Treatment for improving the crystalline quality of thin films by hydrogen plasma and polarization

Abstract

The present invention relates to a method for treating thin films made of conductive or semiconductive materials to improve their crystalline quality, which comprises: - provision of a substrate comprising on one of its faces a thin film of said material; and - biased plasma treatment of an assembly formed by a substrate and a thin film at a predetermined temperature and for a predetermined time, the biased plasma treatment comprising an electrical bias of the thin film and exposure of the thin film thus biased to a hydrogen plasma in order to obtain a crystalline reconstruction throughout the depth of the thin film, the biased plasma treatment being carried out at a temperature below the melting point of the thin film and the substrate.

Description

Technical Field

[0001] The present invention relates to the reconstruction of thin-film crystals. The present invention is applicable to the technical fields using thin films, and the characteristics of these thin films are important for required applications such as in microelectronics, optics, engineering, etc. for the manufacture of anti-rust coatings and the like.

Background Art

[0002] In many applications, particularly in the fields of microelectronics, optics and engineering, it is necessary to be able to manufacture thin films of high crystal quality. In the context of the present invention, a thin film is generally stated to mean a layer having a thickness of less than 10 μm, generally between several tens of nanometers and several micrometers.

[0003] As an example of an application requiring a thin film of high crystal quality, mention can be made of a synthetic film of heteroepitaxial diamond (i.e., quasi-single crystal diamond).

[0004] This synthesis is carried out on a silicon substrate covered with a thin layer of strontium titanate (SrTiO3) (30 - 40 nm) on which a thin film of iridium (100 - 200 nm) is deposited. The crystal quality of the diamond film is affected by the quality of the layers constituting the substrate (Ir / SrTiO3 / Si), more specifically the quality of the iridium layer.

[0005] Currently, a thin film of iridium is deposited by physical vapor deposition on a substrate (SrTiO3 / Si) heated to about 700 °C.

[0006] The quality of the thin film of iridium can be improved by thermal annealing, but since the melting point of silicon (1414 °C) is much lower than the melting point of iridium (2410 °C), thermal annealing is not possible.

[0007] It is advantageous that the atoms of the iridium layer can be reconfigured (i.e., recrystallized) at a temperature lower than the melting point of silicon, thereby improving the crystal quality of the iridium layer.

[0008] However, generally, at present, there is no method capable of recrystallizing a material having a melting point exceeding 2000 °C at a temperature below 1000 °C.

[0009] Furthermore, in some cases, an excessively high annealing temperature for the film causes deterioration of the substrate on which the film is deposited. When the thin film is a metal, an interdiffusion phenomenon occurring between the metal thin film and the substrate can be observed, causing the formation of an alloy.

Summary of the Invention

Means for Solving the Problems

[0010] An object of the present invention is to overcome the drawbacks of the prior art. To do this, a method for treating a thin film made of a conductive or semiconductive material for improving the crystal quality of the thin film is proposed. The method includes - supplying a substrate including a thin film of the material on one of the surfaces, - bias plasma treating the assembly formed by the substrate and the thin film at a predetermined temperature and for a predetermined time, wherein the bias plasma treatment includes an electrical bias of the thin film and exposure of the thus-biased thin film to a hydrogen plasma to obtain crystal reconfiguration throughout the depth of the thin film, and the bias plasma treatment is carried out at a temperature below the melting points of the thin film and the substrate, The hydrogen plasma is obtained from a gas containing only hydrogen and optionally helium, the gas lacking a precursor of the conductive or semiconductive material of the film, and the exposure time of the biased film to the hydrogen plasma is at least 10 minutes.

[0011] Since the hydrogen plasma is obtained from a gas containing only hydrogen and optionally helium, the gas does not contain precursors of conductive or semiconductive materials for the film, and thus there is no risk of nucleation on the thin film.

[0012] The crystal quality of the epitaxial thin film can be improved by reducing its mosaicity and / or having better selectivity of its crystal orientation, and / or reducing structural defects (dislocations, macules, etc.). The crystal quality of the polycrystalline thin film can be improved by increasing the particle size and / or reducing the presence of structural defects (dislocations, macules, etc.).

