An atomic layer etching process for silicon nitride (Si 3 N 4 ) has been developed in which ammonium fluorosilicate [(NH 4 ) 2 SiF 6 ] is formed and desorbed using infrared annealing. The cycle of forming and removing ammonium fluorosilicate was repeated, demonstrating that the Si 3 N 4 etching depth was accurately controlled with high selectivity to SiO 2 by changing the number of cycle. An X-ray photoelectron spectroscopy peak, which had been previously assigned as N-H bond of an ammonium salt, was observed after radical exposure, indicating that the ammonium fluorosilicate-based modified layer had formed. This peak disappeared after infrared annealing for 10 s, demonstrating desorption of the modified layer. In thermal desorption spectroscopy, NH 3 , HF, and SiF x were detected, providing further evidence for the formation of the ammonium fluorosilicate-based modified layer. In addition, this layer has a multilayer structure, protecting the Si 3 N 4 from exposure to reactive radicals.

Twin-image elimination in the context of optical scanning holography has recently been proposed. The proposed technique involves simultaneously acquiring sine and cosine Fresnel holograms. A complex hologram is then formed by complex addition of the holograms, and twin-image rejection is predicted by computer simulations. An experimental verification of the technique by optical acquisition of the two holograms and subsequent reconstruction of the complex hologram digitally is reported. Three-dimensional image reconstruction without twin-image noise is demonstrated.

Luminescent markers play a key role in imaging techniques for life science since they provide a contrast mechanism between signal and background. We describe a new type of marker using second harmonic generation (SHG) from noncentrosymmetric BaTiO 3 nanocrystals. These nanoparticles are attractive due to their stable, non-saturating and coherent signal with a femtosecond-scale response time and broad flexibility in the choice of excitation wavelength. We obtained monodispersed BaTiO 3 nanoparticles in colloidal suspensions by coating the particle surface with amine groups. We characterized the SHG efficiency of 90-nm BaTiO 3 particles experimentally and theoretically. Moreover, we use the coherent SHG signal from BaTiO 3 nanoparticles for three-dimensional (3D) imaging without scanning. We built a harmonic holographic (H 2) microscope which records digital holograms at the second harmonic frequency. For the first time, high-resolution 3D distributions of these SHG markers in mammalian cells are successfully captured and interpreted by the H 2 microscope.

Novel selective cyclic etching of SiN over SiO2 via the formation and desorption of ammonium fluorosilicate was developed. The formation of ammonium fluorosilicate was observed using X-ray photoelectron spectroscopy after hydrofluorocarbon-based radical exposure. Etching of SiN was observed after ammonium fluorosilicate was removed by thermal annealing. Cyclic etching tests were carried out by repeated radical exposure and thermal annealing. The etching depth increased on increasing the number of cycles. It was found that the cyclic etching is self-limiting because the etching depth does not depend on the radical exposure time but on the number of cycles.

Self-limiting reactions of ammonium salt in CHF3/O2 downstream plasma were demonstrated for thermal-cyclic atomic layer etching (ALE) of Si3N4. In situ x-ray photoelectron spectroscopy analysis shows that an (NH4)2SiF6 by-product of the same thickness forms on Si3N4 in a wide gas composition range. The (NH4)2SiF6 layer prevents etching of Si3N4 during continuous plasma exposure in that wide range. The (NH4)2SiF6 layer was sublimated by heating, which was consistent with the result of the thermodynamic calculation. The reactions of the (NH4)2SiF6 layer in CHF3/O2 downstream plasma are used for thermal-cyclic ALE of Si3N4 with a newly developed 300-mm tool equipped with an in situ ellipsometer. It was confirmed that the amount etched per cycle saturates with respect to both plasma exposure time and infrared irradiation time.