CMUTvsPZT

CMUT vs PZT TRANSDUCERS

Engin Dikici

[email protected]


PIEZOELECTRIC TRANSDUCER (PZT)

• What is piezoelectricity?

- Some materials can generate electric potential when they are under mechanical stress (If the material is not short circuited, voltage across the material is generated).

- Piezoelectric effect is reversible; electric potential generates stress. Direct & reverse piezoelectric effects are both observed.

- Some examples, natural crystals including quartz (SiO2), several types of ceramics including barium titanate (BaTiO3), and natural ones including bone, silk…


A simple scheme for understanding the piezoelectricity

Refs: [6]

PIEZOELECTRIC TRANSDUCER (PZT)

• What does PZT stand for and, why do we use ceramic in PZTs?

- Piezoelectricity is used for converting electrical signals into mechanical vibrations (transmit mode) and, mechanical vibrations into electrical signals (receive mode) .

- As an active element, today most of the PZT transducers use piezoelectric ceramic. (1) They can be cut into different shapes to create different wave modes, (2) they can be used for low voltages and relatively higher temperatures. Also, piezo-polymers are used in some latest tools too…


A simplified schematic of a PZT

Refs: [6]

CAPACITOR MICROMACHINED ULTRASOUND TRANSDUCER (CMUT) CMUT vs PZT TRANSDUCERS

• What is CMUT transducer?

- CMUTs have emerged as an alternative to conventional piezoelectric transducers in 90s.

- A CMUT is composed of a thin membrane supported by sidewalls over a thin cavity. Typically, many membranes are connected in parallel to form a single element. These membranes are coated with a layer of metal, and form capacitors with the back plate. The membranes are biased with a DC voltage, and vibrate when an AC voltage is applied, thus generating ultrasonic waves. These capacitors are also used to sense impinging ultrasonic waves. If the electric field across the cavity of the CMUT reaches the magnitude of about 108 V/m, the CMUT becomes an efficient electromechanical transduction mechanism.

A simplified schematic of a CMUT

1D CMUT array

Refs: [3]


PROS and CONS OF PZT AND CMUT

• Fractional Bandwidth

- Using piezoelectric devices in medical applications is problematic due to the impedance mismatch between the piezoelectric materials and media; for example, the characteristic impedance of piezoelectric ceramic is (~33 MRayl) is significantly higher than that of various tissues like blood and fat (~1.6 MRayl). For PZT, it makes the matching layer difficult to identify, and coupling efficiency poor.

- CMUTs offer better acoustic matching to the propagation medium, resulting in broader fractional bandwidth, therefore an improved axial image resolution.

- Due to high axial resolution, CMUTs might be more applicable for the imaging of microcalcifications, small thyroid abnormalities, and vessels.

Short axis view of the carotd artery and thyroid gland from PZT array (left) and CMUT array (right)

Refs: [1], [4]

Images:[8]

PROS and CONS OF PZT AND CMUTCMUT vs PZT TRANSDUCERS

• Transduction Efficiency

- For transducer design, one of the most important concerns is to image deep-laying structures. Therefore, improved transduction efficiency, boosting both output pressure and reception sensitivity is required.

- For CMUTs, increased imaging depth (higher transduction efficiency) can be achieved by increasing the intensity of electrical field in the CMUT vacuum cavities (high bias voltage).

- The dielectric layers in a CMUT can trap surface and interface charges. This trapped charges lead to major reliability issues; creates voltage drifts. Two major sources of the trapped charges are the fabrication process and the strong electrical field within the transducer cavities.

- This reliability issue limits the sensitivity of CMUTs (-10dB less compared to PZT); they can generate shallower penetration.

- Different design schemes are developed currently for CMUTs to overcome this problem (generally complicating the fabrication process).

Refs: [2]


• Transduction Efficiency

- This reliability issue limits the sensitivity of CMUTs (-10dB less compared to PZT); they can generate shallower penetration.

Short and long axis view of the carotid artery and thyroid gland from PZT array (left) and CMUT array (right)

Images:[8]


• Manufacturing

- Conventional PZT arrays are fabricated by using ”dice and fill” approach; a plate of piezoelectric material is seperated by mechanical dicing and a polymer is infiltrated and cured within the kerfs.

• Element-to-element pitch must be inferior to half the wavelength (remember the grating lobes). The current limit at element width is 100 mu, and 50 mu for kerf; complicated manufacturing.

• Require ceramic manufacturing processes that are different from the processing technologies used to manufacture other parts of the US system. This leads to difficulty in integrating front-end electronics; limiting the performance and SNR.

• Difficult to integrate piezoelectric transducers of different resonant frequencies on a single chip.

- The fabrication process of CMUTs is based on standard silicon micromachining tehniques (or semiconductor fabrication processes). Micromachining is the formation of microscopic structures using a combination or subset of (1) Patterning tools, (2) Deposition tools, (3) Etching tools.

• Taking the advantage of well established micromachining technology for more flexibility in the design of complex 1D and 2D arrays.

