Improving safety in dopant gas delivery systems

Dopant gases used in ion implantation are poisonous and flammable (hydrides) and toxic and corrosive (fluorides). In the last 20 years, Safe Gas Delivery Systems were developed to reduce the risks associated with handling and using dopant sources in ion implanters. Two types of technologies are currently in use to achieve these safety and environmental improvements: nano-porous materials to adsorb the dopant gases (SAGS1) and the use of mechanical devices to lower the delivery pressure from high-pressure vessels (SAGS2). Both technologies provide substantial safety advancements compared to conventional high-pressure cylinders albeit SAGS1 systems are recognized as intrinsically safe as the dopant gases are stored at sub-atmospheric pressures. This feature is a major safety advantage as an accidental valve opening of a high-pressure vessel would result in a rapid and catastrophic gas release that could harm humans, the environment, or lead to a costly and disruptive fab evacuation. To date, two SAGS1 products are technically differentiated by the type of adsorbent being used: activated carbon (beaded or monolithic shaped) or metal organic frameworks (MOFs), a new class of high-surface area materials used in ION-X (R) products. One area of safety concern is the impact of accidentally opening or forgetting to close a SAGS1 cylinder valve after use: As opposed to high pressure gas releases, this event results in air rushing into the cylinder reacting with the stored dopant gas and adsorbent. In this paper, the effects of opening full (650 Torr) and nearly empty (50 Torr) ION-X (R) cylinders to air will be reported. The results are compared to published arsine release tests using carbon-based products where full cylinder release tests resulted in measurable quantities of arsine leaking out moments after opening the cylinders. Using similar conditions and timed events, the release test experiments using full and nearly empty ION-X (R) cylinders resulted in non-measurable levels of arsine released from the cylinder. In addition to the experimental details and results, this article rationalizes the different outcomes based on the fundamental physical and chemical differences between carbon and MOF adsorbent.