Abstract: A test key for a semiconductor structure is provided for in-line defecting defects of the contact. The test key is disposed on a scribe line of a wafer substrate, and includes conductive structures and contacts under test. The conductive structures are electrically connected with the substrate and the contacts under test are not electrically connected with the substrate. The conductive structures and the contacts under test are regularly arranged in array. When an electronic beam is utilized to perform in-line monitoring, the normal contacts under test will be shown as bright dots and the bright dots are regularly arranged in the array; any contact under test with defect will be shown as a dark dot which results in an irregular arrangement of the bright dots.