Cost-Effective Strategy for Developing Small Sized High Frequency PMUTs Toward Phased Array Imaging Applications

Abstract

This article presents the development of small sized piezoelectric micromachined ultrasound transducer (PMUT) with cost effective fabrication process. Improved ultrasound imaging quality requires small sized, high frequency and high performance PMUT cell and further high density arrays. Fabrication with currently well developed and cost effective sacrificial process benefits their mass production and application. To determine small sized AlN PMUT cell, commonly reported elastic layer materials including Si, Si 3 N 4 , SiO 2 are compared by mathematic derivation under the same stack thickness and device frequency. SiO 2 has the smallest Young modulus and it serving as elastic layer can yield the expected one. Arranging SiO 2 layer at the top of the device is compatible with phosphor silicate glass (PSG) sacrificial manufacturing process for cost effective fabrication. This fabrication strategy further downsizes cell size since extra device protection layer is omissible. We make 12MHz PMUT linear arrays with cell diameter, thickness and element pitch to be 40µm, 1.9µm and 120µm. Their electrical and ultrasound results are quite uniform at both device and wafer levels. One PMUT line with 20 parallelly connected units can produce pulseecho signal with over 25dB SNR at 10mm distance. Lateral and axial ultrasound imaging resolutions of ∼1 mm and ∼0.3 mm are obtained in practical verification. These indicate the good ultrasound performance of our developed PMUT array. Plus the advantages of small size and cost effective fabrication, using SiO 2 elastic layer and arranging it at the top of the device is a competitive strategy to develop phased array PMUTs. [2022-0184]

Efficient strategy for frequency design and bandwidth extension of curved piezoelectric ultrasonic micromachined transducers

“…This is due to process limitations, the maximum value of b is a. Similar to f min , f max is also determined by the diaphragm configuration and a, as given by equation (27). Tests also show that when b/a < 0.5, f exhibits almost linear behavior with respect to b.…”

“…General curved PMUTs still utilize the same diaphragm configuration. The structure of the planar ones is based on the scheme proposed by Yu et al [27]. The ratio of the thickness of the elastic layer to the thickness of the piezoelectric layer is optimized to increase the sensitivity.…”

PMUT and CMUT Devices for Biomedical Applications: A Review

“…The downside of the sol-gel technique is the heating associated with it, which can cause cracks and delaminations. The cavity of the PMUT must be formed with either front-side etching [35], backside etching [36] or sacrificial layer release [37]. In the first technique, the cavity is defined by etching the front side of the membrane via a hole in the piezoelectric layer.…”