| dc.description.abstract | In the past half-century, there has been tremendous growth in the area of silicon integrated circuits. Utilizing standard CMOS manufacturing processes, integrated, large-scale photonic circuits are available with low-cost fabrication. In addition to the mature of telecommunication industry, silicon photonic applications have now expanded into sensing, nonlinear optics, quantum optics, opto-mechanics, and even neuroscience. Among all the photonic functions, microresonators are mostly applied as modulators, sensors, filters, and cavities for enhancing pump power. The coupling strength of microresonator is typically determined by the available gap between the bus- and resonator-waveguide and limited by the lithography resolution for patterning. Although resonance with high extinction ratio can be achieved by advanced electron-beam lithography (EBL), it is time-consuming and costly. For the first part of this work, the author shows the capability to achieve sub-micron coupling gap between the bus- and microresonator-waveguides with the conventional contact UV lithography. Using the proposed double exposure technology, effective coupling can be achieved in which an extinction ratio 8 dB and gap size < 1 μm is demonstrated. This method exhibits low-cost lithography and yields a sub-micron gap which can be only previously achieved by high-resolution exposure techniques, such as i-line UV stepper lithography or EBL. The patterning of photoresist is based on image reversal process, providing negative slope of the patterning sidewall in the lift off process. In addition, this process gives better resolution, comparing to the direct image projection. However, this process is sensitivity to the fabrication parameters. For instance, temperature of processes will strongly affect the fabricated gap. We will discuss the effect on the temperature change of soft bake, postexposure bake, and the subsequent waiting time, and obtain the relatively stable fabrication process. We show that the adjustment of flood exposure time is especially critical to the coupling gap, and the resolution can be significantly improved by the proposed double exposure technique.
In the second part, we study the capability to fabricate microresonators by i-line UV stepper lithography, which has better resolution (~400 nm) than the contact lithography. I-line Steppers are commonly used in fabrication of microresonator, due to its longer mask lifetime, the repeatability, high productivity, and large-scale manufacturing. In particular, the productivity paves the way for integrated photonics and shows comparable performance with the EBL for microresonator fabrication. Here, we will discuss different waveguide dimensions and investigate the highest available intrinsic quality factor Qi. In this work, the maximum Qi is demonstrated at 2105. Furthermore, it is found that, when the input waveguide width is set at 1 μm, the background interference can be suppressed in comparing to the 2 μm bus. Also, the observed transmission loss at 2 μm is 0.08 dB/mm. To improve the quality of the fabricated waveguides, we performed annealing with oxide cladding in the previous experiment, and then performed annealing with air cladding, and observed a significant change in Qi from 8.1*104 to 1.3*105. Comparing to the results in the first part, a high extinction ratio > 10 dB, high Q, repeatability can be achieved by the i-line UV stepper lithography.
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