Minoru Toriumi

Image contrast slope and line edge roughness of chemically amplified resists for postoptical lithography

The reduction of line edge roughness (LER) or linewidth roughness is the most challenging issue associated with the development of resist processes for the next generation lithography. The extent of LER has been reported to correlate with the image contrast slope. Although the postoptical lithography such as electron beam and extreme ultraviolet lithographies can create high contrast images, low-energy electrons degrade image contrasts in these lithographies. The precise prediction of image contrasts requires the incorporation of the effects of low-energy electrons into process simulators. For the reduction of LER, better understanding of LER formation is needed. In this work, the authors proposed a simulation method for the calculation of latent images for postoptical lithographies on the basis of the sensitization mechanisms of chemically amplified resists.

Effects of Rate Constant for Deprotection on Latent Image Formation in Chemically Amplified Extreme Ultraviolet Resists

The resolution of chemically amplified extreme ultraviolet (EUV) resists has reached sub-30 nm. However, the sensitivity and the reduction in the line edge roughness of such resists is still inadequate for the realization of EUV lithography. The pattern formation of chemically amplified resists is based on chemical reactions, which start with energy deposition from exposure tools and are terminated by quenchers or by cutting the heat supply for chemical reactions. In this study, the effects of rate constant for deprotection on latent image formation in chemically amplified EUV resists were investigated from the viewpoint of the difference between high- and low-activation-energy (Ea)-type resists. In chemically amplified resists, neutralization and deprotection competitively proceed. In high-Ea-type resists, in which neutralization proceeds before deprotection, the contrast of pattern edges is improved by suppressing deprotection in the low-dose region. In low-Ea-type resists, in which a fast deprotection is expected, the difference in the extent of deprotection between the high- and low-dose regions can be enhanced by promoting deprotection over neutralization. For the improvement in high- or low-Ea-resist performance, it is effective to enhance or balance all advantages of chemical reactions.

Feasibility Study of Chemically Amplified Extreme Ultraviolet Resists for 22 nm Fabrication

The trade-off between sensitivity, resolution, and line edge roughness is the most important concern for the development of high-performance resists based on chemical amplification. Below sub-30 nm patterns, a chemical balance between acids and quenchers basically controls the quality of latent images. Under this situation, an ideal chemically amplified resist is the one in which the neutralization between acids and quenchers proceeds before the start of acid catalytic reactions and chemical reactions occur with a diffusion-controlled rate. In this study, the qualities of latent images were compared among exposure doses of 2, 5, 10, and 20 mJ cm-2 under the ideal condition. The authors found that chemically amplified resists with a 22 nm resolution and a 10 mJ cm-2 sensitivity are feasible under the ideal condition. However, a high-quality image is unlikely to be obtained upon 5 mJ cm-2 exposure without increasing polymer absorption or acid generation efficiency per photon.

Dissolution characteristics and reaction kinetics of molecular resists for extreme-ultraviolet lithography

Molecular resist of polyphenol was evaluated as an extreme-ultraviolet resist compared with a polymer resist of p(tert-butoxycarbonyl-hydroxystyrene). The molecular resist shows higher sensitivity than the polymer resist. The dissolution behavior was studied by quartz crystal microbalance method. The molecular resist shows thinner swelling layer than the polymer resist. The deprotection mechanism was approximated by simple reaction equations, and Fourier-transform infrared spectra were interpreted to give the products of a quantum yield and a deprotection rate constant as 6.2×10−8 and 6.0×10−8cm3∕moleculess for molecular and polymer resists. Both deprotection efficiencies are almost same. The higher sensitivity of the molecular resist is due to the dissolution behavior not the reaction mechanism.

Fluoropolymer-based resists for a single-resist process of 157 nm lithography

We investigated two types of main-chain fluorinated polymers and found that incorporating fluorine into their backbones reduced their absorption coefficients to less than 1 μm−1 at 157 nm and showed good dry-etching resistance. Fluoropolymer dissolution during development was studied using a quartz-crystal microbalance and evaluated from the standpoint of molecular structures and polymer acidity. The fluorine and hydrophobic effects, rather than the polymer acidity effect, were found to play the dominant role in forming the swelling layer during development. The use of main-chain fluorinated polymers for the base resin of positive-tone resists results in fine imaging with a film thickness of 300 nm. The incorporation of fluorine into the backbones will thus enable these polymers to be used as single-layer resists for 157 nm lithography.

Absorption effects on total-internal-reflection fluorescence spectroscopy

A theoretical consideration is made for transverse-electric polarization vectors of a transmitted field in an optically absorbing sample to derive a fluorescence intensity formula as a function of the absorption index of the sample as well as incident and observation angles in total-internal-reflection fluorescence spectroscopy. These equations are experimentally confirmed. It is shown that the angular spectrum of fluorescence intensity is the Laplace transformation of the effective penetration depth, even for the absorbing sample. High resolution of the depth profile is expected for an absorbing sample compared with a nonabsorbing one.

Structure of poly(p-hydroxystyrene) film

Pyrene-doped poly(p-hydroxystyrene) (PHST) thin films prepared by spin- coating method are studied by time-resolved total-internal-reflection fluorescence spectroscopy. We observed inhomogeneity such as a concentration gradient of doped pyrene molecules and a gradient of polarity (hydrophobicity), and also the existence of isolated pyrene molecules. Stable ground-state dimers of pyrene are found in surface, bulk, and interface layers of a PHST film. These features cause the complicated rise and decay of fluorescence. These results also depend upon the preparing process such as the baking condition.

Depth Profile of Pyrene Dopant in Polymer Films by Variable-Angle Total Internal Reflection Fluorescence Spectroscopy

The incident and observation angular dependences of the fluorescence from pyrene-doped polymer films are measured by variable-angle total internal reflection fluorescence spectroscopy. These angular profiles depend on refractive index, film thickness, observation wavelength, pyrene concentration, absorption coefficient of the polymer film, and so on. The profiles are in good agreement with the results calculated by fitting the parameters and by considering interference effects and concentration profiles. Step profiles of the pyrene concentration explain the results for the segmented poly(urethaneurea) film, while, in poly(methyl methacrylate) and polystyrene films, pyrene is distributed homogeneously.

Characterization of fluoropolymers for 157 nm chemically amplified resist

Fluoropolymers were characterized for 157 nm lithography resist. We obtained superior optical transparencies, with absorption coefficient of 0.01–2 μm−1 at 157 nm wavelength. The resists based on the fluoropolymers characterized exhibited suitable dissolution behavior into alkaline developer of 0.26 N tetramethylammonium hydroxide solution. Dry-etching resistance of the resist film showed the almost equivalent to conventional ArF resist based on an acrylic polymer. The lithographic evaluation showed that high sensitivities of 1.0–10.0 mJ/cm2 for 157 nm exposure, and high dissolution contrast. Furthermore, the resolution capability of 95 nm lines and spaces was obtained with proper pattern profiles. These results indicate that 157 nm resist based on fluoropolymers we characterized have enough potential for practical use of 157 nm lithography.

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.