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Evaluation of etching on amorphous carbon films in nitric acid
Diamond & Related Materials 24 (2012) 104–106
Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Evaluation of etching on amorphous carbon films in nitric acid☆ Yasushi Sasaki a, Aoi Takeda a, Kiyoto Ii a, Shigeo Ohshio a, Hiroki Akasaka a,⁎, Masayuki Nakano b, Hidetoshi Saitoh a a b
Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan Department of Chemical Science and Engineering, Tokyo National College of Technology, 1220-2 Kunugida, Hachiouji, Tokyo 193-0997, Japan
a r t i c l e
i n f o
Available online 20 December 2011 Keywords: Etching detection Amorphous carbon film Surface plasmon resonance
a b s t r a c t Etching on hydrogenated amorphous carbon (a-C:H) films by nitric acid was evaluated in situ using surface plasmon resonance (SPR) device with a multilayer structure consisting of an a-C:H layer on Au. A flow cell was used to introduce the nitric acid onto the a-C:H top layer of the multilayer structure. Nitric acid solutions with various concentrations were injected into the flow cell. The SPR angle was determined as the angle with minimum reflectivity. The obtained SPR angle decreased with increasing duration of nitric acid injection into the flow cell. The observed behavior of the SPR angle indicated that the a-C:H film surface was etched by nitric acid. The thickness of the film was calculated from the SPR angle, and an etching rate of 3.21 nm/h was obtained in the case of using 2.0 M nitric acid. The obtained shift in the SPR angle depended on the concentration of nitric acid solution. These results indicate that in situ evaluation using the SPR device with a multilayer structure can be used to detect the etching of amorphous carbon films in a liquid. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, hydrogenated amorphous carbon (a-C:H) films have been studied because of their high chemical resistance [1,2]. It has been shown that a-C:H films can potentially be used as coatings for microfluidic devices. The etching of a-C:H films deposited by plasma chemical vapor deposition, which are expected to be used as protective coatings inside micro total analysis systems, was investigated [1]. In studies on the chemical resistance of such films, samples coated with an a-C:H film are usually dipped in an etchant for etching, and cross sections of the etched samples are observed by a scanning electron microscope. In these studies, the process used to evaluate the etching requires a long time and cannot be used to estimate the decrease in film thickness. Hence, a quantitative method for evaluating the chemical resistance of a-C:H in a short time is required. Moreover, in situ detection is the best method. To meet the above requirement, in this paper we propose the use of the surface plasmon resonance (SPR) phenomenon as a method of detecting changes in the thickness of films in liquids. SPR is the resonance between a surface plasmon wave and an evanescent wave on a metal surface [3–8], as shown in Fig. 1. At a metal film on a glass prism, light from a laser is irradiated from the glass and an evanescent wave penetrates through the metal film [3–5]. Plasmon waves are also excited on the reverse side of the film. SPR occurs when the wave numbers of the surface plasmon and evanescent waves are ☆ Presented at NDNC 2011, the 5th International Conference on New Diamond and Nano Carbons, Suzhou, China. ⁎ Corresponding author. E-mail address: [email protected] (H. Akasaka). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.12.006
similar. These waves depend on the laser incident angle; thus, the conditions required for SPR also depend on this angle [8]. When the laser irradiates the back side of a metal film, the intensity of the laser light reflected from the back side of the metal film markedly decreases at the SPR angle. The intensity of a plasmon wave depends on the refractive index of the metal surface. Hence, the refractive index changes with a change in the configuration on the metal surface, leading to a shift in the SPR angle. Generally, the area in which this change in the refractive index can be detected is less than 200 nm above the metal film; this distance is mainly determined by the penetration distance of the evanescent wave [9]. Thus, the proposed system used to detect a change on a metal surface such as a decrease in film thickness is based on the concept of detecting a change in the refractive index. Therefore, the system can detect a change in the thickness of a film on a metal film. We have already reported the relationship between the film thickness on a Au layer and the SPR angle shift, and we proved its validity by a comparison of the film thicknesses obtained by X-ray reflectivity and SPR measurements [10]. According to the above discussion, a change in film thickness can be detected when a film with a thickness of a few nanometers is deposited on a metal. When this film is an amorphous carbon film, a multilayer structure consisting of an amorphous carbon film deposited on a metal will detect a change in the thickness of the film on the metal, as shown in Fig. 1. It is well known that graphite can be etched by nitric acid solution. Since amorphous carbon contains graphitic components, we chose nitric acid as the etchant in this study, in which we attempted to detect the etching of a-C:H by nitric acid through the detection of SPR on the a-C:H/metal multilayer structure shown in as Fig. 1. Furthermore,
Y. Sasaki et al. / Diamond & Related Materials 24 (2012) 104–106
105
Fig. 3. Thickness change due to etching by nitric acid calculated from SPR angles changes.
