Characterization of Acoustic Cavitation from a Megasonic Nozzle Transducer for Photomask Cleaning

Megasonic agitation continues to be used in advanced 193i and EUV photomask cleaning processes. The trend to adopt higher frequencies is driven by the need to control a tighter process window with shrinking feature sizes and a zerotolerance defect requirement. Despite continued use of megasonics, the effect of the acoustic field applied to the mask substrate remains unclear. Photomask cleaning is a dynamic process with many parameters that contribute to the particle removal efficiency and pattern damage. These include transducer type, transducer position, drive frequency, power setting, flow rate, chemistry, gas concentration, etc. To add to the complexity, when the acoustic waves from the transducer interact with a quartz mask the energy may reflect from, transmit through, or couple into the substrate. An in-situ measurement of the acoustic field, as present at the feature location, is required to correlate acoustic parameters with cleaning performance. This work introduces a photomask-shaped cavitation sensor capable of measuring the absolute pressure from the direct field, stable cavitation, and transient cavitation present at the surface. In contrast to previous work characterizing skirt-type transducers, this measurement instrument is sensitive to higher drive frequencies while withstanding concentrated pressure levels from nozzle transducers. Here, a megasonic system with dual nozzle transducers at 5 MHz and 3 MHz was evaluated. The aim of the study is to better understand the acoustic properties from different types of transducers for process development and monitoring in the quest to correlate with photomask cleaning. © 2017 SPIE.