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