Characterization of Sub-micron Metal Line Arrays Using Picosecond Ultrasonics

Characterization of patterned nanostructures in modern nanoelectronic memory and logic devices using traditional optical critical dimension (OCD) metrology technique faces challenges as the structures become optically opaque. In this paper, we present the picosecond ultrasonic (PU) based acoustic metrology technique as a viable option to characterize periodically patterned nanostructures. Specifically, we evaluate the sensitivity of PU to metal line arrays of various geometries and show that the frequency profile of generated acoustics is dependent on the sample geometry including the pitch and width of the metal lines exposed. We also demonstrate that the signal is sensitive to the presence of the lines embedded under a blanket layer of the same metal.