In extreme ultraviolet lithography (EUVL), the technology specific requirements on the mask are a direct consequence of the utilization of radiation in the spectral region between 10 and 15 nm. At these wavelengths, all condensed materials are highly absorbing and efficient radiation transport mandates the use of all-reflective optical systems. Reflectivity is achieved with resonant, wavelength-matched multilayer (ML) coatings on all of the optical surfaces -- including the mask. The EUV mask has a unique architecture -- it consists of a substrate with a highly reflective ML coating (the mask blank) that is subsequently over-coated with a patterned absorber layer (the mask). Particulate contamination on the EUVL mask surface, errors in absorber definition and defects in the ML coating all have the potential to print in the lithographic process. While highly developed technologies exist for repair of the absorber layer, no viable strategy for the repair of ML coating defects has been identified. In this paper the state- of-the-art in ML deposition technology, optical inspection of EUVL mask blank defects and candidate absorber patterning approaches are reviewed.

We address in this report a set of key questions tied to the implementation of liquid immersion lithography, from the perspective of the resist materials. We discuss the broad question of whether chemically amplified resists are capable of achieving the spatial resolution that ultimately will be required for the most advanced immersion scenario. Initial studies undertaken using model 193 nm resist materials provide some insight into how an aqueous liquid immersion process can affect the resist material.

Particles are a serious concern in the fabrication of reticles for extreme ultraviolet lithography because they nucleate perturbations in the reflective multilayer film that can print in the lithographic image. We call these perturbations defects. It has been suggested that reticle substrates can be planarized and the high-spatial-frequency roughness of the multilayer film can be reduced by using an ion-assisted Mo–Si deposition process. In this article we discuss the cleanliness of this ion-assisted deposition process. Within one cleaning cycle, we improved the cleanliness of the deposition process without ion assist from 3–4 to 0.26 particles/cm2. Of these, 0.09 particles/cm2 are due to manual handling of the wafers. We found that in our experimental setup, the added particle density using a process with ion assist is more then six times larger than the added density without ion-assist, suggesting that further work is necessary to develop a clean ion-assisted deposition process. We characterized the chemical composition of some of the added particles and found that they contain Al, C, Fe, Mo, and Si.

The ITRS roadmap for lithography aggressively shrinks many features on the mask from the critical dimension to the size of defect that must be eliminated. When a defect is larger than 30% of the minimum image size, it is considered detrimental to the mask performance. Unfortunately, the sensitivity of current inspection systems does not keep the pace dictated by ITRS for development work. The 65 nm node mask development teams are largely unaware of defects smaller than 90 run. This gap only widens for NGL masks. The scarcity of high-sensitivity inspection systems is leaving many mask makers and their customers unsure of the impact, or even presence of, defects on the reticle. This paper will provide an:analysis of mask blank defects using,several different inspection systems with both low and high sensitivity levels. Defect sources from carriers and shipping will be explored. Finally, the likelihood of removing these particles once they have been lodged on the mask surface is discussed.

Results of at-wavelength inspection of EUVL mask substrate defects which were smoothed using 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 of the mask substrates were performed. The smoothing process was found to be effective in significantly suppressing the EUV visibility of the defects.

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.

We present our study on actinic detection of defects on extreme ultraviolet (EUV) lithography mask blanks with broadband EUV illumination from a synchrotron light source. A mirror blank was substituted for the monochromator grating in the synchrotron beamline. This increases the spectral bandwidth that is taken from the broadband synchrotron beam by 1 order of magnitude, leading to a commensurate increase in the total available flux. The detection sensitivity of this actinic inspection system is able to reach approximately 20 nm in equivalent defect size. This has been determined through cross correlation experiments done with commercial visible-light inspection tools. Experimental results also show that this broadband EUV inspection system has a capability of detecting phase defects with 5 nm surface height.

We present recent experimental results from an actinic defect inspection system for extreme ultraviolet (EUV) lithography mask blanks. The current actinic inspection system has demonstrated the ability to detect 50 run defect in cross correlation experiments with visible-light inspection tool. We found that native defects as small as 60 nm with only 3 nm height were detectable by the actinic tool. These defects are just below the detection limit of current commercial visible-light inspection tools. A new class of defect was discovered, which is quite large, in the several micrometer range, and shows suppressed non-specular EUV scattering intensity as compared to the intrinsic background scatter from the multilayer blanks. Despite their large physical dimensions, these defects are also near the detection limit of current visible-light inspection tools.

