Solid particles in the abatement exhaust must be properly managed, and in some cases, substantially reduced from the gas stream before it is released into the environment.
BY CHRIS JONES, Edwards Vacuum, Ltd., Clevedon, U.K.
Many semiconductor manufacturing processes create solid particles in the process exhaust. Like other exhaust contaminants, these must be properly managed, and in many cases, removed from the stream before it is released into the environment. The permitted release levels vary for particles of different sizes and compositions, depending on their toxicity or potential to damage the environment. Regulations governing particle releases are evolving rapidly. However, the management of particulate flows in process exhaust is also important due to its potential impact on the process itself. Left unmanaged, particulate accumulations can result in shut downs for unplanned maintenance, excessive and premature wear and costly repairs, all of which directly affect the profitability of the manufacturing operation.
Solids may be formed in the exhaust stream of a semiconductor manufacturing process from a number of sources. One important source, though not the focus of this discussion, is the condensation of process gases in vacuum pump exhausts. If not controlled with a thermal management system (e.g. Smart TMS, Edwards) that maintains the pipe surfaces at a sufficiently high temperature, this condensation can quickly accumulate and force a halt to the manufacturing process. This article will discuss issues further downstream in the abatement process, where toxic volatile compounds are converted to more benign forms, some of which form solid particles that must then be removed from the exhaust gases. Many of these solids are oxides formed when gases, such as tungsten hexaflu- oride, silane, organo- and halo- silanes and others, are exposed to heat, air, and water. The particles are typically amorphous, i.e. non-crystalline. Many abatement processes use combustion to supply the heat needed to decompose toxic compounds and chemically convert them to a more harmless form. The particles thus formed have varying sizes and may be hydrophilic (formed from halosilanes), hydrophobic (formed from organsilanes) or mixed (mixed chlorides or silicon, aluminum and boron, for example), depending on the species combusted and the nature of the combustion process. Particle sizes can range from tens of nanometers to tens of microns. As shown in FIGURE 1, the size of the particles depends on, among other factors, the length of the combustion flame. Longer flames maintain the components at high temperature for a longer periods and result in the formation of larger particles.
FIGURE 1. A longer flame maintains the combusting components at higher temperature for a longer time and results in the formation of larger particles.
The behavior of particles once released into the environment varies depending on their sizes. Coarse particles, with diameters ranging from 2.5μm to 10μm, result largely from processes such as erosion, agriculture, or mining and include crustal dust, pollens, fungal spores, biological debris and sea salt. Because of their large size, these particles persist in the atmosphere for only a few hours or days. Fine particles, which range from 2.5μm to 0.1μm and include the particles of concern in semiconductor manufacturing exhaust, may be the direct result of a combustion processes or may also be formed by photochemical reactions between volatile organic compounds (VOC) and oxides in the presence of sunlight. Fine particles can stay suspended in ambient air for days to weeks. Ultrafine particles, less than 0.1μm, are generated by high temperature combustion or formed from the nucleation of atmospheric gases. Ultrafine particles are quickly removed from the atmosphere (minutes to hours) via diffusion to surfaces or coagulation, adsorption and condensing into fine particles.
Regulatory environment
Regulations governing the release of particles into the atmosphere are developing quickly worldwide as scientists expand their knowledge of the particles’ impacts on health and the environment. In addition to regulations governing emissions by particle size, there are specific regulations regarding especially harmful species, such as heavy metals, carcinogens and toxics. For example, the presence of an adsorbed species, like hydrofluoric acid (HF), on oxide particles increases the toxicity of the parent material.
In 2013 the United States Environmental Protection Agency specified an average daily limit of 150μg/m3 for coarse particles and 35μg/m3 for fine particles, and an average annual limit of 12μg/m3 for fine particles (down from 15μg/m3 in 2006). China, as of 2012, imposed limits based on both particle size and type, with permitted daily levels for coarse particles of 50μg/m3 and 150μg/m3 for type I and type II, respectively and 35μg/m3 and 75μg/m3, respectively for fine particles. China also limits annual averages for both sizes and types. The European Commission, the World Health Organization and the Australian National Environmental Council, among others, all specify their own limits. It is clearly incumbent on manufacturers to know and satisfy their local regulations. [1]
Health considerations
The health of employees in manufacturing facilities and people living near manufacturing operations is clearly a high priority for our industry. Epidemiological studies have provided plausible evidence that exposure to particulate material (PM) can impact health in a number of ways, including pulmonary and systemic inflammation, oxidative stress response, protein modification, stimulation of the autonomic nervous system, exaggerated allergic reactions, pro-coagulation activity, and suppression of immune response in the lungs.
Some studies have provided good news as well, specifically, that the amorphous silica particles produced during the abatement of gases used in semiconductor manufacturing have much less impact on lung function than the crystalline silica particles more often encountered in mining and building industries. These studies looked specifically at the effects of pure silica particles, an important caveat. Silica and other dusts that may have acids, such as HF, adsorbed on the particle surface constitute substantially greater health risks than the simple oxide. Other particulate oxides also represent serious health challenges. These include oxides of antimony, arsenic, barium, chromium, cobalt, nickel, phosphorus, tellurium and selenium.
