Micromachining through silicon substrates by ultrafast laser at 1552 nm

Micromachining of silicon (Si) using lasers is attracting much interest in the field of modern microfabrications. Because Si absorbs majority of industrial lasers in near-IR to uv region, laser processing of Si is usually performed using linear absorption processes of the laser radiation. Micromachining by non-linear absorption processes induced by short pulse lasers has many unique features and has been applied to transparent materials in many fields. The application of these non-linear processes to the micromachining of Si would be very advantageous. In this study, ultrafast pulses from a fiber-based infrared laser at 1552.5 nm were irradiated through silicon substrates such that they were focused at the front or the rare surfaces of the substrates under an infrared microscope. Focused outputs of the laser made sharp and deep trenches on the front surface. When the laser was focused on a gold (Au) film on the surface of the second substrate placed at the back of the first Si substrate, the Au film was ablated by irradiation through the Si substrate. This was applied successfully to an up-conversion of the frequency of crystal oscillator in a Si package. When the focus was placed at rear surface of a Si substrate and aberration of the laser light caused by large refractive index of Si was compensated, an Au film on the rear surface was ablated and deposited on another substrate placed at the back of the substrate.