Edward L. Ginzton Lab, Stanford University, CA

www-kyg.stanford.edu

© M. Kupnik 21 May 2010

Mario Kupnik and Butrus T. Khuri-Yakub

2010 CMOS Emerging

Technology Workshop,

Whistler, Canada

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Outline

  Capacitive Micromachined Ultrasonic Transducers

o  Background, how it works, how it’s made, and for what it can be used.

o  What was done for CMOS integration so far.

  Latest 2D array fabrication process (THICK-BOX)

o  Research towards high-reliability CMUTS with high performance.

  Integration to CMOS via low-temperature bonding.

  “Substrate-less” CMUT fabrication – CMOS will help.

  Conclusions and Outlook

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The singing condensor – Historical Perspective

“After a month of careful study, during which both magnetostriction and piezoelectricity were considered and then rejected, Langevin decided that it would be safer to fall back on the “singing condenser”… (March 1915). Numerical estimates indicated that, if electric field strengths of the order of a million volts per centimeter (108 Volt per meter or 100 V per micron) could be maintained, electrostatic forces as large as a kilogram per square centimeter would (theoretically) come into play…”

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Capacitive Transducer – The basic idea is simple

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Capacitive Transducer – Why we need a DC bias voltage

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The first CMUT was an air transducer (Haller, Khuri-Yakub, 1994)

The complete transducer is micromachined, i.e. also the moving part (“membrane”).

Gold Nitride

Oxide

Si

1 venting channel per

cell

Frequency range: 1.8 MHz … 4.6 MHz

3 dB fractional bandwidth: 20 %

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CMUTs can be used for both airborne ultrasound and immersion

Equivalent Circuit Model:

Plat

e Im

peda

nce

(MR

ayl)

Frequency (MHz)

Zplate

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Microelectronics industry gave us all tools that we needed

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Using this technology, various devices were fabricated …

Circular cells Hexagons Tents

100 nm-thick plate 200 nm-thick plate 300 nm-thick plate Devices fabrication and photos by Prof. Arif Sanli Ergun.

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Monolithically integrated CMUTS so far

However, CMUTs fabricated with sacrificial release process suffer from several drawbacks, in particular when low temperature processes are used.

E.g.: Non-uniformities, low reproducibility, intrinsic stress, gap height limits due to roughness, etc.

Chip-bonding after CMUT is finished is a good approach, but there are still weak points present related to sacrificial release process.

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Let’s get the best out of the CMUT first before we think CMOS

Grow thermal oxidize on silicon wafer

Pattern oxide, this step defines cell diameter

Grow thin oxide at bottom of cavity

Perform fusion Bonding step to SOI wafer

Remove handle wafer and BOX layer

Huang, Ergun, Haeggstrom, Khuri-Yakub, Proc. of the MEMS Conference, pp.:522-525, 2003

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Wafer-bonded CMUTs and electronics integration so far

Steve Zhuang, et al, 2009Robert Wodnicki, et al, 2009

Elvis Lin, et al, 2009

Trench-frame 2D CMUT arraysolder bumped to IC

Trench-frame CMUTssolder bumped to interposer

solder bumped to IC

This way, many arrays can form a large imaging device

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However, this is problematic for thin-gap devices

Electrical breakdown and high parasitic capacitance

10 nm average displacement at 10 MHz in immersion translates

into 1 MPa acoustic pressure

Kupnik, Ergun, Huang, Khuri-Yakub, Proc. of the IEEE Ultras. Symp, pp.: 511-514, 2007

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This fabrication process solves the issue and features many advantages

Kupnik and Khuri-Yakub, “A direct wafer bonded 2-D CMUT array“, US patent pending, 2008.

SOI substrate Etch horizontal gap

Etch vertical trenches

Oxidize

Bonding to SOI Thin down substrate (optional)

Etch via hole to each cell Fill via hole with

conductive material

Prepare backside for trench etch for

CMOS integration step

Etch (define) elements of 2D array

Bond to CMOS IC or PCB & remove handle

Ground connection for plate at the edge of the array

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One of the main advantages is a high electrical breakdown voltage

CMOS IC

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Several options how to merge CMUT wafer and CMOS wafer

?

-  Under bump metallization for chip bonding (die or wafer level)

-  Metal layers for eutectic bonding (Au-Si, 365°C) [13], [14]

-  Metal layers for thermo compression bonding (Ti-Au, 300°C) [15]

-  Low temperature fusion bonding, e.g. [16] (Mitsubishi and AML sell such tools, Ziptronix)

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Low temperature fusion bonding is state-of-the-art:

For example: This fully-automated room-temperature bonding tool (8’’) from Mitsubishi Heavy Industries Ltd., Japan, features covalent bond strength at room temperature.

It uses ion beam surface activation technique under high vacuum condition.

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We also pursue a second approach – substrateless CMUT, for example:

Kupnik and Khuri-Yakub, “Monolithic integrated CMUTs fabricated by low-temperature wafer bonding“, US patent pending, 2008.

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Conclusions

  Wafer-bonded CMUTs and CMOS are compatible!

  Required to get the best from both worlds –

Wafer-bonded CMUTs monolithically integrated.