Abatement performance
Just as condensed material deposited in the vacuum lines can shut down the production process, the accumulation of combustion-generated particulates can degrade the performance of the whole facility. In a typical point-of-use (POU) abatement system, after combustion the exhaust gases pass through a series of operations designed to remove particulates and other by-products. In the example shown in FIGURE 2 these include a water weir, quench tanks, a packed-bed scrubber and an atomized spray. Atomizing spray systems, in particular, have been shown to improve solids removal performance from 50 to 75 percent. Blockages can occur at the damper, in duct spurs leading from the abatement to the main duct, in the main duct, before or within the scrubber. In addition to blockages, failure to remove particulate at the primary abatement unit can also lead to environmental discharges and visible plumes at stacks. Any blockage will result in a process shutdown for system maintenance, lasting from a few hours to an entire day.
FIGURE 2. The accumulation of combustion generated particulates can degrade abatement system performance.
Mitigation options
A number of approaches exist for removing particulates downstream of the abatement system. One solution does not fit all and it is important to pick the one that best addresses the specific challenges. FIGURE 3 shows performance characteristics for various technologies. For example, highly toxic particles may require much higher removal rates than less harmful particles.
FIGURE 3. Performance characteristics for various particle removal technologies downstream of the abatement system. Courtesy: Waste-to-Energy Research and Technology Council (greyed out area not relevant to solids).
Edwards’ standard solution (FIGURE 4) for POU removal of fine particles is a wet electrostatic precipitator (WESP). A WESP uses electrostatic forces to remove particles. It requires power, water and pneumatics and can remove up to 95 percent of silica particles at flow rates of 1m3/ min, 85% at 2m3/min. WESP technology can be scaled to handle an entire facility. In one example, Edwards partnered in the installation of a large scale dual WESP integrated with a packed-bed wet scrubber and designed it to meet the specific challenges of arsenic abatement. The system ultimately demonstrated a 99 percent removal rate to meet the stringent requirements of the Chinese government for this highly toxic substance.
FIGURE 4. POU WESP uses electrostatic forces to remove particulates from the exhaust stream. It can remove up to 95 percent of silica particles at a flow rate of 1m3/min.
Alternative technologies that may be appropriate, but have not been evaluated for use in the management of waste gases from semiconductor manufacturing, are the Rotoclone family (from AAF International). POU units handle flow rates of 30m3/min, removing >97 percent of 1μm particles and >99.8 percent of 10μm particles. Duct-based Rotoclones with flow rates up to 1250m3/ min remove as much as 86 percent of 1μm particles and 99 percent of 10μm particles. Rotoclones require power, water, pneumatics and a drain.
More conventionally, a Venturi scrubber can be configured for various flow and removal rates. As a rule, smaller units controlling a low concentration waste stream will be much more expensive per unit of volumetric flow than larger units cleaning high pollutant-load flows. Venturi scrubbers can handle mists and flammable or explosive dusts. They have relatively low maintenance requirements, are simple in design and easy to install. Their collection efficiency can be varied. They can cool hot gases and neutralize corrosive gases. They are susceptible to corrosion and must be protected from freezing. Treated gases may require reheating to avoid a visible water plume. The collected particulate material may be contaminated and not recyclable, requiring expensive disposal of the waste sludge.
Filtration is another alternative for particle removal. It is normally restricted to the management of dry dusts at flow rates of 5 to 250m3/min. Removal rates higher than 99.9 percent are achievable. We have seen a limited number of large filter installations for the removal of hydrophobic silica solids at relative humidities as high as 80 percent. It is not clear how the presence of hydrophilic powder might impact the performance of these facilities.
In cases of highly toxic particles, high efficiency air particle (HEPA) filters can provide very high removal rates, higher than 99.999 percent. However, HEPA filters are appropriate only for very low contaminant concentrations. Edwards has been partnering with third-party suppliers regarding HEPA filtration for highly toxic dusts such as those generated during arsine management. These solutions are often used for highly toxic materials so they are often designed with bag-in-bag-out capability to eliminate potential exposure of maintenance personnel to the removed contaminants. Typically, these critical installations are also designed as dual systems with auto turnover to allow continuous operation of one system while the redundant system is serviced. HEPA technology can scale from POU to full facility.
Conclusion
All of these technologies are available now, but not all have been demonstrated in semicon- ductor manufacturing. Semiconductor manufacturers have long used POU WESPs and Venturi scrubbers and are very familiar with HEPA filtration systems, but primarily for particulate removal for air conditioning. Conventional filters are in operation on flat panel display exhausts (mainly on burner only dry abated CVD processes). Some of the technologies we have described, however, have not been proven in semiconductor applications, but are well developed and widely accepted in other industries. Rotoclone systems, for instance, are UL and CE certified, but have not been SEMI qualified. As semiconductor manufacturing processes continue to evolve, it will behove manufacturers to stay current on available technol- ogies and consider alternatives as performance and cost requirements dictate.
References
1. Review of the health impacts of emission sources, types and levels of particulate matter air pollution in ambient air in NSW; December, 2015; Produced for the NSW Environment Protection Authority and NSW Ministry of Health, Environmental Health Branch.
