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MEMS made Electron Emission Membranes

Final Report Summary - MEMBRANE (MEMS made Electron Emission Membranes)

We have successfully developed a new device called ‘tynode’, as successor of the well-known ‘dynode’, which is still widely used, eighty years after its invention.

Essentially, a tynode is a very thin layer of material, placed in vacuum. An electron, impinging on one surface, causes the emission of a multiple of about 5 electrons at the other surface. A stack of 5 of these ‘transmission dynodes’ tynodes produces 5^5 = 3 k electrons, appearing at the bottom of the last tynode, after the arrival of a single electron onto the top tynode. This charge signal can activate circuitry in the pixel of a CMOS pixel chip, recording the timing of the signal, and the signal amplitude. Thanks to the straight, small and identical electron trajectories the rise time of the charge signal is in the order of a few pico-seconds, enabling a superb time resolution of this electron detector. The spatial resolution can be as good as the granularity of the pixel chip, and the amplification-by multiplication of the tynode stack is free of noise. This single electron detector has therefore zero dark current noise, outperforming state-of the-art Si-photomultipliers.

For this project, the measurement of the secondary electron yield of specific surfaces was essential. We have developed a multi-electrode system which, when placed in the e-beam of a SEM microscope, can characterise all relevant parameters of a tynode. This includes the measurement of charge-up effects: the replenishing of electrons is essential for thin film tynodes.

Starting off with Si-Nitride as thin-layer material, we found the highest secondary electron yield using Atomic Layer Deposition ALD MgO. Parallel to this we performed Monte Carlo simulations yielding knowledge on the interaction of low energy electrons with specific matter. We found good agreement between the results of the simulations and the yields measured in the SEM set-up. Our routines are made available for the GEANT-4 community.

The experience with Atomic Layer Deposition ALD has resulted in a new project with the development of a photocathode with a high quantum efficiency QE as goal. This value has approached asymptotically the value of 45 %. We propose an ALD-made active layer with much higher QE.

At present, the development of prototype TipsyZero is under way in a joint effort with Photonis. A stack of tynodes, placed on top of the TimePix 1 pixel chip, is inserted in a Photonis Planacon detector. In the near future a new pixel chip with time resolution of 20 ps or better, will be required for the ultimate Tipsy detector.