A large-area flexible wireless power transmission sheet
using printed plastic MEMS switches and organic field-effect transistors
Tsuyoshi Sekitani1, Makoto Takamiya2, Yoshiaki Noguchi1, Shintaro Nakano1, Yusaku Kato1,Kazuki Hizu1, Hiroshi Kawaguchi3, Takayasu Sakurai3, and Takao Someya1*
1Quantum-Phase Electronics Center, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan
2 VLSI Design and Education Center, the University of Tokyo, Tokyo 153-8505, Japan
3Center for Collaborative Research, The University of Tokyo, Tokyo 153-8904, Japan*Email: someya@ap.t.u-tokyo.ac.jp, Phone: +81(Japan)-3-5841-6820, Fax: +81(Japan)-3-5841-6828

Abstract
We have successfully manufactured a large-area
power transmission sheet by using printing technologies.
The position of electronic objects on this sheet can becontactlessly sensed by electromagnetic coupling using
an organic transistor active matrix. Power is selectively
fed to the objects by an electromagnetic field using aplastic MEMS-switching matrix.

of 8 × 8 cells comprising position-sensing and powertransmission units. The position of electronic objects on
the sheet can be contactlessly sensed by electromagneticcoupling using an organic transistor active matrix. Then,power is selectively fed to the objects by anelectromagnetic field using a two-dimensional array ofcopper coils that are driven by a printed plastic
MEMS-switching matrix. Due to selective powertransmission, we achieved a coupling efficiency ofpower transmission of 62.3%, and a power of 29.3 W
was wirelessly received. The thickness and weight of theentire sheet are 1 mm and 50 g, respectively.

Introduction
Ubiquitous electronics or ambient intelligence isattracting attention because of its potential to open up a
new class of applications. This paper reports the first
implementation of a large-area wireless powertransmission system (Fig. 1) using organic transistors(1-9) and MEMS switches. The system realizes a
low-cost sheet-type wireless power source of more thanseveral watts. This is the first step toward building
infrastructure for ubiquitous electronics where multiple
electronic objects are scattered over desks, floors, walls,
and ceilings and need to be powered. These objects may
be mobile or located in the dark and therefore solar cells
cannot be used to power them. On the other hand, theperiodic replacement of the primary batteries could betedious since there may be too many objects. The
proposed wireless power transmission sheet may directly
drive electronic objects and/or charge a rechargeable
battery in the objects without a connector, thereby
providing an easy-to-use and reliable power source.
The wireless power transmission sheet has been
manufactured on a plastic film by using printing
technologies. The effective power transmission area is
21 × 21 cm2. The sheet contains a two-dimensional array1-4244-0439-8/06/$20.00 ©2006 IEEE

Device manufacturing process
The entire system comprising 8 × 8 cells is
manufactured by integrating the position-sensing andpower transmission sheets, as shown in Fig. 1. The
periodicity is 25.4 mm.
The contactless position-sensing sheet (Fig. 2)
comprises sheets of the position-sensing coil array andorganic FET active matrix.
An organic FET active matrix is fabricated on apolyimide film. Silver gate electrodes and polyimide
gate dielectric layers are patterned by using inkjet
printing. A pentacene channel layer and goldsource/drain electrodes are deposited in a vacuum. The
channel length and width are 13 Pm and 48 mm,respectively.
A position-sensing coil array is manufactured byscreen printing. The inner diameter of the copper coils is10 mm. Both the width and spacing of the copper lines
are 100 Pm. The number of turns is 38. The inductanceand resistance are 20 PH and 17 :, respectively.

The power transmission sheet (Fig. 3) comprises
sheets of the printed MEMS-switching matrix and powertransmission coil array.
A MEMS-switching matrix is formed by using inkjet
printing and screen printing. The electrodes for powertransmission and those for electrostatic attraction are
patterned on a 25-Pm-thick polyimide membrane.
The power transmission coil array comprises copper
coils with an inner diameter of 10 mm. Both the widthand spacing of the copper lines are 300 Pm. The numberof turns is 13. The inductance and resistance are 3 PH
and 1 :, respectively.

