Precise thermal control; of multi-zone batch furnaces has become increasingly important in semiconductor industry to prevent defect generation during device fabrication. This work analyzes fundamental heat transfer processes during fast furnace ramping. Model simulations show two significant thermal features : (1) the radial temperature distribution on wafers follows a universal parabolic profile; and (2) a wafer stack produces a strong cavity effect for thermal radiation. An engineering model is developed based on these features to characterize the dependence of wafer temperature nonuniformity on processing conditions, including the ramp rate and wafer spacing. This simple yet accurate model is useful for real-time process control.