AMC focus of new wafer storage environment

Sheila Galatowitsch

FRANKLIN, MA — A growing concern about airborne molecular contamination (AMC) in semiconductor processing has led to the development of a wafer storage environment specifically designed to filter molecular contaminates in photolithography processes.

Extraction Systems Inc. (Franklin, MA) has been awarded a U.S. patent for an ultraclean storage environment that protects photoresist-coated semiconductor wafers from AMC. The company also manufactures filters and instrumentation for monitoring and controlling AMC, which poses a threat to deep ultraviolet (DUV) lithography processes.

AMC results from elevated concentrations of bases, such as ammonia, NMP (a solvent used in lithography processes) and organic amines, which exist in cleanroom air and process tools. They typically enter cleanrooms via make-up air, processes where base-containing solvents are used, and personnel.

AMC reacts with chemically amplified photoresists to cause image degradation, and ultimately, wafer scrap. Often, lithography engineers have no idea when AMC is the culprit behind a shift in the critical dimensions of wafer processing, says John Higley, Extraction Systems` vice president for sales and marketing. Awareness of AMC has grown in recent years, leading manufacturers to attempt to measure and quantify AMC levels in cleanrooms and install chemical filters on process tools.

The wafer storage environment minimizes AMC that occurs when wafers are moved from one unclustered tool to another in either closed or open wafer cassettes. The environment combines a traditional wafer container with a molecular air filter that ensures AMC concentrations below 5 ppb total base inside the container. AMC inside traditional wafer cassettes can be 50 ppb to 100 ppb total base when packaged in contaminated cleanroom air, Higley says.

The chemical filter, key to Extraction Systems` design, uses a fan-like element to move air through the container. “In the past filters were good for ammonia, but poor for organic amines or vice versa. The filter technology has improved to the point where we can now filter the total molecular base,” Higley says.

The company plans to market the environment to end users either directly or through reps. Higley expects it will be quickly adopted.