Firas Sammoura

CMOS-compatible AlN piezoelectric micromachined ultrasonic transducers

Piezoelectric micromachined ultrasonic transducers for air-coupled ultrasound applications were fabricated using aluminum nitride (AlN) as the active piezoelectric layer. The AlN is deposited via a low-temperature sputtering process that is compatible with deposition on metalized CMOS wafers. An analytical model describing the electromechanical response is presented and compared with experimental measurements. The membrane deflection was measured to be 210 nm when excited at the 220 kHz resonant frequency using a 1Vpp input voltage.

Highly active ruthenium oxide coating via ALD and electrochemical activation in supercapacitor applications

Highly active ruthenium oxide was uniformly coated on vertically aligned carbon nanotube forests for pseudocapacitor electrodes in enhanced energy storage applications. Atomic layer deposition (ALD) was designed to realize the conformal coating process onto porous structures and an electrochemical oxidation process was developed to achieve highly active ruthenium oxide. Results show 100× and 170× higher specific capacitance after the ALD coating and further electrochemical oxidation process, respectively, as compared with that of pure CNT electrodes. Furthermore, the measured capacitance value was close to the theoretical limit of ruthenium oxide at 644 F g−1 with a high power density at 17 kW kg−1. The electrode performance was tested over 10 000 charge–discharge cycles with gradually improved capacitance of 17% higher than the starting value and at ultra-high scan rates of up to 20 V s−1.

Theoretical modeling and equivalent electric circuit of a bimorph piezoelectric micromachined ultrasonic transducer

An electric circuit model for a circular bimorph piezoelectric micromachined ultrasonic transducer (PMUT) was developed for the first time. The model was made up of an electric mesh, which was coupled to a mechanical mesh via a transformer element. The bimorph PMUT consisted of two piezoelectric layers of the same material, having equal thicknesses, and sandwiched between three thin electrodes. The piezoelectric layers, having the same poling axis, were biased with electric potentials of the same magnitude but opposite polarity. The strain mismatches between the two layers created by the converse piezoelectric effect caused the membrane to vibrate and, hence, transmit a pressure wave. Upon receiving the echo of the acoustic wave, the membrane deformation led to the generation of electric charges as a result of the direct piezoelectric phenomenon. The membrane angular velocity and electric current were related to the applied electric field, the impinging acoustic pressure, and the moment at the edge of the membrane using two canonical equations. The transduction coefficients from the electrical to the mechanical domain and vice-versa were shown to be bilateral and the system was shown to be reversible. The circuit parameters of the derived model were extracted, including the transformer ratio, the clamped electric impedance, the spring-softening impedance, and the open-circuit mechanical impedance. The theoretical model was fully examined by generating the electrical input impedance and average plate displacement curves versus frequency under both air and water loading conditions. A PMUT composed of piezoelectric material with a lossy dielectric was also investigated and the maximum possible electroacoustical conversion efficiency was calculated.

Optimizing the electrode size of circular bimorph plates with different boundary conditions for maximum deflection of piezoelectric micromachined ultrasonic transducers

The effect of plate electrode area on the deflection of a symmetric circular bimorph piezoelectric micromachined ultrasonic transducer (pMUT) with clamped and simply supported boundary conditions was studied for the first time. Distinct plate displacement shape functions were defined for the regions underneath and outside the active electrodes. The plate shape functions were solved analytically using classic plate theory in conjunction with the external boundary conditions and the internal ones between the two regions in order to calculate the exact plate displacement under both external voltage stimulus and acoustic pressure. The model was used to study the effect of the electrode area on the overall plate deflection per unit input voltage such that the electromechanical coupling is optimized. While the center plate deflection increased monotonically with the electrode area for a simply supported plate, it followed a parabolic shape for a clamped one with a maximum deflection when the electrode radius covered 60% of the total plate radius. The simply supported plate exhibited four times the plate deflection capability of its clamped counterpart, when both are operating at their optimal electrode size. Both an experimental clamped bimorph aluminum nitride (AlN) pMUT, recently reported in the literature, and Finite Element Modeling (FEM) were used to verify the developed model. The theoretical model predicted a static displacement per unit voltage of 10.9nm/V and a resonant frequency of 196.5kHz, which were in excellent agreement with the FEM results of 10.32nm/V and 198.5kHz, respectively. The modeling data matched well with the experimental measurements and the error ranged from 2.7-22% due to process variations across the wafer. As such, the developed model can be used to design more sensitive pMUTs or extract the flexural piezoelectric coefficient using piezoelectrically actuated circular plates.