[0013] The reconstruction of the crystals in the thin film can occur over the depth of the thin film spreading from the surface of the thin film.

[0014] The recrystallization of the thin film occurs regardless of whether the film thickness is large or small, and can even protrude across the interface, and a part of the underlying substrate also undergoes recrystallization. Therefore, depending on the film thickness and the processing time, it is possible to obtain partial recrystallization of the film thickness or complete recrystallization of the film, and the recrystallization can extend to the interface between the thin film and the support below it.

[0015] Certain preferred but non-limiting aspects of this method are as follows: - The temperature of the biased plasma treatment is less than 1200 °C. - The electrical bias of the thin film is obtained by applying a negative potential of -10 V to -1000 V with respect to ground (usually with respect to the side wall of the reactor connected to ground) to the film, to the substrate, or to the substrate carrier if the substrate is placed on the substrate carrier. - The time for which the biased film is exposed to the hydrogen plasma is between 10 minutes and several hours. - The biased plasma treatment is carried out at a pressure maintained at a value of 10 - 200 mbar. - The thin film is made of iridium or molybdenum. - The plasma is generated by microwaves of 2.45 GHz, and the power injected into the plasma is 200 to 2000 W. - In the supply step, the surface of the substrate is single crystal, the thin film present on the surface is polycrystalline, and a bias plasma treatment process is carried out until the polycrystalline film becomes an epitaxial film.

[0016] During the step of supplying a substrate including a thin film on one of its surfaces, the thin film is generally deposited on the main surface of the substrate. It is possible to use any deposition method commonly used for depositing thin films, which is generally a deposition method such as chemical vapor deposition (CVD deposition) or physical vapor deposition (PVD deposition).

[0017] To form a hydrogen plasma, it is possible to use any known method. The plasma may be generated using an energy source such as microwaves, radio frequencies, or a thermal filament. In the method according to the present invention, the deposition of the film and the bias plasma treatment (hydrogen plasma + bias) are preferably carried out in the same reactor. In the following exemplary embodiment, an MPCVD (abbreviation for "microwave plasma chemical vapor deposition") microwave reactor with a pressure between 10 and 200 mbar is used. The parameters for forming the hydrogen plasma are those commonly used to clean the iridium surface before the bias nucleation method (bias enhanced nucleation or BEN step) for forming an epitaxial diamond film.

[0018] Advantageously, this method further includes exposing the unbiased film to a hydrogen plasma between the supply step and the bias plasma treatment step, whereby the temperature of the film can be stabilized before the bias is applied. The unbiased film and the biased film are preferably exposed to the same hydrogen plasma, that is, the same conditions of the power reaching the plasma, the gas flow rate, and the pressure.

[0019] The present invention will be better understood by reading the description of exemplary embodiments, given by way of illustration only and not by way of limitation, with reference to the accompanying drawings.

Brief Description of the Drawings

[0020]

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Embodiments for Carrying Out the Invention

[0021] To explain the present invention, several exemplary embodiments in which the thin film is made of iridium or molybdenum will be described in detail here. However, it is clear that the method according to the present invention can be applied to other types of conductive or semiconductive materials.

[0022] In all of the following exemplary embodiments, it is also stated that the following method was followed.

[0023] 7×7mm 2 A substrate of this size is introduced into an MPCVD (abbreviation for "microwave plasma chemical vapor deposition") reactor equipped with a bias system to apply a bias to the substrate.

[0024] First, a pressure of less than 2×10 -5 mbar is achieved and the reactor chamber is evacuated to limit the presence of chemical impurities (nitrogen and oxygen) in the gas phase.

[0025] Next, the substrate is exposed to a hydrogen plasma without bias. This step is optional and can stabilize the surface temperature of the film. The procedure for this optional step is carried out under the following conditions. - Microwave plasma with a power of 600W - Gas flow rate of 250 sccm - Hydrogen pressure of 18 mbar - Time of 20 minutes

[0026] At the end of this step, and continuing from there, the substrate is exposed to a hydrogen plasma with a bias applied according to the following conditions. However, a bias voltage of -280V is applied to the substrate for 1 hour.

