Reliability Study of Polymers Used in Sub-4-μm Pitch RDL Applications

Reliability results obtained on a photosensitive polymer-based redistribution layer (RDL) process with two-metal layers and a target pitch below 4~\mu \text{m} are presented. Fully processed samples have been subjected to 1000 h of high-temperature storage (HTS) performed at 150 °C when a second set of samples endured a temperature-humidity (TH) stress executed at 85 °C and 85 % RH. A thermal cycling (TC) stress between - 50\,\,^{\circ }\text{C} and 125 °C for 1000 cycles was also performed on a third sample set. After 1000 h spent at 150 °C, an increase in the metal line resistivity resulting from copper oxidation by atmospheric oxygen diffusing through the polymer is observed. The oxidation of the top part of the metal lines is further confirmed by failure analyses performed on the stressed samples. Eight different polymers used as a potential capping solution to block the diffusion of oxygen toward the metal lines are investigated. The most promising approach for a reliable polymer-based RDL process involves the use of a mold layer.