Optical characterization of gaps in directly bonded Si compound optics using infrared spectroscopy

TL;DR

This paper presents a precise optical method using infrared spectroscopy to detect and measure nanometer-scale gaps in directly bonded silicon optics, improving quality control in optical manufacturing.

Contribution

It introduces a novel bond inspection technique that employs slit spectroscopy and Fabry-Pérot models to accurately measure sub-20 nm gaps in silicon bonds.

Findings

Detects gaps as small as 14 nm with high accuracy

Quantifies the impact of gaps on optical transmission

Validates method with microlithography-produced gaps

Abstract

Silicon direct bonding offers flexibility in the design and development of Si optics by allowing manufacturers to combine subcomponents with a potentially lossless and mechanically stable interface. The bonding process presents challenges in meeting the requirements for optical performance because air gaps at the Si interface cause large Fresnel reflections. Even small (35 nm) gaps reduce transmission through a direct bonded Si compound optic by 4% at $\lambda = 1.25 \; \mu$m at normal incidence. We describe a bond inspection method that makes use of precision slit spectroscopy to detect and measure gaps as small as 14 nm. Our method compares low finesse Fabry-P\'{e}rot models to high precision measurements of transmission as a function of wavelength. We demonstrate the validity of the approach by measuring bond gaps of known depths produced by microlithography.