Possibility for the form correction of X-ray mirrors by reactive ion-beam etching

Key parameters for the reactive ion-beam etching of glass, quartz and silicon are studied. The dependence of the etching rate on chamber pressure and etching time is investigated. The etching zone has the form of a long (??50 mm), narrow (??1 mm) strip with depth homogeneity at a level of 0.2%. Such a zone is a handy tool for correcting the shape of cylindrical surfaces.     

Device for the precise shape correction of optical surfaces by ion-beam and reactive plasma etching

A device for the precise shape correction of optical surfaces of any irregular shape (convex, concave) up to 300 mm in diameter by means of ion-beam and reactive plasma etching is described. The possibilities of the device allow the fabrication of optical elements with a root-mean-square deviation from the desired shape on the level of 0.3 to 1.0 nm.     

Post-growth tailoring of quantum-dot saturable absorber mirrors by chemical etching

We have designed and grown a resonant, low-finesse quantum-dot saturable absorber mirror and subsequently modified the important parameters using chemical etching. The modulation depth and saturation fluence at the design wavelength of 1064 nm were modified by etching the sample to tune the cavity resonance. The device properties were characterised using normal incidence spectroscopic reflectivity measurements, intensity dependent reflectivity measurements and modelled using a transfer matrix approach. The saturable absorber mirror was used to facilitate self-starting, passively mode locked pulses in a neodymium vanadate laser operating at 1064 nm. The etching was found to affect the duration of the pulses, leading to temporal width tuning over a range of 94 ps. The shortest pulse duration of 84 ps was achieved for the cavity resonance close to 1064 nm, with an output power of 3 W. This method is an effective technique for post-growth engineering of the properties of semiconductor saturable absorber mirrors (SESAMs) with nanometre precision.     

Design and fabrication of low-cost X-ray mirrors

Traditional X-ray mirrors made from Zerodur and silicon carbide are quite expensive because of the fabrication cost involved in achieving a very high surface finish of the order of 15 Å or better. The cost of X-ray mirrors can be greatly reduced by using replication methods for producing the grazing-incidence type mirrors. This paper presents the optical and optomechanical design for a Wolter type I mirror and its mount, and the fabrication method used to produce a low-cost replicated nickel mirror. The finite element analysis results for this mirror are also presented. The design and fabrication steps for the aluminium master mandrel used for replication are also discussed. A surface finish of better than 15 Å was achieved for a such an electroformed mirror with a wall thickness of only 1 mm.     

Influence of resists on reactive ion etching

In the course of plasma etching we can observe a loading effect, i.e. the etch rate depends on the size of the etched surface exposed to the plasma. This phenomenon was explained according to Mogab by the plasma active etch species depletion via a rapid etch reaction. But there exist more coomplicated systems, for example SiO₂-photoresist SCR17-CHF₃, where the SiO₂ surface can be etched and a polymer layer can grow on the photoresist surface. The etching of SiO₂ is also influenced by different resists in the case of differences in their chemical structure. The degree of electrode coating with a resist influences both the etch rate of the masking layer. This may be used for the control of the etching selectivity in the SiO₂-resist system independently of other process parameters.The author is grateful to Mr. Z. Pokorný for his help in preparing the SiO₂ layers used in all experiments.     

The comparison of reactive ion etching and plasma etching in a parallel-plate reactor

Reactive ion etching (RIE) and plasma etching (PE) of different materials (GaAs, Si₃N₄ SiO₂ and photoresist Microposit 1350 H) in freon 116 are compared in the present article. The importance of ion bombardment for the etching rate is evident from the experimental results. GaAs is etched only by RIE due to ion milling, the etching rates of Si₃N₄ and SiO₂ are 4 to 5 times higher by RIE than PE.     

Optimization of platinum-carbon multilayer mirrors for hard X-ray optics

The design of hard X-ray optics for astrophysical technology is one of the key technologies for investigating active galaxies and clusters of galaxies. In the present paper, we have optimized multilayer mirrors of platinum-carbon layer pairs for the hard X-ray region at different grazing angles. The Luus-Jaakola optimization procedure has been implemented for the global optimization of multialyer mirrors. With this algorithm it is not necessary to specify initially the number of layers present in a design.     

