Two kinds of 128 channels pMUT-based phased array ultrasound transducers are described in this paper, one with a high frequency of around 6 MHz and another with a low frequency of around 1.5 MHz. The active area of the transducer is around 25 mm long and 10 mm wide. There are in total 6270 pMUT elements in the reported arrays. For the transducer with a center frequency of 6 MHz, each pMUT has a membrane diameter of 85 $\mu \text{m}$ and the pitch between every two channels is 200 $\mu \text{m}$ . The transducer benefits from a high transmission and receive sensitivity of 44 kPa/V/Channel @ 3 cm and 204 mV/MPa, respectively. For the transducer with a center frequency of 1.5 MHz, each pMUT has a membrane diameter of $160~\mu \text{m}$ and the pitch between every two elements is 214 $\mu \text{m}$ . The proposed transducer obtained a transmit and receive sensitivity of 430 Pa/V/Channel @ 3 cm and 190 mV/MPa, respectively. The transducer has a −3dB and −6dB bandwidth of 118% and 184%, respectively. The bandwidth is higher than any previously reported transducer in any technology, such as bulk-PZT, pMUT, or capacitive MUT (cMUT). The functionality of the transducer arrays is confirmed by obtaining B-mode images in water medium.

The use of bulk piezoelectric transducer arrays in medical imaging is a well-established technology that operates based on thickness mode piezoelectric vibration. Meanwhile, advancements in fabrication techniques have led to the emergence of micromachined alternatives, namely, piezoelectric micromachined ultrasound transducer (PMUT) and capacitive micromachined ultrasound transducer (CMUT). These devices operate in flexural mode using piezoelectric thin films and electrostatic forces, respectively. In addition, the development of flexible ultrasound transducers based on these principles has opened up new possibilities for biomedical applications, including biomedical imaging, sensing, and stimulation. This review provides a detailed discussion of the need for flexible micromachined ultrasound transducers (MUTs) and potential applications, their specifications, materials, fabrication, and electronics integration. Specifically, the review covers fabrication approaches and compares the performance specifications of flexible PMUTs and CMUTs, including resonance frequency, sensitivity, flexibility, and other relevant factors. Finally, the review concludes with an outlook on the challenges and opportunities associated with the realization of efficient MUTs with high performance and flexibility.

This paper presents the design and fabrication process of a spherical-omnidirectional ultrasound transducer for underwater sensor network applications. The transducer is based on the vibration of two hemispheres with a thickness of 1 mm and an outer diameter of 10 mm, which are actuated by two piezoelectric ring elements. Since the ultrasound wave is generated by the vibration of the two hemispheres, a matching layer is not required. Silicon Carbide (SiC) is used as the material of the hemispherical shells of the transducer. The shells were fabricated by laser sintering as an additive manufacturing method, in which the hemispheres were built layer by layer from a powder bed. All manufactured transducers with an outer dimension of $10\times 14.2$ mm and a center frequency of 155 kHz were measured in a water tank by a hydrophone or in mutual communication. The circumferential source level was measured to vary less than 5dB. The power consumption and the insertion loss of the transducer, ranging from 100 $\mathsf{\mu }$ W to 2.4 mW and 21.2 dB, respectively, along with all other measurements, prove that the transducer can transmit and receive ultrasound waves omnidirectionally at tens of centimeters intervals with a decent power consumption and low actuation voltage.

This paper presents the design and fabrication strategy of a lead zirconate titanate (PZT)-based piezoelectric micromachined ultrasound transducers (pMUT) with the objective of maximizing its performance. Choosing the most suitable thickness of the PZT layer, the radius of the top electrode, and the effect of the residual stress on the resonance frequency of a pMUT are investigated by finite element method (FEM) simulation and analytical equations. Three different architectures of pMUTs are presented: (i) a pMUT with a top center electrode, (ii) a pMUT with a top center and ring electrode, (iii) a pMUT with a patterned PZT layer. All three pMUT architectures were fabricated and their displacement response, resonance frequency, and Q-factor are compared. The fabricated pMUT in all three architectures has a radius of 205 m, a Si membrane thickness of 6 m, and a PZT thickness of 1 m. The experimental results were compared to FEM simulation. The proposed architectures have a displacement response of 6.6 m V ⁻¹ , 7.5 m V ⁻¹ , and 8.6 m V ⁻¹ , respectively, for their optimum design parameters.

This paper presents the design and fabrication method to render a silicon-on-insulator (SOI)-based pMUT array bendable. The proposed method can be applied to silicon-based pMUT arrays with different acoustical specifications. A bendable array based on the proposed method is highly conformal to the target structure. The bendable structure is made out of several silicon islands containing a pMUT array and are connected to each other by silicon springs. The silicon springs are realized by deep reactive ion etching (DRIE) that was also used to create the pMUT membranes. The fabrication process requires one additional photomask step compared to a process that realizes only pMUTs. Since the pMUT arrays are fabricated on silicon islands with the thickness of the entire SOI wafer, therefore their performance is not compromised. As a case study, a bendable array was designed and fabricated to be wrapped around a 5x5x5 mm³ 3D printed cube. Therefore, each bendable array has 6 islands with a dimension of 3x3 mm² including 3x3 pMUT array, of which each pMUT has a diameter of 410 μm and a thickness of 6 μm with a 1 μm Lead Zirconate Titanate (PZT) layer as the piezoelectric material. The silicon springs in between each island have a spring constant of 2.3 (N/m) in their folding direction. This compliant structure enables a 90° bending with a bending radius of 1 mm. The pMUT elements in the array resulted in a displacement response of 2.1 μm/V and a Q-factor of 104 at 426 kHz, when all arrays were excited all together. [2019-0224]

Aluminum nitride (AlN) thin film was grown by plasma enhanced atomic layer deposition using trimethylaluminum and ammonia precursors. A method was found to have crystalline thin film AlN with almost zero thickness variation and a truly one layer deposition of atoms per each cycle of the process. The growth rate saturated at ∼ 1 Å/cycle, and the thickness was proportional to the number of reaction cycles. The preferred crystal orientation, uniformity of the nucleation and the surface roughness of the grown AlN were investigated. X-ray diffraction (XRD), atomic focused microscopy (AFM) and scanning electron microscopy (SEM) were carried out to analyze the crystallinity and properties of the films.