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Resists are organic compound (generally thermoplastic polymers) whose solubility is modified by UV radiations. Several resists are available according to the desired process.
When they affect the resists, UV radiations induce different behaviours: concerning negative resists, they cause a polymerization of the exposed zones, rendering them insoluble on contact with the development solvent whereas non-exposed areas disappear in this solvent (for example, SU-8 resist). For positive resists, UV radiations chemically transform the resist so that the exposed areas become soluble in the developer (AZ9260 and S1813 resists). Image reversal resists have the possibility to change their polarity after an annealing step called reversal bake (AZ5214, TI09XR).
* Positive resists :
* Negative resists :
* Reversal resists :
Example of use :
Photoresists are used in uniform and adhesive thin films (from a few hundred nanometres to several microns).
- For the LIGA mould fabrication, high-viscosity resists become necessary
because they provide more important thicknesses (about ten micrometers
(AZ9260) up to several hundred micrometers (SU-8)). These resists are
characterized by their capacity to produce high aspect ratio patterns
(height over the smallest dimension ratio).
Particular resists for LIFT-OFF: to achieve metallic patterns, one
method consists in depositing metal onto resist patterns whose sidewall
slope is reversed compared to the one usually obtained in positive
resist development. The final step consists in removing the polymer
covered by the metal: it is called the Lift-off technique.
• Particular resist for plasma etching: specific resists like SPR220 have been developed in order to be used as masks for plasma etching, especially for BOSCH and DRIE processes.