Adhesion studies of metals on fluorocarbon polymer films

Abstract

Adhesion of metal films to several fluorocarbon polymer films is studied for Cu, Cr, Ti, Al, and Au. The polymers include polytetrafluoroethylene (PTFE), fluoroethylenepropylene (FEP), and a copolymer containing a perfluoroalkoxy group (PFA), all deposited on Cr/SiO2 substrates by spin coating. Peel strengths of the metal strips on these polymers are compared and are taken as measures of the metal–polymer adhesion. Among the polymers, FEP gives the highest peel strengths to metals, with PTFE the lowest. Among the metals, Ti gives the highest peel strength for each polymer, followed by Cr, with Cu being the lowest. The peel strengths for Ti on FEP, PFA, and PTFE are 85, 75, and 20 g/mm, respectively. Those for Cu to the polymers are around 5 g/mm or less. The strong adhesion for the metals on both FEP and PFA is attributed to the high concentration of carbon sites with three fluorine neighbors, by considering the electronegativities among the various carbon sites in the polymer chains. Among the metals, some correlation with the electronegativity is seen. Ti shows, in addition, penetration into the polymers, contributing to the adhesion with all three polymers and resulting in shifts in binding energy for the C 1s electrons of the CF2 sites. An enhanced adhesion is also seen using a presputtering treatment of the PTFE films prior to the deposition of metal. The increase in peel strength, however, is smaller than that of the bulk PTFE.