Daniel M. Potrepka

The objective of this work is to describe the current state of the rapidly evolving field of 3D piezoelectric microelectromechanical systems (piezoMEMS), and where it needs to go to fully leverage the potential performance benefits offered by atomic layer deposition (ALD). We define 3D piezoMEMS as the application of piezoelectric ALD films to 3D, high aspect-ratio, mechanically pliable structures. Since there are so few existing reports of 3D piezoMEMS, a literature review of ALD films applied to conventional microelectromechanical system (MEMS) devices is given. ALD processes for piezoelectric thin films are reviewed in the context of relevant applications such as transducers and actuators. Examples include aluminum nitride, hafnium zirconate, doped-hafnia, lead zirconate-titanate, lead hafnate, and lead hafnate-titanate. New concepts for ALD-enabled 3D piezoMEMS actuators are presented with supporting theoretical calculations that show that chip-scale mechanical work densities could be improved by ≫10× compared to conventional planar piezoMEMS. 3D fabrication methods are also discussed, while the future needs of atomic layer processing are highlighted.

Sputtered Pb(Zr ₅₂ Ti ₄₈ )O ₃ (PZT) thin films grown on 150 mm Silicon-on-Sapphire (SOS) substrates for RF MEMS applications. Film properties are compared to similar films deposited on 150 mm Si/SiO ₂ wafers. PZT films deposited on SOS show improvement in P _MAX (46.2 vs. 42.1 μV/cm ² ) and P _REM (22.0 vs. 16.5 μV/cm ² ) over films deposited on Si. A decrease in maximum ε _r from 1027 (Si) to 927 (SOS) is also noted. Crystal structure is examined using X-ray diffraction, while ferroelectric P-E hysteresis curves, dielectric constant tuning, and loss tangent are studied using fabricated capacitor structures.

Atomic layer deposition (ALD) processes were demonstrated for lead-titanate (PbTiO3) and lead-zirconium-titanate (PZT) films. The metal organic precursors were tetraethyl lead Pb(Et)4 reacted with ozone; along with tetrakis ethylmethylamino zirconium and tetrakis dimethylamino titanium reacted with either water or ozone. These precursors were selected based on compatibility with ALD processes for the component oxides. Single oxide films of PbO, ZrO2, and TiO2 were easily deposited using the selected precursors. ALD of the complex oxide films was done by combining the ALD processes for the component oxide films. The compositions of Pb, Zr, and Ti in the films could be predictably controlled by the relative ratio of Pb, Zr, and Ti precursor doses during the ALD process, and controlled composition for both PbTiO3 and PZT films was demonstrated.

Direct-write laser grayscale lithography has been used to facilitate a single-step patterning technique for multilayer lead zirconate titanate (PZT) thin films. A 2.55-μm-thick photoresist was patterned with a direct-write laser. The intensity of the laser was varied to create both tiered and sloped structures that are subsequently transferred into multilayer PZT(52/48) stacks using a single Ar ion-mill etch. Traditional processing requires a separate photolithography step and an ion mill etch for each layer of the substrate, which can be costly and time consuming. The novel process allows access to buried electrode layers in the multilayer stack in a single photolithography step. The grayscale process was demonstrated on three 150-mm diameter Si substrates configured with a 0.5-μm-thick SiO ₂ elastic layer, a base electrode of Pt/TiO ₂ , and a stack of four PZT(52/48) thin films of either 0.25-μm thickness per layer or 0.50-μm thickness per layer, and using either Pt or IrO ₂ electrodes above and below each layer. Stacked capacitor structures were patterned and results will be reported on the ferroelectric and electromechanical properties using various wiring configurations and compared to comparable single layer PZT configurations.