Jeffrey W. Keister

Performance of a Thin-Window Silicon Drift Detector X-Ray Fluorescence Spectrometer

Several sets of hexagonal Silicon Drift Detector (SDD) arrays were produced by Brookhaven National Laboratory (BNL) and by the commercial vendor, KETEK. These detector arrays were tested at BNL. Each array consists of 14 independent SDD detectors (pixels) and two additional test pixels located at two corners of the array. The side of the detector upon which the X-ray radiation is incident (window side) has a thin junction covering the entire active area. The opposite side (device side) contains a drift-field electrode structure in the form of a hexagonal spiral and an electron collecting anode. There are four guard rings surrounding the 14-pixel array area on each side of the detector. Within each array, seven pixels have aluminum field plates - interrupted spirals that stabilize the electric potential under the Si- SiO2 interface, while the other seven do not. Three bias voltages are applied to control the drift field in the silicon volume; one is applied to a rectifying contact surrounding the central anode (one for each pixel), one is applied to the detector entrance window (common to the full array), and a third bias is applied to a contact on the outer portion of the spiral, common to all pixels in the array. Some arrays were recently tested in NSLS beam line U3C at BNL. For this work, we installed the complete assemblies in the vacuum and cooled them to -27degC. During this beam run, we collected spectra for energies ranging between 350 and 900 eV in several pixels, some with field plates and others without. The detailed testing results of several arrays are reported here.

Multiscale three-dimensional simulations of charge gain and transport in diamond

A promising new concept of a diamond-amplified photocathode for generation of high-current, high-brightness, and low thermal emittance electron beams was recently proposed and is currently under active development. Detailed understanding of physical processes with multiple energy and time scales is required to design reliable and efficient diamond-amplifier cathodes. We have implemented models, within the VORPAL computational framework, to simulate secondary electron generation and charge transport in diamond in order to facilitate the investigation of the relevant effects involved. The models include inelastic scattering of electrons and holes for generation of electron-hole pairs, elastic, phonon, and charge impurity scattering. We describe the integrated modeling capabilities we developed and present results on charge gain and collection efficiency as a function of primary electron energy and applied electric field. We compare simulation results with available experimental data. The simulations show an overall qualitative agreement with the observed charge gain from transmission mode experiments and have enabled better understanding of the collection efficiency measurements.

Transmission-mode diamond white-beam position monitor at NSLS

Two transmission-mode diamond X-ray beam position monitors installed at National Synchrotron Light Source (NSLS) beamline X25 are described. Each diamond beam position monitor is constructed around two horizontally tiled electronic-grade (p.p.b. nitrogen impurity) single-crystal (001) CVD synthetic diamonds. The position, angle and flux of the white X-ray beam can be monitored in real time with a position resolution of 500 nm in the horizontal direction and 100 nm in the vertical direction for a 3 mm × 1 mm beam. The first diamond beam position monitor has been in operation in the white beam for more than one year without any observable degradation in performance. The installation of a second, more compact, diamond beam position monitor followed about six months later, adding the ability to measure the angular trajectory of the photon beam.

ASIC for SDD-Based X-Ray Spectrometers

We present an application-specific integrated circuit (ASIC) for high-resolution x-ray spectrometers (XRS). The ASIC reads out signals from pixelated silicon drift detectors (SDDs). The pixel does not have an integrated field effect transistor (FET); rather, readout is accomplished by wire-bonding the anodes to the inputs of the ASIC. The ASIC dissipates 32 mW, and offers 16 channels of low-noise charge amplification, high-order shaping with baseline stabilization, discrimination, a novel pile-up rejector, and peak detection with an analog memory. The readout is sparse and based on custom low-power tristatable low-voltage differential signaling (LPT-LVDS). A unit of 64 SDD pixels, read out by four ASICs, covers an area of 12.8 cm2 and dissipates with the sensor biased about 15 mW/cm2. As a tile-based system, the 64-pixel units cover a large detection area. Our preliminary measurements at -44°C show a FWHM of 145 eV at the 5.9 keV peak of a 55Fe source, and less than 80 eV on a test-pulse line at 200 eV.

Tracing X-rays through an L-shaped laterally graded multilayer mirror: a synchrotron application

A theoretical model to trace X-rays through an L-shaped (nested or Montel Kirkpatrick-Baez mirrors) laterally graded multilayer mirror to be used in a synchrotron application is presented. The model includes source parameters (size and divergence), mirror figure (parabolic and elliptic), multilayer parameters (reflectivity, which depends on layer material, thickness and number of layers) and figure errors (slope error, roughness, layer thickness fluctuation Deltad/d and imperfection in the corners). The model was implemented through MATLAB/OCTAVE scripts, and was employed to study the performance of a multilayer mirror designed for the analyzer system of an ultrahigh-resolution inelastic X-ray scattering spectrometer at National Synchrotron Light Source II. The results are presented and discussed.

