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Micro/Nano Lithography

Improving extreme UV lithography mask repair

A new generation of electron-beam-based repair technology can successfully restore defects in extreme-UV lithography masks.
1 May 2012, SPIE Newsroom. DOI: 10.1117/2.1201204.004159

Driven by the consumer market and keeping pace with Moore's law, integrated circuit components are continuously shrinking in dimension. To make smaller circuit features, the microelectronics industry is developing next-generation extreme-UV (EUV) lithography for high-volume chip manufacturing.1 The technology's 13.5nm wavelength imposes very strict requirements on the quality of the involved optical elements, such as the light source, mirrors, and masks.

In switching from the transmissive 193nm optics used in conventional photolithography to reflective EUV optics, the mask architecture has had to evolve. EUV masks include a multilayer Bragg mirror onto which an absorber pattern is defined. Defects can arise in these masks from multiple sources: optically imperfect mirrors due to local deviations from flatness; wrongly written absorber patterns; and even dust particles lying on the mask. A mask suitable for a production environment must be free of defects, both at blank level and absorber pattern level, so that they do not get printed onto the integrated circuit. However, even when meeting very strict quality standards, it is hard to avoid some defects. A solution to this quality gap can be mask repair.

Electron-beam-based repair for transmissive photomasks has been around for 10 years.2 Since then, the microelectronics industry has extensively used the Carl Zeiss MeRiT® tool line based on this technology for the production of defect-free masks. Two different generations of the instrument addressed chip technology based on active component dimensions of 65 and 45nm.3 We are now upgrading the tool for 32nm component sizes for EUV masks.4

To start, an automated inspection machine visually maps defects on the EUV mask with respect to reference coordinates.5 The MeRiT tool navigates to the listed coordinates, and then reviews and fixes them. It usually performs the review by top-view scanning electron microscopy (SEM). The repair, meanwhile, relies on the technique of focused electron-beam-induced chemistry.6 This method introduces a high-resolution electron beam and suitable precursors at the mask surface. The electrons induce a chemical reaction, leading to either a fragmentation and deposition of precursor molecules or to a reaction between the adsorbed precursor molecules and the surface material that creates volatile compounds and hence etches the surface. The reaction is confined to the area exposed by the electron beam, resulting in high-resolution repair.

Opaque absorber defects are fixed by etching away the surplus absorber. As an example, we repaired a series of 30 defects on a 32nm EUV mask (see Figure 1). We printed the repaired mask in an NXE:3100 EUV scanner, and inspected the printed wafers with SEM. The repair success rate was over 95% at first try. One particular defect, comparably larger than the others, was repaired marginally and required a second processing step.

Figure 1. Example repair results for real opaque absorber defects on a 32nm extreme-UV (EUV) mask reticle. Scanning electron microscopy (SEM) top views of the defects, and views of the NXE:3100 EUV scanner wafer prints from the defect areas before (left) and after electron-beam etch repair (right).

The masks can also have blank level defects. Distortions of the EUV multilayer in the form of pits and bumps on the order of λ/4 (i.e., around 3–4nm height) affect its reflectivity. That is because they induce destructive interference between the 13.5nm light reflected with a wave shift of λ/2 from these defects and the light reflected from the rest of the flat surface. Such deviations from the nominal flatness are too shallow to be seen by SEM. Visualizing them requires an alternative method such as atomic force microscopy (AFM). The MeRiT platform is equipped with an in situ vacuum AFM to allow for this step. Around the defect location, removing some absorber compensates for the reflectivity loss (see Figure 2). Within certain limits, we can successfully recover the lithographic process windows of both multilayer pits and bumps.7

Figure 2. Demonstration of a successful through-focus compensation repair on a real multilayer bump in 40nm lines and spaces. Reticle atomic force microscopy (AFM) imaging of the defect before (top left) and after repair (top center). MeRiT top-view SEM imaging after compensation repair (top right). SEM views of wafers printed at the EUV Alpha Demo Tool scanner before and after repair (bottom), at different focus positions around best focus. BF: Bright field.

This is the first qualification for the compensation repair strategy of EUV reticles on the NXE:3100 scanner. Future work will aim at demonstrating systematic electron-beam repair success on masks with smaller absorber patterns. This process will provide the confidence to lithographers that affordable, defect-free EUV masks are available for high-volume manufacturing.

Tristan Bret, Christof Baur, Markus Waiblinger, Gabriel Baralia
Carl Zeiss SMS
Rossdorf, Germany

After a degree in physics and chemistry at the Industrial Physics and Chemistry Higher Educational Institution, Paris, Tristan Bret completed a PhD in microengineering at the Swiss Federal Institute of Technology, Lausanne. He joined Carl Zeiss SMS in 2006, and is currently project manager of EUV mask repair-related processes.

Rik Jonckheere, Dieter Van den Heuvel
Interuniversity Microelectronics Center (IMEC)
Leuven, Belgium

1. B. Wu, A. Kumar, Extreme ultraviolet lithography: a review, J. Vac. Sci. Technol. B 25(6), p. 1743-1761, 2007.
2. K. Edinger, H. Becht, J. Bihr, V. Boegli, M. Budach, T. Hofmann, H. W. P. Koops, A novel electron-beam-based photomask repair tool, Proc. SPIE 5256, p. 1222, 2003. doi:10.1117/12.532866
3. A. Garetto, J. Oster, M. Waiblinger, K. Edinger, Challenging defect repair techniques for maximizing mask repair yield, Proc. SPIE 7488, p. 74880H, 2009. doi:10.1117/12.833404
4. M. Waiblinger, K. Kornilov, T. Hofmann, K. Edinger, EMLC 2010 best paper: E-beam induced EUV photomask repair: a perfect match, Proc. SPIE 7545, p. 75450P, 2010. doi:10.1117/12.863542
5. D. Van den Heuvel, R. Jonckheere, J. Magana, T. Abe, T. Bret, E. Hendrickx, S. Cheng, K. Ronse, Natural EUV mask blank defects: evidence, timely detection, analysis, and outlook, Proc. SPIE 7823, p. 78231T, 2010. doi:10.1117/12.865812
6. I. Utke, P. Hoffmann, J. Melngailis, Gas assisted focused electron beam and ion beam processing and fabrication, J. Sci. Vac. Technol. B 26, p. 1197-1276, 2008.
7. R. Jonckheere, T. Bret, D. Van den Heuvel, J. Magana, W. Gao, M. Waiblinger, Repair of natural EUV reticle defects, Proc. SPIE 8166, p. 81661G, 2011. doi:10.1117/12.898864