In the high density semi-conductor fabrication technology, the role of lithography technology is becoming increasingly important, and the various efforts are being made to obtain the higher resolution in lithographic optical system. There have been two important milestones in optical lithography technologies; one is the fabrication technology of 64M DRAM with 0.35㎛ resolution which uses Hg-i line(0.365㎛) and modified mask technology(for example, phase shift mask) or modified illumination, and the second is that of 256M DRAM with 0.25㎛resolution which uses excimer laser stepper(0.248㎛ or 0.193㎛). For the production of GBit DRAM the further improved resolution of 0.18㎛(1G DRAM), 0.13㎛(4G DRAM), 0.10 ㎛(16G DRAM), 0.07㎛(64G DRAM) is required. Currently, it is expected that soft X-ray projection lithography will make the production of GBit DRAM possible. In soft X-ray region$(\approx10^2Å)$, absorption is so severe that we must use the reflective optics in which the multi-layer quantum wells, such as Mo/Si material(Reflectance=66% for 40 bilayers), are employed on the mirror surface.
Several types of mirror system for soft X-ray projection lithography have been reported. For example, Schwarzschild system, consists of two concentric spherical mirrors. As the number of design-parameters is not enough, the system can't overcome aberrations such as astigmatism, so that image size is made very small(25㎛×50㎛). The two types of the ring-field optical system have been reported; one is the modified 1:1 ring-field optical system with ring-field of 10mm ×0.1mm in the image plane, and the second is NTT's two-aspherical-mirror system(magnification; 1/5) with ring -field of 20mm×0.4mm in the image plane.
In our works, our primary attention is on axially symmetric system, because the axially non-symmetric system such as off-set 3 mirror system is difficult to eliminate residual off-axial aberrations. The highest performance (resolution; 0.1 ㎛, image size; 2.5cm×2.5cm) imaging system using soft X-ray which was reported by V. K. Viswanathan and Brian E. Newnam is composed of the axially symmetric aspherical two-mirrors where reflections take place four times at four different annuli, and the inter-surface distances are aligned with the accuracy of less than 0.05㎛.
We already reported on the aspherized UV(ArF excimer laser) four-mirror system with transverse magnification of +1 and +1/5. In the present work, we aim to derive the superb optical system with the performance required for the GBit lithography. For this purpose, first of all we have to use soft X-ray(13nm) for the reflective lithography. As the four-mirror system has 8 design-parameters(4 curvatures and 4 distances), but having 7 constraints from nearly zero third-order aberrations$(∑_{j=1}^V |S_j|<10^{-6})$, a given reduction magnification, and scaling factor, the number of the remaining free design-parameter is one, for instance, $d_0$(the distance of object to the first mirror surface) in this work. We report solutions satisfying 7 constraints and axial vignetting less than 50% for the reduction magnification of +1/2, +1/3,…, +1/10, +1/15, +1/20. After we optimize the four-spherical mirror system which is the best initial solution for each reduction magnification, the resolution behavior according to the variation of reduction magnification is obtained for NA of 0.15 and the object size of 1.5cm. For magnification of +1/10, we have also investigated resolution behavior according to the variation of the object size(1.5cm~3.5cm) for the given NA of 0.15. The two systems finally obtained have the performance of spatial frequency of 3,950cycles/mm (resolution=0.127㎛) for the object size of 1.5cm and 3,600cycles/mm (resolution=0.139㎛) for the object size of 3.0cm respectively at which MTF values for all fields are over 0.4 and DOF is 0.7㎛. These performances are sufficient for giga-bit lithography.
