Patent Application
20150349191
The present invention relates to a method for producing a thick crystalline layer such as a layer of Si, Ge or a SiGe alloy, in particular dedicated to photovoltaic applications. According to another aspect, the present invention relates to an initial structure which may originate from an intermediate step of the method according to the invention and according to another aspect, the present invention relates to a detached structure which may originate from another intermediate step of the method according to the invention.
Photovoltaic applications require using crystalline substrates of silicon, germanium or SiGe alloy sufficiently thick so that the photonic efficiency is optimal, but also sufficiently thin so that the cost of the material is not too high. A substrate thickness of around a few dozen micrometers, such as a thickness between 20 and 50 micrometers is suitable.
Producing this type of substrate may be obtained from silicon or germanium ingots or even thick epitaxial layers on host substrates but the ingot cutting for forming each of the substrates implies a loss of material from the cutting (thickness of the saw cut) and preparing the surface of the substrates (thinning by lapping, chemical attack, polishing). In total, producing a silicon substrate having a thickness of 20 to 50 μm, leads to losing around a hundred micrometers of crystalline silicon from the ingot. This loss of material represents an appreciable part of the cost of each substrate.
Currently, other techniques of producing such substrates involve deep implantations (at several dozens of micrometers in depth) of gaseous ions (hydrogen, helium, . . . ) alone or combined with thermal treatments allowing a fracture at the implanted area in such a manner as to form substrates of a few dozen micrometers. The drawback of this approach is that it is based on using implantation equipment capable of providing energies of the order of a few MeV. These equipments are expensive, not available commercially such that an approach with such equipments remains difficult for a high-volume manufacture with a view to applications in the field of photovoltaics.
Other techniques based on silicon metallurgy (tapping, direct forming . . . ) have been developed in order to manufacture silicon substrates of a few dozen (or even hundred) micrometers in thickness. However, these forming techniques are also tricky to implement for high-volume manufacturing such as is expected in the case of photovoltaic applications.
One of the purposes of the invention is to overcome one or several of these drawbacks. To this end, and according to a first aspect, the present invention proposes a method for producing a thick crystalline layer, in particular intended for photovoltaic applications, comprising the steps consisting in
a) Implanting ionic species through a surface of a substrate including at least on the surface a crystalline layer of SixGe1-x with 0≦x≦1 in such a manner as to form a weakened plane in said layer delimiting a seed film under the surface of the substrate,
b) Depositing an amorphous layer of SiyGe1-y with 0≦x≦1 and y equal to or different from x on the seed film leading to the formation of a weakened composite structure,
c) Applying a fracture treatment in such a manner as to cause a fracture of the substrate according to the weakened plane and obtain a detached structure including the seed film and the amorphous layer of SiyGe1-y on the one hand, and a negative (4) of the substrate (1) on the other hand, and
d) Applying to the detached structure a thermal treatment for bringing about the crystallization of the amorphous layer of SiyGe1-y from the seed film, in such a manner as to obtain a thick crystalline layer (9) of a thickness higher than 10 micrometers and separate from the negative (4)
In the present document the expression “thick layer” means a layer having a thickness of at least a dozen micrometers, for example a thickness between 10 and 50 micrometers. In the present document, the terminology “amorphous layer” means a layer of a material of which the crystalline structure is mainly without order at long distance. In the present document, the term “weakened plane” means, the area impacted by the ion implantation and in which the fracture occurs.
Advantageously x is equal to y in such a manner as to keep the same lattice constant and prevent generating crystallographic defects during the crystallization of the amorphous layer.
When x and y are different, it is worth noting that the closer x and y are, the less the crystallographic defects generated during the crystallization of the amorphous layer.
Thus, the present invention allows producing a thick crystalline layer by using steps which do not lead to a great loss of used crystalline material, easily reproducible and suitable for large volume manufacture. In fact, the implantation step is carried out with a relatively low depth, it may hence be carried out with equipment with low implantation energy (around 250 keV), available commercially, in such a manner that the costs of this step are reasonable. Furthermore, the deposition of an amorphous layer of SiyGe1-y may be accomplished at a low cost and at a speed which is compatible with a high volume manufacture. Moreover, the thermal treatment may be operated by using standard heating equipments at temperatures below 1250° C., preferentially within the range of 300 to 1200° C., suitable for a manufacture of large volumes at a low cost. Furthermore, after the step c), the negative of the implanted substrate remaining after the separation of the detached structure may be recycled in order to become a new substrate based on which the method of the present invention may be reproduced. This participates in a low production cost.
