rus
Issue #2/2023
A.V.Smirnov
DETERMINATION OF THIN METAL FILMS THICKNESS BY INDIRECT METHOD USING THE INTERFERENCE EFFECT
DETERMINATION OF THIN METAL FILMS THICKNESS BY INDIRECT METHOD USING THE INTERFERENCE EFFECT
DOI: https://doi.org/10.22184/1993-8578.2023.16.2.124.129
Methods for determining of thin metal films thicknesses are considered. Thin films of amorphous selenium, silver and silver layers on a selenium film are synthesized. Optical transmission spectra were taken on a UV-spectrophotometer. A technique of thin metal films preparing based on the interference effect on certain thicknesses of selenium films is proposed. The appropriate calculations have been made.
Methods for determining of thin metal films thicknesses are considered. Thin films of amorphous selenium, silver and silver layers on a selenium film are synthesized. Optical transmission spectra were taken on a UV-spectrophotometer. A technique of thin metal films preparing based on the interference effect on certain thicknesses of selenium films is proposed. The appropriate calculations have been made.
Теги: interference maxima metal films selenium films silver spectrophotometry thin-film systems интерференционные максимумы пленки металлов пленки селена серебро спектрофотометрия тонкопленочные системы
INTRODUCTION
Thin films are thin layers of material ranging in thickness from fractions of a nanometre (multi-atom layer) to a few microns. Interference of light is redistribution of light intensity because of superposition of several coherent light waves. Modern trends in thin-film technology and semiconductor industry are inevitably leading to decrease in size of the structures. This leads to increased requirements for analytical tools to control the parameters of layered structures during their production: layer composition, crystalline perfection of materials and, especially, their geometrical characteristics – layer thicknesses. This paper considers the use of the interference effect in film structures thickness estimation.
RESEARCH METHODS
Film thickness measurements during vacuum deposition process are controlled using a quartz resonator.
Film thickness measurements in microelectronics after deposition stage are measured using the following methods:
ellipsometric method;
step measurement (sputtering-substrate interface) on an atomic force microscope;
X-ray photoelectron spectroscopy and reflectometry;
interference methods;
various spectrophotometric methods, etc.
The analytical apparatus developed based on these methods makes it possible to measure film thicknesses from units (or even less than) nanometres to hundreds of nanometres (and more). A disadvantage of these methods is their expensiveness and relative technological complexity.
The ellipsometric method of study is as follows: a plane-polarised wave is incident on the sample in question, which after reflection becomes in general an elliptically polarised wave. Parameters of the polarisation ellipse, i.e. the orientation of its axes and eccentricity, are determined by the optical properties of the reflecting structure and the angle of light incidence. In the experiment, the ratio of complex reflection coefficients for two types of light wave polarisation is measured: in the plane of incidence (p) and perpendicular to it (s).
Under certain conditions, reflection of light from a film structure is accompanied by an interference effect, which can be used to measure thickness of its layers. For interference to occur, the incident radiation must be reflected not only from the surface layer, but also from its boundary with the substrate. This means that the outer layer must be transparent in the wavelength range in use and the optical constants of the layer in that spectral range must differ from those of the substrate.
RESULTS AND DISCUSSION
Selenium is a grey, metallic, brittle non-metal (Fig.1). At atmospheric pressure, there are several dozens of modifications of selenium. The most stable is grey selenium, γ-Se, with a hexagonal lattice (a = 0.436388 nm, c = 0.495935 nm). Melting point is 221 °C, boiling point is 685 °C, density is equal to 4.807 kg/dm3. Density of liquid selenium at 221 °C is 4.06 kg/dm3. Grey selenium is obtained from other forms by prolonged heating and slow cooling of selenium melt or vapour. Its structure consists of parallel helical chains.
