Abstract
A practical methodology for conducting a comparative risk assessment is applied to the problem of identifying a less toxic casting solvent(s) for positive photoresist. Conventional solvents used in photoresists are ethylene glycol-derived ethers, principally 2-ethoxyethyl acetate, commonly known as CEIXOSOLVE® Acetate.A
Concern has grown in recent years over possible reproductive risks of 2-ethoxyethyl acetate and structurally related glycol ethers. Animal toxicology studies indicate that these solvents may induce teratogenic and fetotoxic effects as well as male reproductive effects in animals exposed to air concentrations ranging from 3 ppm to 100 ppm.
Three general concepts in glycol ether toxicology have emerged from the animal data:
| 1. | Toxicological equivalence, mole for mole, of a glycol ether and its acetate. | ||||
| 2. | Inverse relationship between reproductive toxicity and length of the substituted alkyl group. | ||||
| 3. | Biologic activation of ethylene glycol ethers to an alkoxy-acetic acid metabolite (the ultimate toxin). | ||||
Recent regulatory scrutiny by the U.S. Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA) resulted in an announcement in December 1986 that OSHA intends to develop a health standard for four ethylene glycol ethers, including 2-ethoxyethyl acetate. The OSHA Permissible Exposure Limits (PELs), which currently range from 25 ppm to 200 ppm, are expected to be lowered significantly, perhaps as low as 0.5 ppm.
Efforts undertaken by several suppliers of photoresists have resulted in the commercial availability of alternative casting solvents for photoresists. This article compares four solvents currently used in photoresists (propylene glycol monomethyl ether acetate, ethyl-3-ethoxypropionate, ethyl lactate, and 4-butyrolactone) with respect to 1) chemical structure and physical properties related to safety and exposure potential, 2) acute toxicity, 3) chronic toxicity, and 4) metabolic disposition. Using a “weight of evidence” evaluative approach, propylene glycol monomethyl ether acetate and ethyl-3-ethoxypropionate are concluded to be viable, less toxic substitutes for 2-ethoxyethyl acetate in positive photoresist.