CAPACITIVE MICROMACHINED ULTRASONIC TRANSDUCERS (CMUT)

Meeting MAXIM-ACULAB – 7 March 2012 ACULAB Confidential

• ACULAB introduction

• Capacitive Micromachined Ultrasonic Transducers (CMUT):

- Motivation and applications

- Operating principles

• ACULAB Reverse Fabrication Process (RFP) technology:

-Microfabrication and packaging concepts

-Development of an ultrasound probe for medical diagnostic imaging

• Conclusions

Outline

Introduction

• Was founded in 1996 by Prof. Massimo Pappalardo at the Dipartimento di Ingegneria Elettronica of the Università degli Studi Roma Tre.

• The main research activity is in the field of Capacitive Micromachined Ultrasound Transducers (CMUT) for medical imaging.

• Has received, since year 2000, grants for more than 1.5M€ for research activity in the CMUT field, thanks to industrial partnerships (Esaote S.p.A.) and national (MIUR) and international (Eureka! and FP6) research programs.

• Has produced 10 international patents and more than 60 international publications in the specific field.

• The facility includes three offices, a small clean room, an assembly and characterization laboratory, a workshop and a computer room (globally 200 square meters).

ACULAB (Acousto-Electronics LAB)

Introduction

• In 2010, ACULAB obtained the admission to a 512 k€ grant from the Italian Ministry of University to set up a small company, a spin-off of Roma Tre University, with the industrial partnership of ESAOTE S.p.A., ST Microelectronics, Elemaster S.p.A. and other private investors that made available funds amounting to 1,6 M€.

• ACULAB researchers have developed 10 patents partly funded by ESAOTE.

• After 2 years of industrial development (2008-2010) in the field of cMUT medical imaging, ESAOTE (from 2010) decided to stop the collaboration and to abandon “Italian cMUTs”. (Hitachi ?)

• The start-up company was not established and funds were LOST

ACULAB INDUSTRIAL ACTIVITY

• CMUT for medical imaging system design: FEM modeling and simulation tools available;

• CMUT packaging and assembly processes and materials;

• Medical imaging ultrasound probe development;

• Analog electronic front-end circuits for CMUTs;

• Electrical and acoustic characterization of ultrasound transducers and probes.

Introduction

ACULAB EXPERTISE

CMUTs are electro-acoustic MEMS transducers operating at ultrasonic frequencies (MHz).

Capacitive Micromachined Ultrasonic Transducers (CMUT)

Advantages

• Performance: broader frequency response, higher reception sensitivity, higher thermal efficiency.

• Technology and materials: silicon microfabricated devices lead to higher suitability for electronic integration (monolithic or hybrid), making die- and wafer-level packaging processes adoptable; lead-free.

• Cost: lower especially for small-size devices for applications requiring large-scale

production quantities.

Capacitive Micromachined Ultrasonic Transducers (CMUT)

Medical ultrasound diagnostic imaging arrays:

• 2D large arrays (for 3D imaging)

• Very high frequency micro-arrays (for high resolution imaging)

• Portable devices

Fluid flow-meters for medical and industrial applications:

• Patient monitoring/telemedicine (e.g. wireless doppler sensor connected to smartphone)

• Drug delivery pumps

Agile multi-scale

beam forming

Tx Rx

object

Screen out

Ceramic PE UT(conventional)

Matching layer

PZT electrodes

cMUT

Membrane

(e.g., Si, SiNy, SiGe, ..)

electrodes

Gap excitation

PE layer (e.g., PZT)

Membrane

(e.g., Si, SiNy, SiGe, ..)

electrodespMUT

Medical

imaging

NDT

Sensing

......applications

Applications

Capacitive Micromachined Ultrasonic Transducers (CMUT)

Basic structure and operating principle Transducer array

cMUT vs Piezo technologies

MEMBRANE

CAVITY

SUBSTRATE

Sacrificial layer – based PECVD Silicon Nitride SixNy

Pros: •Low temperature •CMOS Compatible •Metals as electrodes Cons: •Limited thickness of films •Low membrane uniformity

Wafer bonding – based LPCVD Silicon Nitride SixNy

Pros: •Good membrane uniformity •Relaxed thickness constraints Cons: •High temperature •Not CMOS compatible •Doped silicon as electrode

CMUT microfabrication technology overview

Two main approaches generally used

• LPCVD SixNy low stress, thickness unlimited, uniform and reliable membrane structure;

• PECVD SixNy low stress used for passivation of electrodes;

• Low temperature process makes it possible to use aluminum as electrode;

• Absence of etch sacrificial holes on the emitting surface;

• Silicon is completely removed: custom backing and interconnection pads on the back side of the device:

- improved performance; - hybrid integration (MEMS/IC) without Trough- Silicon Via (TSV); - curved devices are feasible.

