If negative photoresist CAR 44 (AR-N 4400) is directly used on copper or copper-containing substrates, the following points must be observed.
Substrates can be coated as normal. If subsequently, as usually recommended, the soft-bake is carried out at a temperature between 85 °C and 95 °C, structures can no longer be completely developed. The structures can still be recognized, but a layer with a thickness of about 1 μm remains in unexposed areas which should in fact be completely developed. Copper causes a complete thermal crosslinking of the lower resist layer. Surprisingly, this effect is avoided if the soft-bake is carried out at temperatures not exceeding 70 °C. This temperature must of course then also be applied during the crosslinking bake (after exposure). This changes the development conditions, but not dramatically. A slight dilution of the recommended developer is usually sufficient.