Atomic layer deposition of zirconium silicate films using zirconium tetra-tert-butoxide and silicon tetrachloride Won-Kyu Kim, Sang-Woo Kang, and Shi-Woo Rheea) Laboratory for Advanced Molecular Processing, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea
共Received 10 January 2003; accepted 2 June 2003; published 14 August 2003兲 A new precursor combination (SiCl4 and Zr(Ot C4 H9 ) 4 ) was used to deposit Zr silicate with Zr(Ot C4 H9 ) 4 as a zirconium source and oxygen source at the same time. SiCl4 and Zr(Ot C4 H9 ) 4 have higher vapor pressures than their counterpart, ZrCl4 and tetra-n-butyl orthosilicate 共TBOS兲, and it was expected that the cycle time would be shorter. The deposition temperature of the new combination was about 150 °C lower than that of ZrCl4 and TBOS. The film was zirconium rich while it was silicon rich with ZrCl4 and TBOS. Growth rate 共nm/cycle兲, composition ratio 关 Zr/共Zr⫹Si)], and chlorine impurity were decreased with increasing deposition temperature from 125 to 225 °C. The composition ratio of the film deposited at 225 °C was 0.53 and the chlorine content was about 0.4 at. %. No carbon was detected by x-ray photoelectron spectroscopy. © 2003 American Vacuum Society. 关DOI: 10.1116/1.1595107兴
As the thickness of the gate oxide in field effect transistors approaches 2 nm, direct tunneling will lead to unacceptable leakage currents, making it necessary to replace the silicon dioxide layer with a material possessing a high dielectric constant. Many of the alternatives, such as Ta2 O5 , 1 TiO2 , 2 and SrTiO3 , 3 are not thermodynamically stable in direct contact with Si substrates. Therefore, they require a barrier layer to prevent possible interfacial reactions, which cause lowering of the dielectric constant and degrading the interfacial quality. Although some oxides, such as Al2 O3 , 4 HfO2 , 5 and ZrO2 , 6 have good stability with Si and high dielectric constants, they suffer from diffusion of boron or oxygen through themselves to the bare Si. Zirconium silicate has recently attracted increasing interest due to its outstanding properties as a gate dielectric. It is thermally stable with Si and a good barrier against oxygen diffusion.7 Zr silicate thin films have been deposited by sputtering8 and atomic layer deposition 共ALD兲.9,10 However, the sputtering process has some drawbacks such as poor step coverage and a damaging effect to the channel region of complementary metal–oxide–semiconductor 共CMOS兲 devices. ALD is a very promising process because ALD enables accurate control of film thickness and composition on one atomic layer level through self-limiting surface reactions. In the ALD process, a purge process is inserted right after a precursor is introduced so that only less than a monolayer of the precursor chemisorbed on the substrate surface remains to be reacted with another reactant introduced. Recently, Ritala et al.9 and Gordon et al.10 suggested a new ALD concept using alkoxides or silanol as an oxygen source and a metal source at the same time instead of using oxygen or water. Ritala et al. obtained uniform Si-rich Zr silicate films with a sharp interface using ZrCl4 and silicon alkoxides, Si共OEt兲4 a兲
