Researchers Develop Non-Contact Technique for Cleaning Wafers
Maria A. Lester, Associate Editor -- Semiconductor International, 9/1/1999
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Figs. 1 and 2 show the before and after results,
respectively, of cleaning with NanoClean technology. (Source: Phrasor
Scientific) |
As a precision cleaning process, this can be used as a supplemental method for removing submicron or smaller contaminants left behind by conventional wet chemical cleaners, such as micron-sized water spots. Trace metallic and organic coatings can be removed. Other surface cleaning applications include wafer backsides, edge bevels, post-CMP cleaning, flat-panel displays and photolithography masks, as well as others in other industries. Cleaning of wafer probe card tips, used in testing individual dice on silicon wafers, is presently done in cleaning chambers manufactured under license from Phrasor Scientific. The cleaning action lowers the contact resistance of probe tips.
A scanning electron microscope (SEM) recently was used to demonstrate the
in-situ removal of submicron particulates from silicon wafers. The microscope
was modified to attach a specially designed NanoClean head to clean prepared
wafer samples. The procedure co nsisted of inserting a sample containing a range
of particles in the SEM. The cleaning action was verified by comparing the
photomicrographs of the same local areas before (Fig. 1) and after (Fig. 2)
cleaning. The SEM photo taken before cleaning shows the wide range of particles
generated from scribing a trench. After exposure of the sample to the
microcluster beam, the second SEM photo shows the cleaning action removed
particles from a few microns down to the resolution of the microscope, estimated
at 0.05 µm. 
Medium Pressure, Speed Help in
Post-CMP Cleaning
An understanding of particle adhesion and removal mechanisms in post-CMP cleaning is needed to develop effective cleaning techniques. Researchers at AlliedSignal Inc. (Sunnyvale, Calif.) with Clarkson University (Potsdam, N.Y.) studied how post-CMP cleaning effectiveness was impacted by the effects of pressure, rotation speed, pad hardness and chemical addition. Detailed results were reported in the July issue of The Journal of the Electrochemical Society. The process removal forces were compared and evaluated with adhesion forces to further explain the cleaning efficiencies of different types of abrasive particles.
Post-CMP cleaning usually consists of a final polishing followed by
mechanical brushing. According to the researchers, three types of forces exist
throughout the process: hydrodynamic forces, contact forces from the asperities
and particle adhesion forces. It was found that particles were removed only if
the removal forces overcame the adhesion forces. If a particle is too deep and
the adhesion force is large, it is very difficult to remove the particle.
Experimental data indicated medium pressure and speed provided favorable
results. Extremely high pressure or speed left more defects on the (TEOS) wafer
surface by causing more scratches or more plastic deformation. Extremely low
pressure or speed did not provide enough force for removal or cleaning.
