Photoresist structures of standard resists possess a softening point of 115 – 130 °C. During subsequent thermal processes (plasma etching, sputtering or others), these temperatures are easily exceeded. Particularly structures of thicker resist films will melt and flow together under these conditions.
UV-curing is one method to prevent a melting of structures. The wafer is placed on a hot plate (resist structures facing the plate), heated and simultaneously irradiated with UV-light with a wavelength of 200 – 300 nm. This high-energy UV-light is able to cleave chemical bonds in all resist components, even those of the novolac, before a new cross-linking process of resist components at higher temperatures takes place. The initial temperature of 110 °C (which is below the softening point) is increased in small steps or using a temperature ramp. Structures will remain thermally stable during this procedure.
This procedure is however quite costly and too time-consuming for mass production. Furthermore, temperatures should not be chosen too high (> 160 °C), since cured resist layers may not be removable any more. Mask aligners for broadband UV-exposure are generally unsuitable for UV-curing, since light of a wavelength > 300 nm does not possess enough energy to cleave chemical bonds. In addition, placing a hot plate in the mask aligner poses a technical challenge.
Overview General Process