Depth of focus (DOF) is the gating factor of resolution and an indicator of the feasibility of a given technology node. However, its meaning is ambiguous. Sometimes it is used to indicate the longitudinal range of a good image governed by diffraction. Other times it is used as a budget for mechanical and optical tolerances. Another source of confusion is that the longitudinal range of a good diffracted image is usually evaluated for an air medium, ignoring the wavelength compression effect of the photoresist, as well as the fact that at least two media have to be within the longitudinal range of the good image. An immersion system with a high-index coupling medium makes the situation even more complicated. We classify the types of DOF into diffraction DOF, required DOF, and available DOF, then show the mutual dependence of these parameters and their significance in subwavelength imaging. The impact and DOF-resolution tradeoff, with and without a high-index coupling medium, is also explored.
"Depth of focus in multilayered media-a long-neglected phenomenon aroused by immersion lithography," Journal of Micro/Nanolithography, MEMS, and MOEMS 3(1), (1 January 2004). https://doi.org/10.1117/1.1637591