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Reducing the Coefficient of Thermal Expansion of Polyimide Films in Microelectronics Processing Using ZnS Particles at Low Concentrations Hyungjoon Jeon, Cheolsang Yoon, Young-Gun Song, Junwon Han, Sujin Kwon, Seungwon Kim, Insu Chang, and Kangtaek Lee ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00259 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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ACS Applied Nano Materials

Reducing the Coefficient of Thermal Expansion of Polyimide Films in Microelectronics Processing Using ZnS Particles at Low Concentrations Hyungjoon Jeona‡, Cheolsang Yoona‡, Young-Geon Songa, Junwon Hanb, Sujin Kwonb, Seungwon Kimb, Insu Changc, and Kangtaek Leea,* a

Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722,

Republic of Korea b

Material Development P/J, Semiconductor R&D Center, Samsung Electronics, Hwaseong-si,

Gyeonggi-do 18448, Republic of Korea c

Radiation Safety Management Division, Korea Atomic Energy Research Institute, Yuseong-gu,

Daejeon 34057, Republic of Korea

KEYWORDS: polyimide film; coefficient of thermal expansion; ZnS particles; nanocomposite; microelectronics

ACS Paragon Plus Environment

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ABSTRACT

We report a reduction in the coefficient of thermal expansion (CTE) of polyimide (PI) film in microelectronics processing by using ZnS particles as nanofillers. To prevent agglomeration of ZnS

particles,

the

surfaces

of

ZnS

particles

were

modified

with

the

(3-

mercaptopropyl)trimethoxysilane (MPTMS), creating surface hydroxyl groups. For means of comparison, SiO2 and ZrW2O8 particles that have widely been studied as fillers for various polymer films were also synthesized. The CTE measurements showed that the ZnS particles produced PI nanocomposite film with a much lower CTE than either SiO2 or ZrW2O8 particles at the same concentration. In particular, the surface-modified ZnS particles showed the lowest CTE (13 ppm/K) at 15 wt%, which is comparable to the largest percentage decrease (70%) in CTE from the bare-PI film to date at a much lower particle concentration. To rationalize the significant reduction in CTE with the surface-modified ZnS particles, we considered the intrinsic CTE and thermal conductivity, thermoluminescence property, interfacial area, and dispersion state of ZnS particles, and found that the intrinsic thermal conductivity and dispersion state of ZnS particles were mainly responsible for the reduction in CTE at low particle concentration. Finally, we demonstrated that the optical and mechanical properties of the PI nanocomposite films containing surface-modified ZnS particles at 15 wt% were comparable to those of the barePI film.


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1. Introduction Since the introduction of polyimides (PIs) in the early 1960s, PIs, which have many promising characteristics such as excellent mechanical properties, high thermal stability, low dielectric constant, and easy processability, have been synthesized and used extensively in microelectronic devices, films, and adhesives.1-4 Recently, as demand for closer and faster circuit increases, the dimensional stability of PI film for thermal cycle has become an important issue for the realization of high-density assemblies. The key factor that dominates the dimensional stability of PI film is the linear coefficient of thermal expansion (CTE) which is a linear strain caused by the change in temperature. Discrepancy in the CTEs of two different materials can result in defects such as cracks, distortions, delamination, and warpage.5,6 Although it is required to closely match CTEs of the copper and the PI layer in advanced electronic devices, many PI systems are not applicable because their CTE values are much higher (40~70 ppm/K) than that of the copper (17 ppm/K). Therefore, there have been many attempts to reduce the CTE of the PI layer by preparing nanocomposites with various inorganic fillers.6-9 Among these fillers, silica particles have been widely used because of their low intrinsic CTE (~0.55 ppm/K), high thermal stability, optical transparency, and easy surface modification. For instance, loading 50 wt% silica particles as a filler could reduce the CTE of PI by 50%.6,7 Other metal oxide particles have also been used as fillers.10-13 Zinc oxide filler particles at 5 wt% were used to reduce the CTE of PI by 12%,10 and aluminum oxide particles ranging from 0 to 20 wt% reduced the CTE of PI from 37.3 to 31.6 ppm/K (~15%).11 Recently, ZrW2O8 particles have been studied as fillers because of their notably low intrinsic CTE (-9 ppm/K). and using the 60 vol% (84 wt%) zirconium tungstate