[0027] According to the first exemplary embodiment, the deposition of a thin film of polycrystalline iridium with a thickness of about 200 nm is carried out simultaneously by PVD (abbreviation for "physical vapor deposition") on three substrates of SrTiO3(40 nm) / Si(001) in order to obtain the same form on the three substrates. In this way, Samples 1, 2, and 3 are obtained.

[0028] It should be noted that recrystallization is more prominent in polycrystalline films, and due to recrystallization, the particle size increases with a rather dramatic effect. Therefore, the deposition of polycrystalline thin films was selected here. However, recrystallization leading to an improvement in crystal quality also occurs when the thin film is epitaxial or single crystal.

[0029] Sample 1 functions as a reference example.

[0030] Sample 2 is annealed at a temperature of 875 °C for 1 hour under secondary vacuum.

[0031] Sample 3 is subjected to hydrogen plasma treatment combined with bias according to the above conditions. The temperature during this treatment was 850 °C.

[0032] The temperatures of Samples 2 and 3 were measured with a pyrometer adjusted to an emissivity of 19% (emissivity of iridium).

[0033] The first observations of the surface and cross-section performed with a scanning electron microscope (SEM; acceleration voltage 20 kV) were carried out at the end of all the treatments for each of these three samples.

[0034] Figures 1a and 2a are the SEM observation results of the surface and cross-section of the reference sample (Sample 1), respectively. Figures 1b and 2b are the SEM observation results of the surface and cross-section of the annealed sample (Sample 2). Figures 1c and 2c are the SEM observation results of the surface and cross-section of the sample subjected to the treatment according to the present invention (hydrogen plasma combined with bias) (Sample 3).

[0035] In these Figures 1a - 1c and 2a - 2c, the thin film is shown by reference 1 and the substrate is shown by reference 2.

[0036] By comparing these SEM observation results, it can be seen that in Sample 2 annealed under secondary vacuum, the particle size of about 10 nanometers corresponds to the size observed at the start of iridium deposition (Sample 1). On the contrary, in Sample 3 (hydrogen plasma + bias), an enlargement of iridium particles is seen and the particle size is close to 100 nanometers.

[0037] To confirm the recrystallization that occurred in Sample 3, supplementary XRD (X - ray diffraction) measurements were carried out on these three samples. These measurement values are shown in Figures 3a - 3c (Figure 3a: Sample 1, Figure 3b: Sample 2, Figure 3c: Sample 3).

[0038] It can be seen that the three samples have an orientation (111) of the thin film and no trace of the orientation (001).

[0039] The width of the diffraction peak of Ir(111) indicates the refinement when plasma combined with bias was carried out (Sample 3). The full width at half maximum (FWHM) of the Ir(111) peak is as follows. 0.7° for Sample 1 0.43° for Sample 2 0.22° for Sample 3

[0040] In these three samples, the diffraction peak of STO(002) under the iridium thin film, that is, SrTiO3, does not change.

[0041] To complete this proof, it was also shown that the crystal lattice of the iridium film cannot be re - configured by plasma alone (i.e., without bias).

[0042] Accordingly, according to the second exemplary embodiment, first, 160 nm of single-crystalline iridium was deposited on a SrTiO3(40 nm) / Si(001) substrate by electron beam evaporation. Next, a 97 nm thick polycrystalline iridium film was deposited by PVD to obtain Sample 4. For this same Sample 4, the following four consecutive processes were performed. - Annealing at 860 °C for 1 hour under vacuum - Hydrogen plasma treatment (600 W, 250 cc, 18 mbar, 800 °C) for 1 hour - Hydrogen plasma treatment + bias (600 W, 250 cc, 18 mbar, -250 V, 820 °C) for 30 minutes - Hydrogen plasma treatment + bias (600 W, 250 cc, 18 mbar, -280 V, 835 °C) for 1 hour

[0043] Before the first treatment (Figure 4a) and after each treatment (Figures 4b - 4e), by returning to the range, SEM observation (acceleration voltage 20 kV) of the morphology of the iridium film was performed. To show the change in the surface morphology, a band-pass filter was applied to the upper right frame of the SEM image. These SEM images made it possible to clarify that changes occur only after the steps of hydrogen plasma combined with bias (Figures 4d and 4e).

