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Characterization of Hafnium-Zirconium-Oxide-Nitride films grown by ion beam assisted deposition
Vacuum 86 (2012) 1078e1082
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Characterization of Hafnium-Zirconium-Oxide-Nitride films grown by ion beam assisted deposition C.G. Jin a, c, T. Yu a, c, Y. Bo a, c, Y. Zhao a, c, H.Y. Zhang a, c, Y.J. Dong a, c, X.M. Wu a, c, d, *, L.J. Zhuge b, c, S.B. Ge a, c, * a
Department of Physics, Soochow University, Suzhou 215006, China Analysis and Testing Center, Soochow University, Suzhou 215006, China The Key Laboratory of Thin Films of Jiangsu, Soochow University, Suzhou 215006, China d State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 June 2011 Received in revised form 27 September 2011 Accepted 2 October 2011
Hafnium-Zirconium-Oxide-Nitride (Hf1xZrxO1yNy) films are prepared by ion beam assisted deposition on peSi and quartz substrates with a composite target of sintered high-purity HfO2 and ZrO2. The thermal stability and microstructure characteristics for Hf1xZrxO1yNy films have been investigated. EDS results confirmed that nitrogen was successfully incorporated into the Hf1xZrxO2 films. XRD and Raman analyses showed that the Hf1xZrxO1yNy films remain amorphous after 1100 C under vacuum ambient, and monoclinic HfO2 and ZrO2 crystals separate from Hf1xZrxO1yNy films with a increase of the annealing temperature up to 1300 C. Meanwhile, the Hf1xZrxO1yNy films can also effectively suppress oxygen diffusion during high temperature annealing. AFM measurements demonstrated that surface roughness of the Hf1xZrxO1yNy films increase slightly. Then the optical properties of the samples were observed in the ultravioletevisible range at room temperature. The variation in Eg from 5.64 to 6.09 eV as a function of annealing temperature has also been discussed briefly. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: High-k dielectrics HfO2 ZrO2 Structure Optical properties
1. Introduction In recently years, metal oxide high-k dielectrics have become a feasible replacement for conventional SiO2 gate oxides in complementary metal oxide semiconductor (CMOS). Over the past few years several high-k materials have been investigated, such as hafnium oxide, zirconium oxide, aluminum oxide, and their silicates. Among these materials, HfO2 is demonstrated to be a promising high-k gate dielectric due to its relatively high dielectric constant, larger band gap, band offset and thermodynamic stability [1e10]. Hf-based high-k gate dielectric was selected as the gate dielectric for 45 nm technology node [11,12]. However, important challenges remain related to further scaling, e.g., threshold voltage (Vt) instability, degraded carrier mobility, and reliability issues [13,14]. Additionally, the dielectric constant of HfO2 (w20) might not be high enough for extreme subnanometer equivalent oxide thickness (EOT) scaling.
* Corresponding authors. Department of Physics, Soochow University, Suzhou 215006, China. Tel.: þ86 512 6511 2251; fax: þ86 512 6511 1907. E-mail addresses: [email protected] (X.M. Wu), [email protected] (S.B. Ge). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.10.001
The addition of metal elements to improve the film quality of high-k oxides without reducing their dielectric constants and increasing their leakage currents is important. HfZrOx formed by adding Zr into HfO2 has been extensively studied as an alternative gate dielectric. HfO2 and ZrO2 have very similar physical and chemical properties and these materials are completely miscible in the solid solution [15]. HfZrOx presents advantages compared to HfO2, such as lower electrical thickness (higher dielectric constant), higher level of film structure stability, higher mobility and lower Vt shift after stress (improved reliability) [15,16]. Nitrogen is a well-known diffusion barrier to oxygen and is regularly used in CMOS processing. Given the diffusion barrier properties and the fact that the nitrides tend to have a better electrical response in comparison to oxides, extensive research has been done on incorporating nitrogen in HfO2. Recently, the incorporation of nitrogen in Hf-based gate dielectrics has attracted increasing interest because of the decrease in EOT [17], increase in thermal stability [18,19], higher crystallization temperature [20] and statistical improvement of the breakdown characteristics [21]. When N is added to HfO2, it is expected to distort the equilibrium of the lattice and produce disordered states. Adding nitrogen results in the reduction of the mobility of
C.G. Jin et al. / Vacuum 86 (2012) 1078e1082
Hf and O atoms as well as increase in the nucleation temperature and consequently the crystalline temperature [20]. All these indicate that nitrogen acts as a crystallization inhibitor and causes an increase in the crystallization temperature in Hf-based gate dielectrics. A fundamental concern in ultrathin high-k materials has been the crystallization of the dielectric after activation annealing, which could result in grain boundaries serving as a path for diffusion of impurities such as dopants or dielectric constituents [22,23]. In this paper, HfO2 films doped with Zr and N were fabricated by ion beam assisted deposition with a composite target of sintered HfO2 and ZrO2. In order to lay a strong foundation for discussing the electrical properties of such materials later, our attention is focused on their structure now. We also report the observation of optical transmission from samples, and the optical properties of the samples, one of the most important properties, have also been discussed.
