Transient analysis of gaseous electron‐ion recombination in the environmental scanning electron microscope

Abstract

SummaryMost of the work carried out in relation to contrast mechanisms and signal formation in an environmental scanning electron microscope has yet to consider the time dependent aspects of image generation at a quantitative level. This paper quantitatively describes gaseous electron-ion recombination (also known as 'signal scavenging') in an environmental scanning electron microscope at a transient level by utilizing the dark shadows/streaks seen in gaseous secondary electron detector images of alumina (Al 2 O 3 ) immediately after a region of enhanced secondary electron emission is encountered by a scanning electron beam. The investigation firstly derives a theoretical model of gaseous electron-ion recombination that takes into consideration transients caused by the time constant of the gaseous secondary electron detector electronics and external circuitry used to generate images. Experimental data of pixel intensity versus time of the streaks are then simulated using the model enabling the relative magnitudes of (i) ionization and recombination rates, (ii) recombination coefficients and (iii) electron drift velocities, as well as absolute values of the total time constant of the gaseous secondary electron detection system and external circuitry, to be determined as a function of microscope operating parameters such as gaseous secondary electron detector bias, sampleelectrode separation, imaging gas pressure, and scan speed. The results revealed, for the first time, the exact dependence that the effects of secondary electron-ion recombination on signal formation has on reduced electric field and time in an environmental scanning electron microscope. Furthermore, the model implicitly demonstrated that signal loss as a consequence of field retardation due to ion space charges, although obviously present, is not the foremost phenomenon causing streaking in images, as previously thought.

Immunogold labelling in environmental scanning electron microscopy: applicative features for complementary cytological interpretation

“…Another question arises about the best choice of gas to be used in ESEM conditions (gaseous electron‐ion) which should be the most appropriate to work in low vacuum, partly because a better signal is obtained when working with low accelerating voltage, in order not to damage the biological sample, especially when observing thin layer cells and unicellular material (Morgan & Phillips, 2006; Danilatos, 2009).…”

Gaseous scintillation detection and amplification in variable pressure scanning electron microscopy

“…Inelastic collisions between the cascading SEs and gas molecules are capable of producing ionization, which generates PIs and additional continuum electrons commonly referred to as environmental SEs ͑ESEs͒. 33,34,50,51 As a consequence of their low mobility and recombination lifetime compared to that of electrons, PIs are also capable of producing a positive ion space charge, which can impede E generated by V GSED , affecting cascade amplification and consequently, image contrast. 29,48 High energy PEs and backscattered electrons ͑BSEs͒ are also capable of ionizing collisions with gas molecules and subsequent cascade amplification of the ESEs produced.…”

“…The SE signals are amplified and detected using a positively biased metallic ring ͑or wire͒ connected to an electronic amplifier that is placed above or near a grounded stage. 34 Image formation in a VPSEM can be achieved not only by detecting electron signals induced in grounded or biased electrodes, but also via the detection of ultraviolet ͑UV͒, visible ͑vis͒, and infrared ͑IR͒ photons generated in the gas through the excitation of atoms and/or molecules by free electrons, known as gaseous scintillation. Electron detection can be performed by extracting induced signals not only from the biased electrode itself but also from the stage or sample itself grounded through a specimen current amplifier, commonly referred to as specimen current, 1,20,26-28 ion current, 2,[29][30][31][32][33] or induced stage current ͑ISC͒ imaging.…”

“…25,37 ͑ii͒ High gainϫ bandwidth productsphotomultiplier tubes ͑PMTs͒ used to detect photon signals are capable of operating at high gains ͑Ϸ10 6 ͒ ͑Ref. Consequently, spurious effects of space charge formation or electron-ion recombination such as contrast reversal, 31,32 darkening at topographical, 33 and charged 2,3,34 asperities, and shadowing in images 31,34 have the possibility of being reduced or even alleviated via scintillation detection. The maximum bandwidth of a typical electronic amplifier used to detect induced signals in a VPSEM is 3.5 MHz.…”

Scintillation SE detector for variable pressure scanning electron microscopes

“…For the detection of SEs in the VPSEM, several new detectors have been developed since the first experiments with the gaseous environment surrounding specimens were published (Robinson, 1978). These detectors can be divided into two groups: (i) the detectors that take advantage of a cascade amplification of signal electrons in the gaseous environment in an electrostatic field at the electric field strength typically from tens to hundreds of V/mm (Danilatos, 1990; Fletcher et al ., 1999; Morgan & Phillips, 2006a; Thiel et al , 2006) and (ii) the scintillation detectors.…”