Crystallization Improvement of Ta2o5 Thin Films by the Addition of Water

ARTICLE IN PRESS

Journal of Crystal Growth 274 (2005) 73–77 www.elsevier.com/locate/jcrysgro

Crystallization improvement of Ta2O5 thin ﬁlms by the addition of water vapor

A.P. Huang, Paul K. ChuÃ

Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong

Received 16 September 2004; accepted 5 October 2004 Communicated by R. Kern Available online 13 November 2004

Abstract

We report that by the addition of water vapor into the working gas, crystalline tantalum pentoxide (Ta2O5) thin films can be fabricated at low substrate temperatures. At 500 1C, by the addition of water vapor, the crystallinity of the thin films was obviously improved. The grains with tetragonal structure were clearly observed by AFM. Total transmission greater than 85% was achieved between wavelengths of 400 and 800 nm and the optical absorption edge of the thin films also shifted to smaller wavelength. Our data suggest that the crystallinity of the thin films can be improved and the crystallized temperature can be decreased with the introduction of water vapor in the fabrication process. The effects of water vapor on the crystallization of Ta2O5 thin films are discussed in details. r 2004 Elsevier B.V. All rights reserved.

PACS: 68.55.Ac; 61.50.Àf

Keywords: A1. Crystal structure; B1. Oxides; B2. Dielectric materials

1. Introduction reached its limits due to the high direct tunneling current. As one of the alternative gate dielectric As the feature size of ultra large-scale integra- materials, tantalum pentoxide (Ta2O5) has been tion (ULSI) devices shrinks, thinner gate oxides studied extensively in applications such as storage are necessary to achieve high performance in metal capacitors in dynamic random access memory oxide silicon ﬁeld effect transistors (MOSFET). (DRAM), gate oxide in ﬁeld effect transistors The conventional SiO2 gate dielectric has almost (FET) due to its high permittivity, high refractive index, excellent step coverage, and tolerable ÃCorresponding author. Tel.: +852 27887724; fax: dielectric strength [1–3]. Recently, promising +852 27889549. applications of Ta2O5 as a dielectric layer in E-mail address: [email protected] (P.K. Chu). metal-oxide-semiconductor (MOS) devices have

74 A.P. Huang, P.K. Chu / Journal of Crystal Growth 274 (2005) 73–77 promoted even greater interests [4]. Although backside of Si substrates. The sputtering chamber much research effort has been devoted to the was pumped down using a turbo-molecular pump structural and electrical properties of the mater- to a base pressure under 3 Â 10À3 Pa. Prior to film ials, as-deposited Ta2O5 thin films still suffer from deposition, the target was pre-sputtered by 99.99% the crystallization difficulty and poor electrical pure Ar plasma (0.1 Pa) to improve the properties properties [5–8]. Generally, it is necessary to of the thin film. After pre-sputtering, the chamber deposit or anneal at high temperature in order to was re-evacuated to a base pressure of 3 Â 10À3 Pa. improve the structure and properties of Ta2O5 thin The working chamber was filled with 20% O2 as films on Si substrates. However, a thick SiO2 layer the reactive gas and 80% Ar mixed with deionized is usually formed between the Si substrate and water vapor as the sputtering enhancing gas. The Ta2O5 thin films at high temperature [9], thereby total working pressure of the mixture was held significantly degrading the dielectric properties of at about 0.5 Pa, and RF power of 50 W was MOS devices and limiting the applications in applied in the sputtering process for a deposition ULSI circuits from a storage characteristic point time of 120 min. of view. Therefore, it is important to decrease the The thickness of the as-deposited Ta2O5 thin as-deposited temperature of Ta2O5 thin films and films was measured with a Seimitzu Surfcom 480A mitigate the formation of the interlayer. Most profiler. Microstructrural analysis with or without recently, a special technique to effectively enhance water vapor was carried out by a Xian–Chinetek the crystallinity of ZnOthin films by using argon Fourier transform infrared (FTIR) spectrometer gas mixed with waater vapor as the working gas and a Philip X-ray diffractometer in a y–2y was reported by Anna et al. [10]. configuration with Cu Ka radiation. The surface In this paper, by introducing water vapor into morphology of the thin films was assessed by the working gas and changing the relative humi- AFM. UV-VIS absorption spectra were taken on a dity, crystalline Ta2O5 thin films were deposited on Shimadzu UV-3010PC system in the wavelength Si(1 0 0) substrates in a magnetron sputtering range of 200–800 nm. system at low substrate temperatures. The crystallinity of the thin films was obviously improved, and the grains with tetragonal structure were 3. Results and discussion clearly observed by atomic force microscopy (AFM). Total transmissions larger than 85% were Fig. 1 shows the FTIR spectra acquired from achieved at wavelengths between 400 and 800 nm the Ta2O5 thin films deposited at 500 1C under and the optical absorption edge of the thin films different relative humidity. Two absorption bands was observed to shift to a lower wavelength. appear at 1000–790 cmÀ1 and around 600 cmÀ1, respectively. It can be clearly seen that the band at around 600 cmÀ1 shifts from 640 to 510 cmÀ1 2. Experimental procedures and the intensity of the peak at 1000–790 cmÀ1 weakens with the introduction of water vapor. Ta2O5 thin films were deposited on n-type, Previously, it was reported [11] that the phonon 4–7 O cm, 100 mm Si(1 0 0) wafers using a radio bands of Ta2O5 are mainly in the range of frequency (RF) magnetron sputtering system. A 1000–200 cmÀ1, and Ono and Koyanagi [12] 99.99% pure Ta disk with a diameter of 50 mm studied carefully the FTIR absorption of crystal- was used as the sputtering target. The distance line and amorphous Ta2O5 thin films, as shown in between the target and substrates was fixed at Table 1. Among these FTIR peaks, it is noted that about 3.5 cm during the experiments. The subst- the absorption peak at about 640 cmÀ1 that rate temperature was controlled by means of the corresponds to the vibration mode of O3Ta in À1 heater assembly mounted below the stainless steel amorphous Ta2O5 thin films shifts to 510 cm as substrate holder and was measured with a the film crystallinity improves. Furthermore, it is chromel–alumel thermocouple attached to the apparent that the peak shifts from 640 cmÀ1 ARTICLE IN PRESS

