- Email: [email protected]
Kinetic study of alcohol-assisted supercritical fluid deposition of TiO2
Thin Solid Films 553 (2014) 184–187
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Kinetic study of alcohol-assisted supercritical fluid deposition of TiO2 Yu Zhao ⁎, Kyubong Jung, Takeshi Momose, Yukihiro Shimogaki Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
a r t i c l e
i n f o
Available online 25 November 2013 Keywords: Alcohol Supercritical fluid deposition Conformal films Titanium dioxide
a b s t r a c t The kinetics of TiO2 thin-film deposition in a supercritical CO2/alcohol environment was studied using titanium di(isopropoxide)bis(tetramethylhexanedionate) [Ti(O-i-Pr)2(tmhd)2] as the precursor. The solvent effects of methanol, ethanol, and isopropanol on the supercritical fluid deposition (SCFD) of TiO2 were examined at deposition temperatures of 150–300 °C. Methanol was the most effective of the three alcohols evaluated; it enhanced the TiO2-SCFD deposition rate 2–3-fold and decreased the activation energy from 49 ± 4 to 28 ± 3 kJ/mol. Conformal deposition of TiO2 on a deep trench (aspect ratio of 30) was demonstrated at a growth rate of N 2 nm/min. © 2013 Elsevier B.V. All rights reserved.
1. Introduction TiO2 is a promising material for dielectrics in dynamic random access memory (DRAM) because of its high dielectric constant. DRAM is currently fabricated on three-dimensional (3D) structures and is monolithically integrated with logic circuits to achieve low power consumption, high integration density, and short response time. The international technology roadmap for semiconductors forecasted that highly conformal TiO2 deposition technology that is needed for high-aspect-ratio (N60) features will be mandatory after 2017 [1]. Conventional deposition techniques based on vacuum technology, such as chemical vapor deposition (CVD), have been unable to conformally deposit TiO2 on high-aspect-ratio features [2–4]. Although atomic layer deposition (ALD) has achieved conformal deposition on trenches having aspect ratios N60, its growth rate is limited to 0.5 nm/min because of the cyclic deposition procedure [5–7]. We previously demonstrated the comparable performance of supercritical fluid deposition (SCFD) with ALD, i.e., conformal deposition onto high-aspect-ratio features at a growth rate of 1 nm/min [8]. SCFD has the potential to further enhance the growth rate by optimizing the reaction chemistry, which will be discussed later. SCFD involves the oxidation/reduction of organic compounds in supercritical CO2 (scCO2), which is a phase of CO2 above its critical temperature and pressure [9–11]. Molecules in the supercritical phase are heterogeneously distributed between a sparse region (contributing to gas-like diffusivity) and a dense region (contributing to liquid-like solubility). This heterogeneous distribution of the scCO2 molecular density benefits SCFD [10]; remarkable conformal deposition is achieved without compromising growth rate. We have conformally formed Cu films on holes with an aspect ratio of 20 and also achieved
⁎ Corresponding author. Tel./fax: +81 3 5841 7131. E-mail address: [email protected] (Y. Zhao). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.11.062
void-free filling with Cu [12,13]. Unlike conventional vacuum-based technologies, such as ALD, that are adequate for high-aspect-ratio features, SCFD generally has growth rates of N1 nm/min. These higher rates are suitable for mass production and derive from the high precursor concentration in scCO2 [14,15]. Additionally, the relatively high dielectric constant of scCO2 (ε = 1.2) is expected to decrease the activation energy of reactions through a solvent effect [16–18]. SCFD is a solution-based technology. It is possible to enhance the growth rate and decrease the process temperature by adding carefully selected organic solvents, such as alcohols, to the scCO2 [19–21]. The customizability of the process is one of the advantages of SCFD. Momose et al. introduced ethanol into Cu-SCFD and reported a reduction in the deposition temperature from 220 to 195 °C [21]. Peng et al. suggested that the addition of an alcohol with a high dielectric constant enhanced the growth rate of Al2O3-SCFD [17]. In this study, the solvent effect of readily available alcohols, such as methanol, ethanol, and isopropyl alcohol (IPA), were examined for TiO2-SCFD. The experimental results showed that methanol effectively promoted the SCFD of TiO2. 