Photoresists (also called photo resist; resist = to withstand) are composed of a binder, a sensitizer, and a solvent (thinner).
- Binder (proportion ~20%)
- Novolacs are frequently used as binders. These are phenolic resins (synthetic resin, plastic), which primarily determine the thermal properties of the resist.
- Sensitizer (proportion ~10%)
- The sensitizer determines the photosensitivity of the resist. Sensitizers are composed of molecules that change the solubility of the resist when exposed to high-energy radiation (in positive resist, the sensitizer forms a carboxylic acid upon exposure. More on this in the chapter Development). So that the resist is not exposed by the lighting in the manufacturing facilities, the lithography processes take place under yellow light, to which the resist is insensitive.
- Solvent (proportion ~70%)
- The solvents determine the viscosity of the resist. By evaporating the solvents on a hot plate, the resist is stabilized and made resistant for subsequent processes.
A resist supplied by the manufacturer has a defined surface tension and density, a specific solids content, and a specific viscosity. Thus, when coating wafers in chip manufacturing, the resulting photoresist thickness depends exclusively on the rotational speed of the coating tool.
Chemically amplified resists
The composition described above applies to the classic resists used at the i-line. At 248 nm and below, the number of incoming photons is no longer sufficient to directly convert enough sensitizer molecules. For this reason, chemically amplified resists (CAR) are used: instead of a sensitizer, they contain a photo acid generator (PAG), which releases a strong acid upon exposure. In a subsequent annealing step (post-exposure bake), this acid cleaves protecting groups from the binder and is not consumed in the process, but instead continues to act catalytically. A single photon can thus trigger hundreds to thousands of reactions.
This amplification mechanism comes at a price: the acid diffuses during the bake step, which blurs the edge of the resulting resist structure. Diffusion length, bake temperature, and bake time are therefore process parameters that determine resolution.
Resists for EUV exposure
At 92 eV, an EUV photon carries roughly fourteen times more energy than a photon at 193 nm. For the same amount of energy delivered, correspondingly fewer photons therefore strike the area, and their arrival is a statistical process. At feature sizes of just a few nanometers, the number of photons per feature fluctuates noticeably – the result is randomly missing or extra structures, as well as a rougher edge. These stochastic defects, rather than the optics, are today the practical resolution limit of EUV lithography.
Countermeasures include higher exposure doses (which cost throughput) and resists with higher absorption. Besides further-developed CARs, tin-based metal oxide resists are being considered for this purpose, as they absorb EUV significantly more strongly than organic systems. Also under development are dry-deposited resists, which are deposited from the gas phase rather than spin-coated.
The layer thicknesses involved are considerably lower than in classical photolithography: for EUV, they lie in the range of a few tens of nanometers, since tall, narrow resist lines would otherwise topple over due to the surface tension of the developer solution (pattern collapse). Mechanical stability and etch resistance are therefore provided by additional auxiliary layers beneath the resist.