The Realization & Characterization of Transmission Dynodes

Abstract

The Timed Photon Counter (TiPC) is a novel vacuum electron multiplier that is based on the operating principles of a PhotoMultiplier Tube (PMT), but outperforms it in terms of time and spatial resolution. The core innovation is the transmission dynode (tynode), which emits secondary electrons (SEs) in reflection as well as in transmission. This distinctive property allows tynodes to be closely stacked on top of each other, which results in a compact planar photodetector. The shorter and more uniform electron paths result in a better timing performance compared to PMTs. In addition, spatial information is gained by using a TimePix chip as read-out. A requirement for the tynodes in TiPC is to achieve a (total) transmission electron yield (TEY) of 4 or higher for sub-1-keV primary electrons (PEs).
The goal of this work is to characterize the secondary electron emission (SEE) properties of tynodes that are fabricated using Micro-ElectroMechanical System (MEMS)- based) techniques. The thinness of the membranes, which is in the order of tens of nm, poses challenges in both the fabrication as well as in the characterization of the tynodes. For the latter, a new method has been developed to measure the SEE properties of tynodes within a scanning electron microscope using a (hemispherical) collector assembly mounted to the moving stage. This method allows the tynodes to be inspected before a TEY measurement, so that the electron beam only targets the active surface of the tynode. For the former, different tynode designs have been used to construct mechanically strong, but ultra-thin membranes. In this work, three different types are presented: single-membrane, square-array and corrugated membrane.
The first type is a single-membrane tynode, which is designed to determine the effect of film thickness on the transmission SEE. A tynode is in essence an ultra-thin membrane that consists of a SEE layer and a conductive layer. In this case, Atomic Layer Deposition (ALD) aluminum oxide (Al2O3) is the SEE material and titanium nitride (TiN) is the conductive material. Two types of membranes have been fabricated: a bi-layered and a tri-layered. The former consists of an Al2O3 membrane on which TiN was sputtered at the end of the fabrication process, while for the latter, the TiN was sputtered in an earlier stage in the fabrication process. By encapsulating the conductive layer, the reliability of the fabrication process improved.
The TEY curves as a function of the PE energy of these membranes have been mea- sured for 0.3 to 10 keV for three different thicknesses. The Al2O3/TiN/Al2O3 membrane with layer thicknesses of 5/2.5/5 nm, respectively, has the highest maximum TEY of 3.1, which is obtained with PEs with an energy of 1.55 keV. For increasing film thickness, the onset of transmission SEE occurs at higher PE energy and the maximum TEY is lower. The role of film thickness on the SEE properties has been further investigated by separating the fast and slow electrons.
The second type is an ALD TiN/magnesium oxide (MgO) square-array tynode, which is designed to be compatible with a TimePix chip. The tynode consists of ultra-thin circualar membranes within a supporting frame. The pixelated design ensures that the ultra- thin membranes can be released reliably during fabrication. They have a diameter of 30 μm and are arranged in a 64 × 64 array with a square pitch of 55 μm, which matches the pixel pad pitch on a TimePix chip.
The transmission electron emission of these TiN/MgO tynodes have been investigated with a planar collector assembly within a scanning electron microscope (SEM). The collector assembly is modified to investigate the effect of a strong electric field near the emission surface of a tynode. The electric field strength is in the same order of mag- nitude as in TiPC. A surface scan method has been used to determine the TEY of individual membranes in the array; the TEY map corresponds with the SEM image. PEs with energies of 0.75 to 5 keV were used. The averaged maximum TEY for a TiN/MgO tynode with layer thicknesses 2/5 nm is 4.6 ± 0.2, which is obtained with PEs with an energy of 1.35 keV. The averaged maximum TEY improves from 4.6 ± 0.2 to 5.0 ± 0.3 when the bias voltage increases from −50 V to −100 V. However, the effect levels off when the bias voltage is further increased.
The third type is a corrugated membrane tynode, which is designed to have a larger active surface. These large-area tynodes are fabricated by depositing an ultra-thin continuous film on a silicon wafer with a 3-dimensional pattern. After removing the silicon, a corrugated membrane with enhanced mechanical properties is formed. The ultra-thin film consists of Al2O3/TiN/Al2O3 deposited by ALD. A corrugated membrane can span a larger surface area in comparison with a flat membrane, while retaining an active sur- face close to 100%. In addition, the octagonal cups have a focusing effect, which can be used to direct transmission secondary electrons (TSEs) onto the pixel pads of a TimePix chip. Both improve the collection efficiency of TiPC, since reabsorption of TSEs within the tynodes stack will be less likely.
The TEYs of these corrugated metamaterial membranes are measured within a SEM using the hemispherical collector assembly for PEs with energies ranging from 0.3 to 10 keV. The surface scan method is used to construct a yield map, which shows that transmission electron multiplication occurs on the entire surface regardless of the features on the corrugated membrane. The TEY does vary, but remains larger than 1. An average maximum yield of 2.15 has been measured by using PEs with an energy of 3.15 keV. This is slightly lower compared to the flat single-membrane with a similar thickness.
This work shows that the film thickness determines the transmission SEE properties of an ultra-thin membrane. TiN/MgO membranes with a thickness of 2.5/5 nm fulfill the requirements for tynodes that will be used in TiPC detectors. The square-array tynodes can be reliably fabricated due to its pixelated design with the small circular membranes, which are compatible with a TimePix chip. If a larger membrane is preferred, then the surface area can be enlarged by two orders of magnitude by using a corrugated membrane.