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and Si(On Bu) 4 . And Gordon et al. used an amido compound as the Zr precursor and t-butoxy silanol as the Si precursor with a high growth rate and low impurity content. These two approaches gave the metal silicate film with excess Si, i.e., 关 metal/(metal⫹Si) 兴 below 0.5. In the present work, the SiCl4 and Zr(Ot C4 H9 ) 4 combination was used and the film was grown in a cold-wall flowtype ALD reactor on 共100兲 oriented p-Si single crystal substrates in the range of 125– 225 °C. Prior to the deposition, the Si substrate was cleaned using the modified RCA method. The pressure in the reactor was fixed at 1 Torr. Argon 共99.99995%兲 was used as a carrier and purging gas. The source vapor was carried from each bubbler by Ar carrier gas. Carrier gas and purge gas were introduced into the reactor for a fixed amount of time separately and sequentially by means of digitized on/off control of solenoid valves. Operating conditions are summarized in Table I. Film thickness was measured by ellipsometry at a wavelength of 632.8 nm, and the composition of the deposited films was analyzed by x-ray photoelectron spectroscopy 共XPS兲. Figure 1 shows the vapor pressure of the precursors, and we compared the vapor pressure of ZrCl4 共Ref. 11兲 and Si(On Bu) 4 共Ref. 12兲 reported in other papers9,13 with that of SiCl4 共Ref. 14兲 and ZTB.11 The vapor pressure of the precursor used in this research is higher than those of ZrCl4 and Si(On Bu) 4 . The dosing time could be shorter for the precursor with higher vapor pressure. A solid precursor (ZrCl4 ) is difficult to handle and feed into the reactor. Both of the precursors 共ZTB and SiCl4 ) used in the new combination are liquid. Table I summarizes the deposition condition for Zr silicate thin films. Deposition rate 共nm/cycle兲 was measured as a function of the deposition temperature at a constant reactor pressure of 1 Torr. Figure 2 shows the effect of the substrate temperature on the growth rate when the other process parameters such as source temperature, flow rate and time of purging, source injection time, and operating pressure were fixed. With SiCl4
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©2003 American Vacuum Society
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Kim, Kang, and Rhee: Atomic layer deposition of zirconium silicate
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TABLE I. Operating conditions for the deposition of Zr silicate film. Condition Parameter
ZrCl4 ⫹TBOS
SiCl4 ⫹ZTB
Substrate temperature
300– 500 °C
125– 225 °C
Bubbler temperature
ZrCl4 : 160 °C, TBOS: 95 °C
SiCl4 : 0 °C, ZTB: 50 °C
Vapor pressure
ZrCl4 : 0.15 Torr, TBOS: 1.1 Torr
ZTB: 0.44 Torr SiCl4 : 77 Torr
Carrier gas 共Ar兲
20 sccm
10 sccm
Purge gas 共Ar兲
500 sccm
Deposition pressure
1 Torr
Optimum solenoid valve on/off time
ZTB:purge:SiCl4 :purge⫽2 s:5 s:2 s:5 s ZrCl4 :purge:TBOS:purge⫽5 s:2 s:2 s:2 s
and ZTB, it can be seen that the growth rate was decreased at temperatures below 175 °C. The maximum growth rate, 0.18 nm/cycle, was achieved at 175 °C. Further increase in the temperature up to 225 °C was accompanied with a decrease in the growth rate down to 0.158 nm/cycle. With ZrCl4 and TBOS, the growth rate decreased to 0.105 nm/cycle with an increase of the substrate temperature from 300 to 500 °C. The deposition temperature range with SiCl4 and ZTB is much lower than with ZrCl4 and TBOS. Figure 3 shows the composition ratio 关 Zr/(Zr⫹Si) 兴 and impurity contents. The composition was analyzed by XPS. In the film deposited with SiCl4 and ZTB, we found that the film is Zr rich with composition ratio decreased from 0.75 to 0.53 as the substrate temperature was increased from 125 to
FIG. 1. Vapor pressure of the precursor as a function of the temperature. JVST A - Vacuum, Surfaces, and Films
225 °C. The amount of zirconium oxide was higher than the film deposited with ZrCl4 and TBOS. This may be due to the fact that the zirconium–chlorine bond strength is higher than the zirconium–alkoxy group. Zr-rich films have a higher dielectric constant than Si-rich films. In the ALD with ZrCl4 and TBOS, the amount of carbon and chlorine in the silicate film were not detected with XPS. Carbon was not detected also with SiCl4 and ZTB and chlorine was reduced with the increase of the deposition temperature from 1 at. % to less than 0.5 at. %. There were reports that ZTB was thermally decomposed above 300 °C. 15,16 It was reported that SiCl4 is a very stable compound and has low reactivity that needs high temperature to deposit oxide. According to a precursor-mediated adsorption model,17 the physisorbed SiCl4 can either desorb back to the gas phase or react with a hydroxyl group on the surface. Given reaction activation energy between SiCl4 and
