Mark A. McKernan

Magnetron sputter deposition of boron and boron carbide

Abstract The fabrication of X-ray optical coatings with greater reflectivity required the development of sputter deposition processes for boron and boron carbide. The use of high density boron and boron carbide (B4C) and a vacuum-brazed target design was required to achieve the required sputter process stability and resistance to the thermal stress created by high rate sputtering. Our results include a description of the target fabrication procedures and sputter process parameters necessary to fabricate B4C and boron modulated thin film structures.

Formation of cubic boron-nitride by the reactive sputter deposition of boron

Boron-nitride films are synthesized by RF magnetron sputtering boron targets where the deposition parameters of gas pressure, flow and composition are varied along with substrate temperature and applied bias. The films are analyzed using Auger electron spectroscopy, transmission electron microscopy, nanoindentation, Raman spectroscopy and x-ray absorption spectroscopy. These techniques provide characterization of film composition, crystalline structure, hardness and chemical bonding, respectively. Reactive, rf-sputtering process parameters are established which lead to the growth of crystalline BN phases. The deposition of stable and adherent boron nitride coatings consisting of the cubic phase requires 400 `C substrate heating and the application of a 300 V negative bias.

Wavelength dependence of the damage threshold of inorganic materials under extreme-ultraviolet free-electron-laser irradiation

We exposed bulk SiC and films of SiC and B4C to single 25 fs long free-electron-laser pulses with wavelengths between 13.5 and 32 nm. The materials are candidates for x-ray free-electron laser optics. We found that the threshold for surface-damage of the bulk SiC samples exceeds the fluence required for thermal melting at all wavelengths. The damage threshold of the film sample shows a strong wavelength dependence. For wavelengths of 13.5 and 21.7 nm, the damage threshold is equal to or exceeds the melting threshold, whereas at 32 nm the damage threshold falls below the melting threshold.

Subnanometer multilayers for x‐ray mirrors: Amorphous crystals

The use of ‘‘natural’’ crystals as Bragg diffractors has been supplemented by the advent of artificial composition‐modulated structures, that is, multilayers. The extension of optical systems using soft x rays to hard x rays is now supplemented by the synthesis of subnanometer repeat period multilayers. These structures consist of layering amorphous planes, each an atomic spacing thick, and may therefore be thought of as ‘‘amorphous crystals.’’

Development, characterization and experimental performance of x-ray optics for the LCLS free-electron laser

This manuscript discusses the development of reflective optics for the x-ray offset mirror systems of the Linac Coherent Light Source (LCLS), a 0.15-1.5 nm free-electron laser (FEL) at the Stanford Linear Accelerator Center (SLAC). The unique properties (such as the high peak brightness) of the LCLS FEL beam translate to strict limits in terms of materials choice, thus leading to an x-ray mirror design consisting of a reflective coating deposited on a silicon substrate. Furthermore, the physics requirements for these mirrors result in stringent surface figure and finish specifications that challenge the state-of-the-art in x-ray substrate manufacturing, thin film deposition, and metrology capabilities. Recent experimental results on the development, optimization, and characterization of the LCLS soft x-ray mirrors are presented in this manuscript, including: precision surface metrology on the silicon substrates, and the development of boron carbide reflective coatings with reduced stress and thickness variation < 0.14 nm rms across the 175-mm clear aperture area of the LCLS soft x-ray mirrors.

RadSensor: Xray Detection by Direct Modulation of an Optical Probe Beam

We present a new x-ray detection technique based on optical measurement of the effects of x-ray absorption and electron hole pair creation in a direct band-gap semiconductor. The electron-hole pairs create a frequency dependent shift in optical refractive index and absorption. This is sensed by simultaneously directing an optical carrier beam through the same volume of semiconducting medium that has experienced an xray induced modulation in the electron-hole population. If the operating wavelength of the optical carrier beam is chosen to be close to the semiconductor band-edge, the optical carrier will be modulated significantly in phase and amplitude. This approach should be simultaneously capable of very high sensitivity and excellent temporal response, even in the difficult high-energy xray regime. At xray photon energies near 10 keV and higher, we believe that sub-picosecond temporal responses are possible with near single xray photon sensitivity. The approach also allows for the convenient and EMI robust transport of high-bandwidth information via fiber optics. Furthermore, the technology can be scaled to imaging applications. The basic physics of the detector, implementation considerations, and preliminary experimental data are presented and discussed.

X-ray optics research for free electron lasers: study of material damage under extreme fluxes

Free electron lasers operating in the 0.1–1.5 nm wavelength range have been proposed for the Stanford Linear Accelerator Center (USA) and DESY (Germany). The unprecedented brightness and associated fluence (up to 30 J cm−2) predicted for pulses < 300 fs pose new challenges for optical components. A criterion for optical component design is required, implying an understanding of X-ray—material interactions at these extreme conditions. In our experimental effort, the extreme conditions are simulated by the currently available sources ranging from optical lasers, through X-ray lasers (XRLs) at 14.7 nm down to K-alpha sources (∼0.15 nm). In this paper, we present an overview of our research project on X-ray—matter interaction, including both computer modeling and preliminary results from optical laser experiments, the COMET tabletop high brightness ps XRL and a K-alpha experimental campaign carried out at the JanUSP laser facility at the Lawrence Livermore National Laboratory.© 2003 Elsevier Science B.V. All rights reserved.PACS: 41.50; 42.70; 41.60. Cr; 42.55. Vc; 07.85. F

The Physics Analysis of a Gas Attenuator with Argon as a Working Gas

A gas attenuator is an important element of the LCLS facility. The attenuator must operate in a broad range of x-ray energies, provide attenuation coefficient between 1 and 10{sup 4} with the accuracy of 1% and, at the same time, be reliable and allow for many months of un-interrupted operation. S. Shen has recently carried out a detailed design study of the attenuator based on the use of nitrogen as a working gas. In this note we assess the features of the attenuator based on the use of argon. We concentrate on the physics issues, not the design features.

X-ray Optics Research for Linac Coherent Light Source: Interaction of Ultra-short X-ray Pulses with Matter

Free electron lasers operating in the 0.1 to 1.5 nm wavelength range have been proposed for the Stanford Linear Accelerator Center (USA) and DESY (Germany). The unprecedented brightness and associated fluence predicted for pulses <300 fs pose new challenges for optical components. A criterion for optical component design is required, implying an understanding of x‐ray ‐ matter interactions at these extreme conditions. In our experimental effort, the extreme conditions are simulated by currently available sources ranging from optical lasers, through x‐ray lasers (at 14.7 nm) down to K‐alpha sources (∼0.15 nm). In this paper we present an overview of our research program, including (a) Results from the experimental campaign at a short pulse (100 fs ‐ 5 ps) power laser at 800 nm, (b) K‐α experiments, and (c) Computer modeling and experimental project using a tabletop high brightness ps x‐ray laser at the Lawrence Livermore National Laboratory.

The use of magnetron arrays for depositing large-area oxide coatings

The application of coatings over large areas can be approached through the use of large deposition sources. A versatile alternative, e.g. to long rectangular magnetrons, are linear arrays of circular planar magnetrons to process coatings over wide path lengths. We investigate the feasibility of using a linear array of 76-mm-diameter magnetron sources operated in the rf mode to deposit oxide target materials across a path in excess of 0.7 m wide. Specific results are given for the case of a 2-μm-thick alumina coating.