Amir Heidari

A High Fill-Factor Annular Array of High Frequency Piezoelectric Micromachined Ultrasonic Transducers

This paper presents a 1.2-mm diameter high fill-factor array of 1261 piezoelectric micromachined ultrasonic transducers (PMUTs) operating at 18.6 MHz in fluid for intravascular ultrasound imaging. At 1061 transducers/mm2, the PMUT array has a 10-20 times higher density than previous PMUT arrays realized to date. Aluminum nitride (AlN)-based PMUTs described in this paper are fabricated using a process compatible with the fabrication of inertial sensors, radio frequency (RF) resonators, and CMOS integrated circuits. The PMUTs are released using a front-side sacrificial etch through etching holes that are subsequently sealed by a thin layer of parylene. Finite element method and analytical results, including resonant frequency, pressure sensitivity, output acoustic pressure, and directivity are given to guide the PMUT design effectively, and are shown to match well with measurement results. Due to the PMUTs thin membrane (750-nm AlN/800-nm SiO2) and small diameter, a single 25-μm PMUT has approximately omnidirectional directivity and no near-field zone with irregular pressure pattern. PMUTs are characterized in both the frequency and time domains. Measurement results show a large displacement response of 2.5 nm/V at resonance and good frequency matching in air, high center frequency of 18.6 MHz and wide bandwidth of 4.9 MHz, when immersed in fluid. Phased array simulations based on measured PMUT parameters show a tightly focused high-output pressure acoustic beam.

Hemispherical wineglass resonators fabricated from the microcrystalline diamond

We present the development of millimeter scale 3D hemispherical shell resonators fabricated from the polycrystalline diamond, a material with low thermoelastic damping and very high stiffness. These hemispherical wineglass resonators with 1.1 mm diameter are fabricated through a combination of micro-electro discharge machining (EDM) and silicon micromachining techniques. Using piezoelectric and electrostatic excitation and optical vibration measurement, the elliptical wineglass vibration mode is determined to be at 18.321 kHz, with the two degenerate wineglass modes having a relative frequency mismatch of 0.03%. A study on the effect of the size and misalignment of the anchor and resonator’s radius variation on both the average frequency and frequency mismatch of the 2θ elliptical vibration modes is carried out. It is shown that the absolute frequency of a wineglass resonator will increase with the anchor size. It is also demonstrated that the fourth harmonic of radius variation is linearly related to the frequency mismatch. (Some figures may appear in colour only in the online journal)

High frequency piezoelectric micromachined ultrasonic transducer array for intravascular ultrasound imaging

This paper presents a 1.2 mm diameter high fill-factor array of 1,261 piezoelectric micromachined ultrasonic transducers (PMUTs) operating at 18.6 MHz for intravascular ultrasound (IVUS) imaging and other medical imaging applications. At 1061 transducers/mm2, the PMUT array has a 10-20× higher density than the best PMUT arrays realized to date. The PMUTs utilize a piezoelectric material, AlN, which is compatible with CMOS processes. Measurements show a large voltage response of 2.5 nm/V and good frequency matching in air, a high center frequency of 18.6 MHz and wide bandwidth of 4.9 MHz when immersed in fluid. Phased array simulations based on measured PMUT parameters show a tightly focused, high output pressure acoustic beam.

Self-curved diaphragms by stress engineering for highly responsive pMUT

A process to make self-curved diaphragms by engineering residual stress in thin films has been developed to construct highly responsive piezoelectric micromachined ultrasonic transducers (pMUT). This process enables high device fill-factor for better than 95% area utilization with controlled formation of curved membranes. The placement of a 0.65 μm-thick, low stress silicon nitride (SiN) film with 650 MPa of tensile residual stress and a low temperature oxide (LTO) film with 180 MPa of compressive stress sitting on top of a 4 μm-thick silicon film has resulted in the desirable self-curved diaphragms. A curved pMUT with 200 μm in nominal radius, 2 μm-thick aluminum nitride (AlN) piezoelectric layer, and 50% SiN coverage has resulted in a 2.7 μm deflection at the center and resonance at 647 kHz. Low frequency and resonant deformation responses of 0.58 nm/V and 40nm/V at the center of the diaphragm have been measured, respectively. This process enables foundry-compatible CMOS process and potentially large fill-factor for pMUT applications.

Microcrystalline diamond cylindrical resonators with quality-factor up to 0.5 million

We demonstrate high quality-factor 1.5 mm diameter batch-fabricated microcrystalline diamond cylindrical resonators (CR) with quality-factors limited by thermoelastic damping (TED) and surface loss. Resonators were fabricated 2.6 and 5.3 μm thick in-situ boron-doped microcrystalline diamond films deposited using hot filament chemical vapor deposition. The quality-factor (Q) of as-fabricated CRs was found to increase with the resonator diameter and diamond thickness. Annealing the CRs at 700 °C in a nitrogen atmosphere led to a three-fold increase in Q, a result we attribute to thinning of the diamond layer via reaction with residual O2 in the annealing furnace. Post-anneal Q exceeding 0.5 million (528 000) was measured at the 19 kHz elliptical wineglass modes, producing a ring-down time of 8.9 s. A model for Q versus diamond thickness and resonance frequency is developed including the effects of TED and surface loss. Measured quality factors are shown to agree with the predictions of this model.

