Pei-g Yan

Progress in extreme ultraviolet mask repair using a focused ion beam

The key challenge in extreme ultraviolet (EUV) mask defect repair is to avoid or limit the damage to the sensitive reflective multilayer (ML) stacks on the mask substrate and repair <55 nm mask defects. Our EUV mask design employs an oxide buffer layer between the ML and the absorber to protect the ML during repair. We have developed both opaque and clear EUV mask defect repair processes using focus ion beam (FIB) based gas-assisted etching (GAE) and ion-induced deposition. The process has been successfully demonstrated on our TiN baseline mask by 10× EUV print tests of 100 nm resist lines/spaces. More importantly we have assessed the current FIB tool performance capability and compared it with the general requirements for repairing the EUV mask for the 70 nm lithography node. The characterization includes minimum “effective” beam size, etch selectivity, and edge placement precision. We discussed the required improvements and future directions in repair tool research and development in order for the mask ...

System integration and performance of the EUV engineering test stand

The Engineering Test Stand (ETS) is a developmental lithography tool designed to demonstrate full-field EUV imaging and provide data for commercial-tool development. In the first phase of integration, currently in progress, the ETS is configured using a developmental projection system, while fabrication of an improved projection system proceeds in parallel. The optics in the second projection system have been fabricated to tighter specifications for improved resolution and reduced flare. The projection system is a 4-mirror, 4x-reduction, ring-field design having a numeral aperture of 0.1, which supports 70 nm resolution at a k1 of 0.52. The illuminator produces 13.4 nm radiation from a laser-produced plasma, directs the radiation onto an arc-shaped field of view, and provides an effective fill factor at the pupil plane of 0.7. The ETS is designed for full-field images in step-and-scan mode using vacuum-compatible, magnetically levitated, scanning stages. This paper describes system performance observed during the first phase of integration, including static resist images of 100 nm isolated and dense features.

First lithographic results from the extreme ultraviolet Engineering Test Stand

The extreme ultraviolet (EUV) Engineering Test Stand (ETS) is a step-and-scan lithography tool that operates at a wavelength of 13.4 nm. It has been developed to demonstrate full-field EUV imaging and acquire system learning for equipment manufacturers to develop commercial tools. The initial integration of the tool is being carried out using a developmental set of projection optics, while a second, higher-quality, projection optics is being assembled and characterized in a parallel effort. We present here the first lithographic results from the ETS, which include both static and scanned resist images of 100 nm dense and isolated features throughout the ring field of the projection optics. Accurate lithographic models have been developed and compared with the experimental results.

Sub-70 nm extreme ultraviolet lithography at the Advanced Light Source static microfield exposure station using the engineering test stand set-2 optic

Static microfield printing capabilities have recently been integrated into the extreme ultraviolet interferometer operating at the Advanced Light Source synchrotron radiation facility at Lawrence Berkeley National Laboratory. The static printing capabilities include a fully programmable scanning illumination system enabling the synthesis of arbitrary illumination coherence (pupil fill). This new exposure station has been used to lithographically characterize the static imaging performance of the Engineering Test Stand Set-2 optic. Excellent performance has been demonstrated down to the 70 nm equal line/space level with focus latitude exceeding 1 μm and dose latitude of approximately 10%. Moreover, equal line/space printing down to a resolution of 50 nm has been demonstrated using resolution-enhancing pupil fills.

High transmission pellicles for extreme ultraviolet lithography reticle protection

The authors present the results of a full-field extreme ultraviolet (EUV) pellicle for reticle protection and defect mitigation. Based on novel microelectromechanical systems based fabrication, it comprises a 50 nm Si membrane attached to a wire-grid. Two types of pellicle fabrication techniques are described. The authors present the first actinic results of extreme ultraviolet lithography reticle with pellicle exposed on IMEC Advanced Demo Tool. The impact of different pellicle types on imaging is evaluated as a function of pellicle standoff distance and mesh geometry. A new prototype pellicle has been developed with a measured transmission of 82% in EUV. Actinic exposures are complemented with aerial image modeling, thermal analysis, vacuum cycling, resist outgas tests, and >5 g repeated scan cycle robustness tests.