Device characteristics
The contactless position-sensing sheet (Fig. 4): The
pentacene transistors in DC characterization exhibit
mobility of 1 cm2/Vs and an on/off ratio of 105. A
voltage of ±10 V at a resonance frequency (2.95 MHz) isapplied to the position-sensing cells. The on/off ratio of
the transistors at 2.95 MHz exceeds 500. When thedistance between the position-sensing coil and thereceiver coil reduces, the change in output voltage
increases and reaches 91%.
A stand-alone plastic MEMS switch (Fig. 5) ischaracterized. When 70 V is applied to the electrodes forelectrostatic attraction, the resistance changes from >106
: to 15 :and the frequency response extends up to 4
Hz. After 300,000 switching cycles, the change in theresistance of the MEMS switch is below 5%.
The power transmission sheet (Fig. 6): Wirelesspower transmission is performed at 13.56 MHz. The
on/off ratio of the MEMS switch in power transmission
exceeds 700. We achieve a coupling efficiency of 62.3%,
and a power of 29.3 W was wirelessly received. The
variation in the power efficiency is less than 5% among
the 8 devices on the same line.
Demonstration of power transmission: Figure 6 (f)shows the power transmission to a Christmas tree
decorated with twenty-one light-emitting diodes that
require a power of 2 W.

Acknowledgements
A part of this work is supported by JST/CREST and IT
program, MEXT.

References:(1) S. P. Lacour and S. Wagner, 2005 IEDM TechnicalDigest, pp. 109-112.
(2) S. K. Park, C.-C. Kuo, J. E. Anthony, and T. N.
Jackson, 2005 IEDM Technical Digest, pp. 113-116.
(3) S. E. Molesa, A. F. Vornbrock P.C. Chang, and V.
Subramanian, 2005 IEDM Technical Digest, pp.
117-120.
(4) C. Lackner, H. Klauk, W. Gut, M. Halik, R. Spilka,U. Zschieschang, A. Tanda, F. Eder, D. Rohde, T.
Ostermann, and G. Schmid, 2005 IEDM TechnicalDigest, pp. 121-124.
(5) T. Someya, T. Sekitani, S. Iba, Y. Kato, H.
Kawaguchi, and T. Sakurai, Proceedings of theNational Academy of Sciences of the United Statesof America, 101, pp. 9966-9970 (2004).
(6) T. Someya, Y. Kato, T. Sekitani, S. Iba, Y. Noguchi, Y.
Murase, H. Kawaguchi, and T. Sakurai, Proceedingsof the National Academy of Sciences of the UnitedStates of America, 102, pp. 12321-12325 (2005).
(7) T. Someya, Y. Kato, S. Iba, T. Sekitani, H.
Kawaguchi, and T. Sakurai, IEEE Transactions on
Electron Devices, 52, pp. 2502-2511 (2005).
(8) H. Kawaguchi, T. Someya, T. Sekitani, and T.
Sakurai, IEEE Journal of Solid-State Circuits, 40,
pp.177- 185 (2005).
(9) H. Kawaguchi, S. Iba, Y. Kato, T. Sekitani, T.
Someya, and T. Sakurai, IEEE SENSORSJOURNAL, 6, pp. 1209-1217 (2006).

(a)

(b)
Power transmission
coil sheet

Power transmission
systemMEMS switching
sheet

Position-sensing
coil sheet

Position-sensing
systemOrganic FET
sheet

Ubiquitous

Sheet electronics embedded in the floor
10 cmFig. 1: Wireless power transmission sheet. (a) An image of the world’s first large-area wireless power transmission sheet embedded in the floor.
A part of the cover layer is peeling off. (b) A picture of the device assembly comprising a power transmission system driven by an 8 u 8 MEMSswitching matrix and a position-sensing system driven by an 8 u 8 organic FET active matrix. The power transmission area is 21 u 21 cm2.

(a) Position-sensing coil sheet

(c)
Cu

Via

PolyimideCu (BL)
Output

10 mmBLCu

Sensing
coilAg-paste(b) Organic FET sheet
Ag1 mmGParylene
Epoxy Au (D)SD

Pentacene
Polyimide
Ag (G)ViaS

Polyimide
Ag (WL)Ag
Pentacene

FETAu (S)SS

AC generator : 2.95 MHz

Input

Fig. 2: The contactless position-sensing sheet comprising (a) position-sensing coil sheet and (b) organic FET sheet. Pictures and crosssectional illustrations are shown. S, D, and G represent the electrodes for source, drain, and gate, respectively. The organic FET sheet islaminated with the position-sensing coil sheet using silver paste islands. (c) The circuit diagram of the position-sensing sheet. A resonancefrequency of 2.95 MHz is used in the position-sensing system. WL and BL represent word-line and bit-line, respectively.

