Advanced EUV Imaging*

Michael D. Shumway
(Professor Jeffrey Bokor)
(DARPA) MDA972-97-1-0010 and (SRC) 96-LC-460

Extreme ultraviolet (EUV) projection lithography is a proposed next-generation lithography technique for manufacturing integrated circuits at high volumes. It is targeted to print critical dimensions of 70 nm and below with a large depth of focus. An EUV microstepper using a 10x Schwarzschild objective is currently in use through modifications of an EUV PS/PDI station (phase-shifting/point diffraction interferometer). This EUV interferometer is located at the Advanced Light Source undulator beamline 12.0.1 (Lawrence Berkeley National Laboratory).

To evaluate resist materials for EUV lithography, it is necessary to expose test patterns with very high spatial resolution (less than 50 nm lines and spaces). Currently, there are a limited number of imaging systems that can achieve this fine feature printing at EUV wavelengths. By developing this synchrotron-based imager using the 10x reduction Schwarzschild optics system, the limits of test resists can be examined. One method to print these fine features is to double the spatial frequency of the object grating.

Placing a grating in the path of the coherent EUV source will create diffracted orders at angles determined by the pitch of the grating. By eliminating the DC term and recombining only the +1 and -1 orders, spatial frequency doubling at the image plane can be achieved with high contrast. The combination of the frequency doubling and the optical system creates a 20x reduction of the object grating pitch. Thus far, we have successfully used this technique to print equal line and space patterns with line widths as small as 30 nm. Line edge roughness measurements have been done on our 50 nm dense lines/space pattern with a three sigma rms value of 4 nm. The printing of even smaller features is currently under investigation. Simulations show that by using the fully extended NA, the system can achieve line widths as small as 12 nm. Using a suitably designed mask, spatially separate grating objects with differing pitch can be simultaneously imaged in a single exposure. A technique to print multiple contrasts during a single exposure is also being developed. These configurations will be of great use in evaluating the ultimate performance and extendibility of resist materials for EUV lithography.

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