Chemistry and treatment of III-V semiconductor wastewater
08/01/2001
Josh H. Golden, Andrew Olds, William Hannan, Microbar Inc., Sunnyvale, California
overview
Viable treatment and removal of contaminants from compound semiconductor fab operations require some familiarity with the aqueous chemistry of III-V elements (i.e., GaAs) on the periodic table. This knowledge will enable foundries and fabs to comply with local discharge limits and EPA regulations governing the semiconductor industry. Waste streams from these operations have come under increased scrutiny by local and federal regulatory agencies, especially in light of the current debate about limits for arsenic.
 Figure 1. Arsenic (arsenate) removal by absorption with ferric hydroxide is highly pH-dependent because like-charge repulsion occurs at pH >8. (Adapted from [16]) |
The rapidly increasing use of III-V compound semiconductors is driven by the demand for high-speed, application-specific integrated circuits (ASICs), solar cells, light-emitting diodes, and laser diodes. One projection shows global consumption of compound semiconductors for optoelectronic applications will increase from $3.55 billion in 2000 to $7.7 billion in 2005 [1]. The market for all ASICs in this time span is predicted to reach $2.9 billion in 2005 [2].
Examples of III-V compounds used in these and other applications include GaN, InGaN, InP, GaAlP, InGaAsP, and GaAs. Of particular interest are semiconductor and semi-insulator devices based on gallium arsenide (GaAs) due to several intrinsic advantages of GaAs over silicon (Si):
- GaAs electron mobility is around six times that of silicon; this results in faster response to external radiation signals and clock speeds two to three times greater than those of comparable silicon-based devices.
- Higher speeds are also indirectly realized from the larger GaAs bandgap (1.424eV) vs. that of Si (1.1eV); this results in reduced parasitic capacitance within the device.
These properties make GaAs devices ideal candidates for high-frequency and high-temperature applications in broadband telecommunications, data and optical communications, and for solar cells.
GaAs-based devices are manufactured by a variety of companies, including Anadigics (Warren, NJ), Cree (Durham, NC), Vitesse (Camarillo, CA), Emcore (Somerset, NJ), and JDS Uniphase (San Jose, CA) [3].
GaAs processing and wastes
GaAs single crystals are typically produced by the Czochralski method in which ingots are pulled from a melt of gallium and arsenic at elevated temperature [4, 5]. Arsine (AH3) gaseous by-product may be suppressed by a low-density barrier layer floating on top of the GaAs melt. Epitaxial growth of extremely pure GaAs is commonly achieved by metal organic chemical vapor deposition (MOCVD) via the reaction [4, 5]
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On the exhaust side of this process, destruction of AH3, phosphine (PH3), and volatile organogallium and indium compounds from III-V semiconductor synthesis, and associated ion implantation processes, is achieved by oxidative combustion in a point-of-use thermal processing unit (TPU) [6, 7]. In a typical TPU, the presence of oxygen and methane serves as the primary fuel to maintain continuous combustion at 750-1000°C. The III-V compound precursors are fed into the TPU at flow rates as high as 1000sccm to produce fully oxidized intermediate products, such as As2O5, P2O5, Ga2O3, and In2O3 [8]. These hot, corrosive intermediate products are then exposed to cold water in a wet scrubber for conversion to hydrated oxides and hydroxides (i.e., group III oxides) and water-soluble acids (i.e., group V oxides). The chemical reaction example here shows the conversion of AH3 to arsenic acid:
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Water scrubbers found in a typical fab are usually packed-tower and sieve-tray types, with recirculation flow rates of up to 50gpm [8]. Chemical species of particular concern in scrubber aqueous waste streams (1-2 liters/min) are phosphoric acid (H3PO4) and arsenic acid (H3AsO4) and their related pH-dependent anions. Respective average concentrations of these species in scrubber effluent are approximately 1000-1700ppm over 24 hr at 1000sccm gaseous precursor flow rate [8].
In addition to wet scrubbers, arsenic-bearing aqueous effluents from compound semiconductor processing are obtained from slicing, dicing, and etch processes. Fluoride ions from HF etch and cleans may be also found in these wastewaters, depending on the fab wastewater segregation schemes and processes. Generally, flow rates of these contaminated waters can vary from ~1-50gpm or more, again depending on dilution factors and wastewater blending schemes [9].