(c)(a) Power transmission coil sheet
Cu

Via

Polyimide
Cu

Cu

10 mm

Ag-paste(b) MEMS switching sheet
BLAg
Polyimide
Ag (to BL)

AAg

Via

B

WL
5 mm

CParylene

Transmission
coil

Polyimide

MEMS switch

Ag
Ag (to WL)Power generator : 13.56 MHz

Polyimide
InputAgFig. 3: The power transmission sheet comprising (a) power transmission coil sheet and (b) MEMS switching sheet. Pictures and crosssectional illustrations are shown. A: Electrodes for electrostatic attraction connected to the word-line (WL) and bit-line (BL) of MEMS switch.
B and C: Electrodes for power transmission connected to coils and a power generator. (c) The circuit diagram of the power transmission sheet.
A frequency of 13.56 MHz is used in the power transmission system.

(a)

(b)

10 M:

(c)

LS

VGS= –60 V

d=fVS

d=f0

VS (V)

VGS= –60 V
VGS = 0 V

50d = 1 mm

4LR3d2-4-5d=f645VS (V)78VGS10LRVS (V)~dFET

1-10On/off ratio = 510012Time (Ps)

91% drop-off-830

12Time (Ps)

03LS051015d (mm)2025Fig. 4: Position sensing. (a) Detected voltages (VS) at gate voltage (VGS) = –60 V and 0 V, where vertical distance d between the positionsensing coil (LS) and the receiver coil (LR) is infinite f  A voltage of r 10 V at a resonance frequency (2.95 MHz) is applied to theposition-sensing cells. (b) VS at d = f and 1 mm, where VGS of –60 V is applied. VS decreases as LR approaches because of electromagneticcoupling. (c) VS is shown as a function of d. A change in VS of 91% is attained at d = 1 mm. The dashed line represents the voltage at d = f

(a)

(b)

MEMS switch(c)

MEMSswitch

OFF

Vop R

100

Vop (V)
Voltage(V)

80
80

R (:) (:)
Resistance

ON

40
40
20

R (:)

Ag-bump

80

6000
6106
>10~~
80

In air6040

In nitrogen802040㪋㪇
4000

50 wm

00

55

1015
1015Time(seconds)
Time(s)0456101010103
The number of switching cycles

2020Fig. 5: Stand-alone plastic MEMS switch. (a) Micrograph of cross-sectional surface of MEMS switch. Top electrodes for powertransmission are connected to bottom electrodes when the operation voltage (Vop) is applied to electrodes for electrostatic attraction. (b)When a rectangular wave of Vop = 70 V is applied to the MEMS switch, the resistance changes from >106 :ҏ to 15 : ҏand the frequency
response extends up to 4 Hz. (c) MEMS switch resistance is shown as a function of the number of switching cycles.

(b)
B

Receiver coil: LR

116 mW

Electronicproducts
Power (mW)80

Power transmission coil: LTMEMS switch(c)Sending power: 198 mW

120

MEMS ON(Vop= 70 V)

4000.15 mW
0.1

MEMS OFF(Vop= 0 V)

010

Power generator : 13.56 MHz(e)60LR

Efficiency (%)

50
4030

LT

20

70

62.3%

60
50
4029.3 W

30
201000102030
40
Sending power (W)50

LR

Twenty-one LEDs
2W

40

LT30

X

20
1010080(f)

50

Z

2060

Efficiency (%)(d)

12141618
Frequency (MHz)In shielded boxReceived power (W) Efficiency (%)(a)

02468Z (mm)1012002468X (mm)1012Fig. 6: Power transmission. (a) Schematic illustration of the power transmission system. (b) Power at the receiver coil (LR) is shown withthe MEMS switch operation voltage (Vop) of 70 V and 0 V. Sending power of 198 mW and frequency of 13.56 MHz are applied from thepower transmission coil (LT). (c) The power efficiency and received power are shown as functions of the sending power. The dashed line
represents the power at which the MEMS switch is broken. (d) and (e) The power efficiency is shown as a function of vertical distance Zand horizontal distance X between LT and LR. (f) Demonstration of power transmission to a Christmas tree decorated with twenty-one lightemitting diodes (LEDs) that require a power of 2 W. The space between the power transmission coil and the receiver coil is 5 mm.