Thermodisplacement imaging of current in thin-film circuits

We present initial results on a novel scheme for measuring current distribution in thin-film circuits. When an AC current passes through a thin-film circuit, the resulting periodic heating sets up a dynamic surface displacement which can be detected using a phase-sensitive laser probe. Results indicate that the present system has a sensitivity of 4 × 10¿3 Å for displacement and 2.7 × 10¿2°C for temperature in a 1 Hz bandwidth. The spatial resolution can be as small as an optical wavelength and is currently limited to 2 ¿m by the optics used. As the surface displacement amplitude depends on the thermal properties of the substrate material, the technique can also be used to detect defects beneath the metallisation in integrated-circuit structures.