Currently, two positive-tone two-layer lift-off systems have successfully been employed for a couple of years; the AR-P 5400-3510 series (Allresist) and PMGI positive resist variant.
These systems have a bottom resist that is not photosensitive but soluble in alkali. The polymers must be composed in such a way that the lacquer coat withstands a coating with a photoresist with the solvent PMA (PGMEA) without any problems. Otherwise there would be a mixing of both resist layers. The dissolving speed can be adjusted by tempering of the bottom layer. The variation of the temperature is between 95 – 150 °C. The higher the temperature, the lower the dissolving rate. A second coating with a photoresist is followed by image-wise exposure and development. The photoresist is quickly developed at the exposed areas, and then the developer begins to dissolve the bottom layer in an isotrop way (in all directions). Thus, it also develops below the photresist, which now (unexposed) is no longer affected by the developer.
By varying the development time, users are able to adjust the desired undercut of the AR-P 5400-3510 series as needed.
Fig. 1: 0.8 µm undercut
Fig. 2: 4.6 µm undercut
Undercut structures are almost exclusively used for lift-off processes. During evaporation and particularly during sputtering however high temperatures (of up to 200 °C) may occur, so that a high thermal stability of the upper layer is mandatory. In large undercuts as shown in the second picture, the upper layer tends to melt and flows off; a lift-off process is no longer possible under these conditions. Experiments to develop a thermally more stable positive resist for two-layer systems resulted in no significant improvement so far.
Investigations with negative-tone resists as upper layer in contrast showed promising results. These two-component resist developments could quickly be optimized and produced the desired undercut. For this purpose, resists of the AR-N 4300 -series and bottom layer AR-P 5400 were varied. Resists of the 4300-series are significantly more thermostable than generally used positive resists (see Alkali- and solvent-stable negative resist)
Overview Photoresists- Negative