Multi Detector System with Ion µSource

The Equipment Turning SEM into Advanced 3D VP/ESEM

  • Fig.1: In-lens 4Q BSE/SE ionizing detector system with Coaxial Ion µSource:  a) basic arrangement in the objective lens hole of VP/E SEM (1 - sample plate, 2 -discharge cup for BSE, 3 - objective lens, 4 - four BSE anode loops, 5 - BSE anode plates, 6 - PLA extension pipe, 7 - BSE anode insulator, 8 - BSE anode leads, 9 - ion source anode, 10 - insulator, 11 - PLA cup);  b) vacuum-detector head for a standard HV SEM (1 - head body, 2 - o-ring, 3 - upper PLA cup, 4 - objective lens, 5 - 4Q BSE/SE detector system from figure 1a, 6 - sample plate, 7 - insulating holder, 8 - electrical throughputs, P1, P2, P3 - pressure in particular sectors, UE - extracting voltage, RP - to a pump). Fig.1: In-lens 4Q BSE/SE ionizing detector system with Coaxial Ion µSource: a) basic arrangement in the objective lens hole of VP/E SEM (1 - sample plate, 2 -discharge cup for BSE, 3 - objective lens, 4 - four BSE anode loops, 5 - BSE anode plates, 6 - PLA extension pipe, 7 - BSE anode insulator, 8 - BSE anode leads, 9 - ion source anode, 10 - insulator, 11 - PLA cup); b) vacuum-detector head for a standard HV SEM (1 - head body, 2 - o-ring, 3 - upper PLA cup, 4 - objective lens, 5 - 4Q BSE/SE detector system from figure 1a, 6 - sample plate, 7 - insulating holder, 8 - electrical throughputs, P1, P2, P3 - pressure in particular sectors, UE - extracting voltage, RP - to a pump).
  • Fig.1: In-lens 4Q BSE/SE ionizing detector system with Coaxial Ion µSource:  a) basic arrangement in the objective lens hole of VP/E SEM (1 - sample plate, 2 -discharge cup for BSE, 3 - objective lens, 4 - four BSE anode loops, 5 - BSE anode plates, 6 - PLA extension pipe, 7 - BSE anode insulator, 8 - BSE anode leads, 9 - ion source anode, 10 - insulator, 11 - PLA cup);  b) vacuum-detector head for a standard HV SEM (1 - head body, 2 - o-ring, 3 - upper PLA cup, 4 - objective lens, 5 - 4Q BSE/SE detector system from figure 1a, 6 - sample plate, 7 - insulating holder, 8 - electrical throughputs, P1, P2, P3 - pressure in particular sectors, UE - extracting voltage, RP - to a pump).
  • Fig. 2: Stem cells as in the head picture and aggregates of cells with a few connections between them (microvilli). (Uo = 15 kV, water 650 Pa, ~1oC, sample after additional washing).
  • Fig. 3: Crater etched by ion bombardment in a bronze foil

The system contains a quadruple ionization detector of backscattered electrons placed in a hole of the objective lens and a pressure limiting aperture extension serving alternatively as an ionization secondary electron detector or the coaxial ion micro-source for sample stripping. The equipment enables environmental techniques and 3D imaging in a classic SEM and may upgrade capabilities of VP/ESEM. This solution (pat. appl. EP2672504 A2) won the Gold Medal at INPEX 2014 in Pittsburgh.

Introduction

Scanning Electron Microscopes (SEM) are vacuum instruments which sets limitations to some properties of the samples. Samples of high vapor pressure like biological samples containing water were particularly restricted. The advent of the Environmental Scanning Electron Microscope (ESEM) removed restrictions concerning hydrated samples [1]. The new instruments preserved a traditional electron optical structure while their vacuum system was enriched and new signal detectors had to be invented to work in a gaseous environment. However the complex nature of various bio-medical samples still calls for new procedures and techniques of imaging which might be promptly satisfied by novel methods and equipment rather than new expensive instruments.

The system described [2,3] is just an equipment which enables environmental techniques and three dimensional imaging (3D) even in a classic high vacuum SEM as it may be combined with a differential pumping unit capable to keep water vapor at pressure exceeding 10 mbar in the sample chamber. The system shown in figure 1 is designed as a detachable equipment for a standard SEM or a Variable Pressure/ Environmental SEM to extend its capabilities without changes in an original structure of the instrument. The equipment was mounted to a JSM-840 microscope to conduct presented experiments.

Method of 3D Imaging
The multi-detector method is based on Lambert‘s angular distribution of backscattered (BSE) or secondary electrons (SE). A system of four directional detectors, placed symmetrically around the specimen (fig.

1), acquires electron signals modulated with local surface inclinations. The term directional means that detectors acquire only electrons emitted in defined directions. This allows reconstruction of surface profiles by integration of relative signal differences of opposite detectors along scanning lines. Output signals of the four quadrants (4Q) of the BSE detector are amplified and digitally stored in the form of four basic images which are processed to obtain a 3D pattern of the surface elevations as a bit-map to be visualized in any desired form. For instance, it may be a perspective view (fig. 2) with level lines and surface profiles shown additionally on side grids (fig. 3). The source of a surface texture overlaying the shape may be a combination of BSE images or advantageously an image taken by the axial SE detector.

