Sulfuric acid/hydrogen peroxide rinsing study Page: 4 of 8
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TXRF data shown in Figure 1 indicate that the concentration of sulfur on the wafer
surface is approximately the same for all wafers rinsed with or without megasonic power.
The TOF-SIMS S/02 peak height ratio data, shown in Figure 2, also indicate that
megasonic power applied during the rinse step does not reduce the amount of residual
sulfur on the wafer surface. However, the number of light point defects measured
immediately after the SPM clean, shown in Figure 3, indicates that megasonic power
applied during the rinse step significantly reduces the amount of particulate contamination
added to the wafers during this cleaning step. All wafers used in these experiments failed
the SFS-6200 scan less than 24 hours after the SPM clean. This failure indicates that
megasonic power applied during the rinse step does not eliminate the growth of particles
on wafers cleaned in SPM. Nor does resistivity data taken during the experiment indicate
an advantage of using megasonic power during the rinse. These data are consistent with
that presented by Hall et al. . If contamination remains bound to the surface and
therefore does not contribute to the conductivity of the water, a high resistivity will not
necessarily be indicative of a clean wafer surface. In summary, megasonic power applied
during the rinse step of SPM-cleaned wafers removes particles added during the clean but
does not aid in the removal of sulfur contamination from the wafer surface.
Effect of Si and SiO2 Wafer Contact Angle Prior to SPM Clean on Ease of Rinse
In order to determine if the diffusion of contaminants trapped in the chemical oxide
grown during the SPM clean is the cause of particulate growth, both Si and SiO2 wafers
were cleaned in the same 5:1 SPM solution and then rinsed together. No chemical oxide
should be grown on thermally oxidized wafers during an SPM clean. These oxide wafers
should not experience growth of particles if the SPM diffusion theory  is correct. The
contact angle, and therefore surface termination, of the wafers prior to the SPM clean was
varied by exposing the wafers to SC1, SC2, BF, or a combination of these solutions. The
SiO2 wafers were exposed to either 1) an SCI clean, 2) an SC2 clean, 3) an SCI
followed by an SC2 clean, or 4) a 1 second HF dip to remove approximately 8 A of oxide.
One group of thermally oxidized wafers did not receive any post furnace processing.
Silicon wafers were exposed to 1) an SCI clean, 2) an HF dip followed by an SC2 clean,
or 3) an SC1 followed by an SC2 clean. Wafers of each type were evenly spaced
throughout the cassette. This full cassette of wafers was cleaned in a 5:1 SPM solution
and rinsed with no megasonic power for 5 dumps followed by an overflow rinse for a total
rinse time of 10 minutes.
Sulfur concentration on the wafers used in this experiment was measured by
TXRF. The results are shown in Figure 4. The thermally oxidized wafers that received no
post-furnace processing (SiO2+none) and the ones that received only an SC2 process
(SiO2+SC2) prior to the SPM clean have significantly less sulfur on the wafer surface than
the other wafers processed in this experiment. These wafers were hydrophobic prior to
the SPM clean and had a water contact angle of 330 and 190, respectively. These wafers
also did not exhibit SPM particle growth even after one month of storage. The remaining
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Clews, P.J.; Nelson, G.C. & Matlock, C.A. Sulfuric acid/hydrogen peroxide rinsing study, article, December 1, 1995; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc621692/m1/4/: accessed January 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.