Michelle M. Cook

Application of photodecomposable base concept to 193 nm resists

This paper reports on the use of trimethyl sulfonium hydroxide as a base additive for 193 nm applications, which is found to stabilize the latent image as well a act as a photodecomposable base. Delay time stability (exposure to post-exposure bake) of formulations consisting of trimethylsulfonium hydroxide is compared to that of a non- photodecomposable base (diethanolamine) in both methacrylate- and cycloolefin-based 193 nm resists. Resist formulations made using the trimethylsulfonium base were stable for more than one hour, while the reference formulation with diethanolamine showed T-top formation within 10 minutes delay time under the same conditions. The trialkylsulfonium hydroxide base additives were found to be photodecomposable by measuring the acid produced upon exposure. Compared to a non- photodecomposable base containing resist, the photodecomposable base containing resist produced more acid in the exposed areas under identical PAG/BASE molar ratios.

Cycloolefin/maleic anhydride copolymers for 193 nm resist compositions

Cycloolefin/maleic anhydride systems are a favorable approach to dry etch resistant resists for 193 nm lithography. This paper reports on poly(BNC/HNC/NC/MA) tetrapolymers, from t- butylnorbornene carboxylate (BNC), hydroxyethyl-norbornene carboxylate (HNC), norbornene carboxylic acid (NC) and maleic anhydride (MA). It was found that moisture has to be excluded in the synthesis of these systems if reproducible results are to be obtained. Lithographic evaluation of an optimized, modified polymer has shown linear isolated line resolution down to 100 nm using conventional 193 nm illumination. Possible reactions of the alcohol and anhydride moieties are discussed, and the effect of the anhydride unit on polymer absorbance is discussed using succinnic anhydride as a model compound.

Low-polydispersity novolak resins for i-line resists

Low polydispersity novolak resins were prepared in a sequential process where low molecular weight (Mw) oligomers were first synthesized in a preliminary stp followed by a second condensation with preformed bis-hydroxymethyl derivatives of phenolic starting materials. Resins prepared in this way were found to have polydispersities comparable to that of fractionated resins made in the standard resin synthetic process. The sequential process afforded higher yields of usable resins since no low Mw fractions were isolated and discarded. In several instances, unique bis- hydroxymethyl phenol monomers or monomer/dimmer mixtures were prepared for use in making the low dispersity resins. These compounds were further condensed with the products of the initially prepared low Mw resins in a second acid catalyzed condensation.

High-yield resin fractionation using a liquid/liquid centrifuge

Resins used in photoresist manufacturing are often relatively expensive once processing steps (fractionation e.g.) and yield losses are factored into the net cost. We have previously reported on the merits of using an economically more attractive fractionation process using a liquid/liquid centrifuge. Further refinements of this method indicate that waste streams could be reduced by recycling the extractant phase and that lower molecular weight fractions removed from the starting resin might be used in making other resist ingredients [speed enhancers, photoactive compound (PAC) backbones e.g.]. Both of these improvements would reduce the overall manufacturing costs of making resist raw materials and the final products made with them.

Alternate novolak resin fractionation

Novolak resins fractionated using a unique method, were compared to resins fractionated with conventional methods. The potential for improving the fractionation/separation process and for making improved or more consistent resist with the resins was identified. Several experimental designs were run to determine optimum conditions needed to achieve better separation. Isolated resins were used to make experimental i-line photoresists which were tested against resists made with the conventional processes.

Novel novolak block copolymers for advanced i-line resists

in order to meet customer driven functional criteria for modern i-line resists, formulations have become increasingly more complicated. Often mixtures of both the photoactive compounds and resins are necessary to balance and optimize dissolution inhibition, photosensitivity and resolution. Mixtures of fractionated novolak resins along with low molecular weight (Mw) speed enhancing resin additives were used to attain desired properties. Scumming tendencies increased as the concentration of the low Mw additives increased. Novel resins were synthesized by incorporating fully formed low Mw additives into the synthetic recipes along with the phenolic monomers and formaldehyde. These resins were characterized by gel permeation chromatography, NMR and by functional comparison to traditional formulations. When formulated with small amounts of low Mw additives, photospeeds could be increased while increasing thermal resistance and reducing scumming tendencies. Additional resist performance enhancements were possible when the new resins were fractionated to remove low Mw oligomers and unreacted starting materials. The need for use of small amounts of low Mw speed enhancers to the formulations with the new resins substantiates earlier work on the optimization of resist formulations.

Effects of structural differences in speed enhancers (dissolution promoters) on positive photoresist composition

Several low molecular weight polyhydroxy phenolic compounds were used to study their effect on DNQ novolak photoresist compositions. These compounds used with fractionated novolak resins in a positive photoresist composition, tend to improve the photospeed, but in some cases degrades the other characteristics of the resist including resolution and depth of focus. Improvements in performance (photospeed, sidewall angle, resolution, and depth of focus) may depend on the structure of the speed enhancer. Speed enhancers were obtained commercially or synthesized, characterized by HPLC and NMR, and were formulated as i-line resists. The goal of these studies was to find out the relationship between the structure of the speed enhancer and the performance of the resist.

Novolak resin for ultrafast high-resolution positive i-line photoresist compositions

An improved process for isolation of novolak resins from phenol-formaldehyde condensation products has been developed. The process results in resins having low polydispersity and higher photospeed while typical phenol/formaldehyde resin syntheses generate a broad distribution of molecular weight fragments with a wide polydispersity. The novolak resins were characterized by NMR and GPC and were formulated to obtain ultra fast high resolution i-line photoresists. The characteristics of the resins and their effect on lithographic properties as i-line photoresist compositions will be discussed in this paper.

Preparation of lower-dispersity fractionated novolak resins by ultracentrifugation

The need to make well characterized resins consistently is paramount to the preparation of high performance photoresists. Solid resins fractionated by selective precipitation have been separated by ultracentrifugation at varying temperatures. At sufficiently high revolutions per minute, solvents and oligomers are efficiently squeezed out leaving behind polymer with higher average molecular weight and lower dispersity than resins obtained by more common isolation techniques. By controlling isolation conditions, resins with desired dissolution rates could be produced. Lithographic test confirmed that resists properties could effectively be controlled by manipulation of process conditions to isolate resins used in the formulations.

Performance of 193-nm resists based on alicyclic methacrylate and cyclo-olefin systems

Among the chemistries/polymers reported for the 193nm photoresist applications, methacrylate copolymers consisting of 2-methyl-2-adamantane methacrylate and mevalonic lactone methacrylate and cycloolefin polymers derived from derivatives of norbornene have shown promising results. We have studied the lithographic properties of these two but different promising chemistries. Both system offer linear resolutions down to 0.13 micrometers using conventional 193 nm illumination and high sensitivity at standard developer conditions. While the methacrylate based system shows best performance on substrates with bottom coats, the cycloolefin-Maleic anhydride alternate copolymer based resists performs well on bare silicon as well as substrates with bottom coats. The etch rates of the methacrylate and cycloolefin based resists were found to be 1.4 and 1.3 times relative to that of KrF resist APEX-E. Further, new polymers consisting of isobornyl and alkyl ether chains on the ester groups of norbornene carboxylate were made in order to decrease the glass-transition temperatures of the norbornene-maleic anhydride type polymers. These results will be included and discussed in detail.