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 o f charge density modulations and temperature changes which accompany e lectric signals in integrated circuits has been developed. Measurement s are performed through the substrate of the device under test. Compar ed to previously introduced measuring schemes the system offers the ad vantage of a versatile and inexpensive positioning system of the probi ng and the reference laser beam, noise reduction due to the use of het erodyn techniques, and the absence of polarizing elements. The tempora l resolution is limited by the width of the laser diode pulse to 500 p s. The probe beam can be focused to a diameter of 1.5 mum through a 40 0-mum-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 f or a growing number of highly integrated circuits the front side of wh ich is masked by multilevel wiring, bonding wires, or lead frames.