(Digital Presentation) Effect of N2-Anneal Temperature on Silicon Nitride Film: (II) Fine Structures of ESR Spectrum and FTIR
Authors: Miyagawa Hayato , Kamigaki Yoshiaki , Kurita Hisatsugu , Nakamura Masataka 
Published: July 2022

SiN film has been one of essential materials for electronic devices for its high dielectric properties as well as both chemical and thermal stabilities. However, it is found that SiN film made by LPCVD(Low Pressure Chemical Vapor Deposition) in certain process conditions has the weakness of dielectric voltage durability, which leads to the charge breakdown(Qbd) in application uses. The occurrence frequency of this Qbd strongly depends on the temperature of N2 anneal which is processed after deposition[Ref.1]. However, the details of the breakdown mechanism have not been cleared. In this study, in order to find the key defect which mainly causes the Qbd, we perform ESR measurements for the SiN samples annealed at several temperatures of 700 – 1150 degree C, and also measure FTIR(Fourier Transform Infrared) spectrum for SiN samples, in order to estimate the N2 anneal effect on the change of crystal bonds.

SiN samples were made by LPCVD using a mixture gas of SiH2Cl2 and NH4 on the Si(0 0 1) substrate at the temperature of 700 degree C. Standard SiN layer thickness were designed to be 54 nm for checking the N2 anneal temperature dependency, and some other samples were grown to have thicker SiN layer of 100 nm and 150 nm for checking the SiN layer thickness dependency. After SiN deposition, the annealing process with N2 atmosphere has been done at 700 – 1150 degree C. We also prepared a non-annealed sample for comparison.

ESR spectrum of each SiN samples have been obtained using JEOL JES-RE2X spectrometer at room temperature with the microwave of 9.4 GHz, 0.4 mW. To get statistically sufficient signals, we accumulated approximately 300 times of field scan in the range of ± 7.5 mT at the center field of 335 – 337 mT. We performed fitting analyses for obtained spectrum using 14 peaks with differential shape of Gaussian for background, and Lorentzian for other signals. Fig.1 shows the ESR spectra after UV irradiation and the fitted curves for SiN sample annealed at 900 degree C. We found K center, which is from SiN layer, E’ center, which is from oxide layer between SiN layer and Si substrate, and Pb center, which is from upper a few atomic layers of Si substrate, and we also found the unknown signal, named M signal, having unignorable amount of peak at the g-value of 2.0037, which locates between Pb and E’ center. We expect this unknown signal to be attributed to the dangling bond of Si partially oxidized. We can infer that the breakdown starts with Pb center and M signal at 900C and increases with increasing temperature observed as E’ center.

We also measured ESR signal peaks for samples with different SiN layer thickness and found the intensities of Pb center and M signal changes irrelevant to SiN layer thickness while the K center increase linearly as the function of SiN thickness. This confirms us that the M signal comes from interface region just above and/or near Si substrate, not from the inside of SiN film.

We measured FTIR spectrum for SiN samples at room temperature by means of transmission mode using Fisher Scientific NICOLET8700. Each FTIR measurement has done after N2 purging for 1 – 2 hours avoiding air contamination. The close-up of Si-H stretching signal in FTIR spectrum is shown in Fig.2. With increasing N2 anneal temperature, Si-H intensity decreases, and at 1150 C, splits into two peaks, which means that partially oxidized back-bond state, H-Si(-O,H), disappeared and became the two states of H-Si(-O), and H-Si(-H). This change corresponds to the decrease of M signal intensity at high N2 anneal temperature observed in ESR measurements.

[Ref.1] “Effect of N2-Anneal Temperature on Silicon Nitride Film: (I) Time-Dependent Dielectric Breakdown and ESR Evaluations” (Consolidated presentation in this conference)

Figure 1