Piezoelectric Aluminum Nitride Films Research Articles

Effects of Electric Bias on Different Sc-Doped AlN-Based Film Bulk Acoustic Resonators

Film bulk acoustic resonators (FBARs) based on aluminum nitride (AlN) and scandium-doped aluminum nitride (AlScN) exhibit tremendous application aspects in the radio frequency front-end due to achievable high-frequency characteristics, superior thermal performances and compatibility with harsh environments. Delicately controlling the resonant frequency (fs) of FBAR is essential for integrating filters or modules. In this work, we provide a practical feasibility in adjusting fs of AlN and AlScN FBAR using external direct current electric bias (EDC). When applying a negative EDC (the direction along the reversed c-axis), fs shifts to a lower frequency, whereas a positive EDC brings a higher fs. In order to extract the equivalent values of the stiffness coefficient (c33), piezoelectric coefficient (e33) and dielectric constant (εzz) of AlN and AlScN piezoelectric materials, we adopted the electromechanical equivalent Mason model. The results show that the equivalent values of c33 increase with the change of EDC from negative to positive, and, on the other hand, those of e33 and εzz decrease. Our work provides a systematic investigation on the electric field-influenced stiffening effect of AlN and AlScN piezoelectric films and opens a feasibility for frequency-tunable resonators.

The Design of Aluminum Nitride-Based Lead-Free Piezoelectric MEMS Accelerometer System

In this study, we successfully deposited ${c}$ -axis-oriented aluminum nitride piezoelectric films via low-temperature dc sputtering method. Based on the X-ray diffraction (XRD) and TEM analyses, deposited films with a ${c}$ -axis monocrystal were identified. The effective ${d}_{{33},{f}}$ value of the aluminum nitride (AlN) films is 5.92 pC/N, which is better than most of the reported data using dc sputtering processing. Using ANSYS software, we simulated and designed MEMS accelerometers based on AlN films. Furthermore, we successfully fabricated MEMS accelerometers. The sensitivity of the MEMS accelerometer is 1.49 mV/g, and the resonance frequency is 7.2 kHz. The MEMS accelerometer was combined with a sensing circuit constituting a module. The sensitivity of the module increases approximately tenfold. Finally, the vibration of spindles was successfully detected using the designed module.

Fabrication and application of flexible AlN piezoelectric film

A fabrication method for high quality flexible aluminum nitride piezoelectric thin films based on micro-nano fabrication technology is reported in this paper. Molybdenum (Mo)/aluminum nitride (AlN)/aluminum (Al) structure was prepared on silicon(100) by sputtering. Then, deep-reactive ion etching technology was used to remove the silicon material used for support, then the Mo/AlN/Al flexible sandwich film was obtained. The full-width at half-maximum of the AlN film (002) peak before and after removal of silicon was only 1.47° and 1.49°, respectively. No obvious cracks in the film under bending conditions were observed with a scanning electron microscope. Capacitance–voltage (C–V) test results showed that the capacitance curve of the sandwich structure was consistent with that of the double-sided Schottky junction. The flexible piezoelectric film with an active area of 2 × 2 mm2 exhibited an average peak-to-peak generated voltage of 0.6 V in energy harvesting tests, which indicates that the method could be used to fabricate high quality flexible energy collectors based on AlN film. The preparation scheme in this paper is compatible with integrated circuit processes and simple to implement, which lays a foundation for the wide application of flexible piezoelectric devices.

Reactive ion beam etching of piezoelectric ScAlN for bulk acoustic wave device applications

Etching piezoelectric Scandium Aluminum Nitride (ScAlN) films containing a high concentration of Scandium with controllable feature profile angle and smooth surface is needed for next-generation Bulk Acoustic Wave (BAW) RF filter applications. This paper reports the facile etching of ScAlN with Scandium concentration of up to 15% using Reactive Ion Beam Etching (RIBE) with a very smooth surface of less than 5 nm average roughness and profile angle between 60 to 80 degrees. Recent studies show that doping AlN with a high concentration of Sc increases the piezoelectric response by five times and the bandwidth of RF filters also improved. However, etching ScAlN with a high concentration of Sc using traditional RIE and ICP based methods are extremely difficult and often results in low etch rate and rough surface. We have developed RIBE processes using a mixture of Reactive Gas and Ar with highly controllable profile angle from 60 to 80 degrees, improved etch rate (~ 36nm/min) and selectivity to photoresist mask (0.7:1), in comparison to conventional IBE. This work has positive implications in fabricating ScAlN based BAW RF filters for next-generation mobile and wireless applications.

