Metrology of adhesive layers using acoustic microscopy

A procedure for metrology of thin adhesive layers via acoustic microscopy is discussed in this paper. The phenomenon of reflection of ultrasonic pulses at a layered interface is exploited to make these measurements. An experimental jig, where the layer thickness could be varied continuously from near zero to a known value, was used to validate a theoretical model that is used to estimate layer properties. The practical application of measuring the thickness of a layer of heat conducting epoxy sandwiched between the die backside and a heat spreader was analyzed. Results from this theoretical experiment suggest that small changes in material parameters result in measurable changes in the ultrasonic parameters. For example, a change of 1 MHz in the measured frequency parameter corresponds to a 0.4 /spl mu/m variation in the adhesive layer thickness. Since frequency can be measured with accuracy greater than 1 MHz, relatively small variations in layer thickness and material properties could be monitored nondestructively using the C-SAM acoustic microscope.