Paper PS+TF-ThA9
Understanding the Challenges in Atomic Layer Deposition of SiN_x through Identification of the Surface Reaction Mechanisms

Thursday, November 2, 2017, 5:00 pm, Room 23

The rapid shrinking of semiconductor devices has created a need for the low-temperature (≤400 °C) atomic layer deposition (ALD) of highly-conformal silicon nitride (SiN_x) and C‑containing SiN_x films. However, to date, the ALD of these films remains challenging. In this work, we report the surface reaction mechanisms during the ALD of SiN_x and C‑containing SiN_x for several ALD processes. Initially, our research focused on a baseline SiN_x ALD process that used alternating exposures of Si₂Cl₆ and NH₃ plasma. This process was subsequently modified by replacing the NH₃ plasma half-cycle with a CH₃NH₂ plasma to simultaneously incorporate both C and N. Finally, to overcome the limitations of SiN_x films deposited using H-containing plasmas, a three-step ALD process was developed that used Si₂Cl₆, CH₃NH₂, and N₂ plasma. The film composition, reactive surface sites, and adsorbed surface species were monitored using in situ attenuated total reflection Fourier transform infrared spectroscopy, which allowed us to elucidate the surface reaction mechanisms. In addition, in situ four-wavelength ellipsometry was used to obtain the growth per cycle (GPC). Ex situ analysis was used to obtain the conformality and elemental composition.

For the baseline Si₂Cl₆ and NH₃ plasma ALD process, our infrared spectra show that on a post‑NH₃-plasma-treated SiN_x growth surface, Si₂Cl₆ reacts with surface –NH₂ species to form –NH and –Si_xCl_2x-1 (x = 1, 2) surface species. In the subsequent NH₃ plasma step, the –Si_xCl_2x-1 surface species are removed and the –NH₂ surface species are restored, allowing for the continuation of the ALD process. Film growth during the Si₂Cl₆ and CH₃NH₂ plasma ALD process occurs via an almost identical reaction mechanism, with the exception that C is incorporated in the form of -N=C=N- species during the CH₃NH₂ plasma step. In the three-step ALD process, Si₂Cl₆ again reacts with surface –NH₂ species, while in the CH₃NH₂ step, the CH₃NH₂ reacts with –Si_xCl_2x-1 surface species via the formation of Si-N linkages to form Si₂N-CH₃ surface species. During the N₂ plasma step, the Si₂N‑CH₃ surface species are removed and the –NH₂ species are restored. When we compare the GPC and conformality (see Fig. 1) of the three-step ALD process to an aminosilane and N₂ plasma ALD process, we observe that the three-step ALD process has a higher conformality (~90%) and a higher GPC (~0.9 Å). However, these values are less than those reported for NH₃- or CH₃NH₂‑plasma-based ALD processes. This suggests that the three-step ALD process behaves as an intermediate between an NH₃- or CH₃NH₂‑plasma-based ALD process and an aminosilane and N₂ plasma ALD process.