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Optical grating recording in ChG thin film by electron beam
NOC-16427; No of Pages 3 Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Optical grating recording in ChG thin film by electron beam V. Kolbjonoks a,⁎, V. Gerbreders a, J. Teteris b, A. Gerbreders b a b
Daugavpils University, IMC, Parādes 1, Daugavpils LV5401, Latvia University of Latvia, CFI, Ķengaraga 8, Rīga LV1063, Latvia
a r t i c l e
i n f o
Article history: Received 22 October 2012 Received in revised form 27 December 2012 Available online xxxx Keywords: Chalcogenides; Amorphous thin films; EB lithography
a b s t r a c t Recording capabilities of As\Se\S chalcogenide (ChG) glasses using electron beam (EB) lithography have been investigated in this study. After exposure with EB the thin films of these inorganic glasses can be etched in alkaline amine solvent with high selectivity. High resolution, smooth shaped structures have been fabricated using ChG thin films. Height of developed pattern can be controlled through changing applied electron dose. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In the field of diffractive grating recording the electron beam (EB) pattering has been developed for exposing the resist to constant electron dose at the desired locations. If a dose variation is implemented during the process of recording, the EB resist accumulates different doses under exposure. In the development process this results in different thicknesses of resist, and, correspondingly in the 3D profile of the final structure. The resultant non-planar topography depends on the resist properties, dose distribution, proximity effect, and specific equipment parameters [1,2]. Amorphous ChG glasses compose a well-known group of inorganic materials with a number of remarkable and unique properties. Potential application of ChG glasses was demonstrated in fabrication of the diffractive optical elements (DOEs) with high refractive index and IR transmitting properties [3,4], of photonic-band-gap structures [5–7] and as high-resolution inorganic resists for lithography. The modern EB lithography technique enables realization of increasingly sophisticated DOEs based on the diffraction of light by micro-structured surface. One of the critical aspects of the EB lithography process is selection of a suitable resist material, especially if highly efficient DOEs with a continuous surface profile are to be fabricated. Irradiation of amorphous ChG semiconductor thin films by EB leads to appreciable changes in their properties. These changes are connected with the short-range order modification as a result of chemical bond transformation in the material. The atomic structure variations modify the electronic structure of the disordered system, leading to changes in the film properties. The electron irradiation can change not only the number of physical properties of ChG films, but also their chemical reactivity, e.g. the dissolution rate in various alkaline inorganic and organic ⁎ Corresponding author. Tel.: +371 29791317. E-mail address: [email protected] (V. Kolbjonoks).
solvents [7]. In turn, the dissolution rate depends on the state and chemical composition of ChG films and their etchants. The etching rates of non-exposed and exposed areas of such ChG films are markedly different, and the etching process of the films can be strongly influenced by the presence of surface active substances in the etchant. Our objective in this work was to access the structuring ability of As38S18Se44 ChG glasses under wet etching. We have specifically examined the effect of EB, which can be readily focused to a spot ~10 nm in diameter thus forming a basis for nanostructuring, i.e. possibility to produce DOEs with very high precision. 2. Experimental Amorphous As–S–Se films were obtained by thermal evaporation in vacuum onto a glass substrate. Glass previously covered with FeO and metallic Cr thin layers for better adhesion and electrical conductivity. As–S–Se film thickness (600 nm) measured during evaporation using the thin film interference technique. Chemical composition of freshly made films determined by an Energy-Dispersive X-ray Spectroscopy Microanalysis System is As38S18Se44. The experiments on the surface relief formation were performed using a scanning electron microscope (SEM) with the installed EB lithography system. The lithographic system operates a Gaussian beam to write point-by-point in a raster manner. In order to achieve, for example, a solid exposed line, the exposure points should be close enough to each other so that there is no area/ space between them. The point separation (called the beam step-size) is user-defined and totally dependent on the physical size of the chosen beam. EB diameter (91 nm) calculated and determined by the SEM system, the beam current 271 pA, measured in the SEM chamber before and after exposure using a Faraday cup. For the experiments, non-aqueous amine based organic solvent was used as a negative developer for As38S18Se44. The films were etched using the wet etching technique. The etching rate was determined by
0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.053
Please cite this article as: V. Kolbjonoks, et al., Optical grating recording in ChG thin film by electron beam, J. Non-Cryst. Solids (2013), http:// dx.doi.org/10.1016/j.jnoncrysol.2013.01.053
2
V. Kolbjonoks et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
Fig. 1. Test boxes pattern: each color corresponds to a particular EB dose. Fig. 3. Squares profilometric graph.
