Paper TF-MoM10
In situ FTIR Study of the Surface Reactions during Plasma-assisted Atomic Layer Deposition of SiN_x from Silicon Amides

Monday, November 7, 2016, 11:20 am, Room 105A

Recently, atomic layer deposition (ALD) of silicon nitride (SiN_x) films has been increasingly researched for applications with stringent conformality and processing temperature (≤ 400°C) requirements, such as conformal spacer or etch stop dielectric material in 3-D transistors and air gap interconnect technologies. The necessity for a low-temperature ALD process has shifted focus toward plasma-assisted ALD, mainly using N₂ or NH₃ plasmas. While Cl-based Si precursors have been widely used in ALD of SiN_x films due to their high reactivity, these precursors also form undesirable corrosive byproducts. Silicon amide precursors can overcome these challenges while maintaining a sufficiently high reactivity for ALD.

In this contribution, the focus will be on the growth mechanism of SiN_x films during ALD using H₂Si(N(C₂H₅)₂)₂ (BDEAS) and N₂ or NH₃ plasma, at substrate temperatures between 200 – 300 °C. Specifically, we have employed in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy to study the film composition, surface reactions during each half-cycle, and the surface species involved in the growth process. From these measurements, we conclude that BDEAS adsorption occurs via a ligand-exchange reaction between one of the diethylamino ligands and surface H, liberating HN(C₂H₅)₂ into the gas phase as the main reaction by-product. During the N₂ plasma based ALD process, the N₂ plasma removes the remaining diethylamino ligands from the surface and restores the surface sites necessary for BDEAS chemisorption during the subsequent cycle. The hydrocarbon species on the surface during the N₂ plasma step also leads to the incorporation of C_xN_y species in the SiN_x film. In contrast to the N₂ plasma-based process, NH₃ plasmas in combination with very similar amide precursors have been reported to inhibit SiN_x growth. While our results ultimately confirm these findings, our infrared measurements show that SiN_x growth can initially be achieved with a NH₃ plasma, but attenuates rapidly after the first 5 cycles. The infrared data however suggests that the NH₃ plasma leads to complete removal of the carbon-containing species leading to C-free SiN_x films. Since the composition of SiN_x films deposited by ALD using amide precursors is affected by the nitrogen source in the plasma, a 3-step ALD process involving a NH₃ plasma (to remove C-containing species) followed by a N₂ plasma (to restore surface reactive sites) can potentially optimize the film composition and growth process.