2. Experimental details Titanium di(isopropoxide)bis(tetramethylhexanedionate) [Ti(O-iPr)2(tmhd)2] (99.99%) was employed as the Ti precursor for all experiments. Methanol, ethanol, and IPA were selected to obtain lower deposition temperatures and higher growth rates for TiO2-SCFD. These alcohols were selected because the precursor was soluble in these alcohols. To evaluate the solvent effect, the precursor concentration was adjusted by diluting a saturated precursor/scCO2 mixture. Alcohols were injected afterward to avoid changing the precursor concentration. A planar Si substrate with a SiO2 surface was used for the kinetic study because of the short incubation time (b 1 min) of our experiments. Si trenches with thermal SiO2 surfaces (600 nm wide,
Y. Zhao et al. / Thin Solid Films 553 (2014) 184–187
aspect ratio of 30) were used to demonstrate the capability for conformal deposition. Deposition of TiO2 on the bottom electrode material of DRAM, such as Ru, is a goal of future work. A cold-wall continuous reactor in the face-down configuration was adopted (Fig. 1). It enabled the use of a relatively low fluid temperature and provided laminar flow without natural convection, both of which eliminated unwanted particle generation in the fluid [15]. Before deposition, all the equipment was pressurized at 10 MPa with scCO2 (99.99%). The substrate was placed face-down in the reactor and heated to 150–300 °C. Deposition was initiated by introducing the Ti precursor dissolved in scCO2, and liquid alcohol, from individual lines. The TiO2 was thereby deposited onto the heated face-down substrate in the cold-wall reactor. The concentrations of the Ti precursor and the alcohol were controlled as follows. An excess of the solid Ti precursor was placed in a hot-wall reservoir, and then scCO2 was flowed into the reactor via the reservoir. This provided a scCO2 stream that was saturated in the Ti precursor. This saturated scCO2 solution was then mixed with another scCO2 stream at 10 MPa and 60 °C to adjust the Ti precursor concentration to the desired concentration. Liquid alcohol was directly injected into the flowing scCO2 using a high-pressure pump. The volumetric impact of the alcohol on the scCO2 was neglected in the concentration calculation because the molar ratio of alcohol to scCO2 in the mixed fluid was about 10−2. The Ti precursor and alcohol concentrations used in this work were estimated in scCO2 at 10 MPa and 60 °C. The microstructure of the obtained TiO2 films and step coverage on high-aspect-ratio trenches were analyzed with a field-emission scanning electron microscope (FE-SEM, JEOL 6340F). A film density of 4 g/cm3 was used to estimate the surface reaction rate constant. The atomic composition was determined by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 1600C). The root-mean-square (RMS) surface roughness was determined by atomic force microscopy (SEIKO NPX100).
3. Results and discussion To investigate the effect of alcohol addition on the growth rate of TiO2-SCFD, we deposited TiO2 on a planar Si substrate having a SiO2 surface at 250 °C in the presence of different alcohol mole fractions (Fig. 2). The precursor concentration was fixed at 2 × 10− 3 mol/L. An amorphous solid TiO2 film with smooth morphology (RMS b 2 nm for a 100-nm-thick film) was deposited (data not shown). XPS results showed that the carbon impurities of these samples were less than 1 at.% (data not shown). All three alcohols promoted the growth rate of TiO2-SCFD at low alcohol concentrations. The growth rates were significantly enhanced in all cases by increasing the alcohol concentration and were saturated at a precursor molar ratio of about 0.008. Excess alcohol addition reduced the growth rate in all cases. The increase in the growth rate was attributed to an enhanced solvent effect; details are discussed below. The decrease in the growth rate at high alcohol concentrations was caused by the reduced precursor concentration contributing to the film growth; the precursor was largely consumed in the fluid phase to form particles. Fine particles were indeed found on the inner wall of the reactor after deposition. The saturation at
185
Fig. 2. Growth rate of TiO2-SCFD at 250 °C as a function of alcohol mole fraction.
