A comparative study on fabrication techniques for on-chip microelectrodes

Abstract

This paper presents an experimental study on different microelectrode fabrication techniques, with particular focus on the robustness of the surface insulation towards typical working conditions required in lab-on-a-chip applications. Pt microelectrodes with diameters of 50 μm, 100 μm and 200 μm are patterned on a Si substrate with SiO₂ film. Sputtered SiO₂, low-pressure chemical vapor deposition (LPCVD) low-temperature oxide (LTO), Parylene C, SU-8, and dry-film were deposited and patterned on top of the chips as the passivation layer. This paper provides the detailed fabrication processes, the adhesion enhancement strategies, and the major advantages and disadvantages of each fabrication technique. Firstly, the quality and adhesion strength of the passivations were investigated by means of hydrolysis tests, in which sputtered SiO₂ and dry-film resist showed serious delamination issues and LTO showed minor defects. Secondly, the reliability of the microelectrodes was tested by impedance measurements after overnight ethanol incubation and self-assembled monolayer (SAM) formation. Thirty chips, representing a total of 300 electrodes, were measured, and statistical analyses of the results were conducted for each passivation technique. All of the electrodes passivated with these five techniques showed consistent impedance values after ethanol incubation. On the other hand, only LTO, Parylene C, and SU-8 ensured uniform electrical behavior after SAM formation. Having used both hydrolysis and impedance tests to verify the superior quality of the Parylene-based passivation, electrochemical experiments were performed to study the long-term stability of the passivation layer. Finally, the electrodes were incubated with electroactive alkanethiols functionalized with ferrocene. Square-wave voltammetry measurements demonstrated reproducible results on electrochemical label detection, which confirms the suitability of the Parylene passivation for charge-transfer-based measurements.