A.P. Huang, P.K. Chu / Journal of Crystal Growth 274 (2005) 73–77 75

500°C 530cm 500°C, with water (001) (201) (310) -1 With H O 2 500°C, no water 640cm Intensity (a.u.) Intensity (a.u.)

-1 No H O 2

15 20 25 30 35 40 45 50 55 60 3000 2500 2000 1500 1000 500 2 Theta (Degree) -1 Wavenumber (cm ) Fig. 2. XRD patterns of Ta2O5 thin ﬁlms deposited under Fig. 1. FTIR spectra of Ta2O5 thin ﬁlms prepared under different relative humidity at 500 1C. different relative humidity at 500 1C.

Table 1 Ta2O5 thin films or post annealing at higher FTIR absorption modes of Ta2O5 thin films [12] temperatures is required to improve the crystal- Vibration mode Amorphous (cmÀ1) Crystalline (cmÀ1) linity based on the high diffusing energy of Ta in the crystal lattice of Ta2O5. In this present work, Ta–O–Ta O3Ta 280 210 the observed enhancement of the crystallinity is O3Ta 640 510 achieved by the addition of water vapor into the Ta–O–Ta 790–1000 810 TaQO2340 2335 working gas. It is believed that water vapor + decomposes into molecules or ions such O2 ,O2, + O3, H, OH, H2Oand H 2O and some of them, such as OH radicals, are highly activated oxidizing without water vapor to 510 cmÀ1 with water agent present in the plasma and can enhance the vapor. As shown in Table 1, the behavior results surface reactivity of incident particles [14]. Hence, from crystallization of the amorphous Ta2O5 thin a crystallized thin film can be obtained at lower films. Thus, it can be suggested that the introduc- substrate temperatures. tion of water vapor is effective in improving the To further investigate the effect of water vapor crystallinity of Ta2O5 thin films. on crystallization, the surface morphology was Fig. 2 presents X-ray diffraction (XRD) pat- observed by AFM. Fig. 3 depicts the AFM images terns of the Ta2O5 thin films. The results show that of representative films prepared under different the thin film without water vapor is amorphous relative humidity. Amorphous particles in the thin since no diffraction peak appears in the spectrum. films are clearly seen in Fig. 3(a). With With water vapor in the working gas, the relative the introduction of water vapor, the crystalline diffraction peaks of crystalline Ta2O5 thin films grains with tetragonal structure appear as shown emerge corresponding to the (0 0 1), (2 0 1) and in Fig. 3(b) and the particle size of the thin films is (3 1 0) planes of the tetragonal Ta2O5 phase [13].It uniform, which is consistent with the XRD results. means that the amorphous thin films can crystal- It further corroborates that the crystallinity of the lize when the water vapor is introduced, and thin films can be improved by the addition of the results are consistent with the FTIR results in water vapor at low substrate temperature. Fig. 1. Usually, a substrate temperature higher The Ta2O5 thin films deposited at different than 750 1C is necessary to obtain crystalline relative humidity were also analyzed by UV-visible ARTICLE IN PRESS

