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Frequency selective IR-filter produced by using EB-lithography
Applied Surface Science 253 (2007) 7826–7830 www.elsevier.com/locate/apsusc
Frequency selective IR-filter produced by using EB-lithography Y.P. Kathuria 1,* Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Available online 28 February 2007
Abstract This paper reports on the fabrication of Jerusalem cross diplexer by direct write electron beam (EB) lithography followed by reactive ion etching (RIE) on a phosphorus doped polished silicon wafer substrate. Such structures can be used as frequency selective components in visible, microwave and near infra-red wavelength region. Replication of the patterns is accomplished by micron or sub-micron order mould fabricated from the silicon (Si) master. Fourier transform infra-red reflectance (FT-IR) measurements were performed to characterize the structured patterns. The spectral reflectance from these patterns clearly show a reflection dip due to surface plasmon excitation in the near infra-red wavelength at about 1.42 and 2.5 mm, respectively. Potential applications such as antireflection surface (ARS) can be realized. # 2007 Elsevier B.V. All rights reserved. Keywords: Lithography; Infra-red; Cross diplexer
1. Introduction In the emerging micro and nano technology, the demands for the micro-optical components and photonic bandgap devices require the increasing use of micron and sub-micron features [1,2]. It is often desirable to generate/fabricate complex microstructures in various materials depending upon their applications, such as frequency selective surfaces (FSSs), micro sensors, actuators and micro fluidics control devices. But as the feature size falls to the micron or sub-micron order (100– 0.1 mm), the conventional mechanical techniques must be replaced by the beam technology. This technology has been divided into two categories: ‘top down’ approach and ‘bottom up’ approach. The top down approach includes micromachining, microlithography, electron beam patterning and contact lithography. However, in the bottom up approach, binding of structures atom by atom or molecule by molecule and their reproducibilty depends upon the functionality of the material [3]. This is a recently developed technology for miniaturization of functional devices in the millimeter range with minimum features and dimensional tolerances less than a micron. The
degree of precision is achieved by fabricating a master using the electron beam (EB) lithography technique. The micron or submicron order mould fabricated from the master can be replicated. This is achieved by moulding a network photopolymer into the patterned feature. In the present work, by using the top down approach, micron and sub-micron order Jerusalem cross diplexer are produced in doped silicon by EB-lithography followed by reactive ion etching (RIE). By using the photopolymer based methylacrylate/acrylate mixture (PMMA–MMA), the pattern etched in silicon with feature size down to <0.3 mm is further replicated following the bottom up approach. In this new process, polymer resin is used to impress into the micron or sub-micron patterned structure in silicon. After thermal treatment and polymerization, the replica pattern is transferred and then lift-off. On the other hand, due to the metallic behaviour of doped semiconductor at T (Hz) frequencies [4,5], it is possible to excite surface plasmon in such structures when patterned in doped silicon and to use them as frequency selective surfaces (FSSs) or spectrally selective components at visible, microwave and infrared wavelengths. 2. Fabrication process
* Tel.: +91 22 2576 7615; fax: +91 22 2572 3480. E-mail address: [email protected]. 1 Part of the work was done when the author was formerly with, Department of Robotics, Faculty of Science and Engineering Ritsumeikan University, Kusatsu-shi 525-8577, Shiga-ken, Japan. 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.141
In this experiment, two types of patterns, type-[A] and type-[B] of the Jerusalem cross diplexer as shown in Fig. 1, were used for lithography and replication process. Various steps were involved in the fabrication of this patterned
Y.P. Kathuria / Applied Surface Science 253 (2007) 7826–7830
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Fig. 1. Block diagram of Jerusalem cross diplexer type-[A] and type-[B]; w: structural gap width between two patterns in type-[A] or between two arrows in type-[B].
