2007 IEEE International Conference on Electron Devices and Solid-State Circuits

Transcription

1 Proceedings 2007 IEEE International Conference on Electron Devices and Solid-State Circuits ~ December 20-22, 2007 Tayih Landis Hotel, Tainan, Taiwan Volume I

2 Aluminium Incorporation in Lanthanum Oxide Films by using Plasma Immersion Ion Implantation Banani Sen, B. L. Yang, Hei Wong, P. K. Chu, A. Huang, K. Kakushima, and H. Iwai Abstract The physics and the effects of aluminium incorporation into lanthanum oxide (La 2 O 3 ) films were studied by using x-ray photoelectron spectroscopy and electrical measurements. We found that trace amount (5% ) of aluminium incorporation in lanthanum oxide film can suppress the leakage current effectively. The bulk oxide traps and interface traps can also reduced. The percentage of aluminium incorporation into the La 2 O 3 films by plasma immersion ion-implantation needs to be optimized to have the maximum reduction of oxide traps and to maintain the lowest leakage current. Index: aluminium, x-ray photoelectron spectroscopy, lanthanum oxide, plasma immersion ion implantation I. INTRODUCTION The key factor behind the microelectronics evolution is the continuous downsizing of device dimensions of Metal- Oxide-Semiconductor Field Effect Transistors (MOSFET) which now requires an ultrathin gate insulating film for controlling the current flow. However, the reduction of the gate dielectric thickness leads to an increase in the gate leakage current, which becomes a serious problem for power consumption and reliability of device operation. Many attempts have been made to replace the conventional silicon oxide (SiO 2 ) or silicon oxynitride (SiON) gate dielectric films with high dielectric constant (high-κ) materials such as transition metal oxides or rare earth metal oxides [1-4]. Among them, lanthanum oxide (La 2 O 3 ) is found to be a promising candidate because of its high dielectric constant of B. Sen, B. L. Yang, and H. Wong are with the Department of Electronic Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong. [ bananisen@ieee.org] P. K. Chu and A. Huang are with the Department of Applied Physics, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong. K. Kakushima and H. Iwai are with Frontier Collaborative Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama , Japan. 27 and large band offset of 2.3 ev from the silicon conduction band. However there are several fundamental problems associated with the lanthanum oxide. The hygroscopic nature of La 2 O 3, thermal stability, and the growth of interfacial layer, brings the suitability of La 2 O 3 for next generation gate dielectric material in question [5]. Solving these fundamental problems is important to attain desirable electrical properties of the material. It has been reported that the nitridation or alumination of HfO 2 can raise the crystallization temperature of HfO 2 and suppresses the growth of the interfacial SiO x layer during high temperature annealing [6-8]. It has also been reported that incorporation of N or Al into the dielectric film can reduce the oxide trap density and therefore improves the reliability of the insulating film. Adding N to La 2 O 3 is found to improve the electrical and material characteristics of the insulating film [9-10]. Thus, adding Al to La 2 O 3 is also considered to be a possible approach for improving the insulating characteristics and reliability of La 2 O 3. This work focus on the effect of aluminium incorporation in lanthanum oxide films on the material and electrical characteristics of the dielectric film. II. EXPERIMENT Lanthanum oxide films about 10 nm thick were deposited on (100) n-si substrates using e-beam evaporation [7]. Plasma immersion ion-implantation (PIII) was then conducted to introduce aluminium atoms. The implantation energy varied between 2 to 3 kev. The samples were then treated with rapid thermal annealing (RTA) in N 2 ambient at temperatures between 400 o C to 800 o C for different durations. To investigate the chemical composition and the physical structure of the dielectric films, X-ray Photoelectron Spectroscopy (XPS) measurements were carried out using Physical Electronics PHI 5600 with a monochromatic Al K α X-ray source to probe the profile and bonding features. The excitation energy was ev and the sputtering rate was about 6.0 nm/min. A 600-nm thick aluminum layer was deposited and patterned with photolithography technique to form a number of MOS capacitors of different sizes for electrical measurements. High-frequency (1MHz) capacitance voltage (C V) measurements using a Keithley

