Shigeo Irie

Theoretical Calculation of Photoabsorption of Various Polymers in an Extreme Ultraviolet Region

We have calculated the linear absorption coefficients of various resist polymers using the mass absorption coefficients at 13 nm and the density obtained from the graph-theoretical treatment derived by Bicerano. The values indicate that the transmittance at 13 nm of conventional resists used in 193-nm, 248-nm and 365-nm lithography is about 30% when the thickness is 3000 A and 60–70% when it is 1000 A. This shows that conventional resists are suitable for an EUVL (extreme ultraviolet lithography) thin-layer resist (TLR) process using a hard-mask layer, but their large photoabsorption makes them unsuitable for a single-layer resist (SLR) process. To design polymers that are suitable for an SLR process, we further calculated the absorption of about 150 polymers. The results suggest that the introduction of aromatic groups into a polymer not only reduces the absorption at 13 nm but also increases the etching resistance.

Theoretical estimation of absorption coefficients of various polymers at 13 nm

The linear absorption coefficients at 13 nm were calculated for more than 150 polymers. The results indicate that an aromatic substitution lowers the absorption coefficient. This is because oxygen has a larger atomic absorption than carbon or hydrogen and the substitution reduces the relative oxygen content. Furthermore, the Ohnishi parameters for the polymers were calculated in order to investigate the relationship between the absorption at 13 nm and the etching resistance. This showed that polymers with aromatic groups tend to exhibit a lower absorption and a higher etching resistance than those without aromatic groups. This suggests that, regarding resist processes for EUV (extreme ultraviolet) lithography, a single-layer resist process employing no hard-mask layer is another promising candidate in addition to one that uses both an unltrathin resist layer (~100 nm thick) and a hard-mask layer

Photoinduced outgassing from the resist for extreme ultraviolet lithography by the analysis of mass spectroscopy

Extreme ultraviolet lithography (EUVL) requires the vacuum environment for exposing the resist. The contamination in the vacuum environment decreases the reflectivity of the reflective mask and that of the imaging optics. The photoinduced outgassing from the resist becomes the contamination in the vacuum environment. Therefore, the outgassing detection investigation is very important. The outgassing from the chemically amplified (CA) resists EUV001 for EUVL, EUV006N for EUVL, UV5 for KrF lithography and the nonchemically amplified resists OEBR2000 and ZEP520 for electron beam lithography were investigated. Based on the photoinduced reactions of the resist, the fragment ions species that were measured by the quadrupole mass spectrometer were identified. It is found that the amount of the photoinduced outgassing such as hydrocarbons from the DQN resist and annealing-type CA positive-tone resist is small.

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.

Control of line edge roughness of ultrathin resist films subjected to EUV exposure

The line edge roughness (LER) of ultrathin chemically amplified (CA) KrF-resist-based films was investigated using exposure to extreme ultraviolet (EUV) radiation (13.5nm). For the films between 0.09micrometers and 0.13micrometers thick, the LER was about 5~7% for a target critical dimension (CD) of 70nm and exposure to coherent illumination ((sigma) =0.01). The LER was found to be smaller in samples containing a relatively strong-acid photo-acid generator (PAG). The use of baking conditions producing greater acid diffusivity and the use of weak developer were very effective in reducing the LER of thin resist films. Atomic force microscope (AFM) observations showed the surface morphology of samples with a small LER to be very uniform. These results suggest that the use of a high-sensitivity resist and a weak developer may help to create an environment promoting uniform dissolution, thus resulting in a smaller LER in thin resist films.

Assembly and alignment of three-aspherical-mirror optics for extreme ultraviolet projection lithography

We have assembled and aligned projection optics for extreme ultraviolet (EUV) lithography. The projection optics consists of three aspherical mirrors. First, the positions of the mirrors were coarsely adjusted using the side and back surface of the mirrors. Next, the mirrors were finely aligned to minimize the wavefront errors which were measured by an interferometer. The adjustable axes were selected according to the results of the analysis of the allowable error range. The compensation values for each adjustable axis were calculated by commercially available ray-tracing software. After the alignment procedure, the wavefront error of 3 nm RMS was achieved.

Lithographic performance and optimization of chemically amplified single layer resists for EUV lithography

The single layer chemically amplified resists are investigated for the extreme ultra-violet lithography. From the results of the sensitivity curve, the positive-tone resist of DP603 and the negative-tone resist of SAL601 have high sensitivities and high gamma values to the EUV exposure wavelength. Furthermore, by the optimization of both the dosage and the wafer focusing position, we succeed in replicating 0.056-micrometer-resist- pattern width on the exposure-field size of 10 mm X 1 mm on an 8-inches-diameter wafer. We confirm the resolution capability of the three-aspherical mirror imaging system that has been developed by the Himeji Institute of Technology.

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.

Performances of resists for 157-nm lithography based on monocyclic fluoropolymers

Fluoropolymers are key materials for the single-layer resists used in 157-nm lithography. We have been studying fluoropolymers to determine their potential for use as the base resin and have developed a monocyclic fluorinated polymer with a blocking group of Cyclohexylcyclohexyloxymethyl (CCOM) that has high transmittance (an absorption coefficient of 0.64 μm-1) at a 157-nm exposure wavelength and high dry-etching resistance (a dry-etching rate of 1.75 times that of KrF resist) under organic bottom anti-reflective coating/hard mark dry-etching conditions. A resist based on our monocyclic fluoropolymer had high sensitivity. Using it, we were able to resolve a 60-nm line-and-space pattern using a 157-nm laser microstepper (numerical aperture = 0.85) with a resolution enhanced technology of an alternating phase-shifting mask. This polymer was demonstrated to simultaneously enable high transparency, high dry-etching resistance, and good imaging performance.

Measurement of Resist Transmittance at Extreme Ultraviolet Wavelength Using the Extreme Ultraviolet Reflectometer

Since the same reduction exposure as that of conventional optical lithography technology is possible and the wavelength is as short as 13 nm, extreme ultraviolet (EUV) lithography is a promising method for the fabrication of semiconductors with feature sizes of 50 nm and below. In order to resolve a fine pattern in a single-layer resist process, a resist material with a small absorption coefficient needs to be developed. We have developed a method of measuring resist transmittance at the EUV wavelength using an EUV reflectometer. The EUV light used by the EUV reflectrometer was emitted from a CO2 plasma. The resist transmittance at the EUV wavelength could be calculated based on two parameters measured by the reflectrometer, namely, the reflectivity (Rtotal) of the resist on a multilayer mirror and the reflectivity (RML) of the Mo/Si multilayer mirror surface. The resist sample measured was polymethylmethacrylate (PMMA), a typical resist polymer. We found that the measured resist transmittance is in good agreement with the calculated resist transmittance and that method developed is very effective for measuring actual resist transmittance at the EUV wavelength.