2. Experiments Hf1xZrxO1-yNy films were deposited at room temperature by employing an ion beam assisted deposition. It consists of a focused Kaufman ion source (main ion beam source) and a broad-beam Kaufman ion beam source (assisting on source). Argon with a purity of 99.999% was used as main beam source gas for main ion beam generation. The main ion source with 10 cm diameter and 45 incident angle served as a sputtering ion beam source. The composite target of sintered high-purity HfO2 and ZrO2 was used as the sputtering target. The sputtering chamber was evacuated to 2 104 Pa before argon was introduced through a mass flow controller. The working pressure of argon was 4 102 Pa. Before deposition, the substrates of p-type Si (1 0 0) with resistivity of 5e8 U/cm were dipped in 10% hydrofluoric acid to remove the surface native oxides, rinsed with a large amount of de-ionized water, dried in a flux of N2, and immediately placed into the chamber. During the deposition, the ion energy and beam current of the main ion beam source, which was used to sputter the composite target, were 800 eV and 50 mA, respectively. An assisting ion source with 10 cm diameter and 60 incident angle was used. Nitrogen with a purity of 99.999% was used as assisting ion beam source gas for assisting ion beam generation. The N ion beam was used to bombard the substrate and provide N to the films. The ion energy and beam current of the assisting ion beam source were 16 mA and 200 eV, respectively. No intentional heating was applied on the substrate during the deposition. All the thicknesses of the films were approximately 80 nm, which were measured with an ET350 surface profilometer (Kosaka Laboratory Ltd). Consequently, the as-deposited films were annealed in a tube furnace in the temperature range of 900 Ce1300 C for 10 min under vacuum ambient. The chemical composition of the films was analyzed by Energy dispersive spectroscopy (EDS) of Hitachi S-4700. The structure of the films was analyzed by a Rigaku D/Max-3C X-ray diffraction (XRD) diffractometer using Cu ka radiation at 40 kV. The surface morphologies of the films were investigated using an NT-MDT Solver P47-PRO scanning probe microscope (SPM) operating in the contact atomic force microscope (AFM) mode. The bonding configurations were measured by and Raman spectra which were obtained in the backscattering configuration between 150 cm1 and 800 cm1 with a JY-H800 using an argon ion laser at a wavelength of 514 nm. The optical transmission spectra were recorded by an ultravioletevisible spectrometer (JASCO V-570) in the range of 190e800 nm. All the measurements were conducted at room temperature.
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3. Results and discussion To check the structure of the sample, typical XRD diffraction patterns of the Hf1xZrxO1-yNy films as a function of annealing temperature are shown in Fig. 1 (a). The film remains amorphous up to 1100 C annealing. When annealing temperature increases to 1300 C, the peaks appear at 2q ¼ 30.1 and 35.0 . According to the Joint Committee on Powder Diffraction Standards (JCPDS) card, the two peaks correspond to monoclinic ZrO2 (111) and HfO2 (200), respectively. ZrO2 and HfO2 crystalline peaks appear, which indicates that ZrO2 and HfO2 phases may separate from Hf-Zr-O-N. To the best of our knowledge [24,25], the pure ZrO2 and HfO2 films demonstrate a well-crystallized structure after annealing at 500 C. These results suggest that nitrogen-incorporated into Hf1xZrxO2 films leads to the increase of the crystallization temperature of hafnium-based gate dielectrics. The present is in agreement with those of investigations by Chio et al. for nitrogen incorporation derived HfO2 films [26,27]. Shang et al. have attributed the increase of crystallization temperature of nitrogen-incorporated HfO2 films to the reduction of the diffusion rate for the initial phase separation and the decrease in average atomic coordination [28]. According to our experimental results, we contribute the increase of crystallization temperature of nitrogen-incorporated Hf1xZrxO2 thin films to the reduction of the mobility of Hf, Zr and O atoms and the increase of the nucleation temperature. In order to mention about
a
b
Fig. 1. XRD spectra of (a): annealed at 0e1300 C Hf1xZrxO1yNy films, (b) annealed at 0e800 C Hf1xZrxO2 films.
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C.G. Jin et al. / Vacuum 86 (2012) 1078e1082
Fig. 2. Raman spectra of (a): as-deposited, (b) annealed at 900 C, (c) annealed at 1100 C, and (d) annealed at 1300 C Hf1xZrxO1yNy films.
the nitrogen effect clearly, the Hf1xZrxO2 thin films were prepared. The XRD diffraction patterns of the Hf1xZrxO2 films as a function of annealing temperature show that monoclinic ZrO2 (111) and HfO2 (200) separate from HfeZreO annealed at 800 C as shown in Fig. 1(b), which is similar with the results of the Hf1xZrxO1yNy films. It demonstrates that nitrogen-incorporated increases the crystallization temperature of the films.
