Effect of 4-point bending test procedure on crack propagation in thin film stacks
Adhesion properties of the ultra low-k films play a critical role in mechanical integrity of the Back-End-Of-Line (BEOL). The 4-point bending (4PB) test method has been widely used to quantify the adhesion energy of interfaces in multilayer thin film stacks. Tasks: The goal is to investigate the influence of various test setups on measured critical adhesion energy of ultra low-k films. The test were supported by FEM analysis. Action: Three different film stacks were prepared and tested. First film stack consisted of 200 nm thick spin-on glass (SOG) dielectric film with k-value close to 2.2 which was first deposited onto the wafer, followed by a 5 nm thick TaN/Ta physical vapor deposition (PVD) metal barrier, 60nm PVD Cu seed and 500 nm electroplated Cu. Second stack had an additional 30 nm SiCN/100 nm SiO2 layer, deposited below the low-k. Third stack consisted of plasma enhanced chemical vapor deposition organosilicate glass (PECVD OSG) dielectric film with k-value close to 2.2, followed by a 5 nm thick SiCN, 25 nm SiCO and 1000 nm SiO2 layer. Results: It has been demonstrated that deeper notch depth in most cases can trigger cohesive failure of the low-k instead of adhesive failure (determined by the XPS analysis), which is the original purpose of the 4PB test. FEM analysis showed that, while there is a difference in force plateau length between symmetric and asymmetric crack propagation, the value is the same, as is crack opening.