Surface plasmon polaritons are collective oscillation of electrons in the surface of the metals such as gold, silver and aluminum and it is caused by the incident light. When the specific conditions such as angle of incidence, wavelength and refractive index of materials, are satisfied, the incident light can be absorbed as a surface plasmon polaritons and the reflectance is rapidly decreased. This phenomenon is called as surface plasmon resonance. Characteristics of the surface plasmon resonance is changed sensitively according to a dielectric constant and refractive index change of the materials near the metal surface. Thus, if target molecules contact on the metal surface, surface plasmon resonance effect is changed and we can use this system as a sensor. The surface plasmon resonance sensor has been widely used in the chemical biology and drug design with advantages of label free detection, room temperature detection, and real time monitoring. There are two types of coupling configuration for surface plasmon resonance system using thin metal film. In the kretschmann configuration, metal film is located on the glass surface. On the other hand, metal film of the Otto configuration is located with a distance of the few micrometers from the glass. Due to the robust structure and convenience of fabrication, most of the surface plasmon resonance sensors are based on the kretschmann configuration. However, Otto configuration has a potential for tunable resonance characteristics according to the air-gap distance. If the air-gap distance between the glass substrate and the metal thin film can be stably adjusted, it will be possible to use a surface plasmon resonance sensor with the structural flexibility of the Otto configuration.
In this study, the Otto configuration based sensor was fabricated through silicon on quartz bonding process. An air-gap distance of the sensor was designed to be 2.2 um and whole size of the sensor is 30*30*1 mm^3. Characteristics of the sensor was measured using a laser module with wavelength of 975.1 nm and the results was compared with FEM simulation results. It was verified that the characteristics can be sensitively changed according to the refractive index of the metal film.
Performance of the sensor for gas sensing application was verified with simulation results and the results was compared with those of the other researches. The sensitivity of the fabricated sensor is 55 degree/RIU where the refractive index of the sensing medium is varied from 1 to 1.008.
To verify the potential of the fabricated sensor as a pressure sensor, resonance characteristics was measured with applied pressure level from 0.23 bar to 1.3 bar. The air-gap distance of the fabricated Otto configuration was varied according to the pressure and the resonance characteristics was also changed. From the measurement results, the sensor has a sensitivity and resolution of about 0.17 degree/bar and 0.029 bar, respectively, as a pressure sensing application. Results show that the fabricated Otto configuration based sensor can be used as a hazardous gas and pressure level of the industrial facility with high sensitivity.
Otto configuration with stepped air-gap (1.86 um, 2.42 um, 3.01 um, 3.43 um) was fabricated and the resonance characteristics at each air-gap was measured with 980 nm laser source. Minimum reflectance is measured to be 0.688, 0.716, 0.766, 0.86 at each section and the measurement results was analyzed with FEM simulation results.
To verify the variability of characteristics of the Otto configuration and its applicability, the air-gap between the prism and the metal film was adjusted by using a commercial piezoelectric actuator. When the wavelength of the incident light was 786 nm, the reflectance was measured as 0.25 where the displacement of the piezoactuator was about 10.5 um, and it was confirmed that the maximum surface plasmon resonance occurred. Likewise, when a light source having a wavelength of 977 nm is used, the minimum reflectance becomes 0.22 where the displacement of the piezoactuator is about 9.3 um. The results shows that the wavelength of incident light for the maximum surface plasmon resonance in the Otto configuration is proportional to the air-gap in the actual measurement results.
With an aim of inducing the variability of characteristics of the Otto configuration, a serpentine spring based MEMS actuator was designed and the driving characteristics were theoretically analyzed through numerical calculation and FEM simulation methods. The MEMS actuator for adjusting the air-gap of the Otto configuration was designed based on low-resistivity silicon. A 200 nm-thick gold thin film is located on the upper surface of the driving part. The initial distance between the silicon driving part and the driving electrode on the surface of the glass substrate is designed to be 18 um, and the distance between the gold film and the glass substrate is adjusted to be about 1 um from the initial value of 6 um.
A resonance frequency of the driving part was calculated to be about 1.34 kHz, and it was confirmed that the driving part of the designed MEMS actuator can maintain a stable state even if an external shock is applied. FEM simulation results show that driving part of the proposed sensor can be moved vertically as much as 5.1 um when a potential difference of 59.2 V is applied between the driving electrode on the glass substrate and the driving part.
It is expected that the results of this study can be used as a background data for the development of Otto configuration based surface plasmon resonance sensors with variable resonance characteristics, which has not been reported so far.