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For applications in the Extreme Ultraviolet (EUV) region, phase-shift structures play an important role in pushing the throughput and performance of optical systems. While EUV optical elements are typically designed and fabricated for use in reflection, there are important applications in transmission as well where phase shift structures can provide substantial throughput gains. Examples are EUV microscopy and interferometry using gratings or zone plates. In the EUV regime, few materials offer a better combination of phase shift and absorption properties than molybdenum (Mo), however, drawbacks for Mo include crystalline growth complicating the etch process, and ease of oxidation which leads to diminished performance with time.
Here we develop a fabrication process for transmission optical elements made of an engineered molybdenum-rich film on free-standing silicon membranes and show the performance of these phase structures in the EUV regime. We chose the fabrication of simple binary gratings of 72nm half pitch (Fig. 1) in order to establish a baseline for performance. We further addressed the oxidation concerns for Mo by developing a process to passivate the surface using atomic layer deposition (ALD) to coat a thin and conformal layer of silicon nitride while incurring minimum throughput loss. The gratings were measured for efficiency in three stages of fabrication at Lawrence Berkeley Laboratory’s Advance Light Source (Beamline 6.3.2) in Berkeley California (Fig. 2). The first measurement was prior to ALD passivation, the second measurement was immediately after passivation, and the third measurement was performed after exposure of the gratings to UV ozone used as an accelerated oxidation test. The conformal coating of silicon nitride was effective in passivating the surface of Mo features. The measurement results show that we were able to achieve a grating efficiency of approximately 18% in the 1st and -1st orders (compared to 8% possible with a conventional absorber grating on Si membrane). The results also demonstrate the effectiveness of the ALD passivation process in mitigating oxidation effects with minimal effect on performance.
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