Measurement and Analysis of Water Adsorption in Porous Silica Films

Journal of The Electrochemical Society, 153 8 G759-G764 2006 0013-4651/2006/153 8 /G759/6/$20.00 The Electrochemical Society Measurement and Analysis of Water Adsorption in Porous Silica Films Shin-Ichiro Kuroki a,z and Takamaro Kikkawa* Research Center for Nanodevices and Systems, Hiroshima University, Higashi-Hiroshima 739-8527, Japan G759 The influence of water adsorption on dielectric constants of the porous silica low-k dielectric films was investigated. The amount of water adsorption inside pores was calculated by a capacitance value, a gas adsorption measurement, the BET Brunauer, Emmett, and Teller adsorption theory, and the Kirkwood microscopic theory of water dielectrics. A hexamethyldisilazane HMDS vapor treatment was introduced to make porous silica low-k films hydrophobic, and the effect of the HMDS vapor treatment was investigated quantitatively. The BET adsorption at a low partial pressure of water vapor p/p 0 0.31 for the HMDS-treated film, p/p 0 0.17 for the nontreated film was investigated by a capacitance value. The results showed that the BET adsorptions of the HMDS-treated and the nontreated films were almost the same. A water cluster formation inside pores, at a high partial pressure of water vapor p/p 0 0.31 for the HMDS-treated film and p/p 0 0.17 for the nontreated film, was investigated by a capacitance value and a gas adsorption measurement. The results showed that the HMDS vapor treatment suppressed the water cluster formation, and this suppression could lower the dielectric constant for the porous silica film. 2006 The Electrochemical Society. DOI: 10.1149/1.2210573 All rights reserved. Manuscript submitted July 18, 2005; revised manuscript received April 21, 2006. Available electronically June 14, 2006. The miniaturization of device dimension in complementarymetal-oxide-semiconductor CMOS ultralarge-scale-integrated circuits ULSI has continuously evolved by using the scaling rule. 1 Due to the scaling down, the miniaturization of interconnection is required. However, the miniaturization induces the increase in wire s resistance and wire-to-wire capacitance. To counter this effect, lowresistance copper wire and low-dielectric constant low-k interlayer dielectric film have been introduced. The porous low-k interlayer dielectric films are developed to achieve a low dielectric constant. The porosity of a porous low-k film controls the dielectric constant, and high porosity of a porous film leads to an ultralow-k film that has a dielectric constant k 2.0, which is required for future ULSI beyond 45 nm node. However, water adsorption inside the pore degrades the film properties; for example, water adsorption inside pores increases dielectric constants of porous low-k films. 2-4 For reliable low-k interlayer dielectric films, hydrophobic treatments are needed to repair surface states inside pores. In this paper, the influence of water adsorption on dielectric constants of the porous silica low-k dielectric films was investigated. The amount of water adsorption inside the pores was estimated. The hexamethyldisilazane HMDS treatment was introduced to make porous silica low-k films hydrophobic, and the effects of HMDS on porous silica low-k dielectric films were investigated. Experimental p-type Si 100 4 off substrates were treated in RCA cleaning solution NH 4 OH/H 2 O 2 /H 2 O = 36:720:1680 and dipped into 0.5% hydrofluoric acid HF solution. They were oxidized in O 2 at 900 C to form 5 nm thick thermal SiO 2. A precursor solution for porous-silica films was prepared by adding both pluronic surfactant, which was polyethylene oxide PEO polypropylene oxide PPO PEO tribrock copolymer, and an acidic silica sol derived from tetraethyl orthosilicate TEOS in ethanol diluted with water. The porous-silica precursor solution was spincoated on a silicon substrate to form a homogeneous thin layer. After prebaking in 100 C for 1 h, the sample was annealed in dry air at 400 C for 3 h to burn out the surfactant and to stabilize the chemical structure of the film. The resulting porous silica film thickness was 200 nm. The porous silica film was treated with HMDS vapor CH 3 3 SiNHSi CH 3 3 at 23 C for 24 h. After this treatment, hydrophilic -OH bonds on the film surface were replaced by hydrophobic -SiCH 3 bonds and the porous silica films became hydrophobic, as shown in Fig. 1. Aluminum electrode was formed in the porous silica film as well as the bottom of the Si substrate by sputtering, in order to measure electrical characterizations. Film thickness and refractive index were measured by spectroscopic ellipsometry wave length: 191.4 994.3 nm at room temperature 23 C and 47.5% relative humidity RH. The film structure was analyzed by transmission electron microscopy TEM and X-ray diffraction measurement. The X-ray source was 0.154 nm Cu K. Pore size distribution was determined by the Ar adsorption isotherm. The Ar