EUV lithography has become the leading candidate for pattern replication at the 32-nm technology node, but several important issues remain unresolved. In particular, the availability of defect-free masks is a critical concern. An intensive investigation of defect repair methods for EUVL mask blanks is required because the mitigation of defects has turned out to be much more difficult than anticipated. So, we investigated the effectiveness of several defect repair methods through accurate simulations employing the FDTD method. We calculated aerial images from masks with structural changes due to repair and compared them with those of a perfect mask. All the methods were found to suppress the degradation in light intensity caused by defects. At the same time, each repair method has some limitations and factors that require special attention. Thus, it is important to choose the most suitable repair method for a given defect.
Since extreme-ultraviolet lithography (EUVL) uses a much shorter wavelength than optical lithography, it should provide better pattern fidelity. In this study, various patterns were printed with a high-numerical-aperture (NA=0.3) small-field EUV exposure tool (HiNA) with new set-3 projection optics, and their fidelity to the mask patterns was evaluated. The set-3 optics have one-half the wavefront error and one-quarter the flare of the set-2 optics, and should thus provide much better image contrast. For EUVL mask fabrication, we used mask blanks with a TaGeN/Cr absorber stack and existing process tools for photomasks, which enable the fabrication of very accurate mask patterns. In the printing experiments, 80-nm line-and-space (L/S) and isolated-line patterns were successfully fabricated simultaneously without any optical proximity correction (OPC) of the mask pattern. For 60-70-nm line patterns, the printed patterns were still of good quality, but were slightly deformed. We speculate that this was caused by the low contrast of the aerial image, mainly due to wavefront error and flare. For 60-80-nm contact-hole (C/H) patterns, both dense and isolated holes were successfully fabricated without OPC or phase-shift mask (PSM). However, the printed patterns were almost circular, even though the mask patterns were square. This might be due to deformation of the aerial image and deformation produced by the resist process. For better pattern fidelity, it will probably be necessary to improve the characteristics of both the projection optics and the resist.
The availability of defect-free masks is a critical concern in EUV lithography. An intensive investigation of defect repair methods for EUVL mask blanks is required because the mitigation of defects has turned out to be much more difficult than anticipated. We investigated the effect of four methods through accurate simulations employing the FDTD method: 1) scooping a multilayer for amplitude defects, 2) EB exposure for phase defects, 3) covering a defect with an absorber pattern, and 4) making intrusions in the absorber pattern near a defect. These methods create structural changes in the masks themselves. We calculated the aerial images of masks with such changes and compared them with that of a perfect mask. It was found that all the methods suppress the degradation in light intensity caused by defects. At the same time, each repair method has some limitations and factors that require special attention. Thus, it is important to choose the most suitable repair method for a given defect.
Mask blank inspection is a critical issue in EUV lithography. Visible-light inspection has the advantages of a high throughput and a low tool cost, while actinic inspection potentially has a high inspection capability. The Hamamatsu Photonics Super Fine Particle Detection System, which employs dark-field scattering optics, has a high detection sensitivity and throughput. It is able to detect PSL spheres with diameters below 50 nm on a Si substrate. So, we tried using it to detect fine PSL spheres on Mo/Si multilayer mask blanks. 60-nm PSL spheres were detected, but noise arising from the surface roughness of the Mo/Si multilayer prevented the accurate detection of PSL spheres with diameters of less than 50 nm. Thus, it is important to reduce the surface roughness of the multilayer in order to improve the inspection capability of visible-light inspection systems.
The printability of a dense line pattern and a model pattern using two configurations of absorber and buffer materials for the mask were examined through simulations. An absorber material with a small extinction coefficient of 0.025 must be thicker than one with a large extinction coefficient fo 0.040 to ensure sufficient reflectance contrast. However, a thick absorber enhances the adverse influence of off-axis incidence on the critical dimensions and pattern fidelity of a dense pattern layout, and the influence increases markedly when the incident angle is over 6.2°. Thus, and absorber material with a large extinction coefficient is advantageous in reducing the influence of off-axis incidence because it allows the use of a thinner layer. Another approach to reducing this influence is mask pattern correction. A newly developed algorithm makes the mask near-field energy for off-axis incidence the same as that for normal incidence. This simple correction compensates effectively for the influence of off-axis incidence and provides excellent pattern fidelity. The algorithm enable the use of absorber and buffer materials with a small extinction coefficient and also a large incident angle of over 6.2°.
The strong smoothing effect resulting from recent progress in multilayer deposition technology has a great influence on the imaging characteristics of mask blank defects. The imaging characteristics of such defects were investigated through accurate simulations employing the FDTD method; and the effect of multilayer smoothing on the aerial image was examined. Strong smoothing was found to suppress the degradation in the aerial image due to phase defects while at the same time giving rise to phase defects that are undetectable iwth a visible-light inspection tool. 3-dimensional simulations also indicated the existence of such defects. Moreover, the aerial image of phase defects after repair with an electron beam was also investigated. Repair was found to be effective when there was no smoothing, but not so effective when there was strong smoothing. Experimental verification of these results will be attempted in the near future.