Atomic layer deposition of hafnium silicate films using hafnium ...

Atomic layer deposition of hafnium silicate films using hafnium tetrachloride and tetra-n-butyl orthosilicate Won-Kyu Kim and Shi-Woo Rheea) Laboratory for Advanced Molecular Processing, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea

Nae-In Lee, Jong-Ho Lee, and Ho-Kyu Kang Advanced Process Development Team, System LSI Division, Samsung Electronics Co., Ltd.San #24, NongseoRee, Kiheung-Eup, Yongin-City, Kyonggi-Do, 499-900, Korea

共Received 30 January 2004; accepted 3 May 2004; published 16 July 2004兲 Atomic layer chemical vapor deposition 共ALCVD兲 of hafnium silicate films with a precursor combination of HfCl4 and TBOS 共tetra-n-butyl orthosilicate兲 was studied for high dielectric gate insulators. The effect of deposition conditions, such as deposition temperature and pulse time for precursor injection on the deposition rate per cycle and composition of the film 共fraction of hafnia phase in the silicate film兲 were studied. The growth rate and composition ratio were saturated with the increase of the injection time of HfCl4 and TBOS and decreased with the increased deposition temperature from 300 to 500 °C. The growth rate was 1.4 Å/cycle and the fraction of hafnium phase in the Hf silicate thin films was 0.19 at the deposition temperature of 500 °C. Impurity content, such as carbon and chlorine was below the detection limit of XPS 共x-ray photoelectron spectroscopy兲 and the impurity level detected by SIMS decreased with increasing deposition temperature. It was found that the incorporation rate of metal from halide source was lower than alkoxide source. © 2004 American Vacuum Society. 关DOI: 10.1116/1.1764819兴

I. INTRODUCTION As the thickness of the gate oxide in field effect transistors approaches 2 nm, direct tunneling will become predominant and it will be necessary to replace the silicon dioxide layer with a material possessing a higher dielectric constant 共high k兲. This will allow the use of the physically thicker gate dielectric with the electrically equivalent oxide thickness 共EOT兲. Many of the alternatives, such as Ta2 O5 , 1 TiO2 , 2 and SrTiO3 , 3 are not thermodynamically stable in direct contact with the silicon substrate and the interfacial quality is degraded. Although some oxides, such as Al2 O3 , 4 HfO2 , 5 and ZrO2 , 6 have good stability with Si and a high dielectric constant, the diffusion coefficient for boron or oxygen through these materials is high. In this case, the compatibility of gate dielectric with boron doped poly silicon gate metal becomes poor and low k interlayer formation due to the oxygen diffusion is a problem. Hafnium silicate is considered to be the most promising alternative gate dielectric due to its outstanding properties, such as thermodynamic stability in direct contact with Si up to high temperature and a good barrier against oxygen diffusion.7 Hafnium silicate thin films have been deposited by sputtering,8,9 chemical vapor deposition 共CVD兲,10–12 and atomic layer deposition 共ALD兲.13,14 The sputtering process has some drawbacks such as poor step coverage and the damaging effect of the plasma to the channel region of the CMOS 共complimentary metal-oxide-semiconductor兲 devices. CVD has also some drawbacks such as difficulty in controlling thickness and composition because the nucleation rate control is not easy. For nano scale thin film applications like a兲

Electronic mail: [email protected]

1285

J. Vac. Sci. Technol. A 22„4…, JulÕAug 2004

gate dielectric, ALD is considered to be an alternative. ALD is promising because it enables accurate control of film thickness and composition at one atomic layer level with uniformity over large area and excellent conformal step coverage through self-limiting surface reactions. But in this case, the selection of chemical precursors is important and chemical reactions associated with ALD process must be well understood and controlled. Ritala et al.15 and Gordon et al.14 suggested a new ALD concept. Instead of using an additional oxygen source such as water, hydrogen peroxide, ozone, or oxygen radical, they used two metal compounds such that at least one of them is an alkoxide and oxygen in alkoxy ligand is directly bonded to a metal without an additional oxidant. Due to the strong bond between metal and oxygen, metal alkoxides are less oxidizing toward silicon than common oxygen sources, thereby making it possible to create sharp silicon–metal oxide interface with no interfacial silicon oxide layer. Ritala et al. obtained very uniform Si-rich Zr silicate films with sharp interface using ZrCl4 and silicon alkoxides such as Si共OEt) 4 and Si共On Bu) 4 . Gordon et al. used amido compounds as the Hf precursor and t-butoxy silanol as the Si precursor with high growth rate and low impurity contents. Also zirconium silicate film was deposited with Zr共Ot Bu) 4 – SiCl4 18 and ZrCl4 – Si共On Bu) 4 22 reaction system. In the ALD process, an inert gas purge is necessary to achieve less than a monolayer chemisorption in each cycle by purging physisorbed molecules on the chemisorbed layer right on top of the solid surface. Also there should be no chemical reaction that leads to the nucleation on the surface before the second reactant is introduced to induce the selflimited surface reaction. It is important to find out the ALD process window and optimized process conditions. To de-

