|Apart from the storage at the recommended temperatures longer than the given expiry date, also high temperatures over a longer time span, a too high dilution and/or unsuitable solvents as well as chemical impurities (water, isopropyl, softener from plastics) cause an (accelerated) ageing of photo resists, that impact on the resist performance: The formation of particles, their growth and conglomeration to larger clusters is induced by precipitation of the photo initiator. With the bare eye one can see a rough surface, until in the advanced stage the coating becomes increasingly inhomogeneous (strong ripples, coverage failures behind larger particles). Since by precipitation photo initiator is lost in the resist, the development rate decreases and/or the dark erosion raises, therefore a filtration of photo resists with intrinsic particles is no alternative to disposal.
With increasing storage time, thermal decomposition of the photo initiator concentration also causes a lower development rate, and a higher dark erosion. At the same time, nitrogen formed which partly solves in the resist and may cause popping/bubbles during/after coating. Gradual darkening of the photo resist originates from the formation of azo dyes, but does not have a strong influence on processing and performance of the resist: Even traces of such converted photo initiator change the color of the resist in the visible part of the spectrum, but only marginally affect the UV absorption responsible for the exposure process.
The decreasing adhesion of the resist after advanced ageing is caused by chemical changes of the resin with time. Very frequent opening of small bottles lets a part of the solvent evaporate. Already 1% solvent loss increases the viscosity and thus the layer thickness obtained by spincoating clearly. As a consequence the necessary exposure dose and development time can be increased.