Safety benefits of using a sub-atmospheric pressure hydride gas source for MOCVD

The reduced risks associated with storing toxic and flammable hydride gases on a high surface area substrate within a cylinder at sub-atmospheric pressures have been assessed. Tests have been performed on 2.2 and 49 L sub-atmospheric pressure cylinders filled to various pressures with arsine and phosphine to quantify the gas release rates that occur under conditions of simulated valve failure. Gas releases from sub-atmospheric pressure cylinders are diffusion rather than pressure-controlled and are found to be discrete rather than continuous. Average release concentrations measured at a ventilation air flow rate of 1.42 m 3/min are well below the permissible exposure limit for both hydride gases over the test period. The results are compared to calculated release rates from high-pressure arsine and phosphine cylinders fitted with a restrictive flow orifice under otherwise similar conditions. The findings show that gas release rates from high-pressure cylinders are approximately four orders of magnitude higher than those from sub-atmospheric pressure cylinders. In addition to lowering the safety risks, benefits of adsorbed phase gas storage include the possibility of having increased volumes of hydride gas on site, improved process repeatability and reduced installation and operating costs.