12 May 2004 Understanding of the depth of focus evolution from an analysis of the iso-focal point
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Abstract
For the 90nm node and below the Depth of Focus (DOF) becomes more and more critical. To increase the DOF lithographers have introduced resolution enhancement techniques (RET) such as sub-resolution assist features (SRAF) which are today largely used by the semiconductor industry for 120nm, 90nm and 65nm technologies. Bruce Smith [1] showed that the improvement of the DOF from the adding of the scatter bars depends on the position of the iso-focal intensity threshold compared to the critical dimension (CD) intensity threshold. When these two points are at the same position the DOF is maximum. This paper shows the theoretical link between the iso-focal point and the evolution of the DOF. It will be shown that the link between these two parameters can be described by a simple equation. The theoretical expression shows a good estimation of the DOF evolution. The theoretical evolution of the iso-focal point is obtained from the expressions of the intensity. We will see that its variation is basically a function of the transmission and of the diffraction orders interfering. The expressions giving the evolution of the iso- focal point follows the trends obtained by conventional lithography simulation. We have studied the theoretical evolution of the iso-focal point for the mask types used by the semiconductor industry such as binary, alternating and attenuated phase shift masks. We will also see how this evolution of the iso-focal point impacts the depth of focus and that the DOF can be improved by an adjustment of the iso-focal point.
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S. Manakli, Y. Trouiller, P. Schiavone, "Understanding of the depth of focus evolution from an analysis of the iso-focal point", Proc. SPIE 5754, Optical Microlithography XVIII, (12 May 2004); doi: 10.1117/12.600970; https://doi.org/10.1117/12.600970
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