Tamio Yamazaki

Fluoropolymer resists for 157-nm lithography

Fluoropolymers are key materials for single-layer resists in 157-nm lithography. We investigated main-chain fluorinated polymers and found that the incorporation of fluorine atoms into polymer backbones such as that in tetrafluoroethylene and monocyclic monomers reduced their absorption coefficients to less than 1 micrometers -1 at 157 nm, while side-chain fluorinated polymers had absorption coefficients of 2 to 3 micrometers -1. The main-chain fluorinated polymers also showed good solubility in a standard alkaline developer and their dry-etching resistance was comparable to that of a ArF resist. Prototype positive-tone resists had good sensitivities of less than 10 mJ/cm2, and they exhibited fine imaging resolution with 80-nm dense patterns. The resists can be used to obtain 300-nm-thick films.

Resist materials for 157-nm lithography

Fluoropolymers are key materials for single layer resists of 157nm lithography. We have been studying fluoropolymers to identify their potential for base resins of 157nm photoresist. Many fluoropolymers showed high optical transparencies, with absorption coefficients of 0.01micrometers -1 to 2micrometers -1 at 157nm, and dry- etching resistance comparable to an ArF resist, and non- swelling solubility in the standard developer. Positive- tone resists were formulated using fluoropolymers that fulfill practical resist requirements. They showed good sensitivities, from 1 mJ/cm(superscript 2 to 10 mJ/cm2, and contrast in the sensitivity curves. They were able to be patterned using a F2 laser microstepper.

Novel main-chain-fluorinated polymers for 157-nm photoresists

Main-chain-fluorinated base-resins, including tetrafluoroethylene and norbornene derivatives, were synthesized and their fundamental properties, such as transparency at 157 nm and solubility in a standard alkaline developer, were characterized. A high transparency, i.e., absorbance of less then 0.5 μm-1, was achieved by optimizing the polymerization conditions with a variety of counter monomers. It was found that the polymerization conditions could also control the dissolution rates of the fluoropolymers and increased the dissolution rate of unprotected fluoropolymers by about three orders of magnitude, which was sufficient for the alkaline developability. Positive-working resists based on fluororesins were developed and showed good transparency of less than 1 μm-1 at 157 nm, and good solubility in a standard alkaline solution of 0.26-N tetramethylammonium (without any swelling behavior). And an acceptable etching rate as resistant as ArF resists was obtained and 65-nm dense lines could be delineated by the exposure at 157-nm wavelength.

Synthesis of novel fluorinated norbornene derivatives for 157-nm application

We have synthesized various main-chain fluorinated polymers and studied their transparency and solubility. The main-chain fluorinated polymers were synthesized by co- or ter-polmerization of tetrafluoeoethylene (TFE) with cyclic monomers, especially TFE with newly synthesized norbornene derivatives. Transparency of the main-chain fluorinated polymers tended to be higher with higher fluorine contents. But exact absorbance of the main-chain fluorinated polymers by modifying the STUPID calculation. Solubility of the main-chain fluorinated polymers functionalized by hydroxyfluoroalkyl groups was also studied. We have developed a model to predict pKa of hydroxyfluoroalkyl groups incorporated in the norbornene derivatives, and studied correlation between pKa(OH) and solubility of the co-polymers of the hydroxyfluoroalkyl-functionalized norbornene derivatives with TFE. pKa Of the hydroxyfluoroalkyl groups were lower with higher fluorine contents, and solubility of the co-polymers tended to be higher with lower pKa of the hydroxyalkyl groups.

157-nm single-layer resists based on advanced fluorinated polymers

Various polymer structures for 157-nm lithography have been vigorously developed for post 193-nm lithography. Polymers with a fluorinated main chain can improve high transparency at the 157-nm wavelength; conversely, norbornene copolymers with fluoride pendant groups do not achieve so high transparency at the 157-nm wavelength. We have developed a novel 157-nm chemically amplified resist. It is composed of a fluorinated monocyclic main-chain polymer, which shows high transparency and can be used for single layer resists. This new resist is referred to as a Small Absorbance Fluorine contAining ResIst (SAFARI). The SAFARI resist exhibited high-resolution patterns of 65-nm line and space patterns using a 0.85 NA microstepper, good sensitivity of 4 mJ/cm2, and high transparency when applied to 250-nm film thickness.

