A high-voltage scanning electron microscopy system for in situ electromigration testing

Abstract

An apparatus has been constructed to conduct electromigration tests on realistic specimens while simultaneously observing them at relatively high magnification. A scanning transmission electron microscope has been converted into a high-voltage scanning electron microscope (HVSEM) with a large specimen chamber. By imaging with high-energy electrons (120 keV) and detecting backscattered electrons, voids in metal lines can be viewed through passivation layers. The HVSEM has a resolution of 50 nm through 1 mu m of passivation. We have constructed instrumentation to heat and pass current through interconnect structures, while they are inside the electron microscope. Presently, the specimen temperature can be as high as 350 degrees C and is maintained constant to within 0.1 degrees C. The resistances of interconnects are measured with a precision of 0.05% during an experiment. Testing the lines at moderately accelerated conditions requires great stability of the microscope and instrumentation as well as full automation of the data collection. These requirements have been met, and metallization lines can be tested for several weeks with minimal operator intervention. Digital images of an entire 300-mu m-long test structure as well as electrical data are stored automatically every few minutes during a test. The hundreds to thousands of pictures are analyzed using digital image processing techniques to extract void positions and sizes as a function of time. We use this system to characterize electromigration failure in advanced interconnect structures and to test existing theories on electromigration phenomenon. (C) 2000 American Institute of Physics. [S0034-6748(00)05007-3].