The nonlinear two-photon absorption (2PA) effect used in two-photon polymerization based 3D printing can also be employed to pattern suitable positive tone resists in a subtractive as well as direct-write, and hence maskless, approach. Here, the writing laser is focused inside the material and will only expose it at the focus. The effective exposure can therefore be controlled and confined in all three dimensions.
This way, a single, thick or thin resist film can be exposed in a layer-by-layer fashion. This process feature has one huge benefit: The exposure parameters depend solely on the resist being used and are independent of the pattern to be exposed, since significant absorption inside the material occurs only in the laser’s focus. 2D, 2.5D and even 3D patterns in thin and thick films can be created this way.
Many of the commercially available positive tone resists (such as the AZ® Resists) are compatible with 2PA-processing, making subsequent pattern transfer using established techniques such as etching, material deposition and lift-off or electroplating as easy as possible.