Large-area femtosecond laser milling of silicon employing trench analysis

TL;DR

This study investigates large-area femtosecond laser milling of silicon, analyzing trench formation and process parameters to optimize cavity fabrication for RF applications with improved performance.

Contribution

It introduces a comprehensive analysis method for femtosecond laser silicon milling, linking process parameters to trench quality and enabling high-rate, low-roughness cavity fabrication.

Findings

Achieved milling depths up to 200 μm with high removal rates.

Identified ablation regimes based on fluence and repetition rate.

Enhanced RF device performance through laser-milled cavities.

Abstract

A femtosecond laser is a powerful tool for micromachining of silicon. In this work, large-area laser ablation of crystalline silicon is comprehensively studied using a laser source of pulse width 300 fs at two wavelengths of 343 nm and 1030 nm. We develop a unique approach to gain insight into the laser milling process by means of detailed analysis of trenches. Laser scribed trenches and milled areas are characterized using optical profilometry to extract dimensional and roughness parameters with accuracy and repeatability. In a first step, multiple measures of the trench including the average depth, the volume of recast material, the average longitudinal profile roughness, the inner trench width and the volume removal rate are studied. This allows for delineation of ablation regimes and associated characteristics allowing to determine the impact of fluence and repetition rate on laser milling. In a second step, additional factors of debris formation and material redeposition that come into play during laser milling are further elucidated. These results are utilized for processing large-area (up to few mm2) with milling depths up to 200 {\mu}m to enable the fabrication of cavities with low surface roughness at high removal rates of up to 6.9 {\mu}m3 {\mu}s-1. Finally, laser processing in combination with XeF2 etching is applied on SOI-CMOS technology in the fabrication of radio-frequency (RF) functions standing on suspended membranes. Performance is considerably improved on different functions like RF switch (23 dB improvement in 2nd harmonic), inductors (near doubling of Q-factor) and LNA (noise figure improvement of 0.1 dB) demonstrating the applicability of milling to radio-frequency applications.