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Four years of research have culminated in the construction of a high-numerical-aperture (NA=0.3), small-field EUV exposure tool (HINA) to accelerate the development of masks, resist processes, and materials for EUV lithography. Three sets of projection optics (Sets 1, 2, and 3) were fabricated to the mark of a wave front error (WFE) of less than 1 nm. The RMS WFE is 7.5 nm for Set 1, 1.9 nm for Set 2, and at most 0.9 nm for Set 3. In addition, the RMS mid-spatial frequency roughness (MSFR), which affects flare, is 0.34 nm for Set 2 and 0.17 nm for Set 3. This paper discusses the current lithographic performance of HINA, especially the evaluation of flare and the replication of fine-pitch patterns. Several EUV masks were fabricated to evaluate the effects of flare and to replicate fine-pitch patterns. In the case of Set 2 optics, 90 nm lines and spaces were barely delineated using a bright-field mask due to the RMS MSFR of 0.34 nm, and replication of 70 nm lines and spaces were achieved using a dark-field mask. Since the RMS WFE and the RMS MSFR for Set 3 optics are half as much as that for Set 2 optics, the lithographic performance of HINA is markedly improved. 50 nm lines and spaces of non-chemically-amplified resist were delineated with the illumination condition of a partial coherence, σ , of 0.8 and 45 nm lines and spaces were delineated with the annular illumination condition of outer σ of 0.8 and inner σ of 0.5. In addition ultimate resolution of 30 nm lines and spaces of chemically-amplified resist was performed under the coherent illumination condition of σ of 0.0.

A Schwarzschild objective (magnification, ×32; numerical aperture, 0.2), which has a 0.1-µm resolution within 30 µm of the object height, was designed and fabricated. We have developed new normalincidence multilayer mirrors for carbon Kα radiation (wavelength, 44.8 Å), NiCr (80-20 wt. %)/C multilayers (thickness period, 22.5 Å; number of layers, 50), which are deposited by ion-beam sputtering with the thickness distribution corrected by deposition masks. Magnified images were taken on photographic film with the Schwarzschild objective by using an electron impact carbon Kα radiation source, and a resolution of < 0.5 µm was confirmed.

Extreme ultraviolet focus sensor design optimizationa)We have developed a three-aspherical mirror system which is capable of replicating in a large exposure area ͑30 mmϫ28 mm͒. This system consists of the synchronized scanning mechanism of a mask and a wafer, the alignment optics between a mask and a wafer, the focus detector of a wafer position, and the load-lock chamber for exchanging wafers. The aspherical mirrors have a figure error of 0.58 nm and a surface roughness of 0.3 nm. To obtain a high efficiency mirror, a couple of mirrors were coated with a graded d spacing Mo/Si multilayer. The peak reflectivity is 65% at the wavelength of 13.5 nm. The wavelength matching of each mirror spans 0.45 nm. The mirrors were aligned with a Fizeau-type phase shift interferometer, and a final wave front error of less than 3 nm was achieved. Exposure experiments carried out at NewSUBARU synchrotron facility and a diffraction limited resolution of 56 nm was obtained in an exposure-field size of 10 mmϫ2 mm in static exposure. Furthermore, fine patterns in an area of 10 mmϫ5.2 mm were obtained using the mask and wafer synchronized scanning stages. These results revealed that this system can be applied to fabricate large scale integrated devices.

We have fabricated Si stencil reticles that are employed by a new type of e-beam projection lithography system ͑EB stepper͒. We applied a stress reduction technique to the Si membrane to improve the pattern placement accuracy. The residual stress of Si membranes which were fabricated by anisotropic etching of B-doped Si wafers in KOH aqueous solution was reduced by annealing at 1150°C. We carried out pattern-displacement measurements for a Si stencil reticle made of a Si membrane where the residual stress was reduced to 10 MPa, and we observed that the pattern displacement error was reduced to less than 20 nm. Furthermore, the pattern displacement in the stencil reticle had a high correlation with the displacement determined from a simulation based on a finite element model. However in the same reticle, we discovered additional, comparatively small displacements in random directions, which was not expected in a membrane that had a homogeneous tensile stress. As a cause of the pattern displacement in random directions, we identified a pattern-width broadening in the dry etching process.