What is the optimal pre-treatment of substrates for photoresists?

The adhesion between substrate and resist is of major importance for the safe processing of resists. Smallest changes in the cleaning procedure or the technology itself can exhibit a significant influence on the adhesive strength. Silicon, silicon nitride and base metals (aluminium, copper) are generally characterised by good resist adhesion properties, while adhesion is reduced on SiO₂, glass, noble metals such as gold and silver or on gallium arsenide. For these substrates, adhesion promoters are absolutely required to improve the adhesion strength. A too high air humidity (> 60 %) also reduces the adhesion substantially. If new clean substrates (wafers) are used, a bake at approximately 200 °C minutes (3 min, hot plate) is sufficient for drying, but substrates should be processed quickly thereafter. A temporary storage in a desiccator is highly recommended in order to prevent rehydration.

Pre-used wafers or wafers which are contaminated with organic agents require previous cleaning steps, e.g. in easy cases cleaning in acetone, followed by isopropanol or ethanol treatment and subsequent bake ( Section 1). This procedure improves the adhesion features of the resist. On no account only acetone should be used, since the evaporative heat loss during the bake causes a condensation of air humidity on the wafer. If only acetone is used for cleaning, the substrate must be dried in a drying oven to remove the condensed moisture.

If a technology involves repeated processing of wafers or subjecting these to various conditions, a thorough cleaning is recommended. The cleaning procedure is however highly process- and substrate-dependent (and also on the structures already deposited). The use of removers or acids (e.g. piranha) with subsequent rinse and bake step ( Section 1) may be required in this case. For very difficult cases, a treatment with ultra- or megasound is advisable.

To improve the adhesion features, adhesive agents such as e.g. adhesion promoter AR 300-80 may be used, which is applied directly before the resist coating by spin coating as a thin layer of approx. 15 nm thickness. It is also possible to evaporate HMDS onto the substrates. The monomolecular layer on the wafer surface improves adhesion features since this layer is hydrophobic and absorbs the resist better.