Two-Photon Method Images Flip Chips

The "face down" mounting of flip chips onto motherboards has provided a more efficient way of packaging processors, but it also has complicated inspection techniques because only the back of the chip is directly observable. Using a phenomenon known as the two-photon optical-beam-induced current effect, researchers at Heriot-Watt University in Edinburgh, UK, have captured two- and three-dimensional images of a 0.35-µm-feature-size flip chip with resolutions down to 1 µm in all three dimensions. The technique, which is reported in the July 1 issue of Applied Physics Letters, uses femtosecond, two-photon excitation at 1.275 µm to induce a photocurrent at specified locations within the chips -- thus using the chip itself as a nonlinear detector.