The stability of photolithography process tool is the fundamental to the fabrication of semiconductor devices. Several process control methods are employed to qualify and then monitor every single process layer at the photolithography stage. The CDSEM (Critical Dimension Scanning Electron Microscope) measurements on metrology features and the optical inspection and DRSEM (Defect Review Scanning Electron Microscope) on device features for Process Window Qualification is part of the conventional process control. Here we employed a novel PSD (Process Stability Diagnosis) solution that provides detailed Bossung plot like analysis on device features using CDSEM or DRSEM images. This provides quick insight into the process behavior and also identifies the root cause for any deviation. In this paper we will discuss about monitoring the depth of focus and the best focus as well as diagnosis for lens parameters like astigmatism and spherical aberration. We describe the method of extracting relevant parameters from high resolution images and establishing an automatic monitor for these critical indicators.