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.

The slowing pace of performance improvements in modern processors along with the breakdown of power scaling forecasts an imminent end to the traditional transistor scaling roadmap. Additionally, meeting the aggressive demands of proliferating applications in big-data processing, machine learning, artificial intelligence, and highly distributed edge computing requires radical advancements in materials, devices, and architectures for future processors. Neuromorphic computing has emerged as the most promising successor to conventional complementary metal oxide semiconductor (CMOS) devices and von Neumann architecture. This work reviews the status of neuromorphic research, compares the traditional CMOS approach with neuromorphic devices for implementing biologically inspired circuits, and provides an outlook into integration schemes for future brain-inspired computing hardware.

We present a conformal method of growing ferroelectric lead hafnate-titanate (PbHfxTi1-xO3, PHT) and lead zirconate-titanate (PbZrxTi1-xO3, PZT) using atomic layer deposition (ALD) precursors. The 4+ cation precursors consist of tetrakis dimethylamino titanium (TDMAT), tetrakis dimethylamino zirconium (TDMAZ) and tetrakis dimethyl amino hafnium (TDMAH) for Ti, Zr, and Hf, respectively. The Pb (2+) precursor was Lead bis(3-N,N-dimethyl-2-methyl-2-propanoxide) [Pb(DMAMP)(2)]. PZT was limited to lead titanate (PTO)-rich compositions, where x <0.25 for PbZrxTi1-xO3, and exhibited a remnant polarization of 26-27 mu C/cm(2) with a coercive field between 150 and 170 kV/cm. The 3D-structure coating capability of PZT was demonstrated by deposition on micromachined trench sidewalls 45 mu m deep. We fabricated Microelectromechanical systems (MEMS) cantilever arrays with PZT thin films grown using the present method and demonstrated piezoelectric actuation. Alternatively, PHT was deposited with Ti and Hf compositions within +/- 1 at.% of the morphotropic phase boundary (MPB). The PHT exhibited a remanent polarization of 7.0-8.7 mu C/cm(2) with a coercive field between 84-100 kV/cm. We applied the same Pb and Hf precursors from the PHT process to grow antiferroelectric lead-hafnate (PHO), which showed the characteristic electric field-induced ferroelectric phase transition at approximately +/- 280 kV/cm and a maximum polarization of approximately +/- 32.8 mu C/cm(2).

PbTiO3 (lead titanate) thin films were deposited by atomic layer deposition (ALD) and crystallized via rapid thermal anneal. The films were grown using lead bis(3-N,N-dimethyl-2-methyl-2-propanoxide) and tetrakis dimethylamino titanium as cation precursors. A combination of H2O and ozone was used as oxidizers. Phase-pure, stoichiometric PbTiO3 was confirmed using x-ray diffraction, Rutherford backscattering spectroscopy, and scanning transmission electron microscopy. Ferroelectric hysteresis loops obtained by patterning circular capacitors with areas of 4.92 × 10−4 cm2 indicate a Pmax = 48 μC/cm2, 2Pr = 60 μC/cm2, Ec1 = −73 kV/cm, Ec2 = 125 kV/cm, and a leakage current density of 15 μA/cm2 at 138 kV/cm. Capacitance versus voltage measurements were used to obtain a maximum dielectric constant of 290 at 85 kV/cm and loss tangent under 4% tested in the range of ±275 kV/cm. ALD PbTiO3 grown with near-ideal cation ratios crystallized into randomly oriented perovskite grains when grown on a sputtered Pt-coated Si substrate. A variation of rapid thermal anneal temperatures, ramp rates, and nucleation layers was investigated and did not have a significant effect on perovskite grain orientation.