In order to investigate the film compositions, EDS analysis has been performed on the Hf1xZrxO1yNy films as-deposited and annealed at 900, 1100 and 1300 C, respectively. The EDS results show that the film compositions have no change with the increasing of annealing temperature. The proportion of Hf1xZrxO1yNy film is Hf:Zr:O:N ¼ 15:4:30:1 (at.% ratio), which indicates that nitrogen has incorporated successfully into Hf1xZrxO2 films. Because Raman scattering is very sensitive to the microstructures of materials, it was employed to further confirm the structures of the Hf1xZrxO1yNy films as a function of annealing temperature as shown in Fig. 2. Raman spectroscopy identifies the structural phase of the nanocrystals. The monoclinic form of HfO2 has bands at 301, 390 498 (strong) and 580 cm1 [29], and the monoclinic form of ZrO2 has bands at 192, 220, 347, 382, 476 (strong), 617 and 638 cm1 [30]. However due to disorder, induced by the deposition technique, the translation symmetry is lost, and thus all the modes can participate in the scattering. Raman spectra will be an “image” of the density of vibrational states (DVS) [31]. Both the as-deposited sample (curve a) and the samples annealed at 900 C (curve b) have a pattern of amorphous structure. With the increase of the annealing temperature up to 1100 C, the spectral sharp continues to change: a shoulder located at around 583 cm1 and an additional peak near 209 cm1 starts to appear, which maybe correspond to monoclinic HfO2 and ZrO2 phonon modes, respectively. It means that ZrO2 phase may separate from Hf1xZrxO1yNy film, however the film still has amorphous structure from XRD result. And further increase of the annealing temperature up to 1300 C, three peaks appear at 398, 488 cm1
Fig. 3. AFM images of (a): as-deposited, (b) annealed at 900 C, (c) annealed at 1100 C, and (d) annealed at 1300 C Hf1xZrxO1yNy films.
C.G. Jin et al. / Vacuum 86 (2012) 1078e1082
corresponding to monoclinic HfO2 and 343 cm1 corresponding to monoclinic ZrO2. At the same time, compared with the curve c, the peak near 209 cm1 becomes sharper and the phonon modes become more intense as the annealing temperature increases, as a result of the increased contribution of monoclinic ZrO2 phase within Hf1xZrxO1yNy film. This means that the separated ZrO2 and HfO2 crystals are formed, when annealed at 1300 C and is consistent with the XRD results above. The surface roughness of both as-deposited and post-annealing films is important because it affects shifts in electronic energy levels and may degrade electrical characteristics [32]. Surface morphology could also reveal crystalline phase formation. AFM surface scans were taken on thin Hf1xZrxO1yNy films to study changes in surface morphology as a function of post-deposition annealing without a capping layer. Fig. 3 shows the surface morphologies of the Hf1xZrxO1yNy samples, which are scanned over areas of 2 mm 2 mm. As-deposited film exhibited smooth surface with RMS (root mean square) is 0.127 nm. It is clear from these images that the amplitude of the surface irregularities increases with an increase annealing temperature. This is further illustrated in Fig. 4, which shows a plot of the RMS (standard deviation of the height) surface roughness as a function of annealing temperature. Upon annealing temperature at 1300 C, the RMS of the films is drastically increased and reaches about 4.4 Å, which influences on the size of the grains (roughness) and is consistent with the XRD and Raman results above. The optical properties of the Hf1xZrxO1yNy films deposited on quartz have also been investigated by using an ultravioletevisible spectrometer. Fig. 5 shows the transmittance spectra and UVS absorption spectra of the Hf1xZrxO1yNy films as a function of annealing temperature. In transmittance spectra as shown in Fig. 5(a), one can see that after annealing at 900 C, the optical absorption edge shifts to the higher energy, indicating that the band gap (Eg) of annealing Hf1xZrxO1yNy films increases. The samples annealed at 1300 C have a much wider absorption edge, which is due to the sample surface roughness, which is consistent with the AFM results above. It is clear that all the samples show a high transparency in the spectral region. For Eg, we employed a ðahyÞ2 vs. Eg plot for the spectra to fit the data assuming a ðahyÞ2 fðhy Eg Þ relationship [33], where a is the absorption coefficient and hn is the photo energy as shown in Fig. 5(b). The variation in Eg from 5.64 to 6.09 eV as a function of annealing
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a
b
Fig. 5. (a) Transmittance spectra; (b) (ahn)2 vs Eg plots for Hf1xZrxO1yNy films as a function of annealing temperature. Linear fits of the absorption data indicate the direct band gap of Hf1xZrxO1yNy films. The extrapolation of the linear region to hn ¼ 0 to determine Eg values is as indicated with lines. The insert shows the variation in Eg from 5.64 to 6.09 eV as a function of annealing temperature.
