Kazuo Tawarayama

Extreme Ultraviolet Lithography Using Small-Field Exposure Tool: Current Status

The small-field exposure tool (SFET) for extreme ultraviolet (EUV) lithography was manufactured by Canon and EUVA and installed in Selete. It is being used for developing mask, resist, and tool technologies. In this paper, we review the current status of SFET development and present some initial results on lithographic performance and tool stability.

Aberration measurement from specific photolithographic images: a different approach

Techniques for measurement of higher-order aberrations of a projection optical system in photolithographic exposure tools have been established. Even-type and odd-type aberrations are independently obtained from printed grating patterns on a wafer by three-beam interference under highly coherent illumination. Even-type aberrations, i.e., spherical aberration and astigmatism, are derived from the best focus positions of vertical, horizontal, and oblique grating patterns by an optical microscope. Odd-type aberrations, i.e., coma and three-foil, are obtained by detection of relative shifts of a fine grating pattern to a large pattern by an overlay inspection tool. Quantitative diagnosis of lens aberrations with a krypton fluoride (KrF) excimer laser scanner is demonstrated.

Light-shield border impact on the printability of extreme-ultraviolet mask

When a thinner absorber mask is applied to extreme ultraviolet (EUV) lithography for chip production, it becomes essential to a introduce light-shield border in order to suppress the leakage of EUV light from the adjacent exposure shots. In this paper, we evaluate the leakage of both EUV and out-of-band from light-shield border and clarify the dependence of lithographic performance on light-shield border structure using a small field exposure tool with/without spectral purify filter (SPF). Then we evaluate the lithographic performance of a thin absorber EUV mask with light-shield border of the etched multilayer type and demonstrate the merit of its structure using a full-field scanner operating under the currently employed condition of EUV source in which SPF is not installed.

Recent progress of EUV full-field exposure tool in Selete

The Selete full-field EUV exposure tool, the EUV1, was manufactured by Nikon and is being set up at Selete. Its lithographic performance was evaluated in exposure experiments with a static slit using line-&-space (L&S) patterns, Selete Standard Resist 03 (SSR3), an NA of 0.25, and conventional illumination (σ = 0.8). The results showed that 25- nm L&S patterns were resolved. Dynamic exposure experiments showed the resolution to be 45 nm across the exposure field and the CD uniformity across a shot to be 3 nm, also 26-nm L&S patterns were resolved. Overlay performance of the EUV1 was showed as processed wafer mark alignment, the repeatability was under 1nm. Overlay accuracy using EGA (Enhanced Global Alignment) was below 4nm at the 3-sigma after liner correction. These results were good enough for an alpha-level lithography tool and test site verification.

Effects of aberration and flare on lithographic performance of SFET

The effects of aberration and flare on the lithographic performance of the EUV small-field exposure tool (SFET) were evaluated. Simulation results indicated that the effect of aberration on the image contrast of line-and-space (L&S) patterns should be small. In exposure experiments, 26-45-nm L&S patterns were successfully fabricated under annular illumination (σ=0.3/0.7). A key factor limiting resolution should be resist performance. Simulation results also indicated that the astigmatic aberration could produce a focal shift of about 60 nm between horizontal and vertical L&S patterns. The experimentally obtained focus shift agreed well with the simulation results. Dense 32-45-nm contact-hole (C/H) patterns were also successfully fabricated under annular illumination (σ=0.3/0.5). Due to astigmatic aberration, the C/H patterns were deformed at defocused positions, but they were almost circular at the best focus position. The flare of the projection optics measured by the Kirk method was 11% over a flare range of 1-100 μm. The effects of the 11% flare were evaluated using dark- and bright-field 32-nm L&S patterns. It was found that the top loss and line-width roughness (LWR) of the resist were larger for bright-field than for dark-field patterns. To reduce the impact of flare, we need EUV resists that are more robust with regard to flare. A comparison of the measured point spread function (PSF) of the flare and the calculated PSF revealed good agreement for long-range flare but some difference for short-range flare.

Evaluation of EUV mask defect using blank inspection, patterned mask inspection, and wafer inspection

The key challenge before EUVL is to make defect-free masks, for which it is important to identify the root cause of defects, and it is also necessary to establish suitable critical mask defect size for the production of ULSI devices. Selete has been developing EUV mask infrastructures such as a full-field actinic blank inspection tool and 199nm wavelength patterned mask inspection tool in order to support blank/mask supplier in reducing blank/mask defects which impact on wafer printing. In this paper, by evaluating the printability of programmed phase defects and absorber defects exposed by full-field scanner EUV1, we demonstrate that defect detection sensitivities of ABI (actinic blank inspection) and PI (patterned mask inspection) are higher than that of WI (wafer inspection) in HP32nm. The evaluations were done by comparing the detection sensitivities of full-field actinic blank inspection tool, 199nm wavelength patterned mask inspection tool, and wafer EB inspection tool. And then, based on the native defect analysis of blank/mask, we ascertained that actinic blank inspection and patterned mask inspection developed at Selete, are effective in detecting killer defects both at the main pattern and at light-shield border area.

