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Introduction

Silicon micromachining tools (Köhler 1999) have been developed for over 50 years with the ambition of sculpting silicon wafers at the microscale, continuously paving the way toward novel research/market devices and applications. In the 1960s, the market request for microfluidic devices, e.g., chromatographic columns, inkjet cartridges, etc., drove micromachining research/industry toward the development of wet etching technology characterized by low-cost, but with limited flexibility in fabrication and low integration density. In the 1980s, novel challenging applications, e.g., microelectromechanical systems (MEMS), and lab-on-a-chip (LoC), pushed micromachining research/industry toward the development of dry etching technology with increased flexibility in fabrication and higher integration density, although higher cost too. Both wet and dry technologies are still under development, with wet technology being used nowadays for the fabrication of undemanding microstructures with low density at lower cost, and dry technology exploited for the fabrication of complex microsystems with increased density at higher cost. In the 2000s, the new challenge for silicon micromachining has been the development of high-aspect-ratio technology that allows the etching of microstructures and microsystems with different shapes and micrometer-sized features to be finely controlled up to very high aspect ratios (over 100), thus enabling the actual exploitation of the room available in the third (out-of-plane) wafer direction and breaking novel research/market grounds, e.g., Through Silicon Via (TSV) fabrication for a new generation of three-dimensional chip stacking.

Electrochemical Micromachining of Silicon

Electrochemical micromachining (ECM) (Bassu et al. 2012) of silicon has been recently demonstrated as a powerful and flexible microstructuring technology that allows the etching of silicon wafers to be controlled up to the higher aspect ratios (over 100) with sub-micrometer accuracy, thus pushing silicon micromachining well beyond any up-to-date both wet and dry microstructuring technology. ECM technology capitalizes on the experimental and theoretical results reported in the literature over the last two decades on the electrochemical etching of n-type silicon under backside illumination in acidic (HF-based) electrolytes (backside-illumination electrochemical etching (BIEE)) (Lehmann 2002). The BIEE was pioneered, in the 1990s, by Lehman and Föll for the controlled etching of high-aspect-ratio macropores (Lehmann and Föll 1990; Lehmann 1993, 2002; Foll et al. 2000; Barillaro et al. 2002a; Barillaro and Pieri 2005; Barillaro and Strambini 2010), and later, in the 2000s, pushed by a number of scientists to the fabrication of a multitude of high-aspect-ratio microstructures for different applications (Ottow et al. 1996; Ohji et al. 1999; Kleimann et al. 2001; Barillaro et al. 2002b, 2003, 2005a, b, 2006, 2007a, b, 2008, 2009a, b; Schilling et al. 2001; Matthias et al. 2004, 2005; Astrova and Nechitaĭlov 2008; Astrova et al. 2009, 2010). For a review of the fabrication of macropores by anodization, see handbook chapter “Macroporous Silicon.” In the 2010s, a novel dynamic control of the electrochemical etching anisotropy as the etching progresses has allowed the silicon dissolution to be switched in real time from the anisotropic to the isotropic regime and has enabled advanced silicon microstructuring to be performed (Bassu et al. 2012; Barillaro et al. 2011; Merlo et al. 2012a, b, 2013; Strambini et al. 2012; Surdo et al. 2012, 2013; Carpignano et al. 2012; Polito et al. 2013).

Fabrication of microstructures and microsystems by ECM technology (Bassu et al. 2012; Surdo et al. 2012, 2013; Polito et al. 2013) is carried out according to the following main technological steps, which are sketched in Fig. 1b with specific reference to the microgripper of Fig. 1a. The starting material is n-type silicon, orientation (100), with a thin silicon dioxide layer on top. The pattern of the microstructure to be fabricated is defined on a photoresist layer by standard lithography, transferred to the silicon dioxide layer by buffered HF (BHF) etching through the photoresist mask (Fig. 1b-1), replicated (seed point formation) into the silicon surface by potassium hydroxide (KOH) etching through the silicon dioxide mask (Fig. 1b-2), and finally grooved into the bulk material by BIEE. The BIEE consists of a single-etching step with an initial anisotropic phase (Fig. 1b-3), used to etch the seed pattern deep in the substrate and create high-aspect-ratio microstructures, and a final isotropic phase (Fig. 1b-4), used to release part of the etched microstructures from the substrate and, eventually, yield them freestanding. A dynamic control of the electrochemical etching anisotropy, which is used to switch the silicon dissolution in real time from the anisotropic to the isotropic regime, is synergically combined with the use of both functional and sacrificial structures. Functional (sacrificial) structures are, by definition, parts of the microsystem that are anchored (not anchored) to the silicon substrate after the isotropic phase of electrochemical etching step (and are, hence, removed). The former are indeed functional to the microsystem operation, while the latter are sacrificed for the accurate microsystem fabrication. All the microsystem parts visible in the SEM image of Fig. 1a are functional structures, which are still connected to the substrate through anchor structures properly designed to withstand the isotropic phase of the BIEE. Sacrificial structures are removed after the isotropic phase of the BIEE and hence not visible in Fig. 1a. If needed, a further thermal oxidation step can also be performed to grow a 100-nm-thick conformal silicon dioxide layer, to be used as an insulation layer, on which a metal contact for actuation of the microgripper (or, in general, of the fabricated microsystem) can be deposited by quasi-conformal metal sputtering.

