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Fraunhofer IZM and the University of Münster have developed a wafer-based manufacturing process for a novel silicon diode that can measure particles depending on their angle of incidence. The energy-filter diode is intended to help distinguish signal electrons from background electrons that strike a detector with nearly the same energy but at different angles. One potential application is the international KATRIN experiment, which aims to determine the neutrino mass. In the cleanroom at Fraunhofer IZM, deep honeycomb-shaped microstructures were introduced into large-area detector wafers, electrically passivated, and transferred to the University of Münster for further investigation.
The KATRIN experiment examines the electron spectrum of tritium beta decay close to its endpoint energy of 18.57 kiloelectronvolts. This can provide information about the mass of the neutrino. The measurement is complicated by certain background electrons arriving at the detector with almost the same kinetic energy as the sought-after signal electrons. One key difference, however, is their angle of incidence: A large proportion of the background electrons reach the detector at angles typically below ten degrees, whereas signal electrons can have angles of up to around 50 degrees.
Conventional detectors do not provide suitable angular information at low particle energies, in strong magnetic fields, and under ultrahigh-vacuum conditions. This is where the active transverse energy filter comes in. Deep hexagonal recesses are etched into the surface of a Si-PIN diode. Electrons arriving at a larger angle are more likely to strike the active sidewalls of the honeycomb structure and generate a measurable signal. Electrons arriving at small angles, by contrast, predominantly reach the inactive bottom of the structure.
Previous studies using small-scale prototypes at the University of Münster have already demonstrated angle-dependent detection efficiency. Simulations also indicate that a structure optimized for the KATRIN experiment could reduce the electron background by a factor of three to five. This estimate depends on the composition of the background.
Fraunhofer IZM transfers the concept to wafer scale
The central task of the Fraunhofer Institute for Reliability and Microintegration IZM was to translate the physical principle into a reproducible, wafer-compatible manufacturing process. The starting point was large-area Si-PIN detector wafers with an unusual 24-sided geometry. As this special format could not be processed directly with all equipment in the 200-millimeter cleanroom, the experts developed a temporary carrier concept: The detector wafers were bonded with wax to 200-millimeter silicon wafers and then structured together.
Because only two detector wafers were available, the complete process chain was first developed using specially manufactured test wafers. The steps included lithography, deep reactive ion etching, removal of process residues, debonding from the carrier wafer, and passivation of the resulting sidewalls. The process was transferred to the actual detectors only after the individual steps had been evaluated and optimized.
250-micrometer-deep honeycomb structures in silicon
A particular challenge was producing sidewalls that were nearly vertical and largely free of defects at a target depth of around 250 micrometers. For lithography, the researchers selected a positive photoresist that provided almost vertical sidewalls and sufficient resistance for the subsequent deep-etching process. To transfer the honeycomb-shaped mask structure into the silicon as precisely as possible, the special exposure conditions created by the elevated position of the detector wafer on the carrier also had to be taken into account.
For deep reactive ion etching, or DRIE, the team at Fraunhofer IZM developed a cyclic process combining passivation with anisotropic and isotropic etching. Five process parameters were systematically varied using a statistical design of experiments. These included process pressure, plasma power, gas flow, and wafer temperature. The results showed a significant correlation between wafer temperature and the formation of unwanted silicon structures known as “silicon grass.” By adjusting the process parameters, the team identified a process window for largely vertical sidewalls with minimized defects.
Passivation protects the sensitive sidewalls
After etching, the sidewalls were passivated with silicon dioxide. This insulating layer is intended to saturate surface states, limit charging effects, and reduce surface-related dark currents. Thermal oxidation was not an option because of the existing metallization and the limited thermal budget of the detector wafers. The team therefore used plasma-enhanced chemical vapor deposition (PECVD) at 175 degrees Celsius.
For deep structures, achieving a uniform coating on the upper, middle, and lower sidewall regions is particularly challenging. By adapting the PECVD process, the team improved the layer coverage. Measurements showed 161 nanometers at the upper sidewall, 80 nanometers in the middle, and 55 nanometers at the lower sidewall. The process was compatible with the already metallized Si-PIN diodes.
In several design stages, the researchers at Fraunhofer IZM produced the first detector wafers, which are now undergoing test measurements at the University of Münster.
Technological foundation for novel particle detectors
By developing the complete process chain, Fraunhofer IZM has created a technological foundation for three-dimensional, angle-sensitive silicon detectors. The work combines application-oriented nanofabrication with the requirements of experimental particle physics. Particularly valuable are not only the successfully established lithography, DRIE, and PECVD processes, but also the insights gained into the interplay between microgeometry, semiconductor layer structure, and signal measurement.
The 3D energy-filter diode is not yet a detector ready for use. However, the manufacturing technology developed and the findings from the first demonstrator provide a robust foundation for systematically refining the device architecture and experimentally exploring the potential of angle-dependent particle detection.
Lucas Herrmann I Fraunhofer Institute for Reliability and Microintegration IZM I Wafer Level System Integration I Phone +49 30 46403-652 I lucas.herrmann@izm.fraunhofer.de
https://www.izm.fraunhofer.de/en/news/katrin-experiment.html
Micrometer-scale honeycombs: Using an optimized deep-etching process, Fraunhofer-researchers produce ...
Copyright: © Fraunhofer IZM
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Micrometer-scale honeycombs: Using an optimized deep-etching process, Fraunhofer-researchers produce ...
Copyright: © Fraunhofer IZM
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