Mechanical characterization of sub-micron polytetrafluoroethylene (PTFE) thin films

This study reports the results of an investigation of the mechanical properties of polytetrafluoroethylene (PTFE) thin films on silicon substrates in the 0.5 to 15 mu m thickness regime using frequency specific depth-sensing indentation All measurements were conducted at an excitation frequency of 45 Hz using a constant (1/P dP/dt) load ramp of 0.1 s(-1). The modulus of the PTFE at a depth of 5% of the film thickness was measured to be approximately 1 GPa (nu = 0.46) independent of film thickness. These values are somewhat higher than the values obtained from free-standing 15 mu m film measurements of 0.4 GPa for the tensile modulus and 0.49 GPa for the storage modulus @ 1.1 Hz. The film hardness at these depths was observed to range between 30 and 55 MPa with no correlation observed between the hardness and respective film thickness. While reliability modeling for interconnects currently uses interlayer dielectric mechanical properties data determined from free-standing films with thicknesses of several microns, these in-situ results should more closely mimic the constrained deformation that occurs during service and perhaps lead to a better understanding of the electromigration resistance of PTFE.