The understanding of focus variation across a wafer is crucial to CD control (both ACLV and AWLV) and pattern fidelity on the wafer and chip levels. This is particularly true for the 65nm node and beyond, where focus margin is shrinking with the design rules, and is turning out to be one of the key process variables that directly impact the device yield. A technique based on the Phase-Shift Focus Monitor (PSFM) is developed to measure realistic across-wafer focus errors on materials processed in actual production flows. With this technique, we are able to extract detailed across-wafer focus performance at critical pattern levels from the front end of line (FEOL) all the way through the back end of line (BEOL). Typically, more than 8,000 data points are measured across a wafer, and the data are decomposed into an intra-field focus map, which captures the across chip focus variation (ACFV), and an inter-field focus map, which describes the across wafer focus variation (AWFV). ACFV and AWFV are then analyzed to understand various components in the overall focus error, including; across slit lens image field, reticle shape and dynamic scan components, local wafer flatness, wafer processing effect, pattern density, and edge die abnormality. The intra-field ACFV lens component is compared with TI's ScatterLith and ASML's FOCAL techniques. Results are consistent with the predictions based on the on-board lens aberration data. Inter-field AWFV is the most interesting, due to lack of detailed understanding of the process impact on scanner focus and leveling. PSFM data is used to characterize the effect of wafer processing such as etch, deposition, and CMP on across wafer focus control. Comparison and correlation of PSFM focus mapping with the wafer height and residual moving average (MA) maps generated by the scanner's optical leveling sensors shows a good match in general. Process induced focus errors are clearly observed on wafers of significant film stack variation and/or pattern density variation. Implications on total focus control and depth of focus (DOF) requirements for 65nm mass production are discussed in this paper using a quantitative pattern yield model. The same technique can be extended to immersion lithography.