Surface contamination on pelliclized photomasks

In late 1985 and into 1986, several users of pelliclized chrome masks reported the appearance of particulate deposits on the masks under the pellicle. They showed up after 10 days to two weeks in one well documented case, and, reportedly, in as little as a few hours, or as much as a year after the mask and pellicle were mated, in others. In the U.S., the particles were first observed during microscopic examination of the masks, but could also be seen with the naked eye when the mask was illuminated with bright specular light. In Japan they were also found by reticle inspection equipment. The deposits were found along the edges of images and on top of the chrome surface, but almost never on the glass substrate, or on the inside surface of the pellicle membrane (Figures 1 and 2). It was noted that the resist type used to make the mask, optical or e-beam, did not have an influence on the occurrence of the problem, and it was not isolated to the use of any particular vendor’s pellicles. In some instances, it did seem that the problem