adsorption was measured by a gas sorption method differential pressure method at 187 C after 200 C baking under vacuum for 1 h. Molecular bonds in the film were measured by transmission Fourier transform infrared spectroscopy FTIR. The wave number range was 650 4000 cm 1 and the resolution was 0.4821 cm 1. All of the sample wafer and optical elements in the FTIR spectrometer were placed in vacuum chambers of 1 10 5 Pa. Before and after 200 C baking for 10 h in a vacuum, FTIR spectroscopy were measured. A time-dependent dielectric breakdown experiment TDDB was carried out after 200 C baking for 10 h in a N 2 ambient environment with 1.0% RH. This measurement was automated using a personal computer. The computer was programmed to use the graphical language LabVIEW to control the experiments. A capacitance voltage C V measurement was carried out after 200 C baking for 10 h in a N 2 ambient environment with 1.5% RH. The measurement was carried out for 67 h continuously, while the humidity was varied slowly from 1.5 to 70% RH at room temperature 23 C. The average differential of humidity was 1.7 * Electrochemical Society Active Member. a Present address: Graduate School of Engineering, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8579, Japan. z E-mail: kuroki@ecei.tohoku.ac.jp Figure 1. Schematic diagram of water adsorption on porous silica film. A stereographic structure of SiCH 3 inside pores of the HMDS-treated porous silica films prevents water-cluster formation.

G760 Journal of The Electrochemical Society, 153 8 G759-G764 2006 Figure 2. TEM micrograph of porous silica low-k dielectric film. 10 2 % RH/min, and the humid environment was varied as quasistatic. The C V and the humidity measurements were also automated using a personal computer with the graphical language LabVIEW. The amount of H 2 O gas adsorption was measured by the gas sorption method differential pressure method at room temperature 25 C after 200 C baking under a vacuum for 1 h. In this H 2 O gas adsorption measurement, the porous silica films were placed in a vacuum chamber. After 200 C baking for 1 h, H 2 O gas was poured into the vacuum chamber, and the partial pressure of H 2 O gas was varied from p/p 0 = 0.025 0.99. The H 2 O adsorption inside the films changed the partial pressure of the vacuum chamber. By measuring the change in partial pressure, the H 2 O adsorption inside the films was calculated. Results and Discussion Characterization of the materials. Figure 2 shows TEM micrographs. It was found that this porous silica film has periodic cylindrical pores. The periodicity of pores was measured by X-ray diffraction, as shown in Fig. 3. The Bragg angle of periodic pores was 1.1392, and the periodicity was 7.75 nm. The average diameter determined by the Ar adsorption isotherm of porous silica film was 4.76 nm, as shown in Fig. 4. The Ar specific surface area was 2.03 10 2 m 2 /g, which was also calculated by the Ar adsorption isotherm. The refractive index for porous silica film measured by spectroscopic ellipsometry was 1.273. The porosity of the films was calculated from refractive index by the Lorentz Lorenz equation Figure 4. Pore-size distribution of porous silica film determined by the Ar adsorption isotherm. x =1 n p 2 1 n 2 p +2 ns 2 +2 n 2 s 1 + y nw 2 1 n 2 w +2 ns 2 +2 n 2 1 s 1 where x and y are porosity and water adsorption volume ratio to the film and n p, n s, and n w are the refractive indices of porous film, silica skeleton, and liquid water, respectively. The last term of this equation is correction from water adsorption. Here, we note that the Lorentz Lorenz equation is the refractive index representation of the Clausius Mossotti equation. In this calculation, the reference refractive indices n s = 1.44 silica skeleton and n w = 1.33 liquid water were used. The water adsorption measured by the gas adsorption method at 47.5% RH was y = 8.2%. The porosity was calculated as 41.2%, as shown in Fig. 5. Figure 6 shows FTIR spectroscopy spectra of the porous-silica films before and after 200 C baking under a vacuum for 10 h. The 1079 cm 1 peak corresponds to the Si O asymmetric stretch vibration. In tetrahedral SiO 2, such as thermal oxide, the Si O asymmetric peak position is at 1080 cm 1 and the Si O Si bond of the porous-silica film was almost the same as tetrahedral SiO 2 with the Si O Si angle of 144. The Si O Si bond angle increases to 144, and the Si O peak shifts upward in energy. The lump at around 1180 cm 1 corresponds to the Si O Si cage-type structure. In the HMDS nontreated film before baking, a broad hydrogenbonded hydroxyl OH band in 3350 3600 cm 1 was observed. After the HMDS treatment, Si C stretching vibration of Si CH 3 3 at Figure 3. X-ray diffraction curve of porous silica film. Figure 5. Porosity curve for the measured refractive index n p = 1.273. The water adsorption measured by gas adsorption method at 47.5% RH was 8.2%, and then the porosity was calculated as 41.2%.

