Zuo-Guang. Ye Advanced Dielectric Piezoelectric and Ferroelectric Materials: Synthesis, Characterisation and Applications

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Handbook of dielectric, piezoelectric and ferroelectric materials530

17.12

Embedded capacitor in an LTCC structure (Randall

et al.

, 2003).

gap is greater than 60% of the gap center frequency, while the second gap is

25% of the center frequency (see Fig. 17.13). The interval between the two

gaps is larger than 2 GHz. The return loss of both gaps is as large as –40 dB.

This interesting multi-bandgap characteristic is very useful for diplexers,

multi-mode tunable dielectric antennas and resonators.

17.5.5 Applications as high-

dielectrics for

integrated circuit

With the trend of electronic devices development towards miniaturization,

the demand for high-K dielectrics in integrated circuits (IC) becomes more

and more imminent. The BZN dielectrics are very promising candidates for

being used as integrated dielectrics in IC, such as integrated capacitor, embedded

capacitors and gate insulator in thin film transistors, because of their high

dielectric constant and low dielectric loss.

Recently, J H Park et al. (2006) fabricated metal–insulator–metal (MIM)

thin film capacitors with Bi

1.5

ZnNb

1.5

(BZN) dielectric films. The BZN

thin films were deposited at room temperature by pulsed laser deposition

and annealed below 200 °C, which is compatible with the polymer-based

substrates. The BZN thin films although being amorphous still exhibit a high

capacitance density of 150nF/cm

and a low leakage current less than 1 µA/

at 5V. These results show that the MIM structures using amorphous

BZN thin films will be a promising candidate for the PCB-embedded capacitors.

1.5

ZnNb

1.5

(BZN) dielectric films were also successfully prepared

via a room temperature process by PLD on polyimide substrates by Kim et

Commercial LTCC or Dupont 951

Ag electrode

Bi-pyrochlore dielectric

Ag electrode

Commercial LTCC or Dupont 951

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Bismuth-based pyrochlore dielectric ceramics 531

al. for the use as gate insulators in pentacene organic thin-film transistor

(OTFT) and ZnO thin film transistor (YW Choi et al., 2005; ID Kim et al.,

2005). Figure 17.14 shows the schematic cross-section of the OTFTs. The

OTFT and ZnO TFT devices exhibited very low operating voltages (<2 V

and <4 V, respectively) due to the high capacitance of the BZN dielectric

(Fig. 17.15). The high optical transparency (>80% for wavelength >400

S21 Simulated

S11 Simulated

468101214161820

Frequency (GHz)

(a)

Parameters (dB)

–10

–20

–30

–40

–50

(b)

17.13

Transmission and reflection coefficients (a) of a honeycomb

sample and (b) of 2D EBGs made of BZN dielectrics.

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Handbook of dielectric, piezoelectric and ferroelectric materials532

nm), low-temperature processing and low operating voltage of the TFT devices

with integrated BZN dielectric offer a promising route for the development

of transparent and flexible electronics.

17.6 Summary and future trends

With the rapid development of wireless hand-held devices, the needs for

passive component devices working in the high-frequency to microwave

range with enhanced reliability, miniaturization and electrical performance

are increasing fast. The motivation for the technological innovation is clearly

reflected in the ratio of passive components count to active integrated circuits,

40:1 in some wireless devices (Randall et al., 2003). This component count

ratio was predicted to increase as functionality of hand-held devices increases.

LTCC technology provides a good solution for integrating the passive

components as a functional module for microwave applications. The electrically

CH CH

Parylene C

(a)

SD (Au)

Active

(pentacene)

Gate insulator

(BZN)

Gate (Cr)

Substrate

(polyimide)

Thin parylene

(~2 nm)

SD (Au)

Active

(pentacene)

Gate insulator

(BZN)

Gate (Cr)

Substrate

(polyimide)

(b)

17.14

Schematic cross-section of the OTFTs with the stacked

parylene/BZN, threshold voltage –2.2 V, (a), and BZN only, threshold

voltage 0.1 V (b), gate dielectric (YW Choi

et al.

, 2005).

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Bismuth-based pyrochlore dielectric ceramics 533

steerable devices demand new dielectrics with high tunability and low dielectric

loss. The BZN cubic pyrochlore thin films exhibit high tunability, up to 55%,

with low dielectric loss, making them promising candidates for microwave

tunable devices. However, the electric bias field required for tuning in the

BZN thin films are still very high (2.4 MV/cm for a tunability of 55%) that

obstructs further practical application. The future R&D on bismuth-based

pyrochlores thus would be driven to further satisfy the potential application

demands. More efforts could be made to improve the fundamental

understanding of the structure and properties relation specifically to the

17.15

Drain current versus drain-to-source voltage curves at various

gate-to-source voltages for OTFT with parylene/BZN stacked gate

dielectric (a) and BZN only gate dielectric (b). Drain current and root

drain current versus gate-to-source voltage curve of OTFTs with BZN

only and parylene/BZN stacked gate dielectrics (c). Threshold

voltages of OTFTs with surface treatment of BZN and Al

gate

dielectrics before and after deposition of parylene film (d) (YW Choi

et al.

, 2005).

Drain current (µA)

–3

–2

–1

0 –1–2–3 –4–5

Drain voltage (V)

(a)

0–1–2–3–4–5

Drain voltage (V)

(b)

42 0–2–4–6

Gate voltage (V)

(c)

Stacked parylene/BZN, 200 nm

W/L

=2000 m/50 µm

=–5V

=–4V

=–3V

=–2V

=–1V

=0.1V

Drain current (µA)

–1.2

–1.0

–0.8

–0.6

–0.4

–0.2

0.0

=–2 V

=–1 V

=–0 V

=2 V

BZN only, 20 nm

W/L

=2000 µm/50 µm

200 m,

=–0.2 V

W/L

=2000 µm/50

µm

Without

paryfene

With

paryiene

Vth=-0.1V

Vth=-2.2V

Drain current (A)

–5

–6

–7

–8

–9

–10

–11

–12

–13

0.0020

0.0015

0.0010

0.0005

0.0000

Drain current (A)

1/2

Threshold voltage (V)

–10

–20

–30

BZN

200 nm

150 nm

With

parylene

Without

parylene

Anneated BZN

As-deposited BZN

No treatment

Oxygen plasma

(HMDS) treatment

(d)

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Handbook of dielectric, piezoelectric and ferroelectric materials534

optical response (Zanetti et al., 2006), to explore new process routes (H

Wang et al., 2004a; Zanetti et al., 2004; Silva et al., 2005) to lower the cost

and improve the microwave dielectric properties, to investigate the co-firing

technology of bismuth-based dielectrics with Ag or Cu electrodes for LTCC

application, lowering the crystallization temperature of thin films (JG Chen

et al., 2005) and thick films to approach the semiconductor technology, and

to lower the bias field applied for tuning, so as to accelerate the applications

in microwave tunable devices.

17.7 Acknowledgements

The authors acknowledge the financial support by the National 973-project

of China under grant 2002CB613302, NSFC project of China under grant

50572085 and National 863-project of China under grant 2006AA03Z0429.

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