Patent Application
20240304460
The present invention relates to a process for producing a roll with submicrometer thick wall and a membrane of submicrometric thickness of Ga2O3 by ion implantation of a semiconductor monocrystal of Ga2O3 using ion beams, at specific conditions of energy, flux and fluence and subsequent thermal treatment. The process of the invention finds application, for example, in the field of microelectronics, micro-electro-mechanics and photonics.
The process of producing rolls and membranes of submicrometric thickness (10-1000 nm thick) from monocrystalline materials has been extensively studied and developed in recent years due to their technological potential in different applications: electronic, optical and mechanical (Mikayla A. Yoder et al., J. Am. Chem. Soc., 140, 9001-9019 (2018) and Minghuang Huang et al., Nanoscale, 3, 96 (2011)).
Patent application U.S. Pat. No. 6,008,126A refers to a general method for producing integrated circuits and interconnected metallization structures from membranes of dielectric and semiconductor materials. This patent application is an unmistakable example of the potential and growing technological interest in membranes of submicrometric thickness of different materials, insulators, semiconductors and conductors.
Mechanical exfoliation is one of the most used processes to obtain membranes of submicrometric thickness and has become extremely popular, namely in obtaining graphene using conventional adhesive tape. This process does not require any pre-treatment or sacrificial layer; however, it has the disadvantage of not having any control over the thickness and area of the layer obtained. In spite of its simplicity, the fact that this process does not allow, in most cases, to control the thickness or the area of the layer obtained, limits its industrial use on a large scale, since for large-scale production, well-controlled and reproducible processes are required.
A relevant example of the production of monocrystalline material membranes with thickness control is the use of silicon in the development of semiconductor-oxide-semiconductor structures (known as “silicon on insulator-SOI”). This method is reported in patent application U.S. Pat. No. 5,882,987A. In this case, ion implantation is performed using a beam of hydrogen ions (M. Bruel, B. Aspar, and A.-J. Auberton-Hervé, Jpn. J. Appl. Phys., 36, 1636 (1997)) or helium (C. Qian and B. Terreault, J. Appl. Phys., 90, 5152 (2001)) to create a layer with microbubbles of hydrogen or helium at a specific depth determined by the energy of the implanted ions. The formation of these microbubbles induces stress that allows the separation of a thin layer from the surface of the implanted material when the implanted materials are subjected to an adequate thermal treatment. From this method, it is possible to transfer large areas of thin silicon layers to other types of substrates. This method has been explored in other semiconductors such as II-VI semiconductors (C. Miclaus, G. Malouf, SM Johnson, and M S Goorsky, J. Electron. Mat., 34(6), 859 (2005)), III-V seminconductor (S. Hayashi, D. Bruno, and MS Goorsky, Appl. Phys. Lett., 85, 236 (2004)) and germanium (C. Miclaus and M S Goorsky, J. Phys. D: Appl. Phys., 36, A177 (2003)) and more recently in Oxide semiconductors (Michael E. Liao et al, ECS J. Solid State Sci. Technol. 8 P673 (2019)).
In the work of Minghuang et al., 2011, 3, 96, different examples are described for the production of rolls by creating strain between the layer that composes the rolls and their physical support (substrate or sacrificial layer). The main examples are: layers deposited on a substrate with different lattice constants (heteroepitaxial deposition); layers deposited on a substrate or another layer (sacrificial layer) without any epitaxial relationship (non-epitaxial deposition); bi-layers of materials with different thermal expansions. In all these examples, the thin layer obtained and which composes the roll, results from a physical or chemical deposition process. For this reason, their electrical, optical, structural and mechanical properties are strongly conditioned by the fact that they result from a heteroepitaxial or non-epitaxial deposition process.
The U.S. Pat. No. 8,313,966 B2 patent reports a process for producing rolls of different materials for the development of optoelectronic applications. In this particular patent for the formation of Rolls it is necessary to deposit a sacrificial layer on a substrate on which at least one additional layer of material will be deposited, that will self-roll as the sacrificial layer is removed by a chemical etching process. The self-rolling process that occurs with the etching of the sacrificial layer is associated with the relaxation of strain between the sacrificial layer and the surface layer (layer that rolls up producing the rolls) as is also described by Schmidt et al., Physica E 13 969 (2002).