Fig. 1
figure 1

(a) Example (SEM image) of a silicon microgripper fabricated by ECM technology; (b) main technological steps for advanced silicon microstructuring by ECM technology, with specific reference to the part highlighted (see circle) of the silicon microgripper in (a).

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Sub-micrometer control of the silicon dissolution at the seed sites, in the X-Y plane and along the Z-direction, is obtained by properly modulating both the etching current density, J etch, and the etching voltage, V etch, as the etch progresses. The J etch and V etch values are kept within the working region: J etch < J peak and V etch > V peak, where J peak and V peak represent the values of the current density and voltage of the electropolishing peak of the electrochemical system under investigation. In the working region, the silicon mass dissolved per unit time around a given X-Y position is proportional to the ratio J etch/J peak (also known as the porosity P), for any etching depth; the J peak value also determines the etching rate in the Z-direction, which is, indeed, proportional to J peak. The silicon dissolution (in terms of both volume and isotropy versus time) is finely tuned as the etching progresses by properly controlling the J etch value over time. On the one hand, if the J etch value is slowly reduced over time according to HF-diffusion kinetics, it is possible to compensate for the slight reduction of the J peak value with etching depth, so that by maintaining the J etch/J peak ratio constant over time, the etching results to be perfectly anisotropic and, in turn, micromachined structures with excellent straight walls are achieved. The reduction of J peak with etching depth is caused by a reduction in C HF inside the etched trenches, due to stationary diffusion of HF molecules from the top to the bottom of the etched structures (Barillaro and Pieri 2005). On the other hand, if the J etch value is suddenly increased over time with respect to the HF-diffusion kinetics, a dramatic decrease of the J peak value occurs, which dynamically increases the J etch/J peak ratio, and, in turn, the silicon mass dissolved per unit time. A higher number of HF molecules per unit time, with respect to that supplied by stationary diffusion, is consumed at the bottom of the etched trenches as a consequence of the sudden increase of the J etch value. The excess of HF molecule consumption leads to a dynamic reduction of the C HF value and, in turn, of the J peak value at the bottom of the etched trenches. The etching rate in the Z-direction, being proportional to J peak, is therefore reduced, and the increased mass of silicon removed per unit time, J etch/J peak, mainly occurs in the X and Y directions, thus switching the etching from anisotropic to isotropic.

So far, ECM technology has been successfully applied to the fabrication of high-aspect-ratio microstructures and microsystems with application in different fields, from optofluidics (Barillaro et al. 2009b, 2011; Surdo et al. 2012) to biosensing (Merlo et al. 2012b, 2013; Surdo et al. 2012) and from photonics (Barillaro et al. 2007b, 2008, 2009a; Astrova et al. 2010; Polito et al. 2013; Surdo et al. 2013) to medicine (Merlo et al. 2012a, b; Strambini et al. 2012; Carpignano et al. 2012; Merlo et al. 2013). Figure 2 shows two examples of high-complexity microsystems fabricated by ECM technology. Figure 2a, b shows SEM images of MEMS structures consisting of freestanding inertial masses equipped with high-aspect-ratio comb fingers and suspended by high-aspect-ratio folded springs (AR about 100 for both fingers and springs) that are fixed to an anchor structure composed of a two-dimensional array of square holes (Bassu et al. 2012). In Fig. 2c, d, SEM images of an optofluidic platform consisting of two large reservoirs in series to a high-aspect-ratio one-dimensional photonic crystal transducer for biosensing applications (detection of C-reactive protein (CRP) has been demonstrated) are shown (Surdo et al. 2012).

Fig. 2
figure 2

Examples (SEM images) of two microsystems fabricated by ECM technology: (a) MEMS structures consisting of freestanding inertial masses suspended from the substrate by folded springs and featuring comb fingers for actuation; (b) magnification of a folded spring of the MEMS structures in (a); (c) optofluidic platform featuring a one-dimensional photonic crystal as sensing transducer; (d) magnification of the photonic crystal transducer of the platform in (c)

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Summary

ECM technology enables the low-cost fabrication of high-complexity silicon microstructures and microsystems with sub-micrometer accuracy at aspect-ratio values (about 100) that are well beyond up-to-date, both wet and dry, micromachining technologies. Among ECM main features there are (i) fabrication of microstructure with very high aspect ratio, which can be three times higher than that of state-of-the-art deep etching techniques employed for microsystem fabrication; (ii) tunability of the etching anisotropy (from zero to one) in real-time as the etching progresses, which enables 3D freestanding microstructure fabrication by one-step etching; (iii) fine control of the etching features at the sub-micrometer scale, for both small (features down to 1 μm in size) and large (length over 1 mm) areas; and (iv) high quality of etched structures in terms of both verticality (percentage error over a depth of 100 μm of about 0.04 %) and surface roughness (about 20 nm). In the near future, ECM technology is expected to be greatly exploited at the lab scale, although not limited to, for silicon microstructuring, with potential impact on both microdevices and applications in different research fields.