rus
Issue #6/2023
A. M. Tarasov, D. V. Novikov, D. V. Gorelov, S. S. Generalov, V. V. Amelichev
Formation of Blackened Aluminium by Vacuum-Thermal Evaporation for IR Emitters
Formation of Blackened Aluminium by Vacuum-Thermal Evaporation for IR Emitters
DOI: 10.22184/1993-7296.FRos.2023.17.6.488.496
Formation of Blackened Aluminium by Vacuum-Thermal Evaporation for IR Emitters
A. M. Tarasov 1, D. V. Novikov 1, D. V. Gorelov 2, S. S. Generalov 2, V. V. Amelichev 2
National Research University of Electronic Technology (MIET), Zelenograd, Moscow, Russia
Scientific-Manufacturing Complex “Technological Centre”, Zelenograd, Moscow, Russia
The creation of materials, such as black coatings, operating in the near and middle IR ranges, is a task of current scientific interest. Layers of such materials can be used to improve the efficiency of IR emitters. One of the perspective materials that is well consistent with MEMS technology is a black alumina coating. This article presents the fabrication results of a black aluminum oxide layer by vacuum-thermal evaporation method, and the study of its absorption features in the IR region of the spectrum. Experimental samples of black alumina layers showed absorption in the range of 2,2 to 28 um at 84% and showed high temperature stability up to 800 °C.
Key words: vacuum thermal evaporation, black aluminum, FTIR spectroscopy, black coatings
Article received: 10.09.2023
Article accepted: 25.09.2023
Introduction
Materials with low reflection coefficient and high absorption of incident radiation in the infrared (IR) region of the spectrum are widely used in many fields, especially in thermophotoelectric systems [1, 2], radiation cooling systems [3], stealth technology [4], etc. Functional materials used in IR emitters are the subjects of a special interest. The emission features of such materials in the IR spectrum range are due to their heating and in such application, the selectivity of the radiated spectrum is an important parameter.
One of the interesting and promising materials is black alumina, obtained by the method of thermal evaporation in low vacuum conditions. This technique is compatible with microelectronics technology, which reduces production costs and increases its processability. By vacuum-thermal evaporation (VTI) it is possible to obtain layers of black aluminum on any substrates, including dielectric ones, without using additional sublayers. This feature advantageously distinguishes VTI in comparison with electrochemical methods, where a conductive layer (catalyst) is needed to form functional layers. In addition, electrolyte contamination and structural changes to the near-surface layers of the substrate are excluded. In result, the layers have high stability, low reflectance, and high absorption in the IR region of the spectrum.
To increase the efficiency of IR emitters, materials should have a high absorption coefficient in the required spectrum range. This article [5] presents the production of an absolute black body. Researchers obtained layers of black platinum by electrochemical synthesis method. Black platinum has a high absorption coefficient over a wide range of wavelengths, including the IR region of the spectrum. Another example is study [6], which demonstrates a multilayer system consisting of Ag / ZnS / Si / Ag / Si layers. Such system selectively absorbs radiation in the range of 3 to 5 um and 8 to 14 um with re-radiation in the range of 5 to 8 um.
In this article, we are presenting the procedure for forming layers of black aluminum by vacuum-thermal evaporation. We obtained the dependence of the layer thickness on the amount of applied material along with the material thermal stability study and the dependence of the absorption of the IR spectrum of incident radiation on the layer thickness.
Methods of formation and measurement
The formation of a layer of black aluminum was carried out by vacuum-thermal evaporation (VTI) at the plant URM‑3279011. Tungsten rods 10 cm long and 2 mm thick were used as an evaporator. Electrodes were fixed close to each other in water-cooled current leads. Granular aluminum with 99.99% purity was used as evaporable material. A 100 mg sample of aluminum was placed in the center of the evaporator. The formation of the black aluminum layer was carried out on a single-crystal silicon plate (100) of one-sided polishing with a SiO2 layer with a thickness of about 600 nm. The substrate holder with samples was fixed above the center of the evaporators at a distance of 30 cm. The process of evaporation of the suspension was carried out at the residual pressure in the chamber equal to 5×10–2 Torr. The URM‑3279011 diagram is shown in Figure 1.
