OBTAINING NANOCRYSTALLINE ZINC FILMS BY MAGNETRON SPUTTERING
OBTAINING NANOCRYSTALLINE ZINC FILMS BY MAGNETRON SPUTTERING
Lisa Dudaeva
Master, Higher School of Physics and Materials Technology Peter the Great Saint Petersburg University,
Russia, Saint Petersburg
Alexandr Semencha
Director, Higher School of Physics and Materials Technology Associate Professor, Department of Applied Chemistry Peter the Great Saint Petersburg University,
Russia, Saint Petersburg
As is known, zinc sulfide can be obtained in the gas phase by the interaction of a zinc-containing salt [1], pure zinc and a sulfur-containing substance. [2,3] In our work, a method of interaction of metallic zinc, previously deposited on a quartz substrate by magnetron sputtering, was proposed, and the gas phase consisted of vapors of pure sulfur entering the reaction zone in a high purity inert gas current. Thus, ZnS films were obtained by chemical vacuum deposition in several stages.
1) The first stage was the selection and preparation of substrates for spraying samples: quartz substrates were thoroughly washed in sulfuric acid, then repeatedly in distilled water, after which they were dried in a vacuum drying cabinet at a temperature of 120 degrees.
Two types of substrates were chosen – quartz and potassium bromide [2] (in this case, the substrate was washed only in ethyl alcohol due to its hygroscopicity).
2) The second stage was the analysis of existing installations and the design of a laboratory suitable for the specified purposes. The comparative results of the authors' publications on similar topics have a high level of experimental achievements, but they have a high cost of equipment, increased complexity and duration of the process. The optimal solution was to use a combined method. Preliminary deposition of zinc on a magnetron installation [3,4], then sulfidization of the zinc substrate in sulfur gas [4].
3) Selection of technological parameters of the zinc sulfidization process in a gas-phase reactor.
1. Zinc deposition technology.
The first step in obtaining ZnS was the use of the PWD (physical vacuum deposition) method for obtaining a thin zinc film on a magnetron installation, which was the link for further development of the technology for obtaining thin-film zinc sulfide, a two-zone laboratory installation operating at low pressures with control of the inert gas flow rate was developed.
The zinc deposition technology occurred as follows:
The deposition was carried out by magnetron deposition on an experimental installation in Fig.1, the design of the installation is shown in Fig.2.
Figure 1. Magnetron installation
Figure 2. Prototype of the magnetron sputtering installation
Main blocks: 1 – BUIP RRG, 2 – vacuum meter AV3599, 3 – RRG, 4 – KITP-5, 5 – diffusion pump, 6 – forvacuum pump, 7 – vaporizer vacuum input unit (magnetron), 8 – table rotation control unit, 9 – magnetron power supply unit (BPM), 10 - three–way valve, 11 – working chamber, 12 – chamber valve, 13 – diffusion pump valve, 14 – frame, 15 - working chamber cover [4,5]
The principle of operation of the prototype magnetron installation in Figure 1 is based on spraying the target material onto the substrate (product) (Figure 2). To ensure the operation of the magnetron, it is necessary to create a reduced pressure - vacuum, by pumping air out of the working chamber pos. 11. For the primary pumping of air from the ambient pressure (101,325 Pa) to a pressure acceptable for starting the diffusion pump pos. 5 (15 Pa), a pre-vacuum pump pos. 6 and the valve system pos.10, 12, 13. The maximum pressure that the pre-vacuum pump can pump out is 4 *10-1 Pa at a pumping speed of 4.4 l/s. The maximum pressure that the diffusion pump can pump out is 6.6*10-5 Pa at a pumping speed of 340 l/s. The air pressure required in the working chamber to ensure high purity of the coating is 5 *10-2. When this pressure is reached, argon can be injected into the chamber. The intake of argon must be strictly controlled, otherwise, the pressure in the system can be greatly exceeded, which will negatively affect the diffusion pump. In the PU, the argon intake is made thanks to the RFG pos. 3, which is controlled by the RFG pos. 1 BUIP. BUIP RRG allows you to adjust the gas flow with an accuracy of 0.01 l /h. The required argon flow rate is selected based on the optimal spray pressure, which, on average, is 1 Pa. The pressure control in the working chamber is carried out thanks to the vacuum gauge pos. 2. [6,7]
When the working pressure is reached, the rotation speed of the table is set using the table rotation control unit pos.8. Having set the required speed, the spraying modes are set on the BP pos. 10, figure.3.
