Novel alignment method for planarized substrates in electron beam lithography

We have developed an alignment mark structure for planarized substrates, that enables high alignment accuracy in electron beam lithography. Not only heavy metals but also light elements are used as marks. For multilevel metalization processes, we confirmed the advantages of buried heavy-metal marks by detection-signal simulation and exposure experiments. This mark structure allows the detection of alignment marks using an electron beam, even when they an: covered by a SiO2 film up to a few micrometers thick or with metal films that prevent detection by an optical method. Buried light-element alignment marks, such as a SiO2 mark in a Si substrate, are also effective. Such marks are suitable for use prior to the gate-fabrication process, especially far the shallow groove isolation (SGI) process, because they do not cause heavy-metal contamination. Marks consisting of periodic narrow lines made with light elements are particularly suitable for wafer planarization processes.