Formation Mechanisms of Needles in Porous Silicon Based Needle-like Surfaces
Authors: Keshavarzi Shervin , Mescheder Ulrich , Reinecke Holger 
Published: February 2020

Formation mechanisms of needle-like surfaces generated by anodization of Si in the transition region (region between pore formation and electropolishing) is studied. Formation of needles is monitored via SEM surface images taken after different anodization times and their formation mechanisms is modeled based on current burst (CB) model and Lehmann macro-pore formation model.
Anodization is a well-known technique for porous silicon (PS) formation or electropolishing of silicon wafer. Although the technique is known for several decades, it has attracted renewed interest since 1990 resulting from discovery of luminescence properties of PS. Anodization has emerged in micromachining applications because of its low cost, easy implementation, and compatibility with standard microelectronic processes [1]. Usage of anodization technique to create rebondable (needle-like) Si surfaces with adequate bond strength for low temperature chip/wafer bonding applications has currently attracted attentions [2].
Needle-like Si surfaces with different needle sizes and distributions are obtained by anodization of lowly doped p-type Si in aqueous HF solutions in the transition region. For 12-17 Ωcm p-type Si, anodized in a 7.2 wt.% aqueous HF solution, morphology of generated surfaces (Fig. 1) were clearly related to positions of current peaks (critical current density (J-PS) and electropolishing current density (J-El)) on the I-V curve (Fig. 2). Formation of needles was studied through SEM surface images taken after different anodization times (Fig. 3). Initiation of needles is started by inhomogeneous dissolution of Si, roughening and pitting of the surface (Fig. 3a), and is followed by nucleation of pores (Fig. 3b). As morphology of PS varies in depth from surface to bulk, it results in two-layers PS, a meso PS above a macro PS [3]. Ununiform dissolution of meso-pores results in formation of small irregular islets (remaining bulk Si) at certain times during the anodization (Fig. 3c). With increasing time, small islets are completely etched away and given access to overlapping and widening macro-pores underneath. Remaining bulk Si between widened and overlapped macro-pores results in large islets (Fig. 3d). Further etching of the surface reshapes islets to pyramid-like structures by slightly narrowing their side walls and increasing their heights (Figs. 3e and f). As the time, increased, side walls are narrowed more and their heights are increased further (Fig. 3g). Further etching of the side walls results in formation of semi-cylindrical needles in which their tips are attached together and made cluster of needles (Fig. 3.h). At this stage, with increasing time, only heights of needles are increased and no significant reduction in diameter of needles are occurred (Fig. 3i).
Nucleation of pores and formation of islet can be explained through the CB model [4]. At current densities above J-PS, CBs begin to correlate in time and space. This results in assembling of CBs and formation of either a meso-pore or a macro-pore depending on density of CB events in a particular area (Fig. 4). In a condition where the oxidation reaction is low, CBs result in macro-pores covered with meso-porous (transition layer). However, for the condition where oxidation reaction is dominant (e.g., in aqueous electrolytes), oxidation takes over CBs and smoothes the area. This results in macro-pores with no meso-pores coverage (complete dissolution of the transition layer). In the transition region, Si surface is not completely covered by oxide; hence, both inhomogeneous and homogenous dissolutions may occur on different parts of the surface [3]. This may result in inhomogeneous dissolution of the transition layer and formation of small islets above macro-pores. With increasing time, the transition layer dissolve entirely and give accesses to underneath macro-pores which have already overlapped and widened (Fig. 5). The remaining bulk Si between overlapped macro-pores results in large islets. Assuming large islets with diameters of ~ 4.5 ± 1.4 µm (obtained from Fig. 3i) as remaining and surrounding walls of widened macro-pores in which their wall thickness is greater than the space charge region length (1 – 1.2 µm for 12-17 Ωcm p-type Si), the Lehmann model [5] can be simply used to describe growth and narrowing of side walls of islets and formation of needles.

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Figure 1