intermediate alcohol concentrations might be due to a balance of these two effects. The solvent effect of the alcohol addition was further studied by the growth rate dependence on temperature at a constant alcohol mole fraction of 0.01. Fig. 3 shows an Arrhenius plot of TiO2-SCFD with and without alcohol assistance at 150–300 °C. The activation energies under methanol-, ethanol-, and IPA-assistance were 28 ± 3, 25 ± 2, and 43 ± 2 kJ/mol, respectively, while SCFD without alcohol was 49 ± 4 kJ/mol. The activation energy with CVD using the same precursor was 85 ± 5 kJ/mol [2]. Accordingly, we conclude that the scCO2 solvent effect served to reduce the activation energy, and the addition of alcohol to SCFD led to a further decrease of the activation energy. These results were analyzed by applying Onsager's reaction field theory to SCFD, which was previously used for solvent-based systems to explain the reaction enhancement in the liquid phase [17,22,23]. The activation energy (Ea) is decreased by a factor ΔEa due to solvent clusters around the solute. The value of ΔEa can be estimated using the following formula:
ΔEa ¼
μ 2 2ðε−1Þ a3 ð2ε−1Þ
where μ is the dipole moment of the polar solute molecule in a cluster of radius a, and ε is the dielectric constant of the solvent. Provided that the CVD and SCFD reactions have the same polar transition state, the dielectric constant of the scCO2 solvent (ε = 1.2) was expected to decrease the potential barrier of the decomposition via an effective reduction in the free energy of the polar transition state [15,17]. Furthermore, the addition of alcohol (methanol ε = 32; ethanol ε = 24; IPA ε = 20) further enhanced the dielectric constant of the reaction medium and resulted in an even lower activation energy despite the low molar fraction of the added alcohols. This is because when polar Ti precursor molecules dissolve in scCO2/alcohol mixtures, the CO2 and alcohol molecules likely form “cluster” structures around the precursor; thus, the local effective
Fig. 1. Schematic diagram of the cold-wall flow-type supercritical fluid deposition system.
186
Y. Zhao et al. / Thin Solid Films 553 (2014) 184–187
Fig. 3. Arrhenius plot of TiO2-SCFD with and without alcohol assistance at 150–300 °C.
dielectric constant can be much higher than the apparent value [16,17]. A relatively large difference in Ea was seen between IPA-assistance and methanol- or ethanol-assistance. This can be explained by differences in the polarity and molecular size of the alcohols: the local dielectric constants are determined by the local concentration of the alcohols, and the large molecular size of IPA is unfavorable for clustering around precursor molecules. The dependence of the growth rate on the precursor concentration with alcohol assistance was subsequently studied. Fig. 4 shows the relationship between the growth rate and the Ti precursor concentration at 250 °C with alcohol added at a mole fraction of 0.01. The growth rate increased linearly with precursor concentration, indicating that the deposition had first-order reaction kinetics regardless of the presence of alcohol. The surface reaction rate constants (ks) were (1.98 ± 0.05) × 10−6, (1.75 ± 0.06) × 10−6, (1.08 ± 0.02) × 10−6, and (0.80 ± 0.03) × 10−6 m/s for the methanol-, ethanol-, and IPAassisted, and non-assisted cases, respectively. The SCFD ks values were compared with that for CVD for the same precursor and deposition temperature by estimating the CVD ks value from its Arrhenius plot [2]. In this way, the ks value of TiO2-CVD at 250 °C was estimated as 0.04 m/s. To confirm the conformal deposition capability, methanol-assisted SCFD was examined for deep trenches having an aspect ratio of 30. Fig. 5 shows the excellent conformability of the TiO2 film. The thickness of the TiO2 film over the entire trench was nearly constant, i.e., 132 ± 3 nm. The growth rate under this condition was quite high at 2.2 nm/min. Concerning deposition on 3D structures, a lower ks value leads to better step coverage because the precursor concentration at the bottom of a 3D structure is likely to be lower than at the top, and a lower ks value means less growth rate dependence on the precursor concentration. Hence, the obtained nearly perfect step coverage was attributed to the low ks value of methanol-assisted TiO2-SCFD; the
Fig. 5. Cross-sectional SEM image of a highly conformal TiO2 film deposited on a deep trench (600 nm wide, aspect ratio of 30) at 250 °C with the addition of methanol at a molar ratio of 0.01.
lower ks values of ethanol and IPA should provide better step coverage compared with methanol. The conformal deposition capability between SCFD and CVD was compared as follows. TiO2-SCFD should show superior step coverage compared with TiO2-CVD because the ks of TiO2-SCFD is four orders of magnitude lower than that of TiO2-CVD. Additionally, the growth rate of TiO2-CVD at 250 °C was estimated at only about 0.1 nm/min [2], which is a consequence of the relatively low ks (0.04 m/s at 250 °C) and a low precursor partial pressure (10−8 mol/L). On the other hand, for SCFD, the extremely low ks values (~10−6 m/s) enabled excellent step coverage, while the high precursor concentration in scCO2 (~10−3 mol/L) provided a high growth rate. We also note that the growth rate is limited to 0.5 nm/min for ALD [5,7], while alcoholassisted SCFD at 250 °C had a growth rate of N2 nm/min. Though ALD can conformally deposit on deeper trenches than we used in this study, we previously simulated the conformal deposition on nano-scale trenches having an aspect ratio of 100 using TiO2-SCFD without alcohol at 300 °C [8]. Note that a ks of 2 × 10−6 m/s was used in that work. Considering that the ks of alcohol-assisted SCFD at 250 °C is even smaller, the conformal deposition on nano-scale trenches having an aspect ratio of 100 should also be achievable using our alcohol-assisted SCFD method at 250 °C.