Sample absorption was measured on an FTIR spectrometer FSN‑2201 (Infraspeck, Russia) with a mirror image attachment with an incident angle of 30° in the range from 390 to 4500 cm‑1 in 0,1 cm‑1 increments. To obtain a single spectrum 10 measurements were taken. Before each measurement, the comparison spectrum was shot using a gold mirror. The gold mirror was chosen as the “reference” mirror for IR radiation.
The thermal stability of the layers was investigated by heating in a muffle furnace in an air atmosphere. The absorption of the samples before and after heating was measured. The samples were heated to 100 °C at a rate of 600 °C/h, then the temperature was maintained for 10 minutes and one sample was withdrawn from the batch. The remaining samples afterwords placed back in the muffle furnace and heated to 200 °C. The cycle was repeated to a temperature of 800 °C inclusive.
The surface morphology of the samples was examined using a JEOL JSM 6010plus/la (JEOL Ltd, Japan) scanning electron microscope. The beam current was 21 pA, accelerating a voltage of 5 kV.
Results and discussions
The study of absorption of the IR spectrum with black aluminum was carried out on two groups of samples. In the first group of samples, black aluminum was formed on the polished side of a silicon wafer. In the second group, the formation of the layer was carried out on the unpolished side. For each group, 6 samples were produced, differing in the number of sputtering. Sputtering was carried out sequentially equal in weight to aluminum. As the number of sputtering increases, the thickness of the layer increases. In this case we see, that the dependence is linear dependence. Figure 2 shows the SEM images of the resulting layers after one and three sputtering 225 and 780 nm thick, respectively. The images show that the resulting layers have a highly developed surface morphology.
Figure 3 shows the reflection spectra of a series of samples on the smooth side of the silicon wafer. It can be seen that as the thickness of the layer increases, the total average absorption increases. The greatest decrease in the intensity of the reflected IR spectrum is observed after the fourth sputtering in the range from 1850 to 4500 cm‑1. The presence of peaks in the ranges from 1300 to 1900 cm‑1 and from 3700 to 3600 cm‑1 can be explained by adsorbed gases (CO2) and water (OH-groups) on the developed surface of the absorbing layer of samples [7]. Peaks in the region of 2200–2400 cm‑1 are characteristic of CO2 in the air.
The spectra of reflected IR radiation obtained on black aluminum formed on the unpolished surface of the silicon substrate are shown in Figure 4.
The main difference from a series of samples on a polished substrate is the significantly lower intensity of reflected radiation. At the same time, after the third application of the black aluminum layer, the increase in absorption occurs slightly. It is important to note the absorption of samples on the unpolished surface of IR radiation in the long-wave region of the spectrum. This can be seen from the plot of the 800 cm‑1 spectrum reflection versus the amount of evaporation on the polished and non-polished surface of the substrates (Figure 5).
After experiments with heating samples of black aluminum in air, a change in the color of the sample was observed when the temperature reached more than 400 °C. The difference in color between the unheated sample and the sample that was heated to 800 °C is clearly seen (Figure 6). The sample, which was not heated, has a characteristic black color, which indicates good absorption in the visible range of the spectrum. After the sample is heated, the black aluminum film becomes gray.
A study of the effect of the heating temperature (Figure 7) showed that the absorption spectra varied slightly at temperatures from 100 to 300 °C. Above 400 °C, a decrease in absorption was observed in the range of 1000 cm‑1 to 2850 cm‑1. Further, with increasing heating temperature, there was an increase in absorption in the long wave region (350 to 1900 cm‑1) and an increase in reflection in the near IR spectrum. Such a sharp change in the absorption pattern of the IR spectrum may be due to either a phase change [8] or a change in the morphology of the sample.
Figure 8 shows the SEM images of layers before and after heating to 500 °C. As can be seen from the figure, no change in surface morphology is observed, which may suggest a change in alumina phase. In this case, such a transition can have a positive effect due to an increase in absorption in the region of medium IR radiation.