FIG. 2. Growth rate as a function of the substrate temperature. ( P 0 ⫽1 Torr, ZrCl4 –purge–TBOS–purge: 5 s–2 s–2 s–2 s, ZTB–purge– SiCl4 –purge: 2 s–5 s–2 s–5 s.兲
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Kim, Kang, and Rhee: Atomic layer deposition of zirconium silicate
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deposition temperature from 125 to 225 °C. The deposition temperature of the new combination was about 150 °C lower than that of ZrCl4 and TBOS. The Zr ratio, chlorine contents of the Zr silicate thin film deposited at 225 °C were 0.53 and less than 0.5 at. %, respectively. No carbon was detected using XPS. With this new precursor combination, we obtained the Zr silicate film with a high composition ratio of 关 Zr/(Zr⫹Si) 兴 over 0.5. P. K. Roy and I. C. Kizilyalli, Appl. Phys. Lett. 72, 2835 共1998兲. C. J. Taylor, D. C. Gilmer, D. Colombo, G. D. Wilk, S. A. Campbell, J. Roberts, and W. L. Gladfelter, J. Am. Chem. Soc. 121, 5220 共1999兲. 3 R. A. McKee, J. J. Walker, and M. F. Chisholm, Phys. Rev. Lett. 81, 3014 共1998兲. 4 M. D. Groner, J. W. Elam, F. H. Fabreguette, and S. M. George, Thin Solid Films 413, 186 共2002兲. 5 J. Park, B. K. Park, M. C. Cho, C. S. Hwang, K. Oh, and D. Y. Yang, J. Electrochem. Soc. 149, G89 共2002兲. 6 K. Kukli, M. Ritala, T. Uustare, J. Aarik, K. Forsgren, T. Sajavaara, M. Leskela, and A. Harsta, Thin Solid Films 410, 53 共2002兲. 7 G. D. Wilk, R. M. Wallace, and J. M. Anthony, J. Appl. Phys. 89, 5243 共2001兲. 8 G. D. Wilk and R. M. Wallace, Appl. Phys. Lett. 76, 112 共2000兲. 9 M. Ritala, K. Kukli, A. Rahtu, P. I. Raisanen, M. Leskela, T. Sajavaara, and J. Keinonen, Science 288, 319 共2000兲. 10 R. G. Gordon, J. Becker, D. Hausmann, and S. Suh, Chem. Mater. 13, 2463 共2001兲. 11 J. Korean, Semicond. Int. 11, 86 共2002兲 共in Korean兲. 12 E. J. Kim and W. N. Gill, J. Electrochem. Soc. 142, 676 共1995兲. 13 W.-K. Kim, S.-W. Kang, S.-W. Rhee, N.-I. Lee, J.-H. Lee, and H. K. Kang, J. Vac. Sci. Technol. A 20, 2096 共2002兲. 14 T. Boublik, V. Fried, and E. Hala, Phys. Sci. Data 17, 933 共1984兲. 15 M. A. Cameron and S. M. George, Thin Solid Films 348, 90 共1999兲. 16 J. P. Chang and Y.-S. Lin, J. Appl. Phys. 90, 2964 共2001兲. 17 W. H. Weinberg, in Kinetics of Interface Reactions, edited by M. Grunze and H. J. Kreuzer 共Springer, New York, 1987兲. 18 O. Sneh, M. L. Wise, A. W. Ott, L. A. Okada, and S. M. George, Surf. Sci. 334, 135 共1995兲. 1 2
FIG. 3. Composition ratio 关 Zr/(Zr⫹Si) 兴 and impurity contents 共at. %兲 analyzed by XPS. ( P 0 ⫽1 Torr, ZrCl4 –purge–TBOS–purge: 5 s–2 s–2 s–2 s, ZTB–purge–SiCl4 –purge: 2 s–5 s–2 s–5 s.兲
a hydroxy group, the model predicts that only about 1 out of 106 physisorbed SiCl4 molecules will react with hydroxy groups at 600 K.18 But with ZTB and SiCl4 , the deposition temperature was decreased due to the high reactivity between chlorine of SiCl4 and O-tert-butyl of ZTB. In conclusion, using a new precursor combination (SiCl4 and ZTB兲, the feasibility for low temperature deposition of Zr silicate was shown. Also, it was shown that Zr-rich silicate film could be obtained with this precursor combination. The composition ratio 关 Zr/共Zr⫹Si)] and impurity contents 共carbon and chlorine兲 were decreased with the increase of the
J. Vac. Sci. Technol. A, Vol. 21, No. 5, SepÕOct 2003