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(ZrW2O8) particles could reduce the CTE of PI by 65%.12,13 However, a high loading of inorganic fillers is usually required to achieve a significant reduction in the CTE of PI, which inevitably changes the basic properties of backbone polymers. In other polymers such as epoxy, more than 90 vol% of silica particles were also needed to decrease the CTE by 90%.14 Therefore, it is important to reduce the CTE of the PI layer at a low filler concentration without altering the other properties of the PI polymer for effective thermal management in microelectronic devices. Herein, we investigate ZnS particles as inorganic nanofillers to reduce the CTE of PI polymer films. ZnS particles were selected because they have much higher thermal conductivity (25.1 W/m·K) than other conventional filler materials and exhibit the thermoluminescence property which converts heat into light and may help dissipation of thermal stress. In addition, ZnS particles have relatively low intrinsic CTE value (6.36 ppm/K) that are comparable to other filler particles, and their sizes as well as surface properties can be easily controlled.15,16 We compared the ZnS particles with the widely-studied silica and ZrW2O8 particles, and demonstrated that a significant reduction in the CTE of PI polymer films can be achieved using ZnS particles at low concentration. These results were then explained based on the intrinsic CTE, thermal conductivity, thermoluminescence, interfacial area, and dispersion state of the ZnS particles.

2. Materials and Methods 2.1. Materials Zinc sulfate monohydrate (ZnSO4·H2O, 99.9%), sodium sulfide nonahydrate (Na2S·9H2O, 98%), tetraethylorthosilicate (TEOS), zirconium oxychloride octahydrate (ZrOCl2·8H2O, 99.5%), ammonium metatungstate hydrate ((NH4)6H2W12O40·xH2O, 99.99%), ammonium


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hydroxide

(NH4OH,

5

M),

ethylenediaminetetraacetic

acid

(EDTA,

99.95%),

3-

mercaptopropyltrimethoxysilane (MPTMS, 95%), 1-methyl-2-pyrrolidinone (NMP, 99.5%), 4,4’-oxydianilline (ODA, 97%), pyromellitic dianhydride (PMDA, 97%) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl, 35%) was purchased from Daejung Chemicals & Metals Co. and ethanol (99.9%) was purchased from Duksan Co. Deionized (DI) water from the Millipore water (18 MΩ·cm) purification system was used for all experiments. All the chemicals were used without further purification.

2.2. Preparation of inorganic particles ZnS particles were prepared by the chemical precipitation method.17,18 Briefly, 10 mL of 0.4 M ZnSO4 solution, 10 mL of 0.1 M EDTA solution, and 26 mL of additional DI water were mixed by magnetic stirring at room temperature. Then, 10 mL of 0.5 M Na2S solution was added dropwise under continuous stirring, which produced ZnS particles after 2 h. For surface modification, 15 µL of MPTMS was added to the suspension, which was allowed to react for 22 h at room temperature with magnetic stirring. The resulting surface-modified ZnS (ZnSMPTMS) particles were purified three times by centrifugation and redispersion in NMP using a rotary evaporator (Figure 1). Sub-micrometer silica particles were synthesized by the Stöber method.19 Briefly, 2.68 mL of TEOS, 8.4 mL of DI water, and 1.44 mL of NH4OH were mixed with 47.78 mL of ethanol, followed by magnetic stirring at room temperature for 24 h. The resulting silica particles were washed by several cycles of centrifugation and redispersion in NMP using a rotary evaporator. Zirconium tungstate particles were synthesized by the hydrothermal route.20,21 Typically, 806 mg of ZrOCl2·8H2O and 1325 mg of (NH4)6H2W12O40·xH2O were dissolved in 10 mL and 20 mL


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of DI water, respectively. Then, the two solutions were slowly added to 10 mL of DI water with magnetic stirring. After stirring for 30 min, 10 mL of 12 M HCl solution was added to the mixture. After 1 h, the resulting homogeneous solution was transferred into a Teflon-lined Parr bomb and heated at 180 °C for 24 h. The resulting white precipitates, ZrW2O7(OH)2·2H2O, were washed by several cycles of centrifugation at 8000 rpm and redispersion in DI water. Finally, ZrW2O8 particles were obtained by calcination of the ZrW2O7(OH)2·2H2O at 630 °C for 30 min and redispersion in NMP using a rotary evaporator.

