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Effect of processing temperature on the properties of sol–gel-derived mesoporous silica films
Thin Solid Films 462 – 463 (2004) 306 – 310 www.elsevier.com/locate/tsf
Effect of processing temperature on the properties of sol–gel-derived mesoporous silica films Suzhu Yu a,*, Terence K.S. Wong a, Xiao Hu b, Tat Kean Goh a a
Photonics I, School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore b School of Materials Engineering, Nanyang Technological University, Singapore 639798, Singapore Available online 4 July 2004
Abstract Mesoporous organically modified silicon oxides were fabricated using multi-step sol – gel technique with tetraethyl orthosilicate (TEOS) and methyltriethoxysilane MTES) as precursors. The characterization of the dielectric films with Fourier transform infrared, thermogravimetric (TG) and differential thermal analysis (DTA) as well as dielectric analyzer showed the effect of annealing temperature on the development of porosity and dielectric properties of the films. The pore size was measured by small angle X-ray scattering (SAXS) and was found that the annealing temperature did not affect the pore size very much during the experimental temperature range. An ultra-low dielectric constant of about 2.1 was realized for about 57% porosity in the silica film. It was shown that the method employed is useful for preparing new kinds of films used as interlayer dielectrics. D 2004 Elsevier B.V. All rights reserved. Keywords: Low dielectric constant; Small angle X-ray scattering; Sol – gel; Porous films
1. Introduction Prediction from the semiconductor industry [1] indicates that 130-nm node will be realized soon. In this node and below, interlevel metal dielectrics (ILD) with dielectric constant (k) V 2.2, which is significantly less than that of currently used silica-based insulators (k = 4.0), will be required to reduce the resistance – capacitance coupling and consequently reduce the propagation delay, cross-talk noise and power dissipation [2,3]. The lower dielectric constant materials can be obtained through incorporation of the matrix material with air because air has the lowest dielectric constant of 1. Among porous materials, porous silica films are more promising, because they usually have good mechanical strength and thermal stability and most importantly, they are compatible with the silicon wafer and related materials used in existing IC technology. For porous materials used as ILD, the porosity and pore size are critical [4]. Higher porosity in the films can lower the dielectric constant, but normally it also has negative impact on other
* Corresponding author. Tel.: +65-67906319; fax: +65-67904161. E-mail address: [email protected] (S. Yu). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.05.022
crucial properties of the films such as mechanical and thermal properties. Consequently, porosity should be no higher than needed to achieve the desired dielectric goals. Pore dimensions being much smaller than the feature size of the devices are desirable to minimize film defects and reduce the risk of short circuits. Therefore, it is necessary to characterize the nature of the porosity to guide the synthetic efforts and to correlate a variety of electrical and mechanical properties. However, a quantitative description of the nanoscaled porous morphology in low-k dielectrics can be difficult to achieve for many reasons, including: complexities in the porous structure, inadequate mathematical descriptors, limitations of existing metrology technology and so on. One possible technique is small angle X-ray scattering (SAXS) because it can achieve film-on-wafer measurement and its sensitivity over a broad range of pore sizes (from 2 to 200 nm) [5]. This paper focus on the synthesis and characterization of porous films with low dielectric constant. The organic modified silica films have been synthesized by sol– gel process, which is a direct way to establish the porous network [6]. The effect of annealing temperature on the structural and dielectric properties of the films will be investigated systematically. The porosity of the films is
S. Yu et al. / Thin Solid Films 462 – 463 (2004) 306–310
obtained from optical reflectometer; the pore size of the films is measured using SAXS and deduced from a theoretic structural model.
