Takeshi Iwai

Light propagation characteristics of straight single-line-defect waveguides in photonic crystal slabs fabricated into a silicon-on-insulator substrate

Straight single-line defect optical waveguides in photonic crystal slabs are designed by the finite difference time-domain method and fabricated into a silicon-on-insulator (SOI) wafer. By employing an airbridge structure, clear light propagation for both polarizations is observed without any leakage along the waveguide. This experimental result is well explained by photonic bands of pure guided modes. Minimum propagation loss is estimated to be 11 dB/mm. This value is lower than that reported so far for three-line-defect waveguides with an SOI slab structure and almost comparable to that for an index confinement waveguide with a rectangular Si core. This propagation loss is dominated by the scattering loss by some irregularities. However, photonic crystal waveguides have the possibility of an essential lower scattering loss than in the index confinement waveguide because of the inhibition of radiation modes by the photonic bandgap.

Changes of chemical nature of photoresists induced by various plasma treatments and their impact on LWR

Changes in chemical nature of an ArF photoresist caused by various plasmas were analyzed, and it was found that the HBr plasma treatment induces a selective detachment of a heterocyclic unit of the photoresist, and the detached unit remains in the photoresist film. Thermomechanical analyses were performed, which showed that the softening temperature of the photoresist decreases by the HBr treatment, indicating that the detached heterocyclic unit acts as a plasticizer in the photoresist film. These results showed that the HBr treatment can be regarded as a softening process of the photoresist. This HBr treatment was applied to the fabrication of line patterns and it was shown that the treatment remarkably improves LWR (line width roughness) after etching. This improvement was more pronounced for the case of an isolated pattern than the case of a dense pattern. Further investigations on the HBr treatment were performed by changing the copolymerization ratio of a monomer containing the heterocyclic unit. It was shown that the reduction of LWR by the HBr treatment becomes more enhanced when the copolymerization ratio increases. However, an intensive HBr treatment was found to deteriorate LWR, showing that there is an optimum condition of the HBr treatment in terms of improving LWR.

Progress of topcoat and resist development for 193nm immersion lithography

193nm immersion lithography is the most promising lithography candidate for 45nm node technology and beyond. However, immersion specific issue, such as the immersion specific defect and the leaching of resists compound into immersion fluid, still exists without any effective countermeasure. To realize a productive 193nm immersion lithography process, we have to develop a cost effective material that might be immersion dedicated resist. In this paper, we investigated the leaching with different polymer protective agents and hydrophobicity. It was found that the leaching amount was strongly related to the activation energy of the protective agent and hydrophobicity of the polymer. Higher activation energy of protective agents and higher hydrophobicity of polymer showed less amount of leaching. In this paper, newly developed developable type topcoat TILCTM-031 demonstrated the excellent ability of immersion defect prevention.

Main chain decomposable star shaped polymer for EUV resist

The Extreme Ultra Violet lithography (EUVL) is expected to be the most promising semiconductor fabrication technology for 22 nm node and beyond. Kozawa and his colleagues have documented that non-constant acid diffusion coefficient have a significant impact on the latent image quality of 22 nm patterns. We prepared a novel main chain decomposable star shaped polymer (STAR polymer) to examine the concept. STAR polymer consists of a core unit and several arm units which connect to the core unit as shown in Fig.1. The arm units are partially protected poly (p-hydroxystyrene) (PHS) base linear polymer. The core unit that attached on the arm units employs easily acid cleavable group. The adoption of living anion polymerization for the arm units of the STAR polymer makes the controlled polymerization of one monomer unit possible. Based on this material design concept, the protecting group on the arm unit is de-protected by the acid generated during exposure and continues its reaction at the Post Exposure Bake (PEB) step and the acid will also cleave the bonding of the core unit which would then result in a lower molecular weight polymer of lower Tg. The concept of the novel polymer, which is the decomposition of the core and protecting group of arm units of the STAR polymer, was confirmed with a gel-permeation-chromatography (GPC) study. The thermal property of the exposed and unexposed area was also investigated through a thermal flow method. The Tg decrease of the exposed area was observed with the STAR polymer, regardless of increase in Tg of the linear polymer. General lithographic performance on EUV exposure for STAR polymer was also discussed.

Resist development to improve flare issue of EUV lithography

This report will introduce novel resist materials including specific photo acid generator (PAG) to improve flare issue from the resist standpoint. We have developed a method to control the acid diffusion length from the PAG. It previously reported that acid diffusion length can be altered by the PAG anion size. In this report, we focused on the effect of the Tg of a resist film. The thermal flow rate of a resist film can suggest the approximate resist Tg. Therefore, we measured the thermal flow rate of the resist. And we found out passivity of acid diffusion control by changing PAG species and volume. Moreover, newly designed PAG tested was confirmed to have uniform distribution in the resist film with no PAG clustering at the resist surface at compared to our conventional PAG. This new positive tone resist formulation shows good performance under flare condition. In addition, we focused on the pattern density variation as one of the key parameters for flare value. Low pattern density indicated less flare value. It is considered that negative tone resist to have advantage for isolated line type features. Novel positive and negative tone type resists were compared side-by-side and discussed for its advantages at varying pattern densities.

