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 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.
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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Overview
The document presents a technical support package from NASA detailing a novel vapor drying technique for preparing InGaAsP substrates for epitaxial regrowth, crucial for semiconductor device fabrication. The primary challenge addressed is the presence of trace amounts of water vapor and other contaminants on patterned substrates after chemical cleaning, which can adversely affect the quality of epitaxial growth.
The proposed solution involves a three-part vapor drying process designed to achieve an extremely dry surface. This method consists of immersing the substrate in hot vapors of acetone (ACE) for five minutes, followed by trichloroethylene (TCE) for another five minutes, and concluding with a final immersion in acetone for an additional five minutes. The rationale behind this sequence is to effectively remove water vapor and organic residues that may remain on the surface, which can lead to defects during epitaxial growth.
The document outlines the experimental setup for the vapor drying process, including the construction of vapor stills filled with the solvents. The results of the tests indicated that the three-part vapor dip (ACE/TCE/ACE) was successful in minimizing condensation on the substrate surface, thereby enhancing the cleanliness required for high-quality epitaxial growth. The initial acetone vapor dip helps to remove water vapor, while the TCE dip targets organic residues. The final acetone dip serves to leave a thin coat of solvent that evaporates quickly, ensuring a clean surface for subsequent growth.
The significance of this technique is underscored by its potential to improve the yield and quality of devices fabricated from epitaxial materials, particularly in applications such as distributed-feedback lasers, where surface morphology is critical. The document emphasizes the importance of achieving a chemically pure surface to mitigate defects that can arise during the growth process.
Overall, this innovative vapor drying technique represents a significant advancement in the preparation of semiconductor surfaces, addressing longstanding challenges in the industry and contributing to the development of more efficient and reliable semiconductor devices. The work was conducted by James Singletery Jr. at NASA's Jet Propulsion Laboratory, highlighting the ongoing efforts in research and development within the field of microelectronics.
