Valery V. Felmetsger

Temperature-compensated aluminum nitride lamb wave resonators

In this paper, the temperature compensation of AlN Lamb wave resonators using edge-type reflectors is theoretically studied and experimentally demonstrated. By adding a compensating layer of SiO2 with an appropriate thickness, a Lamb wave resonator based on a stack of AlN and SiO2 layers can achieve a zero first-order temperature coefficient of frequency (TCF). Using a composite membrane consisting of 1 ¿m AlN and 0.83 ¿m SiO2, a Lamb wave resonator operating at 711 MHz exhibits a first-order TCF of -0.31 ppm/°C and a second-order TCF of -22.3 ppb/°C2 at room temperature. The temperature-dependent fractional frequency variation is less than 250 ppm over a wide temperature range from -55°C to 125°C. This temperature-compensated AlN Lamb wave resonator is promising for future applications including thermally stable oscillators, filters, and sensors.

AlN/3C-SiC composite plate enabling high-frequency and high-Q micromechanical resonators.

An AlN/3C-SiC composite layer enables the third-order quasi-symmetric (QS(3)) Lamb wave mode with a high quality factor (Q) characteristic and an ultra-high phase velocity up to 32395 ms(-1). A Lamb wave resonator utilizing the QS(3) mode exhibits a low motional impedance of 91 Ω and a high Q of 5510 at a series resonance frequency (f(s)) of 2.92 GHz, resulting in the highest f(s)·Q product of 1.61 × 10(13) Hz among the suspended piezoelectric thin film resonators reported to date.

Thermally compensated aluminum nitride Lamb wave resonators for high temperature applications

In this letter, temperature compensation for aluminum nitride (AlN) Lamb wave resonators operating at high temperature is presented. By adding a compensating layer of silicon dioxide (SiO2), the turnover temperature can be designed for high temperature operation by varying the normalized AlN film thickness (hAlN/λ) and the normalized SiO2 film thickness (hSiO2/λ). With different designs of hAlN/λ and hSiO2/λ, the Lamb wave resonators were well temperature-compensated at 214 °C, 430 °C, and 542 °C, respectively. The experimental results demonstrate that the thermally compensated AlN Lamb wave resonators are promising for frequency control and sensing applications at high temperature.

Piezoelectric aluminum nitride nanoelectromechanical actuators

This letter reports the implementation of ultrathin (100 nm) aluminum nitride (AlN) piezoelectric layers for the fabrication of vertically deflecting nanoactuators. The films exhibit an average piezoelectric coefficient (d31∼−1.9 pC/N), which is comparable to its microscale counterpart. This allows vertical deflections as large as 40 nm from 18 μm long and 350 nm thick multilayer cantilever bimorph beams with 2 V actuation. Furthermore, in-plane stress and stress gradients have been simultaneously controlled. The films exhibit leakage currents lower than 2 nA/cm2 at 1 V, and have an average relative dielectric constant of approximately 9.2 (as in thicker films). These material characteristics and actuation results make the AlN nanofilms ideal candidates for the realization of nanoelectromechanical switches for low power logic applications.

AlN thin films grown on epitaxial 3C–SiC (100) for piezoelectric resonant devices

Highly c-axis oriented heteroepitaxial aluminum nitride (AlN) films were grown on epitaxial cubic silicon carbide (3C–SiC) layers on Si (100) substrates using alternating current reactive magnetron sputtering at temperatures between approximately 300–450 °C. The AlN films were characterized by x-ray diffraction, scanning electron microscope, and transmission electron microscopy. A two-port surface acoustic wave device was fabricated on the AlN/3C–SiC/Si composite structure, and an expected Rayleigh mode exhibited a high acoustic velocity of 5200 m/s. The results demonstrate the potential of utilizing AlN films on epitaxial 3C–SiC layers to create piezoelectric resonant devices.

