0. Adhesion - substrate pretreatment
Adhesion – substrate pre-treatment
The adhesion between substrate and resist is of major importance for a safe processing of resists. Smallest changes of the cleaning procedure or the technology 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. High air humidity (> 60 %) also reduces 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 a previous cleaning in acetone, followed by isopropanol or ethanol treatment and subsequent drying. This procedure will improve adhesion of the resist. If only acetone is used for cleaning, the substrate must be dried in a drying oven to remove the condensed moisture which is due to the cooling effect caused by acetone evaporation.
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 depends also on the structures already deposited). The use of removers or acids (e.g. piranha) for removal, followed by rinsing and tempering, may be required. In very difficult cases, an ultra- or megasonic cleaning may be helpful.
To improve the adhesion features, adhesion-enhancing agents such as e.g. adhesion promoter AR 300-80 may be used which is applied immediately before resist coating in a very simple procedure by spin coating as thin layer of approx. 15 nm thickness and tempered. It is also possible to evaporate HMDS onto the substrates. The monomolecular layer on the wafer surface has an adhesion-promoting effect due to its hydrophobic properties which facilitate adsorption of the resist.