Fig. 1. Schematic illustration of SPR measurement device of the a-C:H multilayer structure, and flow cell. The scheme does not respect the scales. For example, the film thicknesses have been enlarged.
the etching rate of a-C:H films by nitric acid was estimated using on SPR device with a multilayer structure. 2. Experimental method Fig. 1 shows the multilayer structure used for SPR detection. S-TIH11 optical glass with dimensions of 20 × 20 × 1 mm [3] and a refractive index of 1.778 was chosen as the substrate for the SPR device. A gold (Au) layer was prepared on the S-TIH11 glass by sputtering in Ar gas using a Au target (99.9%). a-C:H films were synthesized on the Au/S-TIH11 glass by magnetron sputtering. These films were deposited by radio-frequency (RF: 13.56 MHz) sputtering from a graphite target in the presence of argon (Ar). The distance between the target and substrate was 20 mm, the pressure was maintained at 20 Pa, and the RF power was set at 150 W. The duration of deposition for each a-C:H film layer was less than 30 min because the film thickness should be less than 200 nm, which is the limit of detection in SPR measurement. The substrate was not damaged by heat during deposition and was sufficiently cool to touch after deposition. Hence, the substrate temperature remained low during deposition. Hydrogen was introduced into the films as a contaminant because the vacuum condition was not ultrahigh. Hence, the obtained film contained approximately 25 at.% hydrogen. The thicknesses of the Au and a-C:H films were approximately 40 and 20 nm, respectively. Finally, the SPR device with the a-C:H/Au/glass structure was integrated with a triangular prism using index-matching oil (n = 1.778). The SPR sensor was set up in the Kretschmann configuration [8–10]. The incident light was a beam from a laser diode with a
a) 0.5 M
b) 1.0 M
wavelength of 635 nm. The intensity of the reflected light was measured using a photodetector. The laser diode and photodetector were mounted on separate rotating stages. A flow cell with a volume of approximately 0.14 mL was attached to top of the SPR device with a multilayer structure to inject the nitric acid onto the a-C:H film. To determine the concentration dependence of the etching of the a-C:H film, we prepared nitric acid with concentrations of 0.5, 1.0, and 2.0 M for the etching tests. These solutions were adjusted to the above concentrations using 60% nitric acid solution (Wako Pure Chemical Industries) and deionized pure water. The procedure for investigating the etching of the a-C:H film with nitric acid of various concentrations was as follows. First, a solution was injected into the flow cell by a syringe pump and the initial SPR angle was immediately measured. The amount of each solution injected into the cell was controlled using the syringe pump. The total amount of solution in each injection was 5 ml and the rate of injection was 2 ml/min. After 15 min, the solution inside the cell was replaced with a new solution with same concentration and the SPR angle was measured again. After that, the solution inside cell was replaced every 15 min and the SPR angle was measured after each replacement. 3. Results Fig. 2 shows SPR curves for each concentration of nitric acid solution. Fig. 2a shows the SPR curves for nitric acid with a concentration of 0.5 M obtained over 150 min at intervals of 15 min. By considering the attenuation of reflected light, the laser incident angle resulting in the weakest reflectivity was defined as θSPR. The reflectivity rapidly decreased at the SPR angle. The SPR angles were 63.49, 63.35, 63.19, 62.98, 62.71, 62.37, 61.93, 61.43, 60.80, 60.06, and 59.39° for etching durations of 0, 15, 30, 45, 60, 75, 90, 105, 120, 135, and 150 min, respectively. The shift of θSPR indicated a decrease in the dielectric constant above the Au film surface caused by the decrease in the
c) 2.0 M
Fig. 2. Reflectivity as a function of incident angles on etching test for nitric acid solution.
