on a soft or flexible substrate. Detailed description of the invention

[0010] The inventors have discovered a new preparation process that solves one or more of the technical problems stated above. This process enables the preparation of semiconductor materials that solve one or more of the technical problems stated above.

[0011] The present invention relates in particular to a method for preparing a semiconductor material comprising at least one fully single-crystal semiconductor layer according to claim 1.

[0012] The deposition of very thin layers of single-crystal silicon (or ultrathin film) by PECVD is already described, for example in Roca i Cabarrocas et al. 2012 (Pere Roca i Cabarrocas, Romain Cariou, Martin Labrune. Low temperature plasma deposition of silicon thin films: From amorphous to crystalline. Journal of Non-Crystalline Solids, Elsevier, 2012, 358 (17), pp.2000-2003. <10.1016 / j.jnoncrysol.2011.12.113>). <hal-00806450>However, in prior art PECVD deposition, the conditions are stationary, meaning that the conditions for generating the plasma do not vary. The present inventors have demonstrated that the epitaxial growth rate gradient allows for the generation of a weak zone in the single-crystal silicon layer, which can be broken to separate the contacting layers on either side of this weak zone. This allows for easy detachment of the single-crystal silicon and the layers deposited on the opposite side of the substrate from the first substrate.

[0013] The term "epitaxial growth rate gradient" refers to PECVD conditions that impose different deposition rates for nanometer-thick layers. These PECVD conditions can be easily observed through variations in the operating parameters of the PECVD equipment during the PECVD step and / or through variations in the mechanical properties and / or chemical composition (particularly hydrogen content) of the deposited layers.

[0014] The process of the invention comprises, after epitaxial growth in step (iii), (iv) the detachment of at least the layer of semiconductor material formed by epitaxial growth in step (iii) for its physical separation from the first substrate and (v) the transfer of at least the layer of semiconductor material formed by epitaxial growth onto a second substrate.

[0015] According to one variant, the preparation (i) of the surface of a first substrate includes the removal of oxides present on the surface of the first substrate intended to receive the silicon layer.

[0016] For the removal of oxides present on the surface of the first substrate, any cleaning method known to a person skilled in the art can be used.

[0017] According to one embodiment, the removal of oxides present on the surface of the first substrate is carried out by a cleaning method comprising the implementation of one or more chemical solutions based on hydrofluoric acid or alkalis and / or fluoride-based plasma (SF6, SiF4, NF3, F2).

[0018] There are no particular limitations regarding the first substrate as long as it is possible to deposit a silicon layer by epitaxial growth on its surface. In one embodiment, the first substrate is chosen from: Si, Ge, SiGe, or another semiconductor material, such as a type III-V semiconductor material.

[0019] Examples of III-V type semiconductor materials include compounds of one or more elements from group III (boron, gallium, aluminum, indium, etc.) and group V (arsenic, antimony, phosphorus, etc.) of Mendeleev's periodic table, such as GaN, GaP, GaAs, GaSb, InN, InP, InAs, BN, BP, Bas, AIN, AIP, or ternary alloys such as In x Ga 1-x As, Al x Ga 1-x As. Semiconductor alloys of elements belonging to group 12 and the chalcogens can also be cited, such as CdS, CdSe, CdTe, ZnO, ZnS, ZnTe, CdZnTe (CZT), or copper indium selenide (CIGS).

[0020] Typically, the first substrate is a silicon wafer, that is, a slice, wafer, plate, or plate of silicon. Silicon wafers can be of various sizes and are generally a disk with a diameter of 100 mm to 300 mm or a square block with sides of 20 mm to 500 mm. The thickness is generally on the order of 0.1 to 1 mm and typically on the order of 0.3 mm.

[0021] Advantageously, after detachment of the material comprising one or more semiconductor layers, the first substrate is recycled for reuse. Typically, the substrate undergoes a cleaning process to remove the epitaxially deposited silicon layer, thus presenting a crystalline silicon surface suitable, for example, for receiving a new silicon layer deposited by PECVD, according to, for example, the process of the present invention. The silicon wafer can therefore be recycled as part of the process of the present invention.

