Plasma nitridation and microstructure of high-k ZrO 2 thin films fabricated by cathodic arc deposition

Transcription

1 Journal of Crystal Growth 277 (2005) Plasma nitridation and microstructure of high-k ZrO 2 thin films fabricated by cathodic arc deposition A.P. Huang a, Ricky K.Y. Fu a, Paul K. Chu a,, L. Wang b, W.Y. Cheung b, J.B. Xu b, S.P. Wong b a Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong b Solid State Laboratory of Electronic Engineering Department, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong Received 20 December 2004; accepted 25 January 2005 Available online 2 March 2005 Communicated by R. Kern Abstract ZrO 2 thin films as high-k gate dielectric materials are deposited by hybrid cathodic arc deposition involving oxygen and nitrogen gases and their properties are investigated. The incorporation of N into the ZrO 2 structure increases the crystallization temperature, and the microstructure of the nitrided ZrO 2 thin films is improved based on X-ray diffraction and atomic force microscopy characterization. The enhancement is believed to be due to the formation of Zr N bonds that increase the degree of particle disorder in the thin films. Our study suggests that a hybrid cathodic arc deposition process conducted in the presence of nitrogen is an effective method to improve the properties of ZrO 2 thin films. The new process can be applied to other gate dielectrics and may accelerate the development of alternative high k dielectric thin films in advanced microelectronic devices and structures. r 2005 Elsevier B.V. All rights reserved. PACS: Ac; f Keywords: A1. Crystal structure; B1. Oxides; B2. Dielectric materials 1. Introduction Advanced gate dielectrics have gained considerable attention, recently, due to the requirement by Corresponding author. Tel.: ; fax: address: paul.chu@cityu.edu.hk (P.K. Chu). the semiconductor roadmaps for a sub-1.5 nm silicon dioxide (SiO 2 ) gate dielectric layer in sub- 120 nm complementary metal-oxide-semiconductor (CMOS) technologies [1 3]. There are, however, significant current leakage and reliability concerns associated with ultra-thin SiO 2 thin films and much effort has been devoted to the fabrica /$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi: /j.jcrysgro

2 A.P. Huang et al. / Journal of Crystal Growth 277 (2005) tion of new-generation gate dielectric materials with high permittivity [4]. As a dielectric material possessing high permittivity and good thermodynamic stability in contact with silicon, zirconia (ZrO 2 ) is considered as one of the most promising materials, and the thin films have also been studied as storage capacitors in dynamic random access memories (DRAMs), gate oxide in field effect transistors (FET), and so on [5 7]. However, ZrO 2 crystallizes at temperatures below 400 1C and it seriously limits the applications of ZrO 2 thin films in ultra large-scale integrated (ULSI) circuits from a storage point of view [8]. Because the grain boundaries in crystallized gate dielectrics can be the fast paths for oxygen and dopant diffusion into the gate dielectric and even to the channel region in the silicon substrate, a significant increase in both the leakage current and surface roughness can result. Consequently, the dielectric properties of MOS devices can degrade making device scaling problematic [9]. It is well known that accumulation of nitrogen (N) atoms at the SiO 2 /Si interface can improve hot carrier resistance, and the use of oxynitride can also suppress boron penetration from the poly-si gate to Si. Thus, nitrided oxides are attracting a great deal of attention for device applications that include not only flash memory but also standard MOS logic circuits [10]. Recently, there have been several reports on incorporating nitrogen into binary metal oxides to increase the crystallization temperature by hightemperature annealing in N 2 O or NH 3 ambient [11,12]. Unfortunately, the high-k dielectric materials nitrided by NH 3 exhibit an increase in the interfacial trapping density and deterioration of mobility due to hydrogen-related traps ( H, OH, and N H) [13]. Besides, N 2 O and NH 3 are both toxic gases. Therefore, the development of novel deposition methods that utilize nitrogen instead of NH 3 and N 2 O is preferred to achieve crystallization control of advanced gate dielectrics for a clean ULSI process. In this work, ZrO 2 thin films were deposited on n-type Si (1 0 0) wafers using a cathodic arc plasma source in the presence of oxygen and nitrogen gases. By introducing nitrogen, ZrO 2 thin films with higher crystallization temperature and better microstructures and interfacial characteristics were produced. This hybrid process is an effective method to improve the properties of ZrO 2 and other high-k dielectric materials. 2. Experimental procedure The nitrogen-doped ZrO 2 thin films were fabricated on n-type, 100 mm Si (1 0 0) wafers with resistivity of 4 7 O cm using a filtered cathodic arc system. The experimental apparatus used in this study mainly included a magnetic duct and cathodic arc plasma source as shown in Fig. 1. A curved magnetic duct was inserted between the plasma source and main chamber to remove macro-particles produced in the cathodic arc plasma. The cathodes used in our experiments were 99.9% pure Zr rods with a diameter of 1 cm, and oxygen gas was bled into the arcing region as shown in Fig. 1. The arc was ignited within the pulse duration of about 300 ms and repetition rate of 60 Hz. The degree of the zirconium discharge was controlled by the main arc current between the cathode and anode. The cathodic arc plasma comprising zirconium and oxygen was guided into the vacuum chamber by an electromagnetic field applied to the curved duct. The duct was biased to 20 V to build up a lateral electric field while the external solenoid coils wrapped around the duct produced the axial magnetic field with the - Ve Sample Gas inlet Anode Coil Plasma stream Zr cathode Trigger Plasma plume Duct Duct Bias Fig. 1. Schematic diagram of the experimental setup illustrating that nitrogen is bled into the vacuum chamber in the vicinity of the Zr plasma plume.