There is a need to provide alternative membrane of submicrometric thickness and roll production processes from a semiconductor monocrystal that eliminate bubble production, the need of a sacrificial layer and the limitation to the use of gas ion beams.
The present invention provides a process for producing membrane of submicrometric thickness of Ga2O3 and roll comprising the steps of:
In a preferred embodiment, the process repeats, in order, steps a) and b).
In a further preferred embodiment, the process further comprises an intermediate step between steps a) and b), wherein the at least one formed roll is transferred to a temperature stable substrate and then heated to the temperature of the thermal treatment of step b). Preferably, the substrate is silicon.
In another preferred embodiment, the process comprises a step prior to step a), wherein at least one additional conductive, semiconducting or insulating layer is deposited on the single crystal of Ga2O3.
In one aspect of the invention, the ion beam is selected from the group comprising Chromium (Cr), Iron (Fe), Nitrogen (N), Carbon (C) and Tungsten (W) ions.
In a preferred embodiment, the heat treatment of step b) is done at an annealing temperature of 700° C. during 30 seconds.
In one embodiment, the ion beam during implantation is in a stationary mode.
In another embodiment, the ion beam during implantation is in a sweeping mode.
The present invention reports a process for producing a membrane of submicrometric thickness and roll of Ga2O3 by ion implantation of a single semiconductor crystal of Ga2O3 in the beta phase, at specific conditions of energy, flux and fluence and subsequent thermal treatment.
In the context of the present description, the term “comprising” is to be understood as “including but not limited to”. As such, this term should not be interpreted as “consisting only of”.
Any X value presented along the present description should be interpreted as an approximate value of the real X value, such approximation to the true value would be reasonably expected by the expert person responsible for the implantation due to experimental and/or measurement conditions that introduce deviations from the real value.
Unless otherwise indicated, the ranges of values presented in the present description are intended to provide a simplified and technically accepted way to indicate each individual value within the respective range. By way of example, the expression “1 to 2” or “between 1 and 2” means any value within this range, for example 1.0; 1.1; 1.2; 1.3; 1.4; 1.5; 1.6; 1.7; 1.8; 1.9; 2.0.
In the context of the present invention, the term “membrane” refers to a material with a submicrometric thickness (10-1000 nm).
In the context of the present invention “thermal treatment” refers to any technical process that applies a temperature equal to or greater than 500° C. on the roll or rolls obtained by the process of the invention.
Still in the context of the present invention, ambient temperature corresponds to a temperature in the range between 15° C. and 25° C.
The process of the present invention comprises the steps of:
Ion implantation is a common technique in the microfabrication industry of silicon-based integrated circuits and serves to incorporate ions of different chemical elements in a matrix material with a well-defined profile. During studies on the potential of the intrinsic properties of chromium ions for the development of optical detectors of ionizing radiation in the red region, by ion implantation of chromium ions in gallium oxide (Ga2O3), surprisingly, the formation of rolls on the surface of a Ga2O3 monocrystal was observed (for implantations performed with low flux, below 2.083×1012 ions/cm2).
During the implantation step of the present invention, ions are accelerated by an applied electric field acquiring a kinetic energy directly proportional to this. The penetration depth of the ions implanted in the single crystal of Ga2O3 depends on its intrinsic properties, namely on its composition and density, but also on the energy of the ion implanted in the studied matrix. In addition, the single crystal of Ga2O3 presents an easy cleavage plane parallel to the surface (100), whereby implantation-induced bending under certain specific energy, fluence and flux conditions unexpectedly creates a breakdown of chemical bonds along the cleavage plane at a certain depth, thus initiating the delamination of a layer of submicrometric thickness, defined between the surface and the plane in which the chemical bonds break, promoting the formation of rolls (
Ga2O3 in the beta phase (β-Ga2O3) is a crystal with a monoclinic crystal structure in which its unit cell is defined by three crystalline axes a, b and c with different lattice parameters 12.03, 3.04 and 5.80 Ångstrom, respectively (1 Ångstrom corresponds to 10-10 m). The angular relationship between a and b and between b and c are 90° and the angular relationship between a and c is 103.7º. The crystalline directions parallel to the crystalline axes a, b and c, applying the nomenclature used in crystallography, are defined as [100], [010] and [001] respectively. The crystal has two easy cleavage planes, the (100) plane and the (001) plane, being the easiest cleavage plane the plane (100).
