Stress-Dependent Optical Extinction in LPCVD Silicon Nitride Measured by Nanomechanical Photothermal Sensing

TL;DR

This study uses nanomechanical photothermal sensing to measure how optical extinction in LPCVD silicon nitride varies with tensile stress, revealing a stress-induced increase in bandgap that reduces optical absorption.

Contribution

It demonstrates a novel application of nanomechanical photothermal sensing to quantify stress-dependent optical extinction in silicon nitride films, linking optical properties to material stress and composition.

Findings

Optical extinction decreases from 10^3 to 10^1 ppm with increasing stress.

Higher tensile stress correlates with increased bandgap and reduced absorption.

Nanomechanical photothermal sensing provides a sensitive, scattering-free measurement platform.

Abstract

Understanding optical absorption in silicon nitride is crucial for cutting-edge technologies like photonic integrated circuits, nanomechanical photothermal infrared sensing and spectroscopy, and cavity optomechanics. Yet, the origin of its strong dependence on film deposition and fabrication process is not fully understood. This Letter leverages nanomechanical photothermal sensing to investigate optical extinction $\kappa_{\mathrm{ext}}$ at 632.8 nm wavelength in LPCVD SiN strings across a wide range of deposition-related tensile stresses ($200-850$ MPa). Measurements reveal a reduction in $\kappa_{\mathrm{ext}}$ from 10$^3$ to 10$^1$ ppm with increasing stress, correlated to variations in Si/N content ratio. Within the band-fluctuations framework, this trend indicates an increase of the energy bandgap with the stress, ultimately reducing absorption. Overall, this study showcases the power and simplicity of nanomechanical photothermal sensing for low absorption measurements, offering a sensitive, scattering-free platform for material analysis in nanophotonics and nanomechanics.