rus
Issue #5/2023
A.I.Arefev, V.N.Gornov, L.E.Peshkicheva, O.S.Yurchenko, L.E.Guss, A.V.Savelyev, A.P.Kotov
FABRICATION OF LASER TARGETS BY VACUUM COATING
FABRICATION OF LASER TARGETS BY VACUUM COATING
INTRODUCTION
At Russian Federal Nuclear Center – All-Russian Research Institute of Technical Physics experiments are conducted to study spectral runs of radiation on high-power laser installations with ultrashort pulse duration. The idea of the experiments is to rapidly heat a thin (0.1–0.4 µm) flat layer of the substance under study (Al, Cu, Au...) with an ultrashort laser pulse and measure the X-ray emission (absorption) spectra of the heated layer. In order to prevent the studied substance layer from evaporating, it is placed in covers made of light refractory material (B, C, CH, Be) with a thickness from 1 to 4 μm, which is fully ionised and transparent to outgoing radiation of the inner buried layer [1]. The B–Al–B design was chosen to work out the fabrication technology of such targets. The targets are a multilayer free-hanging film fixed in a stainless steel holder with a 1 mm diameter hole (Fig.1). The number and variants of layer thicknesses are summarised in Table 1. The works [2, 3] were used to evaluate the necessary variants of targets by layer thicknesses.
DEVELOPMENT OF MULTILAYER THIN-FILM LASER TARGETS MANUFACTURING TECHNOLOGY
Thin targets layers were obtained by magnetron sputtering (for Al) and electron beam evaporation (for B). The NaCl buffer layer was deposited by resistive evaporation.
At the initial stage of technology development, it was decided to deposit a boron cladding layer on a substrate with an applied NaCl buffer layer. This method would allow to obtain a free-hanging boron film on which the aluminium coating and the second boron cladding layer would be subsequently deposited. However, when attempts were made to remove the boron film from the glass substrate using flotation method, complete destruction occurred, presumably due to excessive stress within the film [4].
Therefore, it was decided to apply boron cladding layers on the free-hanging aluminium film. However, at attempts to apply cladding layers on Al with thickness of 0.1 μm and 0.2 μm it was deformed and partially destroyed in the process of boron layer formation due to the stresses arising in the film. The application of cladding layers on a free-hanging Al film with a thickness of 0.4 μm showed that this film thickness allows to apply boron with a thickness of more than 1 μm on each side of the film without violating its integrity.
A disc of KU glass was used as a substrate for target fabrication. The NaCl buffer layer was sputtered resistively on a vacuum sputtering unit UVN-2M. When the working pressure was reached inside the vacuum chamber, salt suspension was evaporated from a box-type evaporator made of molybdenum foil by heating to the salt evaporation temperature. Sputtering of the NaCl buffer layer was performed immediately before Al deposition on the substrate, because during long-term storage of substrates with a sputtered buffer layer, absorption of water molecules from the atmosphere by the NaCl film occurs, leading to deterioration of the surface structure.
Magnetron sputtering of aluminium target on the VUMR-1 unit was used to obtain aluminium film. A substrate with a pre-applied NaCl layer was installed in a metal substrate holder directly above the target sputtering zone. A metal foil mask with holes of 4 mm diameter (target diameter) was installed in front of the substrate. Argon was used as the working gas. To exclude overheating of the applied Al layer, sputtering was carried out in several stages with breaks. The temperature was monitored by a thermocouple sensor attached to the substrate holder.
The final step in the fabrication of free-hanging aluminium film was its removal from the glass substrate and fixation in the holder. For this purpose, the substrate with the film was carefully immersed in distilled water, after dissolution of the buffer layer of salt, the film separated and floated to the surface. It was then pulled out onto one part of a 0.1 mm thick stainless steel holder with a 1 mm hole in the centre. The film was fixed in the holder by clamping it between two parts of the holder with subsequent spot welding.
Thin boron cladding layers were applied on the VU-2M unit by electron-beam method. To fix the target blanks (Al film fixed in a holder) on the substrate holder placed in the chamber of the unit, legs made of 0.8 mm thick nichrome wire were spot welded to them, which were removed after completion of application of boron cladding layers. The substrate holder with targets and witnesses was placed directly above the evaporator. Graphite crucibles with indirect cooling were used as the evaporator. Since boron films are not pure when using carbon evaporators, it is necessary to cool the crucibles to prevent interaction of the crucible material with the evaporated material [5]. Powdered boron (99.99% purity) was used as the vaporised material. To exclude splashing of the material from the crucible in the process of evaporation, its sintering was carried out beforehand. To avoid overheating of the aluminium film, boron sputtering on each side was carried out in several stages with breaks. During the boron deposition process, the sputtering rate and film thickness were monitored with an acousto-optical spectrophotometer AOS-3S.
