Waveguide-mode Sensor Chip with Si/SiO 2 /SiO x Structure

A waveguide-mode sensor detects changes in the complex refractive index on the surface of a planar waveguide chip with high sensitivity. We developed a waveguide-mode sensor chip with the Si/SiO 2 structure. It was predicted from simulations that a third layer of high refractive index added to the outermost surface of the planar waveguide may improve the sensitivity of the waveguide-mode sensor. In this research, we designed and manufactured a borosilicate glass substrate chip with a Si/SiO 2 /SiO x structure via sputtering deposition, and we improved the sensitivity of the waveguide-mode sensor chip. The Si/SiO 2 /SiO x structure chip was approximately 1.5 times more sensitive to refractive index changes than the Si/SiO 2 structure chip.


Introduction
Various fields, such as the medical and pharmaceutical fields, require highly sensitive biological substance detection technology. The enhancement of electric fields of evanescent light, such as that used in surface plasmon resonance sensors, is a widely applied method for realizing highly sensitive detection. (1)(2)(3) The excitation of waveguide modes is one way to obtain an enhanced electric field of evanescent light. (4)(5)(6) A waveguide mode sensor detects changes in the complex refractive index on a planar waveguide surface made from stacked thin films, through electric field enhancement by multiple reflections at a specific wavelength. (5)(6)(7)(8) For wavelengths with an enhanced electric field, a decrease in reflectance occurs and appears as a dip in the reflectance spectrum. When the refractive index on a waveguide surface increases, the dip position shifts toward the longer wavelength side. Using this property of the waveguidemode, various substances, such as proteins, viruses, and biomarkers, adsorbed on the waveguide surface can be detected with high sensitivity. (9)(10)(11)(12)(13)(14)(15) Waveguide-mode sensors using a planar waveguide, with single-crystalline Si (c-Si) and SiO 2 layers sequentially stacked on a silica glass substrate, have been developed. (5,(9)(10)(11) However, the waveguide-mode sensor chip fabricated using the silica glass substrate is expensive for commercial use. Inexpensive optical glass, such as borosilicate glass, is desirable for use as a substrate, but borosilicate glass has a drawback in that its refractive index is higher than that of silica glass. If the refractive index of the substrate becomes higher, the sensitivity becomes worse.
In this study, numerical simulations predicted that increasing the refractive index n of the material of the outermost surface of a planar waveguide improves the sensitivity of the device, which can compensate for the drawback of borosilicate glass. To prove this prediction, we developed a sensor chip having an additional high-refractive-index layer. As an example of the material for the high-refractive-index layer, we chose SiO x whose refractive index is higher than that of SiO 2 . We designed and fabricated planar waveguides with the three-layer structure of μc-Si/SiO 2 /SiO x , and examined the performance of the waveguide-mode sensor.
For the fabrication of a Si/SiO 2 stacked film structure on an optical glass substrate, radio frequency (RF) magnetron sputtering is a desirable fabrication method. (16)(17)(18) Several techniques have been reported for forming an amorphous Si (a-Si) or a microcrystalline Si (μc-Si) at low temperatures that can be adopted for optical glass substrates having a low strain point, such as borosilicate glass. (17,18) Figure 1 shows the schematic of the optical system of the waveguide-mode sensor based on a parallel-incident-type sensor. (11) Borosilicate glass (BK-7) with a trapezoidal prism shape was used as the substrate of the sensor chip. The base angle on the upper base side of the substrate was 35°. The upper base length, width, and height of the substrate were 21, 8, and 2 mm, respectively. A RF magnetron sputtering device (i-miller, CFS-4EP-LL, Shibaura Mechatronics Corporation, Japan) was used to fabricate the waveguide-mode sensor chip. Si (99.999%, Kyodo International Inc., Japan) was used as the target for μc-Si film deposition. The sputter deposition of μc-Si was performed under an atmosphere of Ar and H 2 with flow rates of 8 and 12 sccm, respectively. SiO 2 (99.99%, Kyodo International Inc., Japan) was used as the target for SiO 2 film deposition. The sputter deposition of SiO 2 was performed under an atmosphere (Color online) Schematic of optical system of waveguide-mode sensor. The optical system is based on a parallel-incident-type waveguide-mode sensor. (11) The symmetrical trapezoidal prism-shaped sensor chip is made of BK-7. The base angle θ on the upper base side of the sensor chip is 35°, and the liquid cell is filled with sample solutions of water or aqueous sodium chloride. White LEDs were used as the light source.

Fabrication of planar waveguides
of Ar and O 2 with flow rates of 9 and 1 sccm, respectively. SiO (99.9%, Kyodo International Inc., Japan) and SiO 2 were used as the targets for SiO x film deposition. The sputter deposition of SiO x was performed under an atmosphere of Ar and H 2 with flow rates of 15 sccm for both gases. In each sputtering process, the initial pressure was 1.5 × 10 −3 Pa, and the temperature of the heater for the stage on which the substrates were fixed was 300 °C. The refractive index n and extinction coefficient k of each sputter-deposited layer were measured using a spectroscopic ellipsometer (UVISEL, M200-FUV-FGMS-HNSTSS, HORIBA Jobin Yvon GmbH, Germany).

