Addition of Surfactant Tween 80 in Coating Solutions for Making

Jun 22, 2010 - were strongly affected by the weight ratio of Tween 80 to TEOS and by the .... produced from coating solutions composed of surfactant T...
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Ind. Eng. Chem. Res. 2010, 49, 6279–6286

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Addition of Surfactant Tween 80 in Coating Solutions for Making Mesoporous Pure Silica Zeolite MFI Low-k Films Hsin-Yan Lu, Chin-Lin Teng, Chun-Wei Yu, Yu-Chiao Liu, and Ben-Zu Wan* Department of Chemical Engineering, National Taiwan UniVersity, Taipei 106, Taiwan

A mesoporous pure-silica-zeolite (PSZ) MFI low dielectric (k) film with smooth surface morphology was successfully synthesized using a centrifuged coating solution composed of a solution with PSZ MFI nanoparticles and surfactant Tween 80. The zeolite nanoparticles were prepared using TEOS as the silica source through a two-stage hydrothermal process. Both the k value and the mechanical strength of the film were strongly affected by the weight ratio of Tween 80 to TEOS and by the hydrothermal period of the second stage. When a higher weight ratio of Tween 80 to TEOS was employed, a lower k value and mechanical strength were obtained. A film coated from a solution prepared under optimal experimental conditions possessed an ultra low k value of 1.83, a hardness of 1.39 GPa, an elastic module of 12.3 GPa, and a leakage current of 1.35 × 10-7 A/cm2, all of which met the needs of the integrated circuits (IC) industry. Moreover, the occurrence of electrical degradation of electronic devices could be minimized because only few pores in the film exceeded 5 nm. 1. Introduction As the packing density of metal lines in semiconductors continues to increase, so does the urgent need for intermetal dielectrics with ultralow dielectric constants (ultra low k value). ITRS 2008 established that the required value of the dielectric constant (k value) will be 1.9 as the gate length of CMOS devices approaches 20 nm in 2015.1 However, the k value of the traditionally used intermetal dielectric, dense silica, is around 3.9, which will soon be too large. Mesoporous silica film, a combination of mesopores filled with air and solid silica, can meet this requirement because the k value of this film can be adjusted using the pore volume. Several previous studies have produced mesoporous silica low k value (low-k) films using the surfactant-template method.2-10 The advantage of this method is its ability to create mesopores with controllable size and volume, and the k value can be reduced to smaller than 2 by simply increasing the porosity of the film. However, the drawback is the decrease of mechanical strength as the film porosity increases.8 The mechanical strength must be strong enough to withstand the process of chemical-mechanicalpolishing (CMP) during IC (integrated circuits) manufacturing. Therefore, the mechanical strength of mesoporous low-k films is in urgent need of improvement to meet the needs of practical applications, and, meanwhile, the k value should be maintained smaller than 2. The requirements of mechanical strength for industrial applications are that the elastic modulus should be larger than 10 GPa and that the hardness should be higher than 1 GPa.11 In recent studies, pure-silica-zeolite (PSZ, a microporous material with high crystallinity) MFI low-k films have been produced.12-15 The researchers adopted two hydrothermal processes to synthesize coating solutions containing PSZ MFI nanoparticles. These coating solutions were then spun on a silicon substrate to form the low-k films. The temperature of one of the hydrothermal processes was 80 °C in the study conducted by Wang et al. in 2001 and Li et al. in 2003.12-14 The other one used a two-stage variation of temperature process in the study conducted by Li et al. in 2004.15 Wang et al. * To whom correspondence should be addressed. E-mail: benzuwan@ ntu.edu.tw.

