SEMICONDUCTOR WASTES
Introduction:Gallium arsenide (GaAs)-based semiconductor devices are used for a multitude of military and commercial applications in throughout the world, including lasers, light-emitting diodes, and communications. Manufacturing processes devoted to the fabrication of these devices generate large volumes of wastes which contain the toxic metal arsenic, as well as the economically valuable metal gallium. Even though many of the wastes currently being disposed by the industry are unlisted (e.g. solid GaAs), the toxic arsenic contained therein is regulated, should it be released to the environment (e.g., through the action of acids, present in many landfills). Current gallium prices make recovery of wastes containing this metal economically viable if the recovery process is sufficiently low cost. Therefore, recovery of these metals (As and Ga) from GaAs processing wastes is economically advantageous.
Compound semiconductors:
Silicon (Si) has been, and will continue to be, the dominant material used for the overwhelming majority of semiconductor device applications. Silicon itself is environmentally-benign, and is toxic only when in the form of gaseous silane or as certain organosilanes. In the last ten to twenty years, however, there has been a tremendous upsurge in the use of compound semiconductors (i.e., semiconductors whose crystalline structure contains two or more elements) for both commercial and military applications, as these materials have moved from the laboratory to specific applications. The usage and demand for compound semiconductors will continue to increase, in much the same way that the demand for silicon-based devices has continued to increase. Many of these compound semiconductors utilize chemical elements or precursor materials that exhibit varying degrees of toxicity (e.g., arsine, phosphine, stibine, etc.) For this reason, compound semiconductors present an opportunity to perform pollution prevention and waste minimization on a materials recovery and reuse basis.
Recovery process development:
Thermal processing of GaAs solid wastes to recover gallium has also been demonstrated in the past. While thermal separation under air has been achieved for GaAs, that procedure results in the formation of arsenic and gallium oxides. These oxide "slags" require an additional processing step (reduction) to obtain reusable metals. Therefore, from an in-plant pollution prevention approach, separating under an inert atmosphere or under vacuum is more desirable in order to minimize the number of processing steps (and thus the overall cost of the recovery operation). This too has been attempted, and many of the processes described are very exact with respect to necessary conditions to achieve thermal separation.
Initial studies of the effects of high temperature conditions (above 950Ò°C) showed that thermal cracking of the GaAs takes place until the partial pressure of arsenic vapor in the head space prevents further sublimation of arsenic. Thus, a conceptual process was proposed in which the GaAs solids would be subjected to high temperatures at reduced pressure with a continual draw-off of released arsenic vapors. Continued operation of such a process would ultimately result in removal of most of the arsenic leaving a residue that would be high in gallium, and which would contain any unmelted (or high-boiling) contaminants. However, it was expected that such thermal separation alone would not produce gallium or arsenic products of sufficient purity for reuse in semiconductor crystal growth. Further processing steps would be required whereby the arsenic-rich vapors and the gallium-rich residue could be further purified to acceptable levels for reuse.
My recommendation:
This process has been developed for the on-site recovery of both arsenic and gallium from gallium arsenide (GaAs) solid wastes. The process described herein first involves the thermal separation of GaAs solid wastes into their constituent elements (with a minimum of energy input or additional handling). Each of the separated elements is then purified to the required levels for further crystal growth using low-cost procedures. Because of this three-step approach, the developed procedure can accommodate a wide range of input material characteristics. Prior work with GaAs thermal separation and constituent element purification provided an outline for the development of this process. A second process was developed for the recovery of both arsenic and gallium from gallium arsenide polishing wastes. The economics associated with the current disposal techniques utilizing ferric hydroxide precipitation dictate that sequential recovery of toxic arsenic and valuable gallium, with subsequent purification and in-house reuse of both, is to the benefit of the gallium arsenide crystal grower. The developed process involves first the removal of the majority of the arsenic and suspended polish as a mixed precipitate of calcium arsenate and polish. This first process step is performed at ambient temperatures and at a pH > 11 using NaOH. At these pH regimes, gallium is retained in solution as a sodium gallate species. Precipitation of virtually pure gallium hydroxide is then accomplished in the next process step through pH adjustment to between 6 and 8 with waste acids. The commonly used ferric hydroxide coprecipitation step is retained as a final treatment step, but because of the removal of the majority of the arsenic, gallium, and polish in the two prior steps, far less waste is land disposed.
Conclusion:
This process can be considered for implementation as in-plant pollution prevention techniques. It is believed to be to the ultimate economic advantage of existing GaAs fabrication companies to minimize or altogether eliminate the amount of toxic arsenic which is disposed of from their manufacturing operations. This not only eliminates "short-term" costs such as manifesting and disposal, but also the much more costly "long-term" liability costs associated with environmental cleanup. Payback for gallium recovery is "immediate", in terms of reduction of operating costs. The payback associated with arsenic recovery is an avoidance of future costs that might be incurred for environmental cleanup. The processes developed will allow recovery and reuse of these materials in a cost-effective and environmentally responsible manner.