The experimental samples were amorphous selenium films, selenium films and silver deposition films. Sputtering was carried out on UVR-3M glass substrates in a vacuum apparatus at a pressure of 10–2–10–3 Pa. The substrates were preliminarily subjected to ion cleaning in a glow discharge (argon ions). The working diagram of the installation is shown in Fig.2.
A Lambda 25 UV/Vis spectrophotometer was used in our experiment. The spectrophotometer and data acquisition was carried out by means of a personal computer equipped with the UV WinLab software. In this device as sources of radiation deuterium lamp and halogen tungsten lamp is used, which allowed to examine the samples in the wavelength range from 190 to 1100 nm [1, 2]. Figures 4–7 show transmittance spectra of amorphous Se, Ag and Ag+Se films.
Note that the interference effect with corresponding interference maxima is observed in selenium films. Due to the dependence of refractive index on wavelength the interference maxima in the Ag+Se films spectra are shifted as compared to those of pure Se films.
Determination of the order of the maximum:
λ2m=2dn(λ2) ( λ1m(m+1))/n(λ1) = λ2m/n(λ2)
λ1m(m+1)=2dn(λ1) m= λ1/(n(λ1)/ /(n(λ1)/(n(λ2))·λ2-1
Determination of Se film thickness:
λ2=739,51 nm
λ3=946,07 nm
n(λ2)=2,385+168385/λ22=2,385+168385/546875,04=2,693
n(λ3)=2,385+168385/λ23=2,385+168385/895048,45=2,573
m= λ2/(n(λ2)/(n(λ3)·/(λ3-λ2)=739,51/216,2=3,42
d=mλ2/2n(λ2)=3,42·739,51/2·2,693=468,4 nm
Determination of Ag+Se thickness:
λ2=725,66 nm
λ3=932,41 nm
d=mλ2/2n(λ2)=4·725/2·2,8=517,9 nm
Calculations of Ag film thickness by subtraction method: d(Ag)=d(AgSe)-d(Se); d(Ag)≈50 nm –calculated; d(Ag)≈54 nm is obtained from the calibration diagram.
CONCLUSIONS
Reference Se, Ag+Se, and Ag films were produced by thermoresistive vacuum deposition. Transmission spectra of these films were obtained. From the interference maxima the Se and AgSe films thicknesses were calculated. By subtraction of film thicknesses, the value of silver films thickness was determined. The proposed technique can be used as a new indirect method for determining thin film thicknesses of different metals and is relatively simple and inexpensive. It can be applied in various technological processes in the field of micro- and nanoelectronics.
ACKNOWLEDGMENTS
This work was supported by Russian Science Foundation Regional Competition Grant 23-29-10211 "Development of a System for Remote Control of Vehicle Tyre Pressure".
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 films are thin layers of material ranging in thickness from fractions of a nanometre (multi-atom layer) to a few microns. Interference of light is redistribution of light intensity because of superposition of several coherent light waves. Modern trends in thin-film technology and semiconductor industry are inevitably leading to decrease in size of the structures. This leads to increased requirements for analytical tools to control the parameters of layered structures during their production: layer composition, crystalline perfection of materials and, especially, their geometrical characteristics – layer thicknesses. This paper considers the use of the interference effect in film structures thickness estimation.
RESEARCH METHODS
Film thickness measurements during vacuum deposition process are controlled using a quartz resonator.
Film thickness measurements in microelectronics after deposition stage are measured using the following methods:
ellipsometric method;
step measurement (sputtering-substrate interface) on an atomic force microscope;
X-ray photoelectron spectroscopy and reflectometry;
interference methods;
various spectrophotometric methods, etc.
The analytical apparatus developed based on these methods makes it possible to measure film thicknesses from units (or even less than) nanometres to hundreds of nanometres (and more). A disadvantage of these methods is their expensiveness and relative technological complexity.