The boron cladding layers were deposited in two cycles, with the same parameters on each side of the targets. Figure 2 shows an image of the finished B–Al–B target, performed on a MEIJI MC50T microscope with 75x magnification.
Thickness of the obtained aluminium and boron films were measured using witness sensors, which were installed in close proximity to the targets. Glass plates were used as a witness. A metal foil mask was placed in front of the witness. Thus, a step was formed on the uncovered surface. Scanning its height on the FRT MicroSpy Profile profilometer with a vertical resolution of 6 nm, the thickness of the resulting layers was determined. Table 2 summarises the results of layer thickness measurements of the fabricated targets. The measurement error was 1.8% for boron and 2% for aluminium.
The errors of film thickness measurements are calculated by the formula:
, (1)
where tγ, n – 1 is Student’s coefficient.
Calculations were performed for a confidence level of 0.95.
Density of the obtained films was measured by weighing the glass witness before and after sputtering, and calculated according to the formula:
, (2)
where V = π · r2 · h.
The measurement error was 9%. The results of film density measurements are given in Table 2.
The film density measurements errors are calculated by the formula:
. (2)
Immediately prior to sputtering, the substrates were dried to remove moisture from the surface and pores of the glass, which significantly affects onto the initial weight of the substrate and, consequently, the measurement results.
STUDY OF STRUCTURE AND ANALYSIS OF CHEMICAL COMPOSITION OF THIN FILMS AND MATERIALS USED FOR THEIR FORMATION
To determine chemical composition of the samples listed in Table 3, the samples were analysed using a scanning electron microscope equipped with an energy dispersive spectrometer.
To determine chemical elemental composition, a polishing section was made from sample No. 1. A characteristic photograph of the microstructure of the material of sample No.1 and X-ray spectra from the surface of different phases are shown in Fig.3. It can be seen that the sample consists of two phases. In the X-ray spectrum from the surface of the dark phase, lines belonging to boron are registered (spectrum 24); of the light phase – boron, hafnium, titanium (spectrum 25).
The presence of hafnium and titanium impurities in sample No. 1 is due to the possibility of their penetration into the crucible during boron sintering from the elements of the in-chamber fittings of the VU-2M unit.
To determine chemical elemental composition of the material of samples No. 2–3, X-ray spectra were obtained from their surface. The error of the analysis is not normalised. The zone of generation of X-ray radiation during the analysis captures the whole thickness in sample No. 3, in sample No. 2 captures the substrate.
In the X-ray spectrum from the coating surface in sample No. 2, lines belonging to boron, aluminium, magnesium, oxygen, iron, nickel, chromium, titanium, and silicon are registered (spectrum 39, Fig.4). At the same time iron, nickel, chromium, titanium and silicon belong to the substrate material (spectrum 36, Fig.4). It is safe to say that boron, aluminium, oxygen and magnesium belong only to the coating. The presence of aluminium and magnesium in the coating is less than 1% (Table 4).
The view of the target in a stainless steel holder (sample No. 3) and X-ray spectra from the surface of the target are shown in Fig.5. Numbers on the image indicate the areas of spectrum acquisition. The size of the spectrum acquisition area at each point was 70 × 50 µm. The chemical elemental composition of the material of sample No. 3 is given in the summary table 4 (the results are averaged over the data from 5 sites).
Figs 6–7 shows images of the surface of samples No. 2–3. The surface of samples has a pronounced relief, represented by spherical particles. The reflected electron images (contrast by atomic number of chemical element) show that there are no separate inclusions in the structure. The lighter areas on the images are caused by protrusions of structural components.
The secondary electron image of the chip surface (sample No. 4) and X-ray spectra from its surface are shown in Fig.8. The chemical elemental composition of the chips includes aluminium, sodium (Table 4).
CONCLUSIONS
As a result of this work, the multilayer laser targets manufacturing technology with cladding layers made of refractory material has been developed. Trial batches of targets were produced. The manufacturing error of layer thicknesses did not exceed 10%, the error of layer thickness measurements did not exceed 2%, the error of layer density measurements 9%. Chemical analysis of targets and material samples was performed.