4. Conclusions
Fig. 4. Relationship between growth rate and Ti precursor concentration at 250 °C with alcohol added at a 0.01 molar ratio.
The addition of alcohols (methanol, ethanol, or IPA) to TiO2-SCFD was studied kinetically. The increased growth rate observed in all cases was attributed to an enhanced solvent effect. Methanol was the most effective of the three alcohols. Alcohol addition also decreased the activation energy of the decomposition reactions. The extremely high conformal deposition capability inherent to SCFD was also observed for alcohol-assisted SCFD and was comparable to ALD, while SCFD exhibited higher growth rates exceeding 2 nm/min. These beneficial aspects of TiO2-SCFD with alcohol assistance are well suited for the production of electronic devices with high-aspect-ratio features, such as DRAM.
Y. Zhao et al. / Thin Solid Films 553 (2014) 184–187
Acknowledgments The high-aspect-ratio trenches used in this work were provided by DENSO Corporation.
[8] [9] [10] [11] [12] [13]
References [1] http://www.itrs.net. [2] S.Y. No, J.H. Oh, C.B. Jeon, M. Schindler, C.S. Hwang, H.J. Kim, J. Electrochem. Soc. 152 (2005) C435. [3] V. Siriwongrungson, S.P. Krumdieck, M.M. Alkaisi, Chem. Vap. Depos. 17 (2011) 327. [4] V. Siriwongrungson, M.M. Alkaisi, S.P. Krumdieck, Surf. Coat. Technol. 201 (2007) 8944. [5] T. Blanquart, J. Niinistö, M. Gavagnin, V. Longo, V.R. Pallem, C. Dussarrat, M. Ritala, M. Leskelä, Chem. Mater. 24 (2012) 3420. [6] I. Jõgi, J. Aarik, M. Laan, J. Lu, K. Kukli, H. Kaambre, T. Sajavaara, T. Uustare, Thin Solid Films 510 (2006) 39. [7] I. Jõgi, M. Pärs, J. Aarik, A. Aidla, M. Laan, J. Sundqvist, L. Oberbeck, J. Heitmann, K. Kukli, Thin Solid Films 516 (2008) 4855.
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
187
K. Jung, Y. Zhao, T. Momose, Y. Shimogaki, ECS Solid State Lett. 2 (2013) 79. C. Erkey, J. Supercrit. Fluids 47 (2009) 517. J.M. Blackburn, D.P. Long, A. Cabanas, J.J. Watkins, Science 294 (2001) 141. A.H. Romang, J.J. Watkins, Chem. Rev. 110 (2010) 459. T. Momose, M. Sugiyama, E. Kondoh, Y. Shimogaki, Appl. Phys. Express 1 (2008) 097002. T. Momose, T. Uejima, H. Yamada, Y. Shimogaki, M. Sugiyama, Jpn. J. Appl. Phys. 51 (2012) 056502. T. Momose, M. Sugiyama, E. Kondoh, Y. Shimogaki, Jpn. J. Appl. Phys. 49 (2010) 05FF01. Y. Zhao, K. Jung, T. Momose, Y. Shimogaki, Sci. Technol. 2 (2013) N191. D.S. Bulgarevich, T. Sako, T. Sugeta, K. Otake, M. Sato, M. Uesugi, M. Kato, J. Chem. Phys. 108 (1998) 3915. Q. Peng, D. Hojo, K. Park, G. Parsons, Thin Solid Films 516 (2008) 4997. K. Jung, T. Momose, Y. Shimogaki, J. Supercrit. Fluids 79 (2013) 244. A. Cabañas, X. Shan, J.J. Watkins, Chem. Mater. 15 (2003) 2910. B. Zhao, T. Momose, T. Ohkubo, Y. Shimogaki, Microelectron. Eng. 85 (2008) 675. T. Momose, T. Ohkubo, M. Sugiyama, Y. Shimogaki, Thin Solid Films 517 (2008) 674. O. Kajimoto, Chem. Rev. 99 (1999) 355. L. Onsager, Electr. Moments Mol. Liq. 58 (1936) 1486.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Kinetic study of alcohol-assisted supercritical fluid deposition of TiO2 Yu Zhao ⁎, Kyubong Jung, Takeshi Momose, Yukihiro Shimogaki Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
a r t i c l e
i n f o
Available online 25 November 2013 Keywords: Alcohol Supercritical fluid deposition Conformal films Titanium dioxide
a b s t r a c t The kinetics of TiO2 thin-film deposition in a supercritical CO2/alcohol environment was studied using titanium di(isopropoxide)bis(tetramethylhexanedionate) [Ti(O-i-Pr)2(tmhd)2] as the precursor. The solvent effects of methanol, ethanol, and isopropanol on the supercritical fluid deposition (SCFD) of TiO2 were examined at deposition temperatures of 150–300 °C. Methanol was the most effective of the three alcohols evaluated; it enhanced the TiO2-SCFD deposition rate 2–3-fold and decreased the activation energy from 49 ± 4 to 28 ± 3 kJ/mol. Conformal deposition of TiO2 on a deep trench (aspect ratio of 30) was demonstrated at a growth rate of N 2 nm/min. © 2013 Elsevier B.V. All rights reserved.