Journal of The Electrochemical Society, 153 8 G759-G764 2006 G761 Figure 6. FTIR spectroscopy spectra of the porous silica films: the HMDStreated and nontreated films before and after 200 C baking under vacuum of 1 10 5 Pa for 10 h were measured. 843 cm 1 and C H stretching vibration of SiCH 3 at 2965 cm 1 appeared, and then the porous silica film became hydrophobic. Also, the broad hydrogen-bonded hydroxyl OH band in 3350 3600 cm 1 decreased. Figure 7 shows a TDDB Weibull plot at electric field strength of 2.8MV/cminN 2 ambient at 200 C. The HMDS treatment enlarged a TDDB lifetime and improved reliability of porous silica film. Dielectric constants and water adsorption. Figure 8 shows the effect of the HMDS treatment on a dielectric constant as a function of relative humidity. The dielectric constant was calculated by using the average value at the accumulation capacitance. The sampling number at accumulation capacitance was 50 points for each dielectric constant. The dielectric constants of the HMDS-treated and nontreated films increased as a function of humidity. Above 25% RH, the increasing rate of the dielectric constant was suppressed by the HMDS treatment compared to the one without HMDS treatment. Figure 9 shows the effect of the HMDS treatment on H 2 O adsorption in the gas adsorption measurement as a function of relative humidity. The HMDS treatment suppressed water adsorption, and water adsorption for the HMDS-treated porous silica film Figure 8. Effect of HMDS treatment on dielectric constant as a function of humidity. decreased. Water molecules were adsorbed inside pores in the cylindrical form, 5 and an effective dielectric constant of a water-adsorbed porous silica film was calculated by the modified Rayleigh equations 2,4 k s + k pore x k s k pore k = k s 2 k s + k pore + x k s k pore and 2+ shell k shell 1 k pore = k shell 3 2k shell shell k shell 1 where k, k s, and k shell are the effective dielectric constants of the film, silica skeleton, and water, respectively, x is the porosity, and v shell is the fraction of the adsorbed-water shell to the pore volume, defined by shell = R2 R r 2 R 2 4 where R is the pore radius and r is an inside radius of water shell as shown in Fig. 10. The modified Rayleigh equations are derived from Laplace equation of electrostatic field. By using the modified Rayleigh equation, the reduction ratio of dielectric constant was esti- Figure 7. Effect of HMDS treatment on the time-dependent dielectric breakdown of porous silica films at 2.8 MV/cm in N 2 ambient at 200 C. Figure 9. Effect of HMDS treatment on water adsorption in gas adsorption measurement as a function of humidity.

G762 Journal of The Electrochemical Society, 153 8 G759-G764 2006 Figure 11. Reduction ratio of dielectric constant as a function of humidity: reduction ratio from a gas-adsorption measurement was calculated by the modified Rayleigh equation. Figure 10. Schematic diagram of a H 2 O adsorption shell. mated from the gas adsorption measurement. Here the reduction ratio is defined by the equation =1 k p2 5 k p1 where k p1 and k p2 are the dielectric constants of the HMDS-treated and nontreated porous silica films, respectively. In this calculation, it was assumed that the dielectric constant of the water shell inside pores was H 2 O dielectric constant liquid of 78.5 25 C. The reduction ratio of the HMDS-treated porous silica film to the nontreated film is shown in Fig. 11. From the gas adsorption measurement, an effective dielectric constant of the film was calculated by using Eq. 2, and a reduction ratio of 10% was predicted. From the C V measurements, however, a reduction ratio of 25% was achieved, and there was a difference between the dielectric constants of the C V measurement and the gas adsorption measurement. To investigate this difference, a microscopic theory of water dielectrics was applied. Formation of water cluster. A microscopic analysis of water adsorption was carried out as follows: first, the thickness of the water adsorption shell was assumed to be water-molecular size, as shown in Fig. 12. According to the hard sphere approximation of water molecules, the diameters of H 2 O molecular and cluster were given as 0.31 nm for the unbonded monomer and 0.55 nm for the pentamer. 