A Decoupling Design with T-Shape Structure for the Aluminum Nitride Gyroscope.

This paper reports a novel design for the decoupling of microelectromechanical systems (MEMS) gyroscopes. The MEMS gyroscope is based on piezoelectric aluminum nitride (AlN) film, and the main structure is a mass hung by T-shape beams. A pair of parallel drive electrodes are symmetrically placed on the surface of the vertical bar for driving the oscillating mass. A serpentine sense electrode is placed on the lateral bar. When the gyroscope is oscillating in drive mode, charges with equal quantity and opposite sign will be polarized and distributed symmetrically along the lateral bar. These charges neutralize each other at the sense electrode. Therefore, no coupling signals can be detected from the sense electrode. This design can realize the decoupling between the drive mode and sense mode. In this work, the T-shape decoupled structure was designed as the key component of an AlN piezoelectric gyroscope and the whole structure was simulated by COMSOL Multiphysics 5.2a. The working principle of the decoupling is described in detail. Electrical properties were characterized by the dynamic signal analyzer. According to the test results, the drive mode and the sense mode are decoupled. The coefficient of orthogonal coupling is 1.55%.

Shear Mode Bulk Acoustic Viscosity Sensor for Blood Coagulation Monitoring in Oral Anticoagulant Therapy.

Frequent monitoring of blood coagulation status is essential for patients receiving treatment with some oral anticoagulant agents. In this paper, we present a microelectromechanical film bulk acoustic resonator for the measurement of blood coagulation parameters. The resonator was made of an aluminum nitride piezoelectric film and operated in thickness shear resonance mode with a frequency of about 2 GHz. The resonant frequency showed a linear relationship with the viscosity of the environmental liquid over the wide range of 1-25 cP, falling within the physiological range of human blood. The coagulation process of human blood was monitored by following the frequency downshift due to the viscosity change. Therefore, the frequency response was used to determine quantitatively three clinically significant parameters being the enzymatic cascade time, the coagulation time and the clot degree. As a practical demonstration, the proposed micro-resonator was applied to monitor coagulation for one month in a patient taking the oral anticoagulant, warfarin, daily. The results measured by the resonator were consistent with those of the standard coagulometer. As a result of the excellent potential for integration, miniaturization and the availability of direct digital signals, the shear mode film bulk acoustic resonator has promising application for clinical and personal coagulation monitoring.

Ultrasonic response of a piezoelectric aluminum nitride film deposited on silicon

ABSTRACTAn aluminum nitride (AlN) thick film for ultrasonic transduction was deposited on a silicon wafer by magnetron sputtering. A columnar structure with uniform elemental depth profiles with 40 at.% to 60 at.% (Al-N) concentrations and with a c-axis preferential orientation was observed by X-ray diffraction. The ultrasonic response of the Al/AlN/Si overmoded resonator was characterized from 1.24 MHz to 2 GHz using a vector network analyzer. The time domain transform and the gate features of the vector network analyzer were used to characterize the AlN thick film capabilities in the time and frequency domains. The results show that the obtained resonator operates over a broad frequency range with a central frequency at approximately 930 MHz and a 738 MHz bandwidth at -6 dB. The results were compared with those obtained from a one-dimensional simulation to highlight and predict the AlN response features. A comparison was performed to validate the experimental measurements and to validate the circuit modeling. Overall agreement between the experiment values and the simulation was observed.

