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.
The image shortening of rectangular array patterns replicated by proximity X-ray lithography was investigated. The shorter pattern width was 100 to 200 nm, and shortening on the longer side was studied. Either a negative or positive tone of the mask was used. The experimental results were compared with the absorbed dose distribution calculated using ToolSet developed at the Center for X-ray Lithography (CXrL). The geometrical features of mask patterns, such as absorber side-wall angle, corner rounding, and pattern resize were taken into account. It was found that image shortening is greater with negative tone masks, while negative tone masks provide higher resolution than positive tone masks. It was also found that the slope of the absorber side wall plays an important role in image shortening. The results suggest that the use of negative tone masks, careful tailoring of the absorber side wall, and proper pattern resizing will provide required dimensions for rectangular array patterns.
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.
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.
We evaluated critical-dimension (CD) control for three types of 130-nm patterns (line-and-space (L/S), isolated line, and hole) in proximity x-ray lithography (PXL). The intra-wafer CD variation was found to be 7.9nm (36) for L/S patterns, 9.9 nm (3σ) for isolated lines, and 18.7nm (3σ) for holes. We divided the CD variations into three components: intra-field CD variation, inter-field CD variation, and random CD errors. For line patterns, the largest component was intra-field CD variation, while the contributions of all three components were almost equal for hole patterns. The largest cause of the intra-field CD variation was the nonuniformity of the exposure dose. The CD variation due to mask-CD variation was less than half the mask-CD variation because of the effect of Fresnel diffraction. For holes, the major causes of inter-field CD variation and random CD errors might be the nonuniformity of the proximity gap and the edge roughness of the resist, respectively.
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.
