By Pete Singer, Editor-in-Chief
“Let no element on the periodic table go un-used!” That may well be the rallying cry of the semiconductor industry moving forward. One problem is that many of the new materials being considered are flammable, corrosive, toxic, pyrophoric, carcinogenic and/or hazardous in general.
“We’re all familiar with silane and hydrogen as flammable materials, but there are many other materials finding their way into manufacturing,” said Andrew Chambers, a Technical Manager at Edwards Ltd. “Disilane springs to mind, which is extremely hazardous and flammable.”
In various manufacturing process such as CVD and etch, materials are introduced into the tool as gases, liquids and solids. They react with each other and what’s on the wafer, and byproducts are pumped out of the chamber and into exhaust pipes. They are often diluted and treated by a gas abatement system, and ultimately vented to the atmosphere.
One of the biggest concerns in the fab is with flammable gas and the catastrophic dam- age that a fire could cause in the fab or sub-fab to equipment and, of course, to personnel. “If you have an exhaust pipe with flammable materials in it, it’s routine to dilute them with nitrogen to keep the concentrations below their lower flammable limit,” explained Chambers. “So even if there’s an escape to the environment, it can’t be ignited.”
A pending problem is that as process gas flows increase, more flammable materials such as hydrogen, silane and disilane are being used. “The amount of nitrogen you need to put into the exhaust pipe to dilute it to a safe level becomes extremely large,” Chambers said. “That has a number of consequences. The consumption rate increases significantly. While nitrogen in itself is not that expensive of a commodity, as you start to approach the nitrogen generation limit of your plant, it suddenly gets a lot more expensive if you have to get into extending the nitrogen supply infrastructure in your fab.”
In addition, the point of use abatement system on the end of your exhaust pipe has to be correspondingly large to deal with the very large volume of nitrogen that goes into the front of it. “The abatement of process gases in high dilution flows is very inefficient. The consequence of that is not only are you using a lot of nitrogen, but you’re having to buy very much more gas abatement capacity than you need. With that goes the additional expense of installing the equipment, providing it with water, natural gas, electrical power and so on,” Chambers said.
Edwards is presently exploring alternatives to using a gas dilution strategy to ensure safety. “We have imaginative ways of keeping the operation of the exhaust pipe safe, while reducing the cost of doing that. This includes treating the gas pipe as part of an entire inte- grated sub-fab system that is comprised of the vacuum pumps, the exhaust pipe, the end-of-pipe abatement system and the support infrastructure that goes with that. We would put in place measures so that the flammable gas is not diluted to the same level that they are accustomed to, but the safety of that gas is assured by doing a number of things. It might be that the concentration in the exhaust pipe is higher than its lower flammable limit, but that in itself is not a problem providing you keep air or oxygen or other oxidants out of the pipe at the same time,” Chambers explained.
Chambers said those kinds of ideas are starting to get some traction, because it allows the end user to regulate nitrogen consumption, reduce the amount of abatement capacity needed and generally provide a lower cost of ownership.
Clearly, properly assembled exhaust pipes with conventional joint seals will exclude air from exhausts containing flammable gases, but the operational risk is assurance of exhaust pipe integrity through years of continuous operation and numerous invasive service interventions. He said that a unified process exhaust design, with a clearly identified owner responsible for safe operation and servicing and appropriate integrated safety features, provides assurance of exhaust system integrity during prolonged operation, including routine servicing.
The same argument applies to process gases which may not be flammable but which are corrosive or toxic and could easily condense into the exhaust pipe during normal process operation. Ammonium chloride (NH4Cl), for example, is a common concern during metal etch. Ammonium hexafluorosilicate ((NH4)2 SiF6) is a nasty byproduct of nitride CVD.
“Over a period of time, they’re going to block the exhaust. Once the exhaust become blocked, you’re into areas where you need to take the tool out of manufacturing, pull the ex- haust pipe to pieces, clean it all out, leak check it and get it back into service again,” Chambers said. “There’s strong motivation for end users to provide ways of avoiding condensation in exhaust pipes.”
Interestingly, Chambers said what is perceived as a common solution to the problem — heater jackets — is not effective. “What we’ve found through a lot of experience is that, in many instances, the heater jackets and the heating systems for exhaust pipes are really badly applied. Heating the exhaust pipe and its maintenance at the required temperature is patchy at best and completely ineffective at worst.”
The best way to address all of this, according to Chambers, is to consider the sub-fab system as a complete integrated package. “We’ve been selling integrated systems comprising pumps and abatement systems for many years. If you join that whole thing together as an integrated system, it enables you to get data out of your system that can be used to provide diagnostic routines. It’s going to tell you about impending problems with your vacuum pumps, abatement system and exhaust pipe,” he said.