Characterization of Sub-Optical-Wavelength Structures through Optically Opaque Films Using Picosecond Ultrasonics

Periodic arrays of nanostructures form an important building block of modern integrated circuits and photonic devices. Functionality of such devices is often critically dependent on the detailed structure. Moreover, multistep lithographic processing requires accurate metrology tools to characterize device morphology noninvasively, often after the deposition of additional layers of material. Here we show that ultrafast picosecond ultrasonics enables the accurate characterization of periodic structures below optically opaque thin films. By optically generating and detecting ultrahigh-frequency ultrasound at the surface of the film, we quantitatively characterize the main features of subsurface gratings with line widths down to 100 nm. We find that the acoustic diffraction is sensitive to the shape of the grating lines on the scale of tens of nanometers.