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Introduction

According to the IUPAC standard, macropores correspond to pores exhibiting characteristic sizes (pore diameter and average distance between pores) larger than 50 nm. The term “macropore” is usually associated with smooth cylindrical pores with characteristic sizes on the order of 1 μm.

This kind of pore can be obtained under a variety of conditions and with differing morphologies (see chapter “Routes of Formation for Porous Silicon”). In this review, we focus on electrochemically etched macropores. The key parameters are the electrolyte type (aqueous (aqu), organic (org), oxidant (ox)) the HF concentration, the surfactant, the Si doping type and level (n, n+, p, p+), and in some cases the illumination (backside illumination (bsi) or frontside illumination (fsi)). Detailed reviews regarding their formation are available (Föll et al. 2002; Lehmann 2005; Chazalviel and Ozanam 2005; Lehmann 2002; and handbook chapter “Porous Silicon Formation by Anodization”).

Current-Line- and Crystallography -Driven Macropores

Two distinct classes of macropores are observed, as summarized in Table 1. Macropores obtained from n-Si always exhibit a strong growth dependence on crystallographic orientation. On p-Si, this dependency is lower (Lehmann and Rönnebeck 1999), and in aqueous conditions at low enough current density and/or high enough HF concentration, the growth turned to be determined by the direction of the current lines in the space-charge region Media et al. 2011.

Table 1 Macropores classes and their main characteristics
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Macropore Formation

Table 2 summarizes the main characteristics of the electrochemically grown macropores on Si, as a function of the formation conditions.

Table 2 General conditions for macropore formation. “Passivation power” denotes the degree to which a given electrolyte can remove interface states in the bandgap of Si by covering a freshly etched surface with hydrogen
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Figure 1 illustrates the variety of pores obtained for p-Si under different conditions.

Fig. 1
figure 1

Morphologies for (100) p-Si in 0.05 M HF, 0.05 M NH4F, and 0.9 M NH4Cl, pH = 3, V = 0.15 V for 48 h: (a) Plan view and (b) cross section (After Slimani et al. 2009). (c) Macropores on p-Si (aqu), view after cleavage, for samples prepared from p-Si (400 Ω cm, (100)-oriented), 100 mA/cm2, 6 min, 15 % ethanolic HF (After Chazalviel et al. 2002). Macropores on (100) n-Si etched in ethanolic hydrofluoric solution with frontside illumination and with an anodization current J = 20 mA/cm2 for t = 45 min. (d) Cross-section and (e) plan view (After Outemzabet et al. 2005). (f) Macropore on p-Si (org) prepared from p-Si (100 Ω cm, 20 mA/cm2, 40 min, HF/ethylene glycol 50/50 by vol)

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Macropore Formation Models

Porous silicon formation models have been reviewed (Smith and Collins 1992; Allongue 1997; Zhang 2001). A conceptual analysis has been attempted (Zhang 2004). Major theoretical contributions applying to macropore formation are listed in Table 3.

Table 3 Major theoretical contributions to macropore formation analysis
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Principal Application of Macropores

Macropore arrays found applications in various fields, some of which are listed in Table 4.

Table 4 Main application domains of macropore arrays
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Design of Regular Macropore Arrays

The fabrication of regular macropore arrays requires prestructuring of the Si substrate using lithography and alkaline etching (Chao et al. 2000; van den Meerakker et al. 2000; Starkov 2003). The pitch of the prestructured hole array has to match the average spacing of random macropore arrays grown on the same substrate under similar electrochemical conditions. The width of the walls of the porous structure (which depends on the pitch structure and the pore lateral size) is mostly determined by the width of the space-charge layer (i.e., mostly dependent on substrate doping level) and the pore diameter by the etching conditions. Figures 2 and 3 give some design rules in the case of p-Si. In the case of n-Si, the pore diameter is mostly determined by the current density, i.e., the illumination level, according to Lehmann’s model (Lehmann 1993). However, diffusion effects in the liquid phase, as theoretically modeled (Barillaro and Pieri 2005), must be taken into account in order to keep the fluoride concentration stationary at the pore tips. Figure 4 gives the typical pore-density range accessible on n-Si under usual backside illumination conditions or p-Si in the dark.

Fig. 2
figure 2

Comparison of characteristic macropore sizes on p-Si in the current-line-driven regime, when changing current density for a substrate resistivity of 100 Ω cm (a) and silicon doping for an applied current density of 10 mA/cm2 (b) (After Chazalviel et al. 2002). Triangles refer to the wall width and diamonds to the pore diameter; the closed (open) symbols refer to the data obtained in 35 % (25 %) ethanolic HF. The solid lines refer to the theoretical prediction (Chazalviel et al. 2002) for the pore diameter, and the dotted line is two times the space-charge width λ

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Fig. 3
figure 3

Effect of changing electrolyte resistivity on average macropore diameter on p-Si 1,500 Ω cm, for J0 = 10 mA/cm2 in an electrolyte made of 50 % aqueous HF mixed with ethylene glycol in variable proportions (After Chazalviel et al. 2002)

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Fig. 4
figure 4

Pore density versus silicon electrode doping density for porous silicon layers of different geometries. Notice that macropores are essentially obtained on low to moderately doped substrates. The dashed line shows the pore density of a triangular pore pattern with a pore pitch equal to two times the SCR width for a 3 V applied bias. Note that only macropores on n-type substrates may show a pore spacing significantly exceeding this limit. The regime of stable macropore array formation on n-Si is indicated by a dot pattern. Doping type and etching current density (in mA/cm2) are indicated in the legend (After Lehmann 1993)

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Conclusions

Since the first report of Theunissen (Theunissen 1972) and the pioneering work of Lehmann in the 1990s, many efforts have been devoted to macropore fabrication by electrochemical etching. Impressive macropore arrays have been achieved, with high aspect ratios and smooth or patterned vertical walls. Examples are shown in Fig. 5. Alternative techniques have been proposed such as galvanic etching (Xia et al. 2000), stain etching (Mills et al. 2005), and metal-assisted (electro)chemical etching (Li et al. 2013). These techniques are separately reviewed in this handbook (see Chapters “Porous Silicon Formation by Galvanic Etching,” “Porous Silicon Formation by Stain Etching,” and “Porous Silicon Formation by Metal Nanoparticle-Assisted Etching”).

Fig. 5
figure 5

Examples of regular and ordered macropore arrays. (a) Two-dimensional macropore array with an intentional line defect (From Grüning et al. 1996); (b) array of pores grown on n-Si (1015 cm−3); the pore initiation pattern shown in the inset has been produced by photolithography and alkaline etching (From Lehmann et al. 2000)

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