Importance of solution chemistry in preparing sol-gel PZT thin films directly on copper surfaces

The ferroelectric material lead zirconate titanate (PZT) has traditionally been considered incompatible with base metal technology because PbO volatility makes conventional thermodynamic equilibrium processing impractical. However, by strategically designing solution chemistry and processing conditions to avoid interfacial reaction, chemical-solution-deposited PZT films can be prepared on copper surfaces without oxidizing the base metal or cracking the oxide film. A limited set of thermal and atmospheric processing conditions to kinetically maintain an unoxidized copper substrate are available and not necessarily optimal for processing sol-gel films. Solutions processed within these confined conditions must form gels with sufficiently reduced organic content and properly consolidated gel networks such that phase-pure and crack-free ceramic films can be crystallized. The current work explores three solution chemistries that use different chelating ligands: alkanolamines, acetylactone, and acetic acid. It is found that the alkanolamine solution frustrates perovskite formation and is prone to cracking under processing conditions compatible with the copper substrate. The introduction of water vapor into the processing atmosphere is moderately successful in resolving these issues. Using a more volatile chelating ligand (acetylacetone or acetic acid) shifts the thermal process window nearer a copper-compatible regime. Because of its weaker chelation strength, acetic acid solutions are most compatible with the processing constraints required for copper substrate compatibility.