Tero Pilvi

Advanced thin film technology for ultrahigh resolution X-ray microscopy

Further progress in the spatial resolution of X-ray microscopes is currently impaired by fundamental limitations in the production of X-ray diffractive lenses. Here, we demonstrate how advanced thin film technologies can be applied to boost the fabrication and characterization of ultrahigh resolution X-ray optics. Specifically, Fresnel zone plates were fabricated by combining electron-beam lithography with atomic layer deposition and focused ion beam induced deposition. They were tested in a scanning transmission X-ray microscope at 1.2 keV photon energy using line pair structures of a sample prepared by metal organic vapor phase epitaxy. For the first time in X-ray microscopy, features below 10nm in width were resolved.

Study of a novel ALD process for depositing MgF2 thin films

Magnesium fluoride is an ultraviolet (UV) transparent material which is widely used in optical applications over a wide wavelength range. We have developed a novel atomic layer deposition (ALD) process for depositing magnesium fluoride thin films for the first time. MgF2 films were grown at 250–400 °C using Mg(thd)2 and TiF4 as precursors. The crystallinity, morphology, composition, thicknesses and refractive indices of the films were analyzed by X-ray diffraction/reflection (XRD/XRR), transmission electron microscopy (TEM), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), time-of-flight elastic recoil detection analysis (TOF-ERDA), and UV-vis spectrophotometry. Electrical properties were also measured. The growth rate was temperature dependent decreasing from 1.6 A cycle−1 at 250 °C to 0.7 A cycle−1 at 400 °C. The films were polycrystalline at 250–400 °C. The refractive indices were between 1.34–1.42 and the permittivity 4.9. The impurity levels were below 0.6 at.% in the films deposited at 350–400 °C.

Atomic layer deposition process with TiF 4 as a precursor for depositing metal fluoride thin films

A novel atomic layer deposition process for the preparation of fluoride thin films in a temperature range of 225 degrees C-400 degrees C is introduced. The crystallinity, morphology, composition, thicknesses, refractive indices, and transmittance of the films are analyzed. Low impurity levels are obtained at 350 degrees C-400 degrees C with good stoichiometry. Refractive indices of 1.34-1.42 for MgF(2), 1.43 for CaF(2), and 1.57-1.61 for LaF(3) films are obtained.

MeCp)Ir(CHD) and molecular oxygen as precursors in atomic layer deposition of iridium

Iridium thin films were grown by atomic layer deposition (ALD) between 225 and 350 °C using (MeCp)Ir(CHD) (MeCp = methylcyclopentadienyl, CHD = cyclohexadiene) and molecular oxygen as precursors. (MeCp)Ir(CHD) precursor was synthesized and characterized in-house. Also the crystal structure of (MeCp)Ir(CHD) is reported. All the ALD grown Ir films passed a common tape test indicating a good adhesion on Al2O3 nucleation layer. Quite untypically, surface roughness was the highest on films deposited at 225–250 °C and decreased strongly by increasing deposition temperature. Partial decomposition of the (MeCp)Ir(CHD) precursor resulted in defects on the film surface at 350 °C. Ir thin films with good quality were obtained at the deposition temperatures of 275 and 300 °C. A 50 nm thick film grown at 275 °C had a roughness of 1.2 nm, contained about 3 at% oxygen, 0.6 at% carbon and 1.6 at% hydrogen impurities, while the resistivity was as low as 9 µΩ cm.

Toward the Understanding of Stacked Al-Based High-k Dielectrics for Passivation of 4H-SiC Devices

Metal insulator semiconductor structures using high-k materials have been prepared and investigated for the passivation of 4H-SiC surfaces. Alternate layers of AlN and Al2O3 were deposited on 8 nm ...

Advanced X-ray diffractive optics

X-ray microscopy greatly benefits from the advances in x-ray optics. At the Paul Scherrer Institut, developments in x-ray diffractive optics include the manufacture and optimization of Fresnel zone plates (FZPs) and diffractive optical elements for both soft and hard x-ray regimes. In particular, we demonstrate here a novel method for the production of ultra-high resolution FZPs. This technique is based on the deposition of a zone plate material (iridium) onto the sidewalls of a prepatterned template structure (silicon) by atomic layer deposition. This approach overcomes the limitations due to electron-beam writing of dense patterns in FZP fabrication and provides a clear route to push the resolution into sub-10 nm regime. A FZP fabricated by this method was used to resolve test structures with 12 nm lines and spaces at the scanning transmission x-ray microscope of the PolLux beamline of the Swiss Light Source at 1.2 keV photon energy.

Characterization of Al-Based High-k Stacked Dielectric Layers Deposited on 4H-SiC by Atomic Layer Deposition

Aluminum-based high-k dielectric materials have been studied for their potential use as passivation for SiC devices. Metal-insulator-semiconductor structures were prepared and their dielectric properties were analyzed using capacitance-voltage and current-voltage measurements. Atomic layer deposition was used for the deposition of dielectric layers consisting of AlN with or without a buffer layer of SiO2, and also a stack of alternating AlN and Al2O3 layers. It has been observed that AlN has a polycrystalline structure which provides leakage paths for the current through the grain boundaries. However, adding alternate amorphous layers of Al2O3 prevent this leakage and give better overall dielectric properties. It is also concluded that the breakdown of the dielectric starts from the degradation of the thin interfacial SiO2 layer.

High Spatial Resolution STXM at 6.2 keV Photon Energy

We report on a zone‐doubling technique that bypasses the electron‐beam lithography limitations for the production of X‐ray diffractive optics and enables the fabrication of Fresnel zone plates with smaller outermost zone widths than other well‐established approaches. We have applied this method to manufacture hard X‐ray Fresnel zone plates with outermost zone widths of 25 and 20 nm. These lenses have been tested in scanning transmission X‐ray microscopy (STXM) at energies up to 6.2 keV, producing images of test structures that demonstrate a spatial resolution of 25 nm. High spatial resolution STXM images of several biological specimens have been acquired in transmission, dark‐field and differential phase contrast modes.

Diffractive Optics in Industry and Research - Novel Components for Optical Security Systems

Design and manufacturing of diffractive optical elements (DOEs) are presented. Mass replication methods for DOEs are explained including UV-replication, micro-injection moulding and reel-to-reel production. Novel applications of diffractive optics including spectroscopic surface relief gratings, antireflection surfaces, infrared light rejection gratings, light incoupling into thin waveguides, and additive diffractive colour mixing are presented.