An active Transverse Energy Filter based on microstructured Si-PIN diodes with an angular-selective detection efficiency

TL;DR

The paper addresses background suppression in low-energy electron detection by introducing an active Transverse Energy Filter (aTEF) based on microstructured Si-PIN diodes to achieve angular-selective detection. The Si-aTEF concept leverages hexagonal microchannels with sensitive sidewalls, enabling preferential detection of high-angle electrons while suppressing low-angle background. Proof-of-principle measurements show angular-dependent signal rates consistent with simulations, but the microfabrication-induced defects degrade energy resolution and charge collection, highlighting the need for improved processing and passivation. If refined, the aTEF could provide significant background reduction in the KATRIN spectrometer and similar MAC-E-filter setups, with ongoing work addressing doping geometry and surface treatment to realize practical performance.

Abstract

Si-PIN detectors can be microstructured to achieve angular-selective particle detection capabilities, which we call active Transverse Energy Filter (aTEF). The microstructuring consists of a honeycomb structure of deep hexagonally-shaped holes with active silicon side walls, while the bottom of the holes is made insensitive to ionizing radiation. The motivation for this kind of detector arises from the need to distinguish background electrons from signal electrons in a spectrometer of MAC-E filter type. We have demonstrated the angular-dependent detection efficiency of self-fabricated aTEF prototypes in a test setup using an angular-selective photoelectron source to illuminate the detector from various incidence angles.

Figures (11)

• Figure 1: Angular distributions of signal (orange), "Rydberg" background electrons (red) and background electrons from autoionizing states (green) at the detector of the KATRIN experiment, simulated using input from PhD-Hinz and PhD-Trost. The simulation considers the energy spectrum of background electrons in highly excited Rydberg states of hydrogen (36 %) and oxygen atoms (64 %), ionized by blackbody radiation at room temperature. These Rydberg atoms are sputtered from the spectrometer walls due to $\upalpha$ decays of implanted $^{210}$Pb progenies from the $^{222}$Rn decay chain Fraenkle_2022. The Rydberg atom's energies from PhD-Trost are corrected for the surface binding energy. After ionization, the electron energy and direction are calculated with the Doppler shift of the Rydberg atoms taken into account.
• Figure 2: The Si-aTEF is a microstructured Si-PIN diode, in which the individual holes feature sensitive sidewalls and insensitive grounds. The detector's surface normal is shown as dashed line. Electrons with high incidence angle $\alpha_\text{2}$ relative to the surface normal (orange arrow) have a higher probability to hit a sidewall and thereby induce a detectable signal, while electrons with low incidence angle $\alpha_\text{1}$ (red arrow) are likely to be absorbed without signal creation.
• Figure 3: Top: Illustration of the simplification of the 3D-microstructured diode, here for the simplified example with microstructured strips in y-direction instead of hexagons, to a 2D case or two 1D cases with different charge densities. Bottom: Electric potential and width of depletion zone in a hypothetical 1-dimensional frontside-illuminated Si-PIN diode featuring a microstructure with $x_\text{aTEF} = 150µm$ and $\delta = 0.1$. The straight orange lines show the curves calculated using the Poisson equation for different $U_\text{bias}$, indicated by the maximum achieved potential. The blue-dashed lines show the same curves simulated using COMSOL. The orange-dotted line shows the potential at which a certain depletion-layer depth is reached. Overall good agreement between calculation and simulation for the 1-dimensional case is found.
• Figure 4: Simulated electric potential profile for a 2D Si-PIN diode which is microstructured in the center and flat at the rim, as illustrated on the right. The straight lines show the profiles along a channel in the microstructure, while the dashed lines indicate the profiles in the flat rim, which basically behaves as a standard PIN diode.
• Figure 5: Fabrication steps for microstructuring of aTEF samples. Photolithography and ICP-RIE etching are performed for all samples, while resist removal and passivation are optional steps.