Controlling Nickel Deposition Depth in Mesoporous Silicon through Wetting and Deposition Kinetics

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A strong mechanical and electrical connection between silicon (Si) and a metal film, such as nickel (Ni) or copper (Cu), is essential for several fields of application such as microelectronics, MEMS, analytical systems, biomedical devices and energy storage and conversion. The strongest connection can be achieved by mechanical interlocking of both materials, i.e. by increasing the adhesive surface area and by creating hooking structures. Functionalizing the surface by etching mesopores into the Si creates an interlocking structure which can drastically increase the mechanical integrity of the Si-metal composite. A complete pore filling of the structure can be achieved by depositing at low current densities and by using an electrolyte with a high micro throwing power, i.e. with a high ability to fill coat through-holes. However, preventing the metal ions from penetrating the porous structure or even controlling the penetration depth of the metallization is much more challenging by electrochemical means.In this contribution, we present an inexpensive and easily scalable method to control the penetration depth of a Ni layer into mesoporous Si (Ni@Si) that is not making use of macromolecular Ni-complexes. Instead, the process exploits the hydrophobic nature of freshly etched, and hydrogen-terminated Si. In combination with an aqueous Ni-electrolyte, such a hydrophobic mesoporous Si leads to the Cassie-Baxter state where the liquid will not enter the pores, leaving them air-filled. A pre-treatment of the porous silicon was shown to restrict the Ni deposition to a tunable depth. The penetration depth should be chosen about twice as deep as the pore diameter to get a strong mechanical connection.In addition to tailoring the surface energy of the porous Si through the pre-treatment, it is of great importance to choose the right plating parameter. In this contribution we will discuss how voltage and current profiles should be adapted to the electrochemical system and how these parameters get influenced by factors such as temperature, electrolyte flow, passivation kinetics, etc. Our method enables a homogeneous nucleation of Ni grains within the pre-treated pore openings.We demonstrate that the resulting interlocked Ni@Si metallization is well suited for a layer transfer process of a full-scale 15.6 x 15.6 cm2 wafer. The interconnection can even withstand the cycling of Si in a battery. Si, being the anode material with the highest known gravimetric capacity for Li-ion batteries, can expand and shrink by a factor of 4 during lithiation and delithiation, respectively. These enormous volumetric changes lead to a fracturing and cohesive failure of the Si, which is the main challenge in commercializing pure Si anodes (without any binder and conductive carbon). The Ni@Si interlocking structure we have developed provides an interface that can withstand these large volumetric changes without showing signs of delamination of the porous Si from the deposited Ni current collector by adhesive failure.