# Nanoscale Depth Profiling of Optoelectronic Devices Using Deep-UV LIBS

## TL;DR

This paper introduces a compact, portable deep-UV LIBS system for nanoscale depth profiling of optoelectronic devices under ambient conditions.

## Contribution

The novel use of deep-UV LIBS enables nanoscale depth profiling with high resolution and portability.

## Key findings

- Deep UV LIBS achieves ablation depths of 20–25 nm per pulse, enabling nanoscale profiling.
- The system successfully profiles silicon photovoltaic junctions and dielectric mirrors with high accuracy.
- A compact LIBS optical head allows ambient operation and real-time nanoscale analysis.

## Abstract

Accurate elemental nanoscale depth profiling is vital
for semiconductor
junctions, optical coatings, and thin-film photonic devices. While
Secondary Ion Mass Spectrometry (SIMS) is capable of providing nanoscale
elemental depth profiling, it requires bulky and complex instrumentation
and is not portable for real-time process monitoring. Alternatively,
the technique of Laser-Induced Breakdown Spectroscopy (LIBS) has been
shown to provide sensitive detection of elements; however, its depth
resolution is limited to approximately μm, hindering nanoscale
profiling. Here, we demonstrate nanoscale elemental depth profiling
(∼10s of nm) using deep UV LIBS under ambient conditions. Using
a fiber-coupled 266 nm wavelength (UV–C) pulsed laser, the
system achieves an ablation depth as low as ∼20–25 nm
per pulse, enabling high-resolution profiling, while maintaining an
elemental detection sensitivity in the parts-per-million range. Depth
profiling of a silicon photovoltaic device’s diffused silicon
PN junction reveals a clear boron dopant profile signal within ∼650
nm, aligning with expected emitter diffusion depths. Similarly, the
depth profiling of a dielectric mirror reveals the nanoscale alternating
layers of optical coatings of Ta2O5 and SiO2 elemental profiles, each measuring respectively ∼100
and ∼145 nm. Furthermore, with deep UV single pulse ablation,
the detection sensitivity of a nanoscale thin (∼1–2
nm) native oxide film on top of a silicon wafer is also demonstrated.
The developed instrument for LIBS is an optical head packaged in a
3 × 2 × 1.5 cm3 (24 g) compact head, allowing
autofocusing for best ablation, while employing a custom ball lens
for tight spot focusing and efficient collection of the plasma emission
light. This is an attractive characterization approach for real-time
nanoscale elemental mapping and depth profiling of optical/electronic
devices, eliminating the need for vacuum or extensive sample preparation,
while allowing for the demonstration of a portable device that can
operate under ambient conditions.