3. What is the optimal pre-treatment of substrates for e-beam resist application?
If new and clean substrates (wafers) are used, a bake at approximately 200 °C for a few minutes is sufficient in order to dry the substrates, which however then have to be processed quickly. A temporary storage in an 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 in acetone with subsequent isopropanol or ethanol treatment, followed by a bake.(→Section 1).
If a technology involves repeated processing of wafers or if these are subjected to various conditions, a thorough cleaning is recommended. The cleaning procedure is however highly process- and substratedependent (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.
PMMA- and copolymer resists are characterised by excellent adhesion properties to silica, glass, and most metal surfaces.
Novolak-based e-beam resists however generally require the use of adhesion promoters such as e.g. adhesion promoter AR 300-80 to improve adhesion features. AR 300-80 is deposited immediately before coating (by spin coating) as a thin film of approximately 15 nm. It is also possible to evaporate HMDS onto the substrates. A monomolecular HMDS layer improves the adhesion properties of the wafer surface (which becomes more hydrophobic and thus more organophilic), which then retains the resist much better.