Improvement of interfacial and dielectric properties of sputtered Ta 2 O 5 thin films by substrate biasing and the underlying mechanism

Transcription

1 JOURNAL OF APPLIED PHYSICS 97, Improvement of interfacial and dielectric properties of sputtered Ta 2 O 5 thin films by substrate biasing and the underlying mechanism A. P. Huang and Paul K. Chu a Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong Received 17 November 2004; accepted 6 April 2005; published online 31 May 2005 The use of tantalum pentoxide Ta 2 O 5 thin films as advanced gate dielectrics in integrated circuits has been hampered by thermodynamic instability at the Ta 2 O 5 /Si interface. We have demonstrated the fabrication of crystalline Ta 2 O 5 thin films on n-type Si 100 at lower substrate temperature by means of substrate biasing. In the work reported here, the influence of the substrate bias on the interfacial and dielectric characteristics of the Ta 2 O 5 thin films is investigated in details. Our results show that by applying a suitable bias to the Si substrate, the dielectric properties of Ta 2 O 5 thin films can be improved. Using a substrate bias of 200 V, the thin film has a permittivity of 34 and leakage current density of 10 7 A/cm 2 at an electric field of 800 kv/cm. The effects and mechanism of the bias on the interfacial and dielectric characteristics are described American Institute of Physics. DOI: / I. INTRODUCTION As system-on-a-chip SOC integration and performance of complementary metal-oxide silicon CMOS devices improve, reduction of device size and operation voltage is inevitable. The fundamental limitations of current gate dielectric materials are driving the search for materials with higher dielectric constant to permit further miniaturization. Tantalum pentoxide Ta 2 O 5 is a potential substitute for silicon dioxide as storage capacitors in dynamic random access memory DRAM and gate oxide in field-effect transistors FETs due to its high permittivity, large refractive index, excellent step coverage, and tolerable dielectric strength 1 4 and has spurred many recent research activities. 5,6 Generally, the dielectric constant of amorphous Ta 2 O 5 thin films is about 26, which is modest compared to other types of high permittivity dielectrics such as SrTiO 3. 7 It is therefore of practical and scientific significance if the dielectric constant of Ta 2 O 5 can be enhanced substantially. Recently, an exceptionally high dielectric constant has been achieved in 001 oriented Ta 2 O 5 films by Lin et al. employing N 2 O plasma postannealing at 800 C. 8 Nonetheless, although much improvement has been made with regard to the deposition process, as-deposited Ta 2 O 5 thin films still suffer from the difficulty of crystallization and poor electrical properties Consequently, high deposition or annealing temperature is utilized to produce crystalline Ta 2 O 5 thin films on Si but such processes aggravate the thermodynamic instability at the Ta 2 O 5 /Si interface. In our previous work, crystalline and highly oriented Ta 2 O 5 thin films deposition on n-type Si 100 at a low substrate temperature was demonstrated by substrate biasing and it was shown that even at a low substrate temperature of 400 C, partially crystallized Ta 2 O 5 thin films could be obtained. 