Vol.2 No.2 2009

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Research paper : Development of high-sensitivity molecular adsorption detection sensors (M. Fujimaki et al.)−145−Synthesiology - English edition Vol.2 No.2 (2009) on these results, we used a substrate called silicon-on-quarts (SOQ) which comprises of single-crystalline Si layer on silica glass substrate[12] to fabricate the sensing plate, and devised a method for fabricating the waveguide by oxidizing the single-crystalline Si layer[13]. We called the plate fabricated with this method monolithic sensing plate. In addition, we were able to achieve dramatic high sensitivity using the property in which the monolithic sensing plate sensitively detects the optical absorption of an adsorbent substance. This will be explained in chapter 4. Figure 4 shows the series of R&D.3 Results of the developmentThe results obtained in this research are presented as follows.3.1 Achievement of high sensitivity through nanopore formation technologyAs mentioned above, we used the selective etching by HF vapor of latent tracks formed by irradiation with swift heavy ions for formation of nanopores in the waveguide. For ion irradiation, the 12 UD Pelletron tandem accelerator at the University of Tsukuba was used. The ion irradiation method is shown in Fig. 5. The Au ions accelerated at 150 MeV were irradiated onto a Al foil with thickness of 0.8 µm. The ions were scattered by the foil, and a uniform ion beam with low current density was formed. The current density of the ion beam was set to be 100 pA/cm2 on the sensing plate. The reason for using low current density was to accurately control ion fluence, since the ion fluence was extremely low at the order of 109 to 1010 per 1 cm2. For the vapor etching, a 20 % HF solution was used. An irradiated sensing plate was placed in a container of HF solution so it would not become immersed in HF solution, yet the sample would be exposed to HF vapor.Figure 6 shows the SEM photographs of the surface and cross section of a thermally grown SiO2 film with thickness of 2.0 µm which was irradiated with Au ion and etched by HF vapor for 60 min. The temperature of HF solution during etching was 21.5 ºC. The figure shows that the formed pores penetrate the SiO2 film. The thickness of the SiO2 film after etching was 1.9 µm. That is, the aspect ratio of the pore was 42. Using this method, nanopores with diameters in several 10 nm order could be formed accurately.Sensitivity improvement was attempted using this method, by forming nanopores in the waveguide layer of the sensing plate with silica glass waveguide. A glass (OHARA, S-LAH66, refractive index of 1.76924 at 632.8 nm) was used as a substrate. The glass was cut and polished into a 20 mm × 20 mm piece with thickness of 1 mm. Au was used as the reflective film. Cr layers were used as adhesive layers between the Au film and the glass substrate and between the Au film and the waveguide layer. These films were formed by the vacuum deposition method. The thickness of the Au film was 53 nm, and that of the Cr layer was 0.8 nm. The waveguide layer was formed by RF magnetron sputtering using a silica glass plate as a sputtering target. The thickness of the waveguide was 550 nm. After the sputtering, a thermal annealing at 600 °C for 24 hrs was applied to the substrate in order to densify the formed waveguide layer. The nanopores were formed on the waveguide surface using the aforementioned method. The Au ion fluence was 5.0 × 109 cm−2, and HF vapor etching was done for 30 min. The temperature of HF solution was 19.0 ºC. Figure 7 shows the SEM photographs of the surface and cross section of the plate after etching. As shown in the figure, nanopores of diameters of about 30 nm can be observed. It can also be seen that the pores penetrated to the Au layer. The thickness of the waveguide layer after the etching was 400 nm.A right triangle prism made of S-LAH66 was optically attached to the fabricated plate using matching oil, and the measurement of incident angle dependence of reflectance was conducted using the Kretschmann configuration. The light source was s-polarized He-Ne laser (632.8 nm). A cuvette was placed on the waveguide to support liquid samples. The detection sensitivity was assessed by modifying the waveguide surface with biotynil group and by observing the change in reflectance by specific adsorption of streptavidinTerm 4 on biotinTerm 5. Streptavidin was dissolved Fig. 5 Ion irradiation method used in nanopore formation.Fig. 4 Synthesis of R&D. Improvementof stabilityHighly-sensitive highly-stable sensor with nanoporesOptical absorption detecting ultra-sensitive sensorUse of monolithic sensing plateImprovementof sensitivityReview of materialsfor sensing platesSelective etching oflatent tracks byHF vaporEvanescent-field-coupledwaveguide mode sensor(core technology)Waveguide layerLow current density beamHigh current density beamSensing plateAu30+ 137 MeVThickness 0.8 µmAl foilAu14+ 150 MeV

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