In order to maintain manageable process windows, mask shapes at the 20nm technology node and below become so complex that mask write times reach 40 hours or might not be writeable at all since the extrapolated write time reaches 80 hours. The recently introduced Model Based Mask Data Preparation (MB-MDP) technique is able to reduce shot count and therefore mask write time by using overlapping shots. Depending on the amount of shot count reduction the contour of the mask shapes is changed leading to the question how the mask contour influences wafer performance.This paper investigates the tradeoff between mask shot count reduction using MB-MDP and wafer performance using lithography simulation. A typical Source-Mask-Optimization (SMO) result for a 20nm technology will be used as an example.
In lithography, forbidden pitch refers to pitch that suffers degradation in the process window due to the application of off-axis illumination ͑OAI͒. Destructive light field interference of diffracted light from a mask at forbidden pitch causes reduction in image contrast and depth of focus ͑DOF͒ and limits the pitch range to be patterned. In this paper, a modification to conventional OAI shape is proposed to minimize the effect of forbidden pitch. The modification is based on the interaction of illumination source with pattern density. The modified source employs double annular illumination. Simulation is carried out to investigate the effect of the modified source for one dimensional line and space pattern with a pitch varying from 130 to 500 nm. Results shows that the maximum critical dimension fluctuation is around 3% compared to 13% in conventional annular illumination. Furthermore, the degradation in DOF is within 21% of DOF compared to 49% in conventional annular illumination.
Step and flash imprint lithography template characterization, from an etch perspectiveIn this article, we have systematically investigated the dependencies of k 1 on illumination conditions such as coherence setting and opening angle in off-axis illumination scheme. As result, conventional Rayleigh's equations are not sufficient to address the effect of the numerical aperature and coherence on the depth of focus. Therefore, a metric called the coherency factor ( c ) is proposed as a complementary metric of the low k 1 lithography. Coherency factor ( c ) is defined as the ratio of areas of captured first order and zero order light. The theory is based on simple geometrical analysis of the diffraction orders in the pupil plane. Areas of different diffraction orders captured by the pupil are evaluated as a function of wavelength, numerical aperture, and pitch. As corresponding to experimental results, a higher c value concurs with a larger depth of focus. Extracting from Fraunhofer diffraction equation for a single slit and incorporating coherency factor c , we have modified and extend the use of Rayleigh's equations for 90 nm processes and below. Results show that the extension of Rayleigh's equations is capable to optimize the depth of focus and map out the forbidden pitch locations for any design rules and illumination conditions. More importantly, it can complement the concept of objective lens pupil filling to provide the theoretical ground for illumination design in order to suppress the forbidden pitch phenomenon.
The demand for steadily decreasing dimensions in semiconductor devices is driving the need for increased resolution in optical lithography. The use of phase shift masks ͑PSMs͒ is among such resolution enhancement techniques. PSM are well known to show prominent diffraction effects, which cannot be described by the assumption of an infinitely thin mask ͑Kirchhoff approach͒ that is used in many commercial photolithography simulators. A correct prediction of sidelobe printability, process window and linearity of an OPC mask requires a rigorous application of diffraction theory. Optical lithography simulation employing a time-domain finite-difference ͑TDFD͒ algorithm ͑TEMPEST͒ has been used effectively to study the problem of aerial image intensity imbalance through focus with alternating phase shift masks ͑altPSMs͒. Using Geometrical Theory of Diffraction ͑GTD͒, and the solutions to canonical problems, we obtained a relationship between mask edge and disturbance in an image space. The main objective to develop useful formulations that can be readily applied to diffraction in mask technology. Rigorous analysis of diffraction effect on altPSMs using the GTD approach is discussed and results show that the effect of shifter edge angle is equivalent to that of shifter width.
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