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The birefringence and polarization effects of amorphous Ge and Si gratings by focused-ion-beam
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Microelectronic Engineering 53 (2000) 627-629 www.elsevier.nl/ locate/ mee
The Birefringence and Polarization Effects of Amorphous Ge and Si Gratings by Focused-Ion-Beam Kyung Shin",Jin-WooKima, Jung-I1Park, Young-JongLeeb, Hyun-YongLeec, Hong-BayChung" aDept, of Electron. Mater. Eng., Kwangwoon Univ. 447-1, Nowonku, Seoul 139-701, KOREA Weo Joo Inst. of Technol., Dept. of Electron. Eng., Kyungki, KOREA cCenter for Terahertz Photonics, Pohang Univ. of Sci. and Technol., Kyungbuk 790-784, KOREA The polarization and birefringence effects in amorphous germanium (a-Ge) and amorphous silicon (a-Si) gratings formed by focused-ion beam (FIB) were investigated by using a linearly polarized He-Ne laser beam (632.8nm). The a-Si and a-Ge thin films were deposited on quartz substrates by plasma-enhanced-chemicalvapor deposition (PECVD) and thermal evaporation, respectively. In order to obtain the optimum grating arrays, an inorganic a-Se75Ge25 resist, top layer, was prepared on each films with the thickness (Zm~n)optimized by Monte Carlo (MC) simulation. As the results of MC simulation, the Zm~nof a-Se75Ge25resist was estimated to be about 600A. The double layers prepared were milled by Ga*-FIB with an accelerating energy of 50 keV and then etched by reactive-ion etching (RIE). Eventually, grating arrays fabricated had linewidth and its separation of 0.5 gm and 0.7 ttm, respectively. The maximum diffraction efficiencies, which were measured at an incident angles (about 16.7°) to satisfy the Bragg condition, exhibited very large values to be about 41% and 44% for a-Si and a-Ge, respectively. Also, the birefringence dn obtained by a simple formula were estimated to be approximately 0.200 and 0.148 for a-Si and a-Ge, respectively. 1. INTRODUCTION Recently, in many studies for the integrated optical device, the transmission grating array studied previously were fabricated by using low dielectric constant materials such as photoresist, fused silica, polymethyl-methacrylate (PMMA), and silicon nitride (Si3N4), ete, giving small refractive index modulation. Also, all previous experiments showed consistency with the simple form birefringence theory or effective medium theory, which predict that for a normal incident light the transmission for the TE wave (the electric field parallel to the grating fingers) should be almost the same as that for the TM wave (the electric field perpendicular to the grating fingers), and that birefringence should be independent of the grating period as long as the period is less than the wavelength [1]. On the other hand, subwavelength transmission gratings (SWTG) utilizing high refractive-index materials such as amorphous (a-) Si or a-Ge can serve as antireflection devices, wave- plates, narrow band
filters, and other many optical elements. According to previous reports, strong polarization effects, in addition to large birefringence, were observed for normal incident light: significantly different transmittance for the TE and TM waves. Furthermore the polarization and birefringence effects strongly depend on the ratio of grating period to wavelength. However, due to the fine period required, most of the previous investigations were theoretical; the experimental studies were few and limited. In spite of this limitation, they are very attractive to future integrated optics, because large arrays of different subwavelength optical elements can be made at different locations of a substrate using simple grating structures in a single fabrication step [2]. The interesting optical properties of diffractive structures and their compatibility with integrated optics make the Diffractive Optical Elements (DOE) very promising to substitute conventional optics. In this study, we investigated the polarization and birefringence effects in high refractive a-Si and a-Ge transmission grating arrays fabricated using Ga
0167-9317/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0167-9317(00)00392-0
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K. Shin et al. / Microelectronic Engineering 53 (2000) 627-629
focused-ion-beam (FIB) milling, and their applicability.
As shown in Fig. 1, the birefringence for transmission gratings array can be obtained by measuring light intensities through each analyzer and utilizing formulas as following [5,6].
2.EXPERIMENT First, the a-Si and a-Ge thin films were prepared by plasma-enhanced-chemical-vapor deposition and thermal evaporation on quartz substrates, respectively. The thicknesses of a-Si and a-Ge thin films monitored to be about 619A and 825A, respectively. In order to remove the film damage by the collisions of high energetic ions, we utilize inorganic a-Se75Ge25 resist, which acts as a buffer layer for energetic GaFIB. In previous papers [3,4], we have reported the results of the three-dimensional Monte-Carlo (MC) simulation for Ga ion penetration in a-Se75Ge25resist, in which the minimum thickness, Zm~,, capable of minimizing a film damage and absorbing the ion beam sufficiently at the same time are also presented. The Zmin of the Se75Ge25 buffer layer optimized by MC simulation was about 600A and then the buffer layer was deposited on a-Si(a-Ge)/quartz by thermal evaporation in vacuum - 1x 105 Torr. The buffer layer (a-S%Ge25) were first milled using a Ga+-FIB with an accelerating energies of 50 keV and both a-Si and a-Ge were developed by reactive-ion-etching (RIE). Then, remaining buffer layers were removed by dipping in a solution of NaOH. The RIE process was performed for 10 sec with CF4 gases. Figure 1 shows a schematic diagram of the experimental setup in order to observe the birefringence and polarization effects at a wavelength of 632.8nm.