106
Y. Sasaki et al. / Diamond & Related Materials 24 (2012) 104–106
Fig. 4. Etching rates calculated from SPR angles.
thickness of the a-C:H film. To determine the film thickness, the obtained profiles were fitted using simulation software based on Fresnel equations (WINSPALL developed by Dr. W. Knoll, MPIP, Germany). This simulation can only be applied to isotropic change in film thickness. The SPR curve 60 min after the initial injection indicated some noise in the range of 48–52°, and film thickness calculation from the simulation curve was difficult because of the noise in the SPR curves obtained by measurement. Since this simulation assumes a uniform and flat film, the thickness of a rough film could not be calculated. However, the film surface became rough after etching for more than 45 min because the curves at the SPR angles were boarded. Hence, the thicknesses of the a-C:H films were determined by simulation in the case of etching for l45 min or less. From the simulation, the film thickness was 19.6 nm after 45 min. For nitric acid concentrations of 1.0 and 2.0 M, film thicknesses were determined by the simulation of SPR curves, as shown Fig. 2b and c, with the exception of curves that included noise. Fig. 3 shows the a-C:H film thicknesses for each nitric acid concentration, which were obtained by simulating the SPR curves. At all concentrations, film thicknesses of up to 2 nm could be determined. Hence, the film surface was flat in the case of etching by up to 2 nm because etching was in one direction, whereas the film surface was rough when the amount of etching exceeded 2 nm because the etching started to occur in several directions. Since this simulation was only valid for a uniform and flat film, the measured SPR profiles for an amount of etching exceeding 2 nm did not match which is obtained by simula-
tion owing to the increase in surface roughness. Thus, using this method, film thicknesses can be determined in the case of etching by up to approximately 2 nm. Because the amorphous carbon contains graphitic and diamond-like components, the etching rate was not uniform. Hence, the initial etching rate seemed the etching rate appeared to be isotropic. After that, the effect of anisotropic etching became large, meaning that the eching rate no longer appeared to be isotropic. Fig. 3 shows that the rate of etching by the injected solution depended on the concentration of nitric acid. The etching rate for each concentration was next calculated from Fig. 3. Fig. 4 shows the etching rate of the a-C:H films for each concentration of nitric acid. The etching rates were 0.6, 1.08, and 3.21 nm/h for concentrations of 0.5, 1.0, and 2.0 M, respectively. Thus, the etching rate clearly increased with the concentration of nitric acid. These results indicate that by evaluating the depth of etching of a-C:H films in a liquid using the device, the initial etching rate of the a-C:H film in the liquid can be estimated. 4. Conclusion Changes in the thickness and etching rate of a-C:H films etched using 0.5, 1.0, and 2.0 M nitric acid were determined using an SPR device with an a-C:H/Au/glass multilayer structure. The etching mode was isotropic up to 2 nm of etching, because SPR curves included noise when etching exceeded 2 nm. In the case of isotropic etching, the etching rates were 0.6, 1.08, and 3.21 nm/h for nitric acid concentrations of 0.5, 1.0, and 2.0 M, respectively. The etching rate clearly increased with the concentration of nitric acid. This method of evaluating etching can be used to estimate the initial etching rate of an a-C:H film in a liquid. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Masahito Ban, Taku Yuhara, Surf. Coat. Technol. 203 (2009) 2587. K. Kurihara, K. Nakamura, K. Suzuki, Sens. Actuators, B 86 (2002) 49–57. B. Liedberg, C. Nylander, I. Lundstrom, Biosens. Bioelectron. 10 (1995) 1–9. R.W. Wood, Philos. Mag. 4 (8) (1902) 396. R.W. Wood, Philos. Mag. 23 (1912) 310. L. Rayleigh, Proc. R. Soc. Lond. A 79 (1907) 399. U. Fano, J. Opt. Soc. Am. 31 (1941) 213. E. Kretschmann, Z. Phys. 241 (1971) 313. S. Kishimoto, S. Ohshio, H. Akasaka, H. Saitoh, Jpn. J. Appl. Phys. 47 (2008) 8106. H. Akasaka, N. Gawazawa, S. Kishimoto, S. Ohshio, H. Saitoh, Appl. Surf. Sci. 256 (2009) 1236.
Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Evaluation of etching on amorphous carbon films in nitric acid☆ Yasushi Sasaki a, Aoi Takeda a, Kiyoto Ii a, Shigeo Ohshio a, Hiroki Akasaka a,⁎, Masayuki Nakano b, Hidetoshi Saitoh a a b
Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan Department of Chemical Science and Engineering, Tokyo National College of Technology, 1220-2 Kunugida, Hachiouji, Tokyo 193-0997, Japan
a r t i c l e
i n f o
Available online 20 December 2011 Keywords: Etching detection Amorphous carbon film Surface plasmon resonance
a b s t r a c t Etching on hydrogenated amorphous carbon (a-C:H) films by nitric acid was evaluated in situ using surface plasmon resonance (SPR) device with a multilayer structure consisting of an a-C:H layer on Au. A flow cell was used to introduce the nitric acid onto the a-C:H top layer of the multilayer structure. Nitric acid solutions with various concentrations were injected into the flow cell. The SPR angle was determined as the angle with minimum reflectivity. The obtained SPR angle decreased with increasing duration of nitric acid injection into the flow cell. The observed behavior of the SPR angle indicated that the a-C:H film surface was etched by nitric acid. The thickness of the film was calculated from the SPR angle, and an etching rate of 3.21 nm/h was obtained in the case of using 2.0 M nitric acid. The obtained shift in the SPR angle depended on the concentration of nitric acid solution. These results indicate that in situ evaluation using the SPR device with a multilayer structure can be used to detect the etching of amorphous carbon films in a liquid. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, hydrogenated amorphous carbon (a-C:H) films have been studied because of their high chemical resistance [1,2]. It has been shown that a-C:H films can potentially be used as coatings for microfluidic devices. The etching of a-C:H films deposited by plasma chemical vapor deposition, which are expected to be used as protective coatings inside micro total analysis systems, was investigated [1]. In studies on the chemical resistance of such films, samples coated with an a-C:H film are usually dipped in an etchant for etching, and cross sections of the etched samples are observed by a scanning electron microscope. In these studies, the process used to evaluate the etching requires a long time and cannot be used to estimate the decrease in film thickness. Hence, a quantitative method for evaluating the chemical resistance of a-C:H in a short time is required. Moreover, in situ detection is the best method. To meet the above requirement, in this paper we propose the use of the surface plasmon resonance (SPR) phenomenon as a method of detecting changes in the thickness of films in liquids. SPR is the resonance between a surface plasmon wave and an evanescent wave on a metal surface [3–8], as shown in Fig. 1. At a metal film on a glass prism, light from a laser is irradiated from the glass and an evanescent wave penetrates through the metal film [3–5]. Plasmon waves are also excited on the reverse side of the film. SPR occurs when the wave numbers of the surface plasmon and evanescent waves are ☆ Presented at NDNC 2011, the 5th International Conference on New Diamond and Nano Carbons, Suzhou, China. ⁎ Corresponding author. E-mail address: [email protected] (H. Akasaka). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.12.006
similar. These waves depend on the laser incident angle; thus, the conditions required for SPR also depend on this angle [8]. When the laser irradiates the back side of a metal film, the intensity of the laser light reflected from the back side of the metal film markedly decreases at the SPR angle. The intensity of a plasmon wave depends on the refractive index of the metal surface. Hence, the refractive index changes with a change in the configuration on the metal surface, leading to a shift in the SPR angle. Generally, the area in which this change in the refractive index can be detected is less than 200 nm above the metal film; this distance is mainly determined by the penetration distance of the evanescent wave [9]. Thus, the proposed system used to detect a change on a metal surface such as a decrease in film thickness is based on the concept of detecting a change in the refractive index. Therefore, the system can detect a change in the thickness of a film on a metal film. We have already reported the relationship between the film thickness on a Au layer and the SPR angle shift, and we proved its validity by a comparison of the film thicknesses obtained by X-ray reflectivity and SPR measurements [10]. According to the above discussion, a change in film thickness can be detected when a film with a thickness of a few nanometers is deposited on a metal. When this film is an amorphous carbon film, a multilayer structure consisting of an amorphous carbon film deposited on a metal will detect a change in the thickness of the film on the metal, as shown in Fig. 1. It is well known that graphite can be etched by nitric acid solution. Since amorphous carbon contains graphitic components, we chose nitric acid as the etchant in this study, in which we attempted to detect the etching of a-C:H by nitric acid through the detection of SPR on the a-C:H/metal multilayer structure shown in as Fig. 1. Furthermore,
Y. Sasaki et al. / Diamond & Related Materials 24 (2012) 104–106
105
Fig. 3. Thickness change due to etching by nitric acid calculated from SPR angles changes.
