Nonlinear Micro-Processing of Silicon by Ultrafast Fiber Laser at 1552 nm
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Nonlinear Micro-Processing of Silicon by Ultrafast Fiber Laser at 1552 nm Yoshiro Ito1, Rie Tanabe1 and Kozo Tada2 1 Department of Mechanical Engineering, Nagaoka University of Technology, Kamitomioka, Nagaoka, Niigata 940-2188, Japan 2 Citizen Finetech Miyota, 353, Yaehara, Tomi, Nagano 289-0406, Japan ABSTRACT Processing of transparent materials by non-linear absorption mechanisms induced by short pulse lasers has been applied in many fields. Silicon (Si) is widely used materials in microelectronics, MEMS and photonics. It is, however, not transparent for commonly used processing lasers in near infrared to ultraviolet spectral range and has not been a subject for the non-linear processing by lasers so far. In this paper, possibilities and capabilities of non-linear processing of Si by 900 fs, 1552nm laser radiation are described with special emphasis on application to frequency adjustment of a crystal oscillator in a package made from Si. INTRODUCTION Processing of transparent materials by non-linear absorption mechanisms induced by short pulse lasers has been applied in many fields such like photonics, inner marking and fiber optics. These applications are limited to transparent dielectrics and polymers. It is not applicable to metals and semiconductors because they are not transparent at commonly used laser wavelengths. Silicon (Si) is a widely used material in micro-electronics, MEMS and photonics. It is not transparent in near infrared to ultraviolet spectral range and has not been a target for the nonlinear processing by lasers so far. However, semiconductors become transparent for photons energy of which are less than band gap energies of them. In this paper, possibilities and capabilities of machining of Si by non-linear absorption mechanisms induced by an ultrafast, 1552 nm laser radiation are described with special emphasis on application to frequency adjustment of crystal oscillators in packages made from Si [1, 2, 3]. Band gap energy of Si corresponds to wavelength of 1128 nm and thus the laser light is expected to transmit the Si [4]. Machining at different positions, from the front surface of the first Si substrate to the back surface of the second substrate placed at the back of the first, is carried out in air. Focused outputs of the laser make trenches on the front surface of both Si substrates. Sharp and deep grooves are machined with some deposition on the edges on the front surface. When it is focused on the front surface of the second substrate placed at the back of the first Si substrate, trenches are made on the surface of the second with no detectable change on the first. When the laser is focused at the back surface, quite different machined features are observed on the back surface. A wide stripe of fine grained structure without any deep trench is formed on the back. Highly position selective machining of surfaces has been achieved by adjusting the focus to each surface with no detectable change on the other surfaces. Gold (Au) film on the second substrate can be ablated without any c
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