Conclusion
In the course of the studies, the obtained results allow us to conclude that the suitability of the layer of black aluminum created by vacuum-thermal evaporation as an emission coating in IR emitters is possible. This technology makes it achievable to obtain layers of different thickness in the range from 0,2 um to 10 um during the formation of IR emitters. The layers of black aluminum showed a fairly good level of intensity absorption of the middle IR range (84%) and high temperature stability. Further research will be aimed at studying the emission properties of films of black aluminum formed on a microheater made using MEMS technology.
Acknowledgements
This article was prepared with the financial support of the Ministry of Education and Science of the Russian Federation as part of the implementation of the FNRM‑2022-0009 Research and Development Program.
AUTHORS
A. M. Tarasov, engineer Institute of PMT National Research University of Electronic Technology (MIET), Zelenograd, Moscow, Russia.
ORCID: 0000-0003-3648-8717
D. V. Novikov, engineer Institute of PMT National Research University of Electronic Technology (MIET), Zelenograd, Moscow, Russia.
ORCID: 0000-0002-9518-1208
D. V. Gorelov, head of Research Laboratory “Integrated optical microsystems” Scientific-Manufacturing Complex “Technological Centre”, Zelenograd, Moscow, Russia.
ORCID: 0000-0002-0887-9406
S. S. Genaralov, head of Research Laboratory “Nano and microsystem technology” Scientific-Manufacturing Complex “Technological Centre”, Zelenograd, Moscow, Russia.
ORCID: 0000-0002-7455-7800
V. V. Amelichev, Cand.of Sc. (Eng.), head of the Microstem Technology Department Scientific-Manufacturing Complex “Technological Centre”, Zelenograd, Moscow, Russia.
ORCID: 0000-0002-4204-2626
Author contributions
The article was prepared on the basis of the work of all members of the author’s team: A. M. Tarasov – conducting experiments, measurements, analysis of collected data, processing and discussion of results; D. V. Novikov – conducting experiments, measurements, analysis of collected data, processing and discussion of results; D. V. Gorelov – organization of work, search and translation of foreign sources, discussion of results; S. S. Generalov – organization of work, discussion of results; V. V. Amelichev – organization of work, discussion of results
Conflict of Interest
The authors declare that there is no conflict of interest. All authors participated in the writing of the manuscript in agreed terms of the contribution of each of them to the research and in line with the full text of the manuscript.
A. M. Tarasov 1, D. V. Novikov 1, D. V. Gorelov 2, S. S. Generalov 2, V. V. Amelichev 2
National Research University of Electronic Technology (MIET), Zelenograd, Moscow, Russia
Scientific-Manufacturing Complex “Technological Centre”, Zelenograd, Moscow, Russia
The creation of materials, such as black coatings, operating in the near and middle IR ranges, is a task of current scientific interest. Layers of such materials can be used to improve the efficiency of IR emitters. One of the perspective materials that is well consistent with MEMS technology is a black alumina coating. This article presents the fabrication results of a black aluminum oxide layer by vacuum-thermal evaporation method, and the study of its absorption features in the IR region of the spectrum. Experimental samples of black alumina layers showed absorption in the range of 2,2 to 28 um at 84% and showed high temperature stability up to 800 °C.
Key words: vacuum thermal evaporation, black aluminum, FTIR spectroscopy, black coatings
Article received: 10.09.2023
Article accepted: 25.09.2023
Introduction
Materials with low reflection coefficient and high absorption of incident radiation in the infrared (IR) region of the spectrum are widely used in many fields, especially in thermophotoelectric systems [1, 2], radiation cooling systems [3], stealth technology [4], etc. Functional materials used in IR emitters are the subjects of a special interest. The emission features of such materials in the IR spectrum range are due to their heating and in such application, the selectivity of the radiated spectrum is an important parameter.
One of the interesting and promising materials is black alumina, obtained by the method of thermal evaporation in low vacuum conditions. This technique is compatible with microelectronics technology, which reduces production costs and increases its processability. By vacuum-thermal evaporation (VTI) it is possible to obtain layers of black aluminum on any substrates, including dielectric ones, without using additional sublayers. This feature advantageously distinguishes VTI in comparison with electrochemical methods, where a conductive layer (catalyst) is needed to form functional layers. In addition, electrolyte contamination and structural changes to the near-surface layers of the substrate are excluded. In result, the layers have high stability, low reflectance, and high absorption in the IR region of the spectrum.