Fig. 1. Schematic illustration of SPR measurement device of the a-C:H multilayer structure, and flow cell. The scheme does not respect the scales. For example, the film thicknesses have been enlarged.
the etching rate of a-C:H films by nitric acid was estimated using on SPR device with a multilayer structure. 2. Experimental method Fig. 1 shows the multilayer structure used for SPR detection. S-TIH11 optical glass with dimensions of 20 × 20 × 1 mm [3] and a refractive index of 1.778 was chosen as the substrate for the SPR device. A gold (Au) layer was prepared on the S-TIH11 glass by sputtering in Ar gas using a Au target (99.9%). a-C:H films were synthesized on the Au/S-TIH11 glass by magnetron sputtering. These films were deposited by radio-frequency (RF: 13.56 MHz) sputtering from a graphite target in the presence of argon (Ar). The distance between the target and substrate was 20 mm, the pressure was maintained at 20 Pa, and the RF power was set at 150 W. The duration of deposition for each a-C:H film layer was less than 30 min because the film thickness should be less than 200 nm, which is the limit of detection in SPR measurement. The substrate was not damaged by heat during deposition and was sufficiently cool to touch after deposition. Hence, the substrate temperature remained low during deposition. Hydrogen was introduced into the films as a contaminant because the vacuum condition was not ultrahigh. Hence, the obtained film contained approximately 25 at.% hydrogen. The thicknesses of the Au and a-C:H films were approximately 40 and 20 nm, respectively. Finally, the SPR device with the a-C:H/Au/glass structure was integrated with a triangular prism using index-matching oil (n = 1.778). The SPR sensor was set up in the Kretschmann configuration [8–10]. The incident light was a beam from a laser diode with a
a) 0.5 M
b) 1.0 M
wavelength of 635 nm. The intensity of the reflected light was measured using a photodetector. The laser diode and photodetector were mounted on separate rotating stages. A flow cell with a volume of approximately 0.14 mL was attached to top of the SPR device with a multilayer structure to inject the nitric acid onto the a-C:H film. To determine the concentration dependence of the etching of the a-C:H film, we prepared nitric acid with concentrations of 0.5, 1.0, and 2.0 M for the etching tests. These solutions were adjusted to the above concentrations using 60% nitric acid solution (Wako Pure Chemical Industries) and deionized pure water. The procedure for investigating the etching of the a-C:H film with nitric acid of various concentrations was as follows. First, a solution was injected into the flow cell by a syringe pump and the initial SPR angle was immediately measured. The amount of each solution injected into the cell was controlled using the syringe pump. The total amount of solution in each injection was 5 ml and the rate of injection was 2 ml/min. After 15 min, the solution inside the cell was replaced with a new solution with same concentration and the SPR angle was measured again. After that, the solution inside cell was replaced every 15 min and the SPR angle was measured after each replacement. 3. Results Fig. 2 shows SPR curves for each concentration of nitric acid solution. Fig. 2a shows the SPR curves for nitric acid with a concentration of 0.5 M obtained over 150 min at intervals of 15 min. By considering the attenuation of reflected light, the laser incident angle resulting in the weakest reflectivity was defined as θSPR. The reflectivity rapidly decreased at the SPR angle. The SPR angles were 63.49, 63.35, 63.19, 62.98, 62.71, 62.37, 61.93, 61.43, 60.80, 60.06, and 59.39° for etching durations of 0, 15, 30, 45, 60, 75, 90, 105, 120, 135, and 150 min, respectively. The shift of θSPR indicated a decrease in the dielectric constant above the Au film surface caused by the decrease in the
c) 2.0 M
Fig. 2. Reflectivity as a function of incident angles on etching test for nitric acid solution.
