Ultra-thin mems fabricated tynodes for electron multiplication

Research output: ThesisDissertation (TU Delft)

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Abstract

For decades, photomultiplier tubes (PMTs) have been the most common choice in single photon detection, covering the spectral range from deep-ultraviolet to nearinfrared. PMT is a vacuum tube with three crucial components: photocathode, chain of dynodes and anode. At the photocathode, photons are converted to electrons in a photoelectric effect, after which they are directed to the dynodes chain. The material and geometry of dynodes are chosen to efficiently amplify the charge through the secondary electron emission (in reflection mode). Finally, created avalanche of electrons is collected and measured by the anode. Timed Photon Counter (TiPC) is a novel vacuum-based photomultiplier proposed to overcome limitations of PMTs in terms of size, speed, spatial resolution and operation in the presence of magnetic field. The key novelty of TiPC is a tynode – a large-size array of ultra-thin, free-standing membranes which, in contrast to dynodes, multiply electrons in the transmission mode. Due to the short and straight crossing paths of electrons between subsequent tynodes, the time resolution of the TiPC can be in the order of 10 -12 s. The set of tynodes is placed under the photocathode, and on top of a CMOS detecting chip. With such design, TiPC represents a light, compact and ultra-fast photodetecting device with a high relevance for solid state, atomic and molecular physics experiments, medical imaging and 3D optical imaging. The focus of this thesis is microelectromechanical systems (MEMS) fabrication of the tynodes. To our knowledge, this is the first time MEMS technology is employed as a powerful tool for the production of large arrays of free-standing membranes, with thicknesses of only a few nanometers, to be used in photodetection. Detailed analysis in terms of mechanical, optical, electrical and structural properties were performed in order to discern the most suitable material for the TiPC application among the investigated candidates. The transmission SEY (TSEY) of the released tynodes is analysed with a dedicated setup, specifically developed in our group, inserted in a scanning electron microscope (SEM). Low pressure chemical vapour deposition (LPCVD) was employed as a technique to grow silicon nitride (SiN) tynodes with varied layout, elemental stoichiometry and thicknesses in the range from 25 to 40 nm. Due to its inability to produce good-quality films with thicknesses lower than 20 nm, LPCVD was replaced by atomic layer deposition (ALD). It was found that SiN performs poorly in terms of secondary electron emission (SEE), and we selected Al2O3 (alumina) as the next tynode material. The ALD of alumina is investigated in the temperature range from 300 down to 100 °C, with the goal to determine its viability in the coating of temperature-sensitive substrates such as photoresist. We demonstrated the fabrication of 5 – 25 nm-thick ALD alumina tynodes which exhibited moderately high TSEY. Apart from SiN and alumina, other materials subjected to SEE analysis in this work were: chemical vapour deposited (CVD) ultrananocrystalline diamond (UNCD), monocrystalline silicon and LPCVD silicon carbide (SiC). Applying atomic layer deposited magnesium oxide (MgO) as the tynode material resulted in a transmission secondary electron yield (TSEY) of up to 5.5, by which it proved to be the most efficient electron multiplier among materials taken into account in this work. During the fabrication of tynodes, SEE films were exposed to different MEMS processing steps, and thus inevitably undewent a surface modification which alters the SEE properties. On that account, we conducted a study on the ALD MgO films subjected to various chemical and thermal treatments and explored the methods to further enhance their SEE. For the final application in the TiPC, stacked tynodes should provide the focusing of electrons. To meet this requirement, the emission film was grown on a pre-patterned substrate, which enabled hemi-spherical shape of the released membranes. Finally, for the vertical stacking and alignment of the tynodes, steps for the formation of V-grooves were added in the standard fabrication flowchart.
Original languageEnglish
Awarding Institution
  • Delft University of Technology
Supervisors/Advisors
  • Sarro, P.M., Supervisor
  • van der Graaf, H., Supervisor
Award date19 Nov 2019
Print ISBNs978-94-6384-085-9
DOIs
Publication statusPublished - 2019

Keywords

  • tynodes
  • ultra-thin membrames
  • timed-photon counter
  • secondary electron emission
  • atomic layer deposition

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