Synchrotron X-ray tests of an L-shaped laterally graded multilayer mirror for the analyzer system of the ultra-high-resolution IXS spectrometer at NSLS-II

Characterization and testing of an L-shaped laterally graded multilayer mirror are presented. This mirror is designed as a two-dimensional collimating optics for the analyzer system of the ultra-high-resolution inelastic X-ray scattering (IXS) spectrometer at National Synchrotron Light Source II (NSLS-II). The characterization includes point-to-point reflectivity measurements, lattice parameter determination and mirror metrology (figure, slope error and roughness). The synchrotron X-ray test of the mirror was carried out reversely as a focusing device. The results show that the L-shaped laterally graded multilayer mirror is suitable to be used, with high efficiency, for the analyzer system of the IXS spectrometer at NSLS-II.

Development of Thin-Window Silicon Drift Detector for X-ray spectroscopy

A new set of thin-window silicon drift detectors composed of an array of hexagonal shaped detectors has been designed, constructed and tested for X-ray spectroscopy. Each individual ThinWinSDD has a thin entrance window on one side and a spiral shaped hexagonal cathode around a center anode on the other side. To produce the thin entrance window a 10 keV implantation of boron through a 500 A silicon dioxide was used. The implantation was followed by an annealing at 700degC for 30 min and a reactive ion etching step to ensure the removal of silicon dioxide from the smallest feature (5 mum). An aluminum layer is coated in the same vacuum system after back-sputtering. This step involves removing the native oxide that has formed on the top of the silicon substrate and then sputtering a 1100 A thick layer of aluminum onto the X-ray entrance window. The aluminum layer must be thick enough to block visible light, but thin enough to be transparent to soft X-rays down to 280 eV. We discuss first test results that include detector leakage current measurements and the response for multiple detectors exposed to the National Synchrotron Light Sources UV beam line U3C located at Brookhaven National Laboratory for X-ray energies as low as 280 eV.

Resonance effects in photoemission from TiO2-capped Mo/Si multilayer mirrors for extreme ultraviolet applications

In the unbaked vacuum systems of extreme ultraviolet (EUV) lithography steppers, oxide formation and carbon growth on Mo/Si multilayer mirrors (MLMs) are competing processes leading to reflectivity loss. A major contribution to this mirror degradation is a series of surface reactions that are thought to be driven in large part by photoemitted electrons. In this paper, we focus on the resonance effects in photoemission from Mo/Si MLMs protected by thin TiO2 cap layers. In the vicinity of the resonant energy of the mirror, the energy flux of the EUV radiation forming standing wave oscillates throughout the multilayer stack. As a result, light absorption followed by the emission of photoelectrons becomes a complex process that varies rapidly with depth and photon energy. The electron emission is characterized as a function of the EUV photon energy, the angle of incidence, and the position of the standing wave with respect to the solid/vacuum interface. In our experiments, the position of the standing wave wa...

ASIC for SDD-based X-ray spectrometers

We present an application-specific integrated circuit (ASIC) for high-resolution x-ray spectrometers (XRS). The ASIC reads out signals from pixelated silicon drift detectors (SDDs). The pixel does not have an integrated field effect transistor (FET); rather, readout is accomplished by wire-bonding the anodes to the inputs of the ASIC. The ASIC dissipates 32 mW, and offers 16 channels of low-noise charge amplification, high-order shaping with baseline stabilization, discrimination, a novel pile-up rejector, and peak detection with an analog memory. The readout is sparse and based on custom low-power tristatable low-voltage differential signaling (LPT-LVDS). A unit of 64 SDD pixels, read out by four ASICs, covers an area of 12.8 cm2 and dissipates with the sensor biased about 15 mW/cml As a tile-based system, the 64-pixel units cover a large detection area. Our preliminary measurements show a FWHM of 145 eV at the 5.9 keV peak of a 55Fe source, and less than 80 eV on a test-pulse line at 200 eV.

Comparison of two different methods to produce thin-window silicon drift detectors

We have developed a new method to produce thin-entrance-window Silicon Drift Detectors. To produce the desired thin-entrance-window a double implantation was used. This implantation consists of Boron ions (dose of 1×1014/cm2 at 10 keV) plus a second implant of Phosphorus ions (with a dose of 4×1012/cm2 at 50 keV or dose of 9×1011/cm2 at 80 keV) through 500 A of silicon dioxide. The second Phosphorus implantation compensates for the tail portion of the Boron ion implantation, so that the net Boron ion distribution will result in a thinner “dead” silicon layer and an elevated electric field near the silicon surface. We will compare test results from this newly developed thin-window with those from our previous development, where the thin junction was created using a single implantation of Boron ions (dose of 1×1014/cm2 at 10 keV) through a 500 Å thick silicon dioxide. All testing was done in the U3C beam line at the National Synchrotron Light Source at Brookhaven National Laboratory.