Switch potential electron beam substrate tester

The present invention describes a method for testing the interconnect networks of a multichip module for opens and shorts. An electron beam lands on a pad of an interconnect network located on a substrate. The electron beam is used to interrogate the pad. An extract grid located above the substrate is maintained at a positive potential. While the electron beam interrogates the pad, the pad emits secondary electrons until such a point that the pad reaches a positive potential near that of the positive potential of the extract grid. The extract grid is then switched to a negative potential. The pad, still being interrogated by the electron beam, then collects secondary electrons until such a point that the pad reaches a negative potential near that of the negative potential of the extract grid. The test time, the length of time it takes for the pad to change from the positive potential to the negative potential, is measured and compared to a reference value. From this comparison it can be determined whether the interconnect network is defect-free, open, or shorted. If the test time is approximately equal to the reference value then the interconnect network is defect-free. If the test time is lower than the reference value then the interconnect network contains an open. If the test time is higher than the reference value then the interconnect network is shorted to another interconnect network or etc. Additionally, the above described invention can be accomplished by maintaining the extract grid at ground potential and switching the potential of a backplate.