Enhanced megasonic processing of wafers in MegPie® using carbonated ammonium hydroxide solutions

Megasonic irradiation of cleaning solutions removes particulate contaminants from wafer surfaces through the mechanisms of acoustic streaming and acoustic cavitation. Uncontrolled cavitation, however, also damages fragile wafer features. It has been hypothesized that damage results primarily from violent transient cavitation while cleaning results from the gentler, stable cavitation and is assisted by substrate etching at alkaline pH. A central challenge in the megasonic cleaning field is to find physical and chemical conditions that would suppress violent cavitation without adversely affecting cleaning efficiency. We have previously reported the strong ability of aqueous CO2 in suppressing sonoluminescence, a measure of transient cavitation, and wafer damage. However, dissolution of CO2 also renders solutions acidic and lowers their ability to remove particles. Here we report the development of two new systems, NH4HCO3/NH4OH and NH4OH/CO2(g) designed to suppress pattern damage and enhance cleaning efficiency. Using megasonic irradiation of bare and line-space patterned wafers in a single wafer spin cleaning tool, MegPie®, we measured the efficiencies of these systems as well as NH4OH, at pH 8.2, in removing SiO2 particles from oxide Si wafers and suppressing damage to wafer features. All systems were found to achieve high particle removal efficiencies but extent of damage was strongly reduced in the newly designed NH4HCO3/NH4OH and NH4OH/CO2(g) compared to NH4OH. This study establishes a means for well controlled generation of CO2(aq) over an extended pH range (4.0–8.5) and its application in strongly reducing wafer damage without compromising megasonic cleaning efficiency.

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► We report development of two new systems, NH4HCO3/NH4OH and NH4OH/CO2(g), for megasonic cleaning.
► Carbonated ammonium hydroxide solutions exhibit high particle removal efficiency.
► Feature damage is significantly reduced in carbonated ammonium hydroxide solutions.