Specific Process Knowledge/Lithography/Pretreatment: Difference between revisions
| Line 14: | Line 14: | ||
'''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. 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:''' | '''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. 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:''' | '''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. | 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:''' | '''Dip/spin-on adhesion promoter:''' | ||
| Line 30: | Line 30: | ||
'''Hard bake:''' | '''Hard bake:''' | ||
If the adhesion between the substrate surface and the resist is sufficient to survive the development process, 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. | ||
==Comparing pretreatment methods== | ==Comparing pretreatment methods== | ||