rus
TS_pub
technospheramag
technospheramag
ТЕХНОСФЕРА_РИЦ
Issue #1/2024
A.V.Smirnov, O.P.Chernova, A.A.Terentyev
SYNTHESIS AND STUDY OF ELECTRICAL CONDUCTIVITY OF COPPER FILMS OBTAINED BY COPPER OXIDE REDUCTION BY CATHODIC SPUTTERING IN HYDROGEN ATMOSPHERE FOR PREPARING OF FILM ELECTRODES WITH VARIABLE CAPACITIES
SYNTHESIS AND STUDY OF ELECTRICAL CONDUCTIVITY OF COPPER FILMS OBTAINED BY COPPER OXIDE REDUCTION BY CATHODIC SPUTTERING IN HYDROGEN ATMOSPHERE FOR PREPARING OF FILM ELECTRODES WITH VARIABLE CAPACITIES
DOI: 10.22184/1993-8578.2024.17.1.18.24
The peculiarities of copper films synthesis by cathode sputtering are considered. These films were oxidized and further reduced the oxide films back to the metal phase. The reduction was carried out by bombardment with copper atoms in a hydrogen atmosphere. This method of copper metal films producing provides better electrical conductivity. Surface resistivity was measured using the four-probe method. UV-vis transmission and absorption spectra have been studied. Practical applications of the identified dependencies are proposed.
The peculiarities of copper films synthesis by cathode sputtering are considered. These films were oxidized and further reduced the oxide films back to the metal phase. The reduction was carried out by bombardment with copper atoms in a hydrogen atmosphere. This method of copper metal films producing provides better electrical conductivity. Surface resistivity was measured using the four-probe method. UV-vis transmission and absorption spectra have been studied. Practical applications of the identified dependencies are proposed.
Теги: cathodic reduction copper film electrodes hydrogen synthesis thin film systems водород катодное восстановление медь пленочные электроды синтез тонкопленочные системы
INTRODUCTION
Thin conductive films are used in the microelectronics industry and for nanoelectronics devices. Copper is an accessible and relatively inexpensive metal, so it is convenient to use, but better applications require studying properties of this metal, especially its thin films. In this paper, we will study the methods of producing conductive and non-conductive films. How to make the film most conductive, and what affects the increase or decrease of conductivity? The object of the study is a copper and copper oxide films. Copper and other conductive metallic thin films (silver, nickel, etc.) are studied with the purpose to make on their basis the film electrodes of various elements for purposes of flexible micro- and nanoelectronics, in particular for the preparing of variable capacitances.
RESEARCH METHODS
Samples of copper-based film electrodes were produced on vacuum units UVR-3M by cathodic sputtering. Thermal annealing was carried out in a programmable furnace MIMP-2. UV-vis spectrophotometer Perkin Elmer Lambda 25 was used to take transmission and absorption spectra. To determine the specific surface resistivity, the studied samples were placed on the object stage, and, carefully lowering the measuring head to avoid mechanical damage to the probes and the surface under study, currents and voltage were automatically selected. Measurement of specific surface resistivity was performed by a precision four-probe method on the "RMS-EL-Z" unit.
RESULTS AND DISCUSSION
Four copper films were deposited on the vacuum unit UVR-3M by cathodic sputtering [1]. Preliminary substrates were subjected to ion cleaning in glow discharge (bombardment with argon ions). The first series of films was obtained, and their images are shown in Fig.1.
The sputtering was performed on K8 cover glasses washed with technical alcohol and fixed in a substrate holder. The substrate holder was fixed in the object holder. Copper plates of 99% purity were placed on the cathode so that it was completely covered with them.
Cathode sputtering was carried out in argon atmosphere at a pressure of 10–11 Pa. The voltage between the cathode and anode was 2 kV. The sputtering time was 20 min [10] and film thickness was ~ 100 nm. The obtained films did not conduct electric current. Then one film of series 1 were heat treated in an oven in air atmosphere at 400 °C for 30 minutes, the second film was heat treated at this regime twice. The films remained without electrical conductivity for 1 and 2 annealed sample. After this, the copper film was annealed at 550 °C for 30 minutes. The specific surface resistivity was measured using the four-probe method. A low conductivity appeared and electrical resistivity was 42.6 MOhm/square.
Next, cathodic reduction of film 1 (annealed twice) and film 2 (not annealed, considered as a support) was carried out. For this purpose, the cathode was completely covered with a copper plate, and film 1 and film 2 were placed on top of them (Fig.2). Reduction was carried out in a hydrogen atmosphere. The pressure was ~ 100 Pa (U = 0.30 V).
The voltage between the cathode and anode was between 0.8–1 kV. The reduction lasted for 12 minutes. The films were reduced to the metallic phase. The films acquired a dark grey colour and metallic lustre. Both films showed good conductivity (Fig.3).
The original reduced film 2 had a resistance of 23.5 Ohm/square, which is 2.4 times higher than annealed reduced film resistance (Fig.3). As we can see, the annealed reduced film has a lower resistance than the original reduced film. The same regularity is observed for annealed and original films without reduction.