monitoring the local thickness of the film by its interference (recorded by a scanning confocal microscope system). The surface roughness control and pattern measurements were performed by atomic force microscope (AFM). The experiments were carried out at room temperature. The test EB exposure was performed on the prepared 600 nm thick sample. The pattern to be exposed was Test Boxes (Fig. 1), size of boxes 30 × 30 μm. The dose factor was in the range 50 to 3050 μC/cm2, with a step of 25 μC/cm2 (in order to determine the resist clearing dose). The EB accelerating voltage was 30 keV. After EB exposure, the As38S18Se44 sample was fixed to the bottom of a Petri dish (working surface upward). The dish was placed on a confocal microscope stage under the objective. While scanning constantly the sample surface with a low-energy laser (λ = 633 nm) to the Petri dish the etchant solution was added to fully cover the sample. The whole etching process was monitored and recorded by the confocal microscope system. The etching rate was determined by monitoring the local thickness of the sample using the thin film interference. The scanned sample surface reflects the light which is then registered by the confocal system detectors. To completely dissolve non-exposed and exposed areas of the sample took 200 and 1800 s, respectively. The etch selectivity, defined as the etching rate ratio of exposed and non-exposed areas of a sample, γ = Vexposed / Vunexposed, was 1/9.
Fig. 2. The etching rate of amorphous As38S18Se44 films vs. EB exposure dose.
The next step was recording of different gratings to establish and demonstrate advantages of ChG as material for fabrication of complex optical structures in a single etching process. Each grating was tested for the diffraction efficiency (DE). The percent of the zero order light beam that can be diffracted to the first order beam. It is determined by the ratio of the power of the diffracted light beam to the incident power of the beam. As the sample is not transparent, before the DE test it was covered with a 40 nm aluminum layer for better light reflection using the magnetron sputtering technique. 3. Results and discussion The As38S18Se44 films obtained in the experiments behave as a negative resist for non-aqueous amine organic developer. The etching rate dependence on the EB-exposure dose for amorphous As38S18Se44 films is presented in Fig. 2. In this figure, three linear segments can be marked (60–140, 150–400 and 410–600 μC/cm 2) as corresponding to the profile height. At point 600 μC/cm2 the curve reaches saturation and does not change with time. Such curve behavior remained invariable in the experiment repeated several times. The exponential shape of the profile depth dependence on the electron dose means that higher EB doses make the films more resistant to
Fig. 4. Resist profile height vs. the electron dose.
Please cite this article as: V. Kolbjonoks, et al., Optical grating recording in ChG thin film by electron beam, J. Non-Cryst. Solids (2013), http:// dx.doi.org/10.1016/j.jnoncrysol.2013.01.053
V. Kolbjonoks et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
a) sinusoidal grating, period = 800 nm Diffraction Efficiency = 20%
b) asymmetrical 90º sawtooth grating, period = 1 µm
3
etching — i.e. more time is needed to dissolve the exposed area. This makes the As38S18Se44 resist useful for recording the multilevel diffractive elements, which was the main goal of our work. Fig. 3 demonstrates the squares height as determined by the intensity and time of irradiation. The ChG resist acts as a negative one, the exposed area of the sample has a lower dissolution rate in the developer than unexposed. As could be seen in Fig. 4, the ratio of the electron dose to the profile height grows very fast up to 1000–1200 μC/cm 2. Then the curve slows down and reaches saturation at 3000 μC/cm2. The most interesting region for our research is from 600 to 2000 μC/cm2, where the electron dose switch leads to a significant change in the profile height of developed pattern. This allows the high-quality 3D pattering to be achieved. The next step mentioned above — i.e. fabrication of different gratings to establish and demonstrate advantages of ChG as material for making complex optical structures in a single etching process — resulted in four types of gratings recorded with different profiles and periods. Fig. 5 presents the AFM graphs of the developed gratings with measured diffraction efficiency. 4. Conclusions
Diffraction Efficiency = 28% The surface relief formation on As38S18Se44 thin films was observed under 90 nm electron beam exposure was observed. The surface relief formation with a depth up to 160 nm by EB lithography in the films was established. The results of our experiments confirm that the ChG glass thin films are good material for the use as high-resolution electron resist. The observed nearly linear dependence of the etch depth on the total electron dose absorbed allows the high-quality 3D pattering to be obtained.