Figure 1. Fabrication of the surface-modified ZnS/PI nanocomposite films

2.3. Preparation of PI nanocomposite films The most widely used procedure for PI synthesis is synthesis poly(amic acid), followed by thermal imidization.22,23 For the synthesis of poly(amic acid) for Kapton type PI, 0.01 mol of ODA, and 36.6 mL of NMP were mixed in a 100 mL three neck flask with magnetic stirring at room temperature. Immediately after dissolution, 0.01 mol of PDMA was added to the mixture, which was allowed to react for 24 h at room temperature with magnetic stirring. The resulting polymeric solution was degassed for 2 h to remove water generated during polymerization. To prepare PI nanocomposite films, the synthesized inorganic particles and poly(amic acid) were


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homogeneously mixed with magnetic stirring at room temperature. The mixture was cast on a silicon wafer to form a film by the drop casting or blade coating. The film was dried in ambient condition for 24 h, then cured in a heating oven at 100 °C for 1 h and 200 °C for 1 h, and finally in a furnace at 300 °C for 1 h to convert poly(amic acid) to polyimide (Figure 1).

2.4. Characterization The size and structure of the synthesized particles were determined by quasi-elastic light scattering (QELS, Malvern Nano-ZS), X-ray diffraction (XRD, Rigaku Miniflex), Fourier transform infrared spectroscopy (FT-IR, Perkin Elmer Spectrum Two), and transmission electron microscopy (TEM, JEOL JEM-2010 and JEM-F200). The thermal diffusivity measurements of nanocomposite films were performed using the light flash method (Netzsch LFA467) in which the specimens were in the form of discs with a diameter of 12.7 mm and a thickness of 0.03 mm. The thermoluminescence of the ZnS particles and nanocomposite films was recorded using an automated luminescence reader (Risoe TL, OSL-DA-20) with a heating rate of 1 °C/s. Irradiation for the TL measurement was done using a built-in Sr-90/Y-90 source with a dose rate of 6 mGy/s. The CTEs of nanocomposite films were determined using a thermal mechanical analyzer (TMA, TA Instruments TMA-Q400). The CTEs were measured with an extension probe under a 0.01 N tension force on the films in the temperature range of 25~400 °C at a heating rate of 5 °C/min under nitrogen atmosphere. The cross-sectional views of the nanocomposite films were analyzed using a scanning electron microscope (SEM, JEOL JEOL-7001F) after the samples were polished using a cross section polisher (JEOL, IB-19510CP). Tensile mechanical tests of the nanocomposite films were performed by a universal testing machine (UTM, WL2100B) with


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a uniform strain rate of 5 mm/min at room temperature, and the transmittances were acquired using a Shimadzu 1650 PC spectrophotometer.

3. Results and Discussion 3.1. Characterization of the synthesized inorganic particles Prior to the introduction of the synthesized particles into the poly(amic acid) solution, QELS was used to measure the average hydrodynamic diameters of the particles in NMP which is the solvent for the poly(amic acid) solution. The average diameters of the ZnS, SiO2, and ZrW2O8 particles were 250 ± 50, 120 ± 20, 500 ± 100 nm, respectively. TEM images of the synthesized particles (Figure 2) show that ZnS particles are agglomerates of much smaller nanoparticles, whereas the SiO2 and ZrW2O8 particles are not agglomerated. In addition, the ZrW2O8 particles exhibited a typical rod-like morphology. The XRD patterns in Figure 3 confirm that the synthesized particles are ZnS, SiO2, and ZrW2O8 particles, respectively. Since the peak broadening at lower angle is more meaningful for the calculation of particle size, mean size of the ZnS nanocrystallites was calculated from the (111) reflection (Figure 3) using the DebyeScherrer formula, which is given by: D=

0.94 λ β cos θ

(1)

where D is the particle size, λ is the wavelength of X-ray, β is the full width at half maximum after correcting the instrument peak broadening, and θ is the Bragg’s angle. Based on the XRD result of the ZnS particles, the mean size of the nanocrystallites in the ZnS particles was estimated to be ~3 nm, which is consistent with the TEM image (Figure S1).24 Therefore, we


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could conclude that ~3 nm ZnS nanocrystals agglomerated to form ZnS particles with a diameter of ~250 nm.

Figure 2. TEM images of the (a) ZnS, (b) SiO2, and (c) ZrW2O8 particles

Figure 3. X-ray diffraction patterns of the unmodified ZnS, SiO2, and ZrW2O8 particles

To improve the dispersion of ZnS particles in NMP, the surface of ZnS particles was modified with MPTMS, during which the thiol group of MPTMS reacted with the particle surface while the methoxy groups were hydrolyzed to produce surface hydroxyl (silanol) groups as in SiO2 and ZrW2O8 particles. To confirm surface modification, FT-IR spectroscopy and zeta-potential measurement were used. The FT-IR spectra (Figure 4) of the surface-modified (ZnS-MPTMS) particles showed the new peaks at around 980 cm-1 (Si-O-Si stretching), 850 cm-1 (Si-CH3


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rocking and Si-C stretching in Si-CH3), and 1250 cm-1 (CH3 deformation in Si-CH3) from the hydrolysis and condensation reactions of the MPTMS ligands on the surface. After surface modification, the zeta-potential of the unmodified ZnS particles decreased from -5 mV to -20 mV, which is close to that of the silica particles. Moreover, the average hydrodynamic diameters of the ZnS particles in NMP decreased by half (120±20 nm) after the surface modification. These results suggest that surfaces of the ZnS particles were successfully modified by the MPTMS to create hydroxyl groups, which improved the dispersion stability of the ZnS nanoparticles and decreased the agglomerate size.

