Tailored Femtosecond Bessel Beam Processing - Application to Through Si Vias for 3D Si ICs -

Abstract

The steady pursuit of high-performance, low-power-consumption, and small-footprint microelectronic devices has made three-dimensional integrated circuits (3D ICs) an attractive replacement for conventional 2D ICs. One of the major challenges to realize 3D ICs is the fabrication of high-aspect-ratio through-silicon vias (TSVs), which is a key technology for the 3D assembly of Si ICs [1]. Here, we propose to employ femtosecond (fs) 1.5-μm Bessel beams for high-speed fabrication of high-quality, high-aspect-ratio through Si holes for TSV application. By performing laser ablation in air using conventional Bessel beams, nearly taper-free through Si holes with diameters of ~3 μm in 50-μm-thick Si substrates, which corresponds to aspect ratios of ~17, can be created. However, severe damage with a concentric structure was produced around created holes due to relatively high sidelobe energy in the Bessel beam. To suppress the severe damage, a fs Bessel beam is tailored by using specially designed binary phase plates (BPPs) for the first time. We theoretically and experimentally demonstrate that this method can generate a fs laser beam with a ~6-μm lateral spot size and ~400-μm focal depth, while reducing the sidelobe ratio (SLR) to ~0.6%, which is much smaller than the ~16% SLR for a conventional Bessel beam. The developed technique successfully eliminates sidelobe-induced damage to ensure high-quality fabrication of through Si holes with a high aspect ratio [2]. Specifically, 2D array of through Si holes with an aspect ratio of ~15 in a 100-μm-thick Si substrate without any sidelobe damage was fabricated as shown in Fig. 1. Our technique is potentially applicable for 3D assembly in the manufacturing of 3D Si ICs.