synthesized a film with a k value of 3, and the mechanical strength was found to be good enough for the IC industry after the film passed a polishing test.12 In their later study,13 an improvement was achieved because the k value was further reduced to 2.1, and the elastic modulus was between 16 and 18 GPa. Moreover, Li et al. in 2003 added a porogen γ-CD into PSZ MFI nanoparticle suspension to form a coating solution.14 The porogen could generate more pore volume in the film, and then reduced the k value of the low-k film, with an elastic modulus of 14.3 GPa, to around 1.7. After the 80 °C hydrothermal process, Li et al. in 2004 used a two-stage hydrothermal process,15 which began with a low hydrothermal temperature of 60 °C to produce nuclei of PSZ MFI nanoparticles, and, subsequently, the temperature was raised to 100 °C so that the nuclei could grow into crystals. The k value of the film synthesized from the so-obtained resulting solution was reported as low as 1.6, while the size of the zeolite nanoparticles increased to 75 nm. The elastic modulus of the film was 16 GP. The PSZ MFI low-k films previously synthesized with the hydrothermal processes seem to meet all of the requirements, k value < 2 and an elastic modulus of 10 GPa, of low-k films urgently needed by the IC industry.12-15 However, Eslava et al. repeated the experiment conducted by Wang et al., their first study in 2001,12 to synthesize PSZ MFI low-k films and obtained some results that could be fatal to the use of this kind of low-k films in the IC industry.16,17 It was found that the k values of the films were reduced mainly by large voids (>5 nm), which formed among the packing of large PSZ MFI nanoparticles. The large voids would cause a sealing problem that the diffusion of Cu metal into the pores of low-k films would occur during the later IC manufacturing process. The problem was an integration failure during the process and would result in electrical degradation of electronic devices in IC. They also demonstrated that the surface roughness of the PSZ MFI low-k films would increase with the particle size of the zeolite nanoparticles. Furthermore, Johnson et al. synthesized zeolite low-k films from coating solutions containing another PSZ zeolite (MEL type) nanoparticle,18 and they also found that the surface roughness of the films increased with the size of the zeolite nanoparticles.

10.1021/ie100203e  2010 American Chemical Society Published on Web 06/22/2010

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A rough surface may lead to poor measurements of electric or mechanical properties. For example, Hata et al. in 2008 observed that the surface of low-k film spin-coated from a coating solution containing only PSZ MFI nanoparticles was too rough to obtain useful data through spectroscopic ellipsometry.19 One drawback of the study was that the researchers did not provide any evidence, such as an image of the surface topography, to support their observation. Furthermore, to reduce the surface roughness of zeolite low-k films, Hata et al. added surfactants, P123 or L44, into a PSZ MEL nanoparticle suspension to form a coating solution. It was reported that the surface flatness of the low-k films spin-coated from the coating solution was improved after the addition of the surfactants. Mixing surfactant with zeolite nanoparticles to form coating solutions has also been used by several researchers to produce mesoporous zeolite low-k films.19-22 They used surfactant to produce mesoporous PSZ MEL low-k film with a k value of 1.96 and an elastic modulus of 5.18 GPa. However, the elastic modulus was too poor to be used in IC industry.20 To enhance the elastic modulus, they used an ultraviolet and a silylation treatment to treat the mesoporous PSZ MEL low-k film. The elastic modulus was further increased to 10 GPa, but the k value became a higher value, 2.18.22 In this study, mesoporous PSZ MFI low k films were produced from coating solutions composed of surfactant Tween 80 and PSZ MFI nanoparticles synthesized using a two-stage hydrothermal process (60 °C for 2 days in the first stage, and 100 °C for 36 or 48 h in the second stage), which was developed by Li et al. in 2004.15 The effects of the addition of surfactant Tween 80 and of the hydrothermal period of the second stage for making the coating solutions on the properties of low-k films were investigated. It was found that a film with a k value < 2, an elastic modulus > 10 GPa, and a hardness > 1 GPa could be successfully produced under the presence of Tween 80. The surface flatness of the films was characterized through optical microscopy, which clearly showed the difference between the films produced with and without the addition of the surfactant. Moreover, the total pore volume, pore size distribution, and the problem of large voids were also examined by means of nitrogen adsorption/desorption analysis. 2. Experimental Section 2.1. Preparation of Coating Solutions and Low-k Films. Coating solutions with PSZ MFI nanoparticles and surfactant were synthesized through the following steps. First, 20 mL of a solution containing TEOS (>99 wt %, Merck), EtOH (Acros Organics), TPAOH (25 wt %, Acros Organics), and DI water was mixed in a molar composition ratio of 1TEOS/ 5.6EtOH/0.36TPAOH/12.2H2O, and then the mixture was stirred at 30 °C for 3 days. After that, a two-stage hydrothermal reaction process, employed by Li et al. in 2004,15 was applied. The first stage was carried out at 60 °C for 2 days, and the second stage was at 100 °C for 36 or 48 h. The resulting solution was named solution HT36 or solution HT48, depending on the hydrothermal period of second stage. Twelve milliliters of solution HT36 or solution HT48 was mixed with a surfactant, Tween 80 (polyoxyethylene(20)sorbitan monooleate, Acros Organics), with a weight ratio of Tween 80 to TEOS (denoted as R in this study) of 0.05, 0.21, or 0.41, and then stirred at 30 °C for 3 h. Ten milliliters of this resulting solution was put into a plastic tube and centrifuged for 24 h with a centrifugation speed of 1000 rpm to remove large particles. Subsequently, 3 mL of the solution in the upper part of the plastic tube was taken as the coating solution to synthesize the mesoporous PSZ MFI low-k