• Few masks and materials: low cost;

• PATENTED - US Patent 7790490 - Issued July 9, 2010.

ACULAB cMUT Reverse Fabrication Process (RFP) Takes the best from the two approaches!

ultrasound

CONCEPTS:

ACULAB Reverse Fabrication Process cMUTs: Process Flow

a) Si Wafer + LPCVD SixNy layer

b) Sputtering of Al-Ti-W top electrode

c) Growth of a passivation layer of DF-PECVD SixNy

d) Thermal evaporation of Cr sacrificial islands

e) Growth of passivation DF-PECVD SixNy

g) Passivation and etching holes opening using RIE

f) Sputtering of Al-Ti-W bottom electrode

h) Etching of Cr sacrificial layer (wet)

i) Growth of DF-PECVD SixNy for cavity sealing and definition of pads for electrical interconnection

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ACULAB Reverse Fabrication Process cMUTs: Device packaging

c) Casting of mechanical and acoustical backing material that acts as a glob-top encapsulant (and/or flip-chip underfill).

d) CMUT device release using die-level chemical etching of silicon substrate (HNA)

a) Diced CMUT chip fabricated on silicon using RFP: the interconnection pads are on the rear part of the MEMS device

b) Electrical interconnection of the cMUT to a PCB using wire-bonding (flip-chip bonding can be used to interconnect to an IC).

e) Application of a polymeric layer for protection and acoustic matching.

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DICING

BONDING

BACKING

RELEASE

PROTECTION

10 mm

ACULAB cMUTs

30 µm

ACULAB’s recent results:

• Development of a 192-element 12 MHz CMUT probe with embedded electronics (MAXIM);

• Medical Imaging system integration.

Presented at IEEE - IUS 2011: A. Savoia, G. Caliano, B. Mauti and M. Pappalardo “Performance Optimization of a High Frequency CMUT Probe for Medical Imaging”

Development of a CMUT probe for medical diagnostic imaging CMUT ultrasound probe-head

CMUT assembled probe 192-channel analog FE electronics

MAX4805 MAX4805A

“Industry's First High Voltage-Protected, Low-Noise Op Amp for In-Probe Use in Ultrasound Imaging Applications”

”Typical applications include high-impedance piezoelectric transducers (PZT) and capacitive micromachined ultrasonic transducers (CMUT) in-probe buffering and amplification”

http://www.maxim-ic.com

Ultra-Small (5mm x 5mm) 32-Pin TQFN Package

Integrated electronics development - Design by MAXIM ITALY Octal High-Voltage-Protected, Low-Power, Low-Noise Operational Amplifiers

•Extremely Low Power Dissipation 8mW/ch

•I/O Protection for TX Burst Up to ±100V

•44MHz -3dB Bandwidth (typ)

•Low Voltage Noise 2.2nV/sqrt(Hz)

•Low Current Noise 1.7pA/sqrt(Hz)

Comparison with a commercial state-of-the-art piezoelectric probe

In-vivo B-MODE Imaging Pulse-echo measurement

Time and frequency response Human carotid artery Commercial ultrasound scanner

ACULAB HF12 (cMUT)

ESAOTE LA435 (PZT)

Center frequency (-6dB) (% RSD)

10.8 MHz (1.5 %)

10.9 MHz (1.8 %)

Fractional bandwidth (-6dB) (% RSD)

103.9 % (2.1 %)

70.1 % (3.2 %)

Amplitude % RSD (@ center frequency)

7.1 % 11.0 %

Sensitivity @ 8.5 MHz -32.4 dB -41.7 dB

Sensitivity @ 10 MHz -31.4 dB -41.1 dB

Sensitivity @ 12 MHz -31.0 dB -41.2 dB

Comparison with a commercial state-of-the-art piezoelectric probe performed at GE Healthcare facility in Nice (France)

Full 192-element array characterization

Conclusions

• We have developed a low cost, high quality and extremely flexible cMUT fabrication process (Reverse Fabrication Process):

-Sacrificial release process with improved uniformity;

-Suitable for MEMS-IC hybrid integration using flip-chip bonding (pads on the rear part);

• We have developed a 12 MHz 192-element linear cMUT probe for vascular and small parts ultrasound imaging:

- High performance

- In-probe electronics;

- Characterization and imaging.