12 In spite of the encouraging a Author to whom correspondence should be addressed; FAX: ; electronic mail: paul.chu@cityu.edu.hk results, the underlying mechanism and detailed effects of the substrate bias on the interfacial and dielectric properties of crystalline Ta 2 O 5 thin films are not known. In this work, the interfacial and dielectric characteristics of the Ta 2 O 5 thin films and their relationship with the substrate bias are studied. Our results show that by applying a suitable bias to the Si substrate, the dielectric properties of the Ta 2 O 5 /Si system can be improved, and the thickness of the interfacial layer remains about the same under different substrate biases. Our results suggest that substrate bias assistance is an effective method to improve the dielectric properties of Ta 2 O 5 /Si while the interfacial characteristics do not degrade. This is helpful to the fabrication of MOS devices with high dielectric materials. II. EXPERIMENT Ta 2 O 5 thin films were deposited on n-type, 100-mm Si 100 wafers with resistivity of 4 7 cm using a radio frequency rf magnetron sputtering system. A 99.9% pure Ta disk with a diameter of 50 mm was used as the sputtering target. The distance between the target and substrate was fixed at about 3.5 cm during the experiments. The substrate temperature that was controlled by a heating assembly mounted below the stainless-steel substrate holder was measured by a chromel-alumel thermocouple attached to the backside of the Si substrate. The sputtering chamber was pumped down using a turbomolecular pump to a base pressure under Pa. Prior to film deposition, the target was presputtered by 99.99% pure Ar plasma 0.1 Pa to improve the properties of the thin film. After presputtering, the chamber was reevacuated to a base pressure of Pa. The chamber was then filled with 20% O 2 as the reactive gas and 80% Ar as the sputtering enhancing gas. The working pressure in the chamber was maintained at about 0.5 Pa and rf power of 50 W was applied for a deposition time of 120 min /2005/97 11 /114106/5/$ , American Institute of Physics

2 A. P. Huang and P. K. Chu J. Appl. Phys. 97, FIG. 3. Ratios of Ta to O vs depth at substrate temperature of 620 C and different substrate biases. FIG. 1. RBS spectra showing the raw experimental and fitted data of the films deposited at 620 C at substrate biases of a 0 and b 200 V. The thickness of the as-deposited Ta 2 O 5 thin films was measured by a Seimitzu Surfcom 480A profiler. The composition of the thin films and elemental distribution at the interface were determined by Rutherford backscattering spectrometry RBS. The dielectric properties of the thin films deposited under different substrate biases were measured by the MDC-capacitance-meter-SCM-16-type impedance analyzer and the leakage current was determined using a I V test system. III. RESULTS A. Interfacial characteristics of sputtered Ta 2 O 5 thin films The elemental composition of the thin film influences the structural and electrical properties and RBS was used to characterize our materials. 13 Figures 1 a and 1 b depict the RBS spectra of the thin films produced using a substrate bias of 0 and 200 V. The results indicate the presence of tantalum and oxygen in the thin films and that the composition of the thin films is basically homogeneous throughout the thickness. Both samples have the same tantalum to oxygen ratio within the RBS precision of a few percent. An interfacial layer with graded composition is clearly shown in Fig. 2 by deconvoluting the RBS data. The ratio of tantalum to oxygen at the interface between the Ta 2 O 5 and Si is different from that of the bulk film, as shown in Fig. 3, and it could influence the dielectric properties. This may be due to more diffusion of Ta into the substrate relative to that of oxygen under negative biasing conditions. Comparing Figs. 2 a 2 d, another interesting phenomenon can be observed. The thickness of the interfacial layer is almost independent FIG. 2. Elemental depth profiles derived from the RBS data for Ta 2 O 5 thin films deposited using different substrate biases.

