SPI Supplies

Robinson™ Backscattered Electron (BSE) Detector for Scanning Electron Microscopes

Specifications and Product Description

Basic Product Specifications
Explained here are the detectors manufactured by ETP Semra Pty. Ltd., Australia, more commonly known as the Robinson detectors, invented and designed by Dr. Vivian Robinson who remains active in the firm and is still busy designing even more innovative detectors. Others manufacture BSE detectors but no one can claim to use the designs of Dr. Robinson that the world knows as the "Robinson Backscattered Electron Detectors". All of the Robinson designed detectors are know for their long life and the manufacturer offers a generous list of warranty terms and conditions. SPI Supplies has been appointed to serve as the worldwide "website distributor" for the Robinson line of BSE detectors.

Robinson Detector
The Series 8 Robinson detectors are wide-angle backscattered electron (BSE) detectors that rely upon the energy of the BSEs to generate the signal from a high performance phosphor material. They use a new format of phosphor surface to generate and capture a very high photon signal from low energy electrons. This high performance extends from voltage less than 1 kV and increases with accelerating voltage - see Figure below. The phosphor is easy to clean and exhibits long life. It works equally well in high pressure and high vacuum.

Type: Wide-angle scintillator photomultiplier backscattered electron detector.
Thickness: 2.5 mm at beam entry point
Performance Characteristics: See curve S8.6 in below Figure 1
Minimum Accelerating Voltage: Less than 1 KV
Minimum Working Distance: 4 mm
Retraction Distance: 160 mm (or as is required to retract the scintillator out of the chamber

Figure 1: Plot of signal, arbitrary units, vs. accelerating voltage,
all other parameters being equal.

For a comparison with other BSE detectors, see curve S6 in the above Figure 1, which shows the signal obtained from a series 6 Robinson detector. Previous comparisons showed that the Series 6 Robinson detector had a slightly improved signal to noise over all other BSE scintillates. With a detector thickness of 2.5 mm, the S8.6 scintillator response is superior to the S6 response by at least a factor of four. The S8.6 signal appears to be stronger than all other BSE detectors by a factor of at least five times, a significant amount.

Two examples of the features of this detector are given in the accompanying micrographs. Figures 2a and 2b are images of the polished surface of an Al/Fe/Si alloy. Figure 2a clearly illustrates surface detail, thought to be grain structure, detectable at low accelerating voltages, which is not detectable at high voltages.

Figure 2a:Polished Al/Fe/Si allow imaged at 2.4 kV, showing grain
structure on the surface. This is only detected in high resolution
low kV BSE imaging.
      Figure 2b: Same sample as Figure 2a, imaged at 12 kV.
No fine surface detail is detected.

Figures 3a and 3b show "bubbles" on the surface of a polished aluminum alloy specimen at 2 kV, which "bubbles" are not seen at higher accelerating voltages. These "bubbles" appear to be due to a slightly harder component in the alloy, which results in it protruding slightly out of the surface and appearing slightly brighter, higher atomic number. The ability to image these surfaces is one of the great advantages of being able to image at low accelerating voltages in the SEM.

Figure 3a: Polished Al alloy surface imaged at 2 kV.
Note the slightly higher Z "bubbles" on the surface of the specimen.
      Figure 3b: Same sample as Figure 3a, imaged at 8 kV.
Note the absence of the "bubble" structure seen at 2 Kv.

Another huge advantage of this scintillator is that it uses a long life, high signal phosphor. The illumination characteristics of this scintillator are set to give less topographic contrast and more atomic number contrast than the previous Robinson detector scintillates. This can be varied upon request at the time of ordering, no extra charge. For all situations involving such applications as gun shot residue and phase detection studies, it has huge advantages. The long life means that it will essentially last the life of the detector, well over ten years. It is thinner than a solid-state diode detector and has a signal to noise far in excess of those detectors. The customer should never again want for signal to discriminate the different phases necessary for these studies.