The multi-detector method of 3D imaging can be applied advantageously for smooth and weakly modulated surfaces observed frequently in biomedical applications. In this case, wet biological samples should be kept in a stable temperature (ca. 1 °C) and water vapor pressure (ca. 650 Pa) to prevent them from drying or inundation. These conditions can be stabilized routinely not only by a cooled Peltier stage but also by an environmental sample holder which enables loading samples through the transfer chamber.

However VP/E SEM can see only a sample very surface usually covered by a thin film of water or mucus (fig. 2) masking the tiny surface features and an internal structure. Here, the cryo-microscopy which allows imaging of hydrated objects in vacuum conditions may be used but the sample surface must be cleaned off condensates to reveal the truth. In this case an ion beam may be very helpful to wipe away all impurities but also to abrade surface layers and see intestines of cells [4,5]. The coaxial ion micro-source [6] was developed mainly for such purposes.

Structure of the System
The multi-detector system for the 3D imaging has been developed as a part of the attachment to SEM, which contains a directional quadruple BSE ionization detector with an axial SE detector (fig. 1a) and a five channel preamplifier as well as a digital frame-grabber and a PC-based processing unit. The discharge cup of the detector unit (10 mm long and 6 mm in diameter) is sealed in the objective lens hole. In the gap inside the cup four BSE anode loops are disposed symmetrically around the axis to attract environmental electrons produced by BSE in gas. The gas amplification gives a satisfying output signal for water pressures from 100 Pa to 1000 Pa. The PLA extension structure serves alternatively as an ionization SE detector or the coaxial ion µ-source for sample stripping. Advantages coming from application of the system in a VP/E SEM, may also be accessible in a classic SEM. For this purpose the vacuum-detector head (fig. 1b) has been arranged to provide an intermediate vacuum space (P2) necessary to separate a high vacuum column (P3) from the sample chamber with elevated pressure P1. The head thickness of 3 mm allows a low working distance to the sample.

The ion µ-source can operate in two modes: as a glow discharge source and as an electron beam impact source. In both cases the PLA extension takes the role of a discharge chamber filled with gas (e.g. Argon) at pressure below 1000 Pa. In the former mode, the PLA cup is biased negatively against the anode to attain a discharge ignition voltage. The sample plate is biased negatively with a high voltage (-4 kV here) and positioned at a distance of 1-2 mm to PLA, so a strong electric field penetrates the PLA cup and extracts positive ions toward the sample. Secondary electrons generated from the sample by ions are focused and accelerated toward the PLA cup where they take part in an ionizing avalanche. Such a positive feedback adds to a flat ion current distribution of PLA diameter a dense core of a narrow half width (ca. 10 µm at a current of 2 µA) which may be the main tool for micromachining. To obtain the electron impact source the electron beam should operate to produce ions in the discharge chamber. The e-beam may be also used for neutralization of positive charges on the sample surface

Conclusions
The multi-detector unit presented above can enrich a scope of functionalities that are accessible both in standard HV and VP/E SEMs making these instruments more universal in applications. Miniature size of the ionizing detector system shortens electron paths in gas and strengthens electric fields in the discharge space which shifts its 3D imaging capabilities toward extended pressures (ca. 1 kPa). The PLA extension design creates new functions of the unit as a SE or ion signal detector for extended pressures or a coaxial ion micro-source for the sample etching. This list is to be enriched.

Acknowledgements
Electronic part of the equipment was designed by Mgr. Jakub Brach. This work was supported with the budged resources for science as a statutory research project in 2014/16.

References
[1] Debbie Stokes: Principles and Practice of Variable Pressure/Environmental Scanning Electron Microscopy (VP-ESEM), John Wiley & Sons, (2008)
[2] Witold Slówko: European patent application EP2672504 (A2)
[3] Jaroslaw Paluszyński, Witold Slówko: Measurements of the surface microroughness with the scanning electron microscopeJournal of Microscopy 233, 10-17 (2009), Doi 10.1111/j.1365-2818.2008.03090.x
[4] Guillaume Desbois, Janos L. Urai, Fabián Pérez-Willard, Zsolt Radi, Sébastian van Offern, Ivo Burkart, Peter Kukla, Uwe Wollenberg.: Argon broad ion beam tomography in a cryogenic scanning electron microscope: a novel tool for the investigation of representative microstructures in sedimentary rocks containing pore fluidJournal of Microscopy 249, 215-235 (2013) Doi 10.1111/jmi.12011
[5] Christopher Parmenter, Tingting Wang, Kevin Webb: Focused Ion Beam Ablation Tomography: A Novel 3D Imaging Method for Biological Samples, Imaging & Microscopy, 2, p. 30 (2013)
[6] Witold Slówko: Patent application PL411755

Authors
Prof. Dr. Witold Slówko
(corresponding author)
Dr. Artur Wiatrowski
Wrocław University of Technology
Faculty of Microsystem Electronics and Photonics
Wrocław, Poland
www.w12.pwr.wroc.pl/zue

Register now!

The latest information directly via newsletter.

To prevent automated spam submissions leave this field empty.