films through the following steps. First, the coating solution was spin-coated on a precleaned 4-in. P-type (100) silicon wafer (Siltronic AG) at 2600 rpm for 30 s, using an SSP-01A spinner (King Polytechnic Engineering Co.). The precleaned wafer was first cleaned with HF (48-51 vol. %, Acros organic) solution and DI water with a volume ratio of 1HF/10H2O for 5 min, then immersed in a mixture of H2O2 (35 wt %, Acros organics), NH4OH (28-30 wt %, Acros organics), and DI water, with a volume ratio of 1H2O2/1NH4OH/50H2O, for 20 min. The spincoated film was then baked at 150 °C for 1 h and calcined at 450 °C for 5 h in air. Finally, after the film cooled to 110 °C, it was immersed into a mixture of hexamethyldisilazane (Acros Organics) and toluene (Acros Organics) at 80 °C for 1.5 h to modify the surface to a hydrophobic state. 2.2. Characterization. The coating solutions were dried at 60 °C for 3 h and 90 °C for 3 h to obtain dried samples, which were then calcined at 550 °C for 5 h to obtain powder samples. The dried samples were characterized by using a Bruker DSX300 NMR spectrometer, equipped with a commercial 4 mm cross-polarization magic-angle-spinning (CP-MAS) probe, to obtain solid-state 29Si spectra. The spectra collected were used for calculating intensity ratios between the Q4 and Q3 lines (Q4 ) Si(OSi)4, Q3 ) Si(OSi)3(OH)), which could determine the amount of the surface silanol groups of the particles in the dried samples. The powder samples were also characterized through nitrogen adsorption/desorption measurements at 77 K using a TriStar 3000 (Micromeritics) apparatus, to determine the pore volume and pore size distribution. Also, they were analyzed using X-ray powder diffraction measurement (X’Pert PRO (PANalytical)) to determine their crystallinity. Furthermore, 0.1 mL of solution HT36 or HT48, or the coating solutions, was diluted 100-fold with DI water. The diluted solutions were analyzed using dynamic light scattering (DLS) measurement (ZetaSizer Nano ZS (Malvern)) to acquire particle size distribution and the average particle size. Optical microscopy (VL-11S, Scalas) was utilized to characterize the surface morphologies of the PSZ MFI low-k films. To measure various physical properties (dielectric constant, leakage current density, hardness, and elastic modulus), the films were analyzed with several types of equipment. The capacitance was measured using a Keithley model 82 CV meter. The frequency and the oscillation level were 1 MHz and 100 mV, respectively. The dielectric constant was calculated on the basis of the capacitance in the accumulation region of the capacitance-voltage curve, the film thickness, and the area of the electrode (which was 0.0052 cm2). The film thickness was measured using cross-sectional scanning on a FE-SEM (LEO 1530). The leakage current density of the film was determined from the current-voltage characteristics, measured using a HP4156 semiconductor parameter analyzer with an electric field of 1 MV/cm. The hardness and the elastic modulus of the film were measured using a Nano Indenter XP (MTS) system. The obtained data at 1/10 film thickness were used for comparison. 3. Results and Discussion 3.1. Effects of Tween 80 Addition on the Surface Morphology of the Mesoporous PSZ MFI Low-k Films. Figure 1 presents the effects of Tween 80 addition on the surface morphologies of the mesoporous PSZ MFI low-k films, investigated with an optical microscope. Figure 1a shows a rough surface with many stripes on the film spin-coated from solution HT36 without containing any Tween 80. The width of these stripes was between 80 and 280 µm, and the color contrast of these stripes was strong, suggesting that the difference in

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Figure 1. Surface morphologies of the mesoporous PSZ MFI low-k films spin-coated from various solutions: (a) solution HT36, (b) a solution after solution HT36 was centrifuged, (c-e) three centrifuged solutions composed of solution HT36 and Tween 80 with Tween 80/TEOS weight ratios of 0.05, 0.21, and 0.41, respectively, and (f and g) two centrifuged solutions composed of solution HT48 and Tween 80 with Tween 80/TEOS weight ratios of 0.05 and 0.21, respectively.