3 A. P. Huang and P. K. Chu J. Appl. Phys. 97, FIG. 4. Forward-and-reverse C V curve of Ta 2 O 5 prepared at 620 C under different substrate biases. of the substrate bias increasing perhaps because the thickness of the interface layer is mainly decided by the deposition temperature. Ono and Koyanagi suggested that the formation of the interfacial layers depended on neither the tantalumoxide thickness nor the annealing atmosphere, but rather the annealing temperature. 14 Therefore, it can be deduced that the substrate bias does not affect the deposition temperature apparently but it aids to increase the nuclei density in the thin films and this will be discussed in more details later in this paper. B. Dielectric characteristics of sputtered Ta 2 O 5 thin films To further investigate the influence of the substrate bias on the characteristics of the Ta 2 O 5 thin films, the dielectric properties of the thin films deposited at low substrate temperature under different biases were measured by C V and I V tests. Figure 4 displays the forward-and-reverse C V curves of the as-deposited Al/Ta 2 O 5 /Si MOS structure at different substrate biases from 0 to 200 V and the following can be observed. First of all, based on the largest capacitance value in the forward-and-reverse, C V curves, as the substrate bias increases, the capacitance value is enhanced, which corresponds to the improvement of the ability to save electrical charges in Al/Ta 2 O 5 /Si MOS structure. It also means a larger permittivity. The enhancement is believed to be due to the improved crystallinity and orientation of the thin film as revealed by our x-ray diffraction XRD and Fourier transform infrared FTIR results and it is consistent with observations in other studies. 12 In addition, it can be seen that the flatband in the C V curves shifts towards the negative voltage direction as the substrate bias increases. It is well known that the flatband shift can arise from many factors such as the difference in the work function W Al-n-Si between the Al electrode and n-type silicon, trapped charges at the interface, and the fixed charges in the oxide. 15 It can be seen in Fig. 4 that the flatband voltage is positive in the absence of sample bias and the flatband shifts towards the negative voltage direction when a negative bias is applied. At zero bias, the major factor that influences the flatband shift is the differential in the work function between the Al electrode and n-type silicon in which W Al is larger than W n-si.in order to eliminate this difference W Al-n-Si, it is necessary to add a positive voltage to the Al electrode. As the substrate bias increases, the flatband shifts towards the negative voltage direction suggesting that more ion bombardment increases the amount of fixed charges in the oxide. Furthermore, the C V hysteresis loop widens as the substrate bias increases and it is probably related to the larger number of slow trapping sites. 16,17 The permittivity of the dielectric thin films can be calculated. Figure 4 c shows the forward-and-reverse C V curves of the as-deposited Al/Ta 2 O 5 /Si MOS structure at the bias of 200 V, acquired by scanning from a negative bias 12 V to positive bias 12 V and then immediately back to the starting point 12 V, and the relative dielectric constant k of the as-deposited Ta 2 O 5 /Si sample for a substrate bias of 200 V is about 34, as shown in Fig. 5, which is higher than that of amorphous Ta 2 O 5 thin films. 18 It can also be seen in Fig. 5 that the use of a suitable substrate bias enhances the dielectric constant of the as-deposited Ta 2 O 5 films. Meanwhile, the flatband voltage V fb, extracted from the C V curve of the as-deposited Ta 2 O 5 at a bias of 200 V is about 1 V. This V fb value means that the MOS structure of the Ta 2 O 5 gate dielectric layer has a larger amount of negative charges than the conventional SiO 2 /Si structure, which is indicative of more positive fixed charges Q f in the gate dielectrics. 19 According to the counterclockwise hysteresis behavior in the C V curve in Fig. 4 c, the density of the defect states can be deduced. It is caused by the trapping of positive charges in the oxide defect states when the MOS capacitor is

