A vapor drying treatment removes traces of water and other contaminant residues that remain on the surface of a patterned semiconductor substrate after chemical cleaning in preparation for subsequent epitaxial growth on the substrate. The treatment was developed in conjunction with attempts at epitaxial regrowth of first-order gratings for distributed-feedback lasers in the InGaAsP material system.

A Simple Vapor Degreasing Still can be made with a 2,000-mL beaker of Pyrex (or equivalent) low-thermal-expansion glass. The bottom of the beaker is covered with polytetrafluoroethylene boiling stones, 600 mL of an organic solvent is brought to a boil, and a cover glass is placed over the top. The substrate to be treated is mounted on a polytetrafluoroethylene holder and lowered into the vapor space through a slit in the cover glass.

A chemical cleaning typically ends with a rinse in deionized water followed by drying in isopropyl alcohol vapor followed by blow drying with nitrogen. Notwithstanding the apparent thoroughness of the drying steps, the amount of water and other contaminants that remain on the cleaned surface is sufficient to give rise to a large number of defects in epitaxial material.

The present vapor drying treatment is essentially a modified, three-step vapor degreasing procedure. It was selected from among a number of similar treatments in experiments based on the conjecture that aggressive vapor degreasing might be capable of removing water vapor that sticks to the surface of a substrate after cleaning and conventional drying. In each step of the treatment, the substrate is lowered into a still filled with a solvent vapor (see figure) and kept there for a few minutes.

Of a number of different combinations of vapor dips that were tested in the experiments, the one that proved most successful in removing water vapor from the surface of a cleaned substrate was acetone followed by trichloroethylene followed again by acetone. The trichloroethylene in this combination was initially chosen with the expectation that it would remove any organic residue that would assist in keeping water vapor on the surface. However, since water does not dissolve easily in trichloroethylene, an initial dip in acetone vapor was added to remove as much water vapor as possible from the surface before immersion in trichloroethylene vapor. Furthermore, since acetone easily mixes with trichloroethylene and is more volatile, acetone was also chosen for the final vapor dip, not only to remove trichloroethylene from the surface but also to leave a thin surface layer of solvent that would either quickly evaporate upon removal from the vapor bath, or else would be quickly desorbed upon heating of the substrate to growth temperature.

The Composition of the Surface of an InGaAsP substrate was determined by XPS before and after the treatment described in the text. The treatment caused a reduction in the carbon and oxygen contents and the appearance of phosphorus — all indications of a very clean surface.

In the experiment on this treatment, a specimen substrate was first dipped in acetone vapor for 5 minutes to ensure removal of water vapor. Condensation — most likely acetone — was observed on the substrate and substrate holder throughout this step of the treatment. Next, the substrate was placed in trichloroethylene vapor for 5 minutes. Once again, condensation — most likely trichloroethylene — was observed on the substrate. Finally, the substrate was again exposed to acetone vapor to remove the trichloroethylene from its surface and saturate the surface with the more volatile acetone. The substrate was left in the acetone vapor for 5 minutes. Surprisingly, about two minutes into this last step, all noticeable drops of condensation vanished from the substrate and substrate holder, leaving behind extremely dry surfaces.

The precise physical mechanisms responsible for the effectiveness of this three-step vapor drying treatment remain unknown. What is known is that the technique is so effective in drying the surface as to prevent most of the defects that would otherwise form in epitaxially deposited material. Moreover, analysis of a substrate by x-ray photoelectron spectroscopy (XPS) reveals that the three-step treatment reduces the amount of carbon and oxygen contaminating the surface (see table). The remaining carbon and oxygen have been tentatively attributed to exposure of the substrate to air during transfer to the XPS apparatus.

This work was done by James Singletery, Jr., of Caltech for NASA's Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

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Refer to NPO-19899

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Vapor drying for preparing in GaAsP for epitaxial regrowth

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