[0022] PECVD is a process typically used to deposit thin films onto a substrate from a gaseous (vapor) state.

[0023] Advantageously, PECVD deposition is implemented in an intermediate regime between the epitaxial and microcrystalline regimes. PECVD deposition according to the present invention thus makes it possible to form at least one brittle layer within the single-crystal silicon layer incorporating a high concentration of hydrogen atoms.

[0024] Advantageously, deposition (ii) by PECVD is implemented for the formation of a plasma forming SiH3 radicals and then a plasma forming silicon clusters (“silicon clusters”).

[0025] Thus, the present invention advantageously allows the formation, in the same step of epitaxial growth by PECVD of silicon, of a zone of fragility due to the deposition conditions.

[0026] In particular, PECVD deposition involves a growth rate gradient for the single-crystal silicon layer. Specifically, this gradient can be achieved through different growth rates. This can be a discontinuous variation in the deposition rate, for example, with two or three different deposition rates, or a continuous variation. Growth rates in PECVD are determined, for example, by the parameter settings of the equipment and / or the deposition conditions. For instance, different plasma power densities, pressures, gas flow rates, or substrate temperatures can be used to obtain a growth rate gradient and thus different growth rates.

[0027] In various possible embodiments, the present invention also relates to the following features which may be considered individually or in all their technically possible combinations and each provide specific advantages for step (ii) and / or (iii) of PECVD: the duration is for example between 10 seconds and 10 minutes, the power density of the radiofrequency plasma is for example from 10 to 250 mW / cm2, the pressure of the gas mixture is for example from 200 to 400 Pa, the deposition temperature is for example from 100 to 400 °C, the gas flow rate of SiH 4 is for example from 0.5 to 5 sccm (meaning "standard cubic centimeter per minute" or cm 3 < / min expressed under standard temperature and pressure conditions), the hydrogen gas flow rate is for example from 10 to 500 Ncm3 / min (expressed under standard temperature and pressure conditions).

[0028] The presence of hydrogen significantly influences the stability and mechanical stress of the deposited layer. According to the present invention, this property is used to subsequently allow for very easy detachment of the layer(s) deposited on the single-crystal silicon layer obtained by PECVD.

[0029] Advantageously, the deposition temperature (ii) by PECVD and the epitaxial growth temperature (iii) are below 400 °C. Such a temperature advantageously reduces the operating cost of the manufacturing process. Advantageously, the deposition temperature by PECVD and the epitaxial growth temperature are below 250 °C. Typically, the temperature is 200 °C.

[0030] Advantageously, the silicon layer obtained by epitaxial growth using PECVD exhibits a compositional gradient. A compositional gradient is understood, for example, as a composition with a concentration of hydrogen atoms varying throughout the thickness of the silicon layer obtained by PECVD. Typically, according to the present invention, the silicon layer obtained by PECVD exhibits a hydrogen concentration peak. This hydrogen concentration peak is generally located at the interface between the substrate and the first epitaxial layer. The hydrogen concentration peak can exceed 1 x 10²¹ atoms / cm³, and for example, exceed 2 x 10²¹ atoms / cm³. The hydrogen concentration is measured, for example, by secondary ion mass spectrometry (SIMS), a surface analysis method.We can also speak of a crystallinity gradient for this silicon layer due to the crystallinity defects introduced by the hydrogen atoms. We can also speak of a density gradient.

[0031] According to one embodiment, the presence of a zone of fragility can be observed in the fully single-crystal material because the substrate / silicon layer interface exhibits oscillations (interferences) by spectroscopic ellipsometry in the photon energy range of 0.7 to 3 eV, and preferably in the photon energy range of 1.5 to 2.5 eV.

[0032] Advantageously, PECVD deposition allows control over the thickness of the deposited layer. The layer can, for example, have a thickness of 1 nm to 10 µm.