Advantageously, the step of depositing the layer of amorphous SiyGe1-y is carried out until obtaining a thickness between 10 micrometers and 50 micrometers.
According to a disposition, the step of implanting ionic species is carried out with hydrogen and/or helium based ions with an energy between a few keV and 250 keV and a dose between 1016 and a few 1017 at/cm2. Thus, thanks to the use of a seed layer that is not thick, the depth of the implantation is low and the costs of this step remain reasonable.
Preferably, the method comprises a step i) of balancing inner stresses generated in the weakened composite structure during step c) of applying a fracture treatment, in such a manner as to prevent deformations of the weakened structure which can decrease the quality of the fracture and crystallization. The risks of damaging the layers are hence reduced and the appearance of macroscopic defects is reduced. The fracture may occur without risk of sudden change of mechanical energy able to cause the break of the negative or the weakened composite structure.
Preferably, the layer of amorphous SiyGe1-y is deposited in such a manner as to hardly be or not be stressed. In the opposite case, the stress is taken into account in step i) for the balancing the stresses.
According to a disposition, the step i) of balancing inner stresses includes the application of a pressure along two opposite directions on either side and perpendicularly to the weakened composite structure during the application of a fracture treatment according to the step c).
Advantageously, the application of pressure is carried out by two pistons with a pressure higher than 1000 daN/m2. Thus, the applied pressure is homogenous over the entire structure and the deformation of the structure is limited. This in particular restricts the risks of damage linked to a sudden change in the mechanical energy during the fracture.
According to another possibility, the step i) of balancing inner stresses includes an implantation of ionic species in the layer of amorphous SiyGe1-y, in particular an implantation of silicon or germanium or other ionic species, in such a manner as to create a region of stress.
In fact, the introduction of ionic species in the amorphous layer allows creating a layer in compression reducing the deformation risks during fracture. In fact, the prior implantation of the ionic species at step a) generates a stress in the crystalline SixGe1-x which would lead to the deformation during fracture if this stress was not compensated by an implantation in the layer of amorphous SiyGe1-y. Furthermore, the use of ions of the same nature as the substrate (silicon for a silicon substrate for example) is particularly appropriate as it does not disrupt the nature of the obtained layer. Ions may be implanted with a small depth in the layer of SiyGe1-y in such a manner that the implantation only requires low energy in such a manner that it remains cheap. Moreover, it is not necessary to implant an important dose as the purpose is not to create a weakened plane.
The implantation of ionic species according to the step i) is preferably carried out prior to the fracture treatment of step c).
According to a disposition, the step i) includes an implantation of boron or phosphorous based ionic species. These ions may advantageously serve as dope to the detached structure for example a doping of type P with the boron (B) and a doping of type N with the phosphorous (P).
According to a disposition, the step of implanting ionic species in the layer of amorphous SiyGe1-y is carried out with an energy between 80 keV and 120 keV and a dose between 4·1014 and 6·1015 Si/cm2.
According to yet another possibility, the step i) of balancing inner stresses includes a deposition of a layer stressed into compression on the layer of amorphous SiyGe1-y, in particular a layer of amorphous SiyGe1-y, or a layer of silicon oxide. This type of deposition is obtained by increasing the speed of deposition of this amorphous SiyGe1-y or silicon oxide, for example by increasing the strength of the deposition, which incorporates within the film part of the carrier gas with in particular hydrogen and argon. The fact of depositing a layer stressed into compression allows balancing, during the fracture step c), the stresses in compression linked to the presence of the ionic species implanted beforehand in the substrate and minimizing or even cancelling the global deformation of the weakened composite structure. Moreover, the stressed layer is compatible with the layer of amorphous SiyGe1-y as it is constituted of the same material or of a material comprising silicon. When the stressed layer is made of amorphous SiyGe1-y, it may thus be part of the layer of amorphous SiyGe1-y, which was deposited beforehand.
The deposition of the layer stressed into compression according to the step i) is preferably carried out prior to the fracture treatment of step c).