Figure 4. FT-IR spectra of the unmodified and surface-modified ZnS particles

3.2. CTE values of the PI nanocomposite films To test the possibility of using the ZnS particles as nanofillers to reduce CTE of PI films, we prepared PI nanocomposite films using the synthesized ZnS particles, and compared their CTEs with those incorporating other inorganic particles (SiO2 and ZrW2O8). We found that while the CTE of the bare-PI film was 40 ppm/K in the temperature range of 50~200 °C, it decreased in all


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the PI nanocomposite films containing the synthesized particles (ZnS, SiO2, and ZrW2O8). The CTEs of nanocomposite films containing inorganic fillers at 15 wt% are summarized in Table 1. It is clear that the nanocomposites containing the ZnS particles showed much lower CTEs than those containing the other particles (SiO2 and ZrW2O8). In addition, the surface-modified ZnS particles exhibited the lowest CTE (13.0 ppm/K), which is close to that of the copper (17 ppm/K) and corresponds to a 67% decrease from the bare-PI film. Note that the 67% decrease in the CTE of the PI film with the surface-modified ZnS particles is comparable to the largest reported value to date at a much lower particle concentration, as the largest percentage decrease in PI film in the literature was 65% with 60 vol% (84 wt%) ZrW2O8 particles!12,13

Table 1. Intrinsic CTE and thermal conductivity of the materials used and the nanocomposite CTE values (at 15 wt%)

Bare PI

SiO2

ZrW2O8

Unmodified ZnS

Modified ZnS

Intrinsic CTEa (ppm/K)

40.0

0.55

-9.0

6.36

6.36

Intrinsic thermal conductivitya (W/m·K)

0.27

1.3 ~ 1.5

0.5

25.1

25.1

Nanocompositeb CTE (ppm/K)

-

30.2

33.0

20.8

13.0

a

Intrinsic CTE and thermal conductivity of the PI and particles were obtained from a handbook15,16,

b

Nanocomposite CTE was determined by the experiments.

3.3 Effect of inorganic particle fillers on the CTEs of nanocomposite films


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CTE values of nanocomposite films containing different types of inorganic particles can be calculated by the simple rule of mixture.7,25,26 Figure 5 shows that the calculated CTEs of all the particles at 15 wt% were higher than the experimental results. In particular, the films containing ZnS particles showed much lower CTE values than the predictions from the rule of mixture even though the intrinsic CTE of ZnS is higher than those of the SiO2 and ZrW2O8 (Table 1). This result suggests that the intrinsic CTE of the ZnS particles is not mainly responsible for the reduction of CTEs of the nanocomposite film. Other factors that could reduce the CTE of the nanocomposite film containing the ZnS particles include the thermoluminescence, intrinsic thermal conductivity, interfacial area, and dispersion state of the ZnS particles.

Figure 5. CTEs of PI nanocomposite films containing inorganic particles at 15 wt% from experiments and the rule of mixture (light color: rule of mixture, solid color: experiments)

ZnS is one of the inorganic materials that exhibit the thermoluminescence (TL) phenomenon, the generation of luminescence by the recombination of the carriers that are released from the surface states or defect sites during heating. Since smaller particles have more accessible TL


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carriers than larger particles because of a larger surface/volume ratio and more surface states, we tested whether ZnS particles that are agglomerates of ZnS nanoparticles could convert heat to radiation, thereby reducing the CTE of nanocomposite films. Figure 6 shows that the TL intensity of ZnS particles was zero without β-radiation, and that the TL peak intensity increased with increasing β-radiation dose because increasing β-radiation dose induces formation of new defect sites inside ZnS particles and hence allows additional electron traps.27,28 Note that even the nanocomposite film with the β-radiated ZnS particles showed a very low TL intensity, indicating that TL from the ZnS particles is negligible. The TL results suggest that reduction in CTE of the ZnS/PI nanocomposite was not caused by the TL property of the ZnS particles.