The further direction in the development of our work on the fabrication of multilayer B-Al-B laser targets is indicated in the selection of optimal modes and parameters of sputtering, excluding damage to the free-hanging aluminium film due to stresses in the formed boron layers. This problem is especially relevant when the aluminium film thickness is reduced to 100 nm and the thickness of boron cladding layers is increased to 4 µm.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
At Russian Federal Nuclear Center – All-Russian Research Institute of Technical Physics experiments are conducted to study spectral runs of radiation on high-power laser installations with ultrashort pulse duration. The idea of the experiments is to rapidly heat a thin (0.1–0.4 µm) flat layer of the substance under study (Al, Cu, Au...) with an ultrashort laser pulse and measure the X-ray emission (absorption) spectra of the heated layer. In order to prevent the studied substance layer from evaporating, it is placed in covers made of light refractory material (B, C, CH, Be) with a thickness from 1 to 4 μm, which is fully ionised and transparent to outgoing radiation of the inner buried layer [1]. The B–Al–B design was chosen to work out the fabrication technology of such targets. The targets are a multilayer free-hanging film fixed in a stainless steel holder with a 1 mm diameter hole (Fig.1). The number and variants of layer thicknesses are summarised in Table 1. The works [2, 3] were used to evaluate the necessary variants of targets by layer thicknesses.
DEVELOPMENT OF MULTILAYER THIN-FILM LASER TARGETS MANUFACTURING TECHNOLOGY
Thin targets layers were obtained by magnetron sputtering (for Al) and electron beam evaporation (for B). The NaCl buffer layer was deposited by resistive evaporation.
At the initial stage of technology development, it was decided to deposit a boron cladding layer on a substrate with an applied NaCl buffer layer. This method would allow to obtain a free-hanging boron film on which the aluminium coating and the second boron cladding layer would be subsequently deposited. However, when attempts were made to remove the boron film from the glass substrate using flotation method, complete destruction occurred, presumably due to excessive stress within the film [4].
Therefore, it was decided to apply boron cladding layers on the free-hanging aluminium film. However, at attempts to apply cladding layers on Al with thickness of 0.1 μm and 0.2 μm it was deformed and partially destroyed in the process of boron layer formation due to the stresses arising in the film. The application of cladding layers on a free-hanging Al film with a thickness of 0.4 μm showed that this film thickness allows to apply boron with a thickness of more than 1 μm on each side of the film without violating its integrity.
A disc of KU glass was used as a substrate for target fabrication. The NaCl buffer layer was sputtered resistively on a vacuum sputtering unit UVN-2M. When the working pressure was reached inside the vacuum chamber, salt suspension was evaporated from a box-type evaporator made of molybdenum foil by heating to the salt evaporation temperature. Sputtering of the NaCl buffer layer was performed immediately before Al deposition on the substrate, because during long-term storage of substrates with a sputtered buffer layer, absorption of water molecules from the atmosphere by the NaCl film occurs, leading to deterioration of the surface structure.
Magnetron sputtering of aluminium target on the VUMR-1 unit was used to obtain aluminium film. A substrate with a pre-applied NaCl layer was installed in a metal substrate holder directly above the target sputtering zone. A metal foil mask with holes of 4 mm diameter (target diameter) was installed in front of the substrate. Argon was used as the working gas. To exclude overheating of the applied Al layer, sputtering was carried out in several stages with breaks. The temperature was monitored by a thermocouple sensor attached to the substrate holder.
The final step in the fabrication of free-hanging aluminium film was its removal from the glass substrate and fixation in the holder. For this purpose, the substrate with the film was carefully immersed in distilled water, after dissolution of the buffer layer of salt, the film separated and floated to the surface. It was then pulled out onto one part of a 0.1 mm thick stainless steel holder with a 1 mm hole in the centre. The film was fixed in the holder by clamping it between two parts of the holder with subsequent spot welding.
Thin boron cladding layers were applied on the VU-2M unit by electron-beam method. To fix the target blanks (Al film fixed in a holder) on the substrate holder placed in the chamber of the unit, legs made of 0.8 mm thick nichrome wire were spot welded to them, which were removed after completion of application of boron cladding layers. The substrate holder with targets and witnesses was placed directly above the evaporator. Graphite crucibles with indirect cooling were used as the evaporator. Since boron films are not pure when using carbon evaporators, it is necessary to cool the crucibles to prevent interaction of the crucible material with the evaporated material [5]. Powdered boron (99.99% purity) was used as the vaporised material. To exclude splashing of the material from the crucible in the process of evaporation, its sintering was carried out beforehand. To avoid overheating of the aluminium film, boron sputtering on each side was carried out in several stages with breaks. During the boron deposition process, the sputtering rate and film thickness were monitored with an acousto-optical spectrophotometer AOS-3S.
