Why does the same photoresist formula fail when switching phenolic resin suppliers?

Why does the same photoresist formula fail when switching phenolic resin suppliers?
In DNQ positive photoresists, phenolic resin (novolac) is not just a film former; it sets the 'dissolution baseline' that determines imaging contrast. Two resins with identical average molecular weight (Mw ~10,000) can behave completely differently if their low-molecular-weight fractions differ. Oligomers, dimers, and residual phenol are preferentially leached by alkaline developer, creating channels that accelerate dissolution of unexposed areas, causing film loss or complete washout. Phenolic hydroxyl content also matters, but more hydroxyl groups do not automatically improve performance. The key metric is the dissolution rate ratio between exposed and unexposed regions. Suppliers must control not only Mw but also PDI, GPC low-molecular tailing, and free monomer content. This is why photoresist-grade resin is a specialty product, not a commodity.