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Atmospheric Pressure Plasma Jet Assisted Synthesis of Zeolite-Based Low-k Thin Films Kai-Yu Huang, Heng-Yu Chi, Peng-Kai Kao, Fei-Hung Huang, Qi-Ming Jian, I-Chun Cheng, Wenya Lee, Cheng-Che Hsu, and Dun-Yen Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16410 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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ACS Applied Materials & Interfaces

Atmospheric Pressure Plasma Jet Assisted Synthesis of Zeolite-Based Low-k Thin Films Kai-Yu Huang,† Heng-Yu Chi,† Peng-Kai Kao,† Fei-Hung Huang,† Qi-Ming Jian,† I-Chun Cheng,‡ Wen-Ya Lee,⊥ Cheng-Che Hsu*,† and Dun-Yen Kang*,† †

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan.

‡

Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617,

Taiwan. ⊥

Department of Chemical Engineering and Biotechnology, National Taipei University of

Technology, Taipei 10608, Taiwan.

ACS Paragon Plus Environment

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ABSTRACT Crystalline microporous zeolites are ideal low-dielectric constant (low-k) materials. This paper reports on a novel plasma-assisted approach to the synthesis of low-k thin films comprising puresilica zeolite MFI. The proposed method involves treating the aging solution using an atmospheric pressure plasma jet (APPJ). The high reactivity of the resulting nitrogen plasma helps to produce zeolite crystals with high crystallinity uniform crystal size distribution. The APPJ treatment also remarkably reduces time for the hydrothermal reaction. The zeolite MFI suspensions synthesized with the APPJ treatment is used for the wet deposition to form thin films. The deposited zeolite thin films possessed dense morphology and high crystallinity, which overcomes the tradeoff between crystallinity and film quality. Zeolite thin films synthesized using the proposed APPJ treatment achieve low leakage current (on the order of 10-8 A/cm2) and high Young’s modulus (12 GPa), outperforming the control sample synthesized without plasma treatment. The dielectric constant of our zeolite thin films was as low as 1.41. The overall performance of the low-k thin films synthesized with the APPJ treatment far exceeds existing low-k films comprising pure-silica MFI. KEYWORDS: zeolite; thin film; low-k material; plasma; wet deposition.


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1.

Introduction

Materials with a dielectric constant lower than 4 are referred to as low-k materials.1 Low-k materials play a key role in microelectronics in the reduction of RC delays and power consumption, as well as the mediation of crosstalk noise.2-6 Nonetheless, low-k materials must exhibit good mechanical properties and high thermal stability to make them practical for electronic devices. Specifically, the Young’s modulus of low-k materials should exceed 10 GPa1 while maintaining operational stability at temperatures of 400-450 °C.1 Organic as well as inorganic materials have been investigated as low-k thin films.4, 7-8 The dielectric constant of most organic (polymeric) materials is between 2 and 39 and they are easily processed;10-12 however, they suffer from the low elastic modulus and/or low thermal stability. For example, fluorinated epoxy resins have a k value of 3.2-3.6, but a low Young’s modulus (1.55-1.92 GPa).9

Inorganic microporous or mesoporous substances have been evaluated as low-k materials.13-16 Porous amorphous silica synthesized using the sol-gel method can achieve low k values;

7-8, 17

however, the introduction of micropores or mesopores greatly weakens the mechanical properties.18-20 Inorganic nanotubes have also been investigated as microporous low-k materials.6, 21

The mechanical strength of crystalline microporous materials, such as zeolites,19, 22-25 generally

exceeds that of amorphous porous materials. Zeolites with a variety of topologies, such as MFI, 26-28

MEL, 29-32 or LTA33-34 have been investigated for low-k applications. Pioneering work on in

situ growth methods for the fabrication of zeolite thin films23 involved immersing silicon wafers in a high-pH solution,35 which corrodes wafers. To address this issue, zeolite thin films prepared via direct deposition using zeolite suspensions have been developed.22, 24, 26, 29


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For zeolite thin films prepared via direct deposition, the samples made from high-crystallinity zeolite crystals35 have a lower k value; however, they also tend to possess defects,36-37 which undermines their mechanical strength and promotes current leakage.38 Surfactants, such as TWEEN® 80, have been introduced to zeolite-cast suspensions to reduce the formation of defects;18 however, this method only works for thin films of low crystallinity.18, 38-39 Fabricating defect-free zeolite thin films of high crystallinity via direct wet deposition remains a challenge.

