Backside photoemission microscopy [1-2] was used to analyze the major yield loss of a communication product fabricated with submicron CMOS process --- functional failures of phase-lock-loop (PLL). The PLL block was covered by five metal layers and three of them were bulk metals. Based upon the backside photoemissions detected on the capacitor structures within the PLL block and the ruptures observed at the emission spots on the polysilicon and gate oxide or of the capacitor after physical deprocessing, the failure was proved due to the capacitor gateoxide breakdown. This was believed to be caused by the plasma-induced-damage during high-density-plasma (HDP) CVD oxide deposition after the front-end processes, as only the lots from one HDP-CVD deposition equipment have very high percentage PLL functional failure. Subsequent machine commonality check did find non-uniform inter layer dielectric (ILD) thickness from this equipment, which indicated the non-uniform plasma intensity occurred during the ILD film deposition. This was further confirmed by the finding of a worn-out gas-shower-head in this system. The abnormal high density of plasma created extra charging and caused the PLL poly capacitor’s gate oxide breakdown due to the antenna effect. After replacing the gasshower- head, the failure was disappeared and yield was back to normal. Through this low yield analysis, we demonstrated an effective application of backside photoemission microscopy to fab yield improvement.