A large-area, flexible, ultrasonic imaging systemwith a printed organic transistor active matrix
Yusaku Kato1, Tsuyoshi Sekitani1, Yoshiaki Noguchi1, Makoto Takamiya2,Takayasu Sakurai2, and Takao Someya1
1

Quantum-Phase Electronics Center, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan
2

Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan

Abstract:
We have successfully fabricated a large-area, flexible,
ultrasonic system by integrating a two-dimensional polymericultrasonic sensing array with a printed organic transistor activematrix. The new sheet-type device can offer a cost-effective
solution for a real-time three-dimensional imaging in freespace and/or a large-area proximity sensor for robot skins.

Robot
UltrasonicUltrasonic imaging sheet
Fig. 2: Position sensing in free space. The ultrasonic system can detectapproaching target objects without making contact with them.
Furthermore, it can detect objects behind a cloth and/or a paper.

This

configuration is suitable for obtaining a viewing angle of 360º for a
medium-length (several meters) proximity skin-like sensor.

Introduction

Fig. 1: Image of a printed organic transistor active matrix forultrasonic imaging sheet.

The sensing cells are formed by integrating aprinted organic transistor sheet and an ultrasonic transducer array sheet.
This system is mechanically flexible and can be wrapped around a bodyof robots.

1-4244-2377-4/08/$20.00 ©2008 IEEE

In the forthcoming ambient electronics era, multiple electronic
devices used in human surroundings are expected to interact
with people, objects, and the environment. In such asituation, the three-dimensional (3D) sensing of the position ofpeople and objects in free space will be very important.
Although position sensing in free space has commonly been
carried out using light, radio, and millimeter waves, ultrasonicimaging can offer complimentary attractive features such as asimple architecture and real-time nondestructive 3D imaging
in free space at an ultralow cost.
In this study, we have successfully fabricated a large-area,
flexible, ultrasonic 3D imaging system, for the first time, byintegrating an ultrasonic transducer array sheet with a printedorganic field-effect transistors (FETs) active matrix. The
printed transistors (µ ~ 0.5 cm2/Vs) with a grounded gate canswitch 40 kHz signals with an on/off ratio of greater than 104.
The system comprises 8 × 8 ultrasonic sensing cells or others,

97with a printed area of 25 × 25 cm2. 3D ultrasonic images canbe obtained for multiple target objects over this sheet. Thissystem can detect target objects behind a cloth and a paper.
It is mechanically flexible and can be wrapped around acylindrical bar, as shown in Fig. 1; this configuration issuitable for obtaining a viewing angle of 360º for a
medium-length (~ several meters) proximity robotic skin-likesensor.

Organic FET sheet
Ultrasonic transducer
array sheet

Device manufacturing process
The ultrasonic sensing cell comprises one organic transistor
and one ultrasonic transducer (Fig. 3). A transistor active
matrix sheet and an ultrasonic transducer array sheet (Fig. 4)
are manufactured separately and electrically connected to eachother. The circuit diagram is shown in Fig. 5.

5 cmBLFETWL
Contact pad
5 mm

A. Printed organic transistor active matrix
The organic transistors (1-3) are fabricated on a 75-µm-thickpolyimide film. The source, drain, and gate electrodes aredeposited by an inkjet printing or a vacuum evaporation. A
500-nm-thick polyimide gate dielectric layer is formed byinkjet or spin coating, and a 50-nm-thick pentacene channel
layer is deposited by vacuum evaporation by using a printed
shadow mask.

Ultrasonictransducer

5 mm

Fig. 4: Photographs of the ultrasonic imaging sheet. The dimensions of
the ultrasonic transducer are 5 mm × 10 mm (W × L); it is patterned usinga numerical controlled cutting system. In order to achieve a low channel
resistance, 16 transistors are connected in parallel.

