We describe a magnetic field assisted, two-stage secondary electron gas amplification process for low vacuum scanning electron microscopy. The field of an ultrahigh resolution magnetic immersion objective lens and the electric field of an annular electrode configuration partition the amplification volume into two regions in which the electric and magnetic fields are parallel and crossed, respectively. The fields confine secondary electrons to axial and radial oscillations within the detector volume, until all of the kinetic energy imparted by an anode is dissipated through inelastic collisions with gas molecules. The electron confinement yields high gas amplification efficiency at short working distances and low gas pressures, facilitating high resolution imaging at low electron beam energies. Charging of insulating specimens is stabilized by positive ions produced in the gas ionization cascade. Furthermore, the signal to background level and bandwidth of this detector are superior to those of earlier generations of environmental secondary electron detectors. The combination of low vacuum, short working distance, and low beam energy is attractive to the semiconductor metrology industry, in particular, for critical dimension measurements on photolithographic masks.
Low vacuum scanning electron microscopy technology is an established solution for imaging insulating samples. In addition to high quality secondary electron imaging it offers a means for stabilizing specimen charging.[1] Additionally, the low vacuum environment significantly reduces specimen contamination which can hamper high magnification imaging. However, the technique has hitherto only been applied to SEMs with a conventional pinhole objective lens. This has restricted the available resolution. There are some major challenges to combine this technique with the highest resolution SEMs. This paper describes these challenges and how they can be addressed using a novel structure which allows charge free imaging on the most difficult samples, while retaining very high resolution. Results will be presented from a range of samples.In the highest resolution high vacuum SEM a strong magnetic or electrostatic field is created at the sample in order to reduce the focal length and aberrations of the objective lens. The sample has to be placed very close to the lens (typically within 1-2 mm) to achieve the resolution improvement. Another benefit of these lenses is that the high field of the lens allows the secondary electrons to be extracted from the sample up into the objective lens, above which they can be detected by a high efficiency detector Secondary electron imaging in the current low vacuum SEMs, variable pressure SEMs and ESEMs uses a gas cascade process to amplify the secondary electron signal and generate ions for charge control. An electrostatic field is created between the sample and an anode. This field accelerates the secondary electrons toward the anode and collisions between these electrons and the gas create extra electrons and ionized gas molecules. This process can be controlled to produce a large, stable, amplification and sufficient ions to eliminate negative sample charging.[2] Effective operation of this process requires certain combinations of pressure and electric field strength.[3] The small spacing demanded by the high resolution lens geometry would normally require a high gas pressure to achieve efficient detector operation. This high pressure degrades image quality at low beam voltages due the interaction of the gas with the primary beam.Excess ions generated in the gas cascade can cause positive sample charging which reduces detector efficiency [4] and causes surface potential shifts that degrade X-ray analysis accuracy [5]. The cited paper uses a cathode electrode between the anode and sample to collect excess ions, which conflicts with a need to minimize the space between the sample and the objective lens.
Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.
A novel detector has been developed for imaging of insulating samples at very high magnification. The detector incorporates a dedicated electrode for the control of the ion flow to ensure correct charge control with any type of sample.It is well known that the gas ions in the gaseous detector are used to stabilize electron charging in environmental and variable pressure SEMs during the imaging of insulating samples with higher electron beam voltages. However, the gaseous detector can generate many more ions than are needed for charge control. The excess ions do not cause the same artifacts as electron charging in high vacuum but the excess ions cause the surface of the insulator to acquire a positive voltage which reduces image contrast and causes errors in x-ray analysis [1].Users of the first commercial SEM to incorporate this type of detector (ElectroScan ESEM™) quickly learned that it was easy to image small insulating samples on a metal stub. However, imaging of large insulators was much more difficult unless a conducting path was placed close the imaging area. Copper tape was common solution. The use of ground wires above the sample surface has also been shown to be useful [2]. The ESEM used a high voltage anode directly above, and close to the sample.A novel detector has been developed to work with magnetic immersion lens for ultra high resolution imaging of insulators [3]. The basic detector is shown in Fig 1. An aperture inside the lens restricts the flow of chamber gas so that the electron column is in high vacuum. The signal collection anode is placed directly between the sample and the lens. Secondary electrons are accelerated by the bias on the anode and follow a complex helical path in the fields from the lens and the anode [3]. The long electron trajectories generate very high gain [4] even though the sample is close to the lens for high resolution imaging. The high gain also creates a large number of excess ions. Imaging of samples larger than 1-2mm is difficult with the basic structure of Fig 1. An enhanced detector is shown in Fig 2. Another electrode, the ion trap, is added between the sample and the anode. Since the ion energy is low (due to collisions with neutral gas molecules) only a small change in sample voltage is required to divert excess ions to the ion trap. The ion trap is normally at ground potential. The Helix detector is very compact, even with the ion trap, allowing imaging at less than 3mm working distance.A simple sample was used to characterize the imaging characteristics of the two detector configurations. This sample consisted of thin PTFE (Teflon) tape covering half of a metal stub. The tape was imaged at various positions relative to the exposed metal stub. Fig 3 shows the variation of image intensity as a function of position (with constant detector settings for each curve). The overall gain for each configuration is different so the data have been normalized. The curves are plotted
A high gain and bandwidth, low noise amplifier has been developed for improved imaging in environmental and variable pressure SEM. The calibrated, precision amplifier is also being used in quantitative studies of the physics of gaseous electron detectors.
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