structure and its replica as FSSs are shown in Fig. 2. The material used for our experiments was commercially available silicon wafer having a diameter of about 100 mm and a thickness of about 400 mm only. It is doped with phosphorus with a doping concentration of N = 1017 cm3. EB-resist with trade mark ZEP 520-22CP was deposited by spin coat to a thickness of about 0.3 mm and then baked at 100 8C for a few minutes. The features were written in an area of 4 mm 4 mm by focused electron beam (EB) at a voltage of 50 kV and a dose of 100 mC cm2. The resist was developed in isopropanal for about 60 s. The patterns so developed, act as a mask on silicon wafer [6]. In order to transfer the array of patterns into the silicon substrate, a parallel plate reactive ion etching (RIE) was performed in an SF6 atmosphere under a pressure of 13.3 Pa. A flow rate of 30 sccm (SF6) for an RF-power of 150 W was sufficient for the desired pattern in an etching time of about 4.5 min. After the required etching, the EBresist was further removed by similar procedure, but in the
O2 atmosphere. The resulting etched pattern shall act as a silicon master (Fig. 2a) [7]. 3. Pattern replication Patterned silicon master with the desired feature to be replicated was first rinsed with distilled water, followed by cleaning with isopropanal. A small amount of pre-prepared photopolymer solution (PMMA–MMA) was added to the surface of the silicon master and pre-heated to about 180 8C. This was done because, above the glass transition temperature, the polymer has higher fluidity than below and therefore, can better fill the micron or sub-micron order pattern [8]. A clean glass plate was brought in contact and pushed against the silicon master and the viscous liquid was then allowed to spread and flow into the desired pattern in the pre-heated silicon master. After the complete polymerization, the glass plate was removed and the polymerized mould was carefully separated from the silicon master yielding the desired pattern in polymer
Fig. 2. (a) Steps in fabrication of pattern in silicon master were used for the lithography and replication process and (b) contact moulding replication.
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mould (Fig. 2b). This mould was then radio frequency (RF) sputter coated with a few tens of angstrom thick layer of gold and subsequently, analyzed with scanning electron microscope. Further replication can also be done by polymer to polymer moulding process [3]. 4. Characterization The two types of patterns: type-[A] and type-[B] of the Jerusalem cross diplexer, thus, made were characterized by high resolution optical microscope (OM) and scanning electron microscopy (SEM). The spectral reflectance analyses of these patterns etched in silicon were performed by using infra-red fourier transform spectrometer (Jasco FT-IR 660plus) with the following experimental conditions: detection: MCT, TGS; incident angle: 458; resolution: 4 cm1. Before measurements, samples were three times rinsed in hot acetone and ethanol to remove organic contamination. The reflection spectra were measured at 458 from the non-patterned and patterned structure area in the range from 1.4 to 20 mm. The reflection intensity (R) is normalized at 100% to a reference position, which is the non-patterned place sufficiently far from the patterned area. 5. Results and discussion Various photographic sections of the patterns etched in silicon and its replica are shown in Figs. 3 and 4, respectively. From the SEM images (Fig. 4a and b), it is observed that, depending upon the design of pattern, a resolution of <0.3 mm can be achieved in producing these patterns. This, however, depends upon the design of the pattern. The limiting factor in the line scan width depends upon the focussing of electron beam, which in the ideal case is limited to about 25 nm. Besides this, etching conditions, material characteristics and the replication process add to the limiting factor and lead to higher line width. Fig. 5 shows the infra-red reflection spectra corresponding to patterns of type-[A] and type-[B] in Fig. 3, respectively. The result shows that for both type-[A] and type-[B] patterns, there is clearly a reflection dip at lower infra-red wavelength. This dip in the reflection spectra is caused by the surface plasmon (SP) excitation due to interaction between the incoming photon and surface plasmon resonance in the patterned surface area [9–11]. As shown in both curves, the infra-red intensity from the patterned area (B: blue line) clearly decreases as the wavelength decreases. No other peak has been detected except small peaks originated from air (such as CO2, H2O and so on). This is due to the fact that when the photon of wave vector (v/ c) is made incident in the plane region outside the structured surface of silicon, it would be specularly reflected. There will not be any optical excitation of SP, because, for each frequency v, the SP wave vector (ksp) is greater than the
Fig. 3. High resolution optical microscope photograph of the Jerusalem cross diplexer in silicon wafer. Bar size: type-[A]: ( ) 10.00 mm and type-[B]: ) 50.00 mm. (
photon wave vector (v/c) [10], as evident from the dispersion relation: ksp ¼
1=2 v e1 ðvÞ c 1 þ e1 ðvÞ
where e1(v) is real part of dielectric function of the medium, which is negative, but large in magnitue for IR-frequencies (18.1 for doped Si). However, larger wave vector for the incident photon can be achieved at the structured surface, because there is a periodic density variation in the region of the surface, and the patterned area may absorb momentum, thus, generating surface reciprocal lattice vectors G. Therefore, when an incident photon impinges on the patterned surface at an angle u, it can couple to a SP at the interface through a surface G vector [12,13]: v sin ðuÞi þ G ¼ ksp c
Y.P. Kathuria / Applied Surface Science 253 (2007) 7826–7830
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Fig. 4. (a) Scanning electron micrograph of the replicated Jerusalem cross diplexer. Bar size: type-[A]: 6.67 mm and type-[B]: 33.3 mm and (b) scanning electron micrograph of the replicated Jerusalem cross diplexer at higher magnification. Bar size: type-[A]: 1.43 mm and type-[B]: 5.0 mm.
