XPS Study of the Thermal Instability of HfO 2 Prepared by Hf Sputtering in Oxygen with RTA Nian Zhan, a M. C. Poon, a C. W. Kok, a K. L. Ng, a and Hei Wong b,z a Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong b Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong Hafnium oxide (HfO 2 ) gate dielectric film was prepared by Hf sputtering in oxygen, and the thermal instability of HfO 2 was investigated by rapid thermal annealing RTA in nitrogen. X-ray photoelectron spectroscopy study reveals that the HfO 2 film is thermally unstable at postmetallization annealing temperatures ( 500°C). The HfO 2 film decomposes and some oxygen atoms are released upon the RTA in nitrogen. In addition, the current-voltage characteristics of the Al/HfO 2 /Si capacitor are also highly unstable at temperatures higher than 300 K. These observations suggest that although HfO 2 has a much higher dielectric constant, it may not be suitable for the gate dielectric application because the postdeposition thermal treatment deteriorates both the physical and the electrical properties of the HfO 2 film. © 2003 The Electrochemical Society. DOI: 10.1149/1.1608006 All rights reserved. Manuscript submitted August 19, 2002; revised manuscript received March 17, 2003. Available electronically August 25, 2003. Silicon oxide gate dielectric is now being pushed to its both technological and theoretical limits. 1,2 According to the recent Inter- national Technology Roadmap for Semiconductors ITRS2001 prediction, 3 the equivalent oxide thickness EOT in the 4-Gbit gen- eration dynamic random access memory DRAM will be scaled down further to 0.22 nm which is very close to the structural limit of silicon dioxide as the Si-O bond in silicon oxide is 0.17 nm. Nu- merous attempts in the search for alternate gate dielectrics have been carried out. Significant achievements in the material structures such as oxynitride and oxide/nitride stacks have been obtained. 2,4-7 These materials, with a slightly higher value of dielectric constant, allow greater dielectric thickness for suppressing the direct tunneling leak- age and keeping larger gate capacitance. It also enhances the resis- tance to boron diffusion and better hot-carrier reliability. 4-7 It was found that the oxynitride will work well down to an equivalent oxide thickness of 1.5 nm, but this is still too thick for the 4-Gbit generation dynamic random access memory DRAM which will be needed in several years’ time. The amount of nitrogen incorporation with the N 2 O nitridation method, in the range of 2-4 atom %, 4 is still too low to improve the hardness for hot carrier irradiation and to suppress the direct tunneling current in ultrathin oxynitride film. 6 The oxide/nitride stacked structure has the advantages of low inter- face state density and better equivalent bulk properties. 6 However, the technology constraint on the ultrathin oxide preparation and the structural limit of silicon oxide present a limit to the minimum achievable value of equivalent oxide thickness of the stacked struc- ture. With this connection, materials with much higher dielectric constant have been investigated extensively. Hafnium oxide has been recognized as one of the most promising candidates for replac- ing the silicon oxide because of its high dielectric constant. 1,8-12 Hafnium oxide has a dielectric constant four times higher than SiO 2 and can be produced with the present silicon technology. However, because the Hf-O bond is more ionic and is prepared at temperatures lower than 700°C, hafnium oxide is found to be thermally unstable. This paper focuses on the effect of the postdeposition annealing on the properties of as-grown thin films on silicon substrates. The experimental details are given in next section. As is presented later, with X-ray photoelectron spectroscopy XPS measurements, sig- nificant changes in the chemical composition and the bonding struc- ture are observed for samples undergoing thermal annealing. By measuring the current-voltage characteristics at temperatures rang- ing from 300 to 500 K, the thermal instability of the current con- duction is also observed at these measurement temperatures. Experimental The starting material for the metal-insulator-semiconductor MIS capacitor fabrication is 100 n-type Si substrate with a resis- tivity in the range of 5-10  cm. Follow the standard cleaning process, a HfO 2 layer of about 250 Å thick was deposited by direct sputtering of hafnium metal in oxygen and argon ambient for 30 min. An ARC-12M dc sputter was used for the sputtering. This thickness is larger than that for practical applications of the high- dielectric in future Si technology. The purpose of using thicker film at the present stage is for the ease of studing both the physical properties of bulk and interface using XPS. The ratio of oxygen and argon was 2:30 and remained unchanged during the sputtering. The sputtered sample was then annealed in nitrogen ambient using a rapid thermal annealing RTA chamber at temperature ranging 500 to 700°C for several different durations. To study the chemical com- position and physical structure of the deposited films, XPS measure- ments were carried out using Physical Electronics PHI5600 X-ray photoelectron spectroscopy with an Al K X-ray source to probe the profile and bonding features. To study the electrical characteristics, an Al layer 600 nm thick was finally deposited. The Al electrodes with diameter of 200 m were patterned using the photolithography technique and a number of MIS capacitors were produced. The current-voltage characteristics were measured with a Keithley source-measurement unit SMU 236 and the sample was put in a Bio-Rad 4600 vacuum cryostat chamber. Results and Discussion The chemical characterization of various HfO 2 was accom- plished by XPS. Figure 1 shows the chemical composition of the as-deposited HfO 2 film. As shown in Fig. 1a, the bulk O/Hf ratio is about 1.9 indicating the film is slightly Hf-rich. After RTA, both the physical thickness of the HfO 2 film and the oxygen content reduced significantly. For samples annealed for 5 min, at either 500 or 600°C, the bulk O/Hf ratio is about 1.42. With prolonged annealing 20 min, the film thickness reduced further, and the bulk O/Hf ratio reduces to about 1.31. These results indicate the densification of the deposited film by an out-diffusing of oxygen atoms. As a result, the annealed films become highly nonstoichiometric. This allegation is validated further with the Hf 4 f and O 1s XPS spectra to be pre- sented below. On the other hand, the silicate-containing interface is rather stable for 5 min annealing at either 500 or 600°C. A notable change at the interface is seen for the sample with 20 min annealing at 600°C. Figure 2 shows the Hf 4 f features for various samples. For the sample without annealing, a feature peak is found in the range of 16-19 eV. This peak is an indication of the forming of Hf-O bonds. 8 For samples with RTA in nitrogen, the feature peak shifts to the z E-mail: eehwong@cityu.edu.hk Journal of The Electrochemical Society, 150 10 F200-F202 2003 0013-4651/2003/15010/F200/3/$7.00 © The Electrochemical Society, Inc. F200