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To avoid contamination and ensure stable and reliable performance for MEMS (micro electro mechanical systems) devices, they may require to be operated in a vacuum environment. Wafer level hermetic bonding is one of the main requirements for successful micro-packaging in a vacuum environment. However, most of the devices are required to guarantee that the atmosphere inside the cavity remains stable during the lifetime of the device. How can one test whether such a package is hermetic? A new leak detection method is required for high density MEMS packages as traditional methods are not suited for ultra-low cavities. Depend on the review and present of traditional hermetic testing methods, an optical leak test is selected as the simplest method to monitor the leak of ultra-low bonding cavities. It is a method to estimate the leaking rate using interferometer equipment. In this paper, the optical leak test structure have been designed and fabricated to estimate the leak rate. This hermetic test method has been successfully performed on wafers bonded using anodic bonding, glass frit. This result provides a starting point for further investigation into wafer level metallic SLID bonding result.