I currently perform photolithography on lithium niobate (transparent) wafers. A friend of mine has a 3 inch disc of some black material (carbon, I think) that I place underneath my wafer during UV exposure to prevent light from passing through the substrate and reflecting, thus exposing undesired areas of the wafer. I would like to purchase one for my lab now. Not him nor anyone in his lab knows where the disc came from or if there's a technical term for it, but no matter what I search, I can't seem to find one. Does anybody know if there's a better term to search for, or where I might be able to buy one?