Burning Beats Scrubbing In LED Fabs
Mike Czerniak, product marketing manager for exhaust gas management at Edwards makes the case for combustion based abatement as a cheaper, more environmentally friendly alternative to wet scrubbing.
The volume of compound semiconductor processing has risen rapidly over the last decade, driven primarily by increasing demand for LEDs. These high-brightness light emiters that are made by a GaN MOCVD process are now widely used for backlighting flat screen televisions and computer monitors, and they are likely to see increasing adoption as an alternative to incandescent and fluorescent lighting. The benefits LEDs offer include longer operating life, a more natural light spectrum and reduced power consumption.
As with solar manufacturing, compound semiconductor manufacturing must address two key concerns to successfully supplant more conventional, established technologies. The first is to lower the price of manufacturing to make LEDs cost competitive with the alternatives. The second is to ensure that the LED manufacturing process has a minimal environmental impact and complies with the increasingly stringent global regulations concerning air emissions and waste water. While the extraction, safe handling and disposal of the gases used in the MOCVD process used to manufacture LEDs significantly adds to manufacturing cost of ownership, the choice of abatement technology can significantly reduce manufacturing costs in terms of energy use, water consumption and treatment. This article will compare the costs and efficiency of traditional water scrubbing abatement technology with combustion technology.
The GaN abatement challenge
The two most common gas by-products of the GaN MOCVD manufacturing process are ammonia (NH3) and hydrogen. Both of these gases are highly flammable, while ammonia is, in addition, toxic and has an objectionable odour. The emission of ammonia is strongly regulated globally, both to water and atmosphere. In the past, hydrogen emissions were not tightly regulated, since this gas was not seen as a pollutant. This is changing in many regions, mostly because of the increasing fire risk this poses. In addition, care must be taken in abating emissions from GaN MOCVD processes, to ensure that nitrogen oxide and nitrogen dioxide (i.e. NOx), both regulated global warming gases, are not created.
Wet scrubbing — the traditional approach
While wet scrubbing is probably still the abatement technology most commonly used with MOCVD processing, it is becoming less popular because of the hidden costs associated with it. In wet scrubbing, emission gases are typically bubbled through a tub of water. Unfortunately, this does not remove hydrogen, the most common MOCVD gas by-product, which has typically been vented into the atmosphere. While hydrogen is not considered a global warming gas, it is highly flammable, and there is always a slight, but potential danger that static electricity could ignite the hydrogen during the abatement process, causing an explosion and fire. In Asia, a formerly popular solution was to place an electrically-heated thermal oxidizer in series with a wet scrubber to incinerate the residual hydrogen. This hot-wet scrubber approach has become less acceptable in Korea, for example, due to new water emission regulations and the increased cost and system footprint.
While water scrubbing can eliminate ammonia, the second most common gas by product, there is a danger in those regions with hard water that ammonia will react with the salts that are present, producing solids that can build up in the abatement system, thereby requiring increased maintenance and raising system cost of ownership. Since hard water occurs anywhere that mountain run-off is a major source of water, this is an issue in most parts of the globe. It is not a problem, incidentally, that is solved by using hot-wet scrubber technology.
In addition, dissolving ammonia in water can result in the presence of nitrogen that can promote biological activity such as algae blooms, which deteriorate water supplies by reducing the oxygen content. As a result, the allowed discharge limits in many regions have been reduced. While there are a number of ways of treating this problem, each has its own drawback. By adjusting the acidity and temperature in a high flow of air, ammonia can be removed from the solution and turned back into a gas. It can then be treated in a number of different ways. It can be diluted with non polluting gases and discharged into the atmosphere. While this is cheap, it typically fails to meet new regional emission standards and pollutes the environment. One can use a catalytic converter, which breaks the ammonia down into nitrogen and hydrogen. While this approach works, it does result in a safety issue due to the pyrophoric nature of the hydrogen, and it is expensive in volume production environments. Finally, one can expose the waste water to an acid like ammonium sulfate, which does have the benefit of producing a byproduct that has value as a fertilizer, but this approach typically fails to meet the new nitrogen-in-water emissions regulations. Each of these approaches, however, increases the cost of abatement, and, in addition, may not meet emission requirements in all regions.
Combustion-based abatement technology addresses the abatement challenges involved in GaN MOCVD manufacturing by burning the ammonia and hydrogen in a controlled way. This approach eliminates the hydrogen safety issue and the maintenance problems caused by ammonium solids at a significantly lower operating cost than that of wet scrub technology. Both ammonia and hydrogen are flammable, so a combustion-based abatement system, such as Edwards’ Spectra G, uses the gases to be abated to fuel the reaction. Figure 1 shows these gases being burned in a combustion-based abatement system. The only outside energy source employed by a combustion-based abatement system operates a small pilot light similar to the one used in home heating systems, with the exhaust gases themselves providing the bulk of the fuel for combustion. As a result, combustion-based systems use considerably less energy than wet scrub systems, which helps lower operating costs and results in a smaller carbon footprint. In addition, heat generated during gas combustion can be recovered and used for facility water heating, which can save up to $20,000 a year in facility utility costs.
