Yoko Takebe

Synthesis of novel fluoropolymers for 157-nm photoresists by cyclopolymerization

Novel fluoropolymers having partially fluorinated monocyclic (5-membered and 6-membered ring) structure have been synthesized with radical cyclo-polymerization, which have C- F bond in the polymer main chain and also possess fluorocontaining acidic alcohol group. These polymers have excellent transparency lower than 1.0 μm-1 at 157nm wave length. The number-average molecular weight (Mn) of the polymers is 4000 to 20000, the glass transition temperature (Tg) is 130 to 155 °C and the decomposition temperature (Td) is about 400 °C. Copolymerization reaction with the other monomers (ex. fluoroolefins,(meth)acrylates and vinyl esters) were also examined. The introduction of protecting group (ex. methoxylmethly, and t-butoxycarbonyl group) to alcohol units of the polymer can be applied before or after polymerization reaction. We also evaluated fundamental resist performances. These have excellent transparency of 0.5 to 1.5 μm-1, good solubility in the standard alkaline solution (0.26N N-tetramethylammonium hydroxide aqueous solution) and relatively high sensitivities below than 10mJ/cm2. The imaging results of the above fluoropolymer based positive- working resists are presented. Under 100-nm line and space pattern are delineated in 200-nm thick film by using the phase shift mask.

Development of fluoropolymer for 193nm immersion lithography

We had already developed several series of fluoropolymers, FPRs and FUGUs, having a partially fluorinated monocyclic structure and having acidic hydroxyl group, which acts as dissolution unit into alkaline solution. Then we have optimized these polymers for top-coat as the developer-soluble type in the 193nm immersion lithography. However the hydrophobicity of these polymers were a little poor due to its hydroxyl group. So we thought that the introduction of water repellent moiety into the these polymers structure is effective to improve the their hydrophobicity though the increase of water repellent unit in the polymer leads to lower dissolution rate in developer. To introduce as much as possible of hydrophobicity unit, we selected FUGU as platform, which has larger dissolution rate in developer than that of FPRs, We copolymerized FUGU with higher water-repellent component and obtained three copolymers, FUGU-CoA, FUGU-CoB, and FUGU-CoC. In this paper, we described characteristics and evaluation of these polymers. Most of these polymer showed an improvement of hydrophobicity, in particular FUGU-CoB had excellent hydrophobicity due to introduction bulky containing-fluorine group. In this study, we also investigated the interaction between the water and various polymers by using QCM method. The difference between FUGU and water repellent polymers for swelling behavior to water became clear by analysis of diffusion coefficient. We found that our new co-polymers have excellent diffusion coefficient than FUGU which was confirmed by QCM method used to evaluate water permeability and water diffusion in the materials.

Dry-etching resistance of fluoropolymers for 157-nm single-layer resists

Novel fluoropolymers having partially fluorinated monocyclic (5-membered and 6-membered ring) structure have been synthesized with radical cyclo-polymerization, which have C-F bond in the polymer main chain and also possess fluorocontaining acidic alcohol group. These polymers have excellent transparency lower than 1.0 μm-1 at 157nm wavelength, a small amount of outgassing, high sensitivity and good adhesion to the wafer. However, this fluoropolymer have lower etching resistance (half of conventional KrF resists) and it must be improved for applying to the single-layer resist. In this paper, we show the new model of the estimation of the dry-etching resistance for designing polymer compositions. It is well known that the model using carbon-atom-density as a parameter is useful for estimating dry-etching resistance. However, these models did not agree with the results of our fluoropolymers. Our new model was focused on the surface area and the volume of the polymer. We succeeded to explain the relationship between the dry-etching resistance and the composition of the fluoropolymer. According to this model, the compositions of fluoropolymer such as protective groups, protective ration and co-polymer units were optimized to improve their etching resistance.

A new monocyclic fluropolymer structure for 157-nm photoresists

We earlier developed a series of fluoropolymers (FPRs) for use as first-generation 157-nm photoresist polymers. These FPRs have a partially fluorinated monocyclic structure and provide excellent transparency. However, their etching resistance is low (half that of conventional KrF resists) and an insufficient dissolution rate in tetramethylammonium hydroxide (TMAH) solution. To improve the characteristics of these polymers, while retaining high transparency, we had to redesign the main chain fluoropolymer structure. In this paper, we describe a new monocyclic fluoropolymer structure for a second-generation 157-nm photoresist polymer. This structure also has a fluorine atom in the polymer main chain, as well as a fluoro-containing acidic alcohol group. We synthesized two types of fluoropolymers, ASF-1 and ASF-2. We found that ASF-1 had transparency of 0.18 μm-1, better than that of the FPRs, and the etching resistance was improved. Unfortunately, the dissolution rate was poor. On the other hand, ASF-2 showed even better transparency of 0.1 μm-1, improved etching resistance, and a dissolution rate of more than 600 nm/s, which is sufficient for use as a resist. The introduction of a protecting group (e.g., the methoxymethyl or adamantylmethoxymethyl group) to the hydroxyl group of ASF-2 can be done after the polymerization reaction. Using partially protected ASF-2 with an appropriate protecting group, we were able to fabricate a sub-60-nm line-and-space pattern.

