Replacement of noble metal electrodes by base metals significantly lowers the cost of ferroelectric, piezoelectric and dielectric devices. Here, we demonstrate that it is possible to process lead zirconate (Pb(Zr0.52Ti0.48)O3, or PZT) thin films directly on base metal copper foils. We explore the impact of the oxygen partial pressure during processing, and demonstrate that high-quality films and interfaces can be achieved through control of the oxygen partial pressure within a narrow window predicted by thermodynamic stability considerations. This demonstration has broad implications, opening up the possibility of the use of low-cost, high-conductivity copper electrodes for a range of Pb-based perovskite materials, including PZT films in embedded printed circuit board applications for capacitors, varactors and sensors; multilayer PZT piezoelectric stacks; and multilayer dielectric and electrostrictive devices based on lead magnesium niobate–lead titanate. We also point out that the capacitors do not fatigue on repeated switching, unlike those with Pt noble metal electrodes. Instead, they appear to be fatigue-resistant, like capacitors with oxide electrodes. This may have implications for ferroelectric non-volatile memories.

Lead zirconate titanate thin films directly on copper electrodes for ferroelectric, dielectric and piezoelectric applications

Properties are presented for random and enhanced 〈100〉 orientation SrTiO3 and BaxSr1−xTiO3 (x=0.33, 0.50, and 0.67) films fabricated on base-metal 〈100〉 Ni tapes using a chemical solution deposition approach. Films were crystallized in a reducing atmosphere, which prevented Ni oxidation, but permitted growth of SrTiO3 and BaxSr1−xTiO3 films with dielectric loss tan δ between 0.003 and 0.015. For randomly oriented BaxSr1−xTiO3 (x=0, 0.33, 0.5, and 0.67) films, zero-field 100 kHz dielectric constants ranged from 250 to 420. Films with enhanced 〈100〉 orientation exhibited zero-field dielectric constants of 980 to 1500, three times higher than random films. Development of such high dielectric constants on Ni suggests a future method for inexpensive, high performance base-metal electrode capacitor fabrication.

Dielectric properties of random and 〈100〉 oriented SrTiO3 and (Ba,Sr)TiO3 thin films fabricated on 〈100〉 nickel tapes

Barium titanate thin films have been prepared by chemical solution deposition on 18 μ m thick, industry standard copper foils in the absence of chemical barrier layers. The final embodiment exhibits randomly oriented BaTiO3 grains with diameters between 0.1 and 0.3 μ m, and an equiaxed morphology. The average film thickness is 0.6 μ m and the microstructure is free from secondary or interfacial phases. The BaTiO3 films are sintered in a high temperature reductive atmosphere such that copper oxidation is avoided. Subsequent lower-temperature, higher oxygen pressure anneals are used to minimize oxygen point defects. Permittivities of 2500 are observed at zero bias and room temperature, with permittivities greater than 3000 at the coercive field. Loss tangents under 1.5% are demonstrated at high fields. The BaTiO3 phase exhibits pronounced ferroelectric switching and coercive field values near 10 kV/cm. Temperature dependent measurements indicate a ferroelectric transition near 100∘C with very diffuse character. Combining the approaches of the multilayer capacitor industry with traditional solution processed thin films has allowed pure barium titanate to be integrated with copper. The high sintering temperature—as compared to typical film processing—provides for large grained films and properties consistent with well-prepared ceramics. Integrating BaTiO3 films on copper foil represents an important step towards high capacitance density embedded passive components and elimination of economic constraints imparted by traditional noble metallization.

Copper Compatible Barium Titanate Thin Films for Embedded Passives

A multilayer printed wiring board comprises insulating layers and conductor layers being stacked alternately on each other. The conductor layers are electrically connected to each other through viaholes formed in the insulating layers. Each of the viaholes is formed to bulge in a direction generally orthogonal to the direction of thickness of the insulating layer. The multilayer printed wiring board is to have electronic components such as a capacitor, IC and the like mounted on the surface layer thereof.

Multilayer printed wiring board

Processing effects on the dielectric properties of sol-gel-derived PbZrO 3 -PbTiO 3 (PZT) films integrated onto Pt/Ti/SiO 2 //Si substrates are reported. Sol-gel synthesis and deposition conditions were designed to produce films of varying thickness (95-500 nm) with consistent chemical composition (Pb (Zr 0.53 Ti 0.47 )O 3 ), phase content (perovskite), grain size (∼110 nm), crystallographic orientation (nominally (111) fiber textured), and measured residual stress. The Stoney method, using laser reflectance to determine wafer curvature, derived biaxial tensile stress values of 150 and 180 MPa for PZT films after a baseline correction for electrode interactions during thermal processing was employed. The PZT films were of high dielectric quality, with low losses and_negligible dispersion. Calculated values of dielectric constant (K') were found to decrease from 960 to 600 with decreasing film thickness. A series-capacitor model successfully recovered a room-temperature K' 1 for the PZT (1,170) in good agreement with bulk reports but was unable to reproduce the expected dielectric anomaly near 380°C. This discrepancy and the resulting diffuse phase transformation were attributed to the biaxial tensile stress present in the PZT films.

Processing Effects for Integrated PZT: Residual Stress, Thickness, and Dielectric Properties

An approach for embedding high-permittivity dielectric thin films into glass epoxy laminate packages has been developed. Lead lanthanum zirconate titanate (Pb0.85La0.15(Zr0.52Ti0.48)0.96O3, PLZT) thin films were prepared using chemical solution deposition on nickel-coated copper foils that were 50 μm thick. Sputter-deposited nickel top electrodes completed the all-base-metal capacitor stack. After high-temperature nitrogen-gas crystallization anneals, the PLZT composition showed no signs of reduction, whereas the base-metal foils remained flexible. The capacitance density was 300–400 nF/cm2, and the loss tangent was 0.01–0.02 over a frequency range of 1–1000 kHz. These properties represent a potential improvement of 2–3 orders of magnitude over currently available embedded capacitor technologies for polymeric packages.

Lead Zirconate Titanate Thin Films on Base-Metal Foils: An Approach for Embedded High-Permittivity Passive Components

A dielectric film is formed on a free-standing conductive metal layer to form a multi-layer foil comprising a conductive metal layer, a barrier layer and a dielectric oxide layer Such multi-layer foils are mechanically flexible, and useful for the manufacture of capacitors Examples of barrier layers include Ni—P or Ni—Cr alloys After a second layer of conductive metal is deposited on a dielectric oxide surface opposing the first conductive metal layer, the resulting capacitor foil is processed into a capacitor The resulting capacitor is a surface mounted capacitor or is formed as a integrated or embedded capacitor within a circuit board

Kevin Cheek

Papers

Lead Zirconate Titanate Thin Films on Base-Metal Foils: An Approach for Embedded High-Permittivity Passive Components

Multi-layer conductor-dielectric oxide structure