Accidental Reaction of Metal Alkyls (Organometallic compounds) with Moisture or Air Causing a Cylinder to Rupture

Metal Alkyls (Organometallic) are finding increasing use in the Silicon Semiconductor Fabrication Industries. The most commonly used are:

Trimethylgallium (TMG)

Trimethylaluminum (TMA)

Trimethylindium (TMI)

Diethylzinc (DEZ)

The III-V device manufacturers typically dispense these from a cylinder configured to function as a bubbler (A carrier gas is bubbled through the liquid/solid to be saturated with the vapor). To maintain a precise concentration the cylinders are immersed in a heated or chilled water bath (glycol). These are located within the tool (MOCVD) and are maintained under positive pressure. A leak if it were to occur would be outward causing a fire.

The silicon device manufacturers on the other hand dispense Trimethylaluminum by pulling a vacuum on the vapor space of the canister in the ALD process (Atomic Layer Deposition). Due to the larger volumes used, a small host container is located close to the tool in the cleanroom. This is constantly filled by bulk and supply containers located in the subfab, these are pressurized to fill the canister. Diethylzinc is used to grow a transparent conductive layer for a thin film photovoltaic cell. The use of Metal Alkyls is forecasted to increase dramatically for all uses.

Pyrophoricity (spontaneous ignition) and fire is probably the most well-known hazard characteristic of these key organometallic compounds. A fire is commonly encountered during accidental release of Metal Alkyls into air. Most current safety precautions for handling these materials are associated with the pyrophoricity.

Violent reaction with liquid water is also a well-known problem. The hydrolysis reaction may generate sufficient heat to trigger a self sustaining decomposition reaction.

Can the ingress of air or water in the ALD process system trigger a self sustaining decomposition reaction that ruptures the container? These Metal Alkyls are also thermally unstable. Heat can trigger a decomposition reaction. Imagine a spill that ignites around a container, water cannot be used to cool the container or put out the fire.

There have been fatalities plus numerous injuries

Known container ruptures

TMG container explosion after glycol is sucked in (date unknown)

TMI container explosion after air sucked in, Dec 11,1998 (2 injuries)

TMI container explosion after it was heated to decomposition temperature, 2012 (1 injury)

TMI container explosion from moisture from valve, Oct 9, 2013 (1 fatality, 1 injury)

TMA container explosion after air is sucked in, Jan 7, 2016 (4 injuries)

TMI explosion, cause unknown, Feb 16, 2017 (3 injuries)

TMI container explosion after air is sucked in, May 2018 (2 fatalities, 1 injury)

TMI container explosion, cause unknown, Sept 19, 2018 (1 fatality, 3 injuries)

A test program is to better understand what can happen with Trimethylaluminum in a typical ampoule (<1 liter) in a worst case incident has been proposed for 2019.

Open valve to suck in air.

Drip 5 cc of water into container.

If any of the tests do not trigger a decomposition reaction, heat container to decomposition temperature

The test program will

Video from 4 angles

High speed video of container rupture

Canister pressure measurement

Canister temperature measurement

Overpressure sensors in a x-y grid

Test Directors:

Prof. Jenq-Renn Chen, Department of Safety, Health and Environmental Engineering, National First University of Science and Technology, Kaohsiung, Taiwan. jrc**At_Symbol_Here**nfust.edu.tw

Eugene Ngai, Chemically Speaking LLC, Whitehouse Station, NJ 08889, eugene_ngai**At_Symbol_Here**comcast.net