Laser-Excited Elastic Guided Waves Reveal the Complex Mechanics of Nanoporous Silicon

Abstract

Nanoporosity in silicon leads to completely new functionalities of this mainstream semiconductor. A difficult to assess mechanics has however significantly limited its application in fields ranging from nanofluidics and biosensorics to drug delivery, energy storage and photonics. Here, we present a study on laser-excited elastic guided waves detected contactless and non-destructively in dry and liquid-infused single-crystalline porous silicon. These experiments reveal that the self-organised formation of 100 billions of parallel nanopores per square centimetre cross section results in a nearly isotropic elasticity perpendicular to the pore axes and an 80% effective stiffness reduction, altogether leading to significant deviations from the cubic anisotropy observed in bulk silicon. Our thorough assessment of the wafer-scale mechanics of nanoporous silicon provides the base for predictive applications in robust on-chip devices and evidences that recent breakthroughs in laser ultrasonics open up entirely new frontiers for in-situ, non-destructive mechanical characterisation of dry and liquid-functionalised porous materials.

12 pages, 8 figures, Supplementary information available as ancillary file, in press

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• Wafer-Scale Fabrication of Hierarchically Porous Silicon and Silica Glass by Active Nanoparticle-Assisted Chemical Etching and Pseudomorphic Thermal Oxidation
• Extreme wave skewing and dispersion spectra of anisotropic elastic plates
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• Adiabatic Lamb modes in 3D tapered waveguides: Cut-off effects and ZGV resonances
• Wafer-scale fabrication of mesoporous silicon functionalized with electrically conductive polymers
• Frequency domain laser ultrasound for inertial confinement fusion target wall thickness measurements
• Microscale architected materials for elastic wave guiding: Fabrication and dynamic characterization across length and time scales