Focusing crystal mirrors for X-ray diagnostics of laser-produced plasma

The paper describes an investigation of the characteristics of spherical focusing Bragg-reflection mirrors. The integral reflectivity of bent crystals at Bragg angles 80–90°, the width of the spectrum of the reflected x-rays, and the spatial resolution of the images formed by the focusing crystals are considered. Results are presented of the calculated integral reflectivity of bent silicon and quartz crystals of different orientation and in different orders of reflection. The feasibility is demonstrated of laser-plasma diagnostics using the images obtained with the aid of a multichannel x-ray microscope based on focusing crystals. Possible schemes of active diagnostics of a dense laser plasma using a monochromatic transilluminating x-ray beam are considered.Laser Plasma Laboratory, Lebedev Physics Institute. Translated from FIAN Preprint No. 143, Lebedev Physics Institute of the Academy of Sciences of the USSR, Moscow, 1989.     

X-ray intensity distribution in the image plane of elliptic multilayer mirrors

The x-ray intensity distribution in the image plane of a multilayer elliptical cylinder and ellipsoid-revolution mirrors was studied Analytical expressions for calculating this distribution were derived as a function of the size of the source, its position with respect to the mirror, and the microroughness of the multilayer structure.     

Reaction of hydrogen with chlorine during reactive ionic etching

The role of the chain mechanism of the interaction of chlorine with hydrogen in the gas phase during the reactive ionic etching (RIE) of GaAs in a CF₂Cl₂ plasma is discussed. In the presence of hydrogen, more volatile products are formed, and, as a result, the rate of etching becomes time-independent, while the boundary of etching becomes less diffuse and the etched surface appears to be smooth up to a depth of etching of >1 μm. The interaction of short-lived free radicals with the substrate is treated as heterogeneous chain termination.     

Influence of a carbon over-coat on the X-ray reflectance of XEUS mirrors

We describe measurements of the X-ray reflectance in the range 2-10 keV of samples representative of coated silicon wafers that are proposed for the fabrication of the X-ray evolving universe spectrometer (XEUS) mission. We compare the reflectance of silicon samples coated with bare Pt, with that for samples with an additional 10 nm thick carbon over-coating. We demonstrate a significant improvement in reflectance in the energy range ∼1-4 keV, and at a grazing incidence angle of 10 mrad (0.57°). We consider the resulting effective area that could be attained with an optimized design of the XEUS telescope. Typically an improvement of 10-60% in effective area, depending on photon energy, can be achieved using the carbon overcoat.     

A kinetic method for correcting the X-ray photoelectron spectra of radiation-sensitive compounds

A new method is proposed for determining the initial XPS spectra of compounds which are sensitive to X-ray irradiation, based on the first-order kinetics of the sample decomposition. The method simultaneously gives a value for the rate constant of the radiation-induced decomposition, which may itself be characteristic of the compound. The method is verified for the compounds cupric bis(8-quinolenethiol) and bis(dimethyl-glyoxime)triphenylphosphinecobalt(III) chloride, and a simple criterion for its use is given.     

Influence of the flow,etching time and reactor cleaning method on the etching rate and selectivity of reactive ion etching of SiO2 — Photoresist system

Reactive ion etching (RIE) in a cold plasma of fluoro- and chlorocarbons is one of the most widespread etching methods used in semiconductor industry. The repeatability of etching is influt enced by many factors like the deposition on the electrodes and reactor walls, adsorption of watevapour from air in the case of reactor bleeding, deposition during etching etc. In the presenr work we study the influence of the flow of the etching gas and of the method of reactor cleaning before etching on the etching rate and selectivity in the system SiO₂ —photoresist; the etching gas was trifluorochloromethane (Freon 23, CHF₃) and the etching method was RIE.     

