Freeze cleaning involves the selective separation of particles from the substrate surface by utilizing the volume expansion that occurs when water in a supercooling state changes to ice. This allows particles to be efficiently and easily removed without causing pattern collapse. However, this method has a long processing time because of the following two factors: (i) the particles tend to remain at the four corners of a rectangular substrate; (ii) particle removal efficiency (PRE) per freeze-thaw cycle is low. These factors necessitated 30 repetitions of the freeze-thaw cycle to obtain sufficient removability over the entire substrate surface, which prolonged the processing time. Therefore, we attempted to improve the removability at the four corners of the substrate and PRE per freeze-thaw cycle. The experimental results showed that removability at the four corners of the substrate was enhanced by improving the discharge efficiency of particles separated from the substrate. Furthermore, the PRE per freeze-thaw cycle was improved by achieving a uniform temperature distribution across the substrate at the end of supercooling. These measures reduced the processing time to 1/6 and allowed us to successfully develop a device for mass production.
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.
Background: Although the wet cleaning process has been widely used in semiconductor device manufacturing due to its convenience, it faces theoretical limits. That is, when the size of the objected particle is smaller than 100 nm, it is buried in the stagnant layer where there is substantially no fluid flow.
Aim: Only small particles below the stagnant layer (<100 nm) is removed without any damage to the fine patterns or substrate: pattern collapse, critical dimension shift, and optical property shift.
Approach: Utilizing unique characteristics of water: volume expansion when freezing, solid (ice) is lighter than liquid (water), and particles adhered the substrate is peeled off from the substrate and rise to the water surface along with the surrounding ice.
Results: By repeating the cycle of cooling, thawing, and rinsing, polystyrene sphere particle of 80 nm in diameter can be removed with high particle removal efficiency (PRE >90 % ) and no negative influences on the pattern or substrate.
Conclusions: A new cleaning method for very small (<100 nm) particles is proposed with high PRE and low damage. This method is thought to be applied to every process if water can infiltrate into the gap between the particles and the substrate.
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