Nanoscale Phase and Orientation Mapping in Multiphase Polycrystalline Hafnium Zirconium Oxide Thin Films Using 4D-STEM and Automated Diffraction Indexing.

Ferroelectric hafnium zirconium oxide (HZO) holds promise for nextgeneration memory and transistors due to its superior scalability and seamless integration with complementary metal-oxide-semiconductor processing. A major challenge in developing this emerging ferroelectric material is the metastable nature of the non-centrosymmetric polar phase responsible for ferroelectricity, resulting in a coexistence of both polar and non-polar phases with uneven grain sizes and random orientations. Due to the structural similarity between the multiple phases and the nanoscale dimensions of the thin film devices, accurate measurement of phase-specific information remains challenging. Here, the application of 4D scanning transmission electron microscopy is demonstrated with automated electron diffraction pattern indexing to analyze multiphase polycrystalline HZO thin films, enabling the characterization of crystallographic phase and orientation across large working areas on the order of hundreds of nanometers. This approach offers a powerful characterization framework to produce a quantitative and statistically robust analysis of the intricate structure of HZO films by uncovering phase composition, polarization axis alignment, and unique phase distribution within the HZO film. This study introduces a novel approach for analyzing ferroelectric HZO, facilitating reliable characterization of process-structure-property relationships imperative to accelerating the growth optimization, performance, and successful implementation of ferroelectric HZO in devices.

High Quality Factors in Superlattice Ferroelectric Hf0.5Zr0.5O2 Nanoelectromechanical Resonators.

The discovery of ferroelectricity and advances in creating polar structures in atomic-layered hafnia-zirconia (HfxZr1-xO2) films spur the exploration of using the material for novel integrated nanoelectromechanical systems (NEMS). Despite its popularity, the approach to achieving high quality factors (Qs) in resonant NEMS made of HfxZr1-xO2 thin films remains unexplored. In this work, we investigate the realization of high Qs in Hf0.5Zr0.5O2 nanoelectromechanical resonators by stress engineering via the incorporation of alumina (Al2O3) interlayers. We fabricate nanoelectromechanical resonators out of the Hf0.5Zr0.5O2-Al2O3 superlattices, from which we measure Qs up to 171,000 and frequency-quality factor products (f × Q) of >1011 Hz through electrical excitation and optical detection schemes at room temperature in vacuum. The analysis suggests that clamping loss and surface loss are the limiting dissipation sources and f × Q > 1012 Hz is achievable through further engineering of anchor structure and built-in stress.

Dual-Mode Scandium-Aluminum Nitride Lamb-Wave Resonators Using Reconfigurable Periodic Poling.

This paper presents the use of ferroelectric behavior in scandium-aluminum nitride (ScxAl1-xN) to create dual-mode Lamb-wave resonators for the realization of intrinsically configurable radio-frequency front-end systems. An integrated array of intrinsically switchable dual-mode Lamb-wave resonators with frequencies covering the 0.45-3 GHz spectrum. The resonators are created in ferroelectric scandium-aluminum nitride (Sc0.28Al0.72N) film and rely on period poling for intrinsic configuration between Lamb modes with highly different wavelengths and frequencies. A comprehensive analytical model is presented, formulating intrinsically switchable dual-mode operation and providing closed-form derivation of electromechanical coupling (kt2) in the two resonance modes as a function of electrode dimensions and scandium content. Fabricated resonator prototypes show kt2s as high as 4.95%, when operating in the first modes over 0.45-1.6 GHz, 2.23% when operating in the second mode of operation over 0.8-3 GHz, and series quality factors (Qs) over 300-800. Benefiting from lithographical frequency tailorability and intrinsic switchability that alleviate the need for external multiplexers, and large kt2 and Q, dual-mode Sc0.28Al0.72N Lamb-wave resonators are promising candidates to realize single-chip multi-band reconfigurable spectral processors for radio-frequency front-ends of modern wireless systems.