1. Introduction TiO2 is a promising material for dielectrics in dynamic random access memory (DRAM) because of its high dielectric constant. DRAM is currently fabricated on three-dimensional (3D) structures and is monolithically integrated with logic circuits to achieve low power consumption, high integration density, and short response time. The international technology roadmap for semiconductors forecasted that highly conformal TiO2 deposition technology that is needed for high-aspect-ratio (N60) features will be mandatory after 2017 [1]. Conventional deposition techniques based on vacuum technology, such as chemical vapor deposition (CVD), have been unable to conformally deposit TiO2 on high-aspect-ratio features [2–4]. Although atomic layer deposition (ALD) has achieved conformal deposition on trenches having aspect ratios N60, its growth rate is limited to 0.5 nm/min because of the cyclic deposition procedure [5–7]. We previously demonstrated the comparable performance of supercritical fluid deposition (SCFD) with ALD, i.e., conformal deposition onto high-aspect-ratio features at a growth rate of 1 nm/min [8]. SCFD has the potential to further enhance the growth rate by optimizing the reaction chemistry, which will be discussed later. SCFD involves the oxidation/reduction of organic compounds in supercritical CO2 (scCO2), which is a phase of CO2 above its critical temperature and pressure [9–11]. Molecules in the supercritical phase are heterogeneously distributed between a sparse region (contributing to gas-like diffusivity) and a dense region (contributing to liquid-like solubility). This heterogeneous distribution of the scCO2 molecular density benefits SCFD [10]; remarkable conformal deposition is achieved without compromising growth rate. We have conformally formed Cu films on holes with an aspect ratio of 20 and also achieved
⁎ Corresponding author. Tel./fax: +81 3 5841 7131. E-mail address: [email protected] (Y. Zhao). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.11.062
void-free filling with Cu [12,13]. Unlike conventional vacuum-based technologies, such as ALD, that are adequate for high-aspect-ratio features, SCFD generally has growth rates of N1 nm/min. These higher rates are suitable for mass production and derive from the high precursor concentration in scCO2 [14,15]. Additionally, the relatively high dielectric constant of scCO2 (ε = 1.2) is expected to decrease the activation energy of reactions through a solvent effect [16–18]. SCFD is a solution-based technology. It is possible to enhance the growth rate and decrease the process temperature by adding carefully selected organic solvents, such as alcohols, to the scCO2 [19–21]. The customizability of the process is one of the advantages of SCFD. Momose et al. introduced ethanol into Cu-SCFD and reported a reduction in the deposition temperature from 220 to 195 °C [21]. Peng et al. suggested that the addition of an alcohol with a high dielectric constant enhanced the growth rate of Al2O3-SCFD [17]. In this study, the solvent effect of readily available alcohols, such as methanol, ethanol, and isopropyl alcohol (IPA), were examined for TiO2-SCFD. The experimental results showed that methanol effectively promoted the SCFD of TiO2. 2. Experimental details Titanium di(isopropoxide)bis(tetramethylhexanedionate) [Ti(O-iPr)2(tmhd)2] (99.99%) was employed as the Ti precursor for all experiments. Methanol, ethanol, and IPA were selected to obtain lower deposition temperatures and higher growth rates for TiO2-SCFD. These alcohols were selected because the precursor was soluble in these alcohols. To evaluate the solvent effect, the precursor concentration was adjusted by diluting a saturated precursor/scCO2 mixture. Alcohols were injected afterward to avoid changing the precursor concentration. A planar Si substrate with a SiO2 surface was used for the kinetic study because of the short incubation time (b 1 min) of our experiments. Si trenches with thermal SiO2 surfaces (600 nm wide,
