Analysis of silicon via hole drilling for wafer level chip stacking by UV laser

Abstract

For stacking wafers/dies, through-silicon-vias (TSVs) need to be created for electrical connection of each wafer/die, which enables better electrical characteristics and less footprints. And for via hole processing, chemical methods such as DRIE (Deep Reactive Ion Etching) are mostly used. These methods suffer the problems of slow processing speed, being environment-unfriendly and damage on the existing electric circuits due to high process temperature. Furthermore, masks are also needed. To find an alternative to the methods, researches on the laser drilling of via holes on silicon wafer are being conducted. This paper investigates the silicon via hole drilling process using laser beam. The percussion drilling method is used for this investigation. It is also examined how the laser parameters-laser power, pulse frequency, the number of laser pulses and the diameter of laser beam-have an influence on the drilling depth, the hole diameter and the quality of via holes. From these results, laser drilling process is optimized. The via hole made by UV laser on the crystal silicon wafer is 100µm deep, has the diameter of 27.2µm on the top, 12.9µm at the bottom. These diameters deviate from the target values by 2.8µm and 0.4µm respectively. These values correspond to the deviation from the target taper angle of the via hole by less than 1°. The processing speed of the laser via hole drilling is 114mm/sec, therefore, etching process can be replaced by this method, if the number of via holes on a wafer is smaller than 470,588. The ablation threshold fluence of silicon is also determined by a FEM model and is verified by experiment.

Laser drilling of via micro-holes in single-crystal semiconductor substrates using a 1070nm fibre laser with millisecond pulse widths

“…Melt ejection is the more energy efficient material removal mechanism, reported as requiring, for metals, approximately 25% of the energy per unit volume compared to vaporization. The majority of laser drilling carried out in semiconductor materials uses short pulses [4,5], with ultra-violet [6] lasers being the most widely used. Some longer pulse work has been done Yu et al [7] used 6.6 µs 1070 nm pulses to drill silicon.…”

Comparison of bending fracture strength of silicon after ablation with nanosecond and picosecond lasers

“…One of the principal TSV procedures is drilling holes in a silicon wafer. To drill these holes, laser ablation and deep reactive ion etching (DRIE) techniques have primarily been used [4,5]. These two methods are selectively employed depending on the size, thickness, and pitch interconnection of the silicon wafer as well as the diameter and pitch of the hole itself [6,7].…”

“…Lasers with pulse durations of some tens of nanoseconds have been commonly used in research on drilling using laser ablation [4,[8][9][10][11]. Lasers with pulse durations ranging from a few picoseconds to femtosecond are currently under development; these lasers are referred to as ultra-short pulse lasers, and research using these lasers for laser ablation drilling has been conducted [12][13][14][15][16][17][18].…”

Investigation of durability of TSV interconnect by numerical thermal fatigue analysis

“…1 As compared to stacked packages, TSV technology presents greatly-increased functional density and significantly-reduced package area and interconnect length, thereby leading to an enhanced electrical performance. High durability of TSV interconnects is influenced by the thermal stress during operation or due to environmental temperature cycling.…”