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 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 with acetone and subsequent isopropanol or ethanol treatment, followed by a bake.
If a technology involves repeated processing of wafers or if wafers are subjected 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 mega-sound is advisable.
PMMA-, copolymer and styrene acrylate resist are characterised by excellent adhesion properties to silica, silicon nitride, glass, and most metal surfaces and thus require only in exceptional cases additional adhesion promoters (AR 300-80, HMDS).
Novolac-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.