Y. Zhao et al. / Thin Solid Films 553 (2014) 184–187
aspect ratio of 30) were used to demonstrate the capability for conformal deposition. Deposition of TiO2 on the bottom electrode material of DRAM, such as Ru, is a goal of future work. A cold-wall continuous reactor in the face-down configuration was adopted (Fig. 1). It enabled the use of a relatively low fluid temperature and provided laminar flow without natural convection, both of which eliminated unwanted particle generation in the fluid [15]. Before deposition, all the equipment was pressurized at 10 MPa with scCO2 (99.99%). The substrate was placed face-down in the reactor and heated to 150–300 °C. Deposition was initiated by introducing the Ti precursor dissolved in scCO2, and liquid alcohol, from individual lines. The TiO2 was thereby deposited onto the heated face-down substrate in the cold-wall reactor. The concentrations of the Ti precursor and the alcohol were controlled as follows. An excess of the solid Ti precursor was placed in a hot-wall reservoir, and then scCO2 was flowed into the reactor via the reservoir. This provided a scCO2 stream that was saturated in the Ti precursor. This saturated scCO2 solution was then mixed with another scCO2 stream at 10 MPa and 60 °C to adjust the Ti precursor concentration to the desired concentration. Liquid alcohol was directly injected into the flowing scCO2 using a high-pressure pump. The volumetric impact of the alcohol on the scCO2 was neglected in the concentration calculation because the molar ratio of alcohol to scCO2 in the mixed fluid was about 10−2. The Ti precursor and alcohol concentrations used in this work were estimated in scCO2 at 10 MPa and 60 °C. The microstructure of the obtained TiO2 films and step coverage on high-aspect-ratio trenches were analyzed with a field-emission scanning electron microscope (FE-SEM, JEOL 6340F). A film density of 4 g/cm3 was used to estimate the surface reaction rate constant. The atomic composition was determined by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 1600C). The root-mean-square (RMS) surface roughness was determined by atomic force microscopy (SEIKO NPX100).
3. Results and discussion To investigate the effect of alcohol addition on the growth rate of TiO2-SCFD, we deposited TiO2 on a planar Si substrate having a SiO2 surface at 250 °C in the presence of different alcohol mole fractions (Fig. 2). The precursor concentration was fixed at 2 × 10− 3 mol/L. An amorphous solid TiO2 film with smooth morphology (RMS b 2 nm for a 100-nm-thick film) was deposited (data not shown). XPS results showed that the carbon impurities of these samples were less than 1 at.% (data not shown). All three alcohols promoted the growth rate of TiO2-SCFD at low alcohol concentrations. The growth rates were significantly enhanced in all cases by increasing the alcohol concentration and were saturated at a precursor molar ratio of about 0.008. Excess alcohol addition reduced the growth rate in all cases. The increase in the growth rate was attributed to an enhanced solvent effect; details are discussed below. The decrease in the growth rate at high alcohol concentrations was caused by the reduced precursor concentration contributing to the film growth; the precursor was largely consumed in the fluid phase to form particles. Fine particles were indeed found on the inner wall of the reactor after deposition. The saturation at
185
Fig. 2. Growth rate of TiO2-SCFD at 250 °C as a function of alcohol mole fraction.