Fig. 5. FT-IR reflection spectra (R) vs. wavelength (l) at 458 from silicon wafer and from patterned structures type-[A], type-[B] in silicon wafer (G: reflection from non-patterned area; B: reflection form patterned area).
where i is a unit vector lying in the surface plane. Thereby, surface plasmon with x-momentum plus x-component of the momentum of the incident photon may be created and hence, excitation of surface plasmon which cause a dip in the reflection spectra. Besides that, in type-[A], the infra-red reflection intensity from patterned area shows a peak at about 1.42 mm (Fig. 5 type-[A]), whereas the corresponding infra-red reflection intensity from the patterned area in type-[B] have broad but clear peak at about 2.5 mm only (Fig. 5 type-[B]). The noise like peaks around 3 and 7 mm correspond to water vapor and a sharp peak around 4 mm is originated from CO2. Interestingly, at shorter wavelength, where incoming l is close to the structural gap width (w), the reflectivity shows a remarkable structure. For the 1.6 mm structural gap width between patterns in Fig. 1 type-[A], a pronounced reflectivity dip occur near 1.42 mm as shown in Fig. 5 type-[A]. Similarly, we observe a broader dip near 2.5 mm (Fig. 5 type-[B]) for the 2.97 mm structural gap width between two arrows in Fig. 1 type-[B]. Besides that, on observing critically (Fig. 3), it is found that due to narrow slit width of type-[A] pattern, the spectral dip at 1.42 mm is towards shorter infra-red wavelength,
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compared to type-[B] with coarse pattern, where the spectral dip at 2.5 mm is sligthly at higher infra-red wavelength. The shift towards shorter wavelength is also attributed to shift in the surface plasmon frequency [11]. Therefore, one concludes that narrow line width in the pattern is responsible for a shift in the spectral dip towards shorter wavelength [14,15], whereas the class of patterns, i.e. type-[A] or type-[B], corresponds to narrowing or broadening of the spectral peak. 6. Conclusion In conclusion, micron and sub-micron order Jerusalem cross diplexer are fabricated in silicon wafer, by electron beam lithography followed by reactive ion etching. These patterned structures are further replicated by thermal polymerization of the impressed resin into the silicon master and then lift-off. The resulting replica patterns are then gold coated and characterized by scanning electron microscope. Features down to 0.3 m are achieved and analysed. The spectral reflectance from these patterns clearly show a reflection dip due to surface plasmon excitation in the near infra-red wavelength at about 1.42 and 2.5 mm, respectively. Acknowledgements The author wishes to express his gratitude to Prof. S. Sugiyama of Ritsumeikan University Japan for the kind support and master candidate Mr. Y. Kwata for the experimental help as
well as thanks to Asst. Profs. Nakada, Li and Dr. Hondoh for their support in characterization.