Trimethylated organogallium and In gaseous precursors are mostly consumed in III-V film formation. However, MOCVD tools may use dual gas lines to eliminate lag time while mass flow controllers equilibrate. This portion of In and Ga precursor gas is lost as waste, as well as gas from purging operations. Typical flow rates for the gaseous group-III precusors range from 100-200sccm [8]. After scrubbing, the hydrated oxides and hydroxide of In and Ga may be found at <100ppm as the oxide in the aqueous effluent, again depending on dilution factors and wastewater blending schemes [8, 9]. These materials have limited toxicity, and display solubility behavior that generally tracks with aluminum (i.e., mostly insoluble hydroxide with a 5.5-7 pH), which allows for easier separation and removal [10].
(Due to a worldwide shortage of gallium, obtained as a by-product from aluminum mining, there may be an opportunity for its recovery from waste precipitates; this warrants further investigation. Refs. 6 and 7 are an excellent general review of TPUs, wet scrubbers, and related water usage concerns.)
Chemistry and treatment issues
Treatment and removal of arsenic in both industrial wastewaters and potable water sources in the US and worldwide has recently gained widespread notice, due to the carcinogenic properties of aqueous arsenic, as well as the debate over a reasonable maximum contaminant level (MCL) [11-13]. The MCL currently imposed by the EPA is 50ppb (i.e., µg/liter) but decreases to 10ppb within the next 1-3 years [12, 13]. The new MCL is expected to bring new and more efficient arsenic removal technologies to the forefront, as well as new commercial opportunities, in both potable and industrial water applications [14-18]. Examination of treatment technologies for arsenic is aided by investigating the basic chemistry of typical arsenic-bearing wastewaters:
Arsenic occurs in four valence states: -3, 0, +3, and +5. In most waters, the +3 and +5 oxidation states are respectively found as AsO33- (arsenite) and AsO43- (arsenate) anions. Arsenate is the most common form of soluble arsenic found in semiconductor processing.
Compounded solid arsenic in the form of GaAs particles may be obtained from ingot processing and dicing operations.
The arsenate anion is negatively charged at low pH values because it is the anion of a strong acid, ortho-arsenic acid (H3AsO4, pKa1 = 2.20). In contrast, arsenite removal by absorption and coagulation is less effective because its main form, arsenious acid (H3AsO3) is a weak acid (pKa1 = 9.23) and is only partially ionized at pH values where removal by absorption occurs most effectively (i.e., pH 5-8) [16].
To ensure that arsenic is in the +5 oxidation state, water may be treated with oxidants, including chlorine or permanganate.
 Figure 3. Efficient arsenic removal by absorption and microfiltration is achieved by dosing algorithms based on flow and models of predicted peak arsenic concentration. |
Absorption of arsenate anions and other negatively charged and partially protonated species by aluminum and ferric hydroxide gels between pH 5 and 8 remains the predominant form of treatment [16-18]. This is typically achieved by direct injection of acidic aluminum or ferric chloride solutions into the wastewater with the appropriate pH adjustment. The arsenate anion remains negatively charged even at low pH values and is thus effectively absorbed and removed by ferric or aluminum hydroxide gels (Fig. 1).
The decrease in arsenate absorption above pH 8 is due to the formation of a negatively charged ferric hydroxide surface that repels negatively charged arsenate. A variety of new fixed-bed absorbants have recently been proposed and are being evaluated. (See Refs. 15-18 for reviews of new and existing arsenic removal technologies.)
Recently, we described an effective coagulation process and single-pass microfiltration system for the treatment of wastewaters containing heavy metals, fluoride, and particles from chemical mechanical polishing (CMP) operations [19]. The installation of such a system (Fig. 2) is currently underway at a major wafer fab in the Northeastern US, using copper interconnect processing.
This low-pressure (<25psig) microfiltration technology (0.5-1µm pore size) was originally developed for the treatment of arsenic-bearing waters obtained from gold-mining operations [15]. One such arsenic removal system, installed at a gold mine in northern Nevada, has effectively treated arsenic-bearing waters at flow rates up to 5000gpm [15]. The single-pass microfiltration technology has recently found utility in the treatment of wastewaters containing arsenic and fluoride at Emcore Corp.'s GaAs solar cell fab in Albuquerque, NM [3].
At the request of our customer, in addition to arsenic removal, we developed a process to treat both arsenic and fluoride simultaneously [20]. The goal was to achieve <10ppm fluoride and <50ppb total arsenic. Preliminary process development was conducted at Microbar's Sunnyvale, CA, laboratory to determine the optimal parameters for co-removal of arsenate and fluoride anions. The challenge was to identify optimal conditions for fluoride removal (as fluorospar or CaF2) in the pH range where arsenate is also effectively absorbed by ferric hydroxide (pH <8). The solubility of CaF2 (ksp = 3.9 x 10-11) is pH-dependent [21] and is in competition with calcium hydroxide formation (ksp = 6.9 x 10-6) also with pH-dependent solubility, as shown by:
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Typically, pH values >7 to 8 are needed to suppress the redissolution of fluorospar and achieve <10ppm aqueous fluoride [21]. Because ferric hydroxide absorption of arsenate becomes ineffective above pH 8 (see Fig. 1), we determined that fine pH control was needed to maximize the insolubility of CaF2 and still effectively absorb both arsenate ions and coagulate fluorospar particles. Fine pH control close to the acid-base equivalence point of pH 7 is challenging because very small amounts of acid or base can cause a dramatic change in pH. We found that it was indeed feasible to finely control pH and simultaneously remove both arsenic and fluoride, or arsenic and fluoride separately, using the coagulation processes.
