Dielectric Properties and Thermal Expansion of ...

Journal of Advanced Physics Vol. 1, pp. 48–53, 2012 (www.aspbs.com/jap)

Copyright © 2012 by American Scientific Publishers All rights reserved. Printed in the United States of America

Dielectric Properties and Thermal Expansion of ZrW2O8/Polyimide Hybrid Films Jun-Wei Zha1 , Jing Lv2 , Tao Zhou2 , Jin-Kai Yuan2 , and Zhi-Min Dang1, 2, ∗ 1

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Department of Polymer Science and Engineering, School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, China 2 State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

Zirconium tungstate (ZrW2 O8 has negative thermal expansion characteristics and was obtained through direct solid-state reaction by sintering at 1200 C for 24 h. Subsequently ZrW2 O8 /polyimide (ZWO/PI) hybrid films were prepared and their coefficient of thermal expansion (CTE) as well as dielectric properties was investigated. In the range of 0–35 wt% ZWO, the valueDelivered of the CTE the ZWO/PI byofIngenta to: hybrid films decreased with increasing loading in ZWO while their dielectric constant increased from 3.26 to 4.01, which remained low. Moreover, Guest User the dielectric constant of the ZWO/PI hybrid IP films containing 15 wt% ZWO exhibited a very small temperature : 166.111.120.71 dependence from −50 to 150 C. All these results suggest the ZWO/PI hybrid films can be used as packaging Sat, 15 Dec 2012 12:13:05 materials for electronic devices owing to their low CTE and low dielectric constant at wide frequency and broad temperature ranges. KEYWORDS: Polyimide, Thermal Expansion, Dielectric Properties, Zirconium Tungstate.

1. INTRODUCTION Ideal packaging materials for electronic devices should possess low thermal expansion, low dielectric constant and loss, good mechanical properties and thermal stability, and a high breakdown field.1 2 Within the working life of electronic components, components and packaging materials will be separated in thermal cycles under the effect of thermal expansion. Too large a difference in coefficient of thermal expansion (CTE) between components and packaging materials may bring about serious cracking or separation. Packaging materials are also required to have a CTE value close to the silicon material (35 × 10−6 –42 × 10−6 K−1 in order to obtain better thermal matching. The dielectric property of electronic packaging materials is another important factor affecting the speed of integrated circuit operation. A high dielectric constant leads to an increased integrated circuit signal transmission delay, while a high dielectric loss brings about serious distortion in signal in the transmission process. Therefore, the capacitance and the heating effect of integrated circuits decrease with decreasing dielectric constant of dielectric materials used in integrated circuits. The development of integrated ∗

Author to whom correspondence should be addressed. Email: [email protected] Received: 12 April 2012 Accepted: 10 May 2012

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J. Adv. Phys. 2012, Vol. 1, No. 1

circuits requires the use of electronic packaging materials of high performance. Polyimide (PI) might be a good candidate. It has good dielectric and mechanical properties, high temperature durability, and good electrical insulation in a wide range of temperature.3–5 However, in comparison to metal or ceramic materials, its CTE is much higher, about 35 × 10−5 to 8 × 10−5 K−1 . When PI films are used as electronic packaging materials, a high temperature may brings about a large thermal stress between PI and electronic components, leading to the crack of PI crack and peel off of the electronic components.6 7 Previous studies in the literature attempted to obtain low CTE upon synthesizing PI with special molecular structures. However, those PI generally strong rigidity and the bonding between PI and inorganic materials is often poor. Recent studies show that loading polyimide matrix with low-CTE fillers, such as ceramics particles and fibers, is an effective way to decrease the CTE of PI. For example, zirconium tungstate (ZrW2 O8 , denoted as ZWO, which can be prepared by a variety of methods,8–11 has been widely used owing to its large isotropic negative thermal expansion over a wide range of temperatures (0.3–1050 K).12–14 Its incorporation in a positive thermal expansion material can produce a composite with a desirable CTE value, such as ZWO/cement, ZWO/epoxy, and ZWO/polyester.15 When the ZWO loading is increased from 0 to 30 vol%, the CTE 2168-1996/2012/1/048/006

doi:10.1166/jap.2012.1011

Zha et al.

