Photolithography of Polytetrafluoroethylene for Tailored Adhesion

Abstract

AbstractIrradiation of polytetrafluoroethylene (PTFE) with Mg(Ka) X-rays is shown to protect the surface against the chemical etching steps used to prepare PTFE for adhesion. Pre-irradiated etched samples of PTFE have adhesion strengths of less than 3% of that for non-irradiated etched samples. The major portion of this decrease in adhesion strength occurs for X-ray exposures of less than 10 min (∼4.8×103 mrads) and failure in every case occurs in PTFE and not in the bonded transition region. XPS measurements (20 Å sampling depth) show little difference in F content between irradiated and non-irradiated samples, but thermal desorption shows increasing fluorocarbon desorption with irradiation time. These results are consistent with the known radiation chemistry of PTFE. Irradiation produced free radicals lead to branching and/or cross-linking, and a surface rich in low molecular weight fluorocarbons. The more rigid cross-linked surface is resistant to deep (10,000 Å) chemical attack and the bond formed is with a surface rich in short chain fluorocarbons. Both a thin boundary region and bonding to short chain species is expected to lead to weak adhesive bonding. Electron irradiation is shown to lead to protection against chemical etching comparable to that obtained with X-rays. With electrons one has the capability of rastering the electron beam and drawing low adhesion strength patterns with resolution limited by the beam diameter.