Figure 2. SEM images: (a) a top view of two of the stripes shown in Figure 1a; (b and c) cross section views of the stripes in image a.

thickness among them was large. For example, Figure 2 displays the thicknesses of two close stripes shown in Figure 1a; the thicker stripe shown in Figure 2b is 265 nm thick, and the thinner one in Figure 2c is 84 nm thick. When this rough surface was examined by means of electronic (i.e., k value and leakage current) and mechanical (i.e., elastic modulus and hardness) measurements, the data fluctuated too seriously to be obtained, revealing that this surface was too rough to be used. This result is in agreement with that reported by Hata et al. in 2008.19 When solution HT36 without Tween 80 was centrifuged for 24 h, which was similar to a centrifugation process used by Wang et al. to remove the large particles in coating solutions,12 a film spin-coated from the resulting solution was still found in a coarse state, as shown in Figure 1b. It shows wide stripes with strong color contrast as that in Figure 1a, suggesting that applying centrifugation process for the coating solution did not improve the surface flatness of the film. Similar results were also reported by Petkov et al., but no film images were provided in their

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papers. In their research, a solution containing PSZ MFI nanoparticles was first centrifuged and then spun onto a silicon substrate. The resulting surface was reported to be not smooth.23 Moreover, as zeolite beta nanoparticles were used for spinning, the resulting film was also rough.24 From the above results (no matter from the literature and this study), it is hard to obtain a smooth film from a coating solution containing only zeolite nanoparticles. By adding Tween 80 with a Tween 80/TEOS weight ratio of 0.05 into solution HT36 and centrifuging the mixture for 24 h, a film with a smooth surface can be obtained from the resulting solution (see Figure 1c). No stripes were observed, and the largest difference in thickness (denoted as δ in Table 1) was 2 nm, which was much smaller than that (δ ) 230 nm) of the surface shown in Figure 1a. Therefore, the addition of Tween 80 can effectively improve the surface flatness of the mesoporous PSZ MFI low-k films. With a higher weight ratio of 0.21 in the centrifuged coating solution, the stripes (as indicated with a red arrow in Figure 1d) on the film seem to appear again, although they were not easily observed because the color contrast was not strong. After the weight ratio further increased to 0.41, the color contrast of the stripes became more apparent (see Figure 1e), indicating that too much Tween 80 in the centrifuged coating solution would negatively affect the surface smoothness. It should be noted that the δ value of the films also increased with the weight ratio from 0.05 to 0.41 (see Table 1). As the weight ratio in the centrifuged solution was 0.41, the δ value of the resulting film was as high as 16 nm. However, it was still much smaller than 230 nm in the film (shown in Figure 1a) spin-coated from solution HT36 without addition of any surfactant. The δ value of the film spin-coated from the centrifuged solution HT36 without Tween 80 addition was not measured because its surface morphology (see Figure 1b) was also very coarse as the film spin-coated from the uncentrifuged solution HT36 without Tween 80 addition (see Figure 1a). When longer hydrothermal time was used for the preparation of solution HT48, the film spin-coated from the solution without addition of Tween 80 cracked seriously after it was baked at 150 °C. Even though the solution was centrifuged, the spincoated film still cracked. Both of the coating situations from solution HT48 were much worse than those from solution HT36. The crystallinity of the particles in solution HT48 should be higher than that in solution HT36, suggesting that the particles with high crystallinity are not suitable for coating. However, the addition of a small amount of Tween 80 into solution HT48 can improve the situation. It can be found in Figure 1f and g that the surface morphologies of two films spin-coated from coating solutions, prepared by adding Tween 80 with Tween 80/TEOS weight ratios of 0.05 and 0.21 into solution HT48 and then centrifuging for 24 h, were smooth. Similar to the films spin-coated from two centrifuged solutions composed of solution HT36 and Tween 80 with the same weight ratios (see Figure 1c and d), no apparent stripes were found from the films with the weight ratio of 0.05; furthermore, the stripes appeared slightly as the ratio increased to 0.21. To sum, as the mesoporous PSZ MFI low-k films were spin-coated from the solution HT36 and solution HT48 containing only PSZ MFI nanoparicles, no matter if the solutions were centrifuged on not, the surfaces were very rough or even cracked. However, after adding Tween 80 into solution HT36 or solution HT48 and then centrifuging for 24 h to obtain coating solutions, surface morphologies of the films spin-coated from the solution became much smoother. To explain that the surface flatness of the films can be improved by adding Tween 80 into the coating solution, some