temperature is shown as an inset in Fig. 5(b). The Eg of pure ZrO2 and HfO2 films are 7.8 and 5.8 eV, respectively, and that of the Eg of as-deposited Hf1xZrxO1yNy film is 5.85 eV, which is between them. With the increase of temperature up to 900 C, the increases significantly to 6.09 eV. It maybe due to that higher temperature annealing can effectively reduce the defects in the film. According to XRD and Raman analysis, it is clear that the Eg decreases with increase of annealing temperature from 1100 C to 1300 C, which maybe due to ZrO2 and HfO2 phases separate from Hf1xZrxO1yNy film. However, further experimental and theoretical investigations are needed to address this issue. 4. Conclusion
Fig. 4. RMS surface roughness of Hf1xZrxO1yNy films as a function of annealing temperature.
High-quality Hf1xZrxO1yNy gate dielectric thin films are deposited by ion beam assisted deposition and followed by vacuum annealing with the temperatures varying from 900 to 1300 C. The chemical compositions, thermal stability, surface morphology and optical properties of Hf1xZrxO1yNy films are discussed in detail. EDS analyses confirm that nitrogen is incorporated into Hf1xZrxO2 films effectively through this PVD method. The films remain amorphous after 1100 C post-deposition annealing, indicating that Hf1xZrxO1yNy films increase the crystallization temperature.
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Raman spectra show that ZrO2 phase may separate from Hf1xZrxO1yNy film, however the film still has amorphous structure annealed at 1100 C, indicating that Hf1xZrxO1yNy films can greatly suppress oxygen diffusion through high-k gate dielectric. AFM images show that the amplitude of the surface irregularities increases with an increase annealing temperature. The variation in Eg from 5.64 to 6.09 eV as a function of annealing temperature has also been discussed briefly. These results clearly suggest that nitrogen-doping Hf1xZrxO2 films using a composite target of sintered high-purity HfO2 and ZrO2 increases the crystallization temperature of the Hf1xZrxO2 films. Acknowledgment This work is supported by National Natural Science Foundation of China (10975106) and the Open Project of State Key Laboratory of Functional Materials for Information, Qing Lan Project and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] Wilk GD, Wallace RM, Anthony JM. J Appl Phys 2001;89:5243. [2] Robertson J. Rep Prog Phys 2006;69:327. [3] Zhao C, Witters T, Brijs B, Bender H, Richard O, Caymax M, et al. Appl Phys Lett 2005;86:132903. [4] Delabie A, Caymax M, Brijs B, Brunco DP, Conared T, Sleeckx E, et al. J Electrochem Soc 2006;153:F180. [5] Triyoso D, Liu R, Roan D, Ramon M, Edwards NV, Gregory R, et al. J Electrochem Soc 2004;151:F220. [6] Gusev EP, Cabral Jr C, Copel M, Emic CD, Gribelyuk M. Microelectron Eng 2003; 69:154. [7] Ferrari S, Scarel G, Wiemer C, Fanciulli M. J Appl Phys 2002;95:7675. [8] Cho M, Degraeve R, Pourtois G, Delabie A, Ragnarsson LA, Kauerauf T, et al. IEEE Trans Electron Devices 2007;54:752.
[9] Shi X, Schaekers M, Date L, Rothschild A, Everaert JL, Van Elshocht S, et al. ECS Trans 2006;3(3):417. [10] L-Å Ragnarsson S, Severi L, Trojman D, Brunco P, D Johnson K, Delabie A, et al. Dig Tech Pap - Symp VLSI Technol; 2005:234. [11] Bohr MT, Chau RS, Ghani T, Mistry K. IEEE Spectr 2007;44:29. [12] Mistry K, Allen C, Auth C, Beattie B, Bergstrom D, Bost M, et al. Tech Dig - Int Electron Devices Meet; 2007:247. [13] Kirsch PD, Quevedo-Lopez MA, Li HJ, Senzaki Y, Peterson JJ, Song SC, et al. J Appl Phys 2006;99:0235081. [14] Zafar S, Kim YH, Narayanan V, Cabral Jr C, Paruchuri V, Doris B, et al. Dig Tech Pap - Symp VLSI Technol; 2006:23. [15] Hegde RI, Triyoso DH, Samavedam SB, White BE. J Appl Phys 2007;101: 074113. [16] Triyoso DH, Hegde a_ R I, Schaeffer JK, Gregory R, Wang X-D, Canonico M, et al. J Vac Sci Technol B 2007;25(3):1071. [17] Kang CS, Cho HJ, Choi R, Kim YH, Kang CY, Rhee SJ, et al. IEEE Trans Electron Devices 2004;51:220. [18] Visokay MR, Chambers JJ, Rotondaro ALP, Shanware A, Colombo L. Appl Phys Lett 