Hexamethyldisilazane: Uses, Reactions and Toxicity

Uses of Hexamethyldisilazane

Hexamethyldisilazane (HMDS) is a flammable organosilicon compound with the appearance of a colourless liquid and a relatively low minimum ignition energy (MIE) of<0.1 mJ. HMDS is used to treat the surface of silicon wafers in photolithography processes (silane coupling agent).HMDS reacts with metal and ceramic surfaces (silane coupling reaction), forming on the surface hydrophilic trimethylsilanol (TMSiOH). This makes it easier to apply photoresist to the surface of silicon wafers. HMDS is also used in gas chromatography mass spectrometry applications and in nanosilicon film research.

Reactions of Hexamethyldisilazane

HMDZ is stable in the absence of air, moisture, oxidizing agents, and acids. When HMDZ is exposed to moisture in the air, the HMDZ and water will react to form ammonia.

In addition, the main products of the reaction of HMDS with silica in the temperature range of 150-450 °C are hexamethyldisiloxane (HMDSO) and ammonia, as well as small amounts of nitrogen and methane. Methane is not produced at reaction temperatures below 300 °C. Ammonia is the first product detected, followed by HMDSO, and as the reaction temperature increases, the production of nitrogen increases and the production of ammonia and HMDSO decreases. The mechanism of silylation is that the reaction of HMDS with silica is continuous. As the reaction temperature increases, the reaction of the pendant silica groups with the acidic hydroxyl groups on the surface eliminates methane and the concentration of residual silica groups on the silica surface increases.

Toxicity of Hexamethyldisilazane

HMDS may cause toxic effects by inhalation or ingestion. Contact with the substance may cause severe burns to the skin and eyes. Due to flammability, highly toxic nitrogen oxide fumes may be released when heated to decomposition. Its vapour may cause dizziness or suffocation. HMDS can be considered slightly to moderately toxic following acute oral, dermal, and inhalation exposure. Studies of acute and short-term inhalation exposure to high doses have demonstrated respiratory tract irritation and CNS depression. Under occlusive conditions, dermal necrosis has been observed. Long term inhalation exposure is associated with neurotoxic effects of ataxia and decreased activity.

References:

[1] M. L. HAIR W. H. Reaction of hexamethyldisilazane with silica[J]. The Journal of Physical Chemistry , 1971. DOI:10.1021/j100683a020.

[2] WU L, WANG S, JI Y, et al. Comparative investigation on organosilicon film growth by cyclonic plasma using hexamethyldisilazane and hexamethyldisilazane/nitrogen gas mixture[J]. Surface and Interface Analysis, 2023. DOI:10.1002/sia.7263.