Overview

Sample cleanliness is extremely important for achieving good results in nanofabrication (hence the need for a cleanroom). However, simply working in the cleanroom does not guarantee cleanliness, proper steps need to be taken at every processing step to ensure a clean result. While some processes have more stringent cleanliness needs than others, some general rules apply for all types of lithography:

• Handle substrates/wafers with cleaned tweezers, not hands.
• Substrates should not be left exposed. Although the cleanroom offers

Solvent Cleaning

Standard Process: TAI clean

1. 5 min sonication in beaker of Toluene.
2. 5 min sonication in beaker of Acetone.
3. 5 min sonication in beaker of Isopropanol.

Background

Standard solvent cleaning is a three step process of ultrasonically agitating the substrate in subsequent solvent solutions to remove organic contamination. The first step is a nonpolar solvent (toluene) to remove any oils or grease present on the sample. The second step is a powerful polar solvent (acetone) to remove polar contaminants such as photoresist. The final step is a polar protic solvent (isopropanol).

Nonpolar Solvent Clean: Toluene

Toluene acts a nonpolar solvent to dissolve any grease or oil on the substrate. Nonpolar contaminants (especially fingerprints) will not be effectively removed by Acetone or IPA.

Polar Solvent Clean: Acetone

Acetone's high dipole moment leads to a powerful cleaning of polar residue (such as photoresist). However its quick evaporation leads to residue redepositing on the substrate if evaporation is allowed to occur.

Polar Solvent Clean: Isopropanol

IPA is used to remove fully wash away acetone from the substrate, and remove any remaining dissolved residue. As a result, IPA is not as much providing additional cleaning after Acetone as it is effectively removing the Acetone without residue.

IPA is prefered as a final step over methanol due to its significantly slower evaporation rate. Slower evaporation results in less residue, and the close dipole moments of the two chemicals (D_IPA=1.66 vs. D_Methanol=1.69) means the difference between the two is negligible. Additionally, unlike Methanol, IPA is nontoxic.

Piranha Clean

Questions & Troubleshooting

Process Library

References

Solvents: Theory and Application (via Microchem)

Cleanliness, Contamination, and Chemical Handling (via SNF)