2002;80:3183. [19] Cho M, Park J, Park HB, Hwang CS, Jeong J, Hyun KS, et al. Appl Phys Lett 2002; 81:3630. [20] Choi KJ, Kim JH, Yoon SG, Shin WC. J Vac Sci Technol B 2004;22:1755e8. [21] Koyama M, Satake H, Koike M, Ino T, Suzuki M, Iijima R, et al. Tech Dig - Int Electron Devices Meet; 2003:931. [22] Quevedo-Lopez MA, Visokay MR, Chambers JJ, Bevan MJ, LiFatou A, Colombo L, et al. J Appl Phys 2005;97:0435308. [23] Sivasubramani P, Kim MJ, Gnade BE, Wallace RM, Edge LF, Schlom DG, et al. Appl Phys Lett 2005;86:201901. [24] Njjljsk Abti, Aarika Jaan, Uustar Teet, Mändar Hugo, Tkache Sergey, Manghnani Murli. Proc SPIE 2005;5946:59460. [25] He G, Fang Q, Liu M, Zhu LQ, Zhang LD. J Cryst Growth 2004;268:155. [26] Chio KJ, Kim JH, Yoon SG, Shin WC. J Vac Sci Technol B 2004;22:1755. [27] Chio KJ, Kim JH, Yoon SG. J Electrochem Soc 2004;151:G262. [28] Shang G, Peacock PW, Robertson J. Appl Phys Lett 2004;84:106. [29] Robinson RD, Tang J, Steigerwald ML, Brus LE, Herman IP. Phys Rev B 2005;71: 115408. [30] Quintard PE, Barbéris P, Mirgorodsky AP, Merle-méjean T. J Am Ceram Soc 2002;85:1745. [31] Christensen AN, Dietrich OW, Kress W, Teuchert WD. Phys Rev B 1979;19: 5699. [32] Young-Bae K, Ahn H, Wu TT. Semicond Sci Technol 2005;20:519. [33] Tauc J, Grigorovici R, Vancu A. Phys Status Solidi 1966;15:627.
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Characterization of Hafnium-Zirconium-Oxide-Nitride films grown by ion beam assisted deposition C.G. Jin a, c, T. Yu a, c, Y. Bo a, c, Y. Zhao a, c, H.Y. Zhang a, c, Y.J. Dong a, c, X.M. Wu a, c, d, *, L.J. Zhuge b, c, S.B. Ge a, c, * a
Department of Physics, Soochow University, Suzhou 215006, China Analysis and Testing Center, Soochow University, Suzhou 215006, China The Key Laboratory of Thin Films of Jiangsu, Soochow University, Suzhou 215006, China d State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 June 2011 Received in revised form 27 September 2011 Accepted 2 October 2011
Hafnium-Zirconium-Oxide-Nitride (Hf1xZrxO1yNy) films are prepared by ion beam assisted deposition on peSi and quartz substrates with a composite target of sintered high-purity HfO2 and ZrO2. The thermal stability and microstructure characteristics for Hf1xZrxO1yNy films have been investigated. EDS results confirmed that nitrogen was successfully incorporated into the Hf1xZrxO2 films. XRD and Raman analyses showed that the Hf1xZrxO1yNy films remain amorphous after 1100 C under vacuum ambient, and monoclinic HfO2 and ZrO2 crystals separate from Hf1xZrxO1yNy films with a increase of the annealing temperature up to 1300 C. Meanwhile, the Hf1xZrxO1yNy films can also effectively suppress oxygen diffusion during high temperature annealing. AFM measurements demonstrated that surface roughness of the Hf1xZrxO1yNy films increase slightly. Then the optical properties of the samples were observed in the ultravioletevisible range at room temperature. The variation in Eg from 5.64 to 6.09 eV as a function of annealing temperature has also been discussed briefly. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: High-k dielectrics HfO2 ZrO2 Structure Optical properties
1. Introduction In recently years, metal oxide high-k dielectrics have become a feasible replacement for conventional SiO2 gate oxides in complementary metal oxide semiconductor (CMOS). Over the past few years several high-k materials have been investigated, such as hafnium oxide, zirconium oxide, aluminum oxide, and their silicates. Among these materials, HfO2 is demonstrated to be a promising high-k gate dielectric due to its relatively high dielectric constant, larger band gap, band offset and thermodynamic stability [1e10]. Hf-based high-k gate dielectric was selected as the gate dielectric for 45 nm technology node [11,12]. However, important challenges remain related to further scaling, e.g., threshold voltage (Vt) instability, degraded carrier mobility, and reliability issues [13,14]. Additionally, the dielectric constant of HfO2 (w20) might not be high enough for extreme subnanometer equivalent oxide thickness (EOT) scaling.
