Time-Resolving Image Analysis of Drilling of Thin Silicon Substrates with Femtosecond Laser Ablation

Abstract

We have analyzed a drilling process with a femtosecond laser on a silicon surface in order to investigate the degree of the thermal effect during the dicing of a very thin silicon substrate (thickness: 50 µm). A femtosecond laser pulse (E=30–500 µJ/pulse, τ=200 fs, λ=780 nm, f=10 Hz) was focused on a thin silicon substrate using a lens with a focal length of 100 mm. An image-intensified charge-coupled device (CCD) camera with a high-speed gate of 200 ps was utilized to take images of a drilled hole during the drilling process. As a result, it was found that the smaller the pulse energy, the faster the formation of the hole. Therefore, we tried to estimate the degree of the thermal effect semi quantitatively by analyzing the rise time of the formation of the hole. By measuring the rise time in 8 kinds of metallic material, it was found that the rise time strongly correlates with the thermal conductivity in these materials. This knowledge is thought to be very important and useful for developing a dicing technique for thin silicon wafers using a femtosecond laser.