Concretely, step c) of applying a fracture treatment includes the application of a thermal fracture treatment, in particular at at least a temperature between room temperature and 600° C., and/or the application of a mechanical fracture treatment according to the weakened plane, such as the application of a flexion, a traction, a lateral insertion of a blade, of a shearing stress, a water jet or an air blast. Thus, the thermal treatment may be completed by a mechanical treatment thus allowing to reduce the thermal budget to be applied for obtaining a fracture. Thus, the deformations in the weakened composite structure, exacerbated by the rise in temperature, are reduced at the moment of fracture. In fact, the stressed layers have different dilation coefficients from that of the amorphous SiyGe1-y, hence a high temperature induces important stresses.
Advantageously, the thermal treatment applied to step c) is carried out in a range between 200 and 500° C. and preferably a range between 250 and 360° C. Thus, the temperature of fracture remains low, thus being favorable for reducing deformations in the structure.
Preferably, step c) of applying the thermal fracture treatment is achieved by applying temperature steps. It is thus possible to avoid too sudden changes of the inner stresses.
Advantageously, the method comprises prior to carrying out all or part of step b) of depositing the layer of amorphous SiyGe1-y, the application of a thermal pre-treatment. This thermal pretreatment allows the partial maturation of the cavities formed by the implantation of step a). It is hence possible to reduce the intensity of the fracture treatment, for example to reduce the thermal budget (time, temperature pair) used for obtaining the fracture at step c). This allows reducing the deformation of the weakened composite structure during fracture.
According to a disposition the step d) of thermal crystallization treatment comprises the application of a temperature within a range of 400° C. and 1200° C. during, a period of time between a few hours and a few days. Thus, the crystallization of the layer of amorphous SiyGe1-y starts from the crystalline imprint of the seed film of crystalline SixGe1-x and propagates in the amorphous layer in order to obtain at the end a thick layer of good crystalline quality.
According to a variant, the step c) of applying a fracture treatment and step d) of applying a thermal crystallization treatment are achieved by a unique thermal treatment ranging between room temperature to 1200° C. by applying temperature steps. This reduces the cycle time of the method and prevents handling the structure between the two steps c) and d) which may be carried out in the same equipment.
In an advantageous manner, the method comprises prior to step d) of applying the thermal crystallization treatment, a step ii) including a deposition of at least one film of amorphous material on the exposed surface of the layer of amorphous SiyGe1-y. The presence of this film of amorphous material, which does not have a crystalline structure and being from a material different from SiyGe1-y, prevents the start of crystallization from the exposed face of the layer of amorphous SiyGe1-y and promotes a start of crystallization from the seed film of crystalline SixGe1-x only. It is understood that for being able to ensure its function, this film of amorphous material is constituted of a material other than SiyGe1-y. In the present document, the expression “exposed face of the layer of amorphous SiyGe1-y” means the face of the layer of amorphous SiyGe1-y which is opposite the one in contact with the seed film.
According to a possibility, the film of amorphous material is deposited prior to the fracture treatment.
Preferably, step ii) including a deposit of at least a film of amorphous material on the surface of the layer of amorphous SiyGe1-y also includes a step of depositing at least a film of the same amorphous material on the free surface of the seed film. The presence of a film of amorphous material on the side of the seed film of the detached structure allows balancing the stresses generated by the presence of the film on the layer of amorphous SiyGe1-y appearing during the thermal crystallization treatment.
In an advantageous manner, the films of amorphous material are achieved in silicon oxide, of general formula SiOx with 0≦x≦2. Thus the deposited material has a material different from that of the layer of amorphous SiyGe1-y while remaining compatible with the later treatments of the amorphous layer, for example during step d).
According to a possibility, the step including the deposition of at least a film of an amorphous material comprises the deposition of at least another film of different nature in such a manner as to create a stacking of several films.
Advantageously, the method comprises prior to step d) of applying the thermal crystallization treatment, a step iii) of additional thermal degassing treatment of the layer of amorphous SiyGe1-y. Thus, it is possible to remove all or part of the gaseous elements, for example hydrogen, incorporated in the layer of amorphous SiyGe1-y when it is being deposited. This prevents the appearance of harmful effects linked to the presence of hydrogen gas during the thermal crystallization treatment, such as exfoliations or bubbling of amorphous SiyGe1-y.
