The danger that lurks below

By Pete Singer, Editor-in-Chief

The semiconductor industry is moving quickly to adopt a variety of new materials in an effort to increase chip performance. These new materials can create a host of safety concerns that must be addressed. Many of the new process chemicals have low vapor pressures, are highly reactive and present serious hazards to personnel and equipment. Many new CVD precursors and their associated reaction by-products are flammable, pyrophoric, toxic, corrosive or otherwise hazardous to personnel or destructive to equipment. “The problem’s always been there. It’s just becoming more acute as new processes emerge,” said Andrew Chambers, Senior Product Manager at Edwards Ltd., Clevedon, UK.

The Danger That Lurks Figure

Process byproducts are pumped through exhaust lines to a gas abatement system. Residual precursor materials or reaction byproducts often have a tendency to condense in pipe-work, including process exhausts. These exhaust pipes must be cleaned regularly, since condensed material will block the pipe, reduce its conductance and cause process problems.

Epitaxial silicon (“epi”) deposition processes, for example, are particularly notorious for the process decomposition products condensing in exhaust pipes or in the foreline of the pump. The hazard is greatest when the exhaust system is dismantled for cleaning. “The condensed material can react violently when it’s exposed to air and will burn vigorously or even explode. That presents a pretty serious hazard to the service engineers who are charged with taking the pipe apart and cleaning it,” Chambers said.

These problems can largely be avoided, however, by keeping the exhaust pipe or the pump foreline at a high temperature to avoid condensation of the material. If the surfaces in the exhaust system are warm enough, the processed products transit through the exhaust pipe and into the abatement system, where they can be combusted and dealt with in a safe fashion.

In many fabs, the heating is done with heating tape, but that’s not always the best (or safest) way to go. “In principle, that works up to a point but it’s quite difficult to apply that kind of technique when you’ve got accessories like a large ball valve in the line, where there are brackets attaching the exhaust pipe to the wall or there’s a system for injecting nitrogen dilution gas into the exhaust. The idea of heater tapes is convenient but not a very effective fix,” Chambers said. “Furthermore, removing heater tape to dismantle and clean the exhaust pipe can be inconvenient and time consuming” he added.

What’s really required is an approach that involves heating the pipe in a uniform fashion so that the pipe is universally at a high temperature to avoid the condensation. “You can’t afford to have cold spots in the pipe where there’s no heater or there’s no insulation because the moment you have a cold spot in the pipe, then material’s going to condense there and cause a local blockage,” he said.

Edwards offers a new Temperature Management System (SMART TMS) that ensures these compounds remain volatile until they enter the abatement device. SMART TMS is designed to heat both forelines and pump exhaust lines uniformly as far as the inlet of the abatement device. Molded high surface area heaters maximize contact with pipes and are designed to maintain them at a constant temperature between ambient and 180°C, recognizing that when choosing the temperature set point, knowledge of what process materials and byproducts are going to be in the exhaust pipe is invaluable.

Chambers said this approach is also superior to other heating methods using custom heater mats with integral insulation. “The difficulty you encounter with those kinds of systems is that the heater mat and jacket tend to be custom-designed to suit the particular installation. You spend a lot of time designing stuff, placing orders and waiting for it to be manufactured. Once it is manufactured and installed, there’s no flexibility. If you change the configuration of the exhaust pipe, you’ve got to go buy a whole set of new pipeline heating components,” he said.

With the Edwards SMART TMS approach, heater mats are provided separately from the insulation. “The heater mats are provided in standard lengths and as shaped components too, for elbows, valves, T-pieces and so on. You basically assemble the heater mats of sufficient length to heat your pipe from one end to the other. Then, since they’re made from low-particulate material, the insulation jackets can be cut to shape on assembly to fit the exhaust pipe. They’re all basically reusable,” Chambers said.

Success in thermal management goes well beyond mechanical considerations, however. “A lot of the skill and judgment in temperature management of exhaust pipes is knowing what factors you need to take care of to get decent temperature control throughout the system,” Chambers said.

When handling flammable gases, for example, nitrogen is often used to dilute them below their lower flammable limit to make them safe. “Typically, you pour a whole lot of nitrogen dilution gas into your exhaust pipe. The way in which you do that has a very significant impact on the temperature of the gas and the temperature of the exhaust pipe,” Chambers said. “If you’ve heated your exhaust pipe up to a temperature based on the process gases flowing through it and then you flow into it a couple of hundred liters per minute of cold nitrogen, then your heating system is no longer going to be fully effective. You start to run into the kind of condensation problem you were trying to avoid in the first place.”

The Edwards solution to that problem is to employ a system to heat up the nitrogen dilution gas. “Providing a nitrogen heater system as an accompaniment to a temperature management system for the exhaust pipe is sometimes a desirable thing to do,” Chambers added.

SMART TMS includes a sophisticated control system. “In our system, we have a controller which takes care of exhaust pipes on a zone by zone basis. The controller can control nine zones. All of those nine zones may be nine separate pipes. It may be one long pipe with nine zones in it over a long distance,” Chambers said.

The controller has useful operational features such as the ability to set and log different control temperatures and user-defined limits in each zone. If a temperature strays outside the user-defined band, an alert is transmitted from the controller to the process tool, the fab central monitoring system or other fault reporting system, depending on the nature of the fault. Furthermore, recognizing that some processes can cause very hazardous byproduct build-up in cold exhausts, SMART TMS includes a “fail-on” function to ensure that in the event of a component failure or loss of temperature indication a high pipeline temperature is maintained until servicing can be scheduled. In these cases the integrated health-check function provides an alert, while dual safety devices in each heater provide intrinsic safety and protection against thermal runaway.

“In the future, we can imagine the process which is running in the tool can be used to inform the set-up of the sub-fab equipment, the dry pumps or even the temperature management system,” Chambers said. “We’ve come a long way from relatively simple electrical heaters installed on an ad hoc basis to a sophisticated combination of process knowledge, a wide range of heater mats and shaped heaters, very efficient insulation materials and intelligent controllers with data acquisition capability.”


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