Oxynitride gate dielectric prepared by thermal oxidation of low-pressure chemical vapor deposition silicon-rich silicon nitride Jackie Chan a, Hei Wong b,*, M.C. Poon a, C.W. Kok a a Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong b Department of Electronic Engineering, City University of Hong Kong, Clear Water Bay, 83 Tat Chee Avenue, Kowloon, Hong Kong Received 16 September 2002; received in revised form 7 January 2003 Abstract Thin oxynitride gate dielectric films were prepared by thermal oxidation of low-pressure chemical vapor deposition silicon-rich silicon nitride at temperature ranging 850–1050 �C. X-ray photoelectron spectroscopy results indicate that the conversion of the as-deposited silicon nitride into oxynitride with different composition or oxide is feasible and the process is governed by the oxidation temperature. For sample oxidized at 1050 �C for 1 h, the nitride film was converted into silicon dioxide with 7 at.% of nitrogen at the interface and the leakage current density can be reduced by several orders of magnitude. By measuring the leakage current, the barrier height (extracted from Fowler–Nordheim plot) at the dielectric/Si interface is found to be in the range of 1.14–3.08 eV for the investigated samples. � 2003 Elsevier Science Ltd. All rights reserved. 1. Introduction Further aggressive down-scaling of the metal-oxide- semiconductor (MOS) devices has pushed the gate dielectric thickness into its both technological and theoretical limits [1–3]. In searching the alternate gate dielectrics for MOS device applications, significant achievements in the material structures such as oxynit- ride and oxide/nitride stacks have been obtained [3–14]. It was found that the oxynitride materials can enhance the resistance to boron diffusion and better hot-carrier reliability [3,7]. Silicon nitride, prepared by chemical vapor deposition (CVD) and because of the inherent strain in the networks of the amorphous structure, is not of good device quality [5]. Oxynitride by NH3 nitrida- tion would introduce a large amount of traps because of the hydrogen incorporation [2,6,7]. Although N2O ni- tridation seems to have the advantage of low hydrogen content, the amount of nitrogen incorporation, in the range of 2–3 at.%, is still not large enough to improve the hardness for hot carrier irradiation [3,9]. In addition, the dielectric constant is very close to silicon dioxide and the physical thickness will be too thin to avoid di- rect tunneling leakage in future MOS devices [1]. It was found recently that the silicon oxynitride prepared by oxidation of silicon-rich silicon nitride (SRN) has sev- eral unique features. It has high nitrogen content and has extremely low hydrogen content and hence has higher dielectric constant and lower trap density [5]. This work reports a detailed study on the mechanism and the properties of silicon oxynitride prepared by oxidation of SRN. X-ray photoelectron spectroscopy (XPS) was used to study the physical structure and chemical compositions. Current–voltage (IV ) measure- ments were conducted to study the charge transport mechanism and the barrier between the dielectric and silicon substrate. Microelectronics Reliability 43 (2003) 611–616 www.elsevier.com/locate/microrel * Corresponding author. E-mail address:
[email protected] (H. Wong). 0026-2714/03/$ - see front matter � 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0026-2714(03)00031-3 2. Experiment The starting materials were p-type h100i silicon wa- fers. After the standard cleaning process, thin SRN layer was first deposited with low-pressure chemical vapor deposition (LPCVD) at temperature of 780 �C. In order to study the bulk structure and the bulk properties of the oxynitride film and to avoid the direct tunneling effects during current–voltage measurement, thicker films were used in the investigation. The thickness of the deposited SRN layer was about 13 nm. The as-deposited SRN was then oxidized at temperature of 850, 950, or 1050 �C for 1 h. A thick aluminum layer was then deposited onto the wafer and electrodes with diameter of 200 lm were patterned with lithography technique. To probe the chemical composition and bonding structures of these samples, XPS measurement was conducted using Phys- ical Electronics PHI 5600 XPS. The X-ray beam was obtained by exciting the monochromatic Ka line of an aluminum target. The energy was 1486.6 eV and the takeoff angle was 90�. The current–voltage characteris- tics were measured with a HP4155A parameter analyzer. All electrical measurements were carried out in a dark shielded chamber at room temperature. 3. Results and discussion Fig. 1 shows the XPS profile of the silicon nitride gate dielectrics. As depicted in Fig. 1(a), the sample without oxidation has silicon-to-nitride ratio of about 1 repre- senting the prepared film is Si-rich. After oxidation at 850 �C for 1 h, the surface nitrogen content reduced greatly and oxygen content increases from about 20–50 Fig. 1. Depth profiles of the LPCVD silicon-rich silicon nitride (a) and with 1 h thermal oxidation at 850 �C (b), 950 �C (c) and 1050 �C (d). 