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Table 1. Film Thickness, Electronic, and Mechanical Properties of the Mesoporous PSZ MFI Low-k Films solutiona/second stage hydrothermal period (h)

Tween 80 to TEOS (weight ratio)

average film thickness (nm)/δb

HT36/36

0 0.05 0.21 0.41 0 0.05 0.21

250/230e 263/2 315/12 316/16 cracke 214/6 161/8

HT48/48

k value

leakage current (10-7 A/cm2)

Hc (GPa)

Ed (GPa)

2.98 ( 0.07 2.49 ( 0.05 1.83 ( 0.04

1.82 1.51 1.35

1.35 ( 0.51 1.47 ( 0.24 1.39 ( 0.31

15.9 ( 1.63 14.0 ( 1.25 12.3 ( 1.94

1.88 ( 0.05 1.81 ( 0.04

4.18 6.73

0.44 ( 0.15 0.37 ( 0.11

8.1 ( 3.04 5.4 ( 1.40

a The initial molar ratios in the solution before hydrothermal processes were 1TEOS/5.6EtOH/0.36TPAOH/12.2H2O. b δ represents the largest difference in measured thickness of the films. c H represents the hardness of the films. d E represents the elastic modulus of the films. e From solution HT36 and solution HT48 without going through the centrifugation process.

Figure 3. Schematic illustrations showing (a) a less hydrophilic surface of a particle; (b) a weak affinity occurred between the particle and the surface of the substrate due to a poor interaction of the less hydrophilic particle surface and the highly hydrophilic substrate; (c) a more hydrophilic surface of the particle because of adsorbing the hydrophobic tails of Tween 80; and (d) a strong affinity occurred between the particle and the substrate due to a strong interaction of the hydrophilic heads of Tween 80 and the -OH groups of the substrate. Table 2. Average Particle Sizes in the Coating Solutions and Total Pore Volumes of the Powder Samples Prepared From the Coating Solutions solutiona/ second stage hydrothermal period (h) HT36/36

HT48/48

Tween 80 to TEOS (weight ratio) c

0 0.05 0.21 0.41 0c 0.05 0.21

size (nm)

total pore volume (cm3/g)

41.9 40.8 38.3 35.4 55.7 40.3 38.0

0.322 0.226 0.318 0.397 0.575 0.463 0.652

b

a The initial molar ratios in the solution before hydrothermal processes were 1TEOS/5.6EtOH/0.36TPAOH/12.2H2O. b Average particle size. c From solution HT36 and solution HT48 without going through the centrifugation process.

schematic illustrations are presented in Figure 3. Figure 3a shows one of the particles in solution HT36 with a particle size between 38 and 42 nm (according to the data listed in Table 2). According to the coarse surface morphologies shown in Figure 1a and b, the affinity between the particle and the surface of a precleaned silicon substrate (which was very hydrophilic) was not strong enough to obtain a smooth surface. It is probable that the particle surface was more hydrophobic than the surface

of the silicon substrate (see Figure 3b), thus resulting in the poor interaction between them. As shown in Figure 3c, after a small amount of Tween 80 (with a weight ratio of 0.05) was added, the hydrophobic tail of Tween 80 could surround the particle surface because the surface contained only a few silanol groups. As a result, the hydrophilic heads of Tween 80 oriented toward the liquid phase composed of EtOH and water, thus leading the surface of the particle to become more hydrophilic. The resulting hydrophilic particles could disperse well in EtOH and water, and the affinity between particles and substrates was much improved because of the hydrophilic surfaces. Therefore, no stripe appeared on the film surface, as shown in Figure 1c and f. As more Tween 80 (with weight ratios of 0.21 and 0.41) was added, the micelles in the solution might be formed. Some of them aggregated and dissolved in EtOH and water. They might form the stripes with weak color contrast (see Figure 1d,e) after EtOH and water vaporized during spin-coating and baking processes. 3.2. k Value and Mechanical Strength of the Mesoporous PSZ MFI Low-k Films. It has been shown that the addition of Tween 80 in coating solutions can make the surfaces of the resulting films become smooth and flat. In addition, more pores should be generated in the films after the removal of Tween 80 during the calcination process at 450 °C. For the IC industry application, the k values and the mechanical strength

