A study of closed-loop application: WLCD-CDC for 32nm and beyond reticles

Abstract

Optical lithography stays at 193nm with a numerical aperture of 1.35 for several more years before moving to EUV lithography. Utilization of 193nm lithography for 32nm and beyond forces the mask maker to produce complex mask designs and tighter lithography specifications which in turn make process control more important than ever. High yield with regards to chip production requires accurate process control. Critical Dimension Uniformity (CDU) is one of the key parameters necessary to assure good performance and reliable functionality of any integrated circuit. There are different contributors which impact the total wafer CDU: mask CD uniformity, resist process, scanner and lens fingerprint, wafer topography, etc. In this study the newly developed wafer level CD metrology tool WLCD of Carl Zeiss SMS is utilized for CDU measurements in conjunction with the CDC tool from Carl Zeiss SMS which provides CD uniformity correction. The WLCD measures CD based on proven aerial imaging technology. The CDC utilizes an ultrafast femto-second laser to write intra-volume shading elements (Shade-In Elements™) inside the bulk material of the mask. By adjusting the density of the shading elements, the light transmission through the mask is locally changed in a manner that improves wafer CDU when the corrected mask is printed. The objective of this study is to evaluate the usage of these two tools in a closed loop process to optimize CDU of the mask before leaving the mask shop and to ensure improved intra-field CDU at wafer level. Main focus of the study is to investigate the correlation of applied attenuation by CDC and the resulting CD change, the impact of CDC process on CD linearity behavior and the correlation of WLCD data and wafer data. Logic and SRAM cells with features having designed line CD’s at wafer level, ranging from 27nm to 42nm have been used for the study. The investigation provides evidence that the applied attenuation by CDC shows a linear correlation to CD change at wafer level measured with WLCD. Additionally, WLCD data shows that the CDC application does not impact the CD linearity for the tested feature range. The WLCD measurement data in turn show an excellent correlation to wafer print CD data indicating cost effective use case of closed loop WLCD/CDC application.