Piezoelectric AlN Films for FPW Sensors with Improved Device Performance

Reactively sputtered piezoelectric aluminum nitride (AlN) films for flexural plate wave (FPW) electroacoustic sensors equipped with buried interdigital transducers (IDTs) for sensing in liquids were fabricated and characterized. In order to assess the crystallographic and elastic film properties, residual film stress, and piezoelectric response, comprehensive analyses were performed on AlN layers and bimorph AlN structures, including X-ray diffraction, atomic force microscopy, and nanoindentation, in combination with analysis of the piezoelectric charge coefficient d33,f. To demonstrate the applicability of the optimized AlN films to electro-acoustic devices, IDT equipped devices were fabricated and characterized by means of laser Doppler vibrometry.

Piezoelectric AlN Films for FPW Sensors with Improved Device Performance

Reactively sputtered piezoelectric aluminum nitride (AlN) films for flexural plate wave (FPW) electroacoustic sensors equipped with buried interdigital transducers (IDTs) for sensing in liquids were fabricated and characterized. In order to assess the crystallographic and elastic film properties, residual film stress, and piezoelectric response, comprehensive analyses were performed on AlN layers and bimorph AlN structures, including X-ray diffraction, atomic force microscopy, and nanoindentation, in combination with analysis of the piezoelectric charge coefficient d33,f. To demonstrate the applicability of the optimized AlN films to electro-acoustic devices, IDT equipped devices were fabricated and characterized by means of laser Doppler vibrometry.

Synthesis and characterization of 10 nm thick piezoelectric AlN films with high c-axis orientation for miniaturized nanoelectromechanical devices

The scaling of piezoelectric nanoelectromechanical systems (NEMS) is challenged by the synthesis of ultrathin and high quality piezoelectric films on very thin electrodes. We report the synthesis and characterization of the thinnest piezoelectric aluminum nitride (AlN) films (10 nm) ever deposited on ultrathin platinum layers (2–5 nm) using reactive sputtering. X-ray diffraction, high-resolution transmission electron microscopy, and fast Fourier transform analyses confirmed the proper crystal orientation, fine columnar texture, and the continuous lattice structure within individual grains in the deposited AlN nanometer thick films. The average extracted d31 piezoelectric coefficient for the synthesized films is −1.73 pC/N, which is comparable to the reported values for micron thick and highly c-axis oriented AlN films. The 10 nm AlN films were employed to demonstrate two different types of optimized piezoelectric nanoactuators. The unimorph actuators exhibit vertical displacements as large as 1.1 μm at 0.7 V for 25 μm long and 30 nm thick beams. These results have a great potential to realize miniaturized NEMS relays with extremely low voltage, high frequency resonators, and ultrasensitive sensors.

Sub-1-volt Piezoelectric Nanoelectromechanical Relays With Millivolt Switching Capability

The design, fabrication, and characterization of the first piezoelectric nanoelectromechanical relays with sub-1-volt actuation (520 mV) are presented. The low actuation voltage is enabled by employing ultrathin piezoelectric aluminum nitride films (10 nm) with high c-axis orientation and controlled residual stress in a stress-compensating geometry. Two different actuation methods based on optimized four-layer unimorph actuators are used to synthesize normally closed relays with high mechanical restoring forces. The first experimental demonstration of few millivolts switching (10 mV) using the realized relays is also presented. The relays exhibit a very low energy dissipation per switching cycle (23 aJ), an extremely small subthreshold slope (0.013 mV/decade), and a low ON-state resistance (2.3 kQ).

Uncooled resonant infrared detector based on aluminum nitride piezoelectric film through charge generations and lattice absorptions

This Letter demonstrates an aluminum nitride (AlN) based uncooled resonant infrared (IR) detector utilizing the photo-sensitive and piezoelectric properties of polycrystalline AlN. The AlN Lamb wave mode resonator is found responsive to IR illuminations by showing a decrease in the S21 magnitude instead of a resonant frequency shift. A −0.08 dB shift of S21 magnitude was observed for an IR incident power of 647 nW, which translates to a responsivity of 124 kdB/W. Photoresponse is proposed for the IR sensing mechanism through additional charge carriers generation rather than thermal effects.

