A. Reyes-Mena

Focusing crystal von Hamos spectrometer for x-ray spectroscopy and x-ray fluorescence applications

The use of bent crystals with high integrated reflectivity in focusing crystal spectrometers (Johann and von Hamos schemes) is considered. It is shown that in a von Hamos scheme mosaic focusing takes place. Thus a mosaic crystal simultaneously provides high spectral resolution and high efficiency. Expressions for the mosaic focusing are obtained. Focusing mica and graphite crystal von Hamos spectrometers (radius of crystal curvature is 20 mm) are investigated: spectral and spatial resolution and absolute efficiency are measured in a spectral range of 2 - 2.6 angstroms using laser-produced plasma and iron isotope x-ray sources. The mica crystal spectrometer showed high spatial (up to 10 micrometers ) and spectral ((lambda) /(delta) (lambda) approximately 1000) resolution, whereas the graphite spectrometer showed very high efficiency (30 - 70 times higher than the mica crystal) and moderate spectral resolution ((lambda) /(delta) (lambda) approximately 500 - 750). In the latter case mosaic focusing is observed: spectral resolution is 10 - 15 times higher than spectral resolution determined by the mosaic spread of the crystal ((lambda) /(delta) (lambda) approximately 50). The results allow one to estimate a maximum efficiency for focusing crystal spectrometers. Prospects for using the von Hamos spectrometers for x-ray spectroscopy and x-ray fluorescence are considered.

RAMAN SPECTROSCOPIC STUDY OF THE FORMATION OF T-MOSI2 FROM MO/SI MULTILAYERS

We have used Raman spectroscopy, large‐ and small‐angle x‐ray diffraction spectroscopy of sputter‐deposited, vacuum‐annealed, soft x‐ray Mo/Si thin‐film multilayers to study the physics of silicide formation. Two sets of multilayer samples with d‐spacing 8.4 and 2.0 nm have been studied. Annealing at temperatures above 800 °C causes a gradual formation of amorphous MoSi2 interfaces between the Si and Mo layers. The transition from amorphous to crystalline MoSi2 is abrupt. The experimental results indicate that nucleation is the dominant process for the early stage and crystallization is the dominant process after nucleation is well advanced. In the thicker multilayer, a portion of the silicon crystallizes during annealing and a strong Raman signal is observed. An advantage of Raman spectroscopy is that the Raman signal of the silicide is observed even before the presence of MoSi2 can be seen using x‐ray diffraction. This study indicates that Raman spectroscopy is an effective technique for characterizing ...

Emitted Current Instability from Silicon Field Emission Emitters Due to Sputtering by Residual Gas Ions

We have fabricated arrays of silicon field emitters using semiconductor lithography techniques. The density of the tips was 105/cm2. The maximum current that can be extracted from each emitter is limited by resistive heating. We have investigated how the electron current emitted changes under constant applied voltage. We found that the current is very sensitive to the vacuum conditions. We attribute this to sputtering of the emitters due to ionized residual gas molecules. The poorer the vacuum, the higher the instability in the current. We studied this phenomenon at 10−6 and 10−8 Torr. The model of two concentric spherical shells is used to obtain the ion energy distribution. This is then used to calculate the rate of ion bombardment and the rate of atoms sputtered. A lifetime of the tip can be deduced from these calculations.

An XRD/XRF instrument for the microanalysis of rocks and minerals

A breadboard setup constructed at MOXTEK, Inc., is capable of capturing both x-ray diffraction (XRD) and x-ray fluorescence (XRF) information simultaneously using a charge-coupled device (CCD) as the x-ray detector. This preliminary setup will lead to a prototype simultaneous XRD/XRF instrument. NASA is funding the instruments construction because of its capabilities and small size; it could be used for future Mars missions for analysis of rocks. The instrument uses a CCD to capture both the energy and the spatial information of an incoming x-ray. This is possible because each pixel acts as a spatially addressable energy-dispersive detector. A powdered sample of material is placed in front of the CCD, which in turn is bombarded by a collimated x-ray beam. The instruments critical features - namely the x-ray source, collimation optics and x-ray transparent windows - allow for the first time, to the best of our knowledge, mounting the sample outside the CCD camera. In this paper the instruments design parameters as well as the properties of both a front-side-illuminated (FSI) CCD and back-side-illuminated (BSI) CCD as x-ray detectors are investigated.

The use of sharp silicon tips as photocathodes and electron sources for X-ray generation

Abstract We have produced arrays of 10,000 sharp p-type silicon points using an etch plus oxidation method. These points were used as electron emitters. No high vacuum cesiation or high temperature cleaning was needed to observe the electron emission. These are seen to be photosensitive sources of electrons at 200 K and 300 K. They were also used to produce AlKα x-rays. This constitutes the first use of etched, point arrays for generating electrons for x-ray sources.

