A new low‐voltage contrast mechanism due to electron‐hole pairs generated in the oxide by an electron beam was observed at electric fields lower than 3.5 MV/cm. This is in addition to the tunneling current microscopy (TCM) contrast mechanism observed at electric fields higher than 3.5 MV/cm. The new contrast mechanism is opposite in sign to the TCM contrast mechanism such that the image of a local thinning defect in the oxide is dark at low bias voltage and bright at higher bias voltage. Good contrast can be obtained at electric field as low as 2.4 MV/cm. Applications include large area imaging of oxide defects and quantitative mapping of small oxide thickness variations. ©1995 American Institute of Physics.