Table 2.

Comparison of etching techniques employed for the fabrication of electrodes for neural recording.

	Wet chemical etching		Dry etching	Electrolytic etching
	Isotropic	Anisotropic
Methods	Wet chemical baths, vapors Chemical	Wet chemical baths, Vapors Chemical	Plasma etching Gas phase etching Reactive ion etching Deep reactive ion etching Physical, chemical-physical	Wet chemical baths with the application of electric potential Electrochemical
Properties	Uniform material removal Rounded shapes High etch rates	Material removal with different rates in areas of different crystallographic orientation or doping level Great process control Etch stop possibility Etch rates dependent on temperature and concentration Undercutting	Isotropic, Anisotropic Deep etching Minor broadening and undercutting CMOS compatible Repeatable Possibility to form structures with vertical sidewalls and small features	Combination of electrical and chemical reactions causing anodic dissolution of metals under the application of voltage between material and counter electrode
Materials/Solutions	Silicon: HF, HF/HNO3/CH3COOH Silicon nitride: H3PO4 Tungsten: HF /HNO3 Aluminum: H2O/HF; HCl/HNO3 /HF Platinum, Iridium: HCl /HNO3	Silicon: KOH, NaOH, EDP, TMAH	Silicon: XeF2; RIE- CF4, SF6; DRIE- SF6/C4F8 Polymers: O2, F2 – based plasmas	Silicon: solutions containing HF Tungsten: KOH, NaOH, Platinum, Iridium: CaCl2, HCl, NaCl, KCl, NaOH, KCN
Application in fabrication of neural electrodes	Two stage etching in the formation of Utah electrode arrays: pillar thinning dynamic etch and static- sharpening etch (Bhandari et al., 2010a) Smoothening of roughness induced during dicing and WEDM in Utah electrode arrays (Rakwal et al., 2009) Etching of buried microfluidic channels in planar probes (Cheung et al., 2003) Edges smoothening (Grand et al., 2011) Releasing electrodes from the substrate (Chen C. H. et al., 2009)	Thickness and shape definition of planar probes (Najafi et al., 1990; Lin and Pisano, 1999; Yao et al., 2007) Releasing planar electrodes from the wafer (Edell et al., 1992) Shaping electrodes accordingly to crystallographic planes (Xiao-Hong et al., 2007)	Thickness and outline definition of SOI-based planar electrodes (Norlin et al., 2002) Main removal technique in polymer-based neural interfaces (Kim and Meng, 2015) Release of planar electrodes from the wafer (Suzuki et al., 2003; Fernández et al., 2009; Chung et al., 2015) Formation of macroporous and lattice structures promoting neuronal ingrowth (Wise et al., 2008) Roughening of probes' surface (Chen et al., 2014; Zhang et al., 2014) Deinsulation of recording sites (Yao et al., 2006)	Sharpening metal wires for microwire electrodes using DC voltage for sharp hyperbolical tips, or AC-voltage for larger, angled conical shapes up to tens of nanometers (Grundfest et al., 1950; Chang et al., 2012) Electropolishing to smoothen surfaces and thin wire-based probes (Lalanne et al., 2011) Formation of porous silicon used as on-probe biomolecular filtering element, sacrificial layer, or dissolvable stiffening material (Bell and Wise, 1998; Hajj-Hassan et al., 2012; Sun et al., 2016)