Printability impact of progressive defects: ammonium sulfate emulation study

Abstract

In the relentless pursuit of device miniaturization and sustainable yield performance, resolution enhancement techniques (RET) such as optical proximity correction (OPC) and sub-resolution assist feature (SRAF) are identified as enabling technologies that fuel the industry. The introduction of advanced reticles, however, considerably augments the mask error enhancement factor (MEEF) where the growth of progressive defects or haze is accelerated by repeated laser exposure, and continues to be a source of reticle degradation threatening device yield. Previous investigations have identified ammonium sulfate, cyanuric acid and ammonium oxalate as the primary and most concerning species found in both mask shop and wafer fabs. In this work, magnesium sulfate is used to emulate crystal growth due to its identical optical properties to ammonium sulfate. A technique has been developed to deposit magnesium sulfate of varying concentrations onto chemically cleaned reticle surfaces. These defects are then inspected with a high resolution reticle inspection system enabled with MEEF detector Litho3. Upon inspection, defects are classified and analyzed with respect to their location relative to device geometry, optical transmission loss as well as the residing surface. Ammonium oxalate crystals are also deposited separately onto reticle surface to comprehend the impact of crystal type and population on defect printability. Compositional analysis are carried out using Raman spectroscopy and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) to correlate the amount of magnesium sulfate and ammonium oxalate crystals with transmission loss. Such emulation study of various crystal formulation mimics progressing stages of crystallization and allows a mechanistic understanding of crystal congregation, transmission loss and defect printability.