Herein, we report on a novel approach to the preparation of high-crystallinity zeolite thin films with few defects. The proposed method involves the use of atmospheric pressure plasma jet (APPJ) to treat the aging solution used in the synthesis of the zeolite. Previous studies on the use of APPJ in materials research40-44 led us to expect that the high reactivity of plasma would facilitate the nucleation of small uniform zeolite crystals in the suspensions. Thus, we subjected a series of pure-silica zeolite MFI suspensions to APPJ treatment for use in thin-film deposition. We also prepared thin-film samples containing pure-silica zeolite MFI synthesized without APPJ treatment for use as control samples. Dynamic light scattering (DLS) was used to characterize the particle size distribution of the zeolite crystals in the suspensions. Nitrogen physisorption and powder X-ray diffraction (XRD) were respectively used to examine the porosity and crystallinity of the zeolite powder samples. The zeolite thin films were also subjected to analysis using scanning electron microscopy (SEM) and grazing-incidence wide-angle X-ray scattering (GIWAXS). We also measured the Young’s modulus, leakage current, and dielectric constant of the resulting thin films. Our aim was to elucidate the effects of APPJ treatment on the microstructure and mechanical and electrical properties of pure-silica zeolite MFI thin films.


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2.

Experimental Section

2.1. Chemicals Tetrapropylammonium hydroxide solution (TPAOH 25 wt.% in water) was purchased from Acros Organics. Tetraethyl orthosilicate (TEOS) was purchased from Merck. TWEEN® 80 was purchased from Sigma Aldrich. Ethanol (99.9%) was purchased from Merck. The deionized water (DI water) used for the synthesis was purified using the ELGATM PURELAB® Classic DI ultra-pure water system.

2.2 Synthesis of zeolite suspensions without APPJ treatment A solution containing 10.00 g of TPAOH, 7.11 g of TEOS, and 4.79 g of DI water was mixed at room temperature and then aged at 30 °C for 3 h in a perfluoroalkoxy alkane (PFA) container with a capacity of 100 mL. The aged solution was transferred to a Teflon® lined autoclave with a maximum capacity of 20 mL for the subsequent hydrothermal reaction, which was performed in an oven at 100 °C for 6, 7, 8, 9, 10, or 11 h. Following the hydrothermal synthesis process, 5 g of the zeolite suspension was mixed with 5 g of ethanol and 1.31 g of TWEEN® 80 as a surfactant. The mixture was then stirred at 30 °C for 6 h to form a cast suspension.

2.3 Synthesis of zeolite suspensions with APPJ treatment The process used in the synthesis of zeolite suspensions with nitrogen plasma treatment was similar to that described in the preceding section; however, plasma treatment was applied between the aging process and the hydrothermal reaction. The aged solution was treated with reactive nitrogen plasma using an atmospheric pressure plasma jet (APPJ) fabricated in-house.


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Prior to plasma treatment, 10 mL of the aged solution was placed in a crystalline borosilicate dish with a capacity of 20 mL. The setup is illustrated in Figure 1. The APPJ system, reported previously,44-45 was driven by a pulsed power source that supplied pulse voltage with a peak amplitude up to 350V, followed by a transformer that raises the voltage up to 15 kV. In this work, the APPJ was operated at 273 V and 2.97 A, with nitrogen gas flow rate set at 30 L/min and a jet-to-sample distance of 12.5 mm. Plasma treatment was applied for 30 to 60 s. DI water was then added to each APPJ treatment solution to bring the total volume back to 10 mL. Then APPJ treatment solution was underwent similar hydrothermal reaction as mentioned in the preceding section, which was performed at 100°C for 6 h. After the hydrothermal reaction, 5 g of the zeolite suspension was mixed with the same amount of ethanol and TWEEN® 80 as mentioned in the preceding section. The mixture was then stirred at 30 °C for 6 h to form a cast suspension as well.

2.4 Preparation of zeolite thin films P-boron type silicon wafer with native oxide and orientation of (100) +/-0.5 degree was used as the substrate for thin-film deposition. The silicon wafer used in this work had resistance of 1.5 to 100 ohm-cm (purchased from SUMMIT-TECH Resource Corp). The 6”-wafer was cut into squares (approximately 2 × 2 cm) prior to deposition. Cast solutions were dropped on the substrate until the entire surface was covered. The substrate was then spun at 2600 rpm for 30 s using a spin coater (Laurell Model-WS-650M2-23NPPB). The resulting thin-film samples were placed in an oven and baked at 100°C overnight. The samples were then transferred to a furnace for calcination. The temperature in the furnace was increased from room temperature to 450 °C


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at an elevation rate of 1 °C/min, where it was held for 5 h. Following calcination, the thin-film samples were ready for material characterization.