Figure 6. TL glow curves of the ZnS particles and nanocomposite film (numbers in parentheses represent radiation dose in mGy)

Next, we considered the effect of the intrinsic thermal conductivity of inorganic particles on CTE (Table 1). The heat transport mechanism in non-metals such as polymers is usually explained by the flow of phonons or lattice vibration energy. Inorganic particles in polymer


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nanocomposites can increase the effective thermal transfer inside the polymer matrix by forming a conductive network, but they can also decrease the effective thermal transfer by forming an interfacial thermal resistance due to phonon scattering.29-31 Many studies have shown that the conductivity of nanocomposites generally increases with increasing filler concentration because conductivity is affected mainly by conductive networks of filler particles.30,32 In addition, increasing the filler concentration is known to increase the thermal conductivity of nanocomposites according to the Nelsen equation and Maxwell theory.29,33 Therefore, it is reasonable to expect that filler particles with a high intrinsic thermal conductivity should increase the thermal conductivity of nanocomposites by forming a conductive network. To compare heat transfer phenomena in nanocomposite films, we measured the thermal diffusivities of the nanocomposite films containing different types of filler particles at the same concentration (15 wt%). Figure 7 shows that the ZnS particles with much higher intrinsic thermal conductivity than the SiO2 and ZrW2O8 particles yielded the nanocomposite films with the higher thermal diffusivities (0.2~0.217 mm2/s). It is noteworthy that the thermal diffusivity of the nanocomposite containing the rod-like ZrW2O8 particles was low because of their low intrinsic thermal conductivity, even though high-aspect ratio particles are known to enhance thermal conductivity.29 We calculated the thermal conductivity of the nanocomposite films from the thermal diffusivity, assuming the rule of mixture for the density and specific heat (Table S1), which also shows higher thermal conductivity for the nanocomposite films with the ZnS particles. Another factor that can increase thermal diffusivity of the ZnS particles is interfacial area. Since the ZnS particles are agglomerates of much smaller (~3 nm) primary particles, ZnS particles should have much higher interfacial area than the SiO2 and ZrW2O8 particles. It has


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been shown that when the primary particle size is small enough, high interfacial area increases effective thermal transfer instead of forming an interfacial thermal resistance.31 Based on these results, ZnS particles with high intrinsic thermal conductivity and interfacial area appear to form a thermally conductive network in nanocomposites, thereby helping dissipation of heat and reducing the CTE of the nanocomposites, which is consistent with the results shown in Table 1. However, reduction in CTE due to a decrease in the filler particle size from 10 µm to 80 nm has been shown to be relatively small (5%).34 Therefore, we believe that the intrinsic thermal conductivity of ZnS particles plays a more important role than the interfacial area on the reduction of CTE of nanocomposites.

Figure 7. Thermal diffusivity of PI nanocomposite films

Even though both unmodified and surface-modified ZnS particles have the same intrinsic thermal conductivities, the surface-modified ZnS particles yielded a higher thermal diffusivity and lower CTE than the unmodified particles. Since the dispersion of particles in both solvent and polymer matrix is an important issue in the fabrication of nanocomposite films, we suspected that the dispersion state as well as the intrinsic thermal conductivity of the particles affected the


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CTE of nanocomposites. To compare the dispersion state of ZnS particles in nanocomposite films, cross-sectional SEM was used. The SEM images in Figure 8 clearly demonstrate that the dispersion state of the surface-modified ZnS particles was more uniform in the PI polymer matrix than that of the unmodified particles. Since the dispersion stability of the ZnS particles in NMP was improved by the surface modification, it is reasonable to expect that the dispersion state of the surface-modified ZnS particles be more uniform in the nanocomposite than that of the unmodified particles. Moreover, the interaction between the PI polymer chain and the ZnS particles could be enhanced by hydrogen bonding between hydroxyl groups on the surface of the surface-modified ZnS particles and the polymer chains, which should improve the dispersion of the particles. It is possible that agglomeration of nanoparticles forms a conductive network in the colloidal suspensions, which can increase thermal conductivity of the suspension through Brownian-induced convection of the agglomerates.35 In the inorganic filler/polymer nanocomposite films, however, agglomerates of the nanoparticles are not mobile as in the colloidal suspension, and increasing interfacial area is known to increase effective thermal transfer.30,32 Therefore, we believe that improving dispersion state enhances effective thermal transfer by increasing the interfacial area between ZnS particles and the polymer matrix, which improves heat dissipation and suppresses thermal expansion of the nanocomposite. Based on these results, we can conclude that the intrinsic thermal conductivity and dispersion state of the surface-modified ZnS particles were mainly responsible for the significant reduction in the CTE of the nanocomposite.


