ELECTRON BEAM TESTING OF INTEGRATED CIRCUITS USING A MODIFIED SCANNING ELECTRON MICROSCOPE

Abstract

Traditionally, the mechanical probe has been used extensively in probing internal voltages of semiconductor devices. However, due to the capacitive loading and large probe diameter. the mechanical probe are no longer capable of probing the VLSI devices which are operating at higher frequencies and have smaller structures. The electron beam has been identified as the only usable alternative for probing VLSI devices. The electron beam is a easily positioned, high resolution, nonloading and nondestructive probe. A conventional scanning electron microscope (SEM) can be used for electron beam probing. This dissertation describes the study of the problems of electron beam probing using a SEM. A primary beam energy of about 1 keV is required to prevent irradiation damage and charge-up of ICs. This requirement leads to significant loss of electron beam brightness and hence a degraded signal-to-noise ratio. In this work, the gun brightness of the SEM at low operating voltage was optimized by modifications to electron optics in the electron gun. This modified SEM was then used to observe static and dynamic voltage contrast phenomena at very low frequencies. Voltage coding as a technique was also implemented. Real time waveform measurement was also performed for signals up to 2 MHz. This frequency limit is imposed by the bandwidth limitation of the secondary electron detector which is designed for operation up to TV rate. Continuous beam irradiation at a spot causes contamination build-up. Contamination problems can be prevented if the beam is allowed to stay at the same spot for less than 4 seconds. This however, is not always possible because long recording times are required for signal to noise ratio improvement using boxcar averager. This contradicting requirement can be overcome by primary beam pulsing. Beam pulsing was achieved by deflecting the beam across a chopping aperture by using a set of parallel plate capacitor. Electron pulse of 5 nonoseconds width could be generated at a clock speed up to 20 megahertz. Primary beam pulsing also overcomes the bandwidth limitation of the secondary electron detector. Time resolved stroboscopic voltage contrast at high frequencies was implemented. The time resolution of such a system is solely dependent on the shortest primary electron pulse that can be generated. The secondary electron transit time, on the other hand, limits the maximum frequency at which the device under test can be clocked. The Philips SEM with beam blanker, the SEMICAPS system and the boxcar averager were linked to form an integrated electron beam tester. The primary beam can be stationed at any point within an area of 512 X 512 pixels for waveform measurement. The acquired secondary electron waveform can be displayed on the computer monitor showing the timing and qualitative voltage information. Semi-quantitative voltage contrast without a secondary electron analyzer could also be performed by calibrating the detector output against a known specimen voltage This integrated electron beam tester is used to test an operational amplifier (741). The waveforms measured with the system are in good agreement with the waveforms measured using a mechanical probe. Semi-quantitative voltage waveforms measurement were also obtained to reveal the actual voltage and timing information.