2.5 Preparation of zeolite powder samples Approximately 2 mL of the cast solution was dropped on silicon wafers (approximately 4 × 4 cm) before undergoing drying in an oven overnight at 100 °C to thicken the layer. The thick layer was then scraped off of the substrate and ground to form a powder. Powder samples were then calcined using the same procedure described in the preceding section.

2.6 Material Characterization Dynamic light scattering (DLS) measurements were applied to the zeolite suspensions using a particle size and zeta potential analyzer (Malvern, Zetasizer Nano) with red light laser (633nm). Suspension samples (0.5g) were diluted with deionized water (5 mL) in vials, whereupon 1 mL of the diluted solution was placed in a disposable cuvette (DTS0012). All measurements were conducted at 25 ± 0.1 °C. The measured autocorrelation function versus time was converted into the particle size distribution by fitting using the non-negative least squares (NNLS) model.46

Powder X-ray diffraction (XRD) patterns were obtained using a Rigaku diffractometer (40 kV, 40 mA) with Cu Kα radiation operating at 40 kV and 40 mA and a scanning rate of 4o min-1 from 5o to 40o 2θ with a step size of 0.02o.


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Grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns of the thin films were obtained using the facilities at beamline 13A station at the National Synchotron Radiation Research Center (NSRRC) in Taiwan. Incidence X-ray with energy of 12.05 keV (1.029 Å) and a sample-to-detector distance of 338.77 mm were employed. The incidence angle of X-ray beam was 0.1°. Scattering patterns were recorded by a Bicron point detector (Mar165 CCD) with a 50second exposure. The raw scattering patterns were processed using GIXSGUI.47

A Hitachi S-4800 field emission scanning electron microscope (SEM) was used to characterize the morphology of the thin-film samples. The samples were coated with platinum via sputtering deposition at an acceleration voltage of 25 V for 40 seconds. SEM imaging was operated under an acceleration voltage of 10 kV. To image the cross section of the samples, the entire thin film was cut along certain specific crystallographic planes of the silicon wafer with a diamond saw. No further polishing was conducted on the samples.

The mechanical properties of the thin-film samples were measured via nanoindentation testing using a Hysitron TI 950 TriboIndenter. A Berkovich pyramidal diamond tip with a half angle of 65.31° was used for indentation with a 10-mN/s loading/unloading rate and a 5-s dwell-time. Indentation tests were conducted in three different spots on each sample. The Young’s modulus of the thin-film samples was deduced from the raw data obtained in the nanoindentation tests.

The capacitance of the thin-film samples was measured using a KEITHLEY 4200A dual-channel source meter under a bias voltage of -1 V at frequencies of 1, 10, 100, and 500 kHz. The


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dielectric constant (k) was calculated based on the relation = / , where C is the measured capacitance, d is the thickness (determined from cross-sectional SEM images), A is the area of the deposited electrode (2 mm2), and is the vacuum permittivity (8.854 × 10-12 Fm-1). Leakage current of thin-film samples was examined at room temperature using a KEITHLEY 4200A dual-channel source meter under voltage of -1 V. All of the above measurements were conducted after baking the samples at 105 °C to ensure the elimination of water from the thin films. All measurements were conducted in three different spots on each sample.

Nitrogen physisorption isotherms of powder samples were obtained using a Micromeritics ASAP 2010 analyzer at 77 K. Prior to physisorption measurements, the samples were placed in an analysis tube and degassed overnight under 0.002 torr at 150 °C.


