Spatial correlation of electron-beam mask errors and the implications for integrated circuit yield

Abstract

Specifications for masks are usually based on the assumptions that pattern errors can be adequately described by a stationary Gaussian distribution of independent point processes. Critical dimension and feature position errors across chrome-on-glass photomask plates were measured, and the results show that a description of these errors in terms of a 3-sigma variation about a mean value based on the above description is inadequate and often misleading. Mask dimensional errors exhibit considerable spatial correlation across a plate, and have a spatial power spectrum that has implications for integrated circuit yield because of the different ways that photolithography systems can transfer reticle errors to the wafer. Depending on the spatial correlations of the errors and the specific lithography system the implications could be either positive or negative. As a result any quantitative consideration of the effect of mask errors on device performance and yield must consider mask error spatial correlations specifically.