0734-2101Õ2004Õ22„4…Õ1285Õ5Õ$19.00

©2004 American Vacuum Society

1285

1286

Kim et al.: Atomic layer deposition of hafnium silicate films

1286

FIG. 1. A schematic diagram of the ALD system with computer-controlled solenoid valves.

velop a successful ALD process, optimization of the process variables such as deposition temperature, purging gas flow rate, and the cycle time of the precursor injection is needed and their effect on the deposition rate and film composition should be clearly identified. In this article, we demonstrate the optimization of operating parameters for atomic layer deposition of Hf silicate using the combination of HfCl4 and Si共On Bu) 4 共TBOS兲共Bu⫽butyl兲 for the first time.

II. EXPERIMENT The films were grown in a cold-wall flow-type ALD reactor on 共100兲 oriented p-Si single crystal substrates in the temperature range of 250–500 °C. Figure 1 shows the schematic diagram of the ALD reactor, where the susceptor is resistively heated. Prior to deposition, the Si substrate was cleaned using the modified RCA method to make the surface hydrogen terminated silicon.23 Cleaning included 共i兲 dipping in a H2 SO4 :H2 O2 3:1 solution for 10 min and rinsing with deionized共DI兲 water, 共ii兲 dipping in a HF:H2 O 1:7 solution for 30 s and rinsing with DI water, and 共iii兲 blowing with N2 gas.

The pressure in the reactor was fixed at 1 Torr by using a throttle valve located between the pump and the reaction chamber. Argon 共99.999 95%兲 was used as a carrier and purging gas. HfCl4 共ACROS, 99%兲 and TBOS 共TCI, 98%兲 were used as reactant gases and the temperature 共vapor pressure兲 of the container of HfCl4 and TBOS were 160 °C 共0.14 Torr兲 and 95 °C 共1.1 Torr兲, respectively. Source vapor was carried from each bubbler by Ar. 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. To control source injection and purge time, we varied valve on–off time from 2 to 15 seconds. The flow rate of the purge gas was fixed at 500 sccm 共standard cubic centimeters per minute兲. The feed line and shower head were held at 250 °C to prevent the condensation of the precursors. Deposition conditions are summarized in Table I. Film thickness was measured by ellipsometry, and the composition of the deposited film was analyzed by x-ray photoelectron spectroscopy 共XPS兲. Impurities such as carbon and chlorine, incorporated below the detection limit of XPS,

TABLE I. Operating conditions for the deposition of hafnium silicate film. Parameter

FIG. 2. Ellipsometric measurement of Hf silicate film thickness 共Å兲 deposited on Si共100兲 at 400 °C as a function of the number of cycles. J. Vac. Sci. Technol. A, Vol. 22, No. 4, JulÕAug 2004

Purge gas 共Ar兲 Carrier gas 共Ar兲 Deposition pressure Substrate temperature Size of the substrate, Diameter of the hotplate Distance from the showerhead to the substrate Bubbler temperature Vapor pressure Showerhead temperature Solenoid valve on/off time

Condition 500 sccm 20 sccm 1 Torr 250–500 °C 2 cm⫻2 cm, Dia.⫽15 cm 2 cm

HfCl4 : 160 °C, TBOS: 95 °C HfCl4 : 0.14 Torr, TBOS: 1.1 Torr 250 °C HfCl4 : 2–15 s, TBOS: 2–15 s, Purge: 10 s

1287


1287

FIG. 3. Growth rate as a function of the TBOS injection time 共deposition temperature: 400 °C, HfCl4 pulse: 10 s, purge: 10 s兲.

FIG. 5. Growth rate as a function of the HfCl4 injection time 共deposition temperature: 400 °C, TBOS pulse: 12 s, purge: 10 s兲.

were characterized by secondary ion mass spectroscopy 共SIMS兲.