Figure 1: 160 slm NH3 + 320 slm H2 being combusted under low-NOx conditions
Spectra G combustion systems are cooled using the air flow generated by the house extraction system. This design feature ensures that combustion by-products are efficiently transported from the system to the factory central scrubber or dust filter. This air-cooled design eliminates many of the fixed and operating costs associated with wet scrub technology, including the cost of water, the capital and operating costs associated with the pumps required to pump the water, the energy required to run the pumps and the costs of water treatment. As a result, combustion-based abatement technology is extremely well-suited for regions with tight water-use regulations.
Compared to wet scrub systems, with their pumps and water treatment subsystems, combustion-based abatement systems are much simpler in design and have far fewer moving parts. As a result, maintenance requirements – as well as maintenance times and spares inventory – are significantly reduced.This produces a much lower operating cost per process tool per year. While the capital costs of wet scrub and combustionbased abatement systems are roughly equivalent, the operating cost per tool per year for a combustion-based system is approximately one-tenth that of a wet scrub system. In addition, combustion-based systems have a smaller footprint, thereby saving valuable fab real estate. A combustion-based abatement system may occupy as little as 1.8 x 1.2 x 0.75 m3, compared to a wet scrub system that may occupy 2-3 times this space.
When burning ammonia in a combustion-based system, there is always a danger of creating NOx emissions, which are strictly regulated in most regions. Careful management of the combustion process, to prevent oxidation of the gases being burned, can avoid the formation of these polluting emissions. If not, a NOx removal plant is required, which adds considerably to both equipment size and annual operating costs. In a nonoxidizing combustion system, however, destruction and removal efficiencies of greater than 99.5 percent can be achieved with resultant NOx levels below threshold limit volumes.
Edwards’ Spectra G systems (see Figure 2) are designed to provide reliable, high performance, low-cost abatement of hazardous exhaust gases from MOCVD processes that use large flows of hydrogen and ammonia. They offer up to six process inlets and a very large gas handling capacity. Typical gas loading for a system is 300 liters of hydrogen and 200 liters of ammonia per minute, which equates roughly to the exhaust output of two to three process tools.
Figure 2: A combustionbased abatement system, such as Edwards’ Spectra G, uses the gases to be abated to fuel the reaction
In addition to the benefits of combustion-based exhaust systems in terms of safety, lower CoO and reduced environmental concerns, combustion-based abatement technology has a well-established track record for reliability. Hundreds of these systems are deployed at a variety of companies in the flat panel and solar cell industries. The efficiency of its gas treatment process has been field-tested and meets the most stringent air emission regulations in Europe, the US and Asia. It is currently experiencing a high rate of adoption by leading LED manufacturers world wide. Edwards offers an additional technology for treating the significant ammonia flows present in the exhausts of nitride-based MOCVD processes by decomposing this gas into its constituent elements, nitrogen and hydrogen, using a heated dry cartridge technology. This technology, which comes in a modular system known as GaNcat, eliminates the possibility of contamination of the cartridge by the metalorganic gases that are also present, such as TMG, TMI and TMA. This is achieved without the need for external traps by including a special pre-cleaning stage in the first portion of the cartridge that results in the formation of stable inorganic solids. Each module treats up to 20 slm of ammonia and multiple modules can be combines together, making it suitable for R&D applications. This kind of low-cost abatement approach eliminates concern about NOx generation and uses neither water nor fuel, making it an ideal approach for regions with water usage restrictions. Nitrogen can be safely vented to the atmosphere, while hydrogen must be treated in another fashion in regions restricting its release to the environment. It is clear that LED technology must overcome two major challenges to successfully supplant conventional established mass lighting technologies — lower the cost per lumen, and ensure that the GaN MOCVD processes used to produce them are not seen as major pollution sources. These manufacturing processes use large flows of hydrogen and ammonia which must be abated for both environmental and safety reasons, since they are both flammable and ammonia is toxic. In addition, it is critical that the formation of NOx be avoided during the abatement process.
While wet scrubbing has traditionally been the abatement technology of choice for MOCVD processes, it is more costly than combustion-based abatement technology in terms of energy consumption, water usage, and it does not treat hydrogen. In regions where hydrogen is regulated and cannot simply be vented to the atmosphere, an electrically heated thermal oxidizer must be used in series with a wet scrubber to address this issue, further increasing the cost of this technology. In addition, there is the danger of the build-up of ammonium solids in the abatement system, which requires increased maintenance, further increasing abatement costs. A further concern involves free nitrogen in the waste water, which can lead to algae blooms, thereby lowering water quality.
Combustion-based abatement systems offer a lower cost and more efficient solution. Since they use no water and use the emission gases themselves as a fuel source, operating costs are much lower than wet scrub systems in terms of operating costs. Combustion-based systems are also simpler in design, requiring less maintenance and occupying less fab real estate, which further contributes to a low cost of ownership. In addition, non-oxidizing combustion-based technology, such as that employed in Edwards’ Spectra G system, eliminates the danger of the formation of NOx, which is tightly regulated in most regions of the world.
Table 1. A comparison of the benefits of different abatement technologies
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