Silicon Electrochemical Micromachining Technology: The good, the bad, and the future

Silicon electrochemical micromachining technology (ECM) has come a long way since the pioneering paper of Lehmann and Foll, dated 1990, which firstly reported about the possibility of controlling the electrochemical dissolution of silicon in HF-aqueous electrolytes by back-side illumination etching for the formation of nicely ordered array of macropores. Currently, ECM is routinely used in different laboratories around the world to fabricate silicon microstructures (e.g. microtips, spirals) and microsystems (e.g. microgrippers, microneedles) of much higher complexity than macropores, with application ranging from (bio)sensing to nanomedicine, from photonics to microelectronics. ECM technology capitalizes on the experimental and theoretical results reported in the literature over the last two decades on the electrochemical dissolution of silicon in HF-aqueous electrolytes. Among others (e.g. aspect-ratio>100, verticality≈90°, surface roughness<10 nm), it is worth mentioning the peculiarity of dynamically controlling the electrochemical etching anisotropy from 1 to 0 in real-time, which has greatly advanced such a technology with respect to state-of-the-art etching technology. Nonetheless, a number of important limitations either remain unsolved or need to be improved towards future industrial applications of ECM technology (e.g. minimum and maximum etching features, etching rate). In this paper, basic and advanced features as well as main limitations of silicon microfabrication by ECM technology are thoroughly reviewed and discussed, both from theoretical and experimental points of view, also with reference to future industrial applications. As case studies, fabrication by ECM technology (and characterization) of all-silicon, integrated microstructures and microsystems for different (bio)-sensing/medical applications will be detailed and discussed, with particular emphasis on (though not limited to): i) photonic crystal optofluidic microsystems for (bio)sensing applications, ii) microneedle-based chips for transdermal glycemic control; iii) electrically-actuable silicon microgrippers for biology/medicine applications; iv) 3D microincubators for selection of cancer cells on the basis of their metastatic potential.