[0033] Once the silicon layer is deposited by PECVD, one or more single-crystal layers of one or more semiconductor materials can be grown on top of the silicon. In one embodiment, the single-crystal layers of the material are not identical and may, for example, comprise different semiconductor materials from one layer to another. In another embodiment, the single-crystal layers of the material are identical in the chemical nature of the semiconductor material used, although the material may differ through doping or physical structuring.

[0034] For the layer(s) of semiconductor material deposited on the silicon layer deposited by PECVD with growth gradient, reference can be made to the semiconductor materials mentioned previously.

[0035] According to one variant, epitaxial growth (iii) is implemented with one or more elements chosen from: Si, Ge, SiGe.

[0036] According to one variant, epitaxial growth (iii) is implemented with a technique chosen from PECVD, CVD, MBE, or any combination thereof.

[0037] The growth conditions of the epitaxial layer (step (iii)) are for example the following (PECVD): temperature: 200°C, pressure: 307 Pa, power: 35 mW / cm2, SiH 4: 4 SCCM (Ncm 3< .min -1< ), H 2: 200 SCCM (Ncm 3< .min -1< ), deposition time: 1800 sec.

[0038] Typically, the transfer technique (v) on the second substrate is chosen from a technique including: ionic bonding (or "anodic bonding" in English), or the use of silicone, polyimide tape or high-temperature adhesive (e.g. Ormostamp ®), or any combination thereof.

[0039] OrmoStamp® is an inorganic-organic hybrid polymer for manufacturing transparent working pads used in nanolithography (Nan-Impression Lithography or NIL) as an economical alternative to quartz or galvanized pads. It can be applied in thermal NIL and / or UV NIL.

[0040] The invention also allows for the creation of stacks of doped and / or undoped layers to form PN, NP, PIN or NIP junctions or combinations of such junctions. The doping can be, for example, phosphorus and / or boron doping with a doping concentration, for example, of < 1 × 10²⁰ at. / cm³.

[0041] Once the layer(s) of semiconductor material have been deposited onto the single-crystal silicon layer obtained by PECVD according to the present invention, it is possible to easily detach the layer(s) of semiconductor material from the first substrate to physically separate them. For example, a heating step (generally called annealing) at a temperature of, for example, 250 to 400 °C can be used. This heating step advantageously further weakens the interface between the substrate and the silicon layer epitaxially treated by PECVD, thus allowing the sample to be separated into two distinct parts: one comprising the first substrate, and the other comprising the layer(s) of semiconductor material.

[0042] Advantageously, multiple layers of material can be deposited on PECVD-epitaxial silicon. This allows for the advantageous detachment of a multilayer material from the initial substrate. This enables the preforming of several semiconductor layers suitable for use in a photovoltaic cell.

[0043] According to one variant, heterojunctions are formed directly on the first substrate. This heterojunction can then be transferred to a second substrate.

[0044] The layer(s) of semiconductor material detached from the first substrate can therefore be deposited on a second substrate.

[0045] According to one variant, the detachment (iv) of the semiconductor material layer is achieved by mechanical or thermal treatment, or any combination thereof, and for example by cleavage at the porous interface through the application of mechanical stress, ultrasound, and / or a water jet. This yields a self-supporting multilayer film. The multilayer film can then be transferred onto a second substrate forming a flexible or rigid mechanical support, for example, a non-crystalline substrate, and the first crystalline substrate can be reused.

[0046] Advantageously, the first substrate is recycled. For example, the first substrate can be recycled by cleaning its surface. The cleaning conditions for the first substrate are known to those skilled in the art and are, for example, the recycling conditions of Smartcut™ technology.

[0047] According to one variant, the second substrate is chosen from: glass, a metal or metal alloy, a polymer, including one chosen from copolymers, a flexible material, an elastomer, and a thermoplastic elastomer.

[0048] According to one embodiment, the second substrate is flexible or supple, that is to say that the mechanical properties of the substrate allow it to be physically manipulated to substantially change its physical shape by using a force (for example by bending) without breaking the substrate.