6 The widths of the monomer shell and the pentamer shell were assumed to be 0.31 and 0.55 nm, respectively. Second, the effective dielectric constants of the water adsorption shell were calculated. In this calculation, the measured dielectric constants shown in Fig. 8 and the modified Rayleigh equations were used. The modified Rayleigh Eq. 2 and 3 are transformed as and k shell = 1 2 shell k pore 1 2 shell + 2 shell 2 k pore 1 2 2 +4 shell k pore 6 1 x k s 1 +x k k pore = k s 7 1 x k 1 +x k s where k shell is the effective dielectric constant of the water adsorption shell, and k pore and k are the effective dielectric constant of the pore and the measured dielectric constant of porous-silica film, respectively. The fraction of water adsorption shell, shell, is 0.244 for the monomer and 0.409 for the pentamer. These values were calculated from a measured pore diameter of 4.76 nm. The effective dielectric constant k shell is determined by dielectric properties of absorbed water, e.g., polarizability, dipole moment, and correlation between water molecules. Then the effective dielectric constant k shell translated into fundamental dielectric properties by using the Kirkwood theory of microscopic dielectric properties for water molecules. 7 Microscopic representation of water dielectrics was given by the Kirkwood equations of water molecules as follows 3 k shell 1 2k shell +1 N = x shell 4 9k shell and + 2 2 +zcos 3k B T 1 2 1 Figure 12. Schematic diagram of a H 2 O adsorption shell microscopic picture. 8

Journal of The Electrochemical Society, 153 8 G759-G764 2006 G763 = 0 1 /2 2 z a 3 cos3 9 where N is the number of water molecules per unit volume of lowk film and, a, z, and are the polarizability, the distance between water molecule neighbors, the Kirkwood z-parameter, and the average angle between the correlated water molecule s dipole moment, respectively. and 0 are the dipole moments of water molecule in the liquid and in the vapor, respectively. The parameters, a, 0, and are given as 1.47 10 3 nm 3, 0.292 nm 25 C, 6.14 10 30 Cm, and 105, respectively. The Kirkwood z-parameter is defined by the equation z cos 2 /2 = 2 10 where is a single molecule s moment vector and is the total moment vector, including a molecule and its neighbor. Kirkwood z-parameter counts the number of neighbor water molecules, e.g., z = 0 means that a H 2 O molecule is free from other water molecules, and z = 4.5 corresponds to liquid state at 25 C. In Eq. 9, it is assumed that 2 z/a 3 cos 2 /2 1, e.g., at a parameter z = 4.5, this term takes 0.20 and then, 0 approximately. Figure 13a and b shows the calculated water adsorption of the HMDS nontreated film based on the Kirkwood theory. In Fig. 13a and b, the monomer and the pentamer shell of water adsorption were assumed, respectively. The intersecting point of the Kirkwood calculations and the gas adsorption measurement varied as a function of humidity. This phenomenon means that the size of the water cluster changed as a function of humidity. The z-parameter as a function of humidity is shown in Fig. 14. For the HMDS nontreated porous silica film, z-parameter saturated at z = 3 when the humidity was increasing. For the HMDS-treated film, z-parameter saturated less than z = 1 when the humidity was increasing. Water clusters were formed in the HMDS nontreated porous silica film, and the hydrogen bond between adsorbed water molecules increased the dielectric constant of the films. Water clusters were not formed in the HMDS-treated porous silica film, and the dielectric constant of the porous silica film increased slowly. These results show that the HMDS treatment suppressed the formation of water cluster inside pores and kept a low dielectric constant. Monomer adsorption: Brunauer, Emmett, and Teller method (BET) analysis. In the region of the Kirkwood parameter z = 0, water molecules had no hydrogen bond with other water molecules, and the BET analysis was carried out. 8 The BET equation is given by the equation P N P 0 P = 1 N m c + c 1 N m P c P 0 11 where p and p 0 are the partial pressure and the saturated vapor pressure of H 2 O, and N and N m are the number of the adsorbed H 2 O and the monolayer adsorption. The constant c is given by the equation c = exp E 1 E L /k B T. Here, E 1 and E L are the adsorption energy for monolayer and additional layers, and k B and T are the Boltzmann constant and the absolute temperature. Figure 15 shows the BET plot of the HMDS-treated and the nontreated porous-silica film. Here the adsorption N was calculated by Kirkwood Eq. 8 with a parameter z = 0. It is found that the BET plots were linear, and then the BET adsorption of water was