High speed imaging of drilling process of thin silicon substrates with double-pulsed femtosecond laser

Femtosecond lasers which have an extremely high peak power are expected to use for micro-processing, such as drilling and cutting. We have analyzed the drilling process by femtosecond laser pulses on surface of various materials including silicon with image-intensified CCD camera with a high speed gate. A femtosecond laser pulses (single and double pulses) were focused on surface of samples using a lens with a focal length of 100 mm. As a result, it was found that the smaller the pulse energy, the faster the formation of the hole. By measuring the rise time in 12 kinds of materials, it was found that the rise time strongly correlates with the thermal conductivity in these materials. Besides, it was also found that double pulsed laser suppressed the heat influence compared to the single pulse laser. These knowledges are thought to be very important and useful for developing microfabrication using a femtosecond laser.