intermediate alcohol concentrations might be due to a balance of these two effects. The solvent effect of the alcohol addition was further studied by the growth rate dependence on temperature at a constant alcohol mole fraction of 0.01. Fig. 3 shows an Arrhenius plot of TiO2-SCFD with and without alcohol assistance at 150–300 °C. The activation energies under methanol-, ethanol-, and IPA-assistance were 28 ± 3, 25 ± 2, and 43 ± 2 kJ/mol, respectively, while SCFD without alcohol was 49 ± 4 kJ/mol. The activation energy with CVD using the same precursor was 85 ± 5 kJ/mol [2]. Accordingly, we conclude that the scCO2 solvent effect served to reduce the activation energy, and the addition of alcohol to SCFD led to a further decrease of the activation energy. These results were analyzed by applying Onsager's reaction field theory to SCFD, which was previously used for solvent-based systems to explain the reaction enhancement in the liquid phase [17,22,23]. The activation energy (Ea) is decreased by a factor ΔEa due to solvent clusters around the solute. The value of ΔEa can be estimated using the following formula:
ΔEa ¼
μ 2 2ðε−1Þ a3 ð2ε−1Þ
where μ is the dipole moment of the polar solute molecule in a cluster of radius a, and ε is the dielectric constant of the solvent. Provided that the CVD and SCFD reactions have the same polar transition state, the dielectric constant of the scCO2 solvent (ε = 1.2) was expected to decrease the potential barrier of the decomposition via an effective reduction in the free energy of the polar transition state [15,17]. Furthermore, the addition of alcohol (methanol ε = 32; ethanol ε = 24; IPA ε = 20) further enhanced the dielectric constant of the reaction medium and resulted in an even lower activation energy despite the low molar fraction of the added alcohols. This is because when polar Ti precursor molecules dissolve in scCO2/alcohol mixtures, the CO2 and alcohol molecules likely form “cluster” structures around the precursor; thus, the local effective
Fig. 1. Schematic diagram of the cold-wall flow-type supercritical fluid deposition system.
186
Y. Zhao et al. / Thin Solid Films 553 (2014) 184–187
Fig. 3. Arrhenius plot of TiO2-SCFD with and without alcohol assistance at 150–300 °C.
dielectric constant can be much higher than the apparent value [16,17]. A relatively large difference in Ea was seen between IPA-assistance and methanol- or ethanol-assistance. This can be explained by differences in the polarity and molecular size of the alcohols: the local dielectric constants are determined by the local concentration of the alcohols, and the large molecular size of IPA is unfavorable for clustering around precursor molecules. The dependence of the growth rate on the precursor concentration with alcohol assistance was subsequently studied. Fig. 4 shows the relationship between the growth rate and the Ti precursor concentration at 250 °C with alcohol added at a mole fraction of 0.01. The growth rate increased linearly with precursor concentration, indicating that the deposition had first-order reaction kinetics regardless of the presence of alcohol. The surface reaction rate constants (ks) were (1.98 ± 0.05) × 10−6, (1.75 ± 0.06) × 10−6, (1.08 ± 0.02) × 10−6, and (0.80 ± 0.03) × 10−6 m/s for the methanol-, ethanol-, and IPAassisted, and non-assisted cases, respectively. The SCFD ks values were compared with that for CVD for the same precursor and deposition temperature by estimating the CVD ks value from its Arrhenius plot [2]. In this way, the ks value of TiO2-CVD at 250 °C was estimated as 0.04 m/s. To confirm the conformal deposition capability, methanol-assisted SCFD was examined for deep trenches having an aspect ratio of 30. Fig. 5 shows the excellent conformability of the TiO2 film. The thickness of the TiO2 film over the entire trench was nearly constant, i.e., 132 ± 3 nm. The growth rate under this condition was quite high at 2.2 nm/min. Concerning deposition on 3D structures, a lower ks value leads to better step coverage because the precursor concentration at the bottom of a 3D structure is likely to be lower than at the top, and a lower ks value means less growth rate dependence on the precursor concentration. Hence, the obtained nearly perfect step coverage was attributed to the low ks value of methanol-assisted TiO2-SCFD; the
Fig. 5. Cross-sectional SEM image of a highly conformal TiO2 film deposited on a deep trench (600 nm wide, aspect ratio of 30) at 250 °C with the addition of methanol at a molar ratio of 0.01.
lower ks values of ethanol and IPA should provide better step coverage compared with methanol. The conformal deposition capability between SCFD and CVD was compared as follows. TiO2-SCFD should show superior step coverage compared with TiO2-CVD because the ks of TiO2-SCFD is four orders of magnitude lower than that of TiO2-CVD. Additionally, the growth rate of TiO2-CVD at 250 °C was estimated at only about 0.1 nm/min [2], which is a consequence of the relatively low ks (0.04 m/s at 250 °C) and a low precursor partial pressure (10−8 mol/L). On the other hand, for SCFD, the extremely low ks values (~10−6 m/s) enabled excellent step coverage, while the high precursor concentration in scCO2 (~10−3 mol/L) provided a high growth rate. We also note that the growth rate is limited to 0.5 nm/min for ALD [5,7], while alcoholassisted SCFD at 250 °C had a growth rate of N2 nm/min. Though ALD can conformally deposit on deeper trenches than we used in this study, we previously simulated the conformal deposition on nano-scale trenches having an aspect ratio of 100 using TiO2-SCFD without alcohol at 300 °C [8]. Note that a ks of 2 × 10−6 m/s was used in that work. Considering that the ks of alcohol-assisted SCFD at 250 °C is even smaller, the conformal deposition on nano-scale trenches having an aspect ratio of 100 should also be achievable using our alcohol-assisted SCFD method at 250 °C.