induced at the low partial pressure p/p 0 0.31 for the HMDS-treated film and p/p 0 0.17 for the HMDS nontreated film. The monomer adsorption N m for the HMDS-treated and the nontreated porous-silica films were 1.545 and 1.401 nm 3, respectively. The adsorption energies E 1 E L for the HMDS-treated and the nontreated films were 0.078 and 0.085 ev, respectively. The specific surface area is calculated by the equation Figure 13. Calculated water adsorption for the porous silica film based on the Kirkwood theory for various Kirkwood z-parameters: a monomer shell and b pentamer shell of water adsorption for the HMDS nontreated porous silica film. It was assumed that the thickness of water adsorption inside the pores was equal to the size of monomer and pentamer. A s = a m N m 12 where A s is the specific surface area and a m is the molecular crosssectional area. For a water molecule H 2 O, the molecular crosssectional area a m is 0.125 nm 2. 9 The H 2 O BET specific surface areas of the HMDS-treated and the nontreated film were 1.50 and 1.36 10 2 m 2 /g, respectively. Here the porous silica films weight density of 1.29 g/cm 3 was used. The BET adsorptions of the HMDS-treated and the nontreated films were almost the same, and the HMDS treatment did not suppress the BET adsorption. In the previous section, the water-cluster formation was suppressed by the HMDS treatment. These results indicate that Si CH 3 3 stereoscopic surface of the HMDS-treated film suppressed the formation of the water-hydrogen bond network. Conclusion The water adsorption of the porous silica low-k film was investigated. The amount of water adsorption was calculated by a capacitance value, a gas adsorption measurement, the BET adsorption theory, and the Kirkwood microscopic theory of water dielectrics. At a low partial pressure p/p 0 0.31 for the HMDS-treated film and p/p 0 0.17 for the nontreated film, the BET analysis

G764 Journal of The Electrochemical Society, 153 8 G759-G764 2006 Figure 14. Kirkwood z-parameter as a function of humidity. Over a humidity of 35% RH, z-parameter increased rapidly. This increase was caused by the water cluster formation. Figure 15. BET plot of the HMDS-treated and nontreated porous silica film. The monomer adsorption N m for the HMDS-treated and the nontreated porous silica films were 1.545 and 1.401 nm 3, respectively. The adsorption energies E 1 E L for the HMDS-treated and the nontreated films were 0.078 and 0.085 ev, respectively. based on capacitance measurements for porous silica films was carried out. The monomer adsorption N m of the HMDS-treated and nontreated porous silica films were 1.545 and 1.401 nm 3, respectively. The adsorption energies E 1 E L of the HMDS-treated and nontreated films were 0.078 and 0.085 ev, respectively. The BET adsorptions of the HMDS-treated and nontreated porous silica films were almost the same. At a high partial pressure p/p 0 0.31 for the HMDS-treated film and p/p 0 0.17 for the nontreated film, the Kirkwood analysis based on a capacitance value and a gas adsorption measurement was carried out, and the Kirkwood z-parameters of the HMDStreated and nontreated porous silica film at 50% RH were 0.8 and 2.8, respectively. When the water cluster was formed inside the pores, the cluster formation increased the dielectric constant of the film rapidly. Si CH 3 3 stereoscopic surface of the HMDS-treated porous silica film suppressed the formation of the water-hydrogen bond network. Acknowledgment Part of this work was supported by NEDO. Hiroshima University assisted in meeting the publication costs of this article. References 1. R. H. Dennard, F. H. Gaensslen, H.-N. Yu, V. L. Rideout, E. Bassous, and A. R. LeBlanc, IEEE J. Solid-State Circuits, 9, 6 1974. 2. S. Sakamoto, S. Kuroki, and T. Kikkawa, The 2003 International Conference on Solid State Devices and Materials, p.478 2003. 3. T. M. Shaw, D. Jimerson, D. Haders, C. E. Murry, A. Grill, and D. C. Edelstein, Advanced Metallization Conference, p.11 2003. 4. T. Kikkawa, S. Kuroki, S. Sakamoto, K. Kohmura, H. Tanaka, and N. Hata, J. Electrochem. Soc., 152, G560 2005. 5. E. W. Hansen, M. Stocker, and R. Schmidt, J. Phys. Chem., 100, 2195 1996. 6. K. Arakawa, K. Tokiwano, and K. Kojima, Bull. Chem. Soc. Jpn., 50, 65 1977. 7. J. G. Kirkwood, J. Phys. Chem., 7, 911 1939. 8. S. Brunauer, P. H. Emmett, and E. Teller, J. Am. Chem. Soc., 60, 309 1938. 9. A. L. McCellan and H. F. Harnsberger, J. Colloid Interface Sci., 23, 577 1967.