The availability of photoresists meeting simultaneous resolution, sensitivity, and line edge roughness performance is a critical challenge for the acceptance of Extreme Ultraviolet Lithography. The Extreme Ultraviolet Resist Test Center (EUV RTC) at SEMATECH-North at the State University of New York at Albany is a state of the art facility to support the development of photoresists for EUV lithography. The facility was opened on September 28, 2005, for customer use. SEMATECH researchers, member companies, resist suppliers, and researchers from universities and institutes worldwide can use this neutral site for EUV resist development. The heart of the EUV RTC is an Exitech 5X EUV microstepper with a 0.3 numerical aperture (NA) lens. This tool has successfully imaged 45 nm dense lines in photoresists, and the ultimate imaging performance of the microstepper based on optics and wavefront quality should be near 25nm dense lines.
The availability of high resolution, low line-edge roughness, high sensitivity resists has recently been determined to be one of the most critical issues for the development of extreme ultraviolet (EUV) lithography. To address this issue, a series of 0.3 numerical aperture EUV microfield exposure tools (METs) has been developed. One of these tools is installed at SEMATECH North as part of its EUV Resist Test Center. The MET will be used as a resist evaluation tool and potentially as a mask evaluation tool; it is important to have an accurate knowledge of the aerial-image performance limits of the tool. Such knowledge enables the user to decouple optic effects from the resist and mask architecture effects being studied. Based on wavefront data provided by Zeiss (the manufacturer of the optic) and the lithographically measured flare data, PROLITH modeling is used to predict system performance under a variety of conditions.
Several masks have been fabricated and exposed with the small-field Micro Exposure Tool (MET) at the Advanced Light Source (ALS) synchrotron in Berkeley using EUV radiation at 13.5 nm wavelength. Investigated mask types include two different absorber masks with TaN absorber as well as an etched multilayer mask. The resulting printing performance under different illumination conditions were studied by process window analysis on wafer level. Features with resolution of 60 nm and below were resolved with all masks. The TaN absorber masks with different stack thicknesses showed a similar size of process window. The differences in process windows for line patterns were analyzed for 60 nm patterns. The implications on the choice of optimum mask architecture are discussed.
Corresponding to the ITRS roadmap, EUV Lithography will in the not-too-far future reach the point, where critical resist dimensions are in the same order of magnitude as polymer chains and acid diffusion lengths, while photon energies will largely exceed the binding energies of all organic molecules. Especially in EUV, where secondary electron side reactions may lead to a higher outgassing of polymer fragments than in 157nm and 193nm lithography, outgassing is agreed to be a critical issue for resist development. In this paper EUV, 193nm and 157nm outgassing is characterized using an online mass spectrometer attached to several different outgassing setups (i.e. synchrotron, laser). The total outgassing and the time dependent outgassing of resist fragments has been characterized for a number of resist polymer platforms. The results are compared and discussed in terms of the applied photon energies and differences in EUV, 157nm and 193nm exposures. Time dependent scanning of selected mass channels was used to differentiate if an outgassing fragment had its origin from the photoacid generator (PAG) or from a photolytic or a photochemical reaction of the polymer matrix. For EUV, correlations are given between resist outgassing and high dose crosslinking and scissioning behaviour of EUV resists.
Three different architectures were compared as candidates for EUV lithography masks. Binary masks were fabricated using two different stacks of absorber materials and using a selective etching process to directly pattern the multilayer of the mask blank. To compare the effects of mask architecture on resist patterning, all three masks were used to print features into photoresist on the EUV micro-exposure tool (MET) at Lawrence Berkeley National Laboratory. Process windows, depth of focus, mask contrast at EUV, and horizontal and vertical line width bias were use as metrics to compare mask architecture. From printing experiments, a mask architecture using a tantalum nitride absorber stack exhibited the greatest depth of focus and process window of the three masks. Experimental results obtained using prototype masks are discussed in relation to simulations. After accounting for CD biasing on the masks, similar performance was found for all three mask architectures.