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Figure 8. Cross-sectional SEM images of nanocomposite films containing (a) unmodified and (b) surface-modified ZnS particles at 15 wt%

Finally, we compared the optical and mechanical properties of the bare and the surfacemodified ZnS-containing PI films. It is well known that mechanical properties of nanocomposites are improved when the inorganic nanoparticles are added to polymer nanocomposites, but that poor dispersion of nanoparticles induced by low interfacial interaction between nanoparticles and polymer chains can deteriorate the mechanical modulus.36,37 When the surface-modified ZnS particles were added to the PI films, the tensile strength and modulus of nanocomposites films increased approximately by 1.6- and 1.9- fold compared to the bare-PI, respectively (Table 2), which results from the uniform dispersion of surface-modified ZnS nanoparticles within PI polymer (Figure 8). On the contrary, elongation slightly decreased compared to the bare-PI because rigid inorganic nanoparticles within the nanocomposites prevented PI from stretching. By improving dispersion state of nanoparticles, we could minimize the decrease in elongation. We also compared transmittance of the bare and the surface-modified ZnS-containing PI films at different particle concentrations (Figure 9). Since ZnS nanoparticles absorb light below 300 nm (result not shown), reduction in transmittance of nanocomposite films in range of 400 ~ 700 nm is mainly caused by the light scattering. Although transmittance


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gradually decreased with increasing particle concentration, reduction in transmittance was not significant up to 15 wt%, as shown in Figure 9. Therefore, the PI nanocomposite film containing the surface-modified ZnS particles at 15 wt% exhibited optical and mechanical properties that were comparable to the bare PI film, and can be applied in fabrication of various optoelectronic devices.

Table 2. Mechanical properties of bare and the surface-modified ZnS-containing (15 wt%) PI films

Sample

Tensile Strength (MPa)

Tensile Modulus (GPa)

Elongation (%)

Bare-PI

56.3 ± 8.6

1.8 ± 0.1

7.2 ± 2.6

ZnSMPTMS/PI

87.7 ± 0.5

3.3 ± 0.2

5.3 ± 0.9


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Figure 9. Transmittance and pictures of the bare and the surface-modified ZnS-containing PI films at different particle concentrations

4. Conclusions Thermal management in microelectronics processing is a key issue in the miniaturization of next-generation devices. Even though use of inorganic particles as fillers in polymer films has been already studied extensively to reduce the CTE of polymer film, high concentrations of inorganic particles are required for a significant reduction in CTE, which may deteriorate other properties of the polymer film. In this work, we successfully prepared PI nanocomposite films using ZnS particles as nanofillers, and compared these PI films with others containing different inorganic (SiO2 and ZrW2O8) particles. We demonstrated that use of the surface-modified ZnS particles reduced the CTE of the PI film by 67% at 15 wt%, which is comparable to the largest percentage decrease in CTE to date at a much lower concentration. Moreover, the surfacemodified ZnS particles at 15 wt% did not deteriorate the optical and mechanical properties of the film. The significant reduction in CTE at such a low particle concentration was mainly attributed to the high intrinsic thermal conductivity and the improved dispersion state of the particles. We believe that the knowledge gained from this research will be indispensable in the design of future microelectronic devices and the improvement of their reliability.

ASSOCIATED CONTENT Supporting Information Available: TEM image of the ZnS particles in high-resolution; Properties of PI nanocomposites at 15 wt% filler concentration


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AUTHOR INFORMATION Corresponding Author * TEL: +82-2-2123-2760; FAX: +82-2-312-6401; E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ACKNOWLEDGMENTS This research has been supported by grants from the Samsung Electronics, Co., LTD, Korea and the National Research Foundation of Korea (No. 2014R1A2A1A11051436 and 2017R1A2B4007534).

REFERENCES (1) Odegard, G. M.; Clancy, T. C.; Gates, T. S., Modeling of the Mechanical Properties of Nanoparticle/Polymer Composites. Polymer 2005, 46, 553-562. (2) Yamada, N.; Yoshinaga, I.; Katayama, S., Synthesis and Dynamic Mechanical Behaviour of Inorganic–Organic Hybrids Containing Various Inorganic Components. J. Mater. Chem. 1997, 7, 1491-1495.