4 A. P. Huang and P. K. Chu J. Appl. Phys. 97, IV. DISCUSSION FIG. 5. Dielectric constants of Ta 2 O 5 thin films prepared at 620 C as function of the substrate bias. stressed. 20 These defect states are also the so-called slow trapping sites. The following equation can be used to calculate the density of the slow trapping sites N st : N st = C i V f0 /AQ, where C i is the capacitance of the insulator layer, V fb is the hysteresis offset of the flatband voltage, Q is the electron charge, and A is the capacitor area here is cm 2. The N st value extracted from Fig. 4 c for the as-deposited Ta 2 O 5 is cm 2, which is slightly higher than the value of cm 2 reported by Pignolet et al. 21 Figure 6 shows the forward-and-reverse J V curves of the Al/Ta 2 O 5 /Si structure under different substrate biasing conditions. The leakage current density is 10 3 A/cm 2 for 0 V biasing, 10 6 A/cm 2 for 100 V biasing, and 10 7 A/cm 2 for 200 V biasing at an electric field of 800 kv/cm. It implies that the leakage current density of the thin films decreases as the substrate bias increases. It can be attributed to that the microstructural and interfacial characteristics of the thin films are improved as the substrate bias is introduced. Besides, when the negative test voltage is applied to the Al electrode, the leakage current density is slightly lower than that measured under a positive voltage. It may be due to the difference in the work function W Al-n-Si between the Al electrode and n-type Si substrate and has been reported by Wolf et al. 22 FIG. 6. Forward-and-reverse J V curve of Ta 2 O 5 prepared at 620 C under different substrate biases. Usually, a substrate temperature higher than 800 C or postannealing at higher temperature is necessary to obtain crystalline and even oriented Ta 2 O 5 thin films. 8 Our experimental results demonstrate unequivocally the effects of a substrate bias on the interfacial and dielectric characteristics of Ta 2 O 5 thin films. The substrate bias thus plays an important role in the fabrication of the thin films. Therefore, a good understanding on the effects of the substrate bias on the evolution of the thin film is important. Under a negative substrate bias, the positive ions obtain higher impact energy, diffusion of positive ions is accelerated, and more nucleation sites are produced on the surface. 23 Hence, the negative bias has three main effects: i enhancement of ion flux and energy; ii more energetic ion bombardment; and iii increased number of nucleation sites. That is to say, the substrate bias increases the number of bombarding ions as well as the ion impact energy. The energetic ions consequently produce more pits and structural defects on the surface thereby increasing the nucleation density. Although the nuclei can grow randomly initially, the growth of the nuclei perpendicular to the substrate surface is much faster than that parallel to it as the 001 plane of the tetragonal Ta 2 O 5 crystal is the densest. That is to say, it is the direction of the fastest growth. In the case of a low nucleation density, the nuclei with the orientation parallel to the substrate surface can grow for a long distance before they connect with the other crystallite. On the other hand, in the case of a high nucleation density induced by substrate biasing, the nuclei of Ta 2 O 5 perpendicular to the substrate surface grow quickly. 24 Therefore, when the density of the nuclei is enhanced by substrate bias assistance, the growth of the other oriented nuclei will be constrained thereby making the growth of those particles of the 001 orientation dominant over those of the other orientations. As a result, the thin film consists of mainly the 001 orientation under the suitable bias, as illustrated by our XRD results. 