I = I0 sin2 (dqb /2)
(1)
= 2~t dn d / ~
(2)
where, I and Io mean transmission intensities passing via the orthogonal polarizer-analyzer pair and via the parallel polarizer-analyzer pair, respectively. Also, d d~ is phase difference, d is the thickness of the sample, oh is the birefringence (n:no). In addition, we estimate the maximum polarization-diffraction efficiency as measuring an intensity of l st-order diffraction pattern at an incident angles to satisfy the Bragg condition.
3.RESULTS AND DISCUSSION Figure 2 shows a top-view photo of the gratings array formed in a-S%Ge2Ja-Ge (619A) just after FIB milling process.
Fig. 2 Top view of a-S%Ge2Ja-Ge gratings array just after FIB milling process.
la~nm)
analyzer
Fig.1 Schematic diagrams to measure the optical properties such as the birefringence and the polarization effect. Arrows represent polarization conditions of the linear polarizer and analyzers.
Photograph of Scanning Electron Microscopy (SEM) of transmission gratings array fabricated in aSi is shown in Fig.3, where linewidth and its separation were estimated to be about 0.7/zm and 0.7 fzm, respectively and the area of gratings was about 60x60/zm 2.
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K. Shin et al. I Microelectronic Engineering 53 (2000) 627-629
Table I. Summary of optical properties for a-Si and a-Ge grating array.
Preparation Thickness, d Substrate Phase difference, 8 qb Birefringence, 5 n Maximum diffraction efficiency
a-Ge
a-Si
Thermal evaporation 619 A Quartz 5.231o 0.148 44%
PECVD 825A Quartz 12.7o 0.200 41%
Since the average period of gratings is about 1.1 can, an incident angles to satisfy the Bragg condition is estimated to be about 16.7° and the polarization diffraction efficiency obtained at this incident angle can be thought to be a maximum value. The maximum diffraction efficiencies of lst-order diffraction pattem measured at 16.70 exhibit very large values to be about 41% and 44% for a-Si and aGe, respectively. Also, for a-Si and a-Ge grating arrays, the transmissions through each polarizeranalyzer pairs were measured using a optical power meter. In the case of a-Si, the I and Io at a normal incident angle (incident power = 4.12 roW) were 1.35mW and 16.5//~V,respectively. Therefore the birefringence for a-Si grating became about 0.200, obtained from eq.(1) and (2). The birefringence for a-Ge grating array was about 0.148. The values of maximum polarization diffraction efficiency and birefringence for gratings arrays fabricated in a-Ge and a-Si are summarized in Table I.
Accordingly, we think that a-Si and a-Ge transmission grating arrays with a relatively high refractive index has a potential to be used as various the optical devices and elements. 4.CONCLUSIONS The polarization and birefringence effects in a-Ge and a-Si subwavelength amorphous transmi-ssion gratings array (SWTG) formed by focused-ionbeam(FIB) have been investigated using a linearly polarized He-Ne laser beam(632,8nm). The linewidth and its separation fabricated were 0.5~tm and 0.7#m, respectively. The maximum diffraction efficiencies measured at an incident angles to satisfy the Bragg condition exhibited very large values to be about 41% and 44% for a-Si and a-Ge, respectively. In addition, the birefrin-gence 5 n were estimated to be approximately 0,200 and 0.148 for aSi and a-Ge, respectively. Accordingly, we think that a-Si and a-Ge transmission grating arrays with a relatively high refractive index has a potential to be used as various the optical devices and elements. REFERENCES
Fig.3. SEM photograph of a-Si transmission gratings array.
1. C. Yang and P. Yeh, Appl. Phys. Lett, 69 (23) 1996. 2. S. Y. Chou and W. Deng, Appl. Phys. Lett 67 (6) 1995. 3. H. Y. Lee, H. B. Chung, J. Vac. Sci. Technol., B, 16(3), 1161, 1998. 4. H. Y. Lee, S. H. Park, J. Y. Chun, H.B. Chung, J. Appl. Phys., 83, 5381, 1998. 5 . F . L . Pedrotti, Introduction to Optics, (Prentice Hall, New Jersey, USA) 1993, Chap 16, pp. 323348 and Chapl7, pp. 349-406. 6. T. Todorov, L.Nikolova, and N.Tomova, Appl. Opt. 23 (23), 4309, 1984.