Minimizing image placement errors due to thermal distortion of the mask is a key requirement for qualifying EUV Lithography as a Next Generation Lithography (NGL). Employing Low Thermal Expansion Materials (LTEMs) for mask substrates is a viable solution for controlling mask thermal distortion and is being investigated by a wide array of researchers, tool makers, photomask suppliers, and material manufacturers. Finite element modeling has shown that an EUVL mask with a Coefficient of Thermal Expansion (CTE) of less than 20 ppb/K will meet overlay error budgets for <EQ 70 nm lithography at a throughput of 80 wafers per hour. In this paper, we describe the functional differences between today's photomask and EUVL masks; some of these differences are EUVL specific, while others are natural consequences of the shrinking critical dimension. We demonstrate that a feasible manufacturing pathway exists for Low Thermal Expansion Material (LTEM) EUVL masks by fabricating a wafer-shaped LTEM mask substrate using the same manufacturing steps as for fabricating Si wafers. The LTEM substrate was then coated with Mo/Si multilayers, patterned, and printed using the 10X Microstepper. The images were essentially indistinguishable from those images acquired from masks fabricated from high quality silicon wafers as substrates. Our observations lend further evidence that an LTEM can be used as the EUVL mask substrate material.

EUVL mask blanks consist of a distributed Bragg reflector made of 6.7nm-pitch bi-layers of Mo and Si deposited upon a precision Si or glass substrate. The layer deposition process has been optimized for low defects, by application of a vendor-supplied but highly modified ion-beam sputter deposition system. This system is fully automated using SMIF technology to obtain the lowest possible environmental- and handling-added defect levels. Originally designed to coat 150mm substrates, it was upgraded in July, 1999 to 200 mm and has coated runs of over 50 substrates at a time with median added defects >100nm below 0.05/cm(2). These improvements have resulted from a number of ion-beam sputter deposition system modifications, upgrades, and operational changes, which will be discussed.Success in defect reduction is highly dependent upon defect detection, characterization, and cross-platform positional registration. We have made significant progress in adapting and extending commercial tools to this purpose, and have identified the surface scanner detection limits for different defect classes, and the signatures of false counts and non-printable scattering anomalies on the mask blank. We will present key results and how they have helped reduce added defects.The physics of defect reduction and mitigation is being investigated by a program on multilayer growth over deliberately placed perturbations (defects) of varying size. This program includes modeling of multilayer growth and modeling of defect printability. We developed a technique for depositing uniformly sized gold spheres on EUVL substrates, and have studied the suppression of the perturbations during multilayer growth under varying conditions. This work is key to determining the lower limit of critical defect size for EUV Lithography. We present key aspects of this work.We will summarize progress in all aspects of EUVL mask blank development, and present detailed results on defect reduction and mask blank performance at EUV wavelengths.

We report on the comparison of defect printability experimental results with at-wavelength defect inspection and printability modeling at extreme ultraviolet (EUV) wavelengths. Two sets of EUV masks were fabricated with nm- scale substrate defect topographies patterned using a sacrificial layer and dry-etch process, while the absorber pattern was defined using a subtractive metal process. One set of masks employed a silicon dioxide film to produce the programmed defects, whereas the other set used chromium films. Line-, proximity- and point-defects were patterned and had lateral dimensions in the range of 0.2 micrometer X 0.2 micrometer to 8.0 micrometer X 1.5 micrometer on the EUV reticle, and a topography in the range of 8 nm - 45 nm. Substrate defect topographies were measured by atomic force microscopy (AFM) before and after deposition of EUV-reflective Mo/Si multilayers. The programmed defect masks were then characterized using an actinic inspection tool. All EUVL printing experiments were performed using Sandia's 10x- reduction EUV Microstepper, which has a projection optics system with a wavefront error less than 1 nm, and a numerical aperture of 0.088. Defect dimensions and exposure conditions were entered into a defect printability model. In this investigation, we compare the simulation predictions with experimental results.