Compensation for imaging errors in EUV lithography

Abstract

In this paper we will examine some of the fundamental imaging effects that must be considered with the intended implementation of Extreme Ultraviolet Lithography (EUVL). The paper will show how simulation can be used to examine issues such as image placement and the effect of mask dimension errors. We will show how the exact structure of an EUV mask need not be simulated, but the use of Fourier boundary conditions may be used as an accurate substitute, considerably speeding up computation time. Further, this technique is used to show the positional error that is inherent in an off-axis reflective optic design such as that proposed for EUV exposure tools. Any dimension error that is produced on the mask will not linearly transfer to the printed wafer, this is known as Mask Error Factor (MEF). We will present simulation data showing that the off-axis nature of the incident light leads to different rates of change of printed CD, at defocus, for features orientated perpendicular to each other. These effects must then be taken into account when we consider reticle error budget for EUVL technology. 1) INTRODUCTIONIn the Extreme Ultraviolet (EUV) portion of the electromagnetic spectrum it is almost impossible to create highly transmissive materials, so in an optical system all optical elements must be reflective. The same principles must therefore be applied to the construction of the photomask, where we use a combination of an absorber and a multilayer stack to project an image from the mask. As a result in an EUV exposure tool, due to this requirement for reflectivity, the incident light onto the photomask is off-axis thus a shadow effect is created at the edge of the absorber on the mask. This 'shadow' then propagates through the imaging system to produce a non-ideal image on the wafer.In this paper we will use simulation based data to present how this shadow phenomenon effects both image placement and measured CD when comparing similar geometry's in both x and y directions. We will also highlight the changes in Mask Error Factor (MEF) [1] in EUV imaging as sigma is changed and through focus, for different CD sizes and densities. Data from line and space patterns will be analysed along with data for contact holes. 2) METHODToday we are at a relatively immature stage of tool development for EUV lithography. Our imaging capability is still at a developmental stage and so it is very difficult to accurately perform experiments to determine any effects that we wish to investigate. Simulation allows us to quickly and inexpensively conduct these types of experiments with a very good degree of accuracy, allowing us to better prepare for actual implementation of a technology. The software and models used for these simulations are, however, very complicated and require a great deal of computing power. In EUV lithography the mask is composed of a series of very thin layers that together form a highly reflective surface, effectively a multilayer 'mirror'. A buffer layer and absorber are then patterned on top of this to produce t...