Growth of C-Axis Textured AlN Films on Vertical Sidewalls of Silicon Microfins.

A fabrication process is developed to grow c -axis textured aluminum nitride (AlN) films on the sidewall of single-crystal silicon (Si) microfins to realize fin bulk acoustic wave resonators (FinBARs). FinBARs enable ultradense integration of high-quality-factor ( Q ) resonators and low-loss filters on a small chip footprint and provide extreme lithographical frequency scalability over ultra- and super-high-frequency regimes. Si microfins with large aspect ratio are patterned and their sidewall surfaces are atomically smoothened. The reactive magnetron sputtering AlN deposition is engineered to optimize the hexagonal crystallinity of the sidewall AlN film with c -axis perpendicular to the sidewall of Si microfins. The effect of bottom metal electrode and surface roughness on the texture and crystallinity of the sidewall AlN film is explored. The atomic-layer-deposited platinum film with (111) crystallinity is identified as a suitable bottom electrode for deposition of c -axis textured AlN on the sidewall with c -axis orientation of 88.5° ± 1.5° and arc-angle of ~12° around (002) diffraction spot over film thickness. A 4.2-GHz FinBAR prototype is implemented showing a Q of 1574 and effective electromechanical coupling ( [Formula: see text]) of 2.75%, when operating in the 3rd width-extensional resonance mode. The lower measured Q and [Formula: see text] compared to simulations highlights the effect of granular texture of sidewall AlN film on limiting the performance of FinBARs. The developed c -axis textured sidewall AlN film technology paves the way for realization and monolithic integration of multifrequency and multiband FinBAR spectral processors for the emerging carrier aggregated wireless communication systems.

Clandestine nanoelectromechanical tags for identification and authentication.

The realization of truly unclonable identification and authentication tags is the key factor in protecting the global economy from an ever-increasing number of counterfeit attacks. Here, we report on the demonstration of nanoscale tags that exploit the electromechanical spectral signature as a fingerprint that is characterized by inherent randomness in fabrication processing. Benefiting from their ultraminiaturized size and transparent constituents, these clandestine nanoelectromechanical tags provide substantial immunity to physical tampering and cloning. Adaptive algorithms are developed for digital translation of the spectral signature into binary fingerprints. A large set of tags fabricated in the same batch is used to estimate the entropy of the corresponding fingerprints with high accuracy. The tags are also examined under repetitive measurements and temperature variations to verify the consistency of the fingerprints. These experiments highlight the potential of clandestine nanoelectromechanical tags for the realization of secure identification and authentication methodologies applicable to a wide range of products and consumer goods.

Dispersion-Engineered Guided-Wave Resonators in Anisotropic Single-Crystal Substrates - Part II: Numerical and Experimental Characterization.

In this part of the paper, numerical and experimental verification of the analytical design procedure is presented. Various waveguide-based test-vehicles, implemented in single crystal silicon and transduced by thin aluminum nitride films, are demonstrated. Silicon resonators with type-I and type-II dispersion characteristics are presented to experimentally verify the analytical mode synthesis technique for realization of high quality-factor silicon Lamb wave resonators.

Dispersion-Engineered Guided-Wave Resonators in Anisotropic Single-Crystal Substrates - Part I: Concept and Analytical Design.

This paper presents an analytical approach for implementation of high quality-factor (Q) resonators with arbitrary cross-sectional vibration mode-shapes in anisotropic single-crystal substrates. A closed-form dispersion relation is analytically derived to characterize the dynamics of guided waves in rectangular waveguides. Three categories of waves with propagating, standing-evanescent and propagating-evanescent dynamics are identified and used for energy localization of acoustic excitations with arbitrary cross-sectional vibration patterns. An analytical design procedure is presented for dispersion engineering of waveguides to realize high-Q resonators without the need for geometrical suspension through narrow tethers or rigid anchors. The effectiveness of the dispersion engineering methodology is verified through development of experimental test-vehicles in 20㯌m-thick single-crystal silicon substrate with 500nm aluminum nitride transducers. Various proof-of-concept resonators, representing guided waves with different dispersion types, are presented and compared to highlight the optimum design procedures for Q enhancement and spurious mode suppression. Part I of this paper presents the operation principle of guided wave resonators based on analytical derivation of dispersion relation followed by a systematic resonator design procedure. Numerical and experimental characterization for verification of the proposed design procedure and extensive measurement data on proof-of-concept resonators are presented in Part II.