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Results and Discussion

Figures 2a and 2b present photographic images of pure-silica zeolite MFI suspensions synthesized with and without the plasma treatment. The sample synthesized in a 6-h hydrothermal reaction without plasma treatment is referred to as zeolite MFI (6hr). The sample synthesized from a 6-h hydrothermal reaction following a 30-s treatment using the atmospheric pressure plasma jet (APPJ) is referred to as zeolite MFI (30s-6hr). The remaining samples were named by analogy. The opaqueness of suspensions prepared without APPJ treatment (Figure 2a) increased with hydrothermal treatment duration. Increased opaqueness is an indication of either the presence of large zeolite particles or an increase in zeolite concentration in the suspension.48 Figure 2b presents photographic images of suspensions synthesized in a 6-h hydrothermal reaction following APPJ treatment for various durations. The opaqueness of suspensions containing zeolite MFI (30s-6hr) was similar to that of zeolite MFI (8hr). This implies that APPJ treatment for just 30 s is sufficient to reduce the duration of the hydrothermal reaction by approximately 2 h. The suspension containing zeolite MFI (60s-6hr) was as turbid as that of zeolite MFI (11hr), which suggests that extending APPJ treatment could further increase the size of zeolite crystals.

The particle size distributions of the zeolite suspensions were determined from dynamic light scattering (DLS) measurements (Figure 2c). Only a single peak was observed in the particle size distribution for all of the suspensions discussed in this work. The average size of the particles in the suspensions prepared without APPJ treatment increased with the duration of the hydrothermal reaction. Specifically, the average particle size increased from 60 nm for zeolite MFI (6hr) to 122 nm for zeolite MFI (11hr). The average particle size of suspensions synthesized


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via a hydrothermal reaction for 6 h following APPJ treatment, increased with the duration of APPJ treatment from 93 nm (zeolite MFI (30s-6hr)) to 112 nm (zeolite MFI (60s-6hr)). APPJassisted synthesis also produced a narrower particle size distribution than did synthesis without APPJ. Zeolite MFI (6hr), zeolite MFI (9hr), and zeolite MFI (11hr) presented particle size distributions with a full width at half maximum (FWHM) of 83, 63, and 69 nm, respectively. Zeolite MFI (30s-6hr) and zeolite MFI (60s-6hr) had a particle size distribution with an FWHM of 55 and 64 nm respectively. The slight difference in the FWHM of the particle size distribution suggests that the APPJ-assisted synthesis yields zeolite particles with crystal size of greater uniformity.

Zeolite powder samples derived from suspensions were subjected to powder X-ray diffraction (XRD) to examine their crystallinity (Figure 3). Two of the samples synthesized without APPJ treatment, zeolite MFI (6hr) and zeolite MFI (7hr), did not present the XRD pattern of zeolite MFI; however, a weak diffraction pattern was observed in zeolite MFI (8hr). The XRD patterns was fully developed in samples synthesized using a hydrothermal reaction exceeding 9 h. The XRD pattern obtained from zeolite MFI (11hr) had a high signal-to-noise ratio indicative of high crystallinity. All samples synthesized via a hydrothermal reaction for 6 h after APPJ treatment presented a clear diffraction pattern of zeolite MFI, regardless of the duration of plasma treatment. The signal-to-noise ratio of the XRD pattern of zeolite MFI (30s-6hr) was similar to that of zeolite MFI (9hr). This agrees with the finding for the zeolite suspensions mentioned in the preceding section – the APPJ treatment for 30 s can effectively save 2 to 3 h of hydrothermal synthesis. The signal-to-noise ratio of the XRD pattern increased with the duration of APPJ


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treatment, which indicates that the nitrogen plasma facilitated the nucleation or crystallization of zeolite MFI. Plasma-assisted nucleation has previously been reported in the synthesis of TiO2.49

The pure-silica zeolite MFI suspensions were deposited on a silicon wafer substrate via spin-on depositoin. We employed grazing-incidence wide-angle X-ray scattering (GIWAXS) using a synchotron radiation source to evaluate the crystallinity of the thin films as well as the crystal orientation. The 2D GIWAXS patterns are presented in Figure 4. Thin-film samples synthesized without APPJ treatment did not present diffraction patterns for zeolite MFI (6hr) and zeolite MFI (7hr), which is in agreement with the powder XRD results. Weak GIWAXS patterns were observed in zeolite MFI (8hr) and zeolite MFI (9hr). Zeolite MFI (10hr) and zeolite MFI (11hr) presented sharp 2D diffraction patterns indicative of high crystallinity. The GIWAXS patterns of all thin-film samples synthesized with APPJ treatment were of high contrast, which supports our findings described in the preceding sections; i.e., APPJ treatment enhances the crystallinity of zeolite MFI and reduces the time required to complete the hydrothermal reaction. It was interesting to note in the GIWAXS patterns that the diffraction intensity was not uniform in the azimuthal direction. The intensity distribution of the GIWAXS patterns suggests that the (200) is preferably oriented parallel to the substrate.