Surface acoustic wave devices on AlN/3C–SiC/Si multilayer structures

Surface acoustic wave (SAW) propagation characteristics in a multilayer structure including a piezoelectric aluminum nitride (AlN) thin film and an epitaxial cubic silicon carbide (3C–SiC) layer on a silicon (Si) substrate are investigated by theoretical calculation in this work. Alternating current (ac) reactive magnetron sputtering was used to deposit highly c-axis-oriented AlN thin films, showing the full width at half maximum (FWHM) of the rocking curve of 1.36° on epitaxial 3C–SiC layers on Si substrates. In addition, conventional two-port SAW devices were fabricated on the AlN/3C–SiC/Si multilayer structure and SAW propagation properties in the multilayer structure were experimentally investigated. The surface wave in the AlN/3C–SiC/Si multilayer structure exhibits a phase velocity of 5528 m s−1 and an electromechanical coupling coefficient of 0.42%. The results demonstrate the potential of AlN thin films grown on epitaxial 3C–SiC layers to create layered SAW devices with higher phase velocities and larger electromechanical coupling coefficients than SAW devices on an AlN/Si multilayer structure. Moreover, the FWHM values of rocking curves of the AlN thin film and 3C–SiC layer remained constant after annealing for 500 h at 540 °C in air atmosphere. Accordingly, the layered SAW devices based on AlN thin films and 3C–SiC layers are applicable to timing and sensing applications in harsh environments.

Innovative technique for tailoring intrinsic stress in reactively sputtered piezoelectric aluminum nitride films

Novel technical and technological solutions enabling effective stress control in highly textured polycrystalline aluminum nitride (AlN) thin films deposited with ac (40kHz) reactive sputtering processes are discussed. Residual stress in the AlN films deposited by a dual cathode S-Gun magnetron is well controlled by varying Ar gas pressure, however, since deposition rate and film thickness uniformity depend on gas pressure too, an independent stress control technique has been developed. The technique is based on regulation of the flux of the charged particles from ac plasma discharge to the substrate. In the ac powered S-Gun, a special stress adjustment unit (SAU) is employed for reducing compressive stress in the film by means of redistribution of discharge current between electrodes of the S-Gun leading to controllable suppression of bombardment of the growing film. This technique is complementary to AlN deposition with rf substrate bias which increases ion bombardment and shifts stress in the compressiv...

Deposition of ultrathin AlN films for high frequency electroacoustic devices

The authors investigate the microstructure, crystal orientation, and residual stress of reactively sputtered aluminum nitride (AlN) films having thicknesses as low as 200 down to 25 nm. A two-step deposition process by the dual cathode ac (40 kHz) powered S-gun magnetron enabling better conditions for AlN nucleation on the surface of the molybdenum (Mo) bottom electrode was developed to enhance crystallinity of ultrathin AlN films. Using the two-step process, the residual in-plane stress as well as the stress gradient through the film thickness can be effectively controlled. X-ray rocking curve measurements have shown that ultrathin films grown on Mo using this technology are highly c-axis oriented with full widths at half maximum of 1.8° and 3.1° for 200- and 25-nm-thick films, respectively, which are equal to or even better than the results previously reported for relatively thick AlN films. High-resolution transmission electron microscopy and fast Fourier transform analyses have confirmed strong grain ...

Characterization of aluminum nitride lamb wave resonators operating at 600°C for harsh environment RF applications

In this paper, aluminum nitride (AlN) Lamb wave resonators (LWR) operating in air from room temperature up to 600°C are demonstrated for the first time. To date, no AlN RF devices have been tested and characterized at extreme temperatures. This paper describes the design, fabrication and characterization of temperature compensated AlN Lamb wave resonators at 600°C. Temperature coefficients of frequency (TCF) of both uncompensated and compensated devices were measured. Quality factors against temperature were also recorded. This supports the use of piezoelectric AlN as the material platform for radio-frequency (RF) components and sensing applications in harsh environments.

Characteristics of AlN Lamb wave resonators with various bottom electrode configurations

The characteristics of aluminum nitride (AlN) Lamb wave resonators utilizing the lowest symmetric (S0) mode with grounded, floating, and open bottom surface configurations are theoretically and experimentally investigated. The Lamb wave resonator without the bottom electrode exhibits a quality factor (Q) as high as 2,573 but a low effective coupling of 0.18% at 949.7 MHz. On the contrary, the Lamb wave resonator with a floating bottom electrode shows an effective coupling of 1.05% but a low Q of 850 at 850.3 MHz because the imperfect interface between the AlN plate and bottom electrode usually degrades the Q. Limited by the larger static capacitance, in contrast to the floating bottom electrode, the Lamb wave resonator with a grounded bottom electrode shows a smaller effective coupling of 0.78% and a low Q of 800 at 850.5 MHz. These results suggest that the resonator with a floating bottom surface is suitable for filter applications, whereas that with an open bottom surface is preferred for sensor and oscillator applications.