Current EUV mask technology uses Ta-based metallic absorber layer, on top of a reflective multilayer mirror. Multiple studies have shown that the optical constants and the required 50-70nm thickness of Ta-based metallic absorber at EUV wavelength, do not offer an optimal wafer image, and, for example, produce images with pitch and illumination dependent best focus shifts for patterns at Foundry N5 dimensions. Alternative metal absorbers with higher absorptivity than Ta, such as Ni and Co have been proposed and, in simulation, show improved imaging at <40nm thickness.
The replacement of a Ta- absorber by a new type of metal is a formidable task for the mask industry. A novel absorber must not only meet the criteria for improved imaging, but also must meet the required material properties that make it compatible with different steps in mask blank and subsequent mask manufacturing, such as a controlled deposition technique, availability of a patterning process for mask patterning, and be compatible with mask inspection, repair, and cleaning.
We have started an experimental evaluation of the properties of thin metal Ni and Co films, and alloys of Ni, considering imaging performance and mask manufacturability. Rigorous lithographic simulations are used to screen potential absorber materials for their imaging properties at Foundry N5 dimensions, and find optimal thickness. The microscopic structure of the thin films was determined using X-ray, X-SEM and X-TEM techniques, and optical constants were measured using ellipsometry at EUV wavelength. Towards mask manufacturing, patterning performance, and resistance to typical mask cleaning chemicals was evaluated experimentally.
Standard deposition of Ni and Co metals yielded polycrystalline thin films, that prove difficult to pattern using a traditional etch process. In addition, Co films were found to be affected by standard mask cleaning chemistry. Hence, if Ni and Co are required as new mask materials, also novel patterning techniques will have to be used, that may be additive rather than subtractive. To illustrate this, we show promising performance for area selective Co deposition techniques.
To identify new materials, that have better properties towards manufacturing than single-element Ni and Co, we have started the evaluation of metal alloys, at different elemental ratios. This allows to combine Ni with an element that has refractive index closer to 1, or with an element that has even higher absorptivity. The films of metal alloys have been characterized in a similar way as the single element metals, so that they can be compared to single element metals as suitable materials for mask manufacturing.