References [1] Y.P. Kathuria, J. Indian Inst. Sci. 84 (May–August) (2004) 77. [2] T. Garino, J. Cesarano, A. Morales, Mater. Res. Soc. Symp. Proc. 741 (2003) 227. [3] T.A. von Werne, D.S. Germack, E.C. Hagberg, V.V. Sheares, C.J. Hawker, K.R. Carter, J. Am. Chem. Soc. 125 (2003) 3831. [4] J. Gomez Rivas, C. Schotsch, P. Haring Boliver, H. Kurz, Phy. Rev. B68 (2003) 201306 (R). [5] C. Janke, J. Gomez Rivas, C. Schotsch, L. Beckmann, P. Haring Boliver, H. Kurz, Phy. Rev. B69 (2004) 20534. [6] Y. Kanamori, M. Sasaki, K. Hane, Opt. Lett. 24 (1999) 1422. [7] I. Puscasu, G. Boreman, R.C. Tiberio, D. Spencer, R.R. Krchnavek, J. Vac. Sci. Technol. B18 (2000) 3578. [8] N. Lee, Y.-K. Kim, S. Kang, J. Phys. D: Appl. Phys. 37 (2004) 1624. [9] R.H. Ritchie, E.T. Arakawa, J.J. Cowan, R.N. Hamm, Phy. Rev. Lett. 21 (November) (1968) 1530. [10] N. Maeschall, B. Fischer, H.J. Queisser, Phy. Rev. Lett. 27 (July) (1971) 95. [11] W.E. Anderson, R.W. Alexander, R.J. Bell, Phy. Rev. Lett. 27 (October) (1971) 1057. [12] M.U. Pralle, N. Moelders, M.P. McNeal, I. Puscasu, A.C. Greenwald, J.T. Daly, E.A. Johnson, T.F. George, D.S. Choi, E. El-Kady, R. Biswas, Appl. Phys. Lett. 81 (2002) 4685. [13] H. Raether, Surface Plasmons, Springer-Verlag, Berlin, New York, 1988. [14] T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, P.A. Wolff, Nature 391 (1998) 667. [15] A. Degiron, T.W. Ebbesen, J. Opt.: Pure Appl. Opt. 7 (2005) S90.
Frequency selective IR-filter produced by using EB-lithography Y.P. Kathuria 1,* Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Available online 28 February 2007
Abstract This paper reports on the fabrication of Jerusalem cross diplexer by direct write electron beam (EB) lithography followed by reactive ion etching (RIE) on a phosphorus doped polished silicon wafer substrate. Such structures can be used as frequency selective components in visible, microwave and near infra-red wavelength region. Replication of the patterns is accomplished by micron or sub-micron order mould fabricated from the silicon (Si) master. Fourier transform infra-red reflectance (FT-IR) measurements were performed to characterize the structured patterns. The spectral reflectance from these patterns clearly show a reflection dip due to surface plasmon excitation in the near infra-red wavelength at about 1.42 and 2.5 mm, respectively. Potential applications such as antireflection surface (ARS) can be realized. # 2007 Elsevier B.V. All rights reserved. Keywords: Lithography; Infra-red; Cross diplexer
1. Introduction In the emerging micro and nano technology, the demands for the micro-optical components and photonic bandgap devices require the increasing use of micron and sub-micron features [1,2]. It is often desirable to generate/fabricate complex microstructures in various materials depending upon their applications, such as frequency selective surfaces (FSSs), micro sensors, actuators and micro fluidics control devices. But as the feature size falls to the micron or sub-micron order (100– 0.1 mm), the conventional mechanical techniques must be replaced by the beam technology. This technology has been divided into two categories: ‘top down’ approach and ‘bottom up’ approach. The top down approach includes micromachining, microlithography, electron beam patterning and contact lithography. However, in the bottom up approach, binding of structures atom by atom or molecule by molecule and their reproducibilty depends upon the functionality of the material [3]. This is a recently developed technology for miniaturization of functional devices in the millimeter range with minimum features and dimensional tolerances less than a micron. The