2. Experiment 2.1. Film preparation Films with composite structure of highly porous clusters embedded into a dense matrix were fabricated using a multiple-step sol – gel process. Two types of sols were prepared: sol A consisting of linear oligomers, which would form a dense matrix, was prepared by adding tetraethyl orthosilicate (TEOS) in ethanol with HCl as a catalyst. Sol B consisting of highly branched clusters, which would form porous clusters, was made by dispersing methyltriethoxysilane (MTES) in ethanol with NH4OH as a catalyst. Both sols were allowed to hydrolyze for about 24 h before mixing them together with molar ratio of sol A/sol B = 1:1. The AB sol was spun onto the silicon substrates by commercial spin coater in an ambient environment. Following spin coating, the as deposited films were baked and soaked into hexamethyldisilazane (HMDS)– toluene solution to conduct the surface modification by replacing the unconverted surface silanol groups with unreactive methyl groups. The surface modified films were then thermally treated in air and further dehydroxylation treated in nitrogen for 30 min, respectively. The AB sol was also poured into a petri dish to form xerogel for thermogravimetric and differential thermal analysis (TGA/DTA). The above-mentioned preparations were all carried out in a class 100 clean room. 2.2. Film characterization The refractive index and thickness of the porous films were measured using thin-film measurement system (Filmmetrics F20). For some samples, these properties were also confirmed by ellipsometry (VB 250, VASE Ellipsometer). The porosity of the films can be calculated from the refractive index based on the Lorentz – Lorenz relationship [7]: ðn2f 1Þ=ðn2f þ 2Þ ¼ ð1 Vf Þðn2s 1Þ=ðn2s þ 2Þ
ð1Þ
where nf, ns is the refractive index of porous film and solid skeleton respectively, Vf is the volume fraction of the pores. FTIR measurements were carried out on a PerkinElmer Spectrum 2000 spectrometer. Thermal stability and chemical reactivity were investigated by TGA/DTA (Perkin-Elmer 7). For dielectric properties measurements, the films were prepared on heavily doped, n type silicon wafers. An array of titanium/gold dots was evaporated onto the film in an e-beam evaporator through a shadow mask. A blanket layer of titanium/gold was also deposited on the
307
backside of the wafer to act as a second contact. The dielectric and leakage current behaviors were evaluated using Keithley CV analyzer at frequency of 1 MHz and HP 4155A semiconductor parameter analyzer in ambient condition. For SAXS measurement, the films were deposited on mica substrates with thickness of 100 Am. The thin substrate increases the transmission of X-ray through the films to obtain stronger signal-to-noise ratio. It is performed by focusing a low divergence X-ray beam onto a sample and observing a coherent scattering pattern that arises from electron density inhomogeneities within the sample. In our case, the inhomogeneity of the distribution of electron density arises from sol – gel-derived SiO2 network and pores. According to Babinet theorem [8,9], an ensemble of cavities in a homogeneous medium could be considered equally well in place of an ensemble of particles. From this, it is possible to consider the pores as the convenient ‘scatterer’. In SAXS experiment, the electron density variation can be expressed by the measured scattered intensity, I(q), as a function of the magnitude of scattering vector, q, by the following model [8]: IðqÞ ¼ NFðqÞSðqÞ
ð2Þ
q ¼ ð4p=kÞsinh
ð3Þ
where N is number of effective scatterers, F(q) is a form factor and S(q) is a structure factor, 2h is the scattering angle and k is the wavelength of the radiation. Form factor F(q) is a function describes shape of the scattering pores. Structure factor S(q) describes the effect of the geometric spatial arrangement of the pores. Assuming weak interaction between pores, S(q) is constant, so I(q) is only dependent of F(q). In the case of spherical pore and the pore sizes have a Gaussian distribution centered at r with a standard deviation r, I(q) can be described as [8]: Z
l
1 sinðqrÞ qr cosðqrÞ pffiffiffiffiffiffiffiffiffiffi 3 IðqÞ ¼ 2 ðqrÞ3 2pr 0 ! ðri rÞ2 exp dr 2r2
!2
ð4Þ
where r is the pore radius, ri is the other pore radius different from r. The experiments were carried out using a Bruker Nano˚ ). The q star instrument. The radiation was Cu Ka (k = 1.54 A range covered in SAXS was from 0.05 to 0.15 nm 1 and the scanning time was 50,000 s per sample. The scattered intensities were corrected for transmission and deducted from parasitic and substrate scattering. After scattering
308
S. Yu et al. / Thin Solid Films 462 – 463 (2004) 306–310
curves were collected, the data were fitted with the proposed model (Eq. (4)) to determine the pore size using FISH software (SAXS software by Heenan, RAL).
