EUV resist outgassing: How much is too much?

Optics contamination is a concern for extreme ultraviolet (EUV) lithography. To protect EUV optics, all materials used in EUV vacuum exposure chambers must be screened prior to use. This paper will briefly describe the methods used to measure resist outgassing and list the most frequently encountered outgassing species of current EUV resists. The first attempts to implement witness plate experiments show that the difference between the resist and the control samples is not substantial and that the Si-capped mirrors are not stable. Actual contamination of microexposure tool illumination optics and masks is described, but the source of the contamination has not yet been determined. Although the materials of construction or resist could cause contamination, resist is probably not the source in this case because of the differential pumping of the exposure chamber and the distance from the resist-coated wafer to the illumination optics and mask.