Dielectric Properties and Thermal Expansion of ZrW2 O8 /Polyimide Hybrid Films

is reduced from 54 × 10−6 to 18 × 10−6 K−1 for epoxymatrix composites, and from 94 × 10−6 to 56 × 10−6 K−1 for polyester-matrix composites. PI has very interesting dielectric properties: its dielectric constant and loss are about 3.4 and 0.001, respectively. On the other hand, ZWO has a dielectric constant of about 10. This is relatively small for metal or ceramic fillers. The dielectric constant of ZWO/PI composites should be relatively small too. It is therefore expected that ZWO/PI composites possess both low CTE and low dielectric constant. This paper reports on the synthesis of ZWO particles and preparation of ZWO/PI hybrid films, on the one hand, and the morphology, thermal expansion and dielectric properties of the hybrid films with various weight fractions of ZWO from 0 to 35%, on the other hand.

2.4. Measurements

2.3. Preparation of ZWO/PI Hybrid Films 2 g of ODA, 30 ml of DMAc and a prescribed amount of ZWO particles were placed in a flask and were stirred for 30 min. Then 2.18 g of PMDA was added to flask in four parts. The mixture was stirred mechanically for 12 h to form a ZWO-polyamic-acid mixture. The resulting homogeneous mixture was cast onto a glass plate to form ZWO-polyamic-acid hybrids. Finally ZWO/PI hybrid films with ZWO loading from 0 to 35 wt% were obtained by curing the ZWO-polyamic-acid hybrid films at five different temperature ranges: 80, 120, 180, 240, and 300 C. They were about 40 micron in thickness. J. Adv. Phys., 1, 48–53, 2012

Intensity (counts)

10000

ZWO

8000 6000 4000 2000 0 10

20

30

40

50

60

70

Theta (deg) Fig. 1. XRD pattern of the product obtained by direct sintering between WO3 and ZrO2 powders at 1200 C.

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A thermomechenical analyzer of type Q400, America, was used to measure the CTE of the ZWO/PI hybrid films at 300 C with a heating rate of 10 C/min and under nitrogen. A scanning electron microscope of type Hitanchi S-4700, Japan, was used to characterize the morphologies of the surfaces and fractured cross-sections of the hybrid films. A Fourier transfer infrared spectrometer of type Nicolet 8700 Spectrometer, America, was used to characterize the pure PI and ZWO/PI hybrid films from 500 to 4000 cm−1 . An impedance analyzer of type Agilent 4294A, America, was used to measure the dielectric properties in a frequency range from 102 to 106 Hz and a temperature range from −50 to 150 C. The surface area of the samples was 1cm2 and the two surfaces were covered with silver electrodes. A thermo-gravimetric analyzer of 2. EXPERIMENTAL DETAILS type TA Instrument Hi-Res TGA 2950 was used to assess thermal films under Delivered by the Ingenta to:stability of the ZWO/PI hybrid 2.1. Materials nitrogen and with a heating rate of 10 C/min to 600 C. Guest User Pyromellitic dianhydride (PMDA) and 4,4 -oxydianilline IP : 166.111.120.71 (ODA) were purchased from Sinopharm Chemical Sat, Reagent 15 Dec 2012 12:13:05 AND DISCUSSION 3. RESULTS Co., China. Prior to use, PMDA was dried in a vacuum oven at 160 C for 4 h while ODA in a vacuum oven 3.1. Analysis of ZWO Powders at 120 C for 4 h. N,N -dimethylacetamide (DMAc) was Figure 1 shows the XRD pattern of the powder prodobtained from Beijing Yili Fine Chemicals Co., China. uct resulting from the reaction by sintering between Tungsten trioxide (WO3 and zirconium dioxide (ZrO2 : WO3 and ZrO2 powders. It corresponds to that of ZWO 99% purity, average diameter 40 nm) were purchased from by comparison with its standard powder diffraction file. Ouhe Technology Co., China. Figure 2 shows the SEM micrographs of the ZWO particles obtained from direct sintering. The ZWO particles are 2.2. Preparation of ZWO Microparticles uniform in size and their average particle size is approximately 5 m. These results show that sintering between The WO3 and ZrO2 powders were milled for 4 h to form WO3 and ZrO2 powders is simple method to prepare unia uniform mixture with a stoichiometric ratio of 2:1. The form and pure ZWO particles. WO3 -ZrO2 mixture was compressed in a cylindrical mold with a pressure of 20 MPa to form a thin cylinder. The latter was put in a sealed quartz tube in order to prevent 3.2. Structure of the ZTO/PI Hybrid Films WO3 from volatizing at high temperature. It was then sinThe SEM morphologies of the surfaces (Fig. 3(a)) and tered to 1200 C with a heating rate of 5 C/min. 24 h fractured cross-sections (Fig. 3(b)) of ZWO/PI hybrid later, the product was moved quickly from the furnace to the open air. It was grinded in powders of about 5 m in diameter.