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of the films are very important, and both of them are strongly affected by the pore volume. Table 1 shows the data of electronic and mechanical strength (i.e., elastic modulus and hardness) of the mesoporous PSZ MFI low-k films spin-coated from coating solutions, prepared by adding Tween 80 with different Tween 80/TEOS weigh ratios into solution HT36 or solution HT48 and then centrifuging for 24 h. It shows a tendency that both the k value and the mechanical strength of the films decreased with the amount of Tween 80 in the coating solutions. As seen in Table 2, which shows the average particle sizes in the coating solutions and the total pore volumes of powder samples made from the coating solutions, the total pore volume increased with the weight ratio of Tween 80/TEOS. The increase of total pore volume can reduce the k value because air, inside the pores in the films, possesses the lowest k value (i.e., k Z 1) among all of the materials. The mechanical strength of the films became weaker when more pore volume was formed due to the removal of Tween 80. Notably, a film spin-coated from a centrifuged solution composed of solution HT36 and Tween 80 with a weight ratio of 0.41 possessed a k value of 1.83, a hardness of 1.39 GPa, and an elastic module of 12.3 GPa that all reached the needs of the IC industry. Moreover, comparing the mesoporous silica low-k films, prepared using the surfactant-template method in the previous research,2-10 with the mesoporous PSZ MFI low-k film prepared in this study, the later possessed not only an ultra low k value (1 GPa, the solutions containing nanoparticles with high crystallinity are not preferred to be used for spin-coating. As mentioned in the first section of this Article, the mesoporous PSZ MEL low-k films obtained by previous research showed a weaker elastic modulus, which were always lower than 10 GPa.19-22 To explain with the result shown in the above paragraph, the crystallinity of PSZ MEL nanoparticles produced in their works might be too high; therefore, the elastic modulus of the films was weak. 3.3. Effects of Tween 80 Addition on the Pore Sizes of the Mesoporous PSZ MFI Materials. According to the previous results in this study, the surfactant Tween 80 could not only improve the surface morphology of the mesoporous PSZ MFI low-k films, but also generate more pore volume to reduce the k values of the films. However, the pores with sizes

smaller than 5 nm are necessary to avoid the sealing problem during the later IC manufacture processes.16,17 Therefore, it is important to check the pore size distribution of the low-k materials. Figure 6a and b presents the nitrogen adsorption/ desorption isotherms and the pore size distribution curves, respectively, of the powder samples prepared from solution HT36 without Tween 80 addition, and centrifuged solutions composed of solution HT36 and Tween 80 with different weight ratios of Tween 80/TEOS. The black lines in both figures are the results of the sample prepared from solution HT36 without Tween 80 addition. A slight hysteresis in the isotherm (see Figure 6a) was observed between relative pressures P/Po of 0.4 and 0.6, indicating that some mesopores (with pore size < 5 nm) formed (see Figure 6b). These mesopores were considered to be purely from particle packing because no surfactant was added. Similar results were also reported in the literature.16-18 Even though the mesopore size did not exceed 5 nm, the surface morphology of the films coated from the coating solutions was very coarse (see Figure 1a and b). For the powder sample prepared from a centrifuged solution composed of solution HT36 and Tween 80 with a weight ratio of 0.05, almost no hysteresis was observed from the isotherm, indicating that only a few mesopores formed (the red lines in Figure 6a and b). The adsorption quantity at the pore sizes between 3 and 4 nm, as shown in Figure 6b, and the total pore volume (see Table 2) were both smaller than those of the sample made from solution HT36 without Tween 80 addition. It was

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Figure 7. (a) Nitrogen adsorption/desorption isotherms and (b) pore size distribution curves from the powder samples prepared from solution HT48 without Tween 80 addition, and the centrifuged solutions composed of solution HT48 and Tween 80 with different weight ratios of Tween 80/TEOS.

