Phase Shift Focus Monitor (PSFM) has been successfully utilized as a focus monitoring tool for scanners and steppers from the g-line era to the most advanced immersion technology nodes. PSFM exhibits high sensitivity, linearity and repeatability for immersion scanners with the illumination conditions of conventional mode and NA0.93. A microlithography model was created using Hyperlith to study the PSFM sensitivity and linearity under the conditions of OAI (off-axis illumination) and high NA (0.95~1.35). The model predicts that a PSFM sensitivity of 700 ~ 1000 nm/um can be achieved when an OAI, 40 nm PSFM target and NA 1.35 are used. The model also studied the influence of various parameters on PSFM sensitivity and linearity. Wafer data verified the simulation results. PSFM linear focus range with the NA1.35 condition is shorter than that of NA0.93. The influence of illumination conditions on PSFM, such as OAI modes (annular, Quadra), NA / Sigma values and PSFM target sizes, has also been investigated by the microlithography model.