Challenges and opportunities for 157-nm mask technology

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Abstract
157 nm lithography is a strong contender for 100 nm and 70 nm technology. Although feasibility studies did not identify any strong evidence that 157 nm lithography was not viable, several technological challenges still must be faced. CaF2 has been found to be an optical material that is practical for lens and illumination elements. Initially, the use of CaF2 materials for the reticle substrate was investigated. Its high thermal expansion, however, virtually excludes its use as a mask material. Recent measurements of modified fused silica have shown sufficient transmission of 157 nm for its use in lithographic masking. The residual absorption in modified- quartz masks is expected to give rise to thermal effects, but the choice of a non-Cr-based absorber may reduce the reticle heating effect. When considering mask design rules of 100 nm and 70 nm lithography, CD requirements become more critical. Even with NA values exceeding 0.7, the k1 value for 70 nm resolution exposed with 157 nm wavelengths will be low. Consequently, the mask error factor (MEF) is expected to increase further resulting in a larger reticle contribution to the overall CD budget. A counter measure for this would be a change in stepper reduction ratio from 4X to 6X. For 6' reticles only, this would result in smaller field sizes; but with 7' or 9' reticles, field sizes could become comparable to those available in current 4X Step & Scan Systems. The advantages and disadvantages of a change in the reduction ratio are discussed.

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