Picosecond noninvasive optical detection of internal electrical signals in flip-chip-mounted silicon integrated circuits

This paper reviews the charge-sensing optical probing system, and shows how it may be used to detect internal current and voltage signals in flip-chip-mounted silicon integrated circuits. Previously, researchers have used this concept to detect both single-shot 200-MHz-bandwidth signals, without averaging, and 8-GHz-bandwidth stroboscopic signals. This system has a high sensitivity: 145nA/√Hz current sensitivity in typical bipolar transistors, and 1.35mV/√Hz voltage sensitivity in typical CMOS circuits (using a semiconductor laser probe). It is noninvasive, has a potential submicron spatial resolution, and should be capable of providing linear and calibrated measurements. Therefore, this probing approach should be a powerful tool for future circuit analysis and testing.