Nanoscale Depth Profiling of Optoelectronic Devices Using Deep-UV LIBS

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.