Femtosecond Laser Micromachining of a-Si:H

Abstract

Femtosecond laser micromachining has been used to write bulk waveguides and photonic devices in glasses, polymers, and crystalline silicon (Gattass and Mazur, Nat Photonics 2:219–225, 2008). Refractive index changes in these materials tend to be less than a percent, which sets limitations on applications due to low light confinement. Hydrogenated amorphous silicon (a-Si:H) presents a unique, versatile material platform with incredible potential. Variations in the hydrogen content can produce refractive index changes as high as 40–80 % (Fortmann et al. Thin Solid Films 395:142–146, 2001). There is potential for integration on a silicon platform. We use femtosecond laser micromachining to locally reduce the hydrogen content of the material, with the goal of increasing the refractive index. We will use this method to directly write three-dimensional (3D) photonic devices in a-Si:H. This novel, simple method for photonic device fabrication in a-Si:H will facilitate many applications and device integration. Femtosecond laser processing enables 3D fabrication through nonlinear interactions. An ultrafast pulsed laser is focused inside the bulk of a transparent material. Due to tight focusing, material modifications only occur at the focal point of the laser. Complex patterns can by written by translating the sample with respect to the laser focus using x-, y- and z-translation. Intensity dependence of nonlinear processes enables fabrication of features with dimensions below the diffraction limit of light through careful selection of laser exposure. 3D fabrication allows greater versatility in devices and can increase device density, which is critical for modern optics and electronics where space is at a premium. Waveguide fabrication through variations in hydrogen content has been demonstrated in 2D using traditional micro- and nano-fabrication techniques (Fortmann et al. Thin Solid Films 430:278–282, 2003; 501:350–353, 2006). This process requires many steps, including ion implantation to locally control hydrogen content. The waveguides consist of regions with low hydrogen content. Fabrication of 3D photonic devices would require stacking many 2D layers. We are developing femtosecond laser processing to directly write complex 3D patterns in a single-step process, introducing greater versatility and processing speed. We have succeeded in lowering the hydrogen concentration in 2D patterns within a 1-μm film of a-Si:H using a near infrared (1,050 nm) femtosecond laser. Contrast is visible between unaltered and laser processed material under optical microscopy, suggesting index changes due to a reduction in hydrogen content. Contrast increase with laser fluence or a greater number of incident laser pulses. AFM measurements verify that regions with reduced hydrogen content do not exhibit changes in surface topography. We used Raman spectroscopy to verify the reduction in hydrogen content by examining the intensity ratio of Si-H bonding peaks (2,000 and 2,100 cm⁻¹) to Si-Si bonding around 480 cm⁻¹ (Fig. 58.1). A reduction in the Si-H intensity corresponds to reduced hydrogen content (Brodsky et al. Phys Rev B 16:3556–3571, 1977). These initial results provide a proof of concept and valuable knowledge for the fabrication of devices.