<title>Evaluation criteria for e-beam mask writing systems</title>

Abstract

Mask makers generally specify Critical Dimension (CD) Uniformity for customer product plates as a composite value. This must include the contributions of machine systematics such as deflection field distortions and stripe butting and external error sources such as resist processing and measurement. This broad definition requires that a line at any location within the quality area must be within spec. Manufacturers of electron beam lithography equipment have traditionally taken a more component-based approach: separating CD Uniformity from other lithography error sources by using a test pattern in which features are written at the center of the writing stripe. This approach removes the effect of field distortion and stripe butting error. These intrastripe and interstripe errors are evaluated in terms of their impact on placement accuracy but not CD control. Another issue that clouds the evaluation of CD Uniformity is the definition of the specification itself. Some in the industry represented CD Uniformity in terms of range while others use a 3(sigma) value. Rarely does a specification include the data sample size. Lepton has adopted a new approach to the evaluation of CD Uniformity for EBES4: CD Uniformity is evaluated in terms of both global and local performance. Global CD Uniformity incorporates effects of beam stability, depth of focus and materials effects while Local CD Uniformity addresses the impacts of deflection distortion and stripe butting. This paper focuses on Local CD Uniformity and the contribution of Stripe Butting to that specification. A methodology will be presented which makes use of a test cell containing features not fractured by stripe or cell boundaries (off-boundary) as well as features bisected by these boundaries (on-boundary). The data for a number of plates are analyzed in terms of range and 3(sigma) as well as being evaluated as a function of sample size. In addition, traditional butting evaluation techniques such as the placement based, `railroad track' method and the `point by point' CD method are used for purposes of comparison.