Dilute Chemistries: Promise and Caution
Alexander E. Braun, Senior Editor -- Semiconductor International, 7/1/2000
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The center attempts to strike the right balance between long-term development, short-term relevance and application to present manufacturing problems. It promotes a mix of projects and activities ranging from high-risk, high-payoff research to smaller projects with more immediate applications. According to the center's director, Professor Farhang Shadman, environmental factors are not always considered in the design and development of new tools and processes. Integrating environmental technology into designs and processes is the technical driver and the common theme of the center's interdisciplinary research efforts, which involve six universities, 27 professors, and more than 73 graduate and 60 undergraduate students.
Work in progress at the center by Professor L.C. Kimerling of MIT indicates the use of dilute chemicals is not as straightforward as it first appeared. In a paper presented at the recent AVS conference, "Environmental Challenges in Semiconductor Manufacturing: Drivers for New Process Technologies," Shadman examined the assumption that if the industry goes to dilute chemistries less chemicals and less energy will be used, resulting in reduced waste. Although at first glance this seems eminently desirable, Shadman pointed out that when there is a move toward dilute chemistries, purity requirements (in terms of how much impurity can be tolerated in the cleaning solution) generally rise. This means standards become more stringent and, environmentally speaking, other penalties are paid because purer chemicals require different handling and packaging.
Shadman's premise is that the environmental envelope of dilute chemistries must be broadened not just around a particular fab or location, but around the entire issue. Then it becomes apparent that chemical manufacturers will end up with more waste and extra energy use, as well as having to cope with other environmental problems created by the need to supply end users with purer chemicals. Therefore the burden - environmental and otherwise - shifts from one location to another, with someone else paying the price. Shadman indicated the net waste might be higher.
Another consideration is that some processes can be done at higher concentrations, in terms of bath life extension and possibly longer use of certain baths. With dilute chemistries, this is often impossible because any change reflects tremendously on such small quantities. Also, a dilute bath is not amenable to being re-purified and recycled. Some of the data show that if SC2 is diluted 10 times, for example, with iron as a contaminant, the SC2 must be 10,000 times purer.
While specific data have been obtained in this area, other conclusions are based on speculation. For example, work at the center shows bath life will be shortened, but it is yet to be determined by how much. Shadman views as intuitive the fact that in a dilute bath a minuscule change in concentration results in a high-percentage change in its fragile chemistry.
Although it might appear the cure could be worse than the ailment, dilute chemistries do offer many advantages. Shadman points out that often the focus is on cost or performance or some other factor, instead of all these dimensions. The result then carries a high price tag. Several years ago, the industry was focused on contamination-driven activities. Many of these have been relaxed since, because then performance, not cost, was the principal consideration. When the industry shifted its view and questioned the need for such expensive chemicals, this was reflected by a change in approach.
Something similar is occurring with the present effort to reduce waste in chemical usage. Shadman urges the industry to consider all three factors simultaneously. He believes there is an optimum level of concentration that, though not as dilute, will satisfy requirements while prolonging bath life and providing a better possibility of reuse.
Another consideration is that by switching to these dilute chemistries, it may become necessary to change their composition or add other compounds to cope with the high-purity requirement. In the case of iron precipitation, the early work by Helms and Kimerling at the center shows certain additives could be used to increase the solubility of metallic impurity and compensate for the effect of the dilute chemistries.
It appears solutions to dilute chemistries problems lie in compromises and optimization.