3 590 CV analyzer were conducted and current voltage (I-V) measurements were done using Keithley 236 Source Measure Units (SMUs). All electrical measurements were conducted in a shielded, dark and at low pressure (~ Torr) chamber to avoid any electromagnetic interference, light illumination and moisture effects. required physical thickness of the gate dielectric is quite low, other implantation techniques are not appropriate as the implant is difficult to control and easily penetrates the substrate. Figure 1 shows the typical concentration profile of aluminium-implanted samples for two different doses processed under different annealing conditions. For low dose implantation, the aluminium content is about 5% near the surface only. For high dose and with higher implantation the energy aluminium concentration is as high as 38-40% near the surface. To have a better insight on the bonding structure of the incorporated aluminium atoms, we conducted a detailed analysis of the Al 2p, La 3d 3/2, Si 2s, and O 1s XPS spectra by using the Gaussian deconvolution technique. (a) (a) Fig.1. Typical concentration profile of aluminium implanted samples: (a) low dose, and high dose implantation, followed by RTA under different condition. III. RESULTS AND DISCUSSION In this work aluminium atoms were incorporated into the lanthanum oxide film using Plasma Immersion Ion Implantation. The basic advantage of this method is the low implantation energy and small penetration depth. As the Fig.2. Typical Al 2p spectra at different locations in the Al-PIII samples: (a) low dose high dose implantation.

4 (a) Fig.5. High-frequency (1 MHz) capacitance-voltage characteristics of low dose Al-PIII samples. Fig. 4. Silicon 2s photoelectron spectra from various Al-PIII samples: (a) low dose, and high dose of implantation. For samples with low dose of Al implantation, Al 2p peak is observed at around ev. This binding energy, which is slightly higher than the reported value (~73.0 ev) for stoichiometric LaAlO 3, is assigned to the Al-rich lanthanum aluminate [11]. The peak is found to shift by 0.45 ev to lower energy side after 600 o C RTA in nitrogen ambient indicating a stoichiometric improvement for La-Al-O bonding. For samples with high-dose Al implantation, Al 2p peak is observed at around ev and have a 0.3 ev shift to the lower energy side after 600 o C RTA. For sample with 800 o C RTA, the Al 2p spectra shows a ev peak which is consistent with the Al 2p energy in stoichiometric LaAlO 3 and at the ev peak is assigned to the formation of Al 2 O 3. For samples with high-dose aluminium implantation such Al 2p peaks are also observed near the interface. Fig. 6. Current-voltage characteristics of Al-PIII samples annealed at different conditions. The La 3d 3/2 XPS spectra (not shown) of the Alimplanted samples processed with different RTA conditions indicates two sharp peaks at ev and ev and a broad peak at ev for all samples. The peak at ev is due to La-O bonding and that at ev is an indication of La-rich La silicate. Figure 4 shows the photoelectron spectra of Si 2s for Al-PIII samples. The samples with low-dose Al implantation shows a peak at ev which is due to the typical Si-Si bond and the other small peak at ev can be attributed to Si-O bond [12]. For samples with high-dose aluminium implantation, no Si 2s peak corresponding to Si-O bond is found indicating that the high Al concentration prevents the growth of low-κ interfacial