[0049] The invention relates to a material that can be obtained by a process according to the present invention.

[0050] The invention makes it possible in particular to use a very thin or so-called ultra-thin silicon layer, that is to say, one of very small thickness. This thickness is typically from 1 nanometer (nm) to 10 micrometers (µm), and for example from 1 nm to 500 nm, from 1 nm to 200 nm or even from 1 µm to 10 µm.

[0051] Examples of devices comprising one or more materials according to the invention include, in particular: PN junctions; PIN junctions; PN / PN or PIN / PIN tandem structures; Heterojunctions; Any combination of the preceding stacks; and 3D electrical circuits.

[0052] For example, one could cite a 3D integrated circuit with chips, say, 5 to 10 µm thick. Such a 3D circuit can be used, for example, as layers in 3D stacks. For instance, one could mention a multilayer material comprising the following layers: glass / ITO / c-Si / ZnO / Ag, where the Si is single-crystal (c-Si).

[0053] Advantageously, the present invention allows the transfer of one or more layers of a semiconductor material usable for various applications.

[0054] For example, a multilayer semiconductor material according to the present invention can be used for the manufacture of silicon-on-insulator (SOI) plates.

[0055] The fabrication of thin, multilayer, single-crystal semiconductor materials (thin film, known as "thin layer" or "ultra-thin layer" if the thickness is very small) is a key step in the development of low-cost and / or flexible electronic or photovoltaic devices. The present invention makes it possible to provide such devices.

[0056] The present invention relates to a process in which the first substrate is recyclable, a process for manufacturing an inexpensive multilayer semiconductor material, a process for manufacturing layers of a multilayer semiconductor material that is easy to implement, particularly for considering various industrial applications, and a process for preparing photovoltaic cells.

[0057] The present invention also relates to providing new multilayer semiconductor materials, for example for applications in photovoltaic cells, in particular for photovoltaic cells whose substrate is not the growth substrate, or materials comprising one or more layers of semiconductors deposited on a soft or flexible substrate.

[0058] The multilayer materials according to the present invention can be used in particular in devices comprising one or more thin layers, electronic and optoelectronic devices, and especially semiconductor devices, notably photovoltaic cells. The invention also relates to the fields of nanotechnology and optoelectronics.

[0059] In the figures: There figure 1 represents a measurement by spectroscopic ellipsometry under the conditions of Example 1. figure 2 represents a SIMS measurement under the conditions of example 1. Examples Example 1

[0060] Two multilayer materials were prepared, one according to a process of the present invention with a gradient of epitaxial growth rate of the single-crystal silicon layer, the other, comparative, with a constant epitaxial growth rate by PECVD.

[0061] A thin layer of silicon 600 nm thick was thus prepared according to these two PECVD processes.

[0062] For both samples (comparative and according to the invention), the silicon layer deposited by PECVD is deposited by direct growth on the cleaned silicon wafer (free of native oxide).

[0063] For the example according to the present invention, three PECVD plasma conditions were used.

[0064] The initial conditions for filing a PECVD deposit are as follows: Temperature: 200 °C; Pressure: 240 Pa; Power: 35 mW / cm²; SiH4 flow rate: 2 sccm; H2 flow rate: 200 sccm; Duration: 60 sec.

[0065] The second set of conditions for PECVD filing are as follows: Temperature: 200°C; Pressure: 227 Pa; Power: 17 mW / cm²; SiH4 flow rate: 1 sccm; H2 flow rate: 200 sccm; Duration: 60 sec.

[0066] The third PECVD filing conditions are as follows: Temperature: 200°C; Pressure: 307 Pa; Power: 35 mW / cm²; SiH4 flow rate: 4 sccm; H2 flow rate: 200 sccm; Duration: 1800 sec.

[0067] For the comparison sample, the silicon layer was deposited by PECVD without epitaxial growth rate gradient only under the third PECVD deposition conditions.

