Interface properties of SiOxNy layer on Si prepared by atmospheric-pressure plasma oxidation-nitridation
© Zhuo et al.; licensee Springer. 2013
Received: 17 January 2013
Accepted: 13 March 2013
Published: 1 May 2013
SiOxNy films with a low nitrogen concentration (< 4%) have been prepared on Si substrates at 400°C by atmospheric-pressure plasma oxidation-nitridation process using O2 and N2 as gaseous precursors diluted in He. Interface properties of SiOxNy films have been investigated by analyzing high-frequency and quasistatic capacitance-voltage characteristics of metal-oxide-semiconductor capacitors. It is found that addition of N into the oxide increases both interface state density (Dit) and positive fixed charge density (Qf). After forming gas anneal, Dit decreases largely with decreasing N2/O2 flow ratio from 1 to 0.01 while the change of Qf is insignificant. These results suggest that low N2/O2 flow ratio is a key parameter to achieve a low Dit and relatively high Qf, which is effective for field effect passivation of n-type Si surfaces.
Silicon oxynitride (SiO x N y ) is a very useful material for applications in microelectronic and optoelectronic devices due to the possibility of tailoring the film composition and property according to the O/N ratio. Recently, considerable attention has been focused on SiO x N y for anti-reflection coatings and surface passivation films for thin crystalline Si solar cells [1–3]. It has been reported that SiO x N y films with high positive fixed charge density (Qf) in the range of 1012 cm−2 is effective for field-effect passivation of n-type Si surfaces .
So far, several methods have been applied to grow SiO x N y films. For example, high-temperature (>900°C) processes such as the direct thermal oxynitridation of Si in NO or N2O ambient [4, 5] and the annealing of SiO2 in nitrogen-containing ambient [6, 7] have been widely used. However, the high-temperature processes suffer a large thermal budget and a redistribution problem of dopant atoms. Plasma-enhanced chemical vapor deposition (PECVD) process is a low-temperature alternative below 400°C [8–10]. However, the PECVD method needs toxic precursor gases, and it is also noted that the interfacial properties prepared by this method are usually inferior to those of thermal oxides , because the deposition method does not consume the substrate Si unlike thermal oxidation. Moreover, in the films prepared by low-temperature PECVD, the concentration of hydrogen atoms in the form of Si-OH and Si-H bonds is high, which are responsible for poor dielectric properties . Nitridation of silicon oxide in low-pressure nitrogen plasma has also been investigated to fabricate SiO x N y at low temperatures [13, 14]. In the case of low-pressure nitrogen plasma, the ion bombardment of the film surface is a serious problem to develop highly reliable ultra-large-scale integrated circuits . Recently, we have studied the plasma oxidation of Si wafers to grow SiO2 films using atmospheric-pressure (AP) plasma generated by a 150-MHz very-high-frequency (VHF) electric field and demonstrated that high-quality SiO2 films can be obtained using He/O2 or Ar/O2 plasma at 400°C [16, 17]. We have also reported that the AP VHF plasma oxidation process at 400°C is capable of producing material quality of SiO2 films comparable to those of high-temperature (>1,000°C) thermal oxides. The SiO2/Si structure with low interface state density (Dit) around the midgap of 1.4 × 1010 cm−2 eV−1 and moderately high Qf of 5.3 × 1011 cm−2 has been demonstrated . Therefore, addition of N into the SiO2 film by AP plasma oxidation-nitridation using O2 and N2 precursor gas mixture is an alternative approach for obtaining SiO x N y films at a low temperature of 400°C.
The purpose of this work is to present a method for preparing SiO x N y films by AP VHF plasma oxidation-nitridation with a detailed analysis of interface properties of SiO x N y layer by capacitance-voltage (C-V) measurements on metal-SiO x N y -Si capacitors.
Oxidation-nitridation conditions for Si wafer
O2 concentration (%)
He flow rate (slm)
O2 flow rate (sccm)
N2 flow rate (sccm)
1,10, and 100
VHF power (W)
1,000 to 1,500
Plasma gap (mm)
0.8 to 1
Substrate temperature (°C)
Oxidation-nitridation time (min)
9 to 25
The substrates used in the present experiments were n-type (001) CZ-Si wafers (4-in. diameter) with a resistivity of 1 to 10 Ω cm. They were cleaned by a room-temperature chemical cleaning method  and were finished by a diluted HF treatment. After AP plasma oxidation-nitridation, some of the samples were subjected to a forming gas anneal (FGA) in 10% H2/He for 30 min at 400°C. In order to investigate Qf and Dit of the SiO x N y film, Al/SiO x N y /Si metal-oxide-semiconductor (MOS) capacitors were fabricated with 0.5-mm-diameter Al pads by vacuum deposition. A back contacting electrode at the rear Si surface was also made by Al deposition.
The thickness of the SiO x N y layer was determined by ellipsometry (Rudolph Auto EL III) with a wavelength of 632.8 nm. The chemical bonding in the material was investigated by Fourier transform infrared absorption (FTIR) spectrometry (Shimadzu FTIR–8600PC) in the wave number range of 400 to 4,000 cm−1. X-ray photoelectron spectroscopy (XPS; ULVAC-PHI Quantum 2000) was used to investigate the depth profile of atomic composition and bonding of atoms in SiO x N y films. High-frequency (HF) and quasistatic (QS) C-V measurements were performed using a 1-MHz C meter/CV plotter (HP 4280A) and quasistatic CV meter (Keithley 595), respectively.
Results and discussion
In Figure 2, the strongest peak in IR spectra corresponds to Si-O-Si stretching mode, indicating that the film consists predominantly of SiO2. The dielectric constant of the film was calculated using the maximum accumulation capacitance obtained by C-V curves. The result showed that the dielectric constant was fairly uniform over the sample area with a variation of about 2% and that the average dielectric constants of the films were 4.26 and 4.01 for N2/O2 flow ratios of 0.01 and 1, respectively. Since the dielectric constants of SiO2 and Si3N4 are 3.9 and 7.5, respectively, nitrogen atoms are considered to be incorporated in the SiO2 structure.
SiO x N y films with a low nitrogen concentration (approximately 4%) have been prepared on n-type (001) Si wafers at 400°C for 9 min by oxidation-nitridation process in AP plasma using O2 and N2 diluted in He gas. Interface properties of SiO x N y films have been investigated by C-V measurements, and it is found that addition of N into the oxide increases both the values of Dit and Qf. After FGA, Dit at midgap decreases from 2.3 × 1012 to 6.1 × 1011 cm−2 eV−1 with decreasing N2/O2 flow ratio from 1 to 0.01, while the decrease of Qf is insignificant from 1.5 × 1012 to 1.2 × 1012 cm−2. These results suggest that a low N2/O2 flow ratio is a key parameter to achieve a low Dit and relatively high Qf, which is useful to realize an effective field-effect passivation of n-type Si surfaces.
This work was supported in part by Grants-in-Aid for Scientific Research (no. 21656039, no. 22246017, and Global COE Program (H08)) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors would like to thank A. Takeuchi of Osaka University for his technical assistance.
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