Microelectronic Engineering 53 (2000) 627-629 www.elsevier.nl/ locate/ mee
The Birefringence and Polarization Effects of Amorphous Ge and Si Gratings by Focused-Ion-Beam Kyung Shin",Jin-WooKima, Jung-I1Park, Young-JongLeeb, Hyun-YongLeec, Hong-BayChung" aDept, of Electron. Mater. Eng., Kwangwoon Univ. 447-1, Nowonku, Seoul 139-701, KOREA Weo Joo Inst. of Technol., Dept. of Electron. Eng., Kyungki, KOREA cCenter for Terahertz Photonics, Pohang Univ. of Sci. and Technol., Kyungbuk 790-784, KOREA The polarization and birefringence effects in amorphous germanium (a-Ge) and amorphous silicon (a-Si) gratings formed by focused-ion beam (FIB) were investigated by using a linearly polarized He-Ne laser beam (632.8nm). The a-Si and a-Ge thin films were deposited on quartz substrates by plasma-enhanced-chemicalvapor deposition (PECVD) and thermal evaporation, respectively. In order to obtain the optimum grating arrays, an inorganic a-Se75Ge25 resist, top layer, was prepared on each films with the thickness (Zm~n)optimized by Monte Carlo (MC) simulation. As the results of MC simulation, the Zm~nof a-Se75Ge25resist was estimated to be about 600A. The double layers prepared were milled by Ga*-FIB with an accelerating energy of 50 keV and then etched by reactive-ion etching (RIE). Eventually, grating arrays fabricated had linewidth and its separation of 0.5 gm and 0.7 ttm, respectively. The maximum diffraction efficiencies, which were measured at an incident angles (about 16.7°) to satisfy the Bragg condition, exhibited very large values to be about 41% and 44% for a-Si and a-Ge, respectively. Also, the birefringence dn obtained by a simple formula were estimated to be approximately 0.200 and 0.148 for a-Si and a-Ge, respectively. 1. INTRODUCTION Recently, in many studies for the integrated optical device, the transmission grating array studied previously were fabricated by using low dielectric constant materials such as photoresist, fused silica, polymethyl-methacrylate (PMMA), and silicon nitride (Si3N4), ete, giving small refractive index modulation. Also, all previous experiments showed consistency with the simple form birefringence theory or effective medium theory, which predict that for a normal incident light the transmission for the TE wave (the electric field parallel to the grating fingers) should be almost the same as that for the TM wave (the electric field perpendicular to the grating fingers), and that birefringence should be independent of the grating period as long as the period is less than the wavelength [1]. On the other hand, subwavelength transmission gratings (SWTG) utilizing high refractive-index materials such as amorphous (a-) Si or a-Ge can serve as antireflection devices, wave- plates, narrow band
filters, and other many optical elements. According to previous reports, strong polarization effects, in addition to large birefringence, were observed for normal incident light: significantly different transmittance for the TE and TM waves. Furthermore the polarization and birefringence effects strongly depend on the ratio of grating period to wavelength. However, due to the fine period required, most of the previous investigations were theoretical; the experimental studies were few and limited. In spite of this limitation, they are very attractive to future integrated optics, because large arrays of different subwavelength optical elements can be made at different locations of a substrate using simple grating structures in a single fabrication step [2]. The interesting optical properties of diffractive structures and their compatibility with integrated optics make the Diffractive Optical Elements (DOE) very promising to substitute conventional optics. In this study, we investigated the polarization and birefringence effects in high refractive a-Si and a-Ge transmission grating arrays fabricated using Ga
0167-9317/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0167-9317(00)00392-0
628
K. Shin et al. / Microelectronic Engineering 53 (2000) 627-629
focused-ion-beam (FIB) milling, and their applicability.
As shown in Fig. 1, the birefringence for transmission gratings array can be obtained by measuring light intensities through each analyzer and utilizing formulas as following [5,6].
2.EXPERIMENT First, the a-Si and a-Ge thin films were prepared by plasma-enhanced-chemical-vapor deposition and thermal evaporation on quartz substrates, respectively. The thicknesses of a-Si and a-Ge thin films monitored to be about 619A and 825A, respectively. In order to remove the film damage by the collisions of high energetic ions, we utilize inorganic a-Se75Ge25 resist, which acts as a buffer layer for energetic GaFIB. In previous papers [3,4], we have reported the results of the three-dimensional Monte-Carlo (MC) simulation for Ga ion penetration in a-Se75Ge25resist, in which the minimum thickness, Zm~,, capable of minimizing a film damage and absorbing the ion beam sufficiently at the same time are also presented. The Zmin of the Se75Ge25 buffer layer optimized by MC simulation was about 600A and then the buffer layer was deposited on a-Si(a-Ge)/quartz by thermal evaporation in vacuum - 1x 105 Torr. The buffer layer (a-S%Ge25) were first milled using a Ga+-FIB with an accelerating energies of 50 keV and both a-Si and a-Ge were developed by reactive-ion-etching (RIE). Then, remaining buffer layers were removed by dipping in a solution of NaOH. The RIE process was performed for 10 sec with CF4 gases. Figure 1 shows a schematic diagram of the experimental setup in order to observe the birefringence and polarization effects at a wavelength of 632.8nm.