A ±0.3 ppm Oven-Controlled MEMS Oscillator Using Structural Resistance-Based Temperature Sensing.

This paper presents a 77.7-MHz silicon microelectromechanical-systems oven-controlled oscillator (MEMS OCXO) that uses the structural resistance ( ) of the resonator as an embedded temperature sensor. The exhibits a large temperature coefficient of resistance and is used as a self-temperature sensor to accurately and locally monitor the temperature of the resonator. A high-Q capacitive cross-sectional Lamé-mode resonator fabricated using the nanogap high aspect-ratio combined poly- and single-crystal silicon process (HARPSS) is used as the frequency selective element. A silicon resistor micro-oven is implemented on the MEMS die adjacent to the resonator and the ensemble is wafer-level packaged in vacuum to yield a 2 mm mm MEMS die. The micro-oven resistor is automatically controlled by the analog loop to provide active temperature stabilization for the resonator. A resistance temperature detector (RTD) circuit, high-gain loop filter, and heater amplifier are implemented as the analog micro-oven control loop. To further boost the frequency stability, a digital feedforward calibration path which uses the digitized RTD output to fine tune the phase shift of the sustaining amplifier is added to the system. The silicon MEMS OCXO achieves ±0.3-ppm frequency stability from -25 °C to 85 °C. The microresonator is interfaced with a sustaining amplifier implemented in Taiwan Semiconductor Manufacturing Company 0.35-s CMOS process, consuming 16 mA from a 3.2-V supply.

Dynamic tuning of MEMS resonators via electromechanical feedback.

This paper introduces an active electrical technique for dynamic tuning of MEMS resonators. The proposed technique is based on using the resonator output current to generate displacement or acceleration signals by integration or differentiation operations, respectively. The resulting signal is then scaled to generate an appropriate tuning signal. When applied to the resonator through additional signal ports, the tuning signal electrically modifies the equivalent mechanical stiffness or mass of the resonator, thereby tuning the resonance frequency in a bidirectional fashion depending on the polarity of the scaling. This tuning scheme has been applied to a piezoelectric AlN-on-Si BAW square resonator to tune its 14.2 MHz resonance frequency by 22 kHz, equivalent to 1550 ppm. The proposed tuning technique can be applied to a wide range of MEMS resonators and resonant sensors, e.g., to compensate for temperature or process-induced variations in their resonance frequencies.

Electrostatically tunable piezoelectric-on-silicon micromechanical resonator for real-time clock.

This paper reports on the design, fabrication, and characterization of a small form factor, piezoelectrically transduced, tunable micromechanical resonator for real-time clock (RTC) applications (32.768 kHz). The device was designed to resonate in an out-of-plane flexural mode to simultaneously achieve low-frequency operation and reduced motional resistance in a small die area. Finite element simulations were extensively used to optimize the structure in terms of size, insertion loss, spurious-mode rejection, and frequency tuning. Microresonators with an overall die area of only 350 × 350 µm were implemented on a thin-film AlN on silicon-on-insulator (SOI) substrate with AlN thickness of 0.5 µm, device layer of 1.5 µm, and an electrostatic tuning gap size of 1 µm. A frequency tuning range of 3100 ppm was measured using dc voltages of less than 4 V. This range is sufficient to compensate for frequency variations of the microresonator across temperature from -20°C to 100°C. The device exhibits low motional impedance that is completely independent of the frequency tuning potential. Discrete electronics were used in conjunction with the resonator to implement an oscillator, verifying its functionality as a timing reference.