Specific Process Knowledge/Lithography/Pretreatment: Difference between revisions
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=Pretreatment= | =Pretreatment= | ||
Pretreatment is done before spin coating in order to ensure the best conditions for adhesion between the substrate surface and the resist. Pretreatment ranges from a simple dehydration bake over etching the native oxide to vapor phase deposition of an adhesion promoter. | Pretreatment, also known as ''priming'', is done before spin coating in order to ensure the best conditions for adhesion between the substrate surface and the resist. Pretreatment ranges from a simple dehydration bake over etching the native oxide to vapor phase deposition of an adhesion promoter. The goal of pretreatment is to remove any moisture that may be adsorbed on the surface of the substrate, and/or to modify the contact angle of the surface to match that of the resist to be coated on the substrate. | ||
All surfaces can be divided to hydrophilic or hydrophobic surfaces. Oxidized surfaces such as | All surfaces can be divided to hydrophilic or hydrophobic surfaces. Oxidized surfaces such as SiO<sub>2</sub> or surfaces with native oxide (e.g. Si or Al substrates) are considered to be hydrophilic and have very bad wetting with hydrophobic resist. The adhesion of most resists on hydrophilic surfaces is deteriorated if moisture is present on the surface. Therefore it is very important to do the pretreatment step before the spinning. This page gives an overview of treatments available at DTU Nanolab to promote photoresist adhesion. | ||
'''Dehydration:''' | '''Dehydration:''' | ||
A dehydration bake immediately before spin coating removes the moisture adsorbed to the surface, and greatly improves the adhesion of resist on most surfaces. For thin hydrophilic layers, a few minutes on a hotplate at or above 100°C may suffice. For thicker layers or bulk oxide samples, a dehydration bake at 250°C over night is recommended. | A dehydration bake immediately before spin coating removes the moisture adsorbed to the surface, and greatly improves the adhesion of resist on most surfaces. For thin hydrophilic layers, a few minutes on a hotplate at or above 100°C may suffice. For thicker layers or bulk oxide samples, a dehydration bake at 250°C over night is recommended. | ||
'''BHF dip:''' | '''BHF dip:''' | ||
Stripping the native oxide using BHF only works if the native oxide of the substrate is etched by BHF, and if resist has good adhesion to the substrate material itself, which basically narrows it down to silicon. | Stripping the native oxide using BHF only works if the native oxide of the substrate is etched by BHF, and if resist has good adhesion to the substrate material itself, which basically narrows it down to silicon. | ||
'''HMDS:''' | '''HMDS:''' | ||
In the HMDS priming process, the -OH groups on the surface of the substrate are replaced with Si(CH<sub>3</sub>)<sub>3</sub>, thus changing the surface from hydrophilic to (more) hydrophobic. Substrates with surfaces of silicon or it's oxides or nitrides all work very well with HMDS pretreatment. Other semiconductors, insulators, or metals that form -OH groups on the surface may be suitable as well. | |||
'''Dip/spin-on adhesion promoter:''' | '''Dip/spin-on adhesion promoter:''' | ||
Adhesion promoters for dip or spin-on application are commercially available. Most are solvent based with some, usually proprietary, additives. They work by firstly cleaning the substrate surface, and subsequently priming the surface with the additives as the solvent evaporates. | |||
==Comparing pretreatment methods== | ==Comparing pretreatment methods== | ||