Dana Durham

Nature and degree of substitution patterns in novolaks by carbon-13 NMR spectroscopy

The nature and degree of substitution patterns on different ring carbons in novolak (cresol- formaldehyde) resins have been determined by carbon-13 NMR spectroscopy. The acquisition of carbon-13 NMR spectra of novolak resins yielded improved S/N ratio through selective irradiation of protonated carbons while suppressing quaternary carbons. For quantitative estimations, carbon nuclear spin-relaxation rates were measured to ensure that carbon nuclei of interest fully relax after each pulse train. NMR signals from quaternary carbons in the aromatic region have been selectively suppressed employing distortionless enhancement via polarization transfer (DEPT) pulse sequence, thus greatly improving the quantitative estimations. An experimental parameter, Un, where n is the carbon position in the ring, has been defined for chain propagation at any given carbon position. For example, in a pure meta cresol novolak resin, polymerization mainly proceeds via 4,6-disubstitution, and the C2 ortho carbons do not seem to participate in the polymerization process as much as C4 and C6 carbons do. However, in novolak resins prepared by mixing meta and para cresols, the chain propagation step greatly involves C2 carbons ortho to the hydroxyl group in meta cresol. The extent of such involvement can easily be determined relative to a given standard sample. The polymerization preference and differences in chemical structure of various polymers can easily be compared by determining their respective Un parameters. The bridged methylene linkages yield weaker NMR signals compared to those in the pure meta or para cresol resins, and thus, the estimations based on these carbons are only approximate. The experimental parameters as defined in this paper are discussed in relation to the physical and lithographic properties of these polymers.

Dielectric and chemical characteristics of electron-beam-cured photoresist

This paper investigates the dielectric properties and chemical changes occurring in electron beam cured photoresist and how they compare with photoresist cured with standard thermal bake. The material used in the investigation is AZ P4620, a positive tone photoresist based on diazonapthoquinone (DNQ)/novolak chemistry and formulated mostly for thick film applications. The photoresist was spun on silicon wafers and exposed at different doses using an electron beam curing system. The dielectric properties and chemical changes of the exposed films were then evaluated and compared with photoresist films cured with a standard thermal cure. The dielectric properties that were evaluated were dielectric constant and breakdown voltage. The chemical changes were analyzed by FT-IR. For the electron beam cured photoresist, preliminary results indicate that the photoactive compound undergoes both decomposition and crosslinking during the process. The dielectric properties as well as the chemical changes as a function of dose are discussed.

Rearrangement of novolak resins

During the course of investigations into the synthesis of novolak resins for use in the microelectronics industry we have observed the rearrangement of the resin. Deeper investigation of this phenomenon has shown it to be a chain scission which leads to a rearrangement of the novolak structure. A possible mechanism is discussed.

Blended novolac resins with improved lithographic performance

Statistically designed experiments were performed to optimize novolac blending techniques which would yield superior photoresists. The best lithographic performance was obtained when two novolacs to be blended had dissimilar relative molecular weights (RMW) while also having matched dissolution rates (DR), (lithographic performance equals (RMWA - RMWB)/(DRA - DRB). A calculated plot of (RMWA - RMWB)/(DRA - DRB) matched the experimental plot of lithographic performance well.