MEMS Operating at High-Temperature

Aluminum Scandium Nitride as a Functional Material at 1000 °C

Aluminum scandium nitride (AlScN) has emerged as a highly promising material for high-temperature applications due to its robust piezoelectric, ferroelectric, and dielectric properties. This study investigates the behavior of Al_0.7Sc_0.3N thin films in extreme thermal environments, demonstrating functional stability up to 1000°C, making it suitable for use in aerospace, hypersonics, deep-well, and nuclear reactor systems. Tantalum silicide (TaSi₂)/Al_0.7Sc_0.3N/TaSi₂ capacitors are fabricated and characterized across a wide temperature range, revealing robust ferroelectric and dielectric properties, along with significant enhancement in piezoelectric performance. At 1000°C, the ferroelectric hysteresis loops showed a substantial reduction in coercive field from 4.3 to 1.2MV/cm, while the longitudinal piezoelectric coefficient increased nearly tenfold, reaching 75.1pm/V at 800°C. Structural analysis via scanning and transmission electron microscopy confirmed the integrity of the TaSi₂/Al_0.7Sc_0.3N interfaces, even after exposure to extreme temperatures. Furthermore, the electromechanical coupling coefficient is calculated to increase by over 500%, from 12.9% at room temperature to 82% at 700°C. These findings establish AlScN as a versatile material for high-temperature ferroelectric, piezoelectric, and dielectric applications, offering unprecedented thermal stability and functional enhancement.

AlScN-on-SiC Thin Film Micromachined Resonant Transducers Operating in High-Temperature Environment up to 600°C

The experimental demonstration of aluminum scandium nitride (AlScN)-on-cubic silicon carbide (SiC) heterostructure thin film micromachined resonant transducers operating in a high-temperature environment up to 600°C is reported. Macroscopic and microscopic vibrations are investigated through a combination of ultrasensitive laser interferometry techniques and Raman spectroscopy. An average linear temperature coefficient of resonance frequency (TCf) of <1ppm/°C within the temperature range from room temperature to 200 °C, and an average linear TCf of −16ppm/°C between 200 and 600°C, from the fundamental-mode resonance of AlScN/SiC circular diaphragm resonator with a thickness of 1.9µm and diameter of 250µm, is obtained. Higher-order modes exhibit much larger TCf, which make them strong candidates as high-temperature-tolerant temperature sensors or ultraviolet detectors. Raman spectroscopy indicates that the turning points of the peak positions of the longitudinal optical phonon modes of both 3C-SiC and AlScN occur in almost the same temperature region where the turnover point of TCf is observed, suggesting that the microscopic vibrations in the crystal lattice and the macroscopic oscillation of the diaphragm are naturally mediated by the residual strain inside the materials at varying temperature.

Frequency-Stable AIScN-On-SiC MEMS Lamb Wave Resonator Operating in Very High Temperature Environment up to 1000°C

We report on the frequency stability (Allan deviation, ADEV) and phase noise performance of an aluminum scandium nitride on silicon carbide (AlScN-on-SiC, or simply AlScN/SiC) Lamb wave resonator (LWR) operating in a high temperature (T) vacuum (~1mTorr) environment up to T = 1000°C. The AlScN/SiC LWR has a resonance frequency of f₀ = 35.53MHz with a quality factor Q » 1015 at T = 27°C, still exhibits robust f₀ = 34.08MHz with Q » 717 when operating at T = 1000°C. A phase-locked loop (PLL) is used to track the f₀ over time to measure both frequency stability (ADEV) and phase noise. The measured ADEV is s_A = 1.73×10^-7 at averaging time t » 71ms, with phase noise of -117.54dBc/Hz at 1kHz offset frequency, measured at T = 1000°C. These experimental results mark the very first experimental study of frequency stability and phase noise performance of a MEMS resonator operating at very high temperatures up to T = 1000°C, far exceeding earlier record of T = 300°C.