• The ability to be integrated with the front-end electronic circuits on the same wafer.

• Lower production cost due to batch production.

• Size reduction: Different from PZTs, which require a film up to more than a millimeter thick (for low frequency application), the CMUTs are made of thin films which are typically of a thickness ranging from from submicrometer to a few micrometers.

Refs: [1], [3], [5]



Piezoelectric composite structure

• Manufacturing

The geometry of a 1-D CMUT array

Images: left [7], right [3]


• Manufacturing


(a) SEM photograph of a 1-D CMUT array. (b) Close-up of the ultrasonic transducer element. (c) Photograph of the probe-shaped CMUT array afterbeing bonded on a printed circuit board.

Images:[5]


• Acoustic Crosstalk

- When the ultrasonic beam is steered to off-axis direction, various elements operate at a different phases. This might lead to acousting crosstalk between transducer elements due to acousting coupling through the fluid medium (silicon substrate) outside the array.

- CMUT transducers have relatively high crosstalk because the membrane vibration may propagate acoustically to neighboring elements through the membrane-fluid interface, and through the low-acoustic-loss silicon substrate. Also, unlike PZTs, CMUTs are lack of isolation cuts between the elements.

- More technically, (1) Stoneley Waves propagating along the interface with the fluid, and the (2) Lamb wave energy propagating in the silicon substrate, can propagate to neighbouring CMUT cells...

Crosstalk in a 3-element CMUT device

Images:[10]

Refs: [9], [10]

Pressure output on the driven and adjacent element


• Concluding Remarks- CMUTs generally provide a wider relative bandwidth than PZT transducers. Therefore CMUTs offer better axial resolution making them favorable for the imaging of relatively smaller targets.

- Currently PZTs have a better transducer efficiency, therefore a higher sensitivity. For CMUTs further improvements in sensitivity are needed to fully compete with piezoelectric arrays, especially in the applications where high depth of penetration is required.

- Acoustic crosstalk is a disadvantage for the CMUTs. There are many designs proposed, which include but not limited to the different spacing design of CMUT elements and introducing the isolation trenches between the CMUT elements. There is an ongoing research about this topic.

- Reliability of CMUTs is an important issue to resolve.

- CMUTs have great manufacturing advantages over PZTs due to the usage of micromachining techniques; flexible design, low cost, smaller size, unique wafer design of front-end electronics with the transducers.

Refs: [1], [4], [5], [8], [10]


[1] Alessandro Caronti, Giosuè Caliano, Riccardo Carotenuto, Alessandro Savoia, Massimo Pappalardo, Elena Cianci, Vittorio Foglietti: Capacitive micromachined ultrasonic transducer (CMUT) arrays for medical imaging. Microelectronics Journal (MJ) 37(8):770-777 (2006) [2] Huang Yongli, Zhuang Xuefeng, Haeggstrom Edward O, Ergun A Sanli, Cheng Ching-Hsiang, Khuri-Yakub Butrus T: Capacitive micromachined ultrasonic transducers (CMUTs) with isolation posts. Ultrasonics 2008;48(1):74-81[3] Zhuang, X. Wygant, I. O. Yeh, D. T. Nikoozadeh, A. Oralkan, O. Ergun, A. S. Cheng, C.-H. Huang, Y. Yaralioglu, G. G. Khuri-Yakub, B. T: Two-dimensional capacitive micromachined ultrasonic transducer (CMUT) arrays for a miniature integrated volumetric ultrasonic imaging system. SPIE Proceedings 2005; 5750: 37-46[4] Daft C, Wagner P, Bymaster B, Panda SA, Patel KA, Ladabaum IA: cMUTs and electronics for 2D and 3D imaging: monolithic integration, in-handle chip sets and system implications. Proc. IEEE Ultrason. Symp. 2005;1:463–474[5] J. Chen, X. Cheng, C.-C. Chen, P.-C. Li, J.-H. Liu, and Y.-T. Cheng: A capacitive micromachined ultrasonic transducer array for minimally invasive medical diagnosis. J. Microelectromech. Syst. 2008;17(3): 599–610[6] Web Source: Introduction to Ultrasonic Testing.http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/cc_ut_index.htm[7] Web Source: Piezoelectric Composites. http://www.strath.ac.uk/eee/research/cue/research/transducers/piezoelectriccomposites/[8] D.M. Mills, L.S. Smith: Real-time in-vivo imaging with capacitive micromachined ultrasound transducer (cMUT) linear arrays, in: Proceedings IEEE Ultrasonics Symposium, 2003; 568–571[9] S. Berg, A. Rønnekleiv: Reducing fluid coupled crosstalk between membranes in CMUT arrays by introducing a lossy top layer. IEEE Ultrasonics Symposium Proceedings 2006: 3-6[10] Zhou S, Hossack JA. Reducing inter-element acoustic crosstalk in capacitive micromachined ultrasound transducers. IEEE Trans. UFFC, 2007;54(6): 1217–1228

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CMUTvsPZT

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