Richard B. Mott

Energy-Dispersive Spectrometry from Then until Now: A Chronology of Innovation

: As part of the Microbeam Analysis Society (MAS) symposium marking 30 years of energy-dispersive spectrometry (EDS), this article reviews many innovations in the field over those years. Innovations that added a capability previously not available to the microanalyst are chosen for further description. Included are innovations in both X-ray microanalysis and digital imaging using the EDS analyzer.

Real-World Electron Detector Performance in Scanning Electron Microscopes

We test the application of a novel figure of merit for electron detectors which is representative of their overall suitability for real-world imaging applications. Traditional performance numbers such as detective quantum efficiency (DQE) [1] and bandwidth each address single aspects of detector performance whereas efficiency, speed, and dynamic range all interact to affect perceived image quality. We show how these variables can be combined to reflect the performance trade-offs of electron imaging. Arriving at a single number allows detectors of different hardware and operating principles to be directly compared for their imaging suitability under any desired set of operating conditions.

A Combination BSE and CL Detector using Silicon Photomultipliers

Considerable advancement in the performance of solid state photomultipliers (SiPMs) has been driven by the need to replace photomultiplier tubes (PMTs) for use in combination MRI/PET medical instrumentation due to the sensitivity of PMTs to magnetic fields [1]. The application of SiPM technology to Backscattered Electron imaging (BSE) in electron microscopes was introduced by Barbi et al [2]. In this case, scintillators are bonded to SiPMs to create compact Scintillator-on-Multiplier (SoM) electron sensors. The multiplicity of possible scintillator materials coupled with the rapid and continual improvement in SiPMs encouraged the development of a laboratory testing methodology which can yield a Figure of Merit for any combination of scintillator and SiPM [3]. The SiPMs are separated into two broad categories: those sensitive primarily to visible light (RGB SiPMs) and those with improved sensitivity to near ultraviolet (NUV SiPMs). One of the interesting corollaries of SiPM electron detector technology is that hybrid BSEDs can be constructed using different scintillators within a single detector, an implementation which will become more useful as scintillators with improved sensitivity to low energy electrons are developed. A different type of hybrid detector, discussed here, comprises SoMs optimized for BSEs and bare or filtered SiPMs for Cathodoluminescence (CL).

Thin Film and Multilayer Scintillators for Low Voltage Backscattered Electron Imaging in the Scanning Electron Microscope

Scintillators produce a number of photons proportional to the average energy of the incident electrons. Low energy electron detection using scintillators therefore starts at a disadvantage compared to imaging at higher energies. The predominant scintillator used in electron microscopy is crystalline Ce-doped Yttrium Aluminum Garnet (YAG), a poor electrical conductor that must be coated with a conductive layer on the impact surface to drain the excess charge induced by the very electrons which it must detect. Fortunately, this conductive layer (Al or ITO) can be very thin, on the order of 10-20 nm, and therefore has little effect on electrons with energy greater than 10kV. At low energies, however, absorption in the conductive film becomes a factor. The combined result of lower yield at low energies and the absorption by the conductive layer is that BSE imaging is seldom used below 5kV.

Quantitative Analysis Using Asymmetric Adaptive Pulse Processing

Previous work [1] demonstrated that pulse-by-pulse adaptive digital filtering improves the precision of X-ray quantitative analysis for a given sample electron dose, with no loss of accuracy compared to conventional pulse processing. The improvement stems from better energy resolution compared to short fixed digital filtering for the same throughput. The gain in precision is greatest for small peaks below about 5 keV near to or overlapping with larger peaks, such as Al in NIST K412 glass, which has favorable implications for analysis at low accelerating voltages.