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

Xiaoqing Wang,Yude Yu,Jin Ning

doi:10.3390/mi7120226

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.

Highlights

Ultrasonic transducers are widely used in medical imaging, gas-flow metering, industrial cleaning and non-destructive testing
The main mechanism of wafer direct bonding at low temperature is to activate the surfaces by creating more hydroxyl (OH) groups, which results in higher hydrophilicity
For the performance of capacitive micromachined ultrasonic transducers (CMUTs) made in the way we describe, the Q-factor is 187, which is higheFrorththaenptehrafot rpmraensecnetoefdCiMn U[2T1s]m(aQd-efaicntothr eowf a3y1.w5)e odres[c2r2ib] e(,Qth-feaQct-ofracotofr1i5s01)8. 7I,nwhadicdhitiisohni,ghtheer tdhiaffnertehnactepbreetsweneetendthine m[2e1a]s(uQr-efdacrteosronofan3t1f.5re)qoure[n2c2y] a(Qnd-fathcteotrhoeof r1e5t0ic)a. l Irnesaodndainttiofnre, qthueendciyffiesreonnclye babetowuet e1n%t.hTehmisevaasulureedisresmsoanlalenrttfhreaqnutehnactyofanthdetCheMtUheTosreatcihcaielvreedsoinna[n2t3f]r,ewquheicnhcyhaiss oandlyiffaebroenucte1%of

Summary

Introduction

Ultrasonic transducers are widely used in medical imaging, gas-flow metering, industrial cleaning and non-destructive testing. This method could bring pollution caused by the adhesion material Another approach to decrease the temperature is to employ low-temperature wafer direct bonding technology, which is enabled by using activation processes of the surfaces. Such a process consists of wet chemical activation or O2 plasma treatment prior to bonding [18,19]. With these surface activation techniques, less energy is required during the thermal annealing steps and they achieve good bonding quality at low temperatures. Both electrical and acoustic characteristics of the fabricated devices are measured and discussed

Contact Angle

Low-Temperature CMUT Fabrication Process

Conclusions