Enhancement of Third Order Nonlinearity Calculated for Two-Dimensional Photonic Crystal

Using the finite difference time domain method, we calculated light propagation characteristics in a two dimensional photonic crystal, which is composed of triangular lattice airholes in a third-order nonlinear medium. The results showed the characteristic change of the transmission spectrum resulting in the strong saturation of the response, and the sufficiently low transmission loss with the help of projection-type airholes at input and output interfaces. The three-dimensional calculation also indicated the low transmission loss obtained in a realistic photonic crystal slab. Such characteristics are expected in a continuous spectral range near the Γ point of the second photonic band, owing to an overlap of two bands, which is unique in the two-dimensional lattice. It is applicable for an all-optical switch, an optical power limiter, a pulse reshaping device, and so forth.

Chemically amplified negative-tone resist using novel acryl polymer for 193-nm lithography

We report the development of a novel acryl polymer with an (alpha) -hydroxymethyl acrylate in the application to 193nm chemically amplified negative-tone resist. This new polymer structure consists of ((alpha) -hydroxymethyl)acrylate and MAA. The ester and alcohol group in the polymer contribute to an intramolecule and/or intermolecular hybrid crosslinking reactions without crosslinker and in the presence of a photo generated acid as a catalysis. In an intramolecular crosslink reaction, the ester group reacts to a neighboring hydroxymethyl group within the polymer chain. As a result, a lactone group is made in the main polymer chain. On the other hand, in an intermolecular crosslink reaction, the ester group reacts to a hydroxymethyl group of another polymer chain to make an ester chain. In this reaction, the new polymer is densely crosslinked and fine resist pattern is obtained without having any swelling problem. Consequently, the resist is optimized and contains the new polymer, photoacid generator and a small amount of crosslinker. Under conventional illumination condition, 180nm line and space pattern are achieved without any kind of swelling problem. The sensitivity is 40 mJ/cm2 with the standard developer, NMD-3 2.38 percent.

Topcoat-free ArF negative tone resist

We report the development and applications of ArF negative tone resist for ArF immersion lithography. New developed topcoat-free ArF negative tone resists has sufficient water repellent capability that is applicable to over 700mm/s scan speed water immersion exposure tool and suitable leaching suppression capability within reaching specification of exposure tool. We demonstrated lithographic performance of topcoat-free negative tone resist utilizing 1.07NA immersion tool and confirmed the lithographic window of 55nm 1L1S and 50nm 1L1S. And 27.4nm of isolated space pattern at over dose condition of 55nm 1L1S patterning. This result shows the possibility of topcoat free negative tone resist for dual trench based litho-etch-litho-etch double patterning. Additionally we have demonstrated contact hole patterning utilizing double exposure and generated 65nm gridded contact hole patterns utilizing 0.92NA ArF scanner with applicable pattern profiles.

Effect of Molecular Structure on Depth Profile of Acid Generator Distribution in Chemically Amplified Resist Films

The acid generator distribution in resist films is an important issue for fine patterning based on chemical amplification. In particular, extreme ultraviolet (EUV) resists require a high acid generator concentration compared with conventional chemically amplified photoresists. In this study, the depth density profiles in partially protected poly(4-hydroxystyrene) films containing dispersed triphenylsulfonium salts were investigated by X-ray reflectivity measurements to clarify the depth profile of acid generator distribution. It was found that the depth profile depends on the molecular structure of acid generators. The hydrogen bonding between polymers and acid generators affected the depth profile of acid generator distribution.

Pattern Freezing Process Free Litho–Litho–Etch Double Patterning

Double patterning technology based on existing ArF immersion lithography is considered as the most viable option for complementary metal oxide semiconductor (CMOS) node of 32 nm and below. Most of double patterning approaches previously described requires intermediate processing step such as hard mask etching, spacer material deposition, and resist pattern freezing. The requirement of these additional steps is now leading way to requests for throughput reduction and low cost for production for double patterning technology applications. In this paper, litho–litho–etch (LLE) double patterning without any intermediate processing steps is investigated to achieve narrow pitch resist imaging. The LLE options examined in this work are combinations of positive tone-negative tone and positive tone-positive tone photoresist double patterning process. These are the alternative processes in pattern freezing process free LLE double patterning. The goals of this work are to determine witch of these approaches is the most viable for future application and to confirm the patterning potential for 32 nm and below half pitch resist imaging.