Heterodyn 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 modulations and temperature changes which accompany electric signals in integrated circuits has been developed. Measurements are performed through the substrate of the device under test. Compared to previously introduced measuring schemes the system offers the advantage of a versatile and inexpensive positioning system of the probing and the reference laser beam, noise reduction due to the use of heterodyn techniques, and the absence of polarizing elements. The temporal resolution is limited by the width of the laser diode pulse to 500 ps. The probe beam can be focused to a diameter of 1.5 μm through a 400-μm-thick silicon substrate which determines the spatial resolution of our measurement system. It should be stressed that probing through the substrate provides the only access to internal measurement nodes for a growing number of highly integrated circuits the front side of which is masked by multilevel wiring, bonding wires, or lead frames.