Fundamentals of mega hertz femtosecond laser ablation and its application in interconnection via drilling

Senthilnathan Panchatsharam, Ryerson University

Date of Award

2009

Degree Type

Thesis

Degree Name

Master of Applied Science (MASc)

Department

Mechanical Engineering

First Advisor

K. Venkatakrishnan

Second Advisor

B. Tan

Abstract

With the ever increasing demands on electronics and its portability, speed and function, the need for lighter and faster processing chips is always there. 3D stacking of chips is enabled through Inter-chip connections or Interconnect vias, which is used to create the electrical connections that facilitate the stacking. Interconnect vias provide the connectivity between the layers of the physical design, resulting in short connection lengths, which improve performance and decrease power consumption significantly.

This thesis proposes a high repetition rate, high power femtosecond laser drilling technique for surpassing the quality and throughput demand posed by the current market. A systematic study of the laser control parameters such as the repetition rate, pulse energy, intensity, wavelength, pulse duration, ablation threshold, quality and throughput rates is analyzed in detail. Decreasing via diameter and increasing via depth at high repetition rate are noted. A smallest via of 8.39 μm and deepest via of 121 μm were obtained at 13 MHz and 4.33MHz respectively. Minimal debris accumulation and exceptional surface qualities were noted at high repetition rates. Theoretical and experimental studies of shockwave effects during laser ablation on via surfaces were analyzed. Systematic process of learning the propagation of shockwaves and generating raised rims in ablated crater is analyzed. Analyzing surface temperature of the target revealed presence of small molten liquid later at 13 MHz. Newly termed observations, primary and secondary rims, were observed and analyzed under the influence of various laser parameters. Rim line width spacings were seen increasing at higher pulse width and higher power density. Gas assisted ablation showed decreasing line width spacing due to forced convection. Hence, with the successful results from the studies, the laser system shows the capability of meeting and surpassing the current market demands and considered for commercial applications.