Nai-Kuei Kuo

Microscale inverse acoustic band gap structure in aluminum nitride

This work presents the design and demonstration of a microscale inverse acoustic band gap (IABG) structure in aluminum nitride (AlN) with a frequency stop band for bulk acoustic waves in the very high frequency range. Conversely to conventional microscale acoustic band gaps, the IABG is formed by a two-dimensional periodic array of unit cells consisting of a high acoustic velocity material cylinder surrounded by a low acoustic velocity medium. The periodic arrangement of the IABG array induces scattering of incident acoustic waves and generates a stop band, whose center frequency is primarily determined by the lattice constant of the unit cell and whose bandwidth depends on the cylinder radius, the film thickness, and the size of the tethers that support the cylinder. A wide band gap (>13% of the center frequency) is formed by the IABG even when thin AlN films are used. The experimental response of an IABG structure having a unit cell of 8.6 μm and an AlN film thickness of 2 μm confirms the existence of a...

Fractal phononic crystals in aluminum nitride: An approach to ultra high frequency bandgaps

This letter reports on the design and experimental demonstration of a microscale fractal-like phononic bandgap (PBG) structure in aluminum nitride (AlN). The micro-fabricated fractal phononic crystals (PnCs) exhibit two frequency stop bands for symmetric lamb waves in the Γ-Χ direction centered about 900 MHz (bandwidth of 11.1%) and 1.10 GHz (bandwidth of 9.1%) with maximum acoustic rejection of 40 dB. Differently from the conventional phononic bandgap designs, the unit cell consists of a center air square scatterer with four side air square scatterers repeating at its corners. The presence of these side squares essentially shortens the scattering distance between the unit cells and translates into the suppression of higher frequency vibrational modes. In other words, this design is capable of extending the frequency of operation of the PBGs for a given unit cell and minimum feature size. For the purpose of the demonstration, AlN lamb wave transducers were utilized to launch symmetric lamb waves into the ...

Demonstration of inverse acoustic band gap structures in AlN and integration with piezoelectric contour mode wideband transducers

This paper presents the first design and demonstration of a novel inverse acoustic band gap (IABG) structure in aluminum nitride (AlN) and its direct integration with contour-mode wideband transducers in the Very High Frequency (VHF) range. This design implements an efficient approach to co-fabricate in-plane AlN electro-acoustic transducers with bulk acoustic waves (BAWs) IABG arrays (10×10). The IABG unit cell consists of a cylindrical high acoustic velocity (V) media, which is held by four thin tethers, surrounded by a low acoustic velocity matrix (air). The center media is formed by 2-µm-thick AlN, which is sandwiched by 200-nm-thick top and bottom platinum (Pt) layers. The experimental results indicate that the designed IABG has a stop band from 185 MHz to 240 MHz and is centered at 218 MHz in the Γ-X direction. This demonstration not only confirms the existence of the frequency band gap in the IABG structure, but also opens possibilities for the integration of ABG structures with RF MEMS devices.

Spurious mode suppression via apodization for 1 GHz AlN Contour-Mode Resonators

This paper reports, for the first time, on the application of apodization techniques to 1 GHz MEMS AlN Contour-Mode Resonators (CMRs [1]) to efficiently suppress spurious modes in close proximity of the main mechanical resonance. This concept has been applied with excellent results to a variety of one port resonators formed by patterned top electrodes made out of Aluminum, and a floating bottom electrode made out of Platinum sandwiching the AlN film. As also predicted by 3D COMSOL simulations, a complete elimination of spurious responses in the admittance plot of these resonators is attained without impacting their quality factor and electromechanical coupling coefficient.

Evidence of acoustic wave focusing in a microscale 630 MHz Aluminum Nitride phononic crystal waveguide

This paper reports on the evidence of acoustic wave focusing in an Aluminum Nitride (AlN) phononic crystal (PC) waveguide by means of a PC-tapered lens at 630 MHz. The PC array exhibits a complete stop band ranging from 560 to 680 MHz. The phononic band gap (PBG) waveguide, which is obtained by opening a pathway having a width equal to one lattice constant of the PC array, forms a pass-band centered around 630 MHz inside the PBG. Experimental results also indicate an enhancement in the pass-band transmission (centered at 630 MHz) when the PC waveguide is cascaded with a PC tapered lens with respect to a PBG waveguide located two lattice constants away from the acoustic source. Acoustic waves were launched in the phononic crystals by means of wideband AlN piezoelectric lamb wave transducers, which were integrated in the same plane of the PC. All the devices were micro-fabricated in a three-mask post-CMOS compatible process.

