HETERODYNE INTERFEROMETER FOR THE DETECTION OF ELECTRIC AND THERMAL SIGNALS IN INTEGRATED-CIRCUITS THROUGH THE SUBSTRATE

A laser diode based probing system for the interferometric detection of charge density and temperature modulations accompanying electric signals in integrated circuits has been developed. Its main advantage compared to previously introduced measuring schemes is a versatile and inexpensive positioning system for the probing and reference laser beams, noise reduction due to the use of heterodyn techniques and the absence of polarizing elements. The temporal resolution is limited by the laser diode pulse width to 500ps. The probe and reference beam are both focussed to a diameter of 1.5mum through a 400mum thick silicon substrate which determines the spatial resolution. The system's abilities are demonstrated in measurements of bipolar and CMOS test structures.