Avoiding fire and explosion risks with proper H2 exhaust management
02/01/2005
Hydrogen is used extensively in wafer fabrication processes for epitaxial deposition in silicon and compound semiconductors. The flammability of H2 is a primary safety issue in piping and exhaust gas management of hydrogen-laden processes. Management of H2 is complicated by the presence of epi process gases, which can present additional fire and environmental hazards. Special considerations for exhaust management are required to deal with hydrogen’s flammability and the risk of explosion at various levels of gas concentrations.
Epi deposition processes are increasingly used in the production of high-speed devices for silicon and compound semiconductor applications. In epi processes, hydrogen is used to scavenge O2 and to reduce oxide inclusions. Due to its low viscosity and high specific heat properties, H2 works well to minimize temperature differences across the wafer while allowing deposition gases ready access to the wafer surface [1].
For epi processes, hydrogen flows of 50 standard liters/min (slm) are common and can be as high as 300slm. Along with high flows of H2, epi processes also employ deposition gases (e.g., silane and germane) for silicon-germanium (SiGe) transistors, and dichlorosilane, arsine, phosphine, and various organometallics (e.g., trimethyl gallium or trimethyl indium) for other device applications. These combinations of gases present challenges to traditional methods of both reduced pressure pumping and exhaust abatement, such as water scrubbing.
The flammability of H2, along with the use of pyrophoric and highly toxic precursors in epi processes, requires special considerations for exhaust gas management. Due to hydrogen’s wide flammability range (4-76% in air or 4-95% in O2), a leak into a vacuum line or outward into the fab can produce a flammable or explosive mixture. Some chlorosilane-based epi processes deposit unstable solids in the exhaust pipework, which can serve as ignition sources. Combustion may also be initiated by discharge of static electricity in some water scrubbers, which are commonly used to abate organometallics, chlorosilanes, and hydrogen chloride (HCl) in many epi processes.
Some issues for proper exhaust-gas management of H2-laden processes include flammability of hydrogen and deposition gases; government regulations and industry safety guidelines; and H2-specific exhaust management requirements.
Understanding H2 flammability
The best strategy to prevent H2 combustion is to keep the hydrogen concentration below its 4% lower flammable limit (LFL). If the H2 concentration is above 4%, self-sustained combustion may occur at the source of a leak. If a leak occurs in a pipe at negative pressure, air could be drawn in and sustain combustion inside the pipeline, resulting in damage to clamps, o-rings, and even the piping itself. For this reason, whenever pipes are assembled or “disturbed” by alterations or maintenance, it must be a matter of policy to rigorously leak-check all pipework through which H2 may flow.
Hydrogen is flammable over a wide range of concentrations and conditions. Temperature, pressure, and the percentage of O2, N2, and other gases all affect such properties as LFL, upper flammable limit (UFL) and flame front velocity. The nature of the diluent gas itself (N2, O2, or Ar) will also affect H2 combustion (see Table 1 in “Properties of hydrogen and various gases,” p. 52). Adding CO2 or other diluent to a gas mixture may raise hydrogen’s LFL by a few percent, depending upon the quantity of each diluent (CO2, N2, and O2) [2]. Water vapor also has a small quenching effect. The UFL of H2 is 76% in air (Fig. 1), but flammability will range from 4% to 95% in pure O2 and 4% to 61% in 20:80 O2:CO2. The energy needed to ignite H2 falls from 0.3mJ in air to 0.003mJ in O2 [4].
 Figure 1. How H2 flammability changes [3]. |
The minimum concentration for combustion of H2 is 4% when the H2/air mixture is diluted with N2,, but it rises to 7.5% if CO2 is present in significant amounts (e.g., 57% of the total gas mixture by volume). Similarly, only 20% dilution with Halon 13B1 (CF3Br) is needed to make the hydrogen/air mixture inert [5]. As the propagation of an H2 flame in air proceeds through a series of free radical steps, these diluents may act as free radical scavengers to attenuate combustion. Also, the 4% LFL of H2 in air is defined for an upward-moving flame, but 10% H2 by volume is required for a downward-moving flame [6, 7]. Horizontal combustion requires a minimum H2 concentration of about 6% [8].
It is important to know that there are many parameters that define the flammability of H2. The only absolutes are that H2 will not ignite if hydrogen is <4% of the gas mixture by volume or if O2 is <5% - regardless of other diluents.