Zha et al.

Transmittance (%)

pure PI

10 wt% ZWO/PI film

20 wt% ZWO/PI film

1000

1500

2000

2500

3000

3500

Wavenumber (cm–1) Fig. 4. FTIR spectrum of the pure PI and ZWO/PI hybrid films. SEM morphology of the ZWO particles.

indicates that the imidization of PAA to PI was complete. films with the mass fraction of 15% are shown. The ZWO In other words, in both cases after the imidization the particles were homogeneously dispersed in the PI matrix polyamic acid mixtures were fully cured, forming the polyDelivered to: without serious aggregation, and the blurry interfaces sup- by Ingenta imide films. port strong interaction between the ZWO particles andGuest the User IP : 166.111.120.71 PI matrix. 3.3.12:13:05 Thermal Expansion of the ZWO/PI Hybrid Films Figure 4 shows the FTIR spectra of the Sat, pure15 PI Dec and 2012 ZWO/PI hybrid films. In both cases, the absorption bands The change in length of the PI hybrid films as a function of at 1778, 1720 and 720 cm−1 are characteristic of the imide temperature was used as a measure of their thermal expangroup. The fact that there is no absorption at 1650 cm−1 sion. The temperature ranged from 25 to 300 C and the heating rate was 10 C/min. From Figure 5(a), the magnitude of increase in the film length, dL, increased with (a) (a) 300 0 wt% ZWO 5 wt% ZWO 20 wt% ZWO 35 wt% ZWO

250 200

dL/um

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Fig. 2.

150 100 50 0 50

100

150

200

250

300

Temperature (ºC)

(b) (b)

Fig. 3. SEM morphologies of the surface (a) and fractured cross-section (b) of the ZWO/PI hybrid film with 15 wt% ZWO loading.

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Fig. 5. Thermal expansion behavior of the pure PI and ZWO/PI hybrid films with different mass percentages of ZWO. J. Adv. Phys., 1, 48–53, 2012

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increasing temperature. Figure 5(b) is a zoom for the temperature range of 40 to 120 C. It is seen that the slopes of the 20 wt% and 35 wt% ZWO/PI films are smaller than that of the pure PI film, while that of the 5 wt% ZWO/PI film is very close to that of the pure PI because of the low ZWO loading. The thermal expansion behavior between 50 to 200 C is used to determine the CTE because the thermal expansion of dielectric materials is often severe in this temperature range. The CTE is calculated in the following manner:

(a) 4.2

Dielectric constant

4.0 3.8 3.6 3.4 3.2

L 1 L − L1 1 = CT E = 2 T 2 − T 1 L0 T L0

0

5

10

15

20

25

30

35

40

25

30

35

40

ZWO (wt%)

Dielectric Loss

(b) 0.010 where T1 and T2 are the higher and lower temperatures, respectively; and L1 and L2 are the lengths of the film at 0.008 T1 and T2 , respectively. Figure 6 shows that the CTE of the ZWO/PI hybrid films decreases with increasing ZWO 0.006 loading. It is mainly because ZWO has a negative CTE. Delivered by Ingenta to: When the mass percentage of ZWO reaches 35 wt%, the Guest User 0.004 CTE value of the ZWO/PI hybrid film is reduced by 11% IP : 166.111.120.71 compared to the pure PI, namely 30 × 10−6 K−1 0.002

3.4. Dielectric Properties of the ZWO/PI Hybrid Films

0.000 0

Figure 7(a) shows the dependence of the dielectric constant of the ZWO/PI hybrid films on the mass percentage of the ZWO at room temperature and 103 Hz. It increases moderately with increasing ZWO loading because the dielectric constant of the latter is only slightly higher than that of the pure PI. When the ZWO load reaches 35 wt%, the dielectric constant of the ZWO/PI hybrid film is merely 4.0. From Figure 7(b), the dielectric loss of the ZWO/PI hybrid film increases with increasing ZWO load. Nevertheless, it remains small and is still less than 0.004 with 35 wt%. Figure 8 shows that the dielectric constant of the ZWO/PI hybrid film at room temperature remains almost

5

10

15

20

ZWO (wt%) Fig. 7. Dependence of the dielectric constant (a) and dielectric loss (b) of the ZWO/PI hybrid films on the mass percentage of the ZWO at room temperature and 103 Hz.

constant over a relatively wide frequency range. This is true for a ZWO load range from 0 to 35 wt%. The effect of temperature on the dielectric properties can be double. First, as temperature increases, the activity of polymer chain segments increases, leading to an increase in the dielectric constant. Second, the thermal expansion of the PI matrix brings about an increase in distance among ZWO particles, damaging the network, leading to