Figure 5 presents top-view SEM images of the zeolite MFI thin-film samples. Among the samples synthesized without APPJ treatment, the size of the zeolite crystals increased with the duration of hydrothermal synthesis. The thin films of zeolite MFI (6hr) and zeolite MFI (7hr) are very dense, due to tightly packed zeolite MFI particles of relatively small size. However, XRD and GIWAXS characterization indicated that these two samples almost entirely lacked


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crystallinity (i.e., they were amorphous). In contrast, the thin films of zeolite MFI (10hr) and zeolite MFI (11hr) presented relatively large zeolite crystals; however, severe cracking was also observed in the thin films. These findings are in agreement with those of a previous study, in which the wet deposition of zeolite thin films was shown to incur a tradeoff between crystallinity and film quality.38 The synthesis of pure-silica zeolite MFI thin films with the assistance of APPJ was able to overcome this tradeoff. Thin-film samples of zeolite MFI (30s-6hr) and zeolite MFI (40s-6hr) presented good crystallinity as well as a dense thin-film morphology. A number of defects were observed in zeolite MFI (50s-6hr) and zeolite MFI (60s-6hr); however, the quality of these two samples still exceeded that of zeolite MFI (10hr) and zeolite MFI (11hr), which were synthesized without APPJ treatment.

The packing of particles in the thin films can be observed clearly in the cross-sectional SEM images (Figure 6). The small particles of zeolite MFI (6hr) were uniformly packed to form a thin film of high uniformity. The particle size evolved with the duration of the hydrothermal reaction, wherein zeolite crystals in samples that underwent the longest hydrothermal reactions were not uniformly packed, which resulted in cracking. The sample that underwent the APPJ treatment for 30 s (zeolite MFI (30s-6hr)) presented a thin-film morphology as dense as that of zeolite MFI (6hr), but with high crystallinity, whereas zeolite MFI (6hr) was completely amorphous. This also supports our claim that APPJ-assisted synthesis can be used to fabricate thin films of high crystallinity with few defects, which cannot be achieved using conventional hydrothermal synthesis. The high quality of these thin films can be attributed to uniformity in crystal size, as discussed in the preceding section on DLS.


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Nanoindentation tests revealed that the Young’s modulus of all zeolite MFI thin films (Figure 7) was approximately 10 GPa, which is similar to previous zeolite MFI thin films of pure silica.38 For the nanoindentation measurements, relatively even spots in thin film samples were opted for the tests (Supporting Information, Figure S1a). The Young’s modulus of thin-film samples synthesized without APPJ treatment decreased with the duration of the hydrothermal reaction. The SEM images mentioned above indicate that the packing density of the particles in the thin film may play a critical role in Young’s modulus. Specifically, a long hydrothermal reaction (zeolite MFI (11hr)) resulted in thin films with a large number of defects, which lowered the Young’s modulus to below that of the other two samples (zeolite MFI (6hr) and zeolite MFI (9hr)). The sample that underwent APPJ treatment for 30 s (zeolite MFI (30s-6hr)) presented a Young’s modulus higher than that of all samples prepared without APPJ treatment. The measurement was not conducted on zeolite MFI (8hr). This sample showed very similar surface morphology and crystallinity to zeolite MFI (9hr), so we expect a similar value of Young’s modulus for zeolite MFI (8hr) and zeolite MFI (9hr).

Zeolite MFI (30s-6hr) produced the smallest error bar associated with the Young’s modulus measurements, indicating high uniformity throughout the sample. Zeolite MFI (60s-6hr) presented a slightly lower Young’s modulus and larger error bar than did zeolite MFI (30s-6hr) due to the presence of large zeolite crystals, which undermined the homogeneity of packing in the thin film. The raw force versus displacement curves of zeolite MFI (6hr) and zeolite MFI (11hr) are presented in the Supporting Information (Figure S1b). The raw data of zeolite MFI (6hr) presents higher repeatability than that of zeolite MFI (11hr), because the longer reaction time leads to rougher surface of the thin films. As a result, the standard deviation in the Young’s


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modulus of zeolite MFI (11hr) is greater. It has been known that the crystal orientation of the thin films can affect their Young’s modulus.50 However, the difference in the degree of preferred orientation among the samples is not pronounced. It is likely that the surface roughness, instead of the degree of preferred crystal orientation, dominates the standard deviation for the measurements of Young’s modulus in this work.

