14. How can undercut patterns (lift-off structures) be produced in one- or two layer systems?
A variety of one- or two layer systems are available which generate undercut patterns. For example,AR-P 5350 is a one layer system suitable for the production of metallic vapour phase structures. The resist contains components which harden the resist surface during the bake step (which is performed at slightly higher temperatures than usual). During the aqueous-alkaline development, the undercut patterns are formed. The optimally adjusted developer for this purpose is AR 300-26.
Lift-off processes can also be employed with the two component system AR-P 5400 – 3510. This system also allows generating thermal stable structures up to temperatures of 230 °C and optically transparent structures up to the IR range. As first layer, the copolymer AR-P 5400 is deposited by spin coating. After a bake step at 150 °C, the photoresist AR-P 3510 is applied onto the cooled copolymer, followed by a bake at 100 °C. For the aqueous-alkaline development, MIF-developer AR 300-47 is diluted 1 : 1 with deionised water. After exposed areas of the upper photoresist layer are developed, the developer begins to dissolve the copolymer in a random manner (isotropic) in all directions. The longer the development time, the more polymer is dissolved underneath the photoresist layer. An undercut as desired can thus be achieved by adjusting the development time accordingly. For strong lift-off effects,the resist layers should be relatively thin (1,0 μm), while the copolymer layer should be thicker (1,5 μm).
For a dimensional stable structure transfer into the copolymer layer, both films should have approximately the same thickness.