Back-end-of-Line (BEOL) Mechanical Integrity Evaluation: A Mixed-Mode Double Cantilever Beam Test for Crackstop Strength Assessment

Crackstops are vital parts of the back-end-of-line (BEOL) chip stack in electronic packages and are often responsible for preventing catastrophic mechanical and electrical breakdown within a semiconductor device. The present work is focused on evaluating the fracture toughness of crack-stops using a mixed-mode bending test structure. The test fixture designed as part of this study enables crack propagation in the BEOL under a wide range of mode-mixities which mimics the stress drivers within an organic flip-chip package. Techniques for optimized sample preparation beginning with a processed wafer to the final, test-ready coupon for mixed-mode testing will be discussed. The designed sample preparation tools and methods allow for the precise driving of a crack to specific features within the BEOL. Rigorous analytical verifications were conducted to validate the results obtained from the tests and it will be demonstrated that the test is highly effective for understanding the factors that can influence the strength of a crackstop and the mechanics of its strength degradation under prolonged moisture exposure.