Development and characterization of new 157-nm photoresists based on advanced fluorinated polymers

Fluorinated polymers show a good transparency at the 157-nm exposure wavelength for single-layer resists. We have developed fluorinated resist polymers for 157-nm lithography. These polymers are main-chain fluorinated polymers synthesized by the co-polymerization of tetrafluoroethylene (TFE) and polymers such as poly(TFE/norbornene/α-fluoroolefin) fluoropolymers (FP1). In this paper, a number of polymerization initiators were evaluated in the polymerization of PF1-type polymers in order to investigate the effect of polymer end groups on optical and dissolution properties. We found that the polymer end group greatly affects the dissolution properties of these polymers when using a standard 0.26N tetramethylammonium hydroxide (TMAH) aqueous developer solution. These end groups also affect the polymer transparencies at 157-nm, and the resulting lithographic performance. The fluorocarbon initiator named “F2” induced the lowered absorbance (~0.4μm-1) and an increase in the dissolution rate (~300 nm/sec) without noticeable amounts of swelling. These polymer-based resists can achieve a resolution of less than 60-nm using a 157-nm laser microstepper (NA=0.85) with a Levenson-type strong phase shifting mask.

Dry-etch resistance of fluorine functionalized polymers

The reactive ion etch (RIE) properties of fluorine funtionalized polymers in which fluorine atoms were incorporated in the main chain were examined. There was a tendency that the etching rates of these polymers were higher as lower the fluorine contents. The existing four models such as the Ohnishi model, the Kunz model, the Ohfuji model and the Kishimura model were applied to explain the correlation between the etching rates and the polymer compositions or structures, but the errors were too large to explain the relationship. A new model has developed to explain the effect of the fluorine incorporation to the dry etch resistance. The model assumed that there would be a correlation between the number of main chain fluorine atoms and the dry etch resistance, and the main chain fluorine incorporation would increase the dry etch resistance. The model could explain the dry etch resistance of the main chain fluorine incorporated polymers with adequate accuracy.

Reticle Critical Dimension Latitude for Fabrication of 0.18 µm Line Patterns

The reticle critical dimension (CD) latitude of 0.18 µm line patterns with various pitches was investigated from the standpoint of mask linearity, optical proximity effect (OPE) and depth of focus (DOF). We propose a global reticle CD latitude which is defined by the common range of allowable reticle CDs for various pitches. The global reticle CD latitude was enhanced under illumination conditions in which the OPE was suppressed, e.g., conventional illumination, and at the optimum numerical aperture (NA) which gave a wider DOF, especially for isolated lines. We found experimentally that the global reticle CD latitude (on x4 reticle, range) of 0.18 µm line patterns was 20 nm at NA=0.60 and σ=0.75 conventional illumination. Furthermore, from aerial image simulation results, the global reticle CD latitude was expected to be enhanced at the optimum NA (~0.68) by KrF, and at 0.60 NA by ArF lithography.

157-nm single layer resists based on main-chain-fluorinated polymers

Fluoropolymers are key materials for single layer resists of 157-nm lithography. We have been studying fluoropolymers to identify their potential for base resins of 157-nm photoresist. New monocyclic fluoropolymer resist which we developed have high optical transparency with an absorption coefficient lower than 1.0/spl mu/m/sup -1/, and non-swelling solubility in standard developer. As the exposed results by 157-nm laser microstepper (numerical aperture = 0.85) using a new fluoropolymer based resist, the 65-nm lines and spaces pattern was obtained at a 200-nm film thickness.

Effect of stage synchronization error of KrF scan on 0.18-μm patterning

The stage vibration effect on imaging performance, such as DOF and CD uniformity is evaluated experimentally and compared with simulation analysis. Various kinds of 0.25 - 0.18 micrometer patterns are investigated by using KrF excimer scanner with 0.6 NA and 0.75 partial coherency and two types of chemically amplified positive resists. In the case of a standard resist for 0.25 micrometer level patterning, the CD at the best focus changed and the DOF decreased rapidly with increasing moving standard deviation (MSD) in 0.18 micrometer level pattern formation. Allowable MSD value of L&S pattern was estimated to be around 25 nm. To improve the stage synchronous error margin, the application of a high resolution resist was effective on L&S and isolated space patterns (about 40 nm), but showed little improvement for isolated line and hole patterns. Therefore, totally allowable MSD value was still about 30 nm. In particular it was found that both isolated line and hole patterns were very sensitive to stage vibration effect. Strict stage control has to be required for 0.18 micrometer patterns even if the high resolution resist is used.