Silicon nitride is a common material in the microelectronic industry, thanks to its dielectric, chemical and mechanical properties. However, when exposed to atmosphere, the silicon nitride layer tends to oxidize. This work presents several efficient ways to remove this native oxide layer while controlling the resulting surface chemistry. Achieving such a control may provide a reliable starting point for further surface functionalization by organic molecules.

The silicon nitride thin films are deposited by magnetron sputtering. The silicon nitride stoichiometry can be tuned by varying the gas fluxes (Ar, N₂) during deposition.  Three wet treatments are studied and optimized: hydrofluoric acid (HF), ammonium fluoride (NH₄F) and potassium hydroxide (KOH). The treatment efficiency is controlled by X-ray Photoelectron Spectroscopy measurements (XPS). The surface chemistry is investigated by combining Attenuated Total Reflection InfraRed (ATR-IR) and XPS measurements.

All of the three chemical treatments yield the removal of the native oxide, albeit at different rates. HF solution is an efficient and fast etching treatment of silicon nitride. On the resulting surface, fluorine is detected by XPS, suggesting the presence of Si-F bonds. No Si-H bonds are detected on the etched surface by ATR-IR, in agreement with the literature results.

NH₄F solution is as efficient as HF in etching the silicon nitride surface, although much slower. This also leads to Si-F bonds, but in a lesser amount than after a HF treatment. KOH can also etch the native oxide of the silicon nitride layer. In this case, the resulting surface contains more oxygen than with the two previous solutions, a likely fingerprint of the presence of surface Si-OH species. Interestingly, the etching rate of the bulk silicon nitride appears much reduced upon etching with KOH as compared to HF.

The formation of Si-H bonds can be forced thanks to a H₂plasma treatment performed after the wet native oxide stripping. As shown in the figure, a broad SiH stretching vibration builds up in the infrared spectra upon submitting the silicon nitride film to such a plasma treatment at room temperature. The detailed analysis of XPS signals of samples submitted to various treatment durations shows that the initial surface Si-F bonds are not broken by the plasma, but that the silicon nitride film is progressively etched during the treatment. The film roughens during its dry etching, accounting for the large increase of the SiH signal. 

An important parameter we also investigated is the silicon-nitrogen ratio. The chemistry of the etched surface is highly dependent on this ratio. As shown in the figure, with a silicon-rich silicon nitride layer, a HF treatment leads to the formation of some Si-H bonds on surface.