[0044] It can also be seen that after the hydrogen plasma steps combined with bias (Figures 4d and 4e), stripe patterns (striations) can be observed in the <110> direction. These are characteristics of the orientation of the iridium (001) film, indicating the change of the polycrystalline iridium film to a single-crystalline iridium film after the treatments described here. Therefore, the thin film of polycrystalline iridium changed to a thin film of single-crystalline iridium by homoepitaxy by having the underlying single-crystalline iridium layer. Therefore, the method that is the object of the present invention can here create an epitaxial relationship between the deposited thin film and the substrate.

[0045] Furthermore, weighing of Sample 4 with a microbalance after each of the four treatments showed that there was no material loss after these treatments.

[0046] According to the third exemplary embodiment, the same method was adopted to produce thin films of polycrystalline molybdenum (about 120 nm) deposited by PVD on two Si3N4 (20 nm) / Si substrates, and two samples (Sample 5 and Sample 6) were obtained.

[0047] Sample 5 was subjected to the method according to the present invention (hydrogen plasma + bias) (gas 100% H2, 600 W, 250 cc, 18 mbar, -280 V) for 1 hour. The temperature during this method was measured using a pyrometer with the same emissivity as before fixed at 19% (emissivity of iridium, not molybdenum). The measured temperature (930 °C) is "hypothetical" (or can be said to be incorrect) because it does not correspond to the emissivity of molybdenum (which is unknown in this temperature range). However, the experimental conditions for generating the plasma are the same as those applied to the exemplary embodiment using iridium, and the temperature during this method is assumed to be equivalent to the temperature of iridium.

[0048] From this hypothetical temperature, it can be seen that in any case, to compare with annealing, it is necessary to heat to a hypothetical temperature above 930 °C. Therefore, annealing under secondary vacuum at a hypothetical temperature of 942 °C for 1 hour was carried out on Sample 6.

[0049] SEM observations (20 kV) by returning to the surface area of the molybdenum film were performed on each of these two samples. Figures 5a and 6a show the morphology of the initial polycrystalline molybdenum films of Sample 5 and Sample 6, respectively. Figures 5b and 6b show the morphology of the molybdenum films after hydrogen plasma treatment combined with bias (Sample 5) and after annealing (Sample 6), respectively.

[0050] By comparing these various figures, the recrystallization of molybdenum becomes apparent. Also, since the particle size was about 10 - 20 nm (Figures 5a and 6a) before this method and became several hundred nanometers (Figure 5b) after treatment, the effect is quite dramatic.

[0051] According to the fourth exemplary embodiment, the treatment according to the present invention was carried out on an epitaxial iridium film having a thickness of about 109 nm deposited by electron beam evaporation on a SrTiO3(40 nm) / Si(001) substrate.

[0052] SEM observations (20 kV) and XRD measurements before and after this treatment showed an improvement in the crystal quality of the film (Figs. 7a (before) and 7b (after) are SEM observations, and Figs. 8a (before) and 8b (after) are XRD measurements).

[0053] In Figs. 8a and 8b, the upper graph is a general graph, and the other graphs represent an enlarged view of a specific part of the general graphs of Ir(111) (central graph) and Ir(002) (lower graph).

[0054] The Ir(002) diffraction peak was used for these analyses, but the (001) orientation will be described below. These two orientations form part of the same plane family.

[0055] By analyzing the XRD measurement values, it is shown that the orientations (001) and (111) coexist at a ratio of 65.2% for Ir(002) or Ir(001) and 34.8% for Ir(111) before the application of the method according to the present invention. After the application of the method according to the present invention (H2 plasma treatment combined with bias), the ratio of the orientation (001) increases to 74.4% and the orientation (111) decreases to 25.6%. By analyzing these measurement values, it is clearly understood that the H2 plasma treatment combined with bias affects the crystal quality of iridium.

