Fabrication and Characterization of Capacitive Micromachined Ultrasonic Transducers with Low-Temperature Wafer Direct Bonding

Abstract

This paper presents a fabrication method of capacitive micromachined ultrasonic transducers (CMUTs) by wafer direct bonding, which utilizes both the wet chemical and O2 plasma activation processes to decrease the bonding temperature to 400 °C. Two key surface properties, the contact angle and surface roughness, are studied in relation to the activation processes, respectively. By optimizing the surface activation parameters, a surface roughness of 0.274 nm and a contact angle of 0° are achieved. The infrared images and static deflection of devices are assessed to prove the good bonding effect. CMUTs having silicon membranes with a radius of 60 μm and a thickness of 2 μm are fabricated. Device properties have been characterized by electrical and acoustic measurements to verify their functionality and thus to validate this low-temperature process. A resonant frequency of 2.06 MHz is obtained by the frequency response measurements. The electrical insertion loss and acoustic signal have been evaluated. This study demonstrates that the CMUT devices can be fabricated by low-temperature wafer direct bonding, which makes it possible to integrate them directly on top of integrated circuit (IC) substrates.

PMUT and CMUT Devices for Biomedical Applications: A Review

“…Conventional wafer bonding technology requires high temperatures (more than 1000 • C) to ensure good bonding quality [56], which constitutes another drawback as it prevents integration of CMUTs [54]; moreover, a high temperature could result in the degradation of thermally sensitive devices and residual stress in processed silicon wafers with different thermal expansion coefficients. However, methods for fabricating CMUTs with low-temperature wafer bonding at 400 • C have been developed by activating the surfaces by creating more hydroxyl (OH) groups, which results in higher hydrophlilicity [57]. Surface activation can be achieved by treating the SOI wafer and the substrate with oxygen plasma or wet chemical or both, however treating the pre-bonded wafers to chemicals containing various reagents to activate their surface, followed by O 2 plasma treatment helped to improve the hydrophilicity [57].…”

“…However, methods for fabricating CMUTs with low-temperature wafer bonding at 400 • C have been developed by activating the surfaces by creating more hydroxyl (OH) groups, which results in higher hydrophlilicity [57]. Surface activation can be achieved by treating the SOI wafer and the substrate with oxygen plasma or wet chemical or both, however treating the pre-bonded wafers to chemicals containing various reagents to activate their surface, followed by O 2 plasma treatment helped to improve the hydrophilicity [57]. Alternatively, also anodic bonding enables CMUT fabrication with low thermal budgets; furthermore, it can also improve the CMUT transparency, thus allowing coupling with optical measurement techniques [31].…”

PMMA-Based Wafer-Bonded Capacitive Micromachined Ultrasonic Transducer for Underwater Applications

“…Anodic bonding, metal bonding, adhesive polymers, and wafer direct bonding are the typical bonding methods used for CMUT fabrication [5,6,22,23,24,25,26]. Wafer bonding encounters yield issues due to the roughness of the wafers’ surface and contamination at the bonding surfaces [22].…”