Particle removal from wafer surfaces can be accomplished by irradiation of cleaning fluid by sound waves in the MHz frequency range. Unfortunately, unless proper cleaning conditions are chosen, megasonic irradiation may also result in damage to fragile wafer features. Here, we demonstrate a strong effect of dissolved CO 2 levels on the reduction of wafer damage during megasonic cleaning. Test structures with L/S patterns were irradiated with 0.93 MHz sound waves at varying power densities and dissolved CO 2 levels, in a single wafer spin cleaning tool, MegPie ® . Dissolution of increasing amounts of CO 2 in air saturated DI water caused a significant decrease in the number of breakages to line structures and also decreased the lengths of the line breakages, at all power densities up to 2.94 W/cm 2 . This ability of dissolved CO 2 to protect against feature damage correlates well with its ability to suppress sonoluminescence in sound irradiated DI water.
Acoustic cavitation is known to be a primary source of both cleaning and damage of wafers during their megasonic processing. Understanding the response of process fluids to variables like acoustic power recipe and dissolved gases is an important first step in achieving damage-free megasonic cleaning of wafers. This paper reports the development of a portable, UV light tight, cavitation threshold (CT) cell to measure sonoluminescence (SL) signal arising from cavitation. The closed cell, integrated with a gas sensor and contactor, allows SL measurements under very controlled conditions. Using the CT cell the effect of the concentration of dissolved O 2 , CO 2 and air on SL signal has been investigated. Results show that SL varies linearly with dissolved O 2 concentration while CO 2 is found to be incapable of supporting SL. This study also demonstrates a novel method for precise control of SL through addition of an O 2 scavenger with fast O 2 removal kinetics.
Light emission in sound-irradiated liquids, known as Sonoluminescence (SL), is associated with the phenomenon of cavitation that affects wafer damage during megasonic processing of wafers. It has been shown that the intensity of SL can be substantially decreased through the dissolution of carbon dioxide in deionized water. However, such dissolution decreases the pH to roughly 4.0, which is not very desirable for the removal of contaminant particles. This paper reports two chemical systems that are capable of taking advantage of the effect of CO2 while allowing the use of slightly higher pH values. Specifically, NH4OH/CO2 and NH4HCO3/dilute HCl systems have been shown to be capable of well controlled reduction in SL at pH 5.7 or 7.0. In order to test whether the free radical scavenging ability of CO2 may be responsible for its strong SL-inhibitory effect, the effect of a well known free radical scavenger, dimethyl sulfoxide (DMSO), on SL produced in DI water has been investigated.
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