No abstract

Projection reduction exposure with variable axis immersion lenses (PREVAIL) represents the high throughput e-beam projection approach to NGL, which IBM is pursuing in cooperation with Nikon as alliance partner; another e-beam projection approach is SCALPEL pursued by Lucent Technologies. This article discusses the challenges and accomplishments of the PREVAIL project. It will focus on the results obtained with the proof of concept (POC) system. This system was developed to demonstrate key technical building blocks required for high throughput, high resolution e-beam step, and scan projection lithography. The supreme challenge facing all e-beam lithography approaches has been and still is throughput. Since the throughput of e-beam projection systems is severely limited by the available optical field size, the key to success is the ability to overcome this limitation. The PREVAIL technique overcomes field-limiting off-axis aberrations through the use of variable axis lenses, which electronically shift the optical axis simultaneously with the deflected beam, so that the beam effectively remains on axis. This technique developed by IBM has been successfully applied to probe-forming shaped beam systems (EL-4). It had to be modified and extended to provide the larger beam deflections and the wider beam images at the wafer plane used in projection reduction systems. The POC system projects sequentially 1×1 mm2 subfields, selected at the reticle, in 4:1 reduction mode onto the wafer, exposing and resolving patterns of 80 nm lines and spaces in resist; each subfield contains 107 pixels. The deflection capability demonstrated permits electronic selection of 20 1 mm subfields at the reticle and projection of these 20 subfields onto the wafer exposing a field with 5 mm scan length. The resist images provide proof that PREVAIL effectively eliminates off-axis aberrations affecting resolution, since the deflected and undeflected images are indistinguishable. PREVAIL also controls off-axis aberrations affecting placement accuracy of pixels, since distortions of the deflected subfield are corrected to within 12 nm. A high emittance gun has been developed to provide uniform illumination of the patterned subfield, and to fill the large numerical aperture of the projection optics required to significantly reduce beam blur caused by Coulomb interaction.

Aberrations calculated from the electron trajectories are compared with those estimated from the third-order aberration coefficients. Field curvature and radial and azimuthal distortions are consistent with each other for moderate radial value R. In order to optimize lens positions, a method is adopted which makes the lens condition under which the principal ray trajectory from the largest objective radius R o crosses the optical axis at its normal position equal to that under which the trajectories diverging from the same R o focus on the image. For the 50 cm object-to-image distance optics, residual aberrations are calculated through the trajectory calculation after the corrections of lens current, crossover and beam position. When the main and subfield sizes are 20 mm and 0.25 mm square, the residual field curvature, astigmatism, and radial and azimuthal distortions are 13.1 nm, 6.4 nm, 19 nm, and 8.35 nm, respectively, when the beam semiangle is 0.1 mrad, and the beam energy is 100 keV.

Demagnifying immersion magnetic lenses used for projection electron beam lithography without crossoversScaled measurements of global space-charge induced image blur in electron beam projection system

In electron beam projection lithography (EPL), one of the most crucial tasks is to develop a data post-processing system, namely, a specific tool to expose a faithful pattern for every subfield on the wafer based on the pattern layout data. This system includes two basic flows. The 1st flow is common for reticle fabrication, and the 2nd flow is unique for EPL. During the 2nd flow, based on the LSI pattern data, electron optics space-charge effect correction will be automatically and rapidly executed and output to the EPL system in order to adjust parameters such as focus, magnification, rotation and astigmatism. In addition, this system should perform such tasks as segmentations of subfields (including complementary division), arrangement of stripes and reticlets, and alignment mark insertion. For proximity effect correction, we will first use a pattern shape modulation first. Shape modification at stitching boundaries is also investigated. In summary, to achieve conformable EPL delivery to customers, a new data post-processing system is developed in collaboration with some suppliers.