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This paper mainly describes a liquid based gravity driven etching stop technique used for cantilever thickness control of atomic force microscope (AFM) probes on the wafer level. The technique makes use of the method of opposite etching trenches or the depth rulers. A pair of opposite trenches surrounds several AFM probes on both sides of the wafer to form probe chips. The trench depth on the cantilever front side is equal to the designed thickness of cantilevers. In the final step of the fabrication process for AFM probes, the wafer is etched by the KOH etchant to form the probe handles. The probe chips will be separated from the wafer simultaneously with the penetration of wafers at the trenches. The separated probes fall into the diiodomethane (CH2I2) solution beneath the KOH etchant and the wet etching stops automatically. The cantilever thickness of the AFM probes can then be wafer level controlled by the proposed etching stop technique.