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Formation mechanisms of self-organized Si-needles generated by anodic etching of lowly doped p-type silicon wafers in an aqueous HF solution in the transition region (region between the pore formation and the electropolishing) is studied through surface SEM images taken after different etch times. Impacts of current density, substrate resistivity range in the lowly doped region, and electrolyte additives on morphology of needles are also investigated. A simple model based on pore formation models is presented to describe formation of self-organized needles during anodic etching of lowly doped p-type silicon in an aqueous HF solution in the transition region. Anodization is a well-known technique for porous silicon (PSi) formation or electropolishing of silicon (Si) wafers. Although the technique is known for several decades, it has attracted renewed interest since 1990 resulting from discovery of luminescence properties of PSi.1 The technique has also emerged in micromachining due to its low cost, easy implementation, and compatibility with standard microelectronic processes.2 Usage of the anodization technique to create needle-like Si surfaces for room temperature Si-Si bonding applications has currently attracted attention.3-5 Additionally, the technique has a good potential to easily replace complicated and costly processes, such as inductively coupled plasma reactive ion etching (ICP-RIE), 6 interference lithography combined with Reactive Ion Etching (RIE) technique, 7 metal-assisted etching (MAE) technique, 8 and laser micro/nano-processing, 9 which are commonly used to generate Si needle-like antireflection coating surfaces for visible and (near infrared) NIR applications.Using the anodization technique, needle-like surfaces can be obtained by anodic etching of lowly doped p-type Si in aqueous HF solutions in the so-called transition region, the region between pore formation and electropolishing.3-5 Although formation of PSi and electropolishing of p-type Si are quite well-understood, formation mechanisms of self-organized needles in the transition region, where both pore formation and electropolishing are competing for control over the surface, 10 is still unclear since instabilities in space and time are occurring in this region.11 Even though formation mechanisms of needles generated by anodization of p-type Si are briefly described in, 12,13 no model for their formations has been reported yet. In this work, formation mechanisms of self-organized needles generated by anodic etching of lowly doped (<10 15 atoms/cm 3 ) 14 p-type Si in an aqueous HF solution is investigated in detail. Impacts of current density and substrate resistivity range in the lowly doped region, and electrolyte additives on morphology of surfaces are also discussed. A simple model based on the Current Burst (CB) model, 15 the Zhang model (distribution of the electric field at different regions of a pore), 16 and the Lehmann macro-pore formation model 17 is then presented to describe formation of needles in the transition region. Exp...

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