Newly Identified Hazards Associated with Silicon Wafer Thinning Operations
(Avago Technologies, Fort Collins, CO)
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An unexpected exothermic reaction in a sealed 55-gallon drum, followed by an “explosive” event, initiated an industrial hygiene case study and frustrating investigation to determine the cause of this reaction. Within semiconductor manufacturing, one process involves thinning the back side of a silicon wafer after the circuitry has been completed on the front side. The wafer thinning operation involves a basic water-wet process and diamond grit. The ground silicon contained in the waste water is removed using polypropylene filters. These filters become loaded and must be replaced. One day it was noted the filters were generating an exothermic reaction when removed. As a precaution, the filters were placed into a steel 55-gallon drum and taken outside. Unfortunately, the lid was sealed on the drum. After conferring with several experts about what exothermic reaction might be occurring, a decision was made to don appropriate protective gear and loosen the drum lid. The lid forcefully (6-8 feet in the air) came off the drum either due to over-pressurization or a deflagration event. An industrial hygiene graduate student and the company CIH were tasked to determine the cause of this event. To date, the exothermic reaction continues to occur with the “explosion” hazard being controlled by not sealing the filters in a drum. Literature searches, numerous air sampling tests with direct reading instruments, standard IH sampling methods, grab-bag sampling with GC-MS analysis have been conducted. Though no individual results conclusively identified the compounds, each result provided a puzzle piece to assist in completing the picture. Some compounds have successfully been identified while others remain elusive. The IH graduate student presented the preliminary data at the SESHA conference in 2007 and received best student presentation award but was not able to present this at the closing session. Since that presentation, the investigation has continued and a NIOSH Health Hazard Evaluation was requested and is on-going. Though e results indicate a significant safety (flammable gas/gases) and health hazard (toxic gases) exists with the byproducts associated with this operation.