c) asymmetrical 90º sawtooth grating, period = 2 µm Diffraction Efficiency = 38%
References [1] M.L. Trunov, S.N. Dub, P.M. Nagy, S. Kokenyesi, J. Phys. Chem. Solids 68 (2007) 1062, http://dx.doi.org/10.1016/j.jpcs.2007.03.008. [2] A. Kikineshi, V. Palyok, M. Shiplyak, I.A. Szabo, D.L. Beke, J. Optoelectron. Adv. Mater. 2 (2000) 95. [3] A. Csik, M. Malyovanika, J. Dorogovicsa, A. Kikineshia, D.L. Beke, I.A. Szabo, G. Langer, J. Optoelectron. Adv. Mater. 3 (2001) 33. [4] S. Kokenyesia, I. Ivánb, V. Takátsa, J. Pálinkása, S. Birib, I.A. Szaboa, J. Non-Cryst. Solids 353 (2007) 1470, http://dx.doi.org/10.1016/j.jnoncrysol.2006.09.064. [5] A.M. Andriesh, M.S. Iovu, S.D. Shutov, J. Optoelectron. Adv. Mater. 4 (2002) 631. [6] A.V. Rodea, A. Zakeryb, M. Samoca, R.B. Chartersa, E.G. Gamalya, B. Luther-Davies, Appl. Surf. Sci. 197 (2002) 481. [7] M. Frumar, Z. Polak, Z. Cernosek, B. Frumarova, T. Wagner, Chem. Pap. 51 (1997) 310.
d) sinusoidal grating, with different height of each line, period = 200 nm Diffraction Efficiency = 52% Fig. 5. AFM profile height of developed gratings and diffraction efficiency of the grating.
Please cite this article as: V. Kolbjonoks, et al., Optical grating recording in ChG thin film by electron beam, J. Non-Cryst. Solids (2013), http:// dx.doi.org/10.1016/j.jnoncrysol.2013.01.053
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Optical grating recording in ChG thin film by electron beam V. Kolbjonoks a,⁎, V. Gerbreders a, J. Teteris b, A. Gerbreders b a b
Daugavpils University, IMC, Parādes 1, Daugavpils LV5401, Latvia University of Latvia, CFI, Ķengaraga 8, Rīga LV1063, Latvia
a r t i c l e
i n f o
Article history: Received 22 October 2012 Received in revised form 27 December 2012 Available online xxxx Keywords: Chalcogenides; Amorphous thin films; EB lithography
a b s t r a c t Recording capabilities of As\Se\S chalcogenide (ChG) glasses using electron beam (EB) lithography have been investigated in this study. After exposure with EB the thin films of these inorganic glasses can be etched in alkaline amine solvent with high selectivity. High resolution, smooth shaped structures have been fabricated using ChG thin films. Height of developed pattern can be controlled through changing applied electron dose. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In the field of diffractive grating recording the electron beam (EB) pattering has been developed for exposing the resist to constant electron dose at the desired locations. If a dose variation is implemented during the process of recording, the EB resist accumulates different doses under exposure. In the development process this results in different thicknesses of resist, and, correspondingly in the 3D profile of the final structure. The resultant non-planar topography depends on the resist properties, dose distribution, proximity effect, and specific equipment parameters [1,2]. Amorphous ChG glasses compose a well-known group of inorganic materials with a number of remarkable and unique properties. Potential application of ChG glasses was demonstrated in fabrication of the diffractive optical elements (DOEs) with high refractive index and IR transmitting properties [3,4], of photonic-band-gap structures [5–7] and as high-resolution inorganic resists for lithography. The modern EB lithography technique enables realization of increasingly sophisticated DOEs based on the diffraction of light by micro-structured surface. One of the critical aspects of the EB lithography process is selection of a suitable resist material, especially if highly efficient DOEs with a continuous surface profile are to be fabricated. Irradiation of amorphous ChG semiconductor thin films by EB leads to appreciable changes in their properties. These changes are connected with the short-range order modification as a result of chemical bond transformation in the material. The atomic structure variations modify the electronic structure of the disordered system, leading to changes in the film properties. The electron irradiation can change not only the number of physical properties of ChG films, but also their chemical reactivity, e.g. the dissolution rate in various alkaline inorganic and organic ⁎ Corresponding author. Tel.: +371 29791317. E-mail address: [email protected] (V. Kolbjonoks).
