Quantification of electron-ion recombination in an electron-beam-irradiated gas capacitor

Toth, Milos; Daniels, D R; Thiel, B. L.; Donald, Athene M.

doi:10.1088/0022-3727/35/14/322

Cited by 32 publications

(18 citation statements)

References 38 publications

(132 reference statements)

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“…Experimental and theoretical discussions into gaseous electron‐ion recombination or SE‐ion recombination can be found in standard texts on ionized gases such as von Engel (1965), and Nasser (1971) as well as review articles such as Hahn (1997) and Seaton & Storey (1976). Experimental studies of steady state gaseous electron‐ion recombination relevant to low vacuum SEM has been recently performed by Toth et al . (2002a,c,d).…”

Section: Theorymentioning

confidence: 99%

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

Morgan

Phillips

2006

Journal of Microscopy

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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.

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Section: Theorymentioning

confidence: 99%

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

Morgan

Phillips

2006

Journal of Microscopy

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“…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.…”

Section: A Electronic Gas Cascade Amplificationmentioning

confidence: 99%

Gaseous scintillation detection and amplification in variable pressure scanning electron microscopy

Morgan

Phillips

2006

Journal of Applied Physics

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Absolute charge calibration of scintillating screens for relativistic electron detection Rev. Sci. Instrum. 81, 033301 (2010); 10.1063/1.3310275Silicon photodiodes for low-voltage electron detection in scanning electron microscopy and electron beam lithography J.This work investigates the generation and detection of gaseous scintillation signals produced in variable pressure scanning electron microscopy through electron-gas molecule excitation reactions. Here a gaseous scintillation detection ͑GSD͒ system is developed to efficiently detect photons produced via excitation reactions in electron cascades. Images acquired using GSD are compared to those obtained using conventional gaseous secondary electron detection ͑GSED͒ and demonstrate that images rich in secondary electron ͑SE͒ contrast can be achieved using the gaseous scintillation signal. A theoretical model, based on existing Townsend theories, is developed. It describes the production and amplification of photon signals generated by cascading SEs, high energy backscattered electrons, and primary beam electrons. Photon amplification ͑the total number of photons produced per sample emissive electron͒ is then investigated and compared to conventional electronic amplification over a wide range of microscope operating parameters, scintillating imaging gases, and photon collection geometries. These studies revealed that argon gas exhibited the largest GSD gain, followed by nitrogen then water vapor, exactly opposite to the trend observed for electron amplification data. It was also found that detected scintillation signals exhibit larger SE signal-to-background levels compared to those of conventional electronic signals detected via GSED. Finally, dragging the electron cascade towards the light pipe assemblage of GSD systems, or electrostatic focusing, dramatically increases the collection efficiency of photons.

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“…The dominant factor will be a function of the exact conditions; (ii) the energy dependence of the scavenging mechanism might give rise to a primitive form of energy filtered contrast; (iii) interpretation of specimen topography from SE images is less straightforward; 23,24 and (iv) the gas cascade is not necessarily a linear amplifier, with a damping effect dependent on the ion production rate. 25 Finally, the neutral gas molecules have a significant influence on images. This is another area where modeling has already achieved some success.…”

Section: Modeling Of Imaging Processes In the Low-vacuum Semmentioning

confidence: 99%

Modeling of imaging processes in the low‐vacuum SEM

Thiel

2005

Surface & Interface Analysis

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The presence of a gas in the specimen chamber of low-vacuum scanning electron microscopes (SEMs) gives rise to a number of interactions that are not found in their high-vacuum counterparts. Many of these interactions are integral to the specimen charge neutralizing capabilities of these instruments. However, these interactions, and the electronic charge state of the specimen, give rise to a number of processes that influence, and sometimes even dominate, secondary electron contrast. As many of the processes are stochastic in nature, and depend on discrete interactions between electrons, gaseous ions, neutral molecules, and the specimen surface, modeling can offer important insights for contrast interpretation. This paper discusses these interactions and the mechanisms through which contrast is influenced. Copyright MODELING OF IMAGING PROCESSES IN THE LOW-VACUUM SEMLow-vacuum scanning electron microscopy is emerging as a powerful new technique for the study of poorly conducting materials and other difficult samples.1 -3 Because of these attributes, it is also being considered as a candidate for the next generation of sub-10 nm critical dimension metrology tools for the semiconductor manufacturing industry. 4,5 Widespread acceptance of the technique is hindered because the image formation process is considerably more complicated than in high-vacuum scanning electron microscopes (SEMs). Interactions between electrons and gas molecules, as well as the behavior of positive gaseous ions, all influence the appearance and information content of secondary electron (SE) images. Furthermore, SE images obtained from dielectric materials appear to contain more information than their high-vacuum counterparts, related to the local dielectric properties of the material. 3,6 Considerable work needs to be done before it will be possible to make routine interpretations of the SE contrast. Many of the processes that influence the information content of SE images have complicated nonlinear interdependencies, which make simple analytic descriptions unlikely. The situation is exacerbated by the commercial introduction of detectors with increasingly complicated physical and electromagnetic field geometries. However, since many of the processes are stochastic in nature, computer modeling can offer insights. The processes that contribute to image formation can be divided into three categories: cascade amplification/ion production, intrinsic SE emission from dielectrics, and modulation of SE emissions by the gaseous ions. Most commercial low-vacuum SEMs use some degree of gas cascade amplification to amplify the secondary electron emission current, regardless of the form of the signal that is actually measured. Another critical function of the cascade is to produce a flux of positive ions sufficient to stabilize electronic charging of insulating samples. In its simplest form, the gas cascade is manipulated by placing a positively biased anode (typically a few hundred volts positive) in the vicinity of the specimen. Secondary electrons ...

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Quantification of electron-ion recombination in an electron-beam-irradiated gas capacitor

Cited by 32 publications

References 38 publications

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

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

Gaseous scintillation detection and amplification in variable pressure scanning electron microscopy

Modeling of imaging processes in the low‐vacuum SEM

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