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(3) Kim, J.; Kwon, J.; Lee, D.; Kim, M.; Han, H., Heat Dissipation Properties of Polyimide Nanocomposite Films. Korean J. Chem. Eng. 2016, 33, 3245-3250. (4) Tsai, C. L.; Yen, H. J.; Liou, G. S., Highly Transparent Polyimide Hybrids for Optoelectronic Applications. React. Funct. Polym. 2016, 108, 2-30. (5) Ishii, J.; Takata, A.; Oami, Y.; Yokota, R.; Vladimirov, L; Hasegawa, M., Spontaneous Molecular Orientation of Polyimides Induced by Thermal Imidization (6). Mechanism of Negative in-plain CTE Generation in Non-Stretched Polyimide Films. Eur. Polym. J. 2010, 46, 681-693. (6) Kim, Y. J.; Kim, J. H.; Ha, S. W.; Kwon, D.; Lee, J. K., Polyimide Nanocomposites with Functionalized SiO2 Nanoparticles: Enhanced Processability, Thermal and Mechanical Properties. RSC Adv. 2014, 4, 43371-43377. (7) Tsai, M. H.; Tseng, I. H.; Huang, S. L.; Hsieh, C. W., Enhancement of Dimensional Stability and Optical Transparency of Colorless Organo-Soluble Polyimide by Incorporation of Silica and Cosolvent. Int. J. Polym. Mater. Polym. Biomater. 2014, 63, 48-56. (8) Li, T. L.; Hsu, S. L. C., Enhanced Thermal Conductivity of Polyimide Films via a Hybrid of Micro-and Nano-Sized Boron Nitride. J. Phys. Chem. B 2010, 114, 6825-6829. (9) Li, L.; Chung, D. D. L., Thermally Conducting Polymer-Matrix Composites Containing Both AlN Particles and SiC whiskers. J. Electron. Mater. 1994, 23, 557-564. (10) Hsu, S. C.; Whang, W. T.; Hung, C. H.; Chiang, P. C.; Hsiao, Y. N., Effect of the Polyimide Structure and ZnO Concentration on the Morphology and Characteristics of Polyimide/ZnO Nanohybrid Films. Macromol. Chem. Phys. 2005, 206, 291-298.


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Page 22 of 26

(11) Wu, J.; Yang, S.; Gao, S.; Hu, A.; Liu, J.; Fan, L., Preparation, Morphology and Properties of Nano-Sized Al2O3/Polyimide Hybrid Films. Eur. Polym. J. 2005, 41, 73-81. (12) Sullivan, L. M.; Lukehart, C. M., Zirconium Tungstate (ZrW2O8)/Polyimide Nanocomposites Exhibiting Reduced Coefficient of Thermal Expansion. Chem. Mater. 2005, 17, 2136-2141. (13) Yamashina, N.; Isobe, T.; Ando, S., Low Thermal Expansion Composites Prepared from Polyimide and ZrW2O8 Particles with Negative Thermal Expansion. J. Photopolym. Sci. Technol. 2012, 25, 385-388. (14) Teh, P. L.; Mariatti, M.; Akil, H. M.; Yeoh, C. K.; Seetharamu, K. N.; Wagiman, A. N. R.; Beh, K. S., The Properties of Epoxy Resin Coated Silica Fillers Composites. Mater. Lett. 2007, 61, 2156-2158. (15) Madelung, O.; Rössler, U.; Schulz, M., II-VI and I-VII Compounds; Semimagnetic Compounds. Landolt-Börnstein - Group III Condensed Matter (Numerical Data and Functional Relationships in Science and Technology): Springer, Berlin, Heidelberg, New York, 1999. (16) Krishman, R. S.; Srinivasan, R.; Devanarayanan, S., Thermal Expansion of Crystals: International Series in the Science of the Solid State: Pergamon, Oxford, 2013. (17) Devi, B. R.; Raveendran, R.; Vaidyan, A. V., Synthesis and Characterization of Mn2+Doped ZnS Nanoparticles. Pramana 2007, 68, 679-687. (18) Liveri, V. T.; Rossi, M.; D’Arrigo, G.; Manno, D.; Micocci, G., Synthesis and Characterization of ZnS Nanoparticles in Water/AOT/n-Heptane Microemulsions. Appl. Phys. A: Mater. Sci. Process. 1999, 69, 369-373.