To increase the efficiency of IR emitters, materials should have a high absorption coefficient in the required spectrum range. This article [5] presents the production of an absolute black body. Researchers obtained layers of black platinum by electrochemical synthesis method. Black platinum has a high absorption coefficient over a wide range of wavelengths, including the IR region of the spectrum. Another example is study [6], which demonstrates a multilayer system consisting of Ag / ZnS / Si / Ag / Si layers. Such system selectively absorbs radiation in the range of 3 to 5 um and 8 to 14 um with re-radiation in the range of 5 to 8 um.
In this article, we are presenting the procedure for forming layers of black aluminum by vacuum-thermal evaporation. We obtained the dependence of the layer thickness on the amount of applied material along with the material thermal stability study and the dependence of the absorption of the IR spectrum of incident radiation on the layer thickness.
Methods of formation and measurement
The formation of a layer of black aluminum was carried out by vacuum-thermal evaporation (VTI) at the plant URM‑3279011. Tungsten rods 10 cm long and 2 mm thick were used as an evaporator. Electrodes were fixed close to each other in water-cooled current leads. Granular aluminum with 99.99% purity was used as evaporable material. A 100 mg sample of aluminum was placed in the center of the evaporator. The formation of the black aluminum layer was carried out on a single-crystal silicon plate (100) of one-sided polishing with a SiO2 layer with a thickness of about 600 nm. The substrate holder with samples was fixed above the center of the evaporators at a distance of 30 cm. The process of evaporation of the suspension was carried out at the residual pressure in the chamber equal to 5×10–2 Torr. The URM‑3279011 diagram is shown in Figure 1.
Sample absorption was measured on an FTIR spectrometer FSN‑2201 (Infraspeck, Russia) with a mirror image attachment with an incident angle of 30° in the range from 390 to 4500 cm‑1 in 0,1 cm‑1 increments. To obtain a single spectrum 10 measurements were taken. Before each measurement, the comparison spectrum was shot using a gold mirror. The gold mirror was chosen as the “reference” mirror for IR radiation.
The thermal stability of the layers was investigated by heating in a muffle furnace in an air atmosphere. The absorption of the samples before and after heating was measured. The samples were heated to 100 °C at a rate of 600 °C/h, then the temperature was maintained for 10 minutes and one sample was withdrawn from the batch. The remaining samples afterwords placed back in the muffle furnace and heated to 200 °C. The cycle was repeated to a temperature of 800 °C inclusive.
The surface morphology of the samples was examined using a JEOL JSM 6010plus/la (JEOL Ltd, Japan) scanning electron microscope. The beam current was 21 pA, accelerating a voltage of 5 kV.
Results and discussions
The study of absorption of the IR spectrum with black aluminum was carried out on two groups of samples. In the first group of samples, black aluminum was formed on the polished side of a silicon wafer. In the second group, the formation of the layer was carried out on the unpolished side. For each group, 6 samples were produced, differing in the number of sputtering. Sputtering was carried out sequentially equal in weight to aluminum. As the number of sputtering increases, the thickness of the layer increases. In this case we see, that the dependence is linear dependence. Figure 2 shows the SEM images of the resulting layers after one and three sputtering 225 and 780 nm thick, respectively. The images show that the resulting layers have a highly developed surface morphology.
Figure 3 shows the reflection spectra of a series of samples on the smooth side of the silicon wafer. It can be seen that as the thickness of the layer increases, the total average absorption increases. The greatest decrease in the intensity of the reflected IR spectrum is observed after the fourth sputtering in the range from 1850 to 4500 cm‑1. The presence of peaks in the ranges from 1300 to 1900 cm‑1 and from 3700 to 3600 cm‑1 can be explained by adsorbed gases (CO2) and water (OH-groups) on the developed surface of the absorbing layer of samples [7]. Peaks in the region of 2200–2400 cm‑1 are characteristic of CO2 in the air.