Highly hydrophobic materials for ArF immersion lithography

In immersion lithography, the impact of water on resist performance and the possibility of damage to the lens by the components eluted from the resist material are seriously concern. And much work has shown that controlling the water-resist interface is critical to enabling high scan rates. Many topcoat materials have been developed to control the aforementioned interfacial properties. Developable topcoats have been particularly investigated as suitable candidates for its applicability to the resist developing process. Achieving the balance between the low surface energy required for higher receding contact angle and the base solubility for topcoat removal is challenging. We have already reported FUGU polymer which have partially fluorinated monocyclic structure and hexafluoroalcohol(HFA) group and showed that its developer solubility was excellent but hydrophobicity was insufficient for high scan rate. We have also reported that co-polymers of FUGU and highly fluorinated monomers which have perfluorinated cyclic structure had sufficient hydrophobicity but lower developer solubility. We have found that it was difficult to use these copolymers in themselves as topcoat. But by blending of moderate amount of these copolymers into FUGU polymer, we have finally obtained highly hydrophobic developer-soluble topcoat. Hydrophobicity can be controlled by blending ratio. Furthermore we have newly successfully synthesized a series of fluoropolymers, FIT polymer partially fluorinated monocyclic structure and having carboxylic acid group as developer-soluble unit. When FIT polymer as well as FUGU polymer, was blended to highly hydrophobic copolymer, the blended polymer also showed higher hydrophobicity keeping sufficient developer solubility.

157-nm single-layer resist based on novel monocyclic fluorinated polymer

Fluorinated polymers are key materials for single-layer resists used in 157-nm lithography. We have been studying fluorinated polymers to determine their potential for use as the base resin and have developed a new monocyclic fluorinated polymer that has high transmittance (an absorption coefficient of 0.1 μm-1) at a 157-nm exposure wavelength and high dry-etching resistance (a dry-etching rate of 1.86 times that of a KrF resist) under hard mask dry-etching conditions. Moreover, it has a high dissolution rate in standard aqueous tetramethylammoniumhydroxide developer (a dissolution rate of more than 600 nm/s). Using this polymer with adamanthylmethoxymethyl as a protecting group, we were able to resolve a 60-nm line-and-space pattern using a 0.90 numerical aperture 157-nm laser micro-stepper along with a resolution-enhancement alternating phase-shift mask technique. This polymer has enabled both high dry-etching resistance (a dry-etching rate equal to 1.43 times that of a KrF resist) and good imaging performance.

Improvements of adhesion and hydrophobicity of wafer bevel in water immersion lithography

In this study, we focus on the controllability of a wafer bevel from adhesion and hydrophobicity viewpoints in order to solve the problems of film peeling and microdroplet formation around wafer bevels, which result in pattern defects. Hexamethyldisilazane (HMDS) treatment is a common solution to these problems. We examine a novel wafer bevel treatment utilizing silane coupling agents (SCAs) for obtaining high adhesion and hydrophobicity. SCAs comprise trimethoxysilanol and organic functional groups. These groups react with inorganic substrates and films just over the surface subjected to a novel chemical treatment (NCT), respectively. Several organic functional groups both with and without fluorine are examined. The hydrophobicity is estimated from the static and receding contact angles of water. The adhesion strength is measured from the stress required for pulling the topcoat film away from the substrate subjected to the NCT. The coating performance of chemicals on the surface by the NCT and the aging stability of the formulated solution of the SCAs are examined for optimizing the composition of the NCT solution. Further, we verify the film peeling behavior and water leakage in wafers having a topcoat, ArF resist, and bottom antireflective coating (BARC) using a quasi-immersion exposure stage.

Development of top coat materials for ArF immersion lithography

We have investigated higher hydrophobic developer-soluble topcoat by combination of developer-soluble unit with higher hydrophobic unit. We have already reported a series of fluoropolymers, FUGU having a partially fluorinated monocyclic structure and having acidic hydroxyl group which act as dissolution unit into alkaline solution. In addition, recently we have developed new series of highly fluorinated monomers which was expected to act as hydrophobic unit. In this paper, we describe results of co-polymersization of FUGU with these hydrophobic monomers and evaluation of them. Some of them showed good hydrophobicity keeping moderate developer solubility. Furthermore, we found that higher hydrophobic developer-soluble materials were achieved by adding small amount of highly hydrophobic polymer to developer-soluble polymer, for example FUGU, and in fact this type of blending polymer showed high hydrophobicity keeping high dissolution. We have obtained various kind of new type of topcoat materials whose receding angle varied from 70-90 degree and patterning profile without dissolution residue could be obtained by using two beam interference.

A new monocyclic fluoropolymer for 157-nm and 193-nm photoresists

We earlier developed new monocyclic fluoropolymers (ASF-2) for F2 resist materials. But, it is necessary for ASF-2 to improve of their characteristics, especially the dry-etching resistance, in order to apply for ArF and F2 lithography at fine design rules. In this study, to improve the dry-etching resistance keeping good characteristics of ASF-2, we examined using two methods. The one is to co-polymerize with ASF-2; the other is to introduce protective groups. We synthesized a new series of fluorinated co-polymers (ASF-2 with various monomers, e.g., methacrylate derivatives and vinyl ester derivatives). We found that the dry-etching resistance was improved by co-polymerization. Especially, the co-polymer with methacrylates containing an adamantyl moiety had a good dry-etching resistance, 1.45 vs. conventional KrF resist. This co-polymer also kept a good transparency at 193 nm. The introduction of various protective groups to the hydroxyl group of ASF-2 was also investigated. As the result of the optimization of protective groups and a protecting ratio, the partially protected ASF-2 with CCOM protecting groups had a good transparency at 157 nm and a good etching resistance (1.42 vs. conventional KrF resist). Using partially CCOM protected ASF-2 with an appropriate protecting ratio, sub-60 nm line and space pattern in 150 nm-thick film was obtained.