Molecular dynamics simulations of reactive etching of SiC by energetic fluorine

Molecular dynamics simulations were performed to investigate F continuously bombarding Si-terminated 3C-SiC(001) surfaces with incident energies of 10, 100 and 200 eV at normal incidence and room temperature. For an energy of 10 eV, deposition only occurs on the surface. For energies larger than 10 eV, accompanying the saturation of F uptake, a balance between F deposition from the incident atoms and F removal from the fluorinated substrate is established, while the steady-state etching is reached. The simulated results demonstrate that Si atoms in SiC are preferentially etched, which is in good agreement with experiments. The preferential etching of Si results in formation of a C-rich interfacial layer whose thickness increases with increasing incident energy. The analysis shows that Si-containing etch products are dominant. PACS 52.65.Yy; 81.65.Cf; 52.77.Dq     

Fabrication of high-aspect-ratio silicon nanopillar arrays with the conventional reactive ion etching technique

We fabricate silicon nanopillar arrays with pillar diameters smaller than 200 nm by using the conventional reactive ion etching (RIE) technique and nickel masks. We use the ratio between the lateral and vertical etching rates as an estimate of the etching anisotropy. The dependence of this ratio on the rf power, the chamber pressure, and the gas mixture is investigated systematically to achieve the largest etching anisotropy. Using the optimized etching parameters in the RIE process, we demonstrate silicon pillars with smooth surface, vertical sidewalls, and aspect ratios higher than 20. In addition, we employ dilute aqua regia to treat the pillars and shrink the diameters to 70 nm. The pillar height remains ∼2500 nm after the treatment. PACS 52.77.Bn; 81.65.Cf; 85.40.Hp     

Influence of the spatial coherence of X-ray radiation on mirror reflection from multilayered mirrors

The influence of the degrees of spatial coherence of random modulated X-ray radiation on the intensity of the diffractive reflection of such radiation from a multilayered mirror, and on the spatial coherence function of the reflected field (depending on the statistical characteristics of the incident radiation and the parameters of the multilayered periodic structure), is considered.     

Monte Carlo simulation studies of sidewall roughening during reactive ion etching

The evolution of sidewall roughness (SWR) during reactive ion etching (RIE) was simulated using a Monte Carlo method. It was discovered that the sidewall roughness established during an earlier etch period represents a historical archive of the moment during which the etch front passes by and is not affected by further etching. We also found that the behavior of SWR follows two distinct trends. At the initial stages, SWR increases with etch time, or its equivalent, the etch depth, but, beyond a certain etch depth, SWR of etched surfaces stabilizes and does not change with further etching. This is related to the change of the number of shadowing and reemitted particles as the etch depth increases. When the shadowing sticking coefficient decreases significantly, SWR increases beyond a certain critical depth due to reemission. Additionally, the noise also increases as the sticking coefficient decreases because of an increased number of reemitted particles. The simulated results support very well the assertions and mechanisms of the experimental findings and an etch model based on shadowing and first-order reemission effects at low RIE pressure. PACS 52.65.Pp; 52.77.Bn; 81.65.Cf     

X-ray diffraction analysis of the silicon (111) surface during alkaline etching

We present a surface X-ray diffraction determination of the silicon (111)-liquid interface structure during alkaline etching. Preparation of an atomically smooth surface was realized by an in-situ procedure using an aqueous NH₄F solution devoid of oxygen. Using diluted aqueous potassium hydroxide (KOH) and ammonium fluoride (NH₄F) etchant, we have observed that the crystal surface is hydrogen terminated and is not reconstructed at open circuit potential. In addition, a partial liquid ordering of two water layers on top of the crystal surface was found, indicating a weak interaction with the hydrophobic, hydrogen terminated surface. We have followed in-situ the development of the oxide layer by a birth and spread mechanism during anodic passivation of the silicon surface.     

Structural characterization of ultrathin Cr and Sc films for soft X-ray mirrors

The structure of multilayers of ultrathin scandium (Sc) and chromium (Cr) films has been characterized by means of transmission electron microscopy (TEM). Face centered cubic Sc was found both in magnetron sputtered thin Sc layers on Si(0 0 1) and in Cr/Sc multilayers for soft X-ray mirrors. The single Sc and Cr layers are polycrystalline with randomly oriented grains, while Sc and Cr within the Cr/Sc multilayer show a strong [0 0 1] texture in the deposition direction. From high-resolution images the orientation-relationship at the Cr/Sc interfaces could be deduced as: Sc_[110]//Cr_[100] and Sc_[010]//Cr_[110], which was confirmed by image simulations.