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/NH ₄OH 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 NH ₄OH, 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.