Design And Improvement Of A One-Step Shallow Etched Silicon Grating Coupler

Since the first grating coupler was integrated on a semiconductor chip in 2002,silicon grating couplers have been widely researched in the last decade.However,these works are almost complicated and expensive.A mature high-efficiency low-cost grating coupler is an indispensable part of silicon transceivers,which is of great researching significance.In this thesis,we propose a one-step shallow etched grating coupler,which is fabrication friendly,low-cost and with quite high efficiency.We then improve the directionality of the grating coupler both by FDTD simulation and by experiment,which is less researched before.This thesis consist of the following parts:We introduces the CMOS processes and makes sure about the fabrication limits.We simulate the performance of the grating coupler by Lumerical FDTD Solution and optimize its parameters such as,period,fulfill factor,etching depth and angle of the tapered waveguide,etc.The final structure is optimized to be with 70nm etching depth,630nm period and 55%fulfill factor.Simulation shows the directionality of it is 61%(-2.15dB).Then we layout and draw the design of the testing structures of the grating couplers with different tapers and send it to the cooperating fabrication unit.We build the silicon passive device testing platform and then test the performance of these devices.Result shows that the highest coupling efficiency is 35%(-4.65dB)at 1557.4nm with 51nm 3dB bandwidth and 31nm 1dB bandwidth.Result also show that the taper has little effect on these grating couplers.After that,we find it by FDTD simulation that Cladding Oxide(COX)thickness has an critical influence on grating couplers' directionality.At different COX thickness,the maximum directionality is 1.07dB larger than its minimum.We thus conduct an experiment etching the COX with ICP-RIE and successfully improve the coupling efficiency to 45%(-3.46dB),which is adequate for most applications.We also find that the maximum eficiency is 1.71dB larger than the minimum efficiency,while the maximum directionality is only 1.07dB larger than its minimum.Another FDTD simulation shows that the fiber with 80 polished facet engaged in the experiment is the reason.The maximum coupling efficiency of 80 tilted such fiber is 2.04dB larger than its minimum.And 7.50 tilted such fiber's maximum coupling efficiency is 1.70dB larger than its minimum,which matched the experiment well.As COX thickness has an influence on capacitance of silicon high speed devices,this work is thus of great significance for silicon photonics integration.