22am2-E4 ステンレス板ばね構造を有するAlN圧電薄膜発電機

In this report, vibrational energy harvesters employing an aluminum nitride (AlN) thin film deposited on the stainless-steel (SUS304) was described. The piezoelectric AlN film was reactively sputtered on 50-μm-thick SUS304 substrate with AC magnetron system. The deposited film had a high c-orientation, and indicated FWHM of 1.34 deg for AlN(0002) peak. The developed energy harvesters with this film achieved the output power of 72.6 μW at 1 g acceleration when the vibration conditions were set to 65 Hz in frequency and 400 kΩ in load resistance.

The fabrication of vibration energy harvester arrays based on AlN piezoelectric film

The fabrication and measurement results of the vibration energy harvester arrays based on the piezoelectric aluminum nitride (AlN) film are presented. The structure design and fabrication process of the device are introduced. The key material, the AlN film with crystal orientation (002), was deposited by pulsed-DC magnetron sputtering and characterized by X-ray diffraction (XRD). The resonant frequency, power out, and the open circuit voltage of the device are detected. The optimized load of 80 kΩ and a remarkable maximum power out of 30.4 μW are obtained when the acceleration of 0.9g (g is standard gravity acceleration) is applied.

Improving aluminum nitride plasma etch process for MEMS applications

We present a new plasma etch process optimized for etching piezoelectric aluminum nitride (AlN) film deposited on thin molybdenum (Mo) metal electrode. Such film stack finds application in the integration of AlN-based RF microelectromechanical systems devices. The process is based on Cl2/BCl3/Ar gas chemistry with added buffer gas in inductively coupled plasma reactive ion etching system. The new gas mixture overcomes a generic problem of etched surface roughness without significant drop in AlN etch rate. Using design of experiment, the process window is optimized for improving selectivity to Mo and reducing microtrenching while maintaining smooth etched surface. Finally, an etching rate of 280 nm min−1 with reliable etch stop on Mo electrode and smooth bottom surface is reported. The integration suitability of the developed etch process is tested by etching 2.0 to 5.0 µm size square shaped via holes in 1.0 µm thick (0 0 2) oriented piezoelectric AlN on 0.2 µm thick Mo electrode while integrating contour mode resonators.

High-Frequency Thin-Film AlN-on-Diamond Lateral–Extensional Resonators

In this paper, low-impedance lateral–extensional microresonators are fabricated on a stack of aluminum nitride (AlN) directly deposited on a polished ultrananocrystalline diamond (UNCD) film. The large acoustic velocity of UNCD is utilized to extend the frequency of such resonators beyond 1 GHz while the frequency-defining features are not reduced excessively. In order to promote the growth of a <jats:formula formulatype="inline"><jats:tex Notation="TeX">$c$</jats:tex> </jats:formula>-plane piezoelectric AlN film, the surface of the UNCD film is polished after deposition. Three different UNCD films with different Young's modulus values were prepared, and frequencies up to two times that of similar devices fabricated on silicon have been achieved. The finite-element analysis is employed to evaluate the effect of various physical parameters on the performance of the thin-film piezoelectric-on-substrate resonators in order to achieve very low motional resistance <jats:formula formulatype="inline"><jats:tex Notation="TeX" >$(R_{m})$</jats:tex></jats:formula>. Several resonators were designed with various lateral dimensions and different numbers of support tethers to evaluate the propositions. The lowest <jats:formula formulatype="inline"><jats:tex Notation="TeX" >$R_{m}$</jats:tex></jats:formula> was measured from a multitethered 29th-order thin-film piezoelectric-on-diamond (TPoD) resonator (22 <jats:formula formulatype="inline"><jats:tex Notation="TeX"> $Omega$</jats:tex></jats:formula>) and <jats:formula formulatype="inline"><jats:tex Notation="TeX">$fcdot Q$</jats:tex></jats:formula> product of <jats:formula formulatype="inline"><jats:tex Notation="TeX">$2.72 ast 10^{12}$</jats:tex></jats:formula> at 888 MHz. The temperature coefficient of frequency of this TPoD resonator is measured to be <jats:formula formulatype="inline"><jats:tex Notation="TeX">$-9.6 hbox{ppm}/^{circ} hbox{C}$</jats:tex></jats:formula>, which is much lower than that of the devices fabricated on silicon. Also, this device can withstand input powers up to <jats:formula formulatype="inline"><jats:tex Notation="TeX">$+$</jats:tex></jats:formula>27 dBm, leading to a delivered power density per unit area of <jats:formula formulatype="inline" ><jats:tex Notation="TeX">$sim!!2.9 muhbox{W}/muhbox{m}^{2}$</jats:tex> </jats:formula>.<jats:formula formulatype="inline"><jats:tex Notation="TeX" >$hfill$</jats:tex></jats:formula>[2012-0099]

