Background and applications of electron beam test techniques

Abstract

The use of electron beams for contactless testing of electrical functions and electrical integrity of different active devices in VLSI - chips - has been demonstrated over the past years. This method of testing electronic networks is based on an electron probe which is moved from point to point in the network and a current of secondary electrons emitted in response to the impingement of the probe is converted to a signal indicating the presence of a voltage or varying potential at the different points. Voltage contrast, electron beam induced current, dual potential approach, stroboscopic techniques and other methods have been developed and are used to detect different functional failures in devices.Very little attention in most of the applications has been paid to the electron optical environment, mostly SEM's were upgraded or converted to do the job of a “voltage contrast” machine. This by no means will satisfy all requirements and new thoughts will have to be given to aspects such as: low voltage electron guns, two lens systems, different means of detection, signal processing and storage are a few of them.Besides the VLSI application, the contactless testing of three dimensional conductor networks of a 10 cm × 10 cm × .8 cm multilayer ceramic poses a different and new application. The mechanism of electron-solid interaction, the secondary electron generation, the energy distribution, the secondary electron yield, are important parameters to understand for any electron beam test technique. According to the secondary electron yield curve in Figure 1, floating samples can be charged positively or negatively with respect to ground, depending on the primary electron energy.The surface potential of a sample in turn determines the impact energy of the primary electrons and also energy of the secondaries that leave the surface. These two effects generate different voltage contrasts to be used in electron beam testing. The test method for packages uses multiple electron beams to both generate and detect voltage differences between the surface terminations of conductors in order to identify opens and short in the package networks.