Zeolite MFI (30s-6hr) presents similar morphology to the zeolite MFI (9hr), as shown in the SEM images. However, defect spots appeared in zeolite MFI (9hr) but not in zeolite MFI (30s6hr). In addition, zeolite MFI (30s-6hr) shows considerably higher crystallinity than zeolite MFI (9hr) from the characterization using GIWAXS. These two facts result in the observation of higher Young’s modulus of zeolite MFI (30s-6hr) than zeolite MFI (9hr). The fact also suggests that the APPJ-assisted synthesis outperforms the conventional synthesis in terms of producing zeolite films with higher elastic modulus.

Leakage current (Figure 8) in the thin-film samples was measured to assess the potential of using pure-silica zeolite MFI thin films as a dielectric layer in electronic devices. The leakage current of zeolite MFI (6hr) and zeolite MFI (9hr) samples was on the order of 10-8 A/cm2, which meets the criterion in semiconductor industry (below 10-8 A/cm2).38 The sample synthesized using a hydrothermal reaction of long duration (zeolite MFI (11hr)) presented far higher leakage current (10-5 A/cm2), which is far beyond the acceptable leakage current of 10-8 A/cm2 in the semiconductor industry. This is consistent with the observations described in the preceding sections, in which zeolite MFI (11hr) was shown to have the largest crystals, the highest


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crystallinity, and the largest number of defects in the thin films. The resulting cracks lowered the Young’s modulus while simultaneously increasing the leakage current. The zeolite MFI samples that underwent APPJ treatment also presented low leakage current (10-8 A/cm2 for zeolite MFI (30s-6hr) and 10-7 A/cm2 for zeolite MFI (60s-6hr)). This supports the fact that the APPJ enables the fabrication of high-crystallinity and high-quality thin films. It has been reported in the previous study that the leakage current of the zeolite MFI thin films falls in the range of 10-6-10-8 A/cm, depending on the synthesis conditions.38

It is worth noting that the density of defects in the thin films was highly correlated with the mechanical and electrical properties of the samples. The presence of defects, including grain boundary defects, has been shown to be serious issue for low-k dielectric due to the stress induced by electromigration,51 and lower the selectivity of zeolite membranes for separations. In the past, several approaches have been developed to minimize the grain boundary defects in the inorganic thin films.52-53 In this work, the low leakage current of APPJ treated thin films demonstrate that using APPJ-assisted synthesis to form uniform nanocrystal of zeolite MFI is an effective approach to minimize the grain boundary defects. However, only qualitative tools, such as SEM54 and confocal microscopy,53, 55-56 were available for the assessment of the defects in zeolite membranes. We propose that the Young’s modulus and leakage current could be used as quantitative tool by which to determine the density of defects in zeolite thin films or membranes.

Figure 9 summarizes the dielectric constants (k) of the thin-film samples. Measurements obtained at a low frequency (10 kHz) and high frequency (500 kHz) both revealed that zeolite thin films that underwent APPJ treatment possessed a lower k value. They also presented a lower


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standard deviation in the measurements of dielectric constant thanks to the uniformity of the thin films. The difference in k values between samples synthesized with and without APPJ treatment can be attributed to the porosity of the samples. In particular, zeolite MFI (6hr) and zeolite MFI (9hr) presented lower crystallinity than zeolite MFI (30s-6hr) and zeolite MFI (60s-6hr). The ordered micropore due to the MFI topology in zeolite MFI (30s-6hr) and zeolite MFI (60s-6hr) results lower k values than samples with lower crystallinity (zeolite MFI (6hr) and zeolite MFI (9hr)). Nitrogen physisorption tests were conducted on some of the powder samples for the characterization of porosity, the results of which are summarized in Figure 10. The samples synthesized with APPJ treatment (zeolite MFI (30s-6hr) and zeolite MFI (60s-6hr)) presented higher adsorption uptake indicative of higher pore volume, compared to the sample without APPJ treatment (zeolite MFI (11hr)). All of the isotherms presented hysteresis between adsorption and desorption, which is indicative of mesopores (2 nm < pore size < 50 nm). Zeolite crystals have intrinsic micropores (pore size < 2 nm); therefore, we surmise that the mesopore can be attributed to interstitial spacing among the packed particles, imparted by the addition of TWEEN® 80.39, 57 The mesopore size distribution as well as the mesopore volume derived from the nitrogen physisorption isotherms are presented in the Supporting Information (Figure S2 and Table S1). The high pore volume of samples with APPJ treatment (mainly mesopores) may explain the low k values of these samples. The k values of MFI (30s-6hr) and MFI (60s-6hr) at 500kHz were 1.73 and 1.41 respectively, which exceeded the benchmark low-k zeolite MFI thin films reported in a previous work.38 Figure 11 summarizes the k values obtained in this work and those reported previously.19,

