Photoacoustic imaging of layered microcircuits for non-destructive evaluation of sub-surface defects

Scanning PhotoAcoustic Microscopy (SPAM) was used for the imaging of surface and sub-surface layers in electronic circuits and ad hoc specimens. Depth-profiling SPAM was used, as it allows the imaging of opaque samples and sub-surface features; the signal contrast comes from variations in the optical, thermal or mechanical properties of the samples. In SPAM an intensity-modulated, highly-focused light beam (ion argon laser, 488 nm, 20 mW) scans the surface of the sample in the photoacoustic cell, with micrometric resolution. By varying the modulation frequency of the light beam, the thermal diffusion length of the heat waves inside the sample is changed, thus probing different layers beneath the sample surface. From the comparison of the SPAM images of multi-layer microcircuits obtained at different modulation frequencies, features of the first layers, circuit tracks and conductive connections between the layers can be detected. The SPAM images also correspond dimensionally with the actual layout of the circuits and may reveal the presence of defective parts. The preliminary results presented here suggest the introduction of the SPAM technique in the production process for NDE of layered microcircuits and, in general, for the quality control of sample parts. © 1995.