25 Furthermore, a negative substrate bias increases ion bombardment and reduces the critical nucleation energy giving rise to the changes in the surface morphology and structure of the thin films, as shown by XRD and atomic force microscopy AFM. Meanwhile, Ta ions in multiple charge states obtain even higher energy leading to deeper penetration into the substrate as compared to oxygen under the same negative biasing conditions. Consequently, crystalline Ta 2 O 5 thin films can be obtained and the dielectric properties are improved. However, this trend is not universally true. For example, at substrate bias over 300 V, excessive sputtering, bond breaking, and damage result in decreased film thickness and poor film characteristics. The different dielectric properties of Ta 2 O 5 thin films mainly stem from the different crystalline and interfacial structures. 26 From our experimental results, it is very clear that a proper substrate bias assistance can improve the dielectric properties of the thin films. It is believed that the introduction of a substrate bias also accelerates the interlayer formation consisting of Ta, Si, and O, and as proposed by Alers et al., it may influence the dielectric properties of the

5 A. P. Huang and P. K. Chu J. Appl. Phys. 97, Al/Ta 2 O 5 /Si MOS structure. 27 Lai et al. have suggested that the Ta Si O interlayer can contain some defects which are likely to be electrically active and result in both significant negative flatband voltage shifts and widening of the C V hysteresis loop. 28 Therefore, further work should be carried out in the direction of controlling the interfacial defects in the Ta 2 O 5 /Si structure by means of substrate biasing and other methods. V. CONCLUSION We have investigated the influence of a substrate bias on the interfacial and dielectric characteristics of Ta 2 O 5 thin films. By applying the proper substrate bias during magnetron sputtering, Ta 2 O 5 thin films with excellent interfacial and dielectric characteristics can be fabricated at low substrate temperature. However, a bias that is too large reduces the growth rate and increases the fixed charges and trapping sites in the Al/Ta 2 O 5 /Si MOS structure. Thus, it can be conluded that the use of a suitable substrate bias is an effective method to fabricate high-quality Ta 2 O 5 gate dielectrics for advanced complementary MOS devices ACKNOWLEDGMENTS Our work was jointly supported by Competitive Earmarked Research Grant CERG Grant No. CityU 1137/03E sponsored by the Hong Kong Research Grants Council RGC and Strategic Research Grant SRG Grant No sponsored by City University of Hong Kong. 1 I. Kim, S. Ahn, B. Cho, J. Y. Lee, J. S. Chun, and W. Lee, Jpn. J. Appl. Phys., Part 1 33, E. Atanassova, N. Novkovski, and A. Paskaleva, Solid-State Electron. 46, Y. M. Li and J. W. Lee, Comput. Phys. Commun. 147, G. B. Alers, R. M. Fleming, and A. Pinczuk, Appl. Phys. Lett. 72, T. Kato and T. Ito, J. Electrochem. Soc. 135, H. Zhang and R. Solanki, J. Appl. Phys. 87, R. M. Fleming et al., J. Appl. Phys. 88, J. Lin, N. Masaaki, and M. Yamada, Appl. Phys. Lett. 74, A. Pignolet, G. M. Rao, and S. B. Kurpanidhi, Thin Solid Films 258, Y. Takaishi, M. Sakao, and H. Watanabe, Tech. Dig. Int. Electron Devices Meet., 1994, S. O. Kim and H. J. Kim, Thin Solid Films 253, A. P. Huang, S. L. Xu, M. K. Zhu, B. Wang, H. Yan, and T. Liu, Appl. Phys. Lett. 83, C. Jeynes, Z. H. Jafri, R. P. Webb, A. C. Kimber, and M. J. Ashwin, Surf. Interface Anal. 25, H. Ono and K. I. Koyanagi, Appl. Phys. Lett. 77, H. K. Henisch, Rectifying Semiconductor Contacts Clarendon, Oxford, J. G. Hwu and M. J. Jeng, J. Electrochem. Soc. 135, H. Kato, K. S. Seol, M. Fujimaki, T. Toyada, Y. Ohki, and M. Takiyama, Jpn. J. Appl. Phys., Part 1 38, M. Houssa, R. Deraeve, P. W. Mertens, M. M. Heyns, J. S. Jcon, A. Halliyal, and B. Ogle, J. Appl. Phys. 86, G. D. Wilk, R. M. Wallace, and J. M. Anthony, J. Appl. Phys. 89, G. B. Alers, R. M. Fleming, and A. Pinczuk, Appl. Phys. Lett. 72, A. Pignolet, G. M. Rao, and S. B. Krupanidhi, Thin Solid Films 258, H. F. Wolf, Semiconductors Wiley, New York, 1971, chap.5p B. Wang, W. Liu, G. J. Wang, M. K. Zhu, and H. Yan, Mater. Sci. Eng., B 98, B. Wang, M. Wang, R. Z. Wang, A. P. Huang, and H. Yan, Mater. Lett. 53, A. P. Huang, S. L. Xu, M. K. Zhu, G. H. Li, T. Liu, B. Wang, and H. Yan, J. Cryst. Growth 255, X. G. Yu, C. Zhu, M. F. Li, A. Chin, A. Y. Du, W. D. Wang, and D. L. Kwong, Appl. Phys. Lett. 85, G. B. Alers, D. J. Werder, Y. Chabal, H. C. Lu, E. P. Gusev, E. Garfunkel, T. Gustafsson, and R. S. Urdahl, Appl. Phys. Lett. 73, Y. S. Lai, K. J. Chen, and J. S. Chen, J. Appl. Phys. 91,