Sidney C. Chao

Method using megasonic energy in liquefied gases

A process for removing undesired sub-micrometer particulates from a chosen substrate (16) comprising the steps of: (a) placing the substrate containing the undesired particulates in a cleaning chamber (12) provided with megasonic energy-producing means (20); (b) introducing a liquefied gas (22), such as liquid carbon dioxide, into the cleaning chamber and contacting the substrate containing the undesired particulates with the liquid carbon dioxide at a temperature below its critical temperature; and (c) exposing the liquid carbon dioxide to the megasonic energy-producing means for a period of time sufficient to remove the undesired particulates from the substrate. The substrate containing the undesired particulates may optionally be contacted with carbon dioxide in the dense phase prior to and/or after the treatment with megasonic energy to aid in removal of the undesired particulates. Further, spent liquid carbon dioxide may be treated to regenerate fresh liquid carbon dioxide which is recycled to the cleaning chamber. Other gases besides carbon dioxide which may be used include nitrous oxide, sulfur hexafluoride, and xenon. Further, gas mixtures and gas mixtures with suitable modifiers may be employed.