Subfab Exhaust Management Evolves to Meet New Challenges
Andreas Neuber, John Dickinson, Dustin Ho, and Andrew Herbert
Subfab utilities and systems aren’t glamorous, and they usually aren’t considered as critical to a fab’s success in the same way production tools are. Yet significant economic and reputational penalties may apply if a fab’s emissions violate increasingly strict environmental standards and expectations—so failing to upgrade the subfab is a risk not worth taking.
Abatement of fluorinated greenhouse gases (F-GHGs) generated during semiconductor manufacturing has come a long way in recent years, and it continues to evolve. In 2009, Applied Materials introduced its first zero footprint pre-pump plasma technology for abatement of F-GHGs from dry etching chambers. The Applied Aeris-G product addresses the need for an effcient abatement capability for this application.
The idea is to convert perfluorocarbon compounds (PFCs) and other oxide/poly etch waste gases into water-soluble species, which then can be removed by the facility scrubber. Applied Aeris-G units focus on greenhouse gas reduction and treat PFCs/F-GHGs upstream of the pump, where there is no pump purge gas present to dilute the effluent.
The smaller volume of gas and the higher concentration of target compounds mean that destruction effciency is higher. Incineration by-products, such as volatile organic compounds (VOCs) and nitrogen oxides (NOx), is also reduced compared to post-pump burning and plasma systems.
In addition, treating the process effluent prior to the addition of pump purge gases results in significantly lower operating costs for abatement compared to traditional solutions. A typical Aeris-G unit uses just 1–3 kW of electrical energy per chamber to accomplish the abatement process, as compared to post-pump solutions that typically use 8–12 kW of equivalent energy, usually because they rely on natural gas to destroy the unwanted compounds via combustion.
Since the introduction of prepump plasma technology, more than 1,000 first-generation Aeris-G abatement units have been installed at fabs around the world, with a demonstrated mean-time-between-failure (MTBF) reliability of more than 100,000 hours.
The Applied Aeris product family has been continually adapted to meet the requirements of a growing number of etch processes and, more recently, deposition technologies. This often involves optimization of the chemical reactions that occur during abatement. In many instances O2 or H2 must be added to achieve the optimum conversion into less harmful chemicals and to avoid recombination. The Aeris-G uses water vapor to provide the needed O and H atoms when converting, for example, CxFy into HF and CO2; NF3 into N2 and HF; or SF6 into SO2 and HF (see figure 1).
Figure 1. The figure shows a chemical reaction that typically occurs during pre-pump plasma abatement of F-GHGs in the gaseous effluent from dry etch chambers. At left are the F-GHG molecules to be treated. In the center, a plasma is applied to dissociate them. At right they combine to form compounds that have less environmental impact.
The Applied Aeris product family has evolved in other ways as well (see figure 2). One development is a higher-power replacement plasma source to address increasing PFC gas flow rates and wider process windows.
Figure 2. The figure shows drawings of the Applied Materials Aeris abatement product family. At left is the Aeris-G greenhouse gas product, with an installed base of >1,000 units. Center, a higher-power replacement plasma source for use with wider process windows. At right is a design under evaluation for abatement of hazardous effluent from deposition chamber-types.
Kits are now available to synchronize Applied Aeris abatement operation with the actual needs of the process to further reduce operating costs. One example is the synchronization of abatement operation to process cooling water (PCW) needs at any given point in time (see figure 3).
PLASMA CLEANING OF DEPOSITION WASTE
|Figure 3. Photo of a PCW kit used to synchronize abatement operation to process cooling water needs.|
A tantalizing question in light of all of this progress is: Can pre-pump plasma systems be used to solve other types of subfab problems? Developmental work on an extension of the Applied Aeris family to address issues resulting from waste generated by deposition processes indicates the answer is yes.
First, a little background. A number of the processes used to build high-aspect-ratio structures, which are based on low thermal budget chemical reactions, generate by-products that can clog forelines, piping, pumps and abatement. The problem isn’t only the clogging itself: these by-products may be pyrophoric or toxic. Consequently, safety concerns arise for people who must interact with the system during maintenance.
While these issues are already familiar to semiconductor manufacturers running high-aspect-ratio process (HARP), CVD and some furnace processes, a look into the Factory Integration and Environmental Safety and Health sections of the ITRS Roadmap shows three major areas of specific concern. They are related to the growing use of:
- Energetic materials such as trimethylaluminum or similar metalorganic precursors
- Low thermal budget processes with low material conversion rates, such as ALD or CVD
- Alternative gate materials such as silicon-germanium or III/V and other compound semiconductors
Regarding these points, although safe material handling of energetic materials is currently addressed in new SEMI Standards activities, solutions for waste handling are needed as well.
Lower thermal budget processes, meanwhile, rely on chemical reactions to deposit atomic layers on the wafer. In most cases the reaction effciency is very low (e.g., in the single-digit percent range), which leads to extensive waste generation. This waste is often highly reactive and can easily lead to clogging or even to uncontrolled chemical reactions.
With regard to alternative gate materials, there are two important issues. Some of these materials are quite toxic and so it is desirable to filter them out early in the waste-treatment process by adsorbing them onto a sorbent. However, the possibility of clogging the foreline, vacuum pump and exhaust system remains a major concern. In addition, some alternative gate materials are valuable, meaning that any solution designed to manage them should also enable them to be recovered efficiently.
With all this in mind, and based on the performance and ruggedness of the Applied Aeris product family, Applied Materials set a goal to develop a high-flow capacity pre-pump plasma treatment technology to avoid or reduce clogging from deposition effluent, with or without the traditional methods of pipe heating and post-pump pipe purging.
The idea, as before, is to convert problematic materials into other materials; in this case, into compounds that can be transported more easily for abatement locally or in the central scrubber, and also to convert hazardous materials into less hazardous ones to reduce the risk to maintenance personnel.
SUCCESSFUL RESULTS WITH HARP PROCESS
A first solution for the HARP process has been studied and tested successfully. Users of this process are no doubt familiar with the foreline and gate valve clogging it can bring, as well as the need for more frequent pump maintenance.
The Applied Materials plasma-cleaning process enabled highly reactive F radicals to be recombined into F2 molecules, with an associated loss in reactivity. Then, a second use of the plasma dissociated the F2 molecules, allowing a reaction to take place with SiO2 deposits in the foreline and the pump. This reaction converted the waste stream into SiF4, which passed through the pump and was later removed in the local scrubber.
Thus, the pump was able to run longer before maintenance was required. Moreover, there was no need to add any reactive gas to the system to facilitate these transformations, although processes other than HARP may call for it.
Pre-pump plasma abatement technology continues to evolve in lockstep with increasingly stringent requirements for greener and more sustainable manufacturing. The next opportunity for this efficient, cost-effective, and space-saving solution will be for so-called “dirty” deposition chemistries.