Metrology methods comparison for 2D structures on binary and embedded attenuated phase-shift masks

Abstract

There are several different methods for printing contact holes on wafers using optical lithography. A preferred resolution enhancement technique for improved contact hole lithography performance is the embedded attenuated phase shift mask (EAPSM). The EAPSM comes in many flavors and forms, but the current preferred form is a film transmission of 6 percent and a phase shift of 180 degrees relative to the clear fused silica areas. It is important to note that the phase shift and transmission values for the phase shift mask are at the actinic exposure wavelength of the wafer stepper/scanner. That is the mask is designed to have a transmission of 6 percent and phase shift of 180-degrees at 248nm or 193nm, depending on the wafer stepper. The resulting transmission of the phase shift mask at the inspection tool wavelength of 365nm is much higher, and the phase shift of the 365nm radiation is significantly less than at the shorter actinic wavelength. The gray-scaled aerial images that are collected by the mask inspection tool could vary significantly for the same size 2-D feature in the binary mask, the 248nm EAPSM, and the 193nm EAPSM. This is also compounded by the fact that the inspection tool calibrates the background transmission of the phase shift material as 0 percent transmission and calibrates the transmission of the fused silica as 100 percent transmission. When these gray-scaled images are used in an energy flux algorithm for contact area measurement, they can be potentially different for each of the three types of masks used to print contact holes. This paper explores the issues involved in using an off-actinic aerial image as the basis for the AVI method of contact sizing.