* Corresponding authors. Department of Physics, Soochow University, Suzhou 215006, China. Tel.: þ86 512 6511 2251; fax: þ86 512 6511 1907. E-mail addresses: [email protected] (X.M. Wu), [email protected] (S.B. Ge). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.10.001
The addition of metal elements to improve the film quality of high-k oxides without reducing their dielectric constants and increasing their leakage currents is important. HfZrOx formed by adding Zr into HfO2 has been extensively studied as an alternative gate dielectric. HfO2 and ZrO2 have very similar physical and chemical properties and these materials are completely miscible in the solid solution [15]. HfZrOx presents advantages compared to HfO2, such as lower electrical thickness (higher dielectric constant), higher level of film structure stability, higher mobility and lower Vt shift after stress (improved reliability) [15,16]. Nitrogen is a well-known diffusion barrier to oxygen and is regularly used in CMOS processing. Given the diffusion barrier properties and the fact that the nitrides tend to have a better electrical response in comparison to oxides, extensive research has been done on incorporating nitrogen in HfO2. Recently, the incorporation of nitrogen in Hf-based gate dielectrics has attracted increasing interest because of the decrease in EOT [17], increase in thermal stability [18,19], higher crystallization temperature [20] and statistical improvement of the breakdown characteristics [21]. When N is added to HfO2, it is expected to distort the equilibrium of the lattice and produce disordered states. Adding nitrogen results in the reduction of the mobility of
C.G. Jin et al. / Vacuum 86 (2012) 1078e1082
Hf and O atoms as well as increase in the nucleation temperature and consequently the crystalline temperature [20]. All these indicate that nitrogen acts as a crystallization inhibitor and causes an increase in the crystallization temperature in Hf-based gate dielectrics. A fundamental concern in ultrathin high-k materials has been the crystallization of the dielectric after activation annealing, which could result in grain boundaries serving as a path for diffusion of impurities such as dopants or dielectric constituents [22,23]. In this paper, HfO2 films doped with Zr and N were fabricated by ion beam assisted deposition with a composite target of sintered HfO2 and ZrO2. In order to lay a strong foundation for discussing the electrical properties of such materials later, our attention is focused on their structure now. We also report the observation of optical transmission from samples, and the optical properties of the samples, one of the most important properties, have also been discussed.
2. Experiments Hf1xZrxO1-yNy films were deposited at room temperature by employing an ion beam assisted deposition. It consists of a focused Kaufman ion source (main ion beam source) and a broad-beam Kaufman ion beam source (assisting on source). Argon with a purity of 99.999% was used as main beam source gas for main ion beam generation. The main ion source with 10 cm diameter and 45 incident angle served as a sputtering ion beam source. The composite target of sintered high-purity HfO2 and ZrO2 was used as the sputtering target. The sputtering chamber was evacuated to 2 104 Pa before argon was introduced through a mass flow controller. The working pressure of argon was 4 102 Pa. Before deposition, the substrates of p-type Si (1 0 0) with resistivity of 5e8 U/cm were dipped in 10% hydrofluoric acid to remove the surface native oxides, rinsed with a large amount of de-ionized water, dried in a flux of N2, and immediately placed into the chamber. During the deposition, the ion energy and beam current of the main ion beam source, which was used to sputter the composite target, were 800 eV and 50 mA, respectively. An assisting ion source with 10 cm diameter and 60 incident angle was used. Nitrogen with a purity of 99.999% was used as assisting ion beam source gas for assisting ion beam generation. The N ion beam was used to bombard the substrate and provide N to the films. The ion energy and beam current of the assisting ion beam source were 16 mA and 200 eV, respectively. No intentional heating was applied on the substrate during the deposition. All the thicknesses of the films were approximately 80 nm, which were measured with an ET350 surface profilometer (Kosaka Laboratory Ltd). Consequently, the as-deposited films were annealed in a tube furnace in the temperature range of 900 Ce1300 C for 10 min under vacuum ambient. The chemical composition of the films was analyzed by Energy dispersive spectroscopy (EDS) of Hitachi S-4700. The structure of the films was analyzed by a Rigaku D/Max-3C X-ray diffraction (XRD) diffractometer using Cu ka radiation at 40 kV. The surface morphologies of the films were investigated using an NT-MDT Solver P47-PRO scanning probe microscope (SPM) operating in the contact atomic force microscope (AFM) mode. The bonding configurations were measured by and Raman spectra which were obtained in the backscattering configuration between 150 cm1 and 800 cm1 with a JY-H800 using an argon ion laser at a wavelength of 514 nm. The optical transmission spectra were recorded by an ultravioletevisible spectrometer (JASCO V-570) in the range of 190e800 nm. All the measurements were conducted at room temperature.