35 4.2 34 4.0

32

ZWO dominant

PI dominant

31

Dielectric constant

CTE (10–6/K)

33

PI and ZWO common dominant

30

3.8 3.6 3.4 3.2

29 28

0% 1% 5% 8% 10% 15% 17% 25% 30% 35%

0

5

10

15

20

25

30

35

ZWO (wt%) Fig. 6. Dependence of CTE values of the ZWO/PI hybrid films on the mass concentration of ZWO fillers. J. Adv. Phys., 1, 48–53, 2012

3.0 102

103

104

105

106

Frequency (Hz) Fig. 8. Dependence of dielectric constant of the ZWO/PI hybrid films on frequencies, measured at room temperature.

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4.0

Table I.

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Thermal properties of the ZWO/PI hybrid films.

103

Dielectric Constant

3.8

104

Sample

Ti ( C)

105 106

pure PI 10 wt% ZWO/PI film 30 wt% ZWO/PI film

559.7 575.3 585.8

3.6

WL

( C)

581.1 593.4 604.8

3.4

3.2

3.0 –50

0

50

100

150

Temperature (ºC) Fig. 9. Temperature dependence of the dielectric constant of the ZWO/PI hybrid film with 15 % ZWO at different frequencies.

The initial decomposition temperature (Ti and the temperature of the ZWO/PI hybrid films at 10 % weight loss (T10% WL are higher than those of the pure PI. The presence of 30 wt% ZWO increased the Ti and T10% WL of the PI from 559.7 to 585.8 C and from 581.1 to 604.8 C, respectively. The increase in the thermal stability of the PI by the inclusion of ZWO characterized by the Ti and T10% WL could be explained by the interactions between the PI and the thermally stable ZWO particles, reducing the segmental movement of the PI chains.

a decrease in the dielectric constant. Figure 9Delivered shows the by Ingenta to: temperature dependence of the dielectric constant ofGuest the User 4. CONCLUSIONS ZWO/PI hybrid film with 15 wt% ZWO at different freIP : 166.111.120.71 quencies. Above room temperature, the dielectric Direct solid state reaction was successfully used to synSat, constant 15 Dec 2012 12:13:05 of the ZWO/PI hybrid film decreases slowly with increase thesize ZWO ceramic particles with a negative coefficient temperature slowly as a result of slight increase in the of thermal expansion (CTE). ZWO/PI hybrid films were activity of polymer chain segments. The dielectric constant then prepared by an imidization process. The ZWO/PI hybrid film with a ZWO load of 35 wt% of Z, the CTE also decreases marginally with increasing frequency. The was reduced by 11% with respect to that of the pure PI, change in the dielectric constant of the ZWO/PI hybrid namely, about 30 × 10−6 K−1 . Its dielectric constant was film is very small over a wide temperature range. As a merely increased to 4.0. Moreover, the dielectric constant matter of fact, it is about 3.1 at −50 C and 3.6 at 150 C. of the ZWO/PI hybrid film remained constant over large The low dielectric constant and loss together the weak ranges of frequency and temperature. The thermal stability frequency and temperature dependence of the dielectric of the ZWO/PI hybrid films was higher than that of the constant as well as low CTE make the ZWO/PI hybrid pure PI. In short, the low CTE as well as low dielectric film a potential new electronic packaging material. constant at wide frequency and broad temperature ranges make ZWO/PI hybrid films a very promising candidate as 3.5. Thermal Stability of the ZWO/PI Hybrid Films packaging materials for electronic devices. Figure 10 shows the TGA curves of the pure PI and the ZWO/PI hybrid films. Table I shows selected TGA results.

100

Weight Loss (%)

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T10%

90

80 C

70

60

50 300

A B C

pure PI 10 wt% ZWO/PI film 30 wt% ZWO/PI film 400

500

600

B

References and Notes

A

700

800

900

Temperature (ºC) Fig. 10. TGA curves of the ZWO/PI hybrid films in nitrogen at a heating rate of 10 C/min.

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Acknowledgments: This work was financially supported by NSF of China (No. 50977001, 51073015), State Key Laboratory of Power System (SKLD11KZ04), State Key Laboratory of Electrical Insulation and Power Equipment (EIPE12208, EIPE12207), the Ministry of Sciences and Technology of China through China-Europe International Incorporation Project (No. 2010DFA51490), China Postdoctoral Science Foundation funded project (grant No. KM201210012010), and the Fundamental Research Funds for the Central Universities.

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