Observation of the internal waveforms in high-speed high-density LSIs using an EOS prober

Electron beam (EB) probing has been used to observe the internal waveforms in ULSLs. However, as the speed of LSIs increases and their power consumption decreases, the temporal and voltage resolution of EB probing is going to be inadequate for measuring these waveforms. Electro-optic sampling (EOS) probing is expected to overcome this limitation, provided its spatial resolution can be improved. In this paper, forming a via on the interconnection under test is proposed for practically improving spatial resolution. The effectiveness of this method and the dependence of the detected signal level on testing pad area are clarified by simulation. To verify these results, vias are formed in ECL SRAMs by focused ion beam trimming, and waveforms with frequency as high as 100 MHz and swing as small as 30 mV are successfully observed. Using this technique, the dependence of the detected signal level on testing pad area is verified, and it is confirmed that the amplitude change below 10 mV and the delay time below 100 ps can be measured. With the improved spatial resolution, EOS probing enables us to observe high-frequency and small-swing waveforms even in high-density fine interconnections. In future, this technology will be crucial for observing internal waveforms in LSIs.