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3. Results and discussion To check the structure of the sample, typical XRD diffraction patterns of the Hf1xZrxO1-yNy films as a function of annealing temperature are shown in Fig. 1 (a). The film remains amorphous up to 1100 C annealing. When annealing temperature increases to 1300 C, the peaks appear at 2q ¼ 30.1 and 35.0 . According to the Joint Committee on Powder Diffraction Standards (JCPDS) card, the two peaks correspond to monoclinic ZrO2 (111) and HfO2 (200), respectively. ZrO2 and HfO2 crystalline peaks appear, which indicates that ZrO2 and HfO2 phases may separate from Hf-Zr-O-N. To the best of our knowledge [24,25], the pure ZrO2 and HfO2 films demonstrate a well-crystallized structure after annealing at 500 C. These results suggest that nitrogen-incorporated into Hf1xZrxO2 films leads to the increase of the crystallization temperature of hafnium-based gate dielectrics. The present is in agreement with those of investigations by Chio et al. for nitrogen incorporation derived HfO2 films [26,27]. Shang et al. have attributed the increase of crystallization temperature of nitrogen-incorporated HfO2 films to the reduction of the diffusion rate for the initial phase separation and the decrease in average atomic coordination [28]. According to our experimental results, we contribute the increase of crystallization temperature of nitrogen-incorporated Hf1xZrxO2 thin films to the reduction of the mobility of Hf, Zr and O atoms and the increase of the nucleation temperature. In order to mention about
a
b
Fig. 1. XRD spectra of (a): annealed at 0e1300 C Hf1xZrxO1yNy films, (b) annealed at 0e800 C Hf1xZrxO2 films.
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Fig. 2. Raman spectra of (a): as-deposited, (b) annealed at 900 C, (c) annealed at 1100 C, and (d) annealed at 1300 C Hf1xZrxO1yNy films.
the nitrogen effect clearly, the Hf1xZrxO2 thin films were prepared. The XRD diffraction patterns of the Hf1xZrxO2 films as a function of annealing temperature show that monoclinic ZrO2 (111) and HfO2 (200) separate from HfeZreO annealed at 800 C as shown in Fig. 1(b), which is similar with the results of the Hf1xZrxO1yNy films. It demonstrates that nitrogen-incorporated increases the crystallization temperature of the films.
In order to investigate the film compositions, EDS analysis has been performed on the Hf1xZrxO1yNy films as-deposited and annealed at 900, 1100 and 1300 C, respectively. The EDS results show that the film compositions have no change with the increasing of annealing temperature. The proportion of Hf1xZrxO1yNy film is Hf:Zr:O:N ¼ 15:4:30:1 (at.% ratio), which indicates that nitrogen has incorporated successfully into Hf1xZrxO2 films. Because Raman scattering is very sensitive to the microstructures of materials, it was employed to further confirm the structures of the Hf1xZrxO1yNy films as a function of annealing temperature as shown in Fig. 2. Raman spectroscopy identifies the structural phase of the nanocrystals. The monoclinic form of HfO2 has bands at 301, 390 498 (strong) and 580 cm1 [29], and the monoclinic form of ZrO2 has bands at 192, 220, 347, 382, 476 (strong), 617 and 638 cm1 [30]. However due to disorder, induced by the deposition technique, the translation symmetry is lost, and thus all the modes can participate in the scattering. Raman spectra will be an “image” of the density of vibrational states (DVS) [31]. Both the as-deposited sample (curve a) and the samples annealed at 900 C (curve b) have a pattern of amorphous structure. With the increase of the annealing temperature up to 1100 C, the spectral sharp continues to change: a shoulder located at around 583 cm1 and an additional peak near 209 cm1 starts to appear, which maybe correspond to monoclinic HfO2 and ZrO2 phonon modes, respectively. It means that ZrO2 phase may separate from Hf1xZrxO1yNy film, however the film still has amorphous structure from XRD result. And further increase of the annealing temperature up to 1300 C, three peaks appear at 398, 488 cm1
Fig. 3. AFM images of (a): as-deposited, (b) annealed at 900 C, (c) annealed at 1100 C, and (d) annealed at 1300 C Hf1xZrxO1yNy films.