It was also found that this unexpected effect during implantation in the semiconductor monocrystal of Ga2O3 also occurs when using ions other than Chromium, such as Iron, Nitrogen, Carbon and Tungsten. It was also found that this effect does not dependent on the charge of the implanted ions.
Furthermore, it was also found that the application of a thermal treatment at a temperature equal to or higher than 500° C. promotes the unrolling of the rolls, removing the defects created in the implantation step. It is thus possible to obtain a membrane of submicrometric thickness of Ga2O3 with a thickness that is closely related to the implantation energy and penetration depth of the implanted ion (
The process of this invention eliminates the need for bubble formation and the limitation to implantation with gas ion beams, as well as the need of a sacrificial layer for the formation of the membrane of submicrometric thickness of Ga2O3 and, therefore, does not present the disadvantages associated with those processes enumerated above.
In one embodiment, the process comprises repeating steps a) and b).
In another embodiment of the invention, the process may comprise an intermediate step between steps a) and b) in which the at least one roll obtained is transferred to a substrate stable at the temperature of the thermal treatment of step b). Preferably, the substrate is silicon.
In another embodiment of the invention, the process may comprise a step prior to step a), in which at least one additional conductive, semiconducting or insulating layer is deposited on the single crystal of Ga2O3. Preferably, this layer is a metal or a sequence of layers of different metals that form electrical contacts of an electrical device. Preferably, this layer is a semiconductor or semiconductor structure that forms a semiconductor device on top of the roll or membrane.
The ion beam used in ion implantation step a) may be selected from the group comprising Chromium (Cr), Iron (Fe), Nitrogen (N), Carbon (C) and Tungsten (W) ions. Preferably, the ion beam consists of Chromium ions.
The parameters of step a) of ion implantation, i. e., energy, fluence and flux, are defined as a function of the type of single crystal to be implanted and the chosen ion beam. In particular, the person expert in ion implantation will understand that the ranges described herein for these parameters may be higher or lower depending on the type of single crystal to be implanted and the ion beam chosen. Thus, the person expert in ion implantation will understand that, for example, choosing ions other than those specifically mentioned herein requires adjusting the values of these parameters, the described ranges only serve as a reference values for those other ions.
Also, in case the process comprises the previous step of deposition of a metallic or semiconducting layer described above, the implantation parameters will have to be adjusted considering the energy loss of the ion beam in this layer, as well as the necessary change in the fluence so to change the concentration and profile of defects and thus control the stress profile for the delamination process to occur. The person expert in ion implantation will easily understand the adaptation of parameters as a function of the composition and thickness of the additional layer.
The ions can have any charge state, the parameters of step a) of ion implantation being defined and adjusted by the person expert in ion implantation. In the range of energies comprised by the present invention, the interaction that causes the crystal structure to change is the nuclear interaction (elastic collision) between the implanted ions and the nuclei of the atoms that compose the target material. This interaction is essentially independent of the initial charge state of the ion and the charge state choice will depend on the technical implementation of the ion sources and the implanter as well as the intended ion energy.
The implantation area can be defined by a hard mask or a deposited polymeric mask on the semiconductor monocrystal of Ga2O3. The fact that implantation regions can be defined makes it possible to develop roll patterns on the surface of the single crystal when complemented with physical and chemical patterning and etching techniques.