Applicability of extreme ultraviolet lithography to fabrication of half pitch 35nm interconnects

Extreme ultraviolet lithography (EUVL) is moving into the phase of the evaluation of integration for device fabrication. This paper describes its applicability to the fabrication of back-end-of-line (BEOL) test chips with a feature size of hp 35 nm, which corresponds to the 19-nm logic node. The chips were used to evaluate two-level dual damascene interconnects made with low-k film and Cu. The key factors needed for successful fabrication are a durable multi-stack resist process, accurate critical dimension (CD) control, and usable overlay accuracy for the lithography process. A multi-stack resist process employing 70-nm-thick resist and 25-nm-thick SOG was used on the Metal-1 (M1) and Metal- 2 (M2) layers. The resist thickness for the Via-1 (V1) layer was 80 nm. To obtain an accurate CD, we employed rulebased corrections involving mask CD bias to compensate for flare variation, mask shadowing effects, and optical proximity effects. With these corrections, the CD variation for various 35-nm trench and via patterns was about ± 1 nm. The total overlay accuracy (|mean| ± 3σ) for V1 to M1 and M2 to V1 was below 12 nm. Electrical tests indicate that the uses of Ru barrier metal and scalable porous silica are keys to obtaining operational devices. The evaluation of a BEOL test chip revealed that EUVL is applicable to the fabrication of hp-35-nm interconnects and that device development can be accelerated.

Resolution capability of SFET with slit and dipole illumination

A high-resolution EUV exposure tool is needed to facilitate the development of EUV resists and masks. Since the EUV small-field exposure tool (SFET) has a high numerical aperture (NA = 0.3), low aberration & flare, and excellent stage stability, it should be able to resolve fine L/S patterns for the half-pitch 22-nm & 16-nm nodes. In this study, we evaluated the resolution capability of the SFET and obtained 22-nm L/S patterns with x-slit illumination and clear modulation of 16-nm L/S patterns with x-dipole illumination. The resolution limit of the SFET seems to be about 15 nm. The main cause of pattern degradation in 16-nm L/S is probably resist blur. To obtain good shapes for this pattern size, the resist blur of less than 3.5 nm (σ) is required. The use of y-slit illumination was found to reduce the linewidth roughness (LWR) of resist patterns. Further reduction of the LWR requires a higher image contrast and a smaller flare. Due to the central obscuration, the image contrast of the SFET is sensitive to the change of pupil fill. The degradation in the collector & DMT should be reduced to ensure stable aerial images. This work was supported in part by NEDO.

Evaluation of extreme ultraviolet mask defect using blank inspection, patterned mask inspection, and wafer inspection

The key challenge before extreme ultraviolet lithography is to make defect-free masks, for which it is important to identify the root cause of defects, and it is also necessary to establish suitable critical mask defect size for the production of ULSI devices. We have been developing extreme ultraviolet (EUV) mask infrastructures such as a full-field actinic blank inspection tool and 199 nm wavelength patterned mask inspection tool in order to support blank/mask supplier in reducing blank/mask defects which impact wafer printing. In this paper, by evaluating the printability of programmed phase defects and absorber defects exposed by full-field scanner EUV1, we demonstrate that defect detection sensitivities of actinic blank inspection and patterned mask inspection are higher than that of wafer inspection in HP32nm. The evaluations were done by comparing the detection sensitivities of full-field actinic blank inspection tool, 199 nm wavelength patterned mask inspection tool, and electron beam (EB) wafer inspection tool. And then, based on the native defect analysis of blank/mask, we ascertained that actinic blank inspection and patterned mask inspection are effective in detecting killer defects both at the main pattern and at the light-shield border area.

Lithographic Performance of Extreme Ultravolet Full-Field Exposure Tool at Selete

The Selete full-field etreme ultraviolet (EUV) exposure tool, the EUV1, was manufactured by Nikon and is being developed at Selete. Its lithographic performance was evaluated in exposure experiments with a static slit using line and space (L&S) patterns, Selete Standard Resist 03 (SSR3), a numerical aperture (NA) of 0.25, and conventional illumination (σ= 0.8). The results showed that 25 nm L&S patterns were resolved. Dynamic exposure experiments showed the resolution to be 45 nm across the exposure field and the critical dimension (CD) uniformity across a shot to be 7 nm, which is sufficient for an alpha-level lithography tool.