3. Results and discussion In order to investigate the effect of processing temperature on the properties of the films, the films were heattreated at 300, 400, and 450 jC, respectively. The FTIR spectra of films heat-treated at different temperatures are shown in Fig. 1. The most intense absorption peak located at 1060 cm 1 is the transverse optical (TO) vibration of Si– O – Si linkage. The peak centered around 1100 cm 1 as a shoulder at the high frequency end of 1060 cm 1 peak assigns to the corresponding longitudinal optical (LO) vibration of Si– O – Si linkage. Though light was incident normal to the sample, Almeida and Pantano [10] explained the appearance of the LO shoulder as scattering due to the porous nature of the samples. The peak located in 775 cm 1 is associated with symmetric stretching of Si– O – Si linkage. The broad peak between 3300 and 3700 cm 1 corresponds to a stretching of – OH and physisorbed moisture on the surface in several modes. There was a very great –OH peak for the as-deposited film indicating the high content of the silanol groups. With increasing temperature, the peak intensity around 3300 – 3700 cm 1 decreased gradually, so more silanol groups were removed at higher temperature. Furthermore, the peak area around 1100 cm 1 was also increased with increasing annealing temperature, suggesting the porosity of the films increased. Note that the peak around 1100 cm 1 was not present for the as-deposited film, so there were not many pores in such a film. The main reasons of higher porosity at higher temperature are: (1) at higher temperature, the sol – gel reactions rates are faster and the reactions are more complete leading to more porous nature; (2) the Si-CH3
Fig. 2. TGA/DTA of silica gel.
bond pyrolyzes at higher temperature leaving pores behind. The later can be confirmed by the TGA/DTA analysis (Fig. 2). Two obvious weight loss stages can be seen in Fig. 2. The weight loss below 100 jC was due to the evaporation of solvent and water molecules trapped in the materials, supporting by the presence of an endothermic peak at 100 jC in DTA. The weight loss around 400 jC corresponding to a sharp exothermic peak around 400 jC in DTA attributed to the pyrolysis of CH3 [11], which was incorporated into the silica network during the sol – gel processing through copolymerization. The removal of organic groups left cavities in the films which increased the porosity. Because the burntoff of the CH3 groups started at around 400 jC, the film treated at 300 jC would retain more organic ligand causing lower porosity compared to the films heat treated at 400 and 450 jC. The refractive index, porosity and dielectric constant of the films annealed at different temperatures are tabulated in Table 1. The porosity of the films increased with temperature, which was consistent with FTIR measurement. The porosity of 300 jC film was found to be much lower and the dielectric constant of it was much higher than that of other two samples; whereas the properties of both 400 and 450 jC samples were more close. The changes of porosity and dielectric constant of the 400 and 450j C film with respect to 300j C film are shown in Fig. 3. The porosity increased about 93.46% and 134.4% and the dielectric constant decreased by 26.28% and 32.37% for 400 and 450 jC
Table 1 Effect of thermal treated temperature on the properties of the films Temperature (jC) Refractive index Porosity (%) Dielectric constanta Fig. 1. FTIR spectra of films annealed at different temperature.
a
300 1.31 24.30 3.12
400 1.22 47.01 2.30
Data were obtained on the third day of fabrication.
