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The design, fabrication, and characterization of a piezoresistive membrane deflection test structure for the electrical evaluation of hermeticity in low cavity volume packages is discussed. This test structure uses the zero-level silicon cap as a deflecting membrane to electrically monitor changes in package cavity pressure over time. The hermeticity of the package can then be determined in real-time and low leak rates can be measured using a pressurization stage, which also accelerates the test. The minimum detectable leak rate of the test structure without acceleration has been measured at 6.9× 10-12 atm·cm3·s-1, which is two orders of magnitude lower than the limit of a traditional helium fine leak test.