76 A.P. Huang, P.K. Chu / Journal of Crystal Growth 274 (2005) 73–77

100 ° 90 500 C With water 80 70 60 50 No water 40 30 % Transmission 20 10 0 -10 (a) 300 400 500 600 700 800 Wavelength (nm)

Fig. 4. UV-Visible spectra of Ta2O5 thin ﬁlms prepared with or without water vapor at 500 1C.

of light on the surface of the crystalline films [15]. A higher crystalline quality allows more light to diffuse (comparing Figs. 2 and 4). Diffusion of the light may also be affected by the thickness of the thin films. However, the thickness of the thin films with or without water vapor is about the same as measured by a Seimitzu Surfcom 480A profiler. Hence, we believe that the dominant effect leading to a higher percentage of the optical transmittance is due to the crystallinity improvement. The band gap of the crystal is also known to decrease with (b) higher defect density and it can lead to a red shift in the optical absorption edge of the Ta2O5 thin films. In our work, the high order degree of the

Fig. 3. AFM images of Ta2O5 thin films prepared with or Ta2O5 thin films cause the absorption edge to shift without water vapor at 500 1C. to a lower wavelength. spectrophotometer. The transmission spectra were 4. Conclusion recorded in the range of 200–800 nm as shown in Fig. 4. The total optical transmission increases and By introducing water vapor, crystalline Ta2O5 the optical absorption edges of the thin films thin films were prepared by magnetron sputtering are found to decrease with the introduction of at low substrate temperature. The crystallinity of water vapor. It should be noted that by comparing the thin films was obviously enhanced and grains Figs. 2 and 4, both the grain size and order degree with tetragonal structure were clearly observed of the particles in the thin films affect the by AFM. The total optical transmission increases refraction of incident light. The larger the grain in the range of 400–800 nm and the optical size, the higher is the order degree corresponding absorption edge of the thin films also shifts to a to greater diffused transmittance. This is consis- lower wavelength. It is believed that the addition tent with the nature of the reflection and refraction of water vapor produces many highly activated ARTICLE IN PRESS

A.P. Huang, P.K. Chu / Journal of Crystal Growth 274 (2005) 73–77 77 oxidizing species in the plasma thereby enhancing [2] E. Atanassova, N. Novkovski, A. Paskaleva, Solid-State the reactivity of the incident particles. The Electron. 46 (2002) 1887. introduction of water vapor reduces the crystal- [3] Y.M. Li, J.W. Lee, Comput. Phys. Comm. 147 (2002) 214. [4] G.B. Alers, R.M. Fleming, A. Pinczuk, Appl. Phys. Lett. lization temperature and improves the crystallinty 72 (1998) 1308. of Ta2O5 thin ﬁlms. [5] S. Yugo, T. Kimura, T. Muto, Appl. Phys. Lett. 58 (1991) 11. [6] H. Zhang, R. Solanki, J. Appl. Phys. 87 (2000) 1921. Acknowledgments [7] R.M. Fleming, D.V. Lang, et al., J. Appl. Phys. 88 (2000) 850. [8] Jun Lin, Nakabayasi Masaaki, Masao Yamada, Appl. Our work was jointly supported by Competitive Phys. Lett. 74 (1999) 2370. Earmarked Research Grant (CERG) ]CityU [9] A. Pignolet, G.M. Rao, S.B. Kurpanidhi, Thin Solid Films 1137/03E sponsored by the Hong Kong Research 258 (1995) 1995. Grants Council (RGC) and Strategic Research [10] Anna Selvan, H. Keppner, S. Shah, Mater. Res. Soc. Symp. Proc. 426 (1996) 497. Grant (SRG) ]7001642 sponsored by City Uni- [11] A.P. Huang, S.L. Xu, M.K. Zhu, B. Wang, H. Yan, versity of Hong Kong. T. Liu, Appl. Phys. Lett. 83 (2003) 3278. [12] H. Ono, K.I. Koyanagi, Appl. Phys. Lett. 77 (2000) 1431. [13] Y.S. Lai, J.S. Chen. Thin Solid Films 420/421 (2002) 117. References [14] T. Nakada, Y. Ohkubo, A. Kunioka, Jpn. J. Appl. Phys. 30 (1991) 3344. [1] Z.J. Luo, X. Guo, T.P. Ma, Appl. Phys. Lett. 79 (2001) [15] Manso Ginting, Jeong Chul Lee, Kihwan Kang, Jinsoo 2803. Song, Korean. J. Phys. Soc. 34 (1999) S343.