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.

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.

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.

There is no dead time in data collection! 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.

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.