The impact of mask topography on CD control

Abstract

Past work on mask topography has documented the effects of the topography on the aerial image intensity and on the responses of CD through defocus and image placement. Device performance, however, is limited by the statistical CD variation in the poly lines that form the logic and memory gates. We have developed a tool that combines fast, rigorous EMF calculations with Monte Carlo simulation to investigate the impact of mask topography on CD control. We have applied it to study the effects of mask topography on through-pitch CD control in 6% EAPSM, AAPSM, and CPL reticles at 90-nm half-pitch design rules. The effects of the topography can be understood by examining the coefficients of the Fourier expansion of the near-field radiation pattern. The magnitude of the 3D effects is not correlated with the amount of mask topography but with the specific details of the Fourier coefficients that pass through the pupil. The topography mainly distributes the energy more evenly and introduces additional phase information. The best imaging results at tight pitch are obtained when the difference between the magnitudes of the two main Fourier coefficients that pass through the pupil is small and their phase difference is close to π. At larger pitches more diffracted orders will pass through the pupil, and the extra phase information from the additional orders will couple with aberrations in a reticle-dependent way and complicate overall RET choice.

Image degradation due to phase effects in chromeless phase lithography

“…In what follows we will derive the essential effects of phase errors on image contrast by means of a simple model based on two beam interference assuming an ideal lens. Similar discussions can be found in, for example, [2,9,10]. Polarization dependent reflection at the resist surface as well as vector effects are neglected.…”

OPC in memory-device patterns using boundary layer model for 3-dimensional mask topographic effect

“…8 (b). It is reported that phase change caused by 3D mask effect can be interpreted as sort of aberration and also coupled with defocus [3]. This implies that the phase of BLM might not be determined correctly.…”

“…1. It has been reported in numerous publications that this so-called 3D mask effect has non-negligible impact on lithographic performances [1][2][3][4]. This 3D mask effect can be simulated if the near-field image of mask is obtained.…”