Electron beam writing methods of x-ray masks for eliminating thermal imageplacement errors

In this article we describe electron beam writing methods to suppress image placement errors due to the thermal distortion and drift of x-ray masks. P arts of device patterns are written outside of the writing area of a mask b efore the device pattern writing step to stabilize the temperature of the m ask (preparatory writing method). The seesaw writing method, which divides a pattern into many slit parts that are written reciprocally, is used to co mpensate for the nonuniformity of the heating rate of the mask according to the mask's position on the stage. The absorber fiducial mark is formed on the mask instead of the cassette or the stage to prevent the drift of the m ask, and furthermore, to reduce mask-to-mask overlay errors. The writing is performed by referring to the absorber coordination marks on the mask afte r the registrations of the marks are measured with an image placement (IP) measurement system (mark-on-the-mask method). The effectiveness of these me thods is evaluated through the fabrication of a full-chip mask of a 64 Mbit dynamic random access memory, and an IP accuracy close to 20 nm is obtaine d. (C) 2001 American Vacuum Society.