The spectra of reflected IR radiation obtained on black aluminum formed on the unpolished surface of the silicon substrate are shown in Figure 4.
The main difference from a series of samples on a polished substrate is the significantly lower intensity of reflected radiation. At the same time, after the third application of the black aluminum layer, the increase in absorption occurs slightly. It is important to note the absorption of samples on the unpolished surface of IR radiation in the long-wave region of the spectrum. This can be seen from the plot of the 800 cm‑1 spectrum reflection versus the amount of evaporation on the polished and non-polished surface of the substrates (Figure 5).
After experiments with heating samples of black aluminum in air, a change in the color of the sample was observed when the temperature reached more than 400 °C. The difference in color between the unheated sample and the sample that was heated to 800 °C is clearly seen (Figure 6). The sample, which was not heated, has a characteristic black color, which indicates good absorption in the visible range of the spectrum. After the sample is heated, the black aluminum film becomes gray.
A study of the effect of the heating temperature (Figure 7) showed that the absorption spectra varied slightly at temperatures from 100 to 300 °C. Above 400 °C, a decrease in absorption was observed in the range of 1000 cm‑1 to 2850 cm‑1. Further, with increasing heating temperature, there was an increase in absorption in the long wave region (350 to 1900 cm‑1) and an increase in reflection in the near IR spectrum. Such a sharp change in the absorption pattern of the IR spectrum may be due to either a phase change [8] or a change in the morphology of the sample.
Figure 8 shows the SEM images of layers before and after heating to 500 °C. As can be seen from the figure, no change in surface morphology is observed, which may suggest a change in alumina phase. In this case, such a transition can have a positive effect due to an increase in absorption in the region of medium IR radiation.
Conclusion
In the course of the studies, the obtained results allow us to conclude that the suitability of the layer of black aluminum created by vacuum-thermal evaporation as an emission coating in IR emitters is possible. This technology makes it achievable to obtain layers of different thickness in the range from 0,2 um to 10 um during the formation of IR emitters. The layers of black aluminum showed a fairly good level of intensity absorption of the middle IR range (84%) and high temperature stability. Further research will be aimed at studying the emission properties of films of black aluminum formed on a microheater made using MEMS technology.
Acknowledgements
This article was prepared with the financial support of the Ministry of Education and Science of the Russian Federation as part of the implementation of the FNRM‑2022-0009 Research and Development Program.
AUTHORS
A. M. Tarasov, engineer Institute of PMT National Research University of Electronic Technology (MIET), Zelenograd, Moscow, Russia.
ORCID: 0000-0003-3648-8717
D. V. Novikov, engineer Institute of PMT National Research University of Electronic Technology (MIET), Zelenograd, Moscow, Russia.
ORCID: 0000-0002-9518-1208
D. V. Gorelov, head of Research Laboratory “Integrated optical microsystems” Scientific-Manufacturing Complex “Technological Centre”, Zelenograd, Moscow, Russia.
ORCID: 0000-0002-0887-9406
S. S. Genaralov, head of Research Laboratory “Nano and microsystem technology” Scientific-Manufacturing Complex “Technological Centre”, Zelenograd, Moscow, Russia.
ORCID: 0000-0002-7455-7800
V. V. Amelichev, Cand.of Sc. (Eng.), head of the Microstem Technology Department Scientific-Manufacturing Complex “Technological Centre”, Zelenograd, Moscow, Russia.
ORCID: 0000-0002-4204-2626
Author contributions
The article was prepared on the basis of the work of all members of the author’s team: A. M. Tarasov – conducting experiments, measurements, analysis of collected data, processing and discussion of results; D. V. Novikov – conducting experiments, measurements, analysis of collected data, processing and discussion of results; D. V. Gorelov – organization of work, search and translation of foreign sources, discussion of results; S. S. Generalov – organization of work, discussion of results; V. V. Amelichev – organization of work, discussion of results
Conflict of Interest
The authors declare that there is no conflict of interest. All authors participated in the writing of the manuscript in agreed terms of the contribution of each of them to the research and in line with the full text of the manuscript.
Readers feedback