degree of precision is achieved by fabricating a master using the electron beam (EB) lithography technique. The micron or submicron order mould fabricated from the master can be replicated. This is achieved by moulding a network photopolymer into the patterned feature. In the present work, by using the top down approach, micron and sub-micron order Jerusalem cross diplexer are produced in doped silicon by EB-lithography followed by reactive ion etching (RIE). By using the photopolymer based methylacrylate/acrylate mixture (PMMA–MMA), the pattern etched in silicon with feature size down to <0.3 mm is further replicated following the bottom up approach. In this new process, polymer resin is used to impress into the micron or sub-micron patterned structure in silicon. After thermal treatment and polymerization, the replica pattern is transferred and then lift-off. On the other hand, due to the metallic behaviour of doped semiconductor at T (Hz) frequencies [4,5], it is possible to excite surface plasmon in such structures when patterned in doped silicon and to use them as frequency selective surfaces (FSSs) or spectrally selective components at visible, microwave and infrared wavelengths. 2. Fabrication process
* Tel.: +91 22 2576 7615; fax: +91 22 2572 3480. E-mail address: [email protected]. 1 Part of the work was done when the author was formerly with, Department of Robotics, Faculty of Science and Engineering Ritsumeikan University, Kusatsu-shi 525-8577, Shiga-ken, Japan. 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.141
In this experiment, two types of patterns, type-[A] and type-[B] of the Jerusalem cross diplexer as shown in Fig. 1, were used for lithography and replication process. Various steps were involved in the fabrication of this patterned
Y.P. Kathuria / Applied Surface Science 253 (2007) 7826–7830
7827
Fig. 1. Block diagram of Jerusalem cross diplexer type-[A] and type-[B]; w: structural gap width between two patterns in type-[A] or between two arrows in type-[B].
structure and its replica as FSSs are shown in Fig. 2. The material used for our experiments was commercially available silicon wafer having a diameter of about 100 mm and a thickness of about 400 mm only. It is doped with phosphorus with a doping concentration of N = 1017 cm3. EB-resist with trade mark ZEP 520-22CP was deposited by spin coat to a thickness of about 0.3 mm and then baked at 100 8C for a few minutes. The features were written in an area of 4 mm 4 mm by focused electron beam (EB) at a voltage of 50 kV and a dose of 100 mC cm2. The resist was developed in isopropanal for about 60 s. The patterns so developed, act as a mask on silicon wafer [6]. In order to transfer the array of patterns into the silicon substrate, a parallel plate reactive ion etching (RIE) was performed in an SF6 atmosphere under a pressure of 13.3 Pa. A flow rate of 30 sccm (SF6) for an RF-power of 150 W was sufficient for the desired pattern in an etching time of about 4.5 min. After the required etching, the EBresist was further removed by similar procedure, but in the
O2 atmosphere. The resulting etched pattern shall act as a silicon master (Fig. 2a) [7]. 3. Pattern replication Patterned silicon master with the desired feature to be replicated was first rinsed with distilled water, followed by cleaning with isopropanal. A small amount of pre-prepared photopolymer solution (PMMA–MMA) was added to the surface of the silicon master and pre-heated to about 180 8C. This was done because, above the glass transition temperature, the polymer has higher fluidity than below and therefore, can better fill the micron or sub-micron order pattern [8]. A clean glass plate was brought in contact and pushed against the silicon master and the viscous liquid was then allowed to spread and flow into the desired pattern in the pre-heated silicon master. After the complete polymerization, the glass plate was removed and the polymerized mould was carefully separated from the silicon master yielding the desired pattern in polymer
Fig. 2. (a) Steps in fabrication of pattern in silicon master were used for the lithography and replication process and (b) contact moulding replication.