3 424 A.P. Huang et al. / Journal of Crystal Growth 277 (2005) magnitude of 100 G. 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. Before deposition, the samples that were positioned about 15 cm away from the exit of the plasma stream were cleaned by argon plasma for 2 min using a sample bias of 500 V. To introduce nitrogen into the films, nitrogen gas was bled into the vacuum chamber at different flow rates at the vicinity of the exit of the metal arc discharge plume. In this way, the cathodic arc plasma was mixed with nitrogen to form hybrid metal gas plasma to deposit nitrogen-doped ZrO 2 thin films on the silicon substrates. The vacuum chamber was about Torr. Rutherford backscattering spectrometry (RBS) was carried out using a 2 MeV 4 He ++ beam and a backscattering angle of 1701 to determine the composition as well as thickness of the thin films. Contact mode atomic force microscopy (AFM) was conducted on a Park Scientific Instrument (PSI) Autoprobe Research System to evaluate the surface morphology over a scanned area of 2 mm 2 mm. Zr, O and N bonding information was acquired using X-ray photoelectron spectroscopy (XPS) employing monochromatic Al K a radiation. Prior to the analyses, the sample surface was cleaned by 4 kev Ar ion bombardment for 1 min to remove atmospheric contaminants. The microstructure of the thin films was determined by X-ray diffraction (XRD) using a Siemens D500/ 501 thin film diffractometer with a Cu Ka source. 3. Results and discussion Fig. 2 depicts the X-ray diffraction (XRD) patterns of the thin films deposited at 450 1C using oxygen and oxygen mixed with nitrogen. The XRD results show that the thin film deposited with only oxygen (and Zr) is crystallized, as indicated by the diffraction peaks at and that can be attributed to the (1 1 1) and (0 0 2) planes of the orthorhombic ZrO 2 phase respectively [14]. With the introduction of nitrogen, the crystallized diffraction peaks almost disappear and only one Intensity (a. u.) (111) (002) 450 C Pure O 2 O 2 +N Theta (Degree) Fig. 2. XRD spectra of ZrO 2 thin films prepared at 450 1C under different conditions. weak and broad band remains as shown in Fig. 2. It suggests that the nitrogen-doped film is almost amorphous. Usually, ZrO 2 crystallizes at temperatures below 400 1C [8]. Our results indicate unequivocally that nitrogen incorporation increases the crystallization temperature of the ZrO 2 thin films. This is believed to arise from the breaking of the periodic crystal arrangement or the inhibition of continuous crystal growth in the materials [9]. The composition of the materials and bonding state of nitrogen were investigated employing RBS and XPS. Figs. 3(a) and (b) display the RBS spectra that show both the experimental and fitted results acquired from the oxygen and oxygen plus nitrogen samples. The results indicate the formation of stoichiometric ZrO 2 thin films and that the composition of the thin films produced under different conditions is basically uniform throughout the thickness. These good results are in part due to the effective elimination of Zr macroparticles that are formed in the cathodic arc using the curved magnetic filter. The process efficacy can be evaluated according to the simulation results of the zirconium contents in the layer [15]. It can clearly be observed from the RBS spectra that the thicknesses of the thin films produced under different conditions are different. The nitrogen