Batch-fabricated high Q-factor microcrystalline diamond cylindrical resonator

This paper reports a 1.5 mm batch-fabricated polycrystalline diamond Cylindrical Resonator (CR) for gyroscope applications. The device is fabricated in a cylindrical shape using silicon on insulator (SOI) wafers and deep reactive ion etching (DRIE), which allows flexibility of choosing different geometries and materials for the resonator structure. A quality factor (Q) of 313,100 is measured at the 23 kHz 2 theta elliptical wineglass modes, producing a ring-down time of 4.32 seconds. Annealing CRs at 700 °C in a nitrogen atmosphere improved Q from 75,000 to over 300,000. The highly symmetric fabrication results in CRs with an excellent frequency mismatch of 3 Hz (130 ppm) between the 2 theta degenerate wineglass modes without applying any tuning voltage.

Quality Factor in Polycrystalline Diamond Micromechanical Flexural Resonators

This paper reports an investigation into the various dissipation mechanisms that can affect polycrystalline diamond micromechanical resonators. Double-ended tuning fork and cantilever resonators were fabricated from 1-5-μm thick microcrystalline diamond films. It is shown that the quality factor of the low frequency (<;500 kHz) resonators is limited by surface loss, whereas the thermoelastic damping limits the quality factor of the higher frequency resonators. In resonators where surface loss is the dominant effect, the dependence of quality factor on resonator thickness is demonstrated. The addition of a lossy surface layer of Al2O3 deposited via atomic layer deposition is shown to degrade quality factor, and an experiment that further demonstrates the effect of surface dissipation and results in a reduction in quality factor that scales with the thickness of the Al2O3 layer. Heat treatment of cantilever resonators in N2 for various times up to 660 min is used to modify the resonator surface and is shown to result in a threefold increase in quality factor up to 365 000 at 26.6 kHz.

Simulation Based Design of Disk Resonator Biosensors Under Fabrication Uncertainty

A high performance and cost effective biosensor is designed using a radial contour-mode disk resonator (RCDR). This sensor measures tiny biological mass attached on a disk vibrating at a high frequency, producing high quality of output signal. A series of analysis and simulation models is developed to predict the mass sensitivity, dynamic stability, and motional resistance of the RCDR biosensor with given geometry and signal input. In order to decrease motional resistance while keeping the fabrication cost low, a layer of dielectric material is deposited within the capacitor gap. In designing the RCDR biosensors, we employ Type I, II, and III robust design approach to design a device that is insensitive to various types of uncertainty associated with the fabrication processes and analysis models. A mathematical construct, error margin index, is employed for this robust design. Traditional optimization and robust design approaches are separately formulated, solved, and compared. From the design results, we observe that the RCDR is a promising bio-sensing device compared to the existing ones.

Piezoelectric MEMS resonant gas sensor for defence applications

Piezoelectric AlN resonators with high potential for gravimetric gas sensor application are reported in this paper. An array of interdigitated electrode fingers is used to excite plate acoustic Lamb waves within a piezoelectric layer. The resonant frequency can be changed by adjusting the size of electrode fingers and hence resonators with multiple frequencies from MHz to GHz can be fabricated on a single wafer. A 773.42MHz resonator is coated with multiple layers of Polyelectrolyte multilayer (PEMs) in order to determine its sensitivity. The mass sensitivity of 4.75Hz/fg, limit of detection (LOD) of 187.5pg/mm2, and the minimum detectable mass of 7.8pg are measured.

Biotin-Streptavidin Binding Interactions of Dielectric Filled Silicon Bulk Acoustic Resonators for Smart Label-Free Biochemical Sensor Applications

Sensor performance of a dielectric filled silicon bulk acoustic resonator type label-free biosensor is verified with biotin-streptavidin binding interactions as a model system. The mass sensor is a micromachined silicon square plate with a dielectric filled capacitive excitation mechanism. The resonance frequency of the biotin modified resonator decreased 315 ppm when exposed to streptavidin solution for 15 min with a concentration of 10−7 M, corresponding to an added mass of 3.43 ng on the resonator surface. An additional control is added by exposing a bovine serum albumin (BSA)-covered device to streptavidin in the absence of the attached biotin. No resonance frequency shift was observed in the control experiment, which confirms the specificity of the detection. The sensor-to-sensor variability is also measured to be 4.3%. Consequently, the developed sensor can be used to observe in biotin-streptavidin interaction without the use of labelling or molecular tags. In addition, biosensor can be used in a variety of different immunoassay tests.