The sputtering of copper films of the second series was carried out in argon atmosphere at a pressure of 12 Pa. The voltage between the cathode and anode was 2 kV. The sputtering time was 30 minutes. Thickness of the obtained films was ~ 300 nm. This is due to a more saturated brown colour than the 1st generation films.
The reduction was carried out in a hydrogen atmosphere. Pressure was 0.29 V. The voltage between the cathode and anode was in the range of 0.8–1 kV. The reduction time was 20 minutes. The reduction time was extended compared to the reduction time of the 1st generation films (12 minutes). The films had an almost black colour. The annealed restored film no. 1 had a brown colour. The annealed film did not recover completely, i.e. not all hydrogen atoms replaced oxygen atoms. Only the upper layers were recovered. However, the original film recovered completely. This may be explained by annealing the films and it is more difficult for hydrogen atoms to diffuse into its deep layers. The original film has an amorphous structure and the annealed film has a crystalline structure. On cooling, the substance crystallises and decrease in resistance after annealing may also be due to the appearance of a crystalline lattice.
After reduction, the films gained lustre and ability to reflect light very well. Both films became conductive for the first time. Before reduction, films no. 1 and 2 were non-conductive. The surface resistivity was measured by the four-probe method. Resistance of the reduced annealed film no. 1 was equal to 4.1 Ohm/square.
The original reduced film no. 2 resistance was 290.2 Ohm/square. The initial reduced film resistance is 71 times higher than resistance of the annealed reduced film. The same pattern was observed for films of series no. 1 – annealed reduced films have lower resistance.
Films of series no. 3 were synthesised similarly to films of series no. 1 and no. 2. Cathodic sputtering time was ~ 25 min. Thickness of films of 3 series was ~ 200 nm. The graph of change in electrical resistivity of heat treated films of series no. 3 at 400 °C for 20 min is presented in Fig.4. It can be seen from Fig.4 that films resistance is time stable (no degradation).
Film no. 1 was annealed in a dental oven. Annealing was carried out similarly at 450 °C for 30 minutes. The oven was preheated from 407 to 450 °C at a rate of 10 degrees/min for 4 minutes. The film turned a reddish shade of brown (Fig.5).
The spectra of obtained films were measured – initial, after annealing, reduced initial, reduced annealed. The transmission spectra of the film after annealing in the short-wave region are lower than the spectrum of the film without annealing, and in the long-wave region – higher. These two spectra intersect at a wavelength equal to 653.6 nm. The results are presented in Fig.6 and Fig.7.
Absorption spectra for film 1 before and after annealing were also plotted.
The absorption spectra of the annealed films are placed above the spectra of the original films to the left of the isobestic point, and below to the right. In our case, the wavelength of the isobestic point is 653.6 nm. This arrangement of spectra is characteristic for samples of copper films [2].
CONCLUSIONS
The films made of Se, Ag+Se, Ag (reference) were synthesised by thermoresistive deposition in vacuum. Transmission spectra were plotted. It is applicable in various technical processes in the field of micro- and nano electronics. Annealing of the films reduces films resistance due to crystallisation. The film melts, then crystallises. It becomes dense. The original film without annealing is amorphous, it has air cavities between clusters, and air does not conduct current, so the film is not conductive. So, after crystallisation the film becomes solid, as a result of this process the annealed reduced films have many times lower resistances than the original reduced ones.
ACKNOWLEDGMENTS
This work was supported by the grant of the Russian Science Foundation № 23-29-10211 and the Chuvash Republic of Russia, https://rscf.ru/project/23-29-10211/
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.
Thin conductive films are used in the microelectronics industry and for nanoelectronics devices. Copper is an accessible and relatively inexpensive metal, so it is convenient to use, but better applications require studying properties of this metal, especially its thin films. In this paper, we will study the methods of producing conductive and non-conductive films. How to make the film most conductive, and what affects the increase or decrease of conductivity? The object of the study is a copper and copper oxide films. Copper and other conductive metallic thin films (silver, nickel, etc.) are studied with the purpose to make on their basis the film electrodes of various elements for purposes of flexible micro- and nanoelectronics, in particular for the preparing of variable capacitances.
RESEARCH METHODS
Samples of copper-based film electrodes were produced on vacuum units UVR-3M by cathodic sputtering. Thermal annealing was carried out in a programmable furnace MIMP-2. UV-vis spectrophotometer Perkin Elmer Lambda 25 was used to take transmission and absorption spectra. To determine the specific surface resistivity, the studied samples were placed on the object stage, and, carefully lowering the measuring head to avoid mechanical damage to the probes and the surface under study, currents and voltage were automatically selected. Measurement of specific surface resistivity was performed by a precision four-probe method on the "RMS-EL-Z" unit.
RESULTS AND DISCUSSION
Four copper films were deposited on the vacuum unit UVR-3M by cathodic sputtering [1]. Preliminary substrates were subjected to ion cleaning in glow discharge (bombardment with argon ions). The first series of films was obtained, and their images are shown in Fig.1.
The sputtering was performed on K8 cover glasses washed with technical alcohol and fixed in a substrate holder. The substrate holder was fixed in the object holder. Copper plates of 99% purity were placed on the cathode so that it was completely covered with them.
