In recent years, anodic dissolution of silicon in HF (hydrofluoric acid)-based electrolytes has been largely used for the preparation of ordered and random forms of porous silicon [1,2], from both p-type and n-type materials. In particular, randomly-distributed meso and nanopores have been prepared from p-type silicon electrochemically etched in dark at high HF concentrations [2]; ordered macropores and complex microstructures [3] have been fabricated by back-side illumination electrochemical etching of n-type silicon at low HF concentrations.

Here we report for the first time on the controlled etching at high anodic voltage (up to 40V) of two-dimensional (2D) arrays of ordered pores with sub-micrometric diameter (down to 800 nm) at high depths (up to 40µm), yielding a maximum aspect ratio (AR) of 50, using low-doped (resistivity 3-8 ٟcm) n-type silicon. Back-side illumination etching of silicon at low HF concentration (5% by vol. in H₂O) is used for the silicon etching, starting from a pre-patterned silicon surface featuring a 2D lattice of holes with size of 1µm and spacing of 1.8µm.

The electrochemical etching of the above-described patterned silicon at low anodic voltages (<3V), which is the standard condition for back-side illumination etching, does not allow to obtain a uniform array of pores with constant diameter and same depth, according to both literature and experimental data obtained in this work. Holes generated at the back-side surface (by illumination) diffuse through bulk silicon and are collected at the silicon/electrolyte interface in correspondence of surface defects (either native or pre-patterned), which are known to act as seed-points for pore growth due to the hole-focusing effect arising from the higher electric field established at their tip. At low V_etch values (<3 V) hole-focusing effect has been already proven to be highly effective when diameter and spacing of defects/pores are greater than a few microns (>2 μm) [3]. Nonetheless, when diameter and spacing of defects/pores is below 2μm, low V_etchvalues (<3 V) lead to an unstable and non-uniform pore growth, where some of the pores stop growing and others pores continue their growth though uncontrolled both in depth and diameter. We argue that such a mechanism is due to overlapping (partially at least) of the depletion regions between adjacent pores, which leads to defocusing of the electric field lines in correspondence of surface defects and, in turn, to an uncontrolled etching.

In this work we show that increasing the V_etch value (from 15 up to 40V) has beneficial effects on the controlled etching of pores with sub-micrometric diameter and spacing below 2μm. Experimental data show that as the anodic voltage is increased over 15V, electric field lines narrow more and more around the defect/pore tips, thus improving the hole-focusing effect and enabling, in turn, the uniform etching of pores with constant diameter and same depth. In fact, the number of missing pores (pores that stop their growth at a given depth) significantly reduces as the anodic voltage approaches 15V and for arrays etched at V_etch=35V no missing pores are visible. According to the literature [4], our experimental results also show that a J_etch threshold value also exists setting the minimum porosity (P) value (P_min about 15% for this work) versus pore density over which stable growth of pores can be achieved at high anodic voltages. For a given V_etch value, as J_etch increases, the array porosity and, in turn, the pore diameter increases. On the other hand, for a given J_etch value, as the anodic voltage increases, the diameter of etched pores reduces and depth increases. For instance, etching of our patterned silicon with J_etch=13.4 mAcm^-2 (P=17%) and V_etch=35V leads to the formation of ordered arrays of sub-micrometric pores with diameter of about 800nm and depth of about 40μm.

Building on these results, controlled electrochemical etching of ordered meso and nanopores and structures in low-doped n-type silicon is envisaged. If successful, this would result to be game-changing in the silicon micro and nanofabrication arena, with applications (though not limited) to nanoelectronics, nanoelectromechanical systems (NEMS), and nanomedicine.

References

[1] V. Lehmann. Electrochemistry of silicon. Wiley-VCH, 2002.

[2] M. J. Sailor. Porous Silicon in Practice. Preparation, Characterization and Applications. Wiley-VCH, 2011.

[3] G. Polito, S. Surdo, V. Robbiano, G. Tregnago, F. Cacialli, G. Barillaro. Two-Dimensional Array Of Photoluminescent Light Sources by Selective Integration of Conjugated Luminescent Polymers into Three-Dimensional Silicon Microstructures. Adv. Optical Mater.,1(12):894-898,2013.

[4] G. Barillaro, L. M. Strambini. Controlling Macropore Formation in Patterned n-type Silicon: Existence of a Pitch Dependent Etching Current Density Lower Bound. Electrochem. Commun.,12(10):1314-1317,2010.