Megasonic cleaner

Details Megasonic cleaner Megasonic cleaner

1998-11-11

2001-11-06

Stinson, Frankie L. (Department: 1746)

Cleaning and liquid contact with solids

Processes

Including application of electrical radiant or wave energy...

C134S025400, C134S184000, C134S902000, C310S335000, C310S323190

Reexamination Certificate

active

06311702

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to cleaning fragile substrates with sonic energy. More specifically, the present invention relates to an improved semiconductor wafer cleaning system which employs megasonic energy.
BACKGROUND OF THE INVENTION
A conventional method for cleaning particles from semiconductor wafers is known as megasonic cleaning. During megasonic cleaning, a transducer oscillates between compressed and strained states at a near 1 MHz rate. The transducer is operatively coupled to a source of fluid, either a fluid filled tank, or fluid flowing through a nozzle. The megasonic oscillation output by the transducer is thereby coupled to the fluid causing pressure oscillation therein. As the pressure in the fluid oscillates between positive and negative, cavitation or bubbles form in the liquid during negative pressure and collapse or shrink during positive pressure. This bubble oscillation gently cleans the surface of the wafer.
In practice, megasonic cleaners experience a number of limitations. For instance, transducers with higher power density assure better cleaning efficiency, but generate considerable heat during operation. Accordingly, transducer cooling systems are often used to prevent degradation of adhesive material that attaches a transducer to materials that couple the transducer's acoustic power to the cleaning fluid.
Such transducer cooling systems, however, undesirably increase the cost and complexity of a megasonic cleaning system.
An alternative approach has been to employ a cycled array of multiplexed transducers in which each transducer is on only 1/Nth of the cycle time, where N is the number of transducers per cleaning vessel.
The reduction of duty cycle by a factor of N, which is usually 8 for batch processing vessels for 8 inch wafers, reduces transducer temperatures and in some cases eliminates the need for transducer cooling systems. A major problem of this approach is the often unacceptable increase in processing time by a factor of N. The increase in processing time is particularly problematic for single wafer processing, where short processing time is an important requirement.
Another problem experienced by megasonic cleaners is the shadowing of the transducer's acoustic field by the wafer carrying cassette. Conventionally, two approaches are employed to address cassette shadowing. The first approach uses wafer rocking to expose shadowed parts of the wafers to acoustic field. This approach reduces the duty cycle on the shadowed parts of the wafers and thus increases the wafer's processing time. This approach also increases system cost and complexity. The second approach uses a convex transducer or convex cylindrical lens to diverge the transducer's acoustic field through the opening in the bottom of the cassette. This approach reduces the power density at the top of the cassette and increases the processing time required to clean the wafers.
Cavitation bubbles formed in the cleaning fluid during megasonic cleaning present additional challenges. Specifically bubble implosion near the surface of a wafer helps to remove particles and thus has a positive effect on cleaning efficiency. However, bubbles in the bulk of cleaning solution (i.e., not near the wafer's surface) scatter the acoustic power and thus cause a decreasing power density along the surface of the wafer as the distance from the transducer increases.
Accordingly, a need exists for an improved method and apparatus for sonic cleaning of semiconductor wafers.
SUMMARY OF THE INVENTION
The present invention provides an assembly wherein an elongated transducer is coupled to a focusing element. The focusing element may be a concave cylindrical lens, a concave cylindrical energy emitting surface of the transducer or a concave cylindrical reflector, etc. The focusing element may be coupled in line with the transducer (see
FIG. 1A
) or in an angular relationship (see FIG.
1
B). Both the elongated transducer and the focusing element preferably extend a distance slightly greater than the diameter of the wafer, so as to megasonically clean a line from one edge of the wafer to the other. The wafer may then be translated or rotated (or the transducer and/or focusing element may be translated or rotated) so that megasonic energy scans the entire surface of the wafer.
Because the megasonic power is focused, the power density increases from the transducer to a focal point which may be positioned, preferably on or beyond the surface of the wafer. Thus, with the present invention, the power to the surface of the transducer may be lower; therefore lowering operating temperatures, and eliminating the need for costly cooling systems. Moreover, the power output by the transducer may be selected such that cavitation occurs only near the wafer surface. In this manner the bubbles do not scatter portions of the transmitted acoustic wave, rendering the transducer assembly highly efficient.
In a first embodiment a megasonic cleaning system which employs the inventive assembly comprises a tank filled with liquid for submerging a wafer. A transducer and focusing element are coupled in a linear relationship (forming a transducer/focuser assembly, see
FIG. 2
) and are positioned adjacent the wafer's first major surface. Preferably at least two such transducer/focuser assemblies are positioned in a spaced relationship adjacent the wafer's first major surface. The spacing between the transducer/focuser assemblies is selected to minimize the movement necessary to scan megasonic energy across the surface of the wafer.
A scanning mechanism is coupled to the transducer/focuser assemblies and/or the wafer to translate or rotate the same so that megasonic energy is scanned across the entire first major surface of the wafer. A similar array of transducer/focuser assemblies can be positioned adjacent the wafer's second major surface so that both the first and second surfaces may be cleaned simultaneously provided each transducer/focuser assembly focuses acoustic energy on or before the nearest major surface of the wafer. Alternatively, a single array of transducer/focuser assemblies can clean both major surfaces of a wafer provided the assemblies acoustic energy is focused on or beyond the farthest major surface of the wafer, and thus travels through the wafer causing cavitation along both major surfaces thereof.
In a second embodiment an inventive megasonic cleaning system which employs the inventive transducer/focuser assembly comprises a tank filled with liquid for submerging a wafer. The transducer/focuser assembly is coupled in an angular relationship, preferably with the transducer positioned adjacent the tank's bottom and the focusing element (e.g. a parabolic reflector) positioned adjacent a major surface of the wafer Preferably, a plurality of focusing elements, each slightly greater in length than the wafer's longest chord, are spaced along the wafer's first major surface such that a small movement of the wafer and/or the transducer/focuser assembly scans megasonic energy across the entire first surface of the wafer. A scanning mechanism is coupled to the wafer and/or the transducer/focuser assemblies to translate or rotate the same. A single array of transducer/focuser assemblies can clean both major surfaces of a wafer provided the assembly's acoustic energy is focused on or beyond the farthest major surface of the wafer, and thus travels through the wafer causing cavitation along both major surfaces thereof.
In a third embodiment a megasonic cleaning system which employs the inventive transducer/focuser assembly comprises a tank containing liquid for partially submerging a wafer. Preferably the tank is sized to submerge half of the wafer in the liquid, and a transducer/focuser assembly coupled in a linear relationship is positioned to focus megasonic energy on a first surface of the wafer slightly below an air/liquid interface. A scanning mechanism coupled to the wafer rotates the wafer such that megasonic energy scans the entir

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