Of course at high accelerating voltages, this detector will outperform the secondary electron (SE) detector in terms of signal to noise and resolution of non-edge related detail. Being much less sensitive to charging artifacts, it enables examination of those specimens for which uniform coating is difficult. It ahs the same excellent response in environmental or variable pressure applications. Its high-performance makes it an excellent alternative to the gaseous and environmental SE images obtainable. In these applications, the Series 8 detector is insensitive to pressure and thus very easy to operate.

The detector is available in two different configurations. The S8.6 version uses a 28 mm diameter photomultiplier tube (PMT) with 31.8 mm diameter housing and knob retraction. The S8.5 version uses 19 mm diameter PMT with 22 mm diameter housing and manual or motorized retraction. The S8.5 detectors have only half the signal of the S8.6 detectors (still greater than any other brand of BSE detectors). Retraction distance is sufficient to enable the scintillator to be retracted out of the way when the detector is not being used. The motorized detectors feature immediate stop if it touches anything grounded (earthed) on the way in or out. If it should hit something not grounded (earthed), it has an overload feature, which stops the motor after a few seconds. The in stop position is adjustable. All detectors feature final stop position adjustment of +/- 2 mm in all directions, with accurate stopping at that position.

Signal mixing boards:
In addition to the basic detector, also offered are two electronic signal processing boards, a signal mixing board and a selected phase imaging board. Both boards plug into the base unit of the detector with power supply. They do not operate with the detector only option.

The signal mixing board takes any combination of the SE and BSE images, SE + BSE, SE-BSE, BSE-SE, going from 100% SE to 100% BSE. The features of these are:

SE + BSE: Adds flat surface contrast to the SE image to show greater detail
BSE + SE: Adds edge contrast to the BSE signal to show greater detail and add that extra flare to the BSE otherwise flat surface
BSE - SE: Removes some topography from the BSE image to produce a flatter looking image
SE - BSE: Subtracts atomic number form the SE image to show a surface more representative of pure topography when both are involved.

Hardware specifications:
Operation requires a convenient SE break point, e.g. a BNC cable which can plug into the Robinson module and a socket that can receive the signal out, as well as a convenient BSE input that does not necessarily include the UDT card.

The frequency response of this card is approximately 3 MHZ for 3db signal loss, that is, only slightly lower than the overall frequency response of the BSE detector.

The selected phase imaging module selects signal by intensity and displays that region in one of two ways:
  1. Blacks out the rest of the image, showing only the intensity of the region of interest, that is, only one phase.
  2. Highlights the phase of interest by making it white, while the rest of the image remains normal intensity, well suited for illustrating low atomic number phases.

This enables the best presentation of the detail of interest. The card also features a gate that will turn on or off another feature of interest. It can send a signal that may be used to activate the EDS so that counts are collected from that phase only when the specimen is being scanned.

This selected phase imaging module requires only the standard BSE input. If you wish to use the gate feature, a gate output from the EDS (or other device) and/or a gate input is required. The signal is the usual +5 volts for a digital circuit. In view of noise considerations, a slow scan is required to enable the device to produce high quality low noise images.

Cathodoluminescence (CL) Detector:
The Robinson family of detectors also includes a family of high performance cathodoluminescence detectors. These are similar in design to the BSE detectors with the exception being that the scintillator is replaced with a suitable light guide. For the S8.6 equivalent, a solid light guide plus parabolic mirror is used. An indication of the performance of this detector is given in Figure 4.

Figure 4a: SE semiconductor surface, 5 kV       Figure 4b: CL image of semiconductor surface, 5 kV. Note the high
quality of the image compared to the SE image in Figure 4a.

Note that the signal from the CL image of the sample has a similar signal to noise to the SE image, at an accelerating voltage of 5 kV. We believe this is quite good because the CL image, like the BSE image, depends on the energy of the beam and being able to get a high signal CL image at 5 kV is quite useful in showing just the surface CL detail.

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