106
Y. Sasaki et al. / Diamond & Related Materials 24 (2012) 104–106
Fig. 4. Etching rates calculated from SPR angles.
thickness of the a-C:H film. To determine the film thickness, the obtained profiles were fitted using simulation software based on Fresnel equations (WINSPALL developed by Dr. W. Knoll, MPIP, Germany). This simulation can only be applied to isotropic change in film thickness. The SPR curve 60 min after the initial injection indicated some noise in the range of 48–52°, and film thickness calculation from the simulation curve was difficult because of the noise in the SPR curves obtained by measurement. Since this simulation assumes a uniform and flat film, the thickness of a rough film could not be calculated. However, the film surface became rough after etching for more than 45 min because the curves at the SPR angles were boarded. Hence, the thicknesses of the a-C:H films were determined by simulation in the case of etching for l45 min or less. From the simulation, the film thickness was 19.6 nm after 45 min. For nitric acid concentrations of 1.0 and 2.0 M, film thicknesses were determined by the simulation of SPR curves, as shown Fig. 2b and c, with the exception of curves that included noise. Fig. 3 shows the a-C:H film thicknesses for each nitric acid concentration, which were obtained by simulating the SPR curves. At all concentrations, film thicknesses of up to 2 nm could be determined. Hence, the film surface was flat in the case of etching by up to 2 nm because etching was in one direction, whereas the film surface was rough when the amount of etching exceeded 2 nm because the etching started to occur in several directions. Since this simulation was only valid for a uniform and flat film, the measured SPR profiles for an amount of etching exceeding 2 nm did not match which is obtained by simula-
tion owing to the increase in surface roughness. Thus, using this method, film thicknesses can be determined in the case of etching by up to approximately 2 nm. Because the amorphous carbon contains graphitic and diamond-like components, the etching rate was not uniform. Hence, the initial etching rate seemed the etching rate appeared to be isotropic. After that, the effect of anisotropic etching became large, meaning that the eching rate no longer appeared to be isotropic. Fig. 3 shows that the rate of etching by the injected solution depended on the concentration of nitric acid. The etching rate for each concentration was next calculated from Fig. 3. Fig. 4 shows the etching rate of the a-C:H films for each concentration of nitric acid. The etching rates were 0.6, 1.08, and 3.21 nm/h for concentrations of 0.5, 1.0, and 2.0 M, respectively. Thus, the etching rate clearly increased with the concentration of nitric acid. These results indicate that by evaluating the depth of etching of a-C:H films in a liquid using the device, the initial etching rate of the a-C:H film in the liquid can be estimated. 4. Conclusion Changes in the thickness and etching rate of a-C:H films etched using 0.5, 1.0, and 2.0 M nitric acid were determined using an SPR device with an a-C:H/Au/glass multilayer structure. The etching mode was isotropic up to 2 nm of etching, because SPR curves included noise when etching exceeded 2 nm. In the case of isotropic etching, the etching rates were 0.6, 1.08, and 3.21 nm/h for nitric acid concentrations of 0.5, 1.0, and 2.0 M, respectively. The etching rate clearly increased with the concentration of nitric acid. This method of evaluating etching can be used to estimate the initial etching rate of an a-C:H film in a liquid. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Masahito Ban, Taku Yuhara, Surf. Coat. Technol. 203 (2009) 2587. K. Kurihara, K. Nakamura, K. Suzuki, Sens. Actuators, B 86 (2002) 49–57. B. Liedberg, C. Nylander, I. Lundstrom, Biosens. Bioelectron. 10 (1995) 1–9. R.W. Wood, Philos. Mag. 4 (8) (1902) 396. R.W. Wood, Philos. Mag. 23 (1912) 310. L. Rayleigh, Proc. R. Soc. Lond. A 79 (1907) 399. U. Fano, J. Opt. Soc. Am. 31 (1941) 213. E. Kretschmann, Z. Phys. 241 (1971) 313. S. Kishimoto, S. Ohshio, H. Akasaka, H. Saitoh, Jpn. J. Appl. Phys. 47 (2008) 8106. H. Akasaka, N. Gawazawa, S. Kishimoto, S. Ohshio, H. Saitoh, Appl. Surf. Sci. 256 (2009) 1236.