Evaluation of fabricated waveguide-mode sensor chip
The sensitivity of the fabricated waveguide-mode sensor chips was evaluated using a waveguide-mode sensor (Eva-M02, C&I Co., Ltd., Japan). The incident light, whose direction was parallel to the bottom surface of the sensor chip, was irradiated onto the waveguide structure deposited on the prism-shaped sensor chip (see Fig. 1). The light was reflected and diffracted at each boundary of the layers in the waveguide structure and totally reflected at the chip surface. A dip was observed where the wavelength in the spectrum of the reflected light directed to the spectrometer has a lower reflectance, which was due to the optical absorption by the Si layer. The dip shifted to the longer-wavelength side owing to the increase in the refractive index at the chip surface. (7,11) The experimental procedure for evaluating the performance of the fabricated sensor chips was as follows. First, pure water was introduced into the liquid cell formed on the fabricated sensor chip, and the reflection spectrum was measured using the spectrometer. Next, sodium chloride aqueous solution was introduced into the liquid cell, and the reflection spectrum was measured. The liquid cell was washed three times with pure water.

Theoretical Simulation
The model of the optical system used for the simulation of the electric field is shown in Fig. 1. The waveguide structure models of the Si/SiO 2 and Si/SiO 2 /SiO x waveguide-mode sensor chips are shown in Fig. 2. The results of complex refractive index measurements by ellipsometry are shown in Fig. 3. Figures 3(a)-3(c) show the results of μc-Si, SiO 2 , and SiO x , respectively. Table 1 shows the complex refractive indices of the fabricated Si layer and those of the a-Si and μc-Si films reported in Refs. 19 and 20, respectively. As shown in Fig. 3(a) and Table 1, the k values of the fabricated Si layer were similar to those of μc-Si and lower than those of a-Si reported in the references.
Using the obtained complex refractive indices of μc-Si, SiO 2 , and SiO x and the referenced complex refractive indices of c-Si and a-Si, (19,21) Si/SiO 2 and Si/SiO 2 /SiO x planar waveguide structures for waveguide-mode sensors were designed. In this study, the planar waveguide structures were optimized to enhance the electric field of light with a wavelength of 650 nm. The outermost surface was assumed to be in contact with water. The simulation was performed using the transfer matrix method for a stratified medium using the Fresnel equations [see Eq. (1)]. (22) 2sin cos sin( where α is the incident angle, β is the reflection angle, t s and r s are the transmission and reflection coefficients of s polarized light, respectively. For the simulations, a conventional Si/SiO 2 waveguide structure was first designed. The enhancement factor of the electric field at the waveguide-water interface as a function of the Si and SiO 2 layer thicknesses is shown in Fig. 4. Figures 4(a)-4(c) show the results of the calculations using the c-Si, a-Si, and μc-Si films as the Si layer, respectively. Table 2 shows the   highest enhancement factors of the simulated c-Si/SiO 2 , a-Si/SiO 2 , and μc-Si/SiO 2 waveguides. The Si and SiO 2 layer thicknesses in each chip are summarized in Table 2. Next, a Si/SiO 2 /SiO x waveguide structure was designed. The result of the calculation of the enhancement factor as a function of the SiO 2 and SiO x layer thicknesses is shown in Fig. 5. Figures 5(a)-5(c) show the results of the calculations using the c-Si, a-Si, and μc-Si films as the Si layer, respectively. The Si layer thicknesses were fixed at values shown in Table 2. Table 3 shows the highest enhancement factors of the simulated c-Si/SiO 2 /SiO x , a-Si/SiO 2 /SiO x , and μc-Si/SiO 2 /SiO x waveguides, and the Si, SiO 2 , and SiO x layer thicknesses. Figure 6 shows the simulations of the dip wavelength changes Δλ with the refractive index change Δn. The values of Δλ was obtained by subtracting the dip wavelength of the reflection spectrum in the measurement with the NaCl solution from that in the measurement with pure water. The calculation results of the designed Si/SiO 2 and Si/SiO 2 /SiO x structure chips are indicated by solid and broken lines, respectively. As shown in Fig. 6, the slopes of the designed c-Si/SiO 2 , a-Si/SiO 2 , and μc-Si/SiO 2 structure chips at Δn = 0 were all 850 nm/RIU, where RIU stands for the refractive index unit. The slopes of the designed c-Si/SiO 2 /SiO x , a-Si/SiO 2 / SiO x , and μc-Si/SiO 2 /SiO x structure chips at Δn = 0 were all 1250 nm/RIU. The designed Si/ SiO 2 /SiO x structure chips were 1.47 times more sensitive than the designed Si/SiO 2 structure chips to the refractive index changes Δn. By adding a high-refractive-index material layer, the sensitivity of the waveguide-mode sensor to the refractive index changes is expected to improve.    Figure 7 shows the reflectance spectra obtained when pure water was measured and the dip wavelength changes Δλ observed when the sodium chloride solutions were measured. The dip wavelengths of the fabricated μc-Si/SiO 2 and μc-Si/SiO 2 /SiO x structure chips in the pure water measurement were 658.9 ± 6.4 and 651.9 ± 6.5 nm, respectively. The refractive indices    of the sodium chloride solutions were measured using an Abbe refractometer. The linear approximations of the results of the fabricated μc-Si/SiO 2 and μc-Si/SiO 2 /SiO x structure chips obtained by the least-squares method are indicated by solid and broken lines, respectively. As shown in Fig. 7, the slopes of the linear approximations of the fabricated μc-Si/SiO 2 and μc-Si/SiO 2 /SiO x structure chips were 1412 and 2125 nm/RIU, respectively. The fabricated μc-Si/SiO 2 /SiO x structure chip was 1.5 times more sensitive than the fabricated μc-Si/SiO 2 structure chip to the refractive index changes Δn.

