Grazing incidence and grazing emission X-ray fluorescence spectroscopy (GI/GE-XRF) are techniques that enable nondestructive, quantitative analysis of elemental depth profiles with a resolution in the nanometer regime. A laboratory setup for soft X-ray GEXRF measurements is presented. Reasonable measurement times could be achieved by combining a highly brilliant laser produced plasma (LPP) source with a scanning-free GEXRF setup, providing a large solid angle of detection. The detector, a pnCCD, was operated in a single photon counting mode in order to utilize its energy dispersive properties. GEXRF profiles of the Ni-L line of a nickel-carbon multilayer sample, which displays a lateral (bi)layer thickness gradient, were recorded at several positions. Simulations of theoretical profiles predicted a prominent intensity minimum at grazing emission angles between 5° and 12°, depending strongly on the bilayer thickness of the sample. This information was used to retrieve the bilayer thickness gradient. The results are in good agreement with values obtained by X-ray reflectometry, conventional X-ray fluorescence and transmission electron microscopy measurements and serve as proof-of-principle for the realized GEXRF setup. The presented work demonstrates the potential of nanometer resolved elemental depth profiling in the soft X-ray range with a laboratory source, opening, for example, the possibility of in-line or even in situ process control in semiconductor industry.
A: DEPFET Active Pixel Sensors (APS) have been introduced as focal plane detectors for X-ray astronomy already in 1996. Fabricated on high resistivity, fully depleted silicon and back-illuminated they can provide high quantum efficiency and low noise operation even at very high read rates. In 2009 a new type of DEPFET APS, the DSSC (DEPFET Sensor with Signal Compression) was developed, which is dedicated to high-speed X-ray imaging at the European X-ray free electron laser facility (EuXFEL) in Hamburg. In order to resolve the enormous contrasts occurring in Free Electron Laser (FEL) experiments, this new DSSC-DEPFET sensor has the capability of nonlinear amplification, that is, high gain for low intensities in order to obtain singlephoton detection capability, and reduced gain for high intensities to achieve high dynamic range for several thousand photons per pixel and frame. We call this property "signal compression".Starting in 2015, we have been fabricating DEPFET sensors in an industrial scale CMOS foundry maintaining the outstanding proven DEPFET properties and adding new capabilities due to the industrial-scale CMOS process. We will highlight these additional features and describe the progress achieved so far. In a first attempt on double-sided polished 725 µm thick 200 mm high resistivity float zone silicon wafers all relevant device related properties have been measured, such as leakage current, depletion voltage, transistor characteristics, noise and energy resolution for X-rays and the nonlinear response. The smaller feature size provided by the new technology allows for an advanced design and significant improvements in device performance. A brief summary of the present status will be given as well as an outlook on next steps and future perspectives.
By combining a low noise fully depleted pnCCD detector with a columnar CsI(Tl) scintillator an energy dispersive spatial resolving detector can be realized with a high quantum efficiency in the range from below 0.5 keV to above 150 keV. The used scintillator system increases the pulse height of gamma-rays converted in the CsI(Tl), due to focusing properties of the columnar scintillator structure by reducing the event size in indirect detection mode (conversion in the scintillator). In case of direct detection (conversion in the silicon of the pnCCD) the relative energy resolution is 0.7% at 122 keV (FWHM = 850 eV) and the spatial resolution is less than 75 μm. In case of indirect detection the relative energy resolution, integrated over all event sizes is about 9% at 122 keV with an expected spatial precision of below 75 μm.
We undertook a comparative study on optimizing the position accuracy of pnCCDs for single X-ray photon measurements. Various methods were analyzed by Monte Carlo simulations and related to experimental data obtained with a focused X-ray beam. Even with low energy photons of 1320 eV, a position accuracy much smaller than the actual pixel size of 48 µm × 48 µm can be achieved. This is possible since signal charges from a single photon interaction spread into more than one pixel, allowing a reconstruction of the original point of interaction. We found that a) making a decision on which pixels to use for the reconstruction and b) choosing a centroiding algorithm for carrying out the reconstruction were particularly crucial. For a) we introduce a new and superior method using a two step analysis with an adaptive pattern. It is compared to using a threshold or a fixed pattern. For b) we present a Center-of-Gravity method with a Gaussian correction taking into account the shape of the signal charge cloud. Both methods are also optimized for fast execution by implementing lookup tables rather than time consuming calculations. Our results show that with the appropriate analysis an uncertainty of the position measurement of better than 3.0 µm rms for 1320 eV photons is possible.
When considering cameras for spectroscopic or imaging applications, one of the first questions asked is, "How many pixels does it have?" It is assumed that the smallest resolvable feature in any camera is equal to the pixel size. Unfortunately, cameras and imagers with many thousands of pixels are expensive, and the rate that data are produced can be difficult to manage. In X-ray imaging, whether for diffraction, fluorescence, microscopy or absorption spectroscopy, imaging systems must carefully balance frame rate, pixel size, camera size and incoming photon flux. If, for instance, the incoming photon flux is too high given the camera's potential charge well, then the resulting image quality will be poor due to charge spilling out into neighboring pixels (i.e., image smearing). Increasing the charge well to account for this may require a redesign of the X-ray detector itself. Assuming that users only want to buy one camera for X-ray work, the best solution is to have a camera that is flexible, and can, for instance, change the effective resolution as the experiment requires.pnCCDs are radiation detectors on high resistivity 450 µm thick fully sensitive silicon [1]. They are back-illuminated devices with an ultra-thin, homogeneous radiation entrance window, enabling the direct detection of X-rays from 100 eV up to 30 keV with high quantum efficiency. When pnCCDs are hit by X-rays, the incoming photons are converted into electron-hole pairs through the photoelectric effect. The photoelectron ionizes silicon atoms in the vicinity until all its energy is dissipated. At an X-ray energy of 1320 eV, 360 ± 7 signal electrons are generated within a radius of less than 1 µm. Then, due to electrostatic repulsion and diffusion, the signal charge cloud widens during its drift from the conversion point inside the silicon into the potential minimum of the pixels. The charge cloud expands to approximately a radius of 13 µm (for an X-ray energy of 1320 eV) once arriving in the potential well of the pixel structure (see Figure 1). This widening of the electron cloud causes single X-ray events to appear as a collection of charges in two or more pixels. While this may seem detrimental to the overall position resolution of the camera, this is actually an opportunity to resolve the initial location of the X-ray event to smaller than the size of one pixel. Calculating the center of gravity of a multi-pixel event, and knowing that the charge distribution has a Gaussian profile given electrostatic repulsion theory, the actual location of the X-ray event can be calculated with extremely high accuracy. We have demonstrated with the help of simulations and measurements (see Fig. 2) that a position precision of better than 3 µm (rms) at an X-ray energy of 1320 eV can be achieved with a pixel size of 48 µm [2]. In these experiments, an X-ray beam was focused on the pnCCD entrance window with a spot size of approximately 0.8 µm and scanned over the pixel area with a step size of 3 µm in the x direction and 10 µm in the y direction. For X-rays...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.