450 1.18 56.96 2.11
S. Yu et al. / Thin Solid Films 462 – 463 (2004) 306–310
309
Fig. 3. The changes of dielectric constant and porosity of the films with respect to the 300 jC film.
films, respectively, compared to that of 300 jC film. The higher porosity with increasing temperature results in lower dielectric constant of the films. The film with dielectric constant of around 2.1 could obtain for the film annealing at 450 jC. With combination of Figs. 1 and 2 and Table 1, it is concluded that the higher porosity and lower silanol content in the films are crucial to obtain lower dielectric constant. Similarly, the leakage current of the silica films at 1 MV/ cm was also reduced with increasing temperature (Fig. 4). The leakage current density at 1 MV/cm for the 300 jC film was as high as 8.4 10 3 A/cm2, it reduced to 8.1 10 6 A/cm2 for 400 jC film and 1.6 10 7 A/cm2 for 450 jC film, respectively. The pore size of the films annealed at different temperature was evaluated by SAXS. In contrast to real space methods such as electron microscopy, the information obtained from SAXS measurements is generated from larger sampling volume, as the X-ray beam width is 1 – 5 mm, the beam height is 100 Am. Thus, these experi-
Fig. 5. Experimental results and data fitting of SAXS measurement for the films annealed at different temperature.
Fig. 4. I – V property of different temperature treated films.
ments provide complementary information on a ‘‘global’’ scale. Fig. 5 shows the experimental and fitting data of SAXS plots for different temperature heat-treated films. For all the samples independent of annealing condition, the strongest scattering intensity appeared at very small of q, then it constantly reduced until q = 0.03 nm 1. For q = 0.03 nm 1 above, the intensity is almost constant. The
310
S. Yu et al. / Thin Solid Films 462 – 463 (2004) 306–310
Table 2 SAXS data fitting results for films heated at different temperature Treatment condition (jC)
Average pore diameter (nm)
300 400 450
32.48 33.54 33.94
very much with annealing temperature of 300 to 450 jC. This work also demonstrates that the SAXS is a relative simple, precise and non-destructive method to measure the pore size of thin films.
References pore size of the films derived from the data fitting is tabulated in Table 2. The annealing temperature in the range of 300– 450 jC did not affect the pore size very much. So the annealing temperature of 450 jC did not cause further interconnection of the pores, which will lead to larger pore dimension. The pore size of 450 jC film is about 34 nm, which is in the range of mesopores.
4. Conclusion Uniform mesoporous organosilicate films with a composite structure of highly porous clusters embedded into a dense matrix were fabricated through sol – gel process. The processing temperature had great effect on the properties of the films. With increasing annealing temperature, porosity in the films increased, dielectric constant of the films decreased. However, the pore size of the films did not change
[1] International Technology Roadmap for Semiconductors, Semiconductor Industry Association, Gaithersburg, MD, 2001. [2] R. Wilson, C.J. Tracy, J.L. Freeman, Handbook of Multilevel Metallization for Integrated Circuits: Materials, Technology, and Applications, Noyes Publications, Park Ridge, USA, 1993. [3] D.C. Edelstein, G.A. Sai-Halasz, Y.J. Mii, IBM J. Res. Develop. 39 (1995) 4. [4] S.S. Prakash, C.J. Brinker, A.J. Hurd, S.M. Rao, Nature (London) 374 (1995) 439. [5] E. Lifshin (Ed.), X-ray Characterization of Materials, Wiley-VCH, New York, 1999. [6] C.J. Brinker, G.W. Scherer, Sol – Gel Science, Academic, San Diego, CA, 1990. [7] M. Born, E. Wolf, Principles of Optics, Pergamon Press, New York, 1983, p. 87. [8] A. Guinier, G. Fournet, Small-angle Scattering of X-rays, Wiley, New York, 1955. [9] P. Mittelbach, G. Porod, Acta Phys. Austria 14 (1961) 185. [10] R.M. Almedia, C.G. Pantano, J. Appl. Phys. 68 (1990) 4225. [11] S. Seraji, Y. Wu, M. Forbess, S.J. Limmer, T. Chou, G. Cao, Adv. Mater. 22 (2000) 12.
