Controlled Fabrication of High-Aspect-Ratio Microstructures in Silicon at Etching Rates Beyond State-of-the-Art Microstructuring Technologies
Authors: Cozzi Chiara , Polito Giovanni , Kolasinski Kurt W , Barillaro Giuseppe 
Published: February 2020

After R. Feynman’s inspiring speech in 1959 “There is plenty of room at the bottom”, silicon microstructuring technologies have been increasingly developed, thus pushing silicon toward novel research topics and market opportunities beyond Moore’s law, a trend well known as More Than Moore. In spite of this, emerging applications in different fields, among which through silicon vias for 3D chip stacking and 3D capacitors with dramatically enhanced value for unit area in microelectronics, and lab-on-chips for point-of-care clinical diagnostics and microneedles for transdermal drug delivery and biosensing in nanomedicine, represent new challenges that silicon microstructuring technologies are facing today. These applications require fabrication of structures with high-aspect-ratio values that are in most cases beyond those attainable by commercial technologies (aspect ratio AR<40). On the other hand, state-of-the-art technologies able to control microfabrication at higher aspect-ratios (AR>100) are restricted to etching rates below 2 μm/min when the aspect-ratio increases over 20.
Here we report on the controlled electrochemical etching of deep (up to 200μm) microstructures in silicon with high-aspect-ratio, from 5 to 100, at high etching rates, from 10 to 3 μm/min respectively, at room temperature. This sets a novel record for the high speed fabrication of microstructures with high-aspect-ratio in silicon, and allows silicon microfabrication technology to enter a region in the parameter space etching-rate Vs aspect-ratio that was formerly prohibited for both commercial and research technologies. Addition of an inert oxidant molecule, namely H2O2, to a standard aqueous hydrofluoric (HF) acid electrolyte is used to reduce the valence of the dissolution process to 1 under anodic biasing, thus making the electrochemical etching more effective, on the one hand, and to catalyze the etching rate by opening a more efficient silicon dissolution path with respect to the well-known Gerischer mechanism, thus increasing etching speed at both shorter and higher depths, on the other hand.
Etching experiments of both regular macropores (with spatial period 3.5µm) and complex microstructures were carried out by back-side illumination electrochemical etching of pre-patterned n-type silicon (3–8Ω cm resistivity, (100) oriented) in HF–H2O electrolytes ([HF]=5% by vol.) containing different concentrations of H2O2, from 0 (control electrolyte) to 25% (by vol.). All the experiments were conducted under anodic biasing of the silicon electrode at 1.2V, for a maximum etching time of 65 minutes. Experimental data highlight that for a given set of etching parameters, the etching speed monotonically increases with the H2O2 concentration, with respect to the control electrolyte, in spite of the fact that the HF concentration is kept constant over the different experiments [1]. For instance, at [H2O2]=25% ordered macropore arrays with depth of about 50µm and AR=25 were etched in 10 minutes, at an average etching rate of 5 µm/min; when the etching time is increased to 60 minutes, macropore arrays with depth of about 180 µm and AR=100 are etched at an average etching rate of 3 µm/min. The etching speed is 2.8 and 2 times larger, respectively, than that of the control electrolyte (without H2O2). The increase in etching rate does not effect control on accuracy of the microfabrication, with respect to control electrolyte[2].
For a given H2O2, experimental data show that etching speed can be further increased by increasing the HF concentration[3]. For instance, for [H2O2]=15% and etching time of 10 minutes, regular macropore array with depth of 40 (4μm/min) and 70μm (7μm/min) are etched increasing the HF concentration from 5 to 8%.
Remarkably, the dissolution valence, i.e. the number of charge carriers required for the dissolution of a single silicon atom, decreased as the H2O2 concentration in the electrolyte increased, thus suggesting that H2O2 was able to promote a change in the stoichiometry of silicon dissolution process by consuming conduction band electrons. The dissolution valence in the presence of [H2O2] = 25% had an average value of 1.12 (standard deviation sd=0.24), whereas had an average value of 2.97 (sd=0.40) in the control electrolyte in good agreement with literature data. We propose that a parallel etching, with respect to the well-known Gerischer mechanism [4], introduced by the addition of H2O2, is triggered by the capture of a conduction band electron released during fluoride-induced etching. This parallel pathway is responsible both for the rate enhancement and the reduction in effective valence.
[1] C. Cozzi, G. Polito, K. Kolasinski, G. Barillaro, Advanced Functional Materials (2016) (DOI:10.1002/adfm.201604310).
[2] M. Bassu, S. Surdo, L. M. Strambini, G. Barillaro, Advanced Functional Materials, 22, 1222, 2012.
[3] V. Lehmann, Electrochemistry of Silicon, Wiley-VCH, Weinheim, Germany, 2002.
[4] H. Gerischer, P. Allongue, V. C. Kieling, Phys. Chem. 97, 753, 1993.