arXiv subjects

H. W. Chan

Publications and source records attributed to H. W. Chan.

6 recordsLinked to original sources

arXiv2024

Ultra-thin corrugated metamaterial film as large-area transmission dynode

Large-area transmission dynodes were 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 was formed. Mechanical materials, such as this corrugated membrane, are engineered to improve its strength and robustness, which allows it to span a larger surface in comparison to flat membranes while the film thickness remains constant. The ultra-thin film consists of three layers (Al$_2$O$_3$ /TiN/Al$_2$O$_3$) and is deposited by atomic layer deposition (ALD). The encapsulated TiN layer provides in-plane conductivity, which is needed to sustain secondary electron emission. Two types of corrugated membranes were fabricated: a hexagonal honeycomb and an octagonal pattern. The latter was designed to match the square pitch of a CMOS pixel chip. The transmission secondary electron yield was determined with a collector-based method using a scanning electron microscope. The highest transmission electron yield was measured on a membrane with an octagonal pattern. A yield of 2.15 was achieved for 3.15 keV incident electrons for an Al$_2$O$_3$ /TiN/Al$_2$O$_3$ tri-layer film with layer thicknesses of 10/5/15 nm. The variation in yield across the surface of the corrugated membrane was determined by constructing a yield map. The active surface for transmission secondary electron emission is near 100%, i.e. a primary electron generates transmission secondary electrons regardless of the point of impact on the corrugated membrane.

physics.ins-det↗

arXiv2024

The construction and characterization of MgO transmission dynodes

In this work we demonstrate that ultra-thin (5 and 15 nm) MgO transmission dynodes (tynodes) with sufficient high transmission electron yield (TEY) can be constructed. These tynodes act as electron amplification stages in a novel vacuum electron multiplier: the Timed Photon Counter (TiPC). The ultra-thin membranes with a diameter of 30 μm are arranged in a square 64-by-64-array. The TEY was determined with a scanning electron microscope (SEM) using primary electrons with primary energies of 0.75 - 5 keV. The method allow us to make a TEY map of the surface while simultaneously imaging the surface. The TEY of individual membranes can be extracted from the TEY map. An averaged maximum TEY of 4.6 +/- 0.2 was achieved by using 1.35 keV primary electrons on a TiN/MgO bi-layer membrane with a layer thickness of 2 and 5 nm, respectively. The TiN/MgO membrane with a layer thickness of 2 and 15 nm, respectively, has a maximum TEY of 3.3 +/- 0.1 (2.35 keV). Furthermore, the effect of the electric field strength on transmission (secondary) electron emission was investigated by placing the emission surface of a tynode in close proximity to a planar collector. By increasing the electric potential between the tynode and the collector, from -50 V to -100 V, the averaged maximum TEY improved from 4.6 +/- 0.2 to 5.0 +/- 0.3 at a primary energy of 1.35 keV with an upper limit of 5.5 on one of the membranes.

physics.ins-det↗

arXiv2024

Improving Rehabilitative Assessment with Statistical and Shape Preserving Surrogate Data and Singular Spectrum Analysis

Time series data are collected in temporal order and are widely used to train systems for prediction, modeling and classification to name a few. These systems require large amounts of data to improve generalization and prevent over-fitting. However there is a comparative lack of time series data due to operational constraints. This situation is alleviated by synthesizing data which have a suitable spread of features yet retain the distinctive features of the original data. These would be its basic statistical properties and overall shape which are important for short time series such as in rehabilitative applications or in quickly changing portions of lengthy data. In our earlier work synthesized surrogate time series were used to augment rehabilitative data. This gave good results in classification but the resulting waveforms did not preserve the original signal shape. To remedy this, we use singular spectrum analysis (SSA) to separate a signal into trends and cycles to describe the shape of the signal and low level components. In a novel way we subject the low level component to randomizing processes then recombine this with the original trend and cycle components to form a synthetic time series. We compare our approach with other methods, using statistical and shape measures and demonstrate its effectiveness in classification.

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arXiv2024

Fidelitous Augmentation of Human Accelerometric Data for Deep Learning

Time series (TS) data have consistently been in short supply, yet their demand remains high for training systems in prediction, modeling, classification, and various other applications. Synthesis can serve to expand the sample population, yet it is crucial to maintain the statistical characteristics between the synthesized and the original TS : this ensures consistent sampling of data for both training and testing purposes. However the time domain features of the data may not be maintained. This motivates for our work, the objective which is to preserve the following features in a synthesized TS: its fundamental statistical characteristics and important time domain features like its general shape and prominent transients. In a novel way, we first isolate important TS features into various components using a spectrogram and singular spectrum analysis. The residual signal is then randomized in a way that preserves its statistical properties. These components are then recombined for the synthetic time series. Using accelerometer data in a clinical setting, we use statistical and shape measures to compare our method to others. We show it has higher fidelity to the original signal features, has good diversity and performs better data classification in a deep learning application.

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arXiv2021

Secondary Electron Emission from Multi-layered TiN/Al$_2$O$_3$ Transmission Dynodes

The (secondary) electron emission from multilayered Al$_2$O$_3$/TiN membranes has been investigated with a hemispherical collector system in a scanning electron microscope for electrons with energies between 0.3 and 10 keV. These ultra-thin membranes are designed to function as transmission dynodes in novel vacuum electron multipliers. Two different types, a bi-layer and a tri-layer, have been manufactured by means of atomic-layer deposition (ALD) of aluminum oxide and sputtering of titanium nitride. The reflection and transmission electron yield ($σ_R$, $σ_T$) have been measured for both types of membranes. In comparison, the tri-layer membranes outperformed the bi-layer membranes in terms of transmission electron yield for films with the same effective thickness. The highest transmission electron yield was measured on an Al$_2$O$_3$/TiN/Al$_2$O$_3$ film with layer thicknesses of 5/2.5/5 nm, which had a maximum transmission electron yield $σ_\text{T}^\text{max}(E_0)$ of 3.1 (1.55 keV). Furthermore, the bi-layer membranes have been investigated more in-depth by performing an additional measurement using a positive sample bias to separate the transmitted fraction $η_T$ and the transmission secondary electron yield $δ_T$. The transmitted fraction was used to determine the transmission parameter $p$, which characterizes the interaction of primary electrons (PEs) in thin films. The transmission secondary electron yield was used to compare the energy transfer of PEs in films with different thicknesses.

physics.ins-det↗

arXiv2020

Measurement of the transmission secondary electron yield of nanometer-thick films in a prototype Timed Photon Counter

We measure the transmission secondary electron yield of nanometer-thick Al$_2$O$_3$/TiN/Al$_2$O$_3$ films using a prototype version of a Timed Photon Counter (TiPC). We discuss the method to measure the yield extensively. The yield is then measured as a function of landing energy between $1.2$ and $1.8$ keV and found to be in the range of $0.1$ ($1.2$ keV) to $0.9$ ($1.8$ keV). These results are in agreement to data obtained by a different, independent method. We therefore conclude that the prototype TiPC is able to characterise the thin films in terms of transmission secondary electron yield. Additionally, observed features which are unrelated to the yield determination are interpreted.

physics.ins-det↗