Improved Waveguide Surface Roughness by Foundry-Processing Techniques for Enhanced Light Delivery to Integrated Ion Trap for Quantum Computing Platforms

Various types of qubits have been proposed and demonstrated for quantum information processing. Amongst the candidates, the trapped ion platform is a forerunner for high-fidelity gates and long coherence times of the qubits. Currently, the most advanced scalable ion trap architecture is based on microfabricated ion trap electrodes integrated with photonic circuits to deliver the laser light of different wavelengths performing the tasks of qubit initialization, gate operations and state determination. These operations require low-loss photonic components for a spectrum of wavelengths to address the trapped ion qubit. Moreover, based on the ion species, the wavelengths also differ. Thus, it requires a whole range of designs to accommodate different wavelengths from infra-red to ultra-violet. A low-loss waveguide is typically constructed using silicon nitride (SiN) as core with silicon oxide (SiO2) as the cladding material. During fabrication, some of the processes may degrade the surface quality of the waveguide core resulting in higher propagation loss. In this study, the relation between the waveguide propagation loss and to morphology of the waveguide core surface is investigated. The effect on waveguide propagation loss from the bottom oxide and SiN by chemical mechanical polishing (CMP) is compared to SiN dry etching. Both plasma- enhanced (PE) and low-pressure (LP) chemical vapor deposited (CVD) SiN on 12" Si wafers are used during evaluation. In the case of a well-designed waveguide, it is found that a reduction of the sidewall roughness by as much as 15% translates to a 20% improvement in the propagation loss at 1.6 mu m. On the contrary, when the waveguide core's top and bottom surfaces are polished by CMP, the roughness improves by a factor of 1.75 and 18 for oxide and nitride surfaces respectively. This process however reduces the propagation loss by about 7 times at 1.6 mu m wavelength. Furthermore, other improvements in processing techniques enable the fabrication of LPCVD SiN waveguides with propagation loss of 0.56 dB/cm at 685nm wavelength. The optimization of foundry processes for the fabrication of SiN waveguides will significantly contribute towards the efficient and high-fidelity gates in in integrated ion trap. It will also improve the performance of the photonic integrated circuits for quantum technology applications.