Environmental and health risk analysis of nitrogen trifluoride (NF3), a toxic and potent greenhouse gas

https://doi.org/10.1016/j.jhazmat.2008.02.023Get rights and content

Abstract

This article aimed at the introduction of nitrogen trifluoride (NF3) and its decomposition products into its hazards to the environment and health because this perfluorocompound is a toxic and potent greenhouse gas not blanketed into the Kyoto Protocol. This paper also predicted the global NF3 emissions from the electronics industry on the basis of the methodologies recommended by the Intergovernmental Panel on Climate Change (IPCC), and further discussed its atmospheric implications according to the estimation of environmental fate for NF3. It showed that the vaporization of NF3 from the water bodies to the atmosphere is very high according to its predicted value (ca. 6.0 × 105 MPa) of Henry's law constant (KH). Furthermore, NF3 emitted from the electronics industry around the world in 2006 was estimated to be between 3.6 and 56 metric tonnes and it will be on increasing trend in the near future. Although the accumulative amount of NF3 in the atmosphere currently should be very negligible based on the predicted ratio (the order of 10−6 to 10−7) of equivalent CO2 emission from NF3 to total equivalent CO2 emissions from potent greenhouse gases, it is necessary to adopt the available abatement and also monitor the concentration of NF3 in the workplaces for reducing the overall environmental and health impacts of various semiconductor processes.

Introduction

Prior to the middle of 1990s, perfluorocompounds such as tetrafluoromethane (CF4), hexafluoroethane (C2F6) and sulfur hexafluoride (SF6) are commonly used in the processes of etching silicon materials and cleaning plasma enhanced chemical vapor deposition (PECVD) chamber [1], [2]. In order to reach the goals of reducing these gases by the Kyoto Protocol, several specialty gases have been used as replacements for perfluorocompounds. Because of its high etching rate, higher effectiveness, relatively chemical stability and excellent electrical characteristics [3], [4], nitrogen trifluoride (NF3) has been enormously used as a fluorine source in the electronics industry [5]. Therefore, its usage has been more and more common over the last decade. In comparison with the global production of less than 100 metric tonnes per year in the early 1990s [6], current global production levels are believed to be about 2300 metric tonnes per year [7]. In Taiwan, the rapid growth of the semiconductor and thin film transistor liquid crystal display (TFT-LCD) manufacturing industries has resulted in the consumption of large quantities of NF3 since the early 2000s. According to the industry figures by the Taiwan Semiconductor Industry Association (TSIA) and the Taiwan TFT-LCD Association (TTLA), the demand for NF3 was estimated to be over 1000 metric tonnes per year due to the local investment in the 12-in. wafer and the sixth generation TFT-LCD manufacturing fabs in recent years [8], [9]. As a result, Taiwan is expected to become the largest market for NF3 in the world.

It should be noted that NF3 is still a radiactively active gas although it was not blanketed into one of the six target greenhouse gases under the Kyoto Protocol in 1997. According to the data adopted by the Intergovernmental Panel on Climate Change (IPCC) [10], its atmospheric lifetime and 100 year time horizon-global warming potentials (GWP) are 740 year and 17,200 (relative to CO2), respectively. Furthermore, it is also a hazardous compound with slight solubility in water and inhalation toxicity by inducing the methemoglobin [11]. Consequently its occupational exposure limit (OEL), based on 8-h time-weighted average (TWA), has been set at 10 ppm by the American Conference of Governmental Industrial Hygienists (ACGIH), the Occupational Safety and Health Administration (OSHA), the National Institute for Occupational Safety and Health (NIOSH), etc. As compared to the OEL value (i.e., 1000 ppm) of SF6 and most of hydrofluorocarbons (HFCs) by the ACGIH [12] and the American Industrial Hygiene Association (AIHA) [13], it shows that NF3 should be more toxic than those perfluorocompounds. Therefore, its emissions from the manufacturing processes used in the semiconductor and optoelectronic industries can pose serious hazards to occupation and environment. On the other hand, NF3 has been recognized as a stable gas at room temperature [14], but its toxic decomposition products while applying and/or abating by electrical discharge (e.g., plasma) or other destruction methods have aroused the concern about their potential hazard, especially in fluorides and nitrogen oxides emissions [15]. The toxic products containing fluorine and/or nitrogen in the NF3 decomposition probably include NF3O, F2, HF, SiF4, NO2, NO, N2O, HNO2, and HNO3 [15], [16], [17], [18], [19], [20], [21], [22], [23]. Although these highly reactive and toxic species are well known as water-soluble products, small quantities of these products, not completely solved in the wet scrubbing, are probably found in the vent gas. Furthermore, the discharges via the best available control technology system still pose a potential hazard to health due to the exposure to the removed fluorides in water bodies and vented exhausts.

