Optimization of electron beam-assisted thin film deposition
The use of a crucible with a metal bar to focus an electron beam for C60 pellet evaporation addresses the inefficiencies of traditional methods, resulting in faster, more stable, and uniform C60 thin film deposition with reduced contamination and degradation, suitable for large-scale applications.
Patent Information
- Authority / Receiving Office
- FR · FR
- Patent Type
- Applications
- Current Assignee / Owner
- INSTITUT PHOTOVOLTA QUE D ILE DE FRANCE (IPVF)
- Filing Date
- 2024-12-31
- Publication Date
- 2026-07-03
AI Technical Summary
Existing thin film deposition methods for carbon chain materials like C60, such as thermal evaporation and electron beam evaporation, suffer from long deposition times, material degradation, contamination, and unstable evaporation rates, leading to inefficient and costly production.
A method involving a crucible with a metal bar to focus an electron beam for heating and evaporating C60 pellets, allowing controlled sublimation and deposition, using a high-voltage electric current to generate the electron beam, and employing C60 pellets instead of powder to minimize contamination and degradation.
Achieves faster, more stable, and uniform thin film deposition with reduced contamination and degradation, enabling reproducible and cost-effective production of high-quality C60 films suitable for large-scale applications.
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Abstract
Description
Title of the invention: Optimization of electron beam-assisted thin film deposition. Technical field
[0001] The present disclosure falls within the field of thin film deposition techniques. Previous technique
[0002] It relates more particularly to thin-film deposition techniques of carbon chain materials such as, for example, fullerene (or "C60" hereafter), especially for perovskite-based solar cells (a thin film with photovoltaic properties being made in this type of material), or for organic solar cells (or "OPV"), or more generally for other applications in which thin films of C60 can be used (such as in OLEDs, sensors, optoelectronic devices, passivation layers, "spintronic" type devices, or in biomedical applications, transistors, microelectronics in general, etc.).
[0003] Typically, a layer of C60 is deposited by a method referred to below as "thermal evaporation" (which is actually sublimation). This method consists of placing the C60 powder in a crucible heated under vacuum to approximately 350°C. The C60 sublimates and passes into the gaseous phase before being condensed onto a substrate. This technique is simple to perform, but presents problems, particularly related to the relatively long deposition time, typically several hours. Indeed, this type of deposition requires a temperature increase in the C60 source, a stabilization of the temperature, and finally the actual deposition of the C60 layer.
[0004] Moreover, pollution of the deposition chamber occurs because C60 in particular is usually in powder form and turns out to be a very powdery material.
[0005] This problem of material dusting has been observed on an even larger scale with another vacuum evaporation technique involving the application of an electron beam to the C60 precursor. Typically, attempts to evaporate a C60 precursor by applying an electron beam have proven unsuccessful. Indeed, the beam bombards the powder, resulting in C60 contamination throughout the chamber by dusting, and ultimately a significant loss of precursor material for a quantity that would have been necessary to fabricate a 10 and 20 nm thin film. Furthermore, with the direct electron beam evaporation method on C60, it is very difficult to achieve a stable and reproducible evaporation rate.
[0006] Furthermore, C60 degrades quite rapidly during heating by electron beam application because the C60 molecule can deteriorate, degrading into another material that may contaminate the thin film being formed. This problem necessitates regularly changing the precursor and protecting the entire reactor with a protective layer such as aluminum foil to prevent contamination. These issues limit the number of possible deposits per day. If these precautions are not taken, contaminants are likely to end up in the fullerene thin film being deposited. Summary
[0007] This disclosure improves the situation.
[0008] A method for depositing a thin film of a material under vacuum is proposed, comprising the following steps: - to have at least one pellet of a precursor of said material, compressed in a crucible, and - apply a high-voltage electric current to generate an electron beam, characterized in that the crucible includes a metal bar on which the electron beam is focused to radiate heat and, from there, evaporate said material.
[0009] Such an embodiment makes it possible to "soften" the heating of the material thanks to the presence of the metal bar, and from there, better control of the sublimation of the material and its deposition rate in a thin film on a substrate or another layer typically.
[0010] To take advantage of this softening effect, in one embodiment, a plurality of pellets of the precursor are arranged in the crucible, around the metal bar.
[0011] In one embodiment, the material to be deposited can be fullerene or “C60”.
[0012] It has been observed for this material in particular that, without the use of the aforementioned bar, this material could degrade too quickly (and take on a yellowish color characterizing its degradation) if the electron beam was directed directly at it (without the bar).
[0013] For example, the aforementioned pellet(s) may be based on C60 compressed using a high-pressure press, greater than or on the order of 5 tonnes.