Effect of processing temperature on the properties of sol–gel-derived mesoporous silica films Suzhu Yu a,*, Terence K.S. Wong a, Xiao Hu b, Tat Kean Goh a a
Photonics I, School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore b School of Materials Engineering, Nanyang Technological University, Singapore 639798, Singapore Available online 4 July 2004
Abstract Mesoporous organically modified silicon oxides were fabricated using multi-step sol – gel technique with tetraethyl orthosilicate (TEOS) and methyltriethoxysilane MTES) as precursors. The characterization of the dielectric films with Fourier transform infrared, thermogravimetric (TG) and differential thermal analysis (DTA) as well as dielectric analyzer showed the effect of annealing temperature on the development of porosity and dielectric properties of the films. The pore size was measured by small angle X-ray scattering (SAXS) and was found that the annealing temperature did not affect the pore size very much during the experimental temperature range. An ultra-low dielectric constant of about 2.1 was realized for about 57% porosity in the silica film. It was shown that the method employed is useful for preparing new kinds of films used as interlayer dielectrics. D 2004 Elsevier B.V. All rights reserved. Keywords: Low dielectric constant; Small angle X-ray scattering; Sol – gel; Porous films
1. Introduction Prediction from the semiconductor industry [1] indicates that 130-nm node will be realized soon. In this node and below, interlevel metal dielectrics (ILD) with dielectric constant (k) V 2.2, which is significantly less than that of currently used silica-based insulators (k = 4.0), will be required to reduce the resistance – capacitance coupling and consequently reduce the propagation delay, cross-talk noise and power dissipation [2,3]. The lower dielectric constant materials can be obtained through incorporation of the matrix material with air because air has the lowest dielectric constant of 1. Among porous materials, porous silica films are more promising, because they usually have good mechanical strength and thermal stability and most importantly, they are compatible with the silicon wafer and related materials used in existing IC technology. For porous materials used as ILD, the porosity and pore size are critical [4]. Higher porosity in the films can lower the dielectric constant, but normally it also has negative impact on other
* Corresponding author. Tel.: +65-67906319; fax: +65-67904161. E-mail address: [email protected] (S. Yu). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.05.022
crucial properties of the films such as mechanical and thermal properties. Consequently, porosity should be no higher than needed to achieve the desired dielectric goals. Pore dimensions being much smaller than the feature size of the devices are desirable to minimize film defects and reduce the risk of short circuits. Therefore, it is necessary to characterize the nature of the porosity to guide the synthetic efforts and to correlate a variety of electrical and mechanical properties. However, a quantitative description of the nanoscaled porous morphology in low-k dielectrics can be difficult to achieve for many reasons, including: complexities in the porous structure, inadequate mathematical descriptors, limitations of existing metrology technology and so on. One possible technique is small angle X-ray scattering (SAXS) because it can achieve film-on-wafer measurement and its sensitivity over a broad range of pore sizes (from 2 to 200 nm) [5]. This paper focus on the synthesis and characterization of porous films with low dielectric constant. The organic modified silica films have been synthesized by sol– gel process, which is a direct way to establish the porous network [6]. The effect of annealing temperature on the structural and dielectric properties of the films will be investigated systematically. The porosity of the films is
S. Yu et al. / Thin Solid Films 462 – 463 (2004) 306–310
obtained from optical reflectometer; the pore size of the films is measured using SAXS and deduced from a theoretic structural model.