  For large and medium size 2D arrays, this approach will allow to develop the next generation medical imaging probes and therapeutic transducers (HIFU) at low cost without expensive and complex interposer solutions.

  At the moment we pursue research for direct monolithic CMUT integration on top of a CMOS-circuitry-containing substrate, i.e. CMOS wafer acts as substrate for a wafer-bonded CMUT fabrication process.

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References [1] P.-C. Eccardt, et al, ”Surface micromachined ultrasound transducer in CMOS technology," Proc. Ultrason. Symp., pp.: 959-962,

1996. [2] P.-C. Eccardt and K. Niederer, ”Micromachined ultrasound transducers with improved coupling factors from a CMOS compatible

process," Ultrasonics, vol. 38, pp.: 774-780, 2000. [3] R. A. Noble, et al, ”A cost-effective and manufacturable route to the fabrication of high-density 2D micromachined ultrasonic

transducer arrays and (CMOS) signal conditioning electronics on the same silicon substrate,” Proc. Ultrason. Symp., pp.: 941-945, 2001.

[4] R. A. Noble, et al, ”Low-temperature micromachined CMUTs with fully-integrated analogue front-end electronics," Proc. Ultrason. Symp., pp.: 1045-1050, 2002.

[5] C. Daft, et al, ”Microfabricated ultrasonic transducers monolithically integrated with high voltage electronics," Proc. Ultrason. Symp., pp.: 493-496, 2004.

[6] G. Gurun, et al, ”Front-end CMOS electronics for monolithic integration with CMUT arrays: circuit design and initial experimental results," Proc. Ultrason. Symp., pp.: 390-393, 2008.

[7] J. Zahorian, et al, ”Single chip CMUT arrays with integrated CMOS electronics: fabrication process development and experimental results," Proc. Ultrason. Symp., pp.: 386-389, 2008.

[8] M. Kupnik and B. T. Khuri-Yakub, "A direct wafer bonded 2-D CMUT array," US patent pending. [9] M. Kupnik and B. T. Khuri-Yakub, "High-temperature electrostatic transducer and fabrication method," US patent pend. [10] M. Kupnik, et al, ”CMUT fabrication based on a thick buried oxide layer," submitted to Proc. Ultrason. Symp., San Diego, 2010. [11] M. Kupnik and B. T. Khuri-Yakub, "Monolithic integrated CMUTs fabricated by low-temperature wafer bonding," US patent pend. [12] Y. Tsuji, et al, ”Low-temperature process for CMUT fabrication with wafer bonding technique," submitted to Proc. Ultrason. Symp.,

San Diego, 2010. [13] R. F. Wolffenbuttel and K. D. Wise, "Low-temperature silicon wafer-to-wafer bonding using gold at eutectic temperature," Sensors

and Actuators A, vol. 43,pp.: 223-229, 1994. [14] J. A. Dziuban, "Bonding in microsystem technology," Springer, 2006 [15] C. H. Tsau, et al, "Fabrication of waver-level thermocompression bonds," Journal of Microelectromechanical Systems,vol. 11, no.

6, pp.: 641-647, 2002. [16] T. Rogers and N. Aitken, "Low temperature bonding using in-situ radical activation," available online at http://www.aml.co.uk/

publications.htm. [17] Q.-Y. Tong, "Method of room temperature covalent bonding," US patent10440.099, 2004. [18] G. Kovacs, et al, "Bulk micromachining of silicon," Proc. of the IEEE, vol. 86, no. 8, 1998. [19] L. A. Donahue, et al, "Development in Si and SiO2 etching for MEMS-based optical applications,” Proc. of SPIE, vol. 5347, pp.:

44-53. [20] R. Wodnicki, et al, ”Multi-Row Linear CMUT Array Using CMUTs and Multiplexing Electronics," Proc. Ultrason. Symp., pp.:

2696-2699, 2009. [21] X. Zhuang, et al, ” Wafer-Bonded 2-D CMUT Arrays Incorporating Through-Wafer Trench-Isolated Interconnects with a

Supporting Frame," TUFFC, pp.: 182-192, 2009. [22] E. Lin, et al, ” PACKAGING OF LARGE AND LOW-PITCH SIZE 2D ULTRASONIC TRANSDUCER ARRAYS," MEMS conference, pp.:

508-511, 2010.

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Acknowledgements

Srikant Vaithilingam, Stanford University

Kazutoshi Torashima, Canon Inc.

Ira O. Wygant, National Semiconductor

Yukihide Tsuji, NEC Inc.

Michael Cernusca, AVL List GmbH

Kudlaty Katarzyna, AVL List GmbH

Steve Vargo, SPP Process Technology Systems, Inc.

This reserch was funded by following research partners:

(in alphabetical order)

AVL List GmbH, Graz, Austria

Canon Inc., Tokyo, Japan

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Thank you and for more information feel free to contact us

Visit: www-kyg.stanford.edu

Butrus (Pierre) T. Khuri-Yakub Professor

E. L. Ginzton Laboratory, room 11 Stanford University Stanford, CA 94305-4088 Office: +1-650-723-0718 khuri-yakub@stanford.edu

Mario Kupnik Senior Research Scientist

E. L. Ginzton Laboratory, room 49 Stanford University Stanford, CA 94305-4088 Office: +1-650-725-4942 kupnik@stanford.edu