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Y.P. Kathuria / Applied Surface Science 253 (2007) 7826–7830
mould (Fig. 2b). This mould was then radio frequency (RF) sputter coated with a few tens of angstrom thick layer of gold and subsequently, analyzed with scanning electron microscope. Further replication can also be done by polymer to polymer moulding process [3]. 4. Characterization The two types of patterns: type-[A] and type-[B] of the Jerusalem cross diplexer, thus, made were characterized by high resolution optical microscope (OM) and scanning electron microscopy (SEM). The spectral reflectance analyses of these patterns etched in silicon were performed by using infra-red fourier transform spectrometer (Jasco FT-IR 660plus) with the following experimental conditions: detection: MCT, TGS; incident angle: 458; resolution: 4 cm1. Before measurements, samples were three times rinsed in hot acetone and ethanol to remove organic contamination. The reflection spectra were measured at 458 from the non-patterned and patterned structure area in the range from 1.4 to 20 mm. The reflection intensity (R) is normalized at 100% to a reference position, which is the non-patterned place sufficiently far from the patterned area. 5. Results and discussion Various photographic sections of the patterns etched in silicon and its replica are shown in Figs. 3 and 4, respectively. From the SEM images (Fig. 4a and b), it is observed that, depending upon the design of pattern, a resolution of <0.3 mm can be achieved in producing these patterns. This, however, depends upon the design of the pattern. The limiting factor in the line scan width depends upon the focussing of electron beam, which in the ideal case is limited to about 25 nm. Besides this, etching conditions, material characteristics and the replication process add to the limiting factor and lead to higher line width. Fig. 5 shows the infra-red reflection spectra corresponding to patterns of type-[A] and type-[B] in Fig. 3, respectively. The result shows that for both type-[A] and type-[B] patterns, there is clearly a reflection dip at lower infra-red wavelength. This dip in the reflection spectra is caused by the surface plasmon (SP) excitation due to interaction between the incoming photon and surface plasmon resonance in the patterned surface area [9–11]. As shown in both curves, the infra-red intensity from the patterned area (B: blue line) clearly decreases as the wavelength decreases. No other peak has been detected except small peaks originated from air (such as CO2, H2O and so on). This is due to the fact that when the photon of wave vector (v/ c) is made incident in the plane region outside the structured surface of silicon, it would be specularly reflected. There will not be any optical excitation of SP, because, for each frequency v, the SP wave vector (ksp) is greater than the
Fig. 3. High resolution optical microscope photograph of the Jerusalem cross diplexer in silicon wafer. Bar size: type-[A]: ( ) 10.00 mm and type-[B]: ) 50.00 mm. (
photon wave vector (v/c) [10], as evident from the dispersion relation: ksp ¼
1=2 v e1 ðvÞ c 1 þ e1 ðvÞ
where e1(v) is real part of dielectric function of the medium, which is negative, but large in magnitue for IR-frequencies (18.1 for doped Si). However, larger wave vector for the incident photon can be achieved at the structured surface, because there is a periodic density variation in the region of the surface, and the patterned area may absorb momentum, thus, generating surface reciprocal lattice vectors G. Therefore, when an incident photon impinges on the patterned surface at an angle u, it can couple to a SP at the interface through a surface G vector [12,13]: v sin ðuÞi þ G ¼ ksp c
Y.P. Kathuria / Applied Surface Science 253 (2007) 7826–7830
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Fig. 4. (a) Scanning electron micrograph of the replicated Jerusalem cross diplexer. Bar size: type-[A]: 6.67 mm and type-[B]: 33.3 mm and (b) scanning electron micrograph of the replicated Jerusalem cross diplexer at higher magnification. Bar size: type-[A]: 1.43 mm and type-[B]: 5.0 mm.
Fig. 5. FT-IR reflection spectra (R) vs. wavelength (l) at 458 from silicon wafer and from patterned structures type-[A], type-[B] in silicon wafer (G: reflection from non-patterned area; B: reflection form patterned area).