The boron cladding layers were deposited in two cycles, with the same parameters on each side of the targets. Figure 2 shows an image of the finished B–Al–B target, performed on a MEIJI MC50T microscope with 75x magnification.
Thickness of the obtained aluminium and boron films were measured using witness sensors, which were installed in close proximity to the targets. Glass plates were used as a witness. A metal foil mask was placed in front of the witness. Thus, a step was formed on the uncovered surface. Scanning its height on the FRT MicroSpy Profile profilometer with a vertical resolution of 6 nm, the thickness of the resulting layers was determined. Table 2 summarises the results of layer thickness measurements of the fabricated targets. The measurement error was 1.8% for boron and 2% for aluminium.
The errors of film thickness measurements are calculated by the formula:
, (1)
where tγ, n – 1 is Student’s coefficient.
Calculations were performed for a confidence level of 0.95.
Density of the obtained films was measured by weighing the glass witness before and after sputtering, and calculated according to the formula:
, (2)
where V = π · r2 · h.
The measurement error was 9%. The results of film density measurements are given in Table 2.
The film density measurements errors are calculated by the formula:
. (2)
Immediately prior to sputtering, the substrates were dried to remove moisture from the surface and pores of the glass, which significantly affects onto the initial weight of the substrate and, consequently, the measurement results.
STUDY OF STRUCTURE AND ANALYSIS OF CHEMICAL COMPOSITION OF THIN FILMS AND MATERIALS USED FOR THEIR FORMATION
To determine chemical composition of the samples listed in Table 3, the samples were analysed using a scanning electron microscope equipped with an energy dispersive spectrometer.
To determine chemical elemental composition, a polishing section was made from sample No. 1. A characteristic photograph of the microstructure of the material of sample No.1 and X-ray spectra from the surface of different phases are shown in Fig.3. It can be seen that the sample consists of two phases. In the X-ray spectrum from the surface of the dark phase, lines belonging to boron are registered (spectrum 24); of the light phase – boron, hafnium, titanium (spectrum 25).
The presence of hafnium and titanium impurities in sample No. 1 is due to the possibility of their penetration into the crucible during boron sintering from the elements of the in-chamber fittings of the VU-2M unit.
To determine chemical elemental composition of the material of samples No. 2–3, X-ray spectra were obtained from their surface. The error of the analysis is not normalised. The zone of generation of X-ray radiation during the analysis captures the whole thickness in sample No. 3, in sample No. 2 captures the substrate.
In the X-ray spectrum from the coating surface in sample No. 2, lines belonging to boron, aluminium, magnesium, oxygen, iron, nickel, chromium, titanium, and silicon are registered (spectrum 39, Fig.4). At the same time iron, nickel, chromium, titanium and silicon belong to the substrate material (spectrum 36, Fig.4). It is safe to say that boron, aluminium, oxygen and magnesium belong only to the coating. The presence of aluminium and magnesium in the coating is less than 1% (Table 4).
The view of the target in a stainless steel holder (sample No. 3) and X-ray spectra from the surface of the target are shown in Fig.5. Numbers on the image indicate the areas of spectrum acquisition. The size of the spectrum acquisition area at each point was 70 × 50 µm. The chemical elemental composition of the material of sample No. 3 is given in the summary table 4 (the results are averaged over the data from 5 sites).
Figs 6–7 shows images of the surface of samples No. 2–3. The surface of samples has a pronounced relief, represented by spherical particles. The reflected electron images (contrast by atomic number of chemical element) show that there are no separate inclusions in the structure. The lighter areas on the images are caused by protrusions of structural components.
The secondary electron image of the chip surface (sample No. 4) and X-ray spectra from its surface are shown in Fig.8. The chemical elemental composition of the chips includes aluminium, sodium (Table 4).
CONCLUSIONS
As a result of this work, the multilayer laser targets manufacturing technology with cladding layers made of refractory material has been developed. Trial batches of targets were produced. The manufacturing error of layer thicknesses did not exceed 10%, the error of layer thickness measurements did not exceed 2%, the error of layer density measurements 9%. Chemical analysis of targets and material samples was performed.
The further direction in the development of our work on the fabrication of multilayer B-Al-B laser targets is indicated in the selection of optimal modes and parameters of sputtering, excluding damage to the free-hanging aluminium film due to stresses in the formed boron layers. This problem is especially relevant when the aluminium film thickness is reduced to 100 nm and the thickness of boron cladding layers is increased to 4 µm.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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