The EUV mask as a system: function breakdown and interface description

Abstract

Masks have always played an important role in lithography to improve resolution and achieve lower k1 values [1]. With EUV lithography now being in High Volume Manufacturing, many parties are looking at the various aspects of the EUV mask. However, the EUV mask as a system is much more complex than the ArFi mask. The EUV mask system roughly consists out of 5 parts which all have multiple functions and therefore interfaces. At the top we have the top-layer which serves to protect during cleaning, but also can help inspection of the absorber after patterning. Then we have the patterned absorber, whose primary function is to steer the diffraction and imaging. Below we have the cap layer that serves to protect the multilayer from EUV photons and environmental gases. Then comes the Multilayer that serves to reflect the maximum amount of EUV light possible. Last but not least we have the core material (ULE) and the back side coating needed to clamp the reticle on the reticle chuck. The EUV mask as such is much more complex than the ArFi mask, also depicted in Figure 1. Two main differences to be noted: - The attention to productivity in EUV as it can be steered with many of the mask properties [2] , - The presence of a cap and top layer is important for mask making and conditioning. For EUV the layers also affect imaging and need to be co-optimized. The cap layer serves many functions. It has also been shown that the cap layer can change its composition through oxidation [3], which can eventually lead to blister formation [4]. We will show experimental data that will enable visualization of the cap and multilayer contribution to state of the art EUV imaging. These experiments comprise contrast evaluation for monopole/dipole exposures and measurement of pattern placement through focus, There is quite some progress in demonstrating how absorber tuning can be used to steer the imaging parameters (contrast [NILS] and dose to size). For the absorber recent work has highlighted the physics and demonstrated that an alternative absorber can have improved imaging properties. We will give an update from our experimental findings using a low n mask ([2]). This might fuel even more appetite to optimize the other parts of the mask system as well. Before we do so we need to make sure all the necessary cross-links between functions and contributing parties are clear, without compromising the ability for an end user to differentiate. We will discuss tuning opportunities beyond absorber from imaging perspective, with focus on the cross-link with other aspects. We will also discuss how to interface the different functions by means of “system” documentation and human interaction (“eco-system workshop”) [1] Jo Finders and Christian Wagner "Imaging enhancement (low k1 imaging) in EUV lithography: current status and future resolution enhancement techniques", Proc. SPIE 11609, Extreme Ultraviolet (EUV) Lithography XII, 1160909 (22 February 2021) [2] Claire van Lare, Frank Timmermans, Jo Finders, Olena Romanets, Cheuk-Wah Man, Paul van Adrichem, Yohei Ikebe, Takeshi Aizawa, and Takahiro Onoue: "Investigation into a prototype extreme ultraviolet low-n attenuated phase-shift mask," Journal of Micro/Nanopatterning, Materials, and Metrology 20(2), 021006 (21 May 202 [3] “The refined EUV mask model”, I.A. MAKHOTKIN, M. WU, V. SOLTWISCH, F. SCHOLZE, V. PHILIPSEN, Journal of Applied Physics, 2019 [4] Jetske Stortelder, Arnold, Storm, Veronique de Rooij-Lohmann, Chien-Ching Wu, and Willem van Schaik "Compatibility assessment of novel reticle absorber materials for use in EUV lithography systems.", Proc. SPIE 10957, Extreme Ultraviolet (EUV) Lithography X, 1095713 (26 March 2019);