The metallic nickel inserted p-/p+ Si substrate used for RF crosstalk reduction in mixed signal ICs

A novel approach for 3-dimensional substrate impedance engineering of p^-/p⁺ Si substrate was studied for mixed-signal integrated circuit applications. For enhanced radio frequency (RF) crosstalk isolation, a highly conductive region analogous to a moat was inserted in the substrate between the noise producing and noise sensitive circuits to serve as grounded shields. Electroless plating was used to metallize the macroporous silicon moat etched beforehand within the p^-/p⁺ substrate to isolate electromagnetic interference and provide "true ground" contacts. The electroless plating was conducted in a concentrated aqueous solution containing Ni²⁺ and NH⁴F. Metallic Ni was rapidly deposited without using a reducing agent or any activation treatment at a slightly elevated temperature. After certain immersion duration, the initially single crystalline Si sidewall of the pores was completely modified by the polycrystalline metallic Ni while the original one-dimensional straight pore structure was still maintained. Thus a highly conductive porous structure was obtained in the substrate and RF crosstalk was reduced to the level limited by that across the air gap between the measurement probes. This technique offers further improvement with regard to RF crosstalk via substrate on the isolation effect achieved by high impedance through-the-wafer porous Si.