The Pt/TiO2/SiO2/Si electrode structure prepared using sputter deposition of Ti, conversion of Ti to TiO2 by thermal oxidation, and sputter deposition of Pt was evaluated by using measurements of sheet resistance and X-ray diffraction. Nonlinearity of the reciprocal sheet resistance dependence on thickness revealed a change in the Ti conductivity that was attributed to a change in the Ti microstructure. Upon conversion to TiO2, it was determined that TiO2 exhibits a critical thickness of 32 nm that minimizes normal strain and {100}-textured misorientation and correlates with the onset of the Ti reciprocal sheet resistance nonlinearity. Both the TiO2 {100}-strain and {100}-texture directly affects the {111}-textured growth of Pt deposited at 500 °C. Pt deposited onto TiO2 films with the critical thickness of 32 nm exhibits a maximization of the normal strain relative to bulk Pt and also displays the narrowest distribution of Pt {111}-textured grain misalignment. The results presented show how Ti deposition conditions, TiO2 anneal conditions, and TiO2 thickness combine to modify the Pt electrode structural properties. Additionally, the measurement techniques demonstrated for the Ti, TiO2, and Pt are applicable to process control monitoring as well as standardized comparison of electrode structures used in integrated piezoelectric and ferroelectric devices.

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- -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- -thick SiO2 elastic layer, a base electrode of Pt/TiO2, and a stack of four PZT(52/48) thin films of either 0.25- thickness per layer or 0.50- thickness per layer, and using either Pt or IrO2 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.

The annealing behavior and electrical properties of lead titanate (PTO) and lead zirconate titanate (PZT) thin films deposited by atomic layer deposition (ALD) were investigated. ALD films were deposited on platinized silicon substrates. The composition of the PTO films ranged from Pb-deficient Pb0.73TiO3-x to Pb-rich Pb2.3TiO3-x, including stoichiometric PbTiO3. The PZT films were all Pb-deficient, with Pb/(Zr + Ti) ratios of 0.40-0.75. Stoichiometric PbTiO3 films showed the perovskite structure, and a well-defined, dense microstructure after crystallization at 600 degrees C for 1 min in 2 slpm O-2 in a rapid thermal annealer (RTA). Pb excess PbTiO3 films developed into perovskite PbTiO3 after annealing but the surface microstructure showed a large grained microstructure with significant porosity. The dielectric constant was 140 at 10 kHz and a ferroelectric polarization - electric field curve was observed. A Pb-deficient Pb0.66Zr0.55Ti0.45O3-x film showed a dense and fine-grained microstructure after annealing at 700 degrees C for 1 min in 2 slpm O-2 in a rapid thermal annealer (RTA). The dielectric constant was 100 at 10 kHz. (C) 2018 Elsevier B.V. All rights reserved.

Pb(Zr 52 Ti 48 )O 3 (PZT) thin films have been grown by chemical solution deposition on 150 mm Silicon-on-Sapphire (SOS) substrates for RF MEMS applications. Film properties are compared to similar films deposited on typical Si/SiO 2 substrates. PZT films deposited on SOS show improvement in P MAX (37.8 vs. 32.1 μC/cm 2 ) and PREM (22.7 vs. 10.9 μC/cm 2 ) over films deposited on Si. A decrease in maximum εr from 1650 (Si) to 1031 (SOS) is also noted. Crystal structure is examined using x-ray diffraction. Ferroelectric polarization hysteresis curves, dielectric constant tuning, and loss tangent are studied using fabricated capacitors. Piezoelectric coefficients calculated from cantilever LDV measurements are also presented.

We report on the room temperature thermal conductivity of atomic layer deposition-grown amorphous TiO2 and Al2O3 thin films as a function of film thickness and atomic density. For films thinner than 50 nm, we measure an effective thermal conductivity that is reduced with decreasing film thickness. This dependence is attributed to the increased influence of thermal boundary resistances as film thickness is reduced. In addition, we fit for a thickness-independent intrinsic thermal conductivity using a series-resistor model. For films thicker than ∼50 nm, there is no significant dependence on thickness or substrate. We observe a dependence of the thermal conductivity on density, which agrees well with a differential effective-medium approximation modified minimum limit model.

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.