The installation is controlled according to the following scheme in Fig.3., and the installation screen is shown in Fig.4.
Figure 3. Screen of the magnetron installation control screen
Figure 4. Magnetron installation screen
In industry, the method of magnetron sputtering of metal is quite popular due to the high accuracy of layer thickness control and acceptable uniformity. [7]
For our process, a vacuum of 10-3 mmHg was used, which can be created by a pump system: the forevacuum is paired with a diffusion of a given performance. Unlike resistive heating, the magnetron sputtering method is more progressive and has a number of advantages. In practice, the process of spraying Er and Zn onto substrates in the chamber of a magnetron installation is shown in Fig. 5.
Figure 5. Application of materials to the substrate by magnetron sputtering
The application of thin layers of material by magnetron sputtering (MN) is based on knocking out the atoms of the magnetron target material. Knocking out is carried out during the impact action by argon ions formed and maintained by a high-voltage discharge.[8]
Magnetron sputtering is realized using a working medium - argon, which is fed into a vacuum chamber. The targets are made of zinc with a purity of 99.95% of the base metal, with a thickness of 6 mm and a diameter of 80 mm.
2. Spraying results
A photo of a thin zinc film on a quartz substrate is shown in Fig. 6. The choice of the type of substrate is due to their features.[9]
Figure 6. Photo of a sample with a zinc coating
The kinetics of zinc growth from the deposition time was calculated at constant parameters of vacuum, power, distance to the substrate. A graph illustrating the change in film thickness from the time of application is shown in Fig. 1. with the installation parameters: P = 0.4 kW, the distance to the substrate is 80 mm, the pressure is 2.2 * 10-2 mm Hg, the working medium is argon, HF grades.
Figure 7. Dependence of the thickness of the zinc film on the deposition time with linear approximation
3. Methods of film research
3.1. Fourier spectroscopy.
The removal of the transmission spectra of the glasses was carried out on the FSM - 1201.
The main element of the optical scheme of the Fourier spectrometer is a two-beam Michelson interferometer consisting of a translucent beam splitter and two flat mirrors. The Fourier spectrometer allows obtaining information about the spectral composition of IR radiation and, consequently, about the optical properties of the studied samples.[10]
The scheme for obtaining spectra is shown in Fig. 8.
The radiation from the emitter falls on the translucent surface of the beam splitter and splits into two beams. After reflection from the corresponding mirrors of the interferometer, the radiation of the two beams is added to the beam splitter and sent to the detector, which converts it into an electrical signal. If one of the mirrors of a two-beam Michelson interferometer is moved, then the optical path for the corresponding beam will change, and at the receiving point the radiation intensity will change due to the interference of beams reflected from a moving and stationary mirror. [10]
Figure 8. The scheme of obtaining spectra [10]
The samples were obtained by applying to a substrate of potassium bromide (Fig. 9.) at an increase in temperature.
Figure 9. Photo plates of potassium bromide with coating
Optical spectroscopy was performed on a Fourier spectrometer - a two–beam Michelson interferometer.
The X-ray phase analysis was performed on an X-ray analytical microprobe-the RAM-30µ microscope, which is designed for the study of objects by methods of local elemental microanalysis with the possibility of micro-mapping, transmission radiography and optical microscopy. [9,10]
4. Experimental results
4.1. IR spectrometry
Fig. 10. shows the absorption spectra of samples 2 (spectrum 2) and a sample of zinc film after exposure to air for more than a day.
Figure 10. Absorption spectra of samples
The decoding of the spectra gives the following results, in the green graph (spectrum 1), the peak of 612 cm-1 corresponds to the valence vibrations of the S-O group, and the peak of 1009 cm-1 corresponds to the valence vibrations of the SO2 group, which means that the surface of zinc sulfide had some acidification and contained SO2 on the surface.
Decoding of the red graph (spectrum 2) shows that the peak of 3450 cm-1 corresponds to the valence vibrations of the O-H group, 1485 and 1376 cm-1 are deformation vibrations of O-H, the band in the region of 851 cm-1 is valence vibrations of Zn-OH, vibrations in the region of 1571 cm-1 most likely correspond to for the Zn-O valence oscillation. As a result, it can be stated that under the conditions of deposition of metallic zinc on hygroscopic potassium bromide there is a catalytic effect on zinc, which leads to accelerated hydrolysis of the latter and the transformation of metallic zinc into its hydroxide, which must be taken into account when conducting experiments with hygroscopic substrates.
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