The Merlin for EM Hybrid Pixel Detector (HPD) is an advanced detector development in the field of Electron Microscopy, combining direct detection of electrons and rapid readout in a pixelated format ideal for applications such as 4D STEM and TEM dynamic imaging. Each sensor pixel is individually bump-bonded to an intelligent chip which uses threshold discriminators to distinguish electrons from the background, effectively eliminating all readout noise. This allows for integral mode imaging where multiple short exposure images are acquired and summed together. Uniquely, neighbouring pixels can communicate to mitigate charge-sharing effects, and this, combined with the direct detection of electrons, yields enhanced performance. As beam energies decrease toward 60 keV, the Merlin for EM has been shown to provide near-ideal DQE and MTF detector response.
Suitable Applications include:
- Electron Diffraction
- 4D STEM
- Pump and probe experiments
- Direct Detection Noise-less detection of single electron events
- Dynamic Range Up to 24-bit counting depth enabling 1:16.7 million intensity range in a single image, ideal for recording diffraction patterns.
- Effectively Noise Free 2 threshold discriminators in each pixel means zero read noise and dark current.
- Charge Summing Mode Communication between pixels designed to mitigate charge sharing effects for maximising both DQE and MTF.
- Rapid Readout Kilohertz frame rates in continuous mode with zero deadtime offers more experimental flexibility than ever before, minimising effects such as sample drift, and enabling single shot and “pump and probe” dynamic experiments.
- Wide energy range and radiation tolerance Minimum 20 keV threshold making low energy EM imaging possible, and radiation tolerant design to 300 keV.
- No requirement for a beamstop – radiation tolerant design means no need for a beamstop in diffraction experiments.
- Mount Static and retractable mounts available to fit many electron microscopes.
The various acquisition modes, as well as many other input parameters for the optimisation of the MERLIN system, are easily chosen by a user friendly Graphical Interface as well as remotely controlled via TCP/IP protocol.
Application notes coming soon.
- What is hybrid pixel detector technology?
MERLIN is a new type of technology in the field of electron microscopy. It is a detector based on a hybrid pixel architecture. The detector assembly consists of a thick, highly resistive semiconductor sensor coupled to a Medipix3 chip. Incoming radiation generates charge in the sensor which diffuses under an applied bias to the CMOS circuitry of the individual pixels (via an array of micro-bump bonds). Each pixel contains >1100 transistors (within the 55 micrometer pitch), enabling on-chip counting of incident electrons and enhanced operation modes such as Charge Summing Mode (more about this later).
The counting process consists of analogue comparison of the collected charge to a user selected energy threshold, and subsequent digital counting at 1 MHz if the threshold is exceeded. Thus, since the data readout relating the number of electrons counted by each pixel is digital, the MERLIN detector operates free of readout noise. This is a feature unique to hybrid pixel technology and strongly differentiates it from analogue integrating detectors, such as CCD technology.
Counting detectors are known to offer highest imaging performance in terms of modulation transfer function (MTF) and detective quantum efficiency (DQE). The MERLIN detector has been shown combine ideal TEM performance at the low energies needed to study 2D materials such as graphene for 60keV electrons 1 with 1000’s per second frame rates.
- What is Charge Summing Mode?
When an electron strikes near the edge of a pixel (illustrated left), the resulting charge may leak into neighbouring pixels, thus reducing the MTF. MERLIN has a unique capability where information is shared between adjoining clusters of four pixels, in what is known at the Charge Summing Mode. When charge spread occurs over the four adjoining pixels, MERLIN recognizes that this information belongs to one event on one pixel rather than up to 4 weaker events on 4 pixels. By reconstructing the event using the diffused charge data, MERLIN increases the resolution and ensures an accurate reading of the event giving improved results quality.
- Does the MERLIN detector bin data?
MERLIN provides high versatility with a variety of intrinsically fast (due to highly parallelized digital readout) large dynamic range acquisition options, namely: 14,400 fps@1 bit depth, 2,400 fps @ 6 bit depth, 1,200fps @12 bit and 600fps@24 bit depth. These readout modes (unlike the speed-up strategies employed with CCD technology) are unbinned and therefore imply no reduction of pixel resolution or field of view. Moreover, due to the electron counting approach of the detection system as well as the fully digital readout, MERLIN adds zero noise allowing a Signal to Noise Ratio (SNR) as high as a 16.7 million:1.
- Is it true there is no dead time with MERLIN’s data collection?
Yes! The advanced pixel architecture implements two readout counters per pixel and provides a continuous read/write acquisition mode with zero detector dead time (CCD technologies rely on non active detector frame store areas to reduce detector dead time). The various acquisition modes, as well as many other input parameters for the optimisation of the MERLIN system, are easily chosen by a user friendly Graphical Interface as well as remotely controlled via TCP/IP protocol.
- How easy is it to install the MERLIN detector?
The MERLIN system is really “Plug and Play”, with the detector simply connected by one or two cables, depending on the type of installation (static or retractable). The Medipix3 chip has a very low (<1 Watt) power consumption, requiring minimal cooling and no need for connection to microscope water supplies or to pneumatics. The readout electronics are connected to the detector head via a 10 meter cable, thus giving ultimate flexibility. Therefore, MERLIN installation is rapid and designed not to impact on other microscope services.
- Ultramicroscopy 182 (2017) 44–53: “Characterisation of the Medipix3 detector for 60 and 80 keV electrons”
J.A. Mir a , R. Clough a , R. MacInnes c , C. Gough c , R. Plackett b , I. Shipsey b , H. Sawada a , e , f , I. MacLaren c , R. Ballabriga d , D. Maneuski c , V. O’Shea c , D. McGrouther c , ∗, A.I. Kirkland a , e
a University of Oxford, Department of Materials, Parks Road, Oxford OX1 3PH, United Kingdom b University of Oxford, Department of Physics, Parks Road, Oxford OX1 3PH, United Kingdom c University of Glasgow, School of Physics and Astronomy, Glasgow G12 8QQ, United Kingdom d CERN, 1211 Geneva 23, Geneva, Switzerland e Electron Physical Sciences Imaging Centre, Diamond Light Source Ltd, Harwell Science & Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom f JEOL UK Ltd. JEOL House, Silvercourt, Watchmead, Welwyn garden City, Herts AL71LT, United Kingdom
- Ultramicroscopy:165(2016)42–50 “Pixelated detectors and improved efficiency for magnetic imaging in STEM differential phase contrast”
Matus Krajnak, Damien McGrouther, Dzmitry Maneuski, Val O’ Shea, Stephen McVitie Scottish Universities Physics Alliance, School of Physics and Astronomy, University of Glasgow, Glasgow G128QQ, United Kingdom
- Nuclear Inst. and Methods in Physics Research, A: “Direct imaging detectors for electron microscopy” A.R. Faruqi *, G. McMullan
MRC Laboratory of Molecular Biology, Cambridge CB2 0QH, UK