A nondestructive methodology for characterizing Cu interconnects in integrated circuits (ICs) has long been sought and has motivated significant efforts regarding metrology tool development. Here, we present investigations of high-frequency heterodyne force microscopy (HFM) to investigate the acoustic response of damascene-processed Cu interconnects in a silicon oxide dielectric. HFM is a phase-resolved mechanical imaging technique that can, in principle, measure acoustic phase variations with nanometer spatial resolution. HFM measurements at carrier frequencies of 10.8 MHz and 72.5 MHz reveal phase differences of 0.5°-0.7° and 3.5°-4.5°, respectively, between the SiO2 and Cu regions of the IC test structure. Such phase differences are consistent with expected variations due to acoustic propagation time differentials between the Cu and SiO2 regions. Coupled with detailed mechanical measurements of the Cu and SiO2 regions this data may be used to extract spatial maps of acoustic phase delay and investigate subsurface features of similar IC test structures.