Liquified and Low Vapor Pressure Gases
The semiconductor industry makes use of a wide variety of gases. While gases are normally distinguished by their safety concerns, i.e., corrosive, toxic, pyrophoric, etc., there are other considerations to keep in mind when handling gas distribution. One key detail for these gases is the cylinder delivery pressure and whether these are liquified gases or compressed gases. Liquified gases are transported with both gas and liquid stored in the cylinder. The liquid evaporates until the vapor pressure reaches equilibrium and this vapor is then used in various processes.
This evaporation process occurs the entire time liquid is in the cylinder. Because of this, the outlet pressure of the cylinder is essentially constant until all the liquid has evaporated and you are using the remaining vapor. This makes it difficult for gauges or transducers to monitor the remainder of the cylinder. To account for this, most liquified gases use a scale to monitor cylinder weight and track gas consumption rather than pressure.
The lower pressure also impacts the components needed on the gas panel. Excess flow devices are not required for gases with a cylinder pressure below 15 psi. In addition to that, regulators that normally regulate source pressure from thousands of psi to a delivery pressure less than a hundred may no longer be necessary. Alternatively, a sub-atmospheric regulator can be used and is the default recommendation for gases like BCl3, HF, SiCl4, SiH2Cl2, several halocarbons, and a few others. You will notice that all of those gases are liquified with low cylinder pressures.
To better control the delivery of these gases, heat tracing the gas panel and providing a heat jacket for the cylinder are common practices. This can increase flow rate as well as help avoid the gas liquifying out along the flow path. This is most common among areas creating pressure drops, like the regulator. Using larger diameter piping, such as 3/8 instead of ¼ inch also helps provide more volume available for flow to improve flow rates. Using heat trace has its issues if not done properly. Having a heater blanket on the gas cylinder and heat trace on the panel is good. However, if the gas line downstream of the gas pane is not heat traced, the gas can condense in the gas lines. This can also happen if the gas goes from a higher temperature to a lower temperature. BCl3 is one gas that is notorious for this.
It is important for the gas distribution lines to have a temperature profile that is constant or increasing as it moves towards the point of use. Gas lines can encounter heating issues as they pass through fire rated wall penetrations and/or floor penetrations since those sections are not likely to be heat traced. It is also important to be sure that gas lines and components inside of valve manifold boxes (VMB’s) are heat traced as well. The end user should coordinate heat trace requirements with the tool vendor. If the tool does not include heat trace, liquefied gases can condense inside of the tools and cause false gas flows through the tool’s mass flow controllers.
There are several liquefied gases and gases with low vapor pressure that are common in the semiconductor industry. With the proper oversight and provisions, these gases can be handled as safely and reliably as standard compressed gases. However, it is important to keep in mind that no matter what gas is being used, the delivery system must be tailored to the best practices for that specific gas.