Cathode sputtering was carried out in argon atmosphere at a pressure of 10–11 Pa. The voltage between the cathode and anode was 2 kV. The sputtering time was 20 min [10] and film thickness was ~ 100 nm. The obtained films did not conduct electric current. Then one film of series 1 were heat treated in an oven in air atmosphere at 400 °C for 30 minutes, the second film was heat treated at this regime twice. The films remained without electrical conductivity for 1 and 2 annealed sample. After this, the copper film was annealed at 550 °C for 30 minutes. The specific surface resistivity was measured using the four-probe method. A low conductivity appeared and electrical resistivity was 42.6 MOhm/square.
Next, cathodic reduction of film 1 (annealed twice) and film 2 (not annealed, considered as a support) was carried out. For this purpose, the cathode was completely covered with a copper plate, and film 1 and film 2 were placed on top of them (Fig.2). Reduction was carried out in a hydrogen atmosphere. The pressure was ~ 100 Pa (U = 0.30 V).
The voltage between the cathode and anode was between 0.8–1 kV. The reduction lasted for 12 minutes. The films were reduced to the metallic phase. The films acquired a dark grey colour and metallic lustre. Both films showed good conductivity (Fig.3).
The original reduced film 2 had a resistance of 23.5 Ohm/square, which is 2.4 times higher than annealed reduced film resistance (Fig.3). As we can see, the annealed reduced film has a lower resistance than the original reduced film. The same regularity is observed for annealed and original films without reduction.
The sputtering of copper films of the second series was carried out in argon atmosphere at a pressure of 12 Pa. The voltage between the cathode and anode was 2 kV. The sputtering time was 30 minutes. Thickness of the obtained films was ~ 300 nm. This is due to a more saturated brown colour than the 1st generation films.
The reduction was carried out in a hydrogen atmosphere. Pressure was 0.29 V. The voltage between the cathode and anode was in the range of 0.8–1 kV. The reduction time was 20 minutes. The reduction time was extended compared to the reduction time of the 1st generation films (12 minutes). The films had an almost black colour. The annealed restored film no. 1 had a brown colour. The annealed film did not recover completely, i.e. not all hydrogen atoms replaced oxygen atoms. Only the upper layers were recovered. However, the original film recovered completely. This may be explained by annealing the films and it is more difficult for hydrogen atoms to diffuse into its deep layers. The original film has an amorphous structure and the annealed film has a crystalline structure. On cooling, the substance crystallises and decrease in resistance after annealing may also be due to the appearance of a crystalline lattice.
After reduction, the films gained lustre and ability to reflect light very well. Both films became conductive for the first time. Before reduction, films no. 1 and 2 were non-conductive. The surface resistivity was measured by the four-probe method. Resistance of the reduced annealed film no. 1 was equal to 4.1 Ohm/square.
The original reduced film no. 2 resistance was 290.2 Ohm/square. The initial reduced film resistance is 71 times higher than resistance of the annealed reduced film. The same pattern was observed for films of series no. 1 – annealed reduced films have lower resistance.
Films of series no. 3 were synthesised similarly to films of series no. 1 and no. 2. Cathodic sputtering time was ~ 25 min. Thickness of films of 3 series was ~ 200 nm. The graph of change in electrical resistivity of heat treated films of series no. 3 at 400 °C for 20 min is presented in Fig.4. It can be seen from Fig.4 that films resistance is time stable (no degradation).
Film no. 1 was annealed in a dental oven. Annealing was carried out similarly at 450 °C for 30 minutes. The oven was preheated from 407 to 450 °C at a rate of 10 degrees/min for 4 minutes. The film turned a reddish shade of brown (Fig.5).
The spectra of obtained films were measured – initial, after annealing, reduced initial, reduced annealed. The transmission spectra of the film after annealing in the short-wave region are lower than the spectrum of the film without annealing, and in the long-wave region – higher. These two spectra intersect at a wavelength equal to 653.6 nm. The results are presented in Fig.6 and Fig.7.
Absorption spectra for film 1 before and after annealing were also plotted.
The absorption spectra of the annealed films are placed above the spectra of the original films to the left of the isobestic point, and below to the right. In our case, the wavelength of the isobestic point is 653.6 nm. This arrangement of spectra is characteristic for samples of copper films [2].
CONCLUSIONS
The films made of Se, Ag+Se, Ag (reference) were synthesised by thermoresistive deposition in vacuum. Transmission spectra were plotted. It is applicable in various technical processes in the field of micro- and nano electronics. Annealing of the films reduces films resistance due to crystallisation. The film melts, then crystallises. It becomes dense. The original film without annealing is amorphous, it has air cavities between clusters, and air does not conduct current, so the film is not conductive. So, after crystallisation the film becomes solid, as a result of this process the annealed reduced films have many times lower resistances than the original reduced ones.
ACKNOWLEDGMENTS
This work was supported by the grant of the Russian Science Foundation № 23-29-10211 and the Chuvash Republic of Russia, https://rscf.ru/project/23-29-10211/
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.
Readers feedback
