Fabrication of Lithography-Free Silicon Sub-Micro-Pyramids

Abstract

In this work we report the fabrication of ordered arrays of sub and micron-scale silicon pyramids and pillars by simple electrochemical etching of a p-type silicon substrate through Pd deposition. The Pd was deposited on Si substrate by evaporation technique using a mask (mPd/Si) and without a mask (wPd/Si). The morphology and dimensions of the silicon nano or micro-structure were controlled by the electrochemical etching time, dimension and shape of Pd structure. Then; ordered pyramids, inverted pyramids or mixed pyramids and pillars structure are formed depending on the etching time. The morphologies obtained have been attributed to two etching: chemical and electrochemical.

Finally, total reflectance spectra show a low reflectance value less than 5% in the region 350–1050 nm for anodized (mPd/Si) structures. Such result would be very interesting for both solar cells and optical filters applications.

Additifs

Generally, it is reported that the ethanol does not participate in the anodization process [5]. However, acetonitrile participates in the anodization process as shown in formula (3).

$$2\mathrm{F}-+\mathrm{CN}-{\mathrm{CH}}_3\dots \dots \dots \dots ..{\mathrm{HF}}^{-2}+\mathrm{CN}-{\mathrm{CH}}^{-2}$$

(3)

In addition, molecular structures of ethanol and acetonitrile are shown in Fig. 8. The nitrogen (N) in the acetonitrile has an unshared electron pair, so that the nitrogen combines with H₂O which has a permanent dipole moment to form a weak hydrogen bond. Therefore, we present the possibility that acetonitrile prevents water molecules from participating in the anodization process as in anhydrous solution [5, 6, 12]. Therefore, with the reaction mechanism of silicon in anhydrous solution, hydrogen gas is produced little in the HF-acetonitrile solution in a HF-ethanol solution. Also, due to electron injection into the silicon conduction band, double currents pass to the silicon wafer so that uniform nano-PSi layers can be acquired in the same etching time.

Metal-assisted chemical etching (MACE), of silicon and nanotechnology applications, Hee Hana, Sufficient experimental evidences have been reported to support a solid mass transfer model [8, 9]. In the scenario, the oxidation of Si surface proceeds at the interface between metal and Si substrate; through HF can diffuse through the channels between Si and metal catalyst to bulk Si to facilitate the oxidative dissolution of Si, and reactants and byproducts diffuse as well through the channels (Fig. 1a). Two sets of etching experiments were carried out for comparison. Metal (Ag) stripes, including those with different lateral sizes while the same thickness and those with identical lateral size but different thicknesses were deposited onto Si substrates, and subsequently the samples were etched in an aqueous etchant comprised of HF and H₂O₂. The experiments showed that the etching rate decreases with the lateral size of Ag stripes, indicating the correlation between the etching rates of Si and the lateral sizes of Ag stripes.