Quantitative evaluation of mask phase defects from through-focus EUV aerial images

We present an improved method of phase retrieval from through-focus image series with higher precision and reduced sensitivity to noise. The previous method, developed for EUV, actinic mask measurements, was based on the Gerchberg-Saxton algorithm and made use of two aerial images recorded in different focal planes. The new technique improves the reconstruction uncertainty and increases the convergence speed by integrating information contained in multiple images from a through focus series. Simulations characterize the new technique in terms of convergence speed, accuracy and stability in presence of photon noise. We have demonstrated the phase-reconstruction method on native, mask-blank phase defects and compared the results with phase predictions made from AFM data collected after the multilayer deposition. Measurements show that a defects top-surface height profile is not a reliable predictor of phase change in all cases. The method and the current results can be applied to improve defect modeling and to enhance our understanding of the detectability and printability of native phase defects.

TaN EUVL mask fabrication and characterization

The EUV mask patterning process development depends on the choice of EUV mask absorber material, which has direct impact on the mask quality or performance such as CD control, defect control, and registration. In the past, several EUV mask absorber material candidates that include Al-Cu, Ti, TiN, Ta, TaN, and Cr have been evaluated. Our research indicated that TaN and Cr are the better candidates among the others evaluated. Cr absorber has been used for many optical lithography generations. Further extending Cr mask absorber to EUV lithography presents minimum impact to the currently mask technology infrastructure. TaN is a new film that has not been used in the currently mask technology. However, Ta based metal compound has been studied previously in x-ray mask technology. Its performance in EUV mask fabrication and printing was found compatible and comparable in many process steps and performance aspects to that of Cr absorber. In this paper, we will present our research and development work on TaN absorber EUV mask fabrication and characterization. The studies include material deposition study, etch development, cleaning compatibility evaluation, and mask printing test. The TaN absorber etch was able to achieve good etch profile and high etch selectivity to the buffer oxide layer. The cleaning benchmarking results showed that TaN absorber is compatible to the currently acid based Cr cleaning procedures and solution. No material damage or loss was found in the case of extreme harsh cleaning conditions used. The TaN thin absorber mask was successfully fabricated and printed in 10x microstepper at Sandia National Lab. Minimum feature of 70nm L/S were obtained.

EUV mask absorber characterization and selection

In this paper, we will present our research work in EUVL mask absorber characterization and selection. The EUV mask patterning process development depends on the choice of EUVL mask absorber material, which has direct impact on the mask quality such as critical dimension (CD) control, and registration. EUVL mask absorber material selection consideration involves many aspects of material properties and processes. These include film absorption at EUV wavelength, film emissivity, film stress, mask CD and defect control, defect inspection contrast, absorber repair selectivity to the buffer layer, etc. The selection of the best candidate is non-trivial since no material is found to be superior in all aspects. In an effort of searching the best absorber materials and processes, we evaluated Al-Cu, Ti, TiN, Ta, TaN, and Cr absorbers. The comparison of material intrinsic properties and process properties allowed us to focus on the most promising absorbers and to further develop the corresponding processes to meet EUVL requirement.

Method for compensation of extreme-ultraviolet multilayer defects

We propose the use of optical proximity correction on absorber features to compensate for the effect of subresolution multilayer defects that would otherwise induce a critical error in linewidth. Initial experiments have been performed which validate this concept. Process window simulations quantify the practical limits of this technique.

At-wavelength detection of extreme ultraviolet lithography mask blank defects

We report the design and operation of an at-wavelength system for extreme ultraviolet lithography mask blank defect detection. Initial results demonstrate sensitivity to submicron size phase defects. The performance of the system is compared with the practical requirements for a mask blank inspection system in terms of the sensitivity and scanning time.