Acoustic cavitation continues to be widely used in advanced 193i and EUV megasonic photomask cleaning. However, process challenges remain complex as patterns become smaller and higher in aspect ratio. Operating within a narrow process window that ensures both full particle removal and pattern damage control is a balancing act that requires many parameters to be optimized and controlled. These include the transducer type, drive frequency, power setting, flow rate, gas concentration, temperature, chemistry, transducer position, not to mention the motion of the photomask. Furthermore, the high cost of an advanced lithography process demands a zero-tolerance defect requirement, a constraint prevalent across the semiconductor industry. To correlate acoustic parameters with cleaning performance, an in-situ measurement of the acoustic field is required. Previous work introduced a photomask-shaped cavitation sensor array wired to a cavitation meter capable of measuring the absolute pressure from the direct field, stable cavitation, and transient cavitation present at the surface. Process variables such as drive frequency, generator power, transducer distance, and sensor position were correlated with cavitation pressure. In this study, the technology was extended by developing a wireless photomask-shaped sensor array to acoustically characterize the dynamic effects while the photomask rotates and the transducer arm sweeps. The aim of the study is to better understand how process variables affect acoustic performance to ultimately establish a process control strategy.

In-situ cavitation measurements with a wireless sensor array: Applications in megasonic photomask cleaning

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