Weilun Chao

Understanding the base development mechanism of hydrogen silsesquioxane

The authors study the dissolution mechanism of hydrogen silsesquioxane in base solutions with the addition of chloride salts to elucidate the development mechanism. The reaction mechanisms are proposed based on the dissolution mechanism of quartz. Development kinetics suggests two dose-dependent development mechanisms. Considering ion sizes, both hydrated and nonhydrated, and ion exchange, they propose that a combination of a surface dominated reaction at higher doses and a matrix dominated reaction at lower doses accounts for the high development contrast with a NaOH base/NaCl salt mixture. The interplay between the hydrated and the nonhydrated ion sizes leads to higher contrast developers, such as tetramethyl ammonium hydroxide with NaCl.

Design and demonstration of tunable soft x-ray lateral shearing and Hartmann wavefront sensors

We describe design guidelines for soft x-ray wavefront sensors and experimentally demonstrate their performance, comparing grating-based lateral shearing interferometry and Hartmann wavefront sensing. We created a compact shearing interferometer concept with a dense array of binary amplitude gratings in a single membrane to support one-dimensional wavefront measurements across a wide wavelength range without the need for longitudinal position adjustment. We find that a common scaling parameter based on wavelength and the distance to the measurement plane guides the design of both systems toward optimal sensitivity. We show preliminary results from recent experiments demonstrating one and two-dimensional wavefront sensing below the Marechal criterion.

Fabrication and performance of transmission engineered molybdenum-rich phase structures in the EUV regime (Conference Presentation)

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. nHere 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.

The SEMATECH high-NA actinic reticle review project (SHARP) EUV mask-imaging microscope

The SEMATECH High Numerical Aperture Actinic Reticle Review Project (SHARP) is a newly commissioned, synchrotron-based extreme ultraviolet (EUV) microscope dedicated to photomask research. SHARP offers several major advances including objective lenses with 4xNA values from 0.25 to 0.625, flexible, lossless coherence control through a Fourier-synthesis illuminator, a rotating azimuthal plane of incidence up to ±25°, illumination central ray angles from 6 to 10°, and a continuously tunable, EUV illumination wavelength. SHARP is now being used to study programmed and native mask defects, defect repairs, mask architecture, optical proximity correction, and the influence of mask substrate roughness on imaging. SHARP has the ability to emulate a variety of current and future lithography tool numerical apertures, and illumination properties. Here, we present various performance studies and examples where SHARP’s unique capabilities are used in EUV mask research.