A 586 MHz microcontroller compensated MEMS oscillator based on ovenized aluminum nitride contour-mode resonators

This work reports on the demonstration of a Microcontroller Compensated MEMS Oscillator based on a UHF ovenized AlN CMR, which shows temperature stability, short-term stability, and acceleration sensitivity comparable to SAW-based oscillators. Over the temperature range -25 ÷ 85°C, the demonstrated 586 MHz oscillator exhibits a temperature stability of 1.7 ppm, stable phase noise values better than -91 dBc/Hz at 1 kHz, and -160 dBc/Hz at 40 MHz offsets, and acceleration sensitivity values better than 30 ppb/g. The design of the integrated heater all-around the top RF electrodes and the new digital temperature controller allowed to simplify the fabrication process compared to our previous demonstration, and to virtually compensate frequency shifts of any origin, i.e. from both the MEMS and the electronic circuitry.

Suppression of spurious modes via dummy electrodes and 2% frequency shift via cavity size selection for 1 GHz AlN MEMS contour-mode resonators

This paper reports on the application to 1 GHz AlN MEMS contour-mode resonators (CMR) of a spurious mode suppression technique based on the introduction of dummy electrodes and a method to shift the resonator center frequency by modifying its cavity size. The realization of wideband filters with CMRs is currently limited by the need to 1) enlarge the device capacitance (so as to minimize the inductive components in the matching network), 2) reduce in-band ripples and out-of-band spurs (which are introduced when the device capacitance is increased), and 3) shift the device center frequency by a large percentage (>; 2%) to synthesize ladder/lattice configurations. This work addresses these 3 main challenges by optimizing the electromechanical response of AlN CMRs having a large static capacitance, synthesized by using thin AlN films of two different thicknesses (500 nm and 1 μm thick), sandwiched by Pt and Al electrodes and having a large number of fingers (up to 45). 3D COMSOL finite element model is used to analyze and/predict the resonators behavior.

1 GHz bulk acoustic wave slanted finger interdigital transducers in aluminum nitride for wideband applications

This paper presents the first experimental demonstration of bulk acoustic wave (BAW) slanted finger interdigital transducer (SFIT) delay line in aluminum nitride (AlN) thin film operating around 1 GHz. The SFIT is used to electro-acoustically transduce the Lamb wave S0 mode. The AlN Lamb SFITs were micromachined and experimentally demonstrated to have a band-pass response with a maximum insertion loss (IL) of 26 dB, fractional bandwidth (6 dB) of 7.9%, and a shape factor (-30 dB) of 1.8. This device displays a small form factor of 150 μm × 421 μm. The transmission response of the measured devices was confirmed to match the impulse response model. However, large pass-band ripples were observed in the device due to the acoustic reflections within the interdigitated fingers. In order to verify the reflection effect, AlN lateral field excitation (LFE) SFITs were also microfabricated. Because of the absence of the bottom floating electrode and the use of a thicker AlN membrane in this configuration, the acoustic mismatch between interdigital transducers (IDTs) is reduced and so is the magnitude of the associated ripple (from 5 dB to 3.5 dB).

Impact of metal electrodes on the figure of merit (k t 2 ·Q) and spurious modes of contour mode AlN resonators

This paper presents an experimental study on the impact of the metal electrodes on the level of the first spurious mode, electromechanical coupling (kt2) and quality factor (Q) of laterally vibrating AlN contour mode resonators (CMRs). Pt, Ni, and Al are selected for the fabrication of the device top electrode since they exhibit a broad range of values of Youngs modulus, density and resistivity, which are the most relevant parameters that have an impact on the electromechanical characteristics of the device. The results on the level of the first spurious mode, and kt2 suggest that the acoustic mismatch between electrode and nonelectroded regions affect mostly these parameters. The extracted Q (after subtracting the effect of the electrode resistance) exhibits a trend in line with the theory of interfacial dissipation.

1 GHZ phononic band gap structure in air/aluminum nitride for symmetric lamb waves

This paper presents the first demonstration of a new class of phononic band gap (PBG) structures in air/aluminum nitride (AlN) at 1 GHz for symmetric lamb waves. The unit cell of this design utilizes an X-shaped air inclusion, instead of the conventional circular ones, in a solid host material. This novel design extends the operating frequency of the AlN PBG to the GHz range by using a structure minimum feature size of 750 nm, instead of 300 nm required by the conventional circular design, which therefore lessens the requirements during the photolithography process. The experimental results confirm the existence of a frequency band gap from 860 MHz to 1.2 GHz with maximum attenuation of 40 dB in the Γ-X direction. The amplitude of rejection and the frequency stop band is verified by COMSOL® finite element method (FEM) simulation, proving, for the first time, that COMSOL® can be employed for the full characterization of these structures. Low-loss acoustic delay lines in AlN were employed to form the reference response to which the PBG is compared. The integration of PBGs with these acoustic elements is a clear evidence of the possibility to synthesize miniaturized RF MEMS platforms in the ultra high frequency (UHF) range.