X-ray imaging non-destructively identifies functional 3D photonic nanostructures

TL;DR

This study demonstrates the use of synchrotron X-ray holographic tomography to non-destructively analyze the internal 3D structure of silicon photonic nanostructures, revealing critical insights into their functionality.

Contribution

It introduces a non-destructive X-ray imaging method to accurately assess the internal 3D structure of nanophotonic crystals, correlating structure with optical performance.

Findings

Identified the 3D periodicity in functional structures

Detected buried voids in non-functional structures

Highlighted fabrication errors affecting performance

Abstract

To investigate the performance of three-dimensional (3D) nanostructures, it is vital to study in situ their internal structure non-destructively. Hence, we perform synchrotron X-ray holographic tomography on exemplary 3D silicon photonic band gap crystals without irreversible preparation steps. Here, we obtain real space 3D density distributions of whole crystals buried on 2 mm^2 beams with 20 nanometer resolution. Our X-ray results identify why structures that look similar in scanning electron microscopy have vastly different nanophotonic functionality: One crystal with a broad photonic gap reveals 3D periodicity as designed ("Good"), a second structure without gap reveals a buried void ("Bad"), a third one without gap is shallow due to fabrication errors ("Ugly"). We conclude that X-ray tomography is a crucial tool to critically assess 3D functional nanostructures.