solvents [7]. In turn, the dissolution rate depends on the state and chemical composition of ChG films and their etchants. The etching rates of non-exposed and exposed areas of such ChG films are markedly different, and the etching process of the films can be strongly influenced by the presence of surface active substances in the etchant. Our objective in this work was to access the structuring ability of As38S18Se44 ChG glasses under wet etching. We have specifically examined the effect of EB, which can be readily focused to a spot ~10 nm in diameter thus forming a basis for nanostructuring, i.e. possibility to produce DOEs with very high precision. 2. Experimental Amorphous As–S–Se films were obtained by thermal evaporation in vacuum onto a glass substrate. Glass previously covered with FeO and metallic Cr thin layers for better adhesion and electrical conductivity. As–S–Se film thickness (600 nm) measured during evaporation using the thin film interference technique. Chemical composition of freshly made films determined by an Energy-Dispersive X-ray Spectroscopy Microanalysis System is As38S18Se44. The experiments on the surface relief formation were performed using a scanning electron microscope (SEM) with the installed EB lithography system. The lithographic system operates a Gaussian beam to write point-by-point in a raster manner. In order to achieve, for example, a solid exposed line, the exposure points should be close enough to each other so that there is no area/ space between them. The point separation (called the beam step-size) is user-defined and totally dependent on the physical size of the chosen beam. EB diameter (91 nm) calculated and determined by the SEM system, the beam current 271 pA, measured in the SEM chamber before and after exposure using a Faraday cup. For the experiments, non-aqueous amine based organic solvent was used as a negative developer for As38S18Se44. The films were etched using the wet etching technique. The etching rate was determined by
0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.053
Please cite this article as: V. Kolbjonoks, et al., Optical grating recording in ChG thin film by electron beam, J. Non-Cryst. Solids (2013), http:// dx.doi.org/10.1016/j.jnoncrysol.2013.01.053
2
V. Kolbjonoks et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
Fig. 1. Test boxes pattern: each color corresponds to a particular EB dose. Fig. 3. Squares profilometric graph.
monitoring the local thickness of the film by its interference (recorded by a scanning confocal microscope system). The surface roughness control and pattern measurements were performed by atomic force microscope (AFM). The experiments were carried out at room temperature. The test EB exposure was performed on the prepared 600 nm thick sample. The pattern to be exposed was Test Boxes (Fig. 1), size of boxes 30 × 30 μm. The dose factor was in the range 50 to 3050 μC/cm2, with a step of 25 μC/cm2 (in order to determine the resist clearing dose). The EB accelerating voltage was 30 keV. After EB exposure, the As38S18Se44 sample was fixed to the bottom of a Petri dish (working surface upward). The dish was placed on a confocal microscope stage under the objective. While scanning constantly the sample surface with a low-energy laser (λ = 633 nm) to the Petri dish the etchant solution was added to fully cover the sample. The whole etching process was monitored and recorded by the confocal microscope system. The etching rate was determined by monitoring the local thickness of the sample using the thin film interference. The scanned sample surface reflects the light which is then registered by the confocal system detectors. To completely dissolve non-exposed and exposed areas of the sample took 200 and 1800 s, respectively. The etch selectivity, defined as the etching rate ratio of exposed and non-exposed areas of a sample, γ = Vexposed / Vunexposed, was 1/9.
Fig. 2. The etching rate of amorphous As38S18Se44 films vs. EB exposure dose.
The next step was recording of different gratings to establish and demonstrate advantages of ChG as material for fabrication of complex optical structures in a single etching process. Each grating was tested for the diffraction efficiency (DE). The percent of the zero order light beam that can be diffracted to the first order beam. It is determined by the ratio of the power of the diffracted light beam to the incident power of the beam. As the sample is not transparent, before the DE test it was covered with a 40 nm aluminum layer for better light reflection using the magnetron sputtering technique. 3. Results and discussion The As38S18Se44 films obtained in the experiments behave as a negative resist for non-aqueous amine organic developer. The etching rate dependence on the EB-exposure dose for amorphous As38S18Se44 films is presented in Fig. 2. In this figure, three linear segments can be marked (60–140, 150–400 and 410–600 μC/cm 2) as corresponding to the profile height. At point 600 μC/cm2 the curve reaches saturation and does not change with time. Such curve behavior remained invariable in the experiment repeated several times. The exponential shape of the profile depth dependence on the electron dose means that higher EB doses make the films more resistant to
Fig. 4. Resist profile height vs. the electron dose.
