Lithographic patterning of metals on flexible plastic foils

In this paper the challenges of patterning electrodes with separations in the micron and sub-micron range onto thin polyethylene naphthalate foils for use in biochips are discussed. It was found that it was necessary to improve the adhesion of the metal electrodes to the foil by using plasma treatment and/or an adhesion layer, and a quantitative measurement of this adhesion was preformed using a peel-off tester. Electrode patterning was performed by optical lithography. Two processing methodologies were compared, namely direct patterning of metal films and patterning using a lift-off process. In addition, the effects of organic solvents, sonication, and temperature on film quality were assessed. After optimization of process conditions, biochips with sub-micrometer sized electrode gaps and stable electrode structures were obtained by using the lift-off process.