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corresponding to monoclinic HfO2 and 343 cm1 corresponding to monoclinic ZrO2. At the same time, compared with the curve c, the peak near 209 cm1 becomes sharper and the phonon modes become more intense as the annealing temperature increases, as a result of the increased contribution of monoclinic ZrO2 phase within Hf1xZrxO1yNy film. This means that the separated ZrO2 and HfO2 crystals are formed, when annealed at 1300 C and is consistent with the XRD results above. The surface roughness of both as-deposited and post-annealing films is important because it affects shifts in electronic energy levels and may degrade electrical characteristics [32]. Surface morphology could also reveal crystalline phase formation. AFM surface scans were taken on thin Hf1xZrxO1yNy films to study changes in surface morphology as a function of post-deposition annealing without a capping layer. Fig. 3 shows the surface morphologies of the Hf1xZrxO1yNy samples, which are scanned over areas of 2 mm 2 mm. As-deposited film exhibited smooth surface with RMS (root mean square) is 0.127 nm. It is clear from these images that the amplitude of the surface irregularities increases with an increase annealing temperature. This is further illustrated in Fig. 4, which shows a plot of the RMS (standard deviation of the height) surface roughness as a function of annealing temperature. Upon annealing temperature at 1300 C, the RMS of the films is drastically increased and reaches about 4.4 Å, which influences on the size of the grains (roughness) and is consistent with the XRD and Raman results above. The optical properties of the Hf1xZrxO1yNy films deposited on quartz have also been investigated by using an ultravioletevisible spectrometer. Fig. 5 shows the transmittance spectra and UVS absorption spectra of the Hf1xZrxO1yNy films as a function of annealing temperature. In transmittance spectra as shown in Fig. 5(a), one can see that after annealing at 900 C, the optical absorption edge shifts to the higher energy, indicating that the band gap (Eg) of annealing Hf1xZrxO1yNy films increases. The samples annealed at 1300 C have a much wider absorption edge, which is due to the sample surface roughness, which is consistent with the AFM results above. It is clear that all the samples show a high transparency in the spectral region. For Eg, we employed a ðahyÞ2 vs. Eg plot for the spectra to fit the data assuming a ðahyÞ2 fðhy Eg Þ relationship [33], where a is the absorption coefficient and hn is the photo energy as shown in Fig. 5(b). The variation in Eg from 5.64 to 6.09 eV as a function of annealing
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a
b
Fig. 5. (a) Transmittance spectra; (b) (ahn)2 vs Eg plots for Hf1xZrxO1yNy films as a function of annealing temperature. Linear fits of the absorption data indicate the direct band gap of Hf1xZrxO1yNy films. The extrapolation of the linear region to hn ¼ 0 to determine Eg values is as indicated with lines. The insert shows the variation in Eg from 5.64 to 6.09 eV as a function of annealing temperature.
temperature is shown as an inset in Fig. 5(b). The Eg of pure ZrO2 and HfO2 films are 7.8 and 5.8 eV, respectively, and that of the Eg of as-deposited Hf1xZrxO1yNy film is 5.85 eV, which is between them. With the increase of temperature up to 900 C, the increases significantly to 6.09 eV. It maybe due to that higher temperature annealing can effectively reduce the defects in the film. According to XRD and Raman analysis, it is clear that the Eg decreases with increase of annealing temperature from 1100 C to 1300 C, which maybe due to ZrO2 and HfO2 phases separate from Hf1xZrxO1yNy film. However, further experimental and theoretical investigations are needed to address this issue. 4. Conclusion
Fig. 4. RMS surface roughness of Hf1xZrxO1yNy films as a function of annealing temperature.
High-quality Hf1xZrxO1yNy gate dielectric thin films are deposited by ion beam assisted deposition and followed by vacuum annealing with the temperatures varying from 900 to 1300 C. The chemical compositions, thermal stability, surface morphology and optical properties of Hf1xZrxO1yNy films are discussed in detail. EDS analyses confirm that nitrogen is incorporated into Hf1xZrxO2 films effectively through this PVD method. The films remain amorphous after 1100 C post-deposition annealing, indicating that Hf1xZrxO1yNy films increase the crystallization temperature.
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Raman spectra show that ZrO2 phase may separate from Hf1xZrxO1yNy film, however the film still has amorphous structure annealed at 1100 C, indicating that Hf1xZrxO1yNy films can greatly suppress oxygen diffusion through high-k gate dielectric. AFM images show that the amplitude of the surface irregularities increases with an increase annealing temperature. The variation in Eg from 5.64 to 6.09 eV as a function of annealing temperature has also been discussed briefly. These results clearly suggest that nitrogen-doping Hf1xZrxO2 films using a composite target of sintered high-purity HfO2 and ZrO2 increases the crystallization temperature of the Hf1xZrxO2 films. Acknowledgment This work is supported by National Natural Science Foundation of China (10975106) and the Open Project of State Key Laboratory of Functional Materials for Information, Qing Lan Project and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] Wilk GD, Wallace RM, Anthony JM. J Appl Phys 2001;89:5243. [2] Robertson J. Rep Prog Phys 2006;69:327. [3] Zhao C, Witters T, Brijs B, Bender H, Richard O, Caymax M, et al. Appl Phys Lett 2005;86:132903. [4] Delabie A, Caymax M, Brijs B, Brunco DP, Conared T, Sleeckx E, et al. J Electrochem Soc 2006;153:F180. [5] Triyoso D, Liu R, Roan D, Ramon M, Edwards NV, Gregory R, et al. J Electrochem Soc 2004;151:F220. [6] Gusev EP, Cabral Jr C, Copel M, Emic CD, Gribelyuk M. Microelectron Eng 2003; 69:154. [7] Ferrari S, Scarel G, Wiemer C, Fanciulli M. J Appl Phys 2002;95:7675. [8] Cho M, Degraeve R, Pourtois G, Delabie A, Ragnarsson LA, Kauerauf T, et al. IEEE Trans Electron Devices 2007;54:752.
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