Laser-Assisted Wet Etching of Silicon Back Surfaces Using 1552 nm Femtosecond Laser

Abstract

Efficient three-dimensional (3D) microfabrication techniques of an Si are in high demand for producing micrometer-scale 3D structures. Here, we report a new method for processing Si back surfaces using a 1552.5 nm femtosecond laser. As the Si is optically transparent at this wavelength, we attempted to machine the Si back surfaces via a nonlinear absorption process using the laser. Given that the etch rate of the back surface would be higher, wet etching was performed using an aqueous KOH solution. The 40% KOH solution was maintained in contact with the Si back surface at 25℃ while the laser was irradiated from the front surface. The laser beam was focused on the back surface and linearly scanned under different conditions. Focusing the laser approximately 15 μm into the liquid yielded deeper grooves as compared to those when it was focused precisely on the Si back surface. Further, the etch rate was significantly higher compared to that during dry etching. We could achieve the maximum etch depth of approximately 6 μm during the wet etching process, in contrast to 0.3 μm during dry etching. However, the groove depth was not constant along the processing path. The results demonstrate a possibility of a new, efficient, and debris-free microfabrication technique.