Specific Process Knowledge/Lithography/Pretreatment: Difference between revisions
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=Pretreatment= | =Pretreatment= | ||
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. | 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 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 spin coating. This page gives an overview of treatments available at DTU Nanolab to promote photoresist adhesion. | 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 spin coating. This page gives an overview of treatments available at DTU Nanolab to promote photoresist adhesion. | ||
==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. Due to the moisture in the cleanroom atmosphere, this priming naturally has a short shelf life, so spin coating should be done as soon as possible. | 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. Due to the moisture in the cleanroom atmosphere, this priming naturally has a short shelf life, so spin coating should be done as soon as possible. | ||
==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 limits it to silicon. A BHF dip leaves the surface of a silicon wafer hydrophobic, and the dangling Si-bonds are passivated by adsorbed H<sup>+</sup>. This means the surface will not oxidize immediately, but the lifetime of this passivation is limited, so it is recommended to spin coat the wafer within 20, maximum 45 minutes. | 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 limits it to silicon. A BHF dip leaves the surface of a silicon wafer hydrophobic, and the dangling Si-bonds are passivated by adsorbed H<sup>+</sup>. This means the surface will not oxidize immediately, but the lifetime of this passivation is limited, so it is recommended to spin coat the wafer within 20, maximum 45 minutes. | ||
==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. The shelf life of substrates primed using HMDS (vapor phase) is long, maybe even several weeks. | 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. The shelf life of substrates primed using HMDS (vapor phase) is long, maybe even several weeks. | ||
==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 first cleaning the substrate surface, and subsequently priming the surface with the additives as the solvent evaporates. | Adhesion promoters for dip or spin-on application are commercially available. Most are solvent based with some, usually proprietary, additives. They work by first cleaning the substrate surface, and subsequently priming the surface with the additives as the solvent evaporates. | ||
==Hard bake== | |||
If the adhesion between the substrate surface and the resist is sufficient to survive the development process without delamination, the adhesion in subsequent process steps can be improved by a so-called hard bake. Baking the substrate a few minutes on a hotplate (or approx. half an hour in a convection oven) at a temperature higher than the soft bake temperature, e.g. 110-130°C, usually improves the adhesion between resist and surface. Depending on the type of resist used, and the temperature of the hard bake, reflow of the resist pattern may be a side effect. | If the adhesion between the substrate surface and the resist is sufficient to survive the development process without delamination, the adhesion in subsequent process steps can be improved by a so-called hard bake. Baking the substrate a few minutes on a hotplate (or approx. half an hour in a convection oven) at a temperature higher than the soft bake temperature, e.g. 110-130°C, usually improves the adhesion between resist and surface. Depending on the type of resist used, and the temperature of the hard bake, reflow of the resist pattern may be a side effect. | ||