The implantation of the ion beam is performed at a temperature below 500° C., by making an ion beam strike the surface of a Ga2O3 semiconductor monocrystal along a non-parallel direction to the cleavage plane. In one embodiment, implantation takes place at room temperature. In another embodiment, implantation is performed along the direction normal to the surface. Preferably, the implantation is performed in a direction perpendicular to the cleavage plane of the single crystal, i.e., the implantation is performed near the direction [201] which is the direction perpendicular to the cleavage plane of the single crystal (100).
Fluence is the number of ions implanted per sample area and is given by the flux (number of ions per second reaching a given area of the crystal) times the time (duration of the implantation process):
Fluence(ions/cm2)=flux(ions/cm2·s)*time(s)
Therefore, fluence, flux and implantation time are correlated. For example, a sample implanted with a flux of 1012 ions/cm2·s for 100 s receives a total fluence of 100 s×1012 ions/cm2·s=1014 ions/cm2.
The implantation step of the process of the invention can be in stationary mode or scanning mode.
The steady-state implantation process is shown in
The implantation process in sweeping mode is represented in
The implantation cycle can be followed by a thermal treatment cycle so that the deformation induced during the roll formation can be reversed in the defined area. The process can then be repeated in different successive areas in order to increase the overall size of the membrane.
The great advantage and innovative character of the present invention is the fact that rolls can be directly obtained by self-rolling (4, 4′) from a mono-crystalline material (1), thus guaranteeing their crystalline quality without the need of any additional deposition or sacrificial layers. Furthermore, in the case of the present invention, the self-rolling process that gives rise to the rolls (4, 4′) can be reversed by carrying out a thermal treatment.
The thermal treatment of step b) of the process can be performed by any suitable technique. The temperature at which the thermal treatment takes place must be adequate to remove the defects created in the membrane during implantation as well as during the rolling during the implantation step and, thus, promoting the total unrolling of the same. For the present invention, the thermal treatment must be performed at a temperature equal to or greater than 500° C. The duration of the thermal treatment will depend on the type of treatment selected and performed at greater or lesser speed in order to achieve full membrane unroll.
Preferably, the thermal treatment can be by radiation, convection or thermal diffusion and can be performed in air or in a controlled atmosphere. In a more preferably mode, thermal treatment means thermal annealing, at a temperature of 700° C. for 30 seconds.
For the production of a membrane of submicrometric thickness of Ga2O3, a bulk monocrystal (with a thickness of several hundred micrometers) of β-Ga2O3 was used, having the surface plane (100) perpendicular to the [201] direction and it was implanted at room temperature with an angle of incidence normal to the surface, with a beam of Cr+ and Cr2+ ions with energy in the range of 100-360 keV, flux in the range of 1×1012 ions/cm2·s-2×1012 ions/cm2·s and a fluence of 1×1015 ions/cm2. The formation of rolls on the surface of the monocrystalline material of β-Ga2O3 was observed with walls of submicrometric thickness and diameters of a few tens of micrometers.
Then, the previously formed rolls were transferred to a silicon substrate and annealed by a thermal annealing process with a holding time of 30 seconds at 700° C. and with a heating rate of 1° C. per second. The thermal treatment promotes the unrolling of these rolls, obtaining membranes with different areas from about one square micron to one square millimeter and with a submicrometric thickness, corresponding to the wall thickness of the rolls.
The process described above was repeated with different energy values, in order to produce membranes having different thicknesses, considering that the defect profile that promotes the strain profile depends on the energy of the implanted ions.
The thickness value of the membranes was determined by two different characterization techniques.
The scanning electron microscopy technique was used to characterize the rolls obtained at the end of the implantation step, namely, to determine the thickness of their walls. The thickness of the enrolled membrane does not change with the thermal treatment to which the rolls are subjected. During the thermal treatment only the unrolling of the rolls occurs, resulting in membranes with the same thickness as the original roll walls.
The Rutherford backscattering spectrometry technique was also used, with which the value of the membrane thickness was measured after heat treatment.
Table 1 summarizes the average membrane thickness values for three implantations of Chromium with different energies (180 keV, 250 keV and 360 keV) performed at a fluence of 1×1015 ions/cm2 and a flux of 2.083×1012 ions/cm2·s.