TBOS has the lowest activation energy for dissociation among tetra-alkoxysilanes and it can react with other metal compounds though the compound itself is not thermally decomposed below 650 °C.16 The reaction mechanism was reported to be somewhat complicated and affected by temperature and other operating conditions.17 In the ALD process, the inert gas purge is important so that less than a monolayer is chemisorbed in top of the surface. Argon purge was introduced after the injection of each precursor. Figure 2 shows the film thickness measured by ellipsometry as a function of the number of deposition cycles at 400 °C with 500 sccm of purge. One of the attractive features of the ALD process was that the film thickness was controlled simply by the number of reaction cycles used. In this figure, ideal linear dependence was found with the constant growth rate, 1.9 Å/cycle, resulting from the self-limited surface reaction. Figure 3 shows the dependence of the growth rate on the TBOS injection time at the substrate temperature of 400 °C. HfCl4 injection time and purging time were fixed at 10 s. Insufficient TBOS injection resulted in a low growth rate due to the lack of chemisorbed TBOS molecules. As the TBOS injection time increased, the growth rate was saturated at 1.9

Å/cycle above 12 s. No significant change in the growth rate was observed when TBOS injection time was longer than 12 s. We obtained the composition ratio 关Hf/共Hf⫹Si兲兴 and impurity contents 共at. %兲 such as carbon and chlorine from XPS. Figure 4 shows that the composition was saturated at 0.21 even at 5 s injection time of TBOS. No carbon and chlorine was detected by XPS. Figure 5 shows the variation of the growth rate with respect to the HfCl4 injection time. Purge time was fixed at 10 s. As the HfCl4 injection time increased, the growth rate was saturated at 1.9 Å/cycle above 10 s. No significant change in the growth rate was observed for HfCl4 injection times longer than 10 s. Figure 6 shows the composition ratio and impurity content. We found that the composition ratio 关Hf– 共Hf⫹Si兲兴 was saturated at 0.21 at 7 s injection time of HfCl4 . The optimum shortest cycle time was 10-10-12-10 s, HfCl4 injection time, first purge time, TBOS injection time, and second purge time, respectively. The incorporation rate of the metal 共Hf, Zr, or Si兲 from chloride source was obviously much lower than from alkoxide source. When we used HfCl4 共or ZrCl4 ) and TBOS 共alkoxide兲, we had Si-rich films, whereas we had Zr-rich films using SiCl4 and zirconium tertiary-butoxide.18 The reaction pathway involved in this process that determined the stoichiometry of the film is not clear.14 According to the precursor-mediated adsorption model, the physisorbed SiCl4 , which has a similar structure to HfCl4 , could either desorb back to the gas phase or react with a hydroxyl group

FIG. 4. Composition ratio 关Hf–共Hf⫹Si兲兴 as a function of the TBOS injection time.

FIG. 6. Composition ratio 关Hf–共Hf⫹Si兲兴 as a function of the HfCl4 injection time.

III. RESULTS AND DISCUSSION

JVST A - Vacuum, Surfaces, and Films

1288


FIG. 7. Growth rate as a function of substrate temperature (HfCl4 pulse: 10 s, TBOS pulse: 12 s, purge: 10 s兲.

on the surface. With reaction activation energy between SiCl4 and a hydroxyl group, the model predicts that only about 1 out of 106 physisorbed SiCl4 molecules should react with hydroxyl groups at 600 K.19 With bond strength weaker than Si–Cl, the fraction of HfCl4 reacting on the surface could be higher than SiCl4 . When the other process parameters such as source temperature, flow rate, time of purge, source injection time, and operating pressure were fixed, the growth rate decreased to 1.4 Å/cycle with an increase in the substrate temperature from 300 to 500 °C as shown in Fig. 7. Suntola pointed out that saturation density decreases with increasing temperature, which often occurs on disordered surfaces such as surfaces of polycrystalline and amorphous materials.20 Alternatively it might have been due to the desorption of film forming species along with the change in the reaction pathway with increased deposition temperature.14,21 The slow growth rate at 250 °C is possibly due to the limitation on the kinetics of the surface reaction. Figure 8 shows the composition ratio of the films analyzed by XPS. In this figure, we found that the fraction of hafnia phase decreased from 0.46 to 0.19 as the substrate temperature increased from 250 to 500 °C. No carbon and chlorine was detected by XPS in this temperature range. Figure 9 shows the trace of impurity contents, which were not detected by XPS, characterized by SIMS. In the figure, it was found that impurities such as carbon and chlorine decreased with increasing temperature. Figure 10 shows SIMS