The ellipsometric method of study is as follows: a plane-polarised wave is incident on the sample in question, which after reflection becomes in general an elliptically polarised wave. Parameters of the polarisation ellipse, i.e. the orientation of its axes and eccentricity, are determined by the optical properties of the reflecting structure and the angle of light incidence. In the experiment, the ratio of complex reflection coefficients for two types of light wave polarisation is measured: in the plane of incidence (p) and perpendicular to it (s).
Under certain conditions, reflection of light from a film structure is accompanied by an interference effect, which can be used to measure thickness of its layers. For interference to occur, the incident radiation must be reflected not only from the surface layer, but also from its boundary with the substrate. This means that the outer layer must be transparent in the wavelength range in use and the optical constants of the layer in that spectral range must differ from those of the substrate.
RESULTS AND DISCUSSION
Selenium is a grey, metallic, brittle non-metal (Fig.1). At atmospheric pressure, there are several dozens of modifications of selenium. The most stable is grey selenium, γ-Se, with a hexagonal lattice (a = 0.436388 nm, c = 0.495935 nm). Melting point is 221 °C, boiling point is 685 °C, density is equal to 4.807 kg/dm3. Density of liquid selenium at 221 °C is 4.06 kg/dm3. Grey selenium is obtained from other forms by prolonged heating and slow cooling of selenium melt or vapour. Its structure consists of parallel helical chains.
The experimental samples were amorphous selenium films, selenium films and silver deposition films. Sputtering was carried out on UVR-3M glass substrates in a vacuum apparatus at a pressure of 10–2–10–3 Pa. The substrates were preliminarily subjected to ion cleaning in a glow discharge (argon ions). The working diagram of the installation is shown in Fig.2.
A Lambda 25 UV/Vis spectrophotometer was used in our experiment. The spectrophotometer and data acquisition was carried out by means of a personal computer equipped with the UV WinLab software. In this device as sources of radiation deuterium lamp and halogen tungsten lamp is used, which allowed to examine the samples in the wavelength range from 190 to 1100 nm [1, 2]. Figures 4–7 show transmittance spectra of amorphous Se, Ag and Ag+Se films.
Note that the interference effect with corresponding interference maxima is observed in selenium films. Due to the dependence of refractive index on wavelength the interference maxima in the Ag+Se films spectra are shifted as compared to those of pure Se films.
Determination of the order of the maximum:
λ2m=2dn(λ2) ( λ1m(m+1))/n(λ1) = λ2m/n(λ2)
λ1m(m+1)=2dn(λ1) m= λ1/(n(λ1)/ /(n(λ1)/(n(λ2))·λ2-1
Determination of Se film thickness:
λ2=739,51 nm
λ3=946,07 nm
n(λ2)=2,385+168385/λ22=2,385+168385/546875,04=2,693
n(λ3)=2,385+168385/λ23=2,385+168385/895048,45=2,573
m= λ2/(n(λ2)/(n(λ3)·/(λ3-λ2)=739,51/216,2=3,42
d=mλ2/2n(λ2)=3,42·739,51/2·2,693=468,4 nm
Determination of Ag+Se thickness:
λ2=725,66 nm
λ3=932,41 nm
d=mλ2/2n(λ2)=4·725/2·2,8=517,9 nm
Calculations of Ag film thickness by subtraction method: d(Ag)=d(AgSe)-d(Se); d(Ag)≈50 nm –calculated; d(Ag)≈54 nm is obtained from the calibration diagram.
CONCLUSIONS
Reference Se, Ag+Se, and Ag films were produced by thermoresistive vacuum deposition. Transmission spectra of these films were obtained. From the interference maxima the Se and AgSe films thicknesses were calculated. By subtraction of film thicknesses, the value of silver films thickness was determined. The proposed technique can be used as a new indirect method for determining thin film thicknesses of different metals and is relatively simple and inexpensive. It can be applied in various technological processes in the field of micro- and nanoelectronics.
ACKNOWLEDGMENTS
This work was supported by Russian Science Foundation Regional Competition Grant 23-29-10211 "Development of a System for Remote Control of Vehicle Tyre Pressure".
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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