Abstract

High performance silicon VLSI circuits are often covered with four or more layers of metal, which makes many internal nodes inaccessible to conventional front-surface probing approaches. Previously, Heinrich et al.^1,2 demonstrated a backside optical probe for measuring signals in silicon circuits. This work uses a two-beam Nomarski interferometer to sense electronically modulated charge at a device or pn junction in the circuit. By focusing one beam on the emitter of a transistor and the other beam on the base of the device, we can sense the diffusion charge in the device (αIc). By focusing on a pn junction, we can sense the capacitively stored charge (αVj). This paper presents measurements of the current and voltage at the output of an internal line driver in a silicon IC, and compares these measurements to prior circuit simulations. The voltage measurement agreed well with the circuit simulations. However, it showed an additional overshoot not seen in the simulations, caused by current transients on the power supply. The measured rising edge of the current waveform agreed well with the simulation. However, the falling edge showed much deeper undershoot, caused by removal of majority carriers under strong reversed biased operation.

© 1991 Optical Society of America