M. Dalil Rahman

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.

Novolak/polyhydroxystyrene copolymer and photoresist compositions

Novolak-polyhydroxystyrene copolymers with a high glass transition temperature were synthesized. Copolymers with different compositions (Novolak/PHS ratio) show improved resin characteristics, compared to either novolak or novolak/PHS blend. The novolak-PHS copolymer formation is confirmed by carbon-13 NMR spectroscopy. This technique has provided detailed information on the substitution characteristic of various carbon atoms in the polymer. For example, the carbons bearing hydroxy groups are observed in the range 148-156 ppm and the methylene carbons at 25-36 ppm. The methyl groups on the aromatic ring show chemical shifts at 16 and 25 ppm. Chemical shifts of carbons in various isomers of model compounds have been assigned to respective carbons, which confirms the structure (structure 1) for this copolymer. Novolak-PHS copolymer can be formulated into a high performance and high thermal stability positive photoresist whereas physical blend of novolak and polyhydroxystyrene is not useful. The synthesis of the copolymer and the micro lithographic properties is discussed. The paper discusses comparative data on Novolak-PHS copolymer and novolak/PHS polymer blends.

Novel DNQ PACs for high-resolution i-line lithography

The use of i-line lithography for the 16 to 64 Mbit DRAM device generations calls for increased performance of i-line resists. This paper reports on investigations on novel sensitizers for advanced i-line lithography, starting out with a discussion of general design criteria, then discussing methodology and results of a screening phase, and examining in greater detail a small number of selected candidates for which resolution, exposure latitude, and depth-of-focus data were obtained. Finally, a new advanced resist for i-line lithography, AZR 7500, is presented, and its performance is evaluated in terms of the above criteria as well as thermal flow resistance.

Novel process of isolating novolac resin fractions

Typical phenol/formaldehyde resin syntheses generate a broad distribution of molecular weight fragments with a wide polydispersity. A process was developed to isolate novolak resin fractions of narrow polydispersity from phenol- formaldehyde condensation products in ethyl lactate without any high temperature distillation. The characterization and the performance of these fractions in i-line photoresist composition are discussed.

Isolation of novolak resin at low temperature

It is a well known fact that novolak resin undergoes chemical structure changes using vigorous synthesis conditions, such as under high acid catalyst loading and high temperature during isolation. Under such conditions, the structure of novolak resin is rearranged through quinone methide intermediate. We have also observed that the rearranged novolak resin does not perform well in photolithography. In this paper we discus the alternate isolation procedure, eliminating high temperature vacuum distillation. In this process a typical resin synthesis is performed using cresylic acids, catalyst and a solvent with the addition of formaldehyde over time. At the end of the condensation period, distillation is applied using a sub-surface high forced steam. Distillation is performed until all the unreacted cresols are removed. At this point the temperature is raised up to 140 degrees Celsius, and the vacuum is slowly drawn to 35 mm Hg to remove residual water. After the volatiles have been distilled off, the vacuum is released and a solvent is added to adjust the solid content. We discuss in this paper, the properties of the resin in the application of photolithography.

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.