I = I0 sin2 (dqb /2)
(1)
= 2~t dn d / ~
(2)
where, I and Io mean transmission intensities passing via the orthogonal polarizer-analyzer pair and via the parallel polarizer-analyzer pair, respectively. Also, d d~ is phase difference, d is the thickness of the sample, oh is the birefringence (n:no). In addition, we estimate the maximum polarization-diffraction efficiency as measuring an intensity of l st-order diffraction pattern at an incident angles to satisfy the Bragg condition.
3.RESULTS AND DISCUSSION Figure 2 shows a top-view photo of the gratings array formed in a-S%Ge2Ja-Ge (619A) just after FIB milling process.
Fig. 2 Top view of a-S%Ge2Ja-Ge gratings array just after FIB milling process.
la~nm)
analyzer
Fig.1 Schematic diagrams to measure the optical properties such as the birefringence and the polarization effect. Arrows represent polarization conditions of the linear polarizer and analyzers.
Photograph of Scanning Electron Microscopy (SEM) of transmission gratings array fabricated in aSi is shown in Fig.3, where linewidth and its separation were estimated to be about 0.7/zm and 0.7 fzm, respectively and the area of gratings was about 60x60/zm 2.
629
K. Shin et al. I Microelectronic Engineering 53 (2000) 627-629
Table I. Summary of optical properties for a-Si and a-Ge grating array.
Preparation Thickness, d Substrate Phase difference, 8 qb Birefringence, 5 n Maximum diffraction efficiency
a-Ge
a-Si
Thermal evaporation 619 A Quartz 5.231o 0.148 44%
PECVD 825A Quartz 12.7o 0.200 41%
Since the average period of gratings is about 1.1 can, an incident angles to satisfy the Bragg condition is estimated to be about 16.7° and the polarization diffraction efficiency obtained at this incident angle can be thought to be a maximum value. The maximum diffraction efficiencies of lst-order diffraction pattem measured at 16.70 exhibit very large values to be about 41% and 44% for a-Si and aGe, respectively. Also, for a-Si and a-Ge grating arrays, the transmissions through each polarizeranalyzer pairs were measured using a optical power meter. In the case of a-Si, the I and Io at a normal incident angle (incident power = 4.12 roW) were 1.35mW and 16.5//~V,respectively. Therefore the birefringence for a-Si grating became about 0.200, obtained from eq.(1) and (2). The birefringence for a-Ge grating array was about 0.148. The values of maximum polarization diffraction efficiency and birefringence for gratings arrays fabricated in a-Ge and a-Si are summarized in Table I.
Accordingly, we think that a-Si and a-Ge transmission grating arrays with a relatively high refractive index has a potential to be used as various the optical devices and elements. 4.CONCLUSIONS The polarization and birefringence effects in a-Ge and a-Si subwavelength amorphous transmi-ssion gratings array (SWTG) formed by focused-ionbeam(FIB) have been investigated using a linearly polarized He-Ne laser beam(632,8nm). The linewidth and its separation fabricated were 0.5~tm and 0.7#m, respectively. The maximum diffraction efficiencies measured at an incident angles to satisfy the Bragg condition exhibited very large values to be about 41% and 44% for a-Si and a-Ge, respectively. In addition, the birefrin-gence 5 n were estimated to be approximately 0,200 and 0.148 for aSi and a-Ge, respectively. Accordingly, we think that a-Si and a-Ge transmission grating arrays with a relatively high refractive index has a potential to be used as various the optical devices and elements. REFERENCES
Fig.3. SEM photograph of a-Si transmission gratings array.
1. C. Yang and P. Yeh, Appl. Phys. Lett, 69 (23) 1996. 2. S. Y. Chou and W. Deng, Appl. Phys. Lett 67 (6) 1995. 3. H. Y. Lee, H. B. Chung, J. Vac. Sci. Technol., B, 16(3), 1161, 1998. 4. H. Y. Lee, S. H. Park, J. Y. Chun, H.B. Chung, J. Appl. Phys., 83, 5381, 1998. 5 . F . L . Pedrotti, Introduction to Optics, (Prentice Hall, New Jersey, USA) 1993, Chap 16, pp. 323348 and Chapl7, pp. 349-406. 6. T. Todorov, L.Nikolova, and N.Tomova, Appl. Opt. 23 (23), 4309, 1984.