Highly responsive curved aluminum nitride pMUT

We have successfully demonstrated highly responsive, curved piezoelectric micromachined ultrasonic transducers (pMUTs) based on a CMOS-compatible fabrication process using AlN (aluminum nitride) as the transduction material. Micro fabrication techniques have been used to control the radius of curvature of working diaphragms from 400~2000 μm and theoretical analysis have been developed for the optimal dimensions of the transducers to boost the electromechanical coupling and acoustic pressure. A prototype device made of a 2μm-thick AlN on a curved diaphragm with a nominal size of 140μm in diameter and a radius of curvature of 1065μm has been fabricated. The measured resonant frequency is 2.19MHz and DC response is 1.1nm/V, which is 50X higher than that of a planar device with the same nominal diameter. As such, this new class of curved pMUTs could dramatically enhance the responses of the state-of-art, planar pMUTs with high electromechanical coupling for various ultrasonic transduction applications, such as gesture recognition and medical imaging.

Analytic solution for N-electrode actuated piezoelectric disk with application to piezoelectric micromachined ultrasonic transducers

In this work, the deflection equation of a piezoelectrically-driven micromachined ultrasonic transducer (PMUT) is analytically determined using a Greens function approach. With the Greens function solution technique, the deflection of a circular plate with an arbitrary circular/ring electrode geometry is explicitly solved for axisymmetric vibration modes. For a PMUT with one center electrode covering ≈60% of the plate radius, the Greens function solution compares well with existing piece-wise and energy-based solutions with errors of less than 1%. The Green¿s function solution is also simpler than them requiring no numerical integration, and applies to any number of axisymmetric electrode geometries. Experimentally measured static deflection data collected from a fabricated piezoelectric micro ultrasonic transducer (PMUT) is further used to validate the Greens function model analysis. The center deflection and deflection profile data agree well with the Greens function solution over a range of applied bias voltages (5 to 21 V) with the average error between the experimental and Greens function data less than 9%.

An analytical analysis of the sensitivity of circular piezoelectric micromachined ultrasonic transducers to residual stress

In this work, a novel Greens function technique is used to solve for plate deflection of a piezoelectric micromachined ultrasonic transducer (pMUT) under the influence of residual stress. The pMUT is modelled as a clamped, circular diaphragm consisting of a stack of structural and piezoelectric material actuated by an arbitrary number of circular electrodes. From classic plate theory, the equation of motion for axisymmetric bending is derived based on residual stress considering external pressure, voltage applied across the piezoelectric material, and electrode geometry. With Greens function, the equation of motion is analytically solved where the plate impedance shows a considerable increase in bandwidth and center deflection for a plate under compressive stress. The static zero-bias deflection of a microfabricated pMUT with a center electrode covering ≈60% of the plate radius agrees well with the model prediction. Based on the model and confirmed by experiment, it is suggested that an outer dummy electrode extending to the plate edge should be added to the pMUT design to prevent zero-bias deflection.

A micromachined W-band iris filter

A micromachined W-band iris filter was successfully demonstrated using micro hot embossing and selective electroplating technologies. The filter design follows the insertion loss method with a Chebyshev polynomial to synthesize the desired spectral responses and numerical simulations were performed using the finite-element tool, high frequency structure simulator (HFSS), to study the effects of iris thickness on the bandwidth and center frequency. The measured prototype filter performance has a transmission loss of -3.49 dB and a return loss of -18 dB at the resonant frequency of 95 GHz. The filter has a bandwidth of 3.5 GHz and a rejection loss of better than -50 dB, while the loaded quality factor is 27.27. This plastic, low-cost manufacturing process opens up opportunities in replacing the expensive metallic components and integrated 3D manufacturing for current and future millimeter-wave systems.

Microfabricated plastic 95-GHz rectangular waveguide

A plastic, 95-GHz rectangular waveguide with an integrated plastic flange has been successfully demonstrated using micro hot embossing and electroplating technologies. The prototype device shows s/sub 11/ and s/sub 21/ values of -20.7 dB and -1.35 dB at 95GHz, respectively, and the signal transmission rate is 73.3% as compared to commercial all-metal waveguides at 85%. The time domain measurement showed the system losses are mainly due to the discontinuity losses at the interfaces between the network analyzer adaptors and the waveguide input/output ports and could be further minimized in future integrated systems. As such, this new class of plastic waveguides has potential applications in replacing the expensive yet bulky/heavy metallic waveguides in current millimeter-wave systems.

Bimorph pMUT with dual electrodes

The concept of “bimorph” piezoelectric micromachined ultrasonic transducers (pMUTs) has been demonstrated by utilizing a two active AlN layers structure constructed in a CMOS-compatible process. The prototype device has two 0.95μm-thick AlN layers sandwiched by three 0.15μm-thick Mo electrodes. In a prototype, both an inner circular and an outer annular electrode are designed on a 230 μm in radius, circular-shape diaphragm. When actuated with the inner electrode of 160μm in radius, the pMUT has a resonant frequency of 198.8 kHz and central displacement of 407.4 nm/V. Under the differential drive scheme using the dual-electrodes for large acoustic outputs at a low frequency, the measured central displacement is 13.0 nm/V, which is about 400% higher than that of a unimorph AlN-pMUT under similar actuation conditions. As such, the dual-electrode bimorph pMUT presents the improved operation as compared with the state-of-the-art flat pMUT design to achieve enhanced acoustic outputs.