As an alternative to the polyhydroxystyrene negative resist, a CAR negative resist based on the copolymer poly(hydroxystyrene- co -MMA) was designed which can also be developed under aqueous-
Allresist now also offers colored negative resists with the designation SX AR-N 8500. The difference to FUJIFILM resists is that only dyes and no pigments are dissolved in the resists. This allows to achieve a high resolution and high edge sharpness.
If negative photoresist CAR 44 (AR-N 4400) is directly used on copper or copper-containing substrates, the following points must be observed.
The desired structure size also largely depends on the exposure dose used. In the case of overexposure, structures begin to widen which is particularly pronounced during laser exposure.
With laser direct exposure, a slight undercut of the structures can be generated: The upper part of the layer is more strongly irradiated due to the absorption of the resist and consequently more intensely crosslinked.
Negative resist AR-N 4400-10 can be structured with laser direct exposure at an exposure wavelength of 405 nm. Different arrays are shown in the figure.
Layers are after exposure treated with H2O2 and developed with water. Allresist has developed similar negative resists within the framework of a research project.
By adding suitable dyes to negative-working CAR resists, even a patterning beyond the usual wavelength range is possible if pulsed lasers with sufficiently high intensity are used for exposure.
PMMA resist are mainly used for electron beam applications or as protective coatings in aggressive wet chemical etching procedures. In principle, also a structuring of PMMA layers with deep UV exposure (220-266 nm) is possible, but the sensitivity is in this case low and long exposure times are consequently required.
If the negative resist AR-N 4400-50 is utilised, resist layers of >200 µ m can be built up in repeated coating steps (see product information of CAR 44). Decisive for the quality of the layers to be obtained is the bake procedure.