2. Tempering / Softbake
Tempering / Softbake
Resists films which have been previously coated still contain, depending on the film thickness, a substantial amount of residual solvent. A subsequent tempering at 90 – 100 °C is performed to dry and to harden the resist films. In addition
to improved resist adhesion properties, also the dark erosion during development is reduced by these means.
The decision if a hot plate or a convection oven should
be preferred depends for thin films (< 5 μm) on the availability, since technically none of the procedures offers a particular advantage. The fast through-put of a hot plate is compensated by the option for batch tempering (approx. 25 wafers in one step) in convection ovens. Drying thicker films in a convection oven is however unfavorable since the dried resist surface inhibits a fast solvent evaporation. In these cases, a hot plate is recommended because more solvent is expelled from the bottom of the resist film.
Insufficiently tempered resist films (either too short or at too low temperatures) entail a variety of further problems. Air bubbles may develop successively which are due to an evaporation of residual solvent. Possible consequences are inaccurate structural images, a roundening of resist profiles as well as unacceptable high dark erosion during development.
If temperature-sensitive substrates are processed it is also possible to work at considerably lower softbake temperatures (< 60 °C). The development regime has to be adjusted accordingly.
If the hard bake of resist films was too rigid (temperature too high or tempered too long), a partial destruction of the light-sensitive component results which significantly increases exposure times and reduces the sensitivity.
After the softbake, substrates are cooled to room temperature prior to further use. Especially thick resists require an appropriate waiting time for rehydration before exposure.