2. Experiment 2.1. Film preparation Films with composite structure of highly porous clusters embedded into a dense matrix were fabricated using a multiple-step sol – gel process. Two types of sols were prepared: sol A consisting of linear oligomers, which would form a dense matrix, was prepared by adding tetraethyl orthosilicate (TEOS) in ethanol with HCl as a catalyst. Sol B consisting of highly branched clusters, which would form porous clusters, was made by dispersing methyltriethoxysilane (MTES) in ethanol with NH4OH as a catalyst. Both sols were allowed to hydrolyze for about 24 h before mixing them together with molar ratio of sol A/sol B = 1:1. The AB sol was spun onto the silicon substrates by commercial spin coater in an ambient environment. Following spin coating, the as deposited films were baked and soaked into hexamethyldisilazane (HMDS)– toluene solution to conduct the surface modification by replacing the unconverted surface silanol groups with unreactive methyl groups. The surface modified films were then thermally treated in air and further dehydroxylation treated in nitrogen for 30 min, respectively. The AB sol was also poured into a petri dish to form xerogel for thermogravimetric and differential thermal analysis (TGA/DTA). The above-mentioned preparations were all carried out in a class 100 clean room. 2.2. Film characterization The refractive index and thickness of the porous films were measured using thin-film measurement system (Filmmetrics F20). For some samples, these properties were also confirmed by ellipsometry (VB 250, VASE Ellipsometer). The porosity of the films can be calculated from the refractive index based on the Lorentz – Lorenz relationship [7]: ðn2f 1Þ=ðn2f þ 2Þ ¼ ð1 Vf Þðn2s 1Þ=ðn2s þ 2Þ
ð1Þ
where nf, ns is the refractive index of porous film and solid skeleton respectively, Vf is the volume fraction of the pores. FTIR measurements were carried out on a PerkinElmer Spectrum 2000 spectrometer. Thermal stability and chemical reactivity were investigated by TGA/DTA (Perkin-Elmer 7). For dielectric properties measurements, the films were prepared on heavily doped, n type silicon wafers. An array of titanium/gold dots was evaporated onto the film in an e-beam evaporator through a shadow mask. A blanket layer of titanium/gold was also deposited on the
307
backside of the wafer to act as a second contact. The dielectric and leakage current behaviors were evaluated using Keithley CV analyzer at frequency of 1 MHz and HP 4155A semiconductor parameter analyzer in ambient condition. For SAXS measurement, the films were deposited on mica substrates with thickness of 100 Am. The thin substrate increases the transmission of X-ray through the films to obtain stronger signal-to-noise ratio. It is performed by focusing a low divergence X-ray beam onto a sample and observing a coherent scattering pattern that arises from electron density inhomogeneities within the sample. In our case, the inhomogeneity of the distribution of electron density arises from sol – gel-derived SiO2 network and pores. According to Babinet theorem [8,9], an ensemble of cavities in a homogeneous medium could be considered equally well in place of an ensemble of particles. From this, it is possible to consider the pores as the convenient ‘scatterer’. In SAXS experiment, the electron density variation can be expressed by the measured scattered intensity, I(q), as a function of the magnitude of scattering vector, q, by the following model [8]: IðqÞ ¼ NFðqÞSðqÞ
ð2Þ
q ¼ ð4p=kÞsinh
ð3Þ
where N is number of effective scatterers, F(q) is a form factor and S(q) is a structure factor, 2h is the scattering angle and k is the wavelength of the radiation. Form factor F(q) is a function describes shape of the scattering pores. Structure factor S(q) describes the effect of the geometric spatial arrangement of the pores. Assuming weak interaction between pores, S(q) is constant, so I(q) is only dependent of F(q). In the case of spherical pore and the pore sizes have a Gaussian distribution centered at r with a standard deviation r, I(q) can be described as [8]: Z
l
1 sinðqrÞ qr cosðqrÞ pffiffiffiffiffiffiffiffiffiffi 3 IðqÞ ¼ 2 ðqrÞ3 2pr 0 ! ðri rÞ2 exp dr 2r2
!2
ð4Þ
where r is the pore radius, ri is the other pore radius different from r. The experiments were carried out using a Bruker Nano˚ ). The q star instrument. The radiation was Cu Ka (k = 1.54 A range covered in SAXS was from 0.05 to 0.15 nm 1 and the scanning time was 50,000 s per sample. The scattered intensities were corrected for transmission and deducted from parasitic and substrate scattering. After scattering
308
S. Yu et al. / Thin Solid Films 462 – 463 (2004) 306–310
curves were collected, the data were fitted with the proposed model (Eq. (4)) to determine the pore size using FISH software (SAXS software by Heenan, RAL).