Preferably, the additional thermal treatment iii) is carried out in a range of temperatures between 350° C. and 500° C.
According to a variant, the method comprises, prior to the implantation step a), a step of depositing a lower portion of the layer of amorphous SiyGe1−y and comprises, after the step a), a step of cleaning the implanted surface of said lower portion, the step b) consisting in depositing an upper portion of the layer of amorphous SiyGe1-y.
Preferably, the accumulated lower and upper portions form the totality of the layer of amorphous SiyGe1-y.
The cleaning of the surface, achieved after the implantation step a), advantageously allows removing any contamination or any oxide on the surface in order to optimize the later depositions, for example optionally that of the upper portion of said amorphous layer, and improve the crystallization according to the step d). A cleaning may be in particular be carried out for removing the organic contaminants (for example, by a sulphuric acid, ozone based solution . . . ), for removing particles (for example, by an ammonia based solution . . . ), for removing metal contaminants (for example, by a hydrochloric acid based solution) for removing oxide (for example, by etching in a solution of hydrofluoric acid), leaving a surface of the lower portion of the layer of amorphous SiyGe1-y capable for the deposition of the upper portion of the layer of amorphous SiyGe1-y and for the generation of a crystallization during the step d).
According to a particular disposition, x is equal to 1 and y is equal to 1 in such a manner that the substrate comprises at least on the surface a crystalline layer of silicon and that the step b) comprises the deposition of a layer of amorphous silicon on the seed film.
Preferably, the layer of SixGe1-x is single-crystal in such a manner that the seed film is formed of a single-crystal material. This allows the crystallization of the amorphous layer into a single-crystal material following step d).
In the present document it is obvious that the crystallization according to the invention allows obtaining a single-crystal material and not a polycrystalline material comprising for example large grains of crystals.
According to a second aspect, the invention proposes a weakened composite structure comprising from its base to its surface, a substrate comprising at least on its surface a layer of crystalline SiXGe1-x having a weakened plane delimiting a seed film having a thickness between around 10 nanometers and 2 micrometers, a layer of amorphous SiyGe1-y having a thickness higher than 10 micrometers on the seed film, the layer of amorphous SiyGe1-y including a region of stress comprising implanted ionic species.
This weakened structure is thus suitable for applying the fracture treatment according to step c) and the thermal crystallization treatment according to step d). In fact, the presence of these ionic species in the superficial layer of amorphous SiyGe1-y allows partially or totally balancing the stresses generated by the ionic species implanted in the substrate, during step a), and contributes in maintaining a structure without any great deformation.
Preferably, the SixGe1-x is single-crystal as well as the seed film in such a manner as to obtain in the end a thick single-crystal layer.
According to a third aspect, the invention proposes a detached structure comprising from its base to its surface, a film of amorphous material of silicon oxide having a thickness of a few dozen nanometers, a seed film of crystalline SiXGe1-x having a thickness between around 10 nanometers and 2 micrometers on the film of amorphous material, a layer of amorphous SiyGe1-y having a thickness higher than 10 micrometers on the seed film and a film of amorphous material of silicon oxide having a thickness of a few dozen nanometers on the layer of amorphous SiyGe1-y. The presence of the two films of amorphous material in silicon oxide advantageously allows balancing the stresses on either side of the seed film and the layer of amorphous SiyGe1-y in such a manner as to prevent a large deformation for example during the application of the thermal crystallization treatment. The film deposited on the layer of amorphous SiyGe1-y further contributes in preventing an initiation of the crystallization from the upper surface of the layer of amorphous SiyGe1-y.
Preferably still, the seed film of SixGe1-x is single-crystal.
Other aspects, purposes and advantages of the present invention will become more apparent upon reading the following description of the different variants of the latter, given by way of non limiting examples and made with reference to the accompanying drawings. The figures do not necessarily respect the scale of all the represented elements in such a manner as to make them readable. The dotted lines symbolize a weakened plane. In the rest of the description, for the sake of simplicity, identical, similar or equivalent elements of the different embodiments bear the same numerical references.
