Room-Temperature Sputtered Ultralow-Loss Silicon Nitride

Silicon nitride films with the best optical quality can be realized with low pressure chemical vapor deposition (LPCVD) [1]. However, high mechanical stress within the LPCVD silicon nitride films leads to formation of cracks and prevents thick films
$(> 400\ \text{nm})$
with high optical quality. A number of clever processes have been developed in order to prevent cracks, such as patterning of crack barriers, multi-step deposition and a Damascene process that deposits silicon nitride into trenches. In addition, to reduce material losses and achieve ultralow propagation losses
$(\sim 1\text{dB}/\mathrm{m})$
, long-time high temperature annealing (1200 °C for 3 hours) is needed, which can cause dopant diffusion and damage of prefabricated temperature-sensitive devices. Hence, LPCVD Si
3
N
4
is very challenging for applications in back-end-of-line processes and foundry compatible processes. In recent years, low-temperature deposited Si
3
N
4
thin films have attracted increasing interest for 3D hybrid integration of Si
3
N
4
with other materials. Here we report the fabrication of ultralow-loss Si
3
N
4
photonics film by reactive sputtering at room temperature [2]. Directly after room-temperature sputtering, the Si
3
N
4
film loss is 32 dB/m without thermal annealing and the intrinsic optical quality factor (Q) of the fabricated microresonators is 1.1 million in the L-band. This propagation loss is already lower than that of annealing-free LPCVD Si
3
N
4
film deposited at 780°C [3]. Fig. 1 (a) Intrinsic q measurement of fundamental te modes of a 200-μm-radius sputtered si3n4 ring resonator after 800°c thermal annealing. (b) Optical spectrum of a single bright soliton generated in the sputtered si3n4 resonator pumped at 1310 nm and a sech2 envelope fit (blue dashed trace). (c) Optical spectrum of a single bright soliton pumped at 1575 nm.