Fabrication and Characterization of Optical Waveguide Modulators Implemented on Silicon Substrates

• Main Authors: Mao-Teng Hsu, 徐懋騰
• Other Authors: Yan-Kuin Su
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/47778564987412182083

碩士 === 國立成功大學 === 微機電系統工程研究所 === 94 === Abstract Until now, the silicon-based Mach-Zehnder (M-Z) optical modulators were mostly fabricated on the silicon-on-insulator (SOI) substrate. However, the cost of fabricating M-Z modulators is still considered relatively high due to the expensive nature of SOI wafers. Therefore, the ultimate purpose of this thesis is to fabricate a cost-effective optical amplitude modulator with high response speed, high modulation index, and most of all, still remains compatible to the current CMOS process. Our proposed technique of fabricating P+-P--N+ based optical amplitude modulator on silicon wafer is somewhat similar to the original work of Huang et al. when their result was first reported in 1992. Our whole intention is to fabricate amplitude-modulating devices which are simpler to the M-Z structure and at the same time to neglect the need of using the SOI wafer. The most effective means to modulate silicon optical modulator is to rely on the carrier injection, or plasma dispersion effect. The technique is based on the p-i-n diode structure; while during the forward bias condition the carriers can be subsequently injected into the intrinsic region to serve the purpose of index modulation. Conventionally, the p- and n-doped regions were achieved via ion implantation method. However, the technique is time consuming and highly expansive. Hence, we introduce a much simpler technique using the spin-on-dopant (SOD) diffusion method to carry out the respective doping in both p and n regions. We then employ the spreading resistance probe system (SRP) housed in the Nano Facility Center of the National Chiao Tung University to verify the doping concentration and diffusion depth. The results later demonstrate that the highest doping concentration achieved is 1019/cm3 and the deepest diffusion depth is 1μm. Next, the I-V measurement is conducted to verify the diode nature of P+-P--N+ SOD-diffused structure. The result shows that the rectification is achieved at forward bias condition and very little or no current is observed at reverse bias condition. The optical characterization is then carried out to verify the free carrier injection effect is in fact modulating the amplitude of light once the light passes through the device. The modulating nature based on the carrier injection effect is successfully observed when the intensity of output light becomes lowest as the diode is forward biased, while the opposite is true when the device is reversely biased. Based on the aforementioned optical measurements we hereby conclude that the optical amplitude modulator is successfully fabricated and the influences of different modulation length and current density on the optical modulation index are also quantified. For the 30μm-wide waveguide modulator with the modulation length of 7 mm and the current density of 6mA/mm, the largest optical modulation index achieved is 1.1% using our device characterization system.