Flammability vs. explosive limits
The terms LFL and UFL often are incorrectly used interchangeably with LEL (lower explosive limit) and UEL (upper explosive limit). These terms are not synonymous. For example, H2 has an LEL-to-UEL range of 18-59% in air and 15-90% in O2 (see Table 1 above). Flammability (also referred to as deflagration) is characterized by a subsonic flame front, while an explosion or detonation is characterized by a supersonic wave front, often exceeding 1000m/sec.
“Detonation waves are shock waves which are sustained by the energy of the chemical reaction that is initiated by the shock compression”[3]. In other words, the shock wave itself initiates the reaction between the fuel and oxidizer, and it becomes self-sustaining while both fuel and oxidant are present in appropriate concentrations. A deflagration can turn into an explosion if:
- the fuel concentration is in the detonable range;
- the fuel/air plume is large enough to allow the deflagration front to accelerate beyond sonic speed; and
- either adequate turbulence or pressure-reflecting structures (such as walls, pipework, or other shock wave-reflecting obstructions) are present.
Flashback containment
It is important to note that flame front velocities depend on the concentrations of fuel and oxidizer, as well as the conditions under which the tests are run. The maximum flame front velocity of H2 in air is 0.265m/sec (see Table 2 above), based on 42% H2 with 21% O2 and laminar flow. Laminar flame front velocities of H2 vary from <0.1m/sec at low concentrations of both H2 and O2 in N2 to >0.9m/sec at 70% H2 in pure O2. The flame front velocity can increase dramatically due to turbulence, which may be induced by a variety of factors (obstructions, air cross-flow, a Reynolds number over 2000, pulsations from a vacuum pump, etc.). Turbulence may induce flashback in pipelines even if the forward gas velocity exceeds the laminar flame front velocity.
Of several approaches to flashback containment, the most obvious is to avoid flammable mixtures in the first place. Intrinsic fire safety for pyrophoric and flammable reactive gases, such as SiH4 or H2, relies upon double containment or sufficient inert gas dilution to below LFL. The challenge to the dilution approach increases with the gas flow. For example, SiH4 flowing at a rate of 2slm and a maximum concentration <1% in N2 requires 200slm N2, while H2 at 200slm and a maximum concentration <4% in N2 requires 5000slm (175cfm) N2. Some guidelines recommend dilution to 25% of LFL, which multiplies these flow requirements by 4.
The cost required to dilute process gases is directly related to the total gas flow from the process. Consequently, the dilution approach becomes progressively more costly as flows increase. Dilution of H2 using large volumes of inert gas may be too expensive, and this treatment strategy does not meet the spirit of the ITRS initiatives toward reduction in utilities consumption [9].
 Figure 2. Cutaway of Helios combustor head showing additional air for H2 combustion. |
In some cases, flashback arrestors may be used. A flashback arrestor is a device that confines the flame on a downstream surface for a period of time so that thermal detection can initiate shutdown procedures. A flashback arrestor may be an orifice, a wire mesh, or a container filled with metal or ceramic through which the process gases (and flashback flame) must pass. The flashback arrestor operates on the theory that the burning mixture flows through passages small enough that the obstructions absorb enough heat to prevent the deflagration from proceeding upstream (Fig. 2). The size of the passages - known as the “quenching gap” - is dependent on the fuel and the flame front velocity. For H2, this value is 0.6mm, and for CH4 and C3H8, it is 2.0mm. Unfortunately, most semiconductor manufacturing processes deposit solids in the exhaust pipework, which readily block small passages.
Combustor/scrubber combination
A strategy of limited dilution of process exhaust with N2 reduces the percentages of H2 and O2 concentrations in the event of a leak. The process exhaust can be diluted and passed through constrictions narrow enough to ensure that the forward velocity of the gas exceeds the flame front velocity. The “flame front” approach may be successful for laminar flows, but it is unreliable for turbulent flows. Through the use of limited dilution with N2, a flashback flame can be caused to “latch” onto a constricting orifice long enough that a temperature rise may be used to initiate a mitigation or shutdown sequence. Flashback arrestors may also be used in some cases.
However, management of H2 is not the only issue in dealing with exhaust gas from epi processes. Deposition gases generate solids when oxidized. Furthermore, many processes also use HCl, which can lead to corrosion, buildups in ductwork, or unstable solids that could act as ignition sources. The most comprehensive approach for exhaust gas management in epi processes applies combustion, followed by wet scrubbing. To address this issue, BOC Edwards has developed a combined combustor/scrubber device specifically for exhaust management of high-H2 processes [10].