3. Results and discussion In order to investigate the effect of processing temperature on the properties of the films, the films were heattreated at 300, 400, and 450 jC, respectively. The FTIR spectra of films heat-treated at different temperatures are shown in Fig. 1. The most intense absorption peak located at 1060 cm 1 is the transverse optical (TO) vibration of Si– O – Si linkage. The peak centered around 1100 cm 1 as a shoulder at the high frequency end of 1060 cm 1 peak assigns to the corresponding longitudinal optical (LO) vibration of Si– O – Si linkage. Though light was incident normal to the sample, Almeida and Pantano [10] explained the appearance of the LO shoulder as scattering due to the porous nature of the samples. The peak located in 775 cm 1 is associated with symmetric stretching of Si– O – Si linkage. The broad peak between 3300 and 3700 cm 1 corresponds to a stretching of – OH and physisorbed moisture on the surface in several modes. There was a very great –OH peak for the as-deposited film indicating the high content of the silanol groups. With increasing temperature, the peak intensity around 3300 – 3700 cm 1 decreased gradually, so more silanol groups were removed at higher temperature. Furthermore, the peak area around 1100 cm 1 was also increased with increasing annealing temperature, suggesting the porosity of the films increased. Note that the peak around 1100 cm 1 was not present for the as-deposited film, so there were not many pores in such a film. The main reasons of higher porosity at higher temperature are: (1) at higher temperature, the sol – gel reactions rates are faster and the reactions are more complete leading to more porous nature; (2) the Si-CH3
Fig. 2. TGA/DTA of silica gel.
bond pyrolyzes at higher temperature leaving pores behind. The later can be confirmed by the TGA/DTA analysis (Fig. 2). Two obvious weight loss stages can be seen in Fig. 2. The weight loss below 100 jC was due to the evaporation of solvent and water molecules trapped in the materials, supporting by the presence of an endothermic peak at 100 jC in DTA. The weight loss around 400 jC corresponding to a sharp exothermic peak around 400 jC in DTA attributed to the pyrolysis of CH3 [11], which was incorporated into the silica network during the sol – gel processing through copolymerization. The removal of organic groups left cavities in the films which increased the porosity. Because the burntoff of the CH3 groups started at around 400 jC, the film treated at 300 jC would retain more organic ligand causing lower porosity compared to the films heat treated at 400 and 450 jC. The refractive index, porosity and dielectric constant of the films annealed at different temperatures are tabulated in Table 1. The porosity of the films increased with temperature, which was consistent with FTIR measurement. The porosity of 300 jC film was found to be much lower and the dielectric constant of it was much higher than that of other two samples; whereas the properties of both 400 and 450 jC samples were more close. The changes of porosity and dielectric constant of the 400 and 450j C film with respect to 300j C film are shown in Fig. 3. The porosity increased about 93.46% and 134.4% and the dielectric constant decreased by 26.28% and 32.37% for 400 and 450 jC
Table 1 Effect of thermal treated temperature on the properties of the films Temperature (jC) Refractive index Porosity (%) Dielectric constanta Fig. 1. FTIR spectra of films annealed at different temperature.
a
300 1.31 24.30 3.12
400 1.22 47.01 2.30
Data were obtained on the third day of fabrication.