[0056] In conclusion, the SEM observations and XRD measurements performed on the above exemplary embodiments make it possible to reveal the recrystallization of thin films of iridium and molybdenum by the use of hydrogen plasma combined with bias as compared to annealing carried out under secondary vacuum. Therefore, it has been shown that the method according to the present invention causes a reconstruction of the crystal lattice of the nanocrystalline films of iridium and molybdenum.

[0057] One of the advantages of the method which is an object of the present invention is that the hydrogen plasma treatment combined with the bias of the substrate is carried out at a temperature much lower than the melting points of the thin films (2410 °C and 2617 °C for iridium and molybdenum, respectively), i.e., less than 1000 °C in the case of iridium and molybdenum. Therefore, the rearrangement of iridium and molybdenum atoms cannot be explained by the addition of annealing thermal energy, but can be explained by different mechanisms (such as ion bombardment by H+ ions, diffusion of hydrogen, chemical reactivity between hydrogen and iridium and molybdenum atoms, etc.).

[0058] Therefore, the method according to the present invention makes it possible to improve the crystalline quality of thin films at relatively low processing temperatures (between 500 °C and 1000 °C), which are considerably lower than the normal temperatures used during "standard" thermal annealing, and can be used according to the melting point of the selected substrate. Some materials, especially high melting point metals (>2000 °C), have high melting points, and it is actually known that thermal annealing methods commonly used for the reconstruction of crystal lattices are not applicable when these metals are deposited in the form of thin films on substrates whose melting points are significantly different from those of the high melting point metals.

[0059] Another advantage of this method is the fact that the crystal reconstruction may occur from the surface of the thin film and over a limited depth of the thin film, depending on the experimental conditions applied. Therefore, if the thin film is thick enough, the underlying substrate is not affected by this reconstruction.

[0060] It is also possible to imagine having a sufficiently thin layer in order to obtain a lattice reconstruction close to the interface, in order to promote the diffusion of elements close to the interface or to promote the epitaxial relationship between the substrate and the film. By promoting the epitaxial relationship between the substrate and the film, there is a new method for synthesizing an epitaxial film (the method according to the present invention for reconstructing a polycrystalline film in order to deposit a polycrystal on a single crystal and then form an epitaxial film with the substrate).

Claims

1. A processing method for improving the crystal quality of a thin film made of a conductive material or a semiconductive material, comprising: - Supplying a substrate including a thin film of the conductive material or the semiconductive material on one of its surfaces; - Bias plasma treating the substrate and the assembly formed by the thin film at a predetermined temperature for a predetermined time, wherein the bias plasma treatment includes an electrical bias of the thin film and exposure of the thus biased thin film to a hydrogen plasma to obtain crystal reconfiguration throughout the depth of the thin film, and the bias plasma treatment is carried out at a temperature below the melting points of the thin film and the substrate; The hydrogen plasma is obtained from a gas containing only hydrogen and optionally helium, the gas lacking a precursor of the conductive material or the semiconductive material of the thin film, and the exposure time of the biased film to the hydrogen plasma is from 10 minutes to several hours; A processing method, wherein the thin film is made of iridium or molybdenum.

2. The method according to claim 1, wherein the temperature of the bias plasma treatment is less than 1200°C.

3. The method according to claim 1 or 2, wherein the electrical bias of the thin film is obtained by applying a negative potential of -10 V to -1000 V with respect to ground to the thin film, to the substrate, or to the substrate carrier if the substrate is disposed on the substrate carrier.

4. The method according to any one of claims 1 to 3, wherein the step of bias plasma treatment is carried out at a pressure maintained at a value between 10 and 200 mbar.

5. The method according to any one of claims 1 to 4, wherein the plasma is generated by microwaves of 2.45 GHz, and the power injected into the plasma is 200 to 2000 W.

6. In the supplying step, the surface of the substrate is a single crystal, the thin film present on the surface is polycrystalline, and the step of bias plasma treatment is carried out until the polycrystalline film becomes an epitaxial film. The method according to any one of claims 1 to 5.

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