Recently, many researches have addressed the decomposition technologies of NF3 and the formation mechanism of its decomposed products [18], [21], [23], the published information on the hazard to occupational health and on environmental risks of exposing to these toxic substances and the fluorides probably produced from the exhaust control in the destruction–wet scrubbing system was scarcely addressed or reviewed in the literature. The manuscript is based on the previous works [24], [25] that addressed the analysis of the environmental and health risk of sulfur hexafluoride (SF6) and its decomposition products in detail. This paper aimed at presenting the information on the emissions estimation on the basis of the methodologies recommended by the Intergovernmental Panel on Climate Change, and the hazards to the environment and health caused by its toxic decomposition products while applying and/or abating by destruction methods. According to the prediction/calculation values of its water solubility, Henry's law constant and current emissions, the environmental fate of NF3 and its atmospheric implications were further addressed in this manuscript.

Section snippets

Estimation of NF3 emissions

With respect to the estimation of NF3 emissions from the electronic industry, it becomes a more significant environmental issue, even if NF3 has not been blanketed into the Kyoto Protocol. Due to the great demand for the electronic products, NF3 consumption in Taiwan ranged from 1200 to 1500 metric tonnes on average each year [8], [9], one-third of the total NF3 consumption in the world. NF3 emissions are dependent on a variety of process parameters and emission reduction technologies. In the

Health hazards of NF3 and its decomposition products

Based on the previous description, the identified products in the NF3 decomposition system were NF3O, F2, HF, SiF4, NO2, NO, N2O, HNO2, and HNO3. For example [14], ignition of a mixture of NF3 and water vapor gives a slow reaction, resulting in the production of HF and HNO2. The latter product may react further to form HNO3 and nitrogen monoxide (NO) in the oxidative medium. It is well known that airborne HNO3 is a potent irritant with both acute and chronic effects [38]. On the other hand, HF

Conclusions

Though NF3 addressed in this paper have not been included in the basket of the six major greenhouse gases in Kyoto Protocol, the perfluorocompound gas still possesses some hazards to environment and health, especially in global warming, and adverse effect of exposure to its toxic decomposition products, including NF3O, F2, HF, SiF4, NO2, NO, N2O, HNO2, and HNO3. In this respect, NF3 emissions from the electronics industry amounted to be around 3.6–56 metric tonnes annually based on the

References (47)

  • A.J. Woytek et al.

    Nitrogen trifluoride—a new dry etchant gas

    Solid State Technol.

    (1984 Mar.)
  • J.A. Barkanic et al.

    A review of plasma etching applications using nitrogen trifluoride

    Solid State Technol.

    (1989 Apr)
  • P.B. Henderson et al.

    Nitrogen-nitrogen trifluoride

  • J.I. Robson et al.

    Revised IR spectrum, radiative efficiency and global warming potential of nitrogen trifluoride

    Geophys. Res. Lett.

    (2006)
  • Taiwan Semiconductor Industry Association (TSIA)

    Overview on Taiwan Semiconductor Industry

    (2007)
  • C.H. Leu et al.
  • Intergovernmental Panel on Climate Change (IPCC)

    Climate Change 2007—The Physical Science Basis

    (2007)
  • American Conference of Governmental Industrial Hygienists (ACGIH)

    Documentation of the Threshold Limit Values and Biological Exposure Indices

    (2002)
  • American Industrial Hygiene Association (AIHA)

    2000 WEELs Complete Set

    (2000)
  • H.H. Rogers

    Dielectric strength and other properties of nitrogen trifluoride

    J. Chem. Eng. Data

    (1961)
  • L. Shen et al.

    Nanosize silicon whiskers produced by chemical vapor deposition: active getters for NF3

    Chem. Mater.

    (1995)
  • S. Raoux et al.

    Remote microwave plasma source for cleaning chemical vapor deposition chambers: technology for reducing global warming gas emissions

    J. Vac. Sci. Technol.

    (1999)
  • J.S. Chang et al.

    Removal of NF3 from semiconductor process flue gases by tandem packed bed plasma-adsorbent hybrid systems

    IEEE Trans. Ind. Appl.

    (2000)
  • Cited by (62)

    View all citing articles on Scopus
    View full text