where i is a unit vector lying in the surface plane. Thereby, surface plasmon with x-momentum plus x-component of the momentum of the incident photon may be created and hence, excitation of surface plasmon which cause a dip in the reflection spectra. Besides that, in type-[A], the infra-red reflection intensity from patterned area shows a peak at about 1.42 mm (Fig. 5 type-[A]), whereas the corresponding infra-red reflection intensity from the patterned area in type-[B] have broad but clear peak at about 2.5 mm only (Fig. 5 type-[B]). The noise like peaks around 3 and 7 mm correspond to water vapor and a sharp peak around 4 mm is originated from CO2. Interestingly, at shorter wavelength, where incoming l is close to the structural gap width (w), the reflectivity shows a remarkable structure. For the 1.6 mm structural gap width between patterns in Fig. 1 type-[A], a pronounced reflectivity dip occur near 1.42 mm as shown in Fig. 5 type-[A]. Similarly, we observe a broader dip near 2.5 mm (Fig. 5 type-[B]) for the 2.97 mm structural gap width between two arrows in Fig. 1 type-[B]. Besides that, on observing critically (Fig. 3), it is found that due to narrow slit width of type-[A] pattern, the spectral dip at 1.42 mm is towards shorter infra-red wavelength,
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compared to type-[B] with coarse pattern, where the spectral dip at 2.5 mm is sligthly at higher infra-red wavelength. The shift towards shorter wavelength is also attributed to shift in the surface plasmon frequency [11]. Therefore, one concludes that narrow line width in the pattern is responsible for a shift in the spectral dip towards shorter wavelength [14,15], whereas the class of patterns, i.e. type-[A] or type-[B], corresponds to narrowing or broadening of the spectral peak. 6. Conclusion In conclusion, micron and sub-micron order Jerusalem cross diplexer are fabricated in silicon wafer, by electron beam lithography followed by reactive ion etching. These patterned structures are further replicated by thermal polymerization of the impressed resin into the silicon master and then lift-off. The resulting replica patterns are then gold coated and characterized by scanning electron microscope. Features down to 0.3 m are achieved and analysed. The spectral reflectance from these patterns clearly show a reflection dip due to surface plasmon excitation in the near infra-red wavelength at about 1.42 and 2.5 mm, respectively. Acknowledgements The author wishes to express his gratitude to Prof. S. Sugiyama of Ritsumeikan University Japan for the kind support and master candidate Mr. Y. Kwata for the experimental help as
well as thanks to Asst. Profs. Nakada, Li and Dr. Hondoh for their support in characterization.
References [1] Y.P. Kathuria, J. Indian Inst. Sci. 84 (May–August) (2004) 77. [2] T. Garino, J. Cesarano, A. Morales, Mater. Res. Soc. Symp. Proc. 741 (2003) 227. [3] T.A. von Werne, D.S. Germack, E.C. Hagberg, V.V. Sheares, C.J. Hawker, K.R. Carter, J. Am. Chem. Soc. 125 (2003) 3831. [4] J. Gomez Rivas, C. Schotsch, P. Haring Boliver, H. Kurz, Phy. Rev. B68 (2003) 201306 (R). [5] C. Janke, J. Gomez Rivas, C. Schotsch, L. Beckmann, P. Haring Boliver, H. Kurz, Phy. Rev. B69 (2004) 20534. [6] Y. Kanamori, M. Sasaki, K. Hane, Opt. Lett. 24 (1999) 1422. [7] I. Puscasu, G. Boreman, R.C. Tiberio, D. Spencer, R.R. Krchnavek, J. Vac. Sci. Technol. B18 (2000) 3578. [8] N. Lee, Y.-K. Kim, S. Kang, J. Phys. D: Appl. Phys. 37 (2004) 1624. [9] R.H. Ritchie, E.T. Arakawa, J.J. Cowan, R.N. Hamm, Phy. Rev. Lett. 21 (November) (1968) 1530. [10] N. Maeschall, B. Fischer, H.J. Queisser, Phy. Rev. Lett. 27 (July) (1971) 95. [11] W.E. Anderson, R.W. Alexander, R.J. Bell, Phy. Rev. Lett. 27 (October) (1971) 1057. [12] M.U. Pralle, N. Moelders, M.P. McNeal, I. Puscasu, A.C. Greenwald, J.T. Daly, E.A. Johnson, T.F. George, D.S. Choi, E. El-Kady, R. Biswas, Appl. Phys. Lett. 81 (2002) 4685. [13] H. Raether, Surface Plasmons, Springer-Verlag, Berlin, New York, 1988. [14] T.W. Ebbesen, H.J. Lezec, H.F. Ghaemi, T. Thio, P.A. Wolff, Nature 391 (1998) 667. [15] A. Degiron, T.W. Ebbesen, J. Opt.: Pure Appl. Opt. 7 (2005) S90.