WL0B. Ultrasonic transducer array sheet

WL1

A 28-µm-thick piezoelectric polyvinylidene fluoride (PVDF)

WL2Organic
FET
Ultrasonictransducer

Common electrode
NiCu
PVDF (28 µm)
NiCu
AdhesiveSpacerPolyimide (75 µm)

AgAg (WL)
Polyimide
Pentacene

WL7

Ag(BL)
BL0 BL1 BL2Parylene

BL7Fig. 3: Cross-sectional view of an ultrasonic sensing cell. An organic

Fig. 5: Circuit diagram. Each sensing cell has one ultrasonictransistor active matrix is laminated with an array of an ultrasonictransducer and one transistor. The sensing cells are spaced by intervals

transducer using a silver paste. The ultrasonic transducer is bended by theof 1 inch.

spacer to improve the sensitivity.

98

(b) 10-15

IDS (A)

01015
1

VGS = −40
-0.8 to 0 V-20-40
VDS (V)0.5
0-0.5
VDS (mV)

15(c)

Vout
VGS VGS = −40V

50-5

VGS = +20V
fin = 1 MHz-10W/L = 330

10-1230 20 10 0 -10 -20 -30 -40
VGS (V)-1Vin

1010-910-11-0.400VDS = −1 mV

10-10-1.25-710-8

IDS (mA)

IDS (nA)

VGS = −40 to 30 V-10W/L = 330-5Vout (mV)(a)-150246Time (μsec)

810Fig. 6: High-speed printed organic FETs. (a) Operation of the organic FETs with a small source-drain voltage (VDS). A plot of the source-drain current (IDS)vs. VDS. (b) IDS vs. gate-source voltage (VGS) of the organic transistors in a linear regime (VDS = –1mV). (c) 1 MHz operation of the organic FETs with agrounded-gate.

0.20-0.2

1~100 μsec

Vout

Vout (V)

Vout (V)

0.4

Reflection
from a cloth

0.50Object-0.5-1-0.4
200 400 600 800
Time (μsec)

Reflection
from an object
behind a cloth(c)
Vout (dB)

(b)

(a)

0-5-10-15-20-30

Angle (o)
030
60
90

Cloth

0.8 1.2 1.6 2Time (msec)

2.4

Fig. 7: Stand-alone ultrasonic transducer. (a) A typical waveform (Vout) received at the transducers. The signal is amplified by a 40-dB gain AC amplifier.
The frequency of the transmitting signal is 40 kHz. (b) Received waveform reflected from a stiff object behind a cloth. (c) Angular dependence of Vout. The
ultrasonic transducers can detect signals in an angle of 180º.

film (4), whose surfaces are covered by metal, is used as asheet-type ultrasonic transducer. The transducers are used in
the bending mode (d31 mode). To adjust the mechanicalresonance frequency to 40 kHz, the transducer is bended toimprove the sensitivity using a spacer sheet.
Device characteristics

B. Stand-alone ultrasonic transducer (Fig. 7)The ultrasonic transducer has an output voltage of ~0.8 Vp-p
and a narrow pulse width of ~100 µs after an amplification of
40 dB by the incidence of an ultrasonic wave. The ultrasonictransducers can detect a stiff object that is hidden inside acloth. It also shows broad directivity (>-5 dB for ± 60º).

A. High-speed printed organic FETs (Fig. 6)

C. Integrated imaging system (Fig. 8)

The mobility in the saturation regime is 0.5 cm2/Vs, while an
on/off ratio is 107 in DC measurements. The transistors
operate efficiently in the small signal regime of the ultrasonictransducers (~1 mVp-p). The transistors with the groundedgate can switch 40 kHz signals with an on/off ratio of greaterthan 104.

The characteristics of the integrated ultrasonic sensing cellcomprising one ultrasonic transducer and one organic FET areexamined. The signal from the ultrasonic transducer isswitched by the organic FET at 40 kHz. Crosstalk ischaracterized (Fig. 8c). The integrated sensing cells have anon/off ratio of 104 at 40 kHz.

