Aqueous foam is generally recognized as the medium of choice for cleaning tasks ranging from household tasks to large industrial applications.^1,2 This fact has not been lost on the semiconductor manufacturing industry, as over the years there have been attempts to couple the advantages of aqueous foam cleaning to the special needs of wafer production.^3,4 The most important experiment, indirectly defining that aqueous foam could be used for cleaning semiconductor wafers, was performed by Leenaars⁵,who showed that a single laser generated bubble could be passed over the surface of a wafer producing nearly quantitative particle removal. A summary of the theory and practice of aqueous foam cleaning as it pertains to semiconductor wafers led to the experiments described in this paper^.6

Objectives

Demonstrate that a liquid composition delivered to a semiconductor wafer substrate in a foam medium can provide cleaning results equivalent to the same composition delivered to the wafer in the liquid phase.

Demonstrate that only collapsing aqueous foam bubbles are capable of cleaning semiconductor wafer substrates, without the use of cleaning compositions, in a manner analogous to ultra- or megasonic cleaning.

Theory

The theory pertaining to the first objective involves the fundamentals of aqueous foams. When a suitable fluid is transformed into foam, energy is added during the process of mixing the expansion gas with the foamable liquid. When the foaming system is fully developed for the mixing conditions utilized, the foam formation rate is equal to the foam decay rate, and the system is in equilibrium. If this completely mixed equilibrium foam is discharged from the mixing system, the formation rate declines to zero, but the drainage rate remains the same. The important feature is the immediate onset of drainage the instant the foam leaves the mixing system^.7

In simple foaming systems—water with low levels of surfactant—the foamable liquid with added expansion gas creates foam, and the draining process, in effect, releases the expansion gas, thereby regenerating the original foamable liquid. If the foaming system does not contain any high vapor pressure components which could escape with the expansion gas, then the initial liquid and the post-drainage liquid will be of the same composition.

Since the semiconductor wafer cleaning processes are generally restricted to the liquids covering the wafer surface, foam discharged onto a wafer surface will instantly generate a liquid surface covering with a composition equal to the original foamed liquid.