4. Conclusions
Fig. 4. Relationship between growth rate and Ti precursor concentration at 250 °C with alcohol added at a 0.01 molar ratio.
The addition of alcohols (methanol, ethanol, or IPA) to TiO2-SCFD was studied kinetically. The increased growth rate observed in all cases was attributed to an enhanced solvent effect. Methanol was the most effective of the three alcohols. Alcohol addition also decreased the activation energy of the decomposition reactions. The extremely high conformal deposition capability inherent to SCFD was also observed for alcohol-assisted SCFD and was comparable to ALD, while SCFD exhibited higher growth rates exceeding 2 nm/min. These beneficial aspects of TiO2-SCFD with alcohol assistance are well suited for the production of electronic devices with high-aspect-ratio features, such as DRAM.
Y. Zhao et al. / Thin Solid Films 553 (2014) 184–187
Acknowledgments The high-aspect-ratio trenches used in this work were provided by DENSO Corporation.
[8] [9] [10] [11] [12] [13]
References [1] http://www.itrs.net. [2] S.Y. No, J.H. Oh, C.B. Jeon, M. Schindler, C.S. Hwang, H.J. Kim, J. Electrochem. Soc. 152 (2005) C435. [3] V. Siriwongrungson, S.P. Krumdieck, M.M. Alkaisi, Chem. Vap. Depos. 17 (2011) 327. [4] V. Siriwongrungson, M.M. Alkaisi, S.P. Krumdieck, Surf. Coat. Technol. 201 (2007) 8944. [5] T. Blanquart, J. Niinistö, M. Gavagnin, V. Longo, V.R. Pallem, C. Dussarrat, M. Ritala, M. Leskelä, Chem. Mater. 24 (2012) 3420. [6] I. Jõgi, J. Aarik, M. Laan, J. Lu, K. Kukli, H. Kaambre, T. Sajavaara, T. Uustare, Thin Solid Films 510 (2006) 39. [7] I. Jõgi, M. Pärs, J. Aarik, A. Aidla, M. Laan, J. Sundqvist, L. Oberbeck, J. Heitmann, K. Kukli, Thin Solid Films 516 (2008) 4855.
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
187
K. Jung, Y. Zhao, T. Momose, Y. Shimogaki, ECS Solid State Lett. 2 (2013) 79. C. Erkey, J. Supercrit. Fluids 47 (2009) 517. J.M. Blackburn, D.P. Long, A. Cabanas, J.J. Watkins, Science 294 (2001) 141. A.H. Romang, J.J. Watkins, Chem. Rev. 110 (2010) 459. T. Momose, M. Sugiyama, E. Kondoh, Y. Shimogaki, Appl. Phys. Express 1 (2008) 097002. T. Momose, T. Uejima, H. Yamada, Y. Shimogaki, M. Sugiyama, Jpn. J. Appl. Phys. 51 (2012) 056502. T. Momose, M. Sugiyama, E. Kondoh, Y. Shimogaki, Jpn. J. Appl. Phys. 49 (2010) 05FF01. Y. Zhao, K. Jung, T. Momose, Y. Shimogaki, Sci. Technol. 2 (2013) N191. D.S. Bulgarevich, T. Sako, T. Sugeta, K. Otake, M. Sato, M. Uesugi, M. Kato, J. Chem. Phys. 108 (1998) 3915. Q. Peng, D. Hojo, K. Park, G. Parsons, Thin Solid Films 516 (2008) 4997. K. Jung, T. Momose, Y. Shimogaki, J. Supercrit. Fluids 79 (2013) 244. A. Cabañas, X. Shan, J.J. Watkins, Chem. Mater. 15 (2003) 2910. B. Zhao, T. Momose, T. Ohkubo, Y. Shimogaki, Microelectron. Eng. 85 (2008) 675. T. Momose, T. Ohkubo, M. Sugiyama, Y. Shimogaki, Thin Solid Films 517 (2008) 674. O. Kajimoto, Chem. Rev. 99 (1999) 355. L. Onsager, Electr. Moments Mol. Liq. 58 (1936) 1486.