Optical detection of multibit logic signals at internal nodes in a flip‐chip mounted silicon static random‐access memory integrated circuit

We report the first optical measurements of multiple bit‐pattern logic signals at internal nodes in a flip‐chip mounted silicon static random‐access memory integrated circuit. This probing system measures signals by interferometrically detecting the electrically induced charge‐density modulation within devices and parasitic pn junctions. The optical measurements, made on an output driver circuit, compared well with those on an external high‐speed oscilloscope. Measurements made at internal points generally compared well with circuit simulations. However, the bit‐sense circuit measurements suggested internal logic‐level problems in the circuit design.