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Subsurface Silicon Processing by Microsphere Focusing of Ultrafast Infrared Laser

2019-01-01
İdikut, Fırat
Borra, M. Zolfaghari
Seyedpour, S. Esmaeilzad
Bek, Alpan
Attention on applications of femtosecond lasers in semiconductor materials processing is ever growing. There has been an increasing research momentum especially towards multi-photon absorption based silicon (Si) processing with infrared lasers in the last decade. Since Si is transparent at wavelengths >1.1 mu m, the processing inside Si is triggered by the nonlinear optical phenomenon of two or more photon absorption which requires laser amplitude to reach and pass beyond threshold conditions. A method that utilizes back-reflection at the Si-air interface was developed and demonstrated to be useful for subsurface processing of Si at the micro-scale. In previous studies, pixels of such processed regions were limited to 5-10 mu m size despite use of high numerical aperture lenses due to strong refraction of light in Si. In order to deem the laser processing of Si useful for photonic applications, pixel size needed to be reduced down to a micron or below. In this work, we demonstrate subsurface modification of Si using microsphere based focusing of a 1.5 mu m wavelength ultrafast laser pulses in Si.