Development of low stress TaN thin film for MEMS/sensor electrode application

TaNTa is the excellent material against copper diffusion, and is widely used as Cu diffusion barrier in CMOS Cu-BEOL process, with compressive stress around 2GPa. In this work, TaN thin film was used as electrode material in micro-bridge structure based on its electrical and optical property for post-interconnect CMOS-MEMS single chip integration application. The micro-bridge structure forms a resonant cavity which is most critical structure to this application. There are only several layers of thin film on the micro-bridge, and the electrode layer play the most important role to determine the performance of the bridge. Additionally sheet resistance of TaN electrode should be controlled to around 377ohm/SQ, which is the optimum value for light absorption in cavity structure but much higher than TaN baseline film used in CMOS Cu BEOL. DOE experiments were designed, and gas ratio/power was tuned to get the low stress and high resistivity TaN film. According to the experimental results, sheet resistance of TaN film can reach 362 ohm/SQ with thickness of 500A and compressive stress of 475MPa, and the standard deviation of 3.53% can be achieved for the process uniformity. But the process was almost touching the lower power limit of the PVD tool with smaller process windows, and cannot meet the tool and mass-production requirements. Finally TaN film with thickness of 200A and compressive stress of about 900MPa was chosen as best condition with enough process windows, and from the electrical and physical data, the optimized TaN film process can well match this application.