The development of defect-free reticle blanks is an important challenge facing the commercialization of extreme ultraviolet lithography (EUVL). The basis of an EUVL reticle are mask blanks consisting of a substrate and a reflective Mo/Si multilayer. Defects on the substrate or defects introduced during multilayer deposition can result in critical phase and amplitude defects. Amplitude- or phase-defect repair techniques are being developed with the goal to repair many of these defects. In this paper we discuss the selection of a capping layer for amplitude-defect repair, and report on experimental results of the reflectance variation over the amplitude-defect repair zone for different capping layers. Our results suggest that carbon and silicon carbide are the leading candidates for capping layer materials. We further performed a quantitative assessment of the yield improvement due to defect repair. We found that amplitude- and phase-defect repair have the potential to significantly improve mask blank yield, and that yield can be maximized by increasing the number of Mo/Si bilayers.
Extreme ultraviolet (EUV) multilayer defects (phase defects) are a defect type unique to extreme ultraviolet lithography (EUVL) masks. A manufacturable inspection capability for these defects is key to the success of EUV lithography. Simulations of EUV scattering from multilayer defects suggest that defect printability is related to the phase error induced by the defect, which is in turn strongly coupled to the size of a multilayer surface protrusion or intrusion. We can adopt a strategy of measuring the multilayer surface to detect phase defects.
During the past year a working group composed of members of Intel Corporation, Lawrence Berkeley and Lawrence Livermore National Laboratories, and International Sematech searched for a commercial tool for EUVL mask substrate and blank inspection. This working group established the tool requirements, methodologies for tool evaluation, collected data and recommended a supplier for further development with International Sematech. We collected data from several vendors and found that a multibeam confocal inspection (MCI) system had a capability significantly better than the tools used today.
We will present our strategy, requirements, methodologies and results. We will discuss in detail our unique programmed substrate and multilayer defect masks used to support the tool selection, including their actinic characterization. We will present data that quantifies the inspection capability of the MCI system.
EUV mask blanks are fabricated by depositing a reflective Mo/Si multilayer film onto super-polished substrates. Small defects in this thin film coating can significantly alter the reflected field and introduce defects in the printed image. Ideally one would want to produce defect-free mask blanks; however, this may be very difficult to achieve in practice. One practical way to increase the yield of mask blanks is to effectively repair multilayer defects, and to this effect we present two complementary defect repair strategies for use on multilayer-coated EUVL mask blanks. A defect is any area on the mask which causes unwanted variations in EUV dose in the aerial image obtained in a printing tool, and defect repair is correspondingly defined as any strategy that renders a defect unprintable during exposure. The term defect mitigation can be adopted to describe any strategy which renders a critical defect non-critical when printed, and in this regard a non-critical defect is one that does not adversely affect device function. Defects in the patterned absorber layer consist of regions where metal, typically chrome, is unintentionally added or removed from the pattern leading to errors in the reflected field. There currently exists a mature technology based on ion beam milling and ion beam assisted deposition for repairing defects in the absorber layer of transmission lithography masks, and it is reasonable to expect that these this technology will be extended to the repair of absorber defects in EUVL masks . However, techniques designed for the repair of absorber layers can not be directly applied to the repair of defects in the mask blank, and in particular the multilayer film. In this paper we present for the first time a new technique for the repair of amplitude defects as well as recent results on the repair of phase defects.
Results of at-wavelength inspection of EUVL mask substrate defects that were smoothed by multilayer coatings are presented. Programmed mask substrate defects were made with 80nm gold (Au) spheres, which were deposited on the mask substrate before the Mo/Si reflective multilayer coating. After coating, at-wavelength and visible-light inspection were then performed. The smoothing process was found to be effective in significantly suppressing the EUV visibility of the defects.
The optimization of the lithographic process requires the definition of a figure of merit to gauge the quality of the image. Several ad-hoc approaches have been used in the past, but there is not yet a method which is clearly the most effective. Here we introduce the concept of energy error map, obtained by comparing the aerial image with an ideal target pattern. One is target aerial image which we want to define on the wafer, the other is aerial image which is calculated at each process conditions. Given a target pattern B, in the form of a matrix, and an aerial intensity map A, we define the energy error map as (A - A (DOT) B) plus (A - A (DOT) B). We also used the total sum of error map between target aerial image and calculated aerial image as figure of merit in quantifying the quality of images. With the calculated energy error map, we also proposed first-order proximity-corrected mask pattern which is based on the distribution of the energy error in error map.
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