Subsurface Engineering of Silicon for 3D Devices

Recently we have demonstrated a new 3D-laser-fabrication method which enabled, for the first time, creating highly-controlled subsurface structural modifications (structural imperfections, or defects) buried deep inside Silicon (Si) wafers [1]. Characterizing the material properties of these subsurface Si structures are very critical towards enabling new optical and micro-mechanical applications inside chips [2,3]. Here, we present optical, chemical and microscopic analysis of these buried structures. Specifically, Transmission Electron Microscopy (TEM) studies, Optical Birefringence Analysis and Selective Chemical Etching analysis of the modifications will be presented. Infrared Transmission Microscopy will be shown to be applicable for subsurface imaging, providing a diagnostic tool without damaging the samples.

Material properties of the disruptions in the crystal lattice are then exploited for fabricating various micro-devices. For instance, oxidation-reduction chemistry on laser-induced modifications enables the creation of highly-controllable, uniform and large-area micropillar arrays for solar cell applications, embedded microfludic channels for chip cooling and thru-Si vias for electrical interconnects in Si. These elements, which are challenging to form with conventional methods, can find use in various MEMS and electronics applications. The optical properties (refractive index change) of the structures are used to fabricate functional components such as lenses and gratings buried in chips. Further, the birefringence effect induced in Si may lead to holograms and other photonic applications, such as creating wave plates and polarizers. These functional optical and MEMS elements created inside Si, may find use in imaging and sensing in the near- and mid-infrared wavelength range, as well as in micro-devices towards micro-surgical tools, micro-motors, and micro-resonators. Thus, these capabilities are leading to a new fabrication approach in Si, which is fully CMOS compatible, rapid and mechanically robust, and builds on the optical,electrical and chemical properties of the modified volumes in Si.

[1] Tokel et. al., arxiv.org/abs/1409.2827
[2] Tokel et. al, Direct Laser Writing of Volume Fresnel Zone Plates in Silicon., CLEO/Europe - EQEC, Munich, Germany, 2015.
[3] Tokel et. al., 3D Functional Elements Deep Inside Silicon with Nonlinear Laser Lithography, APS March Meeting, Baltimore, USA, 2016.