Overcoming mask blank defects in EUV lithography

Extreme ultraviolet lithography (EUVL) has been identified as the most promising technique for generating semiconductor wafers with a 22nm half-pitch or less, according to surveys taken at SEMATECH’s Lithography Forum last year as well as at the 2008 International EUVL Symposium.1, 2 The same groups ranked mask defectivity as the second highest challenge – after source power – that needs to be overcome in order to ensure the success of this technology. EUV light (with a wavelength, λ, of ∼13.5nm) is absorbed by all known materials; therefore, one cannot simply make a transmissive mask as is commonly used in optical lithography employing longer wavelengths. It is, however, possible to build Bragg mirrors for EUV light by using Molybdenum/Silicon (MoSi) bilayers of 3nm and 4nm thickness, respectively. The process then becomes one of reflectance rather than transmittance. MoSi multilayers are now commonly used in EUV optic systems. The mask blanks are made of 6-inch square, low thermal expansion material glass substrates coated with MoSi multilayers and surmounted with a capping layer. Typically, 40 to 50 bilayers are required to achieve a reflectivity of 62-70% at λ ∼ 13.5nm. They are then coated with a 70nm tantalum nitride-based layer that will be patterned to form the absorbing masks. EUV patterned mask defects are, to a degree, similar to those on patterned optical masks, and they can be dealt with in the same way. However, reducing blank defects or rendering them non-printable remains a challenge. Blank defects are a combination of defects on the substrate and those generated during the multilayer deposition process. Over the past three years, progress has been made to reduce the total numbers of defects by a factor of 3, primarily through removing smaller particles (see Figure 1). However, the ratio of defects shows no substantial change. The majority of the total arise from Figure 1. Defect Pareto diagram of champion EUV mask blanks at SEMATECH in 2005 (left) and 2008 (right). Total defects are 18 defects ≥73nm in 2005 and 8 defects ≥53nm in 2008.