The processes were successfully introduced at Emcore's facility in Albuquerque early this year. Data presented here show the results of arsenic (Fig. 3) and fluoride (Fig. 4) removal at Emcore, at flow rates approaching 50gpm and a peak filter pressure of 12psig.
Phosphate, while a less significant concern than arsenic, may be of sufficiently high concentration to warrant removal before discharge. Phosphate, as post-scrubber phosphoric acid and related pH-dependent salts, may be removed by co-precipitation with fluoride:
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Because phosphate from ion implantation processes may be mixed with other III-V contaminants, we have studied the simultaneous removal of phosphate, fluoride, and arsenic [22, 23]. To maintain maximum insolubility, careful pH control is again an issue, as well as the potential buffering capacity of phosphate and related species. The results of these studies will be published at a later date.
Conclusion
Telecommunications and optoelectronics industry growth has increased the use of compound semiconductor materials. Waste streams from these operations have come under scrutiny by local and federal regulatory agencies, especially in light of the debate over a lower MCL for arsenic, and its occurrence and toxicity.
Viable treatment and removal of contaminants from groups III and V of the periodic table are indeed feasible, but require some familiarity with the aqueous chemistry of these elements. This knowledge will enable foundries and fabs engaging in compound semiconductor processing to comply with local discharge limits and the EPA regulations governing the semiconductor industry (i.e., 40 CFR 433 and 469, subpart A [24]). It is prudent to be familiar with local regulations that may be more stringent than federal EPA regulations and may have both a mass-based discharge limit and a peak concentration limit.
References
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- See www.compoundsemi.com/documents/view/cldoc.php3?id=527#top.
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- Personal communication, Joseph Van Gompel, BOC Edwards, May 2001.
- Microbar Inc. data.
- M.J.N. Pourbaix, Atlas of Chemical Equilibria in Aqueous Solutions, Natl. Assoc. Corrosion Engineers, Houston, TX, 1974.
- R. Eisler, US Fish and Wildlife Service Biological Report 85 (1.12), 1988.
- See www.epa.gov/fedrgstr/EPA-WATER/2001/May/Day-22/w12878.htm.
- M.M. Frey, M.A. Edwards, Jour. AWWA, 89, pp.105-117, 1997.
- M.M. Frey et al., Jour. AWWA, 90, pp. 89-102, 1998.
- Proc. of the Inorganic Contaminants Workshop, AWWA, Feb. 27-29, 2000: www.mining-technology.com/projects/carlin/.
- J. Hering et al., Jour. AWWA, 88, pp. 155-167, 1996.
- J. Hering, M. Elimelech, Arsenic Removal by Enhanced Coagulation and Membrane Processes, AWWA Research Foundation, Denver, CO, 1996.
- L. G. Twidwell et al., Proc. of the REWAS Global Symposium on Recycling, Waste Treatment, and Clean Technology, Minerals, Metals, and Materials Society, Warrendale, PA, pp. 1715-1726, 1999.
- J.H. Golden et al., "Evaluating, Treating CMP Wastewater," SI, 85, Oct. 2000.
- J.H. Golden et al., US pat. pend. "Process and Apparatus for the Simultaneous Removal of Arsenic and Fluoride." Filed USPTO, April 2001.
- D.C. Harris, Quantitative Chem. Anal., 2d ed., W.H. Freeman, NY, 107, 1987.
- J.H. Golden et al., US pat. pend. "Process, Apparatus for Simultaneous Removal of Arsenic, Fluoride, Phosphate." Provisional filed USPTO, April 2001.
- US pat. 4,145,282, "Process Purifying Wastewater Containing Fluoride Ion."
- See www.epa.gov/docs/epacfr40/chapt-I.info/subch-N/.
Josh H. Golden is director of process technology at Microbar Inc., 1252 Orleans Dr., Sunnyvale, CA 94089; ph 408/542-9069, fax 408/541-9158, e-mail [email protected].
Andrew Olds is an applications engineer at Microbar Inc.
William Hannan is a technical support engineer at Microbar Inc.