4 A.P. Huang et al. / Journal of Crystal Growth 277 (2005) Yield (counts/ch.) Pure oxygen O Zr Relative Density (a.u.) 450 C Zr3d 5/2 Zr3d 3/2 Relative Density (a.u.) Zr- -N N1s Binding Energy (ev) Pure O2 0 (a) Channel number 7000 Oxygen mixed nitrogen O 2 +N (a) Binding Energy (ev) Yield (counts/ch.) O Zr Relative Density (a.u.) 450 C Zr-O O1s Pure O (b) Channel number O 2 +N 2 Fig. 3. RBS spectra acquired from the ZrO 2 deposited at 450 1C using oxygen or mixed oxygen and nitrogen. (b) Binding Energy (ev) concentration in the nitrided sample is below the detection limit of RBS, i.e., less than several atomic % [16]. The composition of the thin films and Zr, O and N bonding information was further determined using X-ray photoelectron spectroscopy (XPS). Prior to the analyses, the sample surface was cleaned by 4 kev Ar ion bombardment for 1 min to remove atmospheric contaminants. The XPS results show the presence of Zr and O in addition to a small amount of nitrogen of about 0.5% atom. Figs. 4(a) and (b) show the Zr 3d and O 1s peaks of the thin films deposited at 450 1C and the N 1s binding information is shown in the inset of Fig. 4(a). The Zr 3d and O 1s binding energies are in Fig. 4. XPS spectra of Zr 3d and O 1s of ZrO 2 thin films prepared at 450 1C under different conditions (XPS spectrum of N 1s is shown in inset plot of Fig. 4(a)). good agreement with those of Zr 2+ and O 2 in ZrO 2, suggesting that the Zr atoms in the thin films are almost completely oxidized and metallic Zr is hardly detectable [17]. It can be seen from the inset of Fig. 4(a) that Zr N bonding has been formed showing physical nitrogen incorporation into the thin film. The introduction of nitrogen may influence the continuous crystal growth and result in the change of the microstructure in the thin films as shown in the XRD spectra. The binding energies of Zr 3d and O 1s are observed to shift to

5 426 A.P. Huang et al. / Journal of Crystal Growth 277 (2005) representative samples prepared under different conditions. Crystallized particles on the thin films are clearly seen in Fig. 5(a). With the addition of nitrogen, an amorphous morphology emerges as shown in Fig. 5(b). As confirmed by the XRD results, the surface roughness of the nitrogendoped thin film decreases substantially. It is well known that the surface roughness of gate dielectric thin films is an important factor influencing the electrical stability of MOS devices [19]. Our results show that the properties of ZrO 2 are improved by the addition of nitrogen and that a hybrid cathodic arc deposition process incorporation of oxygen and nitrogen gases is an effective method to increase the crystallization temperature and improve the microstructure of ZrO 2 thin films. Owing to the simplicity of this method, it can be readily applied to the synthesis of other dielectric materials. 4. Conclusion Fig. 5. AFM images of ZrO 2 thin films prepared at 450 1C (a) with and (b) without nitrogen. higher values as shown in Fig. 4, and it can be interpreted as the increase of the degree of particle disorder in the thin films in the presence of nitrogen. It should be noted that the bonding in ZrO 2 is mainly ionic, but the formation of a small amount of covalent bonding can also result in the increased binding energies [18]. The surface morphology and roughness of the thin films were assessed by atomic force microscopy (AFM). Fig. 5 depicts the AFM images of ZrO 2 thin films as an alternative gate dielectric deposited by a hybrid cathodic arc system containing nitrogen have been demonstrated. The incorporation of N into ZrO 2 increases the crystallization temperature, and the microstructure of ZrO 2 /Si thin films have obviously been improved. It is believed that the addition of nitrogen can result in the breaking of the periodic crystal arrangement or the inhibition of continuous crystal growth in the gate dielectrics, thereby, increasing the disorder degree of the deposited particles. Our study suggests that cathodic arc deposition by adding nitrogen is an effective method to improve the microstructure, which may be significant in many candidate gate dielectrics. It will accelerate the application of alternative high k thin films in advanced gate dielectric fields. Acknowledgments Our work was jointly supported by Competitive Earmarked Research Grant (CERG) #CityU 1137/03E sponsored by the Hong Kong Research

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