Integration of piezoelectric aluminum nitride and ultrananocrystalline diamond films for implantable biomedical microelectromechanical devices

The physics for integration of piezoelectric aluminum nitride (AlN) films with underlying insulating ultrananocrystalline diamond (UNCD), and electrically conductive grain boundary nitrogen-incorporated UNCD (N-UNCD) and boron-doped UNCD (B-UNCD) layers, as membranes for microelectromechanical system implantable drug delivery devices, has been investigated. AlN films deposited on platinum layers on as grown UNCD or N-UNCD layer (5–10 nm rms roughness) required thickness of ∼400 nm to induce (002) AlN orientation with piezoelectric d33 coefficient ∼1.91 pm/V at ∼10 V. Chemical mechanical polished B-UNCD films (0.2 nm rms roughness) substrates enabled (002) AlN film 200 nm thick, yielding d33 = 5.3 pm/V.

Micromechanical Resonators Based on Silicon Two-Dimensional Phononic Crystals of Square Lattice

Phononic crystal (PnC) resonators of Bloch-mode resonance made by replacing periodically arranged two or three rows of air holes with one row of air holes on a two-dimensional (2-D) silicon slab with air holes of square lattice have been investigated. Piezoelectric aluminum nitride (AlN) film is employed as the interdigital transducers to transmit and detect acoustic waves, thus making the whole microfabrication process CMOS compatible. We also fabricate a PnC structure which has a stopband of 140 MHz <; f <; 195 MHz which agrees well with the simulation results. From our experimental results, we found that the two kinds of microfabricated PnC resonators have different optimization conditions in terms of resonant frequency and Q factor, as well as insertion loss, despite their similar design approach. As compared to PnC resonators of hexagonal lattice, the proposed Bloch-mode PnC resonators of square lattice demonstrated higher resonant frequency, higher Q factor, and a smaller device area. The promising acoustic characteristics may be further optimized for applications such as microfluidics, biomedical devices, and radio-frequency communications in the gigahertz range.

Experimental Investigation of a Cavity-Mode Resonator Using a Micromachined Two-Dimensional Silicon Phononic Crystal in a Square Lattice

A 2-D silicon phononic crystal (PnC) slab of a square array of cylindrical air holes in a 10-μm-thick freestanding silicon plate with line defects is characterized as a cavity-mode PnC resonator. A piezoelectric aluminum nitride (AlN) film is employed as the interdigital transducers to transmit and detect acoustic waves, thus making the whole microfabrication process CMOS compatible. Both the band structure of the PnC and the transmission spectrum of the proposed PnC resonator are analyzed and optimized using finite-element method. The measured quality factor (Q factor) of the microfabricated PnC resonator is over 1000 at its resonant frequency of 152.46 MHz. The proposed PnC resonator shows promising acoustic resonance characteristics for radio-frequency communications and sensing applications.

A Novel Micromechanical Resonator Using Two-Dimensional Phononic Crystal Slab

Two-dimensional (2-D) Silicon phononic crystal (PnC) slab of a square array of cylindrical air holes in a 10μm thick free-standing silicon plate with line defects is characterized as a cavity-mode PnC resonator. Piezoelectric aluminum nitride (AlN) film is deployed as the inter-digital transducers (IDT) to transmit and detect acoustic waves, thus making the whole microfabrication process CMOS-compatible. Both the band structure of the PnC and the transmission spectrum of the proposed PnC resonator are analyzed and optimized using finite element method (FEM). The measured quality factor (Q factor) of the microfabricated PnC resonator is over 1,000 at its resonant frequency of 152.46MHz. The proposed PnC resonator shows promising acoustic resonance characteristics for RF communications and sensing applications.