The abatement device, called Helios, uses an inward-fired porous ceramic combustor followed by a three-stage wet scrubber to remove particulate and acid gases. A lean mixture of natural gas and air fuels the combustor. Process gases, including H2, flow into the combustor and are oxidized. After combustion, the hot gases enter a water quench section where a cross-spray pattern cools the combustor exhaust and entrains the solids. Water-soluble gases are removed in the packed tower.
 Figure 3. Cutaway of Helios combustor head showing additional air for H2 combustion. |
Because adequate O2 in the combustor is critical for abatement of H2, an intelligent interface to the process tool detects high H2 flows. When the H2 exceeds 100slm, the combustor increases airflow through a sleeve around the process inlet nozzles (Fig. 3). In addition to O2 for combustion, the air provides N2 that moderates the combustor temperature under high H2 loads.
Altering combustor settings based on hydrogen flows assures optimal H2 destruction removal efficiency while minimizing carbon monoxide from incomplete combustion of natural gas fuel. For example, above 100slm, H2 and natural gas compete for O2, leading to increased CO emissions. Conversely, if too much air is admitted at low H2 flows, the combustor operates below optimal temperature. The tool interface controls the two-step combustor settings to assure that device cleanly abates H2 from 0 to 200slm. In addition to H2, the system has been designed to efficiently abate all common process gases and by-products. Narrow inlets to the combustor increase the forward gas velocity for flashback suppression in the case of an air leak into the exhaust stream.
Summary
Hydrogen abatement must be approached with a detailed understanding of the flammability of H2. Rigorous leak checking is critical with any H2 process. Avoiding a flammable mixture (H2+O2) is the most important step to H2 exhaust gas management. Controlled combustion to ensure concentrations below the LFL is the only way to safely abate H2.
Acknowledgment
Helios is a registered trademark of BOC Edwards.
References
- S. Wolf, Microchip Manufacturing, Lattice Press, p. 308, 2004.
- I. Drell, F. Belles, “Survey of Hydrogen Combustion Properties,” National Advisory Committee on Aeronotics, Report 1383, 1958.
- B. Lewis, G. von Elbe, Combustion, Flames and Explosions of Gases, Academic Press, New York and London, p. 695, 1961.
- Kutchta, US Bureau of Mines Bulletin 680, 1985, as referenced in Gexcon’s Gas Combustion Handbook, Section 4.12, www.gexcon.com/index.php.
- Gexcon’s Gas Combustion Handbook, Section 4.12, www.gexcon.com/index.php.
- M. Berman, “A Critical Review of Recent Large-scale Experiments on Hydrogen/Air Detonations,” Nuclear Science and Engineering, Vol. 93, pp. 321-347, 1986.
- M.N. Swain, et al., “Gaseous Fuel Transport Line Leakage - Natural Gas Compared to Hydrogen,” in Alternative Fuels: Alcohols, Hydrogen, Natural Gas and Propane (SP-982), Proceedings of the 1993 Society of Automotive Engineers’ Future Transportation Tech. Conference, pp. 161-170, Aug. 9-12, 1993.
- H.F. Coward, G.W. Jones, US Bureau of Mines, Bulletin 279, 1939.
- Semiconductor Industry Association, “Factory Integration,” International Technology Roadmap for Semiconductors, 2003 Edition, p. 4.
- K. Takahashi, A. Seeley, P. Mawle, “Addressing the Challenges of LP-Epi Exhaust Management,” Solid State Technology, p. 73, May 2001.
Joe Van Gompel received his BS in chemistry from Carroll College, Waukesha, WI, and his PhD in organic chemistry from the U. of Illinois, Urbana-Champaign. He is a senior product specialist for exhaust management systems at BOC Edwards, 8201 E. Riverside Dr., Bldg. 4, Suite 125, Austin, TX 78744; ph 800/848-9800 or 512/491-6622 ext. 6111, e-mail [email protected].
Properties of hydrogen and various gases
As demonstrated in the article, it is important to understand and factor in the many parameters that define H2 flammability; controlled combustion ensuring concentrations below the lower flammable limit (LFL) will result in safely abated H2 (see Table 1).
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There are many “minimum flammable concentrations” of H2, depending on the other gases present. Two examples are cited here (see Table 2).
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References
- B. Lewis, G. von Elbe, Combustion, Flames and Explosion of Gases, Academic Press, New York and London, 1961.
- T. Nejat Veziroglu, www.iahe.org/Hydrogen_energy_system.htm, Table VI.
- Matheson Gas Data Book, Sixth Edition, 1980.