LER (line edge roughness) is becoming increasingly critical for manufacturers and efforts to understand and control it have given disappointing results. We propose that LER is due to a combination of coherent optical effects, mask LER, and chemical processes during exposure, PEB (post exposure bake) and development. Different sources of LER have similar scaling laws and PSD (Power spectral density) distribution, and the causes of LER are easily misidentified. High sensitivity, thin resist, and low image log-slope generally give more LER. No single-effect model is going to be adequate to give quantitative predictive guidance how to reduce LER. Since LER is shared between chemistry, optics and metrology, a cross-disciplinary model is needed. We propose such an LER budget model with a unified analysis of the metrology and consequences of LER, but with models for source effects plugged in by experts from the relevant domains.
Dynamic speckle is caused by the finite pulse length and limited spectral linewidth of the partially coherent radiation from the excimer lasers used in optical projection lithography. One effect of the dynamic speckle is that the energy delivered to a certain position at the wafer is a stochastic quantity and cannot be precisely controlled, which fundamentally limits the dose control in the lithographic system. Further, the spatial distribution of the dynamic speckle is shown to depend on illumination conditions, contributing to unwanted effects such as line edge roughness (LER). We present a formulation for the dynamic speckle of the partially coherent optical field, based on a temporal degrees of freedom approach. We show theoretically and by numerical simulation how the choice of the illuminator intensity distribution influences LER. We also point to some pitfalls in the struggle to combat dynamic speckle. It is fundamentally important to realize that instantaneously any combination of uncorrelated fields adds to the total field on a complex amplitude (not intensity) basis just like correlated fields. Therefore, any spatial redistribution of light, e.g., with the help of microlens arrays, does not reduce the dynamic speckle.
Dynamic speckle is caused by the finite pulse length and limited spectral linewidth of the partially coherent radiation from the excimer lasers used in optical projection lithography. One effect of the dynamic speckle is that the energy delivered to a certain position at the wafer is a stochastic quantity and cannot be precisely controlled, fundamentally limiting the dose control in the lithographic system. Further, the spatial distribution of dynamic speckle fluctuations is shown to depend on illumination conditions, contributing to unwanted effects such as line edge roughness (LER).
In this work we show, theoretically and by numerical simulation, how the choice of the illuminator intensity distribution influences LER. In particular, it is noted that speckle-induced LER is a prominent cause of the long-range changes in the position of the line edge, as evidenced in the calculations of the power spectral density (PSD) of the LER.
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