because the large particles in the coating solution were removed by using the centrifugation process. The average particle size was reduced to 40.8 nm, as shown in Table 2, resulting in that some of the pores formed by particle packing disappeared. As the weight ratio increased to 0.21 and 0.41, type-E hysteresis loops (see Figure 6a) were observed from the isotherms of the powder samples,25 indicating that mesopores formed due to Tween 80 addition. The majority of the mesopores of both samples distributed uniformly between 2 and 4 nm, as shown in Figure 6b. However, for the sample with a weight ratio of 0.41, a few pores were between 4 and 5 nm and only few of them exceeded 5 nm (which can cause the sealing problem, an integration failure, in the later Cu interconnect process). Therefore, the occurrence of electrical degradation of electronic devices in the films prepared in this research could be minimized. Figure 7a and b presents isotherms and pore size distribution curves, respectively, of the powder samples prepared from solution HT48 without Tween 80 addition, and centrifuged solutions composed of solution HT48 and Tween 80 with different weight ratios of Tween 80/TEOS. The black lines in both figures are the analyzed results from the sample prepared from solution HT48 without Tween 80 addition. A very large hysteresis can be observed in Figure 7a. The pore size (shown in Figure 7b) distributes between 2 and 13.5 nm. Certainly, the pores were formed only by the particle packing. For the powder sample prepared from a centrifuged solution composed of solution HT48 and Tween 80 with a weight ratio of 0.05, the pore size distributed (the red line in Figure 7b) mainly between 3 and 5 nm, and some pores were larger than 5 nm. The distribution was much narrower than that of the sample prepared from solution HT48 without Tween 80 addition because of removing the large particles by using the centrifugation process. As the weight ratios further increased to 0.21, the pore size distribution, the blue line in Figure 7b, shows bimodal distribution peaks where one was between 3 and 4 nm and the other was between 6.5 and 9 nm. The latter was too large to avoid the sealing problem in the later IC manufacture processes. Even though the k values of the films coated from the centrifuged solutions composed of solution HT48 and Tween 80 with two weight ratios were much smaller than 2, their mechanical strength was very poor, and both of the pores were too large (larger than 5 nm) to avoid the occurrence of electrical degradation of electronic devices.16,17

4. Conclusions In this study, the solutions with PSZ MFI nanoparticles were synthesized using a two-stage hydrothermal process (60 °C for 2 days in the first stage, and 100 °C for 36 or 48 h in the second stage). The so-obtained solutions were mixed with the surfactant Tween 80 and then centrifuged to obtain coating solutions. The addition of surfactant Tween 80 not only improved the surface flatness but also generated more pores with mesopore size in the resulting mesoporous PSZ MFI low-k films. When the films were spin-coated from a solution containing only PSZ MFI nanoparicles, the resulting surfaces were very rough or even cracked, and the steady electronic and mechanical strength data could not be obtained. However, after the addition of Tween 80, the surface morphology of the films spin-coated from the resulting coating solution was much improved, and the electronic and mechanical strength properties could be measured. The effects of the hydrothermal periods of the second stage (i.e., 36 and 48 h) on coating solution (with Tween 80) and on film properties were investigated. It was found that the crystallinity and the pore volume of the nanoparticles increased substantially with the increase of the second stage hydrothermal period. The films prepared from the coating solutions with longer hydrothermal period possessed lower k values, very weaker mechanical strength, and higher leakage current. Therefore, only the coating solution from the shorter hydrothermal period (i.e., 36 h) is suggested to be used for spin-coating the low k films, and by adding Tween 80 with a Tween 80/TEOS weight ratio of 0.41 into the solution, the spin-coated film possesses a k value of 1.83, a hardness of 1.39 GPa, an elastic module of 12.3 GPa, and a leakage current of 1.35 × 10-7 A/cm2, all of which reach the needs of the IC industry in the future. Moreover, the film has a minor sealing problem in the later IC manufacturing processes because only few pores exceeded 5 nm. Acknowledgment We would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research, the National Nano Device Laboratory, Hsinchu, Taiwan, for the support of electronic properties and mechanical strength measurement, and the Particulate Technology Laboratory in the Department of Chemical Engineering, National Taiwan University, for the support of surface morphology and particle size characterizations.

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ReceiVed for reView January 28, 2010 ReVised manuscript receiVed May 19, 2010 Accepted June 7, 2010 IE100203E