450 1.18 56.96 2.11
S. Yu et al. / Thin Solid Films 462 – 463 (2004) 306–310
309
Fig. 3. The changes of dielectric constant and porosity of the films with respect to the 300 jC film.
films, respectively, compared to that of 300 jC film. The higher porosity with increasing temperature results in lower dielectric constant of the films. The film with dielectric constant of around 2.1 could obtain for the film annealing at 450 jC. With combination of Figs. 1 and 2 and Table 1, it is concluded that the higher porosity and lower silanol content in the films are crucial to obtain lower dielectric constant. Similarly, the leakage current of the silica films at 1 MV/ cm was also reduced with increasing temperature (Fig. 4). The leakage current density at 1 MV/cm for the 300 jC film was as high as 8.4 10 3 A/cm2, it reduced to 8.1 10 6 A/cm2 for 400 jC film and 1.6 10 7 A/cm2 for 450 jC film, respectively. The pore size of the films annealed at different temperature was evaluated by SAXS. In contrast to real space methods such as electron microscopy, the information obtained from SAXS measurements is generated from larger sampling volume, as the X-ray beam width is 1 – 5 mm, the beam height is 100 Am. Thus, these experi-
Fig. 5. Experimental results and data fitting of SAXS measurement for the films annealed at different temperature.
Fig. 4. I – V property of different temperature treated films.
ments provide complementary information on a ‘‘global’’ scale. Fig. 5 shows the experimental and fitting data of SAXS plots for different temperature heat-treated films. For all the samples independent of annealing condition, the strongest scattering intensity appeared at very small of q, then it constantly reduced until q = 0.03 nm 1. For q = 0.03 nm 1 above, the intensity is almost constant. The
310
S. Yu et al. / Thin Solid Films 462 – 463 (2004) 306–310
Table 2 SAXS data fitting results for films heated at different temperature Treatment condition (jC)
Average pore diameter (nm)
300 400 450
32.48 33.54 33.94
very much with annealing temperature of 300 to 450 jC. This work also demonstrates that the SAXS is a relative simple, precise and non-destructive method to measure the pore size of thin films.
References pore size of the films derived from the data fitting is tabulated in Table 2. The annealing temperature in the range of 300– 450 jC did not affect the pore size very much. So the annealing temperature of 450 jC did not cause further interconnection of the pores, which will lead to larger pore dimension. The pore size of 450 jC film is about 34 nm, which is in the range of mesopores.
4. Conclusion Uniform mesoporous organosilicate films with a composite structure of highly porous clusters embedded into a dense matrix were fabricated through sol – gel process. The processing temperature had great effect on the properties of the films. With increasing annealing temperature, porosity in the films increased, dielectric constant of the films decreased. However, the pore size of the films did not change
[1] International Technology Roadmap for Semiconductors, Semiconductor Industry Association, Gaithersburg, MD, 2001. [2] R. Wilson, C.J. Tracy, J.L. Freeman, Handbook of Multilevel Metallization for Integrated Circuits: Materials, Technology, and Applications, Noyes Publications, Park Ridge, USA, 1993. [3] D.C. Edelstein, G.A. Sai-Halasz, Y.J. Mii, IBM J. Res. Develop. 39 (1995) 4. [4] S.S. Prakash, C.J. Brinker, A.J. Hurd, S.M. Rao, Nature (London) 374 (1995) 439. [5] E. Lifshin (Ed.), X-ray Characterization of Materials, Wiley-VCH, New York, 1999. [6] C.J. Brinker, G.W. Scherer, Sol – Gel Science, Academic, San Diego, CA, 1990. [7] M. Born, E. Wolf, Principles of Optics, Pergamon Press, New York, 1983, p. 87. [8] A. Guinier, G. Fournet, Small-angle Scattering of X-rays, Wiley, New York, 1955. [9] P. Mittelbach, G. Porod, Acta Phys. Austria 14 (1961) 185. [10] R.M. Almedia, C.G. Pantano, J. Appl. Phys. 68 (1990) 4225. [11] S. Seraji, Y. Wu, M. Forbess, S.J. Limmer, T. Chou, G. Cao, Adv. Mater. 22 (2000) 12.