Using a charge-coupled device (CCD) to simultaneously gather x-ray fluorescence (XRF) and x-ray diffraction (XRD) information

A breadboard setup constructed at MOXTEK, Inc., is capable of capturing both x-ray diffraction (XRD) and x-ray fluorescence (XRF) information simultaneously using a charge-coupled device (CCD) as the x-ray detector. This preliminary setup will lead to a prototype simultaneous XRD/XRF instrument. NASA is funding the project because it could be used for future Mars missions for analysis of rocks. The instrument uses a CCD to capture both the energy and the position of an incoming x-ray. This is possible because each pixel acts as a spatially addressable energy- dispersive detector. A powdered sample of material is placed in front of the CCD, which in turn is bombarded by a collimated x-ray beam. The instruments critical features, the x-ray source, collimation optics and x-ray transparent windows need to be optimized in the size and power to allow the instrument to be portable. In this paper the instruments design parameters as well as the properties of both the CCD as x-ray detector and the low-power consumption tube are investigated.

X-ray Diode Using a Silicon Field Emission Photocathode

We have produced arrays of 10,000 sharp p-type silicon points using an etch plus oxidation method. These points were used as electron emitters. No high vacuum caseation or high temperature cleaning was needed to observe the electron emission. These are seen to be photosensitive sources of electrons at 200 K and 300 K. They were also used to produce AlK(alpha ) x rays. This constitutes the first use of etched, point arrays for generating electrons for x-ray sources.

Multilayer phase diffraction gratings modeled as a structure in three dimensions

X-ray phase diffraction gratings can be designed to behave in a fashion similar to blazed gratings, directing the majority of the energy into certain desired orders. They should be easy to fabricate using conventional semiconductor production technology, and offer advantages in design flexibility and efficiency over conventional amplitude grating or blazed grating structures. As a multilayered structure, a phase grating has structure in depth as well as across the surface. Most theoretical analyses in the literature treat the embedded structure through simplifying approximations or assumptions. We will discuss a model which treats the embedded structure explicitly using the Fresnel-Kirchhoff integral in the Fraunhofer diffraction limit. This approach produces a set of equations which are identical to the result for an amplitude diffraction grating except for an additional factor which depends on the phase relationships of the various surfaces in the multilayer stack.

Use of Raman Spectroscopy in Characterizing Soft X-ray Multilayers: Tools in Understanding Structure and Interfaces

Our group is studying the structure and interfaces of soft x-ray multilayers by various techniques including x-ray diffraction and Raman spectroscopy. Raman spectroscopy is particularly useful since it is sensitive to the identity of individual bonds and thus can potentially characterize the abruptness of interfaces in multilayers. Blocking interfacial mixing is very important in achieving and maintaining high reflectivity. We report our studies of the as-deposited and postannealed structure of Mo/Si and W/C multilayers. A high normal- incidence, peak reflectance is mandatory for imaging applications that involve many reflections. The reported theoretical and achieved reflectances of the Mo/Si system are 80% and 65%, respectively. This loss of 15% can bring about a six-fold loss in system throughput in the eight-reflection system contemplated. The interfaces in the Mo/Si system are thought to play a significant role in the degrading reflectance so characterization techniques which have interfacial sensitivity are particularly important. The Mo/Si multilayer system is susceptible to Raman characterization since both the a-Si spacer layer and the MoSi2 compound which forms at the interface have Raman active modes. In this paper we report the first Raman studies, to the best of our knowledge, of the a-Si layers and their crystallization and the crystallization of the Mo/Si interface of the multilayer brought about by a one-hour 1000 degree(s)C anneal. These changes are apparent in the Raman spectra before they can be unambiguously detected by x-ray diffraction.

Raman spectroscopic analysis of MoSi multilayers

Raman spectra are reported from MoSi2 polycrystalline powder and soft x-ray Mo/Si multilayers. The sharp lines at 323 and 438 cm-1 are all due to crystalline MoSi2. These lines in the powder sample intensify with annealing. The Raman spectra of as-deposited multilayers shows a broad asymmetric peak, highest at about 480 cm-1. We attribute this to α-Si which is highly disordered. In contrast to α-Si in semiconductor/semiconductor and semiconductor/dielectric multilayers, in the Mo/Si samples the Raman signal can vanish after modest heating. This provides evidence that the composition of the silicon component of the multilayer changes even with 200°C annealing. Further annealing also produces the signature for crystalline MoSi2 in the multilayer samples. This is the first report of the characterization of Mo/Si soft x-ray multilayers by Raman spectroscopy, and it indicates that Raman spectroscopy may be an effective technique for characterizing these soft x-ray multilayers and may be useful in studying their interfaces.