Megasonic cleaning tool seeks a clean sweep at 65nm and below


At advanced technology nodes (65nm and below) there is a need for cleaning technology that better manages the ever-shrinking cleaning process window. Mechanical damage and defect levels — particularly gate and capacitor structures that become increasingly fragile — become more challenging with each node. It’s this sweetspot that ACM Research Shanghai Ltd. is targeting with its new Ultra C 300mm single-wafer megasonic cleaning tool, which debuted at SEMICON China in March.

Figure 1. PRE test results with 0.17µm polystyrene particles. (Source: ACM Research)
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David Wang, founder and CEO of ACM Research, explained to SST that the proprietary megasonic technology — called space-alternated phase shift (SAPS) — allows tight and uniform control of the megasonic energy distribution across the wafer (2% at 1σ). This enables a particle removal efficiency (PRE) of up to 99.2% using a diluted SC1 cleaning chemical, and 98.3% using de-ionized (DI) water only, without damaging the patterned wafer (Fig. 2). The result is a uniform megasonic power density within-wafer (WIW) and wafer-to-wafer (WTW) non-uniformity of <2% (Fig. 2).

Figure 2. Wafer map using SAPS megasonic technology. (Source: ACM Research)
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The key, Wang said, is to control the mechanical process window of the megasonic energy so that there is enough energy to cause cavitation, but not so much that it causes damage to a patterned wafer (Fig. 3).

Figure 3. The key to megasonics is controlling the mechanical process window so that there is enough energy to cause cavitation (formation of bubbles), but not enough to cause damage to a patterned wafer. (Source: ACM Research)
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“When megasonic power is not uniformly distributed, hot spots form on the wafer where megasonic energy is higher, causing bubbles to collapse,” said Wang. “When a megasonic bubble collapses, it releases a high-pressure (1000atm) microjet that can have a temperature of up to 4000°C that can easily damage fragile structures on the wafer.”

The new technology uses what Wang describes as a “stable cavitation bubble oscillation method” that allows the bubbles to continuously inflate and deflate without collapsing, providing an ultra-uniform energy distribution. The energy of the continuously inflating/deflating bubbles causes the particles to be moved from the acoustic boundary layer, to the thermal diffusion layer, and on to the hydrodynamic boundary layer where they can be carried away by the cleaning solution.

Although the company reports 98.3% PRE rates with DI water alone, the new tool is able to connect up to five chemicals simultaneously, each with its own customizable separation and reclaim functions. —D.V.