Freeze point monitoring system for freeze cleaning method

Abstract

We propose freeze cleaning as a method of photomask cleaning in which particles small enough to be embedded in the region less than 100 nm from the substrate surface, where there is virtually no fluid flow, are selectively removed without causing pattern collapse. In freeze cleaning, a high particle removal efficiency is achieved by repeating the sequence of liquid (deionized water) being poured onto the substrate, freezing, and thawing (rinsing) multiple times. Based on the mechanism of particle removal, the timings at which the water freezes, ice growth, and freezing of the entire surface are important parameters that govern freeze cleaning performance. In contrast, when these timings were monitored during repeated processing, a maximum variation of about 16% was observed. The most significant cause of these fluctuations is attributed to the process performed in a system that is open to the atmosphere at room temperature, despite the use of cryogenic N2 at -120°C. Even with these timing fluctuations, by developing and applying an algorithm that monitors individual changes and automatically determines step switching using this monitor information, it is possible to construct a stable and highly efficient processing system without any tool modification.