For the production of a membrane of submicrometric thickness of Ga2O3, a bulk monocrystal (with a thickness of several hundred micrometers) of β-Ga2O3 was used, having a surface plane (100) perpendicular to the [201] direction and implanted at room temperature, with a beam of Fet ions with an energy of 180 keV, a flux of 1×1012 ions/cm2·s and a fluence of 8×1014 ions/cm2 and with an angle of incidence normal to the surface. The formation of rolls on the surface of the monocrystalline β-Ga2O3 material was observed. Then, the previously formed rolls were transferred to a silicon substrate and annealed by a thermal annealing process with a holding time of 30 seconds at 700° C. and with a heating rate of 1° C. per second.
The process described above is repeated with different energy values, in order to produce membranes with different thicknesses, considering that the defect profile that promotes the strain profile depends on the energy of the implanted ions.
For the production of a membrane of submicrometric thickness of Ga2O3, a bulk monocrystal (with a thickness of several hundred microns) of β-Ga2O3 was used, having a surface plane (100) perpendicular to the [201] direction and implanted at room temperature, with an N+ ion beam with an energy of 95 keV, a flux of 1×1012 ions/cm2·s and a fluence of 6.3×1012 ions/cm2 and with an angle of incidence normal to the surface. The formation of rolls on the surface of the monocrystalline β-Ga2O3 material was observed. Then, the previously formed rolls were transferred to a silicon substrate and annealed by a thermal annealing process with a holding time of 30 seconds at 700° C. and with a heating rate of 1° C. per second.
The process described above is repeated with different energy values, in order to produce membranes having different thicknesses, considering that the defect profile that promotes the strain profile depends on the energy of the implanted ions.
For the production of a membrane of submicrometric thickness of Ga2O3, a bulk monocrystal (with a thickness of several hundred micrometers) of β-Ga2O3 was used, having a surface plane (100) perpendicular to the [201] direction and implanted at room temperature, with a beam of C+ ions with an energy of 65 keV, a flux of 1×1012 ions/cm2·s and a fluence of 7.5×1012 ions/cm2 and with an angle of incidence normal to the surface. The formation of rolls on the surface of the monocrystalline β-Ga2O3 material was observed. Then, the previously formed rolls were transferred to a silicon substrate and annealed by a thermal annealing process with a holding time of 30 seconds at 700° C. and with a heating rate of 1° C. per second.
The process described above is repeated with different energy values, in order to produce membranes having different thicknesses, considering that the defect profile that promotes the strain profile depends on the energy of the implanted ions.
For the production of a membrane of submicrometric thickness of Ga2O3, a bulk monocrystal (with a thickness of several hundred micrometers) of β-Ga2O3 was used, having a surface plane (100) perpendicular to the [201] direction and implanted at room temperature, with a beam of W2+ ions with an energy of 350 keV, a flux of 1×1012 ions/cm2·s and a fluence of 2.8×1014 ions/cm2 and with an angle of incidence normal to the surface. The formation of rolls on the surface of the monocrystalline β-Ga2O3 material was observed. Then, the previously formed rolls were transferred to a silicon substrate and annealed by a a thermal annealing process with a holding time of 30 seconds at 700° C. and with a heating rate of 1° C. per second.
The process described above is repeated with different energy values, in order to produce membranes having different thicknesses, considering that the defect profile that promotes the strain profile depends on the energy of the implanted ions.
As verified by the previous examples, the process of the present invention is successfully performed using beams of different ions.
In addition, the semiconductor monocrystal to be used in the process of the present invention, as an alternative to Ga2O3, may be any material that presents cleavage planes and in which it is possible to induce a curvature that promotes the exfoliation of a surface layer. The application of the present invention in other materials only requires adjustment of the implantation conditions.
Lisbon, 11 Mar. 2022.
| Number | Date | Country | Kind |
|---|---|---|---|
| 117063 | Feb 2021 | PT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/PT2022/050006 | 2/14/2022 | WO |