[0014] Furthermore, the term "precursor of the material" above refers to either the pure material to be deposited or a mixture of this pure material with another material. Typically, in the case of C60 fullerene, traces of C70 or C120 (typical impurities) may be present in the aforementioned "precursor," with C60 being the predominant component.
[0015] In one embodiment, the bar can be made of tungsten (a highly irradiating material which only evaporates at very high temperatures, higher than those involved here).
[0016] Furthermore, the term "high voltage" above means a voltage typically favorable to the generation of an electron beam, for example greater than IkV or on the order of a few kilovolts.
[0017] In one embodiment, the electric current is variable and on the order of a few mA for a constant high voltage, greater than or on the order of 5 kV.
[0018] In such an embodiment, a current of less than 3 mA is applied in a first phase, then the current intensity is increased during a second phase and is between 3 mA and 10 mA to start the thin film deposition.
[0019] In such an embodiment, a stable deposition rate can be achieved during the second phase and it is on the order of 0.1 Å / s.
[0020] In such an embodiment, a thin layer obtained may exhibit a thickness inhomogeneity of less than 10% over the entire surface of a glass substrate with sides of 20cm.
[0021] According to another aspect, the present description also relates to a device for implementing the above process, and comprising a crucible equipped with a metal bar in the center of the crucible and capable of receiving an electron beam to irradiate heat.
[0022] In such an embodiment, the metal bar can be cylindrical, truncated conical or conical in shape.
[0023] According to another aspect, the present description also aims at a use of the above process for the manufacture of a solar cell comprising at least one active layer with photovoltaic properties, on which said thin fullerene layer is deposited by implementing the process.
[0024] In one embodiment, the aforementioned active layer can typically be perovskite-based. Brief description of the drawings
[0025] Other features, details and advantages will become apparent from reading the detailed description below and from analyzing the accompanying drawings, in which: Fig. 1
[0026] [Fig.1] illustrates the steps of a process according to an embodiment for the deposition of C60 without pollution of the deposition chamber. Fig. 2
[0027] [Fig.2] shows the uniformity of a deposit within the meaning of this description on a large plate. Fig. 3
[0028] [Fig.3] shows a Raman (a) and photoluminescence (b) spectrum of the thin films of C60 deposited by thermal evaporation MB (in the sense of the prior art) and by electron beam PL2 and PL3 (by two consecutive electron beam evaporations as defined in this description). Fig. 4
[0029] [Fig.4] shows all the elements of the device used according to one embodiment for the deposition of C60 by way under vacuum, by evaporation by application of an electron beam. Fig. 5
[0030] [Fig.5] shows the architecture of a PIN type photovoltaic device (part a), the performance of solar cells with C60 “MB” and C60 “PL” evaporated by thermal evaporation in the sense of the prior art and by electron beam, respectively (part b). Description of the implementation methods
[0031] A method for depositing C60 by vacuum, by evaporation by application of an electron beam, is proposed, as illustrated in [Fig.1] described below and using a device illustrated in [Fig.4].
[0032] First, 1 cm diameter C60 PST pellets are manufactured using a high-pressure press of more than 6 tons (step S1). A crucible CR is then filled with these pellets (step S2). The crucible is made of graphite, typically round in shape, and has a capacity of approximately 100 cm³. A metal rod BM, preferably made of tungsten due to its high-temperature heat irradiation capabilities, is placed in the center of the crucible. In particular, at step S3, the pellets are distributed around the rod BM to ensure satisfactory homogeneity of the quantity of the C60 precursor around the rod.
[0033] By way of example, the dimensions of the crucible may be as follows: - Outer top diameter 52.9 mm - Height 19.5 mm - Wall thickness ~ 2.4 mm - Wall angles: 15° (i.e. 30° total opening).
[0034] The tungsten bar, for its part, can be cylindrical and: - of a height of 20mm, and - of a diameter of 5mm.
[0035] Next, a low-intensity current (between 2 and 3 mA) at a constant high voltage, for example 5 or 6.0 kV, is applied to generate an electron beam (step S4). The beam's impact can be controlled manually, for example, to focus it on the tungsten rod. A bright spot appearing on the rod indicates that it is beginning to radiate heat (the spot at the top of the rod, here cylindrical BM of [Fig.4] described later). The C60 is then heated and evaporated by irradiation of the tungsten rod.