99

Vout (V)

0.5(b)

VGS

0.4

Vout0.30.210-7>104
VGS = −100 V

10-90 -20 -40 -60
VGS (V)

10-130VGS = 0 V

50 100 150 200
Frequency (kHz)>104
VGS = −40 V

10-2
10-4
10-6

W/L = 330
20100 array

Vout

10-11

0.10(c) 102 1 x 8

VGS

10-5

10-8

0.5
0-0.5
0.5
0-0.5
0.2 0.4 0.6 0.8 1.0
Time (msec)

on

Vout (mV)Stand-alone

Power0.6

Power(a)off+20 V
106
Frequency (Hz)

104

105

Fig. 8: Ultrasonic sensing cells comprising organic FETs and ultrasonic transducers. (a) Switching characteristics of the ultrasonic sensing cell withchanging VGS of the organic transistor. (b) Power spectrum of the ultrasonic sensing cell for VGS = –100 V (solid line) and 0 V (dashed line). The on/off ratio
exceeds 104. (c) An ultrasonic sensing cell connected to a 1 × 8 ultrasonic sensing array also exhibits an on/off ratio exceeding 104.
z (mm)
300(a)(b) 2D image

ObjectUltrasonicwave

Ultrasonicimaging sheet(c) 3D image

250

200 mm
250 mmz (mm)265200Real view

150
100

Real view

210 mm-500Object
200050100x (mm)150Object 1Object 2200x (mm)400 0400200y (mm)Fig. 9: Ultrasonic imaging. (a) Experimental setup of ultrasonic imaging. A 3D image of the object can be reconstructed (synthetic aperture method) using
all the received signals. (b) 2D image obtained by a 1 × 8 ultrasonic sensing array comprising ultrasonic transducers and organic transistors. (c) 3D image
obtained by 17 × 11 stand-alone ultrasonic transducers.

D. Imaging (Fig. 9)(1)

T. Sekitani, Y. Noguchi, S. Nakano, K. Zaitsu, Y. Kato, M. Takamiya,T. Sakurai, and T. Someya, “Communication sheets using printed

We have successfully constructed clear 2D and 3D images by
the manufactured sheet devices. The imaging was exploredat 40 kHz that is suitable for free-space sensing; however, thesimilar system will be feasible for a ultrasonic medical
diagnostic (5) if on/off ratio of the transistors are optimized at
the carrier frequency of 2 MHz.

organic nonvolatile memories,” International Electron Devices Meeting(IEDM) Technical Digest, 9.3, pp. 221-224, December 2007.
(2)

S. K. Park, D. A. Mourey, I. Kim, D. Zhao, S. Subramanian, J. E.
Anthony, and T. N. Jackson, “F-TES ADT Organic Integrated Circuits
on Glass and Plastic Substrates,” International Electron Devices
Meeting (IEDM) Technical Digest, 9.4, pp.225-228, December 2007.

(3)

Acknowledgement

Q. Zhang and V. Subramanian, “Label-free low-cost disposal DNA
hybridization detection systems using organic TFTs,” International
Electron Devices Meeting (IEDM) Technical Digest, 9.5, pp.229-232,
December 2007.

This study was partially supported by the Grant-in-Aid for Scientific Research(Kakenhi, Wakate S), the Special Coordination Funds for Promoting and(4)A. S. Fiorillo, “Design and Characterization of a PVDF Ultrasonic
Range Sensors,” IEEE Trans. on Ultrasonics, Ferroelectrics, andTechnology, and The New Energy and Industrial Technology Development

Frequency Control, vol. 39, no. 6, pp. 688-692, November. 1992.

Organization (NEDO).
(5)

References

A. Austeng and S. Holm, “Sparse 2-D Arrays for 3-D phased array
imaging - Design methods,” IEEE Trans. on Ultrasonics, Ferroelectrics,and Frequency Control, vo. 49, no. 8, pp. 1073-1086, August. 2002.

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