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 nearly particle free air, particles are generated by the movement, plastic wafer carrier, talking, etc. Even in the cleanest areas of the cleanroom, particles will eventually find their way onto exposed surfaces.
Solvent Cleaning
Standard Process: TAI clean
- 5 min sonication in beaker of Toluene.
- 5 min sonication in beaker of Acetone.
- 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 Aprotic 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 Protic 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 mainly used to removing the Acetone without residue. However, unlike Acetone, IPA will form hydrogen bonds, and thus may provide some additional cleaning effects.
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 (DIPA=1.66 vs. DMethanol=1.69) means the difference between the two is negligible. Additionally, unlike Methanol, IPA is not considered toxic.
If the clean is done
Deionized Water Rinse (usually optional, required for samples to be placed in furnaces)
A powerful final step may be a rinse in the nanopure deionized water, which will assist in the removal of any remaining particles, solvent residues, and acid/base residues. However, if this step is employed before spinning photoresist, a dehydration bake must be performed to properly remove bound surface water molecules. This step is required for samples to be placed in any high temperature equipment or furnaces, as even slight solvent residue will combust at temperatures above 400 C.
Piranha Clean
Standard Process: Piranha Clean
Questions & Troubleshooting
Process Library