Discussion
As shown in Figs. 6 and 7, the Δλ/Δn slopes of the Si/SiO 2 /SiO x structure chips were approximately 1.5 times those of the Si/SiO 2 structure chips in both theoretical and experimental results. The deposition of the third layer with a high refractive index is effective in improving the sensitivity of the waveguide-mode sensor. The Δλ/Δn slopes of the fabricated μc-Si/SiO 2 and μc-Si/SiO 2 /SiO x structure chips were approximately 1.7 times larger than those of the designed chips in the simulations. This might be caused by the unevenness of the fabricated chip surfaces. Unlike the ideally flat surface assumed in the simulations, the sputter-deposited surfaces have irregularities. Nanoholes in the waveguide structure surface can effectively enhance the sensitivity of the waveguide-mode sensor. (9) The unevenness of the fabricated chip surfaces might similarly enhance the sensitivity of the waveguide-mode sensor. The standard deviation of the dip wavelength with respect to pure water of the three-layer waveguide structure chips was less than 1%, and the dip wavelength did not change for two months (data not shown). These results indicate that the reproducibility and stability of the fabricated SiO x layer is good enough for practical use.
In this study, we investigated and developed sensor chips having an outermost SiO x layer. Such higher sensitivity can be obtained regardless of the material of the third layer as long as the material has a higher refractive index than SiO 2 . Figure 8 shows calculation result of Δλ of the waveguide-mode sensor with the refractive index of the outermost layer, where Δn of the solution was assumed to be 0.01. In the calculation, the thickness of the Si layer was 54 nm. The thicknesses of the outermost layer and the SiO 2 layer were optimized to make the sensor having the initial dip position of 650 nm and to maximize the Δλ for each material. The refractive indices of BK-7, SiO x , Ta 2 O 5 , C (Diamond) are indicated by open circles in Fig. 8. As shown in Fig. 8, Δλ increases with increasing n of the outermost layer, indicating that a highrefractive-index material is desirable for the outermost layer. Figures 9(a) and 9(b) show intensities of the enhanced electric field of the two-layer (μc-Si/SiO 2 ) and three-layer (μc-Si/SiO 2 /SiO x ) waveguides. The distance from the chip surface of the waveguide structure and the enhancement factor of the electric field are indicated on the y-axis and color bar, respectively. The interfaces of each layer of the waveguides are indicated  . (Color online) Intensities of enhanced electric field of (a) two-layer (μc-Si/SiO 2 ) and (b) three-layer (μc-Si/SiO 2 /SiO x ) waveguides. The enhancement factor indicates the value of electric field intensity, assuming that the electric field intensity of incident light is 1. The interfaces of each layer of the waveguides are indicated by the blue broken lines. The thicknesses of layers are the same as those in Tables 2 and 3. by the blue broken lines. As can be seen, the outermost SiO x layer makes the peak position of the enhanced electric field closer to the chip surface, and as a result, the intensity of the electric field at the surface of the three-layer waveguide is higher than that of the two-layer waveguide. It seems that the increase in the intensity of the electric field by the third layer improves the sensitivity of the waveguide-mode sensor.

Conclusions
In this study, we improved the sensitivity of waveguide-mode sensors with the Si/SiO 2 /SiO x structure in which a high-refractive-index SiO x layer is added to the outermost surface of a Si/SiO 2 structure waveguide. The sensitivities of the Si/SiO 2 -and Si/SiO 2 /SiO x -structure waveguide-mode sensor chips using c-Si, a-Si, and μc-Si as the Si layers were compared through simulations. In all the cases, the Si/SiO 2 /SiO x structure was 1.5 times more sensitive than the Si/SiO 2 structure in a waveguide-mode sensor chip. This result was in agreement with the experimental results obtained where the sensor chip was fabricated by the RF sputtering method. These results indicate that the addition of a high-refractive-index layer is an easy and effective way to improve the sensitivity of waveguide-mode sensors.