In piezoelectric microelectromechanical system devices with PbZr x Ti1−xO3 as the ferroelectric, the bottom electrode can provide a template for oriented PbZr x Ti1−xO3 growth. IrO2/Pt, IrO2, and Pt bottom electrode layers were sputter deposited onto TiO2 and were used as growth templates for oriented PbZr0.52Ti0.48O3 growth. The IrO2 and Pt were found to be {100}- and {111}-oriented, respectively, by X-ray diffraction. Scanning/transmission electron microscopy results indicate that the bottom electrodes are textured; however, the PbZr0.52Ti0.48O3 layer is partially textured. The impact of the bottom electrode type on the electrical properties is investigated by dielectric, ferroelectric, and piezoelectric measurements on circular capacitors formed on blanket PbZr0.52Ti0.48O3 films and unimorph cantilevers. For devices with PbZr0.52Ti0.48O3 on IrO2/Pt bottom electrodes, values for the dielectric constant of 1103 ± 28, loss tangent of 0.070 ± 0.004, maximum polarization of 0.399 ± 0.003 C/m2 at 38 MV/m, and leakage current of 5.4 ± 5.8 nA at 20 MV/m were obtained. Values of normalized strain of 0.0030 ± 0.0001 at 20 MV/m, and effective piezoelectric coefficient, d31,f, of 100 ± 25 pm/V at 15 MV/m were obtained on cantilever unimorphs with electrode area 16 µm × 123 µm and PZT area 16 µm × 125 µm. These values are comparable to results obtained for PbZr0.52Ti0.48O3 on 100 nm thick Pt-only bottom electrodes.

This study examines the effects of sputter deposition growth conditions on IrO2 thin films developed as electrode layers for Pb(ZrxTi1-x)O3 piezoelectric Micro-Electro-Mechanical Systems applications. For IrO2 thin-film bottom electrodes sputter deposited on {100}-textured, rutile structure, TiO2 template layers, the effects of substrate temperature, Ar and O2 flow rates, and post-deposition anneal were studied. Additionally, the impact of various IrO2/Pt bilayer structures on electrode properties were investigated. IrO2 bottom electrodes grown on 100-oriented TiO2 at 500°C with an Ar flow rate of 100sccm, O2 flow rate of 60sccm, and a nominal 100nm thickness exhibited a film sheet resistance of 9.2±0.2Ω/sq., surface roughness of 2.3±0.1nm measured over a 2μm×2μm area, and a rutile structure, preferred-{100} crystalline fiber texture normal to the substrate plane. Furnace-anneal treatments on this electrode at 650°C for 30min in 3 SLM O2 improved its sheet resistance to 7.5±0.2Ω/sq., and surface roughness was 4.0±0.1nm. Bilayers of IrO2/Pt grown on TiO2, with IrO2 and Pt thicknesses in the 0–100nm range, were sputter deposited at 500°C. The electrodes had composite sheet resistances in the range of 1.34±0.03Ω/sq. to 9.92±0.25Ω/sq., and surface roughnesses ranging from 1.5±0.1nm to 2.0±0.1nm for 2μm×2μm measurement areas. Scanning Transmission Electron Microscopy verified preferred IrO2 and Pt crystalline fiber textures of {100} and {111}, respectively.

Ba(Ga,Ta)(0.05)Ti0.90O3, a B-site dipole-like substituted material is investigated for structural phase transitions over the temperature range 30 to 900 degrees C using x-ray diffraction. Rietveld refinement of the data suggests the material to be Pm[GRAPHICS]m (cubic) from 200 to 900 degrees C, P4/mmm (tetragonal) from 30 to 200 degrees C with phases similar to those of BaTiO3 [12]. Average grain size determined using scanning electron microscopy is 650nm. Measurements of dielectric properties from -50 to 120 degrees C and over a frequency range of 10Hz to 2MHz show a relatively flat dielectric constant that is electric-field tunable indicating that Ba(Ga,Ta)(0.05)Ti0.90O3 is a reasonable candidate for frequency agile components.