612 J. Chan et al. / Microelectronics Reliability 43 (2003) 611–616 at.%. A thin silicon dioxide layer appeared on the sur- face (see Fig. 1(b)). As shown in Fig. 1(c), oxidation at 950 �C would further increase the oxygen content to about 60 at.% and a noticeable thin silicon dioxide layer was found on the surface. No pronounced increase in the thickness of the dielectric film is found for samples that underwent 850 or 950 �C oxidation. After oxidizing at 1050 �C for 1 h, the Si-rich nitride layer was almost completely converted into SiO2. The surface oxygen concentration saturated at 60 at.%. About 7 at.% of nitrogen presents at the Si/dielectric interface (see Fig. 1(d)). This heavy oxidation also resulted in a significant increase in the oxide thickness to about 18.5 nm. The amount nitrogen at the interface is still larger than that of the oxide film prepared by N2O nitridation. By growing N2O oxide at 1000 �C, Green et al. [12] found that the nitrogen incorporation is about one monolayer (7� 1014/cm2) in Si(1 0 0). Assuming that the N is uni- formly distributed at a thin layer of 15 �AA-thick at the interface, the interface N concentration is about 6.6 at.%. In forming oxynitride, it was believed that converting from silicon nitride is more difficult than from silicon oxide in thermodynamic point of view. Hence, most of proposed oxynitride preparation methods involve con- verting silicon oxide into silicon oxynitride by nitrida- tion. However, the amount of incorporated nitrogen atom by nitridation was found to be very low ( SiOmN4�m (where m ¼ 0,1,2,3 and 4) [15]. It was found in CVD oxynitride that there is only one broadened peak in the XPS spectra for different chemical composition and can only be fitted with the RB model [15]. It was further suggested that the Si3N4and SiO2 phases could not be coexist in the bulk under equilibrium conditions and Si2N2O is the only thermodynamically stable form of oxynitride [3,15]. In RB model, since the chemical shift of Si 2p level for each of the five tetrahedral is proportional to the partial charge on the Si atom, the resultant Si 2p feature appears as a single broadened peak at a position in the range of 102–103 eV depending on the composition. In addition, since no spatial local fluctuations of dielectric permittivity exist in the ran- domly distributed oxynitride tetrahedral, MOS devices with oxynitride as the gate dielectric will not have any significant surface potential fluctuations [15]. This fea- ture is very important for nanometer-size devices. Fig. 4 shows the oxygen 1s peak of sample with 1-h oxidation at 1050 �C. In both surface and bulk regions, the peak is located at around 533 eV. When sputtered into the Si/dielectric interface, the O 1s peak shifts from 533 to 532 eV. The change in binding energy suggests that the oxygen at the interface is in the form of SiOxNy structure. Fig. 5 shows the nitrogen content at the in- terface for different samples. For samples without oxi- dation or with light oxidation (850 or 950 �C), N 1s peak is located at 398 eV indicating the interface is still in the form of Si–N bonding. For heavy oxidation, the inter- face is converted into oxynitride and the nitrogen peak shifts to 398.5 eV. The SRN consists of Si–N, Si–H, and Si–Si bonds and the chemical reactions for the oxidation of silicon- rich nitride could be very complicated. However, the key reactions should involve the following processes [5]: 2BSi–HþO2 ! BSi–O–SiBþH2 ð1Þ 2BSiNH2 þBSiAHþO2 ! BSiOHþBSi2NHþH2O ð2Þ BSiNH2 þH2O ! BSiOHþNH3 ð3Þ 2NSi3 þH2O ! BSiOHþBSi2NH ð4Þ BSi2NHþH2O ! BSi2OþNH3 ð5Þ Reactions (1)–(5) describe, on the atomic scale, the possible reactions that take place during the silicon nitride oxidation in terms of chemical bonds rear- rangement. For samples with light oxidation (e.g. 850 �C, 1 h), the oxidation mainly results in the consumption of excess silicon in SRN film (reactions (1) and (2)); namely, the presence of excess silicon atoms enables the conversion of the dielectric film into oxynitride film at low temperature. The conversion efficiency is very low for pure silicon nitride at low temperature. Heavy oxi- dation will result in the conversion of Si–N bonds to Si–O (reactions (3)–(5)) and the nitrogen together with hydrogen (in NH3) will diffuse out of the dielectric film. Note that the reactions will also involve the elimination of hydrogen-containing species and result in the removal of potential trap centers [2,5]. Fig. 6 shows the Fowler–Nordheim (FN) tunneling characteristics at the high field region for these samples. For non-oxidized sample and sample with light oxida- tion (850 �C, 1 h), the leakage current is large and the current magnitude of these samples are almost the same. The large leakage current is a consequence of Si-rich content which lowers the Si/dielectric interface barrier. As the oxidation proceeds, the excess Si atoms were consumed and the barrier height at the interface in- creases. The amount of defects created during the LPCVD process would also be reduced and the leakage current reduced remarkably. As shown in Fig. 6, no Fig. 4. Oxygen 1s XPS spectra at different depths for sample oxidized at 1050 �C for 1 h. Fig. 5. Nitrogen 1s XPS spectra of different samples at the Si/ dielectric interface. 