Silicon carbide (SiC) nanoelectromechanical switches and logic gates with long cycles and robust performance in ambient air and at high temperature

We demonstrate nanoelectromechanical contact-mode switches and logic gates with high performance, enabled by cantilever-structured SiC nanoelectromechanical systems (NEMS). In full-cycle recording measurements (complete time-domain trace of every single switching cycle recorded), we show that in ambient air, SiC NEMS switches with nanocontacts have operated >1×10⁷ cycles of “hot-switching” without failure (devices still alive). When only recording valid “on”/”off” states (without the complete trace, to avoid overflowing data recording and to speed up acquisition), >2×10¹⁰ cycles have been measured. These clearly exhibit the unique properties and advantages of SiC NEMS, amongst all contact-mode, genuinely nanoscale switches. We also show robust switching events at high temperature T » 500°C.

AlScN‐on‐SiC microelectromechanical Lamb wave resonators operating at high temperature up to 800°C

We report on the experimental demonstration of aluminum scandium nitride (AlScN)-on-cubic silicon carbide (3C-SiC) Lamb wave resonators (LWRs) realized via microelectromechanical systems (MEMS) technology, operating at high temperature (T) up to T = 800°C, while retaining robust electromechanical resonances at ∼27MHz and good quality factor of Q ≈ 900 even at 800°C. Measured resonances exhibit clear consistency and stability during heating and cooling processes, validating the AlScN-on-SiC LWRs can operate at high-T up to 800°C without noticeable degradation in moderate vacuum (∼20mTorr). Even after undergoing four complete thermal cycles (heating from 23-800°C and then cooling down to 23°C), the devices exhibit robust resonance behavior, suggesting excellent stability and suitability for high-temperature applications. Q starts to decline as the temperature exceeds 400°C, which can be attributed to energy dissipation mechanisms stemming from thermoelastic damping and intrinsic material loss originating from phonon–phonon interactions.

Electrothermally Tunable Graphene Resonators Operating at Very High Temperature up to 1200K

The unique negative thermal expansion coefficient and remarkable thermal stability of graphene make it an ideal candidate for nanoelectromechanical systems (NEMS) with electrothermal tuning. We report on the first experimental demonstration of electrothermally tuned single- and few-layer graphene NEMS resonators operating in the high frequency (HF) and very high frequency (VHF) bands. In single-, bi-, and trilayer (1L, 2L, and 3L) graphene resonators with carefully controlled Joule heating, we have demonstrated remarkably broad frequency tuning up to Δf/f₀ ≈ 310%. Simultaneously, device temperature variations imposed by Joule heating are monitored using Raman spectroscopy; we find that the device temperature increases from 300K up to 1200K, which is the highest operating temperature known to date for electromechanical resonators. Using the measured frequency and temperature variations, we further extract both thermal expansion coefficients and thermal conductivities of these devices. Comparison with graphene electrostatic gate tuning indicates that electrothermal tuning is more efficient. The results clearly suggest that the unique negative thermal expansion coefficient of graphene and its excellent tolerance to very high temperature can be exploited for engineering highly tunable and robust graphene transducers for harsh and extreme environments.

Gallium Nitride (GaN) MEMS Lamb Wave Resonators Operating At High Temperature Up To 800°C

We report on the first experimental demonstration of gallium nitride microelectromechanical Lamb wave resonators operating at high temperature up to 800°C, while retaining robust electromechanical resonances at ~32MHz and good quality factor of Q = 450 at 800°C. The resonance frequency decreases linearly as temperature is being increased, demonstrating an excellent temperature coefficient of resonance frequency of -10 to -20 ppm/°C. The quality factor gradually decreases from ~2000 to ~300 as temperature increases from room temperature to 700°C, then slightly increases to Q = 450 as temperature reaches 800°C. Measured resonances exhibit clear repeatability and consistency during heating and cooling processes, without observable hysteresis. This study helps pave the way for advancing gallium nitride electromechanical transducers into high-temperature and hostile environments.