Seiichiro Shirai

Determination of high-order lens aberration using phase/amplitude linear algebra

New approaches to measuring lens aberration of optical exposure tools in semiconductor manufacturing environment are discussed. First, plural image quantities are measured and expressed by linear combinations of contributions from several Zernike aberration terms. Then, the linear problem is solved to find the best combination of each term to explain the experimental results. Using side-lobe intensity near octagonal aperture edges in an attenuated phase-shifting mask as a detector, third-order Coma and trifoil aberrations were successfully measured.

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.

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.

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.

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.

Evaluation result of Selete's exposure tool: impact of the source performance

We have installed a small-field exposure tool (SFET) manufactured by Canon and EUVA with a discharge-producedplasma EUV source that employs Xenon gas. We investigated how the performance of the source affects lithographic performance. Electrode life has relation to the illumination uniformity of the exposure field on wafer surface. Also source power at the wafer surface has relation to the electrode life. Electrode life makes EUV power decreasing and larger illumination uniformity number. We examine the pupilgram test using high sensitivity resist. Actual pupil fill shape was observed and there was non-uniform distribution. Pupil fill shape was changed after exchanging electrode, also resist CD bias between parallel and horizontal line of the field. That was comparable to the simulation result. The source electrode requires periodic replacement, which impacts not only the performance of the source, but also the lithographic performance of the tool, such as the CD of resist patterns.

Performance evaluation of source collector module for extreme ultraviolet small-field exposure tool

The source collector module (SoCoMo) for the extreme ultraviolet (EUV) small-field exposure tool (SFET) had been operated for 1,700,000,000 pulse radiations using more than 20 electrodes, four debris mitigation tools (DMT), and three collector mirrors. After 600 million pulse radiations, it was found that the EUV light intensity at wafer plane had been decreased to less than 10% of the initial value and the pupilgram exposed by SFET had been drastically deteriorated. In order to study on the cause of the intensity reduction and to maintain the intensity highly, three new evaluation tools were introduced; collector position monitors, an intermediate focus (IF) spot position monitor, and an attachment of screen tool. Utilizing these tools, it was clarified that the main causes of EUV light intensity reduction at IF plane were the plasma fluctuation with electrode erosion, the degradation of DMTs optical transmissivity, and the degradation of collection efficiency. These results indicated that it would be necessary for future SoCoMo not only to achieve highpower, stable and long-life performance, but also to equip with the functions that these tools provided in order to maximize its own performance.

Recent progress in developing an extreme ultraviolet full-field exposure tool at Selete

The Selete full-field EUV exposure tool, EUV1, manufactured by Nikon, is being set up 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 of 0.25, and conventional illumination (σ=0.8). The results show that 25-nm L&S patterns were resolved. Dynamic exposure experiments demonstrate that the resolution is 45 nm across the exposure field. The CD uniformity across a shot is 3 nm. Evaluation of the overlay performance of the EUV1 using alignment marks on a processed wafer revealed the repeatability to be better than 1 nm. The overlay accuracy obtained with enhanced global alignment was less than 4 nm (3σ) after linear correction. These results show that the EUV1 has attained the quality level of a typical alpha-level lithography tool and is suitable for test site verification.

Optimization of exposure procedures for sub-quarter-micron CMOS applications

We investigated various exposure procedures to minimize the Critical Dimension (CD) variation for the patterning of sub- quarter micron gates. To examine dependence of the CD variation on the pattern pitch and defocus conditions, the light intensity profiles of four different mask structures: (1) a binary mask with clear field, (2) a binary mask with dark field, (3) a phase-edge type phase-shifting mask (a phase-edge PSM) with clear field, and (4) a halftone phase- shifting mask (a halftone PSM) were compared, where exposure wavelength was 248 nm and numerical aperture (NA) of KrF stepper was 0.55. For 200-nm gate patterns, dependence of the CD variation on the pattern pitch and defocus conditions was minimized by a phase-edge PSM with clear field. By optimizing the illumination condition for a phase-edge PSM exposure, we obtained the CD variation of 10 nm at the minimum gate pitch of 0.8 micrometer and the defocus condition of plus or minus 0.4 micrometer. Applying the optimized exposure procedure to the device fabrication process, we obtained the total CD variation of plus or minus 27 nm.