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Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Stöber, W.; Fink, A.; Bohn, E., Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62-69. (20) Closmann, C.; Sleight, A. W.; Haygarth, J. C., Low-Temperature Synthesis of ZrW2O8 and Mo-Substituted ZrW2O8. J. Solid State Chem. 1998, 139, 424-426. (21) Xing, Q.; Xing, X.; Yu, R.; Du, L.; Meng, J.; Luo, J.; Wang, D.; Liu, G., Single Crystal Growth of ZrW2O8 by Hydrothermal Route. J. Cryst. Growth 2005, 283, 208-214. (22) Chen, Y.; Iroh, J. O., Synthesis and Characterization of Polyimide/Silica Hybrid Composites. Chem. Mater. 1999, 11, 1218-1222. (23) Liaw, D. J.; Wang, K. L.; Huang, Y. C.; Lee, K. R.; Lai, J. Y.; Ha, C. S., Advanced Polyimide Materials: Syntheses, Physical Properties and Applications. Prog. Polym. Sci. 2012, 37, 907-974. (24) Ayodhya, D.; Venkatesham, M.; Kumari, A. S.; Mangatayaru, K. G.; Veerabhadram, G., Synthesis, Characterization of ZnS Nanoparticles by Coprecipitation Method Using Various Capping Agents-Photocatalytic Activity and Kinetic Study, J. Appl. Chem. 2013, 6, 101-109. (25) Shi, X.; Lian, H.; Yan, X.; Qi, R.; Yao, N.; Li, T., Fabrication and Properties of Polyimide Composites Filled with Zirconium Tungsten Phosphate of Negative Thermal Expansion. Mater. Chem. Phys. 2016, 179, 72-79. (26) Tani, J. I.; Kimura, H.; Hirota, K.; Kido, H., Thermal Expansion and Mechanical Properties of Phenolic Resin/ZrW2O8 Composites. J. Appl. Polym. Sci. 2007, 106, 3343-3347.


23


Page 24 of 26

(27) Chandrakar, R. K.; Baghel, R. N.; Chandra, B. P., Synthesis, Characterization and Thermoluminescence Studies of Mn‐Doped ZnS Nanoparticles. Luminescence 2016, 31, 317322. (28) Ortiz-Hernández, A. A.; García, V. H. M.; Arrieta, M. L. P.; Sígala, J. J. O.; Ibarra, J. D. J. A.; Vega-Carrillo, H. R.; Guajardo, C. F., Thermoluminescent Properties of ZnS: Mn Nanocrystalline Powders. Appl. Radiat. Isot. 2015, 99, 105-109. (29) Jiajun, W.; Xiao-Su, Y., Effects of Interfacial Thermal Barrier Resistance and Particle Shape and Size on the Thermal Conductivity of AlN/PI Composites, Compos. Sci. Technol. 2004, 64, 1623-1628. (30) Gong, F.; Bui, K.; Papavassiliou, D. V.; Duong, H. M., Thermal Transport Phenomena and Limitations in Heterogeneous Polymer Composites Containing Carbon Nanotubes and Inorganic Nanoparticles, Carbon 2014, 78, 305-316. (31) Nan, C. W.; Birringer, R.; Clarke, D. R.; Gleiter, H., Effective Thermal Conductivity of Particulate Composites with Interfacial Thermal Resistance, J. Appl. Phys. 1997, 81, 6692-6699. (32) Wang, S.; Liang, Z.; Gonnet, P.; Liao, Y. H.; Wang, B.; Zhang, C., Effect of Nanotube Functionalization on the Coefficient of Thermal Expansion of Nanocomposites, Adv. Funct. Mater. 2007, 17, 87-92. (33) Lee, G. W.; Park, M.; Kim, J.; Lee, J. I.; Yoon, H. G., Enhanced Thermal Conductivity of Polymer Composites Filled with Hybrid Filler. Composites, Part A 2006, 37, 727-734.


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Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34) Jang, J. S.; Bouveret, B.; Suhr, J.; Gibson, R. F., Combined Numerical/Experimental Investigation of Particle Diameter and Interphase Effects on Coefficient of Thermal Expansion and Young’s Modulus of SiO2/Epoxy Nanocomposites, Polym. Compos. 2012, 33, 1415-1423. (35) Prasher, R.; Phelan, P. E.; Bhattacharya, P., Effect of Aggregation Kinetics on the Thermal Conductivity of Nanoscale Colloidal Solutions (Nanofluid), Nano Lett., 2006, 6, 15291534. (36) Majdzadeh-Ardakani, K.; Navarchian, A. H.; Sadeghi, F., Optimization of Mechanical Properties of Thermoplastic Starch/Clay Nanocomposites, Carbohydr. Polym., 2010, 79, 547554. (37) Yan, S.; Yin, J.; Yang, Y.; Dai, Z.; Ma, J.; Chen, X., Surface-Grafted Silica Linked with L-lactic Acid Oligomer: A Novel Nanofiller to Improve the Performance of Biodegradable Poly(L-lactide), Polymer, 2007, 48, 1688-1694.


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