According to a non illustrated variant, it is possible to deposit a protective layer prior to the implantation step a), in amorphous silicon for example, on the surface of the single-crystal layer 10 of silicon on the surface of the substrate 1 in order to protect the surface from contaminations which may take place during implantation or for preventing a damaging of the single-crystal layer. This protective amorphous layer may have a thickness of a few dozen micrometers. It is obvious that the energy for implanting ionic species is determined by taking into account this thickness in order to attain the required depth of the weakened plane 2 in the single-crystal silicon 10 layer. A required thickness of the seed film 3 is at least 10 nanometers. Preferably, this thickness is at least 100 nanometers so as to slightly shift the implantation peak in the layer 10 of silicon on the surface of the substrate 1 and thus keep the single-crystal quality on the surface for the future seed film 3. Furthermore, this protective layer being in amorphous material, it is not or is hardly damaged by the implantation. This protective layer may hence be kept in order to carry out the step b) of depositing a layer of amorphous silicon 5 on the seed film 3.
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According to a variant, the thermal crystallization treatments are carried out by applying a temperature gradient so that the advance of the crystallization front be preferentially from the seed film 3 towards the surface of amorphous layer 5.
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According to a non illustrated variant, the fracture treatment is obtained by applying a thermal treatment at a temperature lower than 320° C. and is completed by an application of a mechanical stress, such as the application of a blade at the weakened plane 2 in the substrate 1, a water jet or an air blast or a shearing stress. Furthermore, the thermal fracture treatment may be carried out at a temperature a little higher than 320° C. and over a reduced duration. Applying a mechanical stress thus completes this thermal budget (duration temperature) with a view to obtaining the fracture.
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Nevertheless, this film of amorphous material 13 may generate stresses in the detached structure 8 during the thermal crystallization treatment. A second film of the same thickness and same nature 14 is hence advantageously deposited on the exposed face of the seed film 3 after fracture. Thus, the generated stresses are symmetrical thereby limiting the appearance of deformations in the detached structure 8 (
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According to a non illustrated variant, an additional thermal treatment according to step iii) of the method is applied to the detached structure 8 prior to applying the thermal crystallization treatment. This additional treatment allows degassing the layer of amorphous silicon 5 for example hydrogen included in the layer during the deposition thereof. This thermal treatment may be carried out at temperatures lower than 600° C. and preferably between around 350° C. and 500° C. in such a manner that the degassing does not lead to exfoliations or bubbling of the silicon of the amorphous layer 5. Furthermore, the hydrogen exodiffusion carried out by this additional thermal treatment may modify the stresses in the seed film 3 of single-crystal silicon of the detached structure 8. Additional means (mechanical means, modification of the natures and thicknesses of the film depositions of amorphous material 13) for preventing the deformation of the structure 8 may hence be used according to the initial concentration of hydrogen present in the layer of amorphous silicon 5.
By way of example, for a layer of amorphous silicon 5 of 20 μm, the degassing may be obtained by a thermal treatment at 470° during 20 days.
According to yet another non illustrated variant, a thermal pretreatment is applied before or during the step b) of depositing the layer of amorphous silicon 5. Typically the thermal pretreatment is carried out at 320° C. during 1 h. This allows the maturation of the crystalline defects formed by the ionic implantation in such a manner that a low fracture thermal budget, such as 12 hours at 320° C., is then necessary in order to obtain the fracture after the deposition of the layer of amorphous silicon 5.
The examples given above are described in reference to silicon but all these examples may be suitable for the materials of type SixGe1-x with 13)(1 in particular for the seed film 3 and SiyGe1-y for the amorphous layer 5. In the case where y is equal to 0, the amorphous layer 5 of germanium is advantageously deposited by PECVD from a phase of germane, within a temperature range between around 100° C. and 300° C., and preferably at a temperature lower than 150° C. in order to prevent beginnings of crystallization during the deposition thereof. The crystallization of the layer of germanium 5 is obtained by applying a thermal treatment at around 600° C. under nitrogen for a duration of 10 hours for a thickness of 25 micrometers.
Thus, the present invention proposes a method for producing a thick crystalline layer and preferably a single-crystal layer 9 which is simple to implement, cheap and reproducible.
It goes without saying that the invention is not limited to the variants described above by way of examples but comprises all the technical equivalents and variants of the described means as well as their combinations.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1262736 | Dec 2012 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2013/053146 | 12/17/2013 | WO | 00 |