[0068] The results of spectroscopic ellipsometry measurements are illustrated on the figure 1 It is noted that at low energy, the imaginary part of the pseudodielectric function εi exhibits oscillations characteristic of the porosity of the interface between the substrate (monocrystalline silicon wafer) and the silicon layer deposited by PECVD. The high porosity of the interface is detected for the sample according to the present invention, as illustrated by the high amplitude of the oscillations at low energy. The oscillation amplitude is significant compared to the comparator sample. It is noted that in the higher energy region (above 3 eV), the spectrum is very similar to that of the silicon wafer, thus demonstrating the crystalline quality of the material according to the present invention.

[0069] On the figure 2 The hydrogen concentration profile characterized by SIMS for the two samples is noted. For the sample according to the present invention, the interface is very porous with a high accumulation of hydrogen on the surface in contact with the silicon wafer (up to 2.5 x 10²¹ atoms / cm³). Such a concentration peak is not observed in the comparison sample, whose hydrogen concentration is approximately 3 x 10²⁰ atoms / cm³.

[0070] Next, for the material according to the invention, the silicon layer was successfully transferred onto glass, for example, by anodic bonding at 200°C for 10 minutes followed by annealing at 200°C for 5 minutes. In contrast, for the comparison sample, under the same conditions, it was not possible to detach the layer deposited by PECVD. No detachment was observed even after annealing at 550°C for 5 minutes, even when the silicon wafer broke.

[0071] Thus, the process according to the present invention allows a simple, low-cost transfer of a semiconductor material onto a substrate that can be less expensive than silicon. Example 2

[0072] Under the same conditions as in example 1, the thin silicon layer was transferred onto a flexible substrate.

[0073] This allows us to obtain a crystalline semiconductor material on a flexible substrate.

Claims

1. Process for the preparation of a semiconductor material comprising at least one entirely monocrystalline semiconductor layer, said process comprising the steps: (i) preparation of the surface of a first substrate to receive a layer of monocrystalline silicon; (ii) the plasma-enhanced chemical vapor deposition of a layer of monocrystalline silicon by epitaxial growth with a growth rate gradient on the first substrate prepared in step (i), to form, during the same PECVD epitaxial growth step of the silicon, a region of brittleness by using different plasma power densities, pressures, gas flow rates, or substrate temperatures; and (iii) epitaxial growth of a monocrystalline layer of a semiconductor material on the monocrystalline silicon layer obtained in step (ii), to thus obtain a material comprising at least one entirely monocrystalline semiconductor layer; and (iv) detaching of at least the layer of semiconductor material formed by epitaxial growth at step (iii) for its physical separation from the first substrate; and (v) the transference of at least the layer of semiconductor material formed by epitaxial growth, onto a second substrate.

2. Process according to claim 1, characterized in that the preparation (i) of the surface of a first substrate comprises the removal of the oxides present on the surface of the first substrate intended to receive the silicon layer.

3. Process according to claim 1 or 2, characterized in that the deposition step (ii) by PECVD is implemented for the formation of a plasma forming SiH3 radicals then of a plasma forming silicon clusters.

4. Process according to any one of claims 1 to 3, characterized in that the deposition temperature step (ii) by PECVD and epitaxial growth step (iii) is less than 400°C.

5. Process according to any one of claims 1 to 4, characterized in that the epitaxial growth step (iii) is implemented with one or more elements chosen from among: Si, Ge, SiGe.

6. Process according to any one of claims 1 to 5, characterized in that the epitaxial growth step (iii) is implemented with a technique chosen from among PECVD, CVD, MBE, or any of their combinations.

7. Process according to claim 1, characterized in that the deposition technique step (v) on the second substrate is chosen from a technique comprising: anodic bonding, or the use of silicone, a polyimide tape or a high temperature glue, or any combination thereof.

8. Process according to claim 1, characterized in that the detachment step (iv) of the layer of semiconductor material is carried out by mechanical or thermal treatment, or any one of their combinations.