Adhesion between coating and substrates is a very sensitive issue. Smallest changes of the cleaning procedures or the process parameters can have a fatal impact on the adhesive strength. Silica, silicon nitride, and base metals (such as aluminium, copper) generally exhibit good resist adhesion properties, while adhesion is reduced on SiO2, glass, noble metals such as gold and silver, as well as on gallium arsenide. For the last-mentioned substrates, adhesion promoters are absolutely required to improve the adhesion strength ( Question 4: Optimal pre-treatment of substrates). If the air humidity is too high (> 60 %), adhesion is also substantially reduced.