4-(4-(tert-Butyl)phenyl)-1,4-oxathian-4-ium 1,1-difluoro-2-oxo-2-((5',5',6',6'-tetrafluorospiro[adamantane-2,2'-[1,3]dioxepan]-5-yl)oxy)ethane-1-sulfonate
[CAS 1371590-55-4]

Identification

• Classification: Chemical reagent >> Organic reagent >> Sulfonate / sulfinate
• Name: 4-(4-(tert-Butyl)phenyl)-1,4-oxathian-4-ium 1,1-difluoro-2-oxo-2-((5',5',6',6'-tetrafluorospiro[adamantane-2,2'-[1,3]dioxepan]-5-yl)oxy)ethane-1-sulfonate
• Molecular Formula: C₃₀H₃₈F₆O₈S₂
• Molecular Weight: 704.74
• CAS Registry Number: 1371590-55-4
• SMILES: CC(C)(C)C1=CC=C(C=C1)[S+]2CCOCC2.C1C2CC3CC(C2)(CC1C34OCC(C(CO4)(F)F)(F)F)OC(=O)C(F)(F)S(=O)(=O)[O-]

chemBlink Chemical Story

CAS 1371590-55-4 is a highly engineered ionic photoacid generator often identified in specialty records as PAG554. Its long systematic name hides a clear division of labor. The positively charged component is a 4-(4-tert-butylphenyl)-1,4-oxathianium ion, while the counterion is a large fluorinated sulfonate containing a rigid spiro-adamantane-derived framework. This combination belongs to the chemistry of chemically amplified photoresists, where light is used to create acid and that acid then catalyzes chemical change in the resist during post-exposure processing.

Chemical amplification transformed lithography because one absorbed photon no longer had to produce only one permanent chemical event. A photoacid generator can undergo photochemical decomposition and create a strong acid; during post-exposure bake, that acid can catalyze repeated deprotection or other reactions in the surrounding polymer. The result is high sensitivity, but amplification introduces a competing problem. If the acid travels too far before or during the bake, chemistry spreads beyond the region that actually absorbed light. At very small feature sizes this diffusion can blur edges, increase line-edge roughness and reduce the fidelity of the printed pattern.

PAG554 shows why the counterion is therefore not merely a spectator. A bulky, rigid and heavily fluorinated sulfonate can alter ion pairing, solubility, compatibility with the resist polymer and the mobility of the acid-derived species. The spiro-adamantane framework supplies substantial molecular volume and rigidity, while fluorination strongly changes the electronic character of the sulfonate environment. Modern photoacid-generator design often uses precisely these structural variables to balance acid strength, diffusion and formulation behavior.

The compound also illustrates a broader evolution in resist chemistry. Early descriptions of photoacid generators often emphasize whether a molecule generates acid efficiently. Advanced lithography demands more: the PAG must dissolve uniformly, avoid crystallization, survive coating and bake steps, respond at the intended wavelength, and generate an acid whose movement is compatible with the desired resolution. As feature dimensions shrink, the dimensions and shape of the ionic components themselves become part of the imaging problem.

Public records identify CAS 1371590-55-4 and the PAG554 designation, while patent literature on modern resist compositions documents closely related oxathianium and bulky sulfonate photoacid generators. Performance claims should be tied to a particular formulation rather than to the isolated salt. The durable chemical lesson is that a photoacid generator is an engineered ion pair. In nanolithography, generating acid is only half of its job; controlling where that acid can act is equally important.

References:
1. ChemicalBook, CAS 1371590-55-4, PAG554 identity record.
2. WO2025038907A1, Photoresist compositions and pattern formation methods.
3. Ito H. Chemical amplification resists: history and development within microlithography. Adv Polym Sci. 2005;172:37-245.

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