Please cite this article as: V. Kolbjonoks, et al., Optical grating recording in ChG thin film by electron beam, J. Non-Cryst. Solids (2013), http:// dx.doi.org/10.1016/j.jnoncrysol.2013.01.053
V. Kolbjonoks et al. / Journal of Non-Crystalline Solids xxx (2013) xxx–xxx
a) sinusoidal grating, period = 800 nm Diffraction Efficiency = 20%
b) asymmetrical 90º sawtooth grating, period = 1 µm
3
etching — i.e. more time is needed to dissolve the exposed area. This makes the As38S18Se44 resist useful for recording the multilevel diffractive elements, which was the main goal of our work. Fig. 3 demonstrates the squares height as determined by the intensity and time of irradiation. The ChG resist acts as a negative one, the exposed area of the sample has a lower dissolution rate in the developer than unexposed. As could be seen in Fig. 4, the ratio of the electron dose to the profile height grows very fast up to 1000–1200 μC/cm 2. Then the curve slows down and reaches saturation at 3000 μC/cm2. The most interesting region for our research is from 600 to 2000 μC/cm2, where the electron dose switch leads to a significant change in the profile height of developed pattern. This allows the high-quality 3D pattering to be achieved. The next step mentioned above — i.e. fabrication of different gratings to establish and demonstrate advantages of ChG as material for making complex optical structures in a single etching process — resulted in four types of gratings recorded with different profiles and periods. Fig. 5 presents the AFM graphs of the developed gratings with measured diffraction efficiency. 4. Conclusions
Diffraction Efficiency = 28% The surface relief formation on As38S18Se44 thin films was observed under 90 nm electron beam exposure was observed. The surface relief formation with a depth up to 160 nm by EB lithography in the films was established. The results of our experiments confirm that the ChG glass thin films are good material for the use as high-resolution electron resist. The observed nearly linear dependence of the etch depth on the total electron dose absorbed allows the high-quality 3D pattering to be obtained.
c) asymmetrical 90º sawtooth grating, period = 2 µm Diffraction Efficiency = 38%
References [1] M.L. Trunov, S.N. Dub, P.M. Nagy, S. Kokenyesi, J. Phys. Chem. Solids 68 (2007) 1062, http://dx.doi.org/10.1016/j.jpcs.2007.03.008. [2] A. Kikineshi, V. Palyok, M. Shiplyak, I.A. Szabo, D.L. Beke, J. Optoelectron. Adv. Mater. 2 (2000) 95. [3] A. Csik, M. Malyovanika, J. Dorogovicsa, A. Kikineshia, D.L. Beke, I.A. Szabo, G. Langer, J. Optoelectron. Adv. Mater. 3 (2001) 33. [4] S. Kokenyesia, I. Ivánb, V. Takátsa, J. Pálinkása, S. Birib, I.A. Szaboa, J. Non-Cryst. Solids 353 (2007) 1470, http://dx.doi.org/10.1016/j.jnoncrysol.2006.09.064. [5] A.M. Andriesh, M.S. Iovu, S.D. Shutov, J. Optoelectron. Adv. Mater. 4 (2002) 631. [6] A.V. Rodea, A. Zakeryb, M. Samoca, R.B. Chartersa, E.G. Gamalya, B. Luther-Davies, Appl. Surf. Sci. 197 (2002) 481. [7] M. Frumar, Z. Polak, Z. Cernosek, B. Frumarova, T. Wagner, Chem. Pap. 51 (1997) 310.
d) sinusoidal grating, with different height of each line, period = 200 nm Diffraction Efficiency = 52% Fig. 5. AFM profile height of developed gratings and diffraction efficiency of the grating.
Please cite this article as: V. Kolbjonoks, et al., Optical grating recording in ChG thin film by electron beam, J. Non-Cryst. Solids (2013), http:// dx.doi.org/10.1016/j.jnoncrysol.2013.01.053