Nickel Displacement Deposition of Porous Silicon with Ultrahigh Aspect Ratio

With its potential applications in displays and sensors, 1 there has been significant interest in metal deposition into porous silicon PS for use as electrical contacts to the PS-based devices. 2-14 Metalized porous Si has also been shown to be an effective way to engineer the Si substrate impedance for microwave crosstalk isolation in mixedsignal integrated circuits. 15 Many approaches have been experimented for depositing metals into porous Si including dry processes, impregnation by contact with a liquid, chemical bath deposition, and electrochemical deposition. None of these were able to achieve deposition into high aspect ratio pores with most of the metal being deposited at the pore openings near the surface of the porous Si film. 2 The studies utilizing wet processes were often focused on the feasibility of plating bath chemistries and deposition mechanism. 3-6,11-14,16 The PS films used in these studies have thickness ranging from 0.5 to 15 m with the maximum demonstrated depth of deposition being 1.3 m. 5-8,11-13 A deposition process that allows uniform deposition of metal into high aspect ratio of up to 200 pores will have a significant impact on technology. Previously, we conducted theoretical investigation into the uniformity of displacement deposition of nickel with the porous Si skeleton of the PS in an aqueous solution containing nickel ions and fluoride. 17 The distribution of nickel along the depth of the pores was treated as a current distribution issue. It was found that both the interfacial kinetics and mass transfer have great influence on the nickel deposition uniformity, and that the interfacial kinetics varies considerably with deposition time. A polarization parameter was defined to describe the effects of the interfacial kinetics and diffusion. The parameter incorporates all the properties of the displacement deposition system and represents a measure of Ni 2+ interfacial reaction rate relative to its diffusion rate toward the pore bottom. The deposition distribution along the pore depth exhibits a strong dependence on the polarization parameter . High values of correspond to the mass transfer limitation leading to non-uniform deposition distribution, while small gives uniform deposition where the process is dominated by the interfacial reaction kinetics. Nonuniform deposition essentially occurs at the initial stage where the process is accompanied by a high deposition rate and nonuniform Ni 2+ concentration distribution. As the deposition process continues, the initial nonuniform deposition becomes progressively more uniform due to reduced deposition rate and consequent shift of the process from the mass transfer limitation to the interfacial reaction control. It was predicted that the nonuniformity at the initial stage could be remedied by increasing the bulk concentration of nickel ions and decreasing the plating bath pH. Introduction of leveling additive coumarin to the plating solution was also shown to significantly improve the uniformity. This paper presents the results of experimental studies on the behavior of nickel displacement deposition into PS and the dependence of the uniformity on plating bath chemistry. The experimental observations are consistent with the theoretical analyses and predictions made in the previous study. Very uniform deposition is attained at greatly reduced deposition time. Experimental