The Chemical Solution Deposition of Lead Zirconate Titanate (PZT) Thin Films Directly on Copper Surfaces

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Title: The Chemical Solution Deposition of Lead Zirconate Titanate (PZT) Thin Films Directly on Copper Surfaces
Author: Losego, Mark Daniel
Advisors: Mark Johnson, Committee Member
Gerry Lucovsky, Committee Member
Jon-Paul Maria, Committee Chair
Abstract: Traditionally, multifunctional complex oxide thin films, like the common ferroelectric materials lead zirconate titanate (PZT) and barium titanate (BaTiO₃) have been limited to substrates with noble metal or conductive oxide bottom electrodes. This constraint originates from the vulnerability of base metals to oxidation when traditional ceramic processing parameters—high temperatures and oxygen rich atmospheres—are used to synthesize ferroelectric films. With current technology, ferroelectric thin films have demonstrated vast applicability as tunable capacitors, sensors, piezoelectric actuators, and non-volatile memories. By integrating ferroelectrics thin films with base metals, the barrier to mass production is lowered through reduced expense and simplified electrode patternability. Moreover, base metals have higher conductivities and offer the possibility for increased functionality by incorporation of ferromagnetic or shape memory alloys. Recent research efforts have adapted 1970s thick film multilayer capacitor technology to process thin films of the (Ba,Sr)TiO₃ family directly on nickel and copper substrates. This methodology relies on processing these materials within a window of temperature and oxygen partial pressure (pO₂) that affords thermodynamic equilibrium between the oxidized perovskite film and unoxidized base metal substrate. Although the family of (Ba,Sr)TiO₃ materials offers excellent dielectric properties, the material PZT could provide a complementary set of functionality to satisfy applications that require an enhanced ferroelectric or piezoelectric response. Unfortunately, fundamental materials differences—particularly PbO volatility and a narrow thermodynamic stability window—make equilibrium processing impractical for PZT/base metal systems. In this thesis, integration of PZT directly on copper surfaces via a chemical solution deposition (CSD) route is investigated. Using this platform a new methodology is developed for achieving perovskite / base metal compatibility. Unlike the traditional equilibrium approach, this new method focuses on using a knowledge of sol-gel science to design a process window that is compatible with the copper substrate while maintaining the integrity of the PZT film. Using this approach, the chelating ligands (organic molecules that impart stability to the metal cations in solution) have been identified as a critical process parameter. If these chelating species cannot provide sufficient gel consolidation and volatilization prior to crystallization within a processing window compatible with the copper substrate, then various complications can result such as substrate oxidation, non-perovskite phase development, or film cracking. By proper chelating agent selection and a unique composite gel architecture, this thesis demonstrates that PZT films can be processed directly on copper substrates with dielectric and ferroelectric properties comparable to films deposited on conventional platinized silicon. Dielectric constants in excess of 800 with tanδ values below 0.02 have been achieved as well as remanent polarization of 33 μC/cm². C-V and P-E loops exhibit classical ferroelectric shapes with well-saturated intrinsic regimes. Electrical fatigue experiments show a classic response with loss of P-E loop squareness and a recoverable remanent polarization upon annealing above the Curie point. Hence, this work demonstrates a methodology for obtaining PZT thin films on copper substrates with remarkable dielectric and ferroelectric properties that are competitive with current noble metal / conductive oxide bottom metal electrode technologies.
Date: 2005-07-13
Degree: MS
Discipline: Materials Science and Engineering

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