We develop three main topics in support of further understanding and specifying wavefront aberrations from the lithographer's point of view. The concept of the Magnitude Weighted Aberration is introduced providing a convenient and rapid numerical method for assessing the interaction of wavefront aberrations with reticle pattern and illumination mode. This analysis suggests that the advanced lithographic lens user will require unprecedented correction on the total wavefront aberration to realize the full potential of the imaging system in high yielding integrated circuit fabrication. Specific details on the required aberration control are provided with a Monte Carlo tolerancing analysis of the RMS wavefront error using lithographic CD control and pattern placement as quality metrics. Pattern placement proves to be as sensitive to wavefront aberrations as CD control forcing a tight specification on the asymmetric aberration components even when a large focus and exposure latitude is available. Based on the wavefront specifications generated, it is imperative that the lithographic lens user be able to independently de-couple and quantify the state of certain aberration coefficients. Toward this goal, we demonstrate an aberration reverse engineering procedure using experimental pattern placement error as the input response.
The impact of alignment mark structure, mark geometry, and stepper alignment optical system on mark signal contrast was investigated using computer simulation. Several sub-wavelength poiy silicon recessed film stack alignment targets of advanced memory products were studied. Simulated alignment mark signals for both darkfield and bright-field systems using the rigorous electromagnetic simulation program TEMPEST showed excellent agreement with experimental data. For a dark-field alignment system, the critIcal parameters affecting signal contrast were found to be mark size and mark recess depth below silicon surface. On the other hand, film stack thickness and mark recess depth below/above silicon surface are the important parameters for a bright-field alignment system. From observed simulation results optimal process parameters are determined. Based on the simulation results some signal enhancement techniques will be discussed.
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