614 J. Chan et al. / Microelectronics Reliability 43 (2003) 611–616 detectable FN current was found at electric fields lower than 7.7 MV/cm for a sample oxidized at 1050 �C for 1 h. At higher electric field (>6.5 MV/cm), for lightly oxidized samples, the leakage currents divert from the FN curves. This observation should be due to Poole– Frenkel (PF) emission. The PF emission was mainly due to the capture and emission process from the structural defects and shallow traps [16]. The FN characteristic can be expressed as [17,18] J ¼ AE2e�B=E ð6Þ where J is the current density in A/cm2, E is the oxide electric field in V/cm. The pre-exponent A and slope B are given by the following equations: A ¼ q 2 8p2�h/b ð7Þ B ¼ 4 ffiffiffiffiffiffiffiffiffiffiffi 2m q p /3=2b 3q�h ð8Þ where q is the electronic charge, /b is the barrier height in electron volts, �h is the reduced Planck�s constant and m is the effective mass of electron in the gate dielectric. By plotting lnðJ=E2Þ versus 1=E as in Fig. 6, and assuming that the effective mass of electron ðm Þ in the conduction band is half of the free electron mass, the barrier at the Si/dielectric can be extracted from the FN plot. Fig. 7 shows the change of interface barrier height extracted from the FN plot. The barrier is about 1.15 eV for sample without oxidation because of the presence of a lot of excess silicon atoms. The excess silicon atoms in the dielectric film act as trap centers which assist the electrons to transport in the Si-rich nitride layer [3]. The SRN film has a lower bandgap. To the first order ap- proximation, its bandgap is expected to vary linearly between 1.12 eV (silicon) and 5 eV (nitride). Thus the bandgap of SRN is narrow and the electron barrier at the SRN/Si interface is much smaller and a large FN current is recorded. As the oxidation proceeds, the barrier height increases. For sample oxidized at 850 �C for 1 h, the electron barrier is about 1.54 eV. This value is still smaller than that of the silicon nitride. For sample oxidized at 950 �C, the barrier height is about 2.17 eV. This value is larger than that of pure silicon nitride (2.0 eV) and smaller than the NH3-nitrided sample (2.36 eV) [16] as a result of the high nitrogen content of present sample. For sample oxidized for 1 h at 1050 �C, the barrier height is 3.08 eV. Heavily oxidized samples have a large barrier because the bandgap of the oxide is much larger than that of the nitride. The bandgap of oxidized nitride should be in the range of 5 eV (nitride) to 9 eV (oxide) and the barrier height should be in the range of 2.0–3.2 eV. The present value (3.08 eV) is within the same range of those parameters of N2O oxide [19] and thermal oxide [17]. For N2O oxide, it was found that the electron barrier at Si/dielectric extracted from the IV curve is 2:9� 0:1 eV [19]. The differences in these values could be due to the different nitrogen compositions and the value of the effective electron mass used for ex- tracting the barrier height. 4. Conclusion In this paper, thin oxynitride gate dielectric films were prepared by thermal oxidation of LPCVD silicon-rich silicon nitride at temperature ranging 850–1050 �C. XPS indicates that converting the as-deposited silicon nitride into oxynitride with different compositions or oxide is feasible and the process is governed by the oxidation Fig. 6. FN plot of the leakage current characteristics for dif- ferent samples. Fig. 7. Plot of electron barrier height at Si/dielectric interface for gate dielectric oxidized at different temperature. J. Chan et al. / Microelectronics Reliability 43 (2003) 611–616 615 temperature. Simliar to LPCVD oxynitride, the oxynit- ride film obtained by SRN oxidation is in a RB struc- ture. This kind of oxynitride structure will have several advantages. In addition to the high nitrogen content (high dielectric constant) and low hydrogen content (low trap density), the random-bonding oxynitride will not have dielectric permittivity and surface potential fluc- tuations and the silicon atoms in this kind of dielectric film will not be over-constrained; thus this structure has low defect density [5,20]. By measuring the leakage current, the electron barrier height (extracted from FN plot) at the dielectric/Si interface is found to in the range of 1.14–3.08 eV for these samples. Acknowledgements The work described in this paper was partially sup- ported by a UGC Competitive Earmarked Research Grant of Hong Kong (project no. HKUST6174/01E) and a grant from CityU (project no. 7001134). References [1] Iwai H, Ohmi S. Silicon integrated circuit technology from past to future. Microelectron Reliab 2002;42:465–91. [2] Wong H, Gritsenko VA. Defects in silicon oxynitride gate dielectric films. Microelectron Reliab 2002;42:597–605. 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