Reduction of Gate Leakage Current of Ultra thin Silicon Oxynitride via RTP-Induced Phonon-Energy-Coupling Enhancement

Abstract

Aggressive scaling of ultra-thin SiO2 and SiOXNy gate dielectric has caused a number of problems, such as high gate leakage current and reliability degradation. Recently, we reported phonon-energy-coupling enhancement (PECE) effect, i.e. the Si-O bonds in SiO2 are strengthened due to energy coupling from Si-O rocking mode to the Si-Si TO mode using rapid thermal processing (RTP) directly on the oxide [1,2]. The energy coupling is further enhanced after the oxide is annealed in deuterium. The gate leakage current was reduced by five orders of magnitude, which is equivalent to that of HfO2 and HfSiON reported elsewhere [3,4]. Fig. 1 shows the FTIR spectra of the vibration modes of the SiO2/Si sample, where the Si-O TO rocking mode are enhanced significantly upon RTP anneal in N2 for a short period of time. The enhancement of the Si-O bonds can be visualized from the schematic illustration shown in Fig. 2. However, the current mainstream gate dielectric used in industry is SiOxNy. Thus, it is very important to study the PECE effect in SiOXNy, because Si-O and Si-Si bonds also exist in SiOXNy. In this work, we fabricated ultra-thin SiOxNy MOS capacitors that exhibit 4 orders magnitude reduction of tunneling leakage current after using RTP induced PECE effect. MOS capacitors were fabricated on 2-inch p-type (100)-oriented wafers (1-10 Q-cm) via process steps shown in Table 1. XPS measurements were performed on one of the control samples to identify the chemical bonding in the as-grown oxynitride film. Fig. 3 shows the deconvolution of N Is spectra using Gaussian curve fitting with four peaks observed at binding energies 397.9, 400.6, 401.9, and 403.6 eV corresponding to NSi3, NSi2O, NSiO2, and NO3 bondings respectively [5]. Fig. 4 shows the 0 Is spectrum with a distinct peak at 531.6 eV. Fig. 5 shows the deconvolution of the Si 2p spectra with peaks of Si, Si-N, and Si-O, at binding energies 99.4, 102.85, and 103.95 eV respectively. From these XPS spectra, nitrogen concentration was calculated to be approximately 4.7 00 atomic composition. We can then approximate the dielectric constant to be 4.5 [6] and apply it to the ellipsometry measurements (M-44 spectroscopic ellipsometer) to obtain the Equivalent Oxide Thickness (EOT). The EOT of the oxynitride film is approximately 21 A as determined by the above method. It should be noted that the thickness measurements on all the samples before and after RTP treatment showed no thickness variations. High-frequency CV measurements at 1 MHz for the pMOS capacitors are shown in Fig. 6. Flat band voltages of -0.9 V, equivalent to the work function difference between the Al gate and the p-Si, were extracted from the CV measurements for the control and RTP-treated samples. This indicates that there is no significant amount of fixed charges in these samples. The oxynitride film thickness ranging from 20.8 to 21.5 A was extracted from the strong accumulation capacitance of the CV measurements using dielectric constant of 4.5. It is consistent with the ellipsometer measurements. Fig. 7 shows direct tunneling current density, JG, versus gate voltage, VG, performed on the MOS capacitors. Leakage current reduction of three to four orders of magnitude was observed from the MOS capacitors that underwent RTP processing and deuterium anneal. This is consistent with our previous results of SiO2 [2]. We suggest that the dramatic reduction of direct tunneling current might be due to the change of the energy band structure and the electron effective mass in the oxide/oxynitride, which might be caused by compress stress and/or enhanced phonon-energy coupling. Fig. 8 shows the projected leakage current density of the RTP-treated oxynitride as a function of EOT, in comparison with the reported values of SiO2 [3], HfO2 [3] and HfSiON [4] respectively. The leakage current density of the RTP-treated and deuterium annealed oxynitride film is almost comparable with that of HfO2 and HfSiON dielectrics and is three orders of magnitude lower than that of the conventional silicon oxide. It should be noted that our SiOxNy can be scaled down to 0.5 nm By employing RTP-induced PECE effect, significant reduction in leakage current (3-4 orders) was observed in ultrathin silicon oxynitride films, indicating a more robust oxynitride film comparable to the HfSiON dielectric. This technique when incorporated into the conventional CMOS processes may have a profound impact on further scaling and reliability improvement of gate oxide/oxynitride. This research is supported by National Science Foundation (ECS-0093156 and EPS0447479) and the Office of Vice President for Research, University of Kentucky.