22, 26, 38

It suggests that the zeolite thin films prepared using the

APPJ-assisted synthesis present lower dielectric constants than the existing zeolite MFI films fabricated using other methods.


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Conclusions

In this study, we demonstrate the efficacy of using atmospheric pressure plasma jet (APPJ) as a tool in the synthesis of pure-silica zeolite MFI suspensions for use in the wet deposition of thin films. The proposed method overcomes the conventional trade-off between high crystallinity and quality of zeolite thin films made via direct wet deposition. Specifically, we applied APPJ treatment to the aging solution used in zeolite synthesis for durations of 30 – 60 s prior to the hydrothermal reaction. The resulting zeolite suspensions possessed zeolite crystals with crystallinity exceeding that of samples without plasma treatment as well as a narrower particle size distribution. The high crystallinity will result in thin films with ordered micropores, and the uniform particle size distribution will lead to ordered packing among particles to create interstitial mesopores. These physical properties will lead to thin films with high elastic modulus and low dielectric constant. The suspensions were then deposited on a silicon wafer substrate via spin-on deposition to form thin films. The zeolite thin films prepared using APPJ-assisted synthesis presented fewer cracks, a higher Young’s modulus, and less leakage current than did samples synthesized without APPJ treatment. APPJ-assisted synthesis also resulted in thin films with higher mesoporosity, which resulted in a lower dielectric constant (k). The low-k thin films prepared after APPJ treatment for 60 s presented a k value of just 1.41.


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ASSOCIATED CONTENT Supporting Information Spots for nanoindentation tests, force versus displacement curves obtained from nanoindentation tests, mesopore size distributions, mesopore volumes of zeolite MFI samples.

AUTHOR INFORMATION Corresponding Authors Cheng-Che Hsu E-mail: [email protected] Dun-Yen Kang E-mail: [email protected]

Funding Sources This work was supported by the Ministry of Science and Technology (MOST) of Taiwan (MOST 104-2628-E-002-009-MY3 and MOST 105-2221-E-002-056-MY2). Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT The authors acknowledge technical support from Dr. Ming-Tao Lee as well as all of the technical staff at beamline 13A in National Synchrotron Radiation Research Center of Taiwan.


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Figures

Figure 1. (a) Illustration of APPJ setup and (b) photograph of APPJ during operation.


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Figure 2. Photographic images showing the zeolite MFI suspensions (a) without APPJ treatment and (b) with APPJ treatment; (c) particle size distributions of zeolite MFI suspensions.


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Figure 3. XRD patterns obtained from powder samples produced using conventional hydrothermal reaction and proposed APPJ-assisted method.


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Figure 4. GIWAXS patterns of zeolite MFI thin films synthesized under various conditions.


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Figure 5. Top-view SEM images of zeolite MFI thin films synthesized under various conditions.


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11hr

60s-6hr

500nm

9hr

500nm

30s-6hr

500nm

500nm

6hr

500nm

Figure 6. Cross-sectional SEM images of zeolite MFI thin films synthesized under various conditions. Thickness of films: 11hr (540 nm), 9hr (290 nm), 6hr (260 nm); 60s-6hr (250 nm), 30s-6hr (200 nm).


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Figure 7. Young’s modulus of zeolite MFI thin films synthesized under various conditions.


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Figure 8. Leakage current in zeolite MFI thin films synthesized under various conditions, as measured at -1 V.


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(a)

(b)

Figure 9. Dielectric constant (k) of zeolite MFI thin films measured at (a) 10 kHz, and (b) 500 kHz.


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Figure 10. Nitrogen physisorption isotherms of zeolite MFI (11hr) (square), zeolite MFI (30s6hr) (circle), and zeolite MFI (60s-6hr) (triangle).


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Figure 11. Young’s Modulus versus dielectric constant of the zeolite MFI films prepared in this work and those reported previously.


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