Development of Palladium Electrolyte for Maskless Electrochemical microfabrication Process

Abstract

A development of a new palladium electrolyte required for use in a maskless electrochemical microfabrication process, Enface, is presented. Prior works showed that a specific conductivity (~2.7 S/m) of electrolytes is crucial to achieve pattern replication. Selection of appropriate electrolyte based on physicochemical properties is crucial to design a process that is theoretically and practically achievable. The electrolyte was electrochemically characterised to determine onset deposition potential and current densities. In this paper, physicochemical properties i.e. pH and conductivity of palladium electrolyte with composition and concentration ranged between 0.014 – 0.013 M were investigated. The electrochemical experiments were carried out in an unagitated verticallycylindrical cell with nickel pattern deposited on copper disk as working electrode. The pattern feature was 5 mm x 4 mm and 5 mm x 2 mm. The diameter of the copper disk was 1 cm. A nickel disk with diameter of 1 cm was used as counter electrode. Platinum wire was used as a pseudo reference electrode. The experiments were carried out at stagnant condition. The result suggested that an ammoniacal palladium solution was preferable for the experimentation. The spectroscopy analysis confirmed that the palladium was present in the form of Pd(NH3)4Cl2 as major species and PdCl2(H2O)2 with concentration ratio 4:1. According to the conductivity, the electrolyte of 0.019 M PdCl2 + 0.188 M NH4Cl was chosen for use in the experiments. The solution contains 2 g/L as Pd metal and 10.08 g/L NH4Cl exhibiting pH of 2.74 ± 0.31 and conductivity of 2.35 ± 0.11 S/m. At these physicochemical properties, electrodeposition of palladium was expected to occur and pattern replication could possibly be achieved. Polarisation curves obtained from electrochemical characterisation experiments showed that the potential for palladium electrodeposition was in the range of -0.3 to -0.45 V vs Pt wire. For the fully exposed anode, the potentials corresponded to current densities of between 0 and -2.57 mA/cm2.