5 layer during annealing. Similar phenomenon was evident from the O 1s XPS spectra of the samples. Again, no evidence of silicide formation is observed in the Si 2s, Al 2p, or La 3d x-ray photoelectron spectra. Figure 5 shows the 1 MHz C-V characteristics of lowdose Al-implanted La 2 O 3 films with different annealing conditions. The Al-PIII samples followed by RTA at 600 o C show a pronounced reduction in bulk oxide traps. Figure 6 shows the current-voltage characteristics of Al-PIII samples. A significant reduction in the leakage current is observed as a result of aluminium incorporation which may be due to the reduction of oxygen vacancies or the removal of hydroxyl groups. However, high-dose aluminium implantation will result in the increase of the leakage current. Hence, in order to improve the reliability and the material characteristics of the insulating film the percentage of aluminium incorporation needs to be optimized. IV. CONCLUSION The effects of aluminium incorporation into the La 2 O 3 films using plasma immersion ion implantation method are studied. It is found that presence of proper amount of aluminium atoms will help to suppress the leakage current and reduce the oxide trap density of the dielectric film by reducing the oxygen vacancies and forming Al 2 O 3 interface layer. However, high-amount of aluminium incorporation will deteriorate the current leakage because of the conductive nature of excess Al atoms. Hence the amount of aluminium incorporation needs to be optimized depending on the amount of oxygen vacancies in the as-deposited film. [7] H. Y. Yu, N. Wu, M. F. Li, C. Zhu, B. J. Cho, D.-L. Kwong, C. H. Tung, J. S. Pan, J. W. Chai, W. D. Wang, D. Z. Chi, C. H. Ang, J. Z. Zheng and S. Ramanathan, Thermal stability of (HfO 2 ) x (Al 2 O 3 ) 1 x on Si, Appl. Phys. Lett., Vol. 81, pp [8] B. Sen, H. Wong, B. L. Yang, A. P. Huang, P. K. Chu, V. Filip and C. K. Sarkar, Nitrogen iuncorporation into hafnium oxide films by plasma immersion ion implantation, Jap. J. App. Phys. vol. 46, pp , [9] N. Kawada, M. Ito and Y. Saito, Thermal stability of lanthanum oxynitride ultrathin films deposited on silicon substrates, Jap. J. Appl. Phys. vol. 45, pp , [10] T. Shimizu, A. Kurokawa, K. Ishii and E. Suzuki, MOCVD of high-dielectric-constant lanthanum oxide thin films, J. Electrochem Soc. vol. 150, pp.g429-g435, [11] L. Miotti, C. Driemeier, F. Tatsch, C. Radtke, V. Edon, M. C. Hugon, O. Voldoire, B. Agius, and I. J. R. Baumvol, Atomic transport in LaAlO3 films on Si induced by thermal annealing, Electrochem. Solid-State Lett., vol. 9, pp. F49-F52, [12] R. Fujitsuka, M. Sakashita, A. Sakai, M. Ogawa1, S. Zaima and Y. Yasuday, Thermal stability and electrical properties of (La 2 O 3 ) 1-x (Al 2 O 3 ) x composite films, Jap. J. Appl. Phys. Vol. 44, pp , ACKNOWLEDGEMENT The work described in this paper was fully supported by a UGC grant of Hong Kong (Project No. CityU ) REFERENCES [1] H. Wong and H. Iwai, On the scaling issues and high-κ replacement of ultrathin gate dielectrics for nanoscale MOS transistors, Microelectron. Eng. vol. 83, pp , [2] D. Misra, H. Iwai and H. Wong, High-k gate dielectrics, ECS Interface, vol. 14, pp , [3] G.D. Wilk, R.M. Wallace, and J.M. Anthony, High-k gate dielectrics: current status and materials properties considerations, J. Appl. Phys. vol. 89, pp , [4] H. Wong and H. Iwai, The road to miniaturization, Phys. World, vol. 18, pp , [5] B. Sen, H. Wong, J. Molina, H. Iwai, J. A. Ng, K. Kakushima and C. K. Sarkar, Electrical characteristics of high-k dielectric film grown by direct sputtering method, Solid-State Electron., vol. 51, pp , [6] P. F. Lee, J. Y. Dai, K. H. Wong, H. L. W. Chan and C. L. Choy, Growth and characterization of Hf aluminate high-k gate dielectric ultrathin films with equivalent oxide thickness less than 10 Å, J. Appl. Phys. vol. 93, pp , 2003.