FIG. 8. Composition ratio 关Hf–共Hf⫹Si兲兴 as a function of the substrate temperature. J. Vac. Sci. Technol. A, Vol. 22, No. 4, JulÕAug 2004

1288

FIG. 9. The effect of growth temperature on the impurity incorporation during hafnium silicate film growth characterized by SIMS.

depth profile of the hafnium silicate sample deposited at 500 °C. As shown in the Fig. 10, incorporated carbon in the film was negligible and all the elements in the film are uniformly distributed in the film.

IV. CONCLUSION Optimization of the ALD process to deposit hafnium silicate films was demonstrated with a precursor combination of HfCl4 and TBOS. The optimum shortest cycle time was found to be 10-10-12-10 s (HfCl4 injection time, first purge time, TBOS injection time, and second purge time兲. At the deposition temperature of 400 °C, the growth rate and composition ratio 关Hf/共Hf⫹Si兲兴 of the hafnium silicate thin film deposited at this optimum condition were 1.9 Å/cycle and 0.21. The growth rate and composition ratio 共hafnia fraction兲 decreased with the increase of the deposition temperature from 250 to 500 °C. Impurity contents, such as carbon and chlorine, were incorporated with negligible amount, which were not detected by XPS. It was found that the incorporation rate of metal 共Hf, Zr, or Si兲 from halide source was

FIG. 10. SIMS depth profiles of the hafnium silicate sample deposited at 500 °C.

1289


lower than alkoxide source. More study is needed to characterize the electric properties of the silicate film and properties of the silicate and silicon interface. 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 E. P. Gusev, M. Copel, E. Cartier, I. J. R. Baumvol, C. Krug, and M. A. Gribelyuk, Appl. Phys. Lett. 76, 176 共2000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 M. Houssa, V. V. Afanas’ev, A. Stesmans, and M. M. Heyns, Appl. Phys. Lett. 77, 1885 共2000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. 74, 2854 共1999兲. 9 G. D. Wilk, R. M. Wallace, and J. M. Anthony, J. Appl. Phys. 87, 484 共2000兲. 10 Y. Ohshita, A. Ogura, M. Ishikawa, A. Hoshino, S. Hiiro, T. Suzuki, and H. Machida, Thin Solid Films 416, 208 共2002兲. 11 H. Kato, T. Nango, T. Miyagawa, T. Katagiri, K. S. Seol, and Y. Ohki, J. Appl. Phys. 92, 1106 共2002兲. 1 2

JVST A - Vacuum, Surfaces, and Films

12

1289

V. Rangarajan, H. Bhandari, and T. M. Klein, Thin Solid Films 419, 1 共2002兲. 13 E. Vainonen-Ahlgren, E. Tois, T. Ahlgren, L. Khriachtchev, J. Marles, S. Haukka, and M. Tuominen, Comput. Mater. Sci. 27, 65 共2003兲. 14 R. G. Gordon, J. Becker, D. Hausmann, and S. Suh, Chem. Mater. 13, 2463 共2001兲. 15 M. Ritala, K. Kukli, A. Rahtu, P. I. Raisanen, M. Leskela, T. Sajavaara, and J. Keinonen, Science 288, 319 共2000兲. 16 E. J. Kim and W. N. Gill, J. Electrochem. Soc. 142, 676 共1995兲. 17 S. Haukka, E.-L. Lakomaa, and T. Suntola, Thin Solid Films 225, 280 共1993兲. 18 W. K. Kim, S. W. Kang, and S. W. Rhee, J. Vac. Sci. Technol. A 21, L16 共2003兲. 19 O. Sneh, M. L. Wise, A. W. Ott, L. A. Okada, and S. M. George, Surf. Sci. 334, 135 共1995兲. 20 T. Suntola, Handbook of Thin Film Process Technology, 1st ed. 共Institute of Physics, London, 1995兲. 21 J. Aarik, A. Aidla, A.-A. Kiisler, T. Uutare, and V. Sammelselg, Thin Solid Films 340, 110 共1999兲. 22 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共6兲, 2096 共2003兲. 23 W. Kern, Handbook of Semiconductor Wafer Cleaning Technology 共Noyes, Park Ridge, NJ, 1993兲.