Spray coating is often used for the coating of complex topologies. There are various manufacturers of spray coating equipment, probably the best known are EV Group and Süss Microtec. Both producers use different strategies and devices for spraying, and therefore ready-to-use spray resists must be adjusted for the particular equipment.
In some applications, the substrate on which the negative resist is to be applied cannot be heated. This may be the case for sensitive glass and especially for very big substrates.
As an alternative to the polyimide negative resist, SX AR-N 4340/6 was developed, a highly sensitive CAR negative resist based on polyhydroxystyrene, which can be developed under aqueous-alkaline conditions.
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.
AR-N 4400-50 was investigated at the TU Braunschweig with particular regard to the fabrication of well-defined resist architectures with vertical flanks.
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.
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.