[0036] Tungsten has the advantage of a very high evaporation temperature, much higher than that reached when subjected to an electron beam as described here. Typically, given the current and potential values used, tungsten is unlikely to evaporate and contaminate the substrate and the forming C6O layer. The dimensions of the rod are adapted to the dimensions of the crucible. The rod is positioned at its center to ensure homogeneous irradiance for the pellets and to the size of the beam (which is a point a few millimeters in diameter at most). The rod is preferably placed in the middle of the crucible so that the isotropic heat diffusion is homogeneous over all the pellets.
[0037] This step S4 consists of gradually heating the tungsten rod so that it can then heat the C60 pellets by diffusion. This step lasts approximately 5 to 10 minutes (gradually and slowly to maintain a slow deposition rate thereafter). It is preferable to observe the beam and the incandescence of the rod through a window in the chamber, so as to observe a view similar to that of [Fig. 4].
[0038] Next, the current generated by the electron beam can be slowly increased from 3.0 mA to 7.0 mA to achieve a stable deposition rate of 0.1 Å / s (step S5). These values are average values and vary depending on the amount of C60 in the crucible. The more C60 consumed, the higher the current value must be, in order to operate at a nearly constant evaporation rate.
[0039] This step S5 depends on the chosen evaporation rate (very low in the case of C60 deposition) and the desired layer thickness. For a standard C60 deposition, the total time (heating of the rod and deposition of a 15nm thick layer) is approximately 45 minutes.
[0040] The current is generated by a current generator. The voltage is constant. The impact of the electron beam can be observed as a red dot focused on the top of the metal rod, as seen through a window in the evaporation chamber, a view of which is shown in [Fig. 4]. It is thus possible to manually center the electron beam FEL to direct it onto the top of the metal rod BM. As illustrated in [Fig. 4], the tungsten rod BM is cylindrical in shape and positioned in the center of the crucible CR. The impact of the electron beam FEL on the top of the cylinder BM generates heat irradiation around the upper surface of the cylinder, causing the C60 PST pellets arranged around the cylinder to evaporate (or more precisely, to sublime, passing directly from the solid to the gaseous state SUB).Thus, as the C60 sublimates, the "level" of C60 remaining in the crucible decreases and it is necessary to... increase the current intensity to generate the FEL beam in order to maintain a constant C60 deposition rate.
[0041] Moreover, another bar shape can be provided, for example conical or frustoconical with the apex of the cone facing upwards, so as to provide more constant irradiation of the pellets, on the one hand, and to be able to target the head of the bar (at the level of the apex of the cone) more reliably, on the other hand. In this alternative shape, the bar can be of the same height, 20 mm, with a width at half-height of the cone of 5 mm and an angle of approximately 30°.
[0042] The deposition time by this method is faster than conventional deposition (by thermal evaporation), almost two to three times faster.
[0043] Furthermore, the C60 is not degraded and remains C60 when it is deposited on the substrate. Indeed, the C60 molecule degrades less than in the prior art where an electron beam would be directed directly onto the C60 precursor. There is thus less risk of contaminating the forming thin film with material resulting from the degradation of the C60 molecule, which typically occurs when the electron beam is directed directly onto the C60, without the use of a metal rod. Thus, the metal rod acts as an intermediary in the heat supplied to the C60 precursor by gentle irradiation before the sublimation of the C60.
[0044] Furthermore, the deposition rate can be controlled because only a portion of the C60 contained in the crucible is heated to the point of sublimation and deposited onto the substrate, without contaminating the vacuum deposition chamber. This limitation of chamber contamination is further achieved by using C60 in pellet form rather than powder.
[0045] Finally, a homogeneous and uniform CM thin film of C60 is obtained reproducibly on rotating substrates (step S6). Typically, such substrates can be made of glass, for example, and have a large surface area, up to 20x20 cm² (as in the example in [Fig. 2]), or larger. Referring to [Fig. 2], the target layer thickness was set at 20 nm, and a thin film thickness was measured at five distant points using a profilometer: the actual thickness is very close to 20 nm, as shown in [Fig. 2]. After several evaporations, the vacuum deposition chamber could be opened, and virtually no C60 contamination was observed in the chamber.
[0046] To verify the quality of the C60 thin films obtained by electron beam evaporation, Raman spectroscopy and steady-state photoluminescence (PL) were performed on three samples.
[0047] Sample “MB” was prepared using the conventional thermal evaporation technique. Samples “PL2” and “PL3” were deposited by two consecutive electron beam evaporations. The Raman spectrum of [Fig. 3] (part a) shows Three vibrational modes characteristic of C60 fullerene, which are consistent with observations in the literature. The photoluminescence spectrum of [Fig. 3] (part b) indicates a sharp peak at 1.65 eV which indeed corresponds to a C60 band gap, again consistent with observations in the literature.
[0048] These results indicate that a pure C60 film is obtained using both thermal evaporation and electron beam evaporation. Here, three spectra for three samples are indeed identical, which means that the C60 films evaporated by electron beams are comparable to the thermal films, and that the deposition is indeed reproducible for both consecutive evaporations.
[0049] An example of the application of the new C60 films deposited according to the process of [Fig. 1] to perovskite solar cells is shown below. Part of the cell's PIN architecture is shown in [Fig. 5] (part a), where the C60 films are deposited on the perovskite layer. SnO2, or "tin dioxide," denotes an example of a material used for the electron transport layer. Au ("gold") denotes an example of a material used for the contact electrode located on the top layer.
[0050] Figure 5 (part b) compares the efficiency of solar cells (PCE index for " Power Conversion Efficiency") with the prior art C60 "MB" and the C60 "PL" as defined herein (evaporated respectively by thermal evaporation and by electron beam evaporation), according to two measurement methods: forward or FW ("Forward") mode or reverse or RV ("Reverse") mode, known to those skilled in the art. Higher performance and less dispersion are obtained for cells with C60 evaporated by electron beam as defined herein. The best cell efficiency is 16%.
[0051] Furthermore, a first large-sized semi-transparent module (here 64 cm²) was fabricated. Its efficiency was found to be 8%, demonstrating that electron beam evaporation of C60 fullerene can be used to increase the size scale of perovskite-based modules. It also turns out that indirect electron beam evaporation of the C60 layer using C60 pellets did not damage (nor degrade or contaminate) the lower perovskite layer. These promising results demonstrate that the process as described herein can be used for the production of large solar cells directly in industrial production. Industrial application
[0052] Compared to other techniques, the process described herein guarantees a better quality of C60 thin film thanks to faster, more precise and more stable evaporation. In addition, it requires less C60 consumption (or of its precursor), considering the high price of the C60 precursor. This more cost-effective technique makes it well suited to meet one of the key requirements of industrial-scale applications in the field of photovoltaics, particularly for thin-film technologies (based on perovskites in particular, or organic cells, or others).
[0053] The implementation of the above process has proven highly efficient on a material such as fullerene in its stable C60 form. This is a molecule in the shape of a hollow sphere composed of 60 carbon atoms forming 20 hexagons and 12 pentagons, with one carbon atom at the apex of each polygon and one bond on each side of the polygon. Each hexagon is adjacent to three other hexagons and three other pentagons. However, the present disclosure is not limited to the application of the process to such a complex molecule. It may be envisaged that the process could be applied to other molecules, particularly those derived from complex carbon chains, which are at risk, on the one hand, of degradation by reaction upon direct application of the electron beam, and, on the other hand, of poorly controlled dusting during thin-film deposition.
Claims
Demands
1. A method for depositing a material in a thin film, by vacuum, comprising the steps: - placing at least one pellet of a precursor of said material, compressed in a crucible, and - applying a high-voltage electric current to generate an electron beam, characterized in that the crucible comprises a metal rod on which the electron beam is focused to radiate heat and, from there, evaporate said material.
2. A method according to claim 1, wherein a plurality of pellets of the precursor are arranged in the crucible, around the metal bar.
3. A method according to any one of the preceding claims, wherein the bar is made of tungsten.
4. A method according to any one of the preceding claims, wherein the material is fullerene or "C60".
5. A method according to claim 4, wherein the pellets are based on C60 compressed using a high-pressure press, greater than or on the order of 5 tonnes.
6. A method according to any one of the preceding claims, wherein the electric current is variable and on the order of a few mA for a constant high voltage, greater than or on the order of 5 kV.
7. A method according to claim 6, wherein a current of less than 3 mA is applied in a first phase, then the current intensity is increased during a second phase and is between 3 mA and 10 mA to start the thin film deposition.
8. A method according to claim 7, wherein a stable deposition rate is achieved during the second phase and is on the order of 0.1 Å / s.
9. A method according to claim 8, comprising obtaining a thin layer having a thickness inhomogeneity of less than 10% over the entire surface of a glass substrate with sides of 20cm.
10. A device for carrying out the process according to any one of the preceding claims, comprising a crucible equipped with a metal bar in the center of the crucible and capable of receiving an electron beam to radiate heat.
11. Device according to claim 10, wherein the metal bar has a shape among a cylindrical shape, a frustoconical shape and a conical shape.
12. Use of the process according to any one of